Impact of Radiation on Hematopoietic Niche

Irradiation is a pretreatment for hematopoietic stem cell transplantation. Much attention has been paid to the effect of radiation on the eradication of pathogenic hematopoietic cells; its influence on other cells has been less studied. However, according to recent findings, radiation may also adversely affect hematopoietic niche cells that are derived from mesenchymal stem cells. Vasculature niche regresses by sublethal and lethal irradiation, and then is repaired within a few weeks. Endosteal niche may also be received damages by irradiation and repaired by unknown mechanism. Cell death and tissue damage induced by radiation yield danger-associated molecular pattern molecules, which are recognized by Toll-like receptors expressed on macrophages. As a result, inflammatory macrophages are transiently activated, and then regulatory and wound-healing macrophages emerge to maintain homeostasis in the microenvironment of the hematopoietic niche. Disruption of this microenvironment may result from the failure of macrophages to convert from inflammatory to wound-healing phenotype. In this chapter, we review the effect of radiation on the cells of the hematopoietic niche and discuss the mechanisms by which this niche is repaired.