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Ulcerative colitis is the most common inflammatory bowel disease, and is associated with increased risk of colon cancer. Understanding the mechanisms by which carcinogenesis occurs in the setting of colitis could lead to both prevention strategies and improved therapies. Matrix metalloproteinase-10 (MMP10), a member of the matrix metalloproteinase family of proteolytic enzymes, is expressed at healing edges of colonic ulcers. Although MMPs are usually considered as destructive players in inflammatory disease, we considered the possibility that MMP10 had a beneficial role in ulcerative colitis. We used dextran sulfate sodium (DSS) treatment in MMP10-null and corresponding control wild-type mice to investigate the consequences of MMP10 deletion on colitis development and recovery. Groups of mice were exposed to DSS for 7-day periods and sacrificed at different timepoints following the cessation of the DSS. Strikingly, MMP10-null mice were more severely affected by the treatment as determined by gross observations such as weight loss and colon length. Histological analysis at the various timepoints indicated that wild-type mice showed almost complete recovery from DSS within 14 days after DSS treatment. In contrast, MMP10-null mice still showed clear signs of damage with persistent inflammatory foci and non-healing ulcers. In situ hybridization indicated MMP10 expression predominantly in inflammatory cells, although some fibroblasts were also positive. Colonic epithelium was negative for MMP10. Bone marrow transplant experiments indicated that wild-type bone marrow partially rescued the MMP10-null phenotype. Measurement of cytokines by ELISA in colonic lavage fluid from MMP10-null and wild-type mice showed that IL-1α was significantly increased in the absence of MMP10. Levels of other cytokines such as TNFα, IL6, IL17 and G-CSF were not different. Multiple exposures to DSS results in conversion of an acute colitis to a chronic inflammatory disease. With only 2 DSS exposures, MMP10-null mice manifested a chronic condition which failed to resolve even after 3 months. Similarly-treated wild-type mice, however, appeared normal at this timepoint. Examination of the colonic tissue from the DSS double exposure showed a significant increase in the number of dysplastic foci present in the MMP10-null mice. Thus, expression of MMP10 appears necessary for resolution of colitis, and in its absence, the development of chronic colitis results in tumor development. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 1351.