Analysis of procoagulant phosphatidylserine‐exposing platelets by imaging flow cytometry

BACKGROUND: Upon platelet activation, a subpopulation of procoagulant platelets is formed, characterized by the exposure of the anionic aminophospholipid phosphatidylserine (PS) on the surface membrane. OBJECTIVE: To evaluate procoagulant PS‐exposing platelets by imaging flow cytometry. METHODS: Platelet ultrastructure was examined by transmission electron microscopy, and a comprehensive analysis of procoagulant platelets was performed using imaging flow cytometry; platelets were fluorescently labeled for the markers glycoprotein (GP)IX, activated integrin αIIbβ3, CD62P, and PS exposure. RESULTS: A subpopulation of platelets stimulated in suspension by the physiological agonists thrombin+collagen, and all platelets stimulated by the calcium ionophore A23187, had a distinct round morphology. These platelets were PS‐exposing, larger in size, had an increased circularity index, and had reduced internal complexity compared with non‐PS‐exposing platelets. They expressed CD62P and αIIbβ3 in an inactive conformation on the surface, and demonstrated depolarized inner mitochondrial membranes. For the first time, using imaging flow cytometry, a large proportion of PS‐exposing platelets possessing platelet‐associated extracellular vesicles (EVs) was observed, which demonstrated heterogeneous platelet marker expression that was different from free released EVs. CONCLUSIONS: Innovative imaging flow cytometry allowed detailed fluorescence‐based, quantitative morphometric analysis of PS‐exposing platelets; in becoming procoagulant, platelets undergo remarkable morphological changes, transforming into spherical “balloons,” almost devoid of their normal internal architecture. Almost all PS‐exposing platelets have associated EVs that are not detectable by traditional flow cytometry. While their functions have yet to be fully elucidated, the heterogeneity of platelet‐associated and released EVs suggests that they may contribute to different aspects of hemostasis and of thrombosis.