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

Abstract Antiphospholipid syndrome (APS) is an immunothrombotic disorder, frequently attributed to autoantibodies that bind β2-glycoprotein I (β2GPI). A study showed that the platelet-specific chemokine, platelet factor 4 (PF4), binds to β2GPI, enhancing recognition of β2GPI by APS antibodies. APS antibodies induce the release of neutrophil extracellular traps (NETs), webs of decondensed chromatin that bind both PF4 and β2GPI. We propose that PF4 bridges β2GPI to NETs (and other PF4-targeted polyanions), leading to the formation of prothrombotic PF4:β2GPI:NET immunotargets in APS. Dynamic light-scattering studies of isolated IgGs from four patients with triple-positive APS show formation of PF4:β2GPI:NET complexes that bind APS antibodies. NETs released in a microfluidic system bound β2GPI, 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 ± FcγRIIA 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 points PF4 enhances β2GPI binding to NETs and these complexes are central to APS immunothrombosis. Anti-PF4 monoclonal antibodies block APS immunothrombosis in microfluidic and murine studies.