Retrovirus classification and cell interactions

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1 Journal of Antimicrobial Chemotherapy (1996) 37, Suppl. B, 1-11 Retrovirus classification and cell interactions Robin A. Weiss Institute of Cancer Research, Chester Beatty Laboratories, 237 Fulham Road, London SW3 6JB, UK Retroviruses cause a variety of diseases such as cancer, AIDS, autoimmunity and diseases of central nervous system, bone and joints. Some retroviruses are apparently harmless and some retroviral genomes are even inherited as host mendelian traits. Whereas oncoviruses generally have a low virus load, so that antiviral therapy would not greatly affect disease progression, lentiviruses like HIV have high load and turnover. The cell interactions of retroviruses largely determine their pathogenesis. Finally, retroviruses are being exploited therapeutically as vectors for gene therapy. Introduction Retroviruses infect a wide variety of vertebrates ranging from fish to humans (Weiss et al., 1985; Levy, ). Some retroviruses cause diseases (Table I) of agricultural and economic importance, e.g. leukosis of cattle and chickens. Of course, the greatest impact has come from AIDS, a disease which was not recognised until Human immunodeficiency virus (HIV) is the topic of this symposium and this article is to introduce retroviruses in general and to explore their interactions with host cells. Retrovirus replication Retroviruses acquired their name when the discovery in 1970 of reverse transcriptase (RNA dependent DNA polymerase) showed that their RNA genomes are copied into DNA. This RNA to DNA conversion was the first example of a 'backwards' direction of genetic information in biological systems. It has formed the basis of the first clinically applied antivirals for retroviruses, because drugs such as azidothymidine, dideoxycytidine and lamivudine (3TC) are selectively incorporated as DNA chain terminators by viral reverse transcriptase and not by host DNA polymerases. Now we know that a broader group of genetic elements than retroviruses utilise reverse transcription at some stage of replication; these include hepadnaviruses (including hepatitis B virus), cauliflower mosaic virus and retrotransposons of eukaryotes and prokaryotes. Indeed lamivudine may find a place in the treatment of hepatitis B infection as well as HIV. A simplified version of the replication cycle of a retrovirus is shown in Figure 1. The virus particle is diploid, i.e. it carries two genome copies in its core. Reverse transcriptase is also packaged in the core but does not become enzymatically active until penetration 03O5-7453/96/37BO $12.00/0 i t The British Society for Antimicrobial Chemotherapy

2 R. A. Weiss Table I. Retroviral diseases Malignancies myeloid leukaemia erythroid leukaemia lymphoid leukaemia lymphoma sarcoma mammary carcinoma renal carcinoma Haematological deficiencies aplastic anaemia haemolytic anaemia autoimmune disease immunodeficiency Bone and joint disease osteopetrosis arthritis Neurological disease peripheral neuropathy encephalopathy degeneration dementia Pulmonary disease pneumonia adenomatosis and uncoating of the virion takes place. The primers for reverse transcription are cellular transfer RNA molecules that hybridize with the viral RNA genome, trnalys for HIV. From these primers nascent DNA chains are formed. Reverse transcriptase also has an RNaseH activity that removes the parental RNA from the DNA, which then forms the template for second strand DNA synthesis to form a double-stranded DNA 'provirus'. This provirus is haploid but has sequences derived from both RNA Receptor Figure 1. Simplified replication cycle of a retrovirus.

3 Retrovirus classification and cell interactions 3 strands through a copy-choice mechanism to provide long terminal repeats (LTR) at each end of the DNA genome (Skalka & Goff, 1993). The provirus then integrates into host chromosomal DNA, catalysed by another viral enzyme, integrase. Integration is not host-site specific but occurs preferentially in euchromatin. The integrated provirus may remain latent and will be replicated along with the rest of host DNA if the infected cell is proliferating. Expression of the provirus, however, provides mrna for translation into viral proteins and full length transcripts provide genomic RNA for packaging into virions. The transcriptional control of viral RNA from the integrated DNA provirus depends on cellular transcriptional regulators, e.g. NFkB for HIV. Complex retroviruses (see below) like HIV also require viral regulatory proteins, such as tat for efficient RNA transcription, and rev which aids transport of the larger viral transcripts from nucleus to cytoplasm. In virion maturation the core and matrix proteins are formed from the gag protein precursor which is cleaved by viral protease at a late stage of virion budding into the major structural proteins. The envelope glycoproteins, gp41 transmembrane (TM) and gpl20 surface (SU) proteins in the case of HIV, are synthesised in the rough endoplasmic reticulum and are cleaved by cellular furin-like enzymes during processing through the Golgi to the cell surface, where they are incorporated into budding virions. Retrovirus classification Retroviruses are broadly classified into three subfamilies, oncoviruses, lentiviruses and spumaviruses (Table II), based on electron microscopic morphology of virions and analysis of genome sequences. Oncoviruses are further subclassified into B- C- and D-type particles morphologically (A-type particles are cytoplasmic cores of B- and D-type particles). Not all retroviruses classed as oncoviruses are oncogenic. Some are apparently harmless in their natural hosts; others such as the D type simian retroviruses related to Mason-Pfizer monkey virus induce an immune deficiency which can be as severe as those caused by HIV and SIV. Foamy viruses (spumaviruses) frequently occur in monkeys and great apes, especially in the central nervous system, but the status of human foamy virus has become uncertain. They are not known to be pathogenic in vivo, Table II. Retrovirus classification Subfamily Oncovirus B-type C-type D-type Lcntivirus Spumavirus Examples Murine mammary tumour virus (MMTV) Human T-ccll leukaemia virus Avian leukosis and sarcoma viruses Salmon lymphoma virus Mason-Pfizer monkey virus Human immunodeficiency viruses Ovine maedi-visna virus Equine infectious anaemia virus Simian foamy viruses (HTLV) (ALSV) (MPMV) (HIV) (MVV) (EIAV) (SFV)

4 4 R. A. Weiss despite the fact that they are more cytopathic in vitro than any other retroviruses, forming large, vacuolated syncytia with a foamy appearance. An alternative broad classification of retroviruses that has recently come into vogue is to divide them according to genome organisation into 'simple' and 'complex' viruses. Simple retroviruses contain only gag pol and env genes, while complex retroviruses contain extra non-structural genes having regulatory or auxiliary functions. The genome organisation of some typical retroviruses is shown in Figure 2. gag, pol and env encode the core proteins, viral enzymes and envelope glycoproteins respectively. The regulatory genes include transcriptional activators such as tat of HIV, tax of human T cell leukaemia virus (HTLV-I) and bel-\ of human foamy virus (HFV) which upregulate viral transcription. They may also transactivate cellular genes; for example, tax protein acts indirectly to enhance expression of the p55 chain of the interleukin-2 receptor which is overexpressed in the T-cell leukaemia induced by this vims. MLV 1 1.i K- - HTLV-1 nef HIV-1 HFV LTR ML.. bell kb Figure 2. Proviral genomes of different classes. The simple oncovirus, mouse leukaemia virus (MLV) possesses only gag, pol and enc genes whereas the complex viruses shown (HTLV-I, H1V-I and HFV) have accessory genes which aid viral replication. All the genes or their products are potential targets for anti-viral drugs.

5 Retrovirus classification and cell interactions S Retrovinis transmission Retroviruses are transmitted horizontally, vertically, and genetically as integrated DNA proviruses in germ cells. HIV can be transmitted horizontally both as a cell-free virus, or as infected cells, whereas HTLV is only cell-associated. Hence the frequent presence of both HTLV and HIV in injecting drug users, but only HIV in haemophiliacs (before screening was introduced). Horizontal transmission is well adapted to the hosts being infected. Thus some retroviruses are transmitted through biting and scratching (e.g. leukaemia vims among cats, SIV among macaques), via water and gill wounds among fish, and via biting flies among horses in equine infectious anaemia virus (EIAV). Vertical transmission of infectious virus can occur prenatally, perinatally and post-natally, as known for HIV. The majority of paediatric infections appear to occur perinatally but milk transmission also occurs, particularly on primary maternal infection during lactation. Milk transmission is the principle means of infection for HTLV-I, and for the murine mammary tumour virus (MMTV). Indeed, MMTV is adapted to high expression in lactating mammary gland cells by possessing enhancer sequences in its LTR that are upregulated by host hormones. Upon infection of suckling mice (probably via gut lymphoid cells), MMTV is carried in T-lymphocytes which are activated to proliferate by a viral gene encoding a superantigen. The virus then colonises the mammary gland of females (and the epididymis of males) upon sexual maturity. Avian leukosis virus is similarly adapted for congenital transmission by its high production in ovalbumen-secreting cells in the hen's oviducts. Mendelian genetic inheritance is perhaps the most ingenious and economical means of viral transmission known. Various oncoviruses have become integrated into germ cells during vertebrate evolution and are thus passed on to the next host generation in the eggs and sperm. Such 'endogenous' retroviral genomes as they are called, may remain latent for many host generations, or they may later be expressed and cause disease. Thus MMTV and murine leukaemia virus have been integrated into certain mouse inbred lines (GR and AKR, respectively) during their derivation and selection as high incidence cancer mouse strains. We have shown that an endogenous leukosis virus genome of chickens entered the germ-line before their domestication thousands of years ago in China, as the virus is present in the red jungle fowl (Gallus gallus), but was acquired after the diversification of the Gallus genus, as it is not present in the DNA of the other extant species. Human DNA contains thousands of endogenous retroviral genomes, most of which were acquired relatively early in primate evolution since they are widely represented in old world monkeys. In general they have become defective, with numerous stop codons in the viral genes. However, a few open reading frames are conserved, and some human cells produce non-infectious virions containing RNA, reverse transcriptase and gag proteins. We have recently studied a human endogenous retroviral genome which has a conserved env gene expressed as glycoprotein in the syncytiotrophoblast of the normal human placenta (Venables el a/., 1995). Human retroviruses Table III lists the known human retroviruses. The first 'human' retrovirus to be isolated in 1971 was human foamy virus (HFV) from a nasopharyngeal carcinoma line.

6 R. A. Weiss Table III. Human retroviruses Virus Oncovirus HTLV-I HTLV-II Lentivirus HIV-1 HIV-2 Spumavirus HFV? Disease Adult T-cell leukaemia Tropical spastic paraparesis Hairy cell leukaemia? Neurodegeneration? Acquired immune deficiency syndrome Acquired immune deficiency syndrome None Immunofluorescence studies indicated that sera collected from humans in East Africa and Polynesia had HFV antibodies, and various claims have been made that HFV is associated with CNS disease and autoimmune thyroid diseases (Graves and De Quervain thyroditis). However, recent PCR and ELISA tests have not upheld the earlier evidence of HFV infection, and the original HFV genome sequence is almost indistinguishable from chimpanzee simian foamy virus (SFV-6). SFVs are common among old world monkeys and apes and there are now a handful of authenticated, PCR positive human infections among primate handlers, all of whom remain in good health. Human T-cell leukaemia virus type I (HTLV-I) was first isolated in 1980 and is found to be endemic in southwest Japan, West Africa and among black people in or from the New World. It causes adult T-cell leukaemia and is associated with some other less well defined malignancies of mature T-helper lymphocytes. It is also causally linked with tropical spastic paraparesis (TSP), known in Japan as HTLV-I associated myelopathy (HAM), and with a variety of other central and peripheral neurological symptoms. Intensive searches for related retroviruses in multiple sclerosis have not reliably detected retrovirus to date despite several positive reports. HTLV-II was first found in 1982 in a T-cell line from a patient with hairy cell leukaemia, and later among injecting drug users. Now it is known to be endemic among many native American communities in North, Central and South America and also among some African pygmy tribes. It is related to HTLV-I in genome organisation with approximately 40% sequence similarity, but it has not as yet definitively been linked to malignant or neurological disease. HIV-1 was first isolated in 1983 by Francoise Barre-Sinoussi and her colleagues in France and rapidly became established as the principal cause of AIDS. HIV-2 was isolated in 1986 from West African and Portuguese patients with AIDS-like symptoms who had been found to carry antibodies cross-reacting with simian immunodeficiency viruses (SIV). Clearly, it does cause AIDS, although it tends to produce a lower virus load, possibly, no maternal-to-child transmission, and some infected persons remain well for many years. Both the HTLV and HIV groups of human retroviruses are closely related to simian counterparts. They probably originated as human endemic and epidemic infections by zoonoses from monkeys (HTLV-I, HIV-2) and possibly from chimpanzees (HIV-1). The

7 Retrovirus classification and cell interactions 7 HTLVs may have been present in humans for thousands of generations, whereas the HIVs appear to be recent introductions during the past few decades. Humans also harbour endogenous oncoviral genomes as already discussed, resembling C-type and D-type retroviruses. Recently, an oncovirus has been found in immunosuppressed bone marrow transplant patients, through screening for recovery of replication-competent retroviral vectors. There is also some evidence of a virus related to D-type retroviruses in Sjogren's syndrome and normal salivary gland biopsies. These candidates for new human retroviruses require much further characterisation. They serve to remind us that they may be further human retroviruses awaiting discovery. Retroviral pathogenesis Retroviruses elicit so many different diseases and symptoms (Table I) that it is not possible to review them in detail here. However, from the antiviral point of view, it is worth distinguishing those diseases that result from an ongoing, persistent viral burden and might therefore be prevented by arresting or slowing viral replication, from those that arise as late sequelae of early retrovirus infection. The latter include several but not all the retroviral malignancies. Viral oncogenesis can be induced by three main molecular mechanisms, depicted in Figure 3. The transduction of cellular oncogenes has thrown much light on human malignancy because it led to the original discovery of oncogenes in chickens, cats and mice but transduction only very rarely occurs in nature and thus is not of practical value in considering antiviral therapy. Retrovirus proviral integration next to cellular oncogenes (thus activating them through the viral LTR) is a special case of insertional mutagenesis. Since viral integration is not specifically targetted to oncogene loci in the LTR gag pol env LTR tax Figure 3. Activation of transforming genes by oncogenic retroviruses. A. Integration of an avian, murine or feline leukaemia virus adjacent to a cellular oncogene (c-onc). Promoter sequences in the LTR leads to uncontrolled expression of c-onc. Cellular oncogenes can also be activated through enhancer sequences in the LTR, in which case the retrovirus could also act when integrated downstream of c-onc. B. A sarcoma virus carrying a viral oncogene (v-onc) in its genome. This genome is defective, lacking full length gag, pol and enc genes; therefore these essential proteins must be provided in 'trans' by a non-defective 'helper' virus such as genome A. By the same principle retroviral vectors are engineered to carry therapeutic genes in place of v-onc. C. HTLV-I (and related HTLV-II and bovine leukosis virus) express the tax protein which acts in trans not only to upregulate the LTR, but also cellular genes for proliferation such as IL-2 receptor (analogous to c-onc) located at other sites and chromosomes.

8 8 R. A. Weiss chromosomes, the higher the virus load in the target tissue, the higher the chance that one cell among millions infected will integrate its provirus in such a dangerous locus that monoclonal malignancy ensues. Studies of leukosis in chickens have shown that this occurs in B-cells in the bursa of Fabricius during primary infection, so subsequent antiviral treatment would presumably be of no avail. In mice and cats, however, multiple recombination of infectiously spreading leukaemia virus with related yet distinct endogenous envelope genes appear to be required before the typical T-cell lymphomas are initiated. Hence antiviral therapy should suppress leukaemogenesis. In fact the best in-vivo mouse model for testing antiretrovirals (it provided the earliest evidence for azidothymidine) is Friend or Rauscher erythroid leukaemia in which splenomegaly is rapidly caused by an oncogene-carrying defective virus coupled with an infectiously spreading helper virus. Human T-cell leukaemia exerts its oncogenic effect neither by transducing an oncogene nor by integrating into DNA next to an oncogene. Rather, its tax gene transactivates cellular genes such as the IL-2 receptor (Figure 3) as already mentioned. Tax activity, however, is clearly insufficient to cause leukaemia since it is expressed in the majority of infected, activated T-lymphoblasts, which must number tens of millions in HTLV-infected persons, yet only about 3% of such individuals develop T-cell leukaemia during a life-time's infection acquired through their mother's milk. The other events, chromosome translocations and the like, remain obscure for this type of leukaemia. Persistent lymphocytosis is seen in a number of patients over the course of many years, but once the malignant clone emerges it is refractory to chemotherapy. Whether antiviral therapy in seropositive individuals would reduce leukaemia incidence is unknown. Viral load of infected cells, however, is several orders of magnitude lower than in lentivirus infections, as is virus turnover. This means that targetting inhibitors to viral replication is less important; on the other hand, the opportunities for the virus to evolve resistance are also much less. Lentiviruses, as their name implies, are slow to cause disease, but infection is not latent. Progress to disease depends on several viral parameters (Table IV). As has become evident from recent analyses of patients treated with non-nucleoside reverse transcriptase inhibitors and protease inhibitors (Ho et al., 1995; Wei et al., 1995), at least by the time of early symptomatic infection there is both a high turnover of virus in the blood and of virus-infected cells. Since approximately one-third of the 10' infected T-lymphocytes are replaced every day. These dynamics of virus and infected cells Table IV. Changing parameters of HIV infection during progression to AIDS Quantitative viral burden viral dynamics Qualitative genome diversity antigen variation cellular tropism virulence

9 Retrovirus classification and cell interactions Table V. Comparative attributes of lentivirus infection Virus (and host) Syndrome Cell tropism Receptor HIV and SIV (primates) Immune deficiency Wasting CNS disease CD4 T-lymphocytes Macrophages, microglia Dendritic cells CD4 FIV (cats) Immune deficiency Wasting CNS disease CD4 T-lymphocytes Macrophages, microglia CD9 MW (sheep) EIAV (horses) Wasting CNS disease Haemolytic anaemia Wasting CNS disease Macrophages, microglia Monocytes, macrophages MHC-II? 7 emphasise the great importance of using therapies that will inhibit viral replication. Indeed, immune restoration therapy may only make things worse by providing more activated T-lymphocytes for HIV to replicate within (Coffin, 1995), unless accompanied by antiviral treatment. Despite these new insights into viral dynamics we still do not know precisely how HIV causes AIDS (Weiss, 1993). HIV infects T-helper lymphocytes, macrophages and also dendritic cells (Patterson el al., 1995). Escape from virus neutralization can change cellular tropism and vice versa (McKnight et al., 1995). Maedi-visna virus (MW), on the other hand, infects just the antigen-presenting cells. MW disease in sheep resembles a subset of the AIDS syndrome, namely the CNS disease and wasting disease (Table V). It seems reasonable, therefore, to attribute the severe immune deficiency seen in HIV and feline immunodeficiency virus infection to the infection and depletion of T-helper lymphocytes, and the CNS and wasting syndrome to infection of macrophages, including the microglia in the brain. Cell tropism of retroviruses is determined by cell surface receptors allowing binding and entry, and further by tissue-specific transcriptional regulation acting on the proviral LTR. Retroviruses use diverse receptors such as CD4 by HIV and SIV, amino-acid and phosphate transporter molecules by mammalian C type retroviruses and low-density lipoprotein related molecules by avian leukosis viruses (Wimmer, 1994). HIV also binds to the glycolipid, galactosylceramide, which may be relevant to CNS disease. The distinctive tropism of HIV-1 for T-lymphocytes versus macrophages is determined in part by the V3 loop of gpl20 which is thought to interact with secondary cellular receptors following the CD4 interaction (Wimmer, 1994; McKnight et al., 1995). Controlling retroviral disease As argued above, controlling lentiviral infection demands efficacious antiviral drugs in order to keep viral load and turnover down. Yet the very high turnover allows the rapid evolution of drug-resistant strains within each infected individual. The hope must therefore lie in combination antiviral therapy and possibly in early treatment during primary infection, if it is apparent.

10 10 R. A. Weiss For the same reason as the limited success of antiviral treatment, vaccines protecting against lentivirus infection will have to show protection against the myriad (quasi-species) of substrains present in the infected host populations. One may roughly calculate that HIV varies over the course of a 10-year infection period within a singly infected person as much as HTLV-I has done over 100,000 years of co-evolution with its human hosts worldwide. Whereas a vaccine protective against HTLV-I, based on env antigens appears quite feasible (as it protects against HTLV-I infection in experimentally infected rabbits and macaques), the development of HIV vaccines will be much more problematic. But that is not to say that efficacious HIV vaccination will be impossible. A small proportion of African prostitutes exposed to HIV have developed cell-mediated immunity which appears to protect them although they are repeatedly at sexual risk of infection (Rowland-Jones et al., 1995). These individuals have never seroconverted to HIV and lack PCR-detectable HIV genomes in the blood, although they may have been infected locally in the genital tract. There are also a few individuals who have seroconverted and who have detectable virus, albeit at relatively low load, who, nevertheless, remain well and maintain high CD4 T-lymphocytes levels (Cao et al., 1995; Pantaleo et al., 1995). Several of the monkey species which naturally harbour SIV (e.g. sooty mangabeys and African green monkeys) also appear to remain well indefinitely. If we could gain a greater understanding of the reason for human and simian long-term non-progression we could perhaps enable the less fortunate majority of HIV-infected people to stay well, and tip the balance of power between the immune system and the lentivirus in favour of the host. Therapeutic retroviruses The foregoing discussion has dealt with naturally occurring retroviruses. It is worth reminding the reader, too, that retroviruses can be turned to therapeutic use as vectors in gene therapy. The property of retroviruses in causing persistent infection with genome integration allows genes to be delivered to cells and tissues. Just as retroviruses have themselves transduced host oncogenes (Figure 3), so can gene therapists insert genes for replacement therapy in inherited disorders (Lever & Goodfellow, 1995). Thus, the initial clinical trials of gene therapy for adenosine deaminase deficiency have used retroviral vectors derived from disabled murine leukaemia virus genomes. For cancer treatment the gene for herpes simplex thymidine kinase has been delivered to the tumour so that systemically administered ganciclovir is selectively activated in the tumour tissue. Thus it is satisfying to virologists that a viral vector delivering a gene activating an antiviral prodrug may prove useful in treating a non-viral disease. However, there is a long way to go to improve both the safety and efficacy of retroviral vectors. This propensity to integrate is both part of their benefit and a cause of anxiety lest they cause tumours via insertional mutagenesis. Their delivery is still not sufficient to infect the majority of the target cells. Improvements are underway to use selective receptors and tissue-specific gene promotors to restrict expression to target cells (Lever & Goodfellow, 1995). In-vivo delivery is limited by titre and by the observation that human complement rapidly inactivates murine leukaemia virus envelopes. We have recently found ways to obviate complement resistance that may lead to a new generation of retroviral vectors (Takeuchi et al., 1994).

11 Retro virus classification and cell interactions 11 Acknowledgements The work of the author is supported by the Medical Research Council and the Cancer Research Campaign. References Note: The retrovirus literature is voluminous. Therefore some key text books and review articles are cited together with selected articles of topical interest. Cao, Y., Qin, L., Zhang, L., Safrit, J. & Ho, D. D. (1995). Virological and immunological characterization of long-term survivors of human immunodeficiency virus type 1 infection. New England Journal of Medicine 332, Coffin, J. M. (1995). HIV population dynamics in vivo: implications for genetic variation, pathogenesis, and therapy. Science 267, Ho, D. D., Neumann, A. U., Perelson, A. S., Chen, W., Leonard, J. M. & Markowitz, M. (1995). Rapid turnover of plasma virions and CD4 lymphocytes in HIV-1 infection. Nature 373, Lever, A. M. L. & Goodfellow, P. N. (1995). Gene therapy. British Medical Bulletin 51, Levy, J. A. (1992^1). The Retroviridae. Plenum Press, New York. McKnight, A., Weiss, R. A., Shotton, C, Takeuchi, Y., Hoshino, H. & Clapham, P. R. (1995). Change in tropism upon immune escape by human immunodeficiency virus. Journal of Virology 69, Pantaleo, G., Menzo, S., Vaccareqzza, M., Graziosi, C, Cohen, O. J., Demarest, J. F. et al. (1995). Studies in subjects with long-term nonprogressive human immunodeficiency virus infection. New England Journal of Medicine 332, Patterson, S., Gross, J., English, N., Stackpole, A., Bedford, P. & Knight, S. C. (1995). CD4 expression on dendritic cells and their infection by human immunodeficiency virus. Journal of General Virology 76, Rowland-Jones, S., Sutton, J., Ariyoshi, K., Dong, T., Gotch, F., McAdam, S. et al. (1995). HIV-specific cytotoxic T-cells in HIV-exposed but uninfected Gambian women. Nature Medicine 1, Skalka, A. M. & Goff, P. S. (1993). Reverse Transcriptase, pp. 1^492. Cold Spring Harbor Laboratory Press, New York. Takeuchi, Y., Cosset, F.-L. C, Lachmann, P. J., Okada, H., Weiss, R. A. & Collins, M. K. L. (1994). Type C retrovirus inactivation by human complement is determined by both the viral genome and the producer cell. Journal of Virology 68, Venables, P. J. W., Brookes, S. M., Griffiths, D., Weiss,' R. A. & Boyd, M. T. (1995). Abundance of an endogenous retroviral envelope protein in placental trophoblasts suggests a biological function. Virology 211, Wei, X. P., Ghosh, S. K., Taylor, M. E., Johnson, V. A., Emini, E. A., Deutsch, P. (1995). Viral dynamics in human immunodeficiency virus type 1 infection. Nature 373, Weiss, R. A. (1993). How does HIV cause AIDS? Science 260, Weiss, R. A., Teich, N. M., Varmus, H. E. & Coffin, J. M. (1985). RNA Tumor Viruses, 2 vols, pp ; Cold Spring Harbor Laboratory Press, New York. Wimmer, E. (1994). Cellular Receptors for Animal Viruses. Cold Spring Harbor Laboratory Press, New York.