In vitro megakaryocyte expansion in patients with delayed platelet engraftment after autologous stem cell transplantation

Abstract.

Increasing the number of megakaryocytic cells in stem cell transplants by ex vivo expansion culture may provide an approach to accelerate platelet engraftment after high-dose chemotherapy. However, it is unknown if a relationship exists between the expansion potential of progenitor cells and the time to platelet engraftment in vivo. Therefore, we questioned if those patients who potentially would benefit most from expanded cell supplements are able to generate megakaryocytic cells efficiently in vitro. The in vitro megakaryocyte proliferation was analyzed from 19 leukapheresis samples from a group of multiple myeloma patients who all showed rapid neutrophil engraftment, but varied from 7 to 115 days post-transplant to achieve platelet levels >20×10⁹/l. CD34⁺ cells were isolated and analyzed for their potential to form megakaryocytic colonies (CFU-Mk) in colony assays and megakaryocytic (CD61⁺) cells in suspension cultures. The frequency and size of CFU-Mk and the expansion potential of CD61⁺ cells varied eightfold between individual patients. A similar range was found with CD34⁺ cells isolated from normal bone marrow (n=9). Rapid platelet engraftment occurred in patients receiving both high or low CFU-Mk doses and with high and low expansion of CD61⁺ cells. Four patients who experienced prolonged (>3 weeks) thrombocytopenia received low CFU-Mk doses, but the expansion potential was around median values or higher. Therefore, we conclude that the megakaryocyte proliferation is not impaired and that in vitro expansion could increase the number of megakaryocytic cells, although other factors could be more relevant in platelet engraftment in this group of patients.