Suppression is cell-contact dependent but cytokine independent. (A) CD4⁺CD25^– T cells were stimulated by CD3/CD28 Dynal beads in the presence or absence of equal numbers of CD4⁺CD25⁺ T cells in the same chamber, or CD4⁺CD25^– and CD4⁺CD25⁺ T cells were stimulated separately in the bottom and top chambers of transwells by CD3/CD28 Dynal beads. After 20 hours of stimulation, IL-2 production in the culture was measured by CTLL-2 cell proliferation. (B) CD4⁺CD25^– T cells were stimulated by HLA-DR7 homozygous DCs pulsed with the A2 (138-170) peptide in the presence of CD4⁺CD25⁺ T cells at a 1:1 ratio. Antibodies (10 μg/mL) were added at the initiation of culture. After 20 hours of stimulation, IL-2 production in the culture was measured by CTLL-2 cell proliferation. Each bar shows the mean cpm ± SD. Results are representative of 2 independent experiments.