Sialic Acid Binding Domains of CD22 Are Required For Negative Regulation of B Cell Receptor Signaling

CD22, a negative regulator of B cell antigen receptor signaling, binds glycoconjugates terminating in α2, 6 sialic acid. The physiological ligand(s) for CD22 remain unknown. We asked whether the sialic acid binding domains are necessary for CD22 to function as a negative regulator. We generated two mutants that lack sialic acid binding activity and expressed them in a novel CD22(−/−) murine B cell line. Anti-IgM activated B cells expressing either CD22 mutant had greater Ca(2+) responses than cells expressing wild-type CD22. Each variant also had reduced CD22 tyrosine phosphorylation and Src homology 2 domain–containing protein tyrosine phosphatase-1 association. These data suggest that the α2, 6 sialic acid ligand binding activity of CD22 is critical for its negative regulatory functions.