Platelet flow cytometric findings in patients undergoing conditioning therapy for allogeneic hematopoietic stem cell transplantation

The conditioning regimen preceding hematopoietic stem cell transplantation (HSCT) causes a rapid decrease in the platelet count and signs of disseminated intravascular coagulation, possibly indicating platelet activation. As impacts during the conditioning regimen may predict later transplantation-associated complications, we investigated changes in platelet membrane glycoproteins (GP) and the liberation of microparticles. Platelet receptors and granules of 49 patients undergoing HSCT were evaluated by flow cytometric analysis before and after the different phases of the conditioning regimen [chemotherapy, total body irradiation (TBI), therapy with antithymocyte globulin (ATG)] and final transplantation. Following chemotherapy a high surface expression of CD62P, a low mepacrine staining, and a reduced surface expression of CD42b (part of the GP Ib/V/IX complex) were found, indicating an irreversible activation of platelets. In addition, elevated levels of circulating microparticles were observed, which may reinforce the thrombosis risk in these patients. Treatment with ATG leads to an elevated surface expression of PAC-1 epitopes, which are neoepitopes appearing after activation of GP IIb/IIIa. However, a significant degranulation was not detectable, which may be the consequence of inhibitory influences on platelets during ATG-induced cytokine release syndrome. TBI and transplantation itself had no influence on platelets. This study was able to demonstrate activating effects on platelets by certain phases of the conditioning regimen in patients receiving HSCT. Chemotherapy, in particular, leads to a strong and irreversible platelet activation and a generation of microparticles, which may cause an increased thrombosis risk. Our findings underline the impact of platelets on the pathogenesis of hemostatic complications during HSCT.