Cooperation between SHP-2, phosphatidyl inositol 3-kinase and phosphoinositol 5-phosphatase in the Fc gamma RIIb mediated B cell regulation

Go-clustering B cell receptors (BCR) and type II receptors binding the Fc p art of IgG (Fc gamma RIIb) inhibits B cell activation and antibody producti on. Tyrosine phosphorylation of an intracellular motif of Fc gamma RIIb has been shown to be a prerequisite of the inhibition. After being phosphoryla ted by BCR-activated tyrosine kinases, the immunoreceptor tyrosine-based in hibitory motif (P-ITIM) of Fc gamma RIIb recruits SH2 domain containing pro tein tyrosine phosphatase(s) (PTPs) and polyphosphoinositol 5-phosphatase ( SHIP) to the vicinity of BCR, which in turn dephosphorylate their specific substrates. This leads to the interruption of signal transduction, conseque ntly to the anergy and/or apoptosis of the cell. The downstream signaling p athways affected by Fc gamma RIIb-BCR co-clustering are not clarified yet, neither the substrates of PTPs are known. We have studied the Fc gamma RIIb mediated B cell inhibition on human Burkitt lymphoma cell line (BL41). Fro m the lysates of BL41 cells SHP-2 and phosphatidylinositol 3-kinase (PI3-K) , as well as the protein tyrosine kinase (PTK) Lyn bind both to the BCR-co- clustered Fc gamma RIIb and to its P-ITIM peptide. Lyn hyperphosphorylates the P-ITIM associated molecules, including SHIP in the in vitro protein tyr osine kinase activity assay. The P-ITIM-compelled multi-phosphoprotein comp lex binds to and activates SHP-2, which in turn dephosphorylates SHIP and S hc and probably other substrates. Subcellular localisation of these signali ng molecules is regulated by the phosphotyrosin-SH2 domain interactions, th us dephosphorylation may result in the re-direction of Shc and SHIP within the cell, consequently, in the modulation of their activity. Finally, co-cl ustering Fc gamma RIIb and BCR or Fc gamma RIIb and CD19 on the intact cell s inhibited PI3-K activity as detected in the anti-phosphotyrosine (anti-PY ) precipitates. The results indicate that SHP-2 bound to and activated by t he BCR co-clustered Fc gamma RIIb, may down-regulate PI3-K activity by deph osphorylating a yet unidentified regulatory molecule, which recruits PI3-K to the cell membrane. (C) 1999 Elsevier Science B.V. All rights reserved.