ObjectivesThe complement system has been proposed to play a significant role in the regulation of T-cell responses. However, the precise mechanism underlying C4-induced immune tolerance remains to be clarified. We recently reported that monomeric C4b inhibits CXCL10 production from blood cells. The purpose of this study was to verify the active site of monomeric C4b.Materials and methodsWe investigated the in vitro effects of a C4b-derived peptide (VPAGSARPVAFSVVPTAAA), named HP2 (highly homologous peptide 2), on the IFN-β-induced production of CXCL10 in human blood and the in vivo effects of the administration of HP2 on Th1/2 cytokine production in the spleen in mice. We also tested whether the administration of HP2 influences symptoms of experimentally induced ulcerative colitis in mice.ResultsHP2 inhibited CXCL10 production in human blood, and the administration of HP2 significantly suppressed the production of Th1 cytokines, such as IL-2, IFN-γ, and TNF-α, in spleen cells isolated from mice. The administration of HP2 in the mice significantly improved the symptoms of colitis, with down-regulation of colitogenic CD4+CD45RBhigh T cells and up-regulation of CD4+LAP/TGF-β1+ T cells.ConclusionThe amino acid sequence described above is suggested to be the active site in C4b for the inhibition of Th1 cytokine production. These results should contribute to the development of new drugs suppressing autoimmune responses.