Intestinal dendritic cells (DCs) continuously migrate through lymphatics to mesenteric lymph nodes where they initiate immunity or tolerance. Recent research has focused on populations of intestinal DCs expressing CD103. Here we demonstrate, for the first time, the presence of two distinct CD103(-) DC subsets in intestinal lymph. Similar to CD103(+) DCs, these intestine-derived CD103(-) DCs are responsive to Flt3 and they efficiently prime and confer a gut-homing phenotype to naive T cells. However, uniquely among intestinal DCs, CD103(-) CD11b(+) CX(3)CR1(int) lymph DCs induce the differentiation of both interferon-γ and interleukin-17-producing effector T cells, even in the absence of overt stimulation. Priming by CD103(-) CD11b(+) DCs represents a novel mechanism for the rapid generation of effector T-cell responses in the gut. Therefore, these cells may prove to be valuable targets for the treatment of intestinal inflammation or in the development of effective oral vaccines.

The identification of intestinal macrophages (mφs) and dendritic cells (DCs) is a matter of intense debate. Although CD103+ mononuclear phagocytes (MPs) appear to be genuine DCs, the nature and origins of CD103− MPs remain controversial. We show here that intestinal CD103−CD11b+ MPs can be separated clearly into DCs and mφs based on phenotype, gene profile, and kinetics. CD64−CD103−CD11b+ MPs are classical DCs, being derived from Flt3 ligand-dependent, DC-committed precursors, not Ly6Chi monocytes. Surprisingly, a significant proportion of these CD103−CD11b+ DCs express CCR2 and there is a selective decrease in CD103−CD11b+ DCs in mice lacking this chemokine receptor. CCR2+CD103− DCs are present in both the murine and human intestine, drive interleukin (IL)-17a production by T cells in vitro, and show constitutive expression of IL-12/IL-23p40. These data highlight the heterogeneity of intestinal DCs and reveal a bona fide population of CCR2+ DCs that is involved in priming mucosal T helper type 17 (Th17) responses.

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Macrophages in the healthy intestine are highly specialized and usually respond to the gut microbiota without provoking an inflammatory response. A breakdown in this tolerance leads to inflammatory bowel disease (IBD), but the mechanisms by which intestinal macrophages normally become conditioned to promote microbial tolerance are unclear. Strong epidemiological evidence linking disruption of the gut microbiota by antibiotic use early in life to IBD indicates an important role for the gut microbiota in modulating intestinal immunity. Here, we show that antibiotic use causes intestinal macrophages to become hyperresponsive to bacterial stimulation, producing excess inflammatory cytokines. Re-exposure of antibiotic-treated mice to conventional microbiota induced a long-term, macrophage-dependent increase in inflammatory T helper 1 (TH1) responses in the colon and sustained dysbiosis. The consequences of this dysregulated macrophage activity for T cell function were demonstrated by increased susceptibility to infections requiring TH17 and TH2 responses for clearance (bacterial Citrobacter rodentium and helminth Trichuris muris infections), corresponding with increased inflammation. Short-chain fatty acids (SCFAs) were depleted during antibiotic administration; supplementation of antibiotics with the SCFA butyrate restored the characteristic hyporesponsiveness of intestinal macrophages and prevented T cell dysfunction. Butyrate altered the metabolic behavior of macrophages to increase oxidative phosphorylation and also promoted alternative macrophage activation. In summary, the gut microbiota is essential to maintain macrophage-dependent intestinal immune homeostasis, mediated by SCFA-dependent pathways. Oral antibiotics disrupt this process to promote sustained T cell–mediated dysfunction and increased susceptibility to infections, highlighting important implications of repeated broad-spectrum antibiotic use.

Dendritic cells (DCs) in the small intestine (SI) and colon are fundamental to direct intestinal immune responses; they migrate to the mesenteric lymph nodes (MLNs) and prime T cells. We demonstrate anatomical segregation of lymphatic drainage from the intestine, specifically that DCs from the SI and colon migrate to different nodes within the MLN, here called the sMLN and cMLN. As a consequence, different frequencies of DC subsets observed in the SI and colon are reflected among the DCs in the sMLN and cMLN. Consistent with the SI's function in absorbing food, fed antigen is presented in the sMLN, but not in the cMLN. Furthermore, the levels of expression of CCR9 and α4β7 are increased on T cells in the sMLN compared with the cMLN. DCs from the cMLN and colon are unable to metabolize vitamin A to retinoic acid (RA); thus, DCs may contribute to the differential expression of tissue homing markers observed in the sMLN and cMLN. In summary, the sMLN and cMLN, and the DCs that migrate to these LNs are anatomically and immunologically separate. This segregation allows immune responses in the SI and colon to be controlled independently.

IL-22 is mainly produced at barrier surfaces by T cells and innate lymphoid cells and is crucial to maintain epithelial integrity. However, dysregulated IL-22 action leads to deleterious inflammation and is involved in diseases such as psoriasis, intestinal inflammation and cancer. IL-22BP is a soluble inhibitory IL-22 receptor and may represent a crucial regulator of IL-22. We show both in rats and mice that, in the steady state, the main source of IL-22BP is constituted by a subset of conventional dendritic cells (DC) in lymphoid and non lymphoid tissues. In mouse intestine, IL-22BP was specifically expressed in lamina propria CD103+CD11b+ DC. In humans, IL-22BP was expressed in immature monocyte-derived DC (MDDC) and strongly induced by retinoic acid (RA) but dramatically reduced upon maturation. Our data suggest that a subset of immature DC may actively participate in the regulation of IL-22 activity in the gut by producing high levels of IL-22BP.

T-helper 2 (Th2) cell responses defend against parasites. Although dendritic cells (DCs) are vital for the induction of T-cell responses, the DC subpopulations that induce Th2 cells in the intestine are unidentified. Here we show that intestinal Th2 responses against Trichuris muris worms and Schistosoma mansoni eggs do not develop in mice with IRF-4-deficient DCs (IRF-4f/f CD11c-cre). Adoptive transfer of conventional DCs, in particular CD11b-expressing DCs from the intestine, is sufficient to prime S. mansoni-specific Th2 responses. Surprisingly, transferred IRF-4-deficient DCs also effectively prime S. mansoni-specific Th2 responses. Egg antigens do not induce the expression of IRF-4-related genes. Instead, IRF-4f/f CD11c-cre mice have fewer CD11b+ migrating DCs and fewer DCs carrying parasite antigens to the lymph nodes. Furthermore, CD11b+CD103+ DCs induce Th2 responses in the small intestine, whereas CD11b+CD103− DCs perform this role in the colon, revealing a specific functional heterogeneity among intestinal DCs in inducing Th2 responses.

Dendritic cells (DCs) migrating via lymph are the primary influence regulating naive T cell differentiation, be it active immunity or tolerance. How DCs achieve this regulation in vivo is poorly understood. Intestinal DCs are in direct contact with harmless or pathogenic luminal contents, but may also be influenced by signals from epithelial cells, macrophages, or other resident or immigrant cells. To understand the role of TLR7 and TLR8 in regulating intestinal DC function, we fed a TLR7/8 ligand (resiquimod (R-848)) to rats and mice and examined DC in pseudoafferent lymph (rat) and mesenteric lymph nodes (MLNs). Oral R-848 induced a 20- to 30-fold increase in DC output from the intestine within 10 h due to a virtually total release of lamina propria DCs. This resulted in an accumulation of DCs in the MLNs that in mice was completely TNF-α dependent. Surprisingly, intestinal lymph DCs (iL-DCs) released by R-848 did not up-regulate CD86, but did up-regulate CD25. In contrast, MLN-DCs from R-848-stimulated rats and mice expressed high levels of CD86. This DC activation in MLNs was dependent on type 1 IFNs. The major source of these rapidly released cytokines is plasmacytoid DCs (pDCs) and not classical DCs, because depletion of pDCs significantly reduces the R-848-stimulated increase in serum cytokine levels as well as the accumulation and activation of DCs in MLNs. These experiments show that TLR-mediated regulation of iL-DC functions in vivo is complex and does not depend only on direct iL-DC stimulation, but can be regulated by pDCs.