Detection of activated platelets in whole blood using activation- dependent monoclonal antibodies and flow cytometry

Platelets may become activated in a number of clinical disorders and participate in thrombus formation. We developed a direct test for activated platelets in whole blood using flow cytometry. Whole blood was incubated with either biotin-PAC1, a monoclonal antibody specific for the fibrinogen receptor on activated platelets, or biotin-S12, an antibody specific for an alpha-granule membrane protein that associates with the platelet surface during secretion. Platelet-bound antibodies were detected with streptavidin conjugated with fluorescein isothiocyanate (FITC) or phycoerythrin (PE). Platelets were differentiated from the larger erythrocytes and WBCs by their light-scatter profile. Alternatively, platelets could be identified with FITC-AP1, an antibody specific for platelet membrane glycoprotein Ib, and analyzed further for PAC1 or S12 binding with PE-streptavidin. No centrifugation or washing steps were required. With gel-filtered platelets, there was a direct correlation between ADP-induced biotin-PAC1 binding and binding determined in a conventional 125I-PAC1 binding assay (r = .99; P less than .001). Furthermore, as few as 0.8% activated platelets could be detected by flow cytometry when activated platelets were mixed with unstimulated platelets. In whole blood, unstimulated platelets demonstrated no PAC1- or S12-specific fluorescence, indicating that they did not bind these antibodies. On stimulation with agonists, however, the platelets demonstrated a dose-dependent increase in fluorescence similar to that observed for platelets in plasma or buffer. Low concentrations of ADP and epinephrine, which induce fibrinogen receptors but little secretion, stimulated near-maximal PAC1 binding but little S12 binding. On the other hand, a concentration of phorbol myristate acetate (TPA) that evokes full platelet aggregation and secretion induced maximal PAC1 and S12 binding. Activated platelets could also be analyzed in whole blood samples that had been fixed with paraformaldehyde. These studies demonstrate that activated platelets can be reliably detected in whole blood using activation-dependent monoclonal antibodies and flow cytometry. This technique may be useful to assess the degree of platelet activation and the efficacy of antiplatelet therapy in clinical disorders.