B-CELL ANTIGEN RECEPTOR-EVOKED CALCIUM INFLUX IS ENHANCED IN CD22-DEFICIENT B-CELL LINES

CD22 is a B cell membrane glycoprotein that, upon Ag receptor engageme nt, becomes rapidly tyrosyl phosphorylated and associates with several signaling molecules including Lyn, Syk, PLC gamma 1, and the protein- tyrosine phosphatase, SHP-1. Two allelic forms of murine CD22 exist: C D22.1 is expressed in strains such as NZB and DBA/2, whereas CD22.2 is expressed in BALB/c and most other strains. WEHI-231 cells, which der ive from a (BALB/c x NZB)F-1 mouse, express one copy of each allele, P revious studies have proposed both positive and negative functions for CD22, We explored the role of CD22 in surface IgM Ag receptor signal transduction by examining signaling in three clonally independent WEHI -231 variants that have lost expression of the CD22.2 allele. This exp erimental design allowed us to assess the signaling functions of CD22 independent of its developmental role. These variants, which exhibit a 50% reduction of total surface CD22, are hyper-responsive to Ag recep tor stimulation: several cellular proteins are hyperphosphorylated on tyrosyl residues and surface IgM-mediated calcium flux is markedly inc reased. Interestingly, the increased calcium response observed in CD22 -deficient cells is due largely to enhanced calcium influx, Reconstitu tion of CD22 expression reduces these changes. The SHP-1/CD22 associat ion is reduced in CD22-deficient cell lines and is restored by re-expr ession of CD22. Our results demonstrate that CD22 is a cell autonomous negative regulator of B cell Ag receptor signaling, and suggest that it regulates calcium entry via a mechanism downstream from or independ ent of calcium release from intracellular stores.