Recombinase-activating gene-2-deficient (Rag2 ؊/؊ ) mice lacking functional lymphocytes provide a useful model of chronic inflammatory bowel disease-emulating events in human colon cancer. Infection of Rag2 ؊/؊ mice with Helicobacter hepaticus led to accumulation of macrophages and neutrophils in the colon, a process temporally related to up-regulation of tissue inducible nitric oxide synthase (iNOS) expression at the site of infection and increased nitric oxide (NO) production, as evidenced by urinary excretion of nitrate. Progressive development of increasingly severe inflammation, hyperplasia, dysplasia, and cancer accompanied these changes. Concurrent administration of an iNOS inhibitor prevented NO production and abrogated epithelial pathology and inhibited the onset of cancer. The presence of Gr-1 ؉ neutrophils and elevated tumor necrosis factor-␣ (TNF-␣) expression in colon were required for increased iNOS expression and cancer, whereas interleukin-10 (IL-10) down-regulated TNF-␣ and iNOS expression and suppressed cancer. Anti-inflammatory CD4 ؉ regulatory lymphocytes also down-regulated iNOS and reduced cancer formation. Collectively, these results confirm essential roles for inflammation, increased TNF-␣ expression, and elevated NO production in colon carcinogenesis.colorectal cancer ͉ IBD ͉ innate immunity C hronic Helicobacter pylori infection in humans leads to gastritis and has been established as a causative agent for human gastric cancer (1). Inflammatory bowel diseases (IBDs), such as Crohn's disease and ulcerative colitis, also increase risk for colon cancer (2, 3). Generation of nitric oxide (NO) by inducible NO synthase (iNOS) is a central feature of chronic inflammatory diseases in the gastrointestinal tract (4-9), but precise mechanistic roles for NO in colon cancer development remain undefined.Colon cancer patients exhibit evidence of nitrosative and oxidative stresses that increase cancer risk (10), resulting from mutagenic reactive oxygen and nitrogen species derived from NO generated by immune cells (6,(11)(12)(13)(14)(15). Roles for chronic bacterial infection in IBD and colon cancer have been identified recently in recombinase-activating gene-2-deficient mice (Rag2 Ϫ/Ϫ ), which completely lack functional lymphocytes (16-18). We have exploited this mouse model of chronic IBDassociated cancer for studies of the role of NO and its products because it emulates naturally occurring inflammatory events in humans (16,19,20).Rag2 Ϫ/Ϫ mice have been used to assess functions of lymphocytes by adoptive transfer. Populations of CD4 ϩ T cells with low or high expression of CD45RB (17,21, 22) or CD25 (16,18,23,24) prevent or accelerate colitis in these animals. In wild-type (wt) mice, protection against inflammatory pathology induced by bacterial infection has been attributed to interleukin-10 (IL-10) and IL-10-dependent functions of CD4 ϩ cells (18,20,25,26). Collectively, this evidence forms the rationale for the hypothesis that NO overproduction comprises a linkage between Helicobacter hepaticus-i...