Late events in B cell activation. Expression of membrane alkaline phosphatase activity.

Alkaline phosphatase (APase) has been previously described as a membrane marker correlating with B cell proliferation after stimulation by selected B cell mitogens. We have found, however, that the appearance of B cell membrane APase correlates more closely with differentiation than with proliferation. This conclusion has been drawn from the following observations: 1) APase activity appears well after peak B cell thymidine uptake, 2) mitogens which stimulate only B cell proliferation (Salmonella typhimurium mitogen) fail to induce expression of the enzyme, and 3) when proliferation of mitogen-activated B cells is inhibited, APase activity is not suppressed and may even be augmented. In addition to membrane expression, APase is also spontaneously shed into the surrounding milieu, perhaps as a result of endogenous phospholipase activity. By using a group of well-characterized inhibitors, the APase activity was shown to belong to class I (similar to the bone/liver/kidney class). Because APase always appears in differentiating but not proliferating cells, we would propose that the enzyme appearance is a late marker of B cell activation, associated with cell progression to differentiation and consequent IgM synthesis.