Human Retroviruses in the Second Decade: a Personal Perspective

Home , Max Essex, Rev (HIV)

Human retroviruses have developed novel strategies for their propagation and survival. A consequence of their success has been the induction of an extraordinarily diverse set of human dlst!ases, including AIDS, cancers and neurological and Inflammatory disorders. Early research focused on their characterization, linkage to these dlst!ases, and the mechanisms Involved. Research should now aim at the eradication of human retroviruses and on treatment of infected people.

Retroviruses are transmitted either geneti- ...... ···...... ·...... discovered". Though its characteristics are cally (endogenous form) or as infectious ROBERT C. GALLO strikingly similar to HTLV-1, HTLV-II is not agents (exogenous form)'·'. As do many so clearly linked to human disease. It is cu other animal species, humans have both forms ..... In general, rious that HTLV-11 is endemic in some American Indians and endogenous retroviruses are evolutionary relics of old infec more prevalent in drug addicts than HTLV-I"·'•. tions and are not known to cause disease. The DNA of many HIV-1 is also most prevalent in equatorial Africa, but in con species, including humans, harbours multiple copies of differ trast to HTLV the demography of the HIV epidemic is still in ent retroviral proviruses. The human endogenous proviral flux, and the virus is new to most of the world. The number of sequences are virtually all defective, and comprise about one infected people worldwide is now estimated to be about 17 mil percent of the human genome, though R. Kurth's group in lion and is predicted to reach 30 to SO million by the year 2000. Frankfurt showed that one, called HERV-K, can code for all typ However, precise predictions are not possible because of varying ical retroviral gene products and form particles•. Future interest human behaviour and cofactors for transmission. The far greater in these sequences will depend on demonstrating some func variability of HIV as compared with HTLV also leaves open the tion or role in disease. possibility of emergence of variants with greater sexual trans The infectious retroviruses are, of course, another story. In hu missibility, as suggested by Max Essex (pers. commun.). mans they may directly cause leukaemias, neurological diseases, Both the HTL Vs and HIVs have counterparts in Old World inflammatory disorders and immune deficiency, as well as en monkeys known as simian T-cell leukaemia virus type I (STLV- hancing the incidence of other cancers (including B-cell 1), and simian immunodeficiency virus (SIV), respectively. As a lymphoma, cancer of the liver, and cervical cancer, indirectly, result, studies of human retroviruses in these animals have presumably by fostering replication of other viruses. There are been of interest". two groups of infectious human retroviruses: the human T-cell In vitro as well as in vivo the most dramatic effects of both leukaemia/lymphotropic viruses (HTLV-1 and HTLV-11) belonging HTLVs and HIVs are seen on the CD4' T cell. Although HTLV to the oncoretroviridae and the human immunodeficiency infects several T-cell subtypes (its receptor remains unknown), viruses (HIV-1 and HIV-2) belonging to the lentiretroviridae'·''. In it chiefly immortalizes the CD4' T cell which is the same cell this review, HTLV refers to HTLV-1; likewise, HIV refers to HIV-1. type to develop leukaemia. The major receptor.for HIV is CD4. Consequently, HIVs infect chiefly the CD4+ T cell and cells of Similarities and contrasts the monocyte/macrophage lineage that also express CD4. As do all retroviruses, human retroviruses replicate through a Other cell types may be infected, perhaps mediated by Fe or DNA intermediate (the provirus), which integrates in the target complement receptors in the presence of antiviral antibodies. cell DNA where it may remain transcriptionally silent. In fact, In contrast to HTLV-1, HIV is cytopathic for CD4' T cells often latency may be the chief means by which the HTLVs escape im inducing massive syncytia formation and abruptly killing the 17 mune detection. HIV infections are, however, often not latent. infected cell after the cell is activated • A difference between Their survival also involves chronic high-level replication and the two groups of human retroviruses is the relative ease of steady destruction of the immune system. The routes of trans transmission by HIV as cell-free virions compared with the mission of HTLV-1, the first discovered human retrovirus'·", HTLVs, which are probably only transmitted by infected cells. were shown to be through sex, from mother to child and via This difference may account for the more rapid spread of HIV. contaminated blood, similar to the routes later shown for HIV- Both HTL Vs and HIVs take years from the time of infection 1. HTL V-1 is endemic in many parts of the world, including to clinical disease, but for HIV there is no real latency. From in equatorial Africa, southern japan, Central and South America, situ studies in 1985 we have known some infected cells are al and in the immigrant descendants of people from these regions. ways expressing virus, and more recently we have learned that Recent molecular epidemiological studies revealed several sub there is much more ongoing HIV-1 replication and cell killing types, a prevalent type called 'cosmopolitan' HTLV-1 and other than previously thought to occur••. .., especially in lymph 21 subtypes endemic in parts of the South Pacific and equatorial nodes"'· • Africa" . Although distinguishing these isolated subtypes of HTLV-I may be only of academic interest, it remains possible HTLV as a cause of leukaemia that they may cause unexpected diseases. A year after the dis HTLV-I was established as the cause of an aggressive adult T-cell covery of HTLV-1, a second human retrovirus, HTLV-II, was leukaemia (ATL) in 1980-81 (refs 10- 12, 22, 23), just before

NATURE MEDICINE, VOLUME I, NUMBER 8, AUGUST 1995 753 © 1995 Nature Publishing Group http://www.nature.com/naturemedicine ········•····· ···· ··· ·· ····· ·· ···· ··· ······ ··· REVIEW detection of the first cases of AIDS. About two to five per cent velop as a result of Tax activation of cytokines in T cells infil of infected people per 70-year life span develop leukaemia. A trating specific tissue sites, autoimmune phenomena, other similar number may develop serious neurological or inflamma disturbances of the immune system or some other mechanism tory disease. It has recently been suggested that leukaemia is unknown and, as forMS, the ultimate importance of HTLV develops in individuals infected in infancy by .maternal trans in these illnesses may be in elucidating molecular mechanisms mission, in contrast to the neurological disorders, which are for their more common (non-HTLV associated) forms. more apt to occur following adult infection by sexual transmis sion><. Because HTLVs immortalize some infected T cells and Therapy and prevention of HTLV-1 disease because the Tax protein transactivates not only HTL V-1 genes Adult T-cell leukaemia is not curable despite extensive infor but several cellular genes, including interleukin-2 (IL-2) and the mation on its cause and pathogenesis and on the properties of alpha chain of the IL-2 receptor (IL-2Ra.), Tax has been invoked the leukaemic cell. Most therapy is based on combination cyto 25 2 as the key to the events leading toT-cell immortalization • " . toxic chemotherapy, although Waldmann and colleagues have However, because IL-2 production is only transient, other pioneered a rational approach to delivering cell-killing radionu events must be involved in the eventuallL-2 independent T cleotides selectively to the ATL cell by targeting their ee!! proliferation. A candidate mechanism involves a newly constitutively expressed 1L-2Ra. chainJO. Others have used a com discovered HTLV-1 protein, pl2', which binds to the beta chain bination of azidothymidine (AZT) and interferon alpha (lFN-a.), 2 of the IL-2R in the cytoplasmic domain of a region connected presumably as antiviral agents"·' • Remission of the leukaemia to a signal transduction pathwayn. This and other newly found occurred in most patients. However, after development of ATL, HTL V proteins are products of messenger RNA (mRNA) formed HTLV-1 replication is not needed to maintain the leukaemia. by using different splice sites from the major mRNA. Tax also Although the logic of this approach might be faulted, the results transactivates genes for many other cytokines and HLA. so far cannot be. AZT and IFN-a. not only interfere with virus Although it is not easy to speculate on the overall effects of the replication, they also affect cell metabolism. It will be of impor activation of a large number of cytokines by Tax, one, lL-7, also tance to elucidate their synergistic effects on ATL cells. promotes T-ee!! proliferation utilizing the gamma chain of the The legacy of HTLV-1 will not only be as the retrovirus forerun IL-2R. Other effects of Tax may lead to genetic diversity of in ner of HIV. In the 15-16 years since its discovery, practical public fected cells, and some of these changes may promote greater health and clinical therapeutic advances have come out of studies cell proliferation and consequently a greater chance for neo of its basic virology. Japan has eliminated or reduced breast feed 27 plastic transformation • Slowly, the events leading to T-ee!! ing in HTLV-1 endemic regions, and a blood test for HTLV-1 is immortalization are being elucidated, but immortalization or required in both the United States and Japan for blood donors to even transformation of a cell is not equivalent to development assure safety of the blood supply. These practices should substan of leukaemia, which undoubtedly requires further genetic tially reduce HTLV-1 infection. Rabbits can be infected with changes. In the absence of evident cofactors or genetic predis HTLV-1, and successful protection of them has been achieved with positions, we can only speculate why some develop adult T-cell a vaccinia virus recombinant expressing the HTLV env gene. A suc leukaemia and others do not. The failure to generate many viri cessful vaccine against HTLV in humans would provide needed ons coupled with its poor infectiousness presents the need for encouragement for the development of a vaccine against HIV. HTLV to expand and perhaps diversify the infected T-cell popu Perhaps more importantly, HTLV-1 has also provided an experi lation for its survival. The sum effects of these activities mental system to study the cause and pathogenesis of a human (increased cell proliferation and genetic instability) increase the leukaemia and of chronic neurological disease with defined ge risk of neoplastic conversion. However, this should indeed be a netic tools. Although it has a far smaller clinical impact than HIV, rare and chance occurrence, as it has to be balanced against Tax in its more subtle role in human disease, HTLV-1 may ultimately activation of HTL V production and consequent immune detec provide us with more fundamental information on the origin and tion and destruction of the cell. pathogenesis of the more enigmatic chronic human diseases.

HTLV-1 and neurological disease HlV replication and selected aspects of pathogenesis A few neurological diseases are associated with HTLV-1 infec No matter the current favourite view or the multiple models of tion. The most common and severe is tropical spastic HIV pathogenesis, virtually all depend on HIV replication. paraparesis (TSP) also referred to as HTLV-1-associated myelopa Consequently, the fine details of each of the steps involved in thy (HAM), a demyelinating disorder resembling multiple HIV replication merit intense study. HIV replication involves 2 sclerosis (MS) "·". Virus is generally not found in cells of the the usual retrovirus stages of cell surface binding, fusion of the central nervous system (CNS), except for some infiltrating T virion and host eel! lipid bilayers, reverse transcription, DNA in cells. People with TSP usually have more HTLV-I replication tegration, long terminal repeat (LTR)-regulated proviral and correspondingly a greater immune response to the virus expression, synthesis and proteolytic processing of structural compared with infected asymptomatic people and ATL pa proteins, assembly of viral RNA and proteins at the cell mem tients. This finding suggests a possible autoimmune brane, packaging, virion maturation and release. Special mechanism, which was supported by the finding of cos· cyto strategies employed by the HTLVs are also used by HIV. The HIV toxic T lymphocytes (CTLs} with specificity for Tax or Env in tat and rev genes encode nuclear proteins that have functional the cerebrospinal fluid. Although there is no clear evidence for similarity to the regulatory genes of HTLVs, allowing produc a retroviral cause of MS, knowing the pathogenesis of TSP/HAM tion of HlV to occur in bursts. Tat and Rev bind to specific sites may provide clues to its pathogenesis. of HIV, and cellular factors are involved in this process. The rel Other HTLV -!-associated diseases include infective dermatitis ative conservation of their sequences, their need by HIV for of children, lung inflammation, uveitis, rheumatoid arthritis significant levels of replication, and the fact that they offer more and polymyositis. Whether these inflammatory disorders de- than one target - themselves or their target sequences - has

754 NATIJRE MEDICINE, VOLUME I, NUMBER 8, AUGUST 1995 © 1995 Nature Publishing Group http://www.nature.com/naturemedicine • ······· · REVIEW · made them prime candidates for anti-HIV therapy and perhaps a state of viral 'latency' exists for many infected cells due to de the most intensely studied HIV genes and gene products"'". fective proviruses and in some instances probably to the unavailability of needed cellular transcription factors. Soluble Other HIV strategies factors such as some cytokines may also contribute. It is impor HIV and other lentiretroviruses have still more genes (including tant to remember that latently infected cells not making virus nef, vpr, vpu, and vif> and other anti-HIV strategies relate to prod may still make and release viral proteins, which contribute to 33 34 ucts of these and the structural genes ' , For example, we first pathogenesis. learned from HIV that special viral proteins may be released by Several different pathologic effects of HIV -1 infection appear acutely infected cells. As extracellular proteins they may poten constant: (1) eventual immune destruction associated with tiate HIV replication of nearby infected cells, facilitate infection both immune deficiency (and associated opportunistic infec of other cells or interfere with some immune functions. For ex tions) and chronic immune activation; (2) neurological ample, Tat, Vpr and Nef may all be released and affect nearby damage; and (3) hyperproliferation of some cell types, which cells. Tat promotes proliferation of some cells" and expression may lead to neoplastic transformation. The differences among of HIV in some latently infected cells, and it was recently shown individuals in the extent of these consequences and the time it to induce apoptosis of other T takes for them to occur de- cells'•. Vpr has been reported pend on the HIV-1 variant, to interfere with progression the age and very likely the ge of the cell cycle by promoting netics of the individual, and a G2 arrest, perhaps thereby the individual's microbial en allowing more provirus ex vironment. It is possible that pression and virus formation some specific cofactors also before the cell divides". Vpr contribute, but other than by also appears to be necessary increasing HIV replication (as for HIV to infect macrophages. some herpes viruses do), there Along with the matrix anti is no evidence that any one is gen, p17, Vpr forms the critical. It has become increas preintegration DNA complex ingly appreciated that it is the and aids its transport to the amount of HIV more than any 38 39 nucleus ' • In dividing cells, other factor that determines nuclear membrane disruption the clinical outcome. occurs, so the viral DNA can Ironically, science has come reach and interact with the The human immunodeficiency virus full circle to the 1984 view chromosome. Non-dividing that a direct effect of HIV in cells, like macrophages, retain nuclear membrane integrity. Vpr fection is the obvious central aspect of HIV pathogenesis. may be essential to macrophage ·infection by facilitating provirus entry into the nucleus in the absence of nuclear mem HIV pathogenesis brane breakdown'•. Vpr is an excellent example of a viral gene There is now abundant information on the mechanisms that product originally believed to be accessory, but which we have may be involved in immune impairment and notably the fall in recently learned may have multiple intra- and extracellular CD4• T cells. The problem is to ascertain which of the many de functions aiding replication of HIV, and therefore is another tar scribed in vitro mechanisms are important in vivo and which in get for our development of drugs that interfere with HIV vivo result is more important than another. HIV can kill acti replication••. Nef is another example. By binding to some cellu vated CD4· T cells. This effect may be mediated by perturbances lar proteins Nef may contribute toT-cell activation or inhibition in cell membrane integrity or by gp120 interaction with CD4 or of activation, thereby influencing both HIV replication and T both. HIV -1 particles that are defective because of improper 41 42 cell survival ' • If Net can do the same as an extracellular processing of core proteins are nonetheless able to enter some protein, modulating uninfected cells, it would obviously cells, and have been reported able to kill T cells (K. Van der markedly facilitate HIV expansion. These so-called auxiliary pro Helm, pers. commun.). Also, isolated HIV proteins can affect T teins are not the only HIV proteins released into the een survival or proliferation. For example, there are multiple extracellular space. Gp120 is easily shed from virions and has mechanisms by which gpl20 may cause loss or dysfunction of been shown capable of binding CD4 of uninfected T cells im uninfected CD4•T cells, such as cross-linking of CD4, triggering pairing their function and survival, while downregulating CD4 apoptotic death after immune activation or CTL.-ttack of unin of the infected cell. Downregulation of CD4 • infected cells also fected CD4• T cells binding gpl20, which may have been shed occurs from another 'auxiliary' protein, Vpu (ref 43). Vpu disso from some virions. Some clinical results favour an interpreta ciates intracellular interactions of CD4 and the HIV envelope tion that viral proteins alone may promote progression to (gp160) so that the gp160 reaches the cell surface for viral for AIDS. For instance, there is consensus that progression corre mation while facilitating CD4 degradation. Downregulation of lates with the plasma levels of HIV-1 RNA (reflecting viraemia), CD4 may prevent superinfection and thus rapid cell death be and there is evidence that a still greater rate of disease may fore sufficient HIV replication. occur in individuals with increasing levels of p24 while viral ge nomic RNA remains in check 0. Goudsmit, pers. commun.). HIV infection This suggests that defective virions such as RNA-free particles Results over the past few years indicate far more HIV -1 replica and/or viral proteins expressed in the absence of particle forma tion than we first supposed. Nonetheless, there is no doubt that tion contribute to AIDS pathogenesis. Molecular mimicry may

IMAGE UNAVAILABLE FOR COPYRIGHT REASONS © 1995 Nature Publishing Group http://www.nature.com/naturemedicine • REVIEW ------··------time of infection, or some aspect of their genetics. Regarding the fact that KS associated wi~h kidney t;~nsplantation (fro~ the latter, most discussions have logically focused on the genet many different donor sources) is also HHV-8 positive, suggests ics controlling the fundamental way they 'see' or respond to that HHV-8 may be much more widespread than suggested, HIV, that is, the type of immune response. However, there are and that the finding of this virus may not explain the very rea many other possibilities. As do all viruses, HIV depends on nu son it was sought: namely, the greater incidence of all forms of merous host factors for its replication, and as any component KS in men and its extraordinary incidence in HIV-1-infected of the cell does, these host factors vary from individual to indi homosexual men. If a causative role of HHV-8 inKS can be as vidual and could influence the tropism of HJV and its rate and cribed, it may be in augmenting release of some cytokines, like level of replication. the ones described above, and therefore as Ior HIV -1 an indirect role in KS but with linkage to all KS forms. HIV-1 and neoplasias Overall, our results indicate that KS evolves from hyperplasia Two unusual concepts in virology have emerged, which if not to malignancy. The malignant clones could develop from original with HIV-1 and its role in neoplasias, have been more chance genetic alterations from the hyperplastic KS spindle emphasized: the role of extracellular viral proteins in pathogen cell. Recently, my colleagues and I made a fortuitous discovery. esis, and the indirect role of a virus in tumorigenesis. We found that both the hyperplastic KS spindle cells and the neoplastic cells undergo apoptosis when treated with a hor Kaposi's sarcoma. Kaposi's sarcoma is rare in the absence of mone of pregnancy, the beta chain of human chorionic HIV infection and occurs in fewer than 1 per 100,000 per gonadotropin. Consequently, human chorionic gonadotropin year, but most frequently in older Ashkenazi jews, is now in clinical trials for KS. Mediterraneans, equatorial Africans and recipients of organ transplants. Associated with HIV-1 infection, the incidence is 8-cell lymphoma. Unlike KS, immune deficiency is clearly increased three- to fourfold in blood transfusion recipients, linked with development of B-celllymphomas in HIV-1- and several-fold in intravenous drug addicts, and much more HIV-2-infected persons and in SlY-infected macaques. As inKS, (thousands-fold) in homosexual males. Strangely, males dom HIV sequences are not found in the tumour cells. The mecha inate all forms of KS. Although many researchers have nism may involve chronic polyclonal B-cell stimulation reported finding microbial links (varying from viruses to my mediated by gp120 and gp41 and high levels of the B-cell coplasma), the recent discovery of nucleotide sequences of a growth-promoting cytokines, IL-6, and IL-4 in HIV-infected new human herpesvirus, HKSV HHV-8 associated with KS is people. Along with other cytokines (such as IL-10, which de the first that has been reproducibly established as a link to KS creases apoptosis of B cells), these factors may increase the and might be an essential aetiological agent'•. This virus is probability of B-cell neoplastic transformation. EBV sequences composed of some sequences related to Epstein-Barr virus are present only in a fraction of these tumours. Whether there (EBV), but more closely to herpes saimiri, a virus of squirrel is a role for another immortalizing virus is unknown. Our hy monkeys. However, the key enigmatic features remain, such pothesis that there might be one led to our search for and as the unexplained male preponderance, apparent multifocal discovery of a new human herpesvirus, HHV-6 (ref. 54), but appearance, composition (early on as a mixture of cell types ironically HHV-6 turned out not to be a B-cell growth-promot with a predominance of vascular spindle-shaped cells and ac ing virus, but one that infects and impairs T cells. companying extensive angiogenesis), the question of whether it is a malignancy or a hyperplasia, which cell is the true tu AIDS research: Is it at a crossroad? mour cell, and the mechanistic role of the viruses HIV-1 and Effective therapy of HIV-infected people has not yet been suc HHV-8. Nonetheless, studies on the pathogenesis of KS and cessful, and no vaccine with high-level success can be promised the possible role of HIV-1 have substantially progressed in the for the immediate future. The great resources for AIDS research past several years following development of cell culture sys and a frustrated vocal activist group combined to recently gen 52 tems and murine in vivo models•• • These results indicate that erate discussions and debates in and outside of the scientific the majority of cultured cells are hyperplastic, not neoplastic. community that have questioned the direction, priorities and However, we recently isolated malignant cells (called KS Y-1) the effectiveness of the scientific administrative leadership of 52 from a late-stage HIV-1-associated KS for the first time • AIDS research. The failures of therapy and vaccines stand in Another earlier example was found in one case of classical KS" . contrast to the rapid advances made in the early period of AIDS These cells provide the first evidence for malignant cells in KS. research (1983-85), a period called the years of intense discov We find the HIV-1 sequences present only in a minority of cells ery by j. Mann. As compared with these results in only a few of a KS tumour and present neither in the hyperplastic spindle year's time, major results of subsequent years (1986-1995) may, cells nor the neoplastic cells. Consequently, any role of HIV-1 in comparison appear meager, particularly given the large num in KS appears to be indirect. Our results suggest that it is at least ber of scientists involved. However, our appreciation of the real twofold: augmenting inflammatory cytokine production, per progress in this period should not be clouded. We have learnt haps especially IFN-y, and the production and release of the much about the role of the HIV 'auxiliary' genes, developed HIV-1 Tat protein, which promotes endothelial cell growth, ad new animal model systems, gained a greater appreciation for hesion, and invasiveness synergistic with bFGF. clinical experiments and of individual variation in resistance to The role of HHV-8 is yet to be defined. It has generally been disease and come to understand the importance of immune pa assumed that it acts as a true tumour virus, transforming its tar rameters that control HIV replication. Also, recent years have get cells. However, our evidence suggests that like HIV-1, witnessed an explosion of information on various cytokines, HHV-8 is not in most KS tumour cells. Moreover, we have the crystallization of some HIV-1 proteins, an appreciation of found HHV -8 sequences in peripheral blood lymphocytes the value of combination therapy against HIV, fostered a much (PBLs) of several normal individuals. That result, coupled with better understanding of KS and of overall immune pathogene-

NATURE MEJ)ICINE, VOLUME I, NUMBER 8, AUGUST 1995 757 © 1995 Nature Publishing Group http://www.nature.com/naturemedicine • ···REVIEW sis. We have seen the development of several new anti-HIV Organization) that are going forward in areas of extreme high agents and anti-KS agents, realize that many approaches will risk. The right vaccine preparation, of course, is not known. not work for an HIV vaccine and now know that there is much From the available information it is difficult to believe that pu more HIV in an infected person than we realized in the early rified HIV proteins will be effective. The desperation in some years. Nonetheless, the frustration remains, because the major countries has led some to propose using 'live' infectious but scientific advances awaited are nothing less than a complete 'crippled' HIV by deleting the nef gene or multiple auxiliary cure of infected people and a preventive vaccine. The response genes. This was based on success with nef-deleted SIV in mon has been either a call for more basic research or for more careful keys. To go forward in a vaccine trial in normal humans with planning and more targeting of some specific issues. The 'argu such attenuated but still infectious HIV is a course I have been ment' for more basic research is not a real issue. No one doubts strongly against. The time period of observation of the mon its need. The questions must be more specific: What will the de keys is far too short, and our experiences from chickens to finition of basic research be and who will decide what is humans have taught us that any infectious retrovirus, even one included as 'basic'? What will be reduced in the budget to ac poorly replicating, may ultimately cause disease. Finally, R. commodate it and who will make those decisions? In my Ruprecht and her colleagues recently showed that such 'im experience major advances in therapy or public health are pre munogens' do indeed cause AIDS when used in infant ceded not only by active laboratory research but also diverse monkeys••. I favour the intermediate course- neither purified clinical trials based on different ideas from different investiga proteins delivered as such nor live attenuated HIV, but other at tors. Consequently, any reduction in such trials must carry the tenuated viral vectors like attenuated poxvirus or adenovirus as greatest concern. recombinants with HIV genes. Results with them have been en couraging. For instance, in collaboration with E. Paoletti and Therapy and vaccines in the future. All of the known mecha ]. Tartaglia we have shown that this approach can be successful nisms by which HIV impairs the immune system depend on in protecting animals against infection with HTLV-I and HIV-2. HIV replication; consequently, the development of many anti However, for HIV-1 we need more fundamental research on HIV agents and their clinical testing alone and in various ways to broaden, increase and sustain the immune response. combinations and as early after infection as possible are the ob It is my impression that some pharmaceutical companies are vious priorities. The recent findings that some anti-HIV agents slowly moving away from major long-term commitments to produce a profound decline in virus levels within a few weeks development of an HIV vaccine. The combination of a lack of of treatment will make effective drug screening in vivo a reality, sufficient success in vaccine development, the threat of law but there must be more rigorous attempts to avoid drug resis suits and criticisms from activists serves to foster their tance. Targeting conserved and critical viral amino acid or disengagement. The lack of sufficient incentives makes vaccine nucleotide sequences is one way. As we recently proposed, tar development a research area that begs for long-term targeted geting cellular factors (like ribonucleotide reductase with research efforts, financial commitment from the nations of the hydroxyurea)", which the virus may need more than the cell world and effective leadership. It is often said that competition and which is not subJect to the variation of HIV, is another. is divisive and cooperation noble even when the former cat Gene therapy offers hope. There is no doubt that cells trans alyzes success and the latter, sometimes nothing more than duced with HIV inhibitory genes can protect those same cells niceties, but in the arena of development of an HIV effective from infection in vitro. Plans for ex vivo transduction of such vaccine cooperation is now mandatory. genes into T cells of patients and reinfusing these cells is ongo ing. There are uncertainties - such as obtaining a sufficient Laboratory of Tumor Cell Biology, Building 37, Room 6A09 number of stem cells successfully transduced. A major advance National Cancer Institute, would be the development of vectors that could deliver the National Institutes ofHealth gene to stem cells by direct injection into the patient. Knowing 37 Convent Drive whether the inhibitory gene will be stable and not toxic is also Bethesda, Maryland 20892-4255 important. If these problems are solved there still remains the problem of infection of the brain. Clinical trials should answer Acknowledgements some of these questions. Manipulation of cytokine levels will It is a pleasure to acknowledge and to thank my colleagues, Flossie Wong· also be a focus of therapy in the coming years, as well as many Staal, Marjorie Robert-Guroff, Genoveffa Franchini, William Blattner and different approaches. Marvin Reitz for reading this manuscript in advance and making helpful Research for an HIV preventive vaccine presents some special comments. I also am grateful to Cecil Fox for his helpful discussions. difficulties: the limits in the utility of the current animal mod Finally, I thank Anna Mazzuca for her editorial assistance. For editorial els, the failure so far to define immune parameters of reasons, it was not possible to cite many other important contributions. protection for humans, the great variability of HIV, the rela 1. Temin, H.M. Origin and general nature of retrovirus. The Retroviridae, vol. 1 tively short-lived immune response to HIV immunogens, the (ed. Levy, J.A.) (Plenum, New York, 1992). possibility of inducing enhancing antibodies that facilitate 2. Coffin, J.M. Structure and classification of retroviruses. The Retroviridae, vol. 1 (ed. Levy, ].A.) (Plenum, New York, 1992). rather than inhibit infection, and the apparent declining inter 3. Gallo, R.C. The story of retroviruses and cancer; from poultry to people. in est by the pharmaceutical and vaccine companies. All of these Vims Hunting 59-81 (Basic Books, New York, 1991). argue strongly to hold back from efficacy field testing, as in 4. Lower, R. eta/. Identification of human endogenous retroviruses with com plex mRNA expression and particle formation. Proc. natn. Acad. Sci. U.S.A. deed has been the decision in the United States. Yet, many of 90, 4480-4484 (1983). the same arguments can be used to argue for such field trials. 5. Barre-Sinoussi, F. eta/. Isolation of a T-lymphotropic retrovirus from a patient For example, without human testing, we may never know the at risk for acquired immune deficiency syndrome (AIDS). Science 220, 868-871 (1993). immune parameters to seek. It is not hard then to sympathize 6. Popovic, M., Sarngadharan, M.G., Read, E. & Gallo, R.C. Detection, isolation, with those companies and agencies (such as the World Health and continuous production of cytopathic retroviruses (HTLV-IIl) from pa-

758 NATURE MEDICINE, VOLUME 1, NUMBER 8, AUGUST 1995 © 1995 Nature Publishing Group http://www.nature.com/naturemedicine • REVIEW tients with AIDS and pre-AIDS. Science 224, 497- 500 (1984). with zidovudine and interferon alta. New Engl. /. Med. 332, 1749-1751 (1995). 7. Gallo, R.C. et at. Frequent detection and isolation of cytopathic retroviruses 33. Haseltine, W.A. & Wong-Staal, F. The molecular biology of the AIDS virus. (HTLV-III) from patients with AIDS and at risk for AIDS. Science 224, 500-503 Sci. Am. 259 (Oct.), 52-62 (1988). (1984). 34. Cullen, B.R. & Greene, W.C. Functions of the auxiliary gene products of the 8. Sarngadharan, M.G., Popovic, M., Bruch, l., Schupbach,]. & Gallo, R.C. human immunodeficiency virus type 1. Virology 178, 1-S (1990). Antibodies reactive with human T-lymphotropic retroviruses (HTLV-Ill) in 35. Ensoli, B., Barillari, G., Salahuddin, S.Z ., Gallo, R.C. & Wong-Staal, F. Tat the serum of patients with AIDS. Science 224, 506-508 (1984). protein of HIV-1 stimulates growth of cells derived from Kaposi's sarcoma le 9. Gallo, R.C. The first human retrovirus. Sci. Am. 255 (Dec.), 88-98 (1986). sions of AIDS patients. Nature 345, 84-86 (1990). 10. Poiesz, B.j . et at. Detection and isolation of type C retrovirus particles from 36. Li, C.j., Friedman, D.]., Wang, C., Metelev, V. & Pardee, A.B. Induction of fresh and cultured lymphocytes of a patient with cutaneous T -cell lym apoptosis in uninfected lymphocytes by HIV-1 Tat protein. Science 268, phoma. Proc. natn. Acad. Sci. U.S.A . 77, 7415-7419 (1980). 429- 431 (1995). I 1. Miyoshi, I. eta/. Type C virus particles in a cord T-cellline derived by co-cul 37. Rogel, M.E., Wu, Ll. & Emerman, M. The human immunodeficiency virus tivating normal human cord leukocytes and human leukaemic T cells. Nature type I vpr gene prevents cell proliferation during chronic infection. f. Viral . 294, 770-771 (1981). 69, 882-888 (1995). 12. Gallo, R.C. Discovery of a cancer virus: The first human retrovirus. in Vims 38. Bukrinsky, M.l. eta/. A nuclear localization signal within HIV-1 matrix pro Hunting 99-115 (Basic Books, New York, 1991). tein that governs infection of non-dividing cells. Nature 365, 666-669 13. Franchini, G. Molecular mechanisms of HTLV-1 infection. Blood (in the (1993). press). 39. Bukrinsky, M.l. eta/. Active nuclear import of human immunodeficiency 14. Kalyanaraman, V.S. eta/. A new subtype of human T-cell leukemia virus virus type 1 preintegration complexes. Proc. natn. Acad. Sci. U.S.A. 89, (HTLV-11) associated with aT-cell variant of'hairy cell leukemia. Science 218, 6580-6584 (1992). 571-573 (1982). 40. Balotta, C., Lusso, P., Crowley, R., Gallo, R. C. & Franchini, G. Antisense 15. Lairmore, M.D. eta/. Isolation of human T-celllymphotropic virus type 2 phosphorothioate oligodeoxynucleotides targeted to the vpr gene inhibit from Guaymi Indians in Panama. Proc. natn. Acad. Sci. U.S.A. 87, 8840-8844 human immunodeficiency virus type 1 replication in primary human (1990). macrophages. J. Viral. 67, 4409-4414 (1993). 16. Robert-Guroff, M. et at. Prevalence of antibodies to HTLV-1, -II, and -JII in in 41. Saksela, K., Cheng, G. & Baltimore, D. Proline-rich (PxxP) motifs in HIV-1 travenous drug abusers from an AIDS endemic region. lAMA 255,3133-3137 Net bind to SH3 domains of a subset of Src kinases and are required for the (1986). enhanced growth of Nef+ viruses but not for down-regulation of CD4. EMBO 17. Zagury, D. eta/. long-term cultures of HTLV-III-infected T cells: A model of f. 14, 484-491 (1995). cytopathology ofT-cell depletion in AIDS. Science 231, 850-853 (1986). 42. Baur, A.S. et al. HIV-1 Net leads to inhibition or activation ofT cells depend 18. Wei, X. eta/. Viral dynamics in human immunodeficiency virus type I in Ing on its intracellular localization. Immunity 1, 373-384 (1994). fection. Nature 373, 117-122 (1995). 43 . Strebel, K., Klimkait, T. & Martin, M.A. A novel gene of HIV-1, vpu, and its 19. Ho, D.O. et al. Rapid turnover of plasma virions and CD4 lymphocytes in 16-kilodalton product. Science 241, 1221-1223 (1988). HIV-1 infection. Nature 373, 123-126 (1995). 44. Zagury, j .F. eta/. Identification of CD4 and major histocompatibility com 20. Pantaleo, G., Graziosi, C. & Fauci, A.S. The role of lymphoid organs in the plex functional peptide sites and their homology with oligopeptides from pathogenesis of HIV infection. Sembz. lmmun. 5, 157-163 (1993). human immunodeficiency virus type 1 glycoprotein gp120: Role in AIDS 21. Embretson, J. et at. Analysis of human immunodeficiency virus-infected tis pathogenesis. Proc. llatll. Acad. Sci. U.S.A . 90, 7573-7577 (1993). sues by amplification and in situ hybridization reveals latent and permissive 45. Finkel, T.H. eta/. Apoptosis occurs predominantly in bystander cells and not infections at single-cell resolution. Proc. natn. Acad. Sci. U.S.A. 90, 357-361 In productively infected cells of HIV- and SlY-infected lymph nodes. Nal!lre (1993). Med. 1, 129-134 (1995). 22. Catovsky, D. eta/. Adult T-cell lymphoma-leukaemia in Blacks from the 46. Pantaleo, G. & Fauci, A.S. Apoptosis in HIV infection. Nature Med. 1, West Indies. Lancet 1, 639-643 (1982). 118- 120 (1995). 23. Robert-Guroff, M. eta/. Natural antibodies to human retrovirus HTLV in a 47. Turano, A. et al. Natural human antibodies to interferon- interfere with the cluster of Japanese patients with adult T cell leukemia. Sciezzce 215, 975-978 immunomodulating activity of the lymphokine. Proc. natzz. A cad. Sci. U.S.A . (1982). 89,4447-4451 (1992). 24. Bartholomew, C. et at. HTLV-1 and tropical spastic paraparesis [letter). Lmzcet 48. Chang, Y. et al. Identification of herpesvirus-like DNA sequences in AIDS-as 2, 99-100 (1986). sociated Kaposi's sarcoma. Science 266, 1865-1869 (1994). 25 . Grassmann, R. et a/. Transformation to continuous growth of primary 49. Nakamura, 5. et al. Kaposi's sarcoma cells: Long-term culture with growth human T lymphocytes by human T-cellleukemia virus type I X-region genes factor from retrovirus-infected CD4. T cells. Science 242, 426-430 (1988). transduced by a Herpesvirus saimiri vector. Proc. zzatn . Acad. Sci. U.S .A . 86, SO. Ensoli, B. et al. Synergy between basic fibroblast growth factor and HIV-1 Tat 3351-3355 (1989). protein in induction of Kaposi's sarcoma. Nature 371, 674--680 (1994). 26. Tanaka, A. et al. Oncogenic transformation by the tax gene of human T-cell 51. Ensoli, B. & Gallo, R.C. AIDS-associated Kaposi's sarcoma: A new perspective leukemia virus type I in vitro. Proc. natn. Acad. Sci. U.S.A. 87, 1071-1075 of its pathogenesis and treatment. Proc. Assoc. Am. Physicians 107, 8-18 (1990). (1995). 27. Saggioro, D. et al. Tax protein of human T-lymphotropic virus type I triggers 52. Lunardi-lskandar, Y. et a/. Tumorigenesis and metastasis of neoplastic DNA damage. Leuk. Lymphoma 12, 281- 286 (1994). Kaposi's sarcoma cell line in immunodeficient mice blocked by a human 28. Gessain, A. eta/. Antibodies to human T-lymphotropic virus type-I in pa pregnancy hormone. Nature 375, 64-68 (1995). tients with tropical spastic paraparesis. Lancet 2, 407-410 (1985). 53. Siegal, B. eta/. Kaposi sarcoma in immunosuppression. Possibly the result of 29. Osame, M. et al. HTLV-1 associated myelopathy, a new clinical entity [letter). a dual viral infection. Cancer 65, 492-498 ( 1990). Lancet 1, 1031-1032 (1986). 54. Salahuddin, S.Z. et a/. Isolation of a new virus, HBLV, in patients with lym 30. Waldmann, T.A. Anti-IL-2 receptor monoclonal antibody (anti-Tac) treat phoproliferative disorders. Science 234, 596-601 (1986). ment of T-celllymphoma. Jmportant Adv. 011Col. 131-141 (1994). 55. Lori, F. et a/. Hydroxyurea as an inhibitor of human immunodeficiency 31. Gill, P.S. eta/. Treatment of adult T-cellleukemia-lymphoma with a combina virus-type I replication. Science 266, 80!- 805 (1994). tion of Interferon alta and zidovudine. New Engl. f. Med. 332, 1744--1748 (1995). 56. Baba, T.W. eta/. Pathogenicity of live, attenuated SIV after mucosal infec 32. Hermine, 0. eta/. Brief report: treatment of adult T-cellleukemia-lymphoma tion of neonatal macaques. Science 267, 1820-1825 (1995).

NATURE MEDICINE, VOLUME 1, NUMBER 8, AUGUST 1995 759