CCL9/CCR1 chemokine signaling plays a protective role in limiting intestinal inflammation by maintaining an anti-inflammatory M2 macrophage pool in the intestinal lamina propria

Inflammatory bowel diseases (IBD) are chronic disorders characterized by dysregulated immune responses and chemokines regulate immune cell trafficking in the intestinal mucosa. CCL9, a murine chemokine mediated by CCR1, is preferentially expressed in the intestinal tract, yet its role in intestinal inflammation remains unclear. We generated Ccl9^-/- and Ccr1^-/- mice using CRISPR/Cas9 and subjected them to dextran sulfate sodium (DSS)-induced colitis to investigate the role of CCL9/CCR1 axis. Both mutants developed more severe colitis than wild-type controls, with greater weight loss, colon shortening, histopathological damage, and enhanced pro-inflammatory cytokine production. In the steady state, most colonic macrophages were M2-like and expressed CCR1, but disruption of CCL9/CCR1 signaling reduced M2-like macrophages and expanded pro-inflammatory M1-like macrophages. In addition, loss of CCL9 or CCR1 induced compensatory upregulation of CCL2, CCL3 and CCL5, contributing to a pro-inflammatory environment. These findings identify CCL9 as an anti-inflammatory chemokine that sustains M2 macrophages, restrains excessive M1 expansion and limits intestinal inflammation. Targeting the CCL9/CCR1 axis may provide new therapeutic opportunities for IBD.