Intestinal Monocyte-Derived Macrophages Control Commensal-Specific Th17 Responses

From the Friend to the Foe-Enterococcus faecalis Diverse Impact on the Human Immune System

Enterococcus faecalis is a bacterium which accompanies us from the first days of our life. As a commensal it produces vitamins, metabolizes nutrients, and maintains intestinal pH. All of that happens in exchange for a niche to inhabit. It is not surprising then, that the bacterium was and is used as an element of many probiotics and its positive impact on the human immune system and the body in general is hard to ignore. This bacterium has also a dark side though. The plasticity and relative ease with which one acquires virulence traits, and the ability to hide from or even deceive and use the immune system to spread throughout the body make E. faecalis a more and more dangerous opponent. The statistics clearly show its increasing role, especially in the case of nosocomial infections. Here we present the summarization of current knowledge about E. faecalis, especially in the context of its relations with the human immune system.

The Maternal Microbiome as a Map to Understanding the Impact of Prenatal Stress on Offspring Psychiatric Health

Stress and psychiatric disorders have been independently associated with disruption of the maternal and offspring microbiome and with increased risk of the offspring developing psychiatric disorders, both in clinical studies and in preclinical studies. However, the role of the microbiome in mediating the effect of prenatal stress on offspring behavior is unclear. While preclinical studies have identified several key mechanisms, clinical studies focusing on mechanisms are limited. In this review, we discuss 3 specific mechanisms by which the microbiome could mediate the effects of prenatal stress: 1) altered production of short-chain fatty acids; 2) disruptions in TH17 (T helper 17) cell differentiation, leading to maternal and fetal immune activation; and 3) perturbation of intestinal and microbial tryptophan metabolism and serotonergic signaling. Finally, we review the existing clinical literature focusing on these mechanisms and highlight the need for additional mechanistic clinical research to better understand the role of the microbiome in the context of prenatal stress.

Intestinal damage is required for the pro-inflammatory differentiation of commensal CBir1-specific T cells

Commensal-specific clusters of differentiation (CD)4+ T cells are expanded in patients with inflammatory bowel disease (IBD) compared to healthy individuals. How and where commensal-specific CD4+ T cells get activated is yet to be fully understood. We used CBir1 TCR-transgenic CD4+ T cells, specific to a commensal bacterial antigen, and different mouse models of IBD to characterize the dynamics of commensal-specific CD4+ T-cells activation. We found that CBir1 T cells proliferate following intestinal damage and cognate antigen presentation is mediated by CD11c+ cells in the colon-draining mesenteric lymph nodes. Using assay for transposase-accessible chromatin sequencing and flow cytometry, we showed that activated CBir1 T cells preferentially acquire an effector rather than regulatory phenotype, which is plastic over time. Moreover, CBir1 T cells, while insufficient to initiate intestinal inflammation, contributed to worse disease outcomes in the presence of other CD4+ T cells. Our results suggest that the commensal-specific T-cell responses observed in IBD exacerbate rather than initiate disease.

Farnesoid X receptor mediates macrophage-intrinsic responses to suppress colitis-induced colon cancer progression

Bile acids (BAs) affect the intestinal environment by ensuring barrier integrity, maintaining microbiota balance, regulating epithelium turnover, and modulating the immune system. As a master regulator of BA homeostasis, farnesoid X receptor (FXR) is severely compromised in patients with inflammatory bowel disease (IBD) and colitis-associated colorectal cancer (CAC). At the front line, gut macrophages react to the microbiota and metabolites that breach the epithelium. We aim to study the role of the BA/FXR axis in macrophages. This study demonstrates that inflammation-induced epithelial abnormalities compromised FXR signaling and altered BAs' profile in a mouse CAC model. Further, gut macrophage-intrinsic FXR sensed aberrant BAs, leading to pro-inflammatory cytokines' secretion, which promoted intestinal stem cell proliferation. Mechanistically, activation of FXR ameliorated intestinal inflammation and inhibited colitis-associated tumor growth, by regulating gut macrophages' recruitment, polarization, and crosstalk with Th17 cells. However, deletion of FXR in bone marrow or gut macrophages escalated the intestinal inflammation. In summary, our study reveals a distinctive regulatory role of FXR in gut macrophages, suggesting its potential as a therapeutic target for addressing IBD and CAC.

Intestinal microbiota-specific Th17 cells possess regulatory properties and suppress effector T cells via c-MAF and IL-10

Commensal microbes induce cytokine-producing effector tissue-resident CD4+ T cells, but the function of these T cells in mucosal homeostasis is not well understood. Here, we report that commensal-specific intestinal Th17 cells possess an anti-inflammatory phenotype marked by expression of interleukin (IL)-10 and co-inhibitory receptors. The anti-inflammatory phenotype of gut-resident commensal-specific Th17 cells was driven by the transcription factor c-MAF. IL-10-producing commensal-specific Th17 cells were heterogeneous and derived from a TCF1+ gut-resident progenitor Th17 cell population. Th17 cells acquired IL-10 expression and anti-inflammatory phenotype in the small-intestinal lamina propria. IL-10 production by CD4+ T cells and IL-10 signaling in intestinal macrophages drove IL-10 expression by commensal-specific Th17 cells. Intestinal commensal-specific Th17 cells possessed immunoregulatory functions and curbed effector T cell activity in vitro and in vivo in an IL-10-dependent and c-MAF-dependent manner. Our results suggest that tissue-resident commensal-specific Th17 cells perform regulatory functions in mucosal homeostasis.

The gut microbiota-induced kynurenic acid recruits GPR35-positive macrophages to promote experimental encephalitis

The intricate interplay between gut microbes and the onset of experimental autoimmune encephalomyelitis (EAE) remains poorly understood. Here, we uncover remarkable similarities between CD4+ T cells in the spinal cord and their counterparts in the small intestine. Furthermore, we unveil a synergistic relationship between the microbiota, particularly enriched with the tryptophan metabolism gene EC:1.13.11.11, and intestinal cells. This symbiotic collaboration results in the biosynthesis of kynurenic acid (KYNA), which modulates the recruitment and aggregation of GPR35-positive macrophages. Subsequently, a robust T helper 17 (Th17) immune response is activated, ultimately triggering the onset of EAE. Conversely, modulating the KYNA-mediated GPR35 signaling in Cx3cr1+ macrophages leads to a remarkable amelioration of EAE. These findings shed light on the crucial role of microbial-derived tryptophan metabolites in regulating immune responses within extraintestinal tissues.

Commensal Microbiota Effects on Craniofacial Skeletal Growth and Morphology

Microbes colonize anatomical sites in health to form commensal microbial communities (e.g., commensal gut microbiota, commensal skin microbiota, commensal oral microbiota). Commensal microbiota has indirect effects on host growth and maturation through interactions with the host immune system. The commensal microbiota was recently introduced as a novel regulator of skeletal growth and morphology at noncraniofacial sites. Further, we and others have shown that commensal gut microbes, such as segmented filamentous bacteria (SFB), contribute to noncraniofacial skeletal growth and maturation. However, commensal microbiota effects on craniofacial skeletal growth and morphology are unclear. To determine the commensal microbiota's role in craniofacial skeletal growth and morphology, we performed craniometric and bone mineral density analyses on skulls from 9-week-old female C57BL/6T germ-free (GF) mice (no microbes), excluded-flora (EF) specific-pathogen-free mice (commensal microbiota), and murine-pathogen-free (MPF) specific-pathogen-free mice (commensal microbiota with SFB). Investigations comparing EF and GF mice revealed that commensal microbiota impacted the size and shape of the craniofacial skeleton. EF versus GF mice exhibited an elongated gross skull length. Cranial bone length analyses normalized to skull length showed that EF versus GF mice had enhanced frontal bone length and reduced cranial base length. The shortened cranial base in EF mice was attributed to decreased presphenoid, basisphenoid, and basioccipital bone lengths. Investigations comparing MPF mice and EF mice demonstrated that commensal gut microbes played a role in craniofacial skeletal morphology. Cranial bone length analyses normalized to skull length showed that MPF versus EF mice had reduced frontal bone length and increased cranial base length. The elongated cranial base in MPF mice was due to enhanced presphenoid bone length. This work, which introduces the commensal microbiota as a contributor to craniofacial skeletal growth, underscores that noninvasive interventions in the gut microbiome could potentially be employed to modify craniofacial skeletal morphology. © 2023 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Dysbiosis and primary B-cell immunodeficiencies: current knowledge and future perspective

According to Elie Metchnikoff, an originator of modern immunology, several pivotal functions for disease and health are provided by indigenous microbiota. Nonetheless, important mechanistic insights have been elucidated more recently, owing to the growing availability of DNA sequencing technology. There are 10 to 100 trillion symbiotic microbes (such as viruses, bacteria, and yeast) in each human gut microbiota. Both locally and systemically, the gut microbiota has been demonstrated to impact immune homeostasis. Primary B-cell immunodeficiencies (PBIDs) are a group of primary immunodeficiency diseases (PIDs) referring to the dysregulated antibody production due to either intrinsic genetic defects or failures in functions of B cells. Recent studies have found that PBIDs cause disruptions in the gut's typical homeostatic systems, resulting in inadequate immune surveillance in the gastrointestinal (GI) tract, which is linked to increased dysbiosis, which is characterized by a disruption in the microbial homeostasis. This study aimed to review the published articles in this field to provide a comprehensive view of the existing knowledge about the crosstalk between the gut microbiome and PBID, the factors shaping the gut microbiota in PBID, as well as the potential clinical approaches for restoring a normal microbial community.

Immunoglobulins at the interface of the gut mycobiota and anti-fungal immunity

The dynamic and complex community of microbes that colonizes the intestines is composed of bacteria, fungi, and viruses. At the mucosal surfaces, immunoglobulins play a key role in protection against bacterial and fungal pathogens, and their toxins. Secretory immunoglobulin A (sIgA) is the most abundantly produced antibody at the mucosal surfaces, while Immunoglobulin G (IgG) isotypes play a critical role in systemic protection. IgA and IgG antibodies with reactivity to commensal fungi play an important role in shaping the mycobiota and host antifungal immunity. In this article, we review the latest evidence that establishes a connection between commensal fungi and B cell-mediated antifungal immunity as an additional layer of protection against fungal infections and inflammation.

Macrophages in intestinal homeostasis and inflammatory bowel disease

Macrophages are essential for the maintenance of intestinal homeostasis, yet appear to be drivers of inflammation in the context of inflammatory bowel disease (IBD). How these peacekeepers become powerful aggressors in IBD is still unclear, but technological advances have revolutionized our understanding of many facets of their biology. In this Review, we discuss the progress made in understanding the heterogeneity of intestinal macrophages, the functions they perform in gut health and how the environment and origin can control the differentiation and longevity of these cells. We describe how these processes might change in the context of chronic inflammation and how aberrant macrophage behaviour contributes to IBD pathology, and discuss how therapeutic approaches might target dysregulated macrophages to dampen inflammation and promote mucosal healing. Finally, we set out key areas in the field of intestinal macrophage biology for which further investigation is warranted.

MHCII expression on gut macrophages supports T cell homeostasis and is regulated by microbiota and ontogeny

Macrophages are traditionally considered antigen-presenting cells. However, their ability to present antigen and the factors regulating macrophage MHCII expression are poorly understood. Here, we demonstrate that MHCII expression on murine intestinal macrophages is differentially controlled by their residence in the small intestine (SI) or the colon, their ontogeny and the gut microbiota. Monocyte-derived macrophages are uniformly MHCII^hi, independently of the tissue of residence, microbial status or the age of the mouse, suggesting a common monocyte differentiation pathway. In contrast, MHCII expression on long-lived, prenatally-derived Tim4⁺ macrophages is low after birth but significantly increases at weaning in both SI and colon. Furthermore, MHCII expression on colonic Tim4⁺, but not monocyte-derived macrophages, is dependent on recognition of microbial stimuli, as MHCII expression is significantly downregulated in germ-free, antibiotic-treated and MyD88 deficient mice. To address the function of MHCII presentation by intestinal macrophages we established two models of macrophage-specific MHCII deficiency. We observed a significant reduction in the overall frequency and number of tissue-resident, but not newly arrived, SI CD4⁺ T cells in the absence of macrophage-expressed MHCII. Our data suggest that macrophage MHCII provides signals regulating gut CD4⁺ T cell maintenance with different requirements in the SI and colon.

Pathophysiology of Inflammatory Bowel Disease: Innate Immune System

Inflammatory bowel disease (IBD), comprising Crohn's disease (CD) and ulcerative colitis (UC), is a heterogeneous state of chronic intestinal inflammation with no exact known cause. Intestinal innate immunity is enacted by neutrophils, monocytes, macrophages, and dendritic cells (DCs), and innate lymphoid cells and NK cells, characterized by their capacity to produce a rapid and nonspecific reaction as a first-line response. Innate immune cells (IIC) defend against pathogens and excessive entry of intestinal microorganisms, while preserving immune tolerance to resident intestinal microbiota. Changes to this equilibrium are linked to intestinal inflammation in the gut and IBD. IICs mediate host defense responses, inflammation, and tissue healing by producing cytokines and chemokines, activating the complement cascade and phagocytosis, or presenting antigens to activate the adaptive immune response. IICs exert important functions that promote or ameliorate the cellular and molecular mechanisms that underlie and sustain IBD. A comprehensive understanding of the mechanisms underlying these clinical manifestations will be important for developing therapies targeting the innate immune system in IBD patients. This review examines the complex roles of and interactions among IICs, and their interactions with other immune and non-immune cells in homeostasis and pathological conditions.

Agr2-associated ER stress promotes adherent-invasive E. coli dysbiosis and triggers CD103+ dendritic cell IL-23-dependent ileocolitis

Endoplasmic reticulum (ER) stress is associated with Crohn's disease (CD), but its impact on host-microbe interaction in disease pathogenesis is not well defined. Functional deficiency in the protein disulfide isomerase anterior gradient 2 (AGR2) has been linked with CD and leads to epithelial cell ER stress and ileocolitis in mice and humans. Here, we show that ileal expression of AGR2 correlates with mucosal Enterobactericeae abundance in human inflammatory bowel disease (IBD) and that Agr2 deletion leads to ER-stress-dependent expansion of mucosal-associated adherent-invasive Escherichia coli (AIEC), which drives Th17 cell ileocolitis in mice. Mechanistically, our data reveal that AIEC-induced epithelial cell ER stress triggers CD103⁺ dendritic cell production of interleukin-23 (IL-23) and that IL-23R is required for ileocolitis in Agr2^-/- mice. Overall, these data reveal a specific and reciprocal interaction of the expansion of the CD pathobiont AIEC with ER-stress-associated ileocolitis and highlight a distinct cellular mechanism for IL-23-dependent ileocolitis.

Small secretory proteins of immune cells can modulate gynecological cancers

Small secretory proteins of immune cells are mostly Cytokines, which include chemokines, interleukins, interferons, lymphokines and tumor necrosis factors but not hormones or growth factors. These secretory proteins are the molecular messengers and primarily involved in autocrine, paracrine and endocrine signaling as immunomodulating agents. Hence, these proteins actually regulate the cells of immune system to communicate with one another to produce a synchronized, robust, still self-regulated response to a specific antigen. Chemokines are smaller secreted proteins that control overall immune cell movement and location; these chemokines are divided into 4 subgroups, namely, CXC, CC, CX3C and C according to the position of 4 conserved cysteine residues. Complete characterization of cytokines and chemokines can exploit their vast signaling networks to develop cancer treatments. These secretory proteins like IL-6, IL-10, IL-12, TNFα, CCL2, CXCL4 & CXCL8 are predominantly expressed in most of the gynecological cancers, which directly stimulate immune effector cells and stromal cells at the tumor site and augment tumor cell recognition by cytotoxic T-cells. Hence; these secretory proteins are the major regulators, which can actually modulate all kinds of gynecological cancers. Furthermore, advancements in adoptive T-cell treatment have relied on the use of multiple cytokines/chemokines to establish a highly regulated environment for anti-tumor T cell growth. A number of in vitro studies as well as animal models and clinical subjects have also shown that cytokines/chemokines have broad antitumor activity, which has been translated into a number of cancer therapy approaches. This review will focus on the foremost cytokines & chemokines involved in the majority of the gynecological malignancies and discuss their basic biology as well as clinical applications.

Sequestration of gut pathobionts in intraluminal casts, a mechanism to avoid dysregulated T cell activation by pathobionts

T cells that express the transcription factor RORγ, regulatory (Treg), or conventional (Th17) are strongly influenced by intestinal symbionts. In a genetic approach to identify mechanisms underlying this influence, we performed a screen for microbial genes implicated, in germfree mice monocolonized with Escherichia coli Nissle. The loss of capsule-synthesis genes impaired clonal expansion and differentiation of intestinal RORγ⁺ T cells. Mechanistic exploration revealed that the capsule-less mutants remained able to induce species-specific immunoglobulin A (IgA) and were highly IgA-coated. They could still trigger myeloid cells, and more effectively damaged epithelial cells in vitro. Unlike wild-type microbes, capsule-less mutants were mostly engulfed in intraluminal casts, large agglomerates composed of myeloid cells extravasated into the gut lumen. We speculate that sequestration in luminal casts of potentially harmful microbes, favored by IgA binding, reduces the immune system's actual exposure, preserving host-microbe equilibrium. The variable immunostimulation by microbes that has been charted in recent years may not solely be conditioned by triggering molecules or metabolites but also by physical limits to immune system exposure.

(Not) Home alone: Antigen presenting cell - T Cell communication in barrier tissues

Priming of T cells by antigen presenting cells (APCs) is essential for T cell fate decisions, enabling T cells to migrate to specific tissues to exert their effector functions. Previously, these interactions were mainly explored using blood-derived cells or animal models. With great advances in single cell RNA-sequencing techniques enabling analysis of tissue-derived cells, it has become clear that subsets of APCs are responsible for priming and modulating heterogeneous T cell effector responses in different tissues. This composition of APCs and T cells in tissues is essential for maintaining homeostasis and is known to be skewed in infection and inflammation, leading to pathological T cell responses. This review highlights the commonalities and differences of T cell priming and subsequent effector function in multiple barrier tissues such as the skin, intestine and female reproductive tract. Further, we provide an overview of how this process is altered during tissue-specific infections which are known to cause chronic inflammation and how this knowledge could be harnessed to modify T cell responses in barrier tissue.

Context-Dependent Regulation of Type17 Immunity by Microbiota at the Intestinal Barrier

T-helper-17 (Th17) cells and related IL-17-producing (type17) lymphocytes are abundant at the epithelial barrier. In response to bacterial and fungal infection, the signature cytokines IL-17A/F and IL-22 mediate the antimicrobial immune response and contribute to wound healing of injured tissues. Despite their protective function, type17 lymphocytes are also responsible for various chronic inflammatory disorders, including inflammatory bowel disease (IBD) and colitis associated cancer (CAC). A deeper understanding of type17 regulatory mechanisms could ultimately lead to the discovery of therapeutic strategies for the treatment of chronic inflammatory disorders and the prevention of cancer. In this review, we discuss the current understanding of the development and function of type17 immune cells at the intestinal barrier, focusing on the impact of microbiota-immune interactions on intestinal barrier homeostasis and disease etiology.

Migrating Type 2 Dendritic Cells Prime Mucosal Th17 Cells Specific to Small Intestinal Commensal Bacteria

Dendritic cells (DCs) are professional APCs equipped with MHC-restricted Ags, costimulations, and cytokines that effectively prime and differentiate naive T cells into distinct functional subsets. The immune signals that DCs carry reflect the route of Ag uptake and the innate stimuli they received. In the mucosal tissues, owing to the great variety of foreign Ags and inflammatory cues, DCs are predominantly activated and migratory. In the small intestine, CD4 Th17 cells are abundant and have been shown to be regulated by DCs and macrophages. Using a mouse commensal bacteria experimental model, we identified that the early priming step of commensal-driven Th17 cells is controlled by bona fide Zbtb46-expressing DCs. CCR7-dependent migration of type 2 DCs (DC2s) from the small intestine to the mesenteric lymph nodes (MLNs) is essential for the activation of naive CD4 T cells. The migratory DC2 population in the MLNs is almost exclusively Esam+ cells. Single-cell RNA sequencing highlighted the abundance of costimulatory markers (CD40 and OX40) and chemokines (Ccl22 and Cxcl16) on MLN migratory DCs. Further resolution of MLN migratory DC2s revealed that the Th17-polarizing cytokine IL-6 colocalizes with DC2s expressing CD40, Ccl17, and Ccl22. Thus, early Th17 cell differentiation is initiated by a small subset of migratory DC2s in the gut-draining lymph nodes.

Group 3 innate lymphoid cells require BATF to regulate gut homeostasis in mice

Group 3 innate lymphoid cells (ILC3s) are crucial for the maintenance of host-microbiota homeostasis in gastrointestinal mucosal tissues. The mechanisms that maintain lineage identity of intestinal ILC3s and ILC3-mediated orchestration of microbiota and mucosal T cell immunity are elusive. Here, we identified BATF as a gatekeeper of ILC3 homeostasis in the gut. Depletion of BATF in ILC3s resulted in excessive interferon-γ production, dysbiosis, aberrant T cell immune responses, and spontaneous inflammatory bowel disease (IBD), which was considerably ameliorated by the removal of adaptive immunity, interferon-γ blockade, or antibiotic treatment. Mechanistically, BATF directly binds to the cis-regulatory elements of type 1 effector genes, restrains their chromatin accessibility, and inhibits their expression. Conversely, BATF promotes chromatin accessibility of genes involved in MHCII antigen processing and presentation pathways, which in turn directly promotes the transition of precursor ILC3s to MHCII+ ILC3s. Collectively, our findings reveal that BATF is a key transcription factor for maintaining ILC3 stability and coordinating ILC3-mediated control of intestinal homeostasis.

Macrophage orchestration of epithelial and stromal cell homeostasis in the intestine

The intestinal tract is a complex ecosystem where numerous cell types of epithelial, immune, neuronal, and endothelial origin coexist in an intertwined, highly organized manner. The functional equilibrium of the intestine relies heavily on the proper crosstalk and cooperation among each cell population. Furthermore, macrophages are versatile, innate immune cells that participate widely in the modulation of inflammation and tissue remodeling. Emerging evidence suggest that macrophages are central in orchestrating tissue homeostasis. Herein, we describe how macrophages interact with epithelial cells, neurons, and other types of mesenchymal cells under the context of intestinal inflammation, followed by the therapeutic implications of cellular crosstalk pertaining to the treatment of inflammatory bowel disease.

T Cell Responses to the Microbiota

The immune system employs recognition tools to communicate with its microbial evolutionary partner. Among all the methods of microbial perception, T cells enable the widest spectrum of microbial recognition resolution, ranging from the crudest detection of whole groups of microbes to the finest detection of specific antigens. The application of this recognition capability to the crucial task of combatting infections has been the focus of classical immunology. We now appreciate that the coevolution of the immune system and the microbiota has led to development of a lush immunological decision tree downstream of microbial recognition, of which an inflammatory response is but one branch. In this review we discuss known T cell-microbe interactions in the gut and place them in the context of an algorithmic framework of recognition, context-dependent interpretation, and response circuits across multiple levels of microbial recognition resolution. The malleability of T cells in response to the microbiota presents an opportunity to edit immune response cellularity, identity, and functionality by utilizing microbiota-controlled pathways to promote human health.

Commensal gut bacterium critically regulates alveolar bone homeostasis

The alveolar bone is a unique osseous tissue due to the presence of the teeth and the proximity of commensal oral microbes. Commensal microbe effects on alveolar bone homeostasis have been attributed to the oral microbiota, yet the impact of commensal gut microbes is unknown. Study purpose was to elucidate whether commensal gut microbes regulate osteoimmune mechanisms and skeletal homeostasis in alveolar bone. Male C57BL/6T germfree (GF) littermate mice were maintained as GF or monoassociated with segmented filamentous bacteria (SFB), a commensal gut bacterium. SFB has been shown to elicit broad immune response effects, including the induction of T_H17/IL17A immunity, which impacts the development and homeostasis of host tissues. SFB colonized the gut, but not oral cavity, and increased IL17A levels in the ileum and serum. SFB had catabolic effects on alveolar bone and non-oral skeletal sites, which was attributed to enhanced osteoclastogenesis. The alveolar bone marrow of SFB vs. GF mice had increased dendritic cells, activated helper T-cells, T_H1 cells, T_H17 cells, and upregulated Tnf. Primary osteoblast cultures from SFB and GF mice were stimulated with vehicle-control, IL17A, or TNF to elucidate osteoblast-derived signaling factors contributing to the pro-osteoclastic phenotype in SFB mice. Treatment of RAW264.7 osteoclastic cells with supernatants from vehicle-stimulated SFB vs. GF osteoblasts recapitulated the osteoclast phenotype found in vivo. Supernatants from TNF-stimulated osteoblasts normalized RAW264.7 osteoclast endpoints across SFB and GF cultures, which was dependent on the induction of CXCL1 and CCL2. This report reveals that commensal gut microbes have the capacity to regulate osteoimmune processes in alveolar bone. Outcomes from this investigation challenge the current paradigm that alveolar bone health and homeostasis is strictly regulated by oral microbes.

Macrophage control of Crohn's disease

The intestinal tract is the body's largest mucosal surface and permanently exposed to microbial and environmental signals. Maintaining a healthy intestine requires the presence of sentinel grounds keeper cells, capable of controlling immunity and tissue homeostasis through specialized functions. Intestinal macrophages are such cells and important players in steady-state functions and during acute and chronic inflammation. Crohn's disease, a chronic inflammatory condition of the intestinal tract is proposed to be the consequence of an altered immune system through microbial and environmental stimulation. This hypothesis suggests an involvement of macrophages in the regulation of this pathology. Within this chapter, we will discuss intestinal macrophage development and highlight data suggesting their implication in chronic intestinal pathologies like Crohn's disease.

T Cell Responses to the Microbiota

The immune system employs recognition tools to communicate with its microbial evolutionary partner. Among all the methods of microbial perception, T cells enable the widest spectrum of microbial recognition resolution, ranging from the crudest detection of whole groups of microbes to the finest detection of specific antigens. The application of this recognition capability to the crucial task of combatting infections has been the focus of classical immunology. We now appreciate that the coevolution of the immune system and the microbiota has led to development of a lush immunological decision tree downstream of microbial recognition, of which an inflammatory response is but one branch. In this review we discuss known T cell-microbe interactions in the gut and place them in the context of an algorithmic framework of recognition, context-dependent interpretation, and response circuits across multiple levels of microbial recognition resolution. The malleability of T cells in response to the microbiota presents an opportunity to edit immune response cellularity, identity, and functionality by utilizing microbiota-controlled pathways to promote human health. Expected final online publication date for the Annual Review of Immunology, Volume 40 is April 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Gut dysbiosis is associated with acceleration of lupus nephritis

The gut microbiota (GM) exerts a strong influence over the host immune system and dysbiosis of this microbial community can affect the clinical phenotype in chronic inflammatory conditions. To explore the role of the GM in lupus nephritis, we colonized NZM2410 mice with Segmented Filamentous Bacteria (SFB). Gut colonization with SFB was associated with worsening glomerulonephritis, glomerular and tubular immune complex deposition and interstitial inflammation compared to NZM2410 mice free of SFB. With SFB colonization mice experienced an increase in small intestinal lamina propria Th17 cells and group 3 innate lymphoid cells (ILC3s). However, although serum IL-17A expression was elevated in these mice, Th17 cells and ILC3s were not detected in the inflammatory infiltrate in the kidney. In contrast, serum and kidney tissue expression of the macrophage chemoattractants MCP-1 and CXCL1 were significantly elevated in SFB colonized mice. Furthermore, kidney infiltrating F4/80+CD206+M2-like macrophages were significantly increased in these mice. Evidence of increased gut permeability or "leakiness" was also detected in SFB colonized mice. Finally, the intestinal microbiome of SFB colonized mice at 15 and 30 weeks of age exhibited dysbiosis when compared to uncolonized mice at the same time points. Both microbial relative abundance as well as biodiversity of colonized mice was found to be altered. Collectively, SFB gut colonization in the NZM2410 mouse exacerbates kidney disease, promotes kidney M2-like macrophage infiltration and overall intestinal microbiota dysbiosis.

Dendritic cell functions in the inductive and effector sites of intestinal immunity

The intestine is constantly exposed to foreign antigens, which are mostly innocuous but can sometimes be harmful. Therefore, the intestinal immune system has the delicate task of maintaining immune tolerance to harmless food antigens while inducing tailored immune responses to pathogens and regulating but tolerating the microbiota. Intestinal dendritic cells (DCs) play a central role in these functions as sentinel cells able to prime and polarize the T cell responses. DCs are deployed throughout the intestinal mucosa but with local specializations along the gut length and between the diffuse effector sites of the gut lamina propria (LP) and the well-organized immune inductive sites comprising isolated lymphoid follicles (ILFs), Peyer's patches (PPs), and other species-specific gut-associated lymphoid tissues (GALTs). Understanding the specificities of each intestinal DC subset, how environmental factors influence DC functions, and how these can be modulated is key to harnessing the therapeutic potential of mucosal adaptive immune responses, whether by enhancing the efficacy of mucosal vaccines or by increasing tolerogenic responses in inflammatory disorders. In this review, we summarize recent findings related to intestinal DCs in steady state and upon inflammation, with a special focus on their functional specializations, highly dependent on their microenvironment.

Depletion of Lipocalin 2 (LCN2) in Mice Leads to Dysbiosis and Persistent Colonization with Segmented Filamentous Bacteria

Lipocalin 2 (LCN2) mediates key roles in innate immune responses. It has affinity for many lipophilic ligands and binds various siderophores, thereby limiting bacterial growth by iron sequestration. Furthermore, LCN2 protects against obesity and metabolic syndrome by interfering with the composition of gut microbiota. Consequently, complete or hepatocyte-specific ablation of the Lcn2 gene is associated with higher susceptibility to bacterial infections. In the present study, we comparatively profiled microbiota in fecal samples of wild type and Lcn2 null mice and show, in contrast to previous reports, that the quantity of DNA in feces of Lcn2 null mice is significantly lower than that in wild type mice (p < 0.001). By using the hypervariable V4 region of the 16S rDNA gene and Next-Generation Sequencing methods, we found a statistically significant change in 16 taxonomic units in Lcn2^-/- mice, including eight gender-specific deviations. In particular, members of Clostridium, Escherichia, Helicobacter, Lactococcus, Prevotellaceae_UCG-001 and Staphylococcus appeared to expand in the intestinal tract of knockout mice. Interestingly, the proportion of Escherichia (200-fold) and Staphylococcus (10-fold) as well as the abundance of intestinal bacteria encoding the LCN2-sensitive siderphore enterobactin (entA) was significantly increased in male Lcn2 null mice (743-fold, p < 0.001). This was accompanied by significant higher immune cell infiltration in the ileum as demonstrated by increased immunoreactivity against the pan-leukocyte protein CD45, the lymphocyte transcription factor MUM-1/IRF4, and the macrophage antigen CD68/Macrosialin. In addition, we found a higher expression of mucosal mast cell proteases indicating a higher number of those innate immune cells. Finally, the ileum of Lcn2 null mice displayed a high abundance of segmented filamentous bacteria, which are intimately associated with the mucosal cell layer, provoking epithelial antimicrobial responses and affecting T-helper cell polarization.

Novel role of zonulin in the pathophysiology of gastro-duodenal transit: a clinical and translational study

We examined the relationship between zonulin and gastric motility in critical care patients and a translational mouse model of systemic inflammation. Gastric motility and haptoglobin (HP) 2 isoform quantification, proxy for zonulin, were examined in patients. Inflammation was triggered by lipopolysaccharide (LPS) injection in C57Bl/6 zonulin transgenic mouse (Ztm) and wildtype (WT) mice as controls, and gastro-duodenal transit was examined by fluorescein-isothiocyanate, 6 and 12 h after LPS-injection. Serum cytokines and zonulin protein levels, and zonulin gastric-duodenal mRNA expression were examined. Eight of 20 patients [14 years, IQR (12.25, 18)] developed gastric dysmotility and were HP2 isoform-producing. HP2 correlated with gastric dysmotility (r = − 0.51, CI − 0.81 to 0.003, p = 0.048). LPS injection induced a time-dependent increase in IL-6 and KC-Gro levels in all mice (p < 0.0001). Gastric dysmotility was reduced similarly in Ztm and WT mice in a time-dependent manner. Ztm had 16% faster duodenal motility than WT mice 6H post-LPS, p = 0.01. Zonulin mRNA expression by delta cycle threshold (dCT) was higher in the stomach (9.7, SD 1.4) than the duodenum (13.9, SD 1.4) 6H post-LPS, p = 0.04. Serum zonulin protein levels were higher in LPS-injected mice compared to vehicle-injected animals in a time-dependent manner. Zonulin correlated with gastric dysmotility in patients. A mouse model had time-dependent gastro-duodenal dysmotility after LPS-injection that paralleled zonulin mRNA expression and protein levels.

Serum amyloid A delivers retinol to intestinal myeloid cells to promote adaptive immunity

[Figure: see text].

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.

Niche-specific functional heterogeneity of intestinal resident macrophages

Intestinal resident macrophages are at the front line of host defence at the mucosal barrier within the gastrointestinal tract and have long been known to play a crucial role in the response to food antigens and bacteria that are able to penetrate the mucosal barrier. However, recent advances in single-cell RNA sequencing technology have revealed that resident macrophages throughout the gut are functionally specialised to carry out specific roles in the niche they occupy, leading to an unprecedented understanding of the heterogeneity and potential biological functions of these cells. This review aims to integrate these novel findings with long-standing knowledge, to provide an updated overview on our understanding of macrophage function in the gastrointestinal tract and to speculate on the role of specialised subsets in the context of homoeostasis and disease.

Tissue-Specific Contributions to Control of T Cell Immunity

T cells are critical for orchestrating appropriate adaptive immune responses and maintaining homeostasis in the face of persistent nonpathogenic Ags. T cell function is controlled in part by environmental signals received upon activation and derived from the tissue environment in which Ag is encountered. Indeed, tissue-specific environments play important roles in controlling the T cell response to Ag, and recent evidence suggests that tissue draining lymph nodes can mirror those local differences. Thus, tissue-specific immunity may begin at priming in secondary lymph nodes, where local signals have an important role in T cell fate. In this study, we discuss the tissue-specific signals that may impact T cell differentiation and function, including the microbiome, metabolism, and tissue-specific innate cell imprinting. We argue that these individual contributions create tissue-specific niches that likely play important roles in T cell differentiation and function controlling the outcome of the response to Ags.

Th17 Immunity in the Colon Is Controlled by Two Novel Subsets of Colon-Specific Mononuclear Phagocytes

Intestinal immunity is coordinated by specialized mononuclear phagocyte populations, constituted by a diversity of cell subsets. Although the cell subsets constituting the mononuclear phagocyte network are thought to be similar in both small and large intestine, these organs have distinct anatomy, microbial composition, and immunological demands. Whether these distinctions demand organ-specific mononuclear phagocyte populations with dedicated organ-specific roles in immunity are unknown. Here we implement a new strategy to subset murine intestinal mononuclear phagocytes and identify two novel subsets which are colon-specific: a macrophage subset and a Th17-inducing dendritic cell (DC) subset. Colon-specific DCs and macrophages co-expressed CD24 and CD14, and surprisingly, both were dependent on the transcription factor IRF4. Novel IRF4-dependent CD14+CD24+ macrophages were markedly distinct from conventional macrophages and failed to express classical markers including CX3CR1, CD64 and CD88, and surprisingly expressed little IL-10, which was otherwise robustly expressed by all other intestinal macrophages. We further found that colon-specific CD14+CD24+ mononuclear phagocytes were essential for Th17 immunity in the colon, and provide definitive evidence that colon and small intestine have distinct antigen presenting cell requirements for Th17 immunity. Our findings reveal unappreciated organ-specific diversity of intestine-resident mononuclear phagocytes and organ-specific requirements for Th17 immunity.

Innate Mechanisms in Selective IgA Deficiency

Selective IgA deficiency (SIgAD), characterized by a serum IgA level below 0.07 mg/ml, while displaying normal serum levels of IgM and IgG antibodies, is the most frequently occurring primary immunodeficiency that reveals itself after the first four years after birth. These individuals with SIgAD are for the majority healthy and even when they are identified they are usually not investigated further or followed up. However, recent studies show that newborns and young infants already display clinical manifestations of this condition due to aberrancies in their immune defense. Interestingly, there is a huge heterogeneity in the clinical symptoms of the affected individuals. More than 50% of the affected individuals do not have clinical symptoms, while the individuals that do show clinical symptoms can suffer from mild to severe infections, allergies and autoimmune diseases. However, the reason for this heterogeneity in the manifestation of clinical symptoms of the individuals with SIgAD is unknown. Therefore, this review focusses on the characteristics of innate immune system driving T-cell independent IgA production and providing a mechanism underlying the development of SIgAD. Thereby, we focus on some important genes, including TNFRSF13B (encoding TACI), associated with SIgAD and the involvement of epigenetics, which will cover the methylation degree of TNFRSF13B, and environmental factors, including the gut microbiota, in the development of SIgAD. Currently, no specific treatment for SIgAD exists and novel therapeutic strategies could be developed based on the discussed information.

From Species to Regional and Local Specialization of Intestinal Macrophages

Initially intended for nutrient uptake, phagocytosis represents a central mechanism of debris removal and host defense against invading pathogens through the entire animal kingdom. In vertebrates and also many invertebrates, macrophages (MFs) and MF-like cells (e.g., coelomocytes and hemocytes) are professional phagocytic cells that seed tissues to maintain homeostasis through pathogen killing, efferocytosis and tissue shaping, repair, and remodeling. Some MF functions are common to all species and tissues, whereas others are specific to their homing tissue. Indeed, shaped by their microenvironment, MFs become adapted to perform particular functions, highlighting their great plasticity and giving rise to high population diversity. Interestingly, the gut displays several anatomic and functional compartments with large pools of strikingly diversified MF populations. This review focuses on recent advances on intestinal MFs in several species, which have allowed to infer their specificity and functions.

Dynamic role of macrophage CX3CR1 expression in inflammatory bowel disease

Inflammatory bowel disease (IBD), consisting of ulcerative colitis (UC) and Crohn's disease (CD), is featured by overactive immune response and enduring course of unrestrained colitis. Genetic predisposition and environmental factors are fundamental in disease progression. Notably, microbiota dysregulation and its interaction with host mucosal barrier perplex disease phenotype. Under experimental setting, distinct mouse models are established to mimic human colitis process, including infection induced dysbiosis, dextran sulfate sodium (DSS) etc. induced barrier destruction, anti-CD40 L induced innate immunity dominant colitis and T cell transfer colitis model. Thus, from a more detailed aspect, IBD is heterogeneous and can be further classified into different subtypes based on the specific etiological pathways. As a typical inflammatory disorder, various immune cell types are involved in IBD pathogenesis. Among them, macrophages are believed to play a pivotal role. CX3CR1⁺ macrophages, deriving from peripheral patrolling CD14⁺ Ly6C^hi monocytes, are specified cell population dwelling in the gut. Accumulating evidence suggests that CX3CR1⁺ macrophages are critical for mucosal homeostasis and IBD pathogenesis, while some conflicts exist in current studies with both protective and harmful effects being revealed. Herein, we reviewed published literatures and found that the observed discrepancies stem from many aspects: the expression level of CX3CR1, the confounding dendritic cell subsets and most importantly, the different colitis stages and subtypes. Overall, CX3CR1 targeting strategy could be powerful weapon in fighting against colitis, but at the same time, the precise etiological and pathological mechanisms should be cautiously examined concerning the appropriate usage of CX3CR1 targeted therapy.

IL-22-dependent dysbiosis and mononuclear phagocyte depletion contribute to steroid-resistant gut graft-versus-host disease in mice

Efforts to improve the prognosis of steroid-resistant gut acute graft-versus-host-disease (SR-Gut-aGVHD) have suffered from poor understanding of its pathogenesis. Here we show that the pathogenesis of SR-Gut-aGVHD is associated with reduction of IFN-γ+ Th/Tc1 cells and preferential expansion of IL-17-IL-22+ Th/Tc22 cells. The IL-22 from Th/Tc22 cells causes dysbiosis in a Reg3γ-dependent manner. Transplantation of IFN-γ-deficient donor CD8+ T cells in the absence of CD4+ T cells produces a phenocopy of SR-Gut-aGVHD. IFN-γ deficiency in donor CD8+ T cells also leads to a PD-1-dependent depletion of intestinal protective CX3CR1hi mononuclear phagocytes (MNP), which also augments expansion of Tc22 cells. Supporting the dual regulation, simultaneous dysbiosis induction and depletion of CX3CR1hi MNP results in full-blown Gut-aGVHD. Our results thus provide insights into SR-Gut-aGVHD pathogenesis and suggest the potential efficacy of IL-22 antagonists and IFN-γ agonists in SR-Gut-aGVHD therapy.

Adherent-invasive E. coli metabolism of propanediol in Crohn's disease regulates phagocytes to drive intestinal inflammation

Adherent-invasive E. coli (AIEC) are enriched in the intestinal microbiota of patients with Crohn's disease (CD) and promote intestinal inflammation. Yet, how AIEC metabolism of nutrients impacts intestinal homeostasis is poorly defined. Here, we show that AIEC encoding the large subunit of propanediol dehydratase (PduC), which facilitates the utilization of fucose fermentation product 1,2-propanediol, are increased in the microbiome of CD patients and drive AIEC-induced intestinal T cell inflammation. In murine models, CX₃CR1⁺ mononuclear phagocytes (MNP) are required for PduC-dependent induction of T helper 17 (Th17) cells and interleukin-1β (IL-1β) production that leads to AIEC-induced inflammatory colitis. Activation of this inflammatory cascade requires the catalytic activity of PduC to generate propionate, which synergizes with lipopolysaccharide (LPS) to induce IL-1β by MNPs. Disrupting fucose availability limits AIEC-induced propionate production and intestinal inflammation. These findings identify MNPs as metabolic sensors linking AIEC metabolism with intestinal inflammation and identify microbial metabolism as a potential therapeutic target in Crohn's disease treatment.

Prostaglandin E2 promotes intestinal inflammation via inhibiting microbiota-dependent regulatory T cells

The gut microbiota fundamentally regulates intestinal homeostasis and disease partially through mechanisms that involve modulation of regulatory T cells (Tregs), yet how the microbiota-Treg cross-talk is physiologically controlled is incompletely defined. Here, we report that prostaglandin E2 (PGE2), a well-known mediator of inflammation, inhibits mucosal Tregs in a manner depending on the gut microbiota. PGE2 through its receptor EP4 diminishes Treg-favorable commensal microbiota. Transfer of the gut microbiota that was modified by PGE2-EP4 signaling modulates mucosal Treg responses and exacerbates intestinal inflammation. Mechanistically, PGE2-modified microbiota regulates intestinal mononuclear phagocytes and type I interferon signaling. Depletion of mononuclear phagocytes or deficiency of type I interferon receptor diminishes PGE2-dependent Treg inhibition. Together, our findings provide emergent evidence that PGE2-mediated disruption of microbiota-Treg communication fosters intestinal inflammation.

Gut-resident CX3CR1hi macrophages induce tertiary lymphoid structures and IgA response in situ

Intestinal mononuclear phagocytes (MPs) are composed of heterogeneous dendritic cell (DC) and macrophage subsets necessary for the initiation of immune response and control of inflammation. Although MPs in the normal intestine have been extensively studied, the heterogeneity and function of inflammatory MPs remain poorly defined. We performed phenotypical, transcriptional, and functional analyses of inflammatory MPs in infectious Salmonella colitis and identified CX3CR1⁺ MPs as the most prevalent inflammatory cell type. CX3CR1⁺ MPs were further divided into three distinct populations, namely, Nos2 ⁺CX3CR1^lo, Ccr7 ⁺CX3CR1^int (lymph migratory), and Cxcl13 ⁺CX3CR1^hi (mucosa resident), all of which were transcriptionally aligned with macrophages and derived from monocytes. In follow-up experiments in vivo, intestinal CX3CR1⁺ macrophages were superior to conventional DC1 (cDC1) and cDC2 in inducing Salmonella-specific mucosal IgA. We next examined spatial organization of the immune response induced by CX3CR1⁺ macrophage subsets and identified mucosa-resident Cxcl13 ⁺CX3CR1^hi macrophages as the antigen-presenting cells responsible for recruitment and activation of CD4⁺ T and B cells to the sites of Salmonella invasion, followed by tertiary lymphoid structure formation and the local pathogen-specific IgA response. Using mice we developed with a floxed Ccr7 allele, we showed that this local IgA response developed independently of migration of the Ccr7 ⁺CX3CR1^int population to the mesenteric lymph nodes and contributed to the total mucosal IgA response to infection. The differential activity of intestinal macrophage subsets in promoting mucosal IgA responses should be considered in the development of vaccines to prevent Salmonella infection and in the design of anti-inflammatory therapies aimed at modulating macrophage function in inflammatory bowel disease.

The Gut Microbiota: How Does It Influence the Development and Progression of Liver Diseases.Biomedicines. 2020;8:501. [PMID: 33207562 DOI: 10.3390/biomedicines8110501]

The gut–liver axis plays important roles in both the maintenance of a healthy liver and the pathogenesis of liver diseases, where the gut microbiota acts as a major determinant of this relationship. Gut bacteria-derived metabolites and cellular components are key molecules that affect the function of the liver and modulate the pathology of liver diseases. Accumulating evidence showed that gut microbiota produces a myriad of molecules, including lipopolysaccharide, lipoteichoic acid, peptidoglycan, and DNA, as well as short-chain fatty acids, bile acids, trimethylamine, and indole derivatives. The translocation of these components to the liver exerts beneficial or pathogenic effects by interacting with liver immune cells. This is a bidirectional relationship. Therefore, the existence of crosstalk between the gut and liver and its implications on host health and diseases are essential for the etiology and treatment of diseases. Several mechanisms have been proposed for the pathogenesis of liver diseases, but still, the mechanisms behind the pathogenic role of gut-derived components on liver pathogenesis remain elusive and not understandable. This review discusses the current progress on the gut microbiota and its components in terms of the progression of liver diseases, and in turn, how liver diseases indirectly affect the intestinal function and induce intestinal inflammation. Moreover, this paper highlights the current therapeutic and preventive strategies used to restore the gut microbiota composition and improve host health.

The colonic macrophage transcription factor RBP-J orchestrates intestinal immunity against bacterial pathogens

Macrophages play pleiotropic roles in maintaining the balance between immune tolerance and inflammatory responses in the gut. Here, we identified transcription factor RBP-J as a crucial regulator of colonic macrophage–mediated immune responses against the enteric pathogen Citrobacter rodentium. In the immune response phase, RBP-J promoted pathogen clearance by enhancing intestinal macrophage-elicited Th17 cell immune responses, which was achieved by maintenance of C/EBPβ-dependent IL-6 production by overcoming miRNA-17∼92–mediated suppressive effects. RBP-J deficiency–associated phenotypes could be genetically corrected by further deleting miRNA-17∼92 in macrophages. In the late phase, noneradicated pathogens in RBP-J KO mice recruited abundant IL-1β–expressing CD64⁺Ly6C⁺ colonic macrophages and thereby promoted persistence of ILC3-derived IL-22 to compensate for the impaired innate and adaptive immune responses, leading to ultimate clearance of pathogens. These results demonstrated that colonic macrophage–intrinsic RBP-J dynamically orchestrates intestinal immunity against pathogen infections by interfacing with key immune cells of T and innate lymphoid cell lineages.

Microbial modulation of intestinal T helper cell responses and implications for disease and therapy

Induction of intestinal T helper cell subsets by commensal members of the intestinal microbiota is an important component of the many immune adaptations required to establish host-microbial homeostasis. Importantly, altered intestinal T helper cell profiles can have pathological consequences that are not limited to intestinal sites. Therefore, microbial-mediated modulation of the intestinal T helper cell profile could have strong therapeutic potentials. However, in order to develop microbial therapies that specifically induce the desired alterations in the intestinal T helper cell compartment one has to first gain a detailed understanding of how microbial composition and the metabolites derived or induced by the microbiota impact on intestinal T helper cell responses. Here we summarize the milestone findings in the field of microbiota-intestinal T helper cell crosstalk with a focus on the role of specific commensal bacteria and their metabolites. We discuss mechanistic mouse studies and are linking these to human studies where possible. Moreover, we highlight recent advances in the field of microbial CD4 T cell epitope mimicry in autoimmune diseases and the role of microbially-induced CD4 T cells in cancer immune checkpoint blockade therapy.

Functionally distinct resident macrophage subsets differentially shape responses to infection in the bladder

Resident macrophages are abundant in the bladder, playing key roles in immunity to uropathogens. Yet, whether they are heterogeneous, where they come from, and how they respond to infection remain largely unknown. We identified two macrophage subsets in mouse bladders, MacM in muscle and MacL in the lamina propria, each with distinct protein expression and transcriptomes. Using a urinary tract infection model, we validated our transcriptomic analyses, finding that MacM macrophages phagocytosed more bacteria and polarized to an anti-inflammatory profile, whereas MacL macrophages died rapidly during infection. During resolution, monocyte-derived cells contributed to tissue-resident macrophage pools and both subsets acquired transcriptional profiles distinct from naïve macrophages. Macrophage depletion resulted in the induction of a type 1-biased immune response to a second urinary tract infection, improving bacterial clearance. Our study uncovers the biology of resident macrophages and their responses to an exceedingly common infection in a largely overlooked organ, the bladder.

At the Forefront of the Mucosal Barrier: The Role of Macrophages in the Intestine

Macrophages are part of the innate immunity and are key players for the maintenance of intestinal homeostasis. They belong to the group of mononuclear phagocytes, which exert bactericidal functions and help to clear apoptotic cells. Moreover, they play essential roles for the maintenance of epithelial integrity and tissue remodeling during wound healing processes and might be implicated in intestinal tumor development. Macrophages are antigen-presenting cells and secrete immune-modulatory factors, like chemokines and cytokines, which are necessary to activate other intestinal immune cells and therefore to shape immune responses in the gut. However, overwhelming activation or increased secretion of pro-inflammatory cytokines might also contribute to the pathogenesis of inflammatory bowel disease. Presently, intestinal macrophages are in the center of intense studies, which might help to develop new therapeutic strategies to counteract the development or treat already existing inflammatory diseases in the gut. In this review, we focus on the origin of intestinal macrophages and, based on current knowledge, discuss their role in the gut during homeostasis and inflammation, as well as during intestinal wound healing and tumor development.

Dendritic Cell Subsets in Intestinal Immunity and Inflammation

The mammalian intestine is a complex environment that is constantly exposed to Ags derived from food, microbiota, and metabolites. Intestinal dendritic cells (DC) have the responsibility of establishing oral tolerance against these Ags while initiating immune responses against mucosal pathogens. We now know that DC are a heterogeneous population of innate immune cells composed of classical and monocyte-derived DC, Langerhans cells, and plasmacytoid DC. In the intestine, DC are found in organized lymphoid tissues, such as the mesenteric lymph nodes and Peyer's patches, as well as in the lamina propria. In this Brief Review, we review recent work that describes a division of labor between and collaboration among gut DC subsets in the context of intestinal homeostasis and inflammation. Understanding relationships between DC subtypes and their biological functions will rationalize oral vaccine design and will provide insights into treatments that quiet pathological intestinal inflammation.

Intestinal resident macrophages: Multitaskers of the gut

BACKGROUND

Intestinal resident macrophages play a crucial role in homeostasis and have been implicated in numerous gastrointestinal diseases. While historically believed to be largely of hematopoietic origin, recent advances in fate-mapping technology have unveiled the existence of long-lived, self-maintaining populations located in specific niches throughout the gut wall. Furthermore, the advent of single-cell technology has enabled an unprecedented characterization of the functional specialization of tissue-resident macrophages throughout the gastrointestinal tract.

PURPOSE

The purpose of this review was to provide a panorama on intestinal resident macrophages, with particular focus to the recent advances in the field. Here, we discuss the functions and phenotype of intestinal resident macrophages and, where possible, the functional specialization of these cells in response to the niche they occupy. Furthermore, we will discuss their role in gastrointestinal diseases.

The Gut Microbiome Regulates Psychological-Stress-Induced Inflammation

Psychological stress has adverse effects on various human diseases, including those of the cardiovascular system. However, the mechanisms by which stress influences disease activity remain unclear. Here, using vaso-occlusive episodes (VOEs) of sickle cell disease as a vascular disease model, we show that stress promotes VOEs by eliciting a glucocorticoid hormonal response that augments gut permeability, leading to microbiota-dependent interleukin-17A (IL-17A) secretion from T helper 17 (Th17) cells of the lamina propria, followed by the expansion of the circulating pool of aged neutrophils that trigger VOEs. We identify segmented filamentous bacteria as the commensal essential for the stress-induced expansion of aged neutrophils that enhance VOEs in mice. Importantly, the inhibition of glucocorticoids synthesis, blockade of IL-17A, or depletion of the Th17 cell-inducing gut microbiota markedly reduces stress-induced VOEs. These results offer potential therapeutic targets to limit the impact of psychological stress on acute vascular occlusion.

Transcriptional regulation of DC fate specification

Dendritic cells function in the immune system to instruct adaptive immune cells to respond accordingly to different threats. While conventional dendritic cells can be subdivided into two main subtypes, termed cDC1s and cDC2s, it is clear that further heterogeneity exists within these subtypes, particularly for cDC2s. Understanding the signals involved in specifying each of these lineages and subtypes thereof is crucial to (i) enable us to determine their specific functions and (ii) put us in a position to be able to target these cells to promote or prevent a specific function in any given disease setting. Although we still have much to learn regarding the specification of these cells, here we review the most recent advances in our understanding of this and highlight some of the next questions for the future.