Studies on selectin-carbohydrate interactions

Recruitment of neutrophils to sites of inflammation is now believed to occur through an initial rolling interaction at the luminal surface of activated endothelium and is mediated by a class of mammalian lectins referred to as the selectins. Selectins recognize carbohydrate determinants on co-receptors. It is generally believed that many selectin molecules must bind to many carbohydrate receptor molecules i.e. multivalent binding, to enable sufficient binding strength to elicit the rolling response between the neutrophil and the endothelial cell. One of the approaches to the generation of more potent molecular antagonists of the selectin-mediated cell-cell interaction is to mimic the multivalent interaction in a single compound. Recent experiments utilising conjugated forms of sialyl Lewis(x)-BSA have explored this feasibility (Welply et al., 1994). In that study, monovalent sLe(x) (sialic acidα2-3Galβ1-4(Fucα1-3)GlcNAc), the minimum binding determinant for E-selectin, as well as monovalent sialyllactosamine (sialic acidα2-3Galβ1-4GlcNAc), a non-binding structure, and the corresponding multivalent BSA-conjugated forms were tested for their ability to inhibit binding of HL-60 cells to immobilised E-selectin. As expected, only sLe(x) and sLe(x)-BSA were found to do so. sLe(x)₁₆-BSA (16 mol tetrasaccharide/mol BSA) showed a dose-dependent inhibition of HL-60 binding with a measured IC₅₀ of 1 μM; demonstrating close to a three-order of magnitude enhancement of inhibitory activity compared to free sLe(x). This result indicated that multivalent forms of sLe(x) are capable of binding to E-selectin with higher affinity than do monovalent glycans. In another study, fluorescent forms of monovalent sLe(x) were synthesized and used to measure a true thermodynamic dissociation constant for the monovalent sLe(x):E-selectin interaction of 120±31 μM.