The B‐Cell Adhesion Molecule CD22 is Cross‐Species Reactive and Recognizes Distinct Sialoglycoproteins on Different Functional T‐Cell Sub‐Populations

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Abstract
CD22 is a B cell lineage restricted cell‐surface adhesion glycoprotein which recognizes ligands on human T and B cells and cell lines. A soluble recombinant form of human CD22 (hCD22Rg) has been used to identify and characterize CD22‐specific ligands on human T cells, one of which has been shown to be the receptor‐linked phosphotyrosine phosphatase CD45. Because CD45 plays a pivotal role in lymphocyte activation, we assessed whether human CD22 might display cross‐species reactivity with CD45. In the study presented here we demonstrate that human CD22Rg recognizes several murine cell‐surface sialoglycoproteins. including CD45, containing sialic acid in a2, 6 linkage. Furthermore, hCD22Rg recognizes different ligands on functionally distinct T helper‐cell subpopulations and selectively binds medullary thymocytes in rivo. Our results confirm and extend previous observations that CD22 is a sialic acid‐binding lectin which interacts with CD45 and other glycoproteins capable of presenting α2, 6‐linked sialic acid in a manner that promotes high affinity binding. The cross‐species reactivity ofCD22 with its ligands underscores the potential physiologic importance ofCD22‐mediated lymphocyte interactions.