Here, we demonstrate that CD22 is efficiently activated in trans by complexes of Ag and soluble IgM (sIgM) due to the presence of glycan ligands on sIgM. This result strongly suggests sIgM as a natural trans ligand for CD22. Also, CD22 appears to serve as a receptor for
sIgM, which induces a negative feedback loop for B-cell activation similar to the Fc receptor for IgG (FcγRIIB). CD22 is a 140 kDa glycoprotein on the surface of B cells that negatively regulates signaling through the B-cell Ag receptor (BCR) 1–3. There are six tyrosine residues within the cytoplasmic portion of CD22, four of which are located within ITIMs 4. These tyrosine residues are phosphorylated upon BCR cross-linking, leading to recruitment of SHP-1 4, 5. SHP-1 subsequently dephosphorylates the BCR-proximal signaling molecules, resulting in downmodulation of BCR signaling. Consistent with this, B cells high throughput screening compounds from CD22-deficient mice are hyperactive 6–9. The extracellular portion of CD22 is composed of seven immunoglobulin (Ig)-like domains, the most distal of which is a V-set Ig-like domain that recognizes α2,6-linked sialic acid (α2,6Sia)-containing glycoconjugates 3, 10. α2,6Sia is common at the terminal of N-linked glycans and is abundantly expressed
on various kinds of cells, including erythrocytes, monocytes, B cells, and T cells. α2,6Sia also exists on soluble plasma proteins such as serum-soluble IgM (sIgM) 11. CD22 is a member of the sialic Neratinib cost acid-binding Ig-like lectin (Siglec) family, and is also referred to as Siglec-2. CD22 appears to interact with various ligands in cis and in trans to modulate B-cell activity 10. Potential CD22 ligands, including IgM, CD45, and CD22 itself, have been identified 12. Among them, only CD22 has been identified as a natural
glycan ligand for CD22 in cis 13. Furthermore, CD22 regulates BCR signaling induced by Ags expressed on other cells in an α2,6Sia-dependent manner 14. It has recently been reported that sialylated multivalent Pregnenolone Ags engage CD22 in trans and inhibit B-cell activation 15. Thus, various interactions between CD22 and its ligands have been shown. However, the overall interactions and the subsequent effects on B-cell activation are not fully understood. In this study, we further evaluated the role of CD22 ligand binding in trans in B-cell activation and propose a novel model of CD22 function. Since sIgM has been shown to bind to recombinant CD22 fusion protein (CD22-Fc) 11, we tested whether sIgM binds to CD22-expressing cells. The mouse myeloma line J558L fails to express the CD22 glycan ligand α2,6Sia at the terminal of N-glycan due to a lack of β-galactoside α2,6-sialyltransferase I (ST6GalI) expression. Introduction of a ST6GalI expression vector can restore α2,6Sia on cell-surface glycoproteins and we showed previously that the soluble CD22 fusion protein (CD22-Fc) bound to J558L cells expressing ST6GalI (J558L/ST6) but not to J558L cells 16.