Damaged Intestinal Epithelial Integrity Linked to Microbial Translocation in Pathogenic Simian Immunodeficiency Virus Infections

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.     

Microbial translocation in simian immunodeficiency virus (SIV)‐infected rhesus monkeys (Macaca mulatta)

Background Chronic immune activation is a hallmark of HIV infection and has been postulated as major factor in the pathogenesis of AIDS. Recent evidence suggests that activation of immune cells is triggered by microbial translocation through the impaired gastrointestinal barrier. Methods To determine the association between microbial translocation and disease progression, we have retrospectively analyzed microbial products, viral load and markers of immune activation in a cohort of 37 simian immunodeficiency virus‐infected rhesus monkeys, divided in two groups with distinct disease courses. Results As seen in HIV‐infected patients, we found elevated levels of lipopolysaccharide (LPS) in infected animals. However, LPS levels or LPS control mechanisms like endotoxin core antibodies or LPS‐binding protein did not differ between groups with different disease progression. In contrast, neopterin, a metabolic product of activated macrophages, was higher in fast progressors than in slow progressors. Conclusion Our data indicate that translocation of microbial products is not the major driving force of immune activation in HIV infection.     

Microbial translocation is a cause of systemic immune activation in chronic HIV infection

Chronic activation of the immune system is a hallmark of progressive HIV infection and better predicts disease outcome than plasma viral load, yet its etiology remains obscure. Here we show that circulating microbial products, probably derived from the gastrointestinal tract, are a cause of HIV-related systemic immune activation. Circulating lipopolysaccharide, which we used as an indicator of microbial translocation, was significantly increased in chronically HIV-infected individuals and in simian immunodeficiency virus (SIV)-infected rhesus macaques (P 相似文献   

Toll-like receptor ligands induce human T cell activation and death, a model for HIV pathogenesis

BACKGROUND: Recently, heightened systemic translocation of microbial products was found in persons with chronic HIV infection and this was linked to immune activation and CD4(+) T cell homeostasis. METHODOLOGY: We examined here the effects of microbial Toll-like receptor (TLR) ligands on T cell activation in vitro. CONCLUSIONS/FINDINGS: We show that exposure to TLR ligands results in activation of memory and effector CD4(+) and CD8(+) T cells. After exposure to each of 8 different ligands that activate TLRs 2, 3, 4, 5, 7, 8, and 9, CD8(+) T cells are activated and gain expression of the C type lectin CD69 that may promote their retention in lymphoid tissues. In contrast, CD4(+) T cells rarely increase CD69 expression but instead enter cell cycle. Despite activation and cell cycle entry, CD4(+) T cells divide poorly and instead, disproportionately undergo activation-induced cell death. Systemic exposure to TLR agonists may therefore increase immune activation, effector cell sequestration in lymphoid tissues and T cell turnover. These events may contribute to the pathogenesis of immune dysfunction and CD4+ T cell losses in chronic infection with the human immunodeficiency virus.     

A Compositional Look at the Human Gastrointestinal Microbiome and Immune Activation Parameters in HIV Infected Subjects

HIV progression is characterized by immune activation and microbial translocation. One factor that may be contributing to HIV progression could be a dysbiotic microbiome. We therefore hypothesized that the GI mucosal microbiome is altered in HIV patients and this alteration correlates with immune activation in HIV. 121 specimens were collected from 21 HIV positive and 22 control human subjects during colonoscopy. The composition of the lower gastrointestinal tract mucosal and luminal bacterial microbiome was characterized using 16S rDNA pyrosequencing and was correlated to clinical parameters as well as immune activation and circulating bacterial products in HIV patients on ART. The composition of the HIV microbiome was significantly different than that of controls; it was less diverse in the right colon and terminal ileum, and was characterized by loss of bacterial taxa that are typically considered commensals. In HIV samples, there was a gain of some pathogenic bacterial taxa. This is the first report characterizing the terminal ileal and colonic mucosal microbiome in HIV patients with next generation sequencing. Limitations include use of HIV-infected subjects on HAART therapy.     

Suppressed Th17 Levels Correlate with Elevated PIAS3, SHP2, and SOCS3 Expression in CD4 T Cells during Acute Simian Immunodeficiency Virus Infection

T helper 17 (Th17) cells play an important role in mucosal immune homeostasis and maintaining the integrity of the mucosal epithelial barrier. Loss of Th17 cells has been extensively documented during human immunodeficiency virus (HIV) and simian immunodeficiency virus (SIV) infections. The lack of effective repopulation of Th17 cells has been associated with chronic immune activation mediated by the translocation of microbial products. Using ex vivo analysis of purified peripheral blood CD4 T cells from SIV-infected rhesus macaques, we show that the suppression of interleukin-17 (IL-17) expression correlated with upregulated expression of negative regulatory genes PIAS3, SHP2, and SOCS3 in CD4 T cells. Suppressed Th17 expression was accompanied by elevated levels of soluble CD14 (sCD14) and lipopolysaccharide binding protein (LBP) in the plasma during early stages of infection. Plasma viral loads rather than sCD14 or LBP levels correlated with acute immune activation. Additionally, we observed a significant increase in the expression of CD14 on peripheral blood monocytes that correlated with IL-23 expression and markers of microbial translocation. Taken together, our results provide new insights into the early events associated with acute SIV pathogenesis and suggest additional mechanisms playing a role in suppression of Th17 cells.     

Circulating Microbial Products and Acute Phase Proteins as Markers of Pathogenesis in Lymphatic Filarial Disease

Lymphatic filariasis can be associated with development of serious pathology in the form of lymphedema, hydrocele, and elephantiasis in a subset of infected patients. Dysregulated host inflammatory responses leading to systemic immune activation are thought to play a central role in filarial disease pathogenesis. We measured the plasma levels of microbial translocation markers, acute phase proteins, and inflammatory cytokines in individuals with chronic filarial pathology with (CP Ag+) or without (CP Ag−) active infection; with clinically asymptomatic infections (INF); and in those without infection (endemic normal [EN]). Comparisons between the two actively infected groups (CP Ag+ compared to INF) and those without active infection (CP Ag− compared to EN) were used preliminarily to identify markers of pathogenesis. Thereafter, we tested for group effects among all the four groups using linear models on the log transformed responses of the markers. Our data suggest that circulating levels of microbial translocation products (lipopolysaccharide and LPS-binding protein), acute phase proteins (haptoglobin and serum amyloid protein-A), and inflammatory cytokines (IL-1β, IL-12, and TNF-α) are associated with pathogenesis of disease in lymphatic filarial infection and implicate an important role for circulating microbial products and acute phase proteins.     

The immunomodulatory properties of probiotic microorganisms beyond their viability (ghost probiotics: proposal of paraprobiotic concept)

The probiotic approach represents a potentially effective and mild alternative strategy for the prevention and treatment of either inflammatory or allergic diseases. Several studies have shown that different bacterial strains can exert their probiotic abilities by influencing the host's immune system, thereby modulating immune responses. However, the emerging concern regarding safety problems arising from the extensive use of live microbial cells is enhancing the interest in non-viable microorganisms or microbial cell extracts, as they could eliminate shelf-life problems and reduce the risks of microbial translocation and infection. The purpose of this review is to provide an overview of the scientific literature concerning studies in which dead microbial cells or crude microbial cell fractions have been used as health-promoting agents. Particular attention will be given to the modulation of host immune responses. Possible mechanisms determining the effect on the immune system will also be discussed. Finally, in the light of the FAO/WHO definition of probiotics, indicating that the word 'probiotic' should be restricted to products that contain live microorganisms, and considering the scientific evidence indicating that inactivated microbes can positively affect human health, we propose the new term 'paraprobiotic' to indicate the use of inactivated microbial cells or cell fractions to confer a health benefit to the consumer.     

Tobacco Smoking Increases Immune Activation and Impairs T-Cell Function in HIV Infected Patients on Antiretrovirals: A Cross-Sectional Pilot Study

Background

The influence of tobacco smoking on the immune system of HIV infected individuals is largely unknown. We investigated the impact of tobacco smoking on immune activation, microbial translocation, immune exhaustion and T-cell function in HIV infected individuals.

Method

HIV infected smokers and non-smokers (n = 25 each) with documented viral suppression on combination antiretroviral therapy and HIV uninfected smokers and non-smokers (n = 15 each) were enrolled. Markers of immune activation (CD38 and HLA-DR) and immune exhaustion (PD1, Tim3 and CTLA4) were analyzed in peripheral blood mononuclear cells (PBMCs) by flow cytometry. Plasma markers of microbial translocation (soluble-CD14 - sCD14 and lipopolysaccharide - LPS) were measured. Antigen specific functions of CD4+ and CD8+ T-cells were measured, by flow cytometry, in PBMCs after 6 hours stimulation with Cytomegalovirus, Epstein-Barr virus and Influenza Virus (CEF) peptide pool.

Results

Compared to non-smokers, smokers of HIV infected and uninfected groups showed significantly higher CD4+ and CD8+ T-cell activation with increased frequencies of CD38+HLA-DR+ cells with a higher magnitude in HIV infected smokers. Expressions of immune exhaustion markers (PD1, Tim3 and CTLA4) either alone or in combinations were significantly higher in smokers, especially on CD4+ T-cells. Compared to HIV uninfected non-smokers, microbial translocation (sCD14 and LPS) was higher in smokers of both groups and directly correlated with CD4+ and CD8+ T-cell activation. Antigen specific T-cell function showed significantly lower cytokine response of CD4+ and CD8+ T-cells to CEF peptide-pool stimulation in smokers of both HIV infected and uninfected groups.

Conclusions

Our results suggest that smoking and HIV infection independently influence T-cell immune activation and function and together they present the worst immune profile. Since smoking is widespread among HIV infected individuals, studies are warranted to further evaluate the cumulative effect of smoking on impairment of the immune system and accelerated disease progression.     

Toll-like receptors and innate immunity

Toll-like receptors have a crucial role in the detection of microbial infection in mammals and insects. In mammals, these receptors have evolved to recognize conserved products unique to microbial metabolism. This specificity allows the Toll proteins to detect the presence of infection and to induce activation of inflammatory and antimicrobial innate immune responses. Recognition of microbial products by Toll-like receptors expressed on dendritic cells triggers functional maturation of dendritic cells and leads to initiation of antigen-specific adaptive immune responses.     

Increased Escherichia coli-Induced Interleukin-23 Production by CD16+ Monocytes Correlates with Systemic Immune Activation in Untreated HIV-1-Infected Individuals

The level of microbial translocation from the intestine is increased in HIV-1 infection. Proinflammatory cytokine production by peripheral antigen-presenting cells in response to translocated microbes or microbial products may contribute to systemic immune activation, a hallmark of HIV-1 infection. We investigated the cytokine responses of peripheral blood myeloid dendritic cells (mDCs) and monocytes to in vitro stimulation with commensal enteric Escherichia coli in peripheral blood mononuclear cells (PBMC) from untreated HIV-1-infected subjects and from uninfected controls. Levels of interleukin 23 (IL-23) produced by PBMC from HIV-1-infected subjects in response to E. coli stimulation were significantly higher than those produced by PBMC from uninfected subjects. IL-23 was produced primarily by CD16⁺ monocytes. This subset of monocytes was increased in frequency and expressed higher levels of Toll-like receptor 4 (TLR4) in HIV-1-infected individuals than in controls. Blocking TLR4 on total CD14⁺ monocytes reduced IL-23 production in response to E. coli stimulation. Levels of soluble CD27, an indicator of systemic immune activation, were elevated in HIV-1-infected subjects and were associated with the percentage of CD16⁺ monocytes and the induction of IL-23 by E. coli, providing a link between these parameters and systemic inflammation. Taken together, these results suggest that IL-23 produced by CD16⁺ monocytes in response to microbial stimulation may contribute to systemic immune activation in HIV-1-infected individuals.     

Microbial immune suppression mediated by direct engagement of inhibitory Fc receptor

A microbial polysaccharide (glucuronoxylomannan (GXM)) exerts potent immunosuppression by direct engagement to immunoinhibitory receptor FcgammaRIIB. Activation of FcgammaRIIB by GXM leads to the recruitment and phosphorylation of SHIP that prevents IkappaBalpha activation. The FcgammaRIIB blockade inhibits GXM-induced IL-10 production and induces TNF-alpha secretion. GXM quenches LPS-induced TNF-alpha release via FcgammaRIIB. The addition of mAb to GXM reverses GXM-induced immunosuppression by shifting recognition from FcgammaRIIB to FcgammaRIIA. These findings indicate a novel mechanism by which microbial products can impair immune function through direct stimulation of an inhibitory receptor. Furthermore, our observations provide a new mechanism for the ability of specific Ab to reverse the immune inhibitory effects of certain microbial products.     

Higher Frequency of NK and CD4+ T-Cells in Mucosa and Potent Cytotoxic Response in HIV Controllers

HIV infection induces immune alterations, mainly in gut mucosa, where the main target cells reside. However, the evolution of the infection is variable among infected individuals, as evidenced by HIV controllers who exhibit low or undetectable viral load in the absence of treatment. The aim of this study was to evaluate the frequency, phenotype and activity of T and NK cells in peripheral blood and gut mucosa in a cohort of Colombian HIV controllers. Blood and gut biopsies were included. The frequency and the activation status of T and NK cells were performed by flow cytometry. In addition, Gag-stimulated CD8⁺ T-cells and cytokine-stimulated NK cells were tested for cytotoxic activity. Finally, microbial translocation was measured by plasma lipopolysaccharide quantification. Compared with HIV-progressors, HIV controllers exhibited higher frequency of CD4⁺ T and NK cells, and lower expression of activation molecules in blood and mucosal immune cells, as well as lower microbial translocation. An increased production of molecules associated with cytotoxic activity of CD8⁺ T-cells in blood and mucosa and a higher percentage of polyfunctional CD8⁺ T cells in blood were also observed in HIV controllers. In addition, an increased activity of NK cells was observed in blood. These findings suggest that HIV controllers have a potent immune response, mainly mediated by cytotoxic cells that control HIV replication, which contribute to reducing alterations at the gut mucosa.     

Cutting edge: Experimentally induced immune activation in natural hosts of simian immunodeficiency virus induces significant increases in viral replication and CD4+ T cell depletion

Chronically SIVagm-infected African green monkeys (AGMs) have a remarkably stable nonpathogenic disease course, with levels of immune activation in chronic SIVagm infection similar to those observed in uninfected monkeys and with stable viral loads for long periods of time. In vivo administration of LPS or an IL-2/diphtheria toxin fusion protein (Ontak) to chronically SIVagm-infected AGMs triggered increases in immune activation and subsequently of viral replication and depletion of intestinal CD4(+) T cells. Our study indicates that circulating microbial products can increase viral replication by inducing immune activation and increasing the number of viral target cells, thus demonstrating that immune activation and T cell proliferation are key factors in AIDS pathogenesis.     

The role of lipopolysaccharide as a marker of immune activation in HIV-1 infected patients: a systematic literature review

ABSTRACT: BACKGROUND: Recent observational studies suggest a role for lipopolysaccharide (LPS) as a marker of immune activation in HIV-infected patients, with potential repercussions on the effectiveness of antiretroviral regimens.ObjectA systematic review of LPS as a marker of immune activation in HIV-1 infected patients. DATA SOURCES: MEDLINE register of articles and international conference proceedings. REVIEW METHODS: Case--control studies comparing the role of plasma LPS as a marker of immune activation in HIV-infected patients versus HIV negative subjects.Data synthesisTwo hundred and six articles were selected using MEDLINE, plus 51 studies presented at international conferences. Plasma LPS is a marker of immune activation in HIV-infected patients, determining the entry of central memory CD4+ T cells into the replication cycle and finally generating cell death. Plasma LPS probably results from immune-mediated alterations of the intestinal barrier, which can occur soon after HIV seroconversion. LPS is a likely marker of disease progression, as it drives chronic monocyte activation, and some studies suggest that hyperexpression of CCR5 receptors, related to LPS plasma levels, could be responsible for monocyte trafficking in the brain compartment and for the appearance of HIV-associated neurocognitive disorders. Long-term combination antiretroviral therapy (cART) generally reduces LPS concentrations, but rarely to the same levels as in the control group. This phenomenon probably depends on ongoing but incomplete repair of the mucosal barrier. Only in patients achieving maximal viral suppression (i.e. viral load < 2.5 cp/ml) are LPS levels comparable to healthy donors. In successfully treated patients who did not restore CD4+ T cells, one hypothesis is that the degree of residual microbial translocation, measured by LPS, alters the turnover of CD4+ T cells. CONCLUSIONS: LPS is a marker of microbial translocation, responsible for chronic immune activation in HIV-infected patients. Even in successfully treated patients, LPS values are rarely normal. Several studies suggest a role for LPS as a negative predictive marker of immune restoration in patients with blunted CD4 T cell gain.     

Negative regulation of toll-like receptor-mediated immune responses

Toll-like receptors (TLRs) are involved in host defence against invading pathogens, functioning as primary sensors of microbial products and activating signalling pathways that induce the expression of immune and pro-inflammatory genes. However, TLRs have also been implicated in several immune-mediated and inflammatory diseases. As the immune system needs to constantly strike a balance between activation and inhibition to avoid detrimental and inappropriate inflammatory responses, TLR signalling must be tightly regulated. Here, we discuss the various negative regulatory mechanisms that have evolved to attenuate TLR signalling to maintain this immunological balance.     

Secretory leukoprotease inhibitor in mucosal lymph node dendritic cells regulates the threshold for mucosal tolerance

The notion that the mucosal immune system maintains a tolerogenic response to harmless Ags while continually being challenged with microbial products seems an enigma. The aim of this study was to unravel mechanisms that are involved in regulating the development of tolerance under constant microbial pressure. The tolerogenic response to Ags administered via the nasal mucosa is dependent on the organized lymphoid tissue of the cervical lymph nodes (LN). We show that cervical LN differentially express secretory leukoprotease inhibitor (SLPI) compared with peripheral LN. SLPI was expressed by dendritic cells (DCs) and because SLPI is known to suppress LPS responsiveness, it was hypothesized that its expression in mucosal DCs may be required to regulate cellular activation to microbial products. Indeed, compared with wild-type controls, bone marrow-derived DCs from SLPI(-/-) mice released more inflammatory cytokines and enhanced T cell proliferation after stimulation with low dose LPS. This increased sensitivity to LPS was accompanied by increased NF-kappaB p65 activation in SLPI(-/-) DCs. In vivo, nasal application of OVA with LPS to SLPI(-/-) mice resulted in enhanced DC activation in the cervical LN reflected by increased costimulatory molecule expression and release of inflammatory cytokines. This led to failure to maintain tolerance to nasal OVA application in the presence of low doses of LPS. We propose that expression of SLPI functions as a rheostat by controlling the level of bacterial stimuli that induce mucosal DC activation. As such, it regulates the quality of the ensuing Ag-specific immune response in the mucosa draining LN.     

Toll-like receptors are key participants in innate immune responses

During an infection, one of the principal challenges for the host is to detect the pathogen and activate a rapid defensive response. The Toll-like family of receptors (TLRs), among other pattern recognition receptors (PRR), performs this detection process in vertebrate and invertebrate organisms. These type I transmembrane receptors identify microbial conserved structures or pathogen-associated molecular patterns (PAMPs). Recognition of microbial components by TLRs initiates signaling transduction pathways that induce gene expression. These gene products regulate innate immune responses and further develop an antigen-specific acquired immunity. TLR signaling pathways are regulated by intracellular adaptor molecules, such as MyD88, TIRAP/Mal, between others that provide specificity of individual TLR- mediated signaling pathways. TLR-mediated activation of innate immunity is involved not only in host defense against pathogens but also in immune disorders. The involvement of TLR-mediated pathways in auto-immune and inflammatory diseases is described in this review article.     

Lack of Evidence for mtDNA as a Biomarker of Innate Immune Activation in HIV Infection

Many human immunodeficiency virus (HIV) infected individuals suffer from persistent immune activation. Chronic inflammation and immune dysregulation have been associated with an increased risk of age-related diseases even among patients on highly active antiretroviral therapy. The factors leading to immune activation are complex, but have been hypothesized to include persistent viral replication with cellular death as well as microbial translocation across the gastrointestinal tract. Both processes may trigger innate immune responses since many native molecules released from dying cells are similar in structure to pathogen associated molecular patterns. These damage associated molecular patterns include mitochondrial DNA and formylated peptides. We hypothesized that circulating mitochondrial nucleic acid could serve as a biomarker for HIV-associated cell death and drive innate immune activation in infected individuals. We developed a quantitative polymerase chain reaction assay for plasma mitochondrial DNA and validated it on normal blood donors. We then measured mitochondrial DNA levels in acute and chronic HIV infection. While the assay proved to be accurate with a robust dynamic range, we did not find a significant association between HIV disease status and circulating mitochondrial DNA. We did, however, observe a negative correlation between age and plasma mitochondrial DNA levels in individuals with well-controlled HIV.