Epithelial relay of microbial signals coordinates intestinal macrophage supported barrier repair

Abstract

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 <i>Escherichia coli</i> isolate increased CCR2⁺ macrophages in the intestine and ameliorated pathology in a colitis model. Using human colonic organoids, we report that <i>E. coli</i> 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 <i>E. coli</i>, we identified that high flagellin expression correlated with epithelial CCL2 production. Demonstrating a requirement for <i>E. coli</i> flagellin, in vivo protection was lost in mice lacking epithelial Toll-like receptor 5 (TLR5) or after colonization with flagellin-deficient <i>E. coli</i>. Thus, epithelial flagellin sensing by TLR5 recruits CCR2⁺ macrophages to the intestine, promoting barrier repair.