Development of hematopoietic stem cells (HSC) and their progeny is maintained by the interaction with cells in the microenvironment. In addition to hematopoietic cells, Id1 is expressed in stromal cells known to support hematopoiesis, and is involved in cell proliferation, differentiation and senescence. Therefore, to investigate the role of Id1 in hematopoiesis, we examined hematologic phenotypes of Id1−/− mice. In this study, we found increased neutrophils and macrophages, and decreased B cells and platelets in peripheral blood, and decreased BM cellularity. While the percentages of hematopoietic stem cells (HSC) in Id1−/− mice were increased relative to the Id1+/+ mice, their total numbers and function appeared normal. For example, Id1 was not required for self-renewal or repopulation of HSC. In contrast, we found that there were increased numbers of hematopoietic progenitor cells (HPC) in S phase of cell cycle in Id1−/− mice BM, suggesting that the loss of Id1 within HPC promotes proliferation. However, purified Id1−/− HPC had the same proliferation potential as Id1+/+ HPC when cultured in vitro. In transplantation experiments, we proved that BM microenvironment in Id1−/− mice is defective by showing that the Id1+/+ HSC showed impaired hematopoietic development in Id1−/− mice, while the Id1−/− HSC had normal repopulation potential in an Id1+/+ microenvironment. In agreement with these findings, Id1−/− BM stromal cell cultures supported enhanced proliferation of hematopoietic progenitors. Furthermore, quantitative PCR showed that SCF, M-CSF, OPN, SDF-1 and TGF-α mRNA expression was decreased in Id1−/− stromal cells relative to Id1+/+ stromal cells, while G-CSF, GM-CSF, and VEGF mRNA expression was significantly increased. Id1−/− BM showed decreased number of mesenchymal stem/progenitor cells. Thus, Id1 does not play a role in maintaining HSC, but is involved in regulating hematopoietic progenitor niche. Funded by NCI contract No. N01-CO-12400.

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