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CD45 Regulates Tyrosine Phosphorylation of CD22 and Its Association with the Protein Tyrosine Phosphatase SHP-1¹

Department of Microbiology, Division of Developmental and Clinical Immunology, University of Alabama, Birmingham, AL 35294

Cross-linking of CD45 induced capping and physical sequestration from CD22 leading to an increase in tyrosine phosphorylation of CD22 and SHP-1 recruitment. Additionally, CD22 isolated from a CD45-deficient B cell line exhibited increased basal/inducible tyrosine phosphorylation and enhanced recruitment of SHP-1 compared with CD22 isolated from CD45-positive parental cells. Subsequent experiments were performed to determine whether enhanced SHP-1 recruitment to CD22 is responsible for attenuation of receptor-mediated Ca²⁺ responses in CD45-deficient cells. Catalytically inactive SHP-1 expressed 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 restored intracellular mobilization of Ca²⁺ in response to MHC class II cross-linking, but did not affect B cell Ag receptor- or class II-mediated Ca²⁺ influx from the extracellular space. These results indicate that CD45 regulates tyrosine phosphorylation of CD22 and binding of SHP-1. The data further indicate that enhanced recruitment and activation of SHP-1 in CD45-deficient cells affect intracellular mobilization of Ca²⁺, but are not responsible for abrogation of receptor-mediated Ca²⁺ influx from the extracellular space.

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