Phorbol ester activates CD5+ leukaemic B cells via a T cell-independent mechanism.

B cell chronic lymphocytic leukaemia (B-CLL) is characterized by the proliferation and accumulation of sIgM+ CD5+ lymphocytes that fail to progress to the final stages of B cell development. Stimulation of unfractionated PBL from three patients with B-CLL with the phorbol ester PMA and the mitogens PHA and PWM resulted in significant increases in cell proliferation, RNA synthesis and IgM secretion when compared to unstimulated cell populations. PMA was the most potent inducer of IgM secretion and this occurred irrespective of the presence of residual T cells. PMA-induced proliferation and RNA synthesis was also independent of T cells. However, in the presence of T cells, these parameters of cellular activation were enhanced during in vitro culture. Thus, the inductive ability of PMA on CD5+ CLL B cells was independent of T cells. In contrast, activation and differentiation of the malignant CD5+ B cells into IgM-secreting cells following culture with mitogens did not occur in the absence of T cells.