Mucosally sourced complement factor B modulates the host response to colitis.

Distinct host factors maintain intestinal homeostasis but are incompletely understood. The complement system is primarily liver-derived and serum-operative. However, there is growing recognition for complement-mediated host defense at mucosal surfaces. The alternative pathway, which is constitutively active at low levels and amplifies complement activation independent of antibodies, requires Complement Factor B (CFB). Despite its evolutionary conservation, the spatial, cellular, and functional roles of CFB in the intestine are poorly understood. Here, we show that CFB is produced in the human colon and is increased in patients with active inflammatory bowel disease. To isolate the role of local CFB in mucosal responses, we interrogated a mouse strain that has no circulating, liver-derived CFB but retains intact CFB expression in the gut. Global CFB-deficient mice succumb to colitis compared to these liver-specific knockout mice, suggesting that locally synthesized CFB mitigates colitis. Single-cell analyses identify enterocytes and fibroblasts as key CFB producers in the gut. Compartment-specific deletion of CFB from epithelial or stromal cells abrogates mucosal protection independent of circulating levels, which corroborates with pharmacological CFB inhibition. These findings redefine complement in the intestine as a locally regulated mucosal defense system and establish gut-derived CFB as a critical determinant of intestinal homeostasis.