Cox, Alison D.; (1991) A study of platelet activation: The role of membrane glycoproteins of the integrin and selectin superfamilies. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Platelet activation results in two major changes at the platelet surface: a change in GPIIb- Illa, exposing the previously cryptic fibrinogen binding site and fusion of platelet and granule membranes, leading to the expression of the two known granule membrane proteins, GMP-140 (α-granule) and GP53 (lysosome). Anti-GPIIb-IIIa monoclonal antibodies (MAbs) were raised and characterised in functional and flow cytometric assays. Competitive binding studies, with these and a panel of anti-GPIIb-IIIa MAbs, identified distinct epitopes on the complex important in influencing fibrinogen receptor exposure. Platelet aggregation was totally inhibited by MAbs binding to two conformation-dependent sites, one on GPIIb-IIIa and one on GPIIIa. A second group of antibodies, heterogeneous in their inhibitory effects on platelet aggregation, bound to overlapping sites on GPIIb-IIIa, GPIIb and GPIIIa. Three conformation-specific sites were identified, which induced platelet aggregation in the absence of any other agonist. All inhibitory MAbs totally inhibited fibrinogen binding to ADP-stimulated platelets and evidence is presented that fibrinogen and von Willebrand Factor bind to GPIIb-IIIa in a comparable and competitive way. Anti-GMP-140and anti-GP53 MAbs, raised against activated platelets, were characterised in Western blotting and phenotypic studies. Anti-GP53 MAbs were heterogeneous in their reactivity with a number of cell types; in particular, one MAb, RFAC-4, failed to bind to platelets from albino patients: a finding that may reflect the homology of GP53 with the melanoma-associated antigen, ME491. Flow cytometric, antibody binding studies showed that whilst the expression of both GMP-140 and GP53 was up-regulated over very similar thrombin concentrations, their expression ADP-stimulated platelets differed: GMP-140 was not detected on these cells but a partial expression of GP53 was seen. Using a whole blood, flow cytometric method, which minimises platelet activation in vitro. GP53 was shown to be a useful marker of platelet activation in the ex vivo analysis of platelets in a several clinical conditions.

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