Receptor patching and capping of platelet membranes induced by monoclonal antibodies

Redistribution of glycoproteins (GP) Ib, glycocalicin, IIb, and IIIa on the surface of human platelets in response to stimulation with corresponding monoclonal antibodies (MoAb) and a polyclonal antiglycocalicin antibody was studied by immunofluorescence, immunoelectron microscopy, and a quantitative radioimmune assay. Immobilization of the antigens by prefixation with formaldehyde showed a uniform distribution over the surface of the platelet. Incubation of unfixed platelets with specific MoAb against various epitopes on GPIIb and/or IIIa resulted in a time-dependent patching, subsequent capping, and after prolonged exposure to the antibody/label complex, internalization of the complex, possibly by endocytosis. In contrast, GPIb could not be shown to cap. From these results we conclude that platelet GPIIb and/or IIIa undergo spatial rearrangement in a manner analogous to that observed in lymphocytes, whereas GPIb does not. Since both GPIb and GPIIb and/or IIIa seem to be transmembraneous GP associated with the cytoskeleton, a special, though unidentified, role of GPIIb/IIIa in the induction of lateral membrane mobility is postulated.