Platelet-neutrophil aggregate formation induces NLRP3 inflammasome activation in vaccine-induced thrombotic thrombocytopenia.

Background: 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 antiplatelet factor 4 antibody titers promotes the formation of immune complexes capable of activating platelets and neutrophils through FcγRIIa.

Objectives: Given that platelet-leukocyte aggregate formation and inflammasome activation are common features of thromboinflammatory diseases, we aimed to evaluate if these are also features of VITT.

Methods: Samples from a cohort of 57 postvaccination thrombosis patients and 28 age- and sex-matched unvaccinated individuals were used for ex vivo investigation of platelet-leukocyte aggregate formation and inflammasome activation.

Results: Patients with clinical features of VITT presented elevated levels of activated caspase-1, interleukin-18, and interleukin-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 αβ, also inhibiting caspase-1 activation. Additionally, MCC950, an NLRP3 inflammasome inhibitor, blocked caspase-1 activation.

Conclusion: Taken together, these data show that VITT plasma induces platelet-neutrophil aggregate formation in a 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.