MoAb90 inhibits CD40-dependent B-cell proliferation. Tonsil B cells (2 × 10⁴/well) were cultured on irradiated CD32-transfected L cells (5 × 10³/well) in the presence of 1 μg/mL anti-CD40 MoAb and (A) with increasing concentrations of MoAb90 or W6/32 added at the onset of the culture; (B) with 5 μg/mL of control IgG1, W6/32 or MoAb90 added on day 0 or 4. Proliferation was measured by [³H]TdR incorporation after a pulse with [³H]TdR during the last 16 hours of the culture on day 6 (A) or at the time indicated (B), and results are expressed as mean ± standard deviation (SD) of triplicate determinations. In each experiment, background of [³H]TdR uptake of CD32-L cells was less than 500 counts per minute (cpm). Arrow in B indicates time of MoAb90 addition. Data presented are representative of three independent experiments. (⋄) indicates IgG1 control, (○) indicates W6/32, and (•) indicates MoAb90. (C) Tonsil B cells vere cultured for 5 days on CD32-L cells in the presence of 1 μg/mL anti-CD40 MoAb with 1 μg/mL of W6/32 or MoAb90. Photos of culture wells (original magnitude ×100) show the formation of B-cell clumps induced by proliferation.