Enhanced cellular immune response and reduced CD8(+) lymphocyte apoptosis in acutely SIV-infected Rhesus macaques after short-term antiretroviral treatment

Revisiting a quarter of a century of simian immunodeficiency virus (SIV)-associated cardiovascular diseases at the German Primate Center

Human immunodeficiency virus (HIV) comorbidities have become clinically more important due to antiretroviral therapy. Although therapy increases life expectancy, it does not completely suppress immune activation and its associated complications. The simian immunodeficiency virus (SIV)-infected rhesus macaque (Macaca mulatta) represents a valuable model for the investigation of SIV-associated diseases. Although cardiovascular (CV) changes are common in HIV-infected patients, there are only a few reports on the incidence of CV findings in SIV-infected animals. In addition, potential associations between pathohistological findings and hematological parameters are still unclear.

We therefore conducted a retrospective analysis of 195 SIV-infected rhesus macaques that were euthanized with AIDS-related symptoms at the German Primate Center, Goettingen, over a 25-year period. Pathological findings were correlated with hematological data.

The main findings included myocarditis (12.8 %), endocarditis (9.7 %), and arteriopathy (10.3 %) in various organs. Thrombocytopenia occurred more frequently in macaques with endocarditis or arteriopathy than in macaques without CV disease (80 % in animals with endocarditis, 60 % in animals with arteriopathy, p<0.0001 and p=0.0016, respectively).

Further investigations of the interaction between coagulation markers, proinflammatory cytokines, and biomarkers associated with endothelial dysfunction (e.g., D-dimers) and histological data (vascular wall structure) may unravel the mechanisms underlying HIV/SIV-associated CV comorbidities.

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.

Safety and adherence to intermittent pre-exposure prophylaxis (PrEP) for HIV-1 in African men who have sex with men and female sex workers

BACKGROUND

Little is known about safety of and adherence to intermittent HIV PrEP regimens, which may be more feasible than daily dosing in some settings. We present safety and adherence data from the first trial of an intermittent PrEP regimen among Kenyan men who have sex with men (MSM) and female sex workers (FSW).

METHODS/PRINCIPAL FINDINGS

MSM and FSW were randomized to daily oral FTC/TDF or placebo, or intermittent (Monday, Friday and within 2 hours after sex, not to exceed one dose per day) oral FTC/TDF or placebo in a 2:1:2:1 ratio; volunteers were followed monthly for 4 months. Adherence was assessed with the medication event monitoring system (MEMS). Sexual activity data were collected via daily text message (SMS) queries and timeline followback interviews with a one-month recall period. Sixty-seven men and 5 women were randomized into the study. Safety was similar among all groups. Median MEMS adherence rates were 83% [IQR: 63-92] for daily dosing and 55% [IQR:28-78] for fixed intermittent dosing (p = 0.003), while adherence to any post-coital doses was 26% [IQR:14-50]. SMS response rates were low, which may have impaired measurement of post-coital dosing adherence. Acceptability of PrEP was high, regardless of dosing regimen.

CONCLUSIONS/SIGNIFICANCE

Adherence to intermittent dosing regimens, fixed doses, and in particular coitally-dependent doses, may be more difficult than adherence to daily dosing. However, intermittent dosing may still be appropriate for PrEP if intracellular drug levels, which correlate with prevention of HIV acquisition, can be attained with less than daily dosing and if barriers to adherence can be addressed. Additional drug level data, qualitative data on adherence barriers, and better methods to measure sexual activity are necessary to determine whether adherence to post-coital PrEP could be comparable to more standard regimens.

TRIAL REGISTRATION

ClinicalTrials.gov NCT00971230.

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.

Evaluation of antiretrovirals in animal models of HIV infection

Animal models of HIV infection have played an important role in the development of antiretroviral drugs. Although each animal model has its limitations and never completely mimics HIV infection of humans, a carefully designed study allows experimental approaches that are not feasible in humans, but that can help to better understand disease pathogenesis and to provide proof-of-concept of novel intervention strategies. While rodent and feline models are useful for initial screening, further testing is best done in non-human primate models, such as simian immunodeficiency virus (SIV) infection of macaques, because they share more similarities with HIV infection of humans. In the early years of the HIV pandemic, non-human primate models played a relatively minor role in the antiretroviral drug development process. Since then, a better understanding of the disease and the development of better drugs and assays to monitor antiviral efficacy have increased the usefulness of the animal models. In particular, non-human primate models have provided proof-of-concept for (i) the benefits of chemoprophylaxis and early treatment, (ii) the preclinical efficacy of novel drugs such as tenofovir, (iii) the virulence and clinical significance of drug-resistant viral mutants, and (iv) the role of antiviral immune responses during drug therapy. Ongoing comparison of results obtained in animal models with those observed in human studies will further validate and improve these animal models so they can continue to help advance our scientific knowledge and to guide clinical trials. This article forms part of a special issue of Antiviral Research marking the 25th anniversary of antiretroviral drug discovery and development, Vol 85, issue 1, 2010.

Prolonged survival of vaccinated macaques after oral SIVmac239 challenge regardless of viremia control in the chronic phase

To evaluate the efficacy of a multigenic vaccine and its protective immunity in the SIVmac239 challenge model, 12 rhesus macaques were divided into two groups. The vaccine group was intramuscularly immunized with multigenic DNA and recombinant adenovirus vaccine, while the control group received buffers. At 16 weeks after the last immunization, all macaques were challenged orally with pathogenic SIVmac239. The mean plasma SIV RNA loads of the vaccine group were significantly lower than those of the placebo control group up to 16 weeks post-challenge. The vaccine-induced Gag-specific IFN-gamma ELISPOT T cell responses inversely correlated with the viral loads before the chronic phase. Two out of six vaccinated macaques with strong and sustained Gag-specific T cell responses showed viremia control and maintained CD4+ T cell percentage. However, the other four vaccinated macaques showed high viral loads and reduced level of CD4+ T cell percentages during the chronic phase, comparable to those in control macaques. Five out of six vaccinated macaques survived for more than 72 weeks, while five out of six controls died of an AIDS-related disease. Therefore, the vaccination conferred not only reduction of viral loads in a portion of vaccinated macaques (2/6), but also prolonged survival of all vaccinated macaques regardless of viremia control. Our results further suggest that new experimental approaches may be needed to assess protective effects from AIDS-associated disease in the immunized macaques after oral SIV challenge.

Attenuated disease in SIV-infected macaques treated with a monoclonal antibody against FasL

Acute SIVmac infection in macaques is accompanied by high levels of plasma viremia that decline with the appearance of viral immunity and is a model for acute HIV disease in man. Despite specific immune responses, the virus establishes a chronic, persistent infection. The destruction of CD4+ and CD4- lymphocyte subsets in macaques contributes to viral persistence and suggests the importance of mechanisms for depleting both infected and uninfected (bystander) cells. Bystander cell killing can occur when FasL binds the Fas receptor on activated lymphocytes, which include T and B cell subpopulations that are responding to the infection. Destruction of specific immune cells could be an important mechanism for blunting viral immunity and establishing persistent infection with chronic disease. We inhibited the Fas pathway in vivo with a monoclonal antibody against FasL (RNOK203). Here we show that treatment with anti-FasL reduced cell death in circulating T and B cells, increased CTL and antibody responses to viral proteins, and lowered the setpoint viremia. By blocking FasL during only the first few weeks after infection, we attenuated SIVmac disease and increased the life span for infected and treated macaques.

Immunogenicity of a DNA prime and recombinant adenovirus boost regime significantly varies between rhesus macaques of Chinese and Indian origins

BACKGROUND

Due to an ever increasing shortage of rhesus macaques of Indian origin (InR) that have been generally used for preclinical AIDS vaccine trials in non-human primates, demand is rising for Chinese rhesus macaques (ChR). However, the immunogenicity of an AIDS vaccine candidate has not been compared in parallel in both rhesus macaque subspecies.

METHODS

ChR and InR were immunized with SIV/HIV DNA and adenovirus vaccine and their immune responses to SIV and HIV evaluated.

RESULTS

SIV Gag- and Env-specific T-cell responses and SIV-specific lymphoproliferative responses measured in ChR were significantly weaker than those in InR (P < 0.05). By contrast, antibody responses to SIV Env, Tat, and Nef in ChR were stronger than those in InR (P < 0.05).

CONCLUSIONS

Immunogenicity of an AIDS vaccine can vary significantly depending on the geographic origin implying genetic differences of macaques. This must be considered when describing and interpreting results of such vaccine studies.

Post-exposure prophylaxis for SIV revisited: animal model for HIV prevention

Background

A 4-week, uninterrupted treatment with 9-(2-phosphonyl-methoxypropyly)adenine (PMPA, commonly called tenofovir) completely prevents simian immunodeficiency virus (SIV_mne) infection in cynomolgus macaques if treatment begins within 24 hours after SIV_mne inoculation, but is less effective if treatment is delayed or duration of treatment is shortened. Critical factors for efficacy include timing and duration of treatment, potency of antiretroviral drug and a contribution from antiviral immune responses. Therefore, we evaluated the impact of one or more treatment interruptions plus SIV_mne re-exposures on efficacy of PMPA treatment to prevent SIV_mne infection in cynomolgus macaques. We also evaluated whether macaques with pre-existing SIV immune responses show increased efficacy of treatment. Eight PMPA-treated, virus-negative and seronegative macaques, and five PMPA-treated, virus-negative but weakly or strongly seropositive macaques were re-inoculated with SIV_mne and treated with PMPA starting 24 hr post inoculation. Thereafter, they received either a 5-week treatment involving one interruption plus one SIV_mne challenge or a 10-week treatment involving six interruptions plus six SIV_mne challenges early during treatment. Parameters measured were plasma SIV RNA, SIV-antibody response, CD4+ T lymphocyte subsets and in vivo CD8+ cell-suppression of virus infection.

Results

All seronegative macaques developed persistent antibody response beginning 4 to 8 weeks after stopping PMPA-treatment in absence of viremia in a majority of macaques and coinciding with onset of intermittent viremia in other macaques. In contrast, all weakly or strongly seropositive macaques showed immediate increase in titers (> 1600) of SIV antibodies, even before the end of PMPA-treatment, and in absence of detectable viremia. However, in vivo CD8+ -cell depletion revealed CD8 cell-suppression of viremia and persistence of virus in the macaques as long as 2 years after PMPA-treatment, even in aviremic macaques. Unlike untreated macaques, a treated macaque controlled viral replication and blocked CD4+ T cell depletion when challenged with a heterologus chimeric SIV/HIV-1 virus called SHIV_89.6P.

Conclusion

A single interruption plus one SIV_mne challenge was as sufficient as six interruptions plus six SIV_mne challenges in reducing efficacy of PMPA, but results in long-term persistence of virus infection suppressed by CD8+ cells. Efficacy of PMPA treatment was highest in macaques with pre-existing SIV immune responses.

Sustained Conservation of CD4+T Cells in Multiprotein Triple Modality-Immunized Rhesus Macaques after Intrarectal Challenge with Simian Immunodeficiency Virus

As part of a European multicenter study designed to determine the optimal combination and order of a mixed-modality vaccine against acquired immunodeficiency syndrome, rhesus monkeys received a combination of three different vectors, all expressing the same Simian Immunodeficiency Virus (SIV) genes followed by mucosal challenge with highly pathogenic SIV. In the study reported here, animals were primed with DNA followed by one booster immunization with Semliki Forest Virus (SFV) and two immunizations with modified Vaccinia Ankara (MVA). To address the relevance of mucosal immunization, we compared systemic versus a combination of systemic and mucosal antigen application. Although all vaccinees became infected after intrarectal challenge with SIV, most (six of eight) were protected from profound loss of CD4+ cells. In addition, vaccinees showed lower viral loads than did controls (p < 0.05). Overall, these protective effects were more pronounced in those animals whose schedule included immunization via the mucosa. In summary, the vaccine regimen used here achieved one important criterion of efficacy: the suppression of disease development as indicated by conservation of CD4+ cells.

Structured treatment interruptions with tenofovir monotherapy for simian immunodeficiency virus-infected newborn macaques

We demonstrated previously that prolonged tenofovir treatment of infant macaques, starting early during infection with virulent simian immunodeficiency virus (SIVmac251), can lead to persistently low or undetectable viremia even after the emergence of mutants with reduced in vitro susceptibility to tenofovir as a result of a K65R mutation in reverse transcriptase; this control of viremia was demonstrated to be mediated by the generation of effective antiviral immune responses. To determine whether structured treatment interruptions (STI) can induce similar immunologic control of viremia, eight newborn macaques were infected with highly virulent SIVmac251 and started on a tenofovir STI regimen 5 days later. Treatment was withdrawn permanently at 33 weeks of age. All animals receiving STI fared much better than 22 untreated SIVmac251-infected infant macaques. However, there was a high variability among animals in the viral RNA set point after complete drug withdrawal, and none of the animals was able to achieve long-term immunologic suppression of viremia to persistently low levels. Early immunologic and viral markers in blood (including the detection of the K65R mutation) were not predictive of the viral RNA set point after drug withdrawal. These results, which reflect the complex interactions between drug resistance mutations, viral virulence, and drug- and immune-mediated inhibition of virus replication, highlight the difficulties associated with trying to develop STI regimens with predictable efficacy for clinical practice.

The clinical benefits of tenofovir for simian immunodeficiency virus-infected macaques are larger than predicted by its effects on standard viral and immunologic parameters

Previous studies have demonstrated that tenofovir (9-[2-(phosphonomethoxy)propyl]adenine; PMPA) treatment is usually very effective in suppressing viremia in macaques infected with simian immunodeficiency virus (SIV). The present study focuses on a subset of infant macaques that were chronically infected with highly virulent SIVmac251, and for which prolonged tenofovir treatment failed to significantly suppress viral RNA levels in plasma despite the presence of tenofovirsusceptible virus at the onset of therapy. While untreated animals with similarly high viremia developed fatal immunodeficiency within 3-6 months, these tenofovir-treated animals had significantly improved survival (up to 3.5 years). This clinical benefit occurred even in animals for which tenofovir had little or no effect on CD4 and CD8 lymphocyte counts and antibody responses to SIV and test antigens. Thus, the clinical benefits of tenofovir were larger than predicted by plasma viral RNA levels and other routine laboratory parameters.

Early antiretroviral therapy for simian immunodeficiency virus infection leads to mucosal CD4+ T-cell restoration and enhanced gene expression regulating mucosal repair and regeneration

Simian immunodeficiency virus (SIV) and human immunodeficiency virus (HIV) infections lead to rapid depletion of CD4(+) T cells from gut-associated lymphoid tissue (GALT). Although the administration of antiretroviral therapy (ART) has been shown to increase CD4(+) T-cell levels in the peripheral blood in both SIV and HIV infections, its efficacy in restoring intestinal mucosal CD4(+) T cells has not been well investigated. To gain insights into the molecular mechanisms of virally induced disruptions in the mucosal immune system, we have evaluated longitudinal changes in viral burden, T-cell subsets, and mucosal gene expression profiles in SIV-infected rhesus macaques in the absence or presence of ART. Our results demonstrate a dramatic suppression of mucosal viral loads and rapid reconstitution of CD4(+) T cells in GALT in animals receiving ART that were not observed in untreated SIV-infected animals. DNA microarray-based gene expression profiling indicated that CD4(+) T-cell restoration in GALT was associated with up regulation of growth factors and genes involved in repair and regeneration of the mucosal epithelium. In contrast, untreated SIV-infected animals increased expression of lymphocyte activation and inflammatory response-associated genes and did not up regulate mucosal growth and repair associated transcription. In conclusion, these data indicate that initiating ART in primary SIV infection may lead to the restoration of the mucosal immune system through reduction of inflammation and promotion of epithelial repair in the intestinal mucosa.

Prognostic value of proliferative responses to HIV-1 antigen in chronically HIV-infected patients under antiretroviral therapy

BACKGROUND

In the chronic stage of HIV infection T cell proliferative responses to HIV antigens are rare, mostly of low level, and the influence of responses on antiretroviral therapy is not known.

OBJECTIVES

To determine a potential correlation between HIV-specific proliferative responses and the subsequent course of infection under antiretroviral therapy.

STUDY DESIGN

Proliferation assays were performed with freshly isolated blood mononuclear cells from 45 chronically HIV-infected HAART treated individuals using HIV-p24, other recall antigens, and mitogens as stimulants. Virus load was monitored at the time of stimulation and during 33 months follow-up.

RESULTS

A proliferative response to HIV antigen stimulation was detectable in 7 of 45 patients (15.5% responders). This group showed elevated reactions against tetanus toxoid and tuberculin, whereas reactions against standard mitogens were equal in the HIV responder and nonresponder groups. None of the seven HIV-specific responders had a blood virus load rebound of more than 1000 genome copies/ml during follow-up, whereas in 50% of the non-responders higher virus rebounds occurred. CD4 cell levels were slightly higher in the responder group, but mostly independent of virus rebound within the non-responders. Only four patients with high and continuous virus rebound experienced a significant CD4 cell decline.

CONCLUSIONS

In patients under HAART, HIV-specific proliferative response is frequently related to anamnestic antigen responses and an enduring control of virus replication.

CD8+-cell-mediated suppression of virulent simian immunodeficiency virus during tenofovir treatment

The ability of tenofovir to suppress viremia in simian immunodeficiency virus (SIV)-infected macaques for years despite the presence of virulent viral mutants with reduced in vitro susceptibility is unprecedented in this animal model. In vivo cell depletion experiments demonstrate that tenofovir's ability to suppress viremia during acute and chronic infection is significantly dependent on the presence of CD8+ lymphocytes. Continuous tenofovir treatment was required to maintain low viremia. Although it is unclear whether this immune-mediated suppression of viremia is linked to tenofovir's direct antiviral efficacy or is due to independent immunomodulatory effects, these studies prove the concept that antiviral immune responses can play a crucial role in suppressing viremia during anti-human immunodeficiency virus drug therapy.

Effective postexposure treatment of retrovirus-induced disease with immunostimulatory DNA containing CpG motifs

Therapeutic strategies for the treatment of acute retroviral infections have relied mainly on antiviral drugs. In this study we used the Friend virus model system to demonstrate that enhancement of the immune system can also have dramatic therapeutic effects. Since resistance to Friend virus-induced leukemia in mice is associated with T helper cell type 1 (Th1) immune responses, we enhanced these responses in susceptible mice by treatment with synthetic oligodeoxynucleotides containing unmethylated CpG motifs (CpG-ODN). Treatments begun at 4 days postinfection increased recovery from 6% in the control group to 74% in the CpG-treated group. CpG-mediated recovery was associated with a significant reduction of viral loads in the blood and spleens of treated mice compared to those of control animals. The treatment promoted Th1-type cytokine production by splenocytes of Friend virus-infected mice and augmented Friend virus-specific cytotoxic T-cell responses, but no influence on the virus-specific neutralizing antibody response was observed. Friend virus-specific CD8(+) T cells were critical for effective treatment with CpG-ODN, since in vivo depletion of these cells from treated mice prevented their recovery. Our results demonstrate that CpG-ODN therapy can significantly enhance virus-specific cellular immune responses and prevent retrovirus-induced disease. These findings may have implications for antiviral therapy in general.

Virological and immunological effects of short-course antiretroviral therapy in primary HIV infection

BACKGROUND

National and international guidelines call for the treatment of primary HIV infection (PHI) with combination antiretroviral therapy, although the ideal timing and duration of this intervention is unknown. Recent immunological studies of antiretroviral therapy on small numbers of patients with PHI have reported preservation of HIV-specific CD4 T-helper responses, ordinarily lost in the absence of intervention. We sought to investigate whether a short course of antiretroviral therapy (SCART) at PHI was sufficient to preserve HIV-specific cellular immunity.

METHODS

Forty-five subjects with confirmed PHI were offered SCART at diagnosis. HIV specific cellular immune responses and virological parameters were assessed at monthly intervals.

RESULTS

Thirty-seven of the subjects chose SCART at PHI, and achieved a plasma viral load < 50 RNA copies/ml by a median of 10 weeks (range, 4-32 weeks). Two of the 45 individuals had evidence of genotypic HIV drug resistance at baseline, and none developed new mutations following therapy. All patients who received SCART at PHI showed preservation of HIV-specific CD4 T-helper responses up to 64 weeks off SCART.

CONCLUSION

SCART at PHI was safe, did not induce the development of drug resistance, and appeared sufficient to preserve HIV-specific CD4 T-helper responses. However, PHI is highly heterogeneous, and a large-scale randomized trial of SCART at PHI is now needed.

ALVAC-SIV-gag-pol-env-based vaccination and macaque major histocompatibility complex class I (A*01) delay simian immunodeficiency virus SIVmac-induced immunodeficiency

T-cell-mediated immune effector mechanisms play an important role in the containment of human immunodeficiency virus/simian immunodeficiency virus (HIV/SIV) replication after infection. Both vaccination- and infection-induced T-cell responses are dependent on the host major histocompatibility complex classes I and II (MHC-I and MHC-II) antigens. Here we report that both inherent, host-dependent immune responses to SIVmac251 infection and vaccination-induced immune responses to viral antigens were able to reduce virus replication and/or CD4+ T-cell loss. Both the presence of the MHC-I Mamu-A*01 genotype and vaccination of rhesus macaques with ALVAC-SIV-gag-pol-env (ALVAC-SIV-gpe) contributed to the restriction of SIVmac251 replication during primary infection, preservation of CD4+ T cells, and delayed disease progression following intrarectal challenge exposure of the animals to SIV(mac251 (561)). ALVAC-SIV-gpe immunization induced cytotoxic T-lymphocyte (CTL) responses cumulatively in 67% of the immunized animals. Following viral challenge, a significant secondary virus-specific CD8+ T-cell response was observed in the vaccinated macaques. In the same immunized macaques, a decrease in virus load during primary infection (P = 0.0078) and protection from CD4 loss during both acute and chronic phases of infection (P = 0.0099 and P = 0.03, respectively) were observed. A trend for enhanced survival of the vaccinated macaques was also observed. Neither boosting the ALVAC-SIV-gpe with gp120 immunizations nor administering the vaccine by the combination of mucosal and systemic immunization routes increased significantly the protective effect of the ALVAC-SIV-gpe vaccine. While assessing the role of MHC-I Mamu-A*01 alone in the restriction of viremia following challenge of nonvaccinated animals with other SIV isolates, we observed that the virus load was not significantly lower in Mamu-A*01-positive macaques following intravenous challenge with either SIV(mac251 (561)) or SIV(SME660). However, a significant delay in CD4+ T-cell loss was observed in Mamu-A*01-positive macaques in each group. Of interest, in the case of intravenous or intrarectal challenge with the chimeric SIV/HIV strains SHIV(89.6P) or SHIV(KU2), respectively, MHC-I Mamu-A*01-positive macaques did not significantly restrict primary viremia. The finding of the protective effect of the Mamu-A*01 molecule parallels the protective effect of the B*5701 HLA allele in HIV-1-infected humans and needs to be accounted for in the evaluation of vaccine efficacy against SIV challenge models.

The simian immunodeficiency virus deltaNef vaccine, after application to the tonsils of Rhesus macaques, replicates primarily within CD4(+) T cells and elicits a local perforin-positive CD8(+) T-cell response

Deletion of the nef gene from simian immunodeficiency virus (SIV) strain SIVmac239 yields a virus that undergoes attenuated growth in rhesus macaques and offers substantial protection against a subsequent challenge with some SIV wild-type viruses. We used a recently described model to identify sites in which the SIVDeltanef vaccine strain replicates and elicits immunity in vivo. A high dose of SIVDeltanef was applied to the palatine and lingual tonsils, where it replicated vigorously in this portal of entry at 7 days. Within 2 weeks, the virus had spread and was replicating actively in axillary lymph nodes, primarily in extrafollicular T-cell-rich regions but also in germinal centers. At this time, large numbers of perforin-positive cells, both CD8(+) T cells and CD3-negative presumptive natural killer cells, were found in the tonsil and axillary lymph nodes. The number of infected cells and perforin-positive cells then fell. When autopsy studies were carried out at 26 weeks, only 1 to 3 cells hybridized for viral RNA per section of lymphoid tissue. Nevertheless, infected cells were detected chronically in most lymphoid organs, where the titers of infectious virus could exceed by a log or more the titers in blood. Immunocytochemical labeling at the early active stages of infection showed that cells expressing SIVDeltanef RNA were CD4(+) T lymphocytes. A majority of infected cells were not in the active cell cycle, since 60 to 70% of the RNA-positive cells in tissue sections lacked the Ki-67 cell cycle antigen, and both Ki-67-positive and -negative cells had similar grain counts for viral RNA. Macrophages and dendritic cells, identified with a panel of monoclonal antibodies to these cells, were rarely infected. We conclude that the attenuated growth and protection observed with the SIVDeltanef vaccine strain does not require that the virus shift its characteristic site of replication, the CD4(+) T lymphocyte. In fact, this immunodeficiency virus can replicate actively in CD4(+) T cells prior to being contained by the host, at least in part by a strong killer cell response that is generated acutely in the infected lymph nodes.

Efficient class I major histocompatibility complex down-regulation by simian immunodeficiency virus Nef is associated with a strong selective advantage in infected rhesus macaques

Substitution of Y223F disrupts the ability of simian immunodeficiency virus (SIV) Nef to down-modulate major histocompatibility complex (MHC) class I from the cell surface but has no effect on other Nef functions, such as down-regulation of CD4, CD28, and CD3 cell surface expression or stimulation of viral replication and enhancement of virion infectivity. Inoculation of three rhesus macaques with the SIVmac239 Y223F-Nef variant revealed that this point mutation consistently reverts and that Nef activity in MHC class I down-modulation is fully restored within 4 weeks after infection. Our results demonstrate a strong selective pressure for a tyrosine at amino acid position 223 in SIV Nef, and they constitute evidence that Nef-mediated MHC class I down-regulation provides a selective advantage for viral replication in vivo.

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.