Programmed cell death and AIDS: from hypothesis to experiment

viewpoint

Programmed cell death and AIDS: from hypothesis to experiment Jean Claude Ameisen Here, Jean Claude Ameisen discusses new findings supporting the hypothesis that abnormal induction of programmed cell death (PCD) is relevant to AIDS patbogenesis. Recent evidence also suggests that the prevention of PCD is a factor in oncogenesis. Together, these ideas provide a framework for the reinterpretation of cell survival disorders in terms of PCD dysregulation, and suggest that in vivo control of cell signalling has wide-ranging therapeutic potential. Programmed cell death (PCD, also known as apoptosis), an active cell suicide process, has long been recognized as an important feature in embryonic development, in particular in shaping the immune and nervous systems 1-3. It also occurs in adult tissues. PCD is regulated by signals provided by the local environment2.3; unlike cell degeneration or necrosis, PCD can be induced or suppressed by the withdrawal or addition of defined activation signals 2,3. In a previous paper in this journal 4, submitted in May 1990, it was proposed that inappropriate induction of PCD may play a central role in the pathogenesis of acquired immune deficiency syndrome (MDS). The idea presented was that most immunological and nonimmunological defects leading to AIDS, including brain atrophy and dementia, could be related to an activation-induced suicide process in CD4 + T cells and neurons, caused by indirect interference of human immunodeficiency virus (HIV) with inter: and intracellular signalling. The hypothesis made several testable predictions, based on the assumption that both early in vitro dysfunction and late in vivo depletion of CD4+ T cells are due to PCD, and that modulation of cell signalling may prevent death and restore normal CD4 + T-cell functions 4. During the last two years, a series of experimental observations have provided support for this model, by indicating that Tcell receptor (TCR) stimulation can lead to PCD in mature T cells in several circumstances, including mouse and human acquired immune deficiencies of retroviral origin.

proliferation to the stimuli, including proliferation of memory T cells to a recall antigen (tetanus) not encountered by the patients since they become infected by HW. Secondly, Montagnier's group reported that poor in vitro survival of lymphocytes from HIV-infected individuals is due to PCD induction 7, and observed activation-induced PCD of CD4 ÷ and CD8* T cells in response to calcium ionophore and of CD4 ÷ T cells in response to superantigens. Third, Miedema and colleagues reported that the failure of T cells from HIVinfected individuals to proliferate in response to suboptimal doses of a particular CD3 mAb of IgE isotype was related to CD4 * and CD8* T-cell PCD induction 8. Fourth, the in vitro cytopathic effect on CD4+ T cells of HIV itself was shown by Carson et al.9 and Hovanessian et al? ° to be due to PCD, a finding that provided an explanation for the previous observation that death of HIV-infected CD4 + T cells in vitro is preceded by the accumulation of histones ~, a consequence of internucleosome cleavage during PCD. Fifth, and finally, extending the observation of Newell et al.~2 on the effect of CD4 crosslinking on murine T cells, FinkeW reported that CD4 ligation by gpl20 plus anti-gpl20 antibodies primes normal human CD4+ T cells for PCD in response to subsequent TCR stimulation. This suggests that both the HIV envelope protein and the immune response to it may play a role in PCD induction in AIDS. PCD induction and AIDS

HIV and CD4* T-cell PCD T-cell PCD and AIDS Several lines of evidence suggest a relationship between T-cell dysfunction, T-cell PCD and AIDS. First, we observed that CD4 + T cells from HIV-infected asymptomatic individuals undergo PCD upon in vitro stimulation by pokeweed mitogen or by TCR stimulation by self major histocompatibility complex (MHC) class II-dependent superantigens s,6. The death of the cells is an active process that can be prevented by protein synthesis inhibitors or by cyclosporin A. Furthermore, the addition of a co-signal, in the form of an anti-CD28 monoclonal antibody (mAb), prevents both CD4 • T-cell suicide and restores normal T-cell

The HIV proteins gpl20 (Ref. 13) and tat (Ref. 14), as well as anti-envelope antibodies that crossreact with MHC class II molecules ~s, inhibit antigen-specific proliferation of normal CD4 + T cells in vitro. Of these, gp120 has been reevaluated in terms of PCD induction; it therefore provides the first, but perhaps not the only, candidate for PCD induction in AIDS. An important question is whether co-signals, such as that provided by anti-CD28 mAb (Ref. 6) or cytokines 7, which prevent PCD in T cells from HIV-infected individuals, also prevent gp120-mediated PCD induction in normal CD4 + T cells. If so, then it is possible that HIV envelope proteins, rather than HIV superinfection 9, accounts for

© 1992, Elsevier Science Publishers Ltd, UK.

Immunology Today

388

vol 13 No. 10 1992

viewpoint PCD in productively infected CD4 + T-cell cultures<"">: envelope proteins expressed on the surface of HIVinfected cells may lead to PCD in neighbouring activated but uninfected cells by crosslinking the CD4 molecule, thus interfering with activation signals that prevent T-cell death. Support for this hypothesis comes from the recent finding that the cytopathic effects of the envelope-CD4 interaction between infected and uninfected CD4* T cells requires intracellular signalling~6; and that such envelope-CD4 interaction leads to PCD (A. Hovanessian, abstract presented at the VIII International Conference on AIDS, Amsterdam, July 1992). A far reaching implication of these findings is that modulation of cell signalling may prevent PCD and allow HIV infection to proceed in the absence of T-cell death. Additional mechanisms, which are not mutually exclusive, may contribute to PCD induction in HIV infection. For example, two series of findings have led Janeway ~" to propose that variable H1V-encoded superantigens exist. First, superantigens induce PCD, in vivo, of CI)4 + T cells that express the appropriate Vb TCR molecule js (after an initial phase of intense T-cell proliferation) leading to T-cell depletion rather than Tcell memory. Secondly, the selective depletion of T cells bearing particular VI, families in murine retroviral (MMTV) infection is caused by retrovirus-encoded superantigens I~. Selective T-cell depletion has been reported in AIDS patients >, but HIV-encoded superantigens have not yet been identified. The possible role of autoimmunity in AIDS pathogenesis-'* should also be taken into consideration. The finding that antibodies against T-cell surface molecules, such as Fas (Ref. 22) induce apoptosis in normal, acti-rated T cells, provides a rationale for the involvement of autoantibodies in PCI) induction. On the basis of similarities between AIDS and graft versus host diseases2~, it has been suggested that autoimmune mechanisms may involve molecular mimicry between gpl60 and MHC class II molecules Is,2<-'4. This mimicry may also cause an inappropriate tolerance to self MHC class II-dependent TCR Iigands, and induce CD4 + Tcell PCD. However, both abnormal self reactivity and inappropriate nonself tolerance could result from viralmediated interference with T-cell signalling, rather than from the antigenic nature of HIV proteins. The two signal model and T-cell PCD

The two signal model of T-cell activation implies that the ability of mature T cells to discriminate between self and nonself does not depend solely on the nature of the TCR, but also on the co-signals provided by antigen presenting cells (APCs) 2>2~. TCR stimulation plus a second signal delivered from an APC induces T-cell proliferation and differentiation into effector and memory T cells; TCR stimulation alone induces a state of unresponsiveness, termed anergy, that can be reversed under certain conditions2% Recent findings in the mouse indicate that TCR stimulation plus inappropriate co-signals might lead to a more dramatic consequence; that of mature T-cell deletion by PCD. PCI) has been induced in mature T cells by lig-

Imm,,,otogy 7od,y

ation of CD4 t Ref. 12), or by ligation of MHC class I molecule's (the veto phenomenon). PCD can also be induced in activated interleukin 2 (IL-2)-dependent CD4 + or CD8 ~ T-cell lines or clones by a variety of methods: withdrawal of exogenous IL-2 (Ref. 29), TCR stimulation in the absence of accessory cells ~'', activation by I1,-2 alone 3j, and consecutive TCR stimulation performed under slightly different conditions ~-~. A general model of cell survival regulation can be drawn from these studies: any isolated activation signal may prime a cell for PCD; multiple signals may be required for cell differentiation and/or proliferation to proceed2'L In this context, the stringent requirement of T cells for accessory cell co-signals, then for coordinated autocrine and paracrine growth factors, may represent only one example of the general control exerted by the environment on cell survival. All of these factors may participate in the maintenance of self tolerance and in the downregulation of successful immune responses. T-cell PCD, anergy and proliferation may thus represent a hierarchy of discrete steps in T-cell responsiveness that depend on the initial degree of Tcell stimulation and on the nature of environmental signals available to prevent T-cell death. Together, these findings suggest that a virus that interacts with both CD4 ~ T cells and APCs could induce PCD by several different mechanisms, not only in CD4 + T cells hut also in CD8 + T cells. Various mechanisms, including CD4" T-cell dysfunction, interference with growth factor availability or requirements, and CD8" T-cell hyperactivation, may account for the reported in vitro PCD of CDS+ T cells from HIVinfected people ~'~. PCD induction and pathogenesis

The observation that in addition to CI)4 + T cells, CD8 + T cells from HIV-infected individuals may also undergo PCD -'s raises the broad question of the relevance of in vitro findings of PCD to the in viw~ fate of the T cells. Do CD8 + T cells die under particular culture conditions but survive in the patients? Do CDS* T cells die in vivo but are replaced? Does impairment of renewal of CD4 + T cells, rather than the death of mature CD4 + T cells, account for the selective loss of this cell population in vivo? Impairment of renewal of CD4 + T cells in HIVinfected individuals is suggested by the recent finding presented by J. McCune at the Keystone Symposium, March 1992, that HIV infection of SCID mice reconstituted with human fetal thymuses leads to profound CD4 + thymocyte depletion by PCD. Whether CD4 ÷ thymocyte apoptosis by preactivation or by printing is due to the cytopathic effect of H1V infection, or is the consequence of an indirect effect of HIV on thymic selection processes remains to be investigated. PCD induction may not be pathogenic per se, unless it interferes with the generation and renewal of effector cells, or the maintenance of memory cells, leading to a permanent and detrimental deletion of T cells. A prediction of our earlier hypothesis was that inappropriate T-cell PCD induction may not be unique to AIDS4. We also proposed that in contrast to chronic PCD

389

v,,t. 1~ No. 10 i992

viewpoint induction, transient T-cell PCD may even exert a beneficial role in the control of acute lymphotropic viral infection, such as measles4. Support for this hypothesis comes from the recent finding that spontaneous in vitro PCD of a very large proportion of in vivo activated CD4 ÷ and CD8 ÷ T cells occurs during acute, benign Epstein-Barr virus-induced infectious mononucleosis in children 3s. An essential question is whether ongoing T-cell PCD is central to AIDS pathogenesis, or merely a consequence of ongoing stimulation of the immune system in a chronic infection. In order to address the question of the importance of PCD in AIDS pathogenesis, animal models provide good examples of pathogenic and nonpathogenic chronic retroviral infections. Feline leukemia virus strains that induce lymphopenia and disease in vivo have been reported to also induce lymphocyte PCD in vitro, whereas weakly in vivo virulent strains do not induce PCD 34. More relevant to AIDS are simian models of lentiviral infections. These include experimentally HIV-l-infected chimpanzees, which are productively infected but do not develop disease; and experimentally SIV-infected macaque rhesus monkeys, that develop a disease closely resembling AIDS. We have found that T-cell PCD did not occur in HIV-1infected chimpanzees; however, CD4* T cells from SIVinfected macaques underwent PCD when stimulated in vitro with mitogens or superantigens (Ref. 35; and H. Groux et al., unpublished). Similar results were presented at the VIII International Conference on AIDS (July 1992) by M.L. Gougeon. These findings support the hypothesis that inappropriate and ongoing T-cell PCD induction is related to AIDS pathogenesis, and suggest that T-cell priming for PCD by HIV or SIV is not simply the consequence of the ability of the virus to infect CD4 ÷ T cells, but requires an additional level of interplay with the immune system. Further studies of pathogenic and nonpathogenic SIV infections will be required in order to confirm these findings and should provide essential information on the mechanisms by which viruses induce T-cell PCD. SIV-infected macaques may also be useful in exploring the consequences of therapeutical prevention of CD4 ÷T-cell PCD.

PCD prevention and oncogenesis There may be a price to pay for the prevention of PCD. In the nematode Caenorhabditis elegans, genes that regulate PCD during development have been identified. Whereas abnormal induction of PCD is lethal in C. elegans, complete prevention of PCD does not lead to any detectable adverse consequences36. In more complex organisms with extended lifetimes, however, PCD prevention may not be devoid of deleterious effects. These may include the breaking of self tolerance, dysregulation of immune responses, and the development of tumors. Recent findings indicate that aberrant cell survival resulting from prevention of PCD may contribute to oncogenesis (reviewed in Refs 37, 38). Expression of the bcl-2 gene inhibits PCD induction in myeloid precursors, B cells and thymocytes. Although no relationship has been found between bcl-2 and cancer in T cells,

mmu oto

oday

this may be related to the selective effect of the gene on PCD regulation. Indeed, in bcl-2 transgenic mouse thymocytes, bcl-2 expression prevents apoptosis induction in response to several signals, including CD3-mediated TCR stimulation, but does not prevent physiological negative selection by self MHC class II-dependent TCR ligands. In contrast, the role of bcl-2 in the development of B-cell tumors is supported by observations of its constitutive expression in human follicular B-cell lymphomas, in Epstein-Barr virus-infected B cells, and by transgenic mice experiments in which B-cell lymphomas occur when additional activation, such as c-myc induction, is provided 3v'3~. Inactivation of the product of a tumor suppressor gene, p53, may also lead to cancer, by abrogating the requirement for growth factors in order to prevent PCD (Ref. 39). The possibility that permanent abnormal expression of genes that block PCD may be involved in tumor cell immortalization is further sugested by the observation that protein synthesis inhibitors, that prevent PCD in normal cells l's, often suffice to induce PCD in tumor cells4°. Together, these findings indicate that inappropriate expression of genes that prevent PCD induction may lead to disease. An important question, that remains to be explored, is whether inappropriate intercellular signalling can also induce such an abnormal and permanent gene expression. An interesting example may be cells from Kaposi's sarcoma, a frequent tumor in HIVinfected patients, that appear to depend on paracrine growth factors released by other cells in order to become transformed 4~. It may be hypothesized, therefore that AIDS is a range of diseases in which cell signalling dysregulation leads, in different cell types, to PCD or to PCD-related cell immortalization.

Conclusion Strategies that interfere with cell signalling and lead to induction or prevention of PCD may have evolved in viruses and other pathogens in order either to escape immune defence mechanism, or to induce immortalized cells. Profound cell loss or development of cancers may represent the most dramatic consequences of such strategies. Interdisciplinary studies will be required in order to assess whether this is the case and whether the ability to restore in vivo physiological PCD regulation could have wide-ranging consequences. An implication of our earlier hypothesis was that therapeutic intervention aimed at the control of PCD may contribute to the prevention of AIDS development in HIV-infected people4. An intriguing possibility that deserves consideration, is that simian models of non pathogenic HIV and SIV infections may represent natural examples of how such host-pathogen equilibria can be achieved. Jean Claude Ameisen is at the Unit~ INSERM U167CNRS 624, Institut Pasteur, 1 rue du Pr. A. Calmette, F-59019 Lille, France.

References 1 Duvall, E. and Wyllie, A.H. (1986) Immunol. Today 7, 115-119

390

Vo .

No.

992

viewpoint 2 Tomei, L.D. and Cope, F.O., eds (1991) Apoptosis: 7he Molecular Basis of Cell Death. Current Communications in Cell and Molecular Biology, Cold Spring Harbor Laboratory Press 3 Raft, M. (1992) Nature 356, 397-400 4 Ameisen, J.C. and Capron, A. (1991) Immunol. Today 12, 102-105 5 Groux, H., Mont6, D., Bourrez J.M., Capron, A. and Ameisen, J.C. (1991) C. R. Acad, Sci. Paris (S&ie IlI) 312, 599-606 6 Groux, H., Torpier, G., Mont& D. etal. (1992)J. Exp. Med. 175, 331-340 7 Gougeon, M.L., Olivier, R., Garcia, S. etal. (199l) C. R. Acad. Sci. Paris (S&ie llI) 312, 529-537 8 Meyaard, L., Otto, S.A., Jonker, R.R. et al. (1992) Science 257, 217-219 9 Terai, C., Kornbluth, R., Pauza, C., Richman, D. and Carson, D. (1991)]. Clin. Invest. 87, 1710-1715 10 Laurent-Crawford, A.G., Krust, B., Muller, S. et al. (1991) Virology 185, 829-839 11 Krust, B., Laurent, A., Cointe, D. etal. (1990) in Retroviruses of Human AIDS and Related Animal Diseases

(Girard, M. and Valette, L., eds), pp. 41-45, Fondation M. M&ieux 12 Newell, M.K., Haughn, L.J., Maroun, C.R. and Julius, M.H. (1990) Nature 347, 286-289 13 Mann, D.L., Lasane, F., Popovic, M. etal. (1987) J. Immunol. 138, 2640-2644 14 Viscidi, R.P., Mayur, K., Lederman, H.M. and Frankel, A.D. (1989) Science 246, 1606-1608 15 Golding, H., Shearer, G.M., Hillman, K. etal. (1989) J. Clin. hwest. 83, 1430-1435 16 Cohen, D.I., Tani, Y., Tian, H. et al. (1992) Science 256, 542-545 17 Janeway, C.A. Jr (1991) Nature 349, 459-461 18 Kawabe, Y. and Oshi, A. (1991) Nature 349, 245-247 19 Choi, Y.W., Kappler, J.W. and Marrack, P. (1991) Nature 350, 203-207 20 lmberti, L., Sottini, A., Bettinardi, A., Puoti, M. and Primi, D. (1991 ) Science 254, 860-862

21 Via, C.S., Morse, H.C. II1 and Shearer, G.M. (1990) Immunol. Today 11,250-255 22 Itoh, N., Yonehara, S., Ishihi, A. et al. ( 1991 ) Cell 66, 233-243 23 Kion, T.A. and Hoffmann, G.W. ( 1991 ) Science 253, 1138-1140 24 Habeshaw, J., Dalgleish, A.G., BountifL L. et al. (1990) lmmunol. Today 11,418--425 25 Bretscher, P. and Cohn, M. (1970)Science 169, 1042-1049 26 Jenkins, M. (1992) lmmunol. Today 13, 69-73 27 Janeway, C.A. Jr (1992) Immunol. Today 13, 11-16 28 Sambhara, S. and Miller, R. (1991) Science 252, 1424-1427 29 Duke, R. and Cohen, J. (1986) Lymphokine Res. 5, 289-299 30 Liu, Y. and Janeway, C.A. Jr (1990) 1. Exp. Med. 172, 1735-1739 31 Lenardo, M.J. (1991) Nature 353,858-861 32 Russel, J,H., White, C.L., Lob, D.Y and Meleedy-Rey, P. ( 1991 ) Proc. Natl Acad. Sci. USA 88, 2151-2155 33 Uehara, T., Miyawaki, T., Ohta, K. et al. (1992) Blood 80, 452-4,58 34 Rojko, J.L., Fulton, R.M., Rezanka, L.J. et al. (1992) Lab. Invest. 66, 418-426 35 Ameisen, J.C., Groux, H., Plouvier, B. et al. (1991)in Retroviruses of Human AIDS and Related Animal Diseases

(Girard, M. and Valette, L., eds), pp. 19-24, Fondation M. M6rieux 36 Hengartner, M., Ellis, R. and Horvitz, H. (1992) Nature 356, 494-499 37 Williams, G. (1991) Cell65, 1097-1098 38 Korsmeyer, S.J. lmmunol. Today (in press) 39 Yonish-Rouach, E., Resnitzky, D., Lotem, J. et al. ( 1991 ) Nature" 352, 345-347 40 Martin, S., Lennon, S., Bonham, A. and Cotter, T. (1990) J. ImmunoL 145, 1859-1867 41 Chandran Nair, B., DeVico, A.L., Nakamura, S. et al. (1992) Science 255, 1430-1432 42 Finkel, T. J. Exp. Med. (in press)

Programmed D N A Rearrangements ~ a Special Issue of Trends in

Genetics

~

The December 1992 issue of TIG will be a special issue devoted to programmed DNA rearrangements. Many of the articles will be of interest to immunologists. The contents of the issue will include: Genetic switches: mechanisms and function, by Ronald

Catalysis by site-specific recombinases, by W. Marshall Stark,

Plasterk

Martin Boocock and David Sherratt

V(D)J recombination gets a break, by Martin Gellert Functional and mechanistic aspects of genetic variation in bacterial pathogens, by Brian Robertson and Thomas Meyer

The unusual organization and processing of genomic DNA in

Chromatin diminution in nematode development, by Heinz

Van der Ploeg. Keit-Gottesdiener and Mary Lee

Tobler, Adrian Etter and Fritz Mueller

Activation of V(D)J recombination by RAG-1 and RAG-2, by Marjorie Oettinger

Mating-type gene switching in Saccaromyces cerevisiae, by James Haber

hypotrichous ciliates, by David Prescott Antigenic variation in African trypanosomes: DNA rearrangement events at telomeric VSG gene expression sites, by Lex Creation of immunoglobulin diversity by intrachromosomal gene conversion, by Craig Thompson DNA inversions in phages and bacteria, by Pieter van de Putte and Nora Goosen

Copies of this issue will be available from our Barking office, at a cost of £8.50 or $15.00 per copy. Discounts are available for multiple orders of 10 copies or more. To obtain copies of the special issue, send your order to: TIG Single Issue Sales, Elsevier Science Publishers Ltd, Crown House, Linton Road, Barking, Essex, UK IG11 8JU. Cheques should be made out to Elsevier.

Immunology Today

391

Vol 13 No. 10 1992