The Role of BMP-4 in the Stromal-Maintenance of Haematopoietic Stem Cells.

Haematopoietic stem cells (HSCs) expand in number during fetal liver hematopoiesis via a process that is not understood. Establishment of conditions supporting HSC maintenance and expansion ex vivo is critical for wider application of cord blood derived HSC. We have shown that bone morphogenic protein (BMP) 4 secreted by mouse foetal liver cells contributes to expansion of cord blood-derived HSCs during in vitro co-culture. Significant levels of BMP-4 mRNA and secreted protein were produced by the supportive murine foetal liver stromal cell line AFT024. Supplementing 14 day co-cultures of AFT024 and human cord blood CD34⁺ cells with the BMP-4 antagonist Noggin, or a neutralising BMP-4 antibody decreased the proportion of cells maintaining a CD34⁺CD38⁻CD33⁻ primitive phenotype (by 27.6% and 37.2% respectively), decreased CFU-GM expansion (by 20.6% and 22.2% respectively), and caused a large reduction in net expansion of long-term culture-initiating cells (LTC-IC) (by 31.7% and 61.5% respectively). The ability of BMP4 to support the multipotency and self-renewal of hematopoietic stem cells is consistent with a role recently shown on embryonic stem cells and suggests that it may act generally as a stem cell maintenance factor. Elucidating further the role of HSC growth factors such as BMP-4 in ex vivo culture may lead to development of defined systems for the routine clinical expansion of HSC.