High concentrations of thrombopoietin activate platelets in vitro

Thrombopoietin (TPO), the ligand for the protooncogene c-mpt, has been cloned and expressed from both human and murine sources. Thrombopoietin increases platelet counts when given in vivo and acts on progenitor cells to increase their proliferation and maturation into megakaryocytes. The effects of TPO on mature platelets were investigated by evaluating platelet aggregation and platelet activation-dependent antigen expression. Platelet aggregation score, a quantitative representation of aggregation, showed potentiation of response to ADP-induced aggregation but no direct agonist response to TPO alone. Soluble c-mpl blocked the effect of TPO on the platelet aggregation score. Flow cytometry showed that TPO at concentrations >250 U/ml (50 ng/ml) caused a minority population of platelets to express the activation markers CD62, CD63 and activated glycoprotein IIb/IIIa. While stem cell factor and interleukins-3 and -6 did not affect platelet activation antigen expression interleukin-ll increased CD62 expression on platelets in vitro. The effects of TPO on antigenic expression and aggregability were partially inhibited in vitro by preincubation with aspirin. We conclude that high concentrations of TPO promote platelet activation antigen expression on a proportion of platelets and potentiate platelet aggregability to ADP in vitro by a process that is partially inhibited by aspirin.