Failure to Achieve a Threshold Dose of CD34+CD110+ Progenitor Cells in the Graft Predicts Delayed Platelet Engraftment after Autologous Stem Cell Transplantation for Multiple Myeloma

To predict platelet engraftment more accurately post autologous stem cell transplantation (SCT), we retrospectively analyzed the CD34+CD110+ (CD110 or c-mpl, thrombopoietin receptor) content in the grafts of 70 patients undergoing transplantation for multiple myeloma (MM) with an in-house flow cytometric assay. We found that infusing at least 3.0 × 104 CD34+CD110+ cells/kg clearly separated the cohort into those who achieved platelet engraftment before or after 21 days. This early megakaryocyte cell marker correlated more closely with early versus delayed platelet engraftment than CD34+ measurements. Of the 70 patients, 4 required ≥ 21 days to achieve platelet transfusion independence. Three of the 4 received a CD34+CD110+ cell dose of <3.0 × 104 cells/kg, whereas 66 of 70 patients who received >3.0 × 104 CD34+CD110+ cells/kg achieved platelet transfusion independence in <21 days (P < .001). Infusing >3.0 × 104 CD34+CD110+ cells/kg was sensitive (100%) and specific (75%) for achieving platelet engraftment within 21 days. Patients with delayed platelet engraftment received a median of 2.28 × 104 CD34+CD110+ cells/kg versus 12.1 × 104 cells/kg in those without this complication (P = .033). No effect was seen with neutrophil engraftment. Patients with early engraftment required a median of 1 platelet transfusion post transplant versus 2.5 in those with late engraftment (P = .009). A subthreshold absolute CD34+CD110+ cell dose in the graft is a reliable predictor of delayed platelet engraftment, and could be used to guide the timing and number of peripheral blood stem cell (PBSC) collections for patients with MM undergoing an SCT.