Interleukin‐6 (IL‐6) and low O 2 concentration (1%) synergize to improve the maintenance of hematopoietic stem cells (pre‐CFC)

Low O 2 concentration (1%) favors the self‐renewal of hematopoietic stem cells and inhibits committed progenitors (CFC). Since IL‐6 influences both stem cells and committed progenitors at 20% O 2 , we studied its effects in cultures at 1% O 2 . The pre‐CFC activity in Lin − population of mouse bone marrow was analyzed following 10 days of serum‐free culture in medium (LC1) supplemented with IL‐3 with and without IL‐6, at 20 and 1% O 2 and phenotypic differentiation and proliferative history monitored. The IL‐6 receptor expression and initiation of VEGF‐A synthesis were also investigated. At 20% O 2 , the effects of IL‐6 on pre‐CFC were negligible but effects on CFC were apparent; conversely, at 1% O 2 , the IL‐6 enhances activity of pre‐CFC but not of CFC. Unlike at 20% O 2 , at 1% O 2 a subpopulation of cells remained Lin − in spite of extensive proliferation. However, the absolute number of Lin − cells, did not correlate with pre‐CFC activity. A relative increase in VEGF transcripts at 1% O 2 in presence of IL‐3 alone was enhanced by the addition of IL‐6. IL‐6 enhanced pre‐CFC activity at 1% O 2 and this was correlated to the induction of VEGF. These data reinforce the concept that physiologically low oxygenation of bone marrow is a regulator of stem cell maintenance. Since the 20% O 2 does not exist in tissues in vivo, further studies in vitro at lower O 2 concentrations should revise our knowledge relating to cytokine effects on stem and progenitor cells. J. Cell. Physiol. 212: 68–75, 2007. © 2007 Wiley‐Liss, Inc.