Background The novel coronavirus SARS-CoV-2 has caused a global pandemic. Vaccines are an important part of the response. Although rare, unusual thrombotic events and thrombocytopenia in recipients 4-16 days after vaccination with the AstraZeneca AZD1222 have been reported. This syndrome of vaccine-induced immune thrombotic thrombocytopenia (VITT) clinically resembles heparin induced thrombocytopenia (HIT), which is caused by platelet activating antibodies against platelet factor 4 (PF4). Here, we investigate the effect of serum from patients with VITT on platelet activation, and assess the ability of clinically available therapeutics to prevent platelet activation. Methods Aggregation responses of healthy donor washed platelets were assessed in response to serum from patients with VITT pre- and post-intravenous immunoglobulin (IVIg) treatment and in the presence of anti-Fc{gamma}RIIA blocking IV.3 F(ab), anti-platelet drugs and kinase inhibitors. Findings Four patients (21-48 years old) presented with thrombosis (three patients: cerebral venous sinus thrombosis, one patient: ischemic stroke) and thrombocytopenia 10-14 days after AZD1222 vaccination. All patients tested positive for anti-PF4 antibody despite no prior heparin exposure. Serum from patients with VITT, but not healthy donor controls, induced platelet aggregation, which was abrogated following IVIg treatment. Aggregation to patient sera was blocked by IV.3 F(ab) which targets Fc{gamma}RIIA, and inhibitors of Src, Syk and Btk kinases downstream of the receptor. Anti-platelet therapies indomethacin and ticagrelor also blocked aggregation. Interpretation In conclusion, serum from patients with VITT activates platelets via the Fc{gamma}RIIA, which can be blocked in vitro by anti-platelet therapies suggesting possible new therapeutic interventions for this rare syndrome. Funding This work was supported by an Accelerator Grant (AA/18/2/34218) from the British Heart Foundation (BHF).
Platelet Activation by Vaccine-Induced Immune Thrombotic Thrombocytopenia (VITT) Patient Serum is Blocked by COX, P2Y12 and Kinase Inhibitors