Thymol as a reciprocal regulator of T cell differentiation: Promotion of regulatory T cells and suppression of Th1/Th17 cells

Regulatory T cells (Tregs) are critical for maintaining immune response and enhancing their differentiation has therapeutic implications for autoimmune diseases. In this study, we investigated the effects of thymol a well-known monoterpene from Thyme on differentiation and function of Tregs. In vitro generation of Tregs from purified naive CD4(+)CD25(-) T cells in the presence of thymol was carried out. Suppressor activity of generated Tregs was examined by changes in the proliferation of CFSE-labeled conventional T cells. Thymol promotes differentiation of naive CD4(+)CD25(-) T cells to CD4(+)CD25(+)Foxp3(+) Tregs [66.9-71.8% vs. control (47%)] and increased intensity of Foxp3 expression on Tregs (p < 0.01). In functional assay, an increased immune suppression by thymol-induced Tregs (approximate to 2.5 times of untreated Tregs) was detected. For in vivo study, thymol was intraperitoneally administered to ovalbumin (Ova)-immunized mice. Flow cytometry assessment of spleens from thymol-treated Ova-immunized mice showed increased number of CD4(+) Foxp3(+) Tregs (> 8%, p < 0.01(and decreased levels of CD4(+)T-bet(+) Th1 and CD4(+)ROR gamma t(+) Th17 cells resulted in significant decreased Th1/Treg and Th17/Treg ratios. In ex vivo Ova challenge of splenocytes from thymol-treated Ova -immunized mice, similarly higher levels of CD4(+) Foxp3(+) Tregs, and also elevated TGF-beta expression in CD4(+) Foxp3(+) population (48.1% vs. 18.9% in untreated Ova-immunized group) and reduced IFN-gamma-producing CD4(+)T-bet(+) T cells and IL-17-producing CD4(+)ROR gamma t(+) T cells were detected. This led to marked decreased ratios of IFN gamma/TGF-beta and IL-17/TGF-beta expressions. In conclusion, this study revealed thymol as a compound with enhancing effects on Treg differentiation and function, which may have potential benefits in treatment of immune-mediated diseases with Th1/Th17 over-activation.