The intestinal microbiota is tolerated by the host, and potentially harmful inflammatory responses to commensals could be actively suppressed. In Nature, Littman and colleagues show that, at steady state, the microbiota inhibits the transport of commensal and pathogenic bacteria from the intestinal lumen to the mesenteric lymph nodes (MLNs). After antibiotic-induced dysbiosis, noninvasive Salmonella enterica or nonpathogenic Escherichia coli, which under normal conditions do not reach the MLNs, are trafficked to the MLNs by CX3CR1hi phagocytes in a chemokine receptor CCR7–dependent manner. Mice deficient in the adaptor Myd88 similarly allow trafficking of bacteria to the MLNs and induction of specific T cell responses and production of IgA, which indicates that sensing the microbiota via TLRs inhibits a pathway that allows access of lamina propria cell populations and phagocytosed bacteria to the MLNs.

Nature (13 January 2013) http://dx.doi.org/10.1038/nature11809