Spontaneous Platelet Aggregation in Blood Is Mediated by FcγRIIA Stimulation of Bruton’s Tyrosine Kinase

Abstract

High platelet reactivity leading to spontaneous platelet aggregation (SPA) is a hallmark of cardiovascular diseases; however, the mechanism underlying SPA remains obscure. Platelet aggregation in stirred hirudin-anticoagulated blood was measured by multiple electrode aggregometry (MEA) for 10 min. SPA started after a delay of 2–3 min. In our cohort of healthy blood donors (n = 118), nine donors (8%) with high SPA (>250 AU*min) were detected. Pre-incubation of blood with two different antibodies against the platelet Fc-receptor (anti-FcγRIIA, CD32a) significantly reduced high SPA by 86%. High but not normal SPA was dose-dependently and significantly reduced by blocking Fc of human IgG with a specific antibody. SPA was completely abrogated by blood pre-incubation with the reversible Btk-inhibitor (BTKi) fenebrutinib (50 nM), and 3 h after intake of the irreversible BTKi ibrutinib (280 mg) by healthy volunteers. Increased SPA was associated with higher platelet GPVI reactivity. Anti-platelet factor 4 (PF4)/polyanion IgG complexes were excluded as activators of the platelet Fc-receptor. Our results indicate that high SPA in blood is due to platelet FcγRIIA stimulation by unidentified IgG complexes and mediated by Btk activation. The relevance of our findings for SPA as possible risk factor of cardiovascular diseases and pathogenic factor contributing to certain autoimmune diseases is discussed.

Chemical degradation of BTK/TEC as a novel approach to inhibit platelet function and thrombosis

“…Oral BTK inhibitors furthermore impair thrombus formation triggered by plaques and collagen, suggesting BTK is a promising target as an anti-platelet drug 3,4 . In addition, BTK contributes to FcRIIa-mediated spontaneous platelet aggregation 5 and responses to bacteria 6 . However, clinically used BTK inhibitors are associated with bleeding, likely to be due to off target effects.…”

Chemical degradation of BTK/TEC as a novel approach to inhibit platelet function

“…Studies on mouse models with megakaryocyte/platelet specific deletion of BTK, pharmacological inhibitors, and platelets from patients with a functional mutation in BTK (X-linked Bruton agammaglobulinemia), demonstrated an essential role of BTK downstream of immunoreceptor tyrosine-based activation motif receptors such as GPVI and CLEC2, whereas having little effect on GPCR-mediated platelet function. 2 , 4 In addition, BTK contributes to FcRIIa-mediated spontaneous platelet aggregation 5 and responses to bacteria. 6 Oral BTK inhibitors furthermore impair thrombus formation triggered by plaques and collagen, proposing BTK is a promising target as an antiplatelet drug.…”