MASKING AND UNMASKING OF THE SIALIC ACID-BINDING LECTIN ACTIVITY OF CD22 (SIGLEC-2) ON B-LYMPHOCYTES

CD22 is a B cell-restricted glycoprotein involved in signal transducti on and modulation of cellular activation. It is also an I-type lectin (nom designated Siglec-2), whose extracellular domain can specifically recognize alpha 2-6-linked sialic acid (Sia) residues. This activity is postulated to mediate intercellular adhesion and/or to act as a cor eceptor in antigen-induced B cell activation. However, studies with re combinant CD22 indicate that the lectin function can be inactivated by expression of alpha 2-6-linked Sia residues on the same cell surface. To explore whether this masking phenomenon affects native CD22 on B c ells, we first developed a probe to detect the lectin activity of reco mbinant CD22 expressed on Chinese hamster ovary cells (which have no e ndogenous alpha 2-6-linked Sia residues). This probe is inactive again st CD22-positive B lymphoma cells and Epstein-Barr virus transformed l ymphoblasts which express high levels of alpha 2-6-linked Sia residues . Enzymatic desialylation unmasks the CD22 lectin activity, indicating that endogenous Sia residues block the CD22 lectin-binding site. Trun cation of the side chains of cell surface Sia residues by mild perioda te oxidation (known to abrogate Sia recognition by CD22) also had this unmasking effect, indicating that the effects of desialylation are no t due to a loss of negative charge. Normal resting B cells from human peripheral blood gave similar findings. However, the lectin is partial ly unmasked during in vitro activation of these cells. Thus, the lecti n activity of CD22 is restricted by endogenous sialylation in resting B cells and may be transiently unmasked during in vivo activation, per haps to modulate intercellular or intracellular interactions at this c ritical stage in the humoral response.