Microbiota Restrict Trafficking of Bacteria to Mesenteric Lymph Nodes by CX(3)CR1(hi) Cells

The intestinal microbiota have critical roles in immune system and metabolic homeostasis, but they must be tolerated by the host to avoid inflammatory responses that can damage the epithelial barrier separating the host from the luminal contents(1-6). Breakdown of this regulation and the resulting inappropriate immune response to commensals are thought to lead to the development of inflammatory bowel diseases (IBDs) such as Crohn's disease and ulcerative colitis(7). We hypothesized that the intestinal immune system is instructed by the microbiota to limit responses to luminal antigens. We demonstrate that, at steady state, the microbiota inhibit the transport of both commensal and pathogenic bacteria from the lumen to a key immune inductive site, the mesenteric lymph node (MLN). However, in the absence of Myd88 or under conditions of antibiotic-induced dysbiosis, non-invasive bacteria trafficked to the MLN in a CCR7-dependent manner and induced both T cell responses and IgA production. Trafficking was carried out by CX(3)CR1(hi) mononuclear phagocytes, an intestinal cell population previously reported to be non-migratory(8). These findings define a central role for commensals in regulating the migration to the MLN of CX(3)CR1(hi) mononuclear phagocytes endowed with the ability to capture luminal bacteria, thereby compartmentalizing the intestinal immune response to avoid inflammation.