Induction of human B cell proliferation and differentiation by the combination of phorbol ester and ionomycin

Phorbol esters and Ca²⁺ ionophores are known to mimic intracellular messengers involved in cell activation. We studied the effects of 12-O-tetradecanoylphorbol 13-acetate (TPA) and ionomycin on tonsil and peripheral blood-derived B lymphocytes. We show that TPA and ionomycin are co-mitogenic and induce B lymphocyte differentiation. Although TPA in high concentrations is mitogenic to B lymphocytes by itself, submitogenic concentrations of TPA in combination with ionomycin trigger 50% of B lymphocytes to synthesize DNA. Stimulation of B lymphocytes with TPA plus ionomycin resulted in increased magnitude and a shift in the kinetics of c-fos and c-myc expression compared with either agent used alone. Activation markers such as the transferrin and interleukin 2 (IL 2) receptors were markedly increased after 24 h incubation with TPA and ionomycin. In parallel to the rapid proliferative burst, we observed evidence for B lymphocyte differentiation with an increase in the number of cells expressing cytoplasmic immunoglobulin (Ig) and the disappearance of the B1 surface marker. Since the cells remained surface Ig⁺ and secreted only small quantities of Ig, our results suggest that the combination of TPA and ionomycin is a potent inducer of B cell proliferation and early differentiation; terminal differentiation to an Ig-secreting state, however, is not achieved.