The CD4⁺CD25⁺ T cells effect “linked” or “unlinked” suppression. (A-B) Freshly isolated CD4⁺CD25^– T cells were stimulated by semiallogeneic HLA-DR10, DR16 DCs or HLA-DR7, DR12 DCs in the presence of equal numbers of CD4⁺CD25⁺ T cells with or without the A2 (137-180) allopeptide. Supernatants were harvested after 5 days of culture and IL-2 secretion by CD4⁺CD25^– T cells was measured by CTLL-2 cell proliferation. (C) Freshly isolated CD4⁺ T cells were used as responders instead of CD4⁺CD25^– T cells as in panels A-B, and different numbers of CD4⁺CD25⁺ T cells were added at the initiation of the culture with (▪) or without (□) the A2 peptide. Each bar shows the mean cpm ± SD. Results are representative of at least 3 independent experiments.