A structure-function study of ligand recognition by CD22 beta

B-cell-specific CD22 is a member of a group of cell adhesion molecules with in the immunoglobulin superfamily that display binding to glycans with term inal sialic acid residues. Binding of endogenous ligands to CD22 triggers B -cell activation and proliferation. It is therefore conceivable that high a ffinity ligands for CD22 may be of value as inhibitors of B-cell activation in allergy and chronic inflammation. In this study, we aimed to delineate the structural requirements for ligand binding to CD22. A library of 20 mon o-, di-, and trisaccharide analogs of the basic binding motif Neu5Ac(alpha2 ,6)Lac was synthesized and screened for affinity for CD22 beta, In general, CD22 ligand recognition appeared to be rather tolerant with respect to str uctural modifications of the anomeric sugar on a mono-, di-, and trisacchar ide level, although affinity was increased by the presence of a nitro aroma tic group at C-2, The most potent monovalent ligand, Neu5Ac-4-nitrobenzoyl- Glc, was selected to generate multivalent ligands based on either a glutama te or Tris cluster core. All multivalent ligands displayed at least a 10-fo ld increased affinity for CD22 compared with the corresponding monovalent g lycoside, Interestingly, a maximal gain in affinity was already obtained fo r bivalent ligands, regardless of the terminal glycoside, A trivalent Tris- based cluster of Neu5Ac-4-nitrobenzoyl-Glc displayed a 300-fold higher affi nity compared with the basic binding motif, which makes it, to our knowledg e, the most potent antagonist for CD22 yet synthesized. As our in vitro flu orescence-activated cell sorting studies demonstrated efficient cellular up take of a CD22 substrate, the most potent ligand in this study may hold pro mise as a homing device for immunomodulatory compounds and cytostatics.