Fc gamma receptor-mediated inhibition of human B cell activation: the roleof SHP-2 phosphatase

Co-clustering of the type II receptors binding the Fc part of IgG (Fc gamma RIIb) and B cell receptors results in the translocation of cytosolic, nega tive regulatory molecules to the phosphorylated immunoreceptor tyrosine-bas ed inhibitory motif (P-ITIM) of the FcyRIIb. SH2 domain-containing protein tyrosine phosphatases (SHP-1 and SHP-2), and the polyphosphoinositol 5'-pho sphatase (SHIP) have been reported earlier to bind to murine Fc gamma RIIb P-ITIM. However, neither the functional substrates of these enzymes, nor th e mechanism of the inhibition are fully resolved. We show here that the hum an Fc gamma RIIb binds SHP-2 when co-clustered with the B cell receptors, w hereas its synthetic P-ITIM peptide bindes SHP-2 and SHIP in lysates of the Burkitt's lymphoma cell line BL41. The P-ITIM peptide binding enhances SHP -2 activity resulting in dephosphorylation and release of P-ITIM-bound SHIP and She. Moreover, P-ITIM-bound SHP-2 dephosphorylates synthetic peptides corresponding to the sites of tyrosine phosphorylation on SHIP and She, ind icating that these proteins are its potential substrates. Thus SHP-2-induce d dephosphorylation may modulate the intracellular localization and/or acti vity of SHIP and She, thereby inhibiting further activation pathways which they mediate.