Antibodies against Platelet Membrane Glycoproteins

S ummary. Platelet membrane glycoproteins have been isolated by lectin-affinity chromatography and antibodies prepared against them. Platelets that have lost glycocalicin no longer respond to ristocetin-human VIIIR:WF, bovine VIIIR: WF, or to glycocalicin or glycoproteins la and lb antibodies but are still agglutinated by glycoproteins IIb and IIIa antibodies. Glycoproteins la and Ib and glycocalicin antibodies, IgG and Fab fragments, inhibited ristocetin-human VIIIR: WF-induced aggregation of fixed, washed platelets and of platelets in plasma while glycoproteins IIb and IIIa antibodies were without effect.
Cross immunoelectrophoretic studies showed that glycocalicin was present on whole platelets in only trace amounts. Glycocalicin antibodies, however, recognized a slower migrating component. Platelets incubated in an EDTA-free medium no longer respond to ristocetin-human VIIIR: WF. Membranes isolated from such platelets contained glycocalicin which cross-reacted with a remnant of the slower migrating component. Glycoproteins la and Ib antibodies gave more complex patterns but it was possible to identify the slower moving component recognized by the glycocalicin antibodies.
These results show that glycocalicin is not normally found as such on whole platelets but is present as a precursor which is most likely glycoprotein Ib. On degradation of this precursor, glycocalicin is released from the membrane and VIIIR: WF-receptor activity is lost.