Biochemical analysis of the immune B cell defect in xid mice.

Previous studies have shown that B cells from xid immune defective CBA/N mice that are unresponsive do not proliferate after stimulation with unconjugated anti-Ig. The experiments in this manuscript demonstrate that dextran-anti-Ig conjugates, which induce extensive and prolonged sIg cross-linking, are able to stimulate proliferation of xid B cells. The ability of these conjugates to stimulate proliferation of xid B cells is not related to their ability to stimulate higher levels of PIP2 breakdown. Thus, high concentrations of unconjugated anti-Ig antibody, which are nonmitogenic for xid B cells, stimulate higher levels of PIP2 breakdown and of calcium transients than lower concentrations of dextran-conjugated anti-Ig, which are mitogenic. Although unconjugated anti-Ig does not provide a fully competent signal to stimulate proliferation of xid B cells, it induces a sufficiently stimulatory signal to enable them to enter DNA synthesis in the presence of the protein kinase C activator, indolactam. This suggests that the extent or duration of activation of protein kinase C by anti-Ig may be limiting in xid B cells. To examine whether another recently described pathway of B cell activation is defective in these mice, we studied the induction of early anti-Ig-mediated tyrosine kinase activity in xid B cells. Both unconjugated and dextran-conjugated anti-Ig antibody stimulated comparable but not identical patterns of tyrosine phosphorylation. These data taken together with other findings that the combination of phorbol ester and calcium ionophore stimulates high levels of proliferation in xid B cells suggests that the immune defect of xid B cells may be distal to surface Ig-mediated activation of tyrosine kinase and of PIP2 breakdown but proximal to PKC activation. Alternatively, the xid immune defect may not result from abnormalities in the early signal transduction pathways, but rather from more distal and/or as yet undefined pathways leading to B cell activation.