Cyclosporine May Inhibit the Effect of Extra-Physiologic Oxygen Shock/Stress on Peripheral Blood Stem Cells

Introduction: Adequate numbers of stem cells with preserved multi-potency and self-renewing capabilities are necessary for successful hematopoietic reconstitution after bone marrow transplantation. Although hematopoietic stem cells (HSC) reside in the bone marrow under a hypoxic microenvironment (1-4% O₂), human HSC are collected and processed in ambient air (21% O₂) (Spencer et al., Nature 508:269-73). Exposure of murine bone marrow and human cord blood to ambient air for as little as 30 minutes triggered stem cell differentiation from quiescent pluripotent long term stem cells into activated multipotent progenitors (MPPs), a phenomenon called extra-physiologic oxygen shock / stress (EPHOSS)(Mantel et al., Cell 161:1553-65). The effect of EPHOSS on HSCs is mediated through reactive oxygen species (ROS) which open the mitochondrial permeability transition pore (MPTP) and trigger stem cell differentiation. Cyclosporine (CSA) inhibits the MPTP regulator cyclophilin D and prevents MPTP opening (Kroemer et al., Physiol Rev 87:99-163). Currently, mobilized peripheral blood stem cells (PBSCs) are the major source of grafts for hematopoietic cell transplantation. We hypothesized that EPHOSS is detrimental to human PBSCs similar to murine bone marrow and human cord blood stem cells and CSA will protect human PBSCs from the effects of EPHOSS and inhibit their differentiation from pluripotent long term HSC into short term MPPs.

Study design and methods. This is a proof-of-concept, prospective, non-interventional study. We obtained G-CSF mobilized PBSCs from healthy related donors under an IRB approved protocol. All donors provided written informed consent. Blood containing HSCs was collected from each donor with minimal exposure to ambient air (<60 seconds). The sample was immediately split and incubated for 60 minutes with and without CSA (50 µg/mL). CSA treated and untreated PBSCs were immunophenotyped by multi-parameter flow cytometry for HSCs (CD34+CD38-CD90+CD45RA-), MPP (CD34+CD38-CD90-CD45RA-) and common lymphoid progenitors (CD34+CD38+CD127+). We used a CD34 ISHAGE-based gating strategy to accurately enumerate HSC and MPP.

Results. In four separate experiments, CSA treated PBSCs had a higher median (range) number of HSCs/10⁶ total nucleated cells (TNC) compared to untreated PBSCs ((297 (51 - 512) versus 185 (47 -392), respectively). CSA treated PBSCs also had a higher median (range) HSC:MPP ratio compared to untreated PBSCs ((0.56 (0.40 - 0.70) versus 0.43 (0.30 - 0.50), respectively) suggesting that the differentiation of HSC to MPP upon exposure to air was decreased in the CSA treated PBSCs.

Conclusions / future directions. This is the first report describing the effect of EPHOSS in human PBSCs. Our preliminary data suggest that EPHOSS promotes the differentiation of human PBSCs from HSC to MPP and that CSA may inhibit this process. Further confirmatory and mechanistic studies exploring the contribution of MPTP and ROS to EPHOSS are ongoing. The results from this research may potentially change the current practice of collecting PBSCs in ambient air.