Monoclonal and oligoclonal anti-platelet factor 4 antibodies mediate VITT

COVID-19 vaccines have been associated with a rare life-threatening thrombotic and thrombocytopenic adverse reaction, vaccine-induced immune thrombotic thrombocytopenia (VITT) characterized by high levels of platelet-activating anti-platelet factor 4 (PF4) antibodies. In this study, we evaluated the pathogenesis and clonality of VITT antibodies from five patients with VITT secondary to ChAdOx1 nCoV-19 (n=1) and Ad26.COV2.S (n=4) vaccination. For comparative studies with heparin-induced thrombocytopenia (HIT), anti-PF4 antibodies were isolated from patients with spontaneous HIT, "classical" HIT, and patients with non-pathogenic anti-PF4 antibodies. Isolated antibodies were subject to ELISA, functional testing, and mass spectrometric evaluation. All five VITT patients had oligoclonal anti-PF4 antibodies (3 monoclonal, one bi- and one tri-clonal antibodies), while HIT anti-PF4 antibodies were polyclonal. Notably, like VITT, anti-PF4 antibodies from a spontaneous HIT patient were monoclonal. Polyclonal anti-PF4 antibodies were detected by mass spectrometry in one patient with non-pathogenic HIT, while none were detected in two others. The ChAdOx1 nCoV-19-associated VITT patient made an excellent recovery on heparin, and in vitro studies demonstrated inhibition of VITT antibody-induced platelet activation with therapeutic concentrations of heparin in this and all four Ad26.COV2.S-associated VITT patients. Persistent platelet-activating antibodies were detected at 1.5 and 2.5 months after acute presentation in two patients tested. This study demonstrates that mono/oligoclonal anti-PF4 antibodies mediate VITT and are frequently persistent. As VITT antibody-mediated platelet activation is inhibited by heparin, additional studies to evaluate a potential role of heparin treatment in VITT may be warranted.