Cross-protective immune responses induced in rhesus macaques by immunization with attenuated macrophage-tropic simian immunodeficiency virus.

The simian immunodeficiency virus (SIV) macaque model of AIDS has provided a valuable system with which to investigate vaccine approaches for protection against human immunodeficiency virus type 1 (HIV-1) infection. In particular, the ability of macaques persistently infected with attenuated infectious molecular clones of SIV to resist challenge with the pathogenic parental swarm has conclusively demonstrated that protective immunity can be achieved by immunization prior to exposure. The breadth of these protective responses and the immunological correlates of protection, however, have not been identified. In addition, vaccine studies have mainly employed lymphocyte-tropic strains of HIV-1 and SIV. Recent studies have implicated macrophage-tropic strains in the transmission of HIV-1 and have suggested that these virus strains should be examined in vaccine strategies. Macrophage-tropic viruses may confer additional advantages in the induction of protective immunity by replication in antigen-presenting cells. In this study, the immune response of rhesus macaques inoculated with an attenuated macrophage-tropic recombinant of SIVmac239 (SIV/17E-Cl) was evaluated with respect to protective immunity by heterologous challenge at various times after infection. Vigorous type-specific neutralizing-antibody responses restricted to SIV/17E-Cl were evident by 2 weeks postinfection. By 7 months, however, cross-reactive neutralizing antibodies emerged which neutralized not only SIV/17E-Cl but also the heterologous primary isolate SIV/DeltaB670. Challenge of SIV/17E-Cl-infected monkeys with SIV/DeltaB670 at various times postinfection demonstrated that protective responses were associated with the appearance of cross-reactive neutralizing antibodies. Furthermore, passive transfer of sera from SIV/17E-Cl-infected animals passively protected two of four naive recipients.     

Vaccine protection against simian immunodeficiency virus by recombinant strains of herpes simplex virus

An effective vaccine for AIDS may require development of novel vectors capable of eliciting long-lasting immune responses. Here we report the development and use of replication-competent and replication-defective strains of recombinant herpes simplex virus (HSV) that express envelope and Nef antigens of simian immunodeficiency virus (SIV). The HSV recombinants induced antienvelope antibody responses that persisted at relatively stable levels for months after the last administration. Two of seven rhesus monkeys vaccinated with recombinant HSV were solidly protected, and another showed a sustained reduction in viral load following rectal challenge with pathogenic SIVmac239 at 22 weeks following the last vaccine administration. HSV vectors thus show great promise for being able to elicit persistent immune responses and to provide durable protection against AIDS.     

Induction of cellular immune responses to simian immunodeficiency virus gag by two recombinant negative-strand RNA virus vectors

A recombinant Newcastle disease virus (rNDV) expressing simian immunodeficiency virus (SIV) Gag protein (rNDV/SIVgag) was generated. The rNDV/SIVgag virus induced Gag-specific cellular immune responses in mice, leading to a specific anti-Gag antiviral immunity. This was evidenced by the inhibition of growth of recombinant vaccinia virus expressing an identical Gag antigen (rVac/SIVgag) but not of wild-type vaccinia virus in rNDV/SIVgag-immunized mice. Among intravenous, intraperitoneal, or intranasal immunization routes, intranasal administration induced the strongest protective response against challenge with rVac/SIVgag. We further demonstrated that these immune responses were greatly enhanced after booster immunization with recombinant influenza viruses expressing immunogenic portions of SIV Gag. The magnitude of the protective immune response correlated with the levels of cellular immune responses to Gag, which were still evident 9 weeks after immunization. These results suggest that rNDV and influenza virus vectors are suitable candidate vaccines against AIDS as well as against other infectious diseases.     

Containment of simian immunodeficiency virus infection: cellular immune responses and protection from rechallenge following transient postinoculation antiretroviral treatment

To better understand the viral and host factors involved in the establishment of persistent productive infection by primate lentiviruses, we varied the time of initiation and duration of postinoculation antiretroviral treatment with tenofovir (9-[2-(R)-(phosphonomethoxy)propyl]adenine) while performing intensive virologic and immunologic monitoring in rhesus macaques, inoculated intravenously with simian immunodeficiency virus SIVsmE660. Postinoculation treatment did not block the initial infection, but we identified treatment regimens that prevented the establishment of persistent productive infection, as judged by the absence of measurable plasma viremia following drug discontinuation. While immune responses were heterogeneous, animals in which treatment resulted in prevention of persistent productive infection showed a higher frequency and higher levels of SIV-specific lymphocyte proliferative responses during the treatment period compared to control animals, despite the absence of either detectable plasma viremia or seroconversion. Animals protected from the initial establishment of persistent productive infection were also relatively or completely protected from subsequent homologous rechallenge. Even postinoculation treatment regimens that did not prevent establishment of persistent infection resulted in downmodulation of the level of plasma viremia following treatment cessation, compared to the viremia seen in untreated control animals, animals treated with regimens known to be ineffective, or the cumulative experience with the natural history of plasma viremia following infection with SIVsmE660. The results suggest that the host may be able to effectively control SIV infection if the initial exposure occurs under favorable conditions of low viral burden and in the absence of ongoing high level cytopathic infection of responding cells. These findings may be particularly important in relation to prospects for control of primate lentiviruses in the settings of both prophylactic and therapeutic vaccination for prevention of AIDS.     

Vaccine protection by a triple deletion mutant of simian immunodeficiency virus.

Twelve rhesus monkeys were vaccinated with SIVmac316 delta nef (lacking nef sequences), and 12 were vaccinated with SIVmac239 delta3 (lacking nef, vpr, and upstream sequences in U3). SIVmac316 and SIVmac239 differ by only eight amino acids in the envelope; these changes render SIVmac316 highly competent for replication in macrophages. Seventeen of the animals developed persistent infections with the vaccine viruses. Seven of the 24 vaccinated animals, however, developed infections that were apparently transient in nature. Six of these seven yielded virus from peripheral blood when tested at weeks 2 and/or 3, three of the seven had transient antibody responses, but none of the seven had persisting antibody responses. The 24 monkeys were challenged in groups of four with 10 rhesus monkey infectious doses of wild-type, pathogenic SIVmac251 at weeks 8, 20, and 79 following receipt of vaccine. None of the seven with apparently transient infections with vaccine virus were protected upon subsequent challenge. Analysis of cell-associated viral loads, CD4+ cell counts, and viral gene sequences present in peripheral blood in the remainder of the monkeys following challenge allowed a number of conclusions. (i) There was a trend toward increased protection with length of time of vaccination. (ii) Solid vaccine protection was achieved by 79 weeks with the highly attenuated SIV239 delta3. (iii) Solid long-term protection was achieved in at least two animals in the absence of complete sterilizing immunity. (iv) Genetic backbone appeared to influence protective capacity; animals vaccinated with SIV239 delta3 were better protected than animals receiving SIV316 delta nef. This better protection correlated with increased levels of the replicating vaccine strain. (v) The titer of virus-neutralizing activity in serum on the day of challenge correlated with protection when measured against a primary stock of SIVmac251 but not when measured against a laboratory-passaged stock. The level of binding antibodies to whole virus by enzyme-linked immunosorbent assay also correlated with protection.     

Potent simian immunodeficiency virus-specific cellular immune responses in the breast milk of simian immunodeficiency virus-infected, lactating rhesus monkeys

Breast milk transmission of HIV is a leading cause of infant HIV/AIDS in the developing world. Remarkably, only a small minority of breastfeeding infants born to HIV-infected mothers contract HIV via breast milk exposure, raising the possibility that immune factors in the breast milk confer protection to the infants who remain uninfected. To model HIV-specific immunity in breast milk, lactation was pharmacologically induced in Mamu-A*01(+) female rhesus monkeys. The composition of lymphocyte subsets in hormone-induced lactation breast milk was found to be similar to that in natural lactation breast milk. Hormone-induced lactating monkeys were inoculated i.v. with SIVmac251 and CD8(+) T lymphocytes specific for two immunodominant SIV epitopes, Gag p11C and Tat TL8, and SIV viral load were monitored in peripheral blood and breast milk during acute infection. The breast milk viral load was 1-2 logs lower than plasma viral load through peak and set point of viremia. Surprisingly, whereas the kinetics of the SIV-specific cellular immunity in breast milk mirrored that of the blood, the peak magnitude of the SIV-specific CD8(+) T lymphocyte response in breast milk was more than twice as high as the cellular immune response in the blood. Furthermore, the appearance of the SIV-specific CD8(+) T lymphocyte response in breast milk was associated with a reduction in breast milk viral load, and this response remained higher than that in the blood after viral set point. This robust viral-specific cellular immune response in breast milk may contribute to control of breast milk virus replication.     

Novel, live attenuated simian immunodeficiency virus constructs containing major deletions in leader RNA sequences

We have constructed a series of simian immunodeficiency virus (SIV) mutants containing deletions within a 97-nucleotide (nt) region of the leader sequence. Deletions in this region markedly decreased the replication capacity in tissue culture, i.e., in both the C8166 and CEMx174 cell lines, as well as in rhesus macaque peripheral blood mononuclear cells. In addition, these deletions adversely affected the packaging of viral genomic RNA into virions, the processing of Gag precursor proteins, and patterns of viral proteins in virions, as assessed by biochemical labeling and polyacrylamide gel electrophoresis. Different levels of attenuation were achieved by varying the size and position of deletions within this 97-nt region, and among a series of constructs that were generated, it was possible to rank in vitro virulence relative to that of wild-type virus. In all of these cases, the most severe impact on viral replication was observed when the deletions that were made were located at the 3' rather than 5' end of the leader region. The potential of viral reversion over protracted periods was investigated by repeated viral passage in CEMx174 cells. The results showed that several of these constructs showed no signs of reversion after more than 6 months in tissue culture. Thus, a series of novel, attenuated SIV constructs have been developed that are significantly impaired in replication capacity yet retain all viral genes. One of these viruses, termed SD4, may be appropriate for study with rhesus macaques, in order to determine whether reversions will occur in vivo and to further study this virus as a candidate for attenuated vaccination.     

Robust vaccine-elicited cellular immune responses in breast milk following systemic simian immunodeficiency virus DNA prime and live virus vector boost vaccination of lactating rhesus monkeys

Breast milk transmission of HIV remains an important mode of infant HIV acquisition. Enhancement of mucosal HIV-specific immune responses in milk of HIV-infected mothers through vaccination may reduce milk virus load or protect against virus transmission in the infant gastrointestinal tract. However, the ability of HIV/SIV strategies to induce virus-specific immune responses in milk has not been studied. In this study, five uninfected, hormone-induced lactating, Mamu A*01(+) female rhesus monkey were systemically primed and boosted with rDNA and the attenuated poxvirus vector, NYVAC, containing the SIVmac239 gag-pol and envelope genes. The monkeys were boosted a second time with a recombinant Adenovirus serotype 5 vector containing matching immunogens. The vaccine-elicited immunodominant epitope-specific CD8(+) T lymphocyte response in milk was of similar or greater magnitude than that in blood and the vaginal tract but higher than that in the colon. Furthermore, the vaccine-elicited SIV Gag-specific CD4(+) and CD8(+) T lymphocyte polyfunctional cytokine responses were more robust in milk than in blood after each virus vector boost. Finally, SIV envelope-specific IgG responses were detected in milk of all monkeys after vaccination, whereas an SIV envelope-specific IgA response was only detected in one vaccinated monkey. Importantly, only limited and transient increases in the proportion of activated or CCR5-expressing CD4(+) T lymphocytes in milk occurred after vaccination. Therefore, systemic DNA prime and virus vector boost of lactating rhesus monkeys elicits potent virus-specific cellular and humoral immune responses in milk and may warrant further investigation as a strategy to impede breast milk transmission of HIV.     

Pseudovirion particle production by live poxvirus human immunodeficiency virus vaccine vector enhances humoral and cellular immune responses

Live-vector-based human immunodeficiency virus (HIV) vaccines are an integral part of a number of HIV vaccine regimens currently under evaluation. Live vectors that carry an intact gag gene are capable of eliciting HIV pseudovirion particle formation from infected host cells. The impact of pseudovirion particle formation on the immune response generated by live HIV vaccine vectors has not been established. In this study, a canarypox HIV vaccine candidate vector expressing HIV gag and env genes, vCP205, was modified by the introduction of a glycine-to-alanine coding change in the N-terminal myristylation site of gag to create Myr- vCP205. This substitution effectively eliminated particle formation without altering the level of protein production. vCP205 and Myr- vCP205 were then directly compared for the ability to induce HIV-specific immune responses in mice. The particle-competent vector vCP205 elicited higher levels of CD8+ T-cell responses, as indicated by gamma interferon enzyme-linked immunospot (ELISPOT) assay and intracellular cytokine staining. Humoral responses to Gag and Env were also markedly higher from animals immunized with the particle-competent vector. Furthermore, HIV-specific CD4+ T-cell responses were greater among animals immunized with the particle-competent vector. Using a human dendritic cell model of antigen presentation in vitro, vCP205 generated greater ELISPOT responses than Myr- vCP205. These results demonstrate that pseudovirion particle production by live-vector HIV vaccines enhances HIV-specific cellular and humoral immune responses.     

Vaccine protection against simian immunodeficiency virus in monkeys using recombinant gamma-2 herpesvirus

Recombinant strains of replication-competent rhesus monkey rhadinovirus (RRV) were constructed in which strong promoter/enhancer elements were used to drive expression of simian immunodeficiency virus (SIV) Env or Gag or a Rev-Tat-Nef fusion protein. Cultured rhesus monkey fibroblasts infected with each recombinant strain were shown to express the expected protein. Three RRV-negative and two RRV-positive rhesus monkeys were inoculated intravenously with a mixture of these three recombinant RRVs. Expression of SIV Gag was readily detected in lymph node biopsy specimens taken at 3 weeks postimmunization. Impressive anti-SIV cellular immune responses were elicited on the basis of major histocompatibility complex (MHC) tetramer staining and gamma interferon enzyme-linked immunospot (ELISPOT) assays. Responses were much greater in magnitude in the monkeys that were initially RRV negative but were still readily detected in the two monkeys that were naturally infected with RRV at the time of immunization. By 3 weeks postimmunization, responses measured by MHC tetramer staining in the two Mamu-A*01(+) RRV-negative monkeys reached 9.3% and 13.1% of all CD8(+) T cells in peripheral blood to the Gag CM9 epitope and 2.3% and 7.3% of all CD8(+) T cells in peripheral blood to the Tat SL8 epitope. Virus-specific CD8(+) T cell responses persisted at high levels up to the time of challenge at 18 weeks postimmunization, and responding cells maintained an effector memory phenotype. Despite the ability of the RRVenv recombinant to express high levels of Env in cultured cells, and despite the appearance of strong anti-RRV antibody responses in immunized monkeys, anti-Env antibody responses were below our ability to detect them. Immunized monkeys, together with three unimmunized controls, were challenged intravenously with 10 monkey infectious doses of SIVmac239. All five immunized monkeys and all three controls became infected with SIV, but peak viral loads were 1.2 to 3.0 log(10) units lower and chronic-phase viral loads were 1.0 to 3.0 log(10) units lower in immunized animals than the geometric mean of unimmunized controls. These differences were statistically significant. Anti-Env antibody responses following challenge indicated an anamnestic response in the vaccinated monkeys. These findings further demonstrate the potential of recombinant herpesviruses as preventive vaccines for AIDS. We hypothesize that this live, replication-competent, persistent herpesvirus vector could match, or come close to matching, live attenuated strains of SIV in the degree of protection if the difficulty with elicitation of anti-Env antibody responses can be overcome.     

A combination DNA and attenuated simian immunodeficiency virus vaccine strategy provides enhanced protection from simian/human immunodeficiency virus-induced disease

Among the most effective vaccine candidates tested in the simian immunodeficiency virus (SIV)/macaque system, live attenuated viruses have been shown to provide the best protection from challenge. To investigate if preimmunization would increase the level of protection afforded by live attenuated SIVmac239Deltanef (Deltanef), macaques were given two priming immunizations of DNA encoding SIV Gag and Pol proteins, with control macaques receiving vector DNA immunizations. In macaques receiving the SIV DNA inoculation, SIV-specific cellular but not humoral responses were readily detectable 2 weeks after the second DNA inoculation. Following boosting with live attenuated virus, control of Deltanef replication was superior in SIV-DNA-primed macaques versus vector-DNA-primed macaques and was correlated with higher levels of CD8+/gamma-interferon-positive and/or interleukin-2-positive cells. Challenge with an intravenous inoculation of simian/human immunodeficiency virus (SHIV) strain SHIV89.6p resulted in infection of all animals. However, macaques receiving SIV DNA as the priming immunizations had statistically lower viral loads than control animals and did not develop signs of disease, whereas three of seven macaques receiving vector DNA showed severe CD4+ T-cell decline, with development of AIDS in one of these animals. No correlation of immune responses to protection from disease could be derived from our analyses. These results demonstrate that addition of a DNA prime to a live attenuated virus provided better protection from disease following challenge than live attenuated virus alone.     

A rapid progressor-specific variant clone of simian immunodeficiency virus replicates efficiently in vivo only in the absence of immune responses

A subset of simian immunodeficiency virus (SIV)-infected macaques progresses rapidly to disease with transient SIV-specific immune responses and high viral loads. Unique SIV variants with convergent Env mutations evolve in these rapid progressor (RP) macaques. To address the pathogenic significance of RP-specific variants, we generated infectious molecular clones from the terminal-phase plasma of an RP macaque. Inoculation of macaques with a representative clone, SIVsmH635FC, resulted in a persistent viremia, comparable to that produced by pathogenic SIVsmE543-3, and a chronic disease with progressive loss of CD4(+) T cells. However, SIVsmH635FC did not reproduce the rapid-disease phenomenon. Molecular analyses of viruses from these macaques revealed rapid reversion to the wild-type SIVsmE543-3 sequence at two RP-specific sites and slower reversion at another three sites. SIVsmH635FC infection was not sufficient to cause rapid progression even following coinoculation with SIVsmE543-3, despite acute depletion of memory CD4(+) T cells. SIVsmH635FC competed efficiently during primary infection in the coinoculated macaques, but SIVsmE543-3 predominated after the development of SIV-specific immune responses. These data suggest that the replication fitness of the RP variant was similar to that of SIVsmE543-3 in a na?ve host; however, SIVsmH635FC was at a disadvantage following the development of SIV-specific immune responses. Consistent with these findings, neutralization assays revealed that SIVsmH635FC was highly sensitive to neutralization but that the parental SIVsmE543-3 strain was highly resistant. This study suggests that the evolution of RP-specific variants is the result of replication in a severely immunocompromised host, rather than the direct cause of rapid progression.     

Induction of positive cellular and humoral immune responses by a prime-boost vaccine encoded with simian immunodeficiency virus gag/pol

It is believed likely that immune responses are responsible for controlling viral load and infection. In this study, when macaques were primed with plasmid DNA encoding SIV gag and pol genes (SIVgag/pol DNA) and then boosted with replication-deficient vaccinia virus DIs recombinant expressing the same genes (rDIsSIVgag/pol), this prime-boost regimen generated higher levels of Gag-specific CD4+ and CD8+ T cell responses than did either SIVgag/pol DNA or rDIsSIVgag/pol alone. When the macaques were i.v. challenged with pathogenic simian/HIV, the prime-boost group maintained high CD4+ T cell counts and reduced plasma viral loads up to 30 wk after viral challenge, whereas the rDIsSIVgag/pol group showed only a partial attenuation of the viral infection, and the group immunized with SIVgag/pol DNA alone showed none at all. The protection levels were better correlated with the levels of virus-specific T cell responses than the levels of neutralization Ab responses. These results demonstrate that a vaccine regimen that primes with DNA and then boosts with a replication-defective vaccinia virus DIs generates anti-SIV immunity, suggesting that it will be a promising vaccine regimen for HIV-1 vaccine development.     

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.     

Virus-specific cellular immune correlates of survival in vaccinated monkeys after simian immunodeficiency virus challenge

Understanding the characteristics of the virus-specific T-lymphocyte response that will confer optimal protection against the clinical progression of AIDS will inform the development of an effective cellular immunity-based human immunodeficiency virus vaccine. We have recently shown that survival in plasmid DNA-primed/recombinant adenovirus-boosted rhesus monkeys that are challenged with the simian immunodeficiency virus SIVmac251 is associated with the preservation postchallenge of central memory CD4(+) T lymphocytes and robust gamma interferon (IFN-gamma)-producing SIV-specific CD8(+) and CD4(+) T-lymphocyte responses. The present studies were initiated to extend these observations to determine which virus-specific T-lymphocyte subpopulations play a primary role in controlling disease progression and to characterize the functional repertoire of these cells. We show that the preservation of the SIV-specific central memory CD8(+) T-lymphocyte population and a linked SIV-specific CD4(+) T-lymphocyte response are associated with prolonged survival in vaccinated monkeys following challenge. Furthermore, we demonstrate that SIV-specific IFN-gamma-, tumor necrosis factor alpha-, and interleukin-2-producing T lymphocytes are all comparably associated with protection against disease progression. These findings underscore the contribution of virus-specific central memory T lymphocytes to controlling clinical progression in vaccinated individuals following a primate immunodeficiency virus infection.     

CD8+ lymphocytes do not mediate protection against acute superinfection 20 days after vaccination with a live attenuated simian immunodeficiency virus

In order to test the hypothesis that CD8+ cytotoxic T lymphocytes mediate protection against acute superinfection, we depleted >99% of CD8+ lymphocytes in live attenuated simian immunodeficiency virus macC8 (SIVmacC8) vaccinees from the onset of vaccination, maintained that depletion for 20 days, and then challenged with pathogenic, wild-type SIVmacJ5. Vaccinees received 5 mg per kg of humanized anti-CD8 monoclonal antibody (MAb) 1 h before inoculation, followed by the same dose again on days 3, 7, 10, 13, and 17. On day 13, peripheral CD8+ T lymphocytes were >99% depleted in three out of four anti-CD8 MAb-treated vaccinees. At this time attenuated SIVmacC8 viral RNA loads in anti-CD8 MAb-treated vaccinees were significantly higher than control vaccinees treated contemporaneously with nonspecific human immunoglobulin. Lymphoid tissue CD8+ T lymphocyte depletion was >99% in three out of four anti-CD8 MAb-treated vaccinees on the day of wild-type SIVmacJ5 challenge. All four control vaccinees and three out of four anti-CD8 MAb-treated vaccinees were protected against detectable superinfection with wild-type SIVmacJ5. Although superinfection with wild-type SIVmacJ5 was detected at postmortem in a single anti-CD8 MAb-treated vaccinee, this did not correlate with the degree of preceding CD8+ T lymphocyte depletion. Clearance of attenuated SIVmacC8 viremia coincided with recovery of normal CD8+ T lymphocyte counts between days 48 and 76. These results support the view that cytotoxic T lymphocytes are important for host-mediated control of SIV primary viremia but do not indicate a central role in protection against acute superinfection conferred by inoculation with live attenuated SIV.     

Improved vaccine protection from simian AIDS by the addition of nonstructural simian immunodeficiency virus genes

An HIV-1 vaccine able to induce broad CD4+ and CD8+ T cell responses may provide long-term control of viral replication. In this study we directly assess the relative benefit of immunization with vaccines expressing three structural Ags (Gag, Pol, and Env), three early regulatory proteins (Rev, Tat, and Nef), or a complex vaccine expressing all six Ags. The simultaneous administration of all six Ags during vaccination resulted in Ag competition manifested by a relative reduction of CD8+ T cell and lymphoproliferative responses to individual Ags. Despite the Ag competition, vaccination with all six Ags resulted in a delay in the onset and a decrease in the extent of acute viremia after mucosal challenge exposure to highly pathogenic SIV(mac251). Reduced levels of acute viremia correlated with lower post-set point viremia and long-term control of infection. In immunized animals, virus-specific CD4+ T cell and lymphoproliferative responses were preserved during acute viremia, and the maintenance of these responses predicted the long-term virological outcome. Taken together, these results suggest that the breadth of the immune response is probably more important than high frequency responses to a limited number of epitopes. These data provide the first clear evidence of the importance of nonstructural HIV Ags as components of an HIV-1 vaccine.     

Vaccine-induced cellular immune responses reduce plasma viral concentrations after repeated low-dose challenge with pathogenic simian immunodeficiency virus SIVmac239

The goal of an AIDS vaccine regimen designed to induce cellular immune responses should be to reduce the viral set point and preserve memory CD4 lymphocytes. Here we investigated whether vaccine-induced cellular immunity in the absence of any Env-specific antibodies can control viral replication following multiple low-dose challenges with the highly pathogenic SIVmac239 isolate. Eight Mamu-A*01-positive Indian rhesus macaques were vaccinated with simian immunodeficiency virus (SIV) gag, tat, rev, and nef using a DNA prime-adenovirus boost strategy. Peak viremia (P = 0.007) and the chronic phase set point (P = 0.0192) were significantly decreased in the vaccinated cohort, out to 1 year postinfection. Loss of CD4(+) memory populations was also ameliorated in vaccinated animals. Interestingly, only one of the eight vaccinees developed Env-specific neutralizing antibodies after infection. The control observed was significantly improved over that observed in animals vaccinated with SIV gag only. Vaccine-induced cellular immune responses can, therefore, exert a measure of control over replication of the AIDS virus in the complete absence of neutralizing antibody and give us hope that a vaccine designed to induce cellular immune responses might control viral replication.     

Mechanisms of protection induced by live attenuated simian immunodeficiency virus: III. Viral interference and the role of CD8+ T-cells and beta-chemokines in the inhibition of virus infection of PBMCs in vitro

In this study, we investigated whether a type of retroviral interference might be one mechanism that mediates the powerful protection induced by live attenuated SIVC8. Our results show that retroviral interference could be demonstrated between SIV and SHIV-HXBc2 in human T-cell lines chronically infected with either SIVC8 or SIVJ5. Lymphocytes from macaques infected with live attenuated SIVC8 were significantly less sensitive (P < 0.05) to in vitro infection by virulent SIVJ5 and SHIV-HXBc2 than were lymphocytes from naive controls. However, this significant difference in the sensitivity of lymphocytes to virus infection was not observed for more efficiently replicating viruses such as SHIVSF33 and SIVsm3. Virus growth was significantly enhanced (P < 0.01) by depletion of CD8+ T-cells, suggesting a role for these cells in the control of SIV replication, both in vitro and in vivo. We found that levels of the beta-chemokines regulated upon activation, normal T-cell expressed and secreted, macrophage inflammatory protein-1alpha and macrophage inflammatory protein-1beta did not correlate with inhibition of virus replication. Taken together, our findings do not support the hypothesis that retroviral interference is the mechanism by which live attenuated SIVC8 induces protection.