Alkaline phosphatase activity is expressed only in B lymphocytes committed to proliferation.

Alkaline phosphatase (APase) activity was measured in murine splenic lymphocytes stimulated with the T lymphocyte mitogens phytohemagglutinin (PHA) and Concanavalin A (Con A) and the B lymphocyte mitogens lipopolysaccharide (LPS) and anti-immunoglobulin (anti-Ig). APase activity was found to be enhanced specifically in mitogen-stimulated B lymphocytes, but not in T lymphocytes. This enhancement starts around 8 h after stimulation with a mitogen. With soluble anti-Ig it was observed that the B cells enter G1 phase as assessed by RNA synthesis and blast transformation. However, these cells fail to synthesize DNA and also do not show any increase in APase activity. When the same anti-Ig coupled to Sepharose was used as a stimulator, cells synthesized DNA and also showed significant increase in APase activity. When hydroxyurea was added, the enhancement in APase activity by the mitogen was not diminished although the cells failed to synthesize DNA. These observations indicate that APase activity is enhanced only in activated B cells committed to proliferation.