The mannose receptor (CD206) identifies a population of colonic macrophages in health and inflammatory bowel disease.

To understand the contribution of mononuclear phagocytes (MNP), which include monocyte-derived intestinal macrophages, to the pathogenesis of inflammatory bowel disease (IBD), it is necessary to identify functionally-different MNP populations. We aimed to characterise intestinal macrophage populations in patients with IBD. We developed 12-parameter flow cytometry protocols to identify and human intestinal MNPs. We used these protocols to purify and characterize colonic macrophages from colonic tissue from patients with Crohn's disease (CD), ulcerative colitis (UC), or non-inflamed controls, in a cross-sectional study. We identify macrophage populations (CD45+CD64+ HLA-DR+) and describe two distinct subsets, differentiated by their expression of the mannose receptor, CD206. CD206+ macrophages expressed markers consistent with a mature phenotype: high levels of CD68 and CD163, higher transcription of IL-10 and lower expression of TREM1. CD206- macrophages appear to be less mature, with features more similar to their monocytic precursors. We identified and purified macrophage populations from human colon. These appear to be derived from a monocytic precursor with high CCR2 and low CD206 expression. As these cells mature, they acquire expression of IL-10, CD206, CD63, and CD168. Targeting the newly recruited monocyte-derived cells may represent a fruitful avenue to ameliorate chronic inflammation in IBD.

Intestinal cDC1 drive cross-tolerance to epithelial-derived antigen via induction of FoxP3+CD8+ Tregs.

Although CD8+ T cell tolerance to tissue-specific antigen (TSA) is essential for host homeostasis, the mechanisms underlying peripheral cross-tolerance and whether they may differ between tissue sites remain to be fully elucidated. Here, we demonstrate that peripheral cross-tolerance to intestinal epithelial cell (IEC)-derived antigen involves the generation and suppressive function of FoxP3+CD8+ T cells. FoxP3+CD8+ Treg generation was dependent on intestinal cDC1, whose absence led to a break of tolerance and epithelial destruction. Mechanistically, intestinal cDC1-derived PD-L1, TGFß, and retinoic acid contributed to the generation of gut-tropic CCR9+CD103+FoxP3+CD8+ Tregs Last, CD103-deficient CD8+ T cells lacked tolerogenic activity in vivo, indicating a role for CD103 in FoxP3+CD8+ Treg function. Our results describe a role for FoxP3+CD8+ Tregs in cross-tolerance in the intestine for which development requires intestinal cDC1.

Publisher Correction: Delphine Parrott, a pioneer of T cell biology.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Proinflammatory Role of Monocyte-Derived CX3CR1int Macrophages in Helicobacter hepaticus-Induced Colitis.

Cells of the monocyte/macrophage lineage play important roles in the pathogenesis of inflammatory bowel diseases, but they are also present in the normal healthy intestine, where they are critical for maintaining homeostasis. It has been unclear whether the proinflammatory roles of intestinal macrophages reflect altered behavior of the existing resident cells, or whether they involve recruitment of a distinct cell type. Here, we have explored these ideas using the model of colitis induced by Helicobacter hepaticus in the context of neutralization or deletion of interleukin-10 (IL-10). Granulocytes and monocytes made up most of the inflammatory myeloid infiltrates found in the colon of H. hepaticus-infected colitic mice, rising to a peak within 2 weeks of H. hepaticus inoculation but taking several months to resolve completely. The inflammatory response was dependent on the combined presence of H. hepaticus and absence of IL-10 and was accompanied by increased production of inflammatory mediators such as IL-1ß, tumor necrosis factor alpha (TNF-α), IL-6, and IL-23p19 by infiltrating myeloid cells, mostly relatively immature cells of the macrophage lineage that express intermediate levels of CX3CR1. In contrast, the population of mature CX3CR1hi macrophages did not expand as markedly during colitis, and these cells made little contribution to inflammatory mediator production. Taking into account their numerical dominance in the myeloid compartment, we conclude that newly recruited monocytes are the main source of proinflammatory mediators in colitis induced in the absence of IL-10 signaling and that altered behavior of mature macrophages is not a major component of this pathology.

Mechanisms of innate immune activation by gluten peptide p31-43 in mice.

Celiac disease (CD) is an immune-mediated enteropathy triggered by gluten in genetically susceptible individuals. Innate immunity contributes to the pathogenesis of CD, but the mechanisms remain poorly understood. Although previous in vitro work suggests that gliadin peptide p31-43 acts as an innate immune trigger, the underlying pathways are unclear and have not been explored in vivo. Here we show that intraluminal delivery of p31-43 induces morphological changes in the small intestinal mucosa of normal mice consistent with those seen in CD, including increased cell death and expression of inflammatory mediators. The effects of p31-43 were dependent on MyD88 and type I IFNs, but not Toll-like receptor 4 (TLR4), and were enhanced by coadministration of the TLR3 agonist polyinosinic:polycytidylic acid. Together, these results indicate that gliadin peptide p31-43 activates the innate immune pathways in vivo, such as IFN-dependent inflammation, relevant to CD. Our findings also suggest a common mechanism for the potential interaction between dietary gluten and viral infections in the pathogenesis of CD.

Regional specialization within the intestinal immune system.

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.

Intestinal macrophages and dendritic cells: what's the difference?

Mononuclear phagocytes (MPs) in the murine intestine, comprising dendritic cells (DCs) and macrophages (Mϕs), perform disparate yet complementary immunological functions. Functional analyses of these distinct MP subsets have been complicated by the substantial overlap in their surface phenotypes. Here, we review recent findings that have enabled more accurate definition of these MP subsets. We discuss these recent advances in the context of the current understanding of the functions of DCs and Mϕs in the maintenance of intestinal homeostasis, and how their functions may alter when homeostasis is disrupted.

Unravelling mononuclear phagocyte heterogeneity.

When Ralph Steinman and Zanvil Cohn first described dendritic cells (DCs) in 1973 it took many years to convince the immunology community that these cells were truly distinct from macrophages. Almost four decades later, the DC is regarded as the key initiator of adaptive immune responses; however, distinguishing DCs from macrophages still leads to confusion and debate in the field. Here, Nature Reviews Immunology asks five experts to discuss the issue of heterogeneity in the mononuclear phagocyte system and to give their opinion on the importance of defining these cells for future research.

Differences in the kinetics, amplitude, and localization of ERK activation in anergy and priming revealed at the level of individual primary T cells by laser scanning cytometry.

One of the potential mechanisms of peripheral tolerance is the unresponsiveness of T cells to secondary antigenic stimulation as a result of the induction of anergy. It has been widely reported that antigenic unresponsiveness may be due to uncoupling of MAPK signal transduction pathways. However, such signaling defects in anergic T cell populations have been mainly identified using immortalized T cell lines or T cell clones, which do not truly represent primary Ag-specific T cells. We have therefore attempted to quantify signaling events in murine primary Ag-specific T cells on an individual cell basis, using laser-scanning cytometry. We show that there are marked differences in the amplitude and cellular localization of phosphorylated ERK p42/p44 (ERK1/2) signals when naive, primed and anergic T cells are challenged with peptide-pulsed dendritic cells. Primed T cells display more rapid kinetics of phosphorylation and activation of ERK than naive T cells, whereas anergic T cells display a reduced ability to activate ERK1/2 upon challenge. In addition, the low levels of pERK found in anergic T cells are distributed diffusely throughout the cell, whereas in primed T cells, pERK appears to be targeted to the same regions of the cell as the TCR. These data suggest that the different consequences of Ag recognition by T cells are associated with distinctive kinetics, amplitude, and localization of MAPK signaling.

A role for dendritic cells in the priming of antigen-specific CD4+ and CD8+ T lymphocytes by immune-stimulating complexes in vivo.

Immune-stimulating complexes (ISCOMS) are adjuvant vectors which are unusual in being able to prime both CD4(+) and CD8(+) T cells by parenteral and mucosal routes. However, their mode of action is unclear and to define better the cellular interactions involved we have studied the ability of ISCOMS containing ovalbumin (OVA) to prime TCR transgenic CD4(+) or CD8(+) T cells in vivo. Immunization with OVA ISCOMS caused activation and clonal expansion of CD4(+) and CD8(+) T cells in the T cell areas of the draining lymph nodes, followed by the migration of both CD4(+) and CD8(+) T cells into the B cell follicle. The T cells were primed to proliferate and secrete IFN-gamma after re-stimulation in vitro with the appropriate OVA peptide and CD8(+) T cell priming occurred in the absence of CD4(+) T cells. Increasing the number of dendritic cells (DC) in vivo with flt3 ligand augmented the expansion and activation of the OVA-specific T cells, particularly CD8(+) T cells. These studies indicate DC play a central role in the priming of both CD4(+) and CD8(+) T cells in vivo, and suggest that an ability to target DC may allow ISCOMS to be powerful vaccine vectors for stimulating protective immunity.