Stimulation of tyrosine phosphorylation without inositol lipid hydrolysis in human B lymphocytes on engaging CD72

Occupancy of CD72 on resting tonsillar B cells by monoclonal antibodies (mAb) promotes entry into the G₁ phase of the cell cycle with an accompanying increase in MHC Class II expression and provides a co-stimulus to immobilized anti-μ for driving DNA synthesis. We now report that engagement of CD72 by mAb stimulates tyrosine phosphorylation in B cells with a peak of activity seen at 5-10 min. Two major substrates of 29 and 57 kDa showed a basal level of phosphorylation which increased with time, while a 40 kDa protein and several other minor components were phosphorylated de novo on the addition of mAb to CD72. Inositol lipid hydrolysis was found to be unperturbed, although a shallow rise in the basal level of intracellular free Ca²⁺ was provoked on engaging CD72. Receptor cross-linking was not a requirement for signaling human B cells through CD72: simple occupancy by univalent antibody was sufficient both to trigger the rise in basal [Ca²⁺]_i and to promote DNA synthesis.