Synergy between two cytokines in promoting DNA synthesis by GC B cells in the presence of CD32-L cells and CD40 MoAb. GC B cells were cultured for 5 days with CD40 MoAb, G28-5 in the presence of CD32-L cells and cytokines or culture medium alone (CM) and then [³H]thymidine uptake was measured. Each cytokine employed in Fig 2 was assessed alone and with a second cytokine from the same panel. Expected [³H]thymidine uptake for cells cultured with pairs of cytokines was calculated from the sum of the responses to each cytokine when used alone. For the cytokine combinations shown, the observed responses (▪) were significantly greater than the expected responses (□) in each of three experiments. Cytokines were used at the following concentrations: IL-1β (0.1 ng/mL), IL-3 (10 ng/mL), IL-4 (20 ng/mL), IL-7 (200 U/mL), IL-10 (5 ng/mL). Data are the mean cpm ± SEM for these three experiments. For further details see Results.