Galectins and Immune Responses-Just How Do They Do Those Things They Do?

Galectins are a family of mammalian carbohydrate-binding proteins expressed by many cell types. Galectins can function intracellularly and can also be secreted to bind to cell surface glycoconjugate counterreceptors. Some galectins are made by immune cells, whereas other galectins are secreted by different cell types, such as endothelial or epithelial cells, and bind to immune cells to regulate immune responses. Galectin binding to a single glycan ligand is a low-affinity interaction, but the multivalency of galectins and the glycan ligands presented on cell surface glycoproteins results in high-avidity binding that can reversibly scaffold or cluster these glycoproteins. Galectin binding to a specific glycoprotein counterreceptor is regulated in part by the repertoire of glycosyltransferase enzymes (which make the glycan ligands) expressed by that cell, and the effect of galectin binding results from clustering or retention of specific glycoprotein counterreceptors bearing these specific ligands.