Platelets of COVID-19 patients are refractory due to thrombin receptor desensitizing in the bloodstream

ABSTRACT

Background: Coagulopathy is among the most alarming drivers of COVID-19-induced pathology. COVID-19 can result in blockade of the microvasculature in the lungs with microthrombi. While most of the studies concern COVID-19 impact on plasma coagulation, platelet dysfunction has been reported as well. However, the mechanism of platelet malfunctioning in COVID-19 has not been described yet. Aims: To determine the mechanism of COVID-19 induced platelet disfunction. Methods: 46 patients with confirmed COVID-19 (11 non-ICU, 26 ICU, and 9 ECMO) and 26 healthy volunteers were studied (independent ethics committee of NMRC PHOI No 3/2020). Citrated whole blood samples were diluted in Tyrode's buffer and activated by collagen and TRAP-6. Platelets of healthy donors were additionally washed and pre-treated by 0.5 nM of thrombin. Samples were analyzed using BC Navios flow cytometer. Additionally, light transmission aggregometry (AP-2110 SOLAR) of citrated platelet-rich plasma (PRP) with TRAP-6 and fucoidan as activators was conducted. Results: Platelet forward scattering parameter (FSC-A) for COVID-19 patients was significantly increased compared to healthy donors. This parameter reversibly correlated with mild thrombocytopenia observed in some patients. The amount of Annexin-V positive platelets was increased in all patients as well. Relative CD42b and CD62p binding upon activation were decreased, being statistically different in non-ICU and ICU patients. Both of the parameters also correlated to CRP concentration in patient blood plasma. Platelet aggregation was reduced as well. Altogether, platelets demonstrated refractoriness resembling desensitization of receptors. To prove this hypothesis, we performed thrombin pre-treatment of healthy donor platelets, which resulted in a phenotype resembling the COVID-19. Conclusions: Platelets of COVID-19 patients demonstrate refractoriness to activation through PAR1 receptor, probably because of a previous activation with thrombin in circulation. Together with their increased size and fraction of necrotic platelets, this suggests that in COVID-19 platelets encounter thrombin in circulation.