Membrane alkaline phosphatase activity: an enzymatic marker of B-cell activation.

The expression of membrane alkaline phosphatase (mAlPase) activity was studied on viable cells from mouse lymphoid organs. The low mAlPase activity level of ex vivo mouse spleen cells was markedly increased by in vitro culture in the presence of the direct B-cell mitogen lipopolysaccharide (LPS). This increase occurred nearly simultaneously with increased uptake of 3H-thymidine and an increased percentage of blasts in the culture. The T-cell-dependent B-cell pokeweed mitogen did not increase the mean level of mAlPase activity per cell, although there was an increase per culture. The T cell mitogen ConA did not cause an increase in mAlPase activity, although it was able to stimulate both cell proliferation and blast transformation. Several other mitogens and differentiating agents were tested, but did not detectably affect mAlPase expression. LPS high responder mouse strains C57BL/6 and CBA/J showed a higher LPS-induced mAlPase expression response to LPS than did LPS low responder strains BALB/c or CBA/N. These data suggest a preferential expression of mAlPase by stimulated cycling B cells. However, mAlPase expression appeared restricted to a subpopulation of cycling B cells and could not be elicited by every B-cell stimulus.