Oxygen Tension Regulates Hematopoietic Stem Cell Migration Into An in-Vitro Niche Beneath Mesenchymal Stromal Cells.

Hematopoietic stem cells (HSCs) are located mainly in the bone marrow interacting with a specific microenvironment called “stem cell niche”. The niche has been proven to be critical for stem cell regulation. Coculture with mesenchymal stromal cells (MSCs) has been used as an in vitro model to investigate the interaction between HSCs and MSCs. In our study we investigated the impact of normoxia and hypoxia on the distribution of HSC subsets with regard to their spatial localization in cocultures during ex-vivo expansion.

Three HSC subsets are defined: (i) cells in supernatant (non-adherent cells); (ii) cells adhering on the MSC layer surface (phase-bright cells); (iii) cells beneath the MSC layer (phase-dim cells). Using pimonidazole binding we investigated the spatial distribution of hypoxic cells in various cell subsets. Cell cycle, cell division, immunophenotype and migratory capacity of the three HSC subsets under distinct oxygen tension were studied. In addition the impact of oxygen tension on HSCs via VEGF-A and SDF-1 were analyzed by ELISA and gene knockdown with siRNA.

First we could show that phase-bright cells contained the highest proportion of cycling progenitors. In contrast, phase-dim cells divided much more slowly and retained a more immature phenotype. Next pimonidazole binding revealed that the most hypoxic area in the coculture is the compartment beneath MSC layer. Then we investigated the impact of hypoxia conditions on HSCs in cocultures. We could demonstrate that under hypoxic conditions phase-bright cells were significantly diminished and phase-dim cells were increased. Interestingly, the migratory capacity of phase-bright cells from cocultures performed under hypoxic conditions was consistently enhanced in comparison to normoxia (32.7 ±2.2.0% vs 17.6 ±2.6%, p<0.01). Surprisingly, the SDF-1 concentration was lower after hypoxic coculture (189 ±33μg/ml vs 352 ±40μg/ml, p<0.05). In contrast, the VEGF-A concentration was significantly increased compared to normoxic conditions (7.7 ±1.2ng/ml vs 4.5 ±1.0ng/ml, p<0.01). In addition we could demonstrate that the lower adhesion and higher migratory capacity of HSCs under hypoxia can be partially inversed by silencing VEGF-A with siRNA in MSCs.

Our data indicate that under our experimental conditions, the MSC surface is the dominant location where HSCs proliferate, whereas the compartment beneath the MSC layer seems to be a hypoxic niche dedicated to the maintenance of HSC stemness. The lower levels of SDF-1 in the supernatant may be explained by the increased internalization of SDF-1 by MSCs when cultured together with HSC. This hypothesis will require the concomitant analysis of protein and SDF-1 mRNA in MSC. In addition our data suggest that low oxygen tension facilitates HSC migration into the in-vitro niche provided by MSCs which preserves immaturity of HSCs and modifies the cytokine profile of MSCs.

No relevant conflicts of interest to declare.