There is mounting evidence that matrix metalloproteinases are the predominant proteinases expressed in the gut mucosa during active inflammatory bowel disease. We investigated the role of metalloproteinase 9 (MMP-9), a secreted gelatinase that is consistently up-regulated in both animal models and human inflammatory bowel disease and is associated with disease severity, in the pathogenesis of colitis by using mice containing a targeted deletion of the MMP-9 gene.Dextran sodium sulfate-induced colitis and Salmonella typhimurium-induced enterocolitis were used as animal models to study colitis.MMP-9 activity and protein expression were absent from normal colonic mucosa but were up-regulated during experimental colitis. MMP-9-/- mice exposed to dextran sodium sulfate or salmonella had a significantly reduced extent and severity of colitis. Immunohistochemical studies showed that MMP-9 was localized to epithelial cells and granulocytes during active colitis. The immune response to systemic administration of Salmonella typhimurium was not affected in MMP-9-/- mice. Neutrophil transmigration studies and bone marrow chimeras showed that neutrophil MMP-9 is neither required for its migration nor sufficient to induce tissue damage during colitis and that epithelial MMP-9 is important for tissue damage. MMP-9 inhibited cell attachment and wound healing in the model intestinal epithelial cell line, Caco2-BBE.Taken together, our data suggest that MMP-9 expressed by epithelial cells may play an important role in the development of colitis by modulating cell-matrix interaction and wound healing. Thus, strategies to inhibit MMP-9 may be of potential therapeutic benefit.