PF4 and myalgic encephalomeyelitis/chronic fatigue syndrome: PF4 modulates endothelial and vascular smooth muscle cells in a proinflammatory manner [66, 83] and is likely an ongoing consequence of platelet hyperactivity in ME/CFS (along with elevations in TSP-1), potentially accounting for signs of endothelial damage observed in patients [12, 16, 18].