The Ligand-binding Domain of CD22 Is Needed for Inhibition of the B Cell Receptor Signal, as Demonstrated by a Novel Human CD22-specific Inhibitor Compound

Abstract

CD22 is a B cell–specific transmembrane protein of the Siglec family. It binds specifically to α2,6-linked sialic acid (Sia) residues, which are also present on glycoproteins on the B cell surface. CD22 acts as a negative regulator in B cell receptor–mediated signaling by recruitment of Src homology 2 domain–containing tyrosine phosphatase (SHP)-1 to its intracellular tail. To analyze how ligand-binding of CD22 influences its intracellular signaling domain, we designed synthetic sialosides as inhibitors for the lectin domain of CD22. One of these compounds inhibited binding of human CD22-Fc to target cells over 200-fold better than Sia and was highly selective for human CD22. When Daudi cells or primary B cells were stimulated with anti-immunoglobulin (Ig)M in presence of this sialoside inhibitor, a higher Ca2+ response was observed, similar to CD22-deficient B cells. Accordingly, a lower tyrosine-phosphorylation of CD22 and SHP-1 recruitment was demonstrated in presence of the sialoside. Thus, by interfering with ligand binding of CD22 on the B cell surface, we have shown for the first time that the lectin domain of CD22 has a direct, positive influence on its intracellular inhibitory domain. Also, we have developed a novel low molecular weight compound which can enhance the response of human B cells.

The murine inhibitory receptor mSiglec‐E is expressed broadly on cells of the innate immune system whereas mSiglec‐F is restricted to eosinophils

“…In each case studied, these motifs can be tyrosine phosphorylated, recruit the protein tyrosine phosphatases SHP-1 and SHP-2, and trigger inhibitory signaling when cross-linked to activating receptors with mAb [3][4][5][6]. This suggests that the natural function of CD33-related siglecs is to modulate leukocyte activation, possibly in an analogous fashion to the inhibitory receptor CD22/ siglec-2 on B cells [7][8][9]. Other studies have indicated that CD33-related siglecs may potentially regulate leukocyte proliferation and apoptosis through unknown mechanisms [10][11][12].…”

Siglec-7 Undergoes a Major Conformational Change When Complexed with the α(2,8)-Disialylganglioside GT1b

“…Siglecs recognize sialic acid, a nine-carbon sugar that often caps the non-reducing end of glycans (4). Engagement of specific sialylated ligands by siglecs is thought to influence cellular adhesion, recognition, and signaling in the immune and nervous systems (5)(6)(7)(8).…”

Masking of CD22 by cis ligands does not prevent redistribution of CD22 to sites of cell contact

“…The Sia␣2-6Gal sequence recognized by CD22 is differentially expressed in a cell typespecific manner (8)(9)(10) and is abundant on B and T lymphocytes (11,12). Several reports have suggested that the ligand-binding domain of CD22 can modulate its activity as a regulator of B cell signaling (5,(13)(14)(15). Although the precise ligand interactions that modulate CD22 function are not well understood, both cis (B cell) and trans (opposing cell) glycoprotein ligands appear to be important.…”

“…Indeed, inhibition of CD22 binding to cis ligands by using small molecule ligand mimics (14) or abrogation of its binding by site directed mutagenesis results in decreased SH2 domain-containing phosphatase 1 (SHP-1) recruitment and increased calcium flux after B cell receptor (BCR) crosslinking in vitro (15). CD22 can bind to a variety of B cell glycoproteins in vitro, including CD45 and IgM, which are often cited as potentially important functional cis ligands of CD22 (28).…”