Th-1-type cytotoxic CD8+ T-lymphocyte responses to simian immunodeficiency virus (SIV) are a consistent feature of natural SIV infection in sooty mangabeys

Natural history of postnatal rhesus cytomegalovirus shedding by dams and acquisition by infant rhesus monkeys

BACKGROUND

Human infants frequently acquire human cytomegalovirus (HCMV) through breastfeeding, resulting in persistent high-level viral shedding in saliva and urine and infectivity to others, including pregnant women. Thus, vaccination to interrupt postnatal HCMV transmission is an attractive strategy to prevent HCMV spread and congenital infection. Rhesus CMV (RhCMV) in nonhuman primates is a valuable model for the study of immune strategies to prevent CMV transmission. Although rhesus monkeys typically acquire RhCMV before 1 year of age, the timing and mode of natural infant RhCMV transmission remain unknown.

METHODS

We followed 5 RhCMV-seropositive dams and their infants from birth until weaning, approximately 6 months later. RhCMV DNA levels in plasma, breast milk, saliva, and urine were measured every 2 weeks by quantitative PCR. RhCMV-specific T cell responses in peripheral blood and breast milk were measured by interferon gamma ELISpot assays. Serum IgG antibody levels were measured by ELISA.

RESULTS

Four of five postpartum RhCMV-seropositive mothers had intermittent, low-level RhCMV shedding in breast milk, whereas all had high-magnitude RhCMV shedding in saliva and urine. The kinetics of maternal blood RhCMV-specific T cell responses and viral shedding in urine and saliva did not strongly associate, though dams with consistently high systemic RhCMV-specific T cell responses tended to have undetectable RhCMV shedding in breast milk. All RhCMV-exposed infants had intermittent, low-level RhCMV shedding in saliva during the lactation period, with minimal systemic RhCMV-specific T cell responses.

CONCLUSIONS

Despite exposure to RhCMV shedding in breast milk and other maternal fluids, postnatal mother-to-child RhCMV transmission appears to be less efficient than that of HCMV. A greater understanding of the determinants of RhCMV transmission and its usefulness as a model of HCMV mucosal acquisition may provide insight into strategies to prevent HCMV infections in humans.

Lymph Node Cellular and Viral Dynamics in Natural Hosts and Impact for HIV Cure Strategies

Combined antiretroviral therapies (cARTs) efficiently control HIV replication leading to undetectable viremia and drastic increases in lifespan of people living with HIV. However, cART does not cure HIV infection as virus persists in cellular and anatomical reservoirs, from which the virus generally rebounds soon after cART cessation. One major anatomical reservoir are lymph node (LN) follicles, where HIV persists through replication in follicular helper T cells and is also trapped by follicular dendritic cells. Natural hosts of SIV, such as African green monkeys and sooty mangabeys, generally do not progress to disease although displaying persistently high viremia. Strikingly, these hosts mount a strong control of viral replication in LN follicles shortly after peak viremia that lasts throughout infection. Herein, we discuss the potential interplay between viral control in LNs and the resolution of inflammation, which is characteristic for natural hosts. We furthermore detail the differences that exist between non-pathogenic SIV infection in natural hosts and pathogenic HIV/SIV infection in humans and macaques regarding virus target cells and replication dynamics in LNs. Several mechanisms have been proposed to be implicated in the strong control of viral replication in natural host's LNs, such as NK cell-mediated control, that will be reviewed here, together with lessons and limitations of in vivo cell depletion studies that have been performed in natural hosts. Finally, we discuss the impact that these insights on viral dynamics and host responses in LNs of natural hosts have for the development of strategies toward HIV cure.

Pathogenic Correlates of Simian Immunodeficiency Virus-Associated B Cell Dysfunction

We compared and contrasted pathogenic (in pig-tailed macaques [PTMs]) and nonpathogenic (in African green monkeys [AGMs]) SIVsab infections to assess the significance of the B cell dysfunction observed in simian (SIV) and human immunodeficiency virus (HIV) infections. We report that the loss of B cells is specifically associated with the pathogenic SIV infection, while in the natural hosts, in which SIV is nonpathogenic, B cells rapidly increase in both lymph nodes (LNs) and intestine. SIV-associated B cell dysfunction associated with the pathogenic SIV infection is characterized by loss of naive B cells, loss of resting memory B cells due to their redistribution to the gut, increases of the activated B cells and circulating tissue-like memory B cells, and expansion of the B regulatory cells (Bregs). While circulating B cells are virtually restored to preinfection levels during the chronic pathogenic SIV infection, restoration is mainly due to an expansion of the "exhausted," virus-specific B cells, i.e., activated memory cells and tissue-like memory B cells. Despite of the B cell dysfunction, SIV-specific antibody (Ab) production was higher in the PTMs than in AGMs, with the caveat that rapid disease progression in PTMs was strongly associated with lack of anti-SIV Ab. Neutralization titers and the avidity and maturation of immune responses did not differ between pathogenic and nonpathogenic infections, with the exception of the conformational epitope recognition, which evolved from low to high conformations in the natural host. The patterns of humoral immune responses in the natural host are therefore more similar to those observed in HIV-infected subjects, suggesting that natural hosts may be more appropriate for modeling the immunization strategies aimed at preventing HIV disease progression. The numerous differences between the pathogenic and nonpathogenic infections with regard to dynamics of the memory B cell subsets point to their role in the pathogenesis of HIV/SIV infections and suggest that monitoring B cells may be a reliable approach for assessing disease progression.IMPORTANCE We report here that the HIV/SIV-associated B cell dysfunction (defined by loss of total and memory B cells, increased B regulatory cell [Breg] counts, and B cell activation and apoptosis) is specifically associated with pathogenic SIV infection and absent during the course of nonpathogenic SIV infection in natural nonhuman primate hosts. Alterations of the B cell population are not correlated with production of neutralizing antibodies, the levels of which are similar in the two species. Rapid progressive infections are associated with a severe impairment in SIV-specific antibody production. While we did not find major differences in avidity and maturation between the pathogenic and nonpathogenic SIV infections, we identified a major difference in conformational epitope recognition, with the nonpathogenic infection being characterized by an evolution from low to high conformations. B cell dysfunction should be considered in designing immunization strategies aimed at preventing HIV disease progression.

The well-tempered SIV infection: Pathogenesis of SIV infection in natural hosts in the wild, with emphasis on virus transmission and early events post-infection that may contribute to protection from disease progression

African NHPs are infected by over 40 different simian immunodeficiency viruses. These viruses have coevolved with their hosts for long periods of time and, unlike HIV in humans, infection does not generally lead to disease progression. Chronic viral replication is maintained for the natural lifespan of the host, without loss of overall immune function. Lack of disease progression is not correlated with transmission, as SIV infection is highly prevalent in many African NHP species in the wild. The exact mechanisms by which these natural hosts of SIV avoid disease progression are still unclear, but a number of factors might play a role, including: (i) avoidance of microbial translocation from the gut lumen by preventing or repairing damage to the gut epithelium; (ii) control of immune activation and apoptosis following infection; (iii) establishment of an anti-inflammatory response that resolves chronic inflammation; (iv) maintenance of homeostasis of various immune cell populations, including NK cells, monocytes/macrophages, dendritic cells, Tregs, Th17 T-cells, and γδ T-cells; (v) restriction of CCR5 availability at mucosal sites; (vi) preservation of T-cell function associated with down-regulation of CD4 receptor. Some of these mechanisms might also be involved in protection of natural hosts from mother-to-infant SIV transmission during breastfeeding. The difficulty of performing invasive studies in the wild has prohibited investigation of the exact events surrounding transmission in natural hosts. Increased understanding of the mechanisms of SIV transmission in natural hosts, and of the early events post-transmission which may contribute to avoidance of disease progression, along with better comprehension of the factors involved in protection from SIV breastfeeding transmission in the natural hosts, could prove invaluable for the development of new prevention strategies for HIV.

Enhanced Th1/Th17 Functions of CD161+ CD8+ T Cells in Mucosal Tissues of Rhesus Macaques

Expression of the C-type lectin-like receptor CD161 by human T cells is associated with type-17 responses, which play critical regulatory roles in immunity and inflammation at mucosal sites. However, the functions of CD161-expressing T cells in macaques, the pre-clinical model of several human diseases, remain unknown. This study examined the phenotypic and functional characteristics of CD161⁺ T cells in peripheral blood, mucosal tissues and lymph nodes of rhesus macaques. Majority of CD161-expressing T cells in peripheral blood and lung/intestinal mucosal tissues of rhesus macaques were found to be CD8⁺CD4^– in phenotype. There was a significant enrichment of CD161⁺CD8⁺ T cells in the lungs and colonic mucosa (16.1%±6.6 and 16.8%±5.7) in comparison to peripheral blood (4.2%±1.2) and mesenteric lymph nodes (1.3%±0.8). Regardless of the tissue compartment, CD161⁺CD8⁺ T cells mainly comprised of γδ T cells and TCR Vα7.2⁺ MAIT cells (up to 80%), and displayed Th1 and Th17 cytokine responses to mitogen stimulation. Mucosal CD161⁺CD8⁺ T cells were characterized by very high expression of CD69, a recent activation marker that is preferentially expressed on tissue resident cells. Furthermore, lung and colonic mucosal CD161⁺CD8⁺ T cells showed enhanced IFN-γ, IL-17, and Perforin production in comparison to those in blood. Thus, macaque CD161⁺CD8⁺ T cells represent mucosal tissue-homing innate-like CD8⁺ T-cell populations with Th1/Th17 type cytokine and cytotoxic effector functions that can potentially enhance the recruitment of adaptive immune cells and control initial pathogen burden/dissemination in tissues. Analysis of their role in early immune responses to mucosal pathogens will be valuable in the design of vaccines and therapeutics.

Novel MHC class I full-length allele and haplotype characterization in sooty mangabeys

Sooty mangabeys (Cercocebus atys) are natural SIV hosts and the presumed source of HIV-2 and SIVmac, which makes them a valuable model for HIV/SIV research. However, like other African primates, little is known about their major histocompatibility complex (MHC) genetics. In this study, we used Roche/454 and Illumina MiSeq deep sequencing in order to determine the MHC class I transcripts in a cohort of 165 sooty mangabeys from the Yerkes National Primate Research Center (YNPRC). We have characterized 121 functionally full-length classical (Ceat-A and Ceat-B) and non-classical (Ceat-F and Ceat-I) alleles and have also identified 22 Ceat-A/Ceat-B haplotype chromosomal combinations. We correlated these Ceat-A/Ceat-B haplotype combinations to recently described microsatellite haplotypes from the YNPRC colony. These newly identified alleles and haplotypes establish a resource for studying cellular immunity in sooty mangabeys and provide a framework for rapidly cataloging MHC class I sequences in an understudied, yet important, nonhuman primate species.

Characterization of MHC class I alleles in sooty mangabeys as a tool for evaluating cellular immunity in natural hosts of SIV infection

Although immune pressure exerted by MHC class I-restricted cytotoxic T lymphocytes (CTL) are an important determinant of outcome in pathogenic HIV and SIV infection, lack of data on MHC class I genes has hampered study of its role in natural hosts with nonpathogenic SIV infection. In this study, we cloned and characterized full-length MHC class I genes derived from the cDNA library of two unrelated naturally infected sooty mangabeys (Cercocebus atys) in whom SIV-specific CTL epitopes were previously mapped. Twenty one full-length MHC class I alleles consisting of five MHC-A (Ceat-A), 13 MHC-B (Ceat-B), and three MHC-E (Ceat-E) alleles were identified. Sequence-specific primers (SSP) for high-throughput screening of genomic DNA by PCR were developed for 16 of the 18 Ceat-A and Ceat-B alleles. Screening of 62 SIV-negative and 123 SIV-infected sooty mangabeys at the Yerkes National Primate Research Center (YNPRC) revealed the presence of up to four MHC-A and eight MHC-B alleles in individual mangabeys, indicating that similar to macaque species, mangabeys have at least two duplications of the MHC-A locus and four duplications of the MHC-B locus in the absence of an MHC-C locus. Using stable transfectants of Ceat MHC Class I alleles in the MHC-null 721.221 cell line, we identified Ceat-B*12:01 as the restricting allele of a previously reported Nef20-28 CTL epitope. Ceat-B*1201/Nef20-28 tetramers identified tetramer-positive CD8+ T lymphocytes in Ceat-B*1201-positive SIV-infected mangabeys. This study has laid the groundwork for comprehensive analysis of CTL and SIV evolution in a natural host of SIV infection.

HIV-associated chronic immune activation

Systemic chronic immune activation is considered today as the driving force of CD4(+) T-cell depletion and acquired immunodeficiency syndrome (AIDS). A residual chronic immune activation persists even in HIV-infected patients in which viral replication is successfully inhibited by anti-retroviral therapy, with the extent of this residual immune activation being associated with CD4(+) T-cell loss. Unfortunately, the causal link between chronic immune activation and CD4(+) T-cell loss has not been formally established. This article provides first a brief historical overview on how the perception of the causative role of immune activation has changed over the years and lists the different kinds of immune activation characteristic of human immunodeficiency virus (HIV) infection. The mechanisms proposed to explain the chronic immune activation are multiple and are enumerated here, as well as the mechanisms proposed on how chronic immune activation could lead to AIDS. In addition, we summarize the lessons learned from natural hosts that know how to 'show AIDS the door', and discuss how these studies informed the design of novel immune modulatory interventions that are currently being tested. Finally, we review the current approaches aimed at targeting chronic immune activation and evoke future perspectives.

Distinct evolutionary pressures underlie diversity in simian immunodeficiency virus and human immunodeficiency virus lineages

Simian immunodeficiency virus (SIV) infection of rhesus macaques causes immune depletion and disease closely resembling human AIDS and is well recognized as the most relevant animal model for the human disease. Experimental investigations of viral pathogenesis and vaccine protection primarily involve a limited set of related viruses originating in sooty mangabeys (SIVsmm). The diversity of human immunodeficiency virus type 1 (HIV-1) has evolved in humans in about a century; in contrast, SIV isolates used in the macaque model evolved in sooty mangabeys over millennia. To investigate the possible consequences of such different evolutionary histories for selection pressures and observed diversity in SIVsmm and HIV-1, we isolated, sequenced, and analyzed 20 independent isolates of SIVsmm, including representatives of 7 distinct clades of viruses isolated from natural infection. We found SIVsmm diversity to be lower overall than HIV-1 M group diversity. Reduced positive selection (i.e., less diversifying evolution) was evident in extended regions of SIVsmm proteins, most notably in Gag p27 and Env gp120. In addition, the relative diversities of proteins in the two lineages were distinct: SIVsmm Env and Gag were much less diverse than their HIV-1 counterparts. This may be explained by lower SIV-directed immune activity in mangabeys relative to HIV-1-directed immunity in humans. These findings add an additional layer of complexity to the interpretation and, potentially, to the predictive utility of the SIV/macaque model, and they highlight the unique features of human and simian lentiviral evolution that inform studies of pathogenesis and strategies for AIDS vaccine design.

Loss of effector and anti-inflammatory natural killer T lymphocyte function in pathogenic simian immunodeficiency virus infection

Chronic immune activation is a key determinant of AIDS progression in HIV-infected humans and simian immunodeficiency virus (SIV)-infected macaques but is singularly absent in SIV-infected natural hosts. To investigate whether natural killer T (NKT) lymphocytes contribute to the differential modulation of immune activation in AIDS-susceptible and AIDS-resistant hosts, we compared NKT function in macaques and sooty mangabeys in the absence and presence of SIV infection. Cynomolgus macaques had significantly higher frequencies of circulating invariant NKT lymphocytes compared to both rhesus macaques and AIDS-resistant sooty mangabeys. Despite this difference, mangabey NKT lymphocytes were functionally distinct from both macaque species in their ability to secrete significantly more IFN-γ, IL-13, and IL-17 in response to CD1d/α-galactosylceramide stimulation. While NKT number and function remained intact in SIV-infected mangabeys, there was a profound reduction in NKT activation-induced, but not mitogen-induced, secretion of IFN-γ, IL-2, IL-10, and TGF-β in SIV-infected macaques. SIV-infected macaques also showed a selective decline in CD4⁺ NKT lymphocytes which correlated significantly with an increase in circulating activated memory CD4⁺ T lymphocytes. Macaques with lower pre-infection NKT frequencies showed a significantly greater CD4⁺ T lymphocyte decline post SIV infection. The disparate effect of SIV infection on NKT function in mangabeys and macaques could be a manifestation of their differential susceptibility to AIDS. Alternately, these data also raise the possibility that loss of anti-inflammatory NKT function promotes chronic immune activation in pathogenic SIV infection, while intact NKT function helps to protect natural hosts from developing immunodeficiency and aberrant immune activation.

Several African nonhuman primate species such as sooty mangabeys are naturally infected with SIV and maintain high levels of viral replication without developing AIDS. SIV-infected natural hosts do not show evidence of increased chronic immune activation, a feature that distinguishes them from AIDS-susceptible SIV-infected Asian macaques. In this study we compared natural killer T (NKT) lymphocytes, a unique subset of innate T lymphocytes with anti-inflammatory properties, in AIDS-resistant and AIDS-susceptible hosts. Sooty mangabey NKT cells retained normal functionality following SIV infection and were more potent than macaque NKT cells in their ability to produce interferon-γ and secrete anti-inflammatory cytokines. In contrast, NKT cells of SIV-infected macaques were markedly hypo-functional with regards to secretion of anti-inflammatory and effector cytokines and showed an association between loss of CD4⁺ NKT cells and increased immune activation. These findings suggest that dysfunctional NKT cells may promote increased immune activation in AIDS-susceptible hosts while intact effector and anti-inflammatory NKT cells could help to prevent immunodeficiency and increased immune activation in natural hosts.

Natural SIV hosts: showing AIDS the door

Many species of African nonhuman primates are naturally infected with simian immunodeficiency viruses (SIVs) in the wild and in captivity. In contrast to HIV-infected humans, these natural SIV hosts typically do not develop AIDS, despite chronic infection with a highly replicating virus. In this Review, we discuss the most recent advances on the mechanisms of protection from disease progression in natural SIV hosts, with emphasis on how they differ from pathogenic HIV/SIV infections of humans and rhesus macaques. These mechanisms include: (i) resolution of immune activation after acute infection, (ii) restricted pattern of target cell infection, and (iii) protection from mother-to-infant transmission. We highlight the areas that should be pursued in future studies, focusing on potential applications for the treatment and prevention of HIV infection.

Mucosal simian immunodeficiency virus transmission in African green monkeys: susceptibility to infection is proportional to target cell availability at mucosal sites

African green monkeys (AGMs) are naturally infected with a simian immunodeficiency virus (SIVagm) that is nonpathogenic in its host. Although SIVagm is common and widespread, little is known about the mechanisms that govern its transmission. Since the earliest virus-host interactions may provide key insights into the nonpathogenic phenotype of SIVagm, we developed a mucosal transmission model for this virus. Using plasma from an acutely infected AGM as the virus inoculum, we exposed adult and juvenile AGMs, as well as pigtailed macaques (PTMs) as a nonnatural host control, by mucosal routes to increasing titers of virus and compared the doses needed to establish a productive infection. Four juvenile and four adult AGMs as well as two PTMs were intrarectally (IR) exposed, while two additional adult female AGMs were intravaginally (IVAG) exposed. No animal became infected following exposure to 10(5) RNA copies. Both PTMs but none of the AGMs became infected following exposure to 10(6) RNA copies. Finally, all adult AGMs and two of the four juvenile AGMs became infected following exposure to 10(7) RNA copies, acquiring either one (2 IR infected juveniles, 1 IR infected adult, 2 IVAG infected adults) or two (3 IR infected adults) transmitted founder viruses. These results were consistent with immunophenotypic data, which revealed a significant correlation between the percentage of CD4(+) T cells expressing CCR5 in the mucosa and the susceptibility to infection, in terms of both the viral dose and the numbers of transmitted founder viruses. Moreover, studies of uninfected AGMs showed that the fraction of CCR5-expressing CD4(+) T cells increased significantly with age. These results indicate that (i) AGMs are readily infected with SIVagm by both intrarectal and intravaginal routes, (ii) susceptibility to infection is proportional to the number of available CCR5(+) CD4(+) target cells in the mucosa, and (iii) the paucity of CCR5(+) CD4(+) target cells in infant and juvenile AGMs may explain the near absence of vertical transmission.

Immunovirological analyses of chronically simian immunodeficiency virus SIVmnd-1- and SIVmnd-2-infected mandrills (Mandrillus sphinx)

Simian immunodeficiency virus (SIV) infection in African nonhuman primate (NHP) natural hosts is usually nonpathogenic, despite high levels of virus replication. We have previously shown that chronic SIV infection in sooty mangabeys (SMs) and African green monkeys (AGMs) is associated with low levels of immune activation and bystander T cell apoptosis. To compare these features with those observed in another natural host, the mandrill (MND), we conducted a cross-sectional survey of the 23 SIV-infected and 25 uninfected MNDs from the only semifree colony of mandrills available worldwide. Viral loads (VLs) were determined and phenotypic and functional analysis of peripheral blood- and lymph node-derived lymphocytes was performed. We found that mandrills chronically infected with SIVmnd-1 or SIVmnd-2 have similar levels of viral replication, and we observed a trend toward lower CD4+ T cell counts in chronically SIVmnd-2-infected MNDs than SIVmnd-1-infected MNDs. No correlation between CD4+ T cell counts and VLs in SIV-infected MNDs could be established. Of note, the levels of T cell activation, proliferation, and apoptosis were comparable between SIVmnd-1- and SIVmnd-2-infected MNDs and to those observed in uninfected animals, with the only exception being an increase in tumor necrosis factor alpha-producing CD8+ T cells in SIVmnd-2-infected MNDs. Overall, these findings recapitulate previous observations in SIV-infected SMs and AGMs and lend further evidence to the hypothesis that low levels of immune activation protect natural SIV hosts from disease progression.

Immunodeficiency lentiviral infections in natural and non-natural hosts

The host immune system is profoundly affected during the acute phase of progressive immunodeficiency lentiviral infections. Studies of these alterations have been quite restricted in humans because of the limited availability of samples from acutely HIV-infected persons. Therefore, numerous studies have turned attention to nonhuman primate models. Specifically, SIV-infected rhesus macaques (RMs) have been informative for understanding the pathogenesis of HIV infection in humans. Indeed, advantages of the nonhuman primate model include the ability to study the very early events after infection and the ability to retrieve copious amounts of tissues. In addition, nonhuman primates allow for comparative studies between non-natural and natural hosts for SIV, in which SIV infection results in progression, or not, to AIDS, respectively. Although SIV infection of RM is the best model for HIV infection, the immunologic and/or virologic phenomena in SIV-infected RM do not always reflect those seen in HIV-infected humans. Here virologic and immunologic aspects of acute HIV infection of humans and SIV infection of Asian and African nonhuman primates are discussed and compared in relation to how these aspects relate to disease progression.

Early induction of polyfunctional simian immunodeficiency virus (SIV)-specific T lymphocytes and rapid disappearance of SIV from lymph nodes of sooty mangabeys during primary infection

Although the cellular immune response is essential for controlling SIV replication in Asian macaques, its role in maintaining nonpathogenic SIV infection in natural hosts such as sooty mangabeys (SM) remains to be defined. We have previously shown that similar to rhesus macaques (RM), SM are able to mount a T lymphocyte response against SIV infection. To investigate early control of SIV replication in natural hosts, we performed a detailed characterization of SIV-specific cellular immunity and viral control in the first 6 mo following SIV infection in SM. Detection of the initial SIV-specific IFN-γ ELISPOT response in SIVsmE041-infected SM coincided temporally with a decline in peak plasma viremia and was similar in magnitude, specificity, and breadth to SIVsmE041-infected and SIVmac239-infected RM. Despite these similarities, SM showed a greater reduction in postpeak plasma viremia and a more rapid disappearance of productively SIV-infected cells from the lymph node compared with SIVmac239-infected RM. The early Gag-specific CD8(+) T lymphocyte response was significantly more polyfunctional in SM compared with RM, and granzyme B-positive CD8(+) T lymphocytes were present at significantly higher frequencies in SM even prior to SIV infection. These findings suggest that the early SIV-specific T cell response may be an important determinant of lymphoid tissue viral clearance and absence of lymph node immunopathology in natural hosts of SIV infection.

Lack of clinical AIDS in SIV-infected sooty mangabeys with significant CD4+ T cell loss is associated with double-negative T cells

SIV infection of natural host species such as sooty mangabeys results in high viral replication without clinical signs of simian AIDS. Studying such infections is useful for identifying immunologic parameters that lead to AIDS in HIV-infected patients. Here we have demonstrated that acute, SIV-induced CD4(+) T cell depletion in sooty mangabeys does not result in immune dysfunction and progression to simian AIDS and that a population of CD3(+)CD4(-)CD8(-) T cells (double-negative T cells) partially compensates for CD4(+) T cell function in these animals. Passaging plasma from an SIV-infected sooty mangabey with very few CD4(+) T cells to SIV-negative animals resulted in rapid loss of CD4(+) T cells. Nonetheless, all sooty mangabeys generated SIV-specific antibody and T cell responses and maintained normal levels of plasma lipopolysaccharide. Moreover, all CD4-low sooty mangabeys elicited a de novo immune response following influenza vaccination. Such preserved immune responses as well as the low levels of immune activation observed in these animals were associated with the presence of double-negative T cells capable of producing Th1, Th2, and Th17 cytokines. These studies indicate that SIV-infected sooty mangabeys do not appear to rely entirely on CD4(+) T cells to maintain immunity and identify double-negative T cells as a potential subset of cells capable of performing CD4(+) T cell-like helper functions upon SIV-induced CD4(+) T cell depletion in this species.

SIV infection in natural hosts: resolution of immune activation during the acute-to-chronic transition phase

SIV-infected natural hosts do not progress to clinical AIDS yet display high viral replication and an acute immunologic response similar to pathogenic SIV/HIV infections. During chronic SIV infection, natural hosts suppress their immune activation, whereas pathogenic hosts display a highly activated immune state. Here, we review natural host SIV infections with an emphasis on specific immune cells and their contribution to the transition from the acute-to-chronic phases of infection.

Where the wild things are: pathogenesis of SIV infection in African nonhuman primate hosts

African nonhuman primates that are natural hosts of simian immunodeficiency virus (SIV) are generally spared from disease progression. Pathogenic and nonpathogenic SIV infections share some major features: high viral replication, massive acute depletion of mucosal CD4(+) T cells, and partial control of the virus by both adaptive and innate immune responses. A key distinction of natural SIV infections is rapid and active control of immune activation and apoptosis of T cells that contributes to the integrity of mucosal barrier and lack of microbial translocation. This allows partial recovery of CD4(+) T cells and preservation of the function of other immune cell subsets. A better understanding of the mechanisms underlying the lack of disease in natural hosts for SIV infection will likely provide important clues as to the therapy of HIV-1 infection.

Experimental depletion of CD8+ cells in acutely SIVagm-infected African Green Monkeys results in increased viral replication

Background

In vivo CD8⁺ cell depletions in pathogenic SIV infections identified a key role for cellular immunity in controlling viral load (VL) and disease progression. However, similar studies gave discordant results in chronically-infected SMs, leading some authors to propose that in natural hosts, SIV replication is independent of cellular immunity. To assess the role of cellular immune responses in the control of SIV replication in natural hosts, we investigated the impact of CD8⁺ cell depletion during acute SIV infection in AGMs.

Results

Nine AGMs were infected with SIVagm.sab and were followed up to day 225 p.i. Four were intravenously infused with the cM-T807 antibody on days 0 (50 mg/kg), 6, and 13 (10 mg/kg, respectively) post infection (p.i.). CD8⁺ cells were depleted for up to 28 days p.i. in peripheral blood and LNs in all treated AGMs. Partial CD8⁺ T cell depletion occurred in the intestine. SIVagm VLs peaked at similar levels in both groups (10⁷-10⁸ RNA copies/ml). However, while VLs were controlled in undepleted AGMs, reaching set-point levels (10⁴-10⁵ RNA copies/ml) by day 28 p.i., high VLs (>10⁶ RNA copies/ml) were maintained by day 21 p.i. in CD8-depleted AGMs. By day 42 p.i., VLs were comparable between the two groups. The levels of immune activation and proliferation remained elevated up to day 72 p.i. in CD8-depleted AGMs and returned to preinfection levels in controls by day 28 p.i. None of the CD8-depleted animals progressed to AIDS.

Conclusion

CD8⁺ cells are responsible for a partial control of postacute viral replication in SIVagm.sab-infected AGMs. In contrast to macaques, the SIVagm-infected AGMs are able to control viral replication after recovery of the CD8⁺ T cells and avoid disease progression.

A five-year longitudinal analysis of sooty mangabeys naturally infected with simian immunodeficiency virus reveals a slow but progressive decline in CD4+ T-cell count whose magnitude is not predicted by viral load or immune activation

Natural simian immunodeficiency virus (SIV) infection in sooty mangabeys (SMs) typically does not result in AIDS, despite high-level viremia and significant depletion of mucosal CD4(+) T cells. Here, we report the results of the first longitudinal study of a large cohort of SMs naturally infected with SIV (n = 78) housed at the Yerkes National Primate Research Center from which samples were obtained three times over a 5-year period. In this study, we observed (i) no signs of simian AIDS, (ii) stable SIV loads, (iii) a slow but progressive decline in CD4(+) T-cell counts (from a mean of 1,067.0 cells/mm(3) at time point 1 to 764.8 cells/mm(3) at time point 3) and increases in the numbers of animals with CD4(+) T-cell levels below 500 and 200 cells/mm(3) (from 8 to 28 of 78 and from 1 to 4 of 78, respectively), (iv) progressive declines in percentages of naïve CD4(+) and CD8(+) T cells (from 37.7 to 24.8% and from 21.0 to 13.0%, respectively), and (v) stably low levels of activated/proliferating T cells as well as CD4(+) CCR5(+) T cells. Since the level of total CD4(+) T cells and the fraction of naïve T cells in SIV-uninfected SMs also declined, it is possible that some of these observations are related to aging, as the SIV-infected animals were significantly older than the uninfected animals. In contrast to the decline in CD4(+) T cell counts in individuals infected with human immunodeficiency virus (HIV), the decline in CD4(+) T cell counts in SMs naturally infected with SIV over a 5-year period was not predicted by either plasma viremia or levels of T-cell activation. Taken together, these results confirm that natural SIV infection is nonprogressive from a clinical, virological, and immunological point of view and that stable levels of viremia associated with persistently low-level immune activation represent key differences from the natural course of HIV infection in humans.

Paucity of CD4+ natural killer T (NKT) lymphocytes in sooty mangabeys is associated with lack of NKT cell depletion after SIV infection

Lack of chronic immune activation in the presence of persistent viremia is a key feature that distinguishes nonpathogenic simian immunodeficiency virus (SIV) infection in natural hosts from pathogenic SIV and HIV infection. To elucidate novel mechanisms downmodulating immune activation in natural hosts of SIV infection, we investigated natural killer T (NKT) lymphocytes in sooty mangabeys. NKT lymphocytes are a potent immunoregulatory arm of the innate immune system that recognize glycolipid antigens presented on the nonpolymorphic MHC-class I-like CD1d molecules. In a cross-sectional analysis of 50 SIV-negative and 50 naturally SIV-infected sooty mangabeys, ligand alpha-galactosylceramide loaded CD1d tetramers co-staining with Valpha24-positive invariant NKT lymphocytes were detected at frequencies >or=0.002% of circulating T lymphocytes in approximately half of the animals. In contrast to published reports in Asian macaques, sooty mangabey NKT lymphocytes consisted of CD8(+) and CD4/CD8 double-negative T lymphocytes that were CXCR3-positive and CCR5-negative suggesting that they trafficked to sites of inflammation without being susceptible to SIV infection. Consistent with these findings, there was no difference in the frequency or phenotype of NKT lymphocytes between SIV-negative and SIV-infected sooty mangabeys. On stimulation with alpha-galactosylceramide loaded on human CD1d molecules, sooty mangabey NKT lymphocytes underwent degranulation and secreted IFN-gamma, TNF-alpha, IL-2, IL-13, and IL-10, indicating the presence of both effector and immunoregulatory functional capabilities. The unique absence of CD4(+) NKT lymphocytes in sooty mangabeys, combined with their IL-10 cytokine-secreting ability and preservation following SIV infection, raises the possibility that NKT lymphocytes might play a role in downmodulating immune activation in SIV-infected sooty mangabeys.

Suppression of adaptive immune responses during primary SIV infection of sabaeus African green monkeys delays partial containment of viremia but does not induce disease

One of the most puzzling observations in HIV research is the lack of pathogenicity in most nonhuman primate species that are natural hosts of simian immunodeficiency virus (SIV) infection. Despite this, natural hosts experience a level of viremia similar to humans infected with HIV or macaques infected with SIV. To determine the role of adaptive immune responses in viral containment and lack of disease, we delayed the generation of cellular and humoral immune responses by administering anti-CD8- and anti-CD20 lymphocyte-depleting antibodies to sabaeus African green monkeys (Chlorocebus sabaeus) before challenge with SIV(sab9315BR). In vivo lymphocyte depletion during primary infection resulted in a brief elevation of viremia but not in disease. Based on the magnitude and timing of SIV-specific CD8(+) T-cell responses in the lymphocyte-depleted animals, CD8(+) T-cell responses appear to contribute to viral containment in natural hosts. We found no evidence for a contribution of humoral immune responses in viral containment. These studies indicate that natural hosts have developed mechanisms in addition to classic adaptive immune responses to cope with this lentiviral infection. Thus, adaptive immune responses in natural hosts appear to be less critical for viral containment than in HIV infection.

Resolution of immune activation defines nonpathogenic SIV infection

Natural nonhuman primate hosts of SIV do not succumb to AIDS despite significant viral replication, a phenomenon attributed to reduced levels of chronic and deleterious "immune activation." Two studies in this issue of the JCI, by Bosinger et al. and Jacquelin et al., now show that SIV induces vigorous immune activation and upregulation of IFN-stimulated genes in both natural and susceptible hosts, but strikingly, the responses resolve only in the former (see the related articles, beginning on pages 3556 and 3544, respectively). Thus, natural hosts for SIV actively engage mechanisms to abort sustained immune activation and its associated harmful effects.

Inhibition of adaptive immune responses leads to a fatal clinical outcome in SIV-infected pigtailed macaques but not vervet African green monkeys

African green monkeys (AGM) and other natural hosts for simian immunodeficiency virus (SIV) do not develop an AIDS-like disease following SIV infection. To evaluate differences in the role of SIV-specific adaptive immune responses between natural and nonnatural hosts, we used SIV_agmVer90 to infect vervet AGM and pigtailed macaques (PTM). This infection results in robust viral replication in both vervet AGM and pigtailed macaques (PTM) but only induces AIDS in the latter species. We delayed the development of adaptive immune responses through combined administration of anti-CD8 and anti-CD20 lymphocyte-depleting antibodies during primary infection of PTM (n = 4) and AGM (n = 4), and compared these animals to historical controls infected with the same virus. Lymphocyte depletion resulted in a 1-log increase in primary viremia and a 4-log increase in post-acute viremia in PTM. Three of the four PTM had to be euthanized within 6 weeks of inoculation due to massive CMV reactivation and disease. In contrast, all four lymphocyte-depleted AGM remained healthy. The lymphocyte-depleted AGM showed only a trend toward a prolongation in peak viremia but the groups were indistinguishable during chronic infection. These data show that adaptive immune responses are critical for controlling disease progression in pathogenic SIV infection in PTM. However, the maintenance of a disease-free course of SIV infection in AGM likely depends on a number of mechanisms including non-adaptive immune mechanisms.

Simian immunodeficiency virus (SIV) is a naturally occurring infection in a wide range of African nonhuman primates, including African green monkeys (AGM), which generally results in a clinically inapparent infection. In contrast, SIV infection of Asian nonhuman primates such as macaques can result in an AIDS-like disease similar to that observed in humans infected with human immunodeficiency virus (HIV). This different pathogenic outcome occurs despite similar levels of viremia. In order to evaluate the contribution of adaptive immune responses to these different outcomes, we transiently inhibited the generation of CD8+ and CD20+ lymphocyte-mediated immune responses in vervet AGM and pigtailed macaques (PTM) during primary SIV infection. PTM experienced higher viremia and accelerated progression to disease, whereas AGM showed only a short prolongation of peak viremia but exhibited no signs of illness. These results demonstrate that protection against development of disease in AGM does not solely rely on adaptive immune responses. Future efforts should aim to determine the underlying mechanisms that enable natural hosts to cope with SIV infection and to apply these findings to develop new treatment modalities for humans infected with HIV.

Effect of B-cell depletion on viral replication and clinical outcome of simian immunodeficiency virus infection in a natural host

Simian immunodeficiency virus (SIV)-infected African nonhuman primates do not progress to AIDS in spite of high and persistent viral loads (VLs). Some authors consider the high viral replication observed in chronic natural SIV infections to be due to lower anti-SIV antibody titers than those in rhesus macaques, suggesting a role of antibodies in controlling viral replication. We therefore investigated the impact of antibody responses on the outcome of acute and chronic SIVagm replication in African green monkeys (AGMs). Nine AGMs were infected with SIVagm.sab. Four AGMs were infused with 50 mg/kg of body weight anti-CD20 (rituximab; a gift from Genentech) every 21 days, starting from day -7 postinfection up to 184 days. The remaining AGMs were used as controls and received SIVagm only. Rituximab-treated AGMs were successfully depleted of CD20 cells in peripheral blood, lymph nodes (LNs), and intestine, as shown by the dynamics of CD20+ and CD79a+ cells. There was no significant difference in VLs between CD20-depleted AGMs and control monkeys: peak VLs ranged from 10(7) to 10(8) copies/ml; set-point values were 10(4) to 10(5) SIV RNA copies/ml. Levels of acute mucosal CD4+ T-cell depletion were similar for treated and nontreated animals. SIVagm seroconversion was delayed for the CD20-depleted AGMs compared to results for the controls. There was a significant difference in both the timing and magnitude of neutralizing antibody responses for CD20-depleted AGMs compared to results for controls. CD20 depletion significantly altered the histological structure of the germinal centers in the LNs and Peyer's patches. Our results, although obtained with a limited number of animals, suggest that humoral immune responses play only a minor role in the control of SIV viral replication during acute and chronic SIV infection in natural hosts.

Lessons learned from the natural hosts of HIV-related viruses

The fact that human immunodeficiency virus (HIV) causes a deadly disease in humans whereas its simian counterparts, the simian immunodeficiency viruses (SIVs), are virtually nonpathogenic in their natural hosts remains a fundamental mystery of modern medicine. Arguably, the pathogenesis of HIV infection will remain poorly understood until the mechanisms responsible for the AIDS resistance of natural SIV hosts are fully explained. Over the past few years, some key features of natural SIV infections have been described in studies conducted predominantly in sooty mangabeys (SMs), African green monkeys (AGMs), and mandrills. Natural SIV hosts are able to avoid the chronic, generalized immune system activation that is associated with disease progression in HIV-infected individuals and have evolved to down-modulate the expression of CCR5 on CD4(+) T cells. Better elucidation of the mechanisms underlying the lack of disease progression of natural SIV infections holds promise for the design of novel preventive and therapeutic approaches to HIV infection.

Toward an AIDS vaccine: lessons from natural simian immunodeficiency virus infections of African nonhuman primate hosts

The design of an effective AIDS vaccine has eluded the efforts of the scientific community to the point that alternative approaches to classic vaccine formulations have to be considered. We propose here that HIV vaccine research could greatly benefit from the study of natural simian immunodeficiency virus (SIV) infections of African nonhuman primates. Natural SIV hosts (for example, sooty mangabeys, African green monkeys and mandrills) share many features of HIV infection of humans; however, they usually do not develop immunodeficiency. These natural, nonprogressive SIV infections represent an evolutionary adaptation that allows a peaceful coexistence of primate lentiviruses and the host immune system. This adaptation does not result in reduced viral replication but, rather, involves phenotypic changes to CD4(+) T cell subsets, limited immune activation and preserved mucosal immunity, all of which contribute to the avoidance of disease progression and, possibly, to the reduction of vertical SIV transmission. Here we summarize the current understanding of SIV infection of African nonhuman primates and discuss how unraveling these evolutionary adaptations may provide clues for new vaccine designs that might induce effective immune responses without the harmful consequences of excessive immune activation.

Limited ability of humoral immune responses in control of viremia during infection with SIVsmmD215 strain

We investigated the impact of rhesus macaque (RM) B-cell depletion before inoculation with the isolate SIVsmmD215. Seven RMs were treated every 3 weeks with 50 mg/kg of an anti-CD20 antibody (rituximab) starting 7 days before inoculation for 2 (n = 4) and 5 (n = 3) months. Four control animals received no antibody. Three animals were completely depleted of CD20(+) B cells, but 4 were only partially depleted of CD20 cells in the LNs and intestine. The decrease in antibody production was consistent with the efficacy of tissue CD20 depletion. Seroconversion and neutralizing antibody production was significantly delayed in animals showing complete tissue CD20 depletion and remained at low titers in all CD20-depleted RMs. Surprisingly, there was no significant difference in acute or chronic viral loads between CD20-depleted and control animal groups. There was a tendency for lower viral set points in CD20-depleted animals. At 6 weeks after inoculation, cellular immune responses were significantly stronger in CD20-depleted animals than in controls. There was no significant difference in survival between CD20-depleted and control animals. Our data suggest that a deficiency of Ab responses did not markedly affect viral replication or disease progression and that they may be compensated by more robust cellular responses.

AIDS pathogenesis: a tale of two monkeys

Despite many years of intense scientific effort, the pathogenic mechanisms underlying the immunodeficiency that follows human immunodeficiency virus (HIV) infection are still poorly understood. This lack of understanding is likely the main reason why at present there is neither a cure nor a vaccine for AIDS. Important clues on the immunopathogenesis of primate lentiviral infections have been provided by comparative studies of two simian models of SIV infection: the non-pathogenic SIV infection of sooty mangabey, an African natural host species, and the pathogenic SIV-infection of non-natural host rhesus macaques, that develop a disease that closely resembles AIDS in humans. While the final mechanisms underlying the difference in infection outcome between these two species are still incompletely understood, a series of recent studies has allowed the identification of key similarities and differences between the two models of infection. In this article we summarize these findings and review the main implications in terms of HIV pathogenesis, therapy, and vaccines.

Kinetics of T lymphocyte apoptosis and the cellular immune response in SIVmac239-infected rhesus macaques

BACKGROUND

Although increased apoptosis is a central feature of AIDS, little is known about its kinetics or relationship to the early host response in acute HIV/SIV infection.

METHODS

Ex vivo apoptosis in freshly isolated peripheral blood and lymph node lymphocytes was monitored longitudinally in SIVmac239-infected rhesus macaques by flow-cytometric detection of active caspase-3, cleaved poly (ADP-ribose) polymerase, and fragmented DNA.

RESULTS

Increased apoptosis of multiple lymphocyte subsets was observed in the first 2 weeks following SIV infection. Apoptosis of CD4+ T lymphocytes was of low magnitude but peaked earlier than other T lymphocyte subsets. A 10- to 36-fold increase in CD8+ T lymphocyte apoptosis coincided temporally with onset of the SIV-specific cellular immune response and enrichment of caspase-3-positive cells within recently proliferating, activated CD8+ T lymphocytes.

CONCLUSIONS

The virus-specific T lymphocyte response to primary infection and generalized non-specific immune activation contribute to the pathogenesis of apoptosis in acute SIV infection.

Simian immunodeficiency virus types 1 and 2 (SIV mnd 1 and 2) have different pathogenic potentials in rhesus macaques upon experimental cross-species transmission

The mandrill (Mandrillus sphinx) is naturally infected by two types of simian immunodeficiency virus (SIV): SIVmnd types 1 and 2. Both of these viruses cause long-term, non-progressive infections in their natural host despite high plasma viral loads. This study assessed the susceptibility of rhesus macaques to infection by these two types of SIVmnd and compared the virological and basic immunological characteristics of the resulting infections with those observed in natural infection in mandrills. Whilst both SIVmnd types induced similar levels of virus replication during acute infection in both mandrills and macaques, they produced a more pronounced CD4(+) T-cell depletion in rhesus macaques that persisted longer during the initial stage of infection. Pro-inflammatory cytokine responses were also induced at higher levels in rhesus macaques early in the infection. During the chronic phase of infection in mandrills, which in this case was followed for up to 2 years after infection, high levels of chronic virus replication did not induce significant changes in CD4(+) or CD8(+) T-cell counts. In rhesus macaques, the overall chronic virus replication level was lower than in mandrills. At the end of the follow-up period, although the viral loads of SIVmnd-1 and SIVmnd-2 were relatively similar in rhesus macaques, only SIVmnd-1-infected rhesus macaques showed significant CD4(+) T-cell depletion, in the context of higher levels of CD4(+) and CD8(+) T-cell activation, compared with SIVmnd-infected mandrills. The demonstration of the ability of both SIVmnd types to induce persistent infections in rhesus macaques calls for a careful assessment of the potential of these two viruses to emerge as new human pathogens.

Is the high virulence of HIV-1 an unfortunate coincidence of primate lentiviral evolution?

In the subset of primate lentiviruses that contain a vpu gene - HIV-1 and its simian precursors - the Nef protein has lost the ability to down-modulate CD3, block T cell activation and suppress programmed death. Vpu counteracts a host restriction factor induced by the inflammatory cytokine interferon-alpha. I propose that the acquisition of vpu may have allowed the viral lineage that gave rise to HIV-1 to evolve towards greater pathogenicity by removing the selective pressure for a protective Nef function that prevents damagingly high levels of immune activation.

Transcriptional profiling in pathogenic and non-pathogenic SIV infections reveals significant distinctions in kinetics and tissue compartmentalization

Simian immunodeficiency virus (SIV) infection leads to AIDS in experimentally infected macaques, whereas natural reservoir hosts exhibit limited disease and pathology. It is, however, unclear how natural hosts can sustain high viral loads, comparable to those observed in the pathogenic model, without developing severe disease. We performed transcriptional profiling on lymph node, blood, and colon samples from African green monkeys (natural host model) and Asian pigtailed macaques (pathogenic model) to directly compare gene expression patterns during acute pathogenic versus non-pathogenic SIV infection. The majority of gene expression changes that were unique to either model were detected in the lymph nodes at the time of peak viral load. Results suggest a shift toward cellular stress pathways and Th1 profiles during pathogenic infection, with strong and sustained type I and II interferon responses. In contrast, a strong type I interferon response was initially induced during non-pathogenic infection but resolved after peak viral load. The natural host also exhibited controlled Th1 profiles and better preservation of overall cell homeostasis. This study identified gene expression patterns that are specific to disease susceptibility, tissue compartmentalization, and infection duration. These patterns provide a unique view of how host responses differ depending upon lentiviral infection outcome.

Simian immunodeficiency virus (SIV) does not cause disease in African green monkeys (a natural host for the virus), whereas experimentally infected Asian macaques (a non-natural host) develop a progressive disease that is similar to that which occurs in HIV-infected humans. Insight into how HIV causes disease and leads to development of AIDS may therefore be gained by comparing the response of natural and non-natural hosts to SIV infection. To this end, we examined changes that occurred in gene expression levels over time and in multiple tissues derived from African green monkeys and Asian macaques experimentally infected with SIV. Infection leads to host-specific gene expression patterns in lymph nodes, blood, and colon. The natural and non-natural hosts differed with respect to the timing, intensity, and duration of infection-induced gene expression changes associated with inflammation and response to stress.

Critical loss of the balance between Th17 and T regulatory cell populations in pathogenic SIV infection

Chronic immune activation and progression to AIDS are observed after SIV infection in macaques but not in natural host primate species. To better understand this dichotomy, we compared acute pathogenic SIV infection in pigtailed macaques (PTs) to non-pathogenic infection in African green monkeys (AGMs). SIVagm-infected PTs, but not SIVagm-infected AGMs, rapidly developed systemic immune activation, marked and selective depletion of IL-17-secreting (Th17) cells, and loss of the balance between Th17 and T regulatory (Treg) cells in blood, lymphoid organs, and mucosal tissue. The loss of Th17 cells was found to be predictive of systemic and sustained T cell activation. Collectively, these data indicate that loss of the Th17 to Treg balance is related to SIV disease progression.

Natural infection by the simian immunodeficiency virus (SIV) in over 40 different species of African non-human primates is not accompanied by progression to acquired immunodeficiency syndrome (AIDS). To understand this phenomenon, we have performed a detailed virologic, immunologic, and gene expression analysis of acute SIV infection of two disparate species: the African green monkey (AGM), in which SIV infection is nonpathogenic, and the Asian pigtailed macaque (PT), in which SIV infection results in AIDS. After experimental infection, animals of both species developed high viral loads. In the PTs, viremia was associated with CD4⁺ T cell depletion in the peripheral blood and multiple signs of persistent immune activation and inflammation. Such pathology was not observed in AGMs. Notably, the AGMs maintained high and balanced levels of two subset populations of CD4⁺ T cells, e.g., the immunosuppressive T regulatory (Treg) and the IL-17 producing (Th17) populations, whereas the PTs did not. Further analysis of the role of Th17 and Treg balance during pathogenic lentiviral infection may provide novel insights into our understanding of SIV and HIV pathogenesis and future thoughts about vaccine development.

Into the wild: simian immunodeficiency virus (SIV) infection in natural hosts

Identifying distinctions between pathogenic HIV and simian immunodeficiency virus (SIV) infections and nonprogressive SIV in natural African primate hosts might provide key insights into HIV pathogenesis. Similar to pathogenic HIV infection in humans, natural SIV infections result in high viral replication and massive acute depletion of mucosal CD4(+) T cells. A key distinction of natural SIV infections is a rapidly developing anti-inflammatory milieu that prevents chronic activation, apoptosis and proliferation of T cells and preserves the function of other immune cell subsets, thus contributing to the integrity of the mucosal barrier and the lack of microbial translocation from the gut to the peritoneum. Immunologic features observed during natural SIV infections suggest approaches for designing new strategies for producing novel second-generation vaccines and therapeutic approaches to inhibit disease progression in HIV-infected humans.

Immunity in natural SIV infections

In stark contrast to human immunodeficiency virus (HIV)-infected individuals who, if left untreated, almost invariably progress to acquired immunodeficiency syndrome (AIDS), natural hosts for the simian immunodeficiency viruses (SIV) remain asymptomatic throughout the course of infection. This observation represents one of the main unresolved puzzles of AIDS research, particularly if one considers that natural SIV infections are characterized by chronically high levels of viraemia as well as intrinsic virus cytopathicity comparable with that of HIV. In this review, I discuss the basic immunological features of natural, nonpathogenic SIV infections, the evidence suggesting that attenuated, rather than extraordinarily strong, immune responses to the virus may favour their benign course, and the implications of these findings in terms of HIV therapy and vaccines.

Gag p27-specific B- and T-cell responses in Simian immunodeficiency virus SIVagm-infected African green monkeys

Nonpathogenic simian immunodeficiency virus SIVagm infection of African green monkeys (AGMs) is characterized by the absence of a robust antibody response against Gag p27. To determine if this is accompanied by a selective loss of T-cell responses to Gag p27, we studied CD4(+) and CD8(+) T-cell responses against Gag p27 and other SIVagm antigens in the peripheral blood and lymph nodes of acutely and chronically infected AGMs. Our data show that AGMs can mount a T-cell response against Gag p27, indicating that the absence of anti-p27 antibodies is not due to the absence of Gag p27-specific T cells.

Differential CD4+ T-lymphocyte apoptosis and bystander T-cell activation in rhesus macaques and sooty mangabeys during acute simian immunodeficiency virus infection

In contrast to pathogenic lentiviral infections, chronic simian immunodeficiency virus (SIV) infection in its natural host is characterized by a lack of increased immune activation and apoptosis. To determine whether these differences are species specific and predicted by the early host response to SIV in primary infection, we longitudinally examined T-lymphocyte apoptosis, immune activation, and the SIV-specific cellular immune response in experimentally infected rhesus macaques (RM) and sooty mangabeys (SM) with controlled or uncontrolled SIV infection. SIVsmE041, a primary SIVsm isolate, reproduced set-point viremia levels of natural SIV infection in SM but was controlled in RM, while SIVmac239 replicated to high levels in RM. Following SIV infection, increased CD8(+) T-lymphocyte apoptosis, temporally coinciding with onset of SIV-specific cellular immunity, and elevated plasma Th1 cytokine and gamma interferon-induced chemokine levels were common to both SM and RM. Different from SM, SIV-infected RM showed a significantly higher frequency of peripheral blood activated CD8(+) T lymphocytes despite comparable magnitude of the SIV-specific gamma interferon enzyme-linked immunospot response. Furthermore, an increase in CD4(+) and CD4(-)CD8(-) T-lymphocyte apoptosis and plasma tumor necrosis factor-related apoptosis-inducing ligand were observed only in RM and occurred in both controlled SIVsmE041 and uncontrolled SIVmac239 infection. These data suggest that the "excess" activated T lymphocytes in RM soon after SIV infection are predominantly of non-virus-specific bystander origin. Thus, species-specific differences in the early innate immune response appear to be an important factor contributing to differential immune activation in natural and nonnatural hosts of SIV infection.

Understanding the benign nature of SIV infection in natural hosts

In striking contrast to HIV infection, natural SIV infection of African nonhuman primates is asymptomatic and usually does not induce significant CD4+ T cell depletion despite high levels of virus replication. Recently, significant progress has been made in understanding the mechanisms underlying the remarkable difference in infection outcome between natural and nonnatural HIV/SIV hosts. These advances include the identification of limited immune activation as a key factor protecting natural SIV hosts from AIDS and the discovery of low CC chemokine receptor 5 expression on CD4+ T cells as a specific and consistent immunologic feature in these animals. Further elucidation of the pathways by which the differences in immune activation between natural and nonnatural hosts are manifest holds promise for the design of novel therapeutic approaches to HIV infection.

Simian immunodeficiency virus SIVagm dynamics in African green monkeys

The mechanisms underlying the lack of disease progression in natural simian immunodeficiency virus (SIV) hosts are still poorly understood. To test the hypothesis that SIV-infected African green monkeys (AGMs) avoid AIDS due to virus replication occurring in long-lived infected cells, we infected six animals with SIVagm and treated them with potent antiretroviral therapy [ART; 9-R-(2-phosphonomethoxypropyl) adenine (tenofovir) and beta-2,3-dideoxy-3-thia-5-fluorocytidine (emtricitabine)]. All AGMs showed a rapid decay of plasma viremia that became undetectable 36 h after ART initiation. A significant decrease of viral load was observed in peripheral blood mononuclear cells and intestine. Mathematical modeling of viremia decay post-ART indicates a half-life of productively infected cells ranging from 4 to 9.5 h, i.e., faster than previously reported for human immunodeficiency virus and SIV. ART induced a slight but significant increase in peripheral CD4(+) T-cell counts but no significant changes in CD4(+) T-cell levels in lymph nodes and intestine. Similarly, ART did not significantly change the levels of cell proliferation, activation, and apoptosis, already low in AGMs chronically infected with SIVagm. Collectively, these results indicate that, in SIVagm-infected AGMs, the bulk of virus replication is sustained by short-lived cells; therefore, differences in disease outcome between SIVmac infection of macaques and SIVagm infection of AGMs are unlikely due to intrinsic differences in the in vivo cytopathicities between the two viruses.

Short-lived infected cells support virus replication in sooty mangabeys naturally infected with simian immunodeficiency virus: implications for AIDS pathogenesis

Sooty mangabeys (SMs) naturally infected with simian immunodeficiency virus (SIV) do not develop AIDS despite high levels of virus replication. At present, the mechanisms underlying this disease resistance are poorly understood. Here we tested the hypothesis that SIV-infected SMs avoid immunodeficiency as a result of virus replication occurring in infected cells that live significantly longer than human immunodeficiency virus (HIV)-infected human cells. To this end, we treated six SIV-infected SMs with potent antiretroviral therapy (ART) and longitudinally measured the decline in plasma viremia. We applied the same mathematical models used in HIV-infected individuals and observed that SMs naturally infected with SIV also present a two-phase decay of viremia following ART, with the bulk (92 to 99%) of virus replication sustained by short-lived cells (average life span, 1.06 days), and only 1 to 8% occurring in longer-lived cells. In addition, we observed that ART had a limited impact on CD4(+) T cells and the prevailing level of T-cell activation and proliferation in SIV-infected SMs. Collectively, these results suggest that in SIV-infected SMs, similar to HIV type 1-infected humans, short-lived activated CD4(+) T cells, rather than macrophages, are the main source of virus production. These findings indicate that a short in vivo life span of infected cells is a common feature of both pathogenic and nonpathogenic primate lentivirus infections and support a model for AIDS pathogenesis whereby the direct killing of infected cells by HIV is not the main determinant of disease progression.

Selective downregulation of rhesus macaque and sooty mangabey major histocompatibility complex class I molecules by Nef alleles of simian immunodeficiency virus and human immunodeficiency virus type 2

Human immunodeficiency virus type 1 (HIV-1) Nef downregulates HLA-A and -B molecules, but not HLA-C or -E molecules, based on amino acid differences in their cytoplasmic domains to simultaneously evade cytotoxic T lymphocyte (CTL) and natural killer cell surveillance. Rhesus macaques and sooty mangabeys express orthologues of HLA-A, -B, and -E, but not HLA-C, and many of these molecules have unique amino acid differences in their cytoplasmic tails. We found that these differences also resulted in differential downregulation by primary simian immunodeficiency virus (SIV) SIV(smm/mac) and HIV-2 Nef alleles. Thus, selective major histocompatibility complex class I downregulation is a conserved mechanism of immune evasion for pathogenic SIV infection of rhesus macaques and nonpathogenic SIV infection of sooty mangabeys.

Dynamics of T- and B-lymphocyte turnover in a natural host of simian immunodeficiency virus

Increased lymphocyte turnover is a hallmark of pathogenic lentiviral infection. To investigate perturbations in lymphocyte dynamics in natural hosts with nonpathogenic simian immunodeficiency virus (SIV) infection, the nucleoside analog bromodeoxyuridine (BrdU) was administered to six naturally SIV-infected and five SIV-negative sooty mangabeys. As a measure of lymphocyte turnover, we estimated the mean death rate by fitting a mathematical model to the fraction of BrdU-labeled cells during a 2-week labeling and a median 10-week delabeling period. Despite significantly lower total T- and B-lymphocyte counts in SIV-infected sooty mangabeys than in SIV-negative mangabeys, the turnover rate of B lymphocytes and CD4(+) and CD8(+) T lymphocytes was not increased in the SIV-infected animals. A small, rapidly proliferating CD45RA(+) memory subset and a large, slower-proliferating CD45RA(-) central memory subset of CD4(+) T lymphocytes identified in the peripheral blood of sooty mangabeys also did not show evidence of increased turnover in the context of SIV infection. Independently of SIV infection, the turnover of CD4(+) T lymphocytes in sooty mangabeys was significantly higher (P < 0.01) than that of CD8(+) T lymphocytes, a finding hitherto not reported in rhesus macaques or humans. The absence of aberrant T-lymphocyte turnover along with an inherently high rate of CD4(+) T-lymphocyte turnover may help to preserve the pool of central memory CD4(+) T lymphocytes in viremic SIV-infected sooty mangabeys and protect against progression to AIDS.

Virally induced CD4+ T cell depletion is not sufficient to induce AIDS in a natural host

Peripheral blood CD4+ T cell counts are a key measure for assessing disease progression and need for antiretroviral therapy in HIV-infected patients. More recently, studies have demonstrated a dramatic depletion of mucosal CD4+ T cells during acute infection that is maintained during chronic pathogenic HIV as well as SIV infection. A different clinical disease course is observed during the infection of natural hosts of SIV infection, such as sooty mangabeys (Cercocebus atys), which typically do not progress to AIDS. Previous studies have determined that SIV+ mangabeys generally maintain healthy levels of CD4+ T cells despite having viral replication comparable to HIV-infected patients. In this study, we identify the emergence of a multitropic (R5/X4/R8-using) SIV infection after 43 or 71 wk postinfection in two mangabeys that is associated with an extreme, persistent (>5.5 years), and generalized loss of CD4+ T cells (5-80 cells/microl of blood) in the absence of clinical signs of AIDS. This study demonstrates that generalized CD4+ T cell depletion from the blood and mucosal tissues is not sufficient to induce AIDS in this natural host species. Rather, AIDS pathogenesis appears to be the cumulative result of multiple aberrant immunologic parameters that include CD4+ T cell depletion, generalized immune activation, and depletion/dysfunction of non-CD4+ T cells. Therefore, these data provide a rationale for investigating multifaceted therapeutic strategies to prevent progression to AIDS, even following dramatic CD4 depletion, such that HIV+ humans can survive normal life spans analogous to what occurs naturally in SIV+ mangabeys.

Depletion of CD8+ cells in sooty mangabey monkeys naturally infected with simian immunodeficiency virus reveals limited role for immune control of virus replication in a natural host species

SIV infection of sooty mangabeys (SMs), a natural host species, does not cause AIDS despite high-level virus replication. In contrast, SIV infection of nonnatural hosts such as rhesus macaques (RMs) induces an AIDS-like disease. The depletion of CD8+ T cells during SIV infection of RMs results in marked increases in plasma viremia, suggesting a key role for CD8+ T cells in controlling levels of SIV replication. To assess the role that CD8+ T cells play in determining the virologic and immunologic features of nonpathogenic SIV infection in SMs, we transiently depleted CD8+ T cells in SIV-infected and uninfected SMs using a CD8alpha-specific Ab (OKT8F) previously used in studies of SIV-infected RMs. Treatment of SMs with the OKT8F Ab resulted in the prompt and profound depletion of CD8+ T cells. However, in contrast to CD8+ cell depleted, SIV-infected RMs, only minor changes in the levels of plasma viremia were observed in most SIV-infected SMs during the period of CD8+ cell deficiency. Those SMs demonstrating greater increases in SIV replication following CD8+ cell depletion also displayed higher levels of CD4+ T cell activation and/or evidence of CMV reactivation, suggesting that an expanded target cell pool rather than the absence of CD8+ T cell control may have been primarily responsible for transient increases in viremia. These data indicate that CD8+ T cells exert a limited influence in determining the levels of SIV replication in SMs and provide additional evidence demonstrating that the absence of AIDS in SIV-infected SMs is not due to the effective control of viral replication by cellular immune responses.

Going wild: lessons from naturally occurring T-lymphotropic lentiviruses

Over 40 nonhuman primate (NHP) species harbor species-specific simian immunodeficiency viruses (SIVs). Similarly, more than 20 species of nondomestic felids and African hyenids demonstrate seroreactivity against feline immunodeficiency virus (FIV) antigens. While it has been challenging to study the biological implications of nonfatal infections in natural populations, epidemiologic and clinical studies performed thus far have only rarely detected increased morbidity or impaired fecundity/survival of naturally infected SIV- or FIV-seropositive versus -seronegative animals. Cross-species transmissions of these agents are rare in nature but have been used to develop experimental systems to evaluate mechanisms of pathogenicity and to develop animal models of HIV/AIDS. Given that felids and primates are substantially evolutionarily removed yet demonstrate the same pattern of apparently nonpathogenic lentiviral infections, comparison of the biological behaviors of these viruses can yield important implications for host-lentiviral adaptation which are relevant to human HIV/AIDS infection. This review therefore evaluates similarities in epidemiology, lentiviral genotyping, pathogenicity, host immune responses, and cross-species transmission of FIVs and factors associated with the establishment of lentiviral infections in new species. This comparison of consistent patterns in lentivirus biology will expose new directions for scientific inquiry for understanding the basis for virulence versus avirulence.

Gene expression profiling of gut mucosa and mesenteric lymph nodes in simian immunodeficiency virus-infected macaques with divergent disease course

BACKGROUND

Although the majority of drug-naïve HIV-infected patients develop acquired immunodeficiency syndrome (AIDS), a small percentage remains asymptomatic without therapeutic intervention.

METHODS

We have utilized the simian immunodeficiency virus (SIV)-infected rhesus macaque model to gain insights into the molecular mechanisms of long-term protection against simian AIDS.

RESULTS

Chronically SIV-infected macaques with disease progression had high viral loads and CD4(+) T-cell depletion in mucosal tissue and peripheral blood. These animals displayed pathologic changes in gut-associated lymphoid tissue (GALT) and mesenteric lymph node that coincided with increased expression of genes associated with interferon induction, inflammation and immune activation. In contrast, the animal with long-term asymptomatic infection suppressed viral replication and maintained CD4(+) T cells in both GALT and peripheral blood while decreasing expression of genes involved in inflammation and immune activation.

CONCLUSIONS

Our findings suggest that reduced immune activation and effective repair and regeneration of mucosal tissues correlate with long-term survival in SIV-infected macaques.