Intestinal macrophages are essential for epithelial barrier repair. In homeostasis, macrophages are continuously replenished by recruitment of circulating C-C chemokine receptor 2 (CCR2) + monocytes into the intestinal lamina propria (LP), a process that requires the commensal microbiota. The specific microbial factors and downstream host pathways that coordinate macrophage replenishment are inadequately understood. Here, we show that colonization with an Escherichia coli isolate increased CCR2 + macrophages in the intestine and ameliorated pathology in a colitis model. Using human colonic organoids, we report that E. coli colonization induced the secretion of C-C chemokine ligand 2 (CCL2) by intestinal epithelial stem cells, which promoted monocyte migration. Protection in vivo was abolished in the absence of epithelial CCL2. By screening a panel of E. coli , we identified that high flagellin expression correlated with epithelial CCL2 production. Demonstrating a requirement for E. coli flagellin, in vivo protection was lost in mice lacking epithelial Toll-like receptor 5 (TLR5) or after colonization with flagellin-deficient E. coli . Thus, epithelial flagellin sensing by TLR5 recruits CCR2 + macrophages to the intestine, promoting barrier repair.
Tsai et al. (Fri,) studied this question.