Previous studies have suggested that intestinal epithelial cells (IECs) have the capacity to function as non-professional antigen presenting cells that in the normal state preferentially activate CD8+ T cells. However, under pathological conditions, such as those found in inflammatory bowel disease (IBD), persistent activation of CD4+ T cells is seen. The aim of this study was to determine whether the IBD-IECs contribute to CD4+ T cell activation. Freshly isolated human IECs were obtained from surgical specimens of patients with or without IBD and co-cultured with autologous or allogeneic peripheral blood T lymphocytes. Co-cultures of normal T cells and IECs derived from IBD patients resulted in the preferential activation of CD4+ T cell proliferation that was associated with significant IFN-, but not IL-2, secretion. Cytokine secretion and CD4+ T cell proliferation was inhibited by pre-treatment of the IBD IECs with the anti-DR mAb L243. In contrast, normal IECs stimulated the proliferation and cytokine secretion of CD4+ T cells to a significantly lesser degree than IBD IECs. Furthermore, blockade of HLA-DR had a lesser effect in the normal IEC-CD4+ T cell co-cultures. We conclude that IECs can contribute to the ongoing CD4+ T cell activation seen in IBD. We suggest that the apparent differences between the secreted levels of IFN- indicate that it may play a dual role in intestinal homeostasis, in which low levels contribute to physiologic inflammation while higher levels are associated with an uncontrolled inflammatory state.