Fas/Fas ligand expression and characteristics of primed CD45RO(+) T cells in the inflamed mucosa of ulcerative colitis

Background: Chronic immune activation in the colon is characteristic of ulc erative colitis (UC). Fas/Fas ligand (FasL) system is a mechanism responsib le for activation-induced cell death (AICD), which maintains homeostasis wi thin the immune system. Thus, Fas/FasL expression on activated colonic T ce lls of UC patients, as well as the susceptibility of such T cells to AICD w as investigated in order to determine the rule of activated colonic T cells in the long lasting inflammation in UC. Methods: Fas, FasL, and CD45RO exp ression on peripheral blood and colonic T cells of UC patients were assayed by flow cytometry. Apoptosis of colonic T cells induced by anti Fas antibo dy was assessed using the terminal deoxynucleotidyl transferase-mediated dU TP-biotin nick end-labeling (TUNEL) assay. Results: The majority of colonic T cells expressed both CD45RO and Fas in the colonic mucosa, a situation t hat was quire different from that in the peripheral blood. The number of CD 45RO(+)CD8(+) and Fas(+)CD8(+) T cells was significantly lower in Ut patien ts than the controls, unlike the number of Fas(+)CD4(+) T cells. In contras t, the number of both CD45RO(+)CD4(+) and CD45RO(+)CD8(+) T cells in UC muc osa expressing FasL was significantly higher than in the controls. While Fa s mediated apoptosis of CD45RO(+)CD8(+) T cells was higher in UC patients t han the controls, the number of apoptotic CD45RO(+)CD4(+) T cells from UC m ucosa was not. Conclusions: In UC patients, CD45RO(+)CD4(+) T cells are les s sensitive to apoptotic signals mediated by Fas. These phenomena may contr ibute to the pathogenesis of UC.