TGF-β requires CTLA-4 early after T cell activation to induce foxP3 and generate adaptive CD4⁺CD25⁺ regulatory cells

Although positive CD28 costimulation is needed for the generation of natural CD4⁺CD25⁺ regulatory T cells, we report that negative CTLA-4 costimulation is necessary for generating phenotypically and functionally similar adaptive CD4⁺CD25⁺ suppressor cells. TGF-β could not induce CD4⁺CD25^- cells from CTLA-4^-/- mice to express normal levels of FoxP3 or to develop suppressor activity. Moreover, blockade of CTLA-4 following activation of wild-type CD4⁺ cells abolished the ability of TGF-β to induce FoxP3-expressing mouse suppressor cells. TGF-β accelerated expression of CTLA-4, and time course studies suggested that CTLA-4 ligation of CD80 shortly after T cell activation enables TGF-β to induce CD4⁺CD25 ^- cells to express FoxP3 and develop suppressor activity. TGF-β also enhanced CD4⁺ cell expression of CD80. Thus, CTLA-4 has an essential role in the generation of acquired CD4⁺CD25⁺ suppressor cells in addition to its other inhibitory effects. Although natural CD4⁺CD25⁺ cells develop normally in CTLA-4^-/- mice, the lack of TGF-β-induced, peripheral CD4⁺CD25⁺ suppressor cells in these mice may contribute to their rapid demise.