CD4⁺CD25⁺ Regulatory T Cells Can Mediate Suppressor Function in the Absence of Transforming Growth Factor β1 Production and Responsiveness

CD4⁺CD25⁺ regulatory T cells inhibit organ-specific autoimmune diseases induced by CD4⁺CD25⁻T cells and are potent suppressors of T cell activation in vitro. Their mechanism of suppression remains unknown, but most in vitro studies suggest that it is cell contact–dependent and cytokine independent. The role of TGF-β1 in CD4⁺CD25⁺ suppressor function remains unclear. While most studies have failed to reverse suppression with anti–transforming growth factor (TGF)-β1 in vitro, one recent study has reported that CD4⁺CD25⁺ T cells express cell surface TGF-β1 and that suppression can be completely abrogated by high concentrations of anti–TGF-β suggesting that cell-associated TGF-β1 was the primary effector of CD4⁺CD25⁺-mediated suppression. Here, we have reevaluated the role of TGF-β1 in CD4⁺CD25⁺-mediated suppression. Neutralization of TGF-β1 with either monoclonal antibody (mAb) or soluble TGF-βRII-Fc did not reverse in vitro suppression mediated by resting or activated CD4⁺CD25⁺ T cells. Responder T cells from Smad3^−/− or dominant-negative TGF-β type RII transgenic (DNRIITg) mice, that are both unresponsive to TGF-β1–induced growth arrest, were as susceptible to CD4⁺CD25⁺-mediated suppression as T cells from wild-type mice. Furthermore, CD4⁺CD25⁺ T cells from neonatal TGF-β1^−/− mice were as suppressive as CD4⁺CD25⁺ from TGF-β1^+/+ mice. Collectively, these results demonstrate that CD4⁺CD25⁺ suppressor function can occur independently of TGF-β1.