Therapeutic options to treat immunothrombosis mediated by platelet activating histone/anti-histone immune complexes

Introduction Platelet activating immune complexes can cause severe immunothrombosis. We recently identified histone/anti-histone immune complexes (His-IC) as new cause of immunothrombosis in patients with severe thrombosis and thrombocytopenia. Heparins may theoretically interfere with His-IC formation by promoting their dissociation in a charge dependent manner. Here, we evaluate two potential therapeutic strategies to treat His-IC mediated platelet activation: I. unfractionated heparin and II. M6229, a heparinoid which lacks the pentasaccharide fraction being responsible for the anticoagulant property. Method His-IC were created by recombinant histone octamers (20 – 0.2µg/ml) and monoclonal IgG1-antibodies against the histones H2A, H2B, H2, H4 (1-0.01µg/ml each]). His-IC or serum from anti-histone IgG positive patients were incubated with washed platelets from healthy donors, in presence or absence of recombinant histone octamers (2µg/ml). Procoagulant platelet activation was measured by phosphatidylserine (PS) exposure based on Annexin V binding and degranulation by CD62P expression using flow cytometry. Activation was inhibited using various concentrations of heparin or M6229. Complex size was determined using dynamic light scattering (DLS) technique. Results Washed platelets treated with histone octamer and anti-histone antibodies were strongly activated as shown by high levels of PS exposure (69.9% PS+) and CD62P expression (MFI CD62P=12827). Addition of at least 0.0125 U/ml of heparin (6.0% PS+, p=0.0001; MFI CD62P=2280, p=0.0046) or 1.1 µg/ml of M6229 (2.6% PS+, p=0.0027; MFI CD62P=1436, p=0.0155) fully inhibited platelet activation by His-IC. Furthermore, at least 47 µg/ml of M6229 fully inhibited platelet activation by high dose histones alone (20µg/ml histone: 14.2% PS+, MFI CD62P=3868; 20µg/ml histone+47 µg/ml M6229: 1.2% PS+, p=0.0195, MFI CD62P=453, p=0.0037). Similar results were obtained for platelet activation induced by patient sera containing anti-histone antibodies (56.4% PS+, MFI CD62P: 6443). Addition of at least 1.25 U/ml of heparin fully inhibited platelet activation (1.9% PS+, p=0.0089; MFI CD62P: 2109, p=0.0276). DLS measurements indicate that addition of heparin and M6229 to His-IC result in complex disassembling. Conclusion Heparin and M6229 are able to inhibit platelet activation induced by Histone/anti-histone IgG complexes because both agents interfere with His-IC formation. Our data suggest, that heparins may be used to treat immunothrombosis caused by histone/anti-histone IgG complexes. Publication History Article published online: 17 February 2026 © 2026. Thieme. All rights reserved. Georg Thieme Verlag KG Oswald-Hesse-Straße 50, 70469 Stuttgart, Germany