IL-10 blocks proliferation of CD4⁺ T cells. (A) CD4⁺ T cells were isolated from PBMCs by negative selection chromatography and were added to 96-well plates at 2.5 × 10⁶ cells/mL along with IL-10 at 10 U/mL or media only. After incubating for 2 hours, SEB at varying concentrations was added and plates were incubated for 4 days. Proliferation was measured by [³H]thymidine incorporation. Data are expressed as the mean CPM ± SD. The percentage of suppression by IL-10 and statistical significance, as determined by the Student’s t-test, for each treatment are indicated above the graphs. (B) CD4⁺ T cells were incubated as indicated above and stimulated with a mixture of ionomycin (100 ng/mL) and varying concentrations of PMA. Proliferation was measured by [³H]thymidine incorporation. Data are expressed as the mean CPM ± SD. The percentage of suppression by IL-10 and statistical significance, as determined by the Student’s t-test, for each treatment are indicated above the graphs. In both (A) and (B), the proliferation of CD4⁺ T cells incubated with PHA at 5 μg/mL is compared with that of whole PBMCs incubated with 3 μg/mL.