TGF-β-mediated suppression by CD4+CD25+ T cells is facilitated by CTLA-4 signaling

CD4(+)CD25(+) T cells play a pivotal role in immunological homeostasis by their capacity to exert immunosuppressive activity. However, the mechanism by which these cells function is still a subject for debate. We previously reported that surface (membrane) TGF-beta produced by CD4(+)CD25(+) T cells was an effector molecule mediating suppressor function. We now support this finding by imaging surface TGF-beta on Foxp3(+)CD4(+)CD25(+) T cells in confocal fluorescence microscopy. Then, using a TGF-beta-sensitive mink lung epithelial cell (luciferase) reporter system, we show that surface TGF-beta can be activated to signal upon cell-cell contact. Moreover, if such TGF-beta signaling is blocked in an in vitro assay of CD4(+)CD25(+) T cell suppression by a specific inhibitor of TGF-beta RI, suppressor function is also blocked. Finally, we address the role of CTLA-4 in CD4(+)CD25(+) T cell suppression, showing first that whereas anti-CTLA-4 does not block in vitro suppressor function, it does complement the blocking activity of anti-TGF-beta. We then show with confocal fluorescence microscopy that incubation of CD4(+)CD25(+) T cells with anti-CTLA-4- and rB7-1/Fc-coated beads results in accumulation of TGF-beta at the cell-bead contact site. This suggests that CTLA-4 signaling facilitates TGF-beta-mediated suppression by intensifying the TGF-beta signal at the point of suppressor cell-target cell interaction.