OC 05.3 - Anti-platelet factor 4-induced procoagulant platelets are key mediators of thrombus formation in vaccine-induced thrombotic thrombocytopenia

Background: Vaccine-induced thrombotic thrombocytopenia (VITT) is a rare but life-threatening prothrombotic syndrome observed in individuals following vaccination with vector-based SARS-CoV-2 vaccines. VITT is caused by antibodies (Abs) that recognize specific heparin-independent epitopes on platelet factor 4 (PF4) which results in the initiation of a multicellular driven prothrombotic response and onset of thrombosis at unusual sites. Although PF4 has been identified as the primary target of VITT Abs, the mechanisms of VITT Ab-mediated prothrombotic conditions are not fully clarified yet.

Aims: This study investigates the contribution of anti-PF4-induced procoagulant PLTs in VITT Ab-mediated thrombus formation.

Methods: For the investigation of VITT patient Ab-induced PLT alterations, a flow cytometry (FC)-based protocol was employed. To study the impact of VITT Ab-mediated PLT alterations on thrombus formation, a microfluidic-based ex vivo model of Ab-mediated thrombosis was developed.

Results: VITT patient IgG induced significant formation of a CD62p/phosphatidylserine (PS) double positive procoagulant PLT phenotype in a PF4-dependent manner as procoagulant PLT formation was only detected in the presence of exogenous PF4 and not in vehicle (p value 0.0184). Of note, PLTs that were incubated with VITT IgG in the presence of exogenous PF4 and developed a procoagulant PLT phenotype mediated dramatic thrombus formation compared to control (p value 0.0022). Most importantly, the relevance of Ab-induced procoagulant PLTs in thrombus was further confirmed as depletion of PLTs in whole blood prior to VITT IgG incubation and specific PLT PS inhibition via dynamin inhibitor MitMab both resulted in a nearly complete abolishment of Ab-induced procoagulant PLT-driven thrombus formation.

Conclusion(s): Taken together, our data indicate that PF4 is the key mediator of VITT Ab-induced procoagulant PLT-driven thrombus formation ex vivo. The finding that PLT depletion and specific PLT PS inhibition results in significant reduction of VITT Ab-induced thrombus directs towards an essential contribution of Ab-induced procoagulant PLTs in VITT.