CD22 forms a quaternary complex with SHIP, Grb2, and Shc - A pathway for regulation of B lymphocyte antigen receptor-induced calcium flux

CD22 is a cell surface molecule that regulates signal transduction in B lym phocytes, Tyrosine-phosphorylated CD22 recruits numerous cytoplasmic effect or molecules including SHP-1, a potent phosphotyrosine phosphatase that dow n-regulates B cell antigen receptor (BCR)- and CD19-generated signals. Para doxically, B cells from CD22-deficient mice generate augmented intracellula r calcium responses following BCR ligation, yet proliferation is decreased. To understand further the mechanisms through which CD22 regulates BCR-depe ndent calcium flux and proliferation, interactions between CD22 and effecto r molecules involved in these processes were assessed. The adapter proteins Grb2 and Shc were found to interact with distinct and specific regions of the CD22 cytoplasmic domain. Src homology-2 domain-containing inositol poly phosphate-5'-phosphatase (SHIP) also bound phosphorylated CD22, but binding required an intact CD22 cytoplasmic domain. Ail three molecules were bound to CD22 when isolated from BCR-stimulated splenic B cells, indicating the formation of a CD22 Grb2 Shc SHIP quaternary complex. Therefore, SHIP assoc iating with CD22 may be important for SHIP recruitment to the cell surface where it negatively regulates calcium influx, Although augmented calcium re sponses in CD22-deficient mice should facilitate enhanced c-Jun N-terminal kinase (JNK) activation, BCR ligation did not induce JNK activation in CD22 -deficient B cells. These data demonstrate that CD22 functions as a molecul ar "scaffold" that specifically coordinates the docking of multiple effecto r molecules, in addition to SHP-1, in a context necessary for BCR-dependent SHIP activity and JNK stimulation.