Detection of simian immunodeficiency virus DNA in macrophages from infected rhesus macaques.

We have examined the frequency of infection of monocyte-derived and alveolar macrophages isolated from rhesus macaques inoculated with simian immunodeficiency virus (SIVmac) utilizing a semiquantitative PCR methodology. Animals were inoculated with either pathogenic (SIVmac239) or nonpathogenic (SIVmac1A11) molecularly cloned viruses of SIVmac, or with uncloned pathogenic SIVmacBIOL. The frequency of SIV DNA in macrophages was highest early after infection and at terminal stages of disease, whereas during the asymptomatic period, SIV DNA was present at very low levels in macrophages.     

Viral determinants of simian immunodeficiency virus (SIV) virulence in rhesus macaques assessed by using attenuated and pathogenic molecular clones of SIVmac.

To identify viral determinants of simian immunodeficiency virus (SIV) virulence, two pairs of reciprocal recombinants constructed from a pathogenic (SIVmac239) and a nonpathogenic (SIVmac1A11) molecular clone of SIV were tested in rhesus macaques. A large 6.2-kb fragment containing gag, pol, env, and the regulatory genes from each of the cloned (parental) viruses was exchanged to produce one pair of recombinant viruses (designated SIVmac1A11/239gag-env/1A11 and SIVmac239/1A11gag-env/239 to indicate the genetic origins of the 5'/internal/3' regions, respectively, of the virus). A smaller 1.4-kb fragment containing the external env domain of each of the parental viruses was exchanged to create the second pair (SIVmac1A11/239env/1A11 and SIVmac239/1A11env/239) of recombinant viruses. Each of the two parental and four recombinant viruses was inoculated intravenously into four rhesus macaques, and all 24 animals were viremic by 4 weeks postinoculation (p.i.). Virus could not be isolated from peripheral blood mononuclear cells (PBMC) of any animals infected with SIVmac1A11 after 6 weeks p.i. but was consistently isolated from all macaques inoculated with SIVmac239 for 92 weeks p.i. Virus isolation was variable from animals infected with recombinant viruses; SIVmac1A11/239gag-env/1A11 and SIVmac239/1A11env/239 were isolated most frequently. Animals inoculated with SIVmac239 had 10 to 100 times more virus-infected PBMC than those infected with recombinant viruses. Three animals infected with SIVmac239 died with simian AIDS (SAIDS) during the 2-year observation period after inoculation, and the fourth SIVmac239-infected animal had clinical signs of SAIDS. Two animals infected with recombinant viruses died with SAIDS; one was infected with SIVmac239/1A11gag-env/239, and the other was infected with SIVmac1A11/239gag-env/1A11. The remaining 18 macaques remained healthy by 2 years p.i., and 13 were aviremic. One year after inoculation, peripheral lymph nodes of some of these healthy, aviremic animals harbored infected cells. All animals seroconverted within the first few weeks of infection, and the magnitude of antibody response to SIV was proportional to the levels and duration of viremia. Virus-suppressive PBMC were detected within 2 to 4 weeks p.i. in all animals but tended to decline as viremia disappeared. There was no association of levels of cell-mediated virus-suppressive activity and either virus load or disease progression. Taken together, these results indicate that differences in more than one region of the viral genome are responsible for the lack of virulence of SIVmac1A11.     

Control of simian immunodeficiency virus SIVmac239 is not predicted by inheritance of Mamu-B*17-containing haplotypes

It is well established that host genetics, especially major histocompatibility complex (MHC) genes, are important determinants of human immunodeficiency virus disease progression. Studies with simian immunodeficiency virus (SIV)-infected Indian rhesus macaques have associated Mamu-B*17 with control of virus replication. Using microsatellite haplotyping of the 5-Mb MHC region, we compared disease progression among SIVmac239-infected Indian rhesus macaques that possess Mamu-B*17-containing MHC haplotypes that are identical by descent. We discovered that SIV-infected animals possessing identical Mamu-B*17-containing haplotypes had widely divergent disease courses. Our results demonstrate that the inheritance of a particular Mamu-B*17-containing haplotype is not sufficient to predict SIV disease outcome.     

In Vivo Replication Capacity Rather Than In Vitro Macrophage Tropism Predicts Efficiency of Vaginal Transmission of Simian Immunodeficiency Virus or Simian/Human Immunodeficiency Virus in Rhesus Macaques

We used the rhesus macaque model of heterosexual human immunodeficiency virus (HIV) transmission to test the hypothesis that in vitro measures of macrophage tropism predict the ability of a primate lentivirus to initiate a systemic infection after intravaginal inoculation. A single atraumatic intravaginal inoculation with a T-cell-tropic molecular clone of simian immunodeficiency virus (SIV), SIVmac239, or a dualtropic recombinant molecular clone of SIV, SIVmac239/1A11/239, or uncloned dualtropic SIVmac251 or uncloned dualtropic simian/human immunodeficiency virus (SHIV) 89.6-PD produced systemic infection in all rhesus macaques tested. However, vaginal inoculation with a dualtropic molecular clone of SIV, SIVmac1A11, resulted in transient viremia in one of two rhesus macaques. It has previously been shown that 12 intravaginal inoculations with SIVmac1A11 resulted in infection of one of five rhesus macaques (M. L. Marthas, C. J. Miller, S. Sutjipto, J. Higgins, J. Torten, B. L. Lohman, R. E. Unger, H. Kiyono, J. R. McGhee, P. A. Marx, and N. C. Pedersen, J. Med. Primatol. 21:99–107, 1992). In addition, SHIV HXBc2, which replicates in monkey macrophages, does not infect rhesus macaques following multiple vaginal inoculations, while T-cell-tropic SHIV 89.6 does (Y. Lu, P. B. Brosio, M. Lafaile, J. Li, R. G. Collman, J. Sodroski, and C. J. Miller, J. Virol. 70:3045–3050, 1996). These results demonstrate that in vitro measures of macrophage tropism do not predict if a SIV or SHIV will produce systemic infection after intravaginal inoculation of rhesus macaques. However, we did find that the level to which these viruses replicate in vivo after intravenous inoculation predicts the outcome of intravaginal inoculation with each virus.     

Early short-term 9-[2-(R)-(phosphonomethoxy)propyl]adenine treatment favorably alters the subsequent disease course in simian immunodeficiency virus-infected newborn Rhesus macaques

Simian immunodeficiency virus (SIV) infection of newborn macaques is a useful animal model of human pediatric AIDS to study disease pathogenesis and to develop intervention strategies aimed at delaying disease. In the present study, we demonstrate that very early events of infection greatly determine the ultimate disease course, as short-term antiviral drug administration during the initial viremia stage significantly delayed the onset of AIDS. Fourteen newborn macaques were inoculated orally with uncloned, highly virulent SIVmac251. The four untreated control animals showed persistently high virus levels and poor antiviral immune responses; they developed fatal immunodeficiency within 15 weeks. In contrast, SIV-infected newborn macaques which were started on 9-[2-(R)-(phosphonomethoxy)propyl]adenine (PMPA) treatment at 5 days of age and continued for either 14 or 60 days showed reduced virus levels and enhanced antiviral immune responses. This short-term PMPA treatment did not induce detectable emergence of SIV mutants with reduced in vitro susceptibility to PMPA. Although viremia increased in most animals after PMPA treatment was withdrawn, all animals remained disease-free for at least 6 months. Our data suggest that short-term treatment with a potent antiviral drug regimen during the initial viremia will significantly prolong AIDS-free survival for HIV-infected infants and adults.     

Simian immunodeficiency virus disease course is predicted by the extent of virus replication during primary infection

To elucidate the relationship between early viral infection events and immunodeficiency virus disease progression, quantitative-competitive and branched-DNA methods of simian immunodeficiency virus (SIV) RNA quantitation were cross-validated and used to measure viremia following infection of rhesus macaques with the pathogenic SIVmac251 virus isolate. Excellent correlation between the methods suggests that both accurately approximate SIV copy number. Plasma viremia was evident 4 days postinfection, and rapid viral expansion led to peak viremia levels of 10(7) to 10(9) SIV RNA copies/ml by days 8 to 17. Limited resolution of primary viremia was accompanied by relatively short, though variable, times to the development of AIDS (81 to 630 days). The persistent high-level viremia observed following intravenous inoculation of SIVmac251 explains the aggressive disease course in this model. Survival analyses demonstrated that the disease course is established 8 to 17 days postinfection, when peak viremia is observed. The most significant predictor of disease progression was the extent of viral decline following peak viremia; larger decrements in viremia were associated with both lower steady-state viremia (P = 0.0005) and a reduced hazard of AIDS (P = 0.004). The data also unexpectedly suggested that following SIVmac251 infection, animals with the highest peak viremia were better able to control virus replication rather than more rapidly developing disease. Analysis of early viral replication dynamics should help define host responses that protect from disease progression and should provide quantitative measures to assess the extent to which protective responses may be induced by prophylactic vaccination.     

Live,attenuated simian immunodeficiency virus SIVmac-M4, with point mutations in the Env transmembrane protein intracytoplasmic domain,provides partial protection from mucosal challenge with pathogenic SIVmac251

Attenuated molecular clones of simian immunodeficiency virus (SIVmac) are important tools for studying the correlates of protective immunity to lentivirus infection in nonhuman primates. The most highly attenuated SIVmac mutants fail to induce disease but also fail to induce immune responses capable of protecting macaques from challenge with pathogenic virus. We recently described a novel attenuated virus, SIVmac-M4, containing multiple mutations in the transmembrane protein (TM) intracytoplasmic domain. This domain has been implicated in viral assembly, infectivity, and cytopathogenicity. Whereas parental SIVmac239-Nef(+) induced persistent viremia and simian AIDS in rhesus macaques, SIVmac-M4 induced transient viremia in juvenile and neonatal macaques, with no disease for at least 1 year postinfection. In this vaccine study, 8 macaques that were infected as juveniles (n = 4) or neonates (n = 4) with SIVmac-M4 were challenged with pathogenic SIVmac251 administered through oral mucosa. At 1 year postchallenge, six of the eight macaques had low to undetectable plasma viremia levels. Assays of cell-mediated immune responses to SIVmac Gag, Pol, Env, and Nef revealed that all animals developed strong CD8(+) T-cell responses to Gag after challenge but not before. Unvaccinated control animals challenged with SIVmac251 developed persistent viremia, had significantly weaker SIV-specific T-cell responses, and developed AIDS-related symptoms. These findings demonstrate that SIVmac-M4, which contains a full-length Nef coding region and multiple point mutations in the TM, can provide substantial protection from mucosal challenge with pathogenic SIVmac251.     

Prior infection with a nonpathogenic chimeric simian-human immunodeficiency virus does not efficiently protect macaques against challenge with simian immunodeficiency virus.

Prior infection with a nef-deleted simian immunodeficiency virus (SIV) protects macaques not only against a homologous pathogenic SIV challenge but also against challenge with a chimeric SIV expressing a human immunodeficiency virus type 1 env gene (SHIV). Since this SHIV is itself nonpathogenic, we sought to explore the use of a nonpathogenic SHIV as a live, attenuated AIDS virus vaccine. Four cynomolgus monkeys infected for greater than 600 days with a chimeric virus composed of SIVmac 239 expressing the human immunodeficiency virus type 1 HXBc2 env, tat, and rev genes were challenged intravenously with 100 animal infectious doses of the J5 clone of SIVmac 32H, an isolate derived by in vivo passage of SIVmac 251. Three of the four monkeys became infected with SIVmac. This observation underlines the difficulty, even with a live virus vaccine, in protecting against an AIDS virus infection.     

Pathogenic conversion of live attenuated simian immunodeficiency virus vaccines is associated with expression of truncated Nef

Rhesus macaques infected with simian immunodeficiency virus (SIV) containing either a large nef deletion (SIVmac239Delta(152)nef) or interleukin-2 in place of nef developed high virus loads and progressed to simian AIDS. Viruses recovered from both juvenile and neonatal macaques with disease produced a novel truncated Nef protein, tNef. Viruses recovered from juvenile macaques infected with serially passaged virus expressing tNef exhibited a pathogenic phenotype. These findings demonstrated strong selective pressure to restore expression of a truncated Nef protein, and this reversion was linked to increased pathogenic potential in live attenuated SIV vaccines.     

Induction of CD8+ cells able to suppress CCR5-tropic simian immunodeficiency virus SIVmac239 replication by controlled infection of CXCR4-tropic simian-human immunodeficiency virus in vaccinated rhesus macaques

Recent recombinant viral vector-based AIDS vaccine trials inducing cellular immune responses have shown control of CXCR4-tropic simian-human immunodeficiency virus (SHIV) replication but difficulty in containment of pathogenic CCR5-tropic simian immunodeficiency virus (SIV) in rhesus macaques. In contrast, controlled infection of live attenuated SIV/SHIV can confer the ability to contain SIV superchallenge in macaques. The specific immune responses responsible for this control may be induced by live virus infection but not consistently by viral vector vaccination, although those responses have not been determined. Here, we have examined in vitro anti-SIV efficacy of CD8+ cells in rhesus macaques that showed prophylactic viral vector vaccine-based control of CXCR4-tropic SHIV89.6PD replication. Analysis of the effect of CD8+ cells obtained at several time points from these macaques on CCR5-tropic SIVmac239 replication in vitro revealed that CD8+ cells in the chronic phase after SHIV challenge suppressed SIV replication more efficiently than those before challenge. SIVmac239 superchallenge of two of these macaques at 3 or 4 years post-SHIV challenge was contained, and the following anti-CD8 antibody administration resulted in transient CD8+ T-cell depletion and appearance of plasma SIVmac239 viremia in both of them. Our results indicate that CD8+ cells acquired the ability to efficiently suppress SIV replication by controlled SHIV infection, suggesting the contribution of CD8+ cell responses induced by controlled live virus infection to containment of HIV/SIV superinfection.     

Temporal Analyses of Virus Replication, Immune Responses, and Efficacy in Rhesus Macaques Immunized with a Live, Attenuated Simian Immunodeficiency Virus Vaccine

Despite evidence that live, attenuated simian immunodeficiency virus (SIV) vaccines can elicit potent protection against pathogenic SIV infection, detailed information on the replication kinetics of attenuated SIV in vivo is lacking. In this study, we measured SIV RNA in the plasma of 16 adult rhesus macaques immunized with a live, attenuated strain of SIV (SIVmac239Δnef). To evaluate the relationship between replication of the vaccine virus and the onset of protection, four animals per group were challenged with pathogenic SIVmac251 at either 5, 10, 15, or 25 weeks after immunization. SIVmac239Δnef replicated efficiently in the immunized macaques in the first few weeks after inoculation. SIV RNA was detected in the plasma of all animals by day 7 after inoculation, and peak levels of viremia (10⁵ to 10⁷ RNA copies/ml) occurred by 7 to 12 days. Following challenge, SIVmac251 was detected in all of the four animals challenged at 5 weeks, in two of four challenged at 10 weeks, in none of four challenged at 15 weeks, and one of four challenged at 25 weeks. One animal immunized with SIVmac239Δnef and challenged at 10 weeks had evidence of disease progression in the absence of detectable SIVmac251. Although complete protection was not achieved at 5 weeks, a transient reduction in viremia (approximately 100-fold) occurred in the immunized macaques early after challenge compared to the nonimmunized controls. Two weeks after challenge, SIV RNA was also reduced in the lymph nodes of all immunized macaques compared with control animals. Taken together, these results indicate that host responses capable of reducing the viral load in plasma and lymph nodes were induced as early as 5 weeks after immunization with SIVmac239Δnef, while more potent protection developed between 10 and 15 weeks. In further experiments, we found that resistance to SIVmac251 infection did not correlate with the presence of antibodies to SIV gp130 and p27 antigens and was achieved in the absence of significant neutralizing activity against the primary SIVmac251 challenge stock.     

Divergent host responses during primary simian immunodeficiency virus SIVsm infection of natural sooty mangabey and nonnatural rhesus macaque hosts

To understand how natural sooty mangabey hosts avoid AIDS despite high levels of simian immunodeficiency virus (SIV) SIVsm replication, we inoculated mangabeys and nonnatural rhesus macaque hosts with an identical inoculum of uncloned SIVsm. The unpassaged virus established infection with high-level viral replication in both macaques and mangabeys. A species-specific, divergent immune response to SIV was evident from the first days of infection and maintained in the chronic phase, with macaques showing immediate and persistent T-cell proliferation, whereas mangabeys displayed little T-cell proliferation, suggesting subdued cellular immune responses to SIV. Importantly, only macaques developed (CD4+)-T-cell depletion and AIDS, thus indicating that in mangabeys limited immune activation is a key mechanism to avoid immunodeficiency despite high levels of SIVsm replication. These studies demonstrate that it is the host response to infection, rather than properties inherent to the virus itself, that causes immunodeficiency in SIV-infected nonhuman primates.     

Intravaginal inoculation of rhesus macaques with cell-free simian immunodeficiency virus results in persistent or transient viremia.

The simian immunodeficiency virus (SIV)-rhesus macaque model of heterosexual human immunodeficiency virus transmission consists of atraumatic application of cell-free SIVmac onto the intact vaginal mucosa of mature female rhesus macaques. This procedure results in systemic infection, and eventually infected animals develop the clinical signs and pathologic changes of simian AIDS. To achieve 100% transmission with the virus stocks used to date, multiple intravaginal inoculations are required. The current titration study utilized two stocks of SIVmac and demonstrated that a single intravaginal dose of cell-free SIV can reliably produce infection in rhesus macaques. This study also demonstrated that some animals intravaginally inoculated with cell-free SIVmac develop transient viremia characterized by a limited ability to isolate virus from peripheral blood mononuclear cells and lymph node mononuclear cells and no seroconversion to SIV antigen. SIV could be isolated from the peripheral lymph nodes of transiently viremic animals only during periods of viremia and not at times when SIV was not detected in circulating mononuclear cells. Thus, peripheral lymphoid tissues were not reservoirs of infection in the transiently viremic animals. Taken together, these results suggest either that the SIV infection was cleared in the transiently viremic animals or that SIV infection is limited to a compartment of the genital mucosal immune system that cannot be assessed by monitoring SIV infection in peripheral blood mononuclear cells and peripheral lymphoid tissue.     

Dynamics of simian immunodeficiency virus SIVmac239 infection in pigtail macaques

Pigtail macaques (PTM) are an excellent model for HIV research; however, the dynamics of simian immunodeficiency virus (SIV) SIVmac239 infection in PTM have not been fully evaluated. We studied nine PTM prior to infection, during acute and chronic SIVmac239 infections, until progression to AIDS. We found PTM manifest clinical AIDS more rapidly than rhesus macaques (RM), as AIDS-defining events occurred at an average of 42.17 weeks after infection in PTM compared to 69.56 weeks in RM (P = 0.0018). However, increased SIV progression was not associated with increased viremia, as both peak and set-point plasma viremias were similar between PTM and RM (P = 0.7953 and P = 0.1006, respectively). Moreover, this increased disease progression was not associated with rapid CD4(+) T cell depletion, as CD4(+) T cell decline resembled other SIV/human immunodeficiency virus (HIV) models. Since immune activation is the best predictor of disease progression during HIV infection, we analyzed immune activation by turnover of T cells by BrdU decay and Ki67 expression. We found increased levels of turnover prior to SIV infection of PTM compared to that observed with RM, which may contribute to their increased disease progression rate. These data evaluate the kinetics of SIVmac239-induced disease progression and highlight PTM as a model for HIV infection and the importance of immune activation in SIV disease progression.     

Isolation and Characterization of a Neuropathogenic Simian Immunodeficiency Virus Derived from a Sooty Mangabey

Transfusion of blood from a simian immunodeficiency virus (SIV)- and simian T-cell lymphotropic virus-infected sooty mangabey (designated FGb) to rhesus and pig-tailed macaques resulted in the development of neurologic disease in addition to AIDS. To investigate the role of SIV in neurologic disease, virus was isolated from a lymph node of a pig-tailed macaque (designated PGm) and the cerebrospinal fluid of a rhesus macaque (designated ROn2) and passaged to additional macaques. SIV-related neuropathogenic effects were observed in 100% of the pig-tailed macaques inoculated with either virus. Lesions in these animals included extensive formation of SIV RNA-positive giant cells in the brain parenchyma and meninges. Based upon morphology, the majority of infected cells in both lymphoid and brain tissue appeared to be of macrophage lineage. The virus isolates replicated very well in pig-tailed and rhesus macaque peripheral blood mononuclear cells (PBMC) with rapid kinetics. Differential replicative abilities were observed in both PBMC and macrophage populations, with viruses growing to higher titers in pig-tailed macaque cells than in rhesus macaque cells. An infectious molecular clone of virus derived from the isolate from macaque PGm (PGm5.3) was generated and was shown to have in vitro replication characteristics similar to those of the uncloned virus stock. While molecular analyses of this virus revealed its similarity to SIV isolates from sooty mangabeys, significant amino acid differences in Env and Nef were observed. This virus should provide an excellent system for investigating the mechanism of lentivirus-induced neurologic disease.     

Early hematologic changes in rhesus macaques (Macaca mulatta) infected with pathogenic and nonpathogenic isolates of SIVmac

Early hematologic changes were studied over a 14 day period in three groups of six rhesus macaques intravenously infected with pathogenic and nonpathogenic isolates of SIVmac. Abnormalities in blood included a mild blood loss anemia, sporadic lymphopenia, and variable CD4+ and CD8 + T lymphocyte numbers. Prominent bone marrow findings in macaques inoculated with pathogenic uncloned SIVmac and molecularly cloned pathogenic SIVmac-239 were hypercellularity, myeloid and megakaryocytic hyperplasia, and lymphoid aggregates. Infrequent mild morphologic abnormalities were present in macaques infected with a nonpathogenic molecular clone, SIVmac-1A11.     

Severe depletion of mucosal CD4+ T cells in AIDS-free simian immunodeficiency virus-infected sooty mangabeys

HIV-infected humans and SIV-infected rhesus macaques experience a rapid and dramatic loss of mucosal CD4+ T cells that is considered to be a key determinant of AIDS pathogenesis. In this study, we show that nonpathogenic SIV infection of sooty mangabeys (SMs), a natural host species for SIV, is also associated with an early, severe, and persistent depletion of memory CD4+ T cells from the intestinal and respiratory mucosa. Importantly, the kinetics of the loss of mucosal CD4+ T cells in SMs is similar to that of SIVmac239-infected rhesus macaques. Although the nonpathogenic SIV infection of SMs induces the same pattern of mucosal target cell depletion observed during pathogenic HIV/SIV infections, the depletion in SMs occurs in the context of limited local and systemic immune activation and can be reverted if virus replication is suppressed by antiretroviral treatment. These results indicate that a profound depletion of mucosal CD4+ T cells is not sufficient per se to induce loss of mucosal immunity and disease progression during a primate lentiviral infection. We propose that, in the disease-resistant SIV-infected SMs, evolutionary adaptation to both preserve immune function with fewer mucosal CD4+ T cells and attenuate the immune activation that follows acute viral infection protect these animals from progressing to AIDS.     

Simian-human immunodeficiency virus SHIV89.6-induced protection against intravaginal challenge with pathogenic SIVmac239 is independent of the route of immunization and is associated with a combination of cytotoxic T-lymphocyte and alpha interferon responses

Attenuated primate lentivirus vaccines provide the most consistent protection against challenge with pathogenic simian immunodeficiency virus (SIV). Thus, they provide an excellent model to examine the influence of the route of immunization on challenge outcome and to study vaccine-induced protective anti-SIV immune responses. In the present study, rhesus macaques were immunized with live nonpathogenic simian-human immunodeficiency virus (SHIV) 89.6 either intravenously or mucosally (intranasally or intravaginally) and then challenged intravaginally with pathogenic SIVmac239. The route of immunization did not affect mucosal challenge outcome after a prolonged period of systemic infection with the nonpathogenic vaccine virus. Further, protection from the SIV challenge was associated with the induction of multiple host immune effector mechanisms. A comparison of immune responses in vaccinated-protected and vaccinated-unprotected animals revealed that vaccinated-protected animals had higher frequencies of SIV Gag-specific cytotoxic T lymphocytes and gamma interferon (IFN-gamma)-secreting cells during the acute phase postchallenge. Vaccinated-protected animals also had a more pronounced increase in peripheral blood mononuclear cell IFN-alpha mRNA levels than did the vaccinated-unprotected animals in the first few weeks after challenge. Thus, innate as well as cellular anti-SIV immune responses appeared to contribute to the SHIV89.6-induced protection against intravaginal challenge with pathogenic SIVmac239.     

Role of CD8(+) lymphocytes in control of simian immunodeficiency virus infection and resistance to rechallenge after transient early antiretroviral treatment

Transient antiretroviral treatment with tenofovir, (R)-9-(2-phosphonylmethoxypropyl)adenine, begun shortly after inoculation of rhesus macaques with the highly pathogenic simian immunodeficiency virus (SIV) isolate SIVsmE660, facilitated the development of SIV-specific lymphoproliferative responses and sustained effective control of the infection following drug discontinuation. Animals that controlled plasma viremia following transient postinoculation treatment showed substantial resistance to subsequent intravenous rechallenge with homologous (SIVsmE660) and highly heterologous (SIVmac239) SIV isolates, up to more than 1 year later, despite the absence of measurable neutralizing antibody. In some instances, resistance to rechallenge was observed despite the absence of detectable SIV-specific binding antibody and in the face of SIV lymphoproliferative responses that were low or undetectable at the time of challenge. In vivo monoclonal antibody depletion experiments demonstrated a critical role for CD8(+) lymphocytes in the control of viral replication; plasma viremia rose by as much as five log units after depletion of CD8(+) cells and returned to predepletion levels (as low as <100 copy Eq/ml) as circulating CD8(+) cells were restored. The extent of host control of replication of highly pathogenic SIV strains and the level of resistance to heterologous rechallenge achieved following transient postinoculation treatment compared favorably to the results seen after SIVsmE660 and SIVmac239 challenge with many vaccine strategies. This impressive control of viral replication was observed despite comparatively modest measured immune responses, less than those often achieved with vaccination regimens. The results help establish the underlying feasibility of efforts to develop vaccines for the prevention of AIDS, although the exact nature of the protective host responses involved remains to be elucidated.     

Diverse Host Responses and Outcomes following Simian Immunodeficiency Virus SIVmac239 Infection in Sooty Mangabeys and Rhesus Macaques

Sooty mangabeys naturally infected with simian immunodeficiency virus (SIV) do not develop immunodeficiency despite the presence of viral loads of 10⁵ to 10⁷ RNA copies/ml. To investigate the basis of apathogenic SIV infection in sooty mangabeys, three sooty mangabeys and three rhesus macaques were inoculated intravenously with SIVmac239 and evaluated longitudinally for 1 year. SIVmac239 infection of sooty mangabeys resulted in 2- to 4-log-lower viral loads than in macaques and did not reproduce the high viral loads observed in natural SIVsmm infection. During acute SIV infection, polyclonal cytotoxic T-lymphocyte (CTL) activity coincident with decline in peak plasma viremia was observed in both macaques and mangabeys; 8 to 20 weeks later, CTL activity declined in the macaques but was sustained and broadly directed in the mangabeys. Neutralizing antibodies to SIVmac239 were detected in the macaques but not the mangabeys. Differences in expression of CD38 on CD8⁺ T lymphocytes or in the percentage of naive phenotype T cells expressing CD45RA and CD62L-selection did not correlate with development of AIDS in rhesus macaques. In macaques, the proportion of CD4⁺ T lymphocytes expressing CD25 declined during SIV infection, while in mangabeys, CD25-expressing CD4⁺ T lymphocytes increased. Longitudinal evaluation of cytokine secretion by flow cytometric analysis of unstimulated lymphocytes revealed elevation of interleukin-2 and gamma interferon in a macaque and only interleukin-10 in a concurrently infected mangabey during acute SIV infection. Differences in host responses following experimental SIVmac239 infection may be associated with the divergent outcome in sooty mangabeys and rhesus macaques.