Flow Cytometry Analysis of Tissue Factor Expression in Human Platelets

A subset of circulating human platelets stores Tissue Factor (TF) intracellularly, the key activator of the blood coagulation cascade and thrombus formation. Upon platelet activation, TF is exposed on the cell membrane, where it binds to FVII, ultimately leading to thrombin generation. Considering that (1) levels of TF-positive platelets increase in various clinical settings, contributing to the patient's prothrombotic phenotype, and (2) different drugs can modulate platelet-associated TF expression, a standardized method for assessing TF-positive platelets is valuable, as its evaluation has been controversial in the past. Here, we outline a protocol for measuring the percentage of TF-positive platelets using flow cytometry in whole blood and platelet-rich plasma (PRP)/washed platelets. This protocol aims to provide detailed instructions for quantifying the percentage of TF-positive platelets by assessing the protein (1) intracellularly in resting conditions, and (2) on the cell surface, in both resting and activated conditions. The first section provides essential information for correctly performing blood withdrawal to ensure that pre-analytical procedures do not affect the results. Next, the protocol focuses on sample preparation and labeling procedures for flow cytometry analysis. Detailed steps for cell stimulation, labeling, fixation, and permeabilization -- where necessary -- are outlined. Finally, instructions for flow cytometry settings to correctly identify the platelet population and analyze TF-positive events are described. Lastly, the method includes the procedure for preparing PRP if TF-positive platelets are to be measured in isolated platelets. Since only a subset of platelets contains TF, it is important to ensure that these platelets are not lost during the centrifugation steps required to obtain PRP.