Long-term ex vivo expansion of human fetal liver primitive haematopoietic progenitor cells in stroma-free cultures

Successful expansion of haematopoietic cells in ex vivo cultures will have important applications in transplantation, gene therapy, immunotherapy and potentially also in the production of non- haematopoietic cell types. Haematopoietic stem cells ( HSC), with their capacity to both self- renew and differentiate into all blood lineages, represent the ideal target for expansion protocols. However, human HSC are rare, poorly characterized phenotypically and genotypically, and difficult to test functionally. Defining optimal culture parameters for ex vivo expansion has been a major challenge. We devised a simple and reproducible stroma- free liquid culture system enabling long- term expansion of putative haematopoietic progenitors contained within frozen human fetal liver ( FL) crude cell suspensions. Starting from a small number of total nucleated cells, a massive haematopoietic cell expansion, reaching > 10(13) - fold the input cell number after similar to 300 d of culture, was consistently achieved. Cells with a primitive phenotype were present throughout the culture and also underwent a continuous expansion. Moreover, the capacity for multilineage lymphomyeloid differentiation, as well as the recloning capacity of primitive myeloid progenitors, was maintained in culture. With its better proliferative potential as compared with adult sources, FL represents a promising alternative source of HSC and the culture system described here should be useful for clinical applications.