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 after vaccination against COVID-19. Vaccine-induced immune thrombotic thrombocytopenia (VITT) is a well-known subentity of TTS, caused by adenovirus vector-based vaccines. VITT is mediated by anti-platelet factor 4 (PF4) immunoglobulin G (IgG) antibodies, activating platelets via Fc-γ IIa receptors (FcγRIIa). We describe clinical and serological features of 18 patients with anti-PF4/heparin enzyme-linked immunosorbent assay (ELISA)-negative TTS in temporal relationship to messenger RNA (mRNA)-based COVID-19 vaccination. Symptoms began at a median of 7 (range 1 - 61) days after vaccination. Patients showed thrombocytopenia (platelet count 59 × 103/μL; range, 0 to 127 × 103/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 FcγRIIa-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. Antibodies binding to histones were found in 8 of 12 platelet-activating sera. Ex vivo-generated histone/antihistone IgG complexes strongly activated platelets via FcγRIIa, whereas antihistone IgG alone did not. Platelet autoantibodies were detected in 7 of 12 sera targeting glycoprotein (GP) IIb/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 patient with Glanzmann thrombasthenia, making it unlikely that these autoantibodies are causative for platelet activation. Finally, 2 of 114 healthy vaccinees developed antihistone antibodies after mRNA-based COVID-19 vaccination. Our data indicate a new subentity of TTS associated with platelet-activating histone/antihistone IgG complexes. Further studies are warranted to characterize the biological and clinical role of post-mRNA-based vaccination antihistone antibodies. The SeCo trial was registered at www.ClinicalTrials.gov as #NCT04370119.