Signaling via interleukin (IL)-10 or transforming growth factor (TGF)-β is disrupted in subpopulations of patients with inflammatory bowel disease, but it is not clear how a T-helper (Th) 1 cell response is induced. We studied conversion of human mucosal innate immune cells into inflammatory cells and the initiation of a Th1 cell response following loss of IL-10 or TGF-β signaling. METHODS: We depleted IL-10 or TGF-β from explant cultures of human normal colonic mucosa using immunoneutralization. Pharmacologic inhibitors and antibodies were used to determine the factors involved in the initiation of an interferon (IFN)-γ response following loss of TGF-β or IL-10 signaling. Cytokines produced by mucosal cells were assessed by enzyme-linked immunosorbent assay and quantitative reverse-transcriptase polymerase chain reaction. The subsets of cells involved in cytokine production were determined by in situ immunofluorescence analysis and flow cytometry after digestion of the explants with collagenase. RESULTS: Depletion of IL-10 from human normal colonic mucosa resulted in an IFN-γ response, characterized by early-stage secretion of mature IL-18 and production of the active form of caspase-1 by macrophages and some epithelial cells. A caspase-1 inhibitor or the IL-18 antagonist IL-18-binding protein blocked this response. By contrast, depletion of TGF-β resulted in an IFN-γ response that was preceded by and required secretion of IL-12 from macrophages, dendritic cells, and epithelial cells. CONCLUSIONS: Innate immune cells (macrophages and epithelial cells) activate a Th1 cell response in explant cultures of human normal colonic mucosa depleted in IL-10 or TGF-β via distinct, nonredundant pathways. These pathways might contribute to the pathogenesis of inflammatory bowel disease.