CD22, a negative regulator of B cell signaling, is a member of the siglec family that binds to ␣2-6-linked sialic acids on glycoproteins. Previous reports demonstrated that binding of multivalent sialoside probes to CD22 is blocked, or ''masked,'' by endogenous (cis) ligands, unless they are first destroyed by sialidase treatment. These results suggest that cis ligands on B cells make CD22 functionally unavailable for binding to ligands in trans. Through immunofluorescence microscopy, however, we observed that CD22 on resting B cells redistributes to the site of contact with other B or T lymphocytes. Redistribution is mediated by interaction with trans ligands on the opposing cell because it does not occur with ligand-deficient lymphocytes from ST6GalI-null mice. Surprisingly, CD45, proposed as both a cis and trans ligand of CD22, was not required for redistribution to sites of cell contact, given that redistribution of CD22 was independent of CD45 and was observed with lymphocytes from CD45-deficient mice. Furthermore, CD45 is not required for CD22 masking as similar levels of masking were observed in the WT and null mice. Comparison of the widely used sialoside-polyacrylamide probe with a sialoside-streptavidin probe revealed that the latter bound a subset of B cells without sialidase treatment, suggesting that cis ligands differentially impacted the binding of these two probes in trans. The combined results suggest that equilibrium binding to cis ligands does not preclude binding of CD22 to ligands in trans, and allows for its redistribution to sites of contact between lymphocytes. C D22 (Siglec-2) is a well-known regulator of B cell signaling (1-3), an activity mediated through its recruitment of SH2 domain-containing phosphatase 1 (also known as SHP-1) to the B cell receptor by immunoreceptor tyrosine-based inhibition motifs in its cytoplasmic domain (4, 5). The siglecs are a subgroup of the Ig superfamily that have in common a NH 2 -terminal Ig domain that binds sialic acid containing carbohydrates of glycoproteins as a ligand. CD22 is unique among the siglecs for its high specificity for sialoside ligands containing the sequence Sia␣2-6Gal that often terminates N-linked carbohydrate groups of glycoproteins (6, 7). The Sia␣2-6Gal sequence recognized by CD22 is differentially expressed in a cell typespecific manner (8-10) 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.CD22 is well documented to bind to endogenous B cell glycoproteins in cis. As elegantly demonstrated by Razi et al. (13), cis ligands ''mask'' human CD22 binding to synthetic multivalent sialoside probes unless the cells are first pretreated with sialidase or periodate. Indeed, masking by endogenous cis ligands has su...