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From www.bloodjournal.org by guest on December 22, 2014. For personal use only. REVIEW ARTICLE Pivotal Role of the B7:CD28 Pathway in Transplantation Tolerance and Tumor Immunity By Eva C. Guinan, John G. Gribben, Vicki A. Boussiotis, Gordon J. Freeman, and Lee M. Nadler T HE CLINICAL INTERFACE between immunology, hematology, and oncology has long been appreciated. Many conditions treated by the hematologist or oncologist have either an autoimmune or immunodeficient component to their pathophysiology that has led to the widespread adoption of immunosuppressive medications by hematologists, whereas oncologists have sought immunologic adjuvants that might enhance endogenous immunity to tumors. To date, these interventions have generally consisted of nonspecific modes of immunosuppression and immune stimulation. In addition to the limited efficacy of these interventions, toxicities secondary to their nonspecificity have also limited their overall success. Therefore, alternative strategies have been sought. Elucidation of the functional role of a rapidly increasing number of cell surface molecules has contributed greatly to the integration of immunology with clinical hematology and oncology. Nearly 200 cell surface antigens have been identified on cells of the immune and hematopoietic systems.' These antigens represent both lineage-restricted andmore widely distributed molecules involved in a variety of processes, including cellular recognition, adhesion, induction and maintenance of proliferation, cytokine secretion, effector function, and even cell death. Recognition of the functional attributes of these molecules has fostered novel attempts to manipulate the immune response. Although molecules involved in cellular adhesion and antigen-specific recognition have previously been evaluated as targets of therapeutic immunologic intervention, recent attention has focused on a subgroup of cell surface molecules termed costimulatory molecules.*" Costimulatory molecules do not initiate but rather enable the generation and amplification of antigenspecific T-cell responses and effector In this review, we highlight one specific costimulatory pathway termed B7:CD28.7"0 Since this ligandxeceptor pathway was discovered 4 years ago, a large body of evidence has accumulated suggesting that B7:CD28 interactions represent one of the critical junctures in determining immune reactivity versus In this review, we focus on preclinical models that suggest ways in which this pathway might be manipulated to induce either antigen-specific immunosuppression or immunostimulation. COSTIMULATORY MOLECULES IN T-CELL ACTIVATION AND FUNCTION Requisite signals for T-cellactivation and function. Once T cells exit the thymus competent to respond to specific antigen and enter the recirculating pool of cells in the peripheral blood and lymphoid tissues, they are capable of participation in both antigen-nonspecific and antigen-specific events.'3-" Antigen-specific T-cell activation requires interaction of the T cell with specialized antigen-presenting cells (~pc~).5,6,16-19 Depending on the microenvironment in which the immune response is initiated, distinct populations of cells Blood, Vol 84, No 10 (November 15), 1994: pp 3261-3282 serve as APCs. In the peripheral blood, for example, dendritic cells, activated B cells, and monocytes can present antigen, whereas, in skin, keratinocytes and Langherans cells serve this function. Because the major function of peripheral blood dendritic cells, activated B cells, and activated macrophages is to process and present antigen, such cells are termed professional A P C S . ' , ' ~ , ~ ~Other , ~ ~ , *cells ' (eg, endothelium) can also present antigen under certain condition^.**^*^ Three distinct stages of cell-cell interaction between APCs and antigen-specificT cells are required to induce an antigenspecific immune response. Figure 1 depicts the known cell interaction molecules that are responsible for progression from one stage to another. In a process termed adhesion, APCs and T cells randomly interact both in the circulation and in lymphoid tissues via cell surface ligands and their receptor^.*^^*^ These ligands and receptors, referred to as adhesion molecules, may be relatively lineage restricted (eg, LFA-3 on APCs and its receptor CD2 on T cells) or, alternatively, they may be bidirectional (eg, ICAM-1 on APCs can bind its receptor LFA-l on T cells and ICAM-1 on T cells can bind LFA-1 on APC).*6.27Progression to the next stage, termed recognition, occurs if the APCs can process, transport, and present sufficient quantity of the specific peptide antigen in the context of the major histocompatibility complex (MHC).'8,29-33 Antigen-MHC will then be recognized by the T cell via the T-cell receptor complex (TCR).30*33*34 Depending on the nature and source of the peptide antigen, endogenous peptides (eg, derived from intracellular proteins) are generally presented to T cells coupled to MHC class I (HLA-A, B, or C), whereas exogenous processed peptide antigens (eg, derived from circulating proteins) are generally presented coupled to MHC class I1 (HLA-DR, DP, or DQ) cell surface molecules.~8~*94~ Although there is a common TCWCD3 complex, specific associative recognition structures on T cells are necessary to interact with the APC class I or class I1 MHC. Antigens coupled to class I MHC molecules are recognized by TCWCD3 in the context of an associated CD8 molecule, whereas recognition of antigens coupled to class I1 requires CD4.4*,43This antigen-specific, MHC-restricted interaction initiates a number of complex signaling events and has been extensively reviewed by others.34.44-48 After ligation (ie, binding and cross-linking) of TCR by antigen-MHC, T cells are competent to respond to a number of potential second signals, termed costimula- From the Division of Hematologic Malignancies and Pediatric Oncology, Dana Farber Cancer Institute, and the Departments of Medicine and Pediatrics, Harvard Medical School, Boston, MA. Submitted June 23, 1994; accepted August 11, 1994. Address reprint requests to Eva C. Guinan, MD, Department of Pediatric Oncology, Dana-Farber Cancer Institute, 44 Binney St, Boston, MA 021 15. 0 1994 by The American Society of Hematology. 0006-4971/94/8410-0036$3.00/0 3261 From www.bloodjournal.org by guest on December 22, 2014. For personal use only. 3262 A-1 (CD GUINAN ET At - LFA-1 ( C D l l d l 8 ) CAM-l (CD54) ICAM-1 ( ~ ~ 5 4 ) VCAM LFA-3 (CD58) ICAM-1 (CD54 ) (CDll418) CD43 VIA-4 (CD49d/29 ) HLA-AWC Recognition HLA-DR/DP/DQ LFA-3 (CD58) CAM-1 (CD54) CD40 CD72 CD24 87-1 (CD80) 87-2 (CD86) n[-] Receptors 27 .; D2 LFA-1 (CD11418) CD40-L CD5 CD28 CD24 CD28 CTLA-4 CTLA-4 tion.3-6.8.49.50Costimulatory molecules provide T cells with additional signals that result in the initiation and enhancement of pr~liferation.~,~' Costimulation by some ligands results in cytokine production that can only be detected at the mRNA level, whereas other costimulatory ligands are capable ofinducing significantsecretionandevenaccumulation of ~ytokine?"'~ Figure 1 depicts a representative butnot exhaustive number of candidate costimulatory molecules expressedon the surfaceof @CS. Some costimulatory molecules appear to be expressed on all professional Apcs, whereas others appear to be more lineage restricted. Increasing evidence supports the notion that several molecules previously considered to be adhesion molecules are also capable of delivering costimulatory signals (eg, LFA-3, LFA-1, and 1CAh4-1)!~27~28~5742 The nature of costimnlatory signals and the potential therapeutic implications of their modulation form the basis of this review. Critical nature of the costimulatorysignal:two-signal model of T-cell activation (Fig 2). Signaling through the TCR is necessary but not sufficient to induce antigen-specific T-cell activation and cytokine secretion.63@' In both murine Fig 1. Antigen-specific interactions between APCs and T cells are mediated byat least three types of cell:cell interactions. A representative tiding of receptors and ligands involved in each of these steps is depicted. and human systems, antigen-specific TCR signaling induces a state of readiness in the T cell yet fails to induce proliferationand effector f ~ n c t i o n . ~ , 'This , ~ ~ ,first ~ signal, termed signal 1, is both antigen-specific and MHC-restricted. Signal 2, which is neither antigen-specific nor MHC-restricted, is necessary to induce cytokine secretion, cellular proliferation, and effector f u n ~ t i o n . ~The , ~ .critical ~ ~ nature of this second costimulatory signal was originally proposed by Bretscher andCohn?' In their two-signal model, later extended by Jenkins and S c h ~ a r t z , 6 ~both - ~ ' a TCR and a costimulatory signal are essential for T-cell clonal expansion, lymphokine secretion, and effector function. If signal 2 is not delivered, T cells enter a state of long-term unresponsiveness to specific antigen-termed One hallmark of this state is the inability of anergized T cells to secrete interleukin-2 (IL2), although they retain their capacity to proliferate in the The molecular nature presence of exogenous IL-2.2*6."3,62.66 of the second signal(s) necessary to induce cytokine secretion and cellular proliferation and thus prevent anergy eluded immunologists for nearly two decade^.',^' From www.bloodjournal.org by guest on December 22, 2014. For personal use only. 3263 PIVOTAL ROLE OF B7 Immune Response Receptor IL-2 A 1 Fig 2. Theupperhalfof the of figuredepictspresentation antigen by a professional APC to a T cell. Because signal 1 and rignal2 ara delivered,11-2 secretion and upregulation of and signalIL-2R occur. The ingthrough lower half of the figure depicts presentation of the same antigen by an APC lacking costimulatory IigandM. Because only signal 1 is delivered, significant 11-2 accumulation and upregulation and signaling throughIL-2R fail to occur, multing in the generation of antigm-specific anergy. Evidence that the B7 molecule expressed on APCs delivers signal 2 through the T-cell sulface molecule CD28. After delivery of signal 1, ligation of the CD28 molecule expressed on T cells results in the delivery of signal 2 and progression to T-cell proliferation, cytokine secretion, and effector funct i ~ n .CD28 ~ . ~ is a homodimeric glycoprotein member of the Ig supergene family of cell surface interaction molecule^^.^.^^ and has a single IgV domain (Fig 3).67 The expression of this molecule is unusual as it is limited to T lymphocytes and plasma CD28 is expressed on virtually all thymocytes that coexpress both CD4 and CD8 molecules and is also expressed on 80% of mature peripheral blood T ~ e l l s . ~In~ the , ~ *human, nearly 95% of CD4+ but only 50% of CD8+ peripheral blood T cells are CD28+.73Both antigenspecific signals via the TCR and nonspecific signals delivered by mitogens significantly upregulate the expression of CD28.8.68,74-76In humans, a very small subset of CD4+ and 50% of CD8+ T cells are CD28-. Most of these CD28- T cells express an antigen, CDllb, often found on myeloid ~ e l l s . The ~ ~ ,function ~~ of these cells is presently unknown, although some reports suggest that they have noncytotoxic, suppressor For nearly a decade, it has been known that the CD28 molecule is involved in the induction of T-cell proliferation and IL-2 secretion. Although cross-linking CD28 on resting T cells with anti-CD28 monoclonal antibody (MoAb) induced no apparent T-cell activation or change in function, cross-linking CD28 after either TCR signaling or nonspecific activation of T cells with mitogen resulted in markedly enhanced T-cell proliferation and cytokine secretion.6837"76*81-83 Cross-linking of the CD28 molecule also provided a costimulatory signal for T cells activated via the CD2 pathway.54*84-87 Neither the signal transduction pathway nor the transcriptional pathway are fully ~nderstood.~~~.~~.~~~'~' However, candidate mechanisms of CD28-mediated signaling have been recently reviewed and suggest an important role for phosphatidylinositol 3-kina~e.~."~ Functionally, CD28 signaling has been found to increase secretion of the cytokines IL-la, L-2, interferon-y, IL-3,granulocyte-macrophage colony-stimulating factor (GM-CSF), and lymphotoxin.3.8.51-53.91.95,104.105 This results from at least twoCD28mediated effects most thoroughly studied in the case of IL-2 in which prolongation of E - 2 mRNA half-life and upregulation of IL-2 transcription result in accumulation of considerable amounts of secreted IL-2.5L.74,90*9'.95.'" Indeed, T cells isolated from mice deficient in the CD28 molecule (so called CD28 knock-out mice) have decreased response to lectin mitogens and absent L - 2 secretion in response to lectin acti~ation.'~' In addition, they have abnormal T-cel1:Bcell interactions as shown by decreased Ig production and From www.bloodjournal.org by guest on December 22, 2014. For personal use only. 3264 GUINAN ET AL CD28 COOH COOH COOH Gene Location 2q33 - 2q34 87-2 I I COOH CTLA-4 l NH2 NH2 COOH Gene Location 2q33 87-1 Gene Location 2q33 B7-X 3 COOH Gene Location 3q13.3- 3q21 COOH Gene Location 3ql3.3 - 3q21 Fig 3. The top panel shows a schematic ofthe predictedstructure CD28 of and CTLAJ. Whereas CD28 is homodimeric, CTLA-4isdepicted in both homodimeric and monomeric configurations. The bottom panel shows the predicted structures of members of the B7 family. Several linesofevidencesuggest that additional CD28KTLA4 counter-receptors exist (B7-x). However, they are omitted becausethesegeneshaveyettobe cloned. abnormal Ig regulation. Despite these defects, the mice appear interaction resulting in costimulation (signal 2).ll3The prototo have relatively normal cellular immunity, including cytotoxic type CD28 counter-receptor expressed on APC, B7,was T-cell and delayed-type hypersensitivity responses, suggesting initially discovered on activated but not resting B cells and the existenceof redundant pathways providing important costiwas thought to be restricted to activated B cells."4."5 Molecular cloning of the original B7 molecule (now termed B7mulatory signals. Further exploration of this model system may yield a greater understanding of the role CD28 plays in both 1, CD80) showed that its gene product was a member of the normal development and immune challenge because the func- Ig supergene family with two Ig-like domains (IgV and IgC, Fig 3).116.117 The B7-l gene was subsequently localized to tional consequences of CD28-mediated signaling and subsechromosomal region 3q13.3-3q21.II8 Further investigation quent IL-2 secretion are likely to be quite complex. showed that the B7 molecule was not, in fact, B-cell reHowever, even without full delineation of these consestricted as it could be induced on monocytes by interferonquences, observations that the CD28 signaling pathway is not inhibited by cyclosporin suggested that reagents capable y.'I9 B7 has also been found on some human T-cell clones of modulating CD28 signaling might be of relevance in preand repetitively activated human T lymphocytes in which it vention of tissue rejection and graft-versus-host disease can be specifically induced by either IL-7 or anti-CD3 MoAb (GVHD).52.'05.'08I12 However, development of such potential cross-linking of the CD3/TCR c ~ m p l e x . ' ~ ~Considerable "~' additional confusion concerning the lineage restriction of the therapeutics awaited the discovery of the CD28 counterB7 molecule arose from the observation that two different receptor on APCs. Subsequent manipulation of this receptor:ligand pair permitted unequivocal demonstration that MoAbs [anti-B7 (133) and BB11 both reacted with the gene product of the B7gene."' Whereas anti-B7 (133) MoAb CD28 represented the T-cell component of the T-cel1:APC From www.bloodjournal.org by guest on December 22, 2014. For personal use only. PIVOTAL ROLE OF B7 3265 human systems suggested that more than one B7 molecule existed. First, anti-B7 (133) MoAb was not as effective as CTLA4-Ig in inhibiting costimulation provided by professional APCs.I5' Second, different MoAbs, thought to identify the same B7 antigen (as discussed above), did not demonstrate equivalent staining of target cells or tissues. For example, keratinocytes could be stained with BB1 but not with anti-B7 (133) MoAb.'2"'26 Further study showed that these BBl-positive keratinocytes didnot express B7-1mRNA, strongly suggesting that these MoAbs defined distinct molecules.Iz6 Finally, APCs isolated from B7-l-deficient (B7 knockout) mice remained capable of both providing a costimulatory signal, albeit somewhat reduced, and binding CD28 MoAbs or cells expressing B7 induced T-cell proliferCTLA4-Ig.153All these lines of evidence argued strongly for ation and cytokine secretion. Moreover, anti-CD28 Fab or additional members of the B7 family. Several investigators anti-B7 MoAb inhibited proliferation and IL-2 secretion inthen showed that murine andhuman APCs expressed duced by APCs or allogeneic cells in mixed leukocyte reactions.56.132,138.13YInterestingly, there is enormous phylogenetic CTLA4-Ig binding ligands that were not the original B7 m o l e c ~ l e .Within ~ ~ ~ -several ~ ~ ~ months, two laboratories had conservation of the binding domains of the molecules in this cloned an identical alternative B7.'4'*158*159 Freeman et a1159.'60 p a t h ~ a y . " ~For * ' ~example, mouse B7binds and signals termed the original B7 molecule B7-1 andthe new B7through CD28 on human T cells and human B7 both binds like molecule B7-2 (CD86). The B7-2 gene has also been to and signals through mouse CD28.1'7.'40.'4' localized to chromosomal region 3q13.3-3q21.161.162 Despite Complexity of the B7:CD28 familiesof cell surface molesharing the identical Ig supergene family structure with an cules. As the importance of this pathway to T-cell activaIg variable (IgV) and Ig constant (IgC) region (Fig 3) and tion was investigated, it readily became apparent that additional molecules were involved. On the T-cell side, Brunet sharing the ability to bind CD28 and CTLA-4, these molecules were not highly homologous at the DNA Level."' et all4'cloned a gene encoding a molecule highly homoloIncreasing evidence in murine and human systems suggous to CD28 that they termed CTLA-4. Sequence analysis gests that B7-1 and B7-2 may have distinct costimulatory predicted that the protein product would be a glycoprotein functions,158.159,163This hypothesis is supported by observamember of the Ig supergene family that, like CD28, would tions detailing differences in the temporal expression of these have a single IgV-like domain (Fig 3). Recent evidence sugmolecules after B-cell activation. In both mouseand human, gests that it may exist as a monomer or homodimer on the cell surface, but little, if any, evidence suggests that it can neither B7-1 nor B7-2 protein is expressed on unstimulated B cells, although low levels of B7-2, but not B7-l, mRNA form a heterodimer with CD28.'43CTLA-4 and CD28 appear to be a gene duplication because both were found very close are p r e ~ e n t . ' ~B7-2 ~ . ' ~appears ~ on the cell surface within 24 together on chromosome 2.'44-147 Linsley et all4*showed that hours of B-cell activation and B7-1 appears later.'4'."7.163 CTLA-4, like CD28, bound to B7 and that a soluble fusion Further discordance has been observed in unstimulated huprotein composed of the extracellular domain of CTLA-4 man monocytes in which B7-2 is constitutively expressed and the human Ig Cy1 chain, termed CTLACIg, blocked whereas B7-1 expression is induced after activation.'" B7-mediated costimulation. Surprisingly, B7 had a much Finally, we and others believe that the story is still not higher affinity for CTLA-4 than for CD28, leading some complete. We have reported thatBB1 MoAb appears to investigators to refer to CTLA-4 as the high-affinity receptor recognize a third CTLA-4 binding molecule, which is now for B7.I4'In contradistinction to CD28, neither CTLA-4 termed B7-3.157This molecule has not yet been molecularly mRNA nor cell surface protein could be detected in resting cloned and, therefore, its structure and function cannot be T cells, although both appeared after a c t i v a t i ~ n .CTLA~ ~ ~ ~ ' ~ ~addressed. Additional members of the B7 and CD28/CTLA4, therefore, appears to be coexpressed with CD28 on the 4 families may also exist. Because CTLA-4 does not appear surface of activated T cells. The functional role of CTLA-4 to provide the redundant costimulatory signal in the CD28has been difficult to clarify. The absence of IL-2 production deficientmouse, investigators are searching for either anin the CD28 knock-out mouse suggests that CTLA-4 neither other counter-receptor for B7 family members or even anfunctions equivalently to CD28 nor does it provide a CD28other distinct costimulatory pathway.Io7 like signal 2 equivalent. These findings are consistent with Model of B7:CD28-mediated costimulation and levels of the hypothesis that CTLA-4 does not appear to be a CD28potential immune intervention. Before addressing the poredundant pathway.lo7 Several investigators have reported tential pathophysiologic relevance of this pathway, we will that, although cross-linking human CTLA-4 does not provide present a hypothetical model (Fig 4) of the molecular interaca primary costimulatory signal, it does enhance the costimutions that occur at distinct stages of APC:T cell interaction, latory signal delivered byCD28.I5' Others suggest that including (I) adhesion and antigen presentation, (11) TCR CTLA-4 might provide a negative signal.'" Therefore, the signaling, (111) costimulation, and (IV) cellular proliferation relationship between CD28 and CTLA-4 in the regulation and function. of T-cell activation is still under active investigation. After random cellular adhesion mediated by adhesion liOn the APC side, several lines of evidence in murine and gands and receptors. APCs that have antigen associated with appeared to stain only professional APCs, BB1 MoAb also stained thymic epithelium, keratinocytes, and Langerhans These data provided support for the thesis that, in the human, anti-B7 (133) and BB1 MoAbs might define distinct, but antigenically cross-reactive, molecules. Linsley et found that cells transfected with CD28 could adhere to cells transfected with the B7 molecule and that binding could be blocked by either anti-CD28 or BB1 MoAbs, definitively showing that these molecules were a receptorligand pair. Evidence that B7 provided signal 2 via CD28 then accumulated rapidly in both human and murine systems,82.117.130-137 Here, ligation of CD28 by either anti- From www.bloodjournal.org by guest on December 22, 2014. For personal use only. 3266 GUINAN ET AL APC T Cell I. Adhesion and antigen presentation IL-2R mRNA 11. T cell receptor signaling IL-2 mRNA 111. Costimulation IV. Cellular proliferation and function their MHC are poised to trigger antigen-specific T cells via the TCR (stage I). Engagement of the TCR by antigen initiates a series of events (signal 1) that upregulates CD28 expression and induces IL-2 receptor (IL-2R) expression and IL-2 mRNA transcription (stage 11). Cells receiving signal 1 are capable of low levels of proliferation and, therefore, appear to secrete IL-2 in an autocrine but not a paracrine fashion. On the APC side, the signal(s) that induces human B7-2 expression are still under investigation. However, once CD28 is upregulated and B7-2 is expressed, costimulatory signaling through the CD28 molecule occurs (signal 2, stage 111). Within 24 hours of signal 2, the T cell begins to secrete cytokines (perhaps most importantly IL-2) in a paracrine fashion and to express surface CTLA-4. Over the next 24 hours, T-cell proliferation is markedly enhanced and B7-1 Fig 4. A hypothetical model of the various stagesof antigenspecific activation of T cells by APCs. In stage 1, adhesion receptodigand interactionsjoin APCs and T cells. Antigen (Ag) is digested, processed, and presented by APCs in the context of MHC. In stage II, the APC delivers signal lto the T cell through the TCR complex initiatingtranscription and expression of low levels of IL-2 and IL-2R. During stage 111, 67-2CD28-mediated costimulation delivers signal 2, thus s i g nificantly upregulating 11-2 production and IL-2Rexpression. Stage IV alludes to further APCT-cell contact-generatedinteractions, most of which are notwell understood, which further amplify or potentially downregulatethe T-cell proliferative and effector response. Additional CD28lCTLA-4 binding molecules lindicated as 67-x) may participate in these interactions. and other CD281CTLA-4 counter-receptors (B7-x) appear on the APC surface. The functional consequences of CTLA4, B7-1, and B7-x expression and their potential interactions are still to be elucidated. Regardless of the precise contribution(s) of each of these molecules, costimulated T cells proliferate and are functionally competent. Which precise signals direct the T cell toward amplification ofan immune response or drive the T cells toward memory are poorly understood. Albeit incomplete, Fig 5 depicts the known interactions between an APC and an antigen-specific T cell and proposes several stages at which intervention might result in inhibition of the immune response. Specifically, it is possible to block either adhesion, TCR signaling, or B7 family-mediated costimulation through CD28. Although each maneuver results From www.bloodjournal.org by guest on December 22, 2014. For personal use only. 3267 PIVOTAL ROLE OF B7 ~~ Outcome Site of Intervention APC T cell Antigen Recognition Proliferation Proliferation on rechallenge + Potential therapeutic reagents alCAM-1 a LFA- 1 a LFA-3 CD2-Ig 8ioc&Adhesion - + aTCR aHLA-DR/DP/DQ a HLA-A/B/C Bfock Signal 7 Bfock Signa/ 2 aB7-1 a 87-2 &D28 Fab CTLA4-Ig CD28-lg Soluble CTIA-4 Fig 5. Three sites of intervention in APC:T-cell interaction are depicted along with available reagents capable of blocking at each site. Reagents blocking either adhesion orTCR signaling (signal 1) prevent antigen recognition and T cells remain naive. In contrast, reagents blocking signal 2 do not affect antigen recognition but do prevent both the primary proliferative response and subsequent antigen-specific proliferation resulting in the induction of anergy. in inhibition of T-cell proliferation, the resulting capacity of T cells to respond to antigen on rechallenge differs significantly (Fig 5). Inhibition of one or more critical adhesion interactions, either by blocking the adhesion ligand or its receptor (stage I in Fig 4), completely abrogates the ability of T cells to receive a signal via their antigen or costimulatory receptor and, therefore, the immune response is totally inhibited. In fact, blockade of adhesion completely prevents the recognition of antigen. Moreover, once the adhesion blockade is removed, these T cells respond on rechallenge asif they had never before encountered the antigen, ie, they behave like naive T cells mounting a primary immune response. Similarly, if adhesion is intact but TCR signaling is prevented (stage I1of Fig 4), no antigen recognition or proliferation occurs and removal of TCR signaling blockade results in a primary response after rechallenge with antigen. Thus, the functional outcome of blockade of adhesion or TCR recognition is immunosuppression. If these T cells are withdrawn from such inhibitory conditions, they are again capable of responding to the initial specific antigen. However, if the blockade is at the level of the B7:CD28 costimulatory pathway, then the outcome is quite different. After effective adhesion and TCR signaling, T cells must receive a B7 family-mediated costimulatory signal (signal 2) to pro- liferate and become functional (stages I11 and IV of Fig 4). Blockade of CD28 or B7 family members is sufficient to inhibit signal 2, resulting in failure of both proliferation and IL-2 secretion (Fig 5 ) . T cells that do not receive a costimulatory signal via CD28 enter a state of long-term antigenspecific unresponsiveness and are unable to respond to rechallenge with the antigen. In summary, blockade of adhesion or TCR signaling results in immunosuppression, whereas blockade of the B7:CD28 pathway results in the induction of anergy. Because it might be advantageous to suppress or even anergize an immune response in a number of clinical settings, a panel of potentially clinically effective reagents have been developed that might inhibit to a greater or lesser extent the receptor:ligand interactions contributing to the above stages of T-cell activation. To date, the most effective agents include MoAbs and fusion proteins. As the signal transduction pathways become better elucidated, small molecules, analogous to cyclosporin, will certainly figure inthe modulation of these events. Figure 5 summarizes the available specific reagents and the stages of APC:T cell interaction that they are likely to interrupt. Extensive evaluation of in vitro and several murine preclinical models has been undertaken with each of these reagents. From www.bloodjournal.org by guest on December 22, 2014. For personal use only. 3268 INDUCTION OF TRANSPLANTATION TOLERANCE BY BLOCKADE OF THE B7:CD28 COSTIMULATORY PATHWAY Inan effort to decrease the morbidity and improve the results of transplantation of both solid organs and bone marrow, investigators have attempted to develop methods to induce a state of long-term antigen-specific unresponsiveness. This state has beentermedanergyin vitro and tolerance in v i ~ o . ~ ~ , ~Specifically, ,~""~ anergy reflects the inability of antigen-specific T cells stimulated through their antigen receptor to mount a secondary antigen-specific response on rechallenge. Tolerance is the identical response reflected in the inability of the intact host to mount an effective secondary antigen-specific response in vivo. Historically, experimental methods to induce anergy involved fixation or chemical modification of APCs in vitro to destroy the ability of APCs to provide signal 2.168.16y These techniques, although effective, were not readily translatable to the clinical arena. On the other hand, clinically available agents such as cyclosporin A and FK506 have been shown to be ineffective in blocking signal 2.1fls*109~11fl~'7fl Therefore, such reagents are capable only of inducing immune suppression but not long-lived antigen-specific tolerance. In this section, we will review preclinical murine and human studies suggesting that blockade of the B7:CD28 costimulatory pathway is sufficient to induce anergy in vitro and tolerance in vivo and propose potential therapeutic reagents and settings that might be appropriate for clinical experimentation. Induction of anergy in vitro. Compelling evidence shows that blockade of the B7:CD28 pathway is involved in the generation of anergy in both human and murine in vitro systems.S6.132.139.152.166 In several experimental settings, artificial APCs (nonhematopoietic cells transfected with MHC and B7 molecules) were able to function effectively as APCs and induce antigen-specific proliferative responses accompanied by IL-2 accumulation in both human T-cell clones and normal human T l y m p h o ~ y t e s . ~ ~ Incubation ,"~.'~~ of these same T cells with the identical specific antigen in the context of an artificial APCs bearing MHC but not B7 was undertaken to model delivery of signal 1 but not signal 2. T cells stimulated in this way did not secrete appreciable amounts of IL-2 and were induced into a state of anergy.","' Anergy could also be induced by blocking the capacity of B7 family members to signal through CD28.56,132,139 This blockade could be effected using inhibitory reagents including (1) combinations of individual blocking MoAbs to B7 family members; (2) CTLA4-Ig, the soluble high-affinity counter-receptor for B7 family members that also interrupts B7 binding to CD28; and (3) anti-CD28 Fab, which similarly prevents B7 binding to CD28. Importantly, anergized cells were capable both of proliferation and secretion of IL-2 after stimulation with mitogen and of proliferation in response to exogenous IL-2, again showing that the anergized cells were functionally intact and that their unresponsiveness was limited to specific antigen." Most importantly, the reciprocal experimental conditions, ie, simultaneous engagement of the TCR by antigen and either ligation of CD28 with MoAb or exposure to cells expressing the B7-1 molecule, averted the GUINAN ET AL induction of anergy.56.'32,'3y These studies provided the first evidence that CD28 costimulation was a signal critical to the induction of a full immune response and, importantly, that its absence during antigen-specific TCR signaling resulted in induction of the anergic state. In addition, these experiments showed that blockade of this pathway was able to induce anergy in previously sensitized T cells, ie, T-cell clones. These observations were surprising yet supportive of the notion that CD28-mediated costimulation isnot only essential for primary T-cell activation but also necessary for antigen-specific T-cell reactivation. Subsequent studies showed that B7 family-mediated costimulation was,in fact, uniquein its ability to prevent T cells from entering the anergic state. To determine the contributions of individual costimulatory molecules toprimary and secondary immune responses, humanmodel systems using transfection of specific alloantigens and costimulatory molecules were developed to study the presentation of isolated alloantigen in the presence of either B7- 1 or ICAM-1 .'(' Although alloantigen in the presence of either costimulatory molecule, B7-1or ICAM-I , induced equivalent levels of proliferation, several lines of evidence suggested that these costimulatory signaling pathways differed. First, B7-1 -mediated costimulation was cyclosporin resistant, whereas ICAM- 1-induced proliferation was abrogated by cyclosporin. Second, B7- 1 costimulation was accompanied by accumulation of IL-2in the culture supernatants, whereas IL-2 accumulation could not be detected after ICAM-I costimulation. Finally, and most importantly, subsequent antigen-specific rechallenge of the T cells resulted in a classic secondary response only when B7-I costimulation had been provided during primary stimulation. In contrast, when ICAM- 1 had delivered the initial proliferative costimulatory signal, T cells were anergic on rechallenge. The addition of third-party alloantigens, exogenous IL-2, or mitogens to the secondary culture resulted in proliferation, showing that the anergized cells were viableand capable of responding to several stimuli but not to specific antigen. Of note, the addition of exogenous IL-2 to primary cultures under conditions normally producing T-cell anergy resulted in development of an antigen-specific T-cell proliferative response on rechallenge. This finding suggested that IL-2 could functionally replace CD28-mediated costimulation, supporting the contention that CD28-mediated regulation of IL-2 secretion and perhaps IL-2R expression are pivotal in the prevention of the anergic state. These CD28-mediated IL-2-related effects may explain why blockade of other costimulatory and/or adhesion molecules appears to limit primary immune responses but only effective blockade or absence of B7:CD28 interaction induces antigen-specific anergy. Additional evidence that blockade of the B7:CD28 pathway during a primary alloantigen-specificsensitization limits responsiveness on rechallenge comes from the experiments of Tan et al,Is2 who studied human mixed-leukocyte responses (MLR) between fully HLA-disparate individuals. The presence of either anti-CD28 Fab or CTLA4-Ig in the primary MLR significantly inhibited response to specific alloantigen on rechallenge whereas responsiveness to third party cells was not modified. Unlike results observed above From www.bloodjournal.org by guest on December 22, 2014. For personal use only. PIVOTAL ROLE OF B7 using alloreactive T-cell clones, interruption of the B7:CD28 pathway did not result in anergy, although a 50% to 85% inhibition of secondary proliferation was observed. Generation of cytolytic T-cell precursors was also inhibited. Of note, anti-B7-l MoAb was less effective (30% inhibition of proliferation). In this system, IL-2 (as assayed by presence of IL-2 mRNA) was completely suppressed, although IL-4 mRNA was detectable. As in the above experiments, exogenous IL-2 induced proliferation of the hyporesponsive T cells. Moreover, the addition of IL-2 to the primary sensitization prevented alloantigen hyporesponsiveness on rechallenge. Gribben et all" have confirmed the above observations. Furthermore, they have demonstrated that the addition of CTLACIg results in substantial decreases in the frequency of precursor helper T cells generated during MLR between both matched and mismatched family donor:recipient pairs. These and the above results provide a rationale for attempting to inhibit the B7:CD28 pathway in vitro or in vivo to prevent allograft rejection or GVHD. Induction of tolerance. In fact, several in vivo organ transplant models support the hypothesis that blockade of the B7:CD28 pathway will induce a long-term state of donorspecific tolerance. In one model of xenogeneic pancreatic islet cell transplantation, mice were treated with streptozotocin to eradicate their pancreatic islet /?-cell f~nction.'~' Human pancreatic cells were then transplanted under the kidney capsule. If the transplant was successful, islet cells would produce sufficient insulin to reverse hyperglycemia. In an attempt to prolong the survival or prevent the rejection of human pancreatic islet cells, mice were treated with either intravenous human CTLACIg (which binds to both human and mouse B7) or control Ig fusion protein from the time of transplant through the ensuing 14 days. CTLACIg treatment resulted in normal functioning of the transplanted islet cells, and biopsies of the islet xenografts showed intact islets without lymphocytic infiltration. In contrast, control animals had a mean graft survival of only 6 days and islet tissue showed significant lymphocytic infiltration and islet destruction. Donor-specific tolerance was demonstrated by nephrectomy of the graft-bearing kidney and transplantation (without CTLA4-Ig) of pancreatic islet cells from either the original xenograft donor or an unrelated donor under the remaining kidney capsule. Xenogeneic islets from the original donor survived and functioned, whereas islets from the unrelated donor were rejected by day 5. Thus, CTLACIg treatment, without the need for any additional immune manipulation, resulted in long-term, donor-specific tolerance to xenoantigen and formally demonstrated that tolerance could result from in vivo blockade of the B7:CD28 pathway. In a second model, CTLA4-Ig was used in an attempt to induce tolerance in a fully mismatched rat cardiac allograft Preliminary experiments showed nearly complete suppression of a primary MLR by human CTLACIg. In contrast to the pancreatic islet cell xenografts, the cardiac allografts were ultimately rejected, although infusion of CTLACIg resulted in prolongation of graft survival.'74The failure of CTLA4-Ig to induce tolerance to alloantigen could be interpreted in a number of ways, including (1) CTLA4Ig was immunosuppressive butdidnot induce anergy, (2) 3269 anergy was induced and then reversed, or (3) the model system did not permit effective tolerization of resident host T cells because of inadequate presentation of alloantigen, particularly because cardiac tissue isunlikelyto function well as an APC. This last hypothesis was tested by modification of the experimental design to include donor splenocyte infusions at the time of cardiac transplantation followed by CTLACIg treatment 2 days later.'73This design represented an attempt to assure that all, or nearly all, host T cells had an opportunity to efficiently "see" donor alloantigen (receive signal 1) by increasing the number of available APCs expressing relevant donor alloantigens. Delivery of signal 2 would then be blocked by infusion of CTLACIg. This maneuver resulted in long-term viability of the cardiac allograft, whereas treatment with either CTLA4-Ig alone or donor splenocytes alone produced graft prolongation but not longterm graft survival. Only mononuclear cells syngeneic to the cardiac allograft were effective in preventing rejection; thirdparty mononuclear cells provided no protective effect. These studies provide compelling evidence that, under optimal circumstances, blockade of the B7:CD28 pathway can lead to antigen-specific tolerance facilitating long-term allograft survival without the need for additional immunosuppression. Early results of experiments investigating the effect of CTLACIg in a murine bonemarrow transplant (BMT) model suggest that blockade of this pathway will have some prophylactic efficacy with respect to GVHD.'75BMT of donors and recipients completely mismatched at both class I and class I1 MHC was undertaken with or without in vivo administration of CTLACIg. Hematopoietic reconstitution was unaffected and CTLACIg treatment consistently reduced the incidence of lethal GVHD in recipients, although most animals had evidence of subclinical disease. These studies suggest that CTLACIg can be well-tolerated in the BMT setting and that its use should be further explored to establish the most efficacious treatment regimen. Furthermore, these observations should be extended to models with less MHC disparity, thus better approximating siblingmatched and family-mismatched BMT. Reversal of anergy. The potential therapeutic benefits of inducing alloantigen specific tolerance in the settings of BM and solid organ transplantation are dependent on the longterm stability of the anergic state. In vitro studies show that anergy to specific antigen can be reversed in murine T-cell clones when anergized clones are cultured for prolonged periods in IL-2 and then exposed tothe original specific antigen presented by professional APCs.17' In humans, similar data in an alloantigen-specific clonal T-cell model bolsters the observation that prolonged exposure to IL-2 may initiate or facilitate reversal of anergy.'77In fact, organ rejection and either onset or recurrence of GVHD are frequently preceded by infections that undoubtedly induce nonspecific inflammation and cytokine re1ea~e.l~'Thus, nonspecific events such as intercurrent infections and, potentially, antigen-specific events such as activation of IL-2 secretion through alternative pathways may both pose formidable obstacles to maintenance of the anergic state. However, even if it proved impossible to maintain the anergic state, it is to be hoped that induction of even a limited period of tolerance From www.bloodjournal.org by guest on December 22, 2014. For personal use only. 3270 would allow naive T cells produced during the quiescent state to be "educated" to see either the allogeneic graft or host as ~ e 1 f . I ~ ~ POTENTIAL ROLEFORB7 FAMILYMEMBERSINTHE INDUCTION OF TUMOR-SPECIFIC IMMUNITY Potential capacity of human tumors to present antigen. Although immunologists have identified, and in certain circumstances expanded, small numbers of antigen-specific T cells capable of inducing cytolysis of human neoplastic cells, there is little evidence that these cells are important for autologous tumor surveillance or tumor r e j e c t i ~ n . ' ~It~is" ~un~ clear why we fail to mount an antigen-specific T-cell-mediated immune response against malignant cells and, if one is mounted, why it fails. Of course, the hypothesis that a host can generate a significant cytotoxic T-cell response directed against its autologous neoplasm presumes the existence of sufficient numbers of antigen-specific helper and cytotoxic T-cell precursor^.'^'.'^^ For the purposes of this review, we will presuppose sufficient numbers of T-cell precursors and, therefore, we will concentrate our discussion on the multiple theoretical reasons why neoplasms may not effectively elicit T-cell-mediated tumor-specific immunity. In keeping with this focus, we will discuss the generation of effective antitumor immunity from the vantage point of the potential capacity of the tumor cell to present antigen and emphasize experiments examining the role of costimulation in the generation of tumor-specific tolerance or cytotoxicity (Fig 6). In the specific case of leukemias and lymphomas, malignant cells are often the neoplastic counterparts of professional APCs, suggesting that these cells ought to have the repertoire necessary to effectively present endogenous tumor antigens to T cells.l14.116.180.187-193However, the failure of any given tumor to elicit an adequate, tumor-specific T-cell response might result from (1) absent or inadequate expression of adhesion molecules; (2) inability to effectively present antigen consequent to absence of a tumor antigen, lack of immunogenicity of the antigen, andor inadequate processing or transport of antigen; (3) absent or limiting cell surface MHC; and (4) absent or limiting cell surface costimulatory molecules, specifically B7. These defects would result in discrete and different sequelae and might be amenable to a variety of therapeutic manipulations, depending on the site of the d e f e ~ t . ' ~ ~ " Potential ~ ~ . ' ~ ~defects . ' ~ ~ in antigen presentation by neoplastic cells are depicted in Fig 6A. If tumor cells lack sufficient numbers of adhesion receptors, MHC molecules, or antigen, then no signal 1 is generated. Therefore, no T-cell-mediated immune response is induced. In contrast, if both adhesion and signal 1 delivery mechanisms are intact, absence of signal 2 results in tumor-specific tolerance. In this section, wewill summarize both theoretical constraints on the ability of neoplastic cells to elicit adequate T-cell-mediated antitumor activity and in vitro and in vivo murine experiments that begin to elucidate the conditions under which either naive or tumor-bearing hosts can be induced to recognize and kill syngeneic tumor. Little is known about the expression of adhesion ligands and receptors by solid tumor cells. In contrast, several hematologic malignancies have been extensively studied for their GUINAN ET AL expression of adhesion molec~les.'~l Different histologic subtypes of non-Hodgkin's lymphomas (NHLs) appear to have relatively characteristic and distinct profiles of adhesion receptor surface expression. Not infrequently, neoplastic B cells lack one or more of the adhesion receptors commonly expressed on either normal pre-B or mature B cells; the extreme example is Burkitt's lymphoma, which has no or limited expression of L-selectin, CD44, LFA-I, ICAM- 1, or LFA-3.I9' Of the other components necessary to facilitate antigen presentation, the expression of class I and class I1 antigens by tumor cells has been most systematically examined. 195.196 A significant proportion of epithelial tumors show complete or partial loss of class I expression in comparison to the tissues from which they a r o ~ e . ' ~This ~ . ' is ~ ~less well studied in hematologic malignancies, although approximately half of B-cell NHLs appear to lack cell surface class *.192,193.19s Class I1 expression, in contrast, may be more frequent in epithelial tumors than in their normal counterparts.'" Hematologic malignancies vary greatly in their expression of class I1 (as ascertained by immunophenotyping)~1Y0.192,193The vast majority of B-cell malignancies are class I1 positive, as are normal B cells, whereas the majority of T-cell malignancies are Expression of class I1 in acute myeloid malignancies is highly variable (eg, more common in French-American-British [FAB] M 1 and M2 and uncommon in M3 and M4) and poorly predictable.'" Like their normal cellular counterparts, neoplastic cells can upregulate class I and class I1 after exposure to interferons.'"-"" The most controversial aspect of the potential ability of malignant cells to generate antigen-specific immunity is the very existence and detection of tumor-specific antigenS.183-IK6.2M).201 Tumor antigens have been functionally identified in experimental systems in which naive mice are inoculated (immunized) with irradiated killed ortumor ce~~s.183,18S,186,194.202 The same mice are subsequently inoculated (rechallenged) with unmanipulated tumor cells. Tumor growth is then assessed. Tumors that are rejected or are limited in their growth in comparison to their growth in unimmunized control animals are defined as immunogenic, whereas those that growequivalently in immunized and nonimmunized hosts are defined as nonimmunogenic. In these models, generation of appreciable antitumor activity against so-called immunogenic tumors generally requires repetitive courses of immunization. This suggests that, although the tumor cell itself may be incapable of functioning efficiently as an APC, shedtumor antigens, if present in sufficient quantity, may be taken up, processed, and presented by normal host professional APCs with subsequent generation of an effective, tumor antigen-specific immune response. The tumor antigens themselves have generally not been identified in these experiments. Until recently, the existence of human solid tumor-specific antigens wasnotwell substantiated,18%186The paradigm of tumor-specific antigens in humans evolved from production of anti-idiotype antibodies directed against idiotypic determinants expressed on clonotypic B- and T-cell surface antigen receptors in hematologic malignancies.2""2MHowever, recent recognition that intracellular proteins can be presented as peptides by class I MHC molecules suggests a number of alternative sources of tumor From www.bloodjournal.org by guest on December 22, 2014. For personal use only. 321 1 PIVOTAL ROLE OF B7 A) Tumor Cells as APC: Common defects ~~ T Cell Response Description Fig 6. This figure depicts the potential defects that might limit the ability of neoplastic cells t o function as APCs. It also suggests the functional consequences of tumor-cell:T-cell interactionswith reapect t o potential of each tumor cell as depicted to elicit an antigen-speciiic T-cell response. In (A), failure of antigen presentation or absence of necessary cell surface structures such as MHC and B7 leads to failure of antigen recog nition; thaefore, noT-cellresponse to antigen is evoked. In contrast, only the absence ofB7mediated costitnulation is predicted to result in tumor-specific anergy. (B) Illustrates potential mechanisms by which tumor cells may be modified to improve their capacity to serve as APCt. These modifications include modulation of both tumor cell surface and function by cytokine treatment,transfection, or cell fusion.Finally, (C)illustrates the alternative strategy of using professional APCs to present tumor antigens. Antigen Recognition Proliferation Cytolytic Function Anergy - B) Tumor Cells as APC: Approaches to repair defects antigens (Table 1).30.36,41,l~3.lSS.186 The ability of various tumor types to process, transport, and present such antigens in the context of MHC is presently under study; therefore, whether one or more of these peptide antigens is immunogenic in vivo is still to be determined. Finally, the ability of neoplastic cells to provide requisite costimulatory signals will determine whether a tumor-specific immune response is generated or, alternatively, whether tumor antigen-specific anergy is induced. Few, if any, solid tumors constitutively express either the B7 family of costimulatory molecules forming the focus of this review or + + + + + + + + + - other costimulatory molecules such as LFA-3 and CD40. In contrast, by immunohistochemistry, B7can be found on manybutnot all B-cell NHLs and on Reed-Stemberg cells in Hodgkin’s disease tissue sections and cell ~ines.l16,128,187,1S8,190,191T-cell NHLs and circulating B-cell tumors rarely appear to express measurable amounts of B7.1l4.l16.128 Potential in vivo induction of B7 expression on neoplasms secondary to systemic inflammatory states or local lymphocyte infiltration has not been studied. Generation of antitumor immunity in vivo. Inlight of these potential deficits in tumor cells as APCs, a number of From www.bloodjournal.org by guest on December 22, 2014. For personal use only. 3272 GUINAN ET AL Table 1. Examples of Tumor Antigens That Are Capable of Eliciting T-cell Responses ~ Where Potential Example Tumor Antigen Activated oncogene products Position Mutated Rearranged Tumor suppressor gene products Reactivated embryonic gene products Viral gene products ldiotypic and epitopes lg Malignancies Human Present in mutation 12 point of ~ 2 1 ' ~ ' bcr-ab1 p53 mutations MAGEl Human papilloma virus antigens (E6 and E7) Epstein-Barr virus EBNA-1 gene products TCR hypervariable regions Surface 10% tumors, including sarcomas, carcinomas, leukemias, multiple myeloma CML, ALL 50% tumors, inc. breast, myelodysplasia 50% melanomas, 25% breast cancers Most cervical carcinomas Hodgkin's, nasopharyngeal carcinoma lg' and B-cell TCR' lymphoid T-cell malignancies Table adapted from Pard~ll''~ and other review^.'^^^'^^^'^^ Abbreviations: EBNA, Epstein-Barr virus nuclear antigen; ALL, acute lymphoblastic leukemia. approaches have been developed to "repair" these defects (Fig 6Band C). If the defects can be repaired, then the outcome should be an effective T-cell-mediated immune response with appropriate antigen recognition, T-cell proliferation, and induction of cytolytic effector T-cell function. As illustrated in Fig 6B, a number of cytokines are capable of upregulating surface expression of adhesion, MHC, and even costimulatory molecules on both normal and malignant ce~~s~22.24.104.197.198For example, progenitor cells from patients with chronic myelogenous leukemia (CML) express low to absent levels of the adhesion molecule LFA-3 Treatment with interferon-a either in vitro or in vivo increases expression of LFA-3 to normal or near-normal levels and, interestingly, results in partial correction of the failure of progenitor cells from patients with CML to generate an autologous T-cell proliferative response. Interferon-y upregulates the expression of class I and I1 MHC as well as some adhesion and even costimulatory molecules. In vitro treatment with interferon-y results inmarked improvement in the ability of many cell types to serve as APCs and has also been shown to decrease tumorigenicity of transformed or malignant cell Transfection of MHC molecules into neoplastic cells provides another potential mechanism enabling tumors to better elicit T-cell responses and present themselves as targets for effector cytotoxic cells. Although it is not completely clear why some tumors appear to present antigen in the context of class I MHC while others present antigen in the context of class 11, there is experimental evidence that absence of MHC antigens may limit the ability of the cell to function as an effective APC for tumor antigens.~95.~96,~99.zn4 For example, the murine Sa1 sarcoma does not express class I1 MHC or B7.2"7 Transfection of class I1 not only provides the MHC molecules necessary to present antigen but also provides a signaling pathway that results in expression of B7 after interaction of the transfected tumor with antigen-specificT cells. Therefore, naive animals injected with tumor that has been transfected with syngeneic class I1 MHC reject the modified tumor while the unmodified tumor grows and is fatal. Immunization of naive animals with the modified tumor also provides protection against subsequent challenge with the original class II-deficient sarcoma, whereas immunization with irradiated, but unmanipulated, tumor does not provide a protective effect. Similar results are seen after transfection of syngeneic class I into a class I-negative melanoma.Z02~'"R~Zw Although little is currently known about the expression of B7 family members on tumors other than B-cell neoplasias, several animal models support the relevance of the B7:CD28 pathway in the generation of a tumor-specific immune response. Murine melanoma or sarcoma lines that did not express B7 were identified and transfected with the B7 gene and expression of B7 at the cell surface was demonstrated. In several different murine models, inoculation of animals with the transfected, B7-positive tumor cells resulted in antigen-specific, MHC-restricted tumor cell rejection in naive hosts in contrast to fatal tumor growth in control animals receiving unmodified (B7-negative) cells.202~207~2'o These models also provide support for the two signal hypothesis with respect to generation of an adequate immune response. In some cases, CD8' T cells mediated tumor cell rejection, whereas CD4' T cells did so in others. In the CD4+ T-cell-mediated model, transfection of B7 into class I1 MHC-negative tumor cells did not enhance the ability of the tumor cell to elicit an immune response and, consequently, tumor growth was notim~aired."~ However, rendering tumor cells both class I1 MHC and B7-positive in the same model resulted in ability to induce a vigorous antitumor effect (as in Fig 6B). Therefore, if tumor cells are to function as APCs and generate an efficient tumor-specific T-cell response, they need to be able to deliver both signal 1 and 2. Further support for this notion is found in similar experiments in which tumors (derived from 2 T-cell NHLs, a mastocytoma and a melanoma) were also rejected after transfectionwithB7 although other tumors (3 sarcomas and a melanoma) continued to grow aggressively even after B7 transfection and cell surface expre~sion.~'~ Although all 8 of the unmodified tumors grew in naive hosts, only the tumors with evidence of immunogenicity before B7 transfection (as assessed in the immunization and rechallenge experiments described above) were capable of eliciting an antigen-specific immune response after being rendered B7 cell surface positive. Again, the most straightforward explanation of these results is that the immunogenic tumors were capable of providing signal 1 to T cells. Transfection with B7 rendered them capable of providing signal 2, thus enabling the tumor cell to function as an APC competent to generate a From www.bloodjournal.org by guest on December 22, 2014. For personal use only. PIVOTAL ROLE OF B7 full, antigen-specific immune response (Fig 6B). That B7 expression did not enhance the ability of the nonimmunogenic tumors to elicit a T-cell response, on the other hand, might be presumed to reflect defects in providing signal 1. Interestingly, in these latter experiments (in which the antitumor effect appeared to be mediated by CD8+ T cells), 3 of 4 nonimmunogenic tumors studied lacked class I cell surface antigens, suggesting at least one reason for failure of the presentatiodrecognition step. Other mechanisms, such as lack of recognizable tumor antigens or deficits in processing and transportation of antigens, might also be invoked to explain the failure of these tumor cells to stimulate an immune response despite B7 transfection. On the whole, therefore, these B7 transfection experiments provide significant support for the hypotheses that (1) tumor cells may be limited in their capacity to function as APCs; ( 2 ) such failure may lead not only to abortive immune responses but also to tumor antigen-specific tolerance; and (3) induction of appropriate cell surface molecules, including the costimulatory ligand B7, on tumor cells may lead to antigen-specific immune responses (tumor rejection) in naive hosts. Other experimental designs have also shown the ability of B7 transfection to alter the capacity of neoplastic cells to elicit protective immunity against tumors. In some experiments, naive animals were inoculated (ie, immunized) with B7-transfected tumor ce11s.202~207~209 This inoculation led not only to primary rejection of the modified tumor cells, as in the experiments described above, but also to a significant degree of protection against subsequent inoculation with unmodified, B7-negative tumor cells. In these systems, the induction of systemic immunity could be shown to be antigen specific and was not observed when the transfected tumor cells were derived from nonimmunogenic tumors.209Equivalent schedules of immunization with equal numbers of unmodified killed or irradiated tumor cells afforded little protective effect. However, repetitive immunization with these untransfected but immunogenic tumor cells was able to afford considerable protection to subsequent challenge with nonirradiated tumor. Again, these experiments support the hypothesis that effective antigen-specific immunity requires delivery of both signal 1 and 2. Furthermore, they suggest that this goal may possibly also be accomplished, albeit less well, by presentation of shed or phagocytosed tumor antigens by professional, nonneoplastic APCs (Fig 6C). Because inoculation with killed or irradiated tumor may facilitate sufficient presentation of shed or phagocytosed tumor antigen by resident professional APCs, identification, isolation, and administration of common tumor-specific antigens may provide an alternative strategy to immunize the host. Yet another mechanism for generation of a specific tumorprotective response is suggested by the experiments of Guo et a1’” in which the requisite elements enabling a tumor cell to function as an APC were established by fusion of the tumor cell with purified activated syngeneic B cells (Fig 6B). In the reported experiments, the tumor cell, a hepatocarcinoma, expressed limited amounts of MHC class I and 11 and limited (ICAM-1) or absent (LFA-1 and B7) amounts of adhesion and costimulatory molecules. In contrast, these molecules were all highly expressed by both the activated 3273 B cells and the hybrid, fusion product of both cells. Inoculation of nude (T-cell-deficient and immunoincompetent) mice with the hybrid cells resulted in tumor formation showing the retained neoplastic potential of the fused cells. However, immunologically normal animals inoculated with the hepatoma-B-cell hybrid were able to reject the cells and remain tumor free. In addition, prior exposure of animals to the hybrid cells rendered them resistant to subsequent challenge withunmodified hepatoma, ie, the hybrid cells were able to immunize naive animals. In these experiments, both helper and cytotoxic T cells were essential for the generation of the antigen-specific antitumor cell response. Thus, rendering the neoplastic cell capable of delivering both signal l and signal 2 to antigen-specific T cells by either transfection of relevant molecules or cell fusion with professional APCs that express requisite molecules can result in generation of effective and tumor-specific immunity after primary immunization. A major limitation of all the above model systems is that modified tumor cells are injected into naive, but not tumorbearing, hosts. For clinical translational experimentation, the more relevant model would examine the ability of modified tumor cells to influence the growth of an established tumor or minimal residual disease. In a melanoma model in which tumor inoculation universally leads to disseminated metastatic disease and death, intravenous injection of B7transfected cells 4 days after inoculation of B7-negative tumors resulted in prolonged survival inall animals and long-term survival in nearly half.*” Similarly, in the hybrid B-cell-hepatoma model, either intrahepatic injection or intrahepatic implantation of tumor followed by subcutaneous injection of hybrid cells 10 days later was associated with 80% to 100%tumor-free survival.’” Although these studies are still far removed from treatment of patients with malignant disease, they provide proofof concept that in some tumor-bearing hosts itmaybe possible to induce tumorspecific immunity. Taken together, these experiments suggest that manipulation of host T-cell response to malignant cells to immunize against or treat tumors may be possible. If certain antigens were common to a sufficiently broad spectrum of malignancies, routine vaccination with either a tumor-APC hybrid or transfected tumor cell might be possible. Possibly, a multivalent vaccine comprising the most frequently occurring malignancies for a given age and sex cohort could be used. Alternatively, patients at identified high-risk for particular malignancies (eg, womenwith p53 mutations and family histories of breast cancer) could be selectively immunized. Efficacy of any given approach is predicated on the ability to overcome possible pre-existing T-cell tolerance to the established tumor. Inother words, in vivo therapy with tumor cells that have had their “deficiencies” as APCs remedied ex vivo by cell fusion or transfection may be efficacious but only if pre-existing tolerance does not exist or can be abrogated. In vivo therapy with cytokines capable of inducing neoplasms to express molecules important in immunogenicity may also be efficacious either independently or as adjunctive therapy. Theoretical approaches that might circumvent pre-existing tolerance in the tumor-bearing host From www.bloodjournal.org by guest on December 22, 2014. For personal use only. 3214 include ex vivo expansion of sorted, naive host T cells stimulated with tumor transfectants or hybridomas as well as administration of similarly expanded and targeted cells derived from an HLA-matched sibling or, potentially, alternative donor. In vivo therapy directed against mature, tolerant T cells followed by infusion of large numbers of immunogenic, manipulated tumor cells might also generate sufficient specific activity in emerging, naive cells. GUINAN ET AL the B7:CD28 pathway in vivo with either antimurine B7-2 MoAb or CTLA4-Ig can prevent the development of experimental allergic encephalitis, the murine model for multiple sclerosis."" In aggregate, these studies implicate the B7:CD28 pathway in the pathogenesis of autoimmune diseases and perhaps also those with a significant reactive component. They also provide reasonably compelling evidence that modulation of the pathway may have a beneficial effect. Although more speculative, inappropriate B7-mediated coPOTENTIAL FOR CLINICAL EXPERIMENTATION: stimulation may also play a role in the pathogenesis of autoA PERSPECTIVE immune cytopenias and even a subset of aplastic anemia.'?' Considering the well-established, albeit poorly predictable, Therapeutic reagents intended to modulate the B7:CD28 efficacy of anti-T-cell antisera and other immunosupprescostimulatory pathway have yet to enter the clinical arena. sive medications in treating these conditions, it would not be However, because several teams of investigators are conteinappropriate to undertake clinical trials to block B7:CD28mplating initiation of clinical trials, it is of more than acamediated costimulation. Certainly, a subgroup of patients demic interest to discuss which hematologic diseases might with aplastic anemia respond to antithymocyte globulin provide the promising clinical settings for such studies. and, in fact, some of these patients are cured by this apImmunosuppression. The greatest therapeutic potential p r o a ~ h .Antibodies ~ ~ ~ - ~ that ~ ~inhibit the B7:CD28 andpotenarising from modulation of the B7:CD28 pathway resides in tially other costimulatory pathways could be amongthe the induction of immunosuppression. Reagents that commajor effective reagents in these heteroantisera. Because pletely block the B7:CD28 pathway will likely be most effimodulation of this pathway has been shown to be effective cient in inducing significant immunosuppression (Fig 5). in both tolerizing previously sensitized cells and downreguVirtually all preclinical in vivo studies have used CTLA4-Ig, lating specific antibody production, modulators of this pathwhich has been extremely effective in blocking B7 familyway may also prove useful intreatment of antibody-mediated mediated costimulation.'72"7s Anti-CD28Fab MoAb also inhibitor states (eg, acquired coagulopathies) or even in treatcompletely blocks costimulation delivered by B7 family ment or prevention of transfusional allosensitization.''5 members and may have equivalent potential to be an effecAnother fertile area for investigation is allogeneic BMT tive immunosuppressive Blocking of onemore or in which graft failure, GVHD, treatment-related toxicity, opmembers of the B7 family with specific MoAbs may potenportunistic infection, and B-cell lymphoproliferative disease tially suppress B7-mediated costimulation, although precliniremain significant problems. Current attempts to increase the cal studies supporting this hypothesis have yet tobe reported. size of the donor pool by using mismatched family or unreFurther, administration of cytokines such as IL-10 that downlated BM donors serve to increase both the incidence and regulate B7 expression or, alternatively, blocking signaling severity of these complications.'2"23xIn these settings, inducby cytokines such as IL-7 or surface structures such as CD40 that upregulate B7 expression may also be u~eful."'~~''~''~tion of host tolerance to donor-specific alloantigens might facilitate engraftment and reduce the incidence of graft rejecHowever, entry level clinical studies will likely involve tion, whereas induction of donor tolerance to host-specific blockade of B7:CD28 using CTLACIg or, potentially, solualloantigens might provide a new, more specific approach to ble CTLA-4. treatment or prevention of GVHD.'7'.''9 Modulation of the For the general hematologist, the ability to induce antigenB7:CD28 pathway is most efficient in preventing immune specific immunosuppression would potentially facilitate the responses by both naive and previously sensitized cells and treatment of a broad range of disorders in which either autoless markedin its ability to turnoff established effector immune or alloimmune responses produce hematologic dysfunction. For example, the B7:CD28pathwayisinvolved function. Increasing evidence suggests that a number of autoin the generation of cytolysis but not in cytolytic effector immune diseases can be prevented or suppressed by functi~n.~~ This " ~ ~finding ~' suggests that it is unlikely that modulating the B7:CD28 p a t h ~ a y . ~ For ~ ~ -example, ~'' expersignificant clinical efficacy would be demonstrated. iments in transgenic who constitutively express susceptibilConsiderably greater potential rests in the prevention of ity genes for diabetes and the B7 gene in pancreatic islet GVHD. One strategy would consist of attempts to specificells suggest that inappropriate expression of B7-1 costimucally anergize alloreactive T cells in the donor BM to host lation may be involved in the pathogenesis of this disease.?" alloantigens. Tolerizing the mature donor T cell would in In a second transgenic murine model, only coexpression of effect reproduce the same functional phenotype as depletion high levels of both B7-1 and class I1 MHC antigens resulted of mature T cells from donor BM withrespect to the immediin autoimmune destruction of islet cells, whereas expression ate inability or limited capability of the graft to induce of either molecule alone was insufficient to overcome in vivo GVHD. However, T cells tolerant to specific alloantigens t o l e r a n ~ e .One ~ ' ~ might postulate that either the susceptibility would retain function against viral and other opportunistic genes or the increased expression of MHC result in presentainfections, whereas this benefit is lost with global T depletion of an antigen which can be seen by autologous T cells ti~n.'~'In addition, the increased incidence of both graft as foreign (signal l), whereas expression of B7-1 enables rejection or failure, relapse, and B-cell lymphoproliferative delivery of signal 2 and evocation ofan anti-islet T-cell disease observed after transplantation of T-cell-depleted response. Recent studies in mice also show that blockade of From www.bloodjournal.org by guest on December 22, 2014. For personal use only. PIVOTAL ROLE OF B7 3215 BM might wellbe mitigated.227-22y.232-2383N3 Advantages of geneity has obvious implications for the generation of an antigen-specific antitumor response. Whether morphologic, this approach in comparison to immunosuppressive therapies arise from the fact that immunosuppressive medications are, phenotypic, andor genetic heterogeneity impacts on the antiby definition, general in their effects, contributing to many genicity of a tumor is presently unknown.'" Additional conof the problems cited above. These drugs also have significerns about the applicability of the approaches discussed cant organ toxicities. Limitations to this approach include here arise from the observation that, although some hematologic malignancies are the neoplastic counterparts of APCs selection of appropriate host tissues that present the antigens and some express B7 family members, these tumors are not necessary to generate tolerance to the classic GVHD targets of skin, bowel, and liver.'78*23y A related concern is whether rejected. The resolution of this apparent paradox may lie in considering the evolution of the malignant clone. For examanergy induced in vitro will be long lived or reversed in vivo. However, even a limited period of tolerance might ple, Ig gene rearrangements and bcl-2 translocations can be found in the earliest pre-B cells at which stage both normal have significant advantages over currently available technoand neoplastic pre-B cells express class I and class I1 MHC logies and approaches. but not B7.188~190~193 Although the Reed-Sternberg cells of Another approach would involve the in vivo administration of reagents to block the B7:CD28 pathway. As was Hodgkin's disease are strongly B7 positive, the clonogenic cell in Hodgkin's disease is ~ n k n o w n . ' ~Therefore, ~ ~ ' ~ ~ in shown in the murine model of allogeneic cardiac transplantalymphohematopoietic malignancies, one could imagine that tion, intravenous administration of CTLA4-Ig can result in tumor-specific tolerance maybe generated very early by immunosuppression or even induction of tolerance to alloantigen.173.174 Importantly, in those studies, the timing of intratumors capable of delivering signal l butnot signal 2.244 venous CTLA4-Ig administration relative to the antigenic Attention to mechanisms by which antigen-specific tumorexposure and the mechanism (timing, dose, and site) by cell tolerance might be reversed will be critical to the clinical which the foreign antigens were introduced into the host efficacy of this approach. had significant impact on the success of the intervention. A long and unsuccessful history has accompanied attempts Preliminary studies in mice suggest that intravenous adminto develop tumor-cell vaccines to treat tumor-bearing hosts~~82,~84-186,2~ istration of CTLA4-Ig can significantly reduce but not elimiTumor cell vaccines have consisted of einate lethal GVHD in fully MHC-mismatched mice.'75As in ther modified whole tumor cells or alternatively extracts dethe cardiac transplant model, refinements in the experimental rived from tumor cells. We will not attempt to review this protocol may improve these results. Alternatively, a combiextensive literature here, but rather briefly propose examples nation of in vitro and in vivo therapy may be more effective of alternative strategies based on manipulation of costimulaor some combination of B7:CD28 blockade and inhibition tory molecules. The success of prophylactic vaccination deof effector cell function (eg, anticytokine antibody) may be pends on generation of sufficient numbers of long-lived cytonecessary. Additional concerns regarding maintenance of anlytic T cells to provide immunologic surveillance. ergy suggest that techniques resulting in clonal deletion of Unfortunately, it appears unlikely that mosthematologic madonor alloreactive T cells might provide an even more effeclignancies will be amenable to such an approach. Unlike tive and potentially less reversible approach. candidate tumor-associated antigens on human solid tumors Immunostimulation. Although modulation of the (Table l), fusion proteins from known translocations comB7:CD28 pathway provides a potential mechanism for inmon to hematologic malignancies are characterized by sufducing tumor-specific immunity, one can readily foresee a ficient heterogeneity that a common candidate tumor antigen number of obstacles to the implementation of this approach. has yet to be identified. If a candidate antigen were discovIn addition to the specific issues of tumor antigenicity deered, gene transfer strategies capitalizing on the function of tailed above, central concerns include ( l ) existence of a sufcostimulatory molecules might be implemented. Genes for ficient number of antigen-specific precursor T cells, (2) stothe tumor antigen as well as genes for adhesion and costimuchastic issues related to tumor cell burden, (3) the ability of latory molecules could be simultaneously transfected into specific class I and class I1 molecules to present each and selected host tissues. For example, vaccinia virus packages every peptide antigen, and (4) tumor-cell heterogeneity could be used to infect skin with both tumor antigen and within a given malignancy. With respect to the T cells, not B7. Alternatively, naked DNA encoding both tumor antigen only may there be an inherent limitation in the number of and costimulatory molecules could be injected into muscle cytotoxic and helper T-cell precursors capable of recognizing leading to expression of the encoded proteins by the "inand responding to a specific malignancy, but conventional fected" muscle cells. These approaches have been promising chemoradiotherapy may further limit the T-cell repertoire by in murine systems. Failure of any of these approaches might depleting both specific and nonspecific T-cell populations. be the result of any of the general concerns listed above as Of course, absolute tumor burden and growth fraction may well as failure of the immune system to maintain sufficient exceed even the most potent cytolytic T-cell antitumor relong-term memory. sponse. In addition, certain tumor peptide antigens may not For the tumor-bearing host, two treatment strategies will be efficiently presented to antigen-specific T-cell precursors be considered, although many variations are possible. The by the patient's unique MHC m o l e c ~ l e s ~ 'Therefore, ~ ' ~ ~ ~ ' ~ ~ first approach, termed adoptive transfer, would entail in vitro failure to generate a T-cell antitumor response might be isolation and expansion of tumor-specific cytolytic T lyminherent in the patient's genetic immunologic repertoire. p h o c y t e ~ . ' ~ ~Currently, " ~ ' . ~ ~ isolation, expansion and adminWith respect to the malignancy itself, tumor-cell heteroistration of T-cell tumor-infiltrating lymphocytes (TIL) is From www.bloodjournal.org by guest on December 22, 2014. For personal use only. 3276 GUINAN ET AL limited to tumors in which sufficient T-cell infiltrate exists such that isolation of cells is practical. This limitation is particularly pertinent for tumors of the lymphohematopoietic system. The proposed strategy differs in that it would not rely on expansion of T cells previously sensitized in vivo but would also attempt to generate and expand additional specific cytolytic T cells from antigen-specific precursors. Isolated, purified leukemia or lymphoma cells could be used as APCs to stimulate this population. Tumor cells could be modified as described in preclinical animal models (Fig 6B) to facilitate adhesion, antigen recognition, and costimulation and improve the capacity of the malignant cells to serve as immunogens. Alternatively, tumor-specific peptide antigen could be presented to T cells by professional APCs that might generate a much moreefficient and effective antitumor response (Fig 6C). The success of both approaches is dependent on the ability of T cells to respond to the antigen; therefore, the T-cell repertoire mustbe intact. If T cells have been tolerized to the tumor in vivo, tolerance must be reversed in vitro to enable induction and expansion of antitumor-specific T cells. Manipulation of the B7:CD28 a n d or other costimulatory pathways provides a potential avenue for reversal of anergy. SUMMARY The above story illustrates the translation of basic scientific discoveries to the clinic. In vitro and preclinical in vivo experimentation suggests that modulation of the B7:CD28 pathway will result in either amplification or suppression of the immune response. Considering the frequency with which diseases characterized by either inadequate or dysregulated immune function present to the practicing hematologist or oncologist, it is not difficult to envisage clinical applications for reagents that modulate this pathway. However, we still have much to learn about the function and clinical potential of this and other potentially redundant costimulatory pathways and therefore we suspect that this story will become considerably more complex over the next few years. REFERENCES 1. Schlossman SF, Boumsell L, Gilks JM, Harlan T, Kishimoto C, Morimoto C, Ritz J, Shaw S, Silverstein RL, Springer TA, Tedder TF, Todd RF: CD antigens 1993. Blood 835379, 1994 2. Bretscher P The two-signal model of lymphocyte activation twenty-one years later. Immunol Today 13:74, 1992 3 . Jenkins MK, Johnson JG: Molecules involved in T-cell costimdation. Curr Opin Immunol 5:361, 1993 4. Geppert T, Davis L, Gur H, Wacholtz M, Lipsky P: Accessory cell signals involved in T-cell activation. Immunol Rev 117:5, 1990 5. Weaver CT, Unanue ER: The costimulatory function of antigen-presenting cells. Immunol Today 11:49, 1990 6. Steinman RM, Young JW: Signals arising from antigen-presenting cells. Curr Opin Immunol 3:361, 1991 7. June CH, Bluestone JA, Nadler LM, Thompson CB: The B7 and CD28 receptor families. Immunol Today 15:321, 1994 8. June CH, Ledbetter JA, Linsley PS, Thompson CB: Role of the CD28 receptor in T-cell activation. Immunol Today 11:21 I, 1990 9. Linsley PS, Ledbetter JA: The role of the CD28 receptor during T cell responses to antigen. Annu Rev Immunol 11:191, 1993 IO. Schwartz RH: Costimulation of T lymphocytes: The role of CD28, CTLA-4, and B7BB 1 in interleukin-2 production and immunotherapy. Cell 71:1065, 1992 1 1. Cohen J: Mounting a targeted strike on unwanted immune responses (news; comment). Science 257:751, 1992 12. Cohen J: New protein steals the show as ‘costimulator’ of T cells (news; comment). Science 262:844, 1993 13. Haynes BF, Denning SM, Le PT, Singer KH: Human intrathymic T cell differentiation. Semin Immunol 2:67, 1990 14. Rothenberg E: The development of functionally responsive T cells. Adv Immunol 51:85, 1992 15. Sprent J, Lo D, Gao E, Ron Y: T-cell selection in the thymus. Immunol Rev 111:173, 1988 16. Steinman RM: The dendritic cell system and its role in immunogenicity. AnnuRev Immunol 9:271, 1991 17. Metlay J, Pure E, Steinman R: Control of the immune response at the level of antigen-presenting cells: A comparison of the function of dendritic cells and B lymphocytes. Adv Immunol47:45, 1989 18. Germain RN, Margulies DH: The biochemistry and cell biology of antigen processing and presentation. AnnuRev Immunol 1 I :403, 1993 19. Braciale T, Braciale V: Antigen presentation: Structural themes and functional variations. Immunol Today 12:124, 1991 20. Pierce S, Moms J, Grusby M, Kaumaya P, Buskirk A, Srinivasan M, Crump B, Smolenski L, Glimscher L: Antigen-presenting function of B lymphocytes. Immunol Rev 106:149, 1988 21. Hauser C, Katz S: Generation and characterization of T helper cells by primary in vitro sensitization using Langerhans cells. Immuno1Rev 117:67, 1990 22. Pober JS, Cotran RS: Immunologic interactions of T lymphocytes with vascular endothelium. Adv Immunol 50:261, 1991 23. Hughes C, Savage C, Pober J: The endothelial cell as a regulator of T-cell function. Immunol Rev 117:85, 1990 24. Shimizu Y, Newman W, Tanaka Y, Shaw S: Lymphocyte interactions with endothelial cells. Immunol Today 13:106, 1992 25. Hemler M: VLA proteins in the integrin family: Structures, functions, and their role on leukocytes. Annu Rev Immunol 8:365, 1990 26. Kishimoto T, Larson R, Corbi A, Dustin M, Staunton D, Springer T: The leukocyte integrins. Adv Immunol46: 149, 199I 27. Mackay CR, Imhof BA: Cell adhesion in the immune system. Immunol Today 14:99, 1993 28. Fougerolles AR, Springer TA: Intercellular adhesion molecule 3 , a third adhesion counter-receptor for lymphocyte functionassociated molecule 1 on resting lymphocytes. J Exp Med 175:185, 1992 29. Rothbard JB, Gefter ML: Interactions between immunogenic peptides and MHC proteins. Annu Rev Immunol 9:527, 1991 30. Yewdell J, Bennink J: Cell biology of antigen processing and presentation to major histocompatibility complex class I moleculerestricted T lymphocytes. Adv Immunol 52:125, 1992 31. Townsend A, Bodmer H: Antigen recognition by class 1restricted T lymphocytes. Annu Rev Immunol 7:601, 1989 32. Harding C, Leyva-Cobian F, Unanue ER: Mechanisms of antigen processing. Immunol Rev 106:77, 1988 33. Berzofsky J, Brett S, Streicher H Z , Takahashi H: Antigen processing for presentation to T lymphocytes: Functional mechanisms and implications for the T-cell repetoire. lmmunol Rev 106:s. 1988 34. Janeway C: The T cell receptor as a multicomponent signalling machine: CD4/CD8 coreceptors and CD45 in T cell activation. Annu Rev Immunol 10:645, 1992 35. Brodsky FM, Guagliardi LE: The cell biology of antigen processing and presentation. Annu Rev Immunol 9:707, 1991 36. Rammensee HG, Falk K, Rotzschke 0: Peptides naturally From www.bloodjournal.org by guest on December 22, 2014. For personal use only. PIVOTAL ROLE OF B7 presented by MHC class I molecules. Annu Rev Immunol 11:213, 1993 37. Lorenz R, Allen P: Processing and presentation of self proteins. Immunol Rev 106:115, 1988 38. Sadegh-Nasseri S, Germain RN: How MHC class molecules work: Peptide-dependent completion of protein folding. Immunol Today 13:43, 1992 39. Monaco J: A molecular model of MHC class-I-restricted antigen processing. Immunol Today 13:173, 1992 40. Neefjes J, Ploegh H: Intracellular transport of MHC class I1 molecules. Immunol Today 13:179, 1992 41. Elliot T: How do peptides associate with MHC class I molecules? Immunol Today 12:386, 1991 42. Miceli MC, Parnes JR: The role of CD4 and CD8 in T cell activation and differentiation. Adv Immunol 5359, 1993 43. Julius M, Maroun CR, Haughn L: Distinct roles for CD4 and CD8 as co-receptors in antigen receptor signaling. Immunol Today 14:177, 1993 44. Jorgensen J, Reay P, Ehrich E, Davis M: Molecular components of T-cell recognition. Annu Rev Immunol 10:835, 1992 45. Leiden E: Transcriptional regulation of T cell receptor genes. Annu Rev Immunol 11539, 1993 46. Ullman K, Northrup JP, Venveij CL, Crabtree GC: Transmission of signals from T lymphocyte antigen receptor to the genes responsible for cell proliferation and immune function. Annu Rev Immunol 8:421, 1990 47. Altman A, Coggeshall KM, Mustelin T: Molecular events mediating T cell activation. Adv Immunol 48:227, 1990 48. Bierer B, Burakoff S: T-lymphocyte activation: the biology and function of CD2 and CD4. Immunol Rev 11 1:267, 1989 49. Fraser JD, Straus D, Weiss A: Signal transduction events leading to T-cell lymphokine gene expression. Immunol Today 14:357, 1993 50. Bretscher P, Cohn M: A theory of self-nonself discrimination. Science 169:1042, 1970 51. June CH, Jackson KM, Ledbetter JA, Leiden JM, Lindsten T, Thompson CB: Two distinct mechanisms of interleukin-2 gene expression in human T lymphocytes. J Autoimmunity 255, 1989 52. Bjomdahl JM, Sung S S , Hansen JA, Fu SM: Human T cell activation: Differential response to anti-CD28 as compared to antiCD3 monoclonal antibodies. Eur J Immunol 19:881, 1989 53. Cerdan C, Martin Y, Brailly H, Courcoul M, Flavetta S, Costello R, Mawas C, Birg F, Olive D: IL-1 alpha is produced by T lymphocytes activated via the CD2 plus CD28 pathways. J Immunol 146560, 1991 54. Cerdan C, Martin Y, Courcoul M, Brailly H, Mawas C, Birg F, Olive D: Prolonged IL-2 receptor alphdCD25 expression after T cell activation via the adhesion molecules CD2 and CD28. Demonstration of combined transcriptional and post-transcriptional regulation. J Immunol 149:2255, 1992 55. Cerdan C, Razanajaona D, Martin Y, Courcoul M, Pavon C, Mawas C, Olive D, Birg F: Contributions of the CD2 and CD28 T lymphocyte activation pathways to the regulation of the expression of the colony-stimulating factor (CSF-1) gene. J Immunol 149:373, 1992 56. Boussiotis VA, Freeman GJ, Gray GS, Gribben JG, Nadler LM: B7 but not intercellular adhesion molecule-l costimulation prevents the induction of human alloantigen-specific tolerance. J Exp Med 178:1753, 1993 57. Damle NK, Klussman K, Leytze G, Ochs HD, Aruffo A, Linsley PS, Ledbetter JA: Costimulation via vascular cell adhesion molecule-l induces in T cells increased responsiveness to the CD28 counter-receptor B7. Cell Immunol 148:144, 1993 58. Damle NK, Klussman K, Linsley PS, Aruffo A: Differential costimulatory effects of adhesion molecules B7, ICAM-I, LFA-3, 3277 and VCAM-1 on resting and antigen-primed CD4+ T lymphocytes. J Immunol 148:1985, 1992 59. Damle NK, Klussman K, Linsley PS, Aruffo A, Ledbetter JA: Differential regulatory effects of intercellular adhesion molecule1 on costimulation by the CD28 counter-receptor B7. J Immunol 149:2541, 1992 60. van Seventer G, Shimuzu Y, Horgan K, Shaw S: The LFAl ligand ICAM-l provides an important costimulatory signal for T cell receptor-mediated activation of resting T cells. J Immunol 144:4579, 1990 61. van Seventer G, Shimizu Y, Horgan K, Ginther Luce G, Webb D, Shaw S: Remote T cell co-stimulation via LFA-I/ICAM-1 and CD2LFA-3: Demonstration with immobilized IigandmAb and implication in monocyte-mediated co-stimulation. Eur J Immunol 21:1711, 1991 62. Koyasu S, Lawton T, Novick D, Recny MA, Siliciano RF, Wallner BP, Reinherz EL: Role of interaction of CD2 molecules with lymphocyte function-associated antigen 3 in T-cell recognition of nominal antigen. Roc Natl Acad Sci USA 87:2603, 1990 63. Schwartz RH: A cell culture model for T lymphocyte clonal anergy. Science 248: 1349, 1990 64. Jenkins MK: The role of cell division in the induction of clonal anergy. Immunol Today 13:69, 1992 65. Schwartz RH, Mueller DL, Jenkins MK, Quill H: T-cell clonal anergy. Cold Spring Harb Symp Quant Biol 54:605, 1989 66. Charton B, Auchincloss H, Fathman CG: Mechanisms of transplantation tolerance. Annu Rev Immunol 12:707, 1994 67. Aruffo A, Seed B: Molecular cloning of a CD28 cDNA by a high-efficiency COS cell expression system. Proc Natl Acad Sci USA 84:8573, 1987 68. Martin PJ, Ledbetter JA, Morishita Y, June CH, Beatty PG, Hansen JA: A 44 kilodalton cell surface homodimer regulates interleukin 2 production by activated human T lymphocytes. J Immuno1 33:81, 1986 69. Kozbor D, Moretta A, Messner HA, Moretta L, Croce CM: Tp44 molecules involved in antigen-independent T cell activation are expressed on human plasma cells. J Immunol 138:4128, 1987 70. Hansen JA, Martin PJ, Nowinski RC: Monoclonal antibodies identifying a novel T-cell antigen and Ia antigens of human lymphocytes. Immunogenetics 10:247, 1980 71. Turka LA, Ledbetter JA, LeeK, June CH, Thompson CB: CD28 is an inducible T cell surface antigen that transduces a proliferative signal in CD3+ mature thymocytes. J Immunol 144:1646, 1990 72. Pierres A, Cerdan C, Lopez M, Mawas C, Olive D: “CD3 low” human thymocyte populations can readily be triggered via the CD2 and/or CD28 activation pathways whereas the CD3 pathway remains nonfunctional. J Immunol 144:1202, 1990 73. Damle NK, Mohagheghpour N. Hansen JA, Engleman EG: Alloantigen-specific cytotoxic and suppressor T lymphocytes are derived from phenotypically distinct precursors. J Immunol 131:2296, 1983 74. June CH, Ledbetter JA, Gillespie MM, Lindsten T, Thompson CB: T-cell proliferation involving the CD28 pathway is associated with cyclosporine-resistant interleukin 2 gene expression. Mol Cell Biol 7:4472, 1987 75. Hara T, Fu SM, Hansen JA: A new activation pathway used by a major T cell population via a disulfide-bonded dimer of a 44 kilodalton polypeptide (9.3 antigen). J Exp Med 161:1513, 1985 76. Ledbetter JA, Martin PJ, Spooner CE, WofsyD,Tsu ‘IT, BeattyPG, Gladstone P: Antibodies to Tp67 and Tp44 augment and sustain proliferative responses of activated T cells. J Immunol 135:2331, 1985 77.Yamada H, Martin PJ,BeanMA,BraunMP,BeattyPG, Sadamoto K, Hansen JA: Monoclonal antibody 9.3 and anti-CD1 1 From www.bloodjournal.org by guest on December 22, 2014. For personal use only. 3278 antibodies define reciprocal subsets of lymphocytes. Eur J Immunol 15:1164, 1985 78. Freedman MS, Ruijs TC, Blain M, Antel J P Phenotypic and functional characteristics of activated CD8+ cells: A CDllbCD28- subset mediates noncytolytic functional suppression. Clin Immunol Immunopathol 60:254, 1991 79. Koide J, Engleman EG: Differences in surface phenotype and mechanism of action between alloantigen-specific CD8+ cytotoxic and suppressor T cell clones. J Immunol 144:32, 1990 80. Li SG, Ottenhoff TH, Van den Elsen P, Koning F, Zhang L, Mak T, De Vries RR: Human suppressor T cell clones lack CD28. Eur J Immunol201281, 1990 81. Baroja ML, Lorre K, Van Vaeck F, Ceuppens JL: The antiT cell monoclonal antibody 9.3 (anti-CD28) provides a helper signal and bypasses the need for accessory cells in T cell activation with immobilized anti-CD3 and mitogens. Cell Immunol 120:205, 1989 82. Jenkins MK, Taylor PS, Norton SD, Urdahl KB: CD28 delivers a costimulatory signal involved in antigen-specific IL-2 production by human T cells. J Immunol 147:2461, 1991 83. Gmunder H, Lesslauer W: A 4.5-kDa humanT-cell membrane glycoprotein functions in the regulation of cell proliferative responses. Eur J Biochem 142:153, 1984 84. de Jong R, Stokkers P, Lamme E, Kool JM, Borst F, Brouwer M, Miedema F, van Lier RA: Regulation of T-cell differentiation by CD2 and CD28 accessory molecules. Immunology 74:175, 1991 85. Carabasi MH, Di Santo JP, Yang SY, Dupont B: Activation of peripheral CD8+ T lymphocytes via CD28 plus CD2: Evidence for IL-2 gene transcription mediated by CD28 activation. Tissue Antigens 37:26, 1991 86. Van Lier RA, Brouwer M, Aarden LA: Signals involved in T cell activation. T cell proliferation induced through the synergistic action of anti-CD28 and anti-CD2 monoclonal antibodies. Eur J Immunol 18:167, 1988 87. Ledbetter JA, Rabinovtich PS, Hellstrom I, Hellstrom KE, Grosmaire LS, June CH: Role of CD2 cross-linking in cytoplasmic calcium responses and T cell activation. Eur J Immunol 18:1601, 1988 88. Civil A, Geerts M, Aarden LA, Venveij CL: Evidence for a role of CD28RE as a response element for distinct mitogenic T cell activation signals. Eur J Immunol 22:3041, 1992 89. Costello R, Lipcey C, Algarte M, Cerdan C, Baeuerle PA, Olive D, Imbert J: Activation of primary human T-lymphocytes through CD2 plus CD28 adhesion molecules induces long-term nuclear expression of NF-kappa B. Cell Growth Differ 4:329, 1993 90. Fraser JD, Irving BA, Crabtree GR, Weiss A: Regulation of interleukin-2 gene enhancer activity by the T cell accessory molecule CD28. Science 251:313, 1991 91. Fraser JD, Weiss A: Regulation of T-cell lymphokine gene transcription by the accessory molecule CD28. Mol Cell Biol 12:4357, 1992 92. Fraser JD, Newton ME, Weiss A: CD28 and T cell antigen receptor signal transduction coordinately regulate interleukin 2 gene expression in response to superantigen stimulation. J Exp Med 175:1131, 1992 93. Ghosh P, Tan TH, Rice NR, Sica A, Young HA: The interleukm 2 CD28-responsive complex contains at least three members of the NF kappa B family: c-Rel, p50, and p65. Proc Natl Acad Sci USA 90:1696, 1993 94. Hutchcroft JE, Bierer BE: Activation-dependent phosphorylation of the T-lymphocyte surface receptor CD28 and associated proteins. Proc Natl Acad Sci USA 91:3260, 1994 95. June CH, Ledbetter JA, Lindsten T, Thompson CB: Evidence for the involvement of three distinct signals in the induction of IL2 gene expression in human T lymphocytes. J Immunol 143:153, 1989 GUINAN ET AL 96. Johnson JG, Jenkins MK: CO-stimulatory functions of antigen-presenting cells. J Invest Dermatol 99:62, 1992 97. Ledbetter JA, Imboden JB, Schieven GL, Grosmaire LS, Rabinovitch PS, Lindsten T, Thompson CB, June CH: CD28 ligation in T-cell activation: Evidence for two signal transduction pathways. Blood 7.5:1.531, 1990 98. Nunes J, Klasen S, Franco MD,Lipcey C, Mawas C, Bagnasco M, Olive D: Signalling through CD28 T-cell activation pathway involves an inositol phospholipid-specific phospholipase C activity. Biochem J 293:835, 1993 99. Prasad KVS,CaiY-C, Rabb M, Duckworth B, Cantley L, Shoelson SE, Rudd CE: T-cell antigen CD28 interacts with the lipid kinase phosphatidylinositol 3-kinase by a cytoplasmic Tyr(P)-MetXaa-Met motif. Proc Natl Acad Sci USA 91:2834, 1994 100. Truitt KE, Hicks CM, Imboden JB: Stimulation of CD28 triggers an association between CD28 and phosphatidylinositol 3kinase in Jurkat T cells. J Exp Med 179:1071, 1994 101. Vandenberghe P, Freeman GJ, Nadler LM, Fletcher MC, Kamoun M, Turka LA, Ledbetter JA, Thompson CB, June CH: Antibody and B7BBl-mediated ligation of the CD28 receptor induces tyrosine phosphorylation inhuman T cells. J Exp Med 17.5:9.51, 1992 102. Ward SG, Westwick J, Hall ND, Sansom DM: Ligation of CD28 receptor by B7 induces formation of D-3 phosphoinositides in T lymphocytes independently of T cell receptorKD3 activation. Eur J Immunol 23:2572, 1993 103. Rudd CE, Janssen 0, Cai Y, daSilva AJ, Raab M, Prasad KVS: Two-step TCRQCD3-CD4 and CD28 signaling in T cells: SH2/SH3 domains, protein-tyrosine and lipid kinases. Immunol Today 15:225, 1994 104. Guba SC, Stella G, Turka LA, June CH, Thompson CB, Emerson SG: Regulation of interleukin 3 gene induction in normal human T cells. J Clin Invest 84:1701, 1989 10.5. Thompson CB, Lindsten T, Ledbetter JA, Kunkel SL, Young HA, Emerson SG, Leiden JM, June CH: CD28 activation pathway regulates the production of multiple T-cell-derived lymphokines/ cytokines. Proc Natl Acad Sci USA 86:1333, 1989 106. Lindstein T, June CH, Ledbetter JA, Stella G, Thompson CB: Regulation of lymphokine messenger RNA stability by a surface-mediated T cell activation pathway. Science 244:339, 1989 107. Shahinian A, Pfeffer K, Lee KP, Kundig TM, Kishihara K, Wakeham A,KawaiK, Ohashi PS, Thompson CB, Mak TW: Differential T cell costimulatory requirements in CD28-deficient mice. Science 261:609, 1993 108. Atluru S, Atluru D: Evidence that genistein, a protein-tyrosine kinase inhibitor, inhibits CD28 monoclonal-antibody-stimulated human T cell proliferation. Transplantation 51:448, 1991 109. Bierer BE, Schreiber SL, Burakoff SJ: The effect of the immunosuppressant FK-506 on alternate pathways of T cell activation. Eur J Immunol 21:439, 1991 110. Bloemena E, Van Oers RH, Weinreich S, Stilma-Meinesz AP, Schellekens PT, Van Lier RA: The influence of cyclosporin A on the alternative pathways of human T cell activation in vitro. Eur J Immunol 19:943, 1989 I 1 1. Galvin F, Freeman GJ, Razi-Wolf Z , Benacerraf B, Nadler L, Reiser H: Effects of cyclosporin A, FK 506, and mycalamide A on the activation of murine CD4+ T cells by the murine B7 antigen. Eur J Immunol 23:283, 1993 112. Hess AD, Bright EC: The effect ofthe CD28 activation pathway on the immunosuppressive action of cyclosporine. Transplantation 51:1232, 1991 1 13. Linsley PS, Clark EA, Ledbetter JA: T-cell antigen CD28 mediates adhesion with B cells by interacting with activation antigen B7BB-1. Proc Natl Acad Sci USA 87:5031, 1990 114. Freedman AS, Freeman G, Horowitz JC, Daley J. Nadler From www.bloodjournal.org by guest on December 22, 2014. For personal use only. PIVOTAL ROLE OF B7 LM: B7, a B-cell-restricted antigen that identifies preactivated B cells. J Immunol 139:3260, 1987 115. Yokochi T, Holly RD, Clark EA: B lymphoblast antigen (BB-l) expressed on Epstein-Barr virus-activated B cell blasts, B lymphoblastoid cell lines, and Burkitt’s lymphomas. J Immunol 128:823, 1982 116. Freeman GJ, Freedman AS, Segil JM, Lee G, Whitman JF, Nadler LM: B7, a new member of the Ig superfamily with unique expression on activated and neoplastic B cells. J Immunol 143:27 14, 1989 117. Freeman GJ, Gray GS, Gimmi CD, Lombard DB, Zhou W, White M, Fingeroth JD, Gribben JG, Nadler LM: Structure, expression, and T cell costimulatory activity of the murine homologue of the human B lymphocyte activation antigen B7. J Exp Med 174525, 1991 118. Freeman GJ, Disteche CM, Gribben JG, Adler DA, Freedman AS, Dougery J, Nadler LM: The gene for B7, a costimulatory signal for T-cell activation, maps to chromosomal region 3q13.33q21. Blood 79:489, 1992 119. Freedman AS, Freeman GJ, Rhynhart K, Nadler LM: Selective induction of B7/BB-I on interferon-gamma stimulated monocytes: A potential mechanism for amplification of T cell activation through the CD28 pathway. Cell Immunol 137:429, 1991 120. Azuma M, Yssel H, Phillips JH, Spits H, Lanier LL: Functional expression of B7BB 1 on activated T lymphocytes. J Exp Med 1772345, 1993 121. Friccius H, Siegels-Hubenthal P, Rehbein A, Schlotz E, Pawelec G: Induction of hyporesponsiveness in human lymphocytesT despite their expression of boththe co-receptor CD28 and its ligand B7. Cell Immunol 151:283, 1993 122. Sansom DM,HallND: B7BB1, the ligand for CD28, is expressed on repeatedly activated human T cells in vitro. Eur J Immunol 23:295, 1993 123. Yssel H, Schneider PV, Lanier LL: Interleukin-7 specifically induces the B7IBBI antigen on human cord blood and peripheral blood T cells and T cell clones. Int Immunol 5:753, 1993 124. Augustin M, Dietrich A, Niedner R, Kapp A, Schopf E, Ledbetter JA, Brady W, Linsley PS, Simon JC: Phorbol-12-myristate-13-acetate-treated human keratinocytes express B7-like molecules that serve a costimulatory role in T-cell activation. J Invest Dermatol 100:275, 1993 125. Fleming TE, Mirando WS, Trefzer U, Tubesing KA, Elmets CA: In situ expression of a B7-like adhesion molecule on keratinocytes from human epidermis. J Invest Dermatol 101:754, 1993 126. Nickoloff BJ, Mitra RS, Lee K, Turka LA, Green J, Thompson C, Shimizu Y: Discordant expression of CD28 ligands, BB-l, and B7 on keratinocytes in vitro and psoriatic cells in vivo. Am J Pathol 142:1029, 1993 127. Symington F W , Brady W, Linsley PS: Expression and function of B7 onhuman epidermal Langerhans cells. J Immunol 150:1286, 1993 128. Munro JM, Freedman AS, Aster JC, Gribben JG, Lee NC, Rhynhart KK, Banchereau J, Nadler LM: In vivo expression of the B7 costimulatory molecule by subsets of antigen-presenting cells and the malignant cells of Hodgkin’s disease. Blood 83:793, 1994 129. Vandenberghe P, Delabie J, de Boer M, De Wolf-Peeters C, Ceuppens JL: In situ expression of B7/BB1 on antigen-presenting cells and activated B cells: An immunohistochemical study. Int Immunol 5:317, 1993 130. Reiser H, Freeman GJ, Razi-Wolf Z, Gimmi CD, Benacerraf B, Nadler LM: Murine B7 antigen provides an efficient costimulatory signal for activation of murine T lymphocytes via the T-cell receptorKD3 complex. Proc Natl Acad Sci USA 89:271, 1992 131. Norton SD, Zuckerman L, Urdahl KB, Shefner R, Miller J, Jenkins MK: The CD28 ligand, B7, enhances IL-2 production by 3279 providing a costimulatory signal to T cells. J Immunol 149:1556, 1992 132. Hading FA, McArthur JG, Gross JA, Raulet DH, Allison JP: CD28-mediated signalling co-stimulates murine T cells and prevents induction of anergy in T-cell clones. Nature 356507, 1992 133. Koulova L, Clark EA, Shu G, Dupont B: The CD28 ligand B7/BB1 provides costimulatory signal for alloactivation of CD4+ T cells. J Exp Med 173:759, 1991 134. Linsley PS, Brady W, Grosmaire L, Aruffo A, Damle NK, Ledbetter JA: Binding of the B cell activation antigen B7 to CD28 costimulates T cell proliferation and interleukin 2 mRNA accumulation. J Exp Med 173:721, 1991 135. Gimmi CD, Freeman GJ, Gribben JG, Sugita K, Freedman AS, Morimoto C, Nadler LM: B-cell surface antigen B7 provides a costimulatory signal that induces T cells to proliferate and secrete interleukin 2. Proc Natl Acad Sci USA 88:6575, 1991 136. Galvin F, Freeman GJ, Razi-Wolf Z, Hall W Jr, Benacerraf B, Nadler L, Reiser H: Murine B7 antigen provides a sufficient costimulatory signal for antigen-specific and MHC-restricted T cell activation. J Immunol 149:3802, 1992 137. Azuma M, Cayabyab M, Buck D, Phillips JH, Lanier LL: CD28 interaction with B7 costimulates primary allogeneic proliferative responses and cytotoxicity mediated by small, resting T lymphocytes. J Exp Med 175:353, 1992 138. Damle NK, Linsley PS, Ledbetter JA: Direct helper T cellinduced B cell differentiation involves interaction between T cell antigen CD28 and B cell activation antigen B7. Eur J Immunol 21:1277, 1991 139. Gimmi CD, Freeman GJ, Gribben JG, Gray G, Nadler LM: Human T-cell clonal anergy is induced by antigen presentation in the absence of B7 costimulation. Proc Natl Acad Sci USA 90:6586, 1993 140. Gross JA, St John T, Allison J P The murine homologue of the T lymphocyte antigen CD28. Molecular cloning and cell surface expression. J Immunol 144:3201, 1990 141. Freeman G, Boniello F, Hodes R, Reiser H, Gribben JG, Ng J, Kim, Goldberg J, Hathcock K, Laszlo G, Nadler LM, Sharpe A: Murine B7-2, an alternative CTLA4 counter-receptor that costimulates T cell proliferation and interleukin 2 production. J Exp Med 178:2185, 1993 142. Brunet J-F, Denizot F, Luciani M-F, Roux-Dosseto M, Suzan M, Mattei M-G, Golstein P: A new member of the immunoglobulin superfamily-CTLA-4. Nature 328:267, 1987 143. Lindsten T, Lee KP, Harris ES, Petryniak B, Craighead N, Reynolds PJ, Lombard DB, Freeman GJ, Nadler LM, Gray GS, Thompson CB, June CH: Characterization of CTLA-4 structure and expression on human T cells. J Immunol 151:3489, 1993 144. Balzano C, Buonavista N, Rouvier E, Golstein P: CTLA-4 and CD28: Similar proteins, neighbouring genes. Int J Cancer Suppl 7:28, 1992 145. Harper K, Balzano C, Rouvier E, Mattei MG, Luciani MF, Golstein P CTLA-4 and CD28 activated lymphocyte molecules are closely related in both mouse and human as to sequence, message expression, gene structure, and chromosomal location. J Immunol 147:1037, 1991 146. Lafage-Pochitaloff M, Costello R, Couez D, Simonetti J, Mannoni P, Mawas C, Olive D: Human CD28 and CTLA-4 Ig superfamily genes are located on chromosome 2 at bands q33-q34. Immunogenetics 31:198, 1990 147. Buonavista N, Balzano C, Pontarotti P, Le Paslier D, Golstein P: Molecular linkage of the human CTLA4 and CD28 Igsuperfamily genes in yeast artificial chromosomes. Genomics 13:856, 1992 148. Linsley PS, Brady W, Urnes M, Grosmaire LS, Damle NK, From www.bloodjournal.org by guest on December 22, 2014. For personal use only. 3280 Ledbetter JA: CTLA-4 is a second receptor for the B cell activation antigen B7. J Exp Med 174:561, 1991 149. Freeman GJ, Lombard DB, Gimmi CD, Brod SA, Lee K, Laning JC, Hafler DA, DorfME,Gray GS, Reiser H, June CH, Thompson CB, Nadler LM: CTLA-4 and CD28 mRNA are coexpressed in most T cells after activation. Expression of CTLA-4 and CD28 mRNA does not correlate with the pattern of lymphokine production. J Immunol 149:3795, 1992 150. Linsley PS, Greene JL, Tan P, Bradshaw J, Ledbetter JA, Anasetti C, Damle NK: Coexpression and functional cooperation of CTLA-4 and CD28 on activated T lymphocytes. J Exp Med 176:1595, 1992 151. Gribben JG, Freeman GJ, Boussiotis VA, Rennert P, Jellis CL, Greenfield E, Barber M, Restivo VA, Ke X-Y, Gray GS, Nadler LM: CTLA4 mediates antigen specific apoptosis of human T cells. Proc Natl Acad Sci USA (in press) 152. Tan P, Anasetti C, Hansen JA, Melrose J, Brunvand M, Bradshaw J, Ledbetter JA, Linsley PS: Induction of alloantigenspecific hyporesponsiveness in human T lymphocytes by blocking interaction of CD28 with its natural ligand B7IBBI. J Exp Med 177:165, 1993 153. Freeman GJ, Bomello F, Hodes RJ, Reiser H, Hathcock KS, Laszlo G, MacKnight AJ, Kim J, Du L, Lomb ard DB, Gray GS, Nadler LM, Sharpe AH: Uncovering of functional alternative CTLA4 counter-receptor in B7-deficient mice. Science 262:907, 1993 154. Wu Y, Guo Y, Liu Y: A major costimulatory molecule on antigen-presenting cells, CTLA4 ligand A, is distinct from B7. J ExpMed 178:1789, 1993 155. Razi-Wolf Z, Galvin F, Gray G, Reiser H: Evidence for an additional ligand, distinct from B7, for the CTLA-4 receptor. Proc Natl Acad Sci USA 90:11182, 1993 156. Lenschow DJ, Su GH-T, Zuckerman LA, Nabavi N, Jellis CL, Gray GS, Miller J, Bluestone JA: Expression and functional significance ofan additional ligand for CTLA-4. ProcNatl Acad Sci USA 90:11054, 1993 157. Boussiotis VA, Freeman GJ, Gribben JG, Daley J, Gray CS, Nadler LM: Activated human B lymphocytes express three CTLA4 counterreceptors that costimulate T-cell activation. Proc Natl Acad Sci USA 90: 11059, 1993 158. Azuma M, Ito D, Yagita H, Okumura K, Phillips JH, Lanier LL, Somoza C: B70 antigen is a second ligand for CTLA-4 and CD28. Nature 36676, 1993 159. Freeman GI, Gribben JG, Boussiotis VA, Ng JW, Restivo V Jr, Lombard LA, Gray GS, Nadler LM: Cloning of B7-2: A CTLA4 counter-receptor that costimulates human T cell proliferation (see comments). Science 262:909, 1993 160. Engel P, Gribben JG, Freeman G, Zhou L-J, Nozawa Y, Abe M, Nadler LM, Wakasa H, Tedder TF: The B7-2 (B70) costimulatory molecule of monocytes and activated B lymphocytes is the CD86 differentiation antigen. Blood 84: 1402, 1994 161. Freeman GJ, Edelhoff S, Disteche CM, Wong S, Jellis C, Rennert P, Gray GS, Nadler LM: Genomic structure and chromosomal localization of human and murine B7-2. (submitted) 162. Selvakumar A, Mohanraj BK, Eddy RL, Shows TB, White PC, Dupont B: Genomic organization and chromosomal location of the human gene encoding the B-lymphocyte activation antigen B7. Immunogenetics 36: 175, 1992 163. Hathcock KS, Laszlo G, Dickler HB, Bradshaw J, Linsley P, Hodes RJ: Identification of an alternative CTLA-4 ligand costimulatory for T cell activation (see comments). Science 262:905, 1993 164. Mueller D, Jenkms MK, Schwartz RH: Clonal expansion vs. functional clonal inactivation. Annu Rev Immunol 7:445, 1989 165. Robey E, Urbain J: Tolerance and immune regulation. Immunol Today 12:175, 1991 GUINAN ET AL 166. Greenen V, Kroemer G: Multiple ways to cellular immune tolerance. Immunol Today 14:573, 1993 167. Miller JFAP, Morahan G: Peripheral T cell tolerance. Annu Rev Immunol 1051, 1992 168. Jenkins M, Schwartz R: Antigen presentation by chemically modified splenocytes induces antigen-specific T cell unresponsiveness in vitro and in vivo. J Exp Med 165:302, 1987 169. Jenkins MK, Ashwell JD, Schwartz RH: Allogeneic non-T spleen cells restore the responsiveness of normal T cell clones stimulated with antigen and chemically modified antigen-presenting cells. J Immunol 140:3324,1988 170. Sigal NH, Dumont FJ: Cyclosporin A, FK-506, and rapamycin: Pharmacologic probes of lymphocyte signal transduction. Annu Rev Immunol 10:519, 1992 171. Gribben JG, Guinan EC, Boussiotis VA, Barber M,Gray G, Freeman G, Nadler LM: Blockade of B7 family costimulatory molecules significantly reduces alloantigen specific precursor helper T cell frequency. (submitted) 172. Lenschow DJ, Zeng Y, Thistlethwaite JR, Montag A, Brady W, Gibson MG, Linsley PS, Bluestone JA: Long-term survival of xenogeneic pancreatic islet grafts induced by CTLA4lg (see comments). Science 257:789, 1992 173. Lin H, Bolling SF, Linsley PS, Wei RQ, Gordon D, Thompson CB, Turka LA: Long-term acceptance of major histocompatibility complex mismatched cardiac allografts induced by CTLA4Ig plus donor-specific transfusion. J Exp Med 178:1801, 1993 174. Turka LA, Linsley PS, Lin H, Brady W, Leiden JM, Wei RQ, Gibson ML, Zheng XG, Myrdal S, Gordon D, Bailey T, Bolling SF, Thompson CB: T-cell activation by the CD28 ligand B7 is required for cardiac allograft rejection in vivo. Proc Natl Acad Sci USA 89:11102, 1992 175. Blazar BR, Taylor PA, Linsley PS, Vallera DA: Invivo blockade of CD281CTLA4: B7BB 1 interaction with CTLA4-lg reduces lethal murine graft-versus-host disease across the major histocompatibility complex barrier in mice. Blood 83:3815, 1994 176. Beverly B, Kang SM, Lenardo MJ, Schwartz RH: Reversal of in vitro T cell clonal anergy by IL-2 stimulation. Int Immunol 4:66 1, 1991 177. Boussiotis VA, Freeman GJ, Griffin JD, Gray GS, Gribben JG, Nadler LM: CD2 is involved in maintenance and reversal of human alloantigen specific clonal anergy. J Exp Med (in press) 178. Ferrara JLM, Deeg HJ: Mechanisms of disease. Graft-versus-host disease. N Engl J Med 324:667, 1991 179. Rosenberg S (ed): Gene Therapy of Cancer. Philadelphia, PA, Lippincott, 1992 180. Russell SJ: Lymphokine gene therapy for cancer. Immunol Today 11:196, 1990 18 1, Topalian SL, Rosenberg SA: Tumor-infiltrating lymphocytes:evidence for specific immune reactions against growing cancers in mice and humans, in DeVita VTJ, Hellman S, Rosenberg SA (eds): Important Advances in Oncology. Philadelphia, PA, Lippincott, 1990, p 19 182. Lanzavecchia A: Identifying strategies for immune intervention. Science 260:937, 1993 183. Urban J, Schreiber H: Tumor antigens. Annu Rev Immunol 10:617, 1992 184. Pardoll DM: Cancer vaccines. Immunol Today 14:310, 1993 185. Boon T, Coulie P, Marchand M, Weynants P, Wolfel T, Brichard V: Genes coding for tumor rejection antigens: perspectives for specific immunotherapy, in DeVita VTJ, Hellman S, Rosenberg SA (eds): Important Advances in Oncology. Philadelphia, PA, Lippincott, 1994, p 53 186. Boon T, Cerottini JC, Eynde B, Bruggen P, Pel AV: Tumor antigens recognized by T lymphocytes. Annu Rev Imrnunol 12:337, 1994 From www.bloodjournal.org by guest on December 22, 2014. For personal use only. PIVOTAL ROLE OF B7 187. Delabie J, Ceuppens JL, Vandenberghe P, de Boer M, Coorevits L, De Wolf-Peeters C: The B7/BBl antigen is expressed by Reed-Stemberg cells of Hodgkin’s disease and contributes to the stimulating capacity of Hodgkin’s disease-derived cell lines. Blood 82:2845, 1993 188. Freedman AS, Nadler LM: Cell surface markers in hematologic malignancies. Semin Oncol 14:193, 1987 189. Freedman AS, Boyd AW, Berrebi A, Horowitz JC, Levy DN, Rosen KJ, Daley J, Slaughenhoupt B, Levine H, Nadler L: Expression of B cell activation antigens on normal and malignant B cells. Leukemia 1:9, 1986 190. Freedman AS, Griffin JD, Nadler LM: Ontogeny of leukocytes and leukemias. New York, NY, Raven (in press) 191. Freedman AS: Expression and function of adhesion receptors on normal B cells and B cell non-Hodgkin’s lymphomas. Semin Hematol 30:318, 1993 192. Griffin JD, Ritz J, Nadler LM, Schlossman SF: Expression of myeloid differentiation antigens on normal and malignant myeloid cells. J Clin Invest 68:932, 1981 193. Anderson KC, Slaughenhoupt B, Bates MP, Pinkus G, O’Hara C, Schlossman SF, Nadler LM: Expression of human B cell associated antigens on leukemia and lymphomas: A model of human B cell differentiation. Blood 63:1424, 1984 194. Chen L, Linsley PS, Hellstrom KE: Costimulation of T cells for tumor immunity. Immunol Today 14:483, 1993 195. Elliot BE, Carlow DA, Rodricks A-M, Wade A: Perspectives on the role of MHC antigens in normal and malignant cell development. Adv Cancer Res 53:181, 1989 196. Gamdo F, Cabrera T, Concha A, Glew S, Ruiz-Cabello F, Stem PL: Natural history of HLA expression during tumor development. Immunol Today 14:491, 1993 197. Nathan C: Interferon and inflammation, in Gallin JI, Goldstein IM, Snyderman R (eds): Inflammation. Basic Principles and Clinical Correlates. New York, NY, Raven, 1992, p 260 198. Borden EC, Sondel PM: Lymphokines and cytokines as cancer treatment. Immunotherapy realized. Cancer 65:800, 1990 199. Watanabe Y, Kuribayashi K, Miyatake S, Nishihara K, Nakayama E, Taniyama T, Sakata T: Exogenous expression of mouse interferon gamma cDNA in mouse neuroblastoma C1300 cells results in reduced tumorigenicity by augmented anti-tumor immunity. Proc Natl Acad Science USA 86:9456, 1989 200. Greenberg P: Adoptive T cell therapy of tumors: Mechanisms operative in the recognition and elimination of tumor cells. Adv Immunol49:281, 1991 201. Beun GDM, van de Velde CJH, Fleuren GJ: T-cell based cancer immunotherapy: Direct or redirected tumor-cell recognition? Immunol Today 15:11, 1994 202. Chen L, Ashe S, Brady WA, Hellstrom I, Hellstrom KE, Ledbetter JA, McGowan P, Linsley PS: Costimulation of antitumor immunity by the B7 counterreceptor for the T lymphocyte molecules CD28 and CTLA-4. Cell 71: 1093, 1992 203.KwakLW, Campbell MJ, Czenvinski DK, Hart S, Miller RA, Levy R: Induction of immune responses in patients with B-cell lymphoma against the surface-immunoglobulin idiotype expressed by their tumors. N Engl J Med 327:1209, 1992 204. Levy R, Miller RA: Therapy of lymphoma directed at idiotypes. Monogr Natl Cancer Inst 10:61, 1990 205. Upadhyaya G, Cuba SC, Sih SA, Feinberg AP, Talpaz M, Kantarjian HM, Deisseroth AB, Emerson SG: Interferon-alpha restores the deficient expression of the cytoadhesion molecule lymphocyte function antigen-3 by chronic myelogenous leukemia progenitor cells. J Clin Invest 88:2131, 1991 206. Bateman WJ, Fiera R, Matthews N, Moms AG: Inducibility of class I1 major histocompatibility complex antigens by interferon 3281 gamma is associated with reduced tumorigenicity in C3H mouse fibroblasts transformed by v-Ki-ras. J Exp Med 173:193, 1991 207. Baskar S, Ostrand-Rosenberg S, NabaviN, Nadler LM, Freeman GJ, Glimcher LH: Constitutive expression of B7 restores immunogenicity of tumor cells expressing truncated major histocompatibility complex class I1 molecules. Proc Natl Acad Sci USA 90:5687, 1993 208. Townsend SE, Allison J P Tumor rejection after direct costimulation of CD8+ T cells by B7-transfected melanoma cells (see comments). Science 259:368, 1993 209. Chen L, McGowan P, Ashe S, Johnston J, Li Y,Hellstrom I, Hellstrom KE: Tumor immunogenicity determines the effect of B7 costimulation on T cell-mediated tumor immunity. J Exp Med 179523, 1994 210. Matulonis U, Dosiou D, Mauch P, Freeman G, Lamont C, Nadler L: Rejection of myeloid leukemia cells after B7-1 transfection. (submitted) 211. Guo Y,Wu M, Chen H,Wang X, Liu G, Ma J, SyM: Effective tumor vaccine generated by fusion of hepatoma cells with activated B cells. Science 263:518, 1994 212. Tan R, Teh SJ, Ledbetter JA, Linsley PS, Teh HS: B7 costimulates proliferation of CD4-8+ T lymphocytes but is not required for the deletion of immature CD4+8+ thymocytes. J Immuno1 149:3217, 1992 213. Ranheim EA, Kipps TJ: Activated T cells induce expression of B7/BB1 on normal or leukemic B cells through a CD4O-dependent signal. J Exp Med 177:925, 1993 214. Ding L, Linsley PS, Huang LY, Germain RN, Shevach EM: IL-IO inhibits macrophage costimulatory activity by selectively inhibiting the up-regulation of B7 expression. J Immunol 151:1224, 1993 C ,Petrella 215. Romano MF, Turco MC, Stanziola A, Giarmsso P A, Tassone P, van Lier R, Venuta S, Formisano S: Defect of interleukin-2 production and T cell proliferation in atopic patients: Restoring ability of the CD28-mediated activation pathway. Cell Immunol 148:455, 1993 216. Silverman ED, Somma C, Khan MM, Melmon KL, Engleman EG: Abnormal T suppressor cell function in juvenile rheumatoid arthritis. Arthritis Rheum 33:205, 1990 217. Wyss-Coray T, Mauri-Hellweg D, Baumann K, Bettens F, Grunow R, Pichler WJ: The B7 adhesion molecule is expressed on activated human T cells: Functional involvement in T-T cell interactions. Eur J Immunol 23:2175, 1993 218. Harlan DM, Hengartner H, Huang ML, Kang YH, Abe R, Moreadith RW, Pircher H, Gray GS, Ohashi PS, Freeman GJ, Nadler LM, June CH, Aichele P: Mice expressing both B7 and viral glycoprotein on pancreatic beta cells along with glycoprotein-specific transgenic T cells develop diabetes due to a breakdown of T lymphocyte unresponsiveness. Proc Natl Acad Sci USA 91:3137, 1994 219. Guerder S, Meyerhoff J, Flavell R: The role of the T cell costimulator B7-1 in autoimmunity and the induction and maintenance of tolerance to peripheral antigen. Immunity 1:155, 1994 220. Zamvil S, Steinman L: The T lymphocyte in experimental allergic encephalomyelitis. Annu Rev Immunol 8:579, 1990 221. Warren RP, Storb R, Thomas ED, Su PJ, Mickelson EM, Weiden PL: Autoimmune and allimmune phenomena in patients with aplastic anemia: Cytotoxicity against autologous lymphocytes and lymphocytes from HLA identical siblings. Blood 56:683, 1980 222. Camitta B, O’Reilly RJ, Sensenbrenner L, Rappeport J, Champlin R, Doney K, August C, Hoffmann RG, Kirkpatrick D, Stuart R, Santos G, Parkman R, Gale RP, Storb R, Nathan D: Antithoracic duct lymphocyte globulin therapy of severe aplastic anemia. Blood 62:883, 1983 223. Champlin R, Ho W, Gale RP: Antithymocyte globulin treat- From www.bloodjournal.org by guest on December 22, 2014. For personal use only. 3282 ment in patients with aplastic anemia: A prospective randomized trial. N Engl J Med 308:113, 1983 224. Gluckman E, Esperou-Bourdeau H, Baruchel A, Boogaerts M, Briere J, Donadio D, Leverger G, Lepomer M, Reiffers J, Janvier M, Michallet M, Stryckmans P: Multicenter randomized study comparing cyclosporine-A alone and antithymocyte globulin with prednisone for treatment of severe aplastic anemia. Blood 79:2540, 1992 225. Linsley PS, Wallace PM, Johnson J, Gibson MG, Greene JL, Ledbetter JA, Singh C, Tepper MA: Immunosuppression in vivo by a soluble form of the CTLA-4 T cell activation molecule. Science 257:792, 1992 226. Anasetti C, Beatty PG, Storb R, Martin PJ, Mori M, Sanders JE, Thomas ED, Hansen JA: Effect of HLA incompatibility on graftversus-host disease, relapse, and survival after marrow transplantation for patients with leukemia and lymphoma. Hum Immunol29:79, 1990 227. Antin J, Bierer N, Smith B, Ferrara J, Guinan EC, Sieff C, Golan DE, Macklis RM, Tarbell NJ, Lynch E, Reichert TA, BlythmanH, Bouloux C, Rappeport JM, Burakoff SJ, Weinstein HJ: Selective depletion of bone marrow T lymphocytes with anti-CD5 monoclonal antibodies: Effective prophylaxis for graft-versus-host disease in patients with hematologic malignancies. Blood 78:1464, 1991 228.AshRC, Casper JT, Chitambar CR, Hansen R, Bunin N. Truitt RL, Lawton C, Murray K, Hunter J, Baxter-Lowe LA, Gottschall JL, Oldham K, Anderson T, Camitta B, Menitove J: Successful allogeneic transplantation of T-cell-depleted bone marrow from closely HLA-matched unrelated donors. N Engl J Med 322:485, 1990 229. Bacigalupo A, Hows J, Gordon-Smith EC, Gluckman E, Van Lint MT, Congiu M, James DCO, Barrett AJ, Gmur J, DePlanque MM, Siimes MA, Toivanen A, Ringden 0, Marmont AM, for the Working Party of the EBMTG: Bone marrow transplantation for severe aplastic anemia from donors other thanHLA identical siblings: A report of the BMT Working Party. Bone Marrow Transplant 3:531, 1988 230. Beatty PG, Clift RA, Mickelson EM, Nisperos BB, Flournoy N, Martin PJ, Sanders JE, Stewart P, Buckner CD, Storb R, Thomas ED, Hansen JA: Marrow transplantation from related donors other than HLA-identical siblings. N Engl J Med 3 13:765, 1985 231. Beatty PG, Anasetti C, Hansen JA, Longton GM, Sanders JE, Martin PJ, Mickelson EM, Choo SY, Petersdorf EW, Pepe MS, Appelbaum FR, Bearman SI, Buckner CD, Clift RA, Petersen FB, Singer J, Stewart PS, Storb RF, Sullivan KM, Tesler MC, Witherspoon RP, Thomas ED: Marrow transplantation from unrelated donors for treatment of hematologic malignancies: effect of mismatching for one HLA locus. Blood 81 :249, 1993 232. Goldman JM, Gale RP, Horowitz MM, Biggs JC, Champlin RE, Gluckman E, Hoffmann RC, Jacobsen SJ, Marmont AM, McGlave PB, Messner HA, Rimm AA, Rozman C, Speck B, Tura S, Weiner RS, Bortin MM: Bone marrow transplantation for chronic GUINAN ET AL myelogenous leukemia in chronic phase: lncreased risk for relapse associated with T-cell depletion. Ann Intern Med 108:806, 1988 233. Herve P, Cahn JY, Flesch M, Plouvier E, Racadot E, Noir A, Couteret Y, Goldstein G, Bernard A, Lenys R, Bresson JL, Leconte des Floris R, Peters A: Successful graft-versus-host disease prevention without graft failure in 32 HLA-identical allogeneic bone marrow transplantations with marrow depleted of T cells by monoclonal antibodies and complement. Blood 69:388, 1987 234. Kernan NA, Bartsch G, Ash RC, Beatty PG, Champlin R. Filipovich A, Gajewski J, Hansen JA, Henslee-Downey J, McCullough J, McGlave P, Perkins HA, Phillips CL, Sanders J, Stroncek D, Thomas ED, Blume KG: Analysis of 462 transplantations from unrelated donors facilitated by the National Marrow Donor Program. N Engl J Med 328:593, 1993 235. Maraninchi D, Gluckman E, Blaise D, Guyotat D,Rio B, Pico JL, LeBlond V, Michaller M, Dreyfus F, Ifrath N, Bordigoni A: Impact of T-cell depletion on outcome of allogeneic bone marrow transplantation for standard-risk leukaemias. Lancet 2: 175, I987 236. Marmont AM, Horowitz MM, Gale RP, Sobocinski K, Ash RC, vanBekkumDW, Champlin RE. Dicke KA,Goldman JM, Good RA, Herzig RH, Hong R, Mdsaoka T, Rimm AA, Ringden 0, Speck B, Weiner RS,BortinMM: T-cell depletion of HLAidentical transplants in leukemia. Blood 78:2120, 1991 237. McGlave P, Bartch G, Anasetti C, Ash R, Beatty P, Gajewski J, Kernan NA: Unrelated donor marrow transplantation therapy for chronic myelogenous leukemia: Initial experience ofthenational marrow donor program. Blood 81:543, 1993 238. Pirsch JD, Maki DG: Infectious complications in adults with bone marrow transplantation and T-cell depletion of donor marrow: Increased susceptibility to fungal infections. Ann Intern Med 104:619, 1986 239. van Bekkum DW, Lowenberg B: Graft-versus host disease. in van Bekkum DW (ed): Bone Marrow Transplantation: Biological Mechanisms and Clinical Practice. New York, NY, Dekker. 1985, p 147 240. Azuma M, Cayabyab M, Phillips JH, Lanier LL: Requirements for CD28-dependent T cell-mediated cytotoxicity. J Immunol 150:2091, 1993 241. Harding FA, Allison JP: CD28-B7 interactions allow the induction of CD8+ cytotoxic T lymphocytes in the absence of exogenous help. J Exp Med 177: I79 I , 1993 242. de Waal Malefyt R, Verma S, Bejarano MT, Ranes-Goldbeg M, Hill M, Spits H: CD2LFA-3 or LFA-I/ICAM-I but not CD28/ B7 interactions can augment cytotoxicity by virus-specitic CD8+ cytotoxic T lymphocytes. Eur J Immunol 23:418, 1993 243. Mitsuyasu RT. Champlin RE, Gale RP, Ho WC, Lenarsky C, Winston D, Selch M, Elashoff R, Giorgi JV, Wells J, Terasaki P, Billing R, Feig S: Treatment of donor bone marrow with monoclonal anti-T-cell antibody and complement for theprevention o f graft-versus-host disease. Ann Intern Med 105:20, 1986 244. Parmiani G: Tumor immunity as autoimmunity: Tumor antigens include normal self proteins which stimulate anergic peripheral T cells. Immunol Today 14:536, 1993 From www.bloodjournal.org by guest on December 22, 2014. For personal use only. 1994 84: 3261-3282 Pivotal role of the B7:CD28 pathway in transplantation tolerance and tumor immunity EC Guinan, JG Gribben, VA Boussiotis, GJ Freeman and LM Nadler Updated information and services can be found at: http://www.bloodjournal.org/content/84/10/3261.full.html Articles on similar topics can be found in the following Blood collections Information about reproducing this article in parts or in its entirety may be found online at: http://www.bloodjournal.org/site/misc/rights.xhtml#repub_requests Information about ordering reprints may be found online at: http://www.bloodjournal.org/site/misc/rights.xhtml#reprints Information about subscriptions and ASH membership may be found online at: http://www.bloodjournal.org/site/subscriptions/index.xhtml Blood (print ISSN 0006-4971, online ISSN 1528-0020), is published weekly by the American Society of Hematology, 2021 L St, NW, Suite 900, Washington DC 20036. 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