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 ulcerative colitis (UC). Fas/Fas ligand (FasL) system is a mechanism responsible for activation-induced cell death (AICD), which maintains homeostasis within the immune system. Thus, Fas/FasL expression on activated colonic T cells of UC patients, as well as the susceptibility of such T cells to AICD was investigated in order to determine the role of activated colonic T cells in the long lasting inflammation in UC. Methods: Fas, FasL, and CD45RO expression on peripheral blood and colonic T cells of UC patients were assayed by flow cytometry. Apoptosis of colonic T cells induced by anti Fas antibody was assessed using the terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end-labeling (TUNEL) assay. Results: The majority of colonic T cells expressed both CD45RO and Fas in the colonic mucosa, a situation that was quite different from that in the peripheral blood. The number of CD45RO⁺CD8⁺ and Fas⁺CD8⁺ T cells was significantly lower in UC patients than the controls, unlike the number of Fas⁺CD4⁺ T cells. In contrast, the number of both CD45RO⁺CD4⁺ and CD45RO⁺CD8⁺ T cells in UC mucosa expressing FasL was significantly higher than in the controls. While Fas mediated apoptosis of CD45RO⁺CD8⁺ T cells was higher in UC patients than the controls, the number of apoptotic CD45RO⁺CD4⁺ T cells from UC mucosa was not. Conclusions: In UC patients, CD45RO⁺CD4⁺ T cells are less sensitive to apoptotic signals mediated by Fas. These phenomena may contribute to the pathogenesis of UC.