Dendritic cells produce CXCL13 and participate in the development of murine small intestine lymphoid tissues

Tertiary lymphoid structural heterogeneity determines tumour immunity and prospects for clinical application

Tertiary lymphoid structures (TLS) are clusters of immune cells that resemble and function similarly to secondary lymphoid organs (SLOs). While TLS is generally associated with an anti-tumour immune response in most cancer types, it has also been observed to act as a pro-tumour immune response. The heterogeneity of TLS function is largely determined by the composition of tumour-infiltrating lymphocytes (TILs) and the balance of cell subsets within the tumour-associated TLS (TA-TLS). TA-TLS of varying maturity, density, and location may have opposing effects on tumour immunity. Higher maturity and/or higher density TLS are often associated with favorable clinical outcomes and immunotherapeutic response, mainly due to crosstalk between different proportions of immune cell subpopulations in TA-TLS. Therefore, TLS can be used as a marker to predict the efficacy of immunotherapy in immune checkpoint blockade (ICB). Developing efficient imaging and induction methods to study TA-TLS is crucial for enhancing anti-tumour immunity. The integration of imaging techniques with biological materials, including nanoprobes and hydrogels, alongside artificial intelligence (AI), enables non-invasive in vivo visualization of TLS. In this review, we explore the dynamic interactions among T and B cell subpopulations of varying phenotypes that contribute to the structural and functional diversity of TLS, examining both existing and emerging techniques for TLS imaging and induction, focusing on cancer immunotherapies and biomaterials. We also highlight novel therapeutic approaches of TLS that are being explored with the aim of increasing ICB treatment efficacy and predicting prognosis.

Translational and oncologic significance of tertiary lymphoid structures in pancreatic adenocarcinoma

Pancreatic adenocarcinoma (PDAC) is an aggressive tumor with poor survival and limited treatment options. PDAC resistance to immunotherapeutic strategies is multifactorial, but partially owed to an immunosuppressive tumor immune microenvironment (TiME). However, the PDAC TiME is heterogeneous and harbors favorable tumor-infiltrating lymphocyte (TIL) populations. Tertiary lymphoid structures (TLS) are organized aggregates of immune cells that develop within non-lymphoid tissue under chronic inflammation in multiple contexts, including cancers. Our current understanding of their role within the PDAC TiME remains limited; TLS are complex structures with multiple anatomic features such as location, density, and maturity that may impact clinical outcomes such as survival and therapy response in PDAC. Similarly, our understanding of methods to manipulate TLS is an actively developing field of research. TLS may function as anti-tumoral immune niches that can be leveraged as a therapeutic strategy to potentiate both existing chemotherapeutic regimens and potentiate future immune-based therapeutic strategies to improve patient outcomes. This review seeks to cover anatomy, relevant features, immune effects, translational significance, and future directions of understanding TLS within the context of PDAC.

Small Intestinal Goblet Cells Control Humoral Immune Responses and Mobilization During Enteric Infection

Humoral immune responses within the gut play diverse roles including pathogen clearance during enteric infections, maintaining tolerance, and facilitating the assemblage and stability of the gut microbiota. How these humoral immune responses are initiated and contribute to these processes are well studied. However, the signals promoting the expansion of these responses and their rapid mobilization to the gut mucosa are less well understood. Intestinal goblet cells form goblet cell-associated antigen passages (GAPs) to deliver luminal antigens to the underlying immune system and facilitate tolerance. GAPs are rapidly inhibited during enteric infection to prevent inflammatory responses to innocuous luminal antigens. Here we interrogate GAP inhibition as a key physiological response required for effective humoral immunity. Independent of infection, GAP inhibition resulted in enrichment of transcripts representing B cell recruitment, expansion, and differentiation into plasma cells in the small intestine (SI), which were confirmed by flow cytometry and ELISpot assays. Further we observed an expansion of isolated lymphoid follicles within the SI, as well as expansion of plasma cells in the bone marrow upon GAP inhibition. S1PR1-induced blockade of leukocyte trafficking during GAP inhibition resulted in a blunting of SI plasma cell expansion, suggesting that mobilization of plasma cells from the bone marrow contributes to their expansion in the gut. However, luminal IgA secretion was only observed in the presence of S. typhimurium infection, suggesting that although GAP inhibition mobilizes a mucosal humoral immune response, a second signal is required for full effector function. Overriding GAP inhibition during enteric infection abrogated the expansion of laminar propria IgA+ plasma cells. We conclude that GAP inhibition is a required physiological response for efficiently mobilizing mucosal humoral immunity in response to enteric infection.

Transcription factor TCF-1 regulates the functions, but not the development, of lymphoid tissue inducer subsets in different tissues

Lymphoid tissue inducer (LTi) cells, a subset of innate lymphoid cells (ILCs), play an essential role in the formation of secondary lymphoid tissues. However, the regulation of the development and functions of this ILC subset is still elusive. In this study, we report that the transcription factor T cell factor 1 (TCF-1), just as GATA3, is indispensable for the development of non-LTi ILC subsets. While LTi cells are still present in TCF-1-deficient mice, the organogenesis of Peyer's patches (PPs), but not of lymph nodes, is impaired in these mice. LTi cells from different tissues have distinct gene expression patterns, and TCF-1 regulates the expression of lymphotoxin specifically in PP LTi cells. Mechanistically, TCF-1 may directly and/or indirectly regulate Lta, including through promoting the expression of GATA3. Thus, the TCF-1-GATA3 axis, which plays an important role during T cell development, also critically regulates the development of non-LTi cells and tissue-specific functions of LTi cells.

Radiofrequency Ablation Remodels the Tumor Microenvironment and Promotes Neutrophil-Mediated Abscopal Immunomodulation in Pancreatic Cancer

Pancreatic ductal adenocarcinoma (PDAC) presents a 5-year overall survival rate of 11%, despite efforts to improve clinical outcomes in the past two decades. Therapeutic resistance is a hallmark of this disease, due to its dense and suppressive tumor microenvironment (TME). Endoscopic ultrasound-guided radiofrequency ablation (EUS-RFA) is a promising local ablative and potential immunomodulatory therapy for PDAC. In this study, we performed RFA in a preclinical tumor-bearing KrasG12D; Trp53R172H/+; Pdx1:Cre (KPC) syngeneic model, analyzed local and abscopal affects after RFA and compared our findings with resected PDAC specimens. We found that RFA reduced PDAC tumor progression in vivo and promoted strong TME remodeling. In addition, we discovered tumor-infiltrating neutrophils determined abscopal effects. Using imaging mass cytometry, we showed that RFA elevated dendritic cell numbers in RFA-treated tumors and promoted a significant CD4+ and CD8+ T-cell abscopal response. In addition, RFA elevated levels of programmed death-ligand 1 (PD-L1) and checkpoint blockade inhibition targeting PD-L1 sustained tumor growth reduction in the context of RFA. This study indicates RFA treatment, which has been shown to increase tumor antigen shedding, promotes antitumor immunity. This is critical in PDAC where recent clinical immunotherapy trials have not resulted in substantial changes in overall survival.

Bioinformatics Analysis of Prognostic Significance and Immune Characteristics of CXC Chemokine Family in Patients with Lung Adenocarcinoma

Objective

To screen CXC chemokines related to the risk of lung adenocarcinoma (LUAD) using bioinformatics and construct a CXC-based prognostic risk model to improve the diagnosis and treatment of LUAD patients.

Methods

The Cancer Genome Atlas (TCGA) database and Gene Expression Omnibus (GEO) database were searched to obtain mRNA expression data and clinicopathological information of LUAD patients. CXC genes differentially expressed in LUAD were screened using the R packages. Further, risk factors significantly associated with the survival of LUAD patients were obtained by the univariate Cox proportional hazard regression, LASSO regression, and multivariate Cox proportional hazard regression analysis, following which a risk prediction model was constructed. The performance of the CXCL13-based model in predicting the prognosis of low-risk and high-risk effect LUAD patients was verified, and the association between the model and the degree of tumor immune cell infiltration was investigated.

Results

CXCL13 was significantly highly expressed in the cancer tissues of LUAD patients. The risk of death in patients with highly expressed CXCL13 was about 1.5 times higher than in those with lowly expressed CXCL13 (HR = 1.5153357). CXCL13-based risk scoring showed that the high-risk score of LUAD patients was significantly correlated with poor prognosis, but no relation between the two was found in the GEO validation sets, suggesting that this risk model may not be accurate enough. In addition, activated B cells, CD4+ T cells, CD8+ T cells, and dendritic cells were significantly positively correlated with the high risk of LUAD.

Conclusions

Although we found that a high expression of CXCL13 was associated with a high risk of death and immune cell infiltration and activation in LUAD patients, the CXCL13-based risk model was not accurate enough for predicting the prognosis of LUAD patients.

The role of dendritic cells in the immune niche of the peritoneum

Dendritic cells (DCs) are professional antigen presenting cells that play an important role in the induction of T cell responses. Different subsets (cDC1s, cDC2s, pDCs, and moDCs) were described based on the expression of different surface markers and functions. In the context of peritoneum, DCs are also a key population cell orchestrating immune responses against pathogens, malignant cells and tissue-damage. Furthermore, they play an important role in the promotion of an anti-inflammatory microenvironment, which is necessary to maintain tolerance and adipocyte homeostasis. The aim of this review is to summarize the current knowledge of the functional and phenotypic features of peritoneal DCs and shed some light on the importance of these cells within this unique cavity and its associated components: the omentum, the mesentery and gut-associated lymphoid tissue (GALT).

Effect of morroniside on the transcriptome profiles of rat in injured spinal cords

We have previously reported that morroniside promoted motor activity after spinal cord injury (SCI) in rats. However, the mechanism by which morroniside induces recovery of injured spinal cord (SC) remains unknown. In the current study, RNA sequencing (RNA-seq) was employed to evaluate changes of gene expressions at the transcriptional level of the injured spinal cords in morroniside-administrated rats. Principal component analysis, analysis of enriched Gene Ontology (GO), enrichment analyses Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway, and other bioinformatics analyses were executed to distinguish differentially expressed genes (DEGs). The results of RNA-seq confirmed the anti-inflammatory and anti-apoptotic effects of morroniside on injured SC tissues, and provided the basis for additional research of the mechanisms involving the protective effects of morroniside on SCI.

Differential Effect of Light and Dark Period Sleep Fragmentation on Composition of Gut Microbiome and Inflammation in Mice

Bi-directional interactions amongst the gut microbiota, immune system, and brain function are thought to be critical mediators of health and disease. The role sleep plays in mediating these interactions is not known. We assessed the effects of sleep fragmentation (SF) on the microbiota–gut–brain axis. Male C57BL/6NCrl mice (4 to 5 per cage, fed standard lab chow) experienced SF via mechanical stimulation at 2 min intervals during the light (SF) and dark (DD, dark disturbances) periods. Home cage (HC) controls were undisturbed. After 10 days, fecal samples were collected at light onset, midday, light offset, and midnight. Samples were also collected after 10 days without SF. Subsequently, the mice were randomized across groups and allowed 20 additional days of recovery followed by 10 days of SF or DD. To assess effects on the microbiota, 16S rRNA sequencing was used, and mesenteric lymph nodes (MLNs) and cortex and medial prefrontal cortex were analyzed using cytokine arrays. SF and DD produced significant alterations in the microbiota compared to HC, and DD had greater impact than SF on some organisms. SF produced marked suppression in MLNs of chemokines that regulate inflammation (CCL3, CCL4 and their receptor CCR5) and maintain the immune mucosal barrier (Cxcl13) at the same time that cortical cytokines (IL-33) indicated neuroinflammation. DD effects on immune responses were similar to HC. These data suggest that SF alters the microbiome and suppresses mucosal immunity at the same time that mediators of brain inflammation are upregulated. The translational implications for potential application to clinical care are compelling.

The Impact of Programmed Cell Death on the Formation of Tertiary Lymphoid Structures

Tertiary lymphoid structures are clusters of lymphoid tissue that develop post-natally at sites of chronic inflammation. They have been described in association with infection, autoimmune disorders, cancer, and allograft rejection. In their mature stage, TLS function as ectopic germinal centers, favoring the local production of autoantibodies and cytokines. TLS formation tends to parallel the severity of tissue injury and they are usually indicative of locally active immune responses. The presence of TLS in patients with solid tumors is usually associated with a better prognosis whereas their presence predicts increased maladaptive immunologic activity in patients with autoimmune disorders or allograft transplantation. Recent data highlight a correlation between active cell death and TLS formation and maturation. Our group recently identified apoptotic exosome-like vesicles, released by apoptotic cells, as novel inducers of TLS formation. Here, we review mechanisms of TLS formation and maturation with a specific focus on the emerging importance of tissue injury, programmed cell death and extracellular vesicles in TLS biogenesis.

Human gut-associated lymphoid tissues (GALT); diversity, structure, and function

Gut-associated lymphoid tissues (GALT) are the key antigen sampling and adaptive immune inductive sites within the intestinal wall. Human GALT includes the multi-follicular Peyer's patches of the ileum, the vermiform appendix, and the numerous isolated lymphoid follicles (ILF) which are distributed along the length of the intestine. Our current understanding of GALT diversity and function derives primarily from studies in mice, and the relevance of many of these findings to human GALT remains unclear. Here we review our current understanding of human GALT diversity, structure, and composition as well as their potential for regulating intestinal immune responses during homeostasis and inflammatory bowel disease (IBD). Finally, we outline some key remaining questions regarding human GALT, the answers to which will advance our understanding of intestinal immune responses and provide potential opportunities to improve the treatment of intestinal diseases.

Spatial distribution of LTi-like cells in intestinal mucosa regulates type 3 innate immunity

Lymphoid tissue inducer (LTi)-like cells are tissue resident innate lymphocytes that rapidly secrete cytokines that promote gut epithelial integrity and protect against extracellular bacterial infections.Here, we report that the retention of LTi-like cells in conventional solitary intestinal lymphoid tissue (SILT) is essential for controlling LTi-like cell function and is maintained by expression of the chemokine receptor CXCR5. Deletion of Cxcr5 functionally unleashed LTi-like cells in a cell intrinsic manner, leading to uncontrolled IL-17 and IL-22 production. The elevated production of IL-22 in Cxcr5-deficient mice improved gut barrier integrity and protected mice during infection with the opportunistic pathogen Clostridium difficile Interestingly, Cxcr5-/- mice developed LTi-like cell aggregates that were displaced from their typical niche at the intestinal crypt, and LTi-like cell hyperresponsiveness was associated with the local formation of this unconventional SILT. Thus, LTi-like cell positioning within mucosa controls their activity via niche-specific signals that temper cytokine production during homeostasis.

Tertiary Lymphoid Structures: Diversity in Their Development, Composition, and Role

Lymph node stromal cells coordinate the adaptive immune response in secondary lymphoid organs, providing both a structural matrix and soluble factors that regulate survival and migration of immune cells, ultimately promoting Ag encounter. In several inflamed tissues, resident fibroblasts can acquire lymphoid-stroma properties and drive the formation of ectopic aggregates of immune cells, named tertiary lymphoid structures (TLSs). Mature TLSs are functional sites for the development of adaptive responses and, consequently, when present, can have an impact in both autoimmunity and cancer conditions. In this review, we go over recent findings concerning both lymph node stromal cells and TLSs function and formation and further describe what is currently known about their role in disease, particularly their potential in tolerance.

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.

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.

Chemokine Ligand 13 (CXCL13) in Neuroborreliosis and Neurosyphilis as Selected Spirochetal Neurological Diseases: A Review of Its Diagnostic Significance

Neuroborreliosis (NB) and neurosyphilis (NS) are abnormal conditions caused by spirochetal bacteria which affect the nervous system. Diagnosis of neuroborreliosis and neurosyphilis is determined by clinical examination of visible symptoms, serum and cerebrospinal fluid (CSF) analysis, and serological detection of antibodies against Borrelia burgdorferi sensu lato and Treponema pallidum, respectively. Establishing a diagnosis may sometimes pose a number of diagnostic difficulties. A potential role of chemokine ligand 13 (CXCL13) as an accurate diagnostic biomarker of intrathecal inflammation has been suggested. In this review, we focused on changes in serum and cerebrospinal fluid concentration of chemokine ligand 13 in selected spirochetal neurological diseases neuroborreliosis and neurosyphilis reported in the available literature. We performed an extensive search of the literature relevant to our investigation via the MEDLINE/PubMed database. It has been proven that CXCL13 determination can provide rapid information regarding central nervous system inflammation in patients with selected spirochetosis. We described that neuroborreliosis and neurosyphilis are associated with an elevated CXCL13 concentration, mainly in the cerebrospinal fluid. Moreover, literature data suggest that CXCL13 determination is the most interesting additional marker for diagnosis and monitoring of neuroborreliosis and neurosyphilis thanks to its high sensitivity. Based on these published findings, we suggest that CXCL13 has high diagnostic utility and may be applied in laboratory diagnostics as a potential diagnostic marker in human spirochetal neurologic diseases.

Immune Profiling of Human Gut-Associated Lymphoid Tissue Identifies a Role for Isolated Lymphoid Follicles in Priming of Region-Specific Immunity

The intestine contains some of the most diverse and complex immune compartments in the body. Here we describe a method for isolating human gut-associated lymphoid tissues (GALTs) that allows unprecedented profiling of the adaptive immune system in submucosal and mucosal isolated lymphoid follicles (SM-ILFs and M-ILFs, respectively) as well as in GALT-free intestinal lamina propria (LP). SM-ILF and M-ILF showed distinct patterns of distribution along the length of the intestine, were linked to the systemic circulation through MAdCAM-1⁺ high endothelial venules and efferent lymphatics, and had immune profiles consistent with immune-inductive sites. IgA sequencing analysis indicated that human ILFs are sites where intestinal adaptive immune responses are initiated in an anatomically restricted manner. Our findings position ILFs as key inductive hubs for regional immunity in the human intestine, and the methods presented will allow future assessment of these compartments in health and disease.

Oxysterols in intestinal immunity and inflammation

Cholesterol is an essential molecule for life. It is a component of the cell membrane, and it is a precursor molecule for bile acids, vitamin D and steroid hormones. Cholesterol is actively metabolized, but the impact of endogenous cholesterol metabolites on immune function, especially in the intestine, is poorly understood. In this review, I focus on oxysterols, hydroxylated forms of cholesterol, and their specialized functions in intestinal immunity. Oxysterols act through various intracellular and extracellular receptors and serve as key metabolic signals, coordinating immune activity and inflammation. Our recent work has identified an unexpected link between cholesterol metabolism, innate lymphoid cell function and intestinal homeostasis. We discovered that oxysterol sensing through the G protein-coupled receptor 183 (GPR183) directs the migration of innate lymphoid cells, which is essential for the formation of lymphoid tissue in the colon. Moreover, we found that the interaction of GPR183 with oxysterols regulates intestinal inflammation. I will discuss the therapeutic potential of oxysterols and future possibilities of treating inflammatory bowel disease through the modulation of cholesterol metabolism.

IL-22-Independent Protection from Colitis in the Absence of Nkx2.3 Transcription Factor in Mice

The transcription factor Nkx2.3 regulates the vascular specification of Peyer patches in mice through determining endothelial addressin preference and may function as a susceptibility factor in inflammatory bowel diseases in humans. We wished to analyze the role of Nkx2.3 in colonic solitary intestinal lymphoid tissue composition and in colitis pathogenesis. We studied the colonic solitary intestinal lymphoid tissue of Nkx2.3-deficient mice with immunofluorescence and flow cytometry. Colitis was induced in mice using 2.5% dextran sodium sulfate, and severity was assessed with histology, flow cytometry, and quantitative PCR. We found that the lack of Nkx2.3 impairs maturation of isolated lymphoid follicles and attenuates dextran sodium sulfate-induced colitis independent of endothelial absence of mucosal addressin cell-adhesion molecule-1 (MAdCAM-1), which was also coupled with enhanced colonic epithelial regeneration. Although we observed increased numbers of group 3 innate lymphoid cells and Th17 cells and enhanced transcription of IL-22, Ab-mediated neutralization of IL-22 did not abolish the protection from colitis in Nkx2.3-deficient mice. Nkx2.3-/- hematopoietic cells could not rescue wild-type mice from colitis. Using LacZ-Nkx2.3 reporter mice, we found that Nkx2.3 expression was restricted to VAP-1+ myofibroblast-like pericryptal cells. These results hint at a previously unknown stromal role of Nkx2.3 as driver of colitis and indicate that Nkx2.3+ stromal cells play a role in epithelial cell homeostasis.

Development and Organization of the Secondary and Tertiary Lymphoid Organs: Influence of Microbial and Food Antigens

Background:

Secondary lymphoid organs (SLO) are distributed in many districts of the body and, especially, lymph nodes, spleen and gut-associated lymphoid tissue are the main cellular sites. On the other hand, tertiary lymphoid organs (TLO) are formed in response to inflammatory, infectious, autoimmune and neoplastic events. </P><P> Developmental Studies: In the present review, emphasis will be placed on the developmental differences of SLO and TLO between small intestine and colon and on the role played by various chemokines and cell receptors. Undoubtedly, microbiota is indispensable for the formation of SLO and its absence leads to their poor formation, thus indicating its strict interaction with immune and non immune host cells. Furthermore, food antigens (for example, tryptophan derivatives, flavonoids and byphenils) bind the aryl hydrocarbon receptor on innate lymphoid cells (ILCs), thus promoting the development of postnatal lymphoid tissues. Also retinoic acid, a metabolite of vitamin A, contributes to SLO development during embryogenesis. Vitamin A deficiency seems to account for reduction of ILCs and scarce formation of solitary lymphoid tissue. </P><P> Translational Studies: The role of lymphoid organs with special reference to intestinal TLO in the course of experimental and human disease will also be discussed. </P><P> Future Perspectives: Finally, a new methodology, the so-called “gut-in-a dish”, which has facilitated the in vitro interaction study between microbe and intestinal immune cells, will be described.

Dendritic Cell Subsets and Effector Function in Idiopathic and Connective Tissue Disease-Associated Pulmonary Arterial Hypertension

Pulmonary arterial hypertension (PAH) is a cardiopulmonary disease characterized by an incurable condition of the pulmonary vasculature, leading to increased pulmonary vascular resistance, elevated pulmonary arterial pressure resulting in progressive right ventricular failure and ultimately death. PAH has different underlying causes. In approximately 30–40% of the patients no underlying risk factor or cause can be found, so-called idiopathic PAH (IPAH). Patients with an autoimmune connective tissue disease (CTD) can develop PAH [CTD-associated PAH (CTD-PAH)], suggesting a prominent role of immune cell activation in PAH pathophysiology. This is further supported by the presence of tertiary lymphoid organs (TLOs) near pulmonary blood vessels in IPAH and CTD-PAH. TLOs consist of myeloid cells, like monocytes and dendritic cells (DCs), T-cells, and B-cells. Next to their T-cell activating function, DCs are crucial for the preservation of TLOs. Multiple DC subsets can be found in steady state, such as conventional DCs (cDCs), including type 1 cDCs (cDC1s), and type 2 cDCs (cDC2s), AXL⁺Siglec6⁺ DCs (AS-DCs), and plasmacytoid DCs (pDCs). Under inflammatory conditions monocytes can differentiate into monocyte-derived-DCs (mo-DCs). DC subset distribution and activation status play an important role in the pathobiology of autoimmune diseases and most likely in the development of IPAH and CTD-PAH. DCs can contribute to pathology by activating T-cells (production of pro-inflammatory cytokines) and B-cells (pathogenic antibody secretion). In this review we therefore describe the latest knowledge about DC subset distribution, activation status, and effector functions, and polymorphisms involved in DC function in IPAH and CTD-PAH to gain a better understanding of PAH pathology.

Lymphoid tissue inducer-A divergent member of the ILC family

Innate lymphoid cells (ILCs) that are capable of producing effector cytokines reminiscent of CD4⁺ T helper (Th) cells during infections and tissue inflammations have drawn much attention in the immunology field in recent years. Within the ILCs, the lymphoid tissue inducer (LTi) cells that play a critical role in lymphoid organogenesis were identified long before the establishment of the ILC concept. LTi cells, developed and functioning mainly at the fetal stage, and LTi-like cells, presumably generated during the adulthood, are regarded as a subset of type 3 ILCs (ILC3s) because they express the ILC3 lineage-defining transcription factor RORγt, and like other ILC3s, can produce an ILC3 signature cytokine IL-22 and initiate protective immune responses against extracellular bacteria. However, LTi/LTi-like cells have a unique gene expression pattern, and they develop from a progenitor that is distinct from the progenitor of all other ILCs and the progenitor of conventional natural killer (cNK) cells. There are also several other unique features of LTi/LTi-like cells comparing to non-LTi ILC3s. In addition to their classical function in lymphoid organogenesis, LTi/LTi-like cells also have specialized functions in association with the adaptive immune system, which include their effects on T and B cell development, activation and function. In this review, we summarize these specific features of LTi/LTi-like cells and propose that these cells should be considered as a separated innate lymphoid lineage in parallel with other non-LTi ILCs and cNK cells.

Microbial antigen encounter during a preweaning interval is critical for tolerance to gut bacteria

We have a mutually beneficial relationship with the trillions of microorganisms inhabiting our gastrointestinal tract. However, maintaining this relationship requires recognizing these organisms as affable and restraining inflammatory responses to these organisms when encountered in hostile settings. How and when the immune system develops tolerance to our gut microbial members is not well understood. We identify a specific preweaning interval in which gut microbial antigens are encountered by the immune system to induce antigen-specific tolerance to gut bacteria. For some bacterial taxa, physiologic encounters with the immune system are restricted to this interval, despite abundance of these taxa in the gut lumen at later times outside this interval. Antigen-specific tolerance to gut bacteria induced during this preweaning interval is stable and maintained even if these taxa are encountered later in life in an inflammatory setting. However, inhibiting microbial antigen encounter during this interval or extending these encounters beyond the normal interval results in a failure to induce tolerance and robust antigen-specific effector responses to gut bacteria upon reencounter in an inflammatory setting. Thus, we have identified a defined preweaning interval critical for developing tolerance to gut bacteria and maintaining the mutually beneficial relationship with our gut microbiota.

The distinct effects of orally administered Lactobacillus rhamnosus GG and Lactococcus lactis subsp. lactis C59 on gene expression in the murine small intestine

The molecular mechanisms of strain-specific probiotic effects and the impact of the oral administration of probiotic strains on the host’s gene expression are not yet well understood. The aim of this study was to investigate the strain-specific effects of probiotic strain intake on gene expression in the murine small intestine. Two distinct strains of lactic acid bacteria, Lactobacillus rhamnosus GG (GG) and Lactococcus lactis subsp. lactis C59 (C59), were orally administered to BALB/c mice, daily for 2 weeks. The total RNA was isolated from the upper (including the duodenum) and lower (the terminal ileum) small intestine, and gene expression was assessed by microarray analysis. The data revealed (1) oral administration of C59 and GG markedly down-regulated the expression of genes encoding fibrinogen subunits and plasminogen in the upper small intestine; (2) administration of more than 1 × 10⁷ CFU/day of GG changed the gene expression of the host ileum. (3) strain- and dose-related effects on various GO biological processes; and (4) enrichment for B cell-related Gene Ontology terms among up-regulated genes in the terminal ileum of mice administered the 1 × 10⁹ CFU/day of GG. The distinct effects of GG and C59 on gene expression in the intact small intestine provide clues to understand how the health beneficial effects of specific strains of probiotic bacteria are mediated by interactions with intestinal cells.

CCR6 promotes steady-state mononuclear phagocyte association with the intestinal epithelium, imprinting and immune surveillance

The intestinal lamina propria (LP) contains antigen-presenting cells with features of dendritic cells and macrophages, collectively referred to as mononuclear phagocytes (MNPs). Association of MNPs with the epithelium is thought to play an important role in multiple facets of intestinal immunity including imprinting MNPs with the ability to induce IgA production, inducing the expression of gut homing molecules on T cells, facilitating the capture of luminal antigens and microbes, and subsequent immune responses in the mesenteric lymph node (MLN). However, the factors promoting this process in the steady state are largely unknown, and in vivo models to test and confirm the importance of LP-MNP association with the epithelium for these outcomes are unexplored. Evaluation of epithelial expression of chemoattractants in mice where MNP-epithelial associations were impaired suggested CCL20 as a candidate promoting epithelial association. Expression of CCR6, the only known receptor for CCL20, was required for MNPs to associate with the epithelium. LP-MNPs from CCR6-/- mice did not display defects in acquiring antigen and stimulating T-cell responses in ex vivo assays or in responses to antigen administered systemically. However, LP-MNPs from CCR6-deficient mice were impaired at acquiring luminal and epithelial antigens, inducing IgA production in B cells, inducing immune responses in the MLN, and capturing and trafficking luminal commensal bacteria to the MLN. These findings identify a crucial role for CCR6 in promoting LP-MNPs to associate with the intestinal epithelium in the steady state to perform multiple functions promoting gut immune homeostasis.

Regionalized Development and Maintenance of the Intestinal Adaptive Immune Landscape

The intestinal immune system has the daunting task of protecting us from pathogenic insults while limiting inflammatory responses against the resident commensal microbiota and providing tolerance to food antigens. This role is particularly impressive when one considers the vast mucosal surface and changing landscape that the intestinal immune system must monitor. In this review, we highlight regional differences in the development and composition of the adaptive immune landscape of the intestine and the impact of local intrinsic and environmental factors that shape this process. To conclude, we review the evidence for a critical window of opportunity for early-life exposures that affect immune development and alter disease susceptibility later in life.

Stromal Fibroblasts in Tertiary Lymphoid Structures: A Novel Target in Chronic Inflammation

Tertiary lymphoid structures (TLS) are organized aggregates of lymphocytes, myeloid, and stromal cells that provide ectopic hubs for acquired immune responses. TLS share phenotypical and functional features with secondary lymphoid organs (SLO); however, they require persistent inflammatory signals to arise and are often observed at target sites of autoimmune disease, chronic infection, cancer, and organ transplantation. Over the past 10 years, important progress has been made in our understanding of the role of stromal fibroblasts in SLO development, organization, and function. A complex and stereotyped series of events regulate fibroblast differentiation from embryonic life in SLOs to lymphoid organ architecture observed in adults. In contrast, TLS-associated fibroblasts differentiate from postnatal, locally activated mesenchyme, predominantly in settings of inflammation and persistent antigen presentation. Therefore, there are critical differences in the cellular and molecular requirements that regulate SLO versus TLS development that ultimately impact on stromal and hematopoietic cell function. These differences may contribute to the pathogenic nature of TLS in the context of chronic inflammation and malignant transformation and offer a window of opportunity for therapeutic interventions in TLS associated pathologies.

Development and Function of Secondary and Tertiary Lymphoid Organs in the Small Intestine and the Colon

The immune system of the gut has evolved a number of specific lymphoid structures that contribute to homeostasis in the face of microbial colonization and food-derived antigenic challenge. These lymphoid organs encompass Peyer’s patches (PP) in the small intestine and their colonic counterparts that develop in a programed fashion before birth. In addition, the gut harbors a network of lymphoid tissues that is commonly designated as solitary intestinal lymphoid tissues (SILT). In contrast to PP, SILT develop strictly after birth and consist of a dynamic continuum of structures ranging from small cryptopatches (CP) to large, mature isolated lymphoid follicles (ILF). Although the development of PP and SILT follow similar principles, such as an early clustering of lymphoid tissue inducer (LTi) cells and the requirement for lymphotoxin beta (LTβ) receptor-mediated signaling, the formation of CP and their further maturation into ILF is associated with additional intrinsic and environmental signals. Moreover, recent data also indicate that specific differences exist in the regulation of ILF formation between the small intestine and the colon. Importantly, intestinal inflammation in both mice and humans is associated with a strong expansion of the lymphoid network in the gut. Recent experiments in mice suggest that these structures, although they resemble large, mature ILF in appearance, may represent de novo-induced tertiary lymphoid organs (TLO). While, so far, it is not clear whether intestinal TLO contribute to the exacerbation of inflammatory pathology, it has been shown that ILF provide the critical microenvironment necessary for the induction of an effective host response upon infection with enteric bacterial pathogens. Regarding the importance of ILF for intestinal immunity, interfering with the development and maturation of these lymphoid tissues may offer novel means for manipulating the immune response during intestinal infection or inflammation.

IL35-Producing B Cells Promote the Development of Pancreatic Neoplasia

A salient feature of pancreatic ductal adenocarcinoma (PDAC) is an abundant fibroinflammatory response characterized by the recruitment of immune and mesenchymal cells and the consequent establishment of a protumorigenic microenvironment. Here, we report the prominent presence of B cells in human pancreatic intraepithelial neoplasia and PDAC lesions as well as in oncogenic Kras-driven pancreatic neoplasms in the mouse. The growth of orthotopic pancreatic neoplasms harboring oncogenic Kras was significantly compromised in B-cell-deficient mice (μMT), and this growth deficiency could be rescued by the reconstitution of a CD1d(hi)CD5(+) B-cell subset. The protumorigenic effect of B cells was mediated by their expression of IL35 through a mechanism involving IL35-mediated stimulation of tumor cell proliferation. Our results identify a previously unrecognized role for IL35-producing CD1d(hi)CD5(+) B cells in the pathogenesis of pancreatic cancer and underscore the potential significance of a B-cell/IL35 axis as a therapeutic target.

SIGNIFICANCE

This study identifies a B-cell subpopulation that accumulates in the pancreatic parenchyma during early neoplasia and is required to support tumor cell growth. Our findings provide a rationale for exploring B-cell-based targeting approaches for the treatment of pancreatic cancer.

Intestinal Antigen-Presenting Cells: Key Regulators of Immune Homeostasis and Inflammation

The microbiota that populate the mammalian intestine are critical for proper host physiology, yet simultaneously pose a potential danger. Intestinal antigen-presenting cells, namely macrophages and dendritic cells (DCs), are integral components of the mucosal innate immune system that maintain co-existence with the microbiota in face of this constant threat. Intestinal macrophages and DCs integrate signals from the microenvironment to orchestrate innate and adaptive immune responses that ultimately lead to durable tolerance of the microbiota. Tolerance is not a default response, however, because macrophages and DCs remain poised to vigorously respond to pathogens that breach the epithelial barrier. In this review, we summarize the salient features of macrophages and DCs in the healthy and inflamed intestine and discuss how signals from the microbiota can influence their function.

Reciprocal regulation of lymphoid tissue development in the large intestine by IL-25 and IL-23

Isolated lymphoid follicles (ILFs) develop after birth in the small and large intestines (SI and LI) and represent a dynamic response of the gut immune system to the microbiota. Despite their similarities, ILF development in the SI and LI differs on a number of levels. We show that unlike ILF in the SI, the microbiota inhibits ILF development in the colon as conventionalization of germ-free mice reduced colonic ILFs. From this, we identified a novel mechanism regulating colonic ILF development through the action of interleukin (IL)-25 on IL-23 and its ability to modulate T regulatory cell (Treg) differentiation. Colonic ILF develop in the absence of a number of factors required for the development of their SI counterparts and can be specifically suppressed by factors other than IL-25. However, IL-23 is the only factor identified that specifically promotes colonic ILFs without affecting SI-ILF development. Both IL-23 and ILFs are associated with inflammatory bowel disease, suggesting that disruption to this pathway may have an important role in the breakdown of microbiota-immune homeostasis.

Purification of dendritic cell and macrophage subsets from the normal mouse small intestine

Mononuclear phagocytes are essential for protecting against pathogens breaching the intestinal mucosa and maintaining the integrity of the gastrointestinal tract. The mononuclear phagocyte family of the healthy intestine is represented by a small population of hematopoietic cells including dendritic cells and macrophages. Distinct mononuclear phagocyte subsets strategically accumulate within and below the mucosal epithelium and are distributed in the submucosa and muscularis externa. Shaped by its unique microenvironment, each mononuclear phagocyte subset is developmentally and functionally unique and phenotypically distinct. Here we summarize our recent advances on identifying and purifying various intestinal mononuclear phagocyte subsets by flow cytometry in the context of their developmental properties and location within the intestinal tissue.

Transcription factors controlling innate lymphoid cell fate decisions

The mucosal epithelium is in direct contact with symbiotic and pathogenic microorganisms. Therefore, the mucosal surface is the principal portal of entry for invading pathogens and immune cells accumulated in the intestine to prevent infections. In addition to these conventional immune system functions, it has become clear that immune cells during steady-state continuously integrate microbial and nutrient-derived signals from the environment to support organ homeostasis. A major role in both processes is played by a recently discovered group of lymphocytes referred to as innate lymphoid cells (ILCs) Innate lymphoid cells (ILCs) that are specifically enriched at mucosal surfaces but are rather rare in secondary lymphoid organs. In analogy to the dichotomy between CD8 and CD4 T cells, we propose to classify ILCs into interleukin-7 receptor α-negative cytotoxic ILCs and IL-7Rα(+) helper-like ILCs. Dysregulated immune responses triggered by the various ILC subsets have been linked to inflammatory diseases such as inflammatory bowel disease, atopic dermatitis and airway hyperresponsiveness. Here, we will review recent progress in determining the transcriptional and developmental programs that control ILC fate decisions.

Age-related differences in human palatine tonsillar B cell subsets and immunoglobulin isotypes

The tonsils provide defense of the upper aerodigestive tract against pathogens. Although long known to undergo functional changes with age, the precise changes occurring within tonsillar B cell populations remain undefined. In the present study, we investigated age-related changes in palatine tonsillar B cell subsets and immunoglobulin (Ig) isotypes. Palatine tonsils were obtained from forty-two tonsillectomy patients without tonsillitis who were divided into three groups: young children (4-9 years), adolescents (10-19 years), and adults (20-60 years). Tonsillar B cells were then analyzed by flow cytometry. Using expression of CD38 and IgD to define B cell subsets, we found that the frequency of germinal center (GC) B cells in the tonsils was significantly higher, and the frequency of memory B cells lower, in young children as compared to adolescents and adults. Within the GC B cell subsets, adults had a higher frequency of IgA(+) cells and a lower frequency of IgM(+) cells as compared to individuals in the younger age groups. Moreover, young children had a higher frequency of IgG(+) cells in the GC B cell subsets than did individuals in the older age groups. We also observed an abundance of IgM(+) cells among memory B cells and plasmablasts in young children and IgA(+) cells in adults. In summary, the proportion of GC B cells in palatine tonsillar B cells decreases with age, while the proportion of memory B cells increases with age. In addition, Ig isotypes in tonsils preferentially switch from IgM to IgA as individuals age.

Microbial sensing by goblet cells controls immune surveillance of luminal antigens in the colon

The delivery of luminal substances across the intestinal epithelium to the immune system is a critical event in immune surveillance, resulting in tolerance to dietary antigens and immunity to pathogens. How this process is regulated is largely unknown. Recently goblet cell-associated antigen passages (GAPs) were identified as a pathway delivering luminal antigens to underlying lamina propria (LP) dendritic cells in the steady state. Here, we demonstrate that goblet cells (GCs) form GAPs in response to acetylcholine (ACh) acting on muscarinic ACh receptor 4. GAP formation in the small intestine was regulated at the level of ACh production, as GCs rapidly formed GAPs in response to ACh analogs. In contrast, colonic GAP formation was regulated at the level of GC responsiveness to ACh. Myd88-dependent microbial sensing by colonic GCs inhibited the ability of colonic GCs to respond to Ach to form GAPs and deliver luminal antigens to colonic LP-antigen-presenting cells (APCs). Disruption of GC microbial sensing in the setting of an intact gut microbiota opened colonic GAPs, and resulted in recruitment of neutrophils and APCs and production of inflammatory cytokines. Thus GC intrinsic sensing of the microbiota has a critical role regulating the exposure of the colonic immune system to luminal substances.

Regional specialization within the intestinal immune system. Nat Rev Immunol. 2014;14:667-685. [PMID: 25234148 DOI: 10.1038/nri3738]

The intestine represents the largest compartment of the immune system. It is continually exposed to antigens and immunomodulatory agents from the diet and the commensal microbiota, and it is the port of entry for many clinically important pathogens. Intestinal immune processes are also increasingly implicated in controlling disease development elsewhere in the body. In this Review, we detail the anatomical and physiological distinctions that are observed in the small and large intestines, and we suggest how these may account for the diversity in the immune apparatus that is seen throughout the intestine. We describe how the distribution of innate, adaptive and innate-like immune cells varies in different segments of the intestine and discuss the environmental factors that may influence this. Finally, we consider the implications of regional immune specialization for inflammatory disease in the intestine.

Differential induction of isolated lymphoid follicles in the gut by 18β-glycyrrhetinic acid

18β-glycyrrhetinic acid (GRA) is a pharmacologically active component of licorice root with documented immunomodulatory properties. We reported that GRA administered orally to mice induces B cell recruitment to isolated lymphoid follicles (ILF) in the small intestine and shortens the duration of rotavirus antigen shedding. ILF are dynamic lymphoid tissues in the gut acquired post-natally upon colonization with commensal bacteria and mature through B cell recruitment to the follicles, resulting in up-regulation of IgA synthesis in response to changes in the composition of microbiota. In this study, we investigated potential mechanisms by which GRA induces ILF maturation in the ileum and the colon using mice depleted of enteric bacteria and a select group of mice genetically deficient in pattern recognition receptors. The data show GRA was unable to induce ILF maturation in ileums of mice devoid of commensal bacteria, MyD88^−/− or NOD2^−/− mice, but differentially induced ILF in colons. Increased expression of chemokine and chemokine receptor genes that modulate B and T cell recruitment to the mucosa were in part dependent on NOD2, TLR, and signaling adaptor protein MyD88. Together the results suggest GRA induces ILF through cooperative signals provided by bacterial ligands under normal conditions to induce B cell recruitment to ILF to the gut, but that the relative contribution of these signals differ between ileum and colon.

Early events in Sjögren's Syndrome pathogenesis: the importance of innate immunity in disease initiation

Sjögren's Syndrome (SS) is a debilitating autoimmune disease that primarily affects women. Patients with SS experience dry eyes and dry mouth in addition to systemic disease manifestations, including arthritis, peripheral neuropathy and pulmonary fibrosis. As in many autoimmune diseases, the inciting factors that precipitate SS are poorly understood. Patients with SS have periductal and perivascular lymphocytic infiltration of salivary and lacrimal tissue, and this is a hallmark of disease. While this infiltration is well characterized, the pathologic events that precede and cause this inflammatory cell recruitment are unknown. Although few studies have examined SS salivary tissue prior to disease onset, there is strong evidence for innate immune hyperactivity. Accordingly, processes such as apoptosis of glandular tissue, heightened inflammatory cytokine and chemokine production, and toll-like receptor (TLR) activation are described in early disease and are each linked to innate immune activation in murine models of disease and SS patients. This review will explore the relationship between innate immunity and SS pathogenesis prior to overt disease onset and discuss therapeutic strategies to mitigate disease progression in SS patients.

Notch2-dependent classical dendritic cells orchestrate intestinal immunity to attaching-and-effacing bacterial pathogens

Defense against attaching-and-effacing bacteria requires the sequential generation of interleukin 23 (IL-23) and IL-22 to induce protective mucosal responses. Although CD4(+) and NKp46(+) innate lymphoid cells (ILCs) are the critical source of IL-22 during infection, the precise source of IL-23 is unclear. We used genetic techniques to deplete mice of specific subsets of classical dendritic cells (cDCs) and analyzed immunity to the attaching-and-effacing pathogen Citrobacter rodentium. We found that the signaling receptor Notch2 controlled the terminal stage of cDC differentiation. Notch2-dependent intestinal CD11b(+) cDCs were an obligate source of IL-23 required for survival after infection with C. rodentium, but CD103(+) cDCs dependent on the transcription factor Batf3 were not. Our results demonstrate a nonredundant function for CD11b(+) cDCs in the response to pathogens in vivo.

Colonic patch and colonic SILT development are independent and differentially regulated events

Intestinal lymphoid tissues have to simultaneously ensure protection against pathogens and tolerance toward commensals. Despite such vital functions, their development in the colon is poorly understood. Here, we show that the two distinct lymphoid tissues of the colon-colonic patches and colonic solitary intestinal lymphoid tissues (SILTs)-can easily be distinguished based on anatomical location, developmental timeframe, and cellular organization. Furthermore, whereas colonic patch development depended on CXCL13-mediated lymphoid tissue inducer (LTi) cell clustering followed by LTα-mediated consolidation, early LTi clustering at SILT anlagen did not require CXCL13, CCR6, or CXCR3. Subsequent dendritic cell recruitment to and gp38(+)VCAM-1(+) lymphoid stromal cell differentiation within SILTs required LTα; B-cell recruitment and follicular dendritic cell differentiation depended on MyD88-mediated signaling, but not the microflora. In conclusion, our data demonstrate that different mechanisms, mediated mainly by programmed stimuli, induce the formation of distinct colonic lymphoid tissues, therefore suggesting that these tissues may have different functions.

Mononuclear phagocyte diversity in the intestine

Present in all organs, mononuclear phagocytes consist of a heterogeneous population of hematopoietic cells whose main role is to ensure tissue homeostasis through their ability to scavenge cell debris, promote tissue repair and maintain tolerance to self-antigens while simultaneously inducing innate and adaptive immune responses against foreign antigens that breach the tissue. The intestinal mucosa is particularly exposed to foreign antigen, through constant exposure to high loads of commensal bacteria and dietary antigens as well as providing a site of entry for viral and bacterial pathogens. The molecular mechanisms that control the intestinal ability to distinguish between "innocuous" and "dangerous" antigens remains poorly understood although it is clear that mononuclear phagocytes play a key role in this process. This review highlights recent advances in our understanding of heterogeneous origin of the mononuclear phagocytes that inhabit the intestinal mucosa and discusses how developmental diversity allows for functional diversity to ensure intestinal integrity.

Isolated Lymphoid Follicles are Dynamic Reservoirs for the Induction of Intestinal IgA

IgA is one of the most important molecules in the regulation of intestinal homeostasis. Peyer's patches have been traditionally recognized as sites for the induction of intestinal IgA responses, however more recent studies demonstrate that isolated lymphoid follicles (ILFs) can perform this function as well. ILF development is dynamic, changing in response to the luminal microbial burden, suggesting that ILFs play an important role providing an expandable reservoir of compensatory IgA inductive sites. However, in situations of immune dysfunction, ILFs can over-develop in response to uncontrollable enteric flora, resulting in ILF hyperplasia. The ability of ILFs to expand and respond to help control the enteric flora makes this dynamic reservoir an important arm of IgA inductive sites in intestinal immunity.

Goblet cells deliver luminal antigen to CD103+ dendritic cells in the small intestine

The intestinal immune system is exposed to a mixture of foreign antigens from diet, commensal flora and potential pathogens. Understanding how pathogen-specific immunity is elicited while avoiding inappropriate responses to the background of innocuous antigens is essential for understanding and treating intestinal infections and inflammatory diseases. The ingestion of protein antigen can induce oral tolerance, which is mediated in part by a subset of intestinal dendritic cells (DCs) that promote the development of regulatory T cells. The lamina propria (LP) underlies the expansive single-cell absorptive villous epithelium and contains a large population of DCs (CD11c(+) CD11b(+) MHCII(+) cells) comprised of two predominant subsets: CD103(+) CX(3)CR1(-) DCs, which promote IgA production, imprint gut homing on lymphocytes and induce the development of regulatory T cells, and CD103(-) CX(3)CR1(+) DCs (with features of macrophages), which promote tumour necrosis factor-α (TNF-α) production, colitis, and the development of T(H)17 T cells. However, the mechanisms by which different intestinal LP-DC subsets capture luminal antigens in vivo remains largely unexplored. Using a minimally disruptive in vivo imaging approach we show that in the steady state, small intestine goblet cells (GCs) function as passages delivering low molecular weight soluble antigens from the intestinal lumen to underlying CD103(+) LP-DCs. The preferential delivery of antigens to DCs with tolerogenic properties implies a key role for this GC function in intestinal immune homeostasis.

AHR and the Transcriptional Regulation of Type-17/22 ILC

Mucosal innate lymphoid cells (ILCs) are an emerging population of diverse and heterogeneous immune cells, all with the unique ability to mount a rapid response against invading pathogens. They are further divided into subsets based on their differing cell surface markers as well as in their functional specialization. In this review, we summarize recent reports describing the importance of the transcription factor aryl hydrocarbon receptor (AHR) in regulating the development of one of these subsets, the Type-17/22 ILCs, as well as in the organization of postnatal lymphoid structures. We discuss the mechanisms behind the AHR dependence for development in Type-17/22 ILCs as well as reviewing the proposed physiological ligands that are mediating this effect.

Epithelial expression of the cytosolic retinoid chaperone cellular retinol binding protein II is essential for in vivo imprinting of local gut dendritic cells by lumenal retinoids

Dendritic cells (DCs) use all-trans retinoic acid (ATRA) to promote characteristic intestinal responses, including Foxp3(+) Treg conversion, lymphocyte gut homing molecule expression, and IgA production. How this ability to generate ATRA is conferred to DCs in vivo remains largely unstudied. Here, we observed that among DCs, retinaldehyde dehydrogenase (ALDH1), which catalyzes the conversion of retinal to ATRA, was preferentially expressed by small intestine CD103(+) lamina propria (LP) DCs. Retinoids induced LP CD103(+) DCs to generate ATRA via ALDH1 activity. Either biliary or dietary retinoids were required to confer ALDH activity to LP DCs in vivo. Cellular retinol-binding protein II (CRBPII), a cytosolic retinoid chaperone that directs enterocyte retinol and retinal metabolism but is redundant to maintain serum retinol, was required to confer ALDH activity to CD103(+) LP DCs. CRBPII expression was restricted to small intestine epithelial cells, and ALDH activity in CRBPII(-/-) DCs was restored by transfer to a wild-type recipient. CD103(+) LP DCs from CRBPII(-/-) mice had a decreased capacity to promote IgA production. Moreover, CD103(+) DCs preferentially associated with the small intestine epithelium and LP CD103(+) DC ALDH activity, and the ability to promote IgA production was reduced in mice with impaired DC-epithelia associations. These findings demonstrate in vivo roles for the expression of epithelial CRBPII and lumenal retinoids to imprint local gut DCs with an intestinal phenotype.

Distinct developmental requirements for isolated lymphoid follicle formation in the small and large intestine: RANKL is essential only in the small intestine

Cryptopatches (CPs) and isolated lymphoid follicles (ILFs) are organized intestinal lymphoid tissues that develop postnatally in mice and include stromal cells expressing the receptor activator of nuclear factor kappa-B ligand (RANKL). We investigated how stromal RANKL influences the development and differentiation of CPs and ILFs by analyzing the development of these lymphoid structures in knockout mice lacking RANKL. We found that RANKL(-/-) mice had a fourfold reduction in the overall density of CPs in the small intestine compared to control mice, with the largest decrease in the proximal small intestine. No B cells were present in CPs from the small intestine of RANKL(-/-) mice and ILF formation was completely blocked. In sharp contrast, colonic ILFs containing B cells were present in RANKL(-/-) mice. Stromal cells within CPs in the small intestine of RANKL(-/-) mice did not express CXCL13 (originally called B lymphocyte chemoattractant) and often lacked other normally expressed stromal cell antigens, whereas colonic lymphoid aggregates in RANKL(-/-) mice retained stromal CXCL13 expression. The CXCL13-dependent maturation of precursor CPs into ILFs is differentially regulated in the small intestine and colon, with an absolute requirement for RANKL only in the small intestine.