Antigen-induced arthritis in rats is suppressed by the inducing antigen administered orally before, but not after immunization.

The effects of oral administration of methylated bovine serum albumin (mBSA) on antigen-induced arthritis (AIA) in rats that was induced by immunization with mBSA followed by the intraarticular injection of the same antigen were investigated. When mBSA was orally given to animals before immunization with the antigen, the development of AIA was markedly suppressed. In contrast, feeding mBSA after immunization or after intraarticular challenge was not effective in affecting the progression of arthritis. Delayed-type hypersensitivity and in vitro T cell proliferative responses to mBSA were also significantly downregulated in rats fed the antigen before, but not after immunization. These results indicate that the suppression of AIA by mBSA given before immunization appears to be due to the induction of tolerance to the oral antigen. No influence of mBSA administered orally after immunization on AIA as well as delayed-type hypersensitivity and proliferative responses to the antigen suggests that animals resist to be tolerant against an oral antigen when immune responses to the antigen already exist.