P039 Interferon regulatory factor-2 regulates hematopoietic stem cell in mouse bone marrow

Introduction Interferon regulatory factor-2 (IRF-2) is a transcriptional repressor in the interferon system and is thought to function by competing with IRF-1. Its physiological role in lymphoid and hematopoietic development has been investigated in IRF-2−/− mice, in which a general bone marrow suppression of hematopoiesis and B lymphopoiesis has been reported. Methods We investigate the role of IRF-2 in mouse bone marrow hematopoietic stem cells using IRF-2−/− mice. Hematopoietic stem cells (as KSL) are isolated from IRF-2−/− mice bone marrow and transplanted to lethally ittadiated mice for chimerismanalysis. Results IRF-2 is strongly expressed in hematopoietic stem cells (HSC) in mouse bone marrow. The population of bone marrow Lin-c-Kit+Sca-1+(KSL) cells is increased in IRF-2−/− mice due to the general enhancement of Sca-1-positive cells. SCs are enriched in KSL CD150+CD48-cells. A very small population of CD150-positive cells was detected in bone marrow from IRF-2−/− mice. Real-time polymerase chain reaction showed reduced CD150 gene expression in bone marrow cells from these mice compared to wild type mice. Although the population of bone marrow KSL cells in IRF-2−/− mice increased, a reduction of HSC was observed due to a decrease of the CD150-positive cell population. In IRF-2−/− interferon-a receptor−/− double-knockout mice, the CD150 expression levels were comparable to wild-type mice, indicating that this gene is regulated by the type [1] interferon (IFN) response via transcriptional regulation. Transplantation of KSL cells from IRF-2−/− mice into wild-type mice together with a competitor resulted in failed to engraft. Even the transplantation of a >20-fold excess of KSL cells from IRF-2−/− mice resulted in engraftment that was still poorer than wild-type cells evaluated at 3–4 months after transplantation. However, transplantation of a higher dose (2 × 10 6 cells) of whole bone marrow mononuclear cells from IRF-2−/− mice rescued recipients from lethal irradiation, although the engraftment was poorer than wild-type cells. This documents the presence of some active long-term repopulating hematopoietic stem cells (LT-HSCs) in IRF-2−/− mice. Conclusion Our results reveal unknown HSC markers in IRF-2−/− mouse bone marrow and demonstrate that IRF-2 acts on LT-HSCs not only through protective type I IFN responses, but also by directly regulating HSC cell surface molecules.