*Space Radiation-Induced Hematopoietic Stem Cell Injury DOI: http://dx.doi.org/10.5772/intechopen.88914*

*Beyond LEO - Human Health Issues for Deep Space Exploration*

contribute to γ-irradiation-induced ROS production in HSCs [66], which was supported by increasing NOX4 expression, rather than other isoforms of NOXs in γ-ray irradiated HSCs [66]. Diphenyliodonium (a selective NOX inhibitor) treatment can partially restore the functional impairment in γ-ray irradiated HSCs by decreasing irradiation-induced ROS production in HSCs [66]. For space radiation circumstance, the higher level of NOX4 expression was observed in proton-irradiated HSCs than that in unirradiated HSCs. These data indicated that NOX enzyme, especially NOX4, might be involved in the induction of ROS production in proton-irradiated HSCs. The importance of ROS production in space radiation-induced HSC injury should further be assessed by using NOX4 inhibitor or other antioxidants, such as N-acetylcysteine and gamma-tocotrienol in future studies. There are some other potential unanswered questions including (1) whether these chronically oxidativestressed HSCs induced by proton radiation experience senescence, (2) whether these space-irradiated HSCs have chromosomal instability, and (3) whether the chromosomal aberrant HSCs after space radiation result in the leukemia development,

which was evidenced in mice after γ-irradiation exposure [38, 65, 68].

risk of loss self-renewal of HSCs.

antioxidants, which should be investigated in the future.

**4. Oxygen radiation and hematopoietic stem cells**

One of HSC properties is its self-renewal ability, which is sustained via its slow cycling and quiescence. By using BrdU-chasing assay and H2B-GFP mice model, it has been shown that dominant HSCs divide only once every 145 days, which ensures self-renewal capacity along with providing whole life blood homeostasis and avoiding HSC exhaustion [69]. It was reported that loss of FOXO3a resulted in increasing ROS production and accelerating HSC cycling, which is along with the defect of HSC self-renewal capacity and the exhaustion of HSCs. N-acetylcysteine (NAC), an antioxidant, can protect FOXO3a mutant HSCs from oxidative stress and restore HSC dormancy. The same phenotype was also seen in the case of loss of Bmi-1 and TSC1 in mice [70–72], which is due to increasing ROS production and cycling in HSCs. Upon proton radiation, data have shown that there were far fewer HSCs in G0 phase and higher numbers of HSCs in G1 phase than nonirradiated controls. Proton radiation-induced HSC cycling is consistent with upregulation of positive cell cycle regulators cyclin D1 and cyclin D3. Although HSC proliferation might compensate for the decreased number of HSCs after proton radiation, it will be at

Additionally, it has demonstrated that the persistent increase of DNA damage in proton-irradiated HSCs, but not in HPCs, was associated with proton radiationinduced ROS production in HSCs. Unrepaired DNA damage in proton-irradiated HSCs might negatively affect HSC self-renewal, proliferation, and differentiation, leading to long-term functional damage in HSCs. Taken together, these findings provide strong evidence showing that proton TBI induces not only acute hematopoietic injury but also long-term BM suppression and HSC damage. These detrimental effects of proton radiation on hematopoietic cells are closely related to the induction of oxidative stress in irradiated HSCs. The proton exposure-induced acute and long-term hematopoietic damage might be ameliorated through using

As we discussed above, GCR contains various HZE particles including 56Fe, 28Si, 16O, and 12C, which have more detrimental effects on normal tissues than do photon and proton radiations during spaceflight. Oxygen (16O) radiation has relatively high-charge and high-linear energy transfer (LET), leading to a high relative biological effectiveness. In this section, we will mainly discuss the biological negative

effects of 16O on hematopoietic stem cells in long-duration space missions.

**86**

Hematopoietic cells in the body are the most radiosensitive cells to radiation [73, 74]. Exposure to γ-irradiation causes both acute and long-term damage in hematopoietic stem and progenitor cells (HSPCs), which is due primarily to radiation-induced cellular apoptosis and senescence in HSPCs [37–39, 65]. Using porcine and mice model, it has documented that proton radiation induced both acute and long-term hematopoietic damage. We have described the acute and residual effects of proton radiation on hematopoietic stem cells showing that numbers and function of bone marrow HSCs in mice were detrimentally affected. The negative effects of proton radiation mainly contribute to increasing the production of oxidative stress and DNA damage in irradiated HSCs [53]. 56Fe radiation causes significant alterations in the expression of repetitive elements and DNA methylation, and 0.1-0.4 Gy of 56Fe radiation resulted in significant epigenetic changes in hematopoietic stem and progenitor cells in a mouse model [75]. Using cultured human hematopoietic stem and progenitor cells, it was found that 12C radiation induced chromosome aberrations and cellular apoptosis [76]. 0.3–0.9 Gy of 28Si radiation triggers a significant increase of cellular apoptosis in irradiated mice HSCs at 4 weeks after the exposure, which results in the deficiency of numbers and clonogenic function of irradiated HSCs [77]. These findings indicate that GCR, including different forms of ionizing radiation, induces acute and residual injury in hematopoietic stem cells. However, it remains elusive whether 16O radiation induces acute and long-term hematopoietic effects and what main factors are involved in the negative effects on HSCs under 16O exposure.

In one of our experiments, C57BL/6 J mice were exposed to 0.1, 0.25, and 1.0 Gy 16O (600 MeV/n) total-body irradiation (TBI) and analyzed the effects of 16O radiation on peripheral blood and BM 2 weeks after the exposure [51] (**Table 3**). Since hematopoietic cells are known to be radiosensitive, it is not surprising that a significant decrease was observed in peripheral WBC and platelet counts in mice exposed to 1.0 Gy of 16O. In comparison to 16O radiation, peripheral blood cell counts, including numbers of WBCs and platelets, were almost recovered to normal levels at 2 weeks after 1.0 Gy of γ-ray radiation in BALB/c mice [78]. This might due to (1) different animal species used and (2) different biological effectiveness of 16O and γ-ray radiation along with high LET properties of 16O. 16O TBI causes cellular apoptosis in hematopoietic progenitors but not hematopoietic stem cells at 2 weeks postexposure. To monitor how fast HPCs recover from 16O TBI, apoptotic assay was performed at 3 months after 0.1, 0.25, and 1.0 Gy of 16O TBI, showing that the apoptotic levels in HPCs and HSCs after the exposure are similar to those in nonirradiated mice. These data suggest that HPCs have a slower recovery than HSCs after 16O TBI.


Previous studies have demonstrated that the functional defect of mouse HSCs was observed under either radiation or chemotherapeutic drug treatment. For example, the reconstitution capacity of mouse HSCs has detrimental effects under 1.0 Gy of total-body γ-ray radiation along with apparent myeloid bias differentiation [78, 79]. One dose of 5-fluorouracil treatment significantly decreased the numbers and engraftment ability of mouse HSCs at day 10 after the exposure [80]. To investigate the effects of low doses of 16O TBI on HSC function, in vitro colonyforming assays using bone marrow cells were performed at 2 weeks after 0.1–1.0 Gy doses of 16O exposure. It shows that low doses of 16O TBI not only decreases numbers of HSCs but also abates the in vitro colony-forming abilities, such as decreased numbers of BFU-E, CFU-GM, and CFU-GEMM from irradiated bone marrow cells. These data suggest that the function of HSC after 16O TBI was negative affected despite low doses of 16O used. Using in vitro cell culture model, previous studies have shown that 16O radiation has more dramatic effects on chromosomal aberrations, micronuclei formation, cell survival, and apoptosis than photon radiation [81, 82]. It has been documented that 6.5 Gy total-body γ-irradiation decreased the numbers of bone marrow HSCs up to 50% at 2 weeks postexposure, which was observed at 2 weeks after 1.0 Gy of total-body 16O radiation [38]. These results indicate that 16O radiation has a higher RBE than photon radiation.

To explore long-term effect of 16O TBI on hematopoietic cells, 0.05, 0.1, 0.25, and 1.0 Gy 16O (600 MeV/n) radiations were used to irradiate C57BL/6 J mice. Irradiated mice were analyzed for the long-term effects of 16O radiation on peripheral blood cells and bone marrow cells 3 months postexposure [83]. Although there are the same numbers of peripheral blood cells at 3 months after 0.05 to 1.0 Gy of 16O TBI as nonirradiated controls, numbers of HPC and HSCs from irradiated mice were significantly lower than those from nonirradiated controls. The changes of peripheral blood cell counts after oxygen radiation are similar to the effects of other types of ionizing radiation, such as 0.5 and 1.0 Gy of γ-TBI [84, 85]. Peripheral blood cell counts were back to normal levels 2 months after sublethal doses of γ-ray exposure [86]. Recovery of peripheral blood cell counts may neglect the effects of irradiation on bone marrow HSCs [84], which will result in overlooking the longterm bone marrow suppression after radiation.

We have demonstrated that 0.1 to 1.0 Gy of 16O TBI, but not 0.05 Gy dose, resulted in a dramatic impairment in both numbers and function of bone marrow HSCs in mice at 3 months after exposure. Comparing to 16O TBI, exposure of mice to 0.5 and 1.0 Gy of γ-TBI did not negatively affect the numbers and function of HSC in mice at 3 months after exposure. The phenotype from 16O- and γ-TBI might be related to their RBE along with higher RBE levels of 16O TBI than γ-TBI [81]. The long-term detrimental effects of 16O TBI on bone marrow hematopoietic stem cells have also been seen in 6.5 Gy sublethal doses of γ-rays and 1.0 Gy low dose of proton radiation, showing a reduction in HSC reserves and a defect in HSC function [48, 85, 87, 88]. When comparing the effects of different radiation sources on HPCs, we have shown that acute exposure to low doses of 16O TBI triggered a significant reduction in numbers of HPCs at 2 weeks after exposure. However, numbers of HPCs in irradiated mice recovered back to normal levels 2 weeks after either γ-ray or proton exposure [51]. These data indicate that 16O-irradiated HPCs have a slower recovery than proton- and photon-irradiated HPCs.

We have previously demonstrated that exposure of mice to 1.0 Gy of 16O TBI leads to an increased rate of apoptosis at 2 weeks postexposure in irradiated HPCs but not HSCs. This is consistent with HPC colony-forming ability assay, showing lower numbers of BFU-E, CFU-GM, and CFU-GEMM when compared to those in nonirradiated controls [51]. When we further examined HPC colony-forming abilities at 3 months after same dose of 16O TBI, it showed that numbers of various

**89**

**Table 4.**

*Space Radiation-Induced Hematopoietic Stem Cell Injury*

colonies were still much lower than those in nonirradiated controls [83]. Notably, the decreased colony-forming abilities after 16O TBI were in a dose-independent manner, which suggests "hit and damage." These data suggest that oxygen irradiation has features with a high-linear energy transfer and strong relative biological

A cobblestone area-forming cell (CAFC) assay is a surrogate in vitro hematopoietic stem cell functional assay. We measured HSC in vitro CAFC-forming ability at 3 months after 0.1, 0.25, and 1.0 Gy of 16O radiation, showing that irradiated mice HSCs had a significant reduction of CAFC numbers independent of radiation doses (**Table 4**). These unusual dose-response curves have also been seen in the studies of 28Si radiation, such as effects of 28Si radiation on synaptic plasticity and contextual fear memory [89, 90]. However, when mice were exposed to 0.05 Gy of 16O TBI, numbers of CAFC were comparable to nonirradiated controls, indicating that HSC function was not affected after exposure to 0.05 Gy 16O TBI [83]. These long-term negative effects of 16O TBI on HSCs are also observed in other different types of ionizing radiation [53, 63, 91]. Mice were exposed to 1.0 Gy of proton total-body irradiation, leading to a significant decrease in the CAFC-forming ability in HSCs at 22 weeks postexposure [53]. 6.5 Gy of γ-ray total-body irradiation caused a reduction in HSC colony-forming ability 2 months postexposure [38]. As we discussed previously, peripheral blood cell counts were back to normal levels at 3 months after low-dose 16O TBI, while the numbers and function of bone marrow HSCs were significantly decreased after exposure. Proliferation of progenitors (such as myeloid, lymphoid, and erythroid progenitors) might contribute to the recovery of peripheral blood cells after radiation. These results suggest that low doses of ionizing radiation can induce long-term HSC suppression, while 16O TBI has stronger abilities to induce

the long-term HSC suppression than other types of ionizing radiation.

*Induction of cellular apoptosis, ROS, and DNA damage after oxygen irradiation.*

Cellular apoptosis and ROS production are crucial mediators in irradiationinduced cell damage. We exposed mice to 1.0 Gy of 16O, and it triggered an aberrant increase in ROS production in HPCs and HSCs 2 weeks after exposure [51]. Meanwhile, increasing levels of apoptosis were significant in irradiated HPCs, but not LSK cells and HSCs, when compared to nonirradiated controls. Whether the different acute responses of HPCs and HSCs to 16O TBI are related to ROS production has yet to be determined. Induction of ROS production persisted up to 3 months after 16O TBI [83], which is congruent with the decreased expression of the antioxidant genes GPX2 and SOD3 in 1.0 Gy 16O-irradiated HSCs when compared to nonirradiated HSCs. Previous studies utilized proton and γ-ray radiation to prove that HSC functional impairment might be attributable to the accumulation of residual ROS [53, 63, 92, 93]. This is evidenced by a decrease in in vivo

*DOI: http://dx.doi.org/10.5772/intechopen.88914*

effectiveness.

*Beyond LEO - Human Health Issues for Deep Space Exploration*

indicate that 16O radiation has a higher RBE than photon radiation.

term bone marrow suppression after radiation.

recovery than proton- and photon-irradiated HPCs.

To explore long-term effect of 16O TBI on hematopoietic cells, 0.05, 0.1, 0.25, and 1.0 Gy 16O (600 MeV/n) radiations were used to irradiate C57BL/6 J mice. Irradiated mice were analyzed for the long-term effects of 16O radiation on peripheral blood cells and bone marrow cells 3 months postexposure [83]. Although there are the same numbers of peripheral blood cells at 3 months after 0.05 to 1.0 Gy of 16O TBI as nonirradiated controls, numbers of HPC and HSCs from irradiated mice were significantly lower than those from nonirradiated controls. The changes of peripheral blood cell counts after oxygen radiation are similar to the effects of other types of ionizing radiation, such as 0.5 and 1.0 Gy of γ-TBI [84, 85]. Peripheral blood cell counts were back to normal levels 2 months after sublethal doses of γ-ray exposure [86]. Recovery of peripheral blood cell counts may neglect the effects of irradiation on bone marrow HSCs [84], which will result in overlooking the long-

We have demonstrated that 0.1 to 1.0 Gy of 16O TBI, but not 0.05 Gy dose, resulted in a dramatic impairment in both numbers and function of bone marrow HSCs in mice at 3 months after exposure. Comparing to 16O TBI, exposure of mice to 0.5 and 1.0 Gy of γ-TBI did not negatively affect the numbers and function of HSC in mice at 3 months after exposure. The phenotype from 16O- and γ-TBI might be related to their RBE along with higher RBE levels of 16O TBI than γ-TBI [81]. The long-term detrimental effects of 16O TBI on bone marrow hematopoietic stem cells have also been seen in 6.5 Gy sublethal doses of γ-rays and 1.0 Gy low dose of proton radiation, showing a reduction in HSC reserves and a defect in HSC function [48, 85, 87, 88]. When comparing the effects of different radiation sources on HPCs, we have shown that acute exposure to low doses of 16O TBI triggered a significant reduction in numbers of HPCs at 2 weeks after exposure. However, numbers of HPCs in irradiated mice recovered back to normal levels 2 weeks after either γ-ray or proton exposure [51]. These data indicate that 16O-irradiated HPCs have a slower

We have previously demonstrated that exposure of mice to 1.0 Gy of 16O TBI leads to an increased rate of apoptosis at 2 weeks postexposure in irradiated HPCs but not HSCs. This is consistent with HPC colony-forming ability assay, showing lower numbers of BFU-E, CFU-GM, and CFU-GEMM when compared to those in nonirradiated controls [51]. When we further examined HPC colony-forming abilities at 3 months after same dose of 16O TBI, it showed that numbers of various

Previous studies have demonstrated that the functional defect of mouse HSCs was observed under either radiation or chemotherapeutic drug treatment. For example, the reconstitution capacity of mouse HSCs has detrimental effects under 1.0 Gy of total-body γ-ray radiation along with apparent myeloid bias differentiation [78, 79]. One dose of 5-fluorouracil treatment significantly decreased the numbers and engraftment ability of mouse HSCs at day 10 after the exposure [80]. To investigate the effects of low doses of 16O TBI on HSC function, in vitro colonyforming assays using bone marrow cells were performed at 2 weeks after 0.1–1.0 Gy doses of 16O exposure. It shows that low doses of 16O TBI not only decreases numbers of HSCs but also abates the in vitro colony-forming abilities, such as decreased numbers of BFU-E, CFU-GM, and CFU-GEMM from irradiated bone marrow cells. These data suggest that the function of HSC after 16O TBI was negative affected despite low doses of 16O used. Using in vitro cell culture model, previous studies have shown that 16O radiation has more dramatic effects on chromosomal aberrations, micronuclei formation, cell survival, and apoptosis than photon radiation [81, 82]. It has been documented that 6.5 Gy total-body γ-irradiation decreased the numbers of bone marrow HSCs up to 50% at 2 weeks postexposure, which was observed at 2 weeks after 1.0 Gy of total-body 16O radiation [38]. These results

**88**

colonies were still much lower than those in nonirradiated controls [83]. Notably, the decreased colony-forming abilities after 16O TBI were in a dose-independent manner, which suggests "hit and damage." These data suggest that oxygen irradiation has features with a high-linear energy transfer and strong relative biological effectiveness.

A cobblestone area-forming cell (CAFC) assay is a surrogate in vitro hematopoietic stem cell functional assay. We measured HSC in vitro CAFC-forming ability at 3 months after 0.1, 0.25, and 1.0 Gy of 16O radiation, showing that irradiated mice HSCs had a significant reduction of CAFC numbers independent of radiation doses (**Table 4**). These unusual dose-response curves have also been seen in the studies of 28Si radiation, such as effects of 28Si radiation on synaptic plasticity and contextual fear memory [89, 90]. However, when mice were exposed to 0.05 Gy of 16O TBI, numbers of CAFC were comparable to nonirradiated controls, indicating that HSC function was not affected after exposure to 0.05 Gy 16O TBI [83]. These long-term negative effects of 16O TBI on HSCs are also observed in other different types of ionizing radiation [53, 63, 91]. Mice were exposed to 1.0 Gy of proton total-body irradiation, leading to a significant decrease in the CAFC-forming ability in HSCs at 22 weeks postexposure [53]. 6.5 Gy of γ-ray total-body irradiation caused a reduction in HSC colony-forming ability 2 months postexposure [38]. As we discussed previously, peripheral blood cell counts were back to normal levels at 3 months after low-dose 16O TBI, while the numbers and function of bone marrow HSCs were significantly decreased after exposure. Proliferation of progenitors (such as myeloid, lymphoid, and erythroid progenitors) might contribute to the recovery of peripheral blood cells after radiation. These results suggest that low doses of ionizing radiation can induce long-term HSC suppression, while 16O TBI has stronger abilities to induce the long-term HSC suppression than other types of ionizing radiation.

Cellular apoptosis and ROS production are crucial mediators in irradiationinduced cell damage. We exposed mice to 1.0 Gy of 16O, and it triggered an aberrant increase in ROS production in HPCs and HSCs 2 weeks after exposure [51]. Meanwhile, increasing levels of apoptosis were significant in irradiated HPCs, but not LSK cells and HSCs, when compared to nonirradiated controls. Whether the different acute responses of HPCs and HSCs to 16O TBI are related to ROS production has yet to be determined. Induction of ROS production persisted up to 3 months after 16O TBI [83], which is congruent with the decreased expression of the antioxidant genes GPX2 and SOD3 in 1.0 Gy 16O-irradiated HSCs when compared to nonirradiated HSCs. Previous studies utilized proton and γ-ray radiation to prove that HSC functional impairment might be attributable to the accumulation of residual ROS [53, 63, 92, 93]. This is evidenced by a decrease in in vivo


**Table 4.**

*Induction of cellular apoptosis, ROS, and DNA damage after oxygen irradiation.*

engraftment capacity and in vitro colony-forming ability using bone marrow cells after radiation. The importance of ROS overproduction on functional HSCs was not only supported under radiation stress condition but also supported by other genetic animal models. Deletion of Foxo3, ATM, TSC1, and Bmi-1 in mice leads to the impairment of numbers and function of HSCs along with increasing ROS production. Application of antioxidants, such as N-acetyl cysteine (NAC), on these mutant mice significantly ameliorated the HSC functional deficiency [71, 94–96]. We have provided data showing 16O-irradiated HSCs had higher levels of ROS production than nonirradiated animals in both acute and long-term studies. It is well accepted that mitochondrial oxidative phosphorylation and NADPH oxidases are two main sources to produce ROS in mammalian cells. Because HSCs reside in hypoxic environmental niche in the bone marrow and have higher expression of HIF1α in response to hypoxia, HSCs produce ROS mainly through glycolysis and NOX enzyme. We have previously shown that proton and γ-ray radiation induced significantly upregulation of NOX4 in irradiated HSCs [53, 66]. The NOX4 inhibitor diphenyliodonium can partially protect functional HSCs from γ-irradiationinduced long-term damage. Therefore, antioxidants, such as NOX4 inhibitors and NAC, should be further tested whether inhibiting ROS production can decrease 16O TBI-induced ROS production to accelerate the functional recovery of HPCs and HSCs after 16O irradiation exposure.

Under radiotherapy and chemotherapy stress conditions, dominant HSCs might be activated from quiescent status to provide the need for stressed hematopoietic system. However, frequent HSC activation might cause its loss of self-renewal ability, differentiation, and death with bone marrow failure syndrome [69, 97]. We have shown that proton and γ-irradiation can efficiently activate quiescent HSCs [53, 65], leading to the redistribution of different cell cycle phases and stem cell functional defects. Data from genetic mice models, such as depletion of FOXO3a and Lkb1, showed that HSCs had fast cycling with loss of HSC self-renewal ability and HSC exhaustion [94, 98–100]. There are fewer numbers of HSCs in G0 and higher numbers in G1/G2SM at 2 weeks after 16O TBI [51]. Additionally, we observed that around 15% of irradiated HSCs had more than two γH2AX foci per cell 2 weeks after 16O exposure [51], which is positively correlated with the increased ROS production in 16O TBI HSCs. Taken together, all of ROS production, DNA damage, and HSC cycling after 16O TBI might contribute to HSC defect induced by oxygen radiation, which will be tested in our future studies.

Note:


**91**

*Space Radiation-Induced Hematopoietic Stem Cell Injury*

high-energy charged particle radiation.

and HZE particle exposure on the hematopoietic system.

China (Grant No. 81460110, 81660123, and 81860026),

The authors declare no conflicts of interest.

, Mengzhen Yue1

\*Address all correspondence to: lshao@ncu.edu.cn

provided the original work is properly cited.

1 Medical College of Nanchang University, Nanchang, China

**5. Conclusion**

**Acknowledgements**

**Conflict of interest**

**Author details**

Huihong Zeng1

Nanchang, China

3.The protons and oxygen nuclei in the studies described here were all delivered within a few minutes, and most charged particle exposures during space flight occur at a very low-dose rate and/or are fractionated. Though we acknowledge that the high-dose rates we used are a limitation of our studies, low-dose rate exposures were not possible because of practical constraints. Future studies with low-dose rate or fractionated exposures should provide further insight into dose-rate dependence of hematopoietic stem/progenitor cell response to

In summary, proton and oxygen space radiation have detrimental effects on the hematopoietic system even with at low doses, which will have potential implications for health outcomes during long-duration space missions. Increasing ROS production might be a major mediator on space radiation-induced HSC damage. Knowledge gained from this chapter could aid in planning countermeasure strategies to protect against hematopoietic effects of radiation exposure during space travel. To minimize the health negative effects of deep-space travel, decreasing oxidative stress might be a good approach to mitigate the adverse effects of proton

The study was in part supported by the National Natural Science Foundation of

and Lijian Shao1,2\*

2 Jiangxi Provincial Key Laboratory of Preventive Medicine, Nanchang University,

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

*DOI: http://dx.doi.org/10.5772/intechopen.88914*

*Space Radiation-Induced Hematopoietic Stem Cell Injury DOI: http://dx.doi.org/10.5772/intechopen.88914*

3.The protons and oxygen nuclei in the studies described here were all delivered within a few minutes, and most charged particle exposures during space flight occur at a very low-dose rate and/or are fractionated. Though we acknowledge that the high-dose rates we used are a limitation of our studies, low-dose rate exposures were not possible because of practical constraints. Future studies with low-dose rate or fractionated exposures should provide further insight into dose-rate dependence of hematopoietic stem/progenitor cell response to high-energy charged particle radiation.
