Platelet factor 4 modulates endothelial cell antimicrobial activity to enhance bacterial clearance and improve sepsis outcomes.

Sepsis is a life-threatening condition characterized by dysregulated host responses to infection. Here, we identify platelet factor 4 (PF4) as a key mediator of vascular antimicrobial defense. In vitro, PF4 enhanced endothelial cell internalization of Escherichia coli via interactions with the PF4 receptor CXCR3 and the endothelial glycocalyx, directing bacteria to clathrin-mediated endocytosis and lysosomal degradation. In vivo, PF4 administration improved survival and reduced sepsis severity, bacterial burden, inflammation, and thrombosis in wild-type (WT) and PF4 knockout (PF4italic-/-) mice challenged with systemic polymicrobial infection. Using intravital microscopy, we observed that infused bacteria were rapidly sequestered in the pulmonary microvasculature. However, PF4 -/- mice exhibited impaired bacterial clearance and increased microvascular platelet adhesion and aggregation. In the liver, following Kupffer cell depletion, PF4 -/- mice had increased sinusoidal platelet accumulation, larger bacterial aggregates, and elevated hepatic bacterial burden compared to WT controls. Collectively, these findings reveal that PF4 promotes bacterial clearance and restrains immunothrombosis during sepsis in part via endothelial cell uptake and destruction of microbes. By enhancing endothelial antimicrobial function, PF4 represents a significant yet previously underrecognized host defense mechanism that limits bacterial spread and alleviates vascular injury during infection.