Although some bacterial strains show potential to prevent colitis, their mechanisms are not fully understood. Here, we investigated the anti-colitic mechanisms of Bifidobacterium longum subsp. infantis JCM 1222(T), focusing on the relationship between interleukin (IL)-17A secreting CD4(+) T cells and intestinal epithelial costimulatory molecules in mice. Oral administration of JCM 1222(T) to mice alleviated dextran sulfate sodium (DSS)-induced acute colitis. The expression of type 1 helper T (Th1)- and IL-17 producing helper T (Th17)-specific cytokines and transcriptional factors was suppressed by JCM 1222(T) treatment. Intestinal epithelial cells (IECs) from colitic mice induced IL-17A production from CD4(+) T cells in a cell-cell contact-dependent manner, and this was suppressed by oral treatment with JCM 1222(T). Using blocking antibodies for costimulatory molecules, we revealed that epithelial costimulatory molecules including CD80 and CD40, which were highly expressed in IECs from colitic mice, were involved in IEC-induced IL-17A response. Treatment of mice and intestinal epithelial cell line Colon-26 cells with JCM 1222(T) decreased the expression of CD80 and CD40. Collectively, these data indicate that JCM 1222(T) negatively regulate epithelial costimulatory molecules, and this effect might be attributed, at least in part, to suppression of IL-17A in DSS-induced colitis.