Research Article

Mechanisms whereby the gut mucosa tolerates commensal bacteria and food antigens without developing inflammation remain elusive. Though traditionally viewed as a nonspecific barrier between the host and the environment, mucus also regulates gut homeostasis. The building block of gut mucus is MUC2, a gel-forming mucin secreted by goblet cells (GCs) (1). In the large intestine (LI), MUC2 prevents inflammation by generating an outer nonattached mucous layer inhabited by the microbio-ta and an inner mucous layer adherent to intestinal epithelial cells (IECs) and impervious to bacteria (1). The structure and function of mucus in the small intestine (SI) are less well understood. The SI harbors bacteria that promote homeostasis by inducing Foxp3+ T regulatory (Treg) cells and B cell production of immunoglobu-lin A (IgA) antibodies (Abs) in Peyer’s patches (PPs) and mesenteric lymph nodes (MLNs) (2, 3). These responses involve sampling of bacte-ria by CD11c+ dendritic cells (DCs) (4), including macrophage-like CD103–CD11b+CX3CR1+ DCs and myeloid CD103+CD11b+CX3CR1– DCs (2, 5, 6). In the lamina propria (LP), myeloid DCs sample soluble antigens from GC-associated passages and cooperate with lymphoid CD103+CD11b–CX3CR1 – DCs to generate Treg cells (7–9). Such anti-gen-sampling activities require a porous mucous barrier, raising ques-tions as to how MUC2 prevents inflammation in the SI. MUC2 Mitigates Inflammatory Re-sponses in DCs We first analyzed the structure of gut mucus in wild type (WT) C57BL/6 mice. While LI mucus formed a dense bilayered barrier that segregated bacte-ria from IECs, SI mucus was less orga-nized and thus permitted the adhesion of bacteria to IECs (fig. S1A-C). Con-sistent with the presence of some MUC2-coated bacteria on IECs and inside DCs (fig. S2A-C), CX3CR1+ DCs from both PPs and SI-LP captured carboxyfluorescein succinimidyl ester (CFSE)-labeled MUC2 bound to fluo-rescent bacteria (fig. S3A-C). In agree-ment with recent studies (6), also some CD103+ DCs captured MUC2-coated bacteria (fig. S3A-D). Similarly, human monocyte-derived DCs internalized MUC2-bound bacteria across IECs sealed by occludin-containing tight junctions (fig. S4A-D). Moreover, MUC2 was detected in human SI-L