Natural Ligands of the B Cell Adhesion Molecule CD22/3 can be Masked by 9-O-Aeetylation of Sialic Acids

CD22/3 is a B cell-restricted phosphoprotein expressed on the surface of mature resting B cells. It mediates interactions with other cells partly or exclusively via recognition of ot2-6-1inked sialic acids on glycoconjugates. The sialylated N-linked oligosaccharides recognized best by CD22/~ are common to many glycoproteins, suggesting that additional regulatory mechanisms may exist. Since the exocyclic side chain of sialic acid is required for recognition, we explored the effects of a naturally occurring modification of the side chain, 9-O-acetylation. Semisynthetic N-linked oligosaccharides terminating with 9-0acetylated, o~2-6-1inked sialic acids showed markedly reduced binding to CD22/~ relative to their non-Oacetylated counterparts. Murine lymphoid cells were probed for natural CD22/~ ligands that might be O-acetylated using recombinant soluble forms of CD22/~ (CD22/3Rg) and influenza C esterase (CHE-Fc, which specifically removes 9-O-acetyl esters from sialic acids). By flow cytometry analysis, CD22/3Rg binding to splenic B cells and a subset of T cells was increased by pretreatment with CHE-Fc, indicating that some potential CD22/3 ligands are naturally "masked" by 9-O-acetylation. Unmasking of these CD22/3 ligands by removal of 9-O-acetyl esters from intact splenocytes substantially increases their CD22/3dependent adhesion in an in vitro adhesion assay. Probing of murine lymphoid tissue sections by CD22/~Rg and CHE-Fc treatment demonstrates regionally restricted and differentially expressed patterns of distribution between masked and unmasked ligands. For example, lymph node-associated follicular B cells express high levels of CD22/~ ligands, none of which are masked by 9-O-acetylation. In contrast, the ligands on lymph node-associated dendritic cells are almost completely masked by 9-O-acetylation, suggesting that masking may regulate interactions between CD22/3positive B cells and dendritic cells. In the thymus, only medullary cells express CD22/3 ligands, and a significant portion of these are masked by 9-0acetylation, particularly at the cortical-medullary junction. Thus, 9-O-acetylation of sialic acids on immune cells is in a position to negatively regulate CD22B adhesion events in a manner depending on both cell type and tissue localization. I NTERCELLULAR adhesion between leukocytes involves a variety of receptor/ligand pairs that are required for eliciting immune responses (37). One such adhesion molecule is a B cell-restricted glycoprotein of the immunoglobulin superfamily, CD22B (7, 8, 38, 39, 49). First expressed within the interior of pre-B cells, CD22B is mobilized to the cell surface during maturation, coinciding temporally with the surface expression of IgD (7). Two human CD22 cDNAs designated CD22c¢ and CD22/~ have been isolated, and their predicted sequences share homology to several adhesion molecules, including the neural cell adhesion molecule, myelin-associated glycoprotein, and carcinoembryonic antigen (38, 49). The two differ only in that Address all correspondence to Ajit Varki, Cancer Center, 0063, University of California at San Diego School of Medicine, La Jolla, CA 92093-0063. CD22/~ contains seven extraceUular Ig domains, whereas CD22o~ lacks Ig domains 3 and 4 (38, 49). While some differences in the binding properties of the two recombinant forms have been reported, CD22/~ appears to be the predominant cell surface form in both human and murine systems (41). Much evidence indicates the involvement of CD22~ in B cell proliferation. A fraction of CD22/~ coexists with surface IgM on the surface of naive B ceils (7), In response to IgM cross-linking, CD22-positive lymphocytes show selective increases in calcium flux (26), specific cytoplasmic tyrosine residues on CD22~/are rapidly phosphorylated (32), and the cell surface expression of CD22/3 is quickly enhanced beyond basal levels. However, over longer time periods, surface expression is strikingly reduced (7, 20, 32). Cross-linking of CD22/~ facilitates the calcium mobilization induced by cross linking surface IgM (25). On T cells, various isoforms of the common leukocyte an© The Rockefeller University Press, 0021-9525/94/07/549/14 $2.00 The Journal of Cell Biology, Volume 126, Number 2, July 1994 549-562 549 on Jne 7, 2017 D ow nladed fom Published July 15, 1994