Our body is covered with tight physical barriers of skin and mucosal tissues. Mucosal surfaces are constantly exposed to the external environment, which includes commensal microbes and exogenous antigens. When pathogenic microbes breach the surface barrier, surveillance systems beneath sense the trespassers and send an alarm to defense headquarters. Mucosal immune tissues comprise lymphoid organs associated with the gastro-intestinal tract (e.g., intestine, oral cavity and pharynx), respiratory tract, and urogenital tract, as well as the glands associated with these tissues, such as the salivary glands and lacrimal glands (1). The lactating breast is also a mucosal immune tissue. Mucosal immunity can maintain peaceful body surface by generating secretory IgA (sIgA) from B cells as well as priming specific T cell immunity. The intestine, especially, harbors an enormous community of commensal microorganisms that may contribute to host defense by enforcing the host's barrier function (2) or by competing against other microorganisms metabolically (3,4). Dendritic cells (DCs) or other phagocytic cells continuously survey the mucosal environment by using innate pattern recognition receptors and sample antigens prior to integrate adaptive immune system. These cells also can adjust suppressive regulation to innocuous antigens by inducing Tregs and keep distance to commensals by producing sIgA. Moreover, these cells protect against pathogenic invasion by generating various kinds of helper T (T_H) and CD8⁺ T cells as well as helping to produce sIgA antibodies. Here, we provide an overview of the gut immune response, focusing on unique functional features of intestinal DCs and other phagocytic cells.

Lamina propria DCs in the small intestine (SI) haves been well studied as one of the intestinal DC subsets. CD11c⁺ major histocompatibility (MHC) class II⁺ cells in the gut comprise DCs as well as phagocytic macrophages. Genuine DCs are a CD11c^high MHC class II^high population, whereas macrophages are a CD11c^low MHC class II^low population (5). Lamina propria phagocytic cells in the gut have different origins and functions (6). CD103-expressing DCs are widely present in non-lymphoid tissues. CD103⁺ DC subsets are differentiated by Flt3 ligand-dependent manner whereas CX3CR1-expressing phagocytic cells are dependent on CSF-1R (6). Peripheral CD103⁺CD11b^- DCs are developmentally dependent on Batf3 and are related to CD8α⁺ conventional DCs (7). DC migration is tightly controlled by the expression of CCR7, and it can be largely classified as non-migratory and migratory (8,9). Non-migratory DCs are generally tissue-resident macrophage-like cells. Migratory DCs travel into draining lymph nodes with sampled antigen and can be infiltrated under inflammation. DC and phagocytic cells in the gut and their functions therein are listed in Table I. In the gut, CD103⁺ CD11b⁺ DCs has been well reported by the function to induce lymphocytes. Gut CD103⁺ DCs comprise two major subsets, CD103⁺CD11b⁺ and CD103⁺CD11b^- DCs (10). CD103⁺ CD11b^- DCs are the dominant population of CD103⁺ DC in the Peyer's patches and colon lamina propria (11). In contrast, CD103⁺CD11b⁺ DCs are the major DC subset in the SI lamina propria (12). In addition, recent reports regarding resident CX3CR1⁺ phagocytic cells are increasing. TNF-α/iNOS-producing DCs (Tip DCs) were initially reported in the spleen, where they released large amounts of nitric oxide (NO) after recognizing commensal bacteria through toll-like receptors (TLRs) (13). Several TLR-expressing DCs are reported to induce IgA production. Gut plasmacytoid DCs (pDCs) can induce IgA production and repress inflammation. The detailed function of each subset will be discussed later.

The SI shows a strong propensity for immune suppression in response to exogenous antigens. The presentation of innocuous food antigens by antigen-presenting cells induces oral tolerance by generation of inducible regulatory T (Treg) cells (14). CD103⁺CD11b⁺ DCs can sample soluble antigens from the intestinal lumen through a process termed goblet cell-associated antigen passages (GAPs) (15) (Fig. 1). These DCs can also patrol among enterocytes while extending dendrites toward the lumen (16). These intraepithelial CD103⁺ DCs are recruited to sample bacterial antigens for presentation. In the SI lamina propria as well as mesenteric lymph node (MLN), these DCs express retinaldehyde dehydrogenase type 2 (RALDH2) which can convert retinal to retinoic acid (RA), a metabolic derivative of vitamin A found in food. This DC subset can induce regulatory Foxp3⁺CD4⁺ T cells dependent on RA and TGF-β (17,18,19); Alternatively, CD11b⁺ F4/80⁺CD11c^- macrophages in the lamina propria has been reported as more potent inducers of Treg cells than DCs (20). CX3CR1⁺ phagocytic cells in the lamina propria support expansion of the Foxp3⁺CD4⁺ Treg cell population by producing IL-10 to harness immune tolerance (21). CX3CR1⁺ phagocytic cells can capture Salmonella by extending dendrites across epithelium in a CX3CR1-dependent manner (22). Antigens captured by CX3CR1⁺ phagocytic cells can be transferred through gap junctions to CD103⁺ DCs in the lamina propria to establish oral tolerance (23). In addition to luminal antigen, lamina propria CX3CR1⁺ cells facilitate the surveillance of circulatory antigens from blood vessels (24). These cells fail to prime naïve CD4⁺ T cells; however, cross-presentation by these cells can induce priming of and differentiation into CD8⁺ T cells that express IL-10, IL-13, and IL-9. These CD8⁺ T cells can suppress pathogen-specific CD4⁺ T cell activation through IL-10 (24). Finally, these CD8⁺ T cells act as a regulatory CD8αβ ⁺TCRαβ ⁺ T cell population in the epithelium. CX3CR1⁺ cells regulate colonic IL-22 producing group 3 innate lymphoid cells (ILC3) to promote mucosal healing and maintain barrier integrity (25). Therefore, CD103⁺ DCs and CX3CR1⁺ phagocytic cells can generate two distinct regulatory T cell subsets by different mechanisms to maintain gut immune homeostasis at steady state (Fig. 1). pDCs may mediate anti-inflammatory responses following TLR2-polysaccharide A signaling (26) and Type I interferon supports Treg function and regulates colitis (27,28).

After DCs sample antigen in the lamina propria, they can present antigenic epitope to naïve T cells in the isolated lymphoid follicles (ILF) or draining MLN. Distinct DCs subset instructs T cell differentiation (Fig. 2). CD103⁺CD11b⁺ DCs, one of the major DC subset in the SI lamina propria, are primarily a migratory population that responds to CCR7 expression and can be infiltrated under inflammatory conditions (29,30). Segmented filamentous bacteria (SFB), murine commensal bacteria, are shown to be sufficient for T_H17 differentiation (31). MHC II-dependent presentation of SFB antigens by intestinal DCs is crucial for T_H17 cell induction (32). Most T_H17 cells recognize antigenic repertoires derived from SFB (33). CD103⁺ CD11b⁺ DCs produce IL-6 with TLR stimuli which enable to induce T_H17 cell differentiation (34). Several studies suggest the possibility that CD103⁺CD11b⁺ DCs might interact with SFB and link signals to induce T_H17. CD103⁺ CD11b⁺ DCs can express high amounts of IL-23 following TLR5 stimuli and then drove IL-22-dependent RegIIIγ production from Paneth cells (35). TLR5⁺ DCs promote the differentiation of antigen-specific T_H17 and T_H1 cells following stimulation by flagellin, a TLR5 ligand (36). CD103⁺CD11b^-CD8α⁺ DCs expressing TLR3, TLR7, and TLR9 can produce IL-6 and IL-12p40 following stimulation of the respective TLR ligands (37). These DCs induce a T_H1 response and cytotoxic T lymphocytes (CTL). CX3CR1⁺ phagocytic cells contribute to intestinal clearance of intracellular bacteria. While their function under conditions of inflammation or infection remains unclear, their suppressive functions are well described at steady state.

A unique feature of the mucosal immune system is local production of sIgA from plasma cells differentiated from B cells. IgA class switching generally occurs in gut-associated lymphoid tissues including Peyer's patches, MLNs, and ILFs within the lamina propria. SFB stimulates the postnatal development of ILFs and tertiary lymphoid tissue in the SI lamina propria, which can substitute for Peyer's patches as inductive sites for intestinal IgA (38). Intestinal IgA coating identifies inflammatory commensals that drive intestinal disease like inflammatory bowel disease (33,39). The sIgA within mucus establishes distance with commensals and forms a barrier between invading and commensal microorganisms. Intestinal DCs support IgA isotype class-switching and differentiation into IgA-secreting plasma cells, either together with the assistance of T_H cells or in a T cell-independent manner by expressing B cell-activating factors (BAFFs) and a proliferation-inducing ligand (APRIL) (Fig. 3). Intestinal pDCs, in particular, have been shown to induce IgA production by expressing these factors (40). When commensal bacteria are recognized through TLRs, Tip DCs release large amounts of NO which enhances IgA class-switch DNA recombination (CSR) in B cells by inducing DC expression of BAFF and APRIL (13). Further, RA can be converted by RALDH2 from dietary vitamin A in DCs, and DCs expressing RALDH2 can also induce IgA CSR (41). Lamina propria CD103⁺CD11b⁺ DCs, Tip DCs, and TLR5⁺ DCs express RALDH2 and produce RA, which in turn can be used for IgA production together with TGF-β (13,36,42). Langerin-expressing DCs in the MLNs that emerge following trans-cutaneous vaccination can also induce RA-dependent antigen-specific IgA production in the SI (43). Moreover, RA confers gut homing signature such as α₄β₇ integrin and CCR9 on lymphocytes (42,44). Therefore, RA is essential to maintaining the intestinal immune environment (45,46). In normal lamina propria, large numbers of eosinophils are present,accounting for approximately 5% of all lamina propria leukocytes (47). Recently, eosinophils have been found to promote the generation and maintenance of IgA-producing plasma cells by inducing B cell activating factors (48) and supporting the function of CD103⁺ DCs (49).

In this review, we focused on the integral role of intestinal DCs in shaping the unique intestinal immunity. The advent of advanced experimental techniques for surveying mucosal tissues and analyzing metagenomic data of commensals, along with the wide availability of germ-free mice has facilitated a growing understanding of this unique mucosal immune environment. Diverse microbiota can drive microbe-dependent CD4 effector T-cell programs. For example, Clostridium strains provide a rich environment of TGF-β and induce Foxp3⁺ Treg in the colon (50,51), and SFB induce the generation of T_H17 cell by IL-6 production from DCs (32). Some pathogens (e.g., Listeria spp.) specifically induce T_H1 cells (33). Thus, microbial signals may induce polarizing cytokine secretion from DCs, other innate cells or stromal cells. Assembling combined information, DCs may coordinate to establish gut immune system.