Hypoxia with Wharton’s jelly mesenchymal stem cell coculture maintains stemness of umbilical cord blood-derived CD34(+) cells

BACKGROUND: The physiological approach suggests that an environment associating mesenchymal stromal cells with low O(2) concentration would be most favorable for the maintenance of hematopoietic stem/progenitor cells (HSPCs). To test this hypothesis, we performed a coculture of cord blood CD34(+) cells with Wharton’s jelly mesenchymal stem cells (WJ-MSCs) under different O(2) concentration to simulate the growth of HSPCs in vivo, and assessed the impacts on stemness maintenance and proliferation of cord blood HSPCs in vitro. METHODS: CD34(+) cells derived from cord blood were isolated and cocultured under 1%, 3%, or 20% O(2) concentrations with irradiated WJ-MSCs without adding exogenous cytokines for 7 days. The cultured cells were harvested and analyzed for phenotype and functionality, including total nuclear cells (TNC), CD34(+)Lin(−) cells, colony forming unit (CFU) for committed progenitors, and long-term culture initiating cells (LTC-ICs) for HSPCs. The cytokine levels in the medium were detected with Luminex liquid chips, and the mRNA expression of hypoxia inducible factor (HIF) genes and stem cell signal pathway (Notch, Hedgehog, and Wnt/β-catenin) downstream genes in cord blood HSPCs were confirmed by quantitative real-time polymerase chain reaction (qRT-PCR). RESULTS: Our results showed that the number of TNC cells, CD34(+)Lin(−) cells, and CFU were higher or similar with 20% O(2) (normoxia) in coculture and compared with 1% O(2) (hypoxia). Interestingly, a 1% O(2) concentration ensured better percentages of CD34(+)Lin(−) cells and LTC-IC cells. The hypoxia tension (1% O(2)) significantly increased vascular endothelial growth factor (VEGF) secretion and decreased interleukin (IL)-6, IL-7, stem cell factor (SCF), and thrombopoietin (TPO) secretion of WJ-MSCs, and selectively activated the Notch, Wnt/β-catenin, and Hedgehog signaling pathway of cord blood HSPCs by HIF-related factors, which may play an important role in stemness preservation and for sustaining HSPC quiescence. CONCLUSIONS: Our data demonstrate that cord blood HSPCs maintain stemness better under hypoxia than normoxia with WJ-MSC coculture, partially due to the increased secretion of VEGF, decreased secretion of IL-6 by WJ-MSCs, and selective activation of stem cell signal pathways in HSPCs. This suggests that the oxygenation may not only be a physiological regulatory factor but also a cell engineering tool in HSPC research, and this may have important translational and clinical implications.