Lactadherin functions as a probe for phosphatidylserine exposure and as an anticoagulant in the study of stored platelets

Background and Objectives  Annexin V, the long established standard method of measuring phosphatidylserine (PS) exposure, is not the most suitable probe for the study of stored platelets because of calcium dependence and low sensitivity under 8% PS exposure. The aim of this study was to show lactadherin as a sensitive probe for PS exposure and an effective anticoagulant for stored platelets. Materials and Methods  PS exposure and the associated procoagulant activity of platelets in 20 platelet concentrate units were investigated by flow cytometry, confocal microscopy, coagulation time analysis and enzymatic assays. Annexin V and lactadherin were utilized separately in this study. Results  Using lactadherin, we identified higher levels of PS exposure on the platelets and microparticles compared to detection using annexin V by flow cytometry. Lactadherin staining showed earlier PS exposure localized to platelets membrane in the earlier stage of storage. Subsequently, PS was distributed on the narrow rim of the plasma membrane and blebbing vesicles. Lactadherin or annexin V (32 nm) prolonged coagulation time 2·4 fold versus twofold. The production of thrombin and intrinsic/extrinsic factor Xase were inhibited approximately 85∼90% and 65∼70% by lactadherin and annexin V, respectively. Conclusion  Lactadherin can provide the quantification and location of PS exposure on banked platelets in the absence of agonists. Furthermore, lactadherin is a more effective anticoagulant for inhibiting the procoagulant activity of stored platelets compared with annexin V.