The role of red blood cells in platelet aggregation in whole blood

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

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.

Spontaneous platelet aggregation in whole blood is increased in non-insulin-dependent diabetes mellitus and in female but not male patients with primary dyslipidemia

Increased platelet aggregability has been shown in hypercholesterolemia, and stirring-induced spontaneous aggregation in whole blood is increased in insulin-dependent diabetes mellitus (DM). We have determined spontaneous aggregation in citrated (10 mM) whole blood, from 27 primary dyslipidemic patients (DYS; 14F, 13M), 16 male non-insulin-dependent DM (NIDDM) patients, and 17 normolipidemic controls (N; 6F, 11M), using platelet counting to quantify aggregation. Spontaneous aggregation was significantly higher, both in the female DYS group (median 30% [interquartile range 25,50], P < 0.005) and the NIDDM group (33% [25,41], P < 0.005), than in the N group (17% [12,27]), but did not differ significantly in the male DYS group (23% [10,33]). Similar results were obtained in the presence of indomethacin (25 mumol/l) to prevent artefactual thromboxane (TX) A2 formation, indicating that increased spontaneous aggregation was TXA2-independent. Interestingly, increased spontaneous aggregation appeared to be independent of serum cholesterol and triglyceride concentrations, as well as age and sex per se. We conclude that spontaneous platelet aggregation was increased both in female primary dyslipidemic patients and NIDDM patients, but not in male DYS patients. The clinical significance of increased spontaneous platelet aggregability is that it may favour shear-induced aggregation which may occur at critical arterial stenoses in vivo leading to thrombus formation.

Sex difference in antithrombotic effect of aspirin

A number of clinical trials suggest that the antithrombotic effect of aspirin is limited to men. To test the possibility that this is due to a sex difference in the inhibitory effect of aspirin on platelet behavior, we studied whole-blood platelet aggregation in men and women and in male patients with carcinoma of the prostate receiving hormone therapy. The in vitro inhibitory effect of aspirin on so-called spontaneous platelet aggregation induced by stirring whole blood and monitored by the decrease in the number of singleton platelets was greater in men (mean +/- SD inhibitory ratio 1.54 +/- 0.30 in men, 1.23 +/- 0.22 in women; p less than 0.001). The inhibitory effect of aspirin was reduced in orchiectomized male patients and was restored by the addition of testosterone to blood samples. Estradiol had no detectable influence on the inhibitory effect of aspirin. Testosterone thus seems to influence platelet aggregation and its inhibition by aspirin as assessed by whole-blood in vitro aggregometry. Possible mechanisms for this effect of testosterone and its relevance to the choice of antithrombotic therapy are discussed.