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REVIEWS MOLECULAR DEFECTS IN T AND BCELL PRIMARY IMMUNODEFICIENCY DISEASES Charlotte Cunningham-Rundles and Prashant P. Ponda Abstract | More than 120 inherited primary immunodeficiency diseases have been discovered in the past five decades, and the precise genetic defect in many of these diseases has now been identified. Increasing understanding of these molecular defects has considerably influenced both basic and translational research, and this has extended to many branches of medicine. Recent advances in both diagnosis and therapeutic modalities have allowed these defects to be identified earlier and to be more precisely defined, and they have also resulted in more promising long-term outcomes. The prospect of gene therapy continues to be included in the armamentarium of treatment considerations, because these conditions could be among the first to benefit from gene-therapy trials in humans. Division of Clinical Immunology, Mount Sinai School of Medicine, 1425 Madison Avenue, Box 1089, New York, New York 10029, USA. Correspondence to C.C.R. e-mail: charlotte. [email protected] doi:10.1038/nri1713 880 | NOVEMBER 2005 The human immune system is confronted with the challenge of host defence. This is accomplished through various innate immune responses (which are non-specific) and adaptive immune responses (which are specific) that work synergistically to achieve this goal. Cells of the adaptive immune system include T and B cells, which are derived from a common multipotent haematopoietic stem cell. Defects involving T and B cells have been described with respect to their development, effector function and roles in immunoregulation1. Although defined primary natural killer (NK)-cell deficiencies are rare among primary immunodeficiency diseases, other cells of the innate immune system, which were previously thought to function independently of adaptive immune responses, are now seen as important partners in the development of adaptive immunity. The clinical presentation of patients with a primary immunodeficiency reflects the complex underpinnings of the immune system and depends on the underlying genetic defect. Patients with severe combined immunodeficiency (SCID) generally present with opportunistic infections and fail to thrive within a few months of life. Recurrent bacterial infections are the hallmark of disease in patients with defects in B cells, phagocytic | VOLUME 5 cells or complement, whereas opportunistic infections with viruses or fungi are particularly common in patients with T-cell deficiencies. A subset of primary immunodeficiencies is associated with inflammatory or autoimmune manifestations, and certain subgroups of patients are susceptible to developing malignancies. This Review focuses on the recent advances in the field, with an emphasis on newly identified genetic deficiencies and therapeutic options for patients. SCID — an immunodeficiency that is characterized by severely reduced numbers or an absence of functional T cells, which in turn results in the absence of an adaptive immune response — is a consequence of a mutation in any one of ten distinct genes that are inherited in an autosomal recessive or an X-linked manner 2–10. Four lymphocyte phenotypes are possible on the basis of the influence of the defective gene on B-cell and NK-cell development TABLE 1. A diagnosis is possible at birth, with most affected infants having lymphopaenia (less than 2,000 lymphocytes per mm3 of blood) and their lymphocytes showing decreased proliferation in vitro after stimulation with mitogen, antigen or allogeneic cells11. Although these infants have a severely reduced amount of thymic www.nature.com/reviews/immunol © 2005 Nature Publishing Group REVIEWS TCELLRECEPTOR EXCISION CIRCLES (TRECs). DNA episomes that are normally produced during the thymic maturation of T cells, specifically during recombination of the T-cellreceptor genes. XLINKED LYMPHOPROLIFERATIVE SYNDROME (XLP). A rare, often fatal, primary immunodeficiency disease that is characterized by an inability to mount an effective immune response to Epstein–Barr virus. This can lead to lymphoma or hypogammaglobulinaemia. tissue, accompanied by the absence of normal thymic architecture, T-cell development is achievable after the introduction of normal haematopoietic stem cells12. At present, bone-marrow transplantation using either unfractionated HLA-identical haematopoietic stem cells or T-cell-depleted haploidentical (parental) haematopoietic stem cells is the standard of care for these infants, with improved survival in patients who receive a transplant within the first 4 weeks of life13. Successful intervention depends on early identification of infants, before the development of opportunistic infections that contribute to the increased morbidity and mortality that is associated with delayed transplantation. Nevertheless, there is no programme in place for screening newborns for SCID; such a programme would allow therapy to be provided within this vital window of opportunity. In the United States, the Centers for Disease Control and Prevention has identified SCID as a candidate for the development of a newborn-screening protocol, because SCID meets many of the accepted screening criteria14. Many modalities of testing have been explored; most recently, the examination of TCELLRECEPTOR EXCISION CIRCLES (TRECs) in DNA isolated from dried blood spots has shown promise15. TRECs are more abundant in T cells from a healthy newborn than from an adult. Their absence has been confirmed in patients with SCID15, and largescale implementation of this screening tool might help to identify affected infants. Defects that involve T-cell immunity Patients with defects that involve T cells do not have adequate cellular immune responses and are predisposed to developing opportunistic infections. These T-cell deficiencies are reflected in reduced absolute cell numbers, defective activation and function, and disrupted immunoregulation (FIG. 1; TABLE 2. DiGeorge syndrome has classically been thought of as the model Table 1 | Aetiologies of severe combined immunodeficiency Type of SCID – + Chromosomal location Reference + T B NK Interleukin-7 receptor α-chain deficiency 5p13 2 CD3 δ-chain deficiency 11q23 3 CD3 ε-chain deficiency 11q23 4 T –B+NK– X-linked recessive SCID (γc deficiency) Xq13.1 5 CD45 deficiency 1q31–1q32 6 JAK3 deficiency 19p13.1 7 Artemis gene-product deficiency 10p13 8 RAG1 and RAG2 deficiency 11p13 9 T –B –NK+ T –B –NK– Adenosine-deaminase deficiency 20q13.11 10 γc, common cytokine-receptor γ-chain; JAK3, Janus kinase 3; NK, natural killer; RAG, recombination-activating gene; SCID, severe combined immunodeficiency. NATURE REVIEWS | IMMUNOLOGY for thymic insufficiency leading to a T-cell deficiency, although it also involves abnormal development of spatially related embryological tissues that leads to cardiac, parathyroid and other abnormalities. DiGeorge syndrome is characterized by a decrease in the number of CD3+ cells or an absence of CD3+ cells as a consequence of hypoplasia or aplasia of the thymus. Depending on the number of peripheral T cells, the immune phenotype falls within a range of immunodeficiencies, from full immunocompetence to a SCID-like phenotype. Unlike other forms of SCID, severe DiGeorge syndrome can be treated effectively by thymic transplantation, which allows for the maturation of recipient T cells. Several candidates for the genetic defect in DiGeorge syndrome have been identified; most recently, a member of the T-box transcription-factor family, TBX1, has been implicated as a cause of most of the main signs of DiGeorge syndrome16,17. Genetic defects also affect the signal-transduction pathways that are essential for T-cell activation. Components of these pathways include the γ-chain of CD3 (CD3γ), CD3ε, MHC class I molecules, MHC class II molecules, LCK, ZAP70 (ζ-chain-associated protein kinase of 70 kDa) and CD8α18–25. The resulting defects are highly variable and range from severe cellular dysfunction (from a deficiency in MHC class II molecules) to negligible dysfunction (from a deficiency in CD8α). In addition to genetic defects that reduce or eliminate T-cell-based immunity, there is a growing list of immune defects that result in overactive or abnormal T-cell function that leads to immunodeficiency. An example of a functional mutation is seen in patients with XLINKED LYMPHOPROLIFERATIVE SYNDROME (XLP); these individuals have a mutation in SH2D1A, which encodes SLAM-associated protein (SAP), a cytoplasmic adaptor protein that binds signalling lymphocytic activation molecule (SLAM) and other SLAM-family molecules26. SLAM is a transmembrane protein that is expressed at low levels at the surface of resting cells and at higher levels after cellular activation; intracytoplasmic binding of SLAM by SAP has an inhibitory role. For reasons that are unclear, defects in SAP result in uncontrolled proliferation of T cells in individuals who are infected with Epstein–Barr virus, as well as ineffective viral elimination, lymphoma or hypogammaglobulinaemia. SH2D1A mutations result in fatal infectious mononucleosis in a high proportion of cases. Another emerging role for T cells is that of regulation of the immune response to prevent the recognition of self. Recent studies have outlined aspects of the molecular basis of T-cell defects in three disease states that are characterized by T-cell immunodysregulation. IPEX. Immunodysregulation, polyendocrinopathy and enteropathy, X-linked syndrome (IPEX) can often be fatal and is a recessive disorder of early childhood that involves the classic clinical triad of endocrinopathy (most commonly in the form of early-onset type 1 diabetes), severe enteropathy with VOLUME 5 | NOVEMBER 2005 | 881 © 2005 Nature Publishing Group REVIEWS the molecular defect29. FOXP3 is mainly expressed by, and is a reliable marker of, CD4 CD25 REGULATORY 30 T CELLS . The role of FOXP3 in the development and function of CD4+CD25hi regulatory T cells, however, is largely an enigma. At present, immunosuppressive agents that are directed at activated T cells, such as CYCLOSPORIN A OR TACROLIMUS (Prograf; Astellas Pharma US, Inc.) (administered together with an optional corticosteroid), are watery, sometimes bloody, diarrhoea, and eczematous dermatitis27, resulting from a severe deficiency or absence of regulatory T cells. This syndrome is associated with autoimmune conditions that affect several organ systems and with moderate to severe recurrent infections with Enterococcus and Staphylococcus species28. Mutation of the forkhead box P3 (FOXP3) gene, which encodes a forkhead (also known as winged helix) transcription factor, has been identified as + hi a DiGeorge syndrome ADA IL-7Rα γc JAK3 NK cells Thymic medulla Thymic cortex Bone marrow RAG1, RAG2 CD3δ, CD3ε artemis CD45 AIRE CD3 complex B cells pTα CD8+ T cell αβ-TCR CD8 CD4 CD8α MHC class I molecules (TAP1, TAP2) LCK ZAP70 TCR-β CD3ζ Lymphoid lineages CD34+ Pro-T cell: CD4– CD8– CD25+ CD44+ HSC Pre-T cell: CD4– CD8– CD25+ CD44– DP T cell: CD4+ CD8+ Positive and negative selection MHC class II molecules (CIITA and RFX-family proteins) CD4+ T cell Myeloid lineages b IPEX ALPS CD4+ CD25+ TReg cell DISC, caspase-8 and caspase-10 Calcineurin inhibitors that are used to prevent transplant rejection and that function by inhibiting nuclear factor of activated T cells (NFAT). 882 | NOVEMBER 2005 CD95L SAP APC TCR CYCLOSPORIN A AND TACROLIMUS SLAM CD95 TEff cell A thymus-derived subpopulation of T cells that expresses the transcription factor forkhead box P3 (FOXP3) and is involved in the suppression of immune responses, either by cell–cell contact or cytokine release. SAP CD95 FOXP3 CD4+CD25hi REGULATORY T CELLS XLP CD95L Caspase-8 Caspase-10 Peptide – MHC class I or class II Proliferation inhibited Figure 1 | Protein and gene defects in T-cell development and function. a | Haematopoietic stem cell (HSC)-derived lymphoid progenitor cells migrate from the bone marrow to the thymus and develop into progenitor (pro)-T cells, which then rearrange their T-cell receptor (TCR) genes and differentiate into either γδ or αβ T cells in the cortex. The latter initially co-express CD8 and CD4, which interact with MHC class I and class II molecules, respectively, at the surface of medullary thymic stromal cells. This interaction allows T cells to be ‘educated’ regarding self-antigens and non-self-antigens (enabling the positive or negative selection of T cells in the thymus) before their migration to the periphery, where they exclusively express CD4 or CD8. Numerous defects in maturation have been elucidated. Defects in the genes encoding the molecules listed in the yellow boxes (and the primary immunodeficiency diseases listed) are known to affect the developmental steps indicated. b | Functional defects are also observed after maturation is complete. In patients with IPEX (immunodysregulation, polyendocrinopathy and enteropathy, X-linked syndrome), self-reactive effector T (TEff) cells are not inhibited, because mutations in the forkhead box P3 gene (FOXP3) result in a loss of CD4+CD25+ regulatory T (TReg)-cell activity. In patients with autoimmune lymphoproliferative syndrome (ALPS), defects in the CD95, CD95 ligand (CD95L), caspase-8 or caspase-10 genes abrogate formation of the deathinducing signalling complex (DISC), thereby interfering with apoptosis of TEff cells. In patients with X-linked lymphoproliferative syndrome (XLP), uncontrolled proliferation of T cells occurs as a result of a mutation in SH2D1A, which encodes SAP (signalling lymphocytic activation molecule (SLAM)-associated protein). ADA, adenosine deaminase; AIRE, autoimmune regulator; APC, antigen-presenting cell; γc, common cytokine-receptor γ-chain; CIITA, MHC class II transactivator; DP, double positive; IL-7Rα, interleukin-7 receptor α-chain; JAK3, Janus kinase 3; NK cell, natural killer cell; pre-T cell, precursor-T cell; pTα, pre-TCR α-chain; RAG, recombination-activating gene; RFX, regulatory factor X; TAP, transporter associated with antigen processing; ZAP70, ζ-chain-associated protein kinase of 70 kDa. | VOLUME 5 www.nature.com/reviews/immunol © 2005 Nature Publishing Group REVIEWS the most effective therapy for the treatment of patients with IPEX31. However, for the most affected patients, no acceptable regimen can maintain long-term remission of the disease. Bone-marrow transplantation has been carried out for several patients with IPEX, although the results have mostly been disappointing32,33. Intriguingly, one patient had improved glucose regulation and reduced diarrhoea during the conditioning regimen before bone-marrow transplantation32, underscoring the role of selectively targeting Table 2 | Defects that involve T cells Name Clinical phenotype Chromosomal location Genetic defect Refs DiGeorge syndrome Thymic, cardiac and parathyroid defects, and decreased numbers or absence of CD3+ cells 22q11.2 Possibly TBX1 WHN defect Congenital alopecia, and nail dystrophy 17q11–17q12 WHN 113 CD3 deficiency Autoimmune haemolytic anaemia and severe infections Recurrent Haemophilus influenzae pneumonia and otitis media 11q23 11q23 CD3G CD3E 18 19 MHC class I deficiency Decreased numbers of CD8+ T cells 6p21.3 TAP1 TAP2 20 21 MHC class II deficiency Persistent diarrhoea, bacterial pneumonia, Pneumocystis carinii pneumonia, viral and candidal infections, and low numbers of CD4+ T cells 1q21.1–1q21.3 13q14 19p12 16p13 RFX5 RFXAP RFXANK CIITA 22 22 22 22 LCK deficiency Bacterial, viral and fungal infections, lymphopaenia and hypogammaglobulinaemia 1p34.3–1p35 LCK 23 ZAP70 deficiency Decreased numbers of CD8+ T cells, normal or decreased numbers of CD4+ T cells, and severe recurrent infections 2q12 ZAP70 24 CD8 deficiency Absence of CD8+ T cells, and recurrent respiratory infections 2p12 CD8A 25 HIGM1 Pneumocystis carinii pneumonia, pyogenic infections, normal or increased level of IgM, and low level or absence of serum IgG, IgA and IgE Xq26–Xq27 CD40L 50 HIGM3 Pneumocystis carinii pneumonia and Cryptosporidium parvum infections 20q12–20q13.2 CD40 AD-EDA-ID Lymphocytosis, absence of memory T cells and unresponsive naive T cells 14q13 NFKBIA Uncontrolled T-cell proliferation in EBV infection, fatal infectious mononucleosis in a high proportion of patients, ineffective viral elimination, lymphoma and hypogammaglobulinaemia Xq25 SH2D1A 26,87 IPEX Triad of endocrinopathy, enteropathy and dermatitis, and Enterococcus and Staphylococcus species infections Xp11.23 FOXP3 27,29 APECED Chronic mucocutaneous candidiasis, and parathyroid and adrenal autoimmunity 21q22.3 AIRE 34 ALPS0 Autoimmunity, hypergammaglobulinaemia, lymphoproliferation, and excessive numbers of CD3+CD4–CD8–αβ-TCR+ T cells 10q24.1 CD95 (homozygous) 41 ALPS1a Autoimmunity, hypergammaglobulinaemia, lymphoproliferation, and excessive numbers of CD3+CD4–CD8–αβ-TCR+ T cells 10q24.1 CD95 (heterozygous, germ line) CD95 (heterozygous, somatic) 42 47 ALPS1b Autoimmunity, hypergammaglobulinaemia, lymphoproliferation, and excessive numbers of CD3+CD4–CD8–αβ-TCR+ T cells 1q23 CD95L 45 ALPS2 Autoimmunity, hypergammaglobulinaemia, lymphoproliferation, and excessive numbers of CD3+CD4–CD8–αβ-TCR+ T cells 2q33–2q34 CASP8 CASP10 43 44 ALPS3 Autoimmunity, hypergammaglobulinaemia, lymphoproliferation, and excessive numbers of CD3+CD4–CD8–αβ-TCR+ T cells ND ND 46 Development 16,17 Activation 53,54 59 Function XLP Regulation AD-EDA-ID, autosomal dominant ectodermal dysplasia with immunodeficiency; AIRE, autoimmune regulator; ALPS, autoimmune lymphoproliferative syndrome; APECED, autoimmune polyendocrinopathy-candidiasis-ectodermal-dystrophy syndrome; CIITA, MHC class II transactivator; CASP, caspase; CD40L, CD40 ligand; CD95L, CD95 ligand; EBV, Epstein–Barr virus; FOXP3, forkhead box P3; HIGM, hyper-IgM syndrome; IPEX, immunodysregulation, polyendocrinopathy and enteropathy, X-linked syndrome; ND, not determined; NFKBIA, gene encoding IκBα (inhibitor-of-nuclear-factor-κB α); RFX5, regulatory factor X, 5; RFXANK, RFXassociated ankyrin-containing protein; RFXAP, RFX-associated protein; SH2D1A, gene encoding SAP (signalling lymphocytic activation molecule (SLAM)-associated protein); TAP, transporter associated with antigen processing; TBX1, T-box 1; TCR, T-cell receptor; WHN, winged-helix nude (also known as FOXN1); XLP, X-linked lymphoproliferative syndrome; ZAP70, ζ-chain-associated protein kinase of 70 kDa. NATURE REVIEWS | IMMUNOLOGY VOLUME 5 | NOVEMBER 2005 | 883 © 2005 Nature Publishing Group REVIEWS activated T cells. Nevertheless, the usefulness of either immunosuppression or bone-marrow transplantation would mainly depend on its early implementation, before the onset of permanent organ damage. Further understanding of the mechanisms that are involved in FOXP3 expression and its influence in patients with IPEX will undoubtedly provide therapeutic targets for patients with IPEX and potentially for individuals with other autoimmune diseases. CLASSSWITCH RECOMBINATION (CSR). A switch in the DNA that encodes the constant region of the immunoglobulin heavy chain, from Cµ (which encodes the constant region of IgM) to DNA that is further downstream and encodes the constant region of another immunoglobulin class: that is, to Cγ, Cα or Cε, which encode the constant region of IgG, IgA and IgE, respectively. This is accomplished through an intrachromosomal deletional rearrangement. SOMATIC HYPERMUTATION (SHM). The introduction of point mutations at a high frequency in the variable regions of immunoglobulin genes. 884 | NOVEMBER 2005 APECED. Autoimmune polyendocrinopathycandidiasis-ectodermal-dystrophy syndrome (APECED; also known as APS1) results from a defect in the autoimmune regulator (AIRE) gene34. Patients with APECED usually have chronic mucocutaneous candidiasis, as well as autoimmune manifestations that most commonly affect the parathyroid or adrenal glands and, to a lesser extent, the thyroid gland, liver and skin35. AIRE is expressed at high levels by purified human thymic stromal cells, especially medullary thymic epithelial cells, and it is thought to regulate the ectopic cell-surface expression of tissue-specific proteins, such as insulin and thyroglobulin36. Expression of these self-proteins allows the negative selection of autoreactive T cells during their development. The absence of this key regulatory step results in the organ-specific autoimmune destruction that is seen in patients with APECED. Thymic stromal lymphopoietin, an interleukin-7 (IL-7)-like cytokine, has been shown to induce human peripheral-blood CD11c+ dendritic cells (DCs) to upregulate AIRE mRNA expression strongly, in conjunction with cell-surface MHC class II molecules and the co-stimulatory molecules CD80 and CD86 REF. 37. These activated DCs were able to induce a 1,000-fold clonal expansion in an autologous, naive CD4+ T-cell population in culture. This further emphasizes the role of AIRE in the presentation of self-peptides, because the CD4+ T-cell proliferation occurred in the absence of exogenous antigen and was therefore attributed to the presentation of self-peptide–MHC complexes by DCs. In vitro studies have elucidated that one role of the AIRE protein is to function as an E3 ubiquitin ligase, indicating its involvement in a ubiquitin–proteasome pathway38. Furthermore, two known disease-causing mutations in the AIRE gene abolished this ligase activity38. The precise ubiquitin–proteasome pathway and ubiquitylation substrates of the AIRE protein have yet to be identified, and the overall significance of this pathway in the establishment and maintenance of T-cell self-tolerance is not well understood at present. ALPS. There are four known genetic defects that have been identified in patients with autoimmune lymphoproliferative syndrome (ALPS), an inherited condition that is associated with dysregulation of apoptosis mediated by CD95 (also known as FAS). CD95 is a cell-surface receptor that is a member of the tumour-necrosis factor (TNF)-receptor superfamily, and after binding CD95 ligand (CD95L; also known as FAS ligand), it initiates a complex signalling pathway | VOLUME 5 that results in the induction of apoptosis. This pathway involves formation of the death-inducing signalling complex in association with caspase-8 and caspase-10. All patients have at least three of the four main features of ALPS: autoimmunity, hypergammaglobulinaemia (of both IgG and IgA), lymphoproliferation and excessive numbers of CD3+CD4–CD8–αβ-TCR+ (double negative) T cells39. In vitro study of lymphocyte sensitivity to CD95induced apoptosis allows for a classification scheme that is based on the underlying genetic defect 40 . Defective CD95-induced apoptosis is observed in homozygous CD95 deficiency 41 (classified as ALPS0), heterozygous dominant CD95 mutations42 (classified as ALPS1a), and signalling pathway defects that involve caspase-8 REF. 43 or caspase-10 REF. 44 (classified as ALPS2). CD95-induced apoptosis is intact in two additional subtypes of patients: those with a CD95L mutation45 (classified as ALPS1b), and those with a clinical ALPS phenotype but in whom a molecular defect has yet to be identified46 (classified as ALPS3). A subset of patients who were previously categorized as having ALPS3 has now been identified to have somatic heterozygous CD95 mutations in unstimulated doublenegative T cells47. Interestingly, all patients in this subset had identical CD95 mutations or mutations that led to identical structural changes in CD95 to those observed in patients with ALPS1a. However, mutant CD95 products could not be detected in T-cell blasts following in vitro activation. It is unclear why the mutations in these newly identified patients do not lead to defective CD95 expression in activated T cells and, subsequently, to defective CD95-induced apoptosis. Hyper-IgM syndromes General aspects. Hyper-IgM syndromes (HIGMs) constitute a group of molecular defects that is characterized by impaired immunoglobulin CLASSSWITCH RECOMBINATION (CSR) and SOMATIC HYPERMUTATION (SHM) or by impaired SHM alone. Patients with these syndromes typically have recurrent bacterial infections and often have lymphoid hyperplasia. They have normal numbers of peripheral B cells, albeit with a low proportion of memory B cells (which are CD27+) and normal or increased levels of serum IgM associated with low levels or absent serum IgG, IgA and IgE. CSR48 and SHM49 occur only after antigen binds B cells displaying cell-surface IgM (that is, the B-cell receptor, BCR), and these are two mechanisms by which the primary antibody repertoire is fine-tuned to generate a highly antigen-specific immune response. These events are T-cell dependent and are facilitated through the interaction of CD40L at the surface of activated T cells with its receptor CD40, which is constitutively expressed by B cells. B cells that produce high-affinity specific antibody as a result of SHM have a survival advantage. Two enzymes — activation-induced cytidine deaminase (AID) and uracil-DNA glycosylase (UNG) — are crucial for this editing process. CSR and SHM work together so that the secondary antibody repertoire has a high affinity. Mutations in the www.nature.com/reviews/immunol © 2005 Nature Publishing Group REVIEWS components that are involved in these processes result in the inherent defects in patients with HIGM. So far, seven defects that are known to be involved in HIGM have been characterized: defects in CD40L, classified as HIGM type 1 (HIGM1; also known as X-linked HIGM, XHIGM); defects in AID, classified as HIGM2; defects in CD40, classified as HIGM3; defective CSR with preserved SHM, classified as HIGM4; defects in UNG; defects in IKKG (IκB (inhibitor of nuclear factor-κB, NF-κB) kinase-γ; also known as NEMO); and defects in NFKBIA (which encodes IκBα)50–59 TABLES 2,3. Patients with defects in CD40L, who comprise the HIGM1 subgroup, account for approximately two-thirds of all patients with HIGM. In these patients, an absence of, or a defect in, binding of CD40L to CD40 is caused by a mutation that affects the extracellular domain of CD40L50. There is no intrinsic B-cell defect observed in these patients: their B cells generate normal immunoglobulin class-switching responses in an appropriate microenvironment50. Furthermore, in contrast to most patients with hypogammaglobulinaemia, individuals with HIGM1 are susceptible to opportunistic infections, especially to pneumonia caused by Pneumocystis carinii, thereby underscoring an inherent T-cell defect. HIGM3 has been described in four patients from three families and is characterized by the absence of CD40 expression at the cell surface of B cells, macrophages and DCs53,54. These patients are also susceptible to developing opportunistic infections. It is important to note that there might be defects in components of the signalling pathway that are downstream of CD40–CD40L interactions to account for other patients in the clinical spectrum of HIGM. In addition, other repair mechanisms that work in conjunction with, or independent of, AID and UNG have yet to be clarified. Patients with defects in AID or UNG have a similar clinical phenotype51,52. Similar to patients with a CD40L deficiency, the level of serum IgM is normal or increased, and this occurs together with low levels or an absence of IgG and IgA. However, owing to intact T-cell function, these patients do not seem to be susceptible to opportunistic infections and might not be recognized as having an immune defect until the second or third decade of life60. The exact process by which AID and UNG mediate CSR and SHM is not known; however, switch-region doublestranded-DNA breaks are required for both to occur, Table 3 | Defects that involve B cells Name Clinical phenotype Chromosomal location Genetic defect HIGM2 Pyogenic infections, lymphoid hyperplasia, decreased CD27+ B-cell numbers, normal or increased serum IgM level, and low level or no serum IgG, IgA and IgE 12p13 12q23–12q24.1 AID UNG 51 52 HIGM4 Pyogenic infections, lymphoid hyperplasia, decreased CD27+ B-cell numbers, normal or increased serum IgM level, and low level or no serum IgG, IgA and IgE ND ND 55 XL-EDA-ID Bacterial and mycobacterial infections, and low levels or no antibody specific for carbohydrates Xq28 IKKG Agammaglobulinaemia Low or no levels pre-B-cell and mature B-cell numbers, low serum immunoglobulin levels, and pyogenic infections Normal pro-B-cell numbers, low pre-B-cell and mature B-cell numbers, low serum immunoglobulin levels, and pyogenic infections Low serum immunoglobulin levels, and pyogenic infections Developmental arrest at the pro-B-cell stage, low serum immunoglobulin levels, and pyogenic infections Xq21.3–Xq22 BTK 10q23.2 BLNK 22q11.22 19q13.2 14q32.2 IGLL1 Iga IGHM CVID Sinopulmonary infections, low IgG and IgA levels, and normal B-cell numbers 16p11.2 22q13.1–22q13.31 17p11.2 2q33 CD19 BAFFR TACI ICOS IgAD Many patients are asymptomatic, although pyogenic infections are possible 17p11 Possibly 6p21.3 TACI IGAD1 (HLA-DQ and HLA-DR) t(9; 20) (q33.2; q12) LRRC8 References 56–58 72 114,115 83 76 77 78,79 Unpublished* Unpublished‡ 116,117 88,89 116,117 99,100 *M. C. van Zelm, personal communication; J. L. Franco, personal communication. ‡ V. Salzer, personal communication. AID, activation-induced cytidine deaminase; BAFFR, B-cell-activating-factor receptor; BLNK, B-cell linker; BTK, Bruton’s tyrosine kinase; CVID, common variable immunodeficiency; HIGM, hyper-IgM syndrome; ICOS, inducible T-cell co-stimulator; Iga, gene encoding Igα; IgAD, selective IgA deficiency; IGAD1, IgA-deficiency susceptibility 1; IGHM, gene encoding µ immunoglobulin heavy chain; IGLL1, gene encoding λ5; IKKG, inhibitor-of-nuclear-factor-κB kinase-γ; LRRC8, leucine-rich-repeat-containing 8; ND, not determined; pre-B cell, precursor-B cell; pro-B cell, progenitor-B cell; TACI, transmembrane activator and calcium-modulating cyclophilin-ligand interactor; UNG, uracil-DNA glycosylase; XL-EDA-ID, X-linked ectodermal dysplasia with immunodeficiency. NATURE REVIEWS | IMMUNOLOGY VOLUME 5 | NOVEMBER 2005 | 885 © 2005 Nature Publishing Group REVIEWS and CSR-induced breaks have been shown to occur much less frequently in AID-deficient B cells than in AID-sufficient B cells61. Three patients have been described to have a deficiency in UNG52. All three had normal expression of CD40, CD40L and AID, although (similar to AIDdeficient B cells) their B cells failed to generate switchregion double-stranded-DNA breaks after activation through CD40. In addition, a skewed SHM pattern was observed that showed mutations biased towards transitions in dG and dC nucleotides, whereas dA and dT nucleotides showed transitions and transversions in similar ratios to control values from normal memory B cells. The current hypothesis to explain these editing processes involves AID-mediated deamination of C residues into U residues, followed by UNGmediated removal of U residues. This would create an abasic site that could be targeted by an endonuclease to create the required DNA breaks that are crucial for CSR and SHM. Lack of either enzyme would therefore destabilize the development of a secondary antibody repertoire. Replication protein A, a ubiquitous singlestranded-DNA-binding protein, has been described as the factor that targets AID to SHM motifs to promote their deamination62. Approximately one-quarter of patients with the HIGM phenotype have normal expression and function of CD40L, CD40, AID and UNG63. A subset of these patients has been described to have defective CSR with preserved SHM, and these individuals have been categorized into the HIGM4 subtype55. B cells from these patients express AID and show appropriate AID-dependent CSR-induced DNA breaks in the switch region of Cµ (which encodes the constant region of IgM), indicating that the molecular defect is downstream of these events: that is, there is a deficiency in the repair process that occurs after the induction of double-stranded-DNA breaks. The precise defect in individuals with HIGM4 remains undefined, and its elucidation should further illuminate the complex process of CSR and DNA repair. HYPOMORPHIC MUTATION A type of mutation that results in either diminished quantity of a normal gene product or diminished function of a gene product. HYPODONTIA The partial congenital absence of one or more teeth. TOLLLIKE RECEPTORS (TLRs). A family of evolutionarily conserved pattern-recognition receptors. These molecules are located intracellularly and at the cell surface of macrophages, dendritic cells, B cells and intestinal epithelial cells. Their natural ligands are conserved molecular patterns, known as pathogen-associated molecular patterns, that are found in bacteria, viruses and fungi. 886 | NOVEMBER 2005 Defects in NF-κB signalling. An increasing number of genetic mutations are being identified that have inappropriate activation of NF-κB as a common defect64. Of these, HYPOMORPHIC MUTATIONS in IKKG are linked to a clinical phenotype of immunodeficiency, ectodermal dysplasia, as well as to susceptibility to pyogenic bacterial infections in early infancy or childhood and to mycobacterial infections in early or late childhood58. However, the clinical phenotype is remarkably heterogeneous: in some patients it involves only conical incisors and HYPODONTIA, whereas in others, it involves osteopetrosis with lymphoedema. The range of infections and defects in antibody production are similarly diverse, although a severely reduced level, or an absence of, antibody specific for carbohydrate antigens seems to be a unifying theme. In addition, whereas NF-κB was first noted in B cells, active NF-κB can be released in the cytoplasm of many cells, indicating that this defect might affect other tissues. Two important target | VOLUME 5 genes of NF-κB-mediated transcription are AID and UNG, so in some patients with IKK-γ deficiency, the processes of CSR and SHM are hindered, leading to the increased IgM levels that have been observed65. An HIGM phenotype similar to IKKG deficiency has been described for patients with a mutation in NFKBIA, which encodes IκBα, an inhibitor of NF-κB. This hypermorphic gain-of-function mutation prevents the phosphorylation of IκBα and, in turn, the activation of NF-κB. Unlike IKKG deficiency, however, this phenotype is associated with a T-cell deficiency that is characterized by naive T cells that are unresponsive to stimulation through CD3 in vitro and by the absence of memory T cells. Interestingly, an abnormal response to TOLLLIKE RECEPTOR (TLR) signalling has also been noted in patients with the IKKG mutation56. On binding and activation of the various TLRs by their ligands, a complex signalling cascade is set in motion, the result of which is the activation of NF-κB and the transcription of genes encoding several pro-inflammatory cytokines and chemokines66. Mutations in IKKG result in a substantially diminished response to lipopolysaccharide, an activator of TLR4. This emphasizes the emerging significance of components of the innate immune system in the aetiology of primary immunodeficiency. Other examples of defects of this type include mutations in the IL-1-receptor-associated kinase 4 (IRAK4) gene and the caspase-12 gene67–69. It is crucial to note that the innate and adaptive immune systems, which were historically thought of as segregated, do not function as distinct entities; instead, they are interdependent and function together to coordinate the host immune response. Defects that involve B-cell immunity Deficiencies in antibody production and function are the hallmark of the primary immunodeficiency diseases that involve B cells. Patients with these conditions are especially vulnerable to recurrent infections with encapsulated pathogens, such as Streptococcus pneumoniae, Haemophilus influenzae and Staphylococcus aureus, and with Gram-negative bacteria, such as Pseudomonas species70. A B-cell defect is defined as a markedly decreased serum level of at least one of the three main immunoglobulin classes: IgG, IgA and IgM; the most marked defects lead to either reduced levels or an absence of antibody production. Antibody deficiencies are the largest group within the primary immunodeficiencies, and multiple molecular defects have been identified throughout the pathways that are involved in B-cell development (FIG. 2; TABLE 3. Forms of agammaglobulinaemia. Several genetic defects have been identified that account for the phenotype of agammaglobulinaemia, which is characterized by a B-cell defect and intact T-cell function. Of all of the forms of agammaglobulinaemia, X-linked agammaglobulinaemia (XLA) provides the prototypical clinical description. XLA was the first antibody-deficiency syndrome that was recognized71, www.nature.com/reviews/immunol © 2005 Nature Publishing Group REVIEWS Bone marrow Periphery RAG1, RAG2 Igα Igµ λ5 BLNK IL-7Rα γc JAK3 BTK Igβ Pre-BCR Pro-B cell Immature B cell IgM Igα CVID ICOS CD19 HIGM4 IgAD AID CD40 UNG IKK-γ IgM BAFFR TACI IgG, IgA or IgE Mature B cell Plasma cell Lymphoid lineages CD34+ B220low CD10+ CD19+ CD34+ HSC CD34+ Pre-B cell • Negative selection • Receptor editing IgD • Class-switch recombination • Somatic hypermutation Memory B cell T cells Myeloid lineages NK cells Figure 2 | Protein and gene defects in B-cell development and function. Haematopoietic stem cells (HSCs) give rise to progenitor (pro)-B cells, which then rearrange their immunoglobulin heavy-chain gene segments to generate precursor (pre)B cells. Pre-B cells subsequently rearrange their immunoglobulin light-chain gene segments to produce a functional cell-surface receptor (IgM). This protein is composed of heavy and light chains that are derived from these gene rearrangements, and it functions as a receptor for responding to stimulation with antigen, resulting in the induction of proliferation and differentiation of the B cell. In the periphery, after stimulation with antigen, mature B cells further develop following class-switch recombination and somatic hypermutation and, ultimately, differentiate into memory B cells or plasma cells. Developmental blocks throughout B-cell maturation and differentiation occur as a result of defects in genes encoding the molecules listed in the yellow boxes. Blocks in the function of mature B cells can also occur. Primary immunodeficiency syndromes that cause these blocks are also listed. AID, activation-induced cytidine deaminase; BAFFR, B-cell-activating-factor receptor; BCR, B-cell receptor; BLNK, B-cell linker; BTK, Bruton’s tyrosine kinase; γc, common cytokine-receptor γ-chain; CVID, common variable immunodeficiency; HIGM4, hyper-IgM syndrome 4; ICOS, inducible T-cell co-stimulator; IgAD, selective IgA deficiency; Igµ, µ immunoglobulin heavy chain; IKK-γ, inhibitor-of-nuclear-factor-κB kinase-γ; IL-7Rα, interleukin-7 receptor α-chain; JAK3, Janus kinase 3; NK cell, natural killer cell; RAG, recombination-activating gene; TACI, transmembrane activator and calcium-modulating cyclophilin-ligand interactor; UNG, uracil-DNA glycosylase. BRONCHIECTASIS A permanent dilation of the bronchi, owing to chronic inflammation, that increases susceptibility to recurrent infections. TERMINAL DEOXY NUCLEOTIDYLTRANSFERASE (TdT). An enzyme that inserts nucleotides into the variable regions of T-cell receptor and immunoglobulin genes, thereby creating junctional diversity. and it results from a mutation in the gene encoding Bruton’s tyrosine kinase (BTK), which has a crucial role in B-cell development. This gene is a member of the SRC family of proto-oncogenes, which encodes protein tyrosine kinases72. These patients have precursor (pre)B cells in their bone marrow; however, the absence of BTK prevents these cells from differentiating into circulating, mature B cells and plasma cells65. Afflicted patients have a low number of circulating B cells and extremely low levels of serum immunoglobulin of all classes. During the first few months of life, patients with XLA are protected by circulating maternal IgG, which crossed the placenta during gestation. Because the concentration of this serum IgG diminishes, the clinical presentation is one of recurrent pyogenic bacterial infections, especially sinopulmonary infections73. BRONCHIECTASIS is the most concerning complication of these recurrent infections and is most commonly found in the middle or lower lobes of the lungs, with the upper lobes being spared74. Germinal-centre formation in these patients is defective, and this leads to the underdevelopment of lymphoid tissues, such as the lymph nodes, Peyer’s patches, spleen, tonsils and adenoids. The standard treatment for patients with XLA is monthly immunoglobulin-replacement therapy to prevent chronic lung disease and to protect against enteroviral meningoencephalitis75. NATURE REVIEWS | IMMUNOLOGY Various autosomal recessive mutations and one translocation have also been described in patients with agammaglobulinaemia. Defects in the individual pre-BCR components λ5 (also known as 14.1, in humans) and Igα have been identified in single patients76,77. The surrogate immunoglobulin light chain is composed of λ5 and VpreB, and it is normally expressed only by progenitor (pro)-B cells and pre-B cells. It escorts the µ immunoglobulin heavy chain (Igµ) to the cell surface and might also assess the capacity of Igµ to bind immunoglobulin light chains. Igα and Igβ form a complex with the surrogate light chain and Igµ and then migrate to the cell surface, where both Igα and Igβ function in transmembrane signal transduction through their immunoreceptor tyrosine-based activation motifs (ITAMs). Lack of these crucial pre-BCR components results in the arrest of B-cell development at the pro-B-cell stage. Defective cell-surface expression of Igµ also results in arrest of B-cell differentiation at the CD19+CD34+ + TERMINAL DEOXYNUCLEOTIDYLTRANSFERASE (TdT) pro-B-cell stage. In contrast to patients with XLA, patients with this defect might have an earlier onset of disease that is associated with more severe complications and no detectable B cells78. In one study, bone-marrow-derived pro-B cells from two patients with distinct mutations in the gene encoding Igµ were used to investigate the VOLUME 5 | NOVEMBER 2005 | 887 © 2005 Nature Publishing Group REVIEWS influence of Igµ on BCR development79. Igµ expression did not have an effect on usage of the gene segments encoding the immunoglobulin heavy chain — the variable-region segment (VH), the diversity segment (D) and the joining segment (JH) — or on immunoglobulin light-chain gene recombination or expression. However, persistent secondary VJ rearrangements of the gene encoding Igκ were noted in Igµ-deficient pro-B cells compared with control pro-B cells. This is probably caused by the lack of signalling through the BCR complex, which is required during normal differentiation of these cells into pre-B cells. Although the substrates for BTK have not been elucidated, IgM at the surface of B cells is thought to be one of its key activators80,81. Collectively, these data underscore the important role of transmembrane IgM in B-cell signalling and in normal B-cell development. BTK has also been implicated in the regulation of B-cell tolerance thresholds82. In these experiments, antibodies were cloned from isolated CD10+CD19+CD27– IgM+ B cells (which are newly emigrated from the bone marrow) from four patients with XLA. These antibodies were found to have a repertoire consisting of specific VH and D gene segments and to undergo extensive secondary recombination on both immunoglobulin light-chain loci compared with antibodies from normal, control B cells. In addition, B cells from patients with XLA were found to produce a considerably higher frequency of self-reactive and polyreactive antibodies than normal, control B cells. Taken together, these data indicate an essential role for BTK in BCR signalling that involves subsequent deletion of cells that produce autoreactive antibodies. Last, karyotypic analysis of the leukocytes of one patient with a novel form of agammaglobulinaemia showed a balanced chromosomal translocation — 46,XX,t(9; 20) (q33.2; q12) — that resulted in a truncated product being encoded by the affected gene, leucine-rich-repeat-containing 8 (LRRC8)83. In a mouse model, retroviral transfection of bone-marrow cells with this mutant gene followed by bone-marrow transplantation led to developmental arrest of B cells at the pro-B cell stage, together with a marked deficiency in pre-B cells. Intriguingly, the unaffected allele in this patient could produce normal LRRC8 protein, indicating that the mutant protein has a dominant-suppressor effect on B-cell development. INDUCIBLE TCELL COSTIMULATOR (ICOS). A homodimeric transmembrane protein that is selectively expressed at the surface of activated T cells. It specifically interacts with ICOS ligand (also known as B7-H2), which is expressed by many cell types, including professional antigen-presenting cells, fibroblasts, epithelial cells and endothelial cells. 888 | NOVEMBER 2005 CVID. Common variable immunodeficiency (CVID) is characterized by a defect in antibody production. Males and females are equally affected, with an incidence between 1 in 10,000 and 1 in 50,000. It is usually diagnosed in the second or third decade of life after a history of recurrent pyogenic sinopulmonary infections84. Serum levels of IgG and IgA are lower than in unaffected individuals; however, approximately onehalf of patients have a normal serum level of IgM. The number of circulating B cells is reduced or normal, and these cells can respond and proliferate appropriately to stimulation with antigen; however, they fail to terminally differentiate into plasma cells, which secrete | VOLUME 5 antibody73. One approach to classifying the B-cell phenotype in patients with CVID involves characterizing the population of class-switched memory B cells (which have the phenotype CD27+IgM–IgD–) in these patients. Two groups can be identified on this basis, using a classification system that was proposed by Warnatz et al.85 Patients in group 1 have a low percentage (less than 0.4%) of class-switched memory B cells, and patients in group 2 have a normal percentage (greater than 0.4%). The former can be subdivided into those patients with an increased proportion of CD19+CD21– peripheral B cells (group 1a) and those with a normal proportion (group 1b). Many patients with splenomegaly and autoimmune cytopaenias were found to segregate into group 1a. Unlike patients with XLA, T-cell proliferation to mitogen is impaired in 40% of patients with CVID, and it is directly associated with the serum level of IgG86. Patients with CVID are at an increased risk of developing numerous associated diseases or conditions, including infections, autoimmune diseases, hepatitis, granulomatous infiltrations, gastrointestinal and pulmonary diseases, and malignancies73,86. The development of structural damage to the lungs that leads to bronchiectasis is also of concern and occurs with a similar distribution to that of patients with XLA, although at a later age of onset74. The mutated genes that produce the CVID phenotype are known only for a minority of patients, and they are diverse in their influence on immune function. They include INDUCIBLE TCELL COSTIMULATOR (ICOS), SH2D1A26,87 (which is involved in XLP), and three genes that have recently been described to be involved: CD19, B-cell-activating factor (BAFF) receptor (BAFFR) and TACI (transmembrane activator and calcium-modulating cyclophilin-ligand interactor). Homozygous loss of ICOS as a result of a large genomic deletion has been characterized in nine patients from four families that are not known to be related 88,89. Binding of ICOS to its ligand induces a marked increase in T-cell proliferation and cytokine production, especially of IL-10, which has been implicated in the differentiation of B cells into plasma cells90. Further genetic testing of these nine individuals showed that they all had identical homozygous haplotypes in the ICOS locus, indicating that the mutation was most probably inherited from the same ancestor. ICOS deficiency has also been investigated as a potential aetiology in patients with HIGM that is caused by an unknown genetic defect91. In this study, 33 patients from 30 families were examined; however, none showed a defect in ICOS expression by activated T cells or a defect in the sequence of the coding region or intron–exon boundaries of ICOS. Therefore, although it is probably involved in B-cell activation and class switching, ICOS could not be identified as the genetic defect that is responsible for the clinical presentation of this subset of patients. Patients with a clinical presentation of CVID have also been identified to have mutations in intermediate components in B-cell signalling and B-cell development www.nature.com/reviews/immunol © 2005 Nature Publishing Group REVIEWS pathways. Specifically, defects in CD19 (J. L. Franco, personal communication, and M. C. van Zelm, personal communication), BAFFR (U. Salzer, personal communication) and TACI 116,117 have recently been identified in patients who have a B-cell-defect phenotype. BAFF is a ligand for BAFFR, TACI and B-cell maturation antigen (BCMA)92,93. Another TNF-family member, a proliferation-inducing ligand (APRIL), also binds BCMA and, with lower affinity, TACI; however, it does not bind BAFFR94. The expression of BAFF and APRIL has been shown to be upregulated by human DCs and monocytes after exposure to interferon-α, interferon-γ or CD40L95. In the presence of IL-10 or transforming growth factor-β, BAFF and APRIL have been shown to induce CSR from Cµ to Cγ and/or Cα gene segments in B cells. Therefore, patients with mutations in BAFFR or TACI probably do not have the B-cell signalling that is provided through interaction with BAFF and APRIL and is required to promote maturation of B cells and generation of a diverse antibody repertoire. IgAD. Selective IgA deficiency (IgAD) is the most common primary immunodeficiency, with a prevalence of between 1 in 400 and 1 in 3,000 in healthy blood donors96 . Ethnicity-specific differences are more disparate, ranging in prevalence from 1 in 500 (in Caucasians) to 1 in 18,000 (in Japanese)97. Although most patients are asymptomatic, recurrent pyogenic sinopulmonary infections are the most frequent illnesses that are associated with IgAD. Several autoimmune diseases that involve multiple organ systems are also associated with IgAD96. The molecular defect that accounts for the absence of class switching to IgA is unknown in most cases, although mutations in TACI can lead to an absence of class switching116,117. Familial studies have implicated the existence of an allelic relationship between IgAD and CVID, indicating that these disorders reflect differential expression of the same molecular aetiology98. In one study of 83 multiply-affected families with IgAD and CVID, increased allele sharing at chromosome 6p21, which is in the proximal region of the MHC, was observed, and this susceptibility locus was designated IGAD1 REF. 99. More sensitive genetic analysis was later carried out in 101 multiple-case families (in which more than one family member is affected) and 110 single-case families, and this further localized the defect to the HLA-DQ and HLA-DR loci100. Therapeutic options Replacement therapy, haematopoietic stem-cell transplantation (using bone marrow, cord blood or peripheral blood) and gene therapy are the available treatments for patients with primary immunodeficiency. Immunoglobulin-replacement therapy is the main treatment for antibody-deficiency disorders and is usually given every 3–4 weeks. It can be given by either an intravenous or a subcutaneous route, and the dose and frequency of administration can be adjusted on the basis of the clinical response of the patient and on the adequacy of the concentration of serum IgG NATURE REVIEWS | IMMUNOLOGY that is maintained in the serum. Enzyme replacement with bovine adenosine deaminase (ADA) modified by polyethylene glycol can be used to treat patients with ADA-deficient SCID who are not candidates for haematopoietic stem-cell or bone-marrow transplantation101. Both modalities of replacement therapy are generally well tolerated and provide these patients with a much improved quality of life. Haematopoietic stem-cell transplantation has been attempted for patients with multiple types of primary immunodeficiency, most successfully to treat patients with SCID102. The largest study of patients with SCID who had received a haematopoietic stem-cell transplant was reported by a multinational registry, the European Group for Blood and Marrow Transplantation and the European Society for Immunodeficiencies103. There were 475 patients who were studied, and each had received a transplant in the previous three decades, including 205 (43%) who did not receive a chemoablative preconditioning regimen. Factors that were associated with poorer outcomes among patients receiving non-HLA-identical transplants included a B– SCID phenotype, absence of a protective environment and/or presence of a pulmonary infection before transplantation. Although the use of a preconditioning regimen was shown to promote functional engraftment in the group of patients with B– SCID, this was not statistically significant compared with the results achieved for other groups of patients. Among patients receiving HLA-identical transplants, survival rates were improved when transplantation occurred at less than 6 months of age and when prophylaxis with the antibiotics trimethoprim and sulphamethoxazole was used. One study of 132 patients with SCID who had received a haematopoietic stem-cell transplant(s) within the previous two decades has also been reported11. Most of these patients received neither pre-transplantation chemotherapeutic conditioning (to aid engraftment) nor post-transplantation graft-versus-host-disease prophylaxis. Normal T-cell function was seen within 2 weeks of transplantation of unfractionated HLA-identical bone marrow but was delayed by up to 4 months in patients who received T-cell-depleted bone marrow, owing to the fact that mature T cells are not transferred to these patients104. The survival rate for these 132 patients was positively correlated with Caucasian race, female gender and younger age at the time of transplantation. It is important to note that, although patients in this study who had not received myeloablative pre-transplantation chemotherapy had higher rates of survival, long-term studies that were carried out more than a decade after transplantation have shown an accelerated rate of decline in the TRECs in T cells from such patients and in the development of oligoclonal populations of T cells12,105. Similar longitudinal studies of T-cell function and thymic output have not been carried out in patients with SCID who received pre-transplantation conditioning. Further long-term follow-up studies are needed to assess the efficacy VOLUME 5 | NOVEMBER 2005 | 889 © 2005 Nature Publishing Group REVIEWS Box 1 | Retroviral insertional mutagenesis The clinical trials of gene therapy for the treatment of X-linked severe combined immunodeficiency (SCID) used a retroviral vector to transduce CD34+ haematopoietic stem cells with the gene encoding the common cytokine-receptor γ-chain (γc). Retroviral vectors can stably integrate themselves into the DNA of the host cell; however, the precise location of their insertion cannot be predicted. This property, combined with the propensity of these vectors to insert themselves into transcriptionally active genes, is the reason why the unfavourable side-effect of insertional mutagenesis is a distinct possibility, and it could result in oncogene activation by the inserted target gene. Two patients in the French clinical trial have developed T-cell acute lymphoblastic leukaemia, which is probably a consequence of retroviral insertion near the LIM domain only 2 (LMO2) oncogene109,110. A third patient developed lymphatic cancer. Increased expression of LMO2 is postulated to block T-cell differentiation, and expression of γc by these cells might then facilitate signalling to induce their division, resulting in clonal proliferation110. The exact mechanism by which the LMO2 locus was selectively targeted for pro-virus integration in these patients is uncertain. Before this trial, the risk of retroviral insertional mutagenesis was thought to be mainly theoretical, because a single round of transduction would not account for the multiple mutations that are generally required for clonal proliferation. This idea was further supported by the absence of this phenomenon in preclinical and clinical trials using replication-incompetent retroviral vectors112. Nevertheless, insertional mutagenesis proved to be a serious adverse event in these patients, and further insight is needed for the design and delivery of retroviral vectors before this life-saving therapy can be provided in the future. of various approaches to haematopoietic stem-cell transplantation and to assist in the identification of patient subgroups that are likely to benefit from one form of therapy over another. Clinical trials have been carried out using gene therapy for the treatment of patients with X-linked, recessive SCID (which is caused by a deficiency in the common cytokine-receptor γ-chain, γc ) and for patients with ADA deficiency106–108. For patients with X-linked, recessive SCID, those without an HLA-identical sibling were infused with autologous CD34+ haematopoietic stem cells (enriched from bone marrow) that had been incubated with cytokines and exposed to a γc-encoding retroviral vector. Initial results were promising, with nine 1. 2. 3. 4. 5. 6. Notarangelo, L. et al. Primary immunodeficiency diseases: an update. J. Allergy Clin. Immunol. 114, 677–687 (2004). This is an updated summary and classification scheme of primary immunodeficiency diseases that is based on the most recent consensus of the Primary Immunodeficiency Disease Classification Committee of the International Union of Immunological Societies. Puel, A., Ziegler, S. F., Buckley, R. H. & Leonard, W. J. Defective IL7R expression in T–B+NK+ severe combined immunodeficiency. Nature Genet. 20, 394–397 (1998). Dadi, H. K., Simon, A. J. & Roifman, C. M. Effect of CD3δ deficiency on maturation of α/β and γ/δ T-cell lineages in severe combined immunodeficiency. N. Engl. J. Med. 349, 1821–1828 (2003). de Saint Basile, G. et al. Severe combined immunodeficiency caused by deficiency in either the δ or the ε subunit of CD3. J. Clin. Invest. 114, 1512–1517 (2004). Noguchi, M. et al. Interleukin-2 receptor γ chain mutation results in X-linked severe combined immunodeficiency in humans. Cell 73, 147–157 (1993). Kung, C. et al. Mutations in the tyrosine phosphatase CD45 gene in a child with severe combined immunodeficiency disease. Nature Med. 6, 343–345 (2000). 890 | NOVEMBER 2005 7. 8. 9. 10. 11. 12. 13. of ten treated patients developing normal T-cell function and not requiring gammaglobulin-replacement therapy. However, ∼3 years after gene therapy, the 2 youngest patients developed clonal proliferation of mature T-cell populations109. A third child also developed an abnormal clonal phenotype. In the first two cases, leukaemia-like proliferation was caused by retroviral-vector insertion in a locus that contains the LIM domain only 2 (LMO2) oncogene, and the expression of LMO2 in mature T cells led to their uncontrolled proliferation110. This phenomenon is known as retroviral insertional mutagenesis BOX 1. Clearly, there is tremendous potential for the use of gene therapy to treat patients with SCID or other selective immune defects, and advances in the selection of additional vectors and the elucidation of the mechanisms of gene transduction might allow its re-implementation for the treatment of these patients. Looking forward As we look to the future of the field that encompasses primary immunodeficiency diseases, emphasis needs to be placed on the early detection of life-threatening SCID, the elucidation of the unknown molecular defects that underlie immune dysfunction and the implementation of new modalities of therapy that aim to correct these genetic defects. Adoption of SCID into newborn-screening programmes will aid in the diagnosis, treatment and long-term management of these patients. Advances in haematopoietic stem-cell isolation protocols allow transplantation of far greater numbers of highly purified haematopoietic stem cells, and in conjunction with more efficient negative selection of T cells, this will undoubtedly improve the outcomes of haematopoietic stem-cell transplantation111. Gene therapy, although in its infancy, has shown remarkable success in some patients, and a greater understanding of the mechanisms that are involved and development of safer methods of gene correction might allow this promising therapy to be used in clinical practice. Russell, S. M. et al. Mutation of Jak3 in a patient with SCID: essential role of Jak3 in lymphoid development. Science 270, 797–800 (1995). Moshous, D. et al. Artemis, a novel DNA double-strand break repair/V(D)J recombination protein, is mutated in human severe combined immune deficiency. Cell 105, 177–186 (2001). Schwarz, K. et al. RAG mutations in human B cell-negative SCID. Science 274, 97–99 (1996). Giblett, E., Anderson, J., Cohen, F., Pollara, B. & Meuwissen, H. Adenosine-deaminase deficiency in two patients with severely impaired cellular immunity. Lancet 2, 1067–1069 (1972). Buckley, R. H. Molecular defects in human severe combined immunodeficiency and approaches to immune reconstitution. Annu. Rev. Immunol. 22, 625–655 (2004). This paper reviews SCID and the outcomes of bone-marrow transplantation of 132 patients over two decades. Patel, D. D. et al. Thymic function after hematopoietic stem-cell transplantation for the treatment of severe combined immunodeficiency. N. Engl. J. Med. 342, 1325–1332 (2000). Myers, L. A., Patel, D. D., Puck, J. M. & Buckley, R. H. Hematopoietic stem cell transplantation for severe combined immunodeficiency in the neonatal period | VOLUME 5 leads to superior thymic output and improved survival. Blood 99, 872–878 (2002). 14. Lindegren, M. L. et al. Applying public health strategies to primary immunodeficiency diseases: a potential approach to genetic disorders. MMWR Recomm. Rep. 53, 1–29 (2004). 15. Chan, K. & Puck, J. M. Development of population-based newborn screening for severe combined immunodeficiency. J. Allergy Clin. Immunol. 115, 391–398 (2005). This paper describes the novel approach of measuring the number of TRECs in DNA isolated from dried blood spots to screen infants for SCID. 16. Yagi, H. et al. Role of TBX1 in human del22q11.2 syndrome. Lancet 362, 1366–1373 (2003). 17. Stoller, J. Z. & Epstein, J. A. Identification of a novel nuclear localization signal in Tbx1 that is deleted in DiGeorge syndrome patients harboring the 1223delC mutation. Hum. Mol. Genet. 14, 885–892 (2005). 18. Arnaiz-Villena, A. et al. Primary immunodeficiency caused by mutations in the gene encoding the CD3-γ subunit of the T-lymphocyte receptor. N. Engl. J. Med. 327, 529–533 (1992). 19. Soudais, C., de Villartay, J.-P., Le Deist, F., Fischer, A. & Lisowska-Grospierre, B. Independent mutations of the human CD3-ε gene resulting in a T cell receptor/CD3 complex immunodeficiency. Nature Genet. 3, 77–81 (1993). www.nature.com/reviews/immunol © 2005 Nature Publishing Group REVIEWS 20. de la Salle, H. et al. HLA class I deficiencies due to mutations in subunit 1 of the peptide transporter TAP1. J. Clin. Invest. 103, R9–R13 (1999). 21. Donato, L. et al. Association of HLA class I antigen deficiency related to a TAP2 gene mutation with familial bronchiectasis. J. Pediatr. 127, 895–900 (1995). 22. Masternak, K., Muhlethaler-Mottet, A., Villard, J., Peretti, M. & Reith, W. Molecular genetics of the bare lymphocyte syndrome. Rev. Immunogenet. 2, 267–282 (2000). 23. Goldman, F. D. et al. Defective expression of p56lck in an infant with severe combined immunodeficiency. J. Clin. Invest. 102, 421–429 (1998). 24. Arpaia, E., Shahar, M., Dadi, H., Cohen, A. & Rolfman, C. M. Defective T cell receptor signaling and CD8+ thymic selection in humans lacking Zap-70 kinase. Cell 76, 947–958 (1994). 25. de la Calle-Martin, O. et al. Familial CD8 deficiency due to a mutation in the CD8α gene. J. Clin. Invest. 108, 117–123 (2001). 26. Engel, P., Eck, M. J. & Terhorst, C. The SAP and SLAM families in immune responses and X-linked lymphoproliferative disease. Nature Rev. Immunol. 3, 813–821 (2003). 27. Ochs, H. D., Ziegler, S. F. & Torgerson, T. R. FOXP3 acts as a rheostat of the immune response. Immunol. Rev. 203, 156–164 (2005). 28. Gambineri, E., Torgerson, T. R. & Ochs, H. D. Immune dysregulation, polyendocrinopathy, enteropathy, and X-linked inheritance (IPEX), a syndrome of systemic autoimmunity caused by mutations of FOXP3, a critical regulator of T-cell homeostasis. Curr. Opin. Rheumatol. 15, 430–435 (2003). 29. Bennett, C. L. et al. The immune dysregulation, polyendocrinopathy, enteropathy, X-linked syndrome (IPEX) is caused by mutations of FOXP3. Nature Genet. 27, 20–21 (2001). 30. Baecher-Allan, C., Brown, J. A., Freeman, G. J. & Hafler, D. A. CD4+CD25high regulatory cells in human peripheral blood. J. Immunol. 167, 1245–1253 (2001). 31. Kobayashi, I., Nakanishi, M., Okano, M., Sakiyama, Y. & Matsumoto, S. Combination therapy with tacrolimus and betamethasone for a patient with X-linked auto-immune enteropathy. Eur. J. Pediatr. 154, 594–595 (1995). 32. Baud, O. et al. Treatment of the immune dysregulation, polyendocrinopathy, enteropathy, X-linked syndrome (IPEX) by allogeneic bone marrow transplantation. N. Engl. J. Med. 344, 1758–1762 (2001). 33. Wildin, R. S., Smyk-Pearson, S. & Filipovich, A. H. Clinical and molecular features of the immunodysregulation, polyendocrinopathy, enteropathy, X-linked (IPEX) syndrome. J. Med. Genet. 39, 537–545 (2002). 34. Aaltonen, J. et al. An autoimmune disease, APECED, caused by mutations in a novel gene featuring two PHDtype zinc-finger domains. Nature Genet. 17, 399–403 (1997). 35. Su, M. A. & Anderson, M. S. Aire: an update. Curr. Opin. Immunol. 16, 746–752 (2004). 36. Gotter, J., Brors, B., Hergenhahn, M. & Kyewski, B. Medullary epithelial cells of the human thymus express a highly diverse selection of tissue-specific genes colocalized in chromosomal clusters. J. Exp. Med. 199, 155–166 (2004). This study investigates the expression of selfproteins at the surface of human thymic epithelial cells, and the authors postulate a role for AIRE in regulating the expression of these self-proteins. 37. Watanabe, N. et al. Human thymic stromal lymphopoietin promotes dendritic cell-mediated CD4+ T cell homeostatic expansion. Nature Immunol. 5, 426–434 (2004). 38. Uchida, D. et al. AIRE functions as an E3 ubiquitin ligase. J. Exp. Med. 199, 167–172 (2004). 39. Le Deist, F. et al. Clinical, immunological, and pathological consequences of Fas-deficient conditions. Lancet 348, 719–723 (1996). 40. Rieux-Laucat, F., Fischer, A. & Le Deist, F. L. Cell-death signaling and human disease. Curr. Opin. Immunol. 15, 325–331 (2003). 41. Rieux-Laucat, F. et al. Mutations in Fas associated with human lymphoproliferative syndrome and autoimmunity. Science 268, 1347–1349 (1995). 42. Vaishnaw, A. K. et al. The molecular basis for apoptotic defects in patients with CD95 (Fas/Apo-1) mutations. J. Clin. Invest. 103, 355–363 (1999). 43. Chun, H. J. et al. Pleiotropic defects in lymphocyte activation caused by caspase-8 mutations lead to human immunodeficiency. Nature 419, 395–399 (2002). 44. Wang, J., Chun, H. J., Wong, W., Spencer, D. M. & Lenardo, M. J. Caspase-10 is an initiator caspase in death receptor signaling. Proc. Natl Acad. Sci. USA 98, 13884–13888 (2001). 45. Wu, J. et al. Fas ligand mutation in a patient with systemic lupus erythematosus and lymphoproliferative disease. J. Clin. Invest. 98, 1107–1113 (1996). 46. Ramenghi, U. et al. Deficiency of the Fas apoptosis pathway without Fas gene mutations is a familial trait predisposing to development of autoimmune diseases and cancer. Blood 95, 3176–3182 (2000). 47. Holzelova, E. et al. Autoimmune lymphoproliferative syndrome with somatic Fas mutations. N. Engl. J. Med. 351, 1409–1418 (2004). 48. Kenter, A. L. Class-switch recombination: after the dawn of AID. Curr. Opin. Immunol. 15, 190–198 (2003). 49. Papavasiliou, F. N. & Schatz, D. G. Somatic hypermutation of immunoglobulin genes: merging mechanisms for genetic diversity. Cell 109, S35–S44 (2002). 50. Aruffo, A. et al. The CD40 ligand, gp39, is defective in activated T cells from patients with X-linked hyper-IgM syndrome. Cell 72, 291–300 (1993). 51. Revy, P. et al. Activation-induced cytidine deaminase (AID) deficiency causes the autosomal recessive form of the hyper-IgM syndrome (HIGM2). Cell 102, 565–575 (2000). 52. Imai, K. et al. Human uracil-DNA glycosylase deficiency associated with profoundly impaired immunoglobulin class-switch recombination. Nature Immunol. 4, 1023–1028 (2003). 53. Ferrari, S. et al. Mutations of CD40 gene cause an autosomal recessive form of immunodeficiency with hyper IgM. Proc. Natl Acad. Sci. USA 98, 12614–12619 (2001). 54. Kutukculer, N. et al. Disseminated Cryptosporidium infection in an infant with hyper-IgM syndrome caused by CD40 deficiency. J. Pediatr. 142, 194–196 (2003). 55. Imai, K. et al. Hyper-IgM syndrome type 4 with a B lymphocyte-intrinsic selective deficiency in Ig class-switch recombination. J. Clin. Invest. 112, 136–142 (2003). 56. Doffinger, R. et al. X-linked anhidrotic ectodermal dysplasia with immunodeficiency is caused by impaired NF-κB signaling. Nature Genet. 27, 277–285 (2001). 57. Jain, A. et al. Specific missense mutations in NEMO result in hyper-IgM syndrome with hypohydrotic ectodermal dysplasia. Nature Immunol. 2, 223–228 (2001). 58. Orange, J. S. et al. The presentation and natural history of immunodeficiency caused by nuclear factor κB essential modulator mutation. J. Allergy Clin. Immunol. 113, 725–733 (2004). 59. Courtois, G. et al. A hypermorphic IκBα mutation is associated with autosomal dominant anhidrotic ectodermal dysplasia and T cell immunodeficiency. J. Clin. Invest. 112, 1108–1115 (2003). 60. Minegishi, Y. et al. Mutations in activation-induced cytidine deaminase in patients with hyper IgM syndrome. Clin. Immunol. 97, 203–210 (2000). 61. Catalan, N. et al. The block in immunoglobulin class switch recombination caused by activation-induced cytidine deaminase deficiency occurs prior to the generation of DNA double strand breaks in switch µ region. J. Immunol. 171, 2504–2509 (2003). 62. Chaudhuri, J., Khuong, C. & Alt, F. W. Replication protein A interacts with AID to promote deamination of somatic hypermutation targets. Nature 430, 992–998 (2004). 63. Etzioni, A. & Ochs, H. D. The hyper IgM syndrome — an evolving story. Pediatr. Res. 56, 1–7 (2004). 64. Puel, A., Picard, C., Ku, C.-L., Smahi, A. & Casanova, J. L. Inherited disorders of NF-κB-mediated immunity in man. Curr. Opin. Immunol. 16, 34–41 (2004). 65. Ochs, H. D. & Notarangelo, L. X-linked immunodeficiencies. Curr. Allergy Asthma Rep. 4, 339–348 (2004). 66. Akira, S. & Takeda, K. Toll-like receptor signalling. Nature Rev. Immunol. 4, 499–511 (2004). 67. Medvedev, A. E. et al. Distinct mutations in IRAK-4 confer hyporesponsiveness to lipopolysaccharide and interleukin-1 in a patient with recurrent bacterial infections. J. Exp. Med. 198, 521–531 (2003). 68. Picard, C. et al. Pyogenic bacterial infections in humans with IRAK-4 deficiency. Science 299, 2076–2079 (2003). This report describes three children with a defect in TLR signalling and their susceptibility to infection. 69. Saleh, M. et al. Differential modulation of endotoxin responsiveness by human caspase-12 polymorphisms. Nature 429, 75–79 (2004). References 67–69 show the emerging role of defects in the innate immune system as an aetiology for primary immunodeficiency diseases. 70. Buckley, R. H. Primary immunodeficiency diseases due to defects in lymphocytes. N. Engl. J. Med. 343, 1313–1324 (2000). 71. Bruton, O. Agammaglobulinemia. Pediatrics 9, 722–728 (1952). 72. Vetrie, D. et al. The gene involved in X-linked agammaglobulinaemia is a member of the src family of protein-tyrosine kinases. Nature 361, 226–233 (1993). NATURE REVIEWS | IMMUNOLOGY 73. Buckley, R. H. Pulmonary complications of primary immunodeficiencies. Paediatr. Respir. Rev. 5, S225–S233 (2004). 74. Curtin, J. J., Webster, A. D., Farrant, J. & Katz, D. Bronchiectasis in hypogammaglobulinaemia — a computed tomography assessment. Clin. Radiol. 44, 82–84 (1991). 75. McKinney, R. E. Jr, Katz, S. L. & Wilfert, C. M. Chronic enteroviral meningoencephalitis in agammaglobulinemic patients. Rev. Infect. Dis. 9, 334–356 (1987). 76. Minegishi, Y. et al. Mutations in the human λ5/14.1 gene result in B cell deficiency and agammaglobulinemia. J. Exp. Med. 187, 71–77 (1998). 77. Minegishi, Y. et al. Mutations in Igα (CD79a) result in a complete block in B-cell development. J. Clin. Invest. 104, 1115–1121 (1999). 78. Yel, L. et al. Mutations in the µ heavy-chain gene in patients with agammaglobulinemia. N. Engl. J. Med. 335, 1486–1493 (1996). 79. Meffre, E. et al. Immunoglobulin heavy chain expression shapes the B cell receptor repertoire in human B cell development. J. Clin. Invest. 108, 879–886 (2001). 80. Aoki, Y., Isselbacher, K. & Pillai, S. Bruton tyrosine kinase is tyrosine phosphorylated and activated in preB lymphocytes and receptor-ligated B cells. Proc. Natl Acad. Sci USA 91, 10606–10609 (1994). 81. Genevier, H. C. & Callard, R. E. Impaired Ca2+ mobilization by X-linked agammaglobulinaemia (XLA) B cells in response to ligation of the B cell receptor (BCR). Clin. Exp. Immunol. 110, 386–391 (1997). 82. Ng, Y.-S., Wardemann, H., Chelnis, J., Cunningham-Rundles, C. & Meffre, E. Bruton’s tyrosine kinase is essential for human B cell tolerance. J. Exp. Med. 200, 927–934 (2004). 83. Sawada, A. et al. A congenital mutation of the novel gene LRRC8 causes agammaglobulinemia in humans. J. Clin. Invest. 112, 1707–1713 (2003). 84. Primary Immunodeficiency Diseases. Report of an IUIS Scientific Committee. International Union of Immunological Societies. Clin. Exp. Immunol. 118, 1–28 (1999). 85. Warnatz, K. et al. Severe deficiency of switched memory B cells (CD27+IgM–IgD–) in subgroups of patients with common variable immunodeficiency: a new approach to classify a heterogeneous disease. Blood 99, 1544–1551 (2002). 86. Cunningham-Rundles, C. & Bodian, C. Common variable immunodeficiency: clinical and immunological features of 248 patients. Clin. Immunol. 92, 34–48 (1999). This is the largest case study so far of patients with CVID. 87. Morra, M. et al. Alterations of the X-linked lymphoproliferative disease gene SH2D1A in common variable immunodeficiency syndrome. Blood 98, 1321–1325 (2001). 88. Grimbacher, B. et al. Homozygous loss of ICOS is associated with adult-onset common variable immunodeficiency. Nature Immunol. 4, 261–268 (2003). 89. Salzer, U. et al. ICOS deficiency in patients with common variable immunodeficiency. Clin. Immunol. 113, 234–240 (2004). References 88 and 89 report that a defect in ICOS is the cause of CVID in nine individuals. 90. Choe, J. & Choi, Y. S. IL-10 interrupts memory B cell expansion in the germinal center by inducing differentiation into plasma cells. Eur. J. Immunol. 28, 508–515 (1998). 91. Lee, W.-I. et al. Inducible CO-stimulator molecule, a candidate gene for defective isotype switching, is normal in patients with hyper-IgM syndrome of unknown molecular diagnosis. J. Allergy Clin. Immunol. 112, 958–964 (2003). 92. Gross, J. A. et al. TACI and BCMA are receptors for a TNF homologue implicated in B-cell autoimmune disease. Nature 404, 995–999 (2000). 93. Thompson, J. S. et al. BAFF-R, a newly identified TNF receptor that specifically interacts with BAFF. Science 293, 2108–2111 (2001). 94. Rennert, P. et al. A soluble form of B cell maturation antigen, a receptor for the tumor necrosis factor family member APRIL, inhibits tumor cell growth. J. Exp. Med. 192, 1677–1684 (2000). 95. Litinskiy, M. B. et al. DCs induce CD40-independent immunoglobulin class switching through BLyS and APRIL. Nature Immunol. 3, 822–829 (2002). 96. Cunningham-Rundles, C. Physiology of IgA and IgA deficiency. J. Clin. Immunol. 21, 303–309 (2001). 97. Burrows, P. D. & Cooper, M. D. IgA deficiency. Adv. Immunol. 65, 245–276 (1997). VOLUME 5 | NOVEMBER 2005 | 891 © 2005 Nature Publishing Group REVIEWS 98. Vorechovsky, I. et al. Family and linkage study of selective IgA deficiency and common variable immunodeficiency. Clin. Immunol. Immunopathol. 77, 185–192 (1995). 99. Vorechovsky, I., Webster, A. D. B., Plebani, A. & Hammarstrom, L. Genetic linkage of IgA deficiency to the major histocompatibility complex: evidence for allele segregation distortion, parent-of-origin penetrance differences, and the role of anti-IgA antibodies in disease predisposition. Am. J. Hum. Genet. 64, 1096–1109 (1999). 100. Kralovicova, J., Hammarstrom, L., Plebani, A., Webster, A. D. B. & Vorechovsky, I. Fine-scale mapping at IGAD1 and genome-wide genetic linkage analysis implicate HLA-DQ/DR as a major susceptibility locus in selective IgA deficiency and common variable immunodeficiency. J. Immunol. 170, 2765–2775 (2003). 101. Hershfield, M. S. PEG-ADA replacement therapy for adenosine deaminase deficiency: an update after 8.5 years. Clin. Immunol. Immunopathol. 76, S228–S232 (1995). 102. Buckley, R. H. A historical review of bone marrow transplantation for immunodeficiencies. J. Allergy Clin. Immunol. 113, 793–800 (2004). 103. Antoine, C. et al. Long-term survival and transplantation of haemopoietic stem cells for immunodeficiencies: report of the European experience 1968–99. Lancet 361, 553–560 (2003). This paper details the outcomes of 475 patients in Europe who, over three decades, received a haematopoietic stem-cell transplant. 104. Buckley, R. H. et al. Hematopoietic stem-cell transplantation for the treatment of severe combined immunodeficiency. N. Engl. J. Med. 340, 508–516 (1999). 892 | NOVEMBER 2005 105. Sarzotti, M. et al. T cell repertoire development in humans with SCID after nonablative allogeneic marrow transplantation. J. Immunol. 170, 2711–2718 (2003). 106. Cavazzana-Calvo, M. et al. Gene therapy of human severe combined immunodeficiency (SCID)-X1 disease. Science 288, 669–672 (2000). This report was the first description of successful gene therapy for patients with X-linked SCID. 107. Hacein-Bey-Abina, S. et al. Sustained correction of X-linked severe combined immunodeficiency by ex vivo gene therapy. N. Engl. J. Med. 346, 1185–1193 (2002). 108. Aiuti, A. et al. Correction of ADA-SCID by stem cell gene therapy combined with nonmyeloablative conditioning. Science 296, 2410–2413 (2002). 109. Hacein-Bey-Abina, S. et al. LMO2-associated clonal T cell proliferation in two patients after gene therapy for SCID-X1. Science 302, 415–419 (2003). 110. McCormack, M. P. & Rabbitts, T. H. Activation of the T-cell oncogene LMO2 after gene therapy for X-linked severe combined immunodeficiency. N. Engl. J. Med. 350, 913–922 (2004). This study details the role of LMO2 in the development of the insertional mutagenesis that was seen in several patients with X-linked SCID after gene therapy. 111. Handgretinger, R. et al. Megadose transplantation of purified peripheral blood CD34+ progenitor cells from HLA-mismatched parental donors in children. Bone Marrow Transplant. 27, 777–783 (2001). 112. Kohn, D. B. et al. American Society of Gene Therapy (ASGT) ad hoc subcommittee on retroviral-mediated gene transfer to hematopoietic stem cells. Mol. Ther. 8, 180–187 (2003). | VOLUME 5 113. Frank, J. et al. Exposing the human nude phenotype. Nature 398, 473–474 (1999). 114. Fu, C., Turck, C. W., Kurosaki, T. & Chan, A. C. BLNK: a central linker protein in B cell activation. Immunity 9, 93–103 (1998). 115. Minegishi, Y. et al. An essential role for BLNK in human B cell development. Science 286, 1954–1957 (1999). 116. Salzer, U. et al. Mutations in TNFRSF13B encoding TACI are associated with common variable immunodeficiency in humans. Nature Genet. 37, 820–828 (2005). 117. Castigli, E. et al. TACI is mutant in common variable immunodeficiency and IgA deficiency. Nature Genet. 37, 829–834 (2005). Competing interests statement The authors declare no competing financial interests. Online links DATABASES The following terms in this article are linked online to: OMIM: http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=OMIM ALPS1a | ALPS1b | APECED | CVID | DiGeorge syndrome | HIGM1 | HIGM2 | HIGM3 | HIGM4 | IPEX | XLA | XLP FURTHER INFORMATION Charlotte Cunningham-Rundles homepage: http://directory. mssm.edu/faculty/facultyInfo.php?id=18843&deptid=6 Access to this interactive links box is free online. www.nature.com/reviews/immunol © 2005 Nature Publishing Group