Circulating levels of platelet, endothelial and red blood cell microvesicles in COVID-19: Association with biomarkers of coagulation, fibrinolysis and angiogenesis

Background: Patients with severe coronavirus disease 19 (COVID-19) are at increased risk of thrombosis, which is associated with enhanced coagulopathy and increased mortality. Previous studies showed enhanced platelet aggregation in patients with COVID-19, however the underlying mechanism and how this contributes to thrombosis is still unclear. Aims: To evaluate the platelet proteome, platelet functional responses and platelet/leukocyte aggregate formation in patients with COVID-19. Methods: This study was approved by the local ethics committee. Platelets were isolated from patients with COVID-19 and healthy controls after providing informed consent. Platelet lysates were subjected to tandem mass tag mass spectrometry (TMT-MS) proteomic analysis. Platelet functional responses such as integrin a IIb b 3 activation, P-selectin expression, platelet/leukocyte aggregates and PS exposure were analysed by FACS analysis. Results: The platelet proteome was altered in COVID-19, with enrichment of ribosomal and mitochondrial proteins, and reduced levels of thrombopoietin (TPO) receptor and signalling proteins. Circulating platelets from COVID-19 patients furthermore showed enhanced basal PS exposure, whereas basal integrin a IIb b 3 activation and P-selectin expression were unaltered. In contrast, agonist stimulated integrin a IIb b 3 activation and PS exposure were significantly decreased in COVID-19 patients. Furthermore, we found high levels of platelet/leukocyte aggregate formation in COVID-19 patients, which was reduced by a blocking anti-P-selectin antibody, suggesting immunothrombosis is part of the COVID-19 phenotype. Interestingly, however platelets in these platelet/leukocyte aggregates did not show enhanced integrin a IIb b 3 activation, suggesting they are generally in a low activation state, or having undergone activation followed by desensitisation. Conclusions: High levels of platelet/leukocyte aggregates are present and given the increased thrombotic tendency in severely ill patients, these findings point to an immunothrombotic pathogenesis. It will be important now to determine whether these changes are the result of direct activation of platelets or leukocytes by viral contact or cellular infection.