Characterization of group specific antibodies in primates: studies with SIV envelope in macaques

Sera from SIV-infected macaques were found to contain antibodies that reacted with conformation-dependent, group-specific determinants on the SIV envelope protein gp130. These conformation-dependent antibodies exhibited virus neutralizing activity; their presence was associated with protection in vaccine studies. The properties of these antibodies are quite similar to those that have been identified in sera from HIV-infected human subjects. These data suggest that the SIV envelope gp130 remains a candidate for subunit vaccine studies.

Evidence for a lentiviral etiology in an epizootic of immune deficiency and lymphoma in stump-tailed macaques (Macaca arctoides)

A retrospective study determined that an epizootic of immune suppression and lymphoma in stump-tailed macaques (Macaca arctoides) that began in 1976 was associated with a horizontally spread lentivirus infection. This conclusion was based on serology, epidemiology, pathology, and virus isolation. The lesions found in the stump-tailed macaques were more compatible with lesions seen in SIV-infected rhesus than those seen in rhesus macaques infected with type D retroviruses. A lentivirus, isolated from a rhesus inoculated with lymph node homogenate from a stump-tailed macaque, was designed SIVstm and was pathogenic for rhesus macaques. The isolate was antigenically related to other SIVs as well as to HIV-1 and HIV-2. Two surviving stump-tailed macaques sent to another colony carried SIVstm latently for at least 7 years and disseminated it throughout that colony.

A highly divergent simian immunodeficiency virus (SIVstm) recovered from stored stump-tailed macaque tissues

We report here the results of molecular analysis of a simian immunodeficiency virus (designated SIVstm) which was isolated from a rhesus monkey inoculated with stored lymph node tissue of an Asian stump-tailed macaque. The latter monkey had died in 1977 during an epidemic of acquired immunodeficiency and lymphoma at the California Regional Primate Research Center (L. J. Lowenstine, N. W. Lerche, P. A. Marx, M. B. Gardner, and N. C. Pedersen, p. 174-176, in M. Girard and L. Valette, ed., Retroviruses of Human AIDS and Related Animal Viruses, 1988). Nucleotide sequence analysis of the gag and env regions indicates that SIVstm is an ancient member of the SIV/human immunodeficiency virus type 2 group; it is quite divergent from known SIVs isolated from African sooty mangabeys as well as from Asian macaques. Furthermore, of all SIV strains described to date, SIVstm is the most closely related to human immunodeficiency virus type 2.

Functional and immunological characterization of SIV envelope glycoprotein produced in genetically engineered mammalian cells

Retroviral envelope glycoproteins interact with cell receptors and are targets for antiviral immune responses in infected hosts. Macaque simian immunodeficiency virus (SIVmac) is a T-lymphocytopathic lentivirus which causes an AIDS-like disease in rhesus macaques. The envelope gene of SIVmac encodes a precursor glycoprotein (gp160) which is cleaved into an external domain (gp130) and a transmembrane domain (gp32). To investigate the functional and immunological properties of the SIV external envelope glycoprotein, we have used genetically engineered mammalian cells to produce recombinant gp130 (rgp130). The rgp130 has the appropriate molecular weight, is glycosylated, and has native conformation as determined by binding to the cell receptor for SIV, the CD4 antigen. Rhesus macaques immunized with purified rgp130 formulated in muramyl dipeptide adjuvant generated high titers of antienvelope antibodies. Antibodies from these macaques were tested for in vitro virus neutralization; very low or undetectable levels of neutralization were observed. In contrast, neutralizing antibodies were readily detected in sera from goats immunized with rgp130. With respect to cell-mediated immunity, proliferative responses to rgp130 were demonstrated in peripheral blood monocyte cells (PBMC) from macaques immunized with the recombinant glycoprotein as well as in PBMC from SIV-infected animals. These results show that rgp130 is functional and immunogenic; the potential of rgp130 for protective immunization remains to be determined.

Experimental vaccine protection against feline immunodeficiency virus

Infection of domestic cats with the feline immunodeficiency virus (FIV) represents an important veterinary health problem and a useful animal model for the development of vaccines against acquired immunodeficiency syndrome (AIDS). Two experimental FIV vaccines have been developed; one consisting of fixed infected cells (Vaccine 1), the other of inactivated whole virus (Vaccine 2). After 4-6 immunizations over 2-5 months, both vaccines induced a strong FIV-specific immune response including neutralizing antibody and T-cell proliferation. Vaccine 1 protected 6 of 9 and Vaccine 2 protected 5 of 6 recipient cats against any detectable infection with a low dose (10 animal ID50) of FIV given intraperitoneally 2 weeks after the final boost. One additional cat in each vaccine group had a transient infection at 5-7 weeks postchallenge following which virus could no longer be detected. Thus, a total of 13 of 15 vaccinated cats were protected against persistent infection. By contrast, 13 of 13 controls were persistently infected by this challenge. The infected cell vaccine failed to protect against a higher dose (5 x 10(4) ID50) of FIV. These results indicate that vaccine prophylaxis against natural FIV infection should be achievable and enhance optimism of the prospect of developing an effective AIDS vaccine for humans.

Characterization of rhesus macaque B-lymphoblastoid cell lines infected with simian type D retrovirus

A simian type D retrovirus designated SRV induces a fatal immunosuppressive disease in rhesus macaques. This syndrome shows many clinical similarities to acquired immunodeficiency syndrome (AIDS) in human immunodeficiency virus-infected individuals. To investigate the mechanisms of immune dysfunction in SRV infection, we have focused on the interactions of SRV serotype 1 (SRV-1) with macaque B-lymphoblastoid cell lines (B-LCL). Procedures were optimized for establishing B-LCL by immortalization of macaque B lymphocytes with rhesus Epstein-Barr virus (EBV). These cell lines express B-cell surface markers, secrete immunoglobulins of the IgG or IgM isotypes, and release EBV which transforms monkey B cells. In vitro cultures of B-LCL supported replication of SRV-1. Several B-LCL infected with SRV-1 showed downregulation of major histocompatibility complex (MHC) class II antigen expression whereas levels of MHC class I antigen remained unchanged. Infection of B-LCL with SRV-1 did not alter the level of secreted immunoglobulin. Rhesus EBV was also used to obtain B-LCL from macaques infected with SRV-1; these cell lines were found to release infectious SRV-1. Investigations on the interactions of SRV-1 with B cells will be useful for elucidating mechanisms involved in the immunopathogenesis of primate retroviruses.

Identification of viral determinants of macrophage tropism for simian immunodeficiency virus SIVmac

Simian immunodeficiency virus (SIV), a lymphocytopathic lentivirus, induces an AIDS-like disease in rhesus macaques (Macaca mulatta). A pathogenic molecular clone of rhesus macaque SIV (SIVmac), SIVmac-239, replicates and induces cytopathology in T lymphocytes but is restricted for replication in macrophages. In contrast, a nonpathogenic molecular clone of SIVmac, SIVmac-1A11, replicates and induces syncytia (multinucleated giant cells) in cultures of both T lymphocytes and macrophages. SIVmac-1A11 does not cause disease in macaques. To map the viral determinants of macrophage tropism, reciprocal recombinant genomes were constructed between molecular clones of SIVmac-239 and SIVmac-1A11. Infectious recombinant viruses were rescued by transfection of cloned viral genomes into permissive lymphoid cells. Analysis of one pair of reciprocal recombinants revealed that an internal 6.2-kb DNA fragment of SIVmac-1A11 was necessary and sufficient for both syncytium formation and efficient replication in macrophages. This region includes the coding sequences for a portion of the gag gene, all of the pol, vif, vpr, and vpx genes, the first coding exons of tat and rev, and the external env glycoprotein gp130. Thus, the transmembrane glycoprotein of env, the nef gene, the second coding exons of tat and rev, and the long terminal repeats are not essential for in vitro macrophage tropism. Analysis of additional recombinants revealed that syncytium formation, but not virus production, was controlled by a 1.4-kb viral DNA fragment in SIVmac-1A11 encoding only the external env glycoprotein gp130. Thus, gp130 env of SIVmac-1A11 is necessary for entry of virus into macrophages but is not sufficient for a complete viral replication cycle in this cell type. We therefore conclude that gp130 env and one or more genetic elements (exclusive of the long terminal repeats, transmembrane glycoprotein of env, and second coding exons of tat and rev, and nef) are essential for a complete replication cycle of SIVmac in rhesus macaque macrophages.

Functional deficiency of polymorphonuclear leukocytes in simian acquired immunodeficiency syndrome

The functional characteristics of polymorphonuclear leukocytes (PMN), considered to be the first line of host defense against infections, from rhesus macaques confirmed to have simian retrovirus (SRV)-induced simian acquired immunodeficiency syndrome (SAIDS), were evaluated. The PMN from SRV antibody-positive macaques without clinical signs were chemotactically responsive. Their phagocytic and killing capabilities were normal, and their cell membranes were highly deformable. However, PMN from SRV antibody-positive macaques and with persistent lymphadenopathy, as well as having at least 3 of the 11 common clinical signs of AIDS, were chemotactically nonresponsive. Their phagocytic and killing capabilities were compromised, and their cell membranes were rigid and nondeformable. In general, PMN from macaques with clinically confirmed SAIDS were functionally deficient. The results are similar to those obtained in other retroviral infections and can be clinically significant, because the host defense deficiency may be responsible for the recurrent and opportunistic infections in SAIDS.

Localization of simian immunodeficiency virus in the central nervous system of rhesus monkeys

Simian immunodeficiency virus (SIV), like the human immunodeficiency virus (HIV), is a lentivirus that is both immunosuppressive and neurovirulent. Rhesus macaques (Macaca mulatta) inoculated with SIV often develop a giant cell encephalitis similar to that seen in humans infected with HIV. The authors examined SIV expression by immunohistochemistry and RNA in situ hybridization in the cerebrum, cerebellum, choroid plexus, and spinal cord from five macaques with and two macaques without giant cell encephalitis. Selected portions of the central nervous system (CNS) also were examined by electron microscopy. Simian immunodeficiency virus was detected in the CNS of all seven monkeys whether or not they had giant cell encephalitis. Both SIV antigen and RNA were present in all levels of the CNS examined. Macrophage/giant cell lesions always contained viral RNA and antigen and were the only sites where viral particles were detected by electron microscopy. However, SIV antigen and RNA also were commonly associated with small vessels, the choroid plexus, and meninges; these were the only locations where virus was detected in animals without giant cell encephalitis. Immunophenotyping showed that the cellular infiltrates consisted primarily of monocyte/macrophages and occasional CD8-positive T cells. Macrophages and T cells also were present in the stroma of the choroid plexus and were intimately associated with vessels in the CNS of SIV-infected but not uninfected macaques. Simian immunodeficiency virus infection of the macaque CNS provides an excellent model for studying the pathogenesis, treatment, and prevention of HIV-1-encephalitis.

SIV of stump-tailed macaque (SIVstm) is a divergent Asian isolate

Analysis of molecularly cloned DNAs of SIVs isolated from Asian rhesus macaque (Macaca mulatta; SIVmac) and pig-tailed macaque (Macaca nemestrina; SIVmne) has indicated a high degree of sequence homology between these viruses. Thus SIVmac and SIVmne might have originated from the same or very closely related viruses. We have cloned and sequenced a PCR-amplified segment containing the LTR sequences of SIV originating from a stump-tailed macaque (Macaca arctoides; SIVstm). Comparative sequence analysis indicates that SIVstm belongs to the SIV/HIV-2 group; however, it is genetically distinct from the other members.

Simian and feline immunodeficiency viruses: animal lentivirus models for evaluation of AIDS vaccines and antiviral agents

Infection of captive macaques with simian immunodeficiency virus (SIV) and domestic cats with feline immunodeficiency virus (FIV), both discovered in the last five years, represent excellent animal models for infection of humans with the human immunodeficiency virus (HIV). Protection against challenge infection and protection against development of simian and feline acquired immunodeficiency syndrome has been achieved in each model by use of inactivated whole virus or virus-cell vaccines. A recombinant SIV envelope peptide vaccine has also proved efficacious. These vaccines have protected against 10-100 animal infectious doses of the homologous cell-free virus given systemically, and, in the simian model, apparently show cross protection against a heterologous strain of SIV. Protected animals appear free of any latent infection although late breakthroughs of infection in a few animals imply that not all vaccinated animals are completely protected. The mechanism of protection in the simian model apparently involves envelope antibody but the role of neutralizing antibody remains unclear. Questions remaining to be answered in both SIV and FIV models are: (1) the duration of immunity, (2) the extent of protection against heterologous strains and mucosal infection, (3) protection against infection with cell-associated virus and (4) the role, if any, of cellular immunity in vaccine protection. Initial attempts at post-infection immunotherapy with SIV vaccines have not yet been successful. The inactivated whole SIV and FIV vaccines offer a promising start and provide hope that a prophylactic AIDS vaccine will be developed. Use of these animal models for antiviral therapy is just now getting underway. Both models should prove especially useful for studies of prophylaxis and therapy, especially during the early stages of infection and for investigations on drug pharmacokinetics or toxicity that can not be done as well in HIV-infected humans. The animals will also be ideal for testing the pathogenicity of drug-induced mutant forms of SIV and FIV. For these purposes it will be necessary to create self-sustaining specific pathogen-free macaque and cat breeding colonies and provide increased housing facilities for infected animals. The future of AIDS research is crucially dependent on the long term availability of these animal models.

Neurobiology of simian and feline immunodeficiency virus infections

Experimental and clinical evidence indicates that all lentiviruses of animals and humans are neurotropic and potentially neurovirulent. The prototypic animal lentiviruses, visna virus in sheep and caprine arthritis encephalitis virus in goats have been known for decades to induce neurologic disease. More recently, infection of the brain with the human immunodeficiency virus (HIV) has been linked to an associated encephalopathy and cognitive/motor complex. While the visna virus and caprine arthritis encephalitis virus are important models of neurologic disease they are not optimal for the study of HIV encephalitis because immune deficiency is only a minor component of the disease they induce. By contrast, the recently isolated lentiviruses from monkeys and cats, the simian and feline immunodeficiency viruses (SIV and FIV respectively), are profoundly immunosuppressive as well as neurotropic. SIV infection of the central nervous system of macaques now provides the best animal model for HIV infection of the human brain due to the close evolutionary relationship between monkeys and man, the genetic relatedness of their respective lentiviruses, and the similarities in the neuropathology. This chapter will compare and contrast the neurobiology of SIV and FIV with HIV.

Elimination of type D retrovirus infection from group-housed rhesus monkeys using serial testing and removal

Type D retrovirus was successfully eliminated from an infected population of group-housed rhesus monkeys by serial testing of all animals for virus and antibody and subsequent removal of positives. This population of 53 rhesus had been housed together for 1 year prior to the initiation of the test and removal program, with six deaths from type D retrovirus-induced immunodeficiency disease occurring during this period. No new infections were detected after four rounds of testing. Of the 47 animals present at the start of the testing program, 17 (35%) remained after the elimination of type D virus from this group. These animals and their offspring have remained healthy and antibody negative for more than 2 years. These results demonstrate that elimination of type D retroviruses from rhesus macaque colonies is feasible, and that the objective of establishing and maintaining retrovirus-free colonies is realistic and achievable.

Simian immunodeficiency virus infection of macaque bone marrow macrophages correlates with disease progression in vivo

The pathogenesis of hematopoietic abnormalities associated with infection of susceptible hosts with either simian immunodeficiency virus (SIV) or human immunodeficiency virus (HIV) is not fully understood. To determine if bone marrow cells are infected with SIV and if the pattern of viral infection is correlated with the severity of disease and abnormalities in hematopoiesis, 23 SIV-infected rhesus monkeys were examined by immunohistochemistry and in situ hybridization. By immunohistochemistry, only four monkeys were positive for SIV core protein p27, while in situ hybridization revealed viral RNA in the bone marrow of 15 monkeys. Simian immunodeficiency virus RNA was consistently expressed in the bone marrow from monkeys with severe lymphoid depletion (11 of 11), but less so in monkeys with follicular hyperplasia (0 of 2) or mild lymphoid depletion (4 of 10). In animals with mild lymphoid depletion, bone marrow cells infected with SIV were mainly mononuclear cells that appeared to be of myelomonocytic lineage. In contrast, monkeys with severe lymphoid depletion had SIV RNA localized to larger mononuclear cells with abundant cytoplasm often located in small lucent areas of the stroma. These SIV RNA-positive mononuclear cells were positive for the macrophage determinant CD68 as demonstrated by immunohistochemistry. Furthermore the stage of simian acquired immune deficiency syndrome, as indicated by lymphoid morphology, and SIV localization in the bone marrow were correlated with the incidence of anemia, bone marrow hyperplasia, and abnormal distribution of macrophages in the bone marrow. These results indicate that, in common with other animal lentiviral infections, the macrophage is a major target of SIV infections in the bone marrow.

The Lake Casitas wild mouse: evolving genetic resistance to retroviral disease

A small colony of feral mice from California continues to flourish in spite of a virulent epizootic of pathological retrovirus. Epidemiological and genetic studies revealed that the viral infection is strongly balanced by the polymorphic resistance locus, Fv-4, a transcriptionally active but truncated provirus that originated in the East Asian ancestors of the Californian mice. The natural history of these populations represents a graphic example of genomic adaptation in free-ranging populations to regulate and delimit infectious disease.

Natural killer cell activity of rhesus macaques against retrovirus-pulsed CD4+ target cells

Rhesus peripheral blood mononuclear cells (PBMC) fail to demonstrate natural killer (NK) activity against the human T-cell lines CEM, CEM x 174, or SUP-T1. However, these cell lines could act as NK-sensitive target cells if they were pulsed with heat-inactivated, whole simian immunodeficiency virus (SIV). The ability of these SIV-pulsed T-cell lines to act as NK-sensitive target cells was directly related to the relative density of CD4 on their surface. Target cell generation was inhibited by preincubation of cell lines with CD4 monoclonal antibody (MAb) with specificity for the SIV binding site. In addition, NK activity was seen against target cells that had been prepared with human immunodeficiency virus type 1 (HIV-1) gp120, nonglycosylated gp120, env A of feline leukemia virus (FeLV), and simian type D retrovirus (SRV). Addition of leupeptin to target cells prior to SIV pulsing did not result in a significant decrease in cytotoxic activity, suggesting that processing is not required for the generation of target cells. The cells that mediate NK activity are nonadherent, do not form rosettes with AET-treated sheep red blood cells (SRBC), and are phenotypically CD16+ and CD8+. NK activity of SIV-infected macaques was significantly decreased against both K562 cells and SIV-pulsed target cells as compared with uninfected animals. However, treatment of PBMC with interleukin-2 (IL-2) resulted in a partial restoration of NK activity.

Inactivated simian immunodeficiency virus vaccine failed to protect rhesus macaques from intravenous or genital mucosal infection but delayed disease in intravenously exposed animals

Eight rhesus macaques were immunized four times over a period of 8 months with a psoralen-UV-light-inactivated whole simian immunodeficiency virus vaccine adjuvanted with threonyl muramyl dipeptide. Eight unvaccinated control animals received adjuvant alone. Only the vaccinated animals made antibodies before challenge exposure to the viral core and envelope as determined by Western blotting (immunoblotting) and virus-neutralizing antibodies. Ten days after the final immunization, one-half of the vaccinated and nonvaccinated monkeys were challenged exposed intravenously (i.v.) and one-half were challenge exposed via the genital mucosa with virulent simian immunodeficiency virus. All of the nonvaccinated control monkeys became persistently infected. In spite of preexisting neutralizing antibodies and an anamnestic antibody response, all of the immunized monkeys also became persistently infected. However, there was evidence that the clinical course in immunized i.v. infected animals was delayed. All four mock-vaccinated i.v. challenge-exposed animals died with disease from 3 to 9 months postchallenge. In contrast, only one of four vaccinated i.v. challenge-exposed monkeys had died by 11 months postchallenge.