Galectin-9 drives thrombo-inflammatory circuits initiating venous thrombosis

Background: Venous thromboembolism is increasingly recognized as a thrombo-inflammatory disease, yet the pathways linking inflammation to venous thrombus initiation remain incompletely defined. Galectin-9 (Gal-9), a β-galactoside-binding lectin elevated in inflammatory disorders, has no defined role in venous thrombosis. Methods: Using plasma from patients with deep vein thrombosis (DVT), Gal-9-deficient mice, an inferior vena cava stenosis model, intravital microscopy, microfluidics, and ex vivo platelet and neutrophil assays, we assessed the functional contribution of Gal-9 to venous thrombosis. Results: Plasma Gal-9 was elevated in DVT patients, correlated with D-dimer at presentation, and declined over 3-month follow-up. Gal-9 deletion did not alter hemostasis but markedly impaired thrombus initiation, with reduced platelet and neutrophil recruitment to the vessel wall and reduced intra-thrombus neutrophil extracellular trap (NET) formation. TNF-α/IFN-γ-stimulated endothelial cells upregulated Gal-9 and promoted platelet adhesion and fibrin(ogen) deposition, which Gal-9 neutralization partially reversed. Under venous shear, immobilized recombinant Gal-9 potentiated platelet activation and thrombus formation, particularly alongside collagen, whereas Gal-9 inhibition suppressed these responses. Gal-9-activated platelets promoted NETosis and shifted thrombus composition toward a NET-rich inflammatory phenotype. Conclusions: Gal-9 is a previously unrecognized driver of venous thrombo-inflammation, linking vascular inflammation to platelet activation, NET formation, and thrombosis. Genetic deletion attenuated these responses in vivo, and antibody-mediated Gal-9 neutralization suppressed platelet activation and thrombus formation in vitro, identifying the Gal-9 glycan-binding pathway as a therapeutic target that disrupts thrombosis without impairing hemostasis.