Platelet activation and platelet-monocyte interaction associate with cognitive impairment in Long COVID and drive brain endothelial permeability through sCD40L

SARS-CoV-2 may lead to long COVID (LC), characterized by persistent symptoms including cognitive impairment. LC patients also show thromboinflammation, although its mechanistic association with cognitive impairment is unclear. We prospectively evaluated platelet and platelet-monocyte aggregate (PMA) formation in LC patients with cognitive impairment at three timepoints (T1=8.4±3.5, T2=13.2±4.0, T3=16.5±3.2 months post-infection) compared to recovered COVID-19 patients (REC, 8.2±2.5 months), as approved by the IRB. Neuropsychological assessment using Trail Making, Stroop, Digit Span, Symbol Digit, Hopkins Verbal Learning, Semantic Verbal Fluency, and Boston Naming tests revealed impairment in memory, language, attention, and executive function, which partially recovered at T3. Flow cytometry revealed increased platelet activation, PMA formation and monocyte Tissue Factor (TF) expression at T1, especially in intermediate monocytes and in monocytes interacting with platelets, which were resolved by T3. Platelet activation correlated with impaired language, attention and executive function, while PMA formation correlated with impaired memory. To explore underlying mechanisms, monocytes were plated on surfaces coated with CD62P and fibrinogen, implicated in PMA formation. LC monocytes adhered more to combined CD62P and fibrinogen. To verify if LC platelets modulate monocyte responses, REC monocytes were cocultured with platelets from LC or REC patients, and only LC platelets enhanced TF expression on REC monocytes. To assess the impact of soluble mediators on brain endothelial permeability, HBMEC were stimulated with supernatants from LC platelets or platelet-monocyte cocultures, increasing permeability through mechanisms dependent on platelet-derived sCD40L, but not endothelial IL-1R. Our data indicate platelet-driven monocyte thromboinflammation associated with cognitive impairment in LC, potentially contributing to brain vasculopathy through platelet sCD40L signaling.