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immunoglobulin class and the capacity to eliminate antigen. There must also be a one-to-one relationship between the B cell producing an immunoglobulin class and these clonally distributed Fc-receptor-bearing cells. In the simplest case a B cell producing a given class of immunoglobulin might express an Fc receptor for that class and occupancy of that receptor by an antigen-antibody complex would then initiate an inhibitory signal. The inverse of a selective mechanism based on inhibition of B cells producing an ineffective antibody class requires the activation of B cells producing an effective antibody class. Intuitively this type of model is unattractive because antibody-directed elimination of antigen must almost certainly lead to the elimination of antibody; in this case the effective antibody is absent and not able to directly provide information. A closer examination of the relationship between a class-specific Fc-receptor-bearing regulatory element and the B cell producing a specific class of immunoglobulin shows that a mechanism is required to establish this as a one-to-one relationship. Thus, to propose that a B cell secreting IgG1 has Fc receptors for IgG~ implies that the genes encoding each are co-ordinately expressed. However, in the absence of any apparent cod!nginformation contiguous with the cHylstructural gene, it is necessary to invoke an intracellular regulatory gene product which ties together CHyland Ferl gene expression.
New
In contrast to the complexity encountered with coordinate expression in a single B cell, the separation of Fc receptor and Ig expression in two different cells (for example T cells and B cells, respectively) allows each to be chosen randomly in a given cell and hence clonally distributed. Then, with a relatively straightforward set of interaction rules, the class of the humoral response can be regulated in a biologically sensible way. The interaction of secreted antibody with antigen is first assayed for ineffective removal of antigen (logical because ineffective classes competitively inhibit the effective classes). Persisting ineffective antigen-antibody complexes activate Ig-class-restricted antigen-specific suppressor T cells which deliver their effector function to those B cells which make the inappropriate antibody. These suppressor T cells are restricted to recognition of Ig class as well as antigen and as such bear a striking similarity with M H C restricted T-cell activities. This provocative line of reasoning raises many questions and, in view of the recent uncertainty about 'I-J'-type antigens that are apparently related to certain suppressor T cells, perhaps there are now clues with which to unravel the 'I-J' mystery. This viewpoint has hopefully drawn attention to new questions which, when answered experimentally, will provide the basis for a detailed model of the class-discrimination mechanisms in immunity.
directions in research
The probable cause of AIDS The acquired immune deficiency syndrome (AIDS) has news appeal as well as considerable scientific fascination, so it is not surprising that much publicity has surrounded (and indeed preceded) the presentation of data apparently identifying the causative agent. An infective aetiology was suspected early on. The evidence included the epidemic nature of the disease with its exponential rise, the pattern of patient groups at risk (suggesting transmission by sexual or blood-toblood contact), the geographical clustering of most cases and direct evidence of case-to-case contact. An apparently related disorder (variant or prodrome) of persistent generalized lymphadenopathy affecting the same risk groups has accompanied or just preceded the AIDS epidemic. Although a number of possible infective agents have been considered, retroviruses - R N A tumour viruses which have long been linked with animal malignancies - were considered among the more likely, being blood-borne, some having lymphotropic properties and some causing immunodeficiency in animals. Yet the search for an aetiological agent has not been without difficulties, given that AIDS patients are severely immunocompromised, with major derangement of humoral and cellular immune mechanisms, and have often had prior exposure to multiple infections agents. In May 1983, the involvement of human T-cell leukaemia/lymphoma virus (HTLV) was suggested. Gallo's group at the National Cancer Institute reported © 1984, ElsevierScienccPubli~hersB.V.,Amsterdam 0167-4919/84/$02.00
that proviral DNA of H T L V was present in blood lymphocytes from 2 of 33 AIDS patients 2 and H T L V itself was isolated from another patient ~. Essex and others reported that 25% (19/75) AIDS patients and a similar proportion of lymphadenopathy patients had antibodies to an HTLV-associated membrane antigen detectable by fluorescence4 but the specificity of this assay was not clear. Other groups have had similar results with the same assay. However, isolates of H T L V from AIDS patients have remained few (10% according to GalloS; mostly H T L V I, but one of the less common but antigenically related H T L V II); with other more specific tests of antibodies to H T L V I and II, seropositivity in AIDS patients has been 10-15 %5 or less 6. Most workers have concluded that H T L V I and II are passengers (or opportunists) in AIDS and not the causative agent. However, the significance of Essex's membrane fluorescence test remained unclear. Montagnier's group at the Pasteur Institute in Paris also reported in May 1983 the isolation from a lymphadenopathy patient of another T-lymphotropic retrovirus, apparently distinct from H T L V I, which they termed lymphadenopathy-associated virus (LAV) 7. In September of last year, they reported at a Cold Spring Harbour Symposium8 the further characterization of the virus, showing its antigenic relatedness to equine infectious anaemia virus (EIAV) and demonstrating its tropism for T4 ÷ lymphocytes, the formation of giant
Immunology Today, vol. 5, No. 7, 1984
cells after infection and a slight cytopathic effect. They noted that anti-a-interferon had to be added to culture medium for infection of normal lymphocytes in coculture. They also reported two other viral isolates from AIDS patients, termed immunodeficiency-associated viruses .(IDAV), which bore close resemblance, if not identity, to LAV. They provided serological evidence of a high prevalence of L A V antibodies in lymphadenopathy patients. In April this year the same group published further data 9 on one of these AIDS patients (with haemophilia B) and his brother, who is also infected but remains well (although now with abnormal proportions of T-cell subsets). Repeated viral isolates in both were associated with evidence of antibodies to LAV, although the AIDS patient no longer has detectable antibody. In this paper, further isolates and seroepidemiological studies on antibody prevalence to the specific retroviral protein p25 were alluded to. These data were presented by Montagnier at a meeting of E G I D .1° in late March showing virus isolation in 11 subjects: 7 of 8 AIDS patients and 4 of 8 lymphadenopathy patients. Notably, these included patients from several distinct risk groups: 4 homosexuals (including one who had lived in Haiti), a Haitian, the haemophilic, 3 Zairians (including a husband and wife pair) and a Caucasian living in Trinidad. Montagnier mentioned four further isolates, two from French and two from American patients. Using an enzyme-linked immunosorbent assay (ELISA) and a radioimmunoprecipitation assay for antibodies to LAV/IDAV, he reported seropositivity in about 50% AIDS patients, 80% lymphadenopathy patients, 20% in symptom-free homosexuals at risk and 1 in 330 (0.3%) normal blood donors ml. Similar results were apparently obtained with North America sera. H e has subsequently noted (unpublished observations) that 5-7% normal donors from Zaire were positive, using sera from 1980 and 1983. Furthermore, 94% (35 of 37) of Zairian patients with AIDS were seropositive. Substantial numbers of haemophiliacs are also seropositive. A 13 month-old transfused baby was seropositive but his mother was negative; however, the mothers of a number of Haitian and Zairian children with AIDS had evidence of infection, either isolated virus or antibodies. The virus has been further characterized and this had confirmed its antigenic relatedness (core protein) to EIAV but not to H T L V P 2. The selective infection of T4 ÷ cells has been elegantly demonstrated by the elaboration of reverse transcriptase and by virus budding from cultured infected lymphocytes of T4 ÷ but not T8 + phenotype. Modulation of T4 antigen in prolonged culture of T4 ÷cells has also been demonstrated. In M a y this year, Gallo and his colleagues published elegant and extensive data on a new virus they have isolated from AIDS patients, which they have called human T lymphotropic virus III ( H T L V III). This retrovirus is strikingly similar to L A V / I D A V and may indeed be the same agent 6. This is further suggested by the overall concordance in the findings of the two *European Group on Immunodeficiencies Meeting organized by Dr C. Griscelli and held at Fillerval, near Paris.
197
groups. Gallo's group have characterized the virus extensivelym4, much helped by finding a neoplastic T cell line permissive for the long term culture and replication of H T L V III. They noted that viral replication and reverse transcriptase production increased on adding anti-a-interferon to the cultures 5. They have isolated the virus5 from 26/72 (36%) AIDS cases, including 13/43 with Kaposi's sarcoma, 10/21 with opportunist infections, 3 of 8 children with AIDS and 3 of 4 mothers of such children. The last two groups are particularly important in that the children with AIDS are less likely to have been infected with multiple agents. Virus has been isolate@ from 18/21 (86%) patients with ' w e - A I D S ' (a term Gallo's group use for lymphadenopathy with leucopenia and abnormal T4/T8 ratio). Notably, virus was isolated from 1 of 22 symptomless homosexuals thought to be at moderate risk for AIDS; 6 months later he developed AIDS. No virus was isolated from 115 normal donors. Using an ELISA for antibodies to H T L V IIP 5 seropositivity was seen in 87% (43/49) AIDS patients, 79% (11/14) of 'pre-AIDS' patients, 3 of 5 i.v. drug abusers, 27% (4/15) asymptomatic homosexuals and a further symptomatic sexual contact of a seropositive AIDS patient. Only one of 164 (0.6%) normal controls and none of 22 disease controls were seropositive. The molecular constitution of H T L V III determined serologically~4 suggests that it is antigenically related to H T L V II and to a lesser extent to H T L V I. This relationship could perhaps explain the membrane fluorescence data of Essex's group 4. Nucleotide sequences are reported to show relatedness between H T L V I, II and l i p 4. The likely relatedness between H T L V III and L A V / I D A V 6 has not yet been established directly and exchanges of materials are essential. If, as seems likely, both groups are studying the same virus, the data are compelling for several reasons. The frequency of virus isolation from AIDS and related disorders is high and isolation has been achieved in widely differing patient groups, including Africans and children. The rate of seropositivity is high in AIDS and related disorders and significant in risk groups. There is direct evidence for sexual, blood-borne and vertical transmission. The T4 lymphotropism of the virus is clear and the suggestion that virus replication in T cells requires their activation is a plausible explanation for the long and variable latent period (with a possible role for cofactors). A similar class of retrovirus causing immunodeficiency in cats (feline leukaemia virus) ~6 and monkeys (Mason-Pfizer virus in simian AIDS) 1719. From the immunopathogenic viewpoint it is of considerable interest that this retrovirus is not only tropic for the T4 ÷ cells that are characteristically depleted in AIDS, but also for activated T4 ÷ cells8-~°. Moreover, according to Montagnier, virus budding and the production of infectious virus particles only occur when T4 + cells are activated8-~8. This is true of both patients' cells and normal cells infected in culture and suggests ~° that in vivo, after infection of T4 + cells by the original inoculum, cofactors which activate these cells (e. g. intercuurent infection, possibly even alloantigens2U) could lead to production of infectious virus with conse-
Immunology Today, vol. 5, No. 7, 1984
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quent spread to a larger proportion of T4* cels. Several cycles like this may be needed before T4 ÷ cell depletion, through lysis or other mechanisms of elimination, leads to clinical immunodeficiency. This may account for the long but varying latent period and indicates ways in which life style changes could modify the expression of the disease. It also implies that attempts at therapeutic intervention during the early stages or in more benign forms of the disease - possibly, for example during lymphadenopathy or in a symptomless phase, could be harmful if the therapy was capable of activating T4 + cells. Thus reconstitution factors such as with interleukin 2, which may be appropriate for AIDS patients who have lost all functional activity in T4 ÷ cells, may actually enhance the infection and the elimination of the remaining T4 + cells in subjects infected at only a low level. In what directions should research on AIDS now proceed? These may be summarized as: (1) validation of aetiological agent; (2) seroepidemiology and screening in the hope of containment; (3) immunopathogenic studies, leading to rational therpy; (4) development of a protective vaccine, leading to prevention. Firstly, the relationship between L A V / I D A V and H T L V III needs to be established and the data from these groups confirmed by others using different patients and methodologies. Once a specific serological test is available for wider research use, we need to establish the detailed seroepidemiology of infection with the agent. In particular, we need to determine whether or not all those who have antibodies will necessarily progress to AIDS. By analogy with other infectious diseases, it seems likely that there will be milder and subclinical variants that do not necessarily progress. Such data can be obtained by making use of the serum banks from existing longitudinal studies that have been collected for exactly this purpose. In short, when we have a generally available 'screening test', we have to know what it signifies. It would be prudent tO exclude seropositive subjects as blood donors for the time being. Similarly, specific advice regarding sexual activities and blood donation should be given to infected members of at risk populations. It may also be that measures which reduce the frequency of intercurrent infection could decrease the likelihood of the expression of infection as full blown AIDS in infected subjects. From the limited early data, virus isolation is a poor prognostic sign in symptornless subjects, particularly women of childbearing age. Subjects from whom virus can be isolated (whether or not they have antibodies) should be regarded as infectious; however, seropositive subjects from whom virus cannot be isolated may not be. Patients with lymphadenopathy may be the most infectious group some perhaps acting as carriers. Existing studies on the pathogenesis of AIDS will be brought into much sharper focus by this new evidence. Although the T4 ÷ lymphotropic nature of the retrovirus suggests that the depletion of this cell type may be the primary defect, many issues have still to be resolved. Some evidence suggests that only the Leu 8 + subset of T4 ÷ cells is affected21, so it is important to establish whether the tropism is similarly restricted, However, the
severity of the clinical disease suggests an immunological defect broader than simply T-cell lack (cf. DiGeorge syndrome). The marked polyclonal B-cell activation in AIDS 22 may be a nonspecific response to viral infection, but Montagnier suggests that EBVvirus-transformed B cells may also be infected. Abnormalities in antigen-presenting cells may also underlie the disorder 23. The precise mechanisms underlying defects in cellular interactions, including T-macrophagC 4, T - B and T - T interactions, need to be established further. Such studies on cellular mechanisms, their interactions and the substances involved in their mediation can soon be conducted on clinical material from subjects, and specifically on cells of known status with respect to infection by H T L V III/LAV/ IDAV. An understanding of pathogenetic mechanisms is essential to the effective treatment and immune reconstitution of AIDS patients. Finally, the nature of the protective immune response, if any, to the causative retrovirus has to be determined, not least through the comparison of seropositive patients who do not progress to AIDS with those that do. With this knowledge and once the virus has been produced on a sufficient scale, there is at last a theoretical prospect of a vaccine. However, no retrovirus vaccines have hitherto been developed; it remains to be seen whether the American health authorities' promise of a vaccine in two years is realistic. The balance of evidence strongly suggests that the aetiological agent of AIDS has been identified. A form of AIDS may have existed in Central Africa for many years. Sporadic, retrospectively identified patients linked to Africa were first seen in Europe in the mid1970s. The first patients in the now substantial US epidemic (over 4 000 cases to date) were seen in 1978/9 and the diease was documented in 1981. Its probable infectious aetiology became clear in 1982, with the first isolate of the causative agent in 1983. This evidence has been considerably strengthened in 1984 and is likely to be confirmed by 1985. During this period, many new insights have been gained which extend far beyond AIDS itself and which enhance our understanding of basic mechanisms of immunoregulation and viral immunopathogenesis. While some may be jealous of the attention that AIDS has received, others may wish to study other diseases with comparable intensity in the hope of achieving equally remarkable, far-reaching and rapid results. ~I] Acknowledgements I am most grateful to Dr K. Gallo and Dr L. Montagnier for providing me with prepublication data to enable the preparation of this
report.
ANTHONY PINCHING Department of Immunology, St Ma~y's Hospital Medical School, London W2 1PG, UK.
References 1 2 3
Pinching, A. J. (1984) Clin. Exp. Immunol. 56, 1-13 Gelman, E. P., Popovic, M., Blayney, D., Masur, H., Sidhu, G., Stahl, R. E. and GaUo, R. C. (I983) Sconce 220, 862-865 Gallo, R. C., Satin, P. S., Gelmann, E. P., Robert-Guroff, M., Richardson, E., Kalyanaraman, V. S., Mann, D., Sidhu, G., Stahl,
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4 5
6 7
8
9
10 11
R. E., Zona-Pazner, S., Leibowitch, J. and Popovic, M. (1983) Science220, 865-867 Essex, M., McLane, M. F., Lee, T. H., Falk, L., Howe, (3. W. S., Mulllns, J. I., Cabradilla, C. and Francis, D. P. (1983) Science 220, 859-862 Gallo, R. C., Salahuddin, S. Z., Popovic, M., Shearer, G. M., Kaplan, M., Haynes, B. F., Palker, T. J., Redfield, R., Oleske, J., Safai, B., While, G., Foster, P. and Markham, P. D. (1984) Scienci 224, 500-503 Weiss, R. (1984) Nature (London) 309, 12-13 Barr&Sinoussi, F., Chermann, J. C., Rey, F., Nugeyre, M. T., Chamaret, S., Gruest, J., Dauguet, C., Axler-Blin, C., V~zinetBrun, F., Rouzioux, C., Rozenbaurn, W. and Montagnier, L. (1983) Science 220, 868-871 Montagnier, L., Chermann, J. C., Barr&Sinoussi, F., Chamarct, S., Gruest, J., Nugueyre, M. T., Rey, F., Dauguet, C., Axler-Blin, C., V6zinet-Brun, F., Rouzioux, C., Saimot, G.-A., Rozenbaum, W., Gluckman, J. C., Klatzmann, D., Vilmer, E., Gfiscelli, C., FoyerGazengel, C. and Brunet, J. B. in: Human T Cell Leukaeraia Virus (Gallo, R. "C., Essex, M. and Gross, L., eds), Cold Spring Harbour Laboratory (in press) Vilmer, E., Barr&Sinoussi, F., Rouzioux, C., Gazengel, C., V~zinct-Brun, F., Dauguet, C., Fischer, A., Manigne, P., Chermann, J. C., Griscelli, C. and Montagnier, L. (1984) Lancet (i), 753-757 Montagnier, L., Barr&Sinoussi, F., Chermann, J. C. in: Proceedingsof EGID Meeting (Griseelli, C., ed.), Elsevier Science Publishers, Amsterdam (in press) Brun-V6zinet, F. et al. Lancet (in press)
12 13 14 15 16 17 18
19 20 21 22 23 24
Montagnier, L., Danguet, C., Axler, C., Charnaret, S., Graest, J., Nugueyre, M. T., Rey, F., Barr&Sinoussi, F. and Chermann, J. C. (1984)Ann. Virol. 135E, 119-134 Popovie, M., Sarngadharan, M. G., Read, E. and Gallo, R. C. (1984) Sciou:e 224, 497-500 Schiipbach, J., Popovlc, M., Gilden, R. V., Gond, M. A., Sarngadharan, M. G. and Gallo, R. C. (1984) Sc/ence224, 503-505 Sarngadharan, M. G., Popovic, M., Bruch, L., SchiSpbach, J. and Gallo, R. C. (1984) Sc/ence224, 506-508 Trainin, Z., Wernicke, D., Ungar-Waron, H. and Essex, M. (1983) Science 220, 858-859 Daniel, M. D., King N. W., Letvin, N. L., Hunt, R. D., Sehgal, P. K. and Desrosiers, R. C. (1984) Science 223, 602-605 Marx, P. A., Maul, D. H., Osborn, K. G., Lercbe, N. W., Moody, P., Lowenstine, L. J., Henrickson, r. V., Arthur, L. O., Gilden, R. V., Gravell, M., London, W. T., Sever, J. L., Levy, J. A., Munn and R. J., Gardner, M. B., (1984) Science 223, 1083-1086 Stromberg, K., Benveniste, R. E., Arthur, L. O., Rabin, H., W. E. Jr, Ochs, H. D., Morton, W. R. and Tsai, C-C (1984) Science 224, 289-292 Shearer, G. (1983) ImmunoL Today 4, 181-185 Nicholson, J. K. A., McDougal, J. S., Spira, T. J., Cross, G. B., Jones. B. M. and Reinherz, E. L. (1984)J. Clin. Invest. 73, 191 201 Lane, H. C., Masur, H., Edgar, L. C. Whalen, G., Rook, A. H. and Fanci, A. S. (1983) iV.. Engl. J. Med. 309, 453-458 Bell-Seto, D. V., Sanchez, M. R., Bayer, R. L., Valentine, F. and Thorbecke, G. J. (1984) N. Engl..f Med. 310, 1279-1282 Murray, H. W., Rubin, B. Y., Masur, H. and Roberts, R. B. (1984) N. Engl. J. Med. 310, 883-889
Immunoregulation in leprosy Leprosy is a chronic infectious disease caused by the bacterium Mycobacterium leprae. The immunological basis for susceptibility to infection withM, leprae is not understood; although many individuals in a highly endemic area may become infected with M. leprae, clinical disease will develop in only a very few. In these individuals, the disease has an immunopathological spectrum ranging from a tubereuloid form, in which infection byM. leprae is limited by a strong cell-mediated immune response, to a lepromatous form where cell-mediated immunity to M. leprae is absent. One of the intriguing features of this anergy is its high degree of specificity; lepromatous leprosy patients are capable of responding normally to other T-cell antigens, including the closely related tuberculosis bacillus. The nature and mechanism of this highly specific immunosuppression is becoming a topic of greatinterest, not only to leprologists, but also to many basic immunologists who see it as an opportunity to study cellular interactions and regulation in a clinically relevant context. Recent studies have shown that peripheral blood lymphocytes from lepromatous patients do not proliferate or produce g-interferon when co-incubated with M. leprae ~, and that proliferation 2 and )'-interferon production I can be at least partly restored by the addition of exogenous interleukin 2 (IL2). Lepromatous leprosy patients thus appear to possess T cells in peripheral blood which can recognizeM, leprae and hypotheses thatM, leprae-reactive lymphocytes are clonally deleted or selectively trapped seem invalid. It should be stressed however, that the invitro responses of lymphocytes from some lepromatous patients cannot be restored by the addition ofIL2, and it is not yet clear if these represent a subpopulation of patients with a different or additional immunoregulatory defect. A number of groups have demonstrated suppressor factors in lepromatous leprosy associated both with T
cells 3'4and macrophages 5'6, and Leu 2a/OKT8 + cells are prominent in lepromatous leprosy lesions 1'7. Work from Bloom's group at Albert Einstein Medical College, New York, suggests that M. leprae might possess determinants capable of inducing suppressor cells. Incubation of lymphocytes from lepromatous patients with preparations of M. leprae reduces their responsiveness to concanavalin A 3 and this seems to be mediated by OKT8 + cells 4. More recently they have shown that one particular epitope, the terminal trisaccharide moiety of an M. leprae-specific phenolic glycolipid antigen, carl cause such suppression 8. This finding not only provides further evidence that T suppressor cells are capable of recognizing carbohydrate determinants but, remarkably, shows that the receptor is exquisitely specific since minor changes in the antigenic molecule resulted in abrogation of suppressive activity. Although M. leprae possesses determinants capable of inducing immunosuppression, apparently through interaction of T suppressor and T helper cells, their precise clinical relevance is, for the present, unclear. The observed suppression, although highly specific in its induction, is non-specific in nature, being manifested as a reduction in responsiveness to concanavalin A. The relationship between this non-specific immunosuppression and the highly specific anergy towards M. leprae seen in lepromatous patients has still to be clarified. Although antigen-specific suppressor cells, are an intriguing explanation for the antigen-specific anergy in leprosy, there are alternative, or additional, explanations. For example, it is known that auto-anti-idiotypic antibodies induced during the immune response to various antigens may suppress specific humoral or cellular responses; it is possible that during infection with M. leprae, an anti-idiotypic response against an idiotype predominant in determining immunity to 34. leprae is © 1984,ElsevierSciencePublist~[~BzV.,Amsterdam 0167- 4919/84/$02.00
Immunolog7 Today, vol. 5, No. 7, 1984
immunoglobulin class and the capacity to eliminate antigen. There must also be a one-to-one relationship between the B cell producing an immunoglobulin class and these clonally distributed Fc-receptor-bearing cells. In the simplest case a B cell producing a given class of immunoglobulin might express an Fc receptor for that class and occupancy of that receptor by an antigen-antibody complex would then initiate an inhibitory signal. The inverse of a selective mechanism based on inhibition of B cells producing an ineffective antibody class requires the activation of B cells producing an effective antibody class. Intuitively this type of model is unattractive because antibody-directed elimination of antigen must almost certainly lead to the elimination of antibody; in this case the effective antibody is absent and not able to directly provide information. A closer examination of the relationship between a class-specific Fc-receptor-bearing regulatory element and the B cell producing a specific class of immunoglobulin shows that a mechanism is required to establish this as a one-to-one relationship. Thus, to propose that a B cell secreting IgG1 has Fc receptors for IgG~ implies that the genes encoding each are co-ordinately expressed. However, in the absence of any apparent cod!nginformation contiguous with the cHylstructural gene, it is necessary to invoke an intracellular regulatory gene product which ties together CHyland Ferl gene expression.
New
In contrast to the complexity encountered with coordinate expression in a single B cell, the separation of Fc receptor and Ig expression in two different cells (for example T cells and B cells, respectively) allows each to be chosen randomly in a given cell and hence clonally distributed. Then, with a relatively straightforward set of interaction rules, the class of the humoral response can be regulated in a biologically sensible way. The interaction of secreted antibody with antigen is first assayed for ineffective removal of antigen (logical because ineffective classes competitively inhibit the effective classes). Persisting ineffective antigen-antibody complexes activate Ig-class-restricted antigen-specific suppressor T cells which deliver their effector function to those B cells which make the inappropriate antibody. These suppressor T cells are restricted to recognition of Ig class as well as antigen and as such bear a striking similarity with M H C restricted T-cell activities. This provocative line of reasoning raises many questions and, in view of the recent uncertainty about 'I-J'-type antigens that are apparently related to certain suppressor T cells, perhaps there are now clues with which to unravel the 'I-J' mystery. This viewpoint has hopefully drawn attention to new questions which, when answered experimentally, will provide the basis for a detailed model of the class-discrimination mechanisms in immunity.
directions in research
The probable cause of AIDS The acquired immune deficiency syndrome (AIDS) has news appeal as well as considerable scientific fascination, so it is not surprising that much publicity has surrounded (and indeed preceded) the presentation of data apparently identifying the causative agent. An infective aetiology was suspected early on. The evidence included the epidemic nature of the disease with its exponential rise, the pattern of patient groups at risk (suggesting transmission by sexual or blood-toblood contact), the geographical clustering of most cases and direct evidence of case-to-case contact. An apparently related disorder (variant or prodrome) of persistent generalized lymphadenopathy affecting the same risk groups has accompanied or just preceded the AIDS epidemic. Although a number of possible infective agents have been considered, retroviruses - R N A tumour viruses which have long been linked with animal malignancies - were considered among the more likely, being blood-borne, some having lymphotropic properties and some causing immunodeficiency in animals. Yet the search for an aetiological agent has not been without difficulties, given that AIDS patients are severely immunocompromised, with major derangement of humoral and cellular immune mechanisms, and have often had prior exposure to multiple infections agents. In May 1983, the involvement of human T-cell leukaemia/lymphoma virus (HTLV) was suggested. Gallo's group at the National Cancer Institute reported © 1984, ElsevierScienccPubli~hersB.V.,Amsterdam 0167-4919/84/$02.00
that proviral DNA of H T L V was present in blood lymphocytes from 2 of 33 AIDS patients 2 and H T L V itself was isolated from another patient ~. Essex and others reported that 25% (19/75) AIDS patients and a similar proportion of lymphadenopathy patients had antibodies to an HTLV-associated membrane antigen detectable by fluorescence4 but the specificity of this assay was not clear. Other groups have had similar results with the same assay. However, isolates of H T L V from AIDS patients have remained few (10% according to GalloS; mostly H T L V I, but one of the less common but antigenically related H T L V II); with other more specific tests of antibodies to H T L V I and II, seropositivity in AIDS patients has been 10-15 %5 or less 6. Most workers have concluded that H T L V I and II are passengers (or opportunists) in AIDS and not the causative agent. However, the significance of Essex's membrane fluorescence test remained unclear. Montagnier's group at the Pasteur Institute in Paris also reported in May 1983 the isolation from a lymphadenopathy patient of another T-lymphotropic retrovirus, apparently distinct from H T L V I, which they termed lymphadenopathy-associated virus (LAV) 7. In September of last year, they reported at a Cold Spring Harbour Symposium8 the further characterization of the virus, showing its antigenic relatedness to equine infectious anaemia virus (EIAV) and demonstrating its tropism for T4 ÷ lymphocytes, the formation of giant
Immunology Today, vol. 5, No. 7, 1984
cells after infection and a slight cytopathic effect. They noted that anti-a-interferon had to be added to culture medium for infection of normal lymphocytes in coculture. They also reported two other viral isolates from AIDS patients, termed immunodeficiency-associated viruses .(IDAV), which bore close resemblance, if not identity, to LAV. They provided serological evidence of a high prevalence of L A V antibodies in lymphadenopathy patients. In April this year the same group published further data 9 on one of these AIDS patients (with haemophilia B) and his brother, who is also infected but remains well (although now with abnormal proportions of T-cell subsets). Repeated viral isolates in both were associated with evidence of antibodies to LAV, although the AIDS patient no longer has detectable antibody. In this paper, further isolates and seroepidemiological studies on antibody prevalence to the specific retroviral protein p25 were alluded to. These data were presented by Montagnier at a meeting of E G I D .1° in late March showing virus isolation in 11 subjects: 7 of 8 AIDS patients and 4 of 8 lymphadenopathy patients. Notably, these included patients from several distinct risk groups: 4 homosexuals (including one who had lived in Haiti), a Haitian, the haemophilic, 3 Zairians (including a husband and wife pair) and a Caucasian living in Trinidad. Montagnier mentioned four further isolates, two from French and two from American patients. Using an enzyme-linked immunosorbent assay (ELISA) and a radioimmunoprecipitation assay for antibodies to LAV/IDAV, he reported seropositivity in about 50% AIDS patients, 80% lymphadenopathy patients, 20% in symptom-free homosexuals at risk and 1 in 330 (0.3%) normal blood donors ml. Similar results were apparently obtained with North America sera. H e has subsequently noted (unpublished observations) that 5-7% normal donors from Zaire were positive, using sera from 1980 and 1983. Furthermore, 94% (35 of 37) of Zairian patients with AIDS were seropositive. Substantial numbers of haemophiliacs are also seropositive. A 13 month-old transfused baby was seropositive but his mother was negative; however, the mothers of a number of Haitian and Zairian children with AIDS had evidence of infection, either isolated virus or antibodies. The virus has been further characterized and this had confirmed its antigenic relatedness (core protein) to EIAV but not to H T L V P 2. The selective infection of T4 ÷ cells has been elegantly demonstrated by the elaboration of reverse transcriptase and by virus budding from cultured infected lymphocytes of T4 ÷ but not T8 + phenotype. Modulation of T4 antigen in prolonged culture of T4 ÷cells has also been demonstrated. In M a y this year, Gallo and his colleagues published elegant and extensive data on a new virus they have isolated from AIDS patients, which they have called human T lymphotropic virus III ( H T L V III). This retrovirus is strikingly similar to L A V / I D A V and may indeed be the same agent 6. This is further suggested by the overall concordance in the findings of the two *European Group on Immunodeficiencies Meeting organized by Dr C. Griscelli and held at Fillerval, near Paris.
197
groups. Gallo's group have characterized the virus extensivelym4, much helped by finding a neoplastic T cell line permissive for the long term culture and replication of H T L V III. They noted that viral replication and reverse transcriptase production increased on adding anti-a-interferon to the cultures 5. They have isolated the virus5 from 26/72 (36%) AIDS cases, including 13/43 with Kaposi's sarcoma, 10/21 with opportunist infections, 3 of 8 children with AIDS and 3 of 4 mothers of such children. The last two groups are particularly important in that the children with AIDS are less likely to have been infected with multiple agents. Virus has been isolate@ from 18/21 (86%) patients with ' w e - A I D S ' (a term Gallo's group use for lymphadenopathy with leucopenia and abnormal T4/T8 ratio). Notably, virus was isolated from 1 of 22 symptomless homosexuals thought to be at moderate risk for AIDS; 6 months later he developed AIDS. No virus was isolated from 115 normal donors. Using an ELISA for antibodies to H T L V IIP 5 seropositivity was seen in 87% (43/49) AIDS patients, 79% (11/14) of 'pre-AIDS' patients, 3 of 5 i.v. drug abusers, 27% (4/15) asymptomatic homosexuals and a further symptomatic sexual contact of a seropositive AIDS patient. Only one of 164 (0.6%) normal controls and none of 22 disease controls were seropositive. The molecular constitution of H T L V III determined serologically~4 suggests that it is antigenically related to H T L V II and to a lesser extent to H T L V I. This relationship could perhaps explain the membrane fluorescence data of Essex's group 4. Nucleotide sequences are reported to show relatedness between H T L V I, II and l i p 4. The likely relatedness between H T L V III and L A V / I D A V 6 has not yet been established directly and exchanges of materials are essential. If, as seems likely, both groups are studying the same virus, the data are compelling for several reasons. The frequency of virus isolation from AIDS and related disorders is high and isolation has been achieved in widely differing patient groups, including Africans and children. The rate of seropositivity is high in AIDS and related disorders and significant in risk groups. There is direct evidence for sexual, blood-borne and vertical transmission. The T4 lymphotropism of the virus is clear and the suggestion that virus replication in T cells requires their activation is a plausible explanation for the long and variable latent period (with a possible role for cofactors). A similar class of retrovirus causing immunodeficiency in cats (feline leukaemia virus) ~6 and monkeys (Mason-Pfizer virus in simian AIDS) 1719. From the immunopathogenic viewpoint it is of considerable interest that this retrovirus is not only tropic for the T4 ÷ cells that are characteristically depleted in AIDS, but also for activated T4 ÷ cells8-~°. Moreover, according to Montagnier, virus budding and the production of infectious virus particles only occur when T4 + cells are activated8-~8. This is true of both patients' cells and normal cells infected in culture and suggests ~° that in vivo, after infection of T4 + cells by the original inoculum, cofactors which activate these cells (e. g. intercuurent infection, possibly even alloantigens2U) could lead to production of infectious virus with conse-
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quent spread to a larger proportion of T4* cels. Several cycles like this may be needed before T4 ÷ cell depletion, through lysis or other mechanisms of elimination, leads to clinical immunodeficiency. This may account for the long but varying latent period and indicates ways in which life style changes could modify the expression of the disease. It also implies that attempts at therapeutic intervention during the early stages or in more benign forms of the disease - possibly, for example during lymphadenopathy or in a symptomless phase, could be harmful if the therapy was capable of activating T4 + cells. Thus reconstitution factors such as with interleukin 2, which may be appropriate for AIDS patients who have lost all functional activity in T4 ÷ cells, may actually enhance the infection and the elimination of the remaining T4 + cells in subjects infected at only a low level. In what directions should research on AIDS now proceed? These may be summarized as: (1) validation of aetiological agent; (2) seroepidemiology and screening in the hope of containment; (3) immunopathogenic studies, leading to rational therpy; (4) development of a protective vaccine, leading to prevention. Firstly, the relationship between L A V / I D A V and H T L V III needs to be established and the data from these groups confirmed by others using different patients and methodologies. Once a specific serological test is available for wider research use, we need to establish the detailed seroepidemiology of infection with the agent. In particular, we need to determine whether or not all those who have antibodies will necessarily progress to AIDS. By analogy with other infectious diseases, it seems likely that there will be milder and subclinical variants that do not necessarily progress. Such data can be obtained by making use of the serum banks from existing longitudinal studies that have been collected for exactly this purpose. In short, when we have a generally available 'screening test', we have to know what it signifies. It would be prudent tO exclude seropositive subjects as blood donors for the time being. Similarly, specific advice regarding sexual activities and blood donation should be given to infected members of at risk populations. It may also be that measures which reduce the frequency of intercurrent infection could decrease the likelihood of the expression of infection as full blown AIDS in infected subjects. From the limited early data, virus isolation is a poor prognostic sign in symptornless subjects, particularly women of childbearing age. Subjects from whom virus can be isolated (whether or not they have antibodies) should be regarded as infectious; however, seropositive subjects from whom virus cannot be isolated may not be. Patients with lymphadenopathy may be the most infectious group some perhaps acting as carriers. Existing studies on the pathogenesis of AIDS will be brought into much sharper focus by this new evidence. Although the T4 ÷ lymphotropic nature of the retrovirus suggests that the depletion of this cell type may be the primary defect, many issues have still to be resolved. Some evidence suggests that only the Leu 8 + subset of T4 ÷ cells is affected21, so it is important to establish whether the tropism is similarly restricted, However, the
severity of the clinical disease suggests an immunological defect broader than simply T-cell lack (cf. DiGeorge syndrome). The marked polyclonal B-cell activation in AIDS 22 may be a nonspecific response to viral infection, but Montagnier suggests that EBVvirus-transformed B cells may also be infected. Abnormalities in antigen-presenting cells may also underlie the disorder 23. The precise mechanisms underlying defects in cellular interactions, including T-macrophagC 4, T - B and T - T interactions, need to be established further. Such studies on cellular mechanisms, their interactions and the substances involved in their mediation can soon be conducted on clinical material from subjects, and specifically on cells of known status with respect to infection by H T L V III/LAV/ IDAV. An understanding of pathogenetic mechanisms is essential to the effective treatment and immune reconstitution of AIDS patients. Finally, the nature of the protective immune response, if any, to the causative retrovirus has to be determined, not least through the comparison of seropositive patients who do not progress to AIDS with those that do. With this knowledge and once the virus has been produced on a sufficient scale, there is at last a theoretical prospect of a vaccine. However, no retrovirus vaccines have hitherto been developed; it remains to be seen whether the American health authorities' promise of a vaccine in two years is realistic. The balance of evidence strongly suggests that the aetiological agent of AIDS has been identified. A form of AIDS may have existed in Central Africa for many years. Sporadic, retrospectively identified patients linked to Africa were first seen in Europe in the mid1970s. The first patients in the now substantial US epidemic (over 4 000 cases to date) were seen in 1978/9 and the diease was documented in 1981. Its probable infectious aetiology became clear in 1982, with the first isolate of the causative agent in 1983. This evidence has been considerably strengthened in 1984 and is likely to be confirmed by 1985. During this period, many new insights have been gained which extend far beyond AIDS itself and which enhance our understanding of basic mechanisms of immunoregulation and viral immunopathogenesis. While some may be jealous of the attention that AIDS has received, others may wish to study other diseases with comparable intensity in the hope of achieving equally remarkable, far-reaching and rapid results. ~I] Acknowledgements I am most grateful to Dr K. Gallo and Dr L. Montagnier for providing me with prepublication data to enable the preparation of this
report.
ANTHONY PINCHING Department of Immunology, St Ma~y's Hospital Medical School, London W2 1PG, UK.
References 1 2 3
Pinching, A. J. (1984) Clin. Exp. Immunol. 56, 1-13 Gelman, E. P., Popovic, M., Blayney, D., Masur, H., Sidhu, G., Stahl, R. E. and GaUo, R. C. (I983) Sconce 220, 862-865 Gallo, R. C., Satin, P. S., Gelmann, E. P., Robert-Guroff, M., Richardson, E., Kalyanaraman, V. S., Mann, D., Sidhu, G., Stahl,
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R. E., Zona-Pazner, S., Leibowitch, J. and Popovic, M. (1983) Science220, 865-867 Essex, M., McLane, M. F., Lee, T. H., Falk, L., Howe, (3. W. S., Mulllns, J. I., Cabradilla, C. and Francis, D. P. (1983) Science 220, 859-862 Gallo, R. C., Salahuddin, S. Z., Popovic, M., Shearer, G. M., Kaplan, M., Haynes, B. F., Palker, T. J., Redfield, R., Oleske, J., Safai, B., While, G., Foster, P. and Markham, P. D. (1984) Scienci 224, 500-503 Weiss, R. (1984) Nature (London) 309, 12-13 Barr&Sinoussi, F., Chermann, J. C., Rey, F., Nugeyre, M. T., Chamaret, S., Gruest, J., Dauguet, C., Axler-Blin, C., V~zinetBrun, F., Rouzioux, C., Rozenbaurn, W. and Montagnier, L. (1983) Science 220, 868-871 Montagnier, L., Chermann, J. C., Barr&Sinoussi, F., Chamarct, S., Gruest, J., Nugueyre, M. T., Rey, F., Dauguet, C., Axler-Blin, C., V6zinet-Brun, F., Rouzioux, C., Saimot, G.-A., Rozenbaum, W., Gluckman, J. C., Klatzmann, D., Vilmer, E., Gfiscelli, C., FoyerGazengel, C. and Brunet, J. B. in: Human T Cell Leukaeraia Virus (Gallo, R. "C., Essex, M. and Gross, L., eds), Cold Spring Harbour Laboratory (in press) Vilmer, E., Barr&Sinoussi, F., Rouzioux, C., Gazengel, C., V~zinct-Brun, F., Dauguet, C., Fischer, A., Manigne, P., Chermann, J. C., Griscelli, C. and Montagnier, L. (1984) Lancet (i), 753-757 Montagnier, L., Barr&Sinoussi, F., Chermann, J. C. in: Proceedingsof EGID Meeting (Griseelli, C., ed.), Elsevier Science Publishers, Amsterdam (in press) Brun-V6zinet, F. et al. Lancet (in press)
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Immunoregulation in leprosy Leprosy is a chronic infectious disease caused by the bacterium Mycobacterium leprae. The immunological basis for susceptibility to infection withM, leprae is not understood; although many individuals in a highly endemic area may become infected with M. leprae, clinical disease will develop in only a very few. In these individuals, the disease has an immunopathological spectrum ranging from a tubereuloid form, in which infection byM. leprae is limited by a strong cell-mediated immune response, to a lepromatous form where cell-mediated immunity to M. leprae is absent. One of the intriguing features of this anergy is its high degree of specificity; lepromatous leprosy patients are capable of responding normally to other T-cell antigens, including the closely related tuberculosis bacillus. The nature and mechanism of this highly specific immunosuppression is becoming a topic of greatinterest, not only to leprologists, but also to many basic immunologists who see it as an opportunity to study cellular interactions and regulation in a clinically relevant context. Recent studies have shown that peripheral blood lymphocytes from lepromatous patients do not proliferate or produce g-interferon when co-incubated with M. leprae ~, and that proliferation 2 and )'-interferon production I can be at least partly restored by the addition of exogenous interleukin 2 (IL2). Lepromatous leprosy patients thus appear to possess T cells in peripheral blood which can recognizeM, leprae and hypotheses thatM, leprae-reactive lymphocytes are clonally deleted or selectively trapped seem invalid. It should be stressed however, that the invitro responses of lymphocytes from some lepromatous patients cannot be restored by the addition ofIL2, and it is not yet clear if these represent a subpopulation of patients with a different or additional immunoregulatory defect. A number of groups have demonstrated suppressor factors in lepromatous leprosy associated both with T
cells 3'4and macrophages 5'6, and Leu 2a/OKT8 + cells are prominent in lepromatous leprosy lesions 1'7. Work from Bloom's group at Albert Einstein Medical College, New York, suggests that M. leprae might possess determinants capable of inducing suppressor cells. Incubation of lymphocytes from lepromatous patients with preparations of M. leprae reduces their responsiveness to concanavalin A 3 and this seems to be mediated by OKT8 + cells 4. More recently they have shown that one particular epitope, the terminal trisaccharide moiety of an M. leprae-specific phenolic glycolipid antigen, carl cause such suppression 8. This finding not only provides further evidence that T suppressor cells are capable of recognizing carbohydrate determinants but, remarkably, shows that the receptor is exquisitely specific since minor changes in the antigenic molecule resulted in abrogation of suppressive activity. Although M. leprae possesses determinants capable of inducing immunosuppression, apparently through interaction of T suppressor and T helper cells, their precise clinical relevance is, for the present, unclear. The observed suppression, although highly specific in its induction, is non-specific in nature, being manifested as a reduction in responsiveness to concanavalin A. The relationship between this non-specific immunosuppression and the highly specific anergy towards M. leprae seen in lepromatous patients has still to be clarified. Although antigen-specific suppressor cells, are an intriguing explanation for the antigen-specific anergy in leprosy, there are alternative, or additional, explanations. For example, it is known that auto-anti-idiotypic antibodies induced during the immune response to various antigens may suppress specific humoral or cellular responses; it is possible that during infection with M. leprae, an anti-idiotypic response against an idiotype predominant in determining immunity to 34. leprae is © 1984,ElsevierSciencePublist~[~BzV.,Amsterdam 0167- 4919/84/$02.00
