Fibronectin Enhances Ex Vivo Expansion and Maturation of Megakaryocyte Progenitors from Umbilical Cord Blood.

Following cord blood (CB) transplant and bone marrow (BM) protracted thrombocytopenia remains a serious clinical problem. Platelet production following transplant depends on the availability of adequate numbers of cytokine responsive stem and megakaryocyte progenitor cells (MK-p). Thrombopoietin (TPO), had no clinical impact on thrombopoiesis when given to patients post BMT due to the paucity of MK-p in the grafts. If expanded, Mk-p would supply the appropriate target cells to maximize the effect of TPO and provide efficient earlier platelet engraftment. We propose a novel strategy to facilitate thrombopoiesis, by expanding MK-p from CB mononuclear cells (MNC) prior to transplantation in short term cultures. While CB CD34+ cells can be expanded by several reported methods, isolation of CD34+ cells from the fresh CB is not practical due to the limited number of stem and progenitor cells in the CB units. Additionally, MK expansion from purified stem cells requires long culture periods which are inappropriate for transplantation. We aimed to improved techniques for enrichment and ex-vivo expansion of MK-p and hematopoietic stem cells, from small aliquots of whole CB, using 7–10 days cultures and new growth conditions. CB progenitors were enriched by separation of MNC from RBC on gelatin followed by centrifugation on ficoll, as we previously reported (1). MNC were expanded on fibronectin (FN) coated dishes in the presence of autologous plasma with various new cytokine combinations. These included r-hu-TPO (10 ng/ml), b- FGF (10 ng/ml), r-hu-SCF (50 ng/ml) and ARP a peptide derived from the stress variant of acetylcholinesterase (AChE-R) recently discovered to have potent hematopoietic stem cell and MK growth factor activity (2). The cell populations, MK and MK-p were characterized by high resolution flow cytometry on day 0 and 10 of culture using SSC, CD41 and CD34. True MK expansion was assessed by appropriate gating out of granulocyte and monocytes, which acquire CD41+ adherent platelets in culture. FN alone, without any other growth supplement increased the viability of cells in culture and expansion of MK-p (CD41^high, SSC^low and FSC^low) by 2.8±1.1 (P < 0.05) fold. The combination of FN with TPO enhanced MK-p number by 4.8±2.7 and the addition of either SCF or b-FGF or ARP further stimulated the expansion of MK-p all producing about a 6 fold increase (P < 0.05). Further analysis was performed on the maturing MKs which were characterized as CD41^high, CD45^low/negative, CD34^negative. Increased Mk ploidy was found when either b-FGF or ARP were added to cultures containing TPO, grown on FN coated plates. Significant MK maturation, as measured by GPIIb/IIIa expression using real time quantitative PCR, was also found. The combination of FN and TPO increased the MK colony forming progenitors in culture by 9 fold and up to 35 fold when other supplements were added. We demonstrate that short term expansion of enriched MK-p from a small fraction of the CB unit is feasible and easy to perform and can comply with GTP regulations. This approach may lead to the development of more effective cell therapy modalities to facilitate platelet production and decrease the time of thrombocytopenia in severely myelosuppressed patients during the extended nadir before platelet engraftment.