P-selectin glycoprotein ligand-1 positive microparticles in allogeneic stem cell transplantation of hematologic malignancies

P-selectin and its receptor P-selectin glycoprotein ligand-1 (PSGL-1) mediate adhesion between leukocytes, tumor cells (including leukemias and lymphomas), and platelets, and play an important role in hematopoiesis, T cell activation, and cancer growth and metastasis. As microparticles (MPs) are released from activated or apoptotic cells, there should be significant numbers of circulating PSGL-1-bearing MPs in the blood of patients undergoing allogeneic stem cell transplantation (alloSCT). We enumerated PSGL-1-expressing MPs in plasma samples from 30 consecutive patients with hematologic disorders at different time points during the course of alloSCT by flow cytometry and analyzed their relation to cell counts, patient characteristics, and clinical outcome. Median follow-up time of surviving patients was 1,772 days (range 1272-1981 days). Nineteen patients (63.3%) died, 10 due to progression of disease (33.3%). The PSGL-1 MPs significantly declined during conditioning therapy but increased again after transfusion of donor cells and even more at the time of engraftment. Numbers >250/μL after graft transfusion were associated with a shorter time to engraftment for patients receiving fresh peripheral stem cell grafts (median, 15 vs. 21 days; p = 0.049). Furthermore, low PSGL-1 MP values at those two time points were associated with a higher risk of progress/relapse in univariate analysis (p = 0.008-0.014; hazard ratio [HR] = 15.0-42.0) with cumulative incidences at 5 years of 81.8% versus 28.6% and 85.7% versus 20.0%, respectively. In conclusion, PSGL-1 microparticles show a characteristic course during alloSCT and their possible association with relapse/progress requires further evaluation of the PSGL-1/P-selectin interaction in leukemias and lymphomas.