Antiretroviral therapy during primary immunodeficiency virus infection can induce persistent suppression of virus load and protection from heterologous challenge in rhesus macaques

Tribute to John C. Martin at the Twentieth Anniversary of the Breakthrough of Tenofovir in the Treatment of HIV Infections

At Bristol-Myers (BM) (1985–1990), John C. Martin started his HIV career with directing the clinical development of didanosine (ddI) and stavudine (d4T). During this period, he became aware of the acyclic nucleoside phosphonates (ANPs), such as (S)-HPMPA and PMEA, as potential antiviral drugs. Under his impulse, BM got involved in the evaluation of these ANPs, but the merger of BM with Squibb (to become BMS) incited John to leave BM and join Gilead Sciences, and the portfolio of the ANPs followed the transition. At Gilead, John succeeded in obtaining the approval from the US FDA for the use of cidofovir in the treatment of cytomegalovirus (CMV) retinitis in AIDS patients, which was reminiscent of John’s first experience with ganciclovir (at Syntex) as an anti-CMV agent. At Gilead, John would then engineer the development of tenofovir, first as TDF (tenofovir disoproxil fumarate) and then as TAF (tenofovir alafenamide) and various combinations thereof, for the treatment of HIV infections (i), TDF and TAF for the treatment of hepatitis B (HBV) infections (ii), and TDF and TAF in combination with emtricitabine for the prophylaxis of HIV infections (iii).

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.

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.

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.

Topical nonnucleoside reverse transcriptase inhibitor MC 1220 partially prevents vaginal RT-SHIV infection of macaques

The availability of an effective vaginal microbicide would be a major step toward containment of HIV transmission as well as allowing women self-protection against HIV infection. Here we evaluated the efficacy of vaginal application of the potent nonnucleoside reverse transcriptase inhibitor (NNRTI) MC 1220 against vaginal challenge of macaques with RT-SHIV, a chimeric simian immunodeficiency virus (SIV) containing the reverse transcriptase (RT) gene of HIV-1. Challenge infection of monkeys with RT-SHIV currently represents the only nonhuman primate model available to test the anti-HIV-1 effects of NNRTIs. Two different gel formulations containing different MC 1220 concentrations were evaluated for efficacy in female rhesus macaques exposed to RT-SHIV. Five groups of five animals each were treated with two different gel compositions containing no drug, 0.1% or 0.5% MC 1220, followed by vaginal RT-SHIV challenge 30 min later. One animal in each group treated with the low concentration of MC 1220 as well as one control animal remained uninfected after vaginal challenge. By contrast, three of the animals receiving 0.5% MC 1220 remained uninfected, suggesting a threshold of the drug. Despite being negative for plasma viral RNA and absence of seroconversion, almost all uninfected animals exhibited SIV-specific T cells, either in the periphery or in lymph nodes draining the portal of virus entry. Our results make MC 1220 a promising compound for further development as a topical microbicide and warrant additional testing with improved formulation, long-lasting vaginal delivery systems, or even combinations with other inhibitors.

Youth-specific considerations in the development of preexposure prophylaxis, microbicide, and vaccine research trials

Preventing HIV infection in adolescents and young adults will require a multimodal targeted approach, including individual-directed behavioral risk reduction, community-level structural change, and biomedical interventions to prevent sexual transmission. Trials testing biomedical interventions to prevent HIV transmission will require special attention in this population due to the unique psychosocial and physiologic characteristics that differentiate them from older populations. For example, microbicide research will need to consider acceptability, dosing requirements, and coinfection rates that are unique to this population. Preexposure prophylaxis studies also will need to consider potential unique psychosocial issues such as sexual disinhibition and acceptability as well as unique pharmacokinetic parameters of antiretroviral agents. Vaccine trials also face unique issues with this population, including attitudes toward vaccines, risks related to false-positive HIV tests related to vaccine, and different immune responses based on more robust immunity. In this article, we will discuss issues around implementing each of these biomedical prevention modalities in trials among adolescents and young adults to help to guide future successful research targeting this population.

Macaca fascicularis are highly susceptible to an RT-SHIV following intravaginal inoculation: a new model for microbicide evaluation

BACKGROUND

Human immunodeficiency virus type 1 (HIV-1) reverse transcriptase (RT) is a major target for antiretroviral strategy to block or curtail HIV infection. A suitable RT-SHIV/macaque model is urgently needed for the evaluation of HIV/AIDS therapies and microbicides specifically targeting HIV-1 RT.

METHODS

Fifteen cynomolgus macaques (Macaca fascicularis) were divided into three groups (n = 5) and intravaginally inoculated with 4800, 1200, or 300 TCID(50) of RT-SHIVtc. Systemic infections of RT-SHIVtc exposed macaques were determined by both virological and immunologic parameters during 24 weeks post-challenge.

RESULTS

Within 2 weeks post-inoculation, 13 of 15 macaques became infected as confirmed by virus isolation, plasma viral RNA, proviral DNA, declined CD4(+)T cell counts in peripheral blood and seroconversion.

CONCLUSIONS

Results serve to validate the infectivity and pathogenicity of RT-SHIVtc following vaginal exposure in M. fascicularis. This RT-SHIVtc/macaque model could be suitable for the pre-clinical evaluation of non-nucleoside RT inhibitor-based anti-HIV microbicides.

RT-SHIV, an infectious CCR5-tropic chimeric virus suitable for evaluating HIV reverse transcriptase inhibitors in macaque models

BACKGROUND

Non-nucleoside reverse transcriptase inhibitors (NNRTIs) are an important category of drugs for both chemotherapy and prevention of human immunodeficiency virus type 1 (HIV-1) infection. However, current non-human primate (NHP) models utilizing simian immunodeficiency virus (SIV) or commonly used chimeric SHIV (SIV expressing HIV-1 envelope) are inadequate due to the insensitivity to NNRTIs. To develop a NHP model for evaluation of NNRTI compounds, we characterized a RT-SHIV virus that was assembled by replacing the SIV mac239 reverse transcriptase (RT) with that of HIV-1HXB2. Since RT-SHIV exhibited in vitro characteristics of high infectivity, CCR5-usage, and sensitivity to HIV-1 specific NNRTIs, this virus was thought to be suitable for mucosal transmission and then was used to carry out a vaginal transmission study in pigtail macaques (Macaca nemestrina).

RESULTS

RT-SHIV exhibited in vitro characteristics of an infectious CCR5-tropic chimeric virus. This virus was not only highly sensitive to HIV-1 RT specific NNRTIs; its replication was also inhibited by a variety of NRTIs and protease inhibitors. For in vivo vaginal transmission studies, macaques were either pretreated with a single dose of DMPA (depot medroxyprogesterone acetate) or left untreated before intravaginal inoculation with 500 or 1,000 TCID50 of RT-SHIV. All macaques became systemically infected by 2 or 3 weeks post-inoculation exhibiting persistent high viremia, marked CD4+T cell depletion, and antiviral antibody response. DMPA-pretreated macaques showed a higher mean plasma viral load after the acute infection stage, highly variable antiviral antibody response, and a higher incidence of AIDS-like disease as compared with macaques without DMPA pretreatment.

CONCLUSION

This chimeric RT-SHIV has exhibited productive replication in both macaque and human PBMCs, predominantly CCR5-coreceptor usage for viral entry, and sensitivity to NNRTIs as well as other anti-HIV compounds. This study demonstrates rapid systemic infection in macaques following intravaginal exposure to RT-SHIV. This RT-SHIV/macaque model could be useful for evaluation of NNRTI-based therapies, microbicides, or other preventive strategies.

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.

Potential impact of antiretroviral chemoprophylaxis on HIV-1 transmission in resource-limited settings

Background

The potential impact of pre-exposure chemoprophylaxis (PrEP) on heterosexual transmission of HIV-1 infection in resource-limited settings is uncertain.

Methodology/Principle Findings

A deterministic mathematical model was used to simulate the effects of antiretroviral PrEP on an HIV-1 epidemic in sub-Saharan Africa under different scenarios (optimistic, neutral and pessimistic) both with and without sexual disinhibition. Sensitivity analyses were used to evaluate the effect of uncertainty in input parameters on model output and included calculation of partial rank correlations and standardized rank regressions. In the scenario without sexual disinhibition after PrEP initiation, key parameters influencing infections prevented were effectiveness of PrEP (partial rank correlation coefficient (PRCC) = 0.94), PrEP discontinuation rate (PRCC = −0.94), level of coverage (PRCC = 0.92), and time to achieve target coverage (PRCC = −0.82). In the scenario with sexual disinhibition, PrEP effectiveness and the extent of sexual disinhibition had the greatest impact on prevention. An optimistic scenario of PrEP with 90% effectiveness and 75% coverage of the general population predicted a 74% decline in cumulative HIV-1 infections after 10 years, and a 28.8% decline with PrEP targeted to the highest risk groups (16% of the population). Even with a 100% increase in at-risk behavior from sexual disinhibition, a beneficial effect (23.4%–62.7% decrease in infections) was seen with 90% effective PrEP across a broad range of coverage (25%–75%). Similar disinhibition led to a rise in infections with lower effectiveness of PrEP (≤50%).

Conclusions/Significance

Mathematical modeling supports the potential public health benefit of PrEP. Approximately 2.7 to 3.2 million new HIV-1 infections could be averted in southern sub-Saharan Africa over 10 years by targeting PrEP (having 90% effectiveness) to those at highest behavioral risk and by preventing sexual disinhibition. This benefit could be lost, however, by sexual disinhibition and by high PrEP discontinuation, especially with lower PrEP effectiveness (≤50%).

Suppression of viremia and evolution of human immunodeficiency virus type 1 drug resistance in a macaque model for antiretroviral therapy

Antiretroviral therapy (ART) in human immunodeficiency virus type 1 (HIV-1)-infected patients does not clear the infection and can select for drug resistance over time. Not only is drug-resistant HIV-1 a concern for infected individuals on continual therapy, but it is an emerging problem in resource-limited settings where, in efforts to stem mother-to-child-transmission of HIV-1, transient nonnucleoside reverse transcriptase inhibitor (NNRTI) therapy given during labor can select for NNRTI resistance in both mother and child. Questions of HIV-1 persistence and drug resistance are highly amenable to exploration within animals models, where therapy manipulation is less constrained. We examined a pigtail macaque infection model responsive to anti-HIV-1 therapy to study the development of resistance. Pigtail macaques were infected with a pathogenic simian immunodeficiency virus encoding HIV-1 reverse transcriptase (RT-SHIV) to examine the impact of prior exposure to a NNRTI on subsequent ART comprised of a NNRTI and two nucleoside RT inhibitors. K103N resistance-conferring mutations in RT rapidly accumulated in 2/3 infected animals after NNRTI monotherapy and contributed to virologic failure during ART in 1/3 animals. By contrast, ART effectively suppressed RT-SHIV in 5/6 animals. These data indicate that suboptimal therapy facilitates HIV-1 drug resistance and suggest that this model can be used to investigate persisting viral reservoirs.

Therapeutic immunization with Modified Vaccinia Virus Ankara (MVA) vaccines in SIV-infected rhesus monkeys undergoing antiretroviral therapy

BACKGROUND

The long-term benefits of highly active antiretroviral therapy in HIV-infected patients are limited by emergence of drug-resistant variants and side effects. Therefore, we studied the concept of therapeutic immunization in 18 rhesus monkeys infected with a highly pathogenic simian immunodeficiency virus (SIV) swarm.

METHODS

Monkeys were treated with the reverse transcriptase inhibitor (R)-9-(2-phosphonylmethoxypropyl)adenine (PMPA) for 19 weeks starting 10 days after infection. After suppression of viremia, one group of monkeys was immunized with recombinant modified vaccinia virus Ankara (MVA) vectors expressing gag-pol and env. A second group received MVA vectors expressing the regulatory genes tat, rev and nef, while a third group was not immunized.

RESULTS

Immunization with gag-pol and env expressing MVA enhanced SIV antibody titers. Following discontinuation of PMPA treatment, a rebound in viral load was observed. However, in three of six monkeys immunized with MVA gag-pol and MVA env, and two of six monkeys immunized MVA expressing regulatory genes set point RNA levels were below or close to a threshold level of 10(4) RNA copies/ml, while only one of six unvaccinated monkeys maintained such low RNA levels.

CONCLUSIONS

Although a subset of animals seem to benefit from therapeutic immunization with MVA vectors, the difference in set point RNA levels between the groups did not reach statistical significance.

Sequential emergence and clinical implications of viral mutants with K70E and K65R mutation in reverse transcriptase during prolonged tenofovir monotherapy in rhesus macaques with chronic RT-SHIV infection

BACKGROUND

We reported previously on the emergence and clinical implications of simian immunodeficiency virus (SIVmac251) mutants with a K65R mutation in reverse transcriptase (RT), and the role of CD8+ cell-mediated immune responses in suppressing viremia during tenofovir therapy. Because of significant sequence differences between SIV and HIV-1 RT that affect drug susceptibilities and mutational patterns, it is unclear to what extent findings with SIV can be extrapolated to HIV-1 RT. Accordingly, to model HIV-1 RT responses, 12 macaques were inoculated with RT-SHIV, a chimeric SIV containing HIV-1 RT, and started on prolonged tenofovir therapy 5 months later.

RESULTS

The early virologic response to tenofovir correlated with baseline viral RNA levels and expression of the MHC class I allele Mamu-A*01. For all animals, sensitive real-time PCR assays detected the transient emergence of K70E RT mutants within 4 weeks of therapy, which were then replaced by K65R mutants within 12 weeks of therapy. For most animals, the occurrence of these mutations preceded a partial rebound of plasma viremia to levels that remained on average 10-fold below baseline values. One animal eventually suppressed K65R viremia to undetectable levels for more than 4 years; sequential experiments using CD8+ cell depletion and tenofovir interruption demonstrated that both CD8+ cells and continued tenofovir therapy were required for sustained suppression of viremia.

CONCLUSION

This is the first evidence that tenofovir therapy can select directly for K70E viral mutants in vivo. The observations on the clinical implications of the K65R RT-SHIV mutants were consistent with those of SIVmac251, and suggest that for persons infected with K65R HIV-1 both immune-mediated and drug-dependent antiviral activities play a role in controlling viremia. These findings suggest also that even in the presence of K65R virus, continuation of tenofovir treatment as part of HAART may be beneficial, particularly when assisted by antiviral immune responses.

Tenofovir treatment augments anti-viral immunity against drug-resistant SIV challenge in chronically infected rhesus macaques

BACKGROUND

Emergence of drug-resistant strains of human immunodeficiency virus type 1 (HIV-1) is a major obstacle to successful antiretroviral therapy (ART) in HIV-infected patients. Whether antiviral immunity can augment ART by suppressing replication of drug-resistant HIV-1 in humans is not well understood, but can be explored in non-human primates infected with simian immunodeficiency virus (SIV). Rhesus macaques infected with live, attenuated SIV develop robust SIV-specific immune responses but remain viremic, often at low levels, for periods of months to years, thus providing a model in which to evaluate the contribution of antiviral immunity to drug efficacy. To investigate the extent to which SIV-specific immune responses augment suppression of drug-resistant SIV, rhesus macaques infected with live, attenuated SIVmac239Deltanef were treated with the reverse transcriptase (RT) inhibitor tenofovir, and then challenged with pathogenic SIVmac055, which has a five-fold reduced sensitivity to tenofovir.

RESULTS

Replication of SIVmac055 was detected in untreated macaques infected with SIVmac239Deltanef, and in tenofovir-treated, naïve control macaques. The majority of macaques infected with SIVmac055 experienced high levels of plasma viremia, rapid CD4+ T cell loss and clinical disease progression. By comparison, macaques infected with SIVmac239Deltanef and treated with tenofovir showed no evidence of replicating SIVmac055 in plasma using allele-specific real-time PCR assays with a limit of sensitivity of 50 SIV RNA copies/ml plasma. These animals remained clinically healthy with stable CD4+ T cell counts during three years of follow-up. Both the tenofovir-treated and untreated macaques infected with SIVmac239Deltanef had antibody responses to SIV gp130 and p27 antigens and SIV-specific CD8+ T cell responses prior to SIVmac055 challenge, but only those animals receiving concurrent treatment with tenofovir resisted infection with SIVmac055.

CONCLUSION

These results support the concept that anti-viral immunity acts synergistically with ART to augment drug efficacy by suppressing replication of viral variants with reduced drug sensitivity. Treatment strategies that seek to combine immunotherapeutic intervention as an adjunct to antiretroviral drugs may therefore confer added benefit by controlling replication of HIV-1, and reducing the likelihood of treatment failure due to the emergence of drug-resistant virus, thereby preserving treatment options.

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.

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.

SIV-specific T lymphocyte responses in PBMC and lymphoid tissues of SIV-infected pigtailed macaques during suppressive combination antiretroviral therapy

There is currently no SIV macaque model in which the effects of combination antiretroviral therapy on tissue immune responses and latent reservoirs have been measured. This study was performed to define the impact of combination therapy on the specificity and distribution of the T lymphocyte response in multiple tissue compartments. Pigtailed macaques (Macaca nemestrina) were infected with SIV/17E-Fr and treated with combination antiretroviral therapy consisting of 9-R-(2-phosphonomethoxypropyl)adenine (PMPA) and beta-2',3'-dideoxy-3'-thia-5-fluorocytidine (FTC). The SIV-specific T lymphocyte response was measured in peripheral blood, spleen and several lymph nodes at necropsy by IFN-gamma Elispot analysis. Two animals (one treated, one untreated) had high acute peak viremia, which was associated with lower SIV-specific T lymphocyte responses in the peripheral blood and lymphoid tissues. In the treated animal, viremia was controlled to low or undetectable for the study duration, and virus-specific responses remained low. The untreated animal remained viremic throughout the study and developed clinical symptoms of AIDS. In contrast, the two animals that had lower acute peak viremia (one treated, one untreated) had more robust T lymphocyte responses, and controlled viral replication. Virus-specific responses were detected in the treated animal despite 6 months of suppressive therapy. These data suggest that in this model, in the context of acute peak viremia and weak T cell responses, combination therapy may be essential to control virus replication and disease progression. Conversely, in the setting of low initial viremia and robust T lymphocyte responses, treatment does not have a detrimental effect on the immune response.

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.

Tenofovir resistance and resensitization

Human immunodeficiency viruses in 321 samples from tenofovir-naïve patients were retrospectively evaluated for resistance to this nucleotide analogue. All virus strains with insertions between amino acids 67 and 70 of the reverse transcriptase (n = 6) were highly resistant. Virus strains with the Q151M mutation were divided into susceptible (n = 12) and highly resistant (n = 8) viruses. This difference was due to the absence or presence of the K65R mutation, which was confirmed by site-directed mutagenesis. Viral clones with various combinations of the mutations M41L, K70R, L210W, and T215F or T215Y were analyzed for cross-resistance induced by thymidine analogue mutations (TAMs). The levels of increased resistance induced by single, double, and triple mutations at the indicated positions could be ranked as follows: for mutants with single mutations, mutations at positions 41 > 215 > 70; for mutants with double mutations, mutations at positions 41 and 215 > 70 and 215 = 210 and 215 > 41 and 70; for mutants with triple mutations, mutations at positions 41, 210, and 215 > 41, 70, and 215. Viral clones with M184V or M184I exhibited slightly increased susceptibilities to tenofovir (0.7-fold). Almost all clones with TAM-induced resistance were resensitized when M184V was present (P < 0.001). Among the viruses in the clinical samples, the rate of tenofovir resistance significantly increased with the number of TAMs both in the samples with 184M and in those with 184V (P = 0.005 and P = 0.003, respectively). A resensitizing effect of M184V was confirmed for all samples exhibiting at least one TAM (P = 0.03). However, accumulation of at least two TAMs resulted in more than 2.0-fold reduced susceptibility to tenofovir, irrespective of the presence of M184V. Decision tree building, a classical machine learning technique, was used to generate models for the interpretation of mutations with respect to tenofovir resistance. The application of previously proposed cutoffs for a reduced response to therapy and treatment failure demonstrated the central roles of positions 215 and 65 for 1.5- and 4.0-fold reduced susceptibilities, respectively. Thus, clinically relevant resistance may be conferred by the accumulation of TAMs, and the resensitizing effect of M184V should be considered only minor.

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.

Kinetics of lymphocyte proliferation during primary immune response in macaques infected with pathogenic simian immunodeficiency virus SIVmac251: preliminary report of the effect of early antiviral therapy

The aim of this study was to evaluate the kinetics of lymphocyte proliferation during primary infection of macaques with pathogenic simian immunodeficiency virus (SIV) and to study the impact of short-term postexposure highly active antiretroviral therapy (HAART) prophylaxis. Twelve macaques were infected by intravenous route with SIVmac251 and given treatment for 28 days starting 4 h postexposure. Group 1 received a placebo, and groups 2 and 3 received combinations of zidovudine (AZT), lamivudine (3TC), and indinavir. Macaques in group 2 received AZT (4.5 mg/kg of body weight), 3TC (2.5 mg/kg), and indinavir (20 mg/kg) twice per day by the oral route whereas macaques in group 3 were given AZT (4.5 mg/kg) and 3TC (2.5 mg/kg) subcutaneously twice per day, to improve the pharmacokinetic action of these drugs, and a higher dose of indinavir (60 mg/kg). The kinetics of lymphocyte proliferation were analyzed by monitoring 5-bromo-2'-deoxyuridine (BrdU) uptake ex vivo and by fluorescence-activated cell sorting analysis. HAART did not protect against SIV infection but did strongly impact on virus loads: viremia was delayed and lowered during antiviral therapy in group 2, with better control after treatment was stopped, and in group 3, viremia was maintained at lower levels during treatment, with virus even undetectable in the blood of some macaques, but there was no evidence of improved control of the virus after treatment. We provide direct evidence that dividing NK cells are detected earlier than dividing T cells in the blood (mostly in CD45RA(-) T cells), mirroring plasma viremia. Dividing CD8(+) T cells were detected earlier than dividing CD4(+) T cells, and the highest percentages of proliferating T cells coincided with the first evidence of partial control of peak viremia and with an increase in the percentage of circulating gamma interferon-positive CD8(+) T cells. The level of cell proliferation in the blood during SIV primary infection was clearly associated with viral replication levels because the inhibition of viral replication by postexposure HAART strongly reduced lymphocyte proliferation. The results and conclusions in this study are based on experiments in a small numbers of animals and are thus preliminary.

Transient early post-inoculation anti-retroviral treatment facilitates controlled infection with sparing of CD4+ T cells in gut-associated lymphoid tissues in SIVmac239-infected rhesus macaques, but not resistance to rechallenge

Like human immunodeficiency virus infection of humans, infection of rhesus macaques with pathogenic simian immunodeficiency virus (SIV) strains typically results in persistent progressive infection, leading to clinically significant immunosuppression. In previous studies, we administered short term anti-retroviral treatment, shortly after intravenous inoculation with SIVsmE660, in an effort to allow immunologic sensitization under conditions not characterized by overwhelming cytopathic infection compromising the developing immune response. We showed that such treatment allowed control of off treatment viremia and was associated with resistance to rechallenge. Control of off treatment viremia was associated, at least in part, with CD8+ lymphocytes, based on in vivo CD8 depletion studies. In the present study, six rhesus macaques were infected intravenously with 100 MID50 of SIVmac239; four then received 30 days of treatment with tenofovir 9-[2-(R)-(phosphonomethoxy)propyl]adenine (PMPA); 20-30 mg/kg, subcutaneously) starting 24 hours post-inoculation. Tenofovir-treated animals showed low (<500 copy Eq/ml) or undetectable (<100 copy Eq/ml) plasma SIV RNA levels during treatment, with undetectable plasma viremia following discontinuation of treatment. Plasma SIV RNA remained <100 copy Eq/ml, even after depletion of CD8+ lymphocytes, 6 weeks after discontinuation of tenofovir treatment. In contrast to untreated infected control animals that showed substantial depletion of CD4+ T cells from gut-associated lymphoid tissues (GALT), tenofovir-treated animals showed sparing of GALT CD4+ T cells both during the treatment period and in the off treatment follow-up period. However, in contrast to earlier results with animals infected with SIVsmE660, in the present study, the animals did not develop readily measurable cellular anti-SIV immune responses, and did not resist homologous rechallenge with SIVmac239, administered 44 weeks after the initial infection. Differences in the animals and virus strains employed may in part account for the differences in results observed. Comparative analysis of virologic and immunologic parameters in this model system may provide important insights for understanding the basis of effective immunologic control of SIV infection.

The critical role of CD4(+) T-cell help in immunity to HIV

Vaccine-induced immunity to HIV/AIDS is a world wide health priority and a necessity in order to prevent or curb the transmission of this infection in the different human populations at risk. Failing to prevent infection, it would be desirable to generate sufficient immunity to control viremia in individuals which become infected, given that this would provide sufficient protection to prevent progression to AIDS. From several different pre-clinical settings data revealed that although CTL or neutralising antibodies were necessary immune responses for protection from infection, they were alone or together insufficient for providing solid protective immunity. What was invariably necessary was a strong specific CD4(+) T-cell response. Protective T-helper responses were not skewed towards an IFN-gamma (Th1) or IL-4 (Th2) type response, but were balanced and characterised by the presence of a strong Ag-specific IL-2 response.

Phenotypic susceptibilities to tenofovir in a large panel of clinically derived human immunodeficiency virus type 1 isolates

Tenofovir is a nucleotide analogue human immunodeficiency virus type 1 (HIV-1) reverse transcriptase (RT) inhibitor, and its oral prodrug, tenofovir disoproxil fumarate, has recently been approved for the treatment of HIV-1 infection in the United States. The objective of this study was to characterize the in vitro susceptibility profiles of a large panel of clinically derived HIV-1 isolates for tenofovir. The distribution of tenofovir susceptibilities in over 1,000 antiretroviral-naive, HIV-1-infected individuals worldwide was determined using the Virco Antivirogram assay. In addition, phenotypic susceptibilities to tenofovir and other RT inhibitors were determined in a panel of nearly 5,000 recombinant HIV-1 clinical isolates from predominantly treatment-experienced patients analyzed as a part of routine drug resistance testing. Greater than 97.5% of isolates from treatment-naive patients had tenofovir susceptibilities <3-fold above those of the wild-type controls by the Antivirogram. The clinically derived panel of 5,000 samples exhibited a broad range of antiretroviral drug susceptibilities, including 69, 43, and 16% having >10-fold-decreased susceptibilities to at least one, two, and three antiretroviral drug classes, respectively. Greater than 88% of these 5,000 clinical isolates were within the threefold susceptibility range for tenofovir, and >99% exhibited <10-fold-reduced susceptibilities to tenofovir. Decreased susceptibility to tenofovir was not directly associated with resistance to other RT inhibitors; r(2) values of log-log linear regression plots of susceptibility to tenofovir versus susceptibility to other RT inhibitors were <0.4. The results suggest that the majority of treatment-naive and treatment-experienced individuals harbor HIV that remains within the normal range of tenofovir susceptibilities and may be susceptible to tenofovir disoproxil fumarate therapy.

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.

Real-time TaqMan PCR as a specific and more sensitive alternative to the branched-chain DNA assay for quantitation of simian immunodeficiency virus RNA

We developed a rapid and highly reproducible assay based on real-time PCR (TaqMan, Applied Biosystems, Foster City, CA) to quantitate simian immunodeficiency virus (SIV) RNA in plasma samples. This assay was compared with the current branched-chain DNA assay (Bayer, Emeryville, CA). Results obtained with the real-time TaqMan PCR assay were comparable to those obtained with the branched-chain DNA assay in overlapping ranges of sensitivities (r = 0.9429, p < 0.05). However, the real-time TaqMan PCR assay was capable of detecting as few as 50 copies of RNA/ml, whereas branched-chain DNA was only sensitive to 1,500 copies of RNA/ml. Therefore, several animals that tested negative by branched-chain DNA were positive by realtime TaqMan PCR. Two false positive tests were also recorded for the branched-chain DNA test. False negative and positive tests were confirmed by cell culture isolation and conventional nested RT-PCR. The SIV TaqMan assay detected a wide range of wild-type, cloned, and recombinant SIV strains with similar amplification efficiency, including SIVmac251, SIVmac239, SIVmac239 containing the 184V mutation in RT, SIV1A11, SIVmac239 delta3, SIVmac-M4, and chimeras (SHIVs) containing specific HIV-1 genes, such as reverse transcriptase (RT-SHIV) or Env (SHIV-E). In conclusion, the high sensitivity, increased specificity, wide dynamic range, simplicity, and reproducibility of the real-time SIV RNA quantitation allow the screening of large numbers of samples and make this method especially suitable for measuring both viral DNA and RNA levels during vaccine and therapy studies.

Development of virus-specific immune responses in SHIV(KU)-infected macaques treated with PMPA

Therapeutic intervention with highly active antiretroviral therapy (HAART) can lead to the suppression of HIV viremia below the threshold of detection for several years. However, impact of HAART on reconstitution of virus-specific immune responses remains poorly understood. In this study, four macaques were infected with pathogenic SHIV(KU). One week postinoculation two of the four animals were treated with PMPA [9-R-(2-phosphophomethoxypropyl)adenine] daily for 83 days. Two other macaques, that did not receive treatment, exhibited explosive virus replication accompanied by a near total loss of CD4(+) T cells and succumbed to AIDS-related complications within 6 months of infection. These animals did not develop any virus-specific immune responses. On the contrary, the animals that received PMPA showed transient loss of CD4(+) T cells that recovered during the treatment period. The virus burden declined below the level of detection that rebounded soon after cessation of PMPA therapy. The virus replicated productively for several weeks before both animals controlled the productive replication of virus. This control of virus replication was found to be associated with the development of virus-specific neutralizing antibodies, T-helper cells, and CTLs. Although PMPA did not eliminate virus from the animals, it provided them with enough time to mount virus-specific immune responses that eventually controlled the virus replication in the blood. Our results suggest that antiretroviral therapy, if initiated early during infection, would help the host in mounting virus-specific immune responses that might control productive replication of the virus.

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

Losing the decisive virus-specific functions of both CD4(+) and CD8(+) T lymphocytes in the first weeks after immunodeficiency virus infection ultimately leads to AIDS. The SIV/rhesus monkey model for AIDS was used to demonstrate that a 4-week chemotherapeutic reduction of viral load during acute SIV infection of macaques allowed the development of a competent immune response able to control virus replication after discontinuation of treatment in two of five monkeys. Increasing SIV-specific CD4(+) T-helper-cell proliferation was found in all macaques several weeks after treatment, independent of their viral load. However, only macaques with low viral loads showed persistent T-cell reactivity of lymph node cells. In contrast to animals with higher viral loads, T-helper-cell counts and memory T-helper cells did not decline in the two macaques controlling viral replication. Lymphocyte apoptosis was consistently low in all treated macaques. In contrast, high CD8(+) lymphocyte death but only slightly increased CD4(+) lymphocyte apoptosis were observed during the first weeks after infection in untreated control animals, indicating that early apoptotic death of virus-specific CTL could be an important factor for disease development. Antiretroviral treatment early after infection obviously retained virus-specific and competent T lymphocytes, whereby a virus-specific immune response could develop in two animals able to control the viral replication after cessation of treatment.

Control of SIV rebound through structured treatment interruptions during early infection

In a randomized controlled trial with acute simian immunodeficiency virus (SIV)-infected macaques, both highly active antiretroviral therapy (HAART) and HAART with fixed-schedule structured treatment interruption (STI-HAART; alternating 3 weeks on and 3 weeks off therapy) suppressed viral load. In the STI-HAART group, T cell virus-specific immune response (VIR) and control of viral rebound increased concurrently during subsequent interruptions. In contrast, VIR did not increase and SIV rebounded after permanent treatment withdrawal in all animals on continuous HAART. Fixed-schedule STI-HAART appears to be an effective alternative to continuous HAART for the early treatment of retroviral infection.

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.