DIFFERENCES BETWEEN HUMAN EOSINOPHILS AND NEUTROPHILS IN THE FUNCTIONAND EXPRESSION OF SIALIC ACID-CONTAINING COUNTERLIGANDS FOR E-SELECTIN

Both neutrophils and eosinophils have been shown to bind to the induci ble endothelial cell adhesion molecule E-selectin. For neutrophils, on e 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 allo wed to adhere to a soluble recombinant form of the molecule immobilize d on plastic plates. Eosinophils, like neutrophils, bound to immobiliz ed E-selectin, but significantly more neutrophils than eosinophils adh ered in this assay. Consistent with the greater ability of neutrophils to bind E-selectin was the observation by flow cytometry that neutrop hils expressed significant levels of sLe(x) and a sialylated dimeric f orm of the Le(x) Ag (sialyl-dimeric Le(x), or sialyl-stage-specific em bryonic 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 w as not altered on eosinophils after induction of L-selectin shedding i n vitro by treatment with platelet-activating factor. For both eosinop hils and neutrophils, treatment with sialidase was associated with the complete elimination of sLe(x) and sialyl-dimeric Le(x) surface expre ssion, and abolished leukocyte adhesion to E-selectin. Another glycosi dase, endo-beta-galactosidase, which specifically cleaves the beta1-4 galactose linkage to N-acetyl-glucosamine when it exists in an extende d 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 eosin ophils, while having little effect on total neutrophil sLe(x) expressi on. For both eosinophils and neutrophils the sialylated ligand did not appear to be a glycoprotein because pretreatment of leukocytes with s everal proteases had no effect on adhesion to E-selectin or on express ion of sLe(x) and sialyl-dimeric Le(x). These data suggest that eosino phils, like neutrophils, use sialylated, protease-resistant structures to bind to E-selectin, although the eosinophil expresses much lower l evels of these structures on its surface. A major proportion of the sL e(x)-containing E-selectin ligand on the surface of eosinophils appear s to be in the form of sialyl-dimeric Le(x), whereas this represents a minor proportion on the surface of neutrophils. Based on results usin g endo-beta-galactosidase, it appears that these cells may rely dispro portionately upon the cell surface sialyl-dimeric Le(x) to bind to E-s electin. Differences between eosinophil and neutrophil sLe(x)-containi ng surface molecules may contribute to variations in their E-selectin- mediated cell recruitment seen during inflammatory responses in vivo.