Antibody to a novel 95-kDa surface glycoprotein on human B cells induces calcium mobilization and B cell activation

These findings characterize a 95-kDa glycoprotein on the surface of B lymphocytes recognized by the mAb G28-8. This protein (designated Bgp95), previously classified as a CD39 molecule, is unique based on functional, cell distribution, and immunochemical criteria. Biochemical analyses revealed that Bgp95 is a 95-kDa glycoprotein with N-linked carbohydrate and is reduced to about 70-kDa after treatment with endoglycopeptidase F. In functional studies, stimulation by G28-8 mAb or its F(ab')2 fragments induced a G0 to G1 cell cycle transition and was synergistic with PMA, anti-mu, or anti-CDw40 in stimulating proliferation of resting B cells. G28-8 mAb also could induce increases of cytoplasmic free calcium concentration [Ca2+]i in a subpopulation of tonsillar or peripheral blood B cells. The G28-8 mAb alone induced a steady increase in [Ca2+]i detectable even 1 h after stimulation. Cross-linking the G28-8 mAb with a second mAb specific for murine kappa light chains induced a more rapid increase of [Ca2+]i which peaked at 10 to 20 min and then declined. At 1 h after stimulation, [Ca2+]i was higher in B cells stimulated with G28-8 alone than in B cells stimulated with G28-8 plus anti-kappa. The same conditions of cross-linking with the anti-kappa which increased the kinetics of the [Ca2+]i response decreased the proliferative response which otherwise followed co-incubation of the mAb with B cell growth factor or PMA. Thus, conditions leading to rapid but transient [Ca2+]i increase via Bgp95 may not be as effective at stimulating B cell proliferation as conditions favoring a slower prolonged [Ca2+]i response. Although the Bgp95 molecule is present on activated buoyant tonsillar B cells, mAb to Bgp95 did not trigger [Ca2+]i increases in these cells. These results suggest that the Bgp95 protein may function in early B cell activation and that its signal mechanisms are altered by the activation state of the cell.