In the bone marrow, we have an army of blood cell-making stem cells called hematopoietic stem cells (HSCs) that make all the blood cells that course through our blood vessels. These cells divide throughout our lifetimes, and they replacement themselves while they generate all the red and white cells found in our blood.
HSCs are also the cells that are harvested during bone marrow aspirations and biopsies. Transplantation of HSCs can save the lives of patients with blood cancers or other types of blood-or bone marrow-based diseased.
Harvesting and transplanting HSCs is, therefore, a very important clinical strategy for treating many different types of blood disorders and diseases. However, this crucial strategy is limited by the relative rarity of HSCs in isolated bone marrow. Additionally, the number and function of HSCs deteriorate both during their collection from the bone marrow (BM) and during their manipulation outside the body. Fortunately, the development of culture conditions that best mimic the environment these cells experience in bone marrow (the so-called “HSC niche environment”) may help to minimize this loss.
One of the most important variables for HSC viability is oxygen concentration, since various studies have shown that the oxygen concentrations found in ambient air seems to be damaging to HSCs, which normally are found in rather oxygen-poor reaches in bone marrow. Researchers from the laboratory of Hal Broxmeyer at the Indiana University School of Medicine have discovered that HSCs suffer from ‘‘extra-physiologic oxygen shock/stress (EPHOSS)” if they are harvested under ambient oxygen conditions. On top of that, treatment of the collected HSCs with the immunosuppressant drug cyclosporin A (CSA) can inhibit this stress, enhance the yield of collected HSCs, and increase their transplantation efficiency.
When Broxmeyer and his colleagues compared mouse BM that had been harvested under normal oxygen concentrations (21% O2) and low-oxygen concentrations (3% O2), they observed that the hypoxic (low-oxygen) treatment caused a 5-fold increase in the number of Long Term (LT) self-renewing HSCs, and a decrease in harmful reactive oxygen species (ROS) and mitochondrial activity. Broxmeyer and others also confirmed the positive effect of hypoxia on HSC collection from human cord blood. When mouse BM collected under different conditions were assayed by competitive transplantation, the “hypoxic HSCs” engrafted more efficiently in recipient mice. This increased engraftment was not due to enhanced homing or reduced cell death. Instead it seems that the stress response to non-physiological oxygen concentrations (EPHOSS) has a rapid and significant damaging effect in HSCs.
Broxmeyer decided to take this study one step further. In mitochondria (the powerhouse of the cell), increased expression of the mitochondrial permeability transition pore (MPTP) seems to be one of the key mechanism by which oxidative stress affects HSCs.
Induction of the MPTP leads to mitochondrial swelling and uncoupled energy production (which leads to the generation of reactive oxygen species, otherwise known as “free radicals). This leads to cell death apoptosis and necrosis, and intermittent MPTP activation may also decrease stem cell function in general without killing the cells. Broxmeyer and his coworkers came upon a rather ingenious idea to use the drug cyclosporin A (CSA) to antagonize MPTP induction, since CSA inhibits the associated CypD (cyclophilin) protein. When HSCs were collected under high-oxygen conditions in the presence of CSA, there was a 4-fold increase in the recovery of LT-HSCs and enhanced engraftment levels compared to HSCs harvested in high-oxygen conditions without CSA. This link was further strengthened by examining the HSCs of mice with a deletion of the CypD gene. In these mice, HSCs collected under high-oxygen conditions showed increased LT-HSC recovery and decreased LT-HSC ROS levels compared to wild-type mice.
How, harvesting and processing HSCs from bone marrow in a low-oxygen environment within a transplant clinic is generally not possible. However, given the observed advantages, the application of CSA may represent an easy and attractive alternative. The authors of this paper (which was published in the journal Cell) note that CSA is already used in the clinic as an immunosuppressant. Therefore, this technique could potentially be rapidly adapted into bone marrow harvesting techniques.
An additional thought is that studies that use other types of stem cells for transplantation might also need to consider the effects of EPHOSS and oxygen concentration while preparing their cells in other model systems.