ABSTRACT
Background: Platelets are effectors of hemostasis and play a major role in coordinating immune and inflammatory activities. Suitable animal models are needed to study COVID-19-associated coagulopathy and platelet effector functions in COVID-19, which are currently poorly understood. Aim(s): We aimed to characterize alterations of platelets isolated from K18-hACE2 transgenic mice infected with SARS-CoV-2. Method(s): Heterozygous K18-hACE2 (human ACE2) and C57BL/6J mice were used to study SARS-CoV-2 infectivity. Lung infection, infiltration, and platelet aggregation were characterized with histology and immunohistochemistry. Platelet response to SARS-CoV-2 infection was quantified by mass spectrometry analysis of proteomics and phosphoproteomics. Western blotting, ELISA, and multiplex plasma profiling were performed to validate the proteomics and phosphoproteomics data. Result(s): SARS-CoV-2 inoculated (10E6PFU, i.n.) K18-hACE2 mice started to lose weight at 4 days post-infection (dpi) and showed 90% lethality at 7-dpi in association with viral neuroinvasion. Histopathologic findings of infected K18-hACE2 mice included progressive lymphohistiocytic interstitial pneumonia with absence of diffuse alveolar damage. Lungs of infected K18-hACE2 mice (2-/ 4-dpi) showed mild increase in CD61+ aggregates compared to sham mice, but no overt tissue thrombosis. Gene ontology and pathway analyses of platelet proteomics and phosphoproteomics revealed that SARS-CoV-2 infection significantly upregulates the complement-coagulation cascades (F2/12/13, Tfpi, C1ra, Cd55, C4bp) and platelet activation-adhesion-degranulation proteins (Vwf, Itgb3/5, Selp, Pecam1) and chemokine (Pf4, Cxcl5/12) signaling at 2-dpi. However, interferon (Ddx58, Trim25, Mapk3) signaling was dominant at 4-dpi. Activation of proteomics and phosphoproteomics protein markers were highly correlated with platelet activation and interferon signaling at 2-/ 4-dpi, respectively. Plasma chemokine (e.g., Ccl8 and Pf4) and cytokines (e.g., IL6) were significantly elevated at 2-/ 4-dpi. SARS-CoV-2 spike protein was abundant at 2-/ 4-dpi in the lungs but not in platelets and kidneys, which correlated with no infectious virus in the serum. Conclusion(s): Platelet re-programming towards activation-degranulation-aggregation is likely attributable to a pneumonia-induced elevated circulatory factors (e.g., cytokines)-driven response rather than direct platelet infection.