Platelet activating histone/antihistone IgG complexes in anti-PF4 negative thrombosis and thrombocytopenia syndrome.

Thrombosis and Thrombocytopenia syndromes (TTS) describe immune mediated thrombotic adverse reactions following vaccination against Covid-19. Vaccine-induced immune thrombotic thrombocytopenia (VITT) is a well-known sub-entity of TTS, caused by adenovirus vector-based vaccines. VITT is mediated by anti-platelet factor 4 (PF4) IgG-antibodies, activating platelets via Fc-gamma IIa receptors (FcgRIIa). We describe clinical and serological features of 18 patients with anti-PF4/heparin ELISA negative TTS in temporal relationship to mRNA-based Covid-19 vaccination. Symptoms began at a median of 7 (range 1-61) days after vaccination. Patients showed thrombocytopenia (59,000/µl, 0-127,000/µl); petechiae (n=7), venous thromboembolism (n=11), arterial thrombosis (n=6), disseminated intravascular coagulation (n=1), and combined arterial and venous thromboses (n=1). Twelve sera induced FcgRIIa dependent and caspase independent procoagulant activation of platelets indicated by phosphatidylserine exposure and CD62P expression. We found histones precipitated with IgG fractions of TTS sera and antibodies binding to histones were found in 8/12 platelet-activating sera. Ex vivo generated histone/anti-histone IgG complexes strongly activated platelets via FcgRIIa, whereas anti-histone-IgG alone did not. Platelet autoantibodies were detected in 7/12 sera targeting GPIIb/IIIa (n=5); GPIb/IX (n=5) and GPIa/IIa (n=3). However, sera containing platelet anti-GPIIb/IIIa autoantibodies activated also platelets from a Glanzmann patient, making it unlikely that these autoantibodies are causative for platelet activation. Finally, 2/114 healthy vaccinees developed anti-histone antibodies after mRNA-based Covid-19 vaccination. Our data indicate a new sub-entity of TTS associated with platelet activating histone/anti-histone IgG complexes. Further studies are warranted to characterize the biological and clinical role of post-mRNA-based vaccination anti-histone antibodies. SeCo, NCT04370119.