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Previous studies have suggested that intestinal epithelial cells (IECs) have the capacity to function as nonprofessional 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 cocultured with autologous or allogeneic peripheral blood T lymphocytes. Cocultures 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-gamma, but not IL-2, secretion. Cytokine secretion and CD4+ T cell proliferation was inhibited by pretreatment of the IBD IECs with the anti-DR MAb L243. In contrast, normal IECs stimulated the proliferation and cytokine secretion by CD4+ T cells to a significantly lesser degree than IBD IECs. Furthermore, blockade of human leukocyte antigen-DR had a lesser effect in the normal IEC-CD4+ T cell cocultures. 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-gamma indicate that it may play a dual role in intestinal homeostasis, in which low levels contribute to physiological inflammation whereas higher levels are associated with an uncontrolled inflammatory state.

Adoptive transfer of CD4+ T cells into scid mice leads to a chronic colitis in the recipients. The transferred CD4+ T cells accumulate in the intestinal lamina propria (LP), express an activated Th1 phenotype and proliferate vigorously when exposed ex vivo to enteric bacterial antigens. As LP CD4+ T cells from normal BALB/c mice do not respond to enteric bacterial antigens, we have investigated whether colonic LP-derived CD4+ T cells from normal mice suppress the antibacterial response of CD4+ T cells from scid mice with colitis. LP-derived CD4+ T cells cocultured with bone marrow-derived dendritic cells effectively suppress the antibacterial proliferative response of CD4+ T cells from scid mice with colitis. The majority of these LP T-reg cells display a nonactivated phenotype and suppression is independent of antigen exposure, is partly mediated by soluble factor(s) different from IL-10 and TGF-beta, and is not prevented by the addition of high doses of IL-2 to the assay culture. Functionally and phenotypically the T-reg cells of the present study differ from previously described subsets of T-reg cells. The presence of T cells with a regulatory potential in the normal colonic mucosa suggests a role for these cells in the maintenance of local immune homeostasis of the gut.

Scid mice transplanted with CD4(+) T cells from congenic donor mice develop a chronic and lethal inflammatory bowel disease (IBD) 2-3 months post-transplantation. In the present study we have investigated the response of CD4(+) T cells from scid mice with colitis against fecal extracts. Our results show that in contrast to CD4(+) T cells from normal BALB/c mice, CD4(+) T cells from scid mice with colitis proliferate strongly in response to antigen-presenting cells (APC) pulsed with fecal extracts. The IBD-associated T cells did not respond to either extracts from food antigens or fecal extracts from germ-free mice, which indicates that they recognize bacterial antigens in the fecal extracts. CD4(+) T cells isolated from the colonic lamina propria of scid mice 3 weeks post transplantation also responded vigorously to fecal extracts, demonstrating that reactive CD4(+) T cells are present in the gut mucosa of transplanted scid mice prior to clinical manifestations of IBD. CD4(+) T cells activated by fecal extracts produced high amounts of IL-2 and IFN-gamma, intermediate amounts of IL-4 and low amounts of IL-10, consistent with a Th1 profile. The proliferative reactivity towards fecal extracts was restricted by MHC class II molecules and dependent on antigen processing, as the response could be blocked by anti-MHC class II antibodies or a short fixation of the APC. This study demonstrates that class II-restricted CD4(+) Th1 cells, which recognize enteric bacterial antigens, infiltrate the gut mucosa and spleen of transplanted scid mice prior to and during the course of colitis.

To investigate the role of IFN-gamma in the immunopathogenesis of inflammatory bowel disease (IBD), severe combined immunodeficient (SCID) mice were transplanted with in vitro activated CD4+ T cells from either wild-type (WT) or IFN-gamma-deficient (IFN-gammaKO) BALB/c mice. In vitro, the two types of T cells displayed comparable proliferation rates and production of tumour necrosis factor-alpha (TNF-alpha), IL-2, IL-4 and IL-10 after concanavalin A (Con A) stimulation. When transplanted into SCID mice, WT CD4+ blasts induced a lethal IBD, whereas IFN-gammaKO blasts induced a less severe intestinal inflammation with moderate weight loss. Intracellular cytokine staining of lamina propria lymphocytes (LPL) revealed comparable fractions of CD4+ T cells positive for TNF-alpha, IL-2 and IL-10 in the two groups of transplanted SCID mice, whereas a two-to-three-fold increase in the fraction of IL-4-positive cells was found in IFN-gammaKO-transplanted SCID mice. Flow cytometric analyses showed strong up-regulation of MHC class II expression of colonic epithelial cells of WT-CD4+ T cell-transplanted compared with IFN-gammaKO-transplanted SCID mice. A significantly higher fraction of CD4+ LPL were found to enter the cell cycle, i.e. to incorporate bromo-deoxy-uridine, and to undergo apoptosis in vivo in WT-transplanted compared with IFN-gammaKO-transplanted SCID mice. These data point towards an important role for IFN-gamma in the development of IBD in SCID mice. The inflammation might be initiated and subsequently enhanced by the ability of IFN-gamma to induce de novo MHC class II expression in the colonic epithelium, a change which could lead to increased antigen processing and production of local proinflammatory cytokines, CD4+ T cell turnover and thereby to exaggeration of disease.