Evidence for dendritic cell-dependent CD4(+) T helper-1 type responses to commensal bacteria in normal human intestinal lamina propria

Microbiota-dependent indole production stimulates the development of collagen-induced arthritis in mice

Altered tryptophan catabolism has been identified in inflammatory diseases like rheumatoid arthritis (RA) and spondyloarthritis (SpA), but the causal mechanisms linking tryptophan metabolites to disease are unknown. Using the collagen-induced arthritis (CIA) model, we identified alterations in tryptophan metabolism, and specifically indole, that correlated with disease. We demonstrated that both bacteria and dietary tryptophan were required for disease and that indole supplementation was sufficient to induce disease in their absence. When mice with CIA on a low-tryptophan diet were supplemented with indole, we observed significant increases in serum IL-6, TNF, and IL-1β; splenic RORγt+CD4+ T cells and ex vivo collagen-stimulated IL-17 production; and a pattern of anti-collagen antibody isotype switching and glycosylation that corresponded with increased complement fixation. IL-23 neutralization reduced disease severity in indole-induced CIA. Finally, exposure of human colonic lymphocytes to indole increased the expression of genes involved in IL-17 signaling and plasma cell activation. Altogether, we propose a mechanism by which intestinal dysbiosis during inflammatory arthritis results in altered tryptophan catabolism, leading to indole stimulation of arthritis development. Blockade of indole generation may present a unique therapeutic pathway for RA and SpA.

Microbiota-dependent indole production is required for the development of collagen-induced arthritis

Altered tryptophan catabolism has been identified in inflammatory diseases like rheumatoid arthritis (RA) and spondyloarthritis (SpA), but the causal mechanisms linking tryptophan metabolites to disease are unknown. Using the collagen-induced arthritis (CIA) model we identify alterations in tryptophan metabolism, and specifically indole, that correlate with disease. We demonstrate that both bacteria and dietary tryptophan are required for disease, and indole supplementation is sufficient to induce disease in their absence. When mice with CIA on a low-tryptophan diet were supplemented with indole, we observed significant increases in serum IL-6, TNF, and IL-1β; splenic RORγt+CD4+ T cells and ex vivo collagen-stimulated IL-17 production; and a pattern of anti-collagen antibody isotype switching and glycosylation that corresponded with increased complement fixation. IL-23 neutralization reduced disease severity in indole-induced CIA. Finally, exposure of human colon lymphocytes to indole increased expression of genes involved in IL-17 signaling and plasma cell activation. Altogether, we propose a mechanism by which intestinal dysbiosis during inflammatory arthritis results in altered tryptophan catabolism, leading to indole stimulation of arthritis development. Blockade of indole generation may present a novel therapeutic pathway for RA and SpA.

Distinct Type I Interferon Subtypes Differentially Stimulate T Cell Responses in HIV-1-Infected Individuals

The expression of type I interferons (IFNs) is one of the immediate host responses during most viral infections. The type I IFN family consists of numerous highly conserved IFNα subtypes, IFNβ, and some others. Although these IFNα subtypes were initially believed to act interchangeably, their discrete biological properties are nowadays widely accepted. Subtype-specific antiviral, immunomodulatory, and anti-proliferative activities were reported explained by differences in receptor affinity, downstream signaling events, and individual IFN-stimulated gene expression patterns. Type I IFNs and increased IFN signatures potentially linked to hyperimmune activation of T cells are critically discussed for chronic HIV (human immunodeficiency virus) infection. Here, we aimed to analyze the broad immunological effects of specific type I IFN subtypes (IFNα2, IFNα14, and IFNβ) on T and NK cell subsets during HIV-1 infection in vitro and ex vivo. Stimulation with IFNα14 and IFNβ significantly increased frequencies of degranulating (CD107a⁺) gut-derived CD4⁺ T cells and blood-derived T and NK cells. However, frequencies of IFNγ-expressing T cells were strongly reduced after stimulation with IFNα14 and IFNβ. Phosphorylation of downstream molecules was not only IFN subtype-specific; also, significant differences in STAT5 phosphorylation were observed in both healthy peripheral blood mononuclear cells (PBMCs) and PBMCs of HIV-infected individuals, but this effect was less pronounced in healthy gut-derived lamina propria mononuclear cells (LPMCs), assuming cell and tissue specific discrepancies. In conclusion, we observed distinct type I IFN subtype-specific potencies in stimulating T and NK cell responses during HIV-1-infection.

Granzyme B+ CD4 T cells accumulate in the colon during chronic HIV-1 infection

Chronic HIV-1 infection results in the sustained disruption of gut homeostasis culminating in alterations in microbial communities (dysbiosis) and increased microbial translocation. Major questions remain on how interactions between translocating microbes and gut immune cells impact HIV-1-associated gut pathogenesis. We previously reported that in vitro exposure of human gut cells to enteric commensal bacteria upregulated the serine protease and cytotoxic marker Granzyme B (GZB) in CD4 T cells, and GZB expression was further increased in HIV-1-infected CD4 T cells. To determine if these in vitro findings extend in vivo, we evaluated the frequencies of GZB⁺ CD4 T cells in colon biopsies and peripheral blood of untreated, chronically infected people with HIV-1 (PWH). Colon and blood GZB⁺ CD4 T cells were found at significantly higher frequencies in PWH. Colon, but not blood, GZB⁺ CD4 T cell frequencies were associated with gut and systemic T cell activation and Prevotella species abundance. In vitro, commensal bacteria upregulated GZB more readily in gut versus blood or tonsil-derived CD4 T cells, particularly in inflammatory T helper 17 cells. Bacteria-induced GZB expression in gut CD4 T cells required the presence of accessory cells, the IL-2 pathway and in part, MHC Class II. Overall, we demonstrate that GZB⁺ CD4 T cells are prevalent in the colon during chronic HIV-1 infection and may emerge following interactions with translocated bacteria in an IL-2 and MHC Class II-dependent manner. Associations between GZB⁺ CD4 T cells, dysbiosis and T cell activation suggest that GZB⁺ CD4 T cells may contribute to gut HIV-1 pathogenesis.

Butyrate directly decreases human gut lamina propria CD4 T cell function through histone deacetylase (HDAC) inhibition and GPR43 signaling

An important function of the gut microbiome is the fermentation of non-digestible dietary fibers into short chain fatty acids (SCFAs). The three primary SCFAs: acetate, propionate, and butyrate, are key mediators of metabolism and immune cell function in the gut mucosa. We previously demonstrated that butyrate at high concentrations decreased human gut lamina propria (LP) CD4 T cell activation in response to enteric bacteria exposure in vitro. However, to date, the mechanism by which butyrate alters human gut LP CD4 T cell activation remains unknown. In this current study, we sought to better understand how exposure to SCFAs across a concentration range impacted human gut LP CD4 T cell function and activation. LP CD4 T cells were directly activated with T cell receptor (TCR) beads in vitro in the presence of a physiologic concentration range of each of the primary SCFAs. Exposure to butyrate potently inhibited CD4 T cell activation, proliferation, and cytokine (IFNγ, IL-17) production in a concentration dependent manner. Butyrate decreased the proliferation and cytokine production of T helper (Th) 1, Th17 and Th22 cells, with differences noted in the sensitivity of LP versus peripheral blood Th cells to butyrate's effects. Higher concentrations of propionate and acetate relative to butyrate were required to inhibit CD4 T cell activation and proliferation. Butyrate directly increased the acetylation of both unstimulated and TCR-stimulated CD4 T cells, and apicidin, a Class I histone deacetylase inhibitor, phenocopied butyrate's effects on CD4 T cell proliferation and activation. GPR43 agonism phenocopied butyrate's effect on CD4 T cell proliferation whereas a GPR109a agonist did not. Our findings indicate that butyrate decreases in vitro human gut LP CD4 T cell activation, proliferation, and inflammatory cytokine production more potently than other SCFAs, likely through butyrate's ability to increase histone acetylation, and potentially via signaling through GPR43. These findings have relevance in furthering our understanding of how perturbations of the gut microbiome alter local immune responses in the gut mucosa.

Gut Bacteria Induce Granzyme B Expression in Human Colonic ILC3s In Vitro in an IL-15-Dependent Manner

Group 3 innate lymphoid cells (ILC3s) in the gut mucosa have long been thought to be noncytotoxic lymphocytes that are critical for homeostasis of intestinal epithelial cells through secretion of IL-22. Recent work using human tonsillar cells demonstrated that ILC3s exposed to exogenous inflammatory cytokines for a long period of time acquired expression of granzyme B, suggesting that under pathological conditions ILC3s may become cytotoxic. We hypothesized that inflammation associated with bacterial exposure might trigger granzyme B expression in gut ILC3s. To test this, we exposed human colon lamina propria mononuclear cells to a panel of enteric bacteria. We found that the Gram-negative commensal and pathogenic bacteria induced granzyme B expression in a subset of ILC3s that were distinct from IL-22-producing ILC3s. A fraction of granzyme B⁺ ILC3s coexpressed the cytolytic protein perforin. Granzyme B expression was mediated, in part, by IL-15 produced upon exposure to bacteria. ILC3s coexpressing all three IL-15R subunits (IL15Rα/β/γ) increased following bacterial stimulation, potentially allowing for cis presentation of IL-15 during bacterial exposure. Additionally, a large frequency of colonic myeloid dendritic cells expressed IL-15Rα, implicating myeloid dendritic cells in trans presentation of IL-15 to ILC3s. Tonsillar ILC3s minimally expressed granzyme B when exposed to the same bacteria or to rIL-15. Overall, these data establish the novel, to our knowledge, finding that human colonic ILC3s can express granzyme B in response to a subset of enteric bacteria through a process mediated by IL-15. These observations raise new questions about the multifunctional role of human gut ILC3s.

Reduced immune-regulatory molecule expression on human colonic memory CD4 T cells in older adults

BACKGROUND

The etiology of the low-level chronic inflammatory state associated with aging is likely multifactorial, but a number of animal and human studies have implicated a functional decline of the gastrointestinal immune system as a potential driver. Gut tissue-resident memory T cells play critical roles in mediating protective immunity and in maintaining gut homeostasis, yet few studies have investigated the effect of aging on human gut T cell immunity. To determine if aging impacted CD4 T cell immunity in the human large intestine, we utilized multi-color flow cytometry to measure colonic lamina propria (LP) CD4 T cell frequencies and immune-modulatory marker expression in younger (mean ± SEM: 38 ± 1.5 yrs) and older (77 ± 1.6 yrs) adults. To determine cellular specificity, we evaluated colon LP CD8 T cell frequency and phenotype in the same donors. To probe tissue specificity, we evaluated the same panel of markers in peripheral blood (PB) CD4 T cells in a separate cohort of similarly aged persons.

RESULTS

Frequencies of colonic CD4 T cells as a fraction of total LP mononuclear cells were higher in older persons whereas absolute numbers of colonic LP CD4 T cells per gram of tissue were similar in both age groups. LP CD4 T cells from older versus younger persons exhibited reduced CTLA-4, PD-1 and Ki67 expression. Levels of Bcl-2, CD57, CD25 and percentages of activated CD38+HLA-DR+ CD4 T cells were similar in both age groups. In memory PB CD4 T cells, older age was only associated with increased CD57 expression. Significant age effects for LP CD8 T cells were only observed for CTLA-4 expression, with lower levels of expression observed on cells from older adults.

CONCLUSIONS

Greater age was associated with reduced expression of the co-inhibitory receptors CTLA-4 and PD-1 on LP CD4 T cells. Colonic LP CD8 T cells from older persons also displayed reduced CTLA-4 expression. These age-associated profiles were not observed in older PB memory CD4 T cells. The decline in co-inhibitory receptor expression on colonic LP T cells may contribute to local and systemic inflammation via a reduced ability to limit ongoing T cell responses to enteric microbial challenge.

Enteric bacteria induce IFNγ and Granzyme B from human colonic Group 1 Innate Lymphoid Cells

Group 1 Innate Lymphoid Cells (which include Natural Killer cells and ILC1s) aid in gut anti-bacterial defense through the production of IFNγ, which is critical for mobilizing protective responses against enteric pathogens. When intestinal epithelial barrier integrity is compromised, commensal bacteria are likely to translocate from the gut lumen into the lamina propria. Few studies have addressed the mechanisms by which commensal bacteria impact the function of gut Group 1 ILCs, especially ILC1s. Utilizing an in vitro human colonic lamina propria mononuclear cell (LPMC) model, we evaluated Group 1 ILC cytokine and cytolytic protein production in response to a panel of enteric Gram-positive and Gram-negative commensal and pathogenic bacteria. IFNγ-production by NK cells and ILC1s was significantly increased after LPMC exposure to Gram-negative commensal or pathogenic bacteria, but not after exposure to the Gram-positive bacteria commensals tested. Stimulation of IFNγ production from Group 1 ILCs was not through direct recognition of bacteria by NK cells or ILC1s, but rather required accessory cells within the LPMC population. Myeloid dendritic cells generated IL-12p70, IL-18, and IL-1β upon exposure to enteric bacteria and these cytokines contributed to Group 1 ILC production of IFNγ. Furthermore, Gram-negative commensal or pathogenic bacteria induced significant expression of Granzyme B in NK cells and ILC1s. Overall, these data demonstrate that some enteric commensal bacteria indirectly induce inflammatory cytokine production and cytolytic protein expression from human colonic Group 1 ILCs, a process which could contribute to inflammation in the setting of microbial translocation.

Quantifying HIV-1-Mediated Gut CD4+ T Cell Deathin the Lamina Propria Aggregate Culture (LPAC) Model

Gut CD4 T cells are major targets of HIV-1 and are massively depleted early during infection. To better understand the mechanisms governing HIV-1-mediated CD4 T cell death, we developed the physiologically-relevant Lamina Propria Aggregate Culture (LPAC) model. The LPAC model is ideal for studying CD4 T cell death induced by clinically-relevant Transmitted/Founder (TF) HIV-1 strains and is also suitable for studying how enteric microbes and soluble factors (e.g., Type I Interferons) impact LP CD4 T cell death and function. Here, we detail the protocol to establish LP CD4 T cell infection using a process of spinoculation, the subsequent evaluation of infection levels using multicolor flow cytometry and the determination of overall LP CD4 T cell death using absolute LP CD4 T cell counts. We also describe the preparation of virus stocks of Transmitted/Founder (TF) HIV-1 infectious molecular clones that were successfully used in the LPAC model.

HIV infection does not alter interferon α/β receptor 2 expression on mucosal immune cells

The innate immune response induced by type I interferons (IFNs) plays a critical role in the establishment of HIV infection. IFNs are induced early in HIV infection and trigger an antiviral defense program by signaling through the IFNα/β receptor (IFNAR), which consists of two subunits, IFNAR1 and IFNAR2. Changes in IFNAR expression in HIV target cells, as well as other immune cells, could therefore have important consequences for initial HIV spread. It was previously reported that IFNAR2 expression is increased in peripheral blood CD4+ CXCR4+ T cells of HIV+ patients compared to HIV uninfected controls, suggesting that HIV infection may alter the IFN responsiveness of target cells. However, the earliest immune cells affected by HIV in vivo reside in the gut-associated lymphoid tissue (GALT). To date, it remains unknown if IFNAR expression is altered in GALT immune cells in the context of HIV infection and exposure to IFNs, including the 12 IFNα subtypes. Here, we analyzed the expression of surface bound and soluble IFNAR2 on Lamina propria mononuclear cells (LPMCs) isolated from the GALT of HIV- individuals and in plasma samples of HIV+ patients. IFNAR2 expression varied between different T cells, B cells and natural killer cells, but was not altered following HIV infection. Furthermore, expression of the soluble IFNAR2a isoform was not changed in HIV+ patients compared to healthy donors, nor in LPMCs after HIV-1 infection ex vivo. Even though the 12 human IFNα subtypes trigger different biological responses and vary in their affinity to both receptor subunits, stimulation of LPMCs with different recombinant IFNα subtypes did not result in any significant changes in IFNAR2 surface expression. Our data suggests that potential changes in the IFN responsiveness of mucosal immune cells during HIV infection are unlikely dictated by changes in IFNAR2 expression.

Age-related alterations in human gut CD4 T cell phenotype, T helper cell frequencies, and functional responses to enteric bacteria

Intestinal lamina propria (LP) CD4 T cells play critical roles in maintaining intestinal homeostasis and in immune responses to enteric microbes, yet little is known regarding whether they contribute to age-associated intestinal immune dysfunction. In this study, we evaluated the direct ex vivo frequency, activation/inhibitory phenotype, death profiles, and in vitro functional responses of human jejunum LP CD4 T cells, including Th1, Th17, and Th22 subsets isolated from younger (<45 years) and older (>65years) persons. Expression of the co-inhibitory molecule CTLA-4 was significantly lower in older CD4 T cells, whereas expression of HLA-DR, CD38, CD57, and PD-1 were not significantly different between groups. Total CD4 T cell frequencies were similar between age groups, but lower frequencies and numbers of Th17 cells were observed directly ex vivo in older samples. Older Th17 and Th1 cells proliferated to a lesser degree following in vitro exposure to bacterial antigens vs. their younger counterparts. Levels of spontaneous cell death were increased in older CD4 T cells; however, cellular death profiles following activation did not differ based on age. Thus, small intestinal CD4 T cells from older persons have altered phenotypic and functional profiles including reduced expression of a co-inhibitory molecule, increased spontaneous cell death, and both reduced frequencies and altered functional responses of specific Th cell subsets. These changes may contribute to altered intestinal homeostasis and loss of protective gut immunity with aging.

Commensal and Pathogenic Bacteria Indirectly Induce IL-22 but Not IFNγ Production From Human Colonic ILC3s via Multiple Mechanisms

Innate lymphoid cells (ILCs) are a diverse family of cells that play critical roles in mucosal immunity. One subset of the ILC family, Group 3 ILCs (ILC3s), has been shown to aid in gut homeostasis through the production of IL-22. IL-22 promotes gut homeostasis through its functional effect on the epithelial barrier. When gut epithelial barrier integrity is compromised, such as in Human Immunodeficiency Virus (HIV) infection and inflammatory bowel disease (IBD), microbes from the gut lumen translocate into the lamina propria, inducing a multitude of potentially pathogenic immune responses. In murine models of bacterial infection, there is evidence that bacteria can induce pro-inflammatory IFNγ production in ILC3s. However, the impact of diverse translocating bacteria, particularly commensal bacteria, in dictating IFNγ versus IL-22 production by human gut ILC3s remains unclear. Here, we utilized an in vitro human lamina propria mononuclear cell (LPMC) model to evaluate ILC3 cytokine production in response to a panel of enteric Gram-positive and Gram-negative commensal and pathogenic bacteria and determined potential mechanisms by which these cytokine responses were induced. The percentages of IL-22-producing ILC3s, but not IFNγ-producing ILC3s, were significantly increased after LPMC exposure to both Gram-positive and Gram-negative commensal or pathogenic bacterial stimuli. Stimulation of IL-22 production from ILC3s was not through direct recognition of bacterial antigen by ILC3s, but rather required the help of accessory cells within the LPMC population. CD11c+ myeloid dendritic cells generated IL-23 and IL-1β in response to enteric bacteria and contributed to ILC3 production of IL-22. Furthermore, ligation of the natural cytotoxicity receptor NKp44 on ILC3s in response to bacteria stimulation also significantly increased the percentage of IL-22-producing ILC3s. Overall, these data demonstrate that human gut microbiota, including commensal bacteria, indirectly modulate colonic ILC3 function to induce IL-22, but additional signals are likely required to induce IFNγ production by colonic ILC3s in the setting of inflammation and microbial translocation.

Cell-free DNA-induced alteration of autophagy response and TLR9-signaling: Their relation to amelioration of DSS-colitis

BACKGROUND

The influence of cell-free DNA (fDNA) administration on the TLR9-autophagy regulatory crosstalk within inflammatory circumstances remains unclear.

AIMS

To examine the immunobiologic effects of iv. fDNA injection on the TLR9-mediated autophagy response in murine DSS-colitis.

METHODS

Different types of modified fDNAs were administered to DSS-colitic mice. Disease and histological activities, spleen index were measured. Changes of the TLR9-associated and autophagy-related gene expression profiles of lamina proprial cells and splenocytes were assayed by quantitative real-time PCR, and validated by immunohistochemistries. Ultrastructural changes of the colon were examined by transmission electron microscopy (TEM).

RESULTS

A single intravenous injection of colitic fDNA (C-DNA) exhibited beneficial clinical and histological effects on DSS-colitis, compared to normal (N-DNA). C-DNA administration displayed a more prominent impact on the outcome of the TLR9-autophagy response than N-DNA. C-DNA resulted in a decreased spleen index in DSS-colitic mice. C-DNA treatment of normal mice resulted in a downregulation of Beclin1 and ATG16L1 mRNA and protein expression in the colon. These as well as LC3B were downregulated in the spleen. In contrast, the Beclin1, ATG16L1 and LC3B gene and protein expressions were upregulated in both the colon and the spleen by C-DNA injection. Moreover, C-DNA administration to DSS-colitic mice resulted in a remarkable increase of epithelial autophagic vacuoles representing an intensified macroautophagy.

CONCLUSIONS

The effect of intravenously administered fDNA on the TLR9-mediated autophagy response is expressly dependent on the origin of fDNA (i.e. inflammatory or not) and on the characteristics of the local immunobiologic milieu (i.e. inflammatory or not, as well).

The transcriptome of HIV-1 infected intestinal CD4+ T cells exposed to enteric bacteria

Global transcriptome studies can help pinpoint key cellular pathways exploited by viruses to replicate and cause pathogenesis. Previous data showed that laboratory-adapted HIV-1 triggers significant gene expression changes in CD4+ T cell lines and mitogen-activated CD4+ T cells from peripheral blood. However, HIV-1 primarily targets mucosal compartments during acute infection in vivo. Moreover, early HIV-1 infection causes extensive depletion of CD4+ T cells in the gastrointestinal tract that herald persistent inflammation due to the translocation of enteric microbes to the systemic circulation. Here, we profiled the transcriptome of primary intestinal CD4+ T cells infected ex vivo with transmitted/founder (TF) HIV-1. Infections were performed in the presence or absence of Prevotella stercorea, a gut microbe enriched in the mucosa of HIV-1-infected individuals that enhanced both TF HIV-1 replication and CD4+ T cell death ex vivo. In the absence of bacteria, HIV-1 triggered a cellular shutdown response involving the downregulation of HIV-1 reactome genes, while perturbing genes linked to OX40, PPAR and FOXO3 signaling. However, in the presence of bacteria, HIV-1 did not perturb these gene sets or pathways. Instead, HIV-1 enhanced granzyme expression and Th17 cell function, inhibited G1/S cell cycle checkpoint genes and triggered downstream cell death pathways in microbe-exposed gut CD4+ T cells. To gain insights on these differential effects, we profiled the gene expression landscape of HIV-1-uninfected gut CD4+ T cells exposed to bacteria. Microbial exposure upregulated genes involved in cellular proliferation, MAPK activation, Th17 cell differentiation and type I interferon signaling. Our findings reveal that microbial exposure influenced how HIV-1 altered the gut CD4+ T cell transcriptome, with potential consequences for HIV-1 susceptibility, cell survival and inflammation. The HIV-1- and microbe-altered pathways unraveled here may serve as a molecular blueprint to gain basic insights in mucosal HIV-1 pathogenesis.

Low abundance of colonic butyrate-producing bacteria in HIV infection is associated with microbial translocation and immune activation

OBJECTIVE

Gut microbial translocation is a major driving force behind chronic immune activation during HIV-1 infection. HIV-1-related intestinal dysbiosis, including increases in mucosa-associated pathobionts, may influence microbial translocation and contribute to mucosal and systemic inflammation. Thus, it is critical to understand the mechanisms by which gut microbes and their metabolic products, such as butyrate, influence immune cell function during HIV-1 infection.

DESIGN

A cross-sectional study was performed to compare the relative abundance of butyrate-producing bacterial (BPB) species in colonic biopsies and stool of untreated, chronic HIV-1-infected (n = 18) and HIV-1-uninfected (n = 14) study participants. The effect of exogenously added butyrate on gut T-cell activation and HIV-1 infection was evaluated using an ex-vivo human intestinal cell culture model.

METHODS

Species were identified in 16S ribosomal RNA sequence datasets. Ex-vivo isolated lamina propria mononuclear cells were infected with C-C chemokine receptor type 5-tropic HIV-1Bal, cultured with enteric gram-negative bacteria and a range of butyrate doses, and lamina propria T-cell activation and HIV-1 infection levels measured.

RESULTS

Relative abundance of total BPB and specifically of Roseburia intestinalis, were lower in colonic mucosa of HIV-1-infected versus HIV-1-uninfected individuals. In HIV-1-infected study participants, R. intestinalis relative abundance inversely correlated with systemic indicators of microbial translocation, immune activation, and vascular inflammation. Exogenous butyrate suppressed enteric gram-negative bacteria-driven lamina propria T-cell activation and HIV-1 infection levels in vitro.

CONCLUSION

Reductions in mucosal butyrate from diminished colonic BPB may exacerbate pathobiont-driven gut T-cell activation and HIV replication, thereby contributing to HIV-associated mucosal pathogenesis.

The gut microbiome and HIV-1 pathogenesis: a two-way street

HIV-1 infection is associated with substantial damage to the gastrointestinal tract resulting in structural impairment of the epithelial barrier and a disruption of intestinal homeostasis. The accompanying translocation of microbial products and potentially microbes themselves from the lumen into systemic circulation has been linked to immune activation, inflammation, and HIV-1 disease progression. The importance of microbial translocation in the setting of HIV-1 infection has led to a recent focus on understanding how the communities of microbes that make up the intestinal microbiome are altered during HIV-1 infection and how they interact with mucosal immune cells to contribute to inflammation. This review details the dysbiotic intestinal communities associated with HIV-1 infection and their potential link to HIV-1 pathogenesis. We detail studies that begin to address the mechanisms driving microbiota-associated immune activation and inflammation and the various treatment strategies aimed at correcting dysbiosis and improving the overall health of HIV-1-infected individuals. Finally, we discuss how this relatively new field of research can advance to provide a more comprehensive understanding of the contribution of the gut microbiome to HIV-1 pathogenesis.

The Phenotypic Characterization of the Human Renal Mononuclear Phagocytes Reveal a Co-Ordinated Response to Injury

Mammalian tissues contain networks of mononuclear phagocytes (MPh) that sense injury and orchestrate the response to it. In mice, this is affected by distinct populations of dendritic cells (DC), monocytes and macrophages and recent studies suggest the same is true for human skin and intestine but little is known about the kidney. Here we describe the analysis of MPh populations in five human kidneys and show they are highly heterogeneous and contain discrete populations of DC, monocytes and macrophages. These include: plasmacytoid DC (CD303⁺) and both types of conventional DC—cDC1 (CD141⁺ cells) and CD2 (CD1c⁺ cells); classical, non-classical and intermediate monocytes; and macrophages including a novel population of CD141⁺ macrophages clearly distinguishable from cDC1 cells. The relative size of the MPh populations differed between kidneys: the pDC population was bi-modally distributed being less than 2% of DC in two kidneys without severe injury and over 35% in the remaining three with low grade injury in the absence of morphological evidence of inflammation. There were profound differences in the other MPh populations in kidneys with high and low numbers of pDC. Thus, cDC1 cells were abundant (55 and 52.3%) when pDC were sparse and sparse (12.8–12.5%) when pDC were abundant, whereas the proportions of cDC2 cells and classical monocytes increased slightly in pDC high kidneys. We conclude that MPh are highly heterogeneous in human kidneys and that pDC infiltration indicative of low-grade injury does not occur in isolation but is part of a co-ordinated response affecting all renal DC, monocyte and macrophage populations.

Enhancement of HIV-1 infection and intestinal CD4+ T cell depletion ex vivo by gut microbes altered during chronic HIV-1 infection

BACKGROUND

Early HIV-1 infection is characterized by high levels of HIV-1 replication and substantial CD4 T cell depletion in the intestinal mucosa, intestinal epithelial barrier breakdown, and microbial translocation. HIV-1-induced disruption of intestinal homeostasis has also been associated with changes in the intestinal microbiome that are linked to mucosal and systemic immune activation. In this study, we investigated the impact of representative bacterial species that were altered in the colonic mucosa of viremic HIV-1 infected individuals (HIV-altered mucosal bacteria; HAMB) on intestinal CD4 T cell function, infection by HIV-1, and survival in vitro. Lamina propria (LP) mononuclear cells were infected with CCR5-tropic HIV-1BaL or mock infected, exposed to high (3 gram-negative) or low (2 gram-positive) abundance HAMB or control gram-negative Escherichia coli and levels of productive HIV-1 infection and CD4 T cell depletion assessed. HAMB-associated changes in LP CD4 T cell activation, proliferation and HIV-1 co-receptor expression were also evaluated.

RESULTS

The majority of HAMB increased HIV-1 infection and depletion of LP CD4 T cells, but gram-negative HAMB enhanced CD4 T cell infection to a greater degree than gram-positive HAMB. Most gram-negative HAMB enhanced T cell infection to levels similar to that induced by gram-negative E. coli despite lower induction of T cell activation and proliferation by HAMB. Both gram-negative HAMB and E. coli significantly increased expression of HIV-1 co-receptor CCR5 on LP CD4 T cells. Lipopolysaccharide, a gram-negative bacteria cell wall component, up-regulated CCR5 expression on LP CD4 T cells whereas gram-positive cell wall lipoteichoic acid did not. Upregulation of CCR5 by gram-negative HAMB was largely abrogated in CD4 T cell-enriched cultures suggesting an indirect mode of stimulation.

CONCLUSIONS

Gram-negative commensal bacteria that are altered in abundance in the colonic mucosa of HIV-1 infected individuals have the capacity to enhance CCR5-tropic HIV-1 productive infection and depletion of LP CD4 T cells in vitro. Enhanced infection appears to be primarily mediated indirectly through increased expression of CCR5 on LP CD4 T cells without concomitant large scale T cell activation. This represents a novel mechanism potentially linking intestinal dysbiosis to HIV-1 mucosal pathogenesis.

Gut dendritic cell activation links an altered colonic microbiome to mucosal and systemic T-cell activation in untreated HIV-1 infection

HIV-1-associated disruption of intestinal homeostasis is a major factor contributing to chronic immune activation and inflammation. Dendritic cells (DCs) are crucial in maintaining intestinal homeostasis, but the impact of HIV-1 infection on intestinal DC number and function has not been extensively studied. We compared the frequency and activation/maturation status of colonic myeloid DC (mDC) subsets (CD1c(+) and CD1c(neg)) and plasmacytoid DCs in untreated HIV-1-infected subjects with uninfected controls. Colonic mDCs in HIV-1-infected subjects had increased CD40 but decreased CD83 expression, and CD40 expression on CD1c(+) mDCs positively correlated with mucosal HIV-1 viral load, with mucosal and systemic cytokine production, and with frequencies of activated colon and blood T cells. Percentage of CD83(+)CD1c(+) mDCs negatively correlated with frequencies of interferon-γ-producing colon CD4(+) and CD8(+) T cells. CD40 expression on CD1c(+) mDCs positively associated with abundance of high prevalence mucosal Prevotella copri and Prevotella stercorea but negatively associated with a number of low prevalence mucosal species, including Rumminococcus bromii. CD1c(+) mDC cytokine production was greater in response to in vitro stimulation with Prevotella species relative to R. bromii. These findings suggest that, during HIV infection, colonic mDCs become activated upon exposure to mucosal pathobiont bacteria leading to mucosal and systemic immune activation.

Human blood CD1c dendritic cells stimulate IL-12-independent IFN-γ responses and have a strikingly low inflammatory profile

Adaptive immune responses are initiated by resident myeloid tissue DC. A major fraction of tissue DC express CD1c⁺ and is thought to be derived from blood CD1c DC, an idea supported here by the observation that they express tissue-homing molecules and rapidly differentiate into cells with a tissue DC phenotype. Responses are thought to be augmented/modulated further by inflammatory moDC. Although much accepted human myeloid DC cell biology is based on moDC studies, we find these 2 DC populations to be functionally distinct. Stimulated moDC produce high levels of IL-10 and the Th1-promoting cytokine IL-12. Under identical conditions, CD1c DC synthesized no IL-10 and no or low levels of IL-12. Despite this, CD1c DC stimulated a strong Th1 response, demonstrated by IL-12 neutralization to be IL-12 independent, whereas the response induced by moDC was IL-12 dependent. This finding was supported by studies on a patient with a highly reduced ability to synthesize IL-12, whose CD1c DC induced a good Th1 response contrasting with the failure of his moDC, which were impaired in IL-12 production, to induce IFN-γ-secreting T cells. The IL-10 and IL-12 data were confirmed by microarray analysis, which also showed that stimulated moDC produced inflammatory-associated chemokines and cytokines, whereas stimulated CD1c DC showed minimal up-regulation of these genes. Thus, moDC, widely used as a human myeloid DC model, do not faithfully reflect the properties of CD1c tissue DC, making the initial response to a pathogen or vaccine.

Preconditioning with intravenous colitic cell-free DNA prevents DSS-colitis by altering TLR9-associated gene expression profile

BACKGROUND

Presence of cell-free-circulating DNA (fcDNA) sequences in sera of patients with inflammatory bowel diseases (IBD) is a well-established phenomenon. Potential roles of fcDNA in diagnosis, prognosis and therapy monitoring of chronic inflammatory colonic disorders have already been examined, albeit its actual biological function still remains unclear.

AIMS AND METHODS

In the present experiment, we studied the immunobiological effects of isolated fcDNA of normal and inflammatory origin administered intravenously to mice prior to induction of dextran sulfate sodium (DSS)-colitis. In addition to evaluate the current disease and histological activity, changes of the gene expression profile in isolated lamina propria cells upon TLR9 ligation were assayed.

RESULTS

A single intravenous dose of fcDNA pretreatment with colitic fcDNA exhibited beneficial response concerning the clinical and histological severity of DSS-colitis as compared to effects of normal fcDNA. Pretreatment with colitic fcDNA substantially altered the expression of several TLR9-related and inflammatory cytokine genes in a clinically favorable manner.

CONCLUSIONS

During the process of acute colitis, the subsequent inflammatory environment presumably results in changes of fcDNA with the potential to facilitate the downregulation of inflammation and improvement of regeneration. Thus, preconditioning of mice with colitis-derived fcDNA via TLR9 signaling could exert a tissue-protective effect and influence beneficially the course of DSS-colitis. Elucidating mechanisms of immune response alterations by nucleic acids may provide further insight into the etiology of IBD and develop the basis of novel immunotherapies.

Intravenous administration of a single-dose free-circulating DNA of colitic origin improves severe murine DSS-colitis

In inflammatory bowel diseases the presence of free-circulating DNA (fcDNA) sequences in the sera is an established phenomenon, albeit its real biological function still remains unclear. In our study the immunobiologic effects of a single-dose, intravenously administered fcDNA of normal and colitic origin were assayed in DSS-colitic and control mice. In parallel with disease and histological activity evaluations changes of the TLR9 and inflammatory cytokine signaling gene expression profiles were assayed in isolated cells of the lamina propria. Intravenously administered colitis-derived fcDNA displayed a more prominent beneficial action regarding the clinical and histological severity of DSS-colitis than that of fcDNA of normal origin. Systemic administration of colitis-derived fcDNA significantly altered the expression of certain TLR9-related and proinflammatory cytokine genes in a clinically favorable manner. Presumably due to induction of severe colitis, the subsequent marked inflammatory environment may result changes in fcDNA with a potential to promote the downregulation of inflammation and improvement of tissue regeneration. Elucidating mechanisms of innate immune alterations by nucleic acids may provide further insight into the etiology of inflammatory bowel diseases, and develop the basis of novel nucleic acid-based immunotherapies.

Contribution of intestinal barrier damage, microbial translocation and HIV-1 infection status to an inflammaging signature

Background

Systemic inflammation is a characteristic of both HIV-1 infection and aging (“inflammaging”). Intestinal epithelial barrier damage (IEBD) and microbial translocation (MT) contribute to HIV-associated inflammation, but their impact on inflammaging remains unclear.

Methods

Plasma biomarkers for IEBD (iFABP), MT (LPS, sCD14), T-cell activation (sCD27), and inflammation (hsCRP, IL-6) were measured in 88 HIV-1 uninfected (HIV^neg) and 83 treated, HIV-1-infected (HIV^pos) adults from 20–100 years old.

Results

Age positively correlated with iFABP (r = 0.284, p = 0.008), sCD14 (r = 0.646, p = <0.0001) and LPS (r = 0.421, p = 0.0002) levels in HIV^neg but not HIV^pos subjects. Age also correlated with sCD27, hsCRP, and IL-6 levels regardless of HIV status. Middle-aged HIV^pos subjects had elevated plasma biomarker levels similar to or greater than those of elderly HIV^neg subjects with the exception of sCD14. Clustering analysis described an inflammaging phenotype (IP) based on iFABP, sCD14, sCD27, and hsCRP levels in HIV^neg subjects over 60 years of age. The IP in HIV^neg subjects was used to develop a classification model that was applied to HIV^pos subjects to determine whether HIV^pos subjects under 60 years of age were IP+. HIV^pos IP+ subjects were similar in age to IP- subjects but had a greater risk of cardiovascular disease (CVD) based on Framingham risk score (p =  0.01).

Conclusions

We describe a novel IP that incorporates biomarkers of IEBD, MT, immune activation as well as inflammation. Application of this novel IP in HIV-infected subjects identified a group at higher risk of CVD.

The natural killer cell interferon-gamma response to bacteria is diminished in untreated HIV-1 infection and defects persist despite viral suppression

OBJECTIVE

Natural killer (NK) cells are important in innate immune responses to bacterial and viral pathogens. HIV-1 infection is associated with opportunistic bacterial infections and with microbial translocation, but the nature of the NK cell response to bacteria during HIV-1 infection has not been studied extensively. The objective of this study was to compare NK cell responses to bacteria in HIV-1-infected versus that in uninfected individuals.

METHODS

Multicolor flow cytometry was used to evaluate the ability of blood NK cell subsets (CD56CD16, CD56CD16, and CD56CD16) from treated, virally suppressed, and untreated viremic subjects with chronic HIV-1 infection and uninfected controls, to secrete interferon gamma (IFN-γ) in response to the in vitro stimulation of peripheral blood mononuclear cells with heat-killed commensal Escherichia coli or pathogenic Salmonella typhimurium.

RESULTS

All 3 NK cell subsets produced IFN-γ in response to bacteria, but CD56CD16 NK cells were least responsive. Untreated HIV-1-infected donors had increased frequencies of CD56CD16 NK cells and lower overall frequencies of IFN-γ-producing NK cells responding to E. coli and S. typhimurium than did NK cells from uninfected donors. These NK cell defects were not fully restored in antiretroviral therapy-treated donors. Monocytes were necessary for NK cells to respond to bacteria, but the HIV-associated defect was intrinsic to NK cells because the addition of normal monocytes did not restore IFN-γ production in response to bacteria.

CONCLUSIONS

Functional defects and numeric alterations of NK cell subsets lead to decreased frequencies of bacteria-reactive, IFN-γ-producing NK cells in HIV-1-infected subjects, even those on antiretroviral therapy.

Antigen-presenting cell candidates for HIV-1 transmission in human distal colonic mucosa defined by CD207 dendritic cells and CD209 macrophages

A common route for HIV-1 infection is sexual transmission across colorectal mucosa, which is thought to be 10-2,000 times more vulnerable to infection than that of the female genital tract. Mucosal surfaces are the first line of defense against many pathogens but the antigen-presenting cells (APCs), key regulators of innate immunity and determinants of adaptive immunity, are not well defined in these target tissues. Using immunohistochemistry, dendritic cells expressing Langerin (CD207(+)), a lectin known to bind and internalize HIV-1, were detected in the periphery of colonic glands and sparsely scattered in the submucosa similarly in colorectal mucosa. This cell type, well known in skin, has generally not been reported in colonic/rectal mucosa. Unexpectedly, the largest APC population observed was a macrophage-like population expressing the well-characterized tissue macrophage markers CD68 and CD163. Confocal microscopy of these cells revealed colocalization of CD209 (DC-SIGN), a presumed dendritic cell marker believed to facilitate HIV-1 transmission, but not other dendritic cell markers. These results show evidence of the unconfirmed presence of Langerhans cells in colorectal mucosa and a predominance of macrophage-like APCs that express CD209 (DC-SIGN). These findings define potential target cells in the pathogenesis of HIV-1 transmission, which may have key implications for the study of early transmission events in normal colorectal mucosa, as well as other infectious diseases and primary immune diseases involving the gut.

Toll-like receptor 3 signalling up-regulates expression of the HIV co-receptor G-protein coupled receptor 15 on human CD4+ T cells

Background

Many HIV-2 and SIV isolates, as well as some HIV-1 strains, can use the orphan 7-transmembrane receptor GPR15 as co-receptor for efficient entry into host cells. GPR15 is expressed on central memory and effector memory CD4⁺ T cells in healthy individuals and a subset of these cells is susceptible to HIV-1 and SIV infection. However, it has not been determined whether GPR15 expression is altered in the context of HIV-1 infection.

Results

Here, we show that GPR15 expression in CD4⁺ T cells is markedly up-regulated in some HIV-1 infected individuals compared to the rest of the infected patients and to healthy controls. Infection of the PM1 T cell line with primary HIV-1 isolates was found to up-regulate GPR15 expression on the infected cells, indicating that viral components can induce GPR15 expression. Up-regulation of GPR15 expression on CD4⁺ T cells was induced by activation of Toll-like receptor 3 signalling via TIR-domain-containing adapter-inducing interferon-β (TRIF) and was more prominent on gut-homing compared to lymph node-homing CD4⁺ T cells.

Conclusion

These results suggest that infection-induced up-regulation of GPR15 expression could increase susceptibility of CD4⁺ T cells to HIV infection and target cell availability in the gut in some infected individuals.

Microbial exposure alters HIV-1-induced mucosal CD4+ T cell death pathways Ex vivo

Background

Early HIV-1 infection causes massive CD4+ T cell death in the gut and translocation of bacteria into the circulation. However, the programmed cell death (PCD) pathways used by HIV-1 to kill CD4+ T cells in the gut, and the impact of microbial exposure on T cell loss, remain unclear. Understanding mucosal HIV-1 triggered PCD could be advanced by an ex vivo system involving lamina propria mononuclear cells (LPMCs). We therefore modeled the interactions of gut LPMCs, CCR5-tropic HIV-1 and a commensal gut bacterial species, Escherichia coli. In this Lamina Propria Aggregate Culture (LPAC) model, LPMCs were infected with HIV-1_BaL by spinoculation and cultured in the presence or absence of heat killed E.coli. CD4+ T cell numbers derived from flow cytometry and viable cell counts were reported relative to mock infection. Viable cells were identified by viability dye exclusion (AqVi), and intracellular HIV-1 Gag p24 protein was used to identify infected cells. Annexin V and AqVi were used to identify apoptotic versus necrotic cells. Caspase-1 and Caspase-3 activities were blocked using specific inhibitors YVAD and DEVD, respectively.

Results

CD4+ T cell depletion following HIV-1 infection was reproducibly observed by 6 days post infection (dpi). Depletion at 6 dpi strongly correlated with infection frequency at 4 dpi, was significantly blocked by Efavirenz treatment, and was primarily driven by p24-negative cells that were predominantly necrotic. HIV-1 infection significantly induced CD4+ T-cell intrinsic Caspase-1 activity, whereas Caspase-1 inhibition, but not Caspase-3 inhibition, significantly blocked CD4+ T cell depletion. Exposure to E.coli enhanced HIV-1 infection and CD4+ T depletion, and significantly increased the number of apoptotic p24+ cells. Notably, CD4+ T cell depletion in the presence of E.coli was partially blocked by Caspase-3, but not by Caspase-1 inhibition.

Conclusions

In the LPAC model, HIV-1 induced Caspase-1 mediated pyroptosis in bystander CD4+ T cells, but microbial exposure shifted the PCD mechanism toward apoptosis of productively infected T cells. These results suggest that mucosal CD4+ T cell death pathways may be altered in HIV-infected individuals after gut barrier function is compromised, with potential consequences for mucosal inflammation, viral dissemination and systemic immune activation.

Alterations in the gut microbiota associated with HIV-1 infection

Understanding gut microbiota alterations associated with HIV infection and factors that drive these alterations may help explain gut-linked diseases prevalent with HIV. 16S rRNA sequencing of feces from HIV-infected individuals revealed that HIV infection is associated with highly characteristic gut community changes, and antiretroviral therapy does not consistently restore the microbiota to an HIV-negative state. Despite the chronic gut inflammation characteristic of HIV infection, the associated microbiota showed limited similarity with other inflammatory states and instead showed increased, rather than decreased, diversity. Meta-analysis revealed that the microbiota of HIV-infected individuals in the U.S. was most similar to a Prevotella-rich community composition typically observed in healthy individuals in agrarian cultures of Malawi and Venezuela and related to that of U.S. individuals with carbohydrate-rich, protein- and fat-poor diets. By evaluating innate and adaptive immune responses to lysates from bacteria that differ with HIV, we explore the functional drivers of these compositional differences.

Differential interleukin-10 (IL-10) and IL-23 production by human blood monocytes and dendritic cells in response to commensal enteric bacteria

Human peripheral blood contains antigen-presenting cells (APC), including dendritic cells (DC) and monocytes, that may encounter microbes that have translocated from the intestine to the periphery in disease states like HIV-1 infection and inflammatory bowel disease. We investigated the response of DC and monocytes in peripheral blood mononuclear cells (PBMC) to a panel of representative commensal enteric bacteria, including Escherichia coli, Enterococcus sp., and Bacteroides fragilis. All three bacteria induced significant upregulation of the maturation and activation markers CD40 and CD83 on myeloid dendritic cells (mDC) and plasmacytoid dendritic cells (pDC). However, only mDC produced cytokines, including interleukin-10 (IL-10), IL-12p40/70, and tumor necrosis factor alpha (TNF-α), in response to bacterial stimulation. Cytokine profiles in whole PBMC differed depending on the stimulating bacterial species: B. fragilis induced production of IL-23, IL-12p70, and IL-10, whereas E. coli and Enterococcus induced an IL-10-predominant response. mDC and monocyte depletion experiments indicated that these cell types differentially produced IL-10 and IL-23 in response to E. coli and B. fragilis. Bacteroides thetaiotaomicron did not induce levels of IL-23 similar to those of B. fragilis, suggesting that B. fragilis may have unique proinflammatory properties among Bacteroides species. The addition of recombinant human IL-10 to PBMC cultures stimulated with commensal bacteria abrogated the IL-23 response, whereas blocking IL-10 significantly enhanced IL-23 production, suggesting that IL-10 controls the levels of IL-23 produced. These results indicate that blood mDC and monocytes respond differentially to innate stimulation with whole commensal bacteria and that IL-10 may play a role in controlling the proinflammatory response to translocated microbes.

HIV-1 infection of human intestinal lamina propria CD4+ T cells in vitro is enhanced by exposure to commensal Escherichia coli

Microbial translocation has been linked to systemic immune activation in HIV-1 disease, yet mechanisms by which microbes may contribute to HIV-associated intestinal pathogenesis are poorly understood. Importantly, our understanding of the impact of translocating commensal intestinal bacteria on mucosal-associated T cell responses in the context of ongoing viral replication that occurs early in HIV-1 infection is limited. We previously identified commensal Escherichia coli-reactive Th1 and Th17 cells in normal human intestinal lamina propria (LP). In this article, we established an ex vivo assay to investigate the interactions between Th cell subsets in primary human LP mononuclear cells (LPMCs), commensal E. coli, and CCR5-tropic HIV-1(Bal). Addition of heat-killed E. coli to HIV-1-exposed LPMCs resulted in increases in HIV-1 replication, CD4 T cell activation and infection, and IL-17 and IFN-γ production. Conversely, purified LPS derived from commensal E. coli did not enhance CD4 T cell infection. E. coli exposure induced greater proliferation of LPMC Th17 than Th1 cells. Th17 cells were more permissive to infection than Th1 cells in HIV-1-exposed LPMC cultures, and Th17 cell infection frequencies significantly increased in the presence of E. coli. The E. coli-associated enhancement of infection was dependent on the presence of CD11c(+) LP dendritic cells and, in part, on MHC class II-restricted Ag presentation. These results highlight a potential role for translocating microbes in impacting mucosal HIV-1 pathogenesis during early infection by increasing HIV-1 replication and infection of intestinal Th1 and Th17 cells.

Enteroantigen-presenting B cells efficiently stimulate CD4(+) T cells in vitro

BACKGROUND

Presentation of enterobacterial antigens by antigen-presenting cells and activation of enteroantigen-specific CD4(+) T cells are considered crucial steps in inflammatory bowel disease (IBD) pathology. The detrimental effects of such CD4(+) T cells have been thoroughly demonstrated in models of colitis. Also, we have previously established an in vitro assay where murine enteroantigen-specific colitogenic CD4(+) CD25(-) T cells are activated by splenocytes pulsed with an enterobacterial extract.

METHODS

CD4(+) CD25(-) T cells were stimulated in vitro with various kinds of enterobacterial extract-pulsed antigen-presenting cells. T-helper phenotypes were detected by flow cytometry.

RESULTS

We found that enteroantigen-pulsed splenic B cells possess a significantly higher and more sustained T cell stimulatory capacity than similarly pulsed splenic dendritic cells (DCs) measured by the level of enteroantigen-specific CD4(+) CD25(-) T cell proliferation. In support of this, we observed upregulation of classic maturation markers in B cells following incubation with enterobacterial antigens. Peritoneal and mesenteric lymph node-derived B cells were equally effective as enteroantigen-presenting stimulator cells. B cells greatly expanded the number of stimulated CD4(+) T cells, which acquired a T(H) 2 phenotype. Interestingly, regulatory T cells were primarily activated by enteroantigen-pulsed B cells but not by similarly pulsed DCs.

CONCLUSIONS

We conclude that B cells are superior stimulators of enteroantigen-specific CD4(+) T cells in vitro, favoring T(H) 2 polarization. Thus, enteroantigen-processing and -presentation by B cells instead of by DCs might have opposing consequences for IBD development.

Intestinal dendritic cells: their role in bacterial recognition, lymphocyte homing, and intestinal inflammation

Dendritic cells (DCs) play a key role in discriminating between commensal microorganisms and potentially harmful pathogens and in maintaining the balance between tolerance and active immunity. The regulatory role of DC is of particular importance in the gut where the immune system lies in intimate contact with the highly antigenic external environment. Intestinal DC constantly survey the luminal microenvironment. They act as sentinels, acquiring antigens in peripheral tissues before migrating to secondary lymphoid organs to activate naive T cells. They are also sensors, responding to a spectrum of environmental cues by extensive differentiation or maturation. Recent studies have begun to elucidate mechanisms for functional specializations of DC in the intestine that may include the involvement of retinoic acid and transforming growth factor-β. Specialized CD103(+) intestinal DC can promote the differentiation of Foxp3(+) regulatory T cells via a retinoic acid-dependent process. Different DC outcomes are, in part, influenced by their exposure to microbial stimuli. Evidence is also emerging of the close interaction between bacteria, epithelial cells, and DC in the maintenance of intestinal immune homeostasis. Here we review recent advances of functionally specialized intestinal DC and their mechanisms of antigen uptake and recognition. We also discuss the interaction of DC with intestinal microbiota and their ability to orchestrate protective immunity and immune tolerance in the host. Lastly, we describe how DC functions are altered in intestinal inflammation and their emerging potential as a therapeutic target in inflammatory bowel disease.

Oral Treatment with Extract of Agaricus blazei Murill Enhanced Th1 Response through Intestinal Epithelial Cells and Suppressed OVA-Sensitized Allergy in Mice

To clarify the mechanism of the antiallergic activity of Agaricus blazei Murill extract (ABME), the present paper used an in vivo allergy model and an in vitro intestinal gut model. During OVA sensitization, the serum IgE levels decreased significantly in ABME group. Interleukin (IL)-4 and -5 produced from OVA-restimulated splenocytes was significantly decreased, and anti-CD3ε/CD28 antibody treatment also reduced IL-10, -4, and -5 production and increased IFN-γ production in ABME group. These results suggest that oral administration of ABME improves Th1/Th2 balance. Moreover, a coculture system constructed of Caco-2 cells and splenocytes from OT-II mice or RAW 264.7 cells indicated that the significant increases in IFN-γ production by ABME treatment. Therefore, it was concluded that the antiallergic activity of ABME was due to the activation of macrophages by epithelial cells and the promotion of the differentiation of naïve T cells into Th1 cells in the immune.

Neurogenic regulation of dendritic cells in the intestine

Antigen presenting cells like dendritic cells (DC) are responsible for the initiation of adaptive immune responses via the T helper cells they activate. The type of T cell responses DC induce is dependent on the local immunological environment where antigen has been taken up. In the gut, resident DC are phenotypically and functionally shaped by epithelial and stromal cell derived signals, the cytokine microenvironment, and neuronal products. These factors can control the activation state of DC thereby inducing tolerance for food and commensal organisms or immunity against pathogenic microbes. The enteric nervous system (ENS) is increasingly recognized as an important regulatory factor in intestinal immune cell control. Neurotransmitters and neuropeptides like acetylcholine (ACh), norepinephrine (NE) and vasoactive intestinal peptide (VIP) are released by neurons of the ENS and can affect the function of DC and subsequent immune responses. The critical balance between tolerance and protective immunity is disrupted in inflammatory bowel disease, which results in an exaggerated immune response against commensal bacteria. In this review we discuss the effects of ACh, VIP, and NE on DC function. DC express various receptors for these neuron derived products and can alter DC co-stimulatory molecule expression, cytokine release and subsequent T cell activation in an anti-inflammatory fashion. Knowledge about these interactions will help find new drug targets and may facilitate the development of specific therapies for diseases like inflammatory bowel disease (IBD).

Human intestinal lamina propria CD1c+ dendritic cells display an activated phenotype at steady state and produce IL-23 in response to TLR7/8 stimulation

Intestinal dendritic cells (DCs) play key roles in mediating tolerance to commensal flora and inflammatory responses against mucosal pathogens. The mechanisms by which intestinal "conditioning" influences human DC responses to microbial stimuli remain poorly understood. Infections with viruses, such as HIV-1, that target mucosal tissue result in intestinal epithelial barrier breakdown and increased translocation of commensal bacteria into the lamina propria (LP). It is unclear whether innate LP DC responses to concurrent viral and bacterial stimuli influence mucosal HIV-1 pathogenesis. In this study, direct ex vivo phenotype and in vitro constitutive cytokine production of CD1c+ DCs in human intestinal LP were compared with those in peripheral blood (PB). To evaluate innate responses to viral and bacterial stimuli, intracellular cytokine production by LP and PB DCs following stimulation with ligands for TLRs 2, 4, 5, and 7/8 was evaluated. At steady state, LP CD1c+ DCs expressed higher levels of activation markers (CD40, CD83, CD86, HLA-DR, and CCR7) than did PB CD1c+ DCs, and higher frequencies of LP CD1c+ DCs constitutively produced IL-6 and -10 and TNF-alpha. LP DCs had blunted cytokine responses to TLR4 ligand and TLR5 ligand stimulation relative to PB DCs, yet similarly produced IL-10 in response to TLR2 ligand. Only synthetic TLR7/8 ligand, a mimic of viral ssRNA, induced IL-23 production by LP CD1c+ DCs, and this proinflammatory cytokine response was synergistically enhanced following combined TLR7/8 and TLR4 stimulation. These findings highlight a potential mechanism by which viruses like HIV-1 may subvert homeostatic mechanisms and induce inflammation in the intestinal mucosa.

IL-27 regulates homeostasis of the intestinal CD4+ effector T cell pool and limits intestinal inflammation in a murine model of colitis

IL-27 limits CD4(+) T(H)17 cell development in vitro and during inflammatory responses in the CNS. However, whether IL-27-IL-27R interactions regulate the homeostasis or function of CD4(+) T cell populations in the intestine is unknown. To test this, we examined CD4(+) T cell populations in the intestine of wild-type and IL-27R(-/-) mice. Naive IL-27R(-/-) mice exhibited a selective decrease in the frequency of IFN-gamma producing CD4(+) T(H)1 cells and an increase in the frequency of T(H)17 cells in gut-associated lymphoid tissues. Associated with elevated expression of IL-17A, IL-27R(-/-) mice exhibited earlier onset and significantly increased severity of clinical disease compared with wild-type controls in a murine model of intestinal inflammation. Rag(-/-)/IL-27R(-/-) mice were also more susceptible than Rag(-/-) mice to development of dextran sodium sulfate-induced intestinal inflammation, indicating an additional role for IL-27-IL-27R in the regulation of innate immune cell function. Consistent with this, IL-27 inhibited proinflammatory cytokine production by activated neutrophils. Collectively, these data identify a role for IL-27-IL-27R interaction in controlling the homeostasis of the intestinal T cell pool and in limiting intestinal inflammation through regulation of innate and adaptive immune cell function.