Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Symposium AIDS: Definition, Epidemiology, and Etiology Jeffrey Laurence, MD A fatal new disorder characterized by a rare malignancy, Kaposi's sarcoma, Pneumocystis carinii pneumonia, and other severe opportunistic infections was reported by the Centers for Disease Control (CDC), Atlanta, in 1981. The profound disruption of cellular immune function underlying these illnesses led to their description as the acquired immunodeficiency syndrome (AIDS). For the purposes of national reporting, a case definition of AIDS was published in 1982.l These stringent clinical criteria encompassed secondary conditions that reliably reflect immune suppression. The etiologic agent of AIDS, a retrovirus, was identified two years later. This discovery was made independently by Luc Montagnier of the Pasteur Institute in Paris and Robert C. Gallo at the National Cancer Institute. The French group called the agent LAV (lymphadenopathy-associated virus), and the American workers called it HTLV-III (human T-cell lymphotropic virus type III). IgG antibodies to the human immunodeficiency virus (HIV, formerly From Cornell University Medical Center, HematologyOncology Division, New York, New York 10021. known as HTLV-III/LAV) structural and envelope antigens were soon noted in a large proportion of persons with AIDS, as well as in individuals with risk factors for the disease. Further evidence for the primary role of this virus came from its frequent isolation from AIDS patients and its ability to induce immune abnormalities typical of AIDS in chimpanzees. The identification of anti-HIV antibodies and the culturing of infectious virions refined the specificity and broadened the scope of the definition of AIDS.2 AIDS Risk Groups By June 1986, twenty-one thousand five hundred seventeen cases of AIDS had been reported to the CDC, of which approximately 1.5% involved children. Over 11,700 persons have died, including 71% of those diagnosed before July 1984. The number of cases continues to rise, although the doubling time has decreased from a mean of six months through 1983 to approximately 11 months in late 1985.3 AIDS was first noted among previously healthy homosexual men in New York City, San Francisco, and Los Angeles. Additional cases were soon associated with other risk factors. Homosexual and bisexual men continue to represent the dominant risk category, with 73% of cases, but the susceptible population has expanded to include heterosexual intravenous drug abusers (17%), recipients of blood or blood components within five years of diagnosis (1.5%), hemophiliacs treated with factor concentrates (0.7%), and the heterosexual partners of AIDS patients or of persons at increased risk for AIDS (1.0%). The remaining 6.8% represent people born outside of the United States, in countries such as Haiti, where many cases have not been linked to specific risk factors. In addition, several thousand cases have been noted in Europe and in 23 African countries. HIV-Related Disorders A significant portion of HlV-associated disease lies outside of the CDC criteria for AIDS. Persistent generalized lymphadenopathy has occurred with increasing frequency among otherwise asymptomatic members of the risk groups previously outlined. A spectrum of disorders, including nodal hyperplasia and systemic symptoms such as fever, night sweats, and fatigue, is thought to represent the first clinical indications of infection in 20% to 40% of AIDS patients. The term AIDS-related complex (ARC) has LABORATORY MEDICINE • VOL. 17, NO. 11, NOVEMBER 1986 6 5 9 Table I: Diagnosis of the AIDS-Related Complex* Fever >3 months Weight loss ( -10% total body weight) Lymphadenopathy, 3 months Diarrhea Fatigue Night sweats Decrease in T-helper cells Decrease in T-helper T-suppressor ratio Increase In serum globulins Decrease in lymphocyte blastogenesis Anergy *Two clinical findings plus two laboratory abnormalities indicates an AIDS-related complex. Table II: Estimated No. of Adults Infected With HIV in the United States* Group Homosexuals or bisexuals Intravenous drug abusers Hemophiliacs Haitians^ Heterosexual contacts of persons at high risk Recipients of blood or blood products Persons at no known risk Total No. of Persons Living With AIDSf Estimated No. of Persons With HIV 4,485 923 31 139 48 1,345,500 270,000 8,970 41,700 14,400 69 214 5,909 20,700 64,200 1,765,470 'Data taken from reference 16. tCenters for Disease Control, surveillance data, July 22, 1985. t-Or persons born in a country where AIDS cases have not been associated with known high risk. been proposed to describe a constellation of signs, symptoms, and laboratory abnormalities that might be predictive of AIDS (Table I). An alternative classification scheme, employing symptoms, HIV serology, and in vitro and in vivo immunologic measurements, has been offered by a group at Walter Reed Army Medical Center, Washington, DC." Critical to an understanding of AIDS epidemiology is a prospective survey of individuals with ARC or other manifestations of early HIV infection. Estimates of the number of such individuals vary widely, depending upon assumptions of risk group size and efficiency of viral transmission. On the basis of one such study, calculations of the number of asymptomatic viral carriers in various risk groups have been given (Table II). One problem with these estimates is the uncertainty of the interval required for seroconversion following infection. In terms of the two main routes by which HIV is spread—parenteral exposure and sexual intercourse—anecdotal reports document antibody development within 40 days of inoculation. The range is wide, extending to over nine months. Certain asymptomatic viral carriers appear not to generate viral-specific antibody. These seronegative carriers represent an obvious problem in terms of delineating epidemiology of viral trans- mission, as well as in the screening of blood products solely by tests for antibody. Serologic Assays for HIV The assays used to determine seroconversion—the enzyme-linked immunosorbent assay (ELISA), Western blot, ELISA competition with HIV components, and immunofluorescence—differ in their sensitivity and specificity. The Western blot is considered by many to be the most specific, with electrophoretic separation of viral proteins (p) and glycoproteins (gp) yielding a profile of bands characteristic of HIV when antibody-positive sera are applied. Antibodies to the viral core protein (p24) and its group antigen (gag) precursor peptide (p55) occur earliest in the course of HIV infection. They may disappear as the clinical syndrome becomes manifest and advances. The primary envelope (env) gene product (gpl60) and its processed fragments (gpl20) and the transmembrane protein (gp41) react with most sera from HIV-infected individuals regardless of clinical stage. Antibodies that recognize these antigens on Western blot analysis or by immunofluorescence using HIV-infected T cells appear the most stable indicators of infection, at least among cohorts in the United States. A diagram of the HIV virion shows the interrelationship of these 6 6 0 LABORATORY MEDICINE • VOL. 17, NO. 11, NOVEMBER 1986 proteins (Fig 1). Also shown are radioimmunoprecipitations of certain envelope and structural components of HIV in sera of two representative AIDS patients. Other immunogenic regions of HIV include tatm {trans-acting, type III) protein pl4; p27 of the 3'-orf (open reading frame); p23 of sor (short open reading frame); pl5, p9, p7 of gag; and p55, p61, and p31, portions of the polymerase (pol) gene product. The functions of these antigens will be discussed later. The previously described protean manifestations of HIV infection are a challenge in the clarification of an incubation period for this agent. HIV is commonly referred to as the "AIDS virus," but this is clearly a misnomer. Clinical presentations consonant with the CDC surveillance definition are quite different; other disorders linked to HIV infection are even more varied (Table III). An acute mononucleosis-like illness characterized by fever, malaise, pharyngitis, and erythematous cutaneous lesions may occur within days to weeks following nosocomial inoculation with blood from an HlV-seropositive person. Lymphadenopathy and, less frequently, splenomegaly may develop some weeks to months thereafter and persist indefinitely. The interval to appearance of an opportunistic infection, Kaposi's sarcoma, or B-cell lymphoma ("clinical AIDS") is currently being defined. In one study of hemophilia A patients in Pennsylvania, recent seroconvertors had a 12.8 ± 4.8% incidence of AIDS within three years. The syndrome developed a median of 27.5 months after seroconversion.5 The incidence of AIDS among a cohort of homosexual males in Manhattan (New York City) followed for the same period was 34.2 ± 8.0%.5 No plateau was apparent for either group. Infection with HIV is probably a lifelong event. In animal models of nononcogenic retroviral disorders, persistence of viral genome and continuous or episodic viral replication occurs uniformly, even in the immunocompetent host. Immune Defects Immunologic sequelae of HIV infection are also variable. The most prominent abnormalities in clinical AIDS are an absolute lymphopenia, a Mojor glycoprotein (gpl20) ' 160— 120— - 64— m 31— — .Tronsmembrone protein (gp41) ID Reverse tronscnptose POL Table III: HIV-Related Disorders AIDS: opportunistic infections, Kaposi's sarcoma, B-cell lymphomas, as defined by the Centers for Disease Control Lymphadenopathy syndrome Acute mononucleosis-like disease: fever, malaise, pharyngitis, rash Neuropsychiatric disorders: aseptic meningitis, myelitis, dementia, psychosis Tetratogenesis, spontaneous abortion Thrombocytopenia Malignancy: carcinomas; lymphoproliferative disorders, including lymphoid interstitial pneumonitis Dermatologic disease: atopic dermatitis, leukoplakia gp!20 gp41 3' LTR -Mojor core protein ( p 2 4 ) ENV-gpl60 Fig 1. At left, Schematic of radioimmunoprecipitation (RIP-SDSPAGE) analysis of serum samples from two AIDS patients. T cells were infected with HTV, metabolically labeled with 36S-methionine and disrupted and electrophoresed on polyacrylamide slab gels together with immune complexes formed with serum and Staph protein A. HIV -specific proteins, conforming to structural and envelope regions of HTV, are apparent. At right, Structure of HTV. The virion has two 35S single-stranded pieces of RNA noncovalently linked at their 5' ends to form the genomic 70S RNA. profound decrement in peripheral and tissue helper/inducer T 4 lymphocytes, i n v e r s i o n of t h e T 4 :T 8 p h e n o t y p i c helper-suppressor T-cell ratio, diminished in vitro lymphocyte responses to antigen, and cutaneous anergy. The antigen-responsive T4 lymphocyte appears to be selectively decreased early in the course of HIV infection, even while normal proliferative responses to nonspecific lectins may occur.6 Other frequent aberrations include elevated serum immunoglobulin and acid labile alpha-interferon levels, depressed production of or response to lymphokines such as interleukin-2 and gamma-interferon, and deficient antibody response to in vivo immunization. Natural killer cell and cytotoxic T-lymphocyte functions may also be impaired, as well as antigen recognition in the autologous and allogeneic m i x e d l y m p h o c y t e c u l t u r e reactions. These latter changes appear secondary to defects in both the EN0OINT 3 -ORF Fig 2. A schematic representation of the HIV genome. The six known genes of HTV: gag (group antigen), pol (polymerase, including reverse transcriptase, protease, and endonuclease), sor (short open reading frame), tat (trans-activating), env (envelope), and 3'-ort(open reading frame) are depicted, together with their translational products. Tat is a bipartite gene, represented by the two exons connected by dashed lines. Post-translational processing of the protein (p) and glycoprotein (gp) products required for virion assembly are given together with their apparent molecular weights. A seventh gene, art, partly overlaps the tat/7/ and env genes. T-lymphocyte and antigen-presenting cell. Alterations in chemotaxis and d i m i n u t i o n of m e m b r a n e HLA-DR antigens of antigen presenting cells have been observed. The pattern of infectious complications in AIDS, primarily involving intracellular parasites, is likewise compatible with HTVmediated defects among T lymphocytes and cells of the monocyte lineage. It is clear that HIV can infect several components of the immune system beside the T cell, including B lymphocytes, monocytes, and possibly neuroglial cells. Yet central to the pathogenesis of HIV is an alteration of the T 4 lymphocyte. How is it that by damaging a single link, HTV causes the immune system as a whole to unravel? The answer lies in the complex series of interactions among the different classes of cells and their secreted p r o d u c t s t h a t t a k e p a r t i n immunity. The T4 lymphocyte, through its surface receptors, activation antigens, and lymphokines, plays the prim a r y c o o r d i n a t i n g role i n t h i s network. 7 The T 4 molecule itself appears to be part of the receptor for HTV. Cytopathic Effects HIV causes the premature maturation and death of the T 4 cell. This effect is mediated by several novel regions of the viral genome. Like all retroviruses, HIV has a group antigen or gag region describing its core or structural proteins; pol coding for the polymerase associated with reverse transcriptase, endonuclease, and protease activities; and env representing envelope glycoproteins. The four novel genes of HIV are tatm, sor, S'-orf, and art (Fig 2). These sequences are presumably responsible for the unique ecologic niche that HIV h a s acquired: the ability to uncouple the requirement for integration with replication, permitting infection of nondividing cells. HIV is highly cytopathic for the T 4 cell. Subsequent to activation of an infected lymphocyte, rapid viral transcription and cell death occur. This is partly a consequence of the virus-associated trans-acting factor that greatly elevates t h e level of gene expression directed by t h e viral long terminal repeat (LTR). The elements of this autostimulatory pathway include an effector, the tatj,, p l 4 protein encoded by the bipartite tatm Sene>an^ a responder element, the trans-activating response sequence (TAR), located w i t h i n t h e LTR. 8 Art ( a n t i repression tams-activator) encodes a second fra/is-acting region t h a t partly overlaps t h e tatm and envelope genes. It acts post-transcriptionally to relieve cis-acting negative regulation of the messenger RNAs for viral capsid and envelope proteins. 9 In terms of genome d i s t r i b u t i o n w i t h i n a n infected cell, the unusual ability of HIV to persist in a cytoplasmic proviral state is also found in other agents, including spleen necrosis and visna viruses, which have strong cytopathic potential. The feedback m e c h a n i s m important to HIV replication is regulated by an increased t r a n s l a t i o n a l efficiency of virus-specific messenger RNA LABORATORY MEDICINE • VOL. 17, NO. 11, NOVEMBER 1986 6 6 1 lipid - QCI've cgent, antibody anti-sense oligomers, interferons Fig. 3. A schematic diagram of possible mechanisms of action of drugs with anti-HTV activity. Four major sites for interference include virus attachment, reverse transcription, viral gene translation, and virion assembly. species and possibly by other pathways of post-transcriptional or transcriptional control. It is also conceivable that alteration of the translational efficiency of cellular messenger RNAs associated with endogenous TAR-like sequences could augment production of normal cellular proteins in physiologic modulation of T-cell function. Molecules capable of inhibiting T-cell-dependent immune responses have been identified in short-term cultures of mononuclear cells derived from AIDS and ARC patients, in virus-free supernatants of HIV-infected cells, and in the sera of AIDS patients. Potent suppressor lymphokines secreted by T4 lymphocytes early in the course of HIV infection could serve to amplify the overall depression in immune responsiveness.10 This reaction would be analogous to suppressor factors identified in several mammalian models of retroviral infection, including feline and murine leukemia viruses.11 Viral Latency Another important control region of HTV is the negative regulatory element (NRE) located within the LTR. Gene deletion experiments have determined the inhibitory effect of this sequence on viral transcription.12 This suggests an active state for the maintenance of viral latency in infected, nonreplicating cells. In addition, lack of tatm or art function might lead to accumulation of viral RNA without synthesis of viral structural proteins, thereby establishing a latent state.9 Models for latency have been established in vitro, as T lymphocytes can be induced to express viral proteins and release infectious virions following exposure to certain chemicals, steroids, or antigen. If this model reflects the situation in vivo, it could account for the ostensible clinical importance of cofactors in the development and progression of AIDS. It is possible that antigenic stimulation by immunization or infection with certain lymphotropic viruses, including cytomegalovirus, Epstein-Barr virus, and hepatitis B virus could activate a T cell, rendering it more susceptible to productive infection by HIV, or induce viral expression among a population of latently infected cells. Intervention Strategies Vaccines Intervention in this tangle of pathology, either to prevent infection, mitigate its effects, or cure the disease, will be difficult. Preventive measures, including behavior modification and education, hold some hope of retarding the spread of HrV infection. Simple reduction in the number of sexual partners may in itself provide little protection, however. This relates to the extraordinary seroprevalence of HrV in blood samples of homosexual males from major urban areas in the United States and Europe. It has reached a level as high as 60% in New York City and 73% in San Francisco among asymptomatic homosexual men.13 Potential spread by heterosexual contact with bisexuals and intravenous drug abusers is another problem, with documented transmission of HIV from male to female as well as female to male.13 Thus, there is a pressing need for a vaccine against HIV. 6 6 2 LABORATORY MEDICINE • VOL. 17, NO. 11, NOVEMBER 1986 Current strategies for vaccine development focus on identification of gpl20 epitopes conserved among HIV isolates. These could be used to engineer a vaccine genetically. The heterogeneity of HIV isolates, based on restriction fragment analysis and primary nucleotide sequence data, will hamper this quest. Other difficulties include the low viral neutralization titers of most serum samples from AIDS and ARC patients, and the scarcity of data correlating potent viral neutralization in vitro with a favorable clinical course. The discovery of a series of T-cell tropic retroviruses in monkeys has opened up a new strategy for vaccine development. Present data on the biology of the HIV-related simian Tlymphotropic virus (STLV-IIIAGM) isolated from the African Green monkey indicate that virus- and antibody-positive monkeys are healthy. This finding is in direct contrast to the biology of STLV-IIImac in the macaque host, where this virus is associated with an immune deficiency syndrome resembling human AIDS.14 There is serologic evidence that a small proportion of persons in west Africa are healthy yet infected with an STLV-IIIAGM-related virus. It has been hypothesized that infection with such an agent may be protective for subsequent HIV infection and disease, and therefore might offer clues to the identity of immunogenic and protective viral epitopes. Drugs All retroviruses require reverse transcriptase in order to replicate. This enzyme is thus an obvious target for drug therapy. Several compounds have shown an effect in vitro against HIV, including ribavirin, azido-deoxythymidine, suramin, phosphonoformate, and ansamycin. Many of these drugs are currently in limited phase I or phase II trials. Other points at which the viral life cycle may be interrupted are depicted in Fig 3. For example, over 40% of the viral capsid is lipid. Agents capable of altering the phospholipid composition of the viral envelope might block viral adsorption or penetration. Inhibition of viral translation is also attractive. This could be accomplished by interferons, or oligodeoxynucleotide "anti-sense" sequences complementary to specific regions of the viral genome. The unique genes of HIV-related to irans-activation present special targets. Anti-sense oligomers, modified to permit direct entry into cells while maintaining resistance to cellular degredative enzymes, or antibiotics which block translational processes, might affect HIV growth through this mechanism. It is hoped that the knowledge gained through work in the serologic and culture identification of HIV and strategies to block its infectivity and replication will be applicable to the study of HTLV-I and II as well. A high incidence of HTLV-I (9%) and HTLVII (18%) was noted among HIV-infected intravenous drug abusers15 and of HTLV-I (6%) among HIV seropositive homosexual men. These related retroviruses are probably transmitted by the same routes as HIV. Their dissemination may be a harbinger of future epidemics of T-cell leukemia, lymphoma, and other syndromes linked to HTLV-I and II with much longer incubation periods than the HIV-related disorders. ery of HIV as the cause of AIDS. New information is beginning to uncover the basic mechanisms by which this agent interferes with the cellular immune system. Longitudinal studies have shown that HIV infection may have a long incubation period and is manifest by myriad clinical signs and symptoms, from an acute mononucleosis-like illness to generalized lymphadenopathy or development of an opportunistic infection or neoplasm defining clinical AIDS. Cofactors for the outcome of HIV infection are being defined. In the absence of a vaccine or antiviral drug, the worldwide incidence of AIDS will continue to increase but, because of education, alterations in life-style among risk group members, and the screening of blood and blood products for HIV antibody, at a slower rate. The rapid development of specific immunologic and virologic reagents as well as preventative and therapeutic strategies offer the promise of future successful interventions in this tragic disease. References Conclusion Data from several disciplines, including epidemiology, virology, and immunology, have led to the discov- 1. Update on acquired immune deficiency syndrome (AIDS)—United States. MMWR 1982;31:507-514. 2. Revision of the case definition of acquired immunodeficiency syndrome for national reporting—United States. MMWR 1985;34:373-375. 3. Curran JW, Morgan WM, Hardy AM, et al: The epidemiology of AIDS: Current status and future prospects. Science 1985;229:1352-1357. 4. Redfield RR, Wright DC, Tramont EC: The Walter Reed staging classification for HTLV-m/LAV infection. N Engl J Med 1986;314:131-132. 5. Goedert JJ, Biggar RJ, Weiss SH, et al: Threeyear incidence of AIDS in five cohorts of HTLVIII-infected risk group members. Science 1986;231:992-995. 6. Lane HC, Depper JM, Greene WC, et al: Qualitative analysis of immune function in patients with the acquired immunodeficiency syndrome. N Engl J Med 1985;313:79-84. 7. Laurence J: The immune system in AIDS. Sci Am 1985;253(6):84-93. 8. Rosen CA, Sodroski JG, Goh WC, et al: Posttranscriptional regulation accounts for the transactivation of the human T-lymphotropic virus type III. Nature 1986;319:555-559. 9. Sodroski J, Goh WC, Rosen C, et al: A second post-transcriptional (rans-activator gene required for HTLV-III replication. N a t u r e 1986;321:412-417. 10. Laurence J, Gottlieb AB, Kunkel HG: Soluble suppressor factors in patients with acquired immune deficiency syndrome and its prodrome: Elaboration in vitro by T lymphocyte-adherent cell interactions. J Clin Invest 1983;72:20722081. 11. Bendinelli M, Matteucci D, Friedman H: Retrovirus-induced acquired immunodeficiencies. Adv Cancer Res 1985;45:125-181. 12. Rosen CA, Sodroski JG, Haseltine WA: The location of cis-acting regulatory sequences in the human T cell lymphotropic virus type HI (HTLVm/LAV) long terminal repeat. Cell 1985;41:813823. 13. De Gruttola V, Mayer K, Bennett W: AIDS: Has the problem been adequately assessed? Rev Infect Dis 1986;8:295-305. 14. Kanki PJ, Alroy J, Essex M: Isolation of T-lymphotropic retrovirus related to HTLV-IIiyLAV from wild-caught African Green monkeys. Science 1985;230:951-954. 15. Robert-Guroff M, Weiss SH, Giron JA, et al: Prevalence of antibodies to HTLV-I, -II, and -III in intravenous drug abusers from an AIDS endemic region. JAMA 1986;255:3133-3137. 16. Sivak SL, Wormser GP: How common is HTLVIII infection in the United States? N Engl J Med 1985;313:1352. LABORATORY MEDICINE • VOL. 17, NO. 11, NOVEMBER 1986 6 6 3