Inflammatory monocytes promote pyogranuloma formation to counteract Yersinia blockade of host defense

Granulomas are organized immune cell aggregates that form in response to chronic infection or antigen persistence. Yersinia pseudotuberculosis ( Yp ) blocks innate inflammatory signaling and phagocytosis, inducing formation of neutrophil-rich pyogranulomas within lymphoid tissues. Here, we uncover that Yp triggers pyogranuloma formation within the murine intestinal mucosa, a site not known to contain such structures. Mice lacking circulating monocytes fail to form defined pyogranulomas, have defects in neutrophil activation, and succumb to Yp infection. Yersinia lacking the virulence factors that block phagocytosis did not induce pyogranulomas, indicating that intestinal pyogranulomas form in response to Yp disruption of phagocytosis. Notably, mutation of a single anti-phagocytic virulence factor, YopH, restored pyogranuloma formation and control of Yp infection in monocyte-deficient mice, demonstrating that monocytes override YopH-dependent blockade of innate immune defense. This work reveals an unappreciated site of Yersinia intestinal invasion, and defines host and pathogen drivers of intestinal granuloma formation. ### Competing Interest Statement The authors have declared no competing interest.