IMMUNOLOGICAL COMPARISONS OF INTEGRIN ALPHA(IIB)BETA(3) (GPIIB-IIIA) EXPRESSED ON PLATELETS AND HUMAN ERYTHROLEUKEMIA-CELLS - EVIDENCE FOR CELL-SPECIFIC DIFFERENCES

Platelet glycoprotein IIb-IIIa (GPIIb-IIIa, alpha(IIb)beta(3)) is expr essed on the cell surface of the human erythroleukemia (HEL) cell line , Previous studies have demonstrated differences in GPIIb-IIIa ligand binding properties of HEL cells when compared to platelets. Although t he mRNA sequences for GPIIb and GPIIIa are identical in platelets and HEL cells, cell specific differences in the conformation states of the GPIIb-IIIa complex may exist and may explain in part the contrasting functional properties. Two monoclonal antibodies (mAbs), an anti-GPIIb mAb C3 and an anti-GPIIIa mAb D3, were used to determine whether diff erences in GPIIb-IIIa conformational states could be measured, Initial studies in a purified system showed that the mAbs' binding to isolate d GPIIb-IIIa conformers was increased to the active GPIIb-IIIa and to dissociated receptor subunits when compared to the inactive form, Furt hermore, soluble active GPIIb-IIIa was a much better inhibitor of D3 b inding to the immobilized receptor compared to soluble inactive GPIIb- IIIa. Extending these studies with intact cells, we detected at least two classes of binding sites for each mAb on each cell type. Differenc es in B-max and in the relative affinities of the mAbs were identified and may represent subpopulations of GPIIb-IIIa conformations, Total H EL cell and platelet GPIIb-IIIa was determined in our binding assays u sing a radiolabeled GPIIb-IIIa complex specific mAb, 10E5. HEL cells e xpress approximately five times more GPIIb-IIIa on a per cell basis. T he percent of total GPIIb-IIIa that represented each class of mAb bind ing sites was determined. In summary, the relative differences in GPII b-IIIa conformation found on platelets and HEL cells may be related to cell-specific ligand binding properties and activation states of the receptor.