Simian immunodeficiency viruses from central and western Africa: evidence for a new species-specific lentivirus in tantalus monkeys.

Although up to 50% of African green monkeys (AGMs) are infected by simian immunodeficiency viruses (SIV) in their natural habitat, they remain asymptomatic carriers of these lentiviruses. They provide an attractive model to study not only the origin but also the link among genetic variation, host-virus adaptation, and pathogenicity of primate lentiviruses. SIVagm have been isolated from three species of AGM: the vervet (Cercopithecus pygerythrus), the grivet (Cercopithecus aethiops), and the sabaeus (Cercopithecus sabaeus) monkey. We studied four new SIVagm isolates from a fourth AGM species, the tantalus monkey (Cercopithecus tantalus), caught in the Central African Republic, and four new isolates from feral sabaeus monkeys from Senegal. Antigenic properties and partial env sequences were used to evaluate the diversity among these isolates. Alignment of env sequences in SIVagm isolated from tantalus and sabaeus monkeys permitted detailed mapping of the variable and conserved domains in the external glycoprotein. Genetic distances indicated that SIVagm isolates from tantalus monkeys are the most divergent among SIVagm in feral AGMs in Africa. The fact that AGMs are infected by four distinct lentiviruses, each specific for a single AGM species, supports the hypothesis of a coevolution of these viruses and their natural hosts and suggests that SIV transmission is a rare event among separated AGM species in the wild.     

Simian immunodeficiency virus SIVagm.sab infection of Caribbean African green monkeys: a new model for the study of SIV pathogenesis in natural hosts

Caribbean-born African green monkeys (AGMs) were classified as Chlorocebus sabaeus by cytochrome b sequencing. Guided by these phylogenetic analyses, we developed a new model for the study of simian immunodeficiency virus (SIV) infection in natural hosts by inoculating Caribbean AGMs with their species-specific SIVagm.sab. SIVagm.sab replicated efficiently in Caribbean AGM peripheral blood mononuclear cells in vitro. During SIVagm.sab primary infection of six Caribbean AGMs, the virus replicated at high levels, with peak viral loads (VLs) of 10(7) to 10(8) copies/ml occurring by day 8 to 10 postinfection (p.i.). Set-point values of up to 2 x 10(5) copies/ml were reached by day 42 p.i. and maintained throughout follow-up (through day 450 p.i.). CD4(+) T-cell counts in the blood showed a transient depletion at the peak of VL, and then returned to near preinfection values by day 28 p.i. and remained relatively stable during the chronic infection. Preservation of CD4 T cells was also found in lymph nodes (LNs) of chronic SIVagm.sab-infected Caribbean AGMs. No activation of CD4(+) T cells was detected in the periphery in SIV-infected Caribbean AGMs. These virological and immunological profiles from peripheral blood and LNs were identical to those previously reported in African-born AGMs infected with the same viral strain (SIVagm.sab92018). Due to these similarities, we conclude that Caribbean AGMs are a useful alternative to AGMs of African origin as a model for the study of SIV infection in natural African hosts.     

High cell-free virus load and robust autologous humoral immune responses in breast milk of simian immunodeficiency virus-infected african green monkeys

The design of immunologic interventions to prevent postnatal transmission of human immunodeficiency virus (HIV) will require identification of protective immune responses in this setting. Simian immunodeficiency virus (SIV)-infected rhesus monkeys (RMs), a species that develops an AIDS-like illness following experimental infection, transmit the virus at a high rate during breastfeeding. In contrast, postnatal transmission of SIV occurs rarely or not at all in natural, asymptomatic primate hosts of SIV. These contrasting transmission patterns provide a unique opportunity to study mechanisms that evolved to protect suckling infants from SIV infection. We compared the virologic and immunologic properties of milk of SIV-infected and uninfected natural hosts of SIV, African green monkeys (AGMs), to that of RMs. Interestingly, despite a low number of milk CD4(+) T lymphocytes in uninfected AGMs, milk virus RNA load in SIV-infected AGMs was comparable to that of SIV-infected RMs and that in AGM plasma. This observation is in contrast to the relatively low virus load in milk compared to that in plasma of SIV-infected RMs and HIV-infected women. Milk of SIV-infected AGMs also displayed robust virus-specific cellular immune responses. Importantly, an autologous challenge virus-specific neutralization response was detected in milk of five of six SIV-infected AGMs that was comparable in magnitude to that in plasma. In contrast, autologous challenge virus neutralization was not detectable in milk of SIV-infected RMs. The autologous virus-specific adaptive immune responses in breast milk of AGMs may contribute to impedance of virus transmission in the infant oral/gastrointestinal tract and the rarity of postnatal virus transmission in natural hosts of SIV.     

DC-SIGN from African green monkeys is expressed in lymph nodes and mediates infection in trans of simian immunodeficiency virus SIVagm

African green monkeys (AGMs) infected by simian immunodeficiency virus (SIV) SIVagm are resistant to AIDS. SIVagm-infected AGMs exhibit levels of viremia similar to those described during pathogenic human immunodeficiency virus type 1 (HIV-1) and SIVmac infections in humans and macaques, respectively, but contain lower viral loads in their lymph nodes. We addressed the potential role of dendritic cell-specific intercellular adhesion molecule 3-grabbing nonintegrin (DC-SIGN; CD209) in viral dissemination. In previous studies, it has been shown that human DC-SIGN and macaque DC-SIGN allow transmission of HIV and SIVmac to T cells. Here, we looked at the ability of DC-SIGN derived from AGM lymph nodes to interact with SIVagm. We show that DC-SIGN-expressing cells are present mainly in the medulla and often within the cortex and/or paracortex of AGM lymph nodes. We describe the isolation and characterization of at least three isoforms of dc-sign mRNA in lymph nodes of AGMs. The predicted amino acid sequence from the predominant mRNA isoform, DC-SIGNagm1, is 92 and 99% identical to the corresponding human and rhesus macaque DC-SIGN amino acid sequences, respectively. DC-SIGNagm1 is characterized by the lack of the fourth motif in the repeat domain. This deletion was also detected in the dc-sign gene derived from thirteen animals belonging to five other African monkey species and from four macaques (Macaca fascicularis and M. mulatta). Despite three- to seven-amino-acid modifications compared to DC-SIGNmac, DC-SIGNagm1 allows transmission of SIVagm to T cells. Furthermore, AGM monocyte-derived dendritic cells (MDDC) expressed at least 100,000 DC-SIGN molecules and were able to transmit SIVagm to T cells. At a low multiplicity of infection (10(-5) 50% tissue culture infective doses/cell), viral transmission by AGM MDDC was mainly DC-SIGN dependent. The present study reveals that DC-SIGN from a natural host species of SIV has the ability to act as an efficient attachment and transmission factor for SIVagm and suggests the absence of a direct link between this ability and viral load levels in lymph nodes.     

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.     

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.     

The host restriction factor APOBEC3G and retroviral Vif protein coevolve due to ongoing genetic conflict

APOBEC3G (A3G) is a host cytidine deaminase that inhibits retroviruses. HIV and related primate lentiviruses encode Vif, which counteracts A3G by inducing its degradation. This Vif-mediated A3G inhibition is species specific, suggesting that the A3G-Vif interaction has evolved as primate lentiviruses have adapted to their hosts. We examined the evolutionary dynamics of the A3G-Vif interaction within four African green monkey (AGM) subspecies, which are each naturally infected with a distinct simian immunodeficiency virus (SIV). We identified single amino acid changes within A3G in two AGM subspecies that render it resistant to Vif proteins, except for Vif from the viruses that naturally infect these subspecies. Moreover, experimental infection of AGMs shows that Vif can rapidly adapt to these arising Vif-resistant A3G genotypes. These data suggest that despite being generally nonpathogenic in its natural host, SIV infection selects for Vif-resistant forms of A3G in AGM populations, driving Vif counterevolution and functional divergence.     

Frequent substitution polymorphisms in African green monkey CCR5 cluster at critical sites for infections by simian immunodeficiency virus SIVagm, implying ancient virus-host coevolution

In contrast to humans, several primate species are believed to have harbored simian immunodeficiency viruses (SIVs) since ancient times. In particular, the geographically dispersed species of African green monkeys (AGMs) are all infected with highly diversified SIVagm viruses at high prevalences (greater than 50% of sexually mature individuals) without evident diseases, implying that the progenitor monkeys were infected prior to their dispersal. If this is correct, AGMs would be expected to have accumulated frequent resistance-conferring polymorphisms in host genes that are important for SIV replication. Accordingly, we analyzed the coding sequences of the CCR5 coreceptors from 26 AGMs (52 alleles) in distinct populations of the four species. These samples contained 29 nonsynonymous coding changes and only 15 synonymous nucleotide substitutions, implying intense functional selection. Moreover, 24 of the resulting amino acid substitutions were tightly clustered in the CCR5 amino terminus (D13N in the vervets and Y14N in the tantalus species) or in the first extracellular loop (Q93R and Q93K in all species). The Y14N substitution was extremely frequent in the 12 wild-born African tantalus, with 7 monkeys being homozygous for this substitution and 4 being heterozygous. Although two of these heterozygotes and the only wild-type homozygote were naturally infected with SIVagm, none of the Y14N homozygotes were naturally infected. A focal infectivity assay for SIVagm indicated that all five tested SIVagms efficiently use CCR5 as a coreceptor and that they also use CXCR6 (STRL33/Bonzo) and GPR15 (BOB) with lower efficiencies but not CXCR4. Interestingly, the D13N, Y14N, Q93R, and Q93K substitutions in AGM CCR5 all strongly inhibited infections by the SIVagm isolates in vitro. The Y14N substitution eliminates a tyrosine sulfation site that is important for infections and results in partial N-linked glycosylation (i.e., 60% efficiency) at this position. Nevertheless, the CCR5(Y14N) component that lacks an N-linked oligosaccharide binds the chemokine MIP-lbeta with a normal affinity and is fully active in signal transduction. Similarly, D13N and Q93R substitutions did not interfere with signal transduction. Thus, the common substitution polymorphisms in AGM CCR5 strongly inhibit SIVagm infections while substantially preserving chemokine signaling. In contrast, polymorphisms of human CCR5 are relatively infrequent, and the amino acid substitutions are randomly situated and generally without effects on coreceptor function. These results support an ancient coevolution of AGMs and SIVagm viruses and establish AGMs as a highly informative model for learning about host proteins that play critical roles in immunodeficiency virus infections.     

Lack of B Cell Dysfunction Is Associated with Functional,gp120-Dominant Antibody Responses in Breast Milk of Simian Immunodeficiency Virus-Infected African Green Monkeys

The design of an effective vaccine to reduce the incidence of mother-to-child transmission (MTCT) of human immunodeficiency virus (HIV) via breastfeeding will require identification of protective immune responses that block postnatal virus acquisition. Natural hosts of simian immunodeficiency virus (SIV) sustain nonpathogenic infection and rarely transmit the virus to their infants despite high milk virus RNA loads. This is in contrast to HIV-infected women and SIV-infected rhesus macaques (RhMs), nonnatural hosts which exhibit higher rates of postnatal virus transmission. In this study, we compared the systemic and mucosal B cell responses of lactating, SIV-infected African green monkeys (AGMs), a natural host species, to that of SIV-infected RhMs and HIV-infected women. AGMs did not demonstrate hypergammaglobulinemia or accumulate circulating memory B cells during chronic SIV infection. Moreover, the milk of SIV-infected AGMs contained higher proportions of naive B cells than RhMs. Interestingly, AGMs exhibited robust milk and plasma Env binding antibody responses that were one to two logs higher than those in RhMs and humans and demonstrated autologous neutralizing responses in milk at 1 year postinfection. Furthermore, the plasma and milk Env gp120-binding antibody responses were equivalent to or predominant over Env gp140-binding antibody responses in AGMs, in contrast to that in RhMs and humans. The strong gp120-specific, functional antibody responses in the milk of SIV-infected AGMs may contribute to the rarity of postnatal transmission observed in natural SIV hosts.     

Simian immunodeficiency virus (SIV)-specific CD8+ T-cell responses in vervet African green monkeys chronically infected with SIVagm

African green monkeys (AGM) do not develop overt signs of disease following simian immunodeficiency virus (SIV) infection. While it is still unknown how natural hosts like AGM can cope with this lentivirus infection, a large number of investigations have shown that CD8(+) T-cell responses are critical for the containment of AIDS viruses in humans and Asian nonhuman primates. Here we have compared the phenotypes of T-cell subsets and magnitudes of SIV-specific CD8(+) T-cell responses in vervet AGM chronically infected with SIVagm and rhesus monkeys (RM) infected with SIVmac. In comparison to RM, vervet AGM exhibited weaker signs of immune activation and associated proliferation of CD8(+) T cells as detected by granzyme B, Ki-67, and programmed death 1 staining. By gamma interferon enzyme-linked immunospot assay and intracellular cytokine staining, SIV Gag- and Env-specific immune responses were detectable at variable but lower levels in vervet AGM than in RM. These observations demonstrate that natural hosts like SIV-infected vervet AGM develop SIV-specific T-cell responses, but the disease-free course of infection does not depend on the generation of robust CD8(+) T-cell responses.     

Genetic diversity of simian immunodeficiency viruses from West African green monkeys: evidence of multiple genotypes within populations from the same geographical locale.

High simian immunodeficiency virus (SIV) seroprevalence rates have been reported in the different African green monkey (AGM) subspecies. Genetic diversity of these viruses far exceeds the diversity observed in the other lentivirus-infected human and nonhuman primates and is thought to reflect ancient introduction of SIV in the AGM population. We investigate here genetic diversity of SIVagm in wild-living AGM populations from the same geographical locale (i.e., sympatric population) in Senegal. For 11 new strains, we PCR amplified and sequenced two regions of the genome spanning the first tat exon and part of the transmembrane glycoprotein. Phylogenetic analysis of these sequences shows that viruses found in sympatric populations cluster into distinct lineages, with at least two distinct genotypes in each troop. These data strongly suggest an ancient introduction of these divergent viruses in the AGM population.     

SIVagm Infection in Wild African Green Monkeys from South Africa: Epidemiology,Natural History,and Evolutionary Considerations

Pathogenesis studies of SIV infection have not been performed to date in wild monkeys due to difficulty in collecting and storing samples on site and the lack of analytical reagents covering the extensive SIV diversity. We performed a large scale study of molecular epidemiology and natural history of SIVagm infection in 225 free-ranging AGMs from multiple locations in South Africa. SIV prevalence (established by sequencing pol, env, and gag) varied dramatically between infant/juvenile (7%) and adult animals (68%) (p<0.0001), and between adult females (78%) and males (57%). Phylogenetic analyses revealed an extensive genetic diversity, including frequent recombination events. Some AGMs harbored epidemiologically linked viruses. Viruses infecting AGMs in the Free State, which are separated from those on the coastal side by the Drakensberg Mountains, formed a separate cluster in the phylogenetic trees; this observation supports a long standing presence of SIV in AGMs, at least from the time of their speciation to their Plio-Pleistocene migration. Specific primers/probes were synthesized based on the pol sequence data and viral loads (VLs) were quantified. VLs were of 10⁴–10⁶ RNA copies/ml, in the range of those observed in experimentally-infected monkeys, validating the experimental approaches in natural hosts. VLs were significantly higher (10⁷–10⁸ RNA copies/ml) in 10 AGMs diagnosed as acutely infected based on SIV seronegativity (Fiebig II), which suggests a very active transmission of SIVagm in the wild. Neither cytokine levels (as biomarkers of immune activation) nor sCD14 levels (a biomarker of microbial translocation) were different between SIV-infected and SIV-uninfected monkeys. This complex algorithm combining sequencing and phylogeny, VL quantification, serology, and testing of surrogate markers of microbial translocation and immune activation permits a systematic investigation of the epidemiology, viral diversity and natural history of SIV infection in wild African natural hosts.     

Early divergence in lymphoid tissue apoptosis between pathogenic and nonpathogenic simian immunodeficiency virus infections of nonhuman primates

The events that contribute to the progression to AIDS during the acute phase of a primate lentiviral infection are still poorly understood. In this study, we used pathogenic and nonpathogenic simian models of simian immunodeficiency virus (SIV) infection of rhesus macaques (RMs) and African green monkeys (AGMs), respectively, to investigate the relationship between apoptosis in lymph nodes and the extent of viral replication, immune activation, and disease outcome. Here, we show that, in SIVmac251-infected RMs, a marked increased in lymphocyte apoptosis is evident during primary infection at the level of lymph nodes. Interestingly, the levels of apoptosis correlated with the extent of viral replication and the rate of disease progression to AIDS, with higher apoptosis in RMs of Indian genetic background than in those of Chinese origin. In stark contrast, no changes in the levels of lymphocyte apoptosis were observed during primary infection in the nonpathogenic model of SIVagm-sab infection of AGMs, despite similarly high rates of viral replication. A further and early divergence between SIV-infected RMs and AGMs was observed in terms of the dynamics of T- and B-cell proliferation in lymph nodes, with RMs showing significantly higher levels of cycling cells (Ki67⁺) in the T-cell zones in association with relatively low levels of Ki67⁺ in the B-cell zones, whereas AGMs displayed a low frequency of Ki67⁺ in the T-cell area but a high proportion of Ki67⁺ cells in the B-cell area. As such, this study suggests that species-specific host factors determine an early immune response to SIV that predominantly involves either cellular or humoral immunity in RMs and AGMs, respectively. Taken together, these data are consistent with the hypotheses that (i) high levels of T-cell activation and lymphocyte apoptosis are key pathogenic factors during pathogenic SIV infection of RMs and (ii) low T-cell activation and apoptosis are determinants of the AIDS resistance of SIVagm-infected AGMs, despite high levels of SIVagm replication.     

Simian immunodeficiency viruses replication dynamics in African non-human primate hosts: common patterns and species-specific differences

METHODS: To define potential common features of simian immunodeficiency virus (SIV) infections in different naturally infected host species, we compared the dynamics of viral replication in 31 African green monkeys (10 sabeus, 15 vervets and seven Caribbean AGMs), 14 mandrills and three sooty mangabeys (SMs) that were experimentally infected with their species-specific viruses. RESULTS: After infection, these SIVs replicated rapidly reaching viral loads (VLs) of 10(5)-10(9) copies/ml of plasma between days 9-14 post-infection (p.i). Set point viremia was established between days 42 and 60 p.i., with levels of approximately 10(5)-10(6) copies/ml in SM and mandrills, and lower levels (10(3)-10(5) copies/ml) in AGMs. VL during the chronic phase did not correlate with viral genome structure: SIVmnd-2 (a vpx-containing virus) and SIVmnd-1 (which does not contain vpu or vpx) replicated to similar levels in mandrills. VL was dependent on virus strain: vervets infected with three different viral strains showed different patterns of viral replication. The pattern of viral replication of SIVagm.sab, which uses both CCR5 and CXCR4 co-receptors was similar to those of the other viruses. CONCLUSIONS: Our results show a common pattern of SIV replication in naturally and experimentally infected hosts. This is similar overall to that observed in pathogenic SIV infection of macaques. This result indicates that differences in clinical outcome between pathogenic and non-pathogenic infections rely on host responses rather than the characteristics of the virus itself.     

Genetic diversity of simian immunodeficiency virus

We have demonstrated that the genetic diversity of simian immunodeficiency virus from African green monkeys (SIVagm) is much greater than that observed previously for individual HIV-1, HIV-2, or SIVmac isolates. Extensive genetic variation among SIVagm isolates and the high prevalence of green monkey infection without disease suggest that the virus has been in the green monkey population for a long time. We have also demonstrated that SIV from a sooty mangabey monkey (isolate SMM-7) is closer to SIVmac and HIV-2 than to HIV-1 and SIVagm. The extensive genetic diversity of SIVagm and the relatedness of SIVsmm to HIV-2 warrant continued examination of SIVagm and SIVsmm isolates from dispersed geographic regions. SIV strains much more closely related to HIV-1, HIV-2, or SIVmac may be found which would be reasonable candidates for recent cross-species transmission.     

Memory CD4+ T Lymphocytes in the Gastrointestinal Tract Are a Major Source of Cell-Associated Simian Immunodeficiency Virus in Chronic Nonpathogenic Infection of African Green Monkeys

Simian immunodeficiency virus (SIV) infection of natural hosts is characterized by nonpathogenic chronic viremia, maintenance of gastrointestinal epithelial barrier integrity, and low numbers of target cells. Assessment of cell-associated virus load in T cell subsets in multiple anatomic compartments of chronically SIV-infected sabeus African green monkeys (AGMs) revealed that gastrointestinal memory CD4⁺ T lymphocytes are a major source of cell-associated virus and a significant contributor to SIV viremia in AGMs.     

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.     

A distinct African lentivirus from Sykes' monkeys.

Asymptomatic infection with simian immunodeficiency virus (SIV) has been demonstrated in African Sykes' monkeys (Cercopithecus mitis albogularis), and virus isolation confirmed infection with a novel SIV from Sykes' monkeys (SIVsyk). Macaques inoculated with SIVsyk became persistently infected but remained clinically healthy. We utilized polymerase chain reaction amplification to generate a full-length, infectious molecular clone of SIVsyk. The genome organization of SIVsyk is similar to that of the other primate lentiviruses, consisting of gag, pol, vif, vpr, tat, rev, env, and nef. A unique feature is the absence of the highly conserved NF-kappa B binding site in the long terminal repeat. SIVsyk is genetically equidistant from other primate lentiviruses. Thus, SIVsyk represents a new group that is distinct from the four previously recognized primate lentivirus groups: human immunodeficiency virus type 1 (HIV-1), SIV from sooty mangabeys (SIVsmm) and HIV-2, SIV from African green monkeys (SIVagm), and SIV from mandrills (SIVmnd). The genetic differences between SIVsyk and SIVagm, isolates derived from monkeys of the same genus, underscore the potential for other distinct SIVs which have yet to be isolated and characterized.     

Transient Compartmentalization of Simian Immunodeficiency Virus Variants in the Breast Milk of African Green Monkeys

Natural hosts of simian immunodeficiency virus (SIV), African green monkeys (AGMs), rarely transmit SIV via breast-feeding. In order to examine the genetic diversity of breast milk SIV variants in this limited-transmission setting, we performed phylogenetic analysis on envelope sequences of milk and plasma SIV variants of AGMs. Low-diversity milk virus populations were compartmentalized from that in plasma. However, this compartmentalization was transient, as the milk virus lineages did not persist longitudinally.     

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.