Pathogenesis of lentivirus infections

Expression of viral antigens in the central nervous system of visna-infected sheep: an immunohistochemical study on experimental visna induced by virus strains of increased neurovirulence

Icelandic sheep were infected by intracerebral inoculation with visna virus strains of increased neurovirulence. The character and severity of pathological lesions were studied in brains from four sheep that developed clinical signs 5 to 12 weeks after infection. Viral antigens were identified by immunostaining using mouse monoclonal antibodies against two core proteins and the Avidin-Biotin method of detection. The pathological lesions were in general more severe than observed following infection with the parent strain K1514. Primary demyelination, a late manifestation of infection with K1514, was detected. Thus, in addition to causing more severe pathological lesions, these neurovirulent strains apparently have an increased potential to induce primary demyelination. Viral antigens were detected in lymphocytes, plasma cells, macrophages, endothelial cells, pericytes, fibroblasts and choroidal epithelial cells. Neurons and glial cells were antigen negative. The spectrum of infected cells in the brain was similar to that observed in infections with human immunodeficiency virus. These results do not support the view that the demyelination is caused by immunological damage to infected oligodendrocytes. A perturbation of the function of oligodendrocytes through a non-productive infection could be the underlying pathogenetic mechanism and/or a non-specific demyelination due to the intense inflammatory reaction.

Multiple sclerosis: possible immunological mechanisms

Multiple sclerosis is the principal demyelinating disease of the central nervous system. Although the prevalence of the disease is moderately low, averaging about 40 cases per 100,000 people in high risk areas, it is a particularly devastating disease. It primarily affects young adults, is chronic, and has an unpredictable course. Most discouraging, the cause of the disease is not known and an effective treatment has not been identified. Recently, however, research has yielded some important findings concerning the etiology of MS. Much evidence now points to an immunological process as one of the major elements in the disease. It is also likely that an environmental influence, possibly an infectious process, may contribute to the disease. Finally, it is now certain that genetic makeup influences susceptibility to the disease. At present, the strongest evidence is for a polygenic effect, not the effect of a single gene or gene locus. This review will examine some of the possible immunologically mediated disease processes that could be involved in MS, especially those that could account for a role for infectious and genetic factors in the disease.

Infection of the SCID-hu mouse by HIV-1

SCID-hu mice with human fetal thymic or lymph node implants were inoculated with the cloned human immunodeficiency virus-1 isolate, HIV-1JR-CSF. In a time- and dose-dependent fashion, viral replication spread within the human lymphoid organs. Combination immunohistochemistry and in situ hybridization revealed only viral RNA transcripts in most infected cells, but some cells had both detectable viral transcripts and viral protein. Infected cells were always more apparent in the medulla than in the cortex of the thymus. These studies demonstrate that an acute infection of human lymphoid organs with HIV-1 can be followed in the SCID-hu mouse.

Genetic structure and function of an early transcript of visna virus

During the early step of the lytic cycle, visna provirus is first transcribed into two small multispliced mRNAs of 1.6 and 1.2 kilobases which may encode factors regulating the replication of visna virus (R. Vigne, V. Barban, G. Quérat, V. Mazarin, I. Gourdou, and N. Sauze, Virology 161:218-227, 1987). By cDNA cloning and nucleotide sequencing, we determined that the 1.2-kilobase mRNA is 1,174 nucleotides long without the 3'-polyadenylated tail and is composed of four exons, two of which originated from the 5' and 3' ends, respectively, of the env gene region. Two overlapping open reading frames are present in each of these two exons. They were translated in vitro and gave rise to three proteins, two of 19 and 17 kilodaltons, termed VEP1, and one of 16.5 kilodaltons, termed STM. Only the VEP1 proteins were recognized by a hyperimmune anti-visna virus serum of infected sheep. Transient-expression assays performed in eucaryotic cells demonstrated that the cDNA clone described here has a trans-acting effect on transcription of the visna virus genes.

Immune responses are required to terminate viremia in equine infectious anemia lentivirus infection

Six normal and four immunodeficient horses were injected with a cloned variant of equine infectious anemia virus (EIAV). The six normal horses had detectable EIAV in their plasma by 7 days postinjection. During their primary viremic episode, which was accompanied by fever and anemia, maximum titers of EIAV in plasma ranged from 10(3.8) to 10(4.8) 50% tissue culture infective doses per ml. All six normal horses cleared detectable virus from their plasma by 21 to 35 days after injection. Horses with combined immunodeficiency became viremic by 9 days postinjection and also developed anemia. In contrast to normal horses, foals with combined immunodeficiency did not eliminate the virus from their plasma.

Detection of human T-cell lymphoma/leukemia virus type I DNA and antigen in spinal fluid and blood of patients with chronic progressive myelopathy

The presence of antibodies to human T-cell lymphoma/leukemia virus Type I (HTLV-I) has been associated with chronic progressive myelopathy. We attempted to isolate the virus from the blood and spinal fluid of patients with chronic progressive myelopathy and to define the clinical, radiologic, and electrophysiologic features of this disease. Ten of 13 patients from tropical countries and 2 of 8 from the United States had serum antibodies to HTLV-I. The virus was detected in cultures of peripheral-blood lymphocytes from three of seven patients by means of Southern blot hybridization. Using a sensitive in vitro enzymatic gene-amplification technique, we detected HTLV-I sequences in fresh peripheral-blood mononuclear cells of all of 11 patients tested who were positive for the antibody, and in cell cultures of the spinal fluid from 3 of the 11 tested. Magnetic resonance imaging of the cranium revealed periventricular lesions in the white matter of 3 of the 12 antibody-positive patients. Five of these patients had mild axonal sensorimotor polyneuropathy, and one had bilateral lumbar radiculopathy. Visual evoked potentials were abnormal in three seropositive patients, and brain-stem evoked responses were abnormal in two. The detection of the DNA and proteins of HTLV-I strengthens the proposition that this virus is involved in the pathogenesis of a subset of cases of chronic progressive myelopathy.

Assessment of human immunodeficiency virus expression in cocultures of peripheral blood mononuclear cells from healthy seropositive subjects

We have evaluated a number of methodological variables effecting the expression of human immunodeficiency virus (HIV) in cultured peripheral blood mononuclear cells (PBMCs) of 93 healthy anti-HIV-positive and 72 healthy seronegative subjects. For optimal HIV recovery, PBMCs had to be freshly separated from whole blood. Short-term freezing of purified PBMCs was practically advantageous and actually resulted in more rapid virus recovery. The minimal number of PBMCs necessary for virus expression was determined by dilutional cultures and varied from 10(2) to 10(6) cells. HIV expression was demonstrated initially at the cellular level by immunocytochemical detection of HIV core and envelope proteins using a mixture of monoclonal antibodies, subsequently confirmed by detection of viral antigens and reverse transcriptase (RT) in the culture supernatants. HIV recovery was not improved following induction with 5-iodo-2'-deoxyuridine (IUDR) and only marginally improved following depletion of the CD8+-suppressor cell population in the PBMC specimens. The overall frequency of HIV detection in cultures was 84% in healthy seropositive subjects, whereas none of the PBMCs from 72 seronegative persons yielded HIV expression.

Simian retrovirus D serogroup 1 has a broad cellular tropism for lymphoid and nonlymphoid cells

Simian acquired immunodeficiency syndrome is a fatal immunosuppressive disease caused by type D retroviruses such as simian acquired immunodeficiency syndrome retrovirus type 1 (SRV-1). The disease is characterized by generalized lymphadenopathy, opportunistic infections, and lymphoid depletion with defects in both humoral and cell-mediated immunity. To understand how SRV-1 infection relates to the immune defect, we studied in vivo-infected lymphocytes from SRV-1-positive macaques with and without clinical signs of immunosuppressive disease. B and T helper/inducer and T suppressor/cytotoxic lymphocytes were purified by panning or by flow cytometry. Neutrophils were purified by dextran sedimentation, and platelets were purified by low-speed centrifugation. In vitro infection studies were also done with HUT78, H9, K562, rhesus lung fibroblast, rhesus monkey kidney, and bat lung cells. SRV-1 in lymphocytes or culture supernatants was detected by the induction of syncytia in cocultivated Raji cells and was confirmed by immunofluorescence, electron microscopy, or reverse transcriptase assay. We found that B and T helper/inducer lymphocytes were infected in all animals tested. The number of infected T suppressor/cytotoxic cells was generally lower than that of the other cell subsets, and not all animals in this subset had SRV-1 infections. All other cells exposed in vitro to SRV-1, except bat lung cells, were able to be infected. These findings show that SRV-1 has a broad cell tropism for lymphoid and nonlymphoid cell types.

Efficient isolation and propagation of human immunodeficiency virus on recombinant colony-stimulating factor 1-treated monocytes

Monocytes were maintained in tissue culture for greater than 3 mo in media supplemented with rCSF-1. These cultures provided susceptible target cells for isolation and propagation of virus from PBMC of HIV-infected patients. HIV isolated into monocytes readily infected other rCSF-1-treated monocytes but only inefficiently infected PHA-stimulated lymphoblasts. Similarly, laboratory HIV strains passaged in T cell lines or virus isolated from patients' leukocytes into PHA-stimulated lymphoblasts inefficiently infected rCSF-1-treated monocytes. Persistent, low-level virion production was detected in macrophage culture fluids by reverse transcriptase activity or HIV antigen capture through 6-7 wk. Marked changes in cell morphology with cell death, syncytia, and giant cell formation were observed in monocyte cultures 2 wk after infection, but at 4-6 wk, all cells appeared morphologically normal. However, the frequency of infected cells in these cultures at 6 wk was 60-90% as quantified by in situ hybridization with HIV RNA probes or by immunofluorescence with AIDS patients' sera. Ultrastructural analysis by EM also showed a high frequency of infected cells; virtually all HIV budded into and accumulated within cytoplasmic vacuoles and virus particles were only infrequently associated with the plasma membrane. Retention of virus within macrophages and the macrophage tropism of HIV variants may explain mechanisms of both virus persistence and dissemination during disease.

Cerebral astrocytoma in association with HIV infection

Cerebral astrocytoma has been found in two patients with AIDS related conditions. The fortuity of this unusual association is discussed, insofar as immunosuppression could favour the growth of certain neoplasms.

Heterogeneity of primate foamy virus genomes. Brief report

DNA of the T-lymphotropic simian foamy virus (SFV) LK-3 was cloned in a plasmid vector and used as a probe in comparative DNA:DNA hybridization studies with primate foamy viruses (SFV serotypes 1, 2, 3, 5, 6, 7, 8, fresh SFV isolates from 12 African green monkeys and one rhesus monkey, as well as human syncytium forming virus). All freshly isolated viruses seemed to be variants of SFV types 2 and 3. Especially LK-3 appeared to be closely related to SFV type 3. No hybridization of the LK-3 clone was observed to DNA sequences of human immunodeficiency virus (HIV-1).

Human immunodeficiency virus (HIV-1) cytotoxicity: perturbation of the cell membrane and depression of phospholipid synthesis

The molecular mechanism(s) by which human immunodeficiency virus (HIV-1) injures a T-cell line was studied. A pathological role for viral env proteins, which are inserted into the plasma membrane, has been previously demonstrated for HIV as well as other retroviruses which are cytopathic. We therefore initiated studies examining whether perturbations of the cell membrane or membrane-associated biochemical events may be occurring in cells acutely infected with HIV and whether such perturbations, if present, may be responsible for cytopathology. A human T-cell line (ERIC), which is sensitive to the cytopathic effects of HIVs, was infected with HTLV-IIIB and its membrane permeability to cations and its lipid metabolism were studied coincident with the peak expression of viral p24 and with the first sign of cytopathology (slowing of cell division) 72 to 96 hr after infection. It was found that the rate of influx of Ca2+ into the cell increased over that of uninfected cells and that phospholipid synthesis, primarily phosphatidylcholine, became depressed. Diacylglycerol, which serves both as an intermediate for synthesis of phospholipids and as a second-messenger for lymphocyte activation, was also greatly reduced. However, triglyceride synthesis was enhanced, indicating that not all lipid metabolic pathways were being shut down. This decreased membrane-synthetic ability and reduced second-messenger for cell division are likely to be important causes of HIV-1 cytopathology in ERIC cells. This hypothesis was supported by our finding that HIV cytopathology of ERIC cells could be partially prevented by treatment with compounds (diacylglyceride or PMA and transiently by oleic acid) which either replenish diacylglycerol in the infected cell and/or activate protein kinase C or phosphocholine cytidyltransferase, the latter being the rate-limiting step in synthesis of the major structural phospholipid in most cells.

Strong sequence conservation among horizontally transmissible, minimally pathogenic feline leukemia viruses

We report the first complete nucleotide sequence (8,440 base pairs) of a biologically active feline leukemia virus (FeLV), designated FeLV-61E (or F6A), and the molecular cloning, biological activity, and env-long terminal repeat (LTR) sequence of another FeLV isolate, FeLV-3281 (or F3A). F6A corresponds to the non-disease-specific common-form component of the immunodeficiency disease-inducing strain of FeLV, FeLV-FAIDS, and was isolated from tissue DNA of a cat following experimental transmission of naturally occurring feline acquired immunodeficiency syndrome. F3A clones were derived from a subgroup-A-virus-producing feline tumor cell line. Both are unusual relative to other molecularly cloned FeLVs studied to date in their ability to induce viremia in weanling (8-week-old) cats and in their failure to induce acute disease. The F6A provirus is organized into 5'-LTR-gag-pol-env-LTR-3' regions; the gag and pol open reading frames are separated by an amber codon, and env is in a different reading frame. The deduced extracellular glycoproteins of F6A, F3A, and the Glasgow-1 subgroup A isolate of FeLV (M. Stewart, M. Warnock, A. Wheeler, N. Wilkie, J. Mullins, D. Onions, and J. Neil, J. Virol. 58:825-834, 1986) are 98% homologous, despite having been isolated from naturally infected cats 6 to 13 years apart and from widely different geographic locations. As a group, their envelope gene sequences differ markedly from those of the disease-associated subgroup B and acutely pathogenic subgroup C viruses. Thus, F6A and F3A correspond to members of a highly conserved family and represent prototypes of the horizontally transmitted, minimally pathogenic FeLV present in all naturally occurring infections.

The brain in AIDS: central nervous system HIV-1 infection and AIDS dementia complex

Infection with human immunodeficiency virus type 1 (HIV-1) is frequently complicated in its late stages by the AIDS dementia complex, a neurological syndrome characterized by abnormalities in cognition, motor performance, and behavior. This dementia is due partially or wholly to a direct effect of the virus on the brain rather than to opportunistic infection, but its pathogenesis is not well understood. Productive HIV-1 brain infection is detected only in a subset of patients and is confined largely or exclusively to macrophages, microglia, and derivative multinucleated cells that are formed by virus-induced cell fusion. Absence of cytolytic infection of neurons, oligodentrocytes, and astrocytes has focused attention on the possible role of indirect mechanisms of brain dysfunction related to either virus or cell-coded toxins. Delayed development of the AIDS dementia complex, despite both early exposure of the nervous system to HIV-1 and chronic leptomeningeal infection, indicates that although this virus is "neurotropic," it is relatively nonpathogenic for the brain in the absence of immunosuppression. Within the context of the permissive effect of immunosuppression, genetic changes in HIV-1 may underlie the neuropathological heterogeneity of the AIDS dementia complex and its relatively independent course in relation to the systemic manifestations of AIDS noted in some patients.

The AIDS dementia complex: some current questions

The acquired immunodeficiency syndrome (AIDS) dementia complex (ADC) commonly complicates the course of human immunodeficiency virus (HIV) infection and AIDS. Although many of its clinical aspects have recently been brought into clearer focus, and pathogenetic evidence has accrued implicating direct HIV brain infection, there remain a number of fundamental aspects of ADC and HIV nervous system infection that require clarification. These include clearer definition of the clinical syndrome and its variants; development of instrumentation for diagnosis and monitoring the disorder; definition of the epidemiology and natural history of both central nervous system HIV infection and ADC, which may seemingly be discordant; and understanding of both the viral pathogenesis and the biology of resultant brain dysfunction. Elucidation of these fundamental issues will enhance rational development and evaluation of therapy.

Neurotropic retroviruses of wild mice and macaques

A neurotropic retrovirus causes a naturally occurring lower-limb paralysis in wild mice, characterized by a noninflammatory spongiform change located primarily in the lower spinal cord. The causative agent is an ecotropic murine leukemia virus, unique to certain wild mice in southern California. The disease is readily transmitted to newborn susceptible laboratory mice. The paralytogenic property is attributed to direct viral injury to motor neurons and glial cells and is associated with unique amino acids in the murine leukemia virus envelope gp70. This murine model may have relevance to both human T-lymphotropic virus type I, and human immunodeficiency virus infection of human brain. It presents a practical model for testing antiviral agents aimed at retrovirus infection of the mammalian central nervous system. Simian acquired immunodeficiency syndrome type D retrovirus causes a silent infection of the brain in infected macaques. Viral nucleic acids are detected in the brain parenchyma in the absence of viral antigen, neurological symptoms, and neuropathology. Infected choroid plexus epithelial cells are the source of cell-free virus in the cerebrospinal fluid of viremic monkeys. This model adds yet another example of retroviral infection of the central nervous system and points to the choroid plexus as a potential source of infectious virus.

Lentivirus-host interactions: lessons from visna and caprine arthritis-encephalitis viruses

The biological properties of the ruminant animal lentiviruses, visna and caprine arthritis-encephalitis viruses, closely resemble those of their human counterparts, the human immunodeficiency viruses (HIV). All of these viruses are morphologically identical and are disseminated from host to host in nature during exchange of body fluids. Artificial conditions that favor excess exchange of such fluids precipitate epidemics by these viruses. The strategy of replication of the animal viruses in tissue culture and in vivo are very similar to that of the human virus. Virus replication is highly productive in tissue culture and leads to cytopathic effects characterized by fusion. In vivo, the rate of virus replication is restricted and lesions, suggestive of an immunopathological origin, develop after prolonged periods of subclinical infection. Similar to the animal viruses, the human viruses have a tropism for macrophages in vivo, and this leads somehow to a loss of T helper lymphocytes and proliferation of cytotoxic lymphocytes. In addition, the viruses are highly neurotropic and this results in acute fulminating disease in neonatal hosts and chronic encephalopathy in adults. Both animal and human viruses cause persistent infections and have similar strategies for eluding host immune responses. These include sequestration of neutralizing epitopes, induction of low titers of neutralizing antibodies, and antigenic drift during persistent infection. Despite close homology between genetic sequences of HIV-I and -II, these two viruses seem to have as much biological disparity from each other as does visna virus from caprine arthritis-encephalitis virus. The latter two viruses induce neutralizing antibodies that are highly strain specific and show no cross protection.(ABSTRACT TRUNCATED AT 250 WORDS)

Long-term administration of 3'-azido-2',3'-dideoxythymidine to patients with AIDS-related neurological disease

3'-Azido-2',3'-dideoxythymidine (AZT) has been administered to 7 patients with human immunodeficiency virus-associated neurological disease: 3 with dementia, 2 with peripheral neuropathy, 1 with dementia and peripheral neuropathy, and 1 with T-10 paraplegia. Six of the patients showed improvement in their neurological dysfunction on being administered AZT, as assessed by clinical evaluation, neuropsychological testing, nerve conduction studies, and/or positron emission tomographic scans. Three of these 6 patients showed sustained improvement 5 to 18 months after the initiation of AZT therapy. These results suggest that certain human immunodeficiency virus-associated neurological abnormalities are at least partially reversible following the administration of antiretroviral therapy and provide a rationale for further studies using antiretroviral chemotherapy.

Localization of sequences responsible for trans-activation of the equine infectious anemia virus long terminal repeat

We used the Escherichia coli chloramphenicol acetyltransferase gene (cat) to study sequences that influence expression of the equine infectious anemia virus (EIAV) genome. The EIAV long terminal repeat (LTR) directed CAT activity in a canine cell line, but at levels much lower than those achieved with other eucaryotic viral promoters. In the same cells infected with EIAV or cotransfected with molecularly cloned EIAV genomic DNA, LTR-directed activity was markedly enhanced. Comparison of cat mRNA and protein levels in these cells indicated that this trans-activating effect could be accounted for by a bimodal mechanism in which both transcriptional and posttranscriptional events are enhanced. trans-Activation but not promoter activity was abolished by deletion of the R-U5 region of the EIAV LTR. EIAV sequences responsible for the trans-activating function could be localized to a region encompassing the 3' and 5' termini of the pol and env genes, respectively (nucleotides 4474 to 5775). Interestingly, this stretch harbors a short open reading frame with some amino acid sequence similarity to the human immunodeficiency virus type I tat gene product.

Neurological and neuropathological features of human immunodeficiency virus infection in children

A progressive encephalopathy occurs in 30 to 50% of infants and children infected with the human immunodeficiency virus (HIV). The expression of HIV antigen in the cerebrospinal fluid appears to correlate with the clinical occurrence of progressive encephalopathy. The signs of progressive encephalopathy in children with HIV infection, including loss of developmental milestones, impaired brain growth, and progressive motor dysfunction, indicate a poor prognosis and almost invariably a fatal outcome. Neuropathological findings in these children, including virus-laden macrophages and multinucleated giant cells are unique to this condition. Opportunistic or reactivated latent infections and neoplasms of brain occur in children with HIV infection but are uncommon. These findings support the hypothesis that the progressive encephalopathy observed in HIV-infected children is caused by primary infection of the brain with this virus. Epidemiological data predict increasing numbers of HIV-infected women and children. Research aimed at an understanding of the mechanism(s) of mother-to-infant transmission of HIV infection is urgently needed so that strategies for the prevention and treatment of such infection in children may be planned.

Immunological findings in neurological diseases associated with antibodies to HTLV-I: activated lymphocytes in tropical spastic paraparesis

A retrovirus involvement in the etiology of certain neurological diseases is currently an area of intense interest. Tropical spastic paraparesis and other chronic progressive myelopathies have been clearly associated with increased serum and cerebrospinal fluid antibody titers to human T-lymphotropic virus type I; however, little is known about the cellular immune response. In the present study, activated T-lymphocytes were found in the peripheral blood of patients with this disorder. There were increased numbers of large CD3-positive cells that also expressed histocompatibility leukocyte Class II (DR) and interleukin 2-receptor molecules. In addition, a significantly elevated spontaneous lymphoproliferative response was demonstrated in all patients. This is consistent with the known in vitro effects of human T-lymphotropic virus type I. In one patient, a defect in the generation of measles virus-specific cytotoxic T cells was identified. These observations indicate abnormalities of the cellular immune response in tropical spastic paraparesis.

Molecular genetics and structure of the human immunodeficiency virus

A novel human lymphotropic virus capable of crippling the immune system by infecting and destroying T4 antigen-positive cells is now known to be the etiologic agent of the acquired immune deficiency syndrome (AIDS). The AIDS or human immunodeficiency virus (HIV) belongs to a family of RNA viruses called retroviruses. Several strains of HIV have been molecularly cloned, and DNA sequence comparisons have established that the proviral DNA genome is 9.7 kilobase pairs. The genome possesses characteristic retrovirus features including structural genes, flanked by long terminal repeats, in the order gag, pol, and env and, in addition, four unique nonstructural genes, several of which appear to be essential in regulating virus replication. Electron microscopy has played an important role in elucidating structural, genetic, and molecular properties of HIV and has aided in its classification as a member of the Lentivirnae retrovirus subfamily. Heteroduplex mapping methodologies pertinent to these findings are described. Although the relationships show considerable divergence, the similarities between HIV and lentiviruses are profound and encompass an indistinguishable morphology, genome sequence homology and topography, genomic diversity, and overlapping biology, including a preference for infecting cells of the immune system, a cytopathic effect in vitro, and the ability to produce a persistent, slowly progressing, degenerative disease in vivo. The newest HIV class (HIV-2) has recently been molecularly characterized. HIV-2 also bears all the hallmarks of a lentivirus but is more closely related to simian immunodeficiency viruses than the previously described HIV-1, despite a similar biology. The HIV-lentivirus phylogenetic relationship has broad implications for the AIDS disease process and has given new importance to the study of the natural history and pathogenesis of animal lentiviruses in searching for clues to prevent the spread of AIDS.

The visna virus genome: evidence for a hypervariable site in the env gene and sequence homology among lentivirus envelope proteins

The complete nucleotide sequence of the visna virus 1514 genome was determined. Our sequence confirms the relationship of visna virus and other lentiviruses to human immunodeficiency virus (HIV) both at the level of sequence homology and of genomic organization. Sequence homology is shown to extend to the transmembrane proteins of lentivirus env genes; this homology is strongest in the extracellular domain, suggesting that close structural and functional similarities may also exist among these envelope proteins. Comparison of our data with the sequence of visna virus LV1-1, an antigenic variant derived from strain 1514, demonstrates that the rate of divergence has been about 1.7 x 10(-3) substitutions per nucleotide per year in vivo. This rate is orders of magnitude higher than that for most DNA genomes, but agrees well with estimates of the rate for HIV. A statistically significant cluster of mutations in the env gene appears to represent a hypervariable site and may correspond to the epitope responsible for the antigenic differences between 1514 and LV1-1. Analysis of the potential RNA folding pattern of the visna virus env gene shows that this hypervariable site falls within a region with little potential for intramolecular base pairing. This correlation of hypervariability with lack of RNA secondary structure is strengthened by the fact that it also holds for a hypervariable site in the env gene of HIV.

Replication and cytopathic effects of ovine lentivirus strains in alveolar macrophages correlate with in vivo pathogenicity

Ruminant lentiviruses share genomic sequences and biologic properties with human immunodeficiency viruses. Four ovine lentivirus strains were assessed for cytopathic effects and virus replication. Lentivirus isolate H/24 produced high virus titers and lysis of synovial cells but replicated slowly and caused no fusion of alveolar macrophages. Lentivirus isolates 84/28 and 85/14 produced low virus titers, less syncytia, and limited or no cell lysis in synovial cells and macrophages. In contrast, ovine lentivirus isolate 85/34 produced early peak virus titers and caused rapid fusion and lysis of both macrophages and synovial cells. Ovine lentivirus isolates which were cytopathic for macrophages induced lymphoproliferative disease when inoculated into lambs.

Heterogeneity of human immunodeficiency virus cell-associated antigens and demonstration of virus type specificities of human antibody responses

We examined the antigens of human immunodeficiency viruses (HIV) expressed on infected H9 cells using live-cell membrane immunofluorescence and immunofluorescence absorption. Application of this nondenaturing serological method permitted analysis of HIV antigenic determinants maintained in their native configurations on the cell surface. Sera from infected individuals were found to react variably with H9 cells productively infected with nine different HIV isolates, and certain sera were completely unreactive with some isolates. Absorption of the sera prior to use in immunofluorescence revealed extensive heterogeneity of HIV cell-surface antigens and multiple type-specific antibodies in patients' sera. Immunoprecipitation and SDS-PAGE analysis of radiolabeled cell-surface proteins indicated that the predominant serological reactions were to env-encoded proteins. The observed antigenic and antibody heterogeneity likely reflects env sequence heterogeneity which has been previously reported for different HIV isolates. The demonstration of antigenic diversity among HIVs and the importance of defining the native antigenic epitopes, particularly those most widely shared, are important issues that must be considered in vaccine development.

Role of the host immune response in selection of equine infectious anemia virus variants

Equine infectious anemia virus was isolated from peripheral blood leukocytes collected during two early febrile cycles of an experimentally infected horse. RNase T1-resistant oligonucleotide fingerprint analyses indicated that the nucleotide sequences of the isolates differed by approximately 0.25% and that the differences appeared randomly distributed throughout the genome. Serum collected in the interval between virus isolations was able to distinguish the isolates by membrane immunofluorescence on live cells. However, no neutralizing antibody was detected in the interval between virus isolations. In fact, multiple clinical cycles occurred before the development of a neutralizing antibody response, indicating that viral neutralization might not be the mechanism for selection of antigenic variants. The ability of early immune sera to recognize variant specific antigens on the surface of infected cells suggested that immune selection occurs through recognition and elimination of certain virus-infected cells. Alternately, the random distribution of the genomic differences observed between the two isolates may indicate that equine infectious anemia virus variants emerge as a result of nonimmunological selection processes.

Molecular pathogenesis of neurotropic viral infections

Classical virologists defined a number of viruses that affect the nervous system and identified tissue tropism, extraneural replication, and viremia as important parameters that determine whether viral infections will affect the central nervous system. Molecular techniques are expanding this knowledge by permitting us to relate specific genes and gene products to two defined phenotypes: neuroinvasion and neurovirulence. Two converging situations make this knowledge particularly useful: (1) the development of antiviral drugs and subunit vaccines, which mandate that pathogenesis be related to specific regions of the viral genome; and (2) the expanding problem of central nervous system infections in immunodeficient states.