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 naïve 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 naïve 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 (≈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⁺RORγt⁺ 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-β expression in CD4⁺Foxp3⁺ population (48.1% vs. 18.9% in untreated Ova-immunized group) and reduced IFN-γ-producing CD4⁺T-bet⁺ T cells and IL-17-producing CD4⁺RORγt⁺ T cells were detected. This led to marked decreased ratios of IFNγ/TGF-β and IL-17/TGF-β 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.