SIV-infected Nonhuman Primates Research Articles

2-Hydroxypropyl-β-Cyclodextrin Treatment Induces Modest Immune Activation in Healthy Rhesus Macaques.

Experimental simian immunodeficiency virus (SIV) infection of Asian macaques is an excellent model for HIV disease progression and therapeutic development. Recent coformulations of nucleoside analogs and an integrase inhibitor have been used for parenteral antiretroviral (ARV) administration in SIV-infected macaques, successfully resulting in undetectable plasma SIV RNA. In a cohort of SIVmac239-infected macaques, we recently observed that administration of coformulated ARVs resulted in an unexpected increase in plasma levels of soluble CD14 (sCD14), associated with stimulation of myeloid cells. We hypothesized that the coformulation solubilizing agent Kleptose (2-hydroxypropyl-β-cyclodextrin [HPβCD]) may induce inflammation with myeloid cell activation and the release of sCD14. Herein, we stimulated peripheral blood mononuclear cells (PBMCs) from healthy macaques with HPβCD from different commercial sources and evaluated inflammatory cytokine production in vitro. Treatment of PBMCs resulted in increased sCD14 release and myeloid cell interleukin-1β (IL-1β) production-with stimulation varying significantly by HPβCD source-and destabilized lymphocyte CCR5 surface expression. We further treated healthy macaques with Kleptose alone. In vivo, we observed modestly increased myeloid cell activation in response to Kleptose treatment without significant perturbation of the immunological transcriptome or epigenome. Our results demonstrate a need for vehicle-only controls and highlight immunological perturbations that can occur when using HPβCD in pharmaceutical coformulations. IMPORTANCE SIV infection of nonhuman primates is the principal model system for assessing HIV disease progression and therapeutic development. HPβCD has recently been incorporated as a solubilizing agent in coformulations of ARVs in SIV-infected nonhuman primates. Although HPβCD has historically been considered inert, recent findings suggest that HPβCD may contribute to inflammation. Herein, we investigate the contribution of HPβCD to healthy macaque inflammation in vitro and in vivo. We observe that HPβCD causes an induction of sCD14 and IL-1β from myeloid cells in vitro and demonstrate that HPβCD stimulatory capacity varies by commercial source. In vivo, we observe modest myeloid cell activation in blood and bronchoalveolar lavage specimens absent systemic immune activation. From our findings, it is unclear whether HPβCD stimulation may improve or diminish immune reconstitution in ARV-treated lentiviral infections. Our results demonstrate a need for vehicle-only controls and highlight immunological perturbations that can occur when using HPβCD in pharmaceutical coformulations.

IL-15 enhances HIV-1 infection by promoting survival and proliferation of CCR5+CD4+ T cells.

HIV-1 usually utilizes CCR5 as its coreceptor and rarely switches to a CXCR4-tropic virus until the late stage of infection. CCR5+CD4+ T cells are the major virus-producing cells in viremic individuals as well as SIV-infected nonhuman primates. The differentiation of CCR5+CD4+ T cells is associated with the availability of IL-15, which increases during acute HIV-1 infection. Here, we report that CCR5 was expressed by CD4+ T cells exhibiting effector or effector memory phenotypes with high expression levels of the IL-2/IL-15 receptor common β and γ chains. IL-15, but not IL-7, improved the survival of CCR5+CD4+ T cells, drove their expansion, and facilitated HIV-1 infection in vitro and in humanized mice. Our study suggests that IL-15 plays confounding roles in HIV-1 infection, and future studies on the IL-15-based boosting of anti-HIV-1 immunity should carefully examine the potential effects on the expansion of HIV-1 reservoirs in CCR5+CD4+ T cells.

Pharmacological Validation of Long-Term Treatment with Antiretroviral Drugs in a Model of SIV-Infected Non-Human Primates.

The development of animal models undergoing long-term antiretroviral treatment (ART) makes it possible to understand a number of immunological, virological, and pharmacological issues, key factors in the management of HIV infection. We aimed to pharmacologically validate a non-human primate (NHP) model treated in the long term with antiretroviral drugs after infection with the pathogenic SIVmac251 strain. A single-dose pharmacokinetic study of tenofovir disoproxil fumarate, emtricitabine, and dolutegravir was first conducted on 13 non-infected macaques to compare three different routes of administration. Then, 12 simian immunodeficiency virus (SIV)-infected (SIV+) macaques were treated with the same regimen for two years. Drug monitoring, virological efficacy, and safety were evaluated throughout the study. For the single-dose pharmacokinetic study, 24-h post-dose plasma concentrations for all macaques were above or close to 90% inhibitory concentrations and consistent with human data. During the two-year follow-up, the pharmacological data were consistent with those observed in humans, with low inter- and intra-individual variability. Rapid and sustained virological efficacy was observed for all macaques, with a good safety profile. Overall, our SIV+ NHP model treated with the ART combination over a two-year period is suitable for investigating the question of pharmacological sanctuaries in HIV infection and exploring strategies for an HIV cure.

Mathematical modeling of N-803 treatment in SIV-infected non-human primates

Immunomodulatory drugs could contribute to a functional cure for Human Immunodeficiency Virus (HIV). Interleukin-15 (IL-15) promotes expansion and activation of CD8+ T cell and natural killer (NK) cell populations. In one study, an IL-15 superagonist, N-803, suppressed Simian Immunodeficiency Virus (SIV) in non-human primates (NHPs) who had received prior SIV vaccination. However, viral suppression attenuated with continued N-803 treatment, partially returning after long treatment interruption. While there is evidence of concurrent drug tolerance, immune regulation, and viral escape, the relative contributions of these mechanisms to the observed viral dynamics have not been quantified. Here, we utilize mathematical models of N-803 treatment in SIV-infected macaques to estimate contributions of these three key mechanisms to treatment outcomes: 1) drug tolerance, 2) immune regulation, and 3) viral escape. We calibrated our model to viral and lymphocyte responses from the above-mentioned NHP study. Our models track CD8+ T cell and NK cell populations with N-803-dependent proliferation and activation, as well as viral dynamics in response to these immune cell populations. We compared mathematical models with different combinations of the three key mechanisms based on Akaike Information Criterion and important qualitative features of the NHP data. Two minimal models were capable of reproducing the observed SIV response to N-803. In both models, immune regulation strongly reduced cytotoxic cell activation to enable viral rebound. Either long-term drug tolerance or viral escape (or some combination thereof) could account for changes to viral dynamics across long breaks in N-803 treatment. Theoretical explorations with the models showed that less-frequent N-803 dosing and concurrent immune regulation blockade (e.g. PD-L1 inhibition) may improve N-803 efficacy. However, N-803 may need to be combined with other immune therapies to countermand viral escape from the CD8+ T cell response. Our mechanistic model will inform such therapy design and guide future studies.

High-fat diet exacerbates SIV pathogenesis and accelerates disease progression.

Consuming a high-fat diet (HFD) is a risk factor for obesity and diabetes; both of these diseases are also associated with systemic inflammation, similar to HIV infection. A HFD induces intestinal dysbiosis and impairs liver function and coagulation, with a potential negative impact on HIV/SIV pathogenesis. We administered a HFD rich in saturated fats and cholesterol to nonpathogenic (African green monkeys) and pathogenic (pigtailed macaques) SIV hosts. The HFD had a negative impact on SIV disease progression in both species. Thus, increased cell-associated SIV DNA and RNA occurred in the HFD-receiving nonhuman primates, indicating a potential reservoir expansion. The HFD induced prominent immune cell infiltration in the adipose tissue, an important SIV reservoir, and heightened systemic immune activation and inflammation, altering the intestinal immune environment and triggering gut damage and microbial translocation. Furthermore, HFD altered lipid metabolism and HDL oxidation and also induced liver steatosis and fibrosis. These metabolic disturbances triggered incipient atherosclerosis and heightened cardiovascular risk in the SIV-infected HFD-receiving nonhuman primates. Our study demonstrates that dietary intake has a discernable impact on the natural history of HIV/SIV infections and suggests that dietary changes can be used as adjuvant approaches for HIV-infected subjects, to reduce inflammation and the risk of non-AIDS comorbidities and possibly other infectious diseases.

The Landscape of Persistent Viral Genomes in ART-Treated SIV, SHIV, and HIV-2 Infections

Evaluation of HIV cure strategies is complicated by defective proviruses that persist in ART-treated patients but are irrelevant to cure. Non-human primates (NHP) are essential for testing cure strategies.However, the persisting proviral landscape in ART-treated NHPs is uncharacterized. Here, we describe viral genomes persisting in ART-treated, simian immunodeficiency virus (SIV)-infected NHPs, simian-human immunodeficiency virus (SHIV)-infected NHPs, and humans infected with HIV-2, an SIV-related virus. The landscapes of persisting SIV, SHIV, and HIV-2 genomes are also dominated by defective sequences. However, there was a significantly higher fraction of intact SIV proviral genomes compared to ART-treated HIV-1 or HIV-2 infected humans. Compared to humans with HIV-1, SIV-infected NHPs had more hypermutated genomes, a relative paucity of clonal SIV sequences, and a lower frequency of deleted genomes. Finally, we report an assay for measuring intact SIV genomes which may have value in cure research.

Neutrophil extracellular trap production contributes to pathogenesis in SIV-infected nonhuman primates.

Neutrophil extracellular traps (NETs) are involved in the pathogenesis of many infectious diseases, yet their dynamics and impact on HIV/SIV infection have not yet been assessed. We hypothesized that SIV infection and the related microbial translocation trigger NET activation and release (NETosis), and we investigated the interactions between NETs and immune cell populations and platelets. We compared and contrasted the levels of NETs between SIV-uninfected, SIV-infected, and SIV-infected antiretroviral-treated nonhuman primates. We also cocultured neutrophils from these animals with either peripheral blood mononuclear cells or platelets. Increased NET production was observed throughout SIV infection. In chronically infected animals, NETs were found in the gut, lung, and liver, and in the blood vessels of kidney and heart. Antiretroviral therapy (ART) decreased NETosis, albeit above preinfection levels. NETs captured CD4+ and CD8+ T cells, B cells, and monocytes, irrespective of their infection status, potentially contributing to the indiscriminate generalized immune cell loss characteristic to HIV/SIV infection, and limiting the CD4+ T cell recovery under ART. By capturing and facilitating aggregation of platelets, and through expression of increased tissue factor levels, NETs may also enhance HIV/SIV-related coagulopathy and promote cardiovascular comorbidities.

Total body CD4+ T cell dynamics in treated and untreated SIV infection revealed by in vivo imaging

The peripheral blood represents only a small fraction of the total number of lymphocytes in the body. To develop a more thorough understanding of T cell dynamics, including the effects of SIV/SHIV/HIV infection on immune cell depletion and immune reconstitution following combination antiretroviral therapy (cART), one needs to utilize approaches that allow direct visualization of lymphoid tissues. In the present study, noninvasive in vivo imaging of the CD4+ T cell pool has revealed that the timing of the CD4+ T cell pool reconstitution following initiation of ART in SIV-infected nonhuman primates (NHPs) appears seemingly stochastic among clusters of lymph nodes within the same host. At 4 weeks following initiation or interruption of cART, the changes observed in peripheral blood (PB) are primarily related to changes in the whole-body CD4 pool rather than changes in lymphocyte trafficking. Lymph node CD4 pools in long-term antiretroviral-treated and plasma viral load-suppressed hosts appear suboptimally reconstituted compared with healthy controls, while splenic CD4 pools appear similar between the 2 groups.

Accumulation of follicular CD8+ T cells in pathogenic SIV infection.

LN follicles constitute major reservoir sites for HIV/SIV persistence. Cure strategies could benefit from the characterization of CD8+ T cells able to access and eliminate HIV-infected cells from these areas. In this study, we provide a comprehensive analysis of the phenotype, frequency, localization, and functionality of follicular CD8+ T cells (fCD8+) in SIV-infected nonhuman primates. Although disorganization of follicles was a major factor, significant accumulation of fCD8+ cells during chronic SIV infection was also observed in intact follicles, but only in pathogenic SIV infection. In line with this, tissue inflammatory mediators were strongly associated with the accumulation of fCD8+ cells, pointing to tissue inflammation as a major factor in this process. These fCD8+ cells have cytolytic potential and can be redirected to target and kill HIV-infected cells using bispecific antibodies. Altogether, our data support the use of SIV infection to better understand the dynamics of fCD8+ cells and to develop bispecific antibodies as a strategy for virus eradication.

Emergence of resistance mutations in simian immunodeficiency virus (SIV)-infected rhesus macaques receiving non-suppressive antiretroviral therapy (ART).

Two SIVmac251-infected rhesus macaques received tenofovir/emtricitabine with raltegravir intensification. Viral rebound occurred during treatment and sequencing of reverse transcriptase and integrase genes identified multiple resistance mutations. Similar to HIV infection, antiretroviral-resistance mutations may occur in SIV-infected nonhuman primates receiving nonsuppressive ART. As ART administration to nonhuman primates is currently dramatically expanding, fueled by both cure research and the study of HIV-related comorbidities, viral resistance should be factored in the study design and data interpretation.

Dysbiotic bacteria translocate in progressive SIV infection.

Infection of gut-resident CD4+ memory T-cells during acute HIV and SIV infection is associated with rapid loss of these cells and damage to the epithelial barrier. Damage to the epithelial barrier allows translocation of microbial products from the intestinal lumen into the body. Immune activation caused by these microbial products has been associated with disease progression. Although microbial translocation has been demonstrated in SIV-infected nonhuman primates, the identity of translocating bacteria has not been determined. In this study we examined the communities of bacteria both within the GI tract and systemic tissues of both healthy and experimentally SIV-infected Asian macaques. While there were only modest changes in the GI tract-associated microbiome resulting from infection, there is substantial dysbiosis after administration of antiretrovirals. Analysis of bacterial DNA isolated from tissues of infected animals revealed a preference for the phylum Proteobacteria, suggesting that they preferentially translocate. Consistent with this finding, we observed increased metabolic activity of Proteobacterial species within the colonic lumen of SIV-infected animals. Overall these data provide insights into disease progression and suggest that therapies aimed at altering the composition and metabolic activity of the GI tract microbiome could benefit chronically-HIV infected individuals particularly those on antiretroviral therapies.

Animal models in HIV cure research

Current HIV antiretroviral therapy (ART) successfully inhibits viral replication in the majority of HIV-infected individuals. However, ART is not curative and lifelong adherence is required. Despite the undisputed benefit of ART, long-lived latently infected cells that carry HIV-integrated DNA remain. Hence, upon ART interruption, HIV-infected subjects experience viral rebound. Interestingly, similar disease course occurs in the well-characterised animal model of SIV-infected non-human primates. Using these animal models to investigate the mechanisms involved in the generation of latently infected cells, define the phenotypic and anatomical nature of persistent viral reservoirs, and test novel interventions for viral eradication, is critical for strengthening our understanding of HIV persistence and developing novel therapeutics aimed at curing HIV.In this review, we discuss the current animal models used in AIDS cure research, with a particular focus on non-human primates, and outline the experimental strategies explored in the quest for virus eradication.

F-104 Non-human primate models for immunological studies of HIV eradication

The mechanisms by which pathogenic HIV and SIV infections cause immune deficiency involve factors related to both the virus and the host immune system. Over the past few years, emphasis has been placed on the role of the chronic immune activation that is apparent during the natural history of the infection and remains as “residual” immune activation in HIV-infected individuals treated with antiretroviral therapy (ART). Several key concepts and hypotheses regarding the pathogenic role of immune activation have been generated in studies involving SIV-infected non-human primates (NHP) such as the sooty mangabeys, in which a natural, non-pathogenic infection is characterized by robust virus replication but low levels of immune activation, and Asian macaques, in which experimental infection with SIV results a progressive infection characterized by CD4+ T cell depletion, high immune activation, and simian AIDS. More recently, the SIV/NHP model has been developed and validated for studies of the HIV-associated “residual” disease under fully suppressive ART, which is characterized by persistent immune dysfunction and presence of a reservoir of latently infected cells which makes it impossible to eradicate the virus. In this presentation, I will briefly review the opportunities presented by NHP models to conduct studies that will improve our understanding of AIDS virology, immunology, and pathogenesis, with specific focus the goal of reducing the residual disease that persists in ART-treated HIV-infected individuals despite suppression of virus replication. I will discuss (1) the key pathogenic aspects of the residual immune activation observed in ART-treated HIV-infected individuals and SIV-infected macaques, (2) the role of type I interferons as both antiviral and pro-inflammatory factors, and (3) the key role played by the pattern of infected CD4+ T cell subsets as a determinant of both residual immune activation and persistent virus reservoir in the setting of ART-mediated suppression of virus replication.

Early microbial translocation blockade reduces SIV-mediated inflammation and viral replication.

Damage to the intestinal mucosa results in the translocation of microbes from the intestinal lumen into the circulation. Microbial translocation has been proposed to trigger immune activation, inflammation, and coagulopathy, all of which are key factors that drive HIV disease progression and non-HIV comorbidities; however, direct proof of a causal link is still lacking. Here, we have demonstrated that treatment of acutely SIV-infected pigtailed macaques with the drug sevelamer, which binds microbial lipopolysaccharide in the gut, dramatically reduces immune activation and inflammation and slightly reduces viral replication. Furthermore, sevelamer administration reduced coagulation biomarkers, confirming the contribution of microbial translocation in the development of cardiovascular comorbidities in SIV-infected nonhuman primates. Together, our data suggest that early control of microbial translocation may improve the outcome of HIV infection and limit noninfectious comorbidities associated with AIDS.

Safety and tolerability of a live oral Salmonella typhimurium vaccine candidate in SIV-infected nonhuman primates

Nontyphoidal Salmonella (NTS) serovars are a common cause of acute food-borne gastroenteritis worldwide and can cause invasive systemic disease in young infants, the elderly, and immunocompromised hosts, accompanied by high case fatality. Vaccination against invasive NTS disease is warranted where the disease incidence and mortality are high and multidrug resistance is prevalent, as in sub-Saharan Africa. Live-attenuated vaccines that mimic natural infection constitute one strategy to elicit protection. However, they must particularly be shown to be adequately attenuated for consideration of immunocompromised subjects. Accordingly, we examined the safety and tolerability of an oral live attenuated Salmonella typhimurium vaccine candidate, CVD 1921, in an established chronic simian immunodeficiency virus (SIV)-infected rhesus macaque model. We evaluated clinical parameters, histopathology, and measured differences in mucosal permeability to wild-type and vaccine strains. Compared to the wild-type S. typhimurium strain I77 in both SIV-infected and SIV-uninfected nonhuman primate hosts, this live-attenuated vaccine shows reduced shedding and systemic spread, exhibits limited pathological disease manifestations in the digestive tract, and induces low levels of cellular infiltration in tissues. Furthermore, wild-type S. typhimurium induces increased intestinal epithelial damage and permeability, with infiltration of neutrophils and macrophages in both SIV-infected and SIV-uninfected nonhuman primates compared to the vaccine strain. Based on shedding, systemic spread, and histopathology, the live-attenuated S. typhimurium strain CVD 1921 appears to be safe and well-tolerated in the nonhuman primate model, including chronically SIV-infected rhesus macaques.

Immunopathogenesis of simian immunodeficiency virus infection in nonhuman primates

Soon after the discovery of HIV-infected humans, rhesus macaques in a colony at the New England Primate Research Center showed similar signs of a progressive immune suppression. The discovery of the simian immunodeficiency virus (SIV)-associated disease opened the door to study an AIDS-like illness in nonhuman primates (NHP). Even after 3 decades, this animal model remains an invaluable tool to provide a greater insight into HIV immunopathogenesis. In this review, recent progress in deciphering pathways of immunopathogenesis in SIV-infected NHP is discussed. The immense diversity of mutations in SIV stocks prepared at different laboratories has recently been realized. The massive expansion of the enteric virome is a key finding in SIV-induced immunopathogenesis. Defining the function of host restriction factors, like the recently discovered SAMHD1, helps to evaluate the impact of the innate immune responses on virus replication. Utilization of pyrosequencing and defining molecular mechanisms of major histocompatibility complex (MHC) class I restriction helps to understand how the virus evades CD8 T-cell responses. The definition of MHC class I molecules in different NHP species provides new animal models to study SIV immunopathogenesis. T follicular helper cells have gained major interest in characterizing humoral immune responses in SIV infection and AIDS vaccine strategies. The ability of natural hosts to remain disease-free despite ongoing replication of SIV is continuing to puzzle the field. The HIV research field continues to realize the immense complexity of the host virus interaction. NHP present an invaluable tool to make progress towards an effective AIDS vaccine.

Coagulation biomarkers predict disease progression in SIV-infected nonhuman primates

AbstractHIV infection is associated with increased risk of cardiovascular complications, the underlying mechanism of which remains unclear. Plasma levels of the coagulation biomarker D-dimer (DD) correlate with increased mortality and cardiovascular events in HIV-infected patients. We compared the incidence of cardiovascular lesions and the levels of the coagulation markers DD and thrombin antithrombin in pathogenic SIV infections of rhesus and pigtailed macaques (PTMs) and in nonpathogenic SIV infection of African green monkeys (AGMs) and sooty mangabeys. Hypercoagulability and cardiovascular pathology were only observed in pathogenic SIV infections. In PTMs infected with SIV from AGMs (SIVagm), DD levels were highly indicative of AIDS progression and increased mortality and were associated with cardiovascular lesions, pointing to SIVagm-infected PTMs as an ideal animal model for the study of HIV-associated cardiovascular disease. In pathogenic SIV infection, DD increased early after infection, was strongly correlated with markers of immune activation/inflammation and microbial translocation (MT), and was only peripherally associated with viral loads. Endotoxin administration to SIVagm-infected AGMs (which lack chronic SIV-induced MT and immune activation) resulted in significant increases of DD. Our results demonstrate that hypercoagulation and cardiovascular pathology are at least in part a consequence of excessive immune activation and MT in SIV infection.

Increased inherent intestinal granzyme B expression may be associated with SIV pathogenesis in Asian non-human primates.

Unlike Asian non-human primates, chronically SIV-infected African non-human primates (NHP) display a non-pathogenic disease course. The different outcomes may be related to the development of an SIV-mediated breach of the intestinal mucosa in the Asian species that is absent in the African animals. To examine possible mechanisms that could lead to the gut breach, we determined whether the colonic lamina propria (LP) of SIV-naïve Asian monkeys contained more granzyme B (GrB) producing CD4 T cells than did that of the African species. GrB is a serine protease that may disrupt mucosal integrity by damaging tight junction proteins. We found that the colonic LP of Asian NHP contain more CD4(+) /GrB(+) cells than African NHP. We also observed reduced CD4 expression on LP T cells in African green monkeys. Both phenotypic differences could protect against SIV-mediated damage to the intestinal mucosa and could lead to future therapies in HIV(+) humans.

An early and robust natural killer cell response during acute SIV infection in sooty mangabeys but not rhesus macaques is correlated with disease resistance

A need for defining optimal reagents and assays to identify the NK cell lineage in non‐human primates (NHP) is required because recent data clearly implicate an important role for NK cells in contributing to disease progression in HIV‐1 infected individuals. This cell lineage is particularly involved in the acute phase of infection which can optimally be studied in SIV infected NHP. A study of the role of NK cells in SIV infected disease resistant sooty mangabeys (SM) as compared with SIV infected disease susceptible rhesus macaques (RM) may provide important clues as to the contributing role of NK cells in the polarized clinical outcome in these two species. The use of previously identified reagents along with newly defined reagents and multi‐color flow cytometry has revealed that a gated population of CD3− CD8+ NKG2A+ cells identify 4 NK cell subsets with a predominant CD16+ CD56− subset primarily contributing to functional NK cytolytic activity. Cross sectional studies revealed a significant decline in this cytolytic subset in SIV‐infected RM but not in SIV‐infected SM. Importantly, longitudinal studies of individual animals pre and post SIV infection showed that while infection induces transient increases in NK cells in both species, there was a marked difference in the kinetics by which such increases occurred with SIV‐infected SM exhibiting a much earlier and more robust NK response than SIV‐infected RM.

Is there a role for Tregs in SIV infected non-human primates? (46.15)

Abstract A study was performed to identify, characterize and compare the level and function of peripheral Tregs in uninfected and SIV-infected rhesus macaques (RM) and sooty mangabeys (SM) to determine if Tregs contribute to the muted immune response that is evident in SIV-infected disease-resistant SM but readily detectable in SIV-infected disease-susceptible RM. Results show that SIV-infected SM maintain the level and function of Tregs but a substantial decline was noted in RM. A strong correlation between plasma VL and the level of Tregs was observed in SIV-infected RM but not SM. RM VL controllers exhibited lower levels of immune activation and recovered pre-infection levels of Tregs that were fully functional when tested for suppressive activity in vitro. ICC assays showed that Treg-depleted PBMC fractions from RM but not SM exhibit increased CD4+ and CD8+ T-cell responses to select SIV peptide pools suggesting that Tregs play a role in the SIV-specific immune response at least in RM. The decline in Tregs in SIV-infected RM as a general consequence of viral pathogenesis may further contribute to the increased immune activation and viral replication exhibited by disease progressors. Although the role of Tregs in SM remains unclear, the data imply that therapeutic manipulationof the frequency and function of Treg-cell in RM and perhaps HIV-infected hosts could improve immune controlof viral infection. NIH RO1 27057, AIDS & Health Sciences Research Grants, Ministry of Health Labor & Welfare in Japan.