Initiation of antiretroviral therapy 48 hours after infection with simian immunodeficiency virus potently suppresses acute-phase viremia and blocks the massive loss of memory CD4+ T cells but fails to prevent disease

[In vivo protective mechanisms of neutralizing antibodies against simian immunodeficiency virus replicatio]

Identifying protective adaptive immune responses against human immunodeficiency virus type 1 (HIV-1), mainly comprising CD8+ cytotoxic T lymphocyte (CTL) and neutralizing antibody (NAb) responses, is crucial for understanding in vivo mechanisms of viral persistence and developing prophylactic/intervention strategies. In HIV-1 and pathogenic simian immunodeficiency virus (SIV) infections, CTL responses play the canonical role in primary viral replication control, whereas NAb responses are impaired. This NAb impairment in early infection conversely highlights the necessity of elucidating anti-HIV/SIV antibody defense/induction mechanisms, and one approach to analyze the impact of NAbs on HIV/SIV infection is passive immunization. We have analyzed a simian AIDS model of highly pathogenic SIVmac239-infected rhesus macaques, and characterized that a single acute-phase passive infusion of SIV-specific polyclonal NAbs drives a synergistic qualitative boosting of virus-specific T-cell responses, resulting in sustained SIV replication control. This in vivo functional augmentation of virus-specific T cells by NAbs in the SIV model provides insights into the design of protective immunity against HIV-1 infection.

Patterns of HIV/SIV Prevention and Control by Passive Antibody Immunization

Neutralizing antibody (NAb) responses are promising immune effectors for control of human immunodeficiency virus (HIV) infection. Protective activity and mechanisms of immunodeficiency virus-specific NAbs have been increasingly scrutinized in animals infected with simian immunodeficiency virus (SIV), chimeric simian/human immunodeficiency virus (SHIV) and related viruses. Studies on such models have unraveled a previously underscored protective potential against in vivo immunodeficiency virus replication. Pre-challenge NAb titers feasibly provide sterile protection from SIV/SHIV infection by purging the earliest onset of viral replication and likely modulate innate immune cell responses. Sufficient sub-sterile NAb titers after established infection also confer dose-dependent reduction of viremia, and in certain earlier time frames augment adaptive immune cell responses and even provide rebound-free viral control. Here, we provide an overview of the obtained patterns of SIV/SHIV protection and viral control by various types of NAb passive immunizations and discuss how these notions may be extrapolated to NAb-based clinical control of HIV infection.

Rate of AIDS progression is associated with gastrointestinal dysfunction in simian immunodeficiency virus-infected pigtail macaques

During HIV/SIV infection, mucosal immune system dysfunction and systemic immune activation are associated with progression to AIDS; however, it is unclear to what extent pre-existing gastrointestinal damage relates to disease progression postinfection. Pigtail macaques (PTM) are an excellent model in which to assess mucosal dysfunction in relation to HIV/SIV pathogenesis, as the majority of these animals have high levels of gastrointestinal damage, immune activation, and microbial translocation prior to infection, and rapidly progress to AIDS upon SIV infection. In this study, we characterized the mucosal immune environment prior to and throughout SIV infection in 13 uninfected PTM and 9 SIV-infected PTM, of which 3 were slow progressors. This small subset of slow progressors had limited innate immune activation in mucosal tissues in the periphery, which was associated with a more intact colonic epithelial barrier. Furthermore, we found that preinfection levels of microbial translocation, as measured by LPS-binding protein, in PTM correlated with the rate of progression to AIDS. These data suggest that pre-existing levels of microbial translocation and gastrointestinal tract dysfunction may influence the rate of HIV disease progression.

Characterization of peripheral and mucosal immune responses in rhesus macaques on long-term tenofovir and emtricitabine combination antiretroviral therapy

BACKGROUND

The goal of antiretroviral therapy (ART) is to suppress virus replication to limit immune system damage. Some have proposed combining ART with immune therapies to boost antiviral immunity. For this to be successful, ART must not impair physiological immune function.

METHODS

We studied the impact of ART (tenofovir and emtricitabine) on systemic and mucosal immunity in uninfected and simian immunodeficiency (SIV)-infected Chinese rhesus macaques. Subcutaneous ART was initiated 2 weeks after tonsillar inoculation with SIVmac239.

RESULTS

There was no evidence of immune dysregulation as a result of ART in either infected or uninfected animals. Early virus-induced alterations in circulating immune cell populations (decreased central memory T cells and myeloid dendritic cells) were detected, but normalized shortly after ART initiation. ART-treated animals showed marginal SIV-specific T-cell responses during treatment, which increased after ART discontinuation. Elevated expression of CXCL10 in oral, rectal, and blood samples and APOBEC3G mRNA in oral and rectal tissues was observed during acute infection and was down regulated after starting ART. ART did not impact the ability of the animals to respond to tonsillar application of polyICLC with increased CXCL10 expression in oral fluids and CD80 expression on blood myeloid dendritic cells.

CONCLUSION

Early initiation of ART prevented virus-induced damage and did not impede mucosal or systemic immune functions.

The acute HIV infection: implications for intervention, prevention and development of an effective AIDS vaccine

Effective preventive measures against HIV must function near the time of virus transmission to prevent the establishment of a chronic infection. Low-dose SIV/SHIV infections by multiple routes lead to remarkably rapid systemic dissemination of virus and large numbers of infected cells during the initial weeks of the acute infection. Here we describe the narrow time-frame during which potent post-exposure interventions such as anti-retroviral therapy or the administration of high-titered neutralizing antibodies can block the establishment of the in vivo infection. This short window of opportunity is applicable to HIV infections and represents a formidable challenge for developing effective chemoprophylaxis and vaccine approaches.

Prolonged tenofovir treatment of macaques infected with K65R reverse transcriptase mutants of SIV results in the development of antiviral immune responses that control virus replication after drug withdrawal

BACKGROUND

We reported previously that while prolonged tenofovir monotherapy of macaques infected with virulent simian immunodeficiency virus (SIV) resulted invariably in the emergence of viral mutants with reduced in vitro drug susceptibility and a K65R mutation in reverse transcriptase, some animals controlled virus replication for years. Transient CD8+ cell depletion or short-term tenofovir interruption within 1 to 5 years of treatment demonstrated that a combination of CD8+ cell-mediated immune responses and continued tenofovir therapy was required for sustained suppression of viremia. We report here follow-up data on 5 such animals that received tenofovir for 8 to 14 years.

RESULTS

Although one animal had a gradual increase in viremia from 3 years onwards, the other 4 tenofovir-treated animals maintained undetectable viremia with occasional viral blips (≤ 300 RNA copies/ml plasma). When tenofovir was withdrawn after 8 to 10 years from three animals with undetectable viremia, the pattern of occasional episodes of low viremia (≤ 3600 RNA/ml plasma) continued throughout the 10-month follow-up period. These animals had low virus levels in lymphoid tissues, and evidence of multiple SIV-specific immune responses.

CONCLUSION

Under certain conditions (i.e., prolonged antiviral therapy initiated early after infection; viral mutants with reduced drug susceptibility) a virus-host balance characterized by strong immunologic control of virus replication can be achieved. Although further research is needed to translate these findings into clinical applications, these observations provide hope for a functional cure of HIV infection via immunotherapeutic strategies that boost antiviral immunity and reduce the need for continuous antiretroviral therapy.

Reduced inflammation and CD4 loss in acute SHIV infection during oral pre-exposure prophylaxis

BACKGROUND

The impact of pre-exposure prophylaxis (PrEP) with antiretrovirals on breakthrough HIV or SHIV infection is not fully documented. We addressed the hypothesis that SHIV(SF162P3) infection despite active PrEP results in altered early immune parameters, compared with untreated infection.

METHODS

Eleven rhesus macaques were infected during repeated, rectal, low-dose SHIV(SF162P3) exposures while receiving concurrent oral PrEP (Truvada [n = 2] or GS7340 [n = 4]) or as untreated controls (n = 5). We measured SHIV RNA, inflammatory cytokines, CD4 cells, and SHIV-specific and memory T cells until 20 weeks after peak viremia.

RESULTS

SHIV infection during PrEP resulted in 100-fold lower peak viremia and lower IL-15, IL-18, and IL-1Ra levels, compared with controls (P < .05; Wilcoxon rank-sum test). Unlike controls, PrEP-treated macaques showed no significant CD4 cell count reduction during acute infection and developed more SHIV-specific central memory T cells, relative to controls. After in vivo CD8 cell depletion, viral load increased to similar levels, indicating that CD8 cells were critical for viral control in both groups.

CONCLUSIONS

PrEP with antiretrovirals has beneficial effects on early SHIV infection even when infection is not prevented. Although long-term immune control could not be examined in this SHIV infection model, our results suggest that PrEP results in improved early disease parameters in breakthrough infections.

The use of nonhuman primate models of HIV infection for the evaluation of antiviral strategies

Several nonhuman primate models are used in HIV/AIDS research. In contrast to natural host models, infection of macaques with virulent simian immunodeficiency virus (SIV) isolates results in a disease (simian AIDS) that closely resembles HIV infection and AIDS. Although there is no perfect animal model, and each of the available models has its limitations, a carefully designed study allows experimental approaches that are not feasible in humans, but that can provide better insights in disease pathogenesis and proof-of-concept of novel intervention strategies. In the early years of the HIV pandemic, nonhuman primate models played a minor role in the development of antiviral strategies. Since then, a better understanding of the disease and the development of better compounds and assays to monitor antiviral effects have increased the usefulness and relevance of these animal models in the preclinical development of HIV vaccines, microbicides, and antiretroviral drugs. Several strategies that were first discovered to have efficacy in nonhuman primate models are now increasingly used in humans. Recent trends include the use of nonhuman primate models to explore strategies that could reduce viral reservoirs and, ultimately, attempt to cure infection. Ongoing comparison of results obtained in nonhuman primate models with those observed in human studies will lead to further validation and improvement of these animal models so they can continue to advance our scientific knowledge and guide clinical trials.

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.

Early short-term antiretroviral therapy is associated with a reduced prevalence of CD8(+)FoxP3(+) T cells in simian immunodeficiency virus-infected controller rhesus macaques

Regulatory T cells contain a mix of CD4 and CD8 T cell subsets that can suppress immune activation and at the same time suppress immune responses, thereby contributing to disease progression. Recent studies have shown that an increased prevalence of CD8(+)FoxP3(+) T regulatory cells was associated with immune suppression and diminished viral control in simian immunodeficiency virus (SIV)-infected rhesus macaques. Preventing an increase in the prevalence of CD8 T regulatory subsets is likely to lead to a better long-term outcome. Here we show that short-term antiretroviral therapy initiated within 1 week after SIV infection was associated with lower viral set point and immune activation after withdrawal of therapy as compared to untreated animals. Early short-term treated controller animals were found to have better SIV-specific immune responses and a significantly lower prevalence of immunosuppressive CD8(+)FoxP3(+) T cells. Lower levels of CD8(+)FoxP3(+) T cells coincided with preservation of CD4(+)FoxP3(+) T cells at homeostatic levels, and significantly correlated with lower immune activation, suggesting a role for viral infection-driven immune activation in the expansion of CD8(+)FoxP3(+) T cells. Interestingly, initiation of continuous therapy later in infection did not reduce the increased prevalence of CD8(+)FoxP3(+) T cells to homeostatic levels. Taken together, our results suggest that early antiretroviral therapy preserves the integrity of the immune system leading to a lower viral set point in controller animals, and prevents alterations in the homeostatic balance between CD4(+) and CD8(+) T regulatory cells that could aid in better long-term outcome.

Antiretroviral-based HIV prevention strategies for women

Almost three decades have elapsed since researchers identified HIV as the cause of AIDS, with current estimates from UNAIDS that 33.4 million adults were living with HIV/AIDS in 2008. Two-thirds of this burden of disease is in Sub-Saharan Africa, and 60% of those infected are women. The disease still remains incurable and current prevention strategies including abstinence, male/female condom use and male circumcision are only partially effective. New strategies to curb the epidemic are urgently needed. Scientists are diligently exploring HIV prevention methods that are safe, effective and affordable. These new biological interventions include oral pre-exposure prophylaxis using oral antiretroviral (ARV) drugs, ARV treatment in HIV-infected persons to reduce transmission and topical ARV-based microbicide formulations.