CD45 regulates tyrosine phosphorylation of CD22 and its association with the protein tyrosine phosphatase SHP-1

Cross-linking of CD45 induced capping and physical sequestration from CD22 lending; to an increase in tyrosine phosphorylation of CD22 and SHP-1 recru itment. Additionally, CD22 isolated from a CD45-deficient B cell Line exhib ited increased basal/inducible tyrosine phosphorylation and enhanced recrui tment of SHP-1 compared with CD22 isolated from CD45-positive parental cell s, Subsequent experiments were performed to determine whether enhanced SHP- 1 recruitment to CD22 is responsible for attenuation of receptor-mediated C a2+ responses in CD45-deficient cells. Catalytically. inactive SHP-1 expres sed in CD45-deficient cells interacted with CD22 and decreased phosphatase activity in CD22 immunoprecipitates to levels that were comparable to those in CD45-positive cells, Expression of catalytically inactive SHP-1 restore d intracellular mobilization of Ca2+ in response to MHC class II cross-link ing, but did not affect B cell Ag receptor- or class Ii-mediated Ca2+ influ x from the extracellular space, These results indicate that CD45 regulates tyrosine phosphorylation of CD22 and binding of SHP-1. The data further ind icate that enhanced recruitment and activation of SHP-1 in CD45-deficient c ells affect intracellular mobilization of Ca2+, but are not responsible for abrogation of receptor-mediated Ca2+ influx from the extracellular spare.