Age at exposure is a critical factor that influences the risk of radiation-induced leukemia, which arises from hematopoietic stem and progenitor cells. However, little is known about the effect of age on the radiation response of these cells. In this study, we examined the radiation response of hematopoietic stem and progenitor cells in infant (1-week-old), juvenile (3-week-old), and adult (8- and 14-week-old) C3H/He mice, which are susceptible to radiation-induced myeloid leukemia. We first observed an age-dependent increase in the radioresistance of hematopoietic stem and progenitor cells after in vivo irradiation. However, in vitro irradiation of progenitor cells did not show any age differences, suggesting that radiation sensitivity in vivo is dependent on the bone marrow microenvironment rather than to intrinsic properties of progenitors themselves. Expression profiles of bone marrow tissues identified chemokine and cytokine family genes, whose expression differed between infant and adult tissues at time points before and after irradiation. Among the selected thirteen cytokines reported to be radioprotective, we observed increased expression of Csf1, Csf2, Cxcl12, Fgf1, Fgf7, Il1a, Il1b and Kitl after irradiation, mostly in adult tissues. Specifically, Csf2, Fgf1 and Il1b expression, as revealed by qPCR, were significantly enhanced in adult bone marrow tissue after irradiation, but were unresponsive to irradiation in infant tissue. These results suggest that the higher susceptibility of infant hematopoietic stem and progenitor cells to the cell killing effect of ionizing radiation may be attributed to a failure to induce a subset of radioprotective cytokines in the immature bone marrow microenvironment.

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