707 Immunoregulatory Role of Increased PD-L1 Expression By Colonic Myofibroblasts/Fibroblasts in Ulcerative Colitis

Abstract

BACKGROUND: The B7 co-stimulator PD-L1 is critical in suppression of activated T cell responses in peripheral tolerance. However, overexpression of PD-L1 may contribute to progression of chronic inflammatory diseases. The role of PD-L1 and the phenotype of PDL1 expressing cells in inflammatory bowel diseases is unclear. Human colonic myofibroblasts and fibroblasts (CMFs) are major PD-L1 expressing cells in normal colonic mucosa and may suppress activated CD4+T cell proliferation and production of the Th1 acute inflammatory cytokine IFN-γ in a PD-L1 dependent manner. We noted a significant increase in PD-L1 surface expression on CMFs isolated from ulcerative colitis (UC). Thus, we hypothesize that overexpression of PD-L1 by CMFs may have a critical role in the chronic inflammatory responses during UC pathogenesis. METHODS: PD-L1 expression in colonic tissue from normal (N), UC and Crohn's Disease (CD) patients was determined by immunostaining followed by confocal microscopy. PD-L1, Tbet and GATA3 expression was analyzed by realtime RT-PCR, Western blot and flow cytometry. Lymphoproliferation assays and ELISA were used to evaluate the role of PD-L1 in the regulation of Th cell activity. RESULTS: Confocal microscopy analysis revealed strong upregulation of PD-L1 in inflamed colonic mucosa from UC when compared to CD or normal controls. The PD-L1 upregulation in UC colon was mostly associated with cells bearing fibroblast/myofibroblast phenotype (CD90+). PD-L1 on CMFs isolated from UC (UC-CMFs) was functional: UC-CMFs were capable of suppressing CD3/CD28-activated Th cell proliferation in a PD-L1 dependent manner. Both Nand UCCMFs inhibited expression of a Th1 transcriptional factor, Tbet, in Th cells. PD-L1 blocking led to increased expression of Tbet in Th cells, but did not affect expression of Th2 transcriptional factor GATA3. Treatment of N-CMFs with IFN-γ (10-1000 U/mL) for 7 days resulted in a dose dependent upregulation of PD-L1. Treatment of N-CMFs with IL-13 (25 ng/mL), the major cytokine associated with the UC atypical Th2 response, also upregulate PD-L1. Our data suggest that increased secretion of IFN-γ by activated T cells during acute intestinal inflammation may contribute to an increase in PD-L1 expression on CMFs resulting in the negative feedback on the Th1 responses, in turn, promoting IL-13 production by Th2 and NK(T) cells, which will further upregulate PD-L1 expression on CMFs. CONCLUSIONS: Overexpression of PD-L1 on CMFs in the UC colonic mucosa may play an important immunoregulatory role promoting the chronicity of the Th2 inflammatory responses during progression of UC. Supported by NIDDK, CCFA and Sealy foundation.