Oberg, J. | Search bioRxiv

bioRxiv2025

Antiphospholipid syndrome (APS) is a platelet factor 4 (PF4)-centric immunothrombotic disorder

Antiphospholipid syndrome (APS) is an immunothrombotic disorder, frequently attributed to autoantibodies that bind {beta}2-glycoprotein I ({beta}2GPI). A study showed that the platelet-specific chemokine, platelet factor 4 (PF4), binds to {beta}2GPI, enhancing recognition of {beta}2GPI by APS antibodies. APS antibodies induce the release of neutrophil extracellular traps (NETs), webs of decondensed chromatin that bind both PF4 and {beta}2GPI. We propose that PF4 bridges {beta}2GPI to NETs (and other PF4-targeted polyanions), leading to the formation of prothrombotic PF4:{beta}2GPI:NET immunotargets in APS. Dynamic light-scattering studies of isolated IgGs from four patients with triple-positive APS show formation of PF4:{beta}2GPI:NET complexes that bind APS antibodies. NETs released in a microfluidic system bound {beta}2GPI, but only in the presence of PF4, forming a multimolecular APS antigenic target. Whole blood infused through a photochemically-injured, endothelium-lined microfluidic channel formed platelet-, fibrin-, and complement-rich thrombi that bound APS antibody only in the presence of PF4. Thrombi were reduced in size if either ADAMTS13 or DNase1 was infused. In a murine APS model, wildtype and transgenic mice expressing platelet human PF4 {+/-} Fc{gamma}RIIA developed more intense neutrophil rolling along veins, and more extensive thrombus formation following laser injury to cremaster arterioles and venules, whereas mice lacking PF4 did not. Three antigenically distinct anti-hPF4 monoclonal antibodies blocked thrombosis in vitro, and neutrophil rolling and thrombosis in vivo. Our studies provide new insights into the basis of APS that has mechanistic parallels to other known PF4 immunothrombotic disorders and offer potential diagnostic and non-anticoagulant therapeutic strategies for clinical management. Key pointsO_LIPF4 enhances {beta}2GPI binding to NETs and these complexes are central to APS immunothrombosis. C_LIO_LIAnti-PF4 monoclonal antibodies block APS immunothrombosis in microfluidic and murine studies. C_LI

cell biology↗

bioRxiv2025

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 (PF4-/-) 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. KEY POINTSO_LIIn vitro, PF4 accelerates bacterial clearance by enhancing endothelial uptake of bacteria and promoting their trafficking to the lysosome. C_LIO_LIIn murine sepsis, PF4 augments pathogen clearance to reduce infection severity, limit organ injury, and improve survival. C_LI

cell biology↗