Murine prolactin-like protein E synergizes with human thrombopoietin to stimulate expansion of human megakaryocytes and their precursors

Objective The aim of this study was to determine the effect of promegakaryocytopoietic murine hormone prolactin-like protein E (PLP-E) on human megakaryocytopoiesis. Materials and Methods Human bone marrow CD34+ cells, cultured in serum-free medium with combinations of thrombopoietin (TPO), stem cell factor (SCF), Flt-3 ligand (Flt-3L), and PLP-E, were analyzed via microscopy, flow cytometry, and clonogenic assay. Results Unlike the situation with mouse cells, PLP-E alone did not promote human megakaryocyte (MK) differentiation, but instead synergizes with TPO to increase colony-forming unit megakaryocyte (CFU-MK), burst-forming unit erythroid (BFU-E), and and colony-forming unit granulocyte erythroid macrophage mixed (CFU-GEMM) expansion, as well as total MK production. These effects can be attributed to an increase in colony frequency, combined with a significantly greater total cell expansion induced by adding PLP-E along with TPO. The number of cells in each CFU-MK colony is an indication of the maturity of the progenitor population, with larger colonies deriving from a more immature progenitor cell. PLP-E significantly expanded immature, intermediate, and mature CFU-MK subsets at 3 days of culture, as well as the intermediate and mature subsets at day 6. PLP-E combined with TPO induced significant expansion of all CFU-MK subsets at all time points. PLP-E further increased the effect of SCF and Flt-3L on TPO-induced total cell and CFU-MK expansion. Conclusions PLP-E may act as a survival factor for primitive human megakaryocytic and erythroid progenitors. It appears to preserve the highly proliferative immature fraction of the progenitor compartment but by itself does not promote total cell proliferation or human MK production. PLP-E may prove useful in combination with TPO and other cytokines for ex vivo expansion of hematopoietic progenitors to be used in a clinical setting.