Sialic Acid Ligand Binding of CD22 and Siglec-G Determines Distinct B Cell Functions but Is Dispensable for B Cell Tolerance Induction

Siglec-G and CD22 are inhibitory receptors on B cells and play an important role in the maintenance of tolerance. Although both molecules are expressed on all B cell populations at a similar level, Siglec-G was found to regulate exclusively B1a cells, whereas CD22 functions as an inhibitory receptor specifically on B2 cells. It is known that the mechanistic function of both Siglecs is regulated by sialic acid binding in a reciprocal manner, although it was not known until now how B cells would act when both Siglec-G and CD22 lack their ability to bind sialic acids. We answered this question by analyzing Siglec-G R120E x CD22 R130E mice. These mice show decreased numbers of mature recirculating B cells in the bone marrow similar to mice with mutations in CD22. Also, they show an increased B1a cell population in peritoneal cavity and a skewed BCR repertoire in peritoneal B1a cells, which is characteristic for mice with mutated Siglec-G. Ca²⁺ mobilization was strongly reduced in B2 cells and was altered in peritoneal B1a cells, whereas B cell survival was neither affected in B2 cells nor in B1a cells. Also, aging Siglec-G R120E x CD22 R130E mice do neither develop a general hyperactivated immune status nor autoimmunity. This demonstrates that Siglec binding to sialic acids as abundant self-ligands cannot be a dominant mechanism for the Siglec-mediated B cell tolerance induction.