The chronic phase of HIV infection is marked by pathological activation of the immune system, the extent of which better predicts disease progression than either plasma viral load or CD4+ T cell count. Recently, translocation of microbial products from the gastrointestinal tract has been proposed as an underlying cause of this immune activation, based on indirect evidence including the detection of microbial products and specific immune responses in the plasma of chronically HIV-infected humans or SIV-infected Asian macaques. We analyzed tissues from SIV-infected rhesus macaques (RMs) to provide direct in situ evidence for translocation of microbial constituents from the lumen of the intestine into the lamina propria and to draining and peripheral lymph nodes and liver, accompanied by local immune responses in affected tissues. In chronically SIV-infected RMs this translocation is associated with breakdown of the integrity of the epithelial barrier of the gastrointestinal (GI) tract and apparent inability of lamina propria macrophages to effectively phagocytose translocated microbial constituents. By contrast, in the chronic phase of SIV infection in sooty mangabeys, we found no evidence of epithelial barrier breakdown, no increased microbial translocation and no pathological immune activation. Because immune activation is characteristic of the chronic phase of progressive HIV/SIV infections, these findings suggest that increased microbial translocation from the GI tract, in excess of capacity to clear the translocated microbial constituents, helps drive pathological immune activation. Novel therapeutic approaches to inhibit microbial translocation and/or attenuate chronic immune activation in HIV-infected individuals may complement treatments aimed at direct suppression of viral replication.

HIV/SIV disease progression is associated with multifocal damage to the GI tract epithelial barrier that correlates with microbial translocation and persistent pathological immune activation but the underlying mechanisms remain unclear. Investigating alterations in mucosal immunity during SIV infection, we found that damage to the colonic epithelial barrier was associated with loss of multiple lineages of IL-17-producing lymphocytes, cells that microarray analysis showed express genes important for enterocyte homeostasis, including IL-22. IL-22-producing lymphocytes were also lost after SIV infection. Potentially explaining coordinate loss of these distinct populations, we also observed loss of CD103+ DCs after SIV infection which associated with loss of IL-17 and IL-22-producing lymphocytes. CD103+ DCs expressed genes associated with promotion of IL-17/IL-22+ cells, and co-culture of CD103+ DCs and naïve T-cells led to increased IL17A and RORc expression in differentiating T-cells. These results reveal complex interactions between mucosal immune cell subsets providing potential mechanistic insights into mechanisms of mucosal immune dysregulation during HIV/SIV infection, and offer hints for development of novel therapeutic strategies to address this aspect of AIDS virus pathogenesis.

African green monkeys (genus Chlorocebus) can be infected with SIVagm, but do not develop AIDS. This natural host of SIV, like sooty mangabeys, maintains high levels of SIV replication but has evolved to avoid immunodeficiency. Elucidating the mechanisms that allow the natural hosts to co-exist with SIV without overt disease may provide crucial information to understand AIDS pathogenesis. Here we show: (1) many CD4+ T cells from African green monkeys down-regulate CD4 in vivo as they enter the memory pool, (2) down regulation of CD4 by memory T cells is independent of SIV infection, (3) the CD4− memory T cells maintain functions which are normally attributed to CD4 T cells including production of IL-2, production of IL-17, expression of FoxP3 and expression of CD40L (4) loss of CD4 expression protects these T cells from infection by SIVagm in vivo, and (5) these CD4− T cells can maintain MHC-II restriction. These data demonstrate that the absence of SIV-induced disease progression in natural hosts species may be partially explained by preservation of a subset of T cells that maintain CD4 T cell function while being resistant to SIV-infection in vivo.

The mechanisms underlying the AIDS resistance of natural hosts for simian immunodeficiency virus (SIV) remain unknown. Recently, it was proposed that natural SIV hosts avoid disease because their plasmacytoid dendritic cells (pDCs) are intrinsically unable to produce alpha interferon (IFN-␣) in response to SIV RNA stimulation. However, here we show that (i) acute SIV infections of natural hosts are associated with a rapid and robust type I IFN response in vivo, (ii) pDCs are the principal in vivo producers of IFN-␣/␤ at peak acute infection in lymphatic tissues, and (iii) natural SIV hosts downregulate these responses in early chronic infection. In contrast, persistently high type I IFN responses are observed during pathogenic SIV infection of rhesus macaques.

Pigtail macaques (PTM) rapidly progress to AIDS after SIV infection. Given the strong association between HIV/SIV disease progression and microbial translocation and immune activation, we assessed whether high basal levels of immune activation and microbial translocation exist in PTM. We found that prior to SIV infection, PTM had high levels of microbial translocation that correlated with significant damage to the structural barrier of the GI tract. Moreover, this increased microbial translocation correlated with high levels of immune activation and was associated with high frequencies of IL-17-producing T cells. These data highlight the relationship between mucosal damage, microbial translocation and systemic immune activation in the absence of HIV/SIV replication and underscore the importance of microbial translocation in the rapid course of disease progression in SIV-infected PTM. Furthermore, these data suggest that PTM may be an ideal model to study therapeutic interventions aimed at decreasing microbial translocation-induced immune activation.

We characterized the acute B cell response in adults with cholera by analyzing the repertoire, specificity, and functional characteristics of 138 monoclonal antibodies (MAbs) generated from single-cell-sorted plasmablasts. We found that the cholera-induced responses were characterized by high levels of somatic hypermutation and large clonal expansions. A majority of the expansions targeted cholera toxin (CT) or lipopolysaccharide (LPS). Using a novel proteomics approach, we were able to identify sialidase as another major antigen targeted by the antibody response to Vibrio cholerae infection. Antitoxin MAbs targeted both the A and B subunits, and most were also potent neutralizers of enterotoxigenic Escherichia coli heat-labile toxin. LPS-specific MAbs uniformly targeted the O-specific polysaccharide, with no detectable responses to either the core or the lipid moiety of LPS. Interestingly, the LPS-specific antibodies varied widely in serotype specificity and functional characteristics. One participant infected with the Ogawa serotype produced highly mutated LPS-specific antibodies that preferentially bound the previously circulating Inaba serotype. This demonstrates durable memory against a polysaccharide antigen presented at the mucosal surface and provides a mechanism for the long-term, partial heterotypic immunity seen following cholera.

Many species of African nonhuman primates are natural hosts for individual strains of simian immunodeficiency virus (SIV). These infected animals do not, however, develop AIDS. Here we show that multiple species of African nonhuman primate species characteristically have low frequencies of CD4 ؉ T cells and high frequencies of both T cells that express only the alpha-chain of CD8 and double-negative T cells. These subsets of T cells are capable of eliciting functions generally associated with CD4 ؉ T cells, yet these cells lack surface expression of the CD4 protein and are, therefore, poor targets for SIV in vivo. These data demonstrate that coevolution with SIV has, in several cases, involved downregulation of receptors for the virus by otherwisesusceptible host target cells. Understanding the genetic factors that lead to downregulation of these receptors may lead to therapeutic interventions that mimic this modulation in progressive infections.

Introduction␥␦ T cells are a minor group of T lymphocytes and are distinct from ␣␤ T cells. 1,2 In humans and nonhuman primates, ␥␦ T cells are composed of 2 predominant subsets based on the differential expression of V␦1 and V␦2 genes. Although the antigen specificity and recognition properties of these 2 subsets have yet to be fully elucidated, 3 ␥␦ T cells can expand during bacterial infections. 4 Indeed, V␦1 T cells can recognize small lipoprotein antigens produced by bacterial pathogens, 5 and V␦2 T cells can respond to several distinct chemical structures such as alkylamines 4 and small phosphoantigens, 6 some of which can be produced either as a byproduct of the microbial nonmevalonate pathway or by altered metabolic pathways in stressed host cells during pathogenic infections. 7 In general, ␥␦ T cells represent approximately 4% of peripheral blood T cells, and the majority of these express the V␦2 gene. 4 However, in the gastrointestinal tract, V␦1 T cells are present at higher frequencies and can comprise up to 40% of intraepithelial lymphocytes. 8 Functionally, ␥␦ T cells are similar to ␣␤ T cells in that they can produce interleukin 17 (IL17), interferon ␥ (IFN␥), and other soluble factors after stimulation through the T-cell receptor (TCR). 9,10 Moreover, ␥␦ T cells have been shown to be critical for the recruitment of neutrophils during bacterial infections. 11 The V␦1 subset also contributes to maintenance of the epidermis and the gastrointestinal (GI) epithelium through the production of keratinocyte and epithelial growth factors. [12][13][14] Alterations of ␥␦ T-cell subsets occur during progressive HIV infection and pathogenic Simian immunodeficiency virus (SIV) infections of rhesus macaques (RMs). [15][16][17][18] Specifically, the V␦1 subset, which is usually localized to the mucosal tissues but not the periphery, becomes prevalent in the peripheral blood relative to the V␦2 subset. 15,16 The mechanisms by which this peripheral V␦1/V␦2 T-cell inversion develops are not well understood, although it has been suggested that preferential loss of V␦2 T cells, thymic dysfunction, and/or V␦1 T-cell expansion may be responsible. [16][17][18][19] However, while the biological consequences of these perturbations remain unclear, therapeutic interventions aimed at expanding the V␦2 T-cell subset have resulted in enhanced neutralizing antibody titers in chronically SHIV-infected RMs. 20 The mechanism leading to enhanced V␦2 T cell cytokine production and elevated neutralizing antibody titers in this study was largely unknown.A better understanding of the mechanisms that underlie alterations in ␥␦ T-cell subsets is crucial for future therapeutic interventions aimed at modulating ␥␦ T cells. Chronic immune activation is closely associated with disease progression in HIV/SIV infection, and microbial translocation is well described as one cause of immune activation. 21,22 As ␥␦ T cells seem to be important in the early stages of innate responses to invading microbes, and V␦1 T cells play an important role in g...