Study on the cis-ligand mediated regulation of CD22

B cells play important roles in immune system including recognition of antigens and producing antibodies. BCR signaling is a major process done by B cells which is regulated precisely, because any impairment may cause hyper-activation that may result in autoimmune diseases. The regulation of BCR signaling is orchestrated by inhibitory receptors including CD22. CD22, an inhibitory receptor expressed in B cells, recognizes α2,6 sialic acid linkages (2,6Sia). It associates with glycoproteins and glycolipids containing 2,6Sia linkages expressed on the same cell (cis-ligand). It is suggested that cis-ligands negatively regulate CD22. In this study we addressed whether CD45, a highly glycosylated receptor type protein tyrosine phosphatase, regulates CD22 as a cis-ligand. Proximity labeling revealed that CD45 is associated with CD22 in B cells in a manner dependent on 2,6Sia, indicating that CD45 is a CD22 cis-ligand. Upon BCR ligation, CD22 phosphorylation required for the CD22-mediated signal inhibition was reduced in CD45 deficient B cells. This result suggests that CD45 is crucial in inhibitory function of CD22. However, analysis using the GSC718 compound, a synthetic sialoside that binds to CD22 with high affinity, showed that CD22 phosphorylation is much reduced but still negatively regulated by cis-ligands in CD45-deficient B cells. Thus, CD45 does not appear to play a role in cis-ligand-induced CD22 regulation. Taken together, CD45 is a cis-ligand of CD22 but enhances CD22 function independently of cis-ligand interaction.