Rationale: COVID-19 is characterized by increased incidence of microthrombosis with hyperactive platelets sporadically containing viral RNA. It is unclear if SARS-CoV-2 directly alters platelet activation or if these changes are a reaction to infection-mediated global inflammatory alterations. Importantly, the direct effect of SARS-CoV-2 on platelets has yet to be studied.

Objective: To characterize the direct SARS-CoV-2-platelet interactions using in vitro studies with purified infectious virions and samples from infected patients.

Methods and Results: Platelet RNA analyzed by ARTIC v3 sequencing for SARS-CoV-2 showed presence of fragmented viral genome in all COVID-19 patients. Immunofluorescent imaging of platelets from COVID-19 patients confirmed presence of SARS-CoV-2 proteins, while there was no detection of viral RNA by RT-qPCR. Transmission electron microscopy (TEM) of platelets incubated with purified SARS-CoV-2 virions demonstrated rapid internalization and digestion leading to distinct morphological changes, and resulted in a release of extracellular vesicles. Interactions between SARS-CoV-2 and platelets occurred with or without ACE2 presence as measured by immunofluorescence. TEM showed that SARS-CoV-2 virions became internalized when they were attached to microparticles, bypassing the need for ACE2. Enrichment analysis of platelet-transcriptome from patients with acute COVID-19, compared to those with clinical thrombosis, suggested upregulation of pathways related to virally mediated cell death, specifically necroptosis and apoptosis. Platelets incubated with infectious virus appeared to undergo cell death in 30 min post-incubation as assessed by TEM and platelets from COVID-19 patients showed evidence of increased markers of apoptosis and necroptosis by WB. Immunofluorescence confirmed colocalization of SARS-CoV-2 with phospho-MLKL and Caspase-3 on non-permeabilized platelets in vitro and in COVID-19 platelets.

Conclusions:Platelets internalize SARS-CoV-2 virions, directly or attached to microparticles, and viral internalization leads to rapid digestion, programmed cell death and extracellular vesicle release. During COVID-19, platelets mediate a rapid response to SARS-CoV-2 and this response can contribute to dysregulated immunity and thrombosis.