Ulcerative colitis (UC) is an inflammatory disease of the colon. IL1R2, which encodes IL-1 receptor type 2 (IL-1R2), was reported as a risk gene for UC. To elucidate the roles of IL-1R2 in the development of colitis, we examined the development of dextran sodium sulfate-induced colitis, a mouse model for UC using Il1r2(-/-) mice. We found the severity score of colitis was milder in Il1r2(-/-) mice compared with wild-type (WT) mice when they were housed separately, however the severity score was similar when they were housed in a cage. In the separate housing condition, relative contents of Actinobacteria and Bacilli in feces of Il1r2(-/-) mice were lower than that of WT mice. Furthermore, IL-1 beta induced the expression of antimicrobial peptides (AMPs) from colon. Thus, we show that IL-1R2 is harmful for the development of colitis, because IL-1R2 promotes the growth of proinflammatory intestinal microbiota by suppressing IL-1 beta-induced AMP production. (C) 2018 Elsevier Inc. All rights reserved.