Background: We previously demonstrated that in inflammatory bowel disease (IBD) there is enhanced production of interleukin (IL)-21, a cytokine that activates multiple pathways that sustain mucosal inflammation. However, the phenotype of IL-21-producing cells in IBD, and the cytokine(s) they coproduce, is not known. We here characterized the cell source of IL-21 and determined which factors regulate IL-21 in the human gut.
Methods: Cytokines were analyzed in CD4+ T intestinal lamina propria lymphocytes (T-LPL) isolated from IBD patients and controls by flow cytometry. Moreover, IL-21 was evaluated in mucosal T follicular cells (TFH). To assess the involvement of IL-12 and IL-23 in the production of IL-21, T-LPL were activated in the presence or absence of IL-12 or IL-23.
Results: The proportion of IL-21-producing CD4+ T-LPL was increased in IBD compared to controls. The majority of IL-21-producing T-LPL coexpressed interferon (IFN)-gamma, and to a lesser extent IL-4 or IL-17A. Activation of CD4+ T-LPL with IL-12 but not IL-23 enhanced the fraction of cells coexpressing IL-21 and IFN-gamma. TFH cells in LPL were identified by CXCR5 expression and expressed IL-21 both in IBD and controls; however, the fraction of IL-21-positive TFH cells was higher in Crohn's disease than in ulcerative colitis and controls. Treatment of CD4+ T-LPL with IL-12 enhanced the frequency of CXCR5+ IL-21-producing TFH cells.
Conclusions: These findings indicate that in IBD IL-21 is mostly produced by CD4+ T-LPL coexpressing IFN-gamma, reinforcing the concept that distinct subsets of T cells can produce IL-21.