PB0877 - Discriminating platelet subpopulations on the expression level of HLA-I highlights the high reactivity of young versus old platelets

Background: High proportion of young platelets has been associated with cardiovascular diseases and impaired responsiveness to thienopyridines. So far, the lack of reliable tools to precisely discriminate various platelet subpopulations according to their age has prevented detailed analysis of platelet reactivity.

Aims: To assess the platelet reactivity according to their age based on HLA-I expression level that we recently showed to be over-represented on young platelets.

Methods: Platelet activation of hirudinated human blood 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).

Results: HLA I expression level allows the identification of three platelet subpopulations regarding to their age (HLA low, dim and high). In response to different soluble agonists, levels of fibrinogen binding, phosphatidylserine exposure showed a two- to four- fold increase in the reactivity of young HLA-I high platelets as compared to HLA-I dim ones whereas old HLA-I low platelets were the least reactive. Moreover, the highest capacity of HLA-I high platelets to simultaneously express annexin V and α-granule factor V or activated αIIbβ3 after coactivation with TRAP and CRP indicated that the procoagulant feature of platelets is age-related. In parallel, we demonstrated that HLA-I is reliable to guide platelet cell sorting without any platelet activation as shown by EM and FC ; stimulation with TRAP and CRP confirmed an intrinsic ability of HLA-I high platelets to become procoagulant.

Conclusion(s): Using HLA-I expression as a way to finely discriminate and isolate platelet subpopulations according to their age, we showed that 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 physio/pathological role of young platelets.