Platelet-Neutrophil aggregate formation induces NLRP3 inflammasome activation in VITT

Although rare, vaccine-induced thrombotic thrombocytopenia (VITT) following adenoviral vector COVID-19 vaccination is a concerning and often severe adverse effect of vaccination. The generation of high anti-platelet factor 4 (PF4) antibody titers, promotes the formation of immune complexes capable of activating platelets and neutrophils through FcγRIIa. Given that Platelet-leukocyte aggregate (PLA) formation and inflammasome activation are common features of thromboinflammatory diseases, we aimed to evaluate if these are also features of VITT. Samples from a cohort of 57 postvaccination thrombosis patients and 28 age- and sex-matched unvaccinated individuals were used for ex-vivo investigation of PLA formation and inflammasome activation. Patients with clinical features of VITT presented elevated levels of activated caspase-1, IL-18 and IL-1β in the plasma. We also found that soluble factors in the plasma of VITT patients induce the formation of platelet-neutrophil aggregates but not platelet-monocyte or platelet-T-cell aggregates, which are associated with increased caspase-1 activation in neutrophils ex-vivo. Platelet-neutrophil aggregate formation was prevented through blockage of FcγRIIa with the neutralizing antibody IV.3, and through blockage of P-selectin or integrin αIIbβ3, also inhibiting caspase-1 activation. Additionally, MCC950, an NLRP3 inflammasome inhibitor, blocked caspase-1 activation. Taken together, these data show that VITT plasma induces platelet-neutrophil aggregate formation in an FcγRIIa-dependent manner and that platelet-neutrophil interactions may contribute to thromboinflammation in VITT patients by supporting NLRP3 inflammasome activation. These data shed light on novel immunopathological events associated with inflammation and thrombosis in VITT patients.