Retargeted class I MHC-restricted Tregs suppress antigen-specific T-cell proliferation. Tregs were transduced with an HLA-A2–restricted TCR specific for NY-ESO-1 and effector T cells (Teffs) with an A2-restricted TCR specific for HIV-1gag. The Teff cells were labeled with CFSE, a dye that intercalates into the cell membrane and is diluted with cell division. The top 2 tracings demonstrate the induction of cell division when Teffs are stimulated with antigen. The bottom tracings demonstrate that in the presence of the NY-ESO-1–specific Tregs, Teff proliferation is inhibited. The Tregs must be stimulated with specific antigen to be able to suppress the Teffs.