Sialomucin ligands for selectins: a new family of cell adhesion molecules

The specific migration of various leukocyte subsets to areas undergoing pathogenic attack is one of the cornerstones of the immune system. Assorted adhesion molecules, chemokines, non-protein signalling molecules, and their respective receptors are utilized during this process. The combinatorial matrix formed by this diversity of agents determines the cell type that migrates to a given class of inflammatory site. The selectins are a family of adhesion molecules that elicit the rolling response that initiates the inflammatory cascade. Selectins contain a type C lectin domain at their N-termini which recognizes sialylated, fucosylated carbohydrate ligands of the sialyl Lewis X (sLex) type. Recently, it has been demonstrated that these carbohydrate ligands are presented by a new class of adhesion molecules that have mucin-like structure and have been termed the sialomucin adhesion family. The function of this new family of adhesion molecules is to act as a scaffold that presents selectin carbohydrate ligands in a clustered, tissue specific manner to allow for higher avidity interactions between leukocytes and endothelial cells during the inflammatory process. In this manner, the appropriate type of leukocyte rolls adjacent to an inflammatory site in a temporally correct manner. This review will focus on the molecular and cellular biology of the sialomucin adhesion family and will discuss the possible implications of these findings on the development of specific inhibitors of selectin function.