Blood leukocytes bind platelet glycoprotein (lIb-Ilia)' but do not express the vitronectin receptor

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Abstract
Within the integrin family of Arg-Gly-Asp(RGD)-binding adhesion receptors, the subfamily defined by the β chain known as β3 or glycoprotein (GP)llla is known to contain two individual receptors. These are the GPIIb Ilia complex of platelets, where the α chain of the heterodimer is GPIIb, and the vitronectin receptor (VnR) containing the αv subunit. The presence of either GPIIb - Ilia and/or the VnR on blood leukocytes has been controversial. We have investigated this problem by performing immunoprecipltation and immunoblotting studies with rabbit and monoclonal antibodies (mAb) to each of the subunits of GPIIb-Ilia and the VnR. On cultured cells of different origin, it was established that almost all expressed the VnR but none had GPIIb-Ilia, and the only polypeptide associated with β3 was αv. Platelets expressed predominantly GPIIb-Ilia, and weakly, the VnR. Monocytes and neutrophils freshly isolated from blood did not express the VnR but bore on their surface a modified form of GPIIb-Ilia. This molecule appeared identical to GPIIb-Ilia but an epitope on GPIIb was masked on the intact cell and was only revealed after immunoblotting. We have termed this modified form of GPIIb-Ilia, GP(llb-llla'. With differentiation in culture, monocytes rapidly lost surface GP(llb-llla)' and concurrently began to express the VnR. Evidence is presented that GP(llb-llla)' is derived from particles released by activated platelets and is bound firmly to the leukocyte membrane. Its primary function does not seem to be to mediate attachment to matrix proteins; thus, although U937 cells bearing platelet-derived GP(llb-llla)' bound fibrinogen in an RGD-dependent manner, isolated blood monocytes did not. It is suggested that this transfer of membrane proteins from platelets to monocytes and neutrophils may regulate the expression of the leukocyte VnR and also serve as a means of facilitating leukocyte procoagulant activity.