AbstractMice deficient in interleukin‐2 production (IL‐2null mice) develop colonic inflammation closely resembling ulcerative colitis in humans. Although this disease is marked by substantial infiltration of the colon by CD8+ and CD4+ T lymphocytes, no function has yet been assigned to these T cell subsets in the development of colitis in the IL‐2null mouse. For the present study, we investigated the involvement of T lymphocytes in the onset of colitis in IL‐2null mice, and examined the possible role played by cytotoxic T cells. Both lamina propria lymphocytes (LPL) and intraepithelial lymphocytes (IEL) of the colon of IL‐2null mice were potently cytotoxic ex vivo in short‐term redirected cytotoxic lymphocyte (CTL) assays. In contrast, colonic T cells of wild‐type animals showed little or no constitutive cytotoxic T cell activity. Colonic CTL were detectable prior to the appearance of disease in IL‐2null animals and CTL activity was confined to the TcRαβ, rather than to the TcRγδ IEL subset. IL‐2null animals crossed with major histocompatibility complex class I‐deficient mice [IL‐2null × β2 microglobulin (β2mnull] mice also developed colitis, which appeared even earlier than in most IL‐2null mice. These findings suggest that neither CD8+ IEL nor LPL were causal in the onset of colitis in IL‐2null animals. In IL‐2null × β2mnull mice, an ulcerative colitis‐like disease was evident from histological studies and immunohistological staining which showed very large numbers of CD4+ lymphocytes within the intestinal mucosa. Significant ex vivo killing by CD4+ T cells was observed in IL‐2null × β2mnull animals, although this required an extended incubation time compared to colonic CD8+ T cells. Peripheral as well as colonic CD4+ T cells in IL‐2null and IL‐2null × β2mnull animals, were activated as judged by their cell surface phenotype (CD45RBlo, L‐selectinlo and CD69+). In light of these findings, we propose that infiltrating CD4+, but not CD8+ T cells are central to the inflammation observed in the intestinal mucosa in IL‐2null colitis.