Monocyte-independent stimulation of human B lymphocytes by phorbol myristate acetate.

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Abstract
Phorbol myristate acetate (PMA) induces a low level of proliferation in purified human B lymphocytes when added in nanomolar concentrations to the culture medium. Much higher levels of thymidine incorporation, however, are obtained in the presence of other B cell stimuli such as anti-IgM antibodies or Staphylococcus aureus Cowan Strain 1 (SAC). The peak activity of PMA was observed on day 3 of B cell cultures containing either anti-IgM or SAC. The rigorous depletion of monocytes as well as add-back experiments indicate that the effect of PMA on anti-IgM-stimulated B cells is not mediated by the stimulation of accessory cells. Thus, PMA acts as a very potent mitogenic agent for human B cells under culture conditions that are commonly used to assess B cell growth factor activity.