Abstract
Introduction: Platelets express functional Toll-like receptors (TLRs), and numerous studies have found that certain TLR agonists and bacteria are capable of inducing thrombotic and inflammatory responses from isolated platelets. Few studies have examined the functionality of the platelet-expressed TLRs in the clinical context or in whole blood. Platelets and their thromboinflammatory functions are central to the pathogenesis of atherosclerosis and the acute coronary syndromes (ACS). As a first step to understanding the effects of TLR agonists on platelets in ACS, we aimed to develop a flow cytometry-based protocol to examine platelet activation responses to TLR agonist stimulation.
Methods: A flow cytometry-based protocol to measure the expression of platelet activation markers PAC-1, P-selectin (CD62p) and CD40-ligand (CD40L) in response to stimulation by TLR agonists was developed. Fourteen patients presenting to Wellington Regional Hospital with ACS pretreated with aspirin and clopidogrel (dual antiplatelet therapy, DAPT) were recruited prospectively. Fourteen age- and sex- matched healthy volunteers not on antiplatelet medications were recruited prospectively as controls. Whole blood samples from ACS patients and controls were each separately stimulated with agonists for TLR4 (lipopolysaccharide, LPS), TLR2/1 (Pam3CysSerLys4, PAM3CSK4) and TLR2/6 (fibroblast stimulating ligand-1, FSL-1). Thrombin Receptor Activating Peptide (TRAP) was used a positive control. Following stimulation, samples were analysed by flow cytometry.
Results: There were no differences in the platelet activation states between groups for the unstimulated samples. Stimulation of whole blood from the control group by the TLR agonists tested resulted in statistically significant increases in PAC-1 and CD62p. Stimulation of whole blood from the ACS patients on DAPT with TLR agonists did not result in statistically significant increases in the expression of the platelet-activation markers tested, although trends towards significance were noted at the higher concentrations of agonists tested. Platelet responses were lower in the ACS group relative to the controls. For the maximal LPS concentration used, the %-positive expressiondifferences between the control group and ACS group for PAC-1 and CD62p were 29.3 (p = 0.002) and 7.2 (p = 0.02), respectively. For the maximal concentration of PAM3CSK4, the differences for PAC-1 and CD62p were 16.4 (p < 0.001) and 8.4 (p = 0.04), respectively. For the maximal concentration of FSL-1, the differences for PAC-1 and P-selectin were 18.7 (p < 0.001) and 7.8 (p = 0.04), respectively.
Conclusions: TLR agonist stimulation increased platelet activation in healthy volunteers, although it was unclear whether this was due to a direct effect on the platelet surface-expressed TLRs or an indirect effect on platelets via leucocytes, erythrocytes or plasma proteins. DAPT appeared to inhibit TLR agonist-induced platelet activation in the ACS patients. Although we found no statistically significant platelet activation responses to the TLR agonists tested in the ACS patients on DAPT, this will require confirmation in larger sample sets as our study may have been underpowered. If so, current DAPT regimens may be insufficient to prevent ischaemic events in settings of infection for patients at risk of arteriothromboses involving platelets.