Discriminating young platelets on human leukocyte antigen-I expression highlights their extremely high reactivity potential

BACKGROUND: The platelet population is heterogeneous, with different subsets that differ on the basis of their function and reactivity. An intrinsic factor participating in this difference of reactivity could be the platelet age. The lack of relevant tools allowing a formal identification of young platelets prevents so far to draw solid conclusions regarding platelet reactivity. We recently reported that human leukocyte antigen-I (HLA-I) molecules are more expressed on human young platelets. OBJECTIVES: The aim of this study was to assess platelet reactivity according to their age based on HLA-I expression level. METHODS: Platelet activation was assessed by flow cytometry (FC) for different platelet subsets based on their HLA-I expression. These populations were further cell sorted and their intrinsic properties were determined by FC and electron microscopy (EM). Statistical analyses were performed with GraphPad Prism 5.02 software using two-way ANOVA followed by a Tukey post hoc test. RESULTS: HLA-I expression level allowed the identification of 3 platelet subpopulations regarding to their age (HLA low, dim, and high). HLA-I was reliable to guide platelet cell sorting and highlighted the features of young platelets in the HLA-I(high) population. In response to different soluble agonists, HLA-I(high) platelets were the most reactive subset as shown by the level of P-selectin secretion and fibrinogen binding assessed by flow cytometry. Moreover, the highest capacity of HLA-I(high) platelets to simultaneously express annexin-V and von Willebrand factor or activated αIIbβ3 after coactivation with TRAP and CRP indicated that the procoagulant feature of platelets was age-related. CONCLUSION: The young HLA-I(high) population is the most reactive and prone to become procoagulant. These results open up new perspectives to investigate deeply the role of young and old platelets.