OBJECTIVE: Chebulagic acid (CHE) from the immature seeds of Terminalia chebula was identified from a natural product library as a potent suppressor of T cell activity. This study examined the effectiveness of CHE against the onset and progression of collagen-induced arthritis (CIA) in mice. METHODS: Arthritis was induced in DBA/1J mice by subcutaneous immunization with bovine type II collagen on days 0 and 21. CHE was administered intraperitoneally for 3 weeks, either as prophylaxis (10 or 20 mg/kg) before disease onset or as therapy (20 mg/kg) after disease onset. Clinical scores, serum antibody levels, and cytokines were measured, and flow cytometric analysis and real-time reverse transcription-polymerase chain reaction were performed to evaluate the knee joints of mice with CIA. RESULTS: In both the prophylactic and therapeutic CHE dosing models, all clinical scores, serum levels of total and anticollagen IgG, and levels of interleukin-10 (IL-10) and IL-6 were reduced, while serum levels of transforming growth factor beta (TGFbeta) were markedly elevated. The number of granulocytes was reduced, but the proportion of CD4+,CD25+ T cells was greater in the knee joints of CHE-treated CIA mice. Expression of Foxp3 and TGFbeta messenger RNA was also augmented significantly in the knee joints of CHE-treated CIA mice in the therapeutic dosing model. CONCLUSION: CHE significantly suppressed the onset and progression of CIA in mice. Immune suppression via the induction of TGFbeta and CD4+,CD25+ T cells may represent a new strategy in the development of therapies for managing rheumatoid arthritis and other inflammatory diseases.