Differences between human eosinophils and neutrophils in the function and expression of sialic acid-containing counterligands for E-selectin

Both neutrophils and eosinophils have been shown to bind to the inducible endothelial cell adhesion molecule E-selectin. For neutrophils, one of the reported ligands for E-selectin is the sialylated Lewis X Ag (sLe(x)). To analyze the counterligands on eosinophils for E-selectin, adhesion assays were performed in which purified leukocytes were allowed to adhere to a soluble recombinant form of the molecule immobilized on plastic plates. Eosinophils, like neutrophils, bound to immobilized E-selectin, but significantly more neutrophils than eosinophils adhered in this assay. Consistent with the greater ability of neutrophils to bind E-selectin was the observation by flow cytometry that neutrophils expressed significant levels of sLe(x) and a sialylated dimeric form of the Le(x) Ag (sialyl-dimeric Le(x), or sialyl-stage-specific embryonic Ag-1, recognized by mAb FH6), whereas the expression of these epitopes on eosinophils was extremely low or undetectable. Expression was similar on eosinophils from allergic and nonallergic donors, and was not altered on eosinophils after induction of L- selectin shedding in vitro by treatment with platelet-activating factor. For both eosinophils and neutrophils, treatment with sialidase was associated with the complete elimination of sLe(x) and sialyl-dimeric Le(x) surface expression, and abolished leukocyte adhesion to E-selectin. Another glycosidase, endo-β-galactosidase, which specifically cleaves the β1-4 galactose linkage to N-acetyl-glucosamine when it exists in an extended chain form such as that found in sialyl-dimeric Le(x), significantly inhibited eosinophil and neutrophil adhesion and expression of sialyl-dimeric Le(x). Such treatment also reduced sLe(x) expression on eosinophils, while having little effect on total neutrophil sLe(x) expression. For both eosinophils and neutrophils the sialylated ligand did not appear to be a glycoprotein because pretreatment of leukocytes with several proteases had no effect on adhesion to E-selectin or on expression of sLe(x) and sialyl-dimeric Le(x). These data suggest that eosinophils, like neutrophils, use sialylated, protease- resistant structures to bind to E-selectin, although the eosinophil expresses much lower levels of these structures on its surface. A major proportion of the sLe(x)-containing E-selectin ligand on the surface of eosinophils appears to be in the form of sialyl-dimeric Le(x), whereas this represents a minor proportion on the surface of neutrophils. Based on results using endo-β- galactosidase, it appears that these cells may rely disproportionately upon the cell surface sialyl-dimeric Le(x) to bind to E-selectin. Differences between eosinophil and neutrophil sLe(x)-containing surface molecules may contribute to variations in their E-selectin-mediated cell recruitment seen during inflammatory responses in vivo.