A Combination of Calcium Ionophore and 12‐O‐Tetradecanoyl‐Phorbol‐13‐Acetate (TPA) Stimulates the Growth of Purified Resting B Cells

In this study we investigated whether the calcium ionophores A23187 and ionomycin can act synergislically with the phorbol ester 12‐O‐tetradecanoyl‐phorbol‐13‐acetate (TPA) to stimulate the growth of resting B lymphocytes purified from human tonsil cells, Ionomycin, A23187, and TPA added separately to cultures at doses of 0.4–1.6 μg/ml, 0.2–0.8 μ/ml, and 0.05–0.25 ng/ml respectively, did not induce DNA synthesis in resting B lymphocytes. In contrast, calcium ionophores at concentrations of 0.4–1.6 μg/ml ionomycin and 0.2–0.8 μg/ml A23187, in the presence of 0.05–4 ng/ml TPA, induced marked DNA synthesis and B‐cell proliferation, as shown by analyses of incorporation of [³H]thymidine, growth kinetics, and the percentage of cells in the S and G₂+ M phases of the cell cycle. These results show that the synergistic effects of calcium ionophores and TPA can bypass the requirement for antigen and exogenous growth factors in B‐cell activation. These observations are similar to those obtained from studies of T lymphocytes by other workers.