Stimulation of protein tyrosine phosphorylation by the B-lymphocyte antigen receptor

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Abstract
Signalling by membrane immunoglobulin, the B-lymphocyte antigen receptor, regulates B-cell maturation and activation. Crosslinking of membrane immunoglobulin by antigen or by anti-immunoglobulin antibodies inactivates immature B cells, eliminating many of the B cells capable of producing auto-antibodies. By contrast, crosslinking of membrane immunoglobulin promotes activation of mature B cells for clonal expansion and antibody production against foreign antigens. Crosslinking membrane IgM on the immature B-cell line WEHI-231 induces growth arrest. This response may be analogous to the deletion or inactivation of immature B cells that is induced by antigen or anti-IgM antibodies. Membrane immunoglobulin crosslinking stimulates phosphoinositide hydrolysis, which leads to increases in intracellular calcium and activation of protein kinase C. The induced phosphoinositide breakdown is important for inhibiting WEHI-231 growth (ref. 7 and D. Page, M.R.G., K. Fahey, L. Matsuuchi and A.L.D., manuscript submitted for publication), but may not be sufficient, as agents that elevate calcium and activate protein kinase C cause only partial growth arrest. We now show that in both mature splenic B cells and the immature B-cell line WEHI-231 crosslinking membrane immunoglobulin also stimulates phosphorylation of protein tyrosine, a reaction that has been implicated as a key regulator of cell growth. Most of these phosphorylations were not a consequence of the phosphoinositide pathway. Thus, tyrosine phosphorylation is a second mode of transmembrane signalling by membrane immunoglobulin.