Allospecific regulatory T cells suppress naive alloreactive T cells and do not confer bystander suppression. Regulatory T cells were generated in a Jagged-1 MLC. (A) Irradiated regulatory T cells (1 to 10 × 10⁴) were added to an MLC containing 5 × 10⁴ responder naive T cells and 1.25 × 10³ irradiated stimulator EBV-LCLs. The proliferative responses were determined from ³H incorporation at day 5 of culture. (B) Specific inhibition of target cell lysis was evaluated at day 7 of secondary MLC using a 2:1 ratio between naive and regulatory T cells (gray bar) and compared with control cultures (containing irradiated T cells cultured with nontransduced LCLs) (black bar). The stimulator cells were either the original stimulator EBV-LCLs (used in the primary MLC), third-party EBV-LCLs (completely HLA-mismatched with the original cell stimulator), or autologous EBV-LCLs. Results are expressed as mean ± SD of triplicate measurements.