Inflammation-induced platelet reactivity can cause thromboembolic disease in immune-mediated hemolytic anemia (IMHA) patients. Ultra–low-dose aspirin (ULDAsp) may increase survival rates in IMHA patients by reducing platelet hyperactivity, systemic inflammation, and thromboembolic complications. In this study, ULDAsp effects on platelet aggregation, P-selectin (CD62P) marker for platelet activation) expression, and platelet-leukocyte aggregate formation were assessed from blood samples from 18 clinically normal dogs. There were 3 study sections: Study 1 evaluated the optimal anticoagulant for experimental in vitro platelet aggregation experiments in blood samples from 4 dogs; Study 2 evaluated platelet aggregation responses before and after ULDAsp administration in samples from 18 dogs; and Study 3 assessed P-selectin/CD62P expression and platelet-leukocyte aggregate formation in a subset of Study 2 samples (n = 10).
Platelet aggregation responses were examined using sodium citrate anticoagulated blood and aggregation agonists (adenosine diphosphate [ADP] at 20, 10, and 5 mcmol/L; collagen at 10, 5, and 2 mcg/mL) with whole blood impedance aggregometry. Thrombin-induced CD62P expression and platelet-leukocyte aggregates were identified using flow cytometry in EDTA-anticoagulated whole blood. Antibodies conjugated to phycoerythrin (PE) were used to label externalized platelet CD62P, and those to fluorescein isothiocyanate (FITC) were used for constitutive CD61 (GPIIIa). Whole blood was labeled with both CD61-FITC and a panleukocyte antibody (CD18-PE) to characterize platelet-leukocyte aggregates.
ULDAsp treatment significantly delayed the onset of platelet aggregation with ADP (20 mcmol/L) by 54% to 104%, reduced maximum aggregation by ≥ 41% with a variety of ADP and collagen concentrations, and slowed aggregation rates with the highest ADP and collagen concentrations by ≥ 39%. A proportion of dogs (30%) did not demonstrate a ULDAsp response. CD62P expression was significantly increased in platelets and platelet-leukocyte aggregates by thrombin stimulation. ULDAsp administration did not alter basal or thrombin-stimulated CD62P expression. ULDAsp treatment of clinically normal dogs impaired platelet aggregation in most dogs, but did not influence CD62P platelet membrane expression.
Commentary: Thromboembolic complications are difficult to diagnose and prevent in veterinary medicine, especially in patients diagnosed with IMHA. This is due to an incomplete understanding of disease pathogenesis. This study attempted to determine appropriate testing to assess platelet activation and the specific effects of ULDAsp on platelet activation. Although the study showed a reduction in platelet aggregation under certain conditions with ADP and collagen stimulation, a percentage of dogs (30%) did not respond. In addition, ULDAsp had no effect on thrombin-stimulated platelet activation. However, a recent study did show improved survival when using ULDAsp in the treatment of IMHA. Further studies are needed to determine the pathogenesis of platelet activation in the diseased state as well the effect of ULDAsp on platelets in various conditions.—Shawn Kearns, DVM, Diplomate ACVIM
Influence of treatment with ultralow-dose aspirin on platelet aggregation as measured by whole blood impedance aggregometry and platelet P-selectin expression in clinically normal dogs. Sharpe KS, Center SA, Randolph JF, et al. AM J VET RES 71:1294-1304, 2010.