Engagement of HLA class I α1 domain inhibits DNA synthesis of CD40-activated total, naı̈ve, GC, and memory B cells in response to B-cell growth factors. Tonsil B cells (2 × 10⁴/well) from each subset were cultured on CD32-L cells (5 × 10³/well) in the presence of 1 μg/mL anti-CD40 MoAb with either IL-10 (20 ng/mL), IL-4 (50 U/mL), IL-2 (20 U/mL) plus IL-10 or IL-4 plus IL-10, and with control IgG1 or MoAb90 at 1 μg/mL added at the onset of the culture. Proliferation was measured by [³H]TdR incorporation after a pulse with [³H]TdR during the last 16 hours of the culture at the time indicated. Results are expressed as means ± SD of culture triplicates. [³H]TdR uptake of CD32-L cells was less than 500 cpm in the different experiments. This figure is representative of three independent experiments.