We describe a mouse strain in which B cell development relies either on the expression of membrane-bound immunoglobulin (Ig) γ1 or μ heavy chains. Progenitor cells expressing γ1 chains from the beginning generate a peripheral B cell compartment of normal size with all subsets, but a partial block is seen at the pro– to pre–B cell transition. Accordingly, γ1-driven B cell development is disfavored in competition with developing B cells expressing a wild-type (WT) IgH locus. However, the mutant B cells display a long half-life and accumulate in the mature B cell compartment, and even though partial truncation of the Igα cytoplasmic tail compromises their development, it does not affect their maintenance, as it does in WT cells. IgG1-expressing B cells showed an enhanced Ca2+ response upon B cell receptor cross-linking, which was not due to a lack of inhibition by CD22. The enhanced Ca2+ response was also observed in mature B cells that had been switched from IgM to IgG1 expression in vivo. Collectively, these results suggest that the γ1 chain can exert a unique signaling function that can partially replace that of the Igα/β heterodimer in B cell maintenance and may contribute to memory B cell physiology.

CD22 is an inhibitory co-receptor of B cell receptor (BCR)-mediated signalling which binds specifically to glycan ligands containing alpha2,6-linked sialic acids. This interaction modulates the CD22 activity by an unknown mechanism. Mice deficient for ST6GalI, the enzyme that generates alpha2,6-linked sialic acids, show an immunodeficient and opposing phenotype to CD22-deficient mice. By generating mice double-deficient for this receptor/ligand pair, we analysed its influence on B cell maturation and signalling. Both ST6GalI-deficient and ST6GalI x CD22-deficient mice showed normal B cell development, but an impaired marginal zone B cell population in the spleen. Both types of mutant mice also showed a reduced population of bone marrow recirculating B cells, a defect previously detected in CD22-/- mice. In adoptive transfer experiments, a migration defect of wild-type B cells to the bone marrow of ST6GalI-deficient mice was found. This suggests a direct involvement of CD22 and its ligands 2,6Sia in a homing process of recirculating B cells to the bone marrow. Interestingly, defective B cell Ca2+ signalling and proliferation of ST6Gal-/- mice was rescued in ST6GalI x CD22-deficient mice. This points to a new mechanism of BCR signal regulation by CD22 and its ligand.

CD22 is an inhibitory coreceptor for B cell receptor (BCR) signaling. The inhibition is most likely mediated by activation of SHP-1. We found that SLP65/BLNK reaches maximal tyrosine-phosphorylation at earlier time points in CD22 -/-than in wild type B cells upon BCR cross-linking, suggesting that SLP65/BLNK is a substrate of SHP-1. However, in contrast to the defective Ca 2+ mobilization of SLP65/BLNK -/-B cells, there was a clear Ca 2+ response in SLP65/BLNK×CD22 double-deficient B cells. This implies that SLP65/BLNK is not the sole target of SHP-1 in the regulation of the Ca 2+ signaling strength. While SLP65 -/-mice show several blocks of B cell differentiation, in SLP65/BLNK×CD22 double-deficient mice the maturation block of B cells in the spleen was partially rescued. However, the proliferative responses of B cells from both SLP65/BLNK -/-and double-deficient mice were defective after IgM-or CD40-stimulation. These results show that SLP65/BLNK is not absolutely essential for Ca 2+ induction in B cells, because the deficiency of this adapter can be bypassed by the additional deletion of an inhibitory receptor. Furthermore, these experiments suggest that B cell maturation in the spleen is directly dependent on the strength of BCRderived Ca 2+ signals.