Thrombopoietin combined with early-acting growth factors effectively expands human hematopoietic progenitor cells in vitro

Thrombopoietin (TPO) is established as a powerful stimulant of megakaryocyt e differentiation and platelet production both in vivo and in vitro. In pre paration for future transplantation of ex vivo expanded CD34(+) hematopoiet ic progenitor cells (HPCs), we have examined the in vitro effect of TPO on cultures of HPC when combined with other early-acting hematopoietic growth factors (GFs) in an attempt to decrease post-transplant thrombocytopenia an d accelerate engraftment. By adding TPO to all possible combinations of GM- CSF, IL-3, and c-kit ligand (CKL) in a suspension culture system, we found a significant increase in both relative and absolute numbers of cells in cu ltures containing TPO of the megakaryocytic lineage and CD34(+) cells after 14 days of culture. The most efficient GF combinations for expansion of cell populations of the megakaryocytic lineage and HPCs were TPO, GM-CSF, and CKL, which increased the number of cells of the megakaryocytic lineage 78 fold and the number o f CD34(+) cells 1.8 fold. The number of CD34(+) cells decreased in the cult ures containing GM-CSF and CKL with no TPO present, and the number of cells of the megakaryocytic lineage was increased merely 27 fold. Based on our f indings, we suggest adding cells from HPCs expanded in cultures containing TPO, GM-CSF, and CKL to unexpanded stem cells for stem cell transplantation .