CCR2(+)CD103(-) intestinal dendritic cells develop from DC-committed precursors and induce interleukin-17 production by T cells

CCR2-dependent CX3CR1+ colonic macrophages promote Enterococcus faecalis dissemination

Enterococci are common commensal bacteria that colonize the gastrointestinal tracts of most mammals, including humans. Importantly, these bacteria are one of the leading causes of nosocomial infections. This study examined the role of colonic macrophages in facilitating Enterococcus faecalis infections in mice. We determined that depletion of colonic phagocytes resulted in the reduction of E. faecalis dissemination to the gut-draining mesenteric lymph nodes. Furthermore, we established that trafficking of monocyte-derived CX3CR1-expressing macrophages contributed to E. faecalis dissemination in a manner that was not reliant on CCR7, the conventional receptor involved in lymphatic migration. Finally, we showed that E. faecalis mutants with impaired intracellular survival exhibited reduced dissemination, suggesting that E. faecalis can exploit host immune cell migration to disseminate systemically and cause disease. Our findings indicate that modulation of macrophage trafficking in the context of antibiotic therapy could serve as a novel approach for preventing or treating opportunistic infections by disseminating enteric pathobionts like E. faecalis.

Therapeutic induction of antigen-specific immune tolerance

The development of therapeutic approaches for the induction of robust, long-lasting and antigen-specific immune tolerance remains an important unmet clinical need for the management of autoimmunity, allergy, organ transplantation and gene therapy. Recent breakthroughs in our understanding of immune tolerance mechanisms have opened new research avenues and therapeutic opportunities in this area. Here, we review mechanisms of immune tolerance and novel methods for its therapeutic induction.

Single-cell profiling reveals kidney CD163+ dendritic cell participation in human lupus nephritis

OBJECTIVES

The current work aimed to provide a comprehensive single-cell landscape of lupus nephritis (LN) kidneys, including immune and non-immune cells, identify disease-associated cell populations and unravel their participation within the kidney microenvironment.

METHODS

Single-cell RNA and T cell receptor sequencing were performed on renal biopsy tissues from 40 patients with LN and 6 healthy donors as controls. Matched peripheral blood samples from seven LN patients were also sequenced. Multiplex immunohistochemical analysis was performed on an independent cohort of 60 patients and validated using flow cytometric characterisation of human kidney tissues and in vitro assays.

RESULTS

We uncovered a notable enrichment of CD163+ dendritic cells (DC3s) in LN kidneys, which exhibited a positive correlation with the severity of LN. In contrast to their counterparts in blood, DC3s in LN kidney displayed activated and highly proinflammatory phenotype. DC3s showed strong interactions with CD4+ T cells, contributing to intrarenal T cell clonal expansion, activation of CD4+ effector T cell and polarisation towards Th1/Th17. Injured proximal tubular epithelial cells (iPTECs) may orchestrate DC3 activation, adhesion and recruitment within the LN kidneys. In cultures, blood DC3s treated with iPTECs acquired distinct capabilities to polarise Th1/Th17 cells. Remarkably, the enumeration of kidney DC3s might be a potential biomarker for induction treatment response in LN patients.

CONCLUSION

The intricate interplay involving DC3s, T cells and tubular epithelial cells within kidneys may substantially contribute to LN pathogenesis. The enumeration of renal DC3 holds potential as a valuable stratification feature for guiding LN patient treatment decisions in clinical practice.

Guidelines for DC preparation and flow cytometry analysis of mouse nonlymphoid tissues

This article is part of the Dendritic Cell Guidelines article series, which provides a collection of state-of-the-art protocols for the preparation, phenotype analysis by flow cytometry, generation, fluorescence microscopy and functional characterization of mouse and human dendritic cells (DC) from lymphoid organs and various nonlymphoid tissues. DC are sentinels of the immune system present in almost every mammalian organ. Since they represent a rare cell population, DC need to be extracted from organs with protocols that are specifically developed for each tissue. This article provides detailed protocols for the preparation of single-cell suspensions from various mouse nonlymphoid tissues, including skin, intestine, lung, kidney, mammary glands, oral mucosa and transplantable tumors. Furthermore, our guidelines include comprehensive protocols for multiplex flow cytometry analysis of DC subsets and feature top tricks for their proper discrimination from other myeloid cells. With this collection, we provide guidelines for in-depth analysis of DC subsets that will advance our understanding of their respective roles in healthy and diseased tissues. While all protocols were written by experienced scientists who routinely use them in their work, this article was also peer-reviewed by leading experts and approved by all coauthors, making it an essential resource for basic and clinical DC immunologists.

Peyer's patch phagocytes acquire specific transcriptional programs that influence their maturation and activation profiles

Peyer's patches (PPs) are secondary lymphoid organs in contact with the external environment via the intestinal lumen, thus combining antigen sampling and immune response initiation sites. Therefore, they provide a unique opportunity to study the entire process of phagocyte differentiation and activation in vivo. Here, we deciphered the transcriptional and spatial landscape of PP phagocyte populations from their emergence in the tissue to their final maturation state at homeostasis and under stimulation. Activation of monocyte-derived Lysozyme-expressing dendritic cells (LysoDCs) differs from that of macrophages by their upregulation of conventional DC (cDC) signature genes such as Ccr7 and downregulation of typical monocyte-derived cell genes such as Cx3cr1. We identified gene sets that distinguish PP cDCs from the villus ones and from LysoDCs. We also identified key immature, early, intermediate, and late maturation markers of PP phagocytes. Finally, exploiting the ability of the PP interfollicular region to host both villous and subepithelial dome emigrated cDCs, we showed that the type of stimulus, the subset, but also the initial location of cDCs shape their activation profile and thus direct the immune response. Our study highlights the importance of targeting the right phagocyte subset at the right place and time to manipulate the immune response.

Classical DC2 subsets and monocyte-derived DC: Delineating the developmental and functional relationship

To specifically tailor immune responses to a given pathogenic threat, dendritic cells (DC) are highly heterogeneous and comprise many specialized subtypes, including conventional DC (cDC) and monocyte-derived DC (MoDC), each with distinct developmental and functional characteristics. However, the functional relationship between cDC and MoDC is not fully understood, as the overlapping phenotypes of certain type 2 cDC (cDC2) subsets and MoDC do not allow satisfactory distinction of these cells in the tissue, particularly during inflammation. However, precise cDC2 and MoDC classification is required for studies addressing how these diverse cell types control immune responses and is therefore currently one of the major interests in the field of cDC research. This review will revise murine cDC2 and MoDC biology in the steady state and under inflammatory conditions and discusses the commonalities and differences between ESAM^lo cDC2, inflammatory cDC2, and MoDC and their relative contribution to the initiation, propagation, and regulation of immune responses.

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.

The intestinal microenvironment shapes macrophage and dendritic cell identity and function

The gut comprises the largest body interface with the environment and is continuously exposed to nutrients, food antigens, and commensal microbes, as well as to harmful pathogens. Subsets of both macrophages and dendritic cells (DCs) are present throughout the intestinal tract, where they primarily inhabit the gut-associate lymphoid tissue (GALT), such as Peyer's patches and isolated lymphoid follicles. In addition to their role in taking up and presenting antigens, macrophages and DCs possess extensive functional plasticity and these cells play complementary roles in maintaining immune homeostasis in the gut by preventing aberrant immune responses to harmless antigens and microbes and by promoting host defense against pathogens. The ability of macrophages and DCs to induce either inflammation or tolerance is partially lineage imprinted, but can also be dictated by their activation state, which in turn is determined by their specific microenvironment. These cells express several surface and intracellular receptors that detect danger signals, nutrients, and hormones, which can affect their activation state. DCs and macrophages play a fundamental role in regulating T cells and their effector functions. Thus, modulation of intestinal mucosa immunity by targeting antigen presenting cells can provide a promising approach for controlling pathological inflammation. In this review, we provide an overview on the characteristics, functions, and origins of intestinal macrophages and DCs, highlighting the intestinal microenvironmental factors that influence their functions during homeostasis. Unraveling the mechanisms by which macrophages and DCs regulate intestinal immunity will deepen our understanding on how the immune system integrates endogenous and exogenous signals in order to maintain the host's homeostasis.

IRF4 expression by lung dendritic cells drives acute but not Trm cell-dependent memory Th2 responses

Expression of the transcription factor interferon regulatory factor 4 (IRF4) is required for the development of lung conventional DCs type 2 (cDC2s) that elicit Th2 responses, yet how IRF4 functions in lung cDC2s throughout the acute and memory allergic response is not clear. Here, we used a mouse model that loses IRF4 expression after lung cDC2 development to demonstrate that mice with IRF4-deficient DCs display impaired memory responses to allergen. This defect in the memory response was a direct result of ineffective Th2 induction and impaired recruitment of activated effector T cells to the lung after sensitization. IRF4-deficient DCs demonstrated defects in their migration to the draining lymph node and in T cell priming. Finally, T cells primed by IRF4-competent DCs mediated potent memory responses independently of IRF4-expressing DCs, demonstrating that IRF4-expressing DCs are not necessary during the memory response. Thus, IRF4 controlled a program in mature DCs governing Th2 priming and effector responses, but IRF4-expressing DCs were dispensable during tissue-resident memory T cell-dependent memory responses.

How cell migration helps immune sentinels

The immune system relies on the migratory capacity of its cellular components, which must be mobile in order to defend the host from invading micro-organisms or malignant cells. This applies in particular to immune sentinels from the myeloid lineage, i.e. macrophages and dendritic cells. Cell migration is already at work during mammalian early development, when myeloid cell precursors migrate from the yolk sac, an extra embryonic structure, to colonize tissues and form the pool of tissue-resident macrophages. Later, this is accompanied by a migration wave of precursors and monocytes from the bone marrow to secondary lymphoid organs and the peripheral tissues. They differentiate into DCs and monocyte-derived macrophages. During adult life, cell migration endows immune cells with the ability to patrol their environment as well as to circulate between peripheral tissues and lymphoid organs. Hence migration of immune cells is key to building an efficient defense system for an organism. In this review, we will describe how cell migratory capacity regulates the various stages in the life of myeloid cells from development to tissue patrolling, and migration to lymph nodes. We will focus on the role of the actin cytoskeletal machinery and its regulators, and how it contributes to the establishment and function of the immune system.

(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.

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.

BATF3 Protects Against Metabolic Syndrome and Maintains Intestinal Epithelial Homeostasis

The intestinal immune system and microbiota are emerging as important contributors to the development of metabolic syndrome, but the role of intestinal dendritic cells (DCs) in this context is incompletely understood. BATF3 is a transcription factor essential in the development of mucosal conventional DCs type 1 (cDC1). We show that Batf3-/- mice developed metabolic syndrome and have altered localization of tight junction proteins in intestinal epithelial cells leading to increased intestinal permeability. Treatment with the glycolysis inhibitor 2-deoxy-D-glucose reduced intestinal inflammation and restored barrier function in obese Batf3-/- mice. High-fat diet further enhanced the metabolic phenotype and susceptibility to dextran sulfate sodium colitis in Batf3-/- mice. Antibiotic treatment of Batf3-/- mice prevented metabolic syndrome and impaired intestinal barrier function. Batf3-/- mice have altered IgA-coating of fecal bacteria and displayed microbial dysbiosis marked by decreased obesity protective Akkermansia muciniphila, and Bifidobacterium. Thus, BATF3 protects against metabolic syndrome and preserves intestinal epithelial barrier by maintaining beneficial microbiota.

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.

Epithelial colonization by gut dendritic cells promotes their functional diversification

Dendritic cells (DCs) patrol tissues and transport antigens to lymph nodes to initiate adaptive immune responses. Within tissues, DCs constitute a complex cell population composed of distinct subsets that can exhibit different activation states and functions. How tissue-specific cues orchestrate DC diversification remains elusive. Here, we show that the small intestine included two pools of cDC2s originating from common pre-DC precursors: (1) lamina propria (LP) CD103+CD11b+ cDC2s that were mature-like proinflammatory cells and (2) intraepithelial cDC2s that exhibited an immature-like phenotype as well as tolerogenic properties. These phenotypes resulted from the action of food-derived retinoic acid (ATRA), which enhanced actomyosin contractility and promoted LP cDC2 transmigration into the epithelium. There, cDC2s were imprinted by environmental cues, including ATRA itself and the mucus component Muc2. Hence, by reaching distinct subtissular niches, DCs can exist as immature and mature cells within the same tissue, revealing an additional mechanism of DC functional diversification.

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.

Macrophages play a role in inflammatory transformation of colorectal cancer

Colorectal cancer (CRC) is one of the most common and fatal cancers worldwide, and it is also a typical inflammatory cancer. The function of macrophages is very important in the tissue immune microenvironment during inflammatory and carcinogenic transformation. Here, we evaluated the function and mechanism of macrophages in intestinal physiology and in different pathological stages. Furthermore, the role of macrophages in the immune microenvironment of CRC and the influence of the intestinal population and hypoxic environment on macrophage function are summarized. In addition, in the era of tumor immunotherapy, CRC currently has a limited response rate to immune checkpoint inhibitors, and we summarize potential therapeutic strategies for targeting tumor-associated macrophages.

Identification of Immune-Related Key Genes in Ovarian Cancer Based on WGCNA

Background: Ovarian cancer (OV) is a fatal gynecologic malignancy and has poor survival rate in women over the age of forty. In our study, we aimed to identify genes related to immune microenvironment regulations and explore genes associated with OV prognosis. Methods: The RNA-seq data of GDC TCGA Ovarian Cancer cohort of 376 patients was retrieved from website. Weighted gene co-expression network analysis (WGCNA) and ESTIMATE algorithm were applied to identify the key genes associated with the immune scores. The correlation between key genes and 22 immune cell types were estimated by using CIBERSORT algorithms. Results: WGCNA showed that the pink module was most correlated with the immune score. Seven of 14 key genes (FCRL3, IFNG, KCNA3, LY9, PLA2G2D, THEMIS, and TRAT1) were significantly associated with the OS of OV patients. Correlation analysis showed our key genes positively related to M1 macrophages, CD8 T cells, plasma cells, regulatory T (Treg) cells and activated memory CD4 T cells, and negatively related to naive CD4 T cells, M0 macrophages, activated dendritic cells (DCs) and memory B cells. Kaplan-Meier survival analysis showed that lower abundances of neutrophils and higher abundances of M1 macrophages, plasma cells, T cells gamma delta (γδT) cells and follicular helper T (Tfh) cells predicted better OV prognosis. Conclusion: Forteen key genes related to the immune infiltrating of OV were identified, and seven of them were significantly related to prognosis. These key genes have potential roles in tumor infiltrating immune cells differentiation and proliferation. This study provided potential prognostic markers and immunotherapy targets for OV.

Recruitment of dendritic cell progenitors to foci of influenza A virus infection sustains immunity

Protection from infection with respiratory viruses such as influenza A virus (IAV) requires T cell–mediated immune responses initiated by conventional dendritic cells (cDCs) that reside in the respiratory tract. Here, we show that effective induction of T cell responses against IAV in mice requires reinforcement of the resident lung cDC network by cDC progenitors. We found that CCR2-binding chemokines produced during IAV infection recruit pre-cDCs from blood and direct them to foci of infection, increasing the number of progeny cDCs next to sites of viral replication. Ablation of CCR2 in the cDC lineage prevented this increase and resulted in a deficit in IAV-specific T cell responses and diminished resistance to reinfection. These data suggest that the homeostatic network of cDCs in tissues is insufficient for immunity and reveal a chemokine-driven mechanism of expansion of lung cDC numbers that amplifies T cell responses against respiratory viruses.

Beyond Immunity: Underappreciated Functions of Intestinal Macrophages

The gastrointestinal tract hosts the largest compartment of macrophages in the body, where they serve as mediators of host defense and immunity. Seeded in the complex tissue-environment of the gut, an array of both hematopoietic and non-hematopoietic cells forms their immediate neighborhood. Emerging data demonstrate that the functional diversity of intestinal macrophages reaches beyond classical immunity and includes underappreciated non-immune functions. In this review, we discuss recent advances in research on intestinal macrophage heterogeneity, with a particular focus on how non-immune functions of macrophages impact tissue homeostasis and function. We delve into the strategic localization of distinct gut macrophage populations, describe the potential factors that regulate their identity and functional heterogeneity within these locations, and provide open questions that we hope will inspire research dedicated to elucidating a holistic view on macrophage-tissue cell interactions in the body's largest mucosal organ.

Deep immunophenotyping at the single-cell level identifies a combination of anti-IL-17 and checkpoint blockade as an effective treatment in a preclinical model of data-guided personalized immunotherapy

BACKGROUND

Although immune checkpoint blockade is effective for several malignancies, a substantial number of patients remain refractory to treatment. The future of immunotherapy will be a personalized approach adapted to each patient's cancer-immune interactions in the tumor microenvironment (TME) to prevent suppression of antitumor immune responses. To demonstrate the feasibility of this kind of approach, we developed combination therapy for a preclinical model guided by deep immunophenotyping of the TME.

METHODS

Gastric cancer cell lines YTN2 and YTN16 were subcutaneously inoculated into C57BL/6 mice. YTN2 spontaneously regresses, while YTN16 grows progressively. Bulk RNA-Seq, single-cell RNA-Seq (scRNA-Seq) and flow cytometry were performed to investigate the immunological differences in the TME of these tumors.

RESULTS

Bulk RNA-Seq demonstrated that YTN16 tumor cells produced CCL20 and that CD8⁺ T cell responses were impaired in these tumors relative to YTN2. We have developed a vertical flow array chip (VFAC) for targeted scRNA-Seq to identify unique subtypes of T cells by employing a panel of genes reflecting T cell phenotypes and functions. CD8⁺ T cell dysfunction (cytotoxicity, proliferation and the recruitment of interleukin-17 (IL-17)-producing cells into YTN16 tumors) was identified by targeted scRNA-Seq. The presence of IL-17-producing T cells in YTN16 tumors was confirmed by flow cytometry, which also revealed neutrophil infiltration. IL-17 blockade suppressed YTN16 tumor growth, while tumors were rejected by the combination of anti-IL-17 and anti-PD-1 (Programmed cell death protein 1) mAb treatment. Reduced neutrophil activation and enhanced expansion of neoantigen-specific CD8⁺ T cells were observed in tumors of the mice receiving the combination therapy.

CONCLUSIONS

Deep phenotyping of YTN16 tumors identified a sequence of events on the axis CCL20->IL-17-producing cells->IL-17-neutrophil-angiogenesis->suppression of neoantigen-specific CD8⁺ T cells which was responsible for the lack of tumor rejection. IL-17 blockade together with anti-PD-1 mAb therapy eradicated these YTN16 tumors. Thus, the deep immunological phenotyping can guide immunotherapy for the tailored treatment of each individual patient's tumor.

Dendritic Cells: Versatile Players in Renal Transplantation

Dendritic cells (DCs) induce and regulate adaptive immunity through migrating and maturing in the kidney. In this procedure, they can adopt different phenotypes—rejection-associated DCs promote acute or chronic injury renal grafts while tolerogenic DCs suppress the overwhelmed inflammation preventing damage to renal functionality. All the subsets interact with effector T cells and regulatory T cells (Tregs) stimulated by the ischemia–reperfusion procedure, although the classification corresponding to different effects remains controversial. Thus, in this review, we discuss the origin, maturation, and pathological effects of DCs in the kidney. Then we summarize the roles of divergent DCs in renal transplantation: taking both positive and negative stages in ischemia–reperfusion injury (IRI), switching phenotypes to induce acute or chronic rejection, and orchestrating surface markers for allograft tolerance via alterations in metabolism. In conclusion, we prospect that multidimensional transcriptomic analysis will revolute researches on renal transplantation by addressing the elusive mononuclear phagocyte classification and providing a holistic view of DC ontogeny and subpopulations.

Group 3 Innate Lymphoid Cells Program a Distinct Subset of IL-22BP-Producing Dendritic Cells Demarcating Solitary Intestinal Lymphoid Tissues

Solitary intestinal lymphoid tissues such as cryptopatches (CPs) and isolated lymphoid follicles (ILFs) constitute steady-state activation hubs containing group 3 innate lymphoid cells (ILC3) that continuously produce interleukin (IL)-22. The outer surface of CPs and ILFs is demarcated by a poorly characterized population of CD11c⁺ cells. Using genome-wide single-cell transcriptional profiling of intestinal mononuclear phagocytes and multidimensional flow cytometry, we found that CP- and ILF-associated CD11c⁺ cells were a transcriptionally distinct subset of intestinal cDCs, which we term CIA-DCs. CIA-DCs required programming by CP- and ILF-resident CCR6⁺ ILC3 via lymphotoxin-β receptor signaling in cDCs. CIA-DCs differentially expressed genes associated with immunoregulation and were the major cellular source of IL-22 binding protein (IL-22BP) at steady state. Mice lacking CIA-DC-derived IL-22BP exhibited diminished expression of epithelial lipid transporters, reduced lipid resorption, and changes in body fat homeostasis. Our findings provide insight into the design principles of an immunoregulatory checkpoint controlling nutrient absorption.

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.

Prenatal Development and Function of Human Mononuclear Phagocytes

The human mononuclear phagocyte (MP) system, which includes dendritic cells, monocytes, and macrophages, is a critical regulator of innate and adaptive immune responses. During embryonic development, MPs derive sequentially in yolk sac progenitors, fetal liver, and bone marrow haematopoietic stem cells. MPs maintain tissue homeostasis and confer protective immunity in post-natal life. Recent evidence - primarily in animal models - highlight their critical role in coordinating the remodeling, maturation, and repair of target organs during embryonic and fetal development. However, the molecular regulation governing chemotaxis, homeostasis, and functional diversification of resident MP cells in their respective organ systems during development remains elusive. In this review, we summarize the current understanding of the development and functional contribution of tissue MPs during human organ development and morphogenesis and its relevance to regenerative medicine. We outline how single-cell multi-omic approaches and next-generation ex-vivo organ-on-chip models provide new experimental platforms to study the role of human MPs during development and disease.

CD137 Signaling Regulates Acute Colitis via RALDH2-Expressing CD11b-CD103+ DCs

CD137, a potent costimulatory receptor for CD8⁺ T cells, is expressed in various non-T cells, but little is known about its regulatory functions in these cells. In this study, we show that CD137 signaling, specifically in intestinal CD11b^-CD103⁺ dendritic cells (DCs), restricts acute colitis progression. Mechanistically, CD137 engagement activates TAK1 and subsequently stimulates the AMPK-PGC-1α axis to enhance expression of the Aldh1a2 gene encoding the retinoic acid (RA) metabolizing enzyme RALDH2. RA can act on CD11b⁺CD103^- DCs and induce SOCS3 expression, which, in turn, suppresses p38MAPK activation and interleukin-23 (IL-23) production. Administration of RA in DC-specific CD137^-/- mice represses IL-23-producing CD11b⁺CD103^- DCs and T_H17 cells, indicating that RA is a major inhibitory effector molecule against intestinal CD11b⁺CD103^- DCs. Additionally, the therapeutic effect of the anti-CD137 antibody is abrogated in DC-specific CD137^-/- mice. Taken together, our results define a mechanism of paracrine immunoregulation operating between adjacent DC subsets in the intestine.

Dendritic Cell Regulation of T Helper Cells

As the professional antigen-presenting cells of the immune system, dendritic cells (DCs) sense the microenvironment and shape the ensuing adaptive immune response. DCs can induce both immune activation and immune tolerance according to the peripheral cues. Recent work has established that DCs comprise several phenotypically and functionally heterogeneous subsets that differentially regulate T lymphocyte differentiation. This review summarizes both mouse and human DC subset phenotypes, development, diversification, and function. We focus on advances in our understanding of how different DC subsets regulate distinct CD4⁺ T helper (Th) cell differentiation outcomes, including Th1, Th2, Th17, T follicular helper, and T regulatory cells. We review DC subset intrinsic properties, local tissue microenvironments, and other immune cells that together determine Th cell differentiation during homeostasis and inflammation.

Dendritic Cells Revisited

Dendritic cells (DCs) possess the ability to integrate information about their environment and communicate it to other leukocytes, shaping adaptive and innate immunity. Over the years, a variety of cell types have been called DCs on the basis of phenotypic and functional attributes. Here, we refocus attention on conventional DCs (cDCs), a discrete cell lineage by ontogenetic and gene expression criteria that best corresponds to the cells originally described in the 1970s. We summarize current knowledge of mouse and human cDC subsets and describe their hematopoietic development and their phenotypic and functional attributes. We hope that our effort to review the basic features of cDC biology and distinguish cDCs from related cell types brings to the fore the remarkable properties of this cell type while shedding some light on the seemingly inordinate complexity of the DC field.

CCR2- and Flt3-Dependent Inflammatory Conventional Type 2 Dendritic Cells Are Necessary for the Induction of Adaptive Immunity by the Human Vaccine Adjuvant System AS01

The Adjuvant System AS01 contains monophosphoryl lipid A (MPL) and the saponin QS-21 in a liposomal formulation. AS01 is included in recently developed vaccines against malaria and varicella zoster virus. Like for many other adjuvants, induction of adaptive immunity by AS01 is highly dependent on the ability to recruit and activate dendritic cells (DCs) that migrate to the draining lymph node for T and B cell stimulation. The objective of this study was to more precisely address the contribution of the different conventional (cDC) and monocyte-derived DC (MC) subsets in the orchestration of the adaptive immune response after immunization with AS01 adjuvanted vaccine. The combination of MPL and QS-21 in AS01 induced strong recruitment of CD26+XCR1+ cDC1s, CD26+CD172+ cDC2s and a recently defined CCR2-dependent CD64-expressing inflammatory cDC2 (inf-cDC2) subset to the draining lymph node compared to antigen alone, while CD26-CD64+CD88+ MCs were barely detectable. At 24 h post-vaccination, cDC2s and inf-cDC2s were superior amongst the different subsets in priming antigen-specific CD4+ T cells, while simultaneously presenting antigen to CD8+ T cells. Diphtheria toxin (DT) mediated depletion of all DCs prior to vaccination completely abolished adaptive immune responses, while depletion 24 h after vaccination mainly affected CD8+ T cell responses. Vaccinated mice lacking Flt3 or the chemokine receptor CCR2 showed a marked deficit in inf-cDC2 recruitment and failed to raise proper antibody and T cell responses. Thus, the adjuvant activity of AS01 is associated with the potent activation of subsets of cDC2s, including the newly described inf-cDC2s.

Antigen-Presenting Cells in Food Tolerance and Allergy

Food allergy now affects 6%-8% of children in the Western world; despite this, we understand little about why certain people become sensitized to food allergens. The dominant form of food allergy is mediated by food-specific immunoglobulin E (IgE) antibodies, which can cause a variety of symptoms, including life-threatening anaphylaxis. A central step in this immune response to food antigens that differentiates tolerance from allergy is the initial priming of T cells by antigen-presenting cells (APCs), primarily different types of dendritic cells (DCs). DCs, along with monocyte and macrophage populations, dictate oral tolerance versus allergy by shaping the T cell and subsequent B cell antibody response. A growing body of literature has shed light on the conditions under which antigen presentation occurs and how different types of T cell responses are induced by different APCs. We will review APC subsets in the gut and discuss mechanisms of APC-induced oral tolerance versus allergy to food identified using mouse models and patient samples.

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.

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.

IRF4 Expression Is Required for the Immunoregulatory Activity of Conventional Type 2 Dendritic Cells in Settings of Chronic Bacterial Infection and Cancer

The lamina propria of the gastrointestinal tract and other mucosal surfaces of humans and mice host a network of mononuclear phagocytes that differ in their ontogeny, surface marker and transcription factor expression, and functional specialization. Conventional dendritic cells (DCs) in particular exist as two major subpopulations in both lymphoid and nonlymphoid organs that can be distinguished based on their surface marker and transcription factor expression. In this study, we show in various Th1- and/or Th17-polarized settings of acute and chronic bacterial infection and of tumor growth that the conditional ablation of Irf4 in CD11c⁺ DCs results in more efficient immune control of Helicobacter pylori, Mycobacterium bovis bacillus Calmette-Guérin, and Citrobacter rodentium and of tumor growth in a syngeneic tumor model. We attribute the phenotype of IRF4^ΔDC mice to unrestricted Th1 responses and in particular to IFN-γ- and TNF-α-expressing CD4⁺ T cells. This activity of IRF4-expressing DCs is linked to a DC-specific immunoregulatory transcriptional program. In contrast, in Th2-polarized settings such as house dust mite-induced allergic airway inflammation, the lack of IRF4 expression in the DC compartment alleviates inflammation and goblet cell metaplasia. The combined data provide evidence for immunoregulatory properties of this versatile DC population in Th1-polarized infection settings.

Inflammatory Type 2 cDCs Acquire Features of cDC1s and Macrophages to Orchestrate Immunity to Respiratory Virus Infection

The phenotypic and functional dichotomy between IRF8⁺ type 1 and IRF4⁺ type 2 conventional dendritic cells (cDC1s and cDC2s, respectively) is well accepted; it is unknown how robust this dichotomy is under inflammatory conditions, when additionally monocyte-derived cells (MCs) become competent antigen-presenting cells (APCs). Using single-cell technologies in models of respiratory viral infection, we found that lung cDC2s acquired expression of the Fc receptor CD64 shared with MCs and of IRF8 shared with cDC1s. These inflammatory cDC2s (inf-cDC2s) were superior in inducing CD4⁺ T helper (Th) cell polarization while simultaneously presenting antigen to CD8⁺ T cells. When carefully separated from inf-cDC2s, MCs lacked APC function. Inf-cDC2s matured in response to cell-intrinsic Toll-like receptor and type 1 interferon receptor signaling, upregulated an IRF8-dependent maturation module, and acquired antigens via convalescent serum and Fc receptors. Because hybrid inf-cDC2s are easily confused with monocyte-derived cells, their existence could explain why APC functions have been attributed to MCs.

•

Type I interferon drives differentiation of inf-cDC2s that closely resemble MCs

•

Inf-cDC2s prime CD4⁺ and CD8⁺ T cells, whereas MCs lack APC function

•

Inf-cDC2s internalize antibody-complexed antigen via Fc receptors

•

IRF8 controls maturation gene module in inf-cDC2s

Kidney dendritic cells: fundamental biology and functional roles in health and disease

Dendritic cells (DCs) are chief inducers of adaptive immunity and regulate local inflammatory responses across the body. Together with macrophages, the other main type of mononuclear phagocyte, DCs constitute the most abundant component of the intrarenal immune system. This network of functionally specialized immune cells constantly surveys its microenvironment for signs of injury or infection, which trigger the initiation of an immune response. In the healthy kidney, DCs coordinate effective immune responses, for example, by recruiting neutrophils for bacterial clearance in pyelonephritis. The pro-inflammatory actions of DCs can, however, also contribute to tissue damage in various types of acute kidney injury and chronic glomerulonephritis, as DCs recruit and activate effector T cells, which release toxic mediators and maintain tubulointerstitial immune infiltrates. These actions are counterbalanced by DC subsets that promote the activation and maintenance of regulatory T cells to support resolution of the immune response and allow kidney repair. Several studies have investigated the multiple roles for DCs in kidney homeostasis and disease, but it has become clear that current tools and subset markers are not sufficient to accurately distinguish DCs from macrophages. Multidimensional transcriptomic analysis studies promise to improve mononuclear phagocyte classification and provide a clearer view of DC ontogeny and subsets.

NOD2 modulates immune tolerance via the GM-CSF-dependent generation of CD103+ dendritic cells

Four decades ago, it was identified that muramyl dipeptide (MDP), a peptidoglycan-derived bacterial cell wall component, could display immunosuppressive functions in animals through mechanisms that remain unexplored. We sought to revisit these pioneering observations because mutations in NOD2, the gene encoding the host sensor of MDP, are associated with increased risk of developing the inflammatory bowel disease Crohn's disease, thus suggesting that the loss of the immunomodulatory functions of NOD2 could contribute to the development of inflammatory disease. Here, we demonstrate that intraperitoneal (i.p.) administration of MDP triggered regulatory T cells and the accumulation of a population of tolerogenic CD103⁺ dendritic cells (DCs) in the spleen. This was found to occur not through direct sensing of MDP by DCs themselves, but rather via the production of the cytokine GM-CSF, another factor with an established regulatory role in Crohn's disease pathogenesis. Moreover, we demonstrate that populations of CD103-expressing DCs in the gut lamina propria are enhanced by the activation of NOD2, indicating that MDP sensing plays a critical role in shaping the immune response to intestinal antigens by promoting a tolerogenic environment via manipulation of DC populations.

Single-Cell Protein and RNA Expression Analysis of Mononuclear Phagocytes in Intestinal Mucosa and Mesenteric Lymph Nodes of Ulcerative Colitis and Crohn's Disease Patients

Inflammatory bowel diseases (IBDs), which include Crohn's disease (CD) and ulcerative colitis (UC), are driven by an abnormal immune response to commensal microbiota in genetically susceptible hosts. In addition to epithelial and stromal cells, innate and adaptive immune systems are both involved in IBD immunopathogenesis. Given the advances driven by single-cell technologies, we here reviewed the immune landscape and function of mononuclear phagocytes in inflamed non-lymphoid and lymphoid tissues of CD and UC patients. Immune cell profiling of IBD tissues using scRNA sequencing combined with multi-color cytometry analysis identifies unique clusters of monocyte-like cells, macrophages, and dendritic cells. These clusters reflect either distinct cell lineages (nature), or distinct or intermediate cell types with identical ontogeny, adapting their phenotype and function to the surrounding milieu (nurture and tissue imprinting). These advanced technologies will provide an unprecedented view of immune cell networks in health and disease, and thus may offer a personalized medicine approach to patients with IBD.

Transcriptomic Analysis and High-dimensional Phenotypic Mapping of Mononuclear Phagocytes in Mesenteric Lymph Nodes Reveal Differences Between Ulcerative Colitis and Crohn's Disease

BACKGROUND AND AIMS

Crohn's disease [CD] and ulcerative colitis [UC] are distinct forms of inflammatory bowel disease. Heterogeneity of HLA-DR+SIRPα + mononuclear phagocytes [MNPs], including macrophages [MΦ], monocyte-derived [Mono] cells, and dendritic cells [DCs], was reported in gut tissue but not yet investigated in mesenteric lymph nodes [MLNs] of IBD patients. We here compared the phenotype, function, and molecular profile of HLA-DR+SIRPα + MNPs in CD and UC MLNs.

METHODS

Cell distribution, morphology, immune function, and transcriptomic [bulk RNAseq] and high-dimensional protein expression profiles [CyTOF] of HLA-DR+SIRPα + MNPs were examined in MLNs of UC [n = 14], CD [n = 35], and non-IBD [n = 12] patients.

RESULTS

Elevated frequencies of CD14+CD64+CD163+ [Mono/MΦ-like] MNPs displaying monocyte/MΦ morphology and phagocytic function were a distinct feature of UC MLNs. In CD, the proportion of CD14-CD64-CD163- [DC-like] cells was augmented relative to Mono/MΦ-like cells; DC-like cells drove naïve T cell proliferation, Th1 polarisation, and Th17 TCM plasticity. Gene expression profile corroborated the nature of DC-like cells, best represented by BTLA, SERPINF, IGJ and, of Mono/MΦ-like cells, defined by CD163, MARCO, MAFB, CD300E, S100A9 expression. CyTOF analysis showed that CD123+ plasmacytoid cells predominated over conventional DCs in DC-like cells. Four CD163+ clusters were revealed in Mono/MΦ-like cells, two of which were enriched in MARCO-CD68dimHLA-DRdim monocyte-like cells and MARCOhiCD68hiHLA-DRhi Mɸ, whose proportion increased in UC relative to CD.

CONCLUSIONS

Defining the landscape of MNPs in MLNs provided evidence for expansion of CD163+ Mono/MΦ-like cells in UC only, highlighting a distinction between UC and CD, and thus the potential contribution of monocyte-like cells in driving colitis.

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.

The ontogenetic path of human dendritic cells

Dendritic cells (DCs) orchestrate adaptive immune responses. In healthy individuals, DCs are drivers and fine-tuners of T cell responses directed against invading pathogens or cancer cells. In parallel, DCs control autoreactive T cells, thereby maintaining T cell tolerance. Under various disease conditions, a disruption of this delicate balance can lead to chronic infections, tumor evasion, or autoimmunity. While great efforts have been made to unravel the origin and development of this powerful cell type in mice, only little is known about the ontogeny of human DCs. Here, we summarize the current understanding of the developmental path of DCs from hematopoietic stem cells to fully functional DCs in their local tissue environment and provide a template for the identification of DCs across various tissues.

Batf3-Dependent Intestinal Dendritic Cells Play a Critical Role in the Control of Cryptosporidium parvum Infection

Understanding the protective immune response to Cryptosporidium parvum infection is of critical importance to reduce the widespread impact caused by this disease in young individuals. Here, we analyzed the various subsets of CD103+ and CD103- intestinal dendritic cells (DCs) of wild-type and Batf3-/- neonatal mice at homoeostasis and investigated their role during infection. Neonatal Batf3-/- mice had a low CD103+/CD103- DC ratio, resulting in higher susceptibility to the acute phase of the infection and they could not cure the infection. Early during infection, CD103- DCs of Batf3-/- neonates had a lower ability to produce interleukin-12 than their wild-type littermates and lower levels of interferon-gamma mRNA were detected in the infected mucosa. Amplification of CD103+ DCs in Batf3-/- neonates prior to infectious challenge reduced their susceptibility to infection. CD103+ DCs thus outperform CD103- DCs in controlling C. parvum infections and represent a primary target of host-directed immunotherapies dedicated to neonates.

Dissecting the Heterogeneity in T-Cell Mediated Inflammation in IBD

Infiltration of the lamina propria by inflammatory CD4+ T-cell populations is a key characteristic of chronic intestinal inflammation. Memory-phenotype CD4+ T-cell frequencies are increased in inflamed intestinal tissue of IBD patients compared to tissue of healthy controls and are associated with disease flares and a more complicated disease course. Therefore, a tightly controlled balance between regulatory and inflammatory CD4+ T-cell populations is crucial to prevent uncontrolled CD4+ T-cell responses and subsequent intestinal tissue damage. While at steady state, T-cells display mainly a regulatory phenotype, increased in Th1, Th2, Th9, Th17, and Th17.1 responses, and reduced Treg and Tr1 responses have all been suggested to play a role in IBD pathophysiology. However, it is highly unlikely that all these responses are altered in each individual patient. With the rapidly expanding plethora of therapeutic options to inhibit inflammatory T-cell responses and stimulate regulatory T-cell responses, a crucial need is emerging for a robust set of immunological assays to predict and monitor therapeutic success at an individual level. Consequently, it is crucial to differentiate dominant inflammatory and regulatory CD4+ T helper responses in patients and relate these to disease course and therapy response. In this review, we provide an overview of how intestinal CD4+ T-cell responses arise, discuss the main phenotypes of CD4+ T helper responses, and review how they are implicated in IBD.

Fungal Symbionts Produce Prostaglandin E2 to Promote Their Intestinal Colonization

Candida albicans is a ubiquitous fungal symbiont that resides on diverse human barrier surfaces. Both mammalian and fungal cells can convert arachidonic acid into the lipid mediator, prostaglandin E2 (PGE2), but the physiological significance of fungus-derived PGE2 remains elusive. Here we report that a C. albicans mutant deficient in PGE2 production suffered a loss of competitive fitness in the murine gastrointestinal (GI) tract and that PGE2 supplementation mitigated this fitness defect. Impaired fungal PGE2 production affected neither the in vitro fitness of C. albicans nor hyphal morphogenesis and virulence in either systemic or mucosal infection models. Instead, fungal production of PGE2 was associated with enhanced fungal survival within phagocytes. Consequently, ablation of colonic phagocytes abrogated the intra-GI fitness boost conferred by fungal PGE2. These observations suggest that C. albicans has evolved the capacity to produce PGE2 from arachidonic acid, a host-derived precursor, to promote its own colonization of the host gut. Analogous mechanisms might undergird host-microbe interactions of other symbiont fungi.

To respond or not to respond - a personal perspective of intestinal tolerance

For many years, the intestine was one of the poor relations of the immunology world, being a realm inhabited mostly by specialists and those interested in unusual phenomena. However, this has changed dramatically in recent years with the realization of how important the microbiota is in shaping immune function throughout the body, and almost every major immunology institution now includes the intestine as an area of interest. One of the most important aspects of the intestinal immune system is how it discriminates carefully between harmless and harmful antigens, in particular, its ability to generate active tolerance to materials such as commensal bacteria and food proteins. This phenomenon has been recognized for more than 100 years, and it is essential for preventing inflammatory disease in the intestine, but its basis remains enigmatic. Here, I discuss the progress that has been made in understanding oral tolerance during my 40 years in the field and highlight the topics that will be the focus of future research.

In vivo Differentiation of Human Monocytes

Circulating monocytes can infiltrate mucosal or inflamed tissues where they differentiate into either macrophages or dendritic cells. This paradigm is supported by numerous studies conducted in mice and in different in vitro settings for human cells. Determining whether it holds true in vivo in humans is essential for the successful design of monocyte-targeting therapies. Despite limitations inherent to working with human samples, there is accumulating evidence of the existence of in vivo-generated monocyte-derived cells in humans. Here, we review recent studies showing the recruitment of human monocytes into tissues and their differentiation into macrophages or dendritic cells, in normal or pathological settings. We examine the methods available in human studies to demonstrate the monocytic origin of infiltrating cells. Finally, we review the functions of human monocyte-derived cells and how they might contribute to pathogeny.

Regulation of IgA Production by Intestinal Dendritic Cells and Related Cells

The intestinal mucosa is a physiological barrier for most microbes, including both commensal bacteria and invading pathogens. Under homeostatic conditions, immunoglobulin A (IgA) is the major immunoglobulin isotype in the intestinal mucosa. Microbes stimulate the production of IgA, which controls bacterial translocation and neutralizes bacterial toxins at the intestinal mucosal surface. In the intestinal mucosa, dendritic cells (DCs), specialized antigen-presenting cells, regulate both T-cell-dependent (TD) and -independent (TI) immune responses. The intestinal DCs are a heterogeneous population that includes unique subsets that induce IgA synthesis in B cells. The characteristics of intestinal DCs are strongly influenced by the microenvironment, including the presence of commensal bacterial metabolites and epithelial cell-derived soluble factors. In this review, we summarize the ontogeny, classification, and function of intestinal DCs and how the intestinal microenvironment conditions DCs and their precursors to become the mucosal phenotype, in particular to regulate IgA production, after they arrive at the intestine. Understanding the mechanism of IgA synthesis could provide insights for designing effective mucosal vaccines.

BATF3-dependent dendritic cells drive both effector and regulatory T-cell responses in bacterially infected tissues

The gastric lamina propria of mice that have been experimentally infected with the pathobiont Helicobacter pylori hosts a dense network of myeloid cells that includes BATF3-dependent CD103⁺ dendritic cells (DCs). We show here that CD103⁺ DCs are strictly required for gastric Th1 responses to H. pylori and for H. pylori infection control. A similar dependence of type 1 immunity on CD103⁺ DCs is observed in a Mycobacterium bovis BCG infection model, and in a syngeneic colon cancer model. Strikingly, we find that not only the expansion and/or recruitment of Th1 cells, but also of peripherally induced, neuropilin-negative regulatory T-cells to sites of infection requires BATF3-dependent DCs. A shared feature of the examined models is the strongly reduced production of the chemokines and CXCR3 ligands CXCL9, 10 and 11 in BATF3-deficient mice. The results implicate BATF3-dependent DCs in the recruitment of CXCR3⁺ effector and regulatory T-cells to target tissues and in their local expansion.

In this work, Arnold & Zhang et al report that CD103⁺ DCs are required for protective Th1 responses, infection control of mucosal and systemic bacterial pathogens, and anti-tumor immunity driven by CD4⁺ Th1 cells and CD8⁺ T cells. CD103⁺ DCs further specifically promote the recruitment of Tbet⁺ peripherally induced Tregs to sites of infection. The results implicate CD103⁺ DCs in the trafficking of CXCR3⁺ Tbet⁺ T-cells to sites of infection and tumorigenesis.

IgG and Fcγ Receptors in Intestinal Immunity and Inflammation

Fcγ receptors (FcγR) are cell surface glycoproteins that mediate cellular effector functions of immunoglobulin G (IgG) antibodies. Genetic variation in FcγR genes can influence susceptibility to a variety of antibody-mediated autoimmune and inflammatory disorders, including systemic lupus erythematosus (SLE) and rheumatoid arthritis (RA). More recently, however, genetic studies have implicated altered FcγR signaling in the pathogenesis of inflammatory bowel disease (IBD), a condition classically associated with dysregulated innate and T cell immunity. Specifically, a variant of the activating receptor, FcγRIIA, with low affinity for IgG, confers protection against the development of ulcerative colitis, a subset of IBD, leading to a re-evaluation of the role of IgG and FcγRs in gastrointestinal tract immunity, an organ system traditionally associated with IgA. In this review, we summarize our current understanding of IgG and FcγR function at this unique host-environment interface, from the pathogenesis of colitis and defense against enteropathogens, its contribution to maternal-fetal cross-talk and susceptibility to cancer. Finally, we discuss the therapeutic implications of this information, both in terms of how FcγR signaling pathways may be targeted for the treatment of IBD and how FcγR engagement may influence the efficacy of therapeutic monoclonal antibodies in IBD.

NLRP3 Controls the Development of Gastrointestinal CD11b+ Dendritic Cells in the Steady State and during Chronic Bacterial Infection

The gastric lamina propria is largely uncharted immunological territory. Here we describe the evolution and composition of the gastric, small intestinal, and colonic lamina propria mononuclear phagocyte system during the steady state and infection with the gastric pathogen Helicobacter pylori. We show that monocytes, CX₃CR1^hi macrophages, and CD11b⁺ dendritic cells are recruited to the infected stomach in a CCR2-dependent manner. All three populations, but not BATF3-dependent CD103⁺ DCs, sample red fluorescent protein (RFP)⁺Helicobacter pylori (H. pylori). Mice reconstituted with human hematopoietic stem cells recapitulate several features of the myeloid cell-H. pylori interaction. The differentiation in and/or recruitment to gastrointestinal, lung, and lymphoid tissues of CD11b⁺ DCs requires NLRP3, but not apoptosis-associated speck-like protein containing a carboxy-terminal CARD (ASC) or caspase-1, during steady-state and chronic infection. NLRP3^-/- mice fail to generate Treg responses to H. pylori and control the infection more effectively than wild-type mice. The results demonstrate a non-canonical inflammasome-independent function of NLRP3 in DC development and immune regulation.