1. Harvesting mouse hematopoietic stem cells (HSCs) in hypoxia (3% oxygen) rather than ambient air (21% oxygen) resulted in greatly increased stem cell numbers, both at the time of collection and after transplantation into recipient mice.
2. Using the drug cyclosporin A (CSA) during ambient air harvest of mouse and human HSCs reduced the negative effects of oxygen exposure, resulting in improved HSC numbers and mouse transplant efficacy.
Evidence Rating Level: 1 (Excellent)
Study Rundown: HSCs represent the earliest stage of undifferentiated blood cell precursors and reside in bone marrow (BM), peripheral blood (PB), and cord blood (CB). HSC transplantation is a risky but commonly used treatment for various blood disorders, including leukemia. However, the low number of transplanted HSCs can limit transplantation efficacy. The researchers of this study hypothesized that exposure to ambient air may reduce HSC numbers, since the physiological HSC environments are extremely hypoxic by comparison.
Harvesting and processing BM from mice in a hypoxic chamber resulted in significantly more HSCs as compared to harvesting in ambient room air. When BM was harvested from donor mice in hypoxia, recipient mice that subsequently received the transplant in hypoxic conditions showed significantly higher numbers of donor cells than recipient mice in ambient air. Since hypoxic harvest and transplantation are impractical in clinical settings, the authors next identified the cellular mechanisms behind the harmful effects of oxygen exposure on HSCs. Oxygen exposure was found to trigger deleterious mitochondrial signaling, which is known to be preventable using CSA. Injecting mice with CSA before BM harvest in ambient air resulted in more HSCs compared to controls. Further, mice transplanted with BM harvested in the presence of CSA showed improved engraftment of donor cells. Excitingly, using CSA when collecting human HSCs from CB improved HSC numbers and engraftment into mice.
This work suggests a powerful way to improve the clinical method of HSC transplantation based on HSC biological mechanisms. Because CSA is currently approved for other indications, adapting it to prevent oxygen stress during HSC transplantation may be less challenging than developing a new drug.
Relevant Reading: Umbilical cord blood transplantation: the first 25 years and beyond
In-Depth [animal study]: Long-term repopulating HSCs (LT-HSCs) were quantified using flow cytometry, based on the presence or absence of several known cell markers. Harvesting mouse BM in hypoxic conditions resulted in a nearly 5-fold increase in LT-HSC numbers compared to ambient air harvest (p=0.002, n=6 experiments). Further experiments showed that ambient air harvest increased numbers of cells which had begun to differentiate (p<0.001 for 3 separate differentiating cell types, n=3 mice from 1 experiment), suggesting that oxygen exposure triggered differentiation. After BM was harvested from mice in hypoxia, recipient mice transplanted with the BM under hypoxia showed superior donor cell engraftment to mice transplanted under ambient air (p=0.014 measured in BM at 7 or 8 months post-transplantation, n=6-8 mice from 2 experiments).
Oxygen-induced cellular damage also occurs in ischemia-reperfusion injury, which is triggered by opening of the mitochondrial permeability transition pore (MPTP). Hypothesizing that similar mechanisms were at play here, the researchers injected mice with 50 µg/mL CSA (which antagonizes MPTP opening) before BM harvesting in air. Compared to injection with control medium, CSA injection resulted in an approximately 4-fold increase in LT-HSC cell numbers (p=0.008, n=3 experiments). Transplantation of this BM into mice showed increased engraftment in recipient mice (p=0.044 measured in BM at 8 months post-transplantation, n=6-10 mice from 2 experiments).
Finally, CSA was tested with human LT-HSCs. Cells were collected via venipuncture using a syringe containing CSA or control medium. Use of CSA resulted in increased cell numbers after collection (p=0.04, n=5 experiments) and after transplantation into mice (p=0.004 measured in PB at 2 months post-transplantation, n=6-10 mice from 2 experiments).
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