Regulatory T cells expanded by rapamycin in vitro suppress colitis in an experimental mouse model

Background: To provide rapid immunosuppression without side effects, we analyzed whether rapamycin alone, and regulatory T cells (Tregs) expanded ex vivo by rapamycin, suppressed colitis in a mouse model. Methods: Severe combined immunodeficiency (SCID) mice reconstituted with naive CD4⁺ T cells were treated with or without intraperitoneal rapamycin. Body weight was evaluated. CD4⁺ T cells were cultured inthe presence of rapamycin for three 7-day rounds of stimulation. The ratio of Tregs to CD4⁺T cells was analyzed by flow cytometry. NaiveCD4⁺T cells were transferred into SCID mice with CD4⁺ T cells expanded in the presence or absence of rapamycin. Clinical symptoms of colitis, histological changes, and cytokine expression were investigated. Results: Systemic rapamycin partially prevented the development of colonic inflammation in a transfer model of colitis, but decreased body weight in control mice. With rapamycin, stimulated CD4 ⁺T cells expanded eightfold in 3 weeks in vitro, and the proportion of Tregs increased to about 40%. Without rapamycin, CD4⁺T cells expanded 20-fold in 3 weeks, but the proportion of Tregs remained at about 15%. CD4⁺ T cells expanded with rapamycin prevented the development of colitis in a naïve CD4⁺ T-cell transfer model, in association with the down regulation of Th1 and Th17 responses. Conclusions: We demonstrated, for the first time, that CD4⁺ T cells expanded with rapamycin in vitro suppressed colitis. Therefore, rapamycin-expanded Treg transfer therapy is expected to be efficacious for inflammatory bowel disease.