Currently, heavy ion radiotherapy most commonly employs ¹²C carbon ions for patient treatment. Clinical feasibility and benefits of carbon ion radiotherapy have been studied in various malignancies such as intracranial tumors, head-and-neck cancers, lung cancer, gastrointestinal malignancies, prostate cancer, sarcomas, gynecological cancers and pediatric malignancies (25).

The acute radiation syndrome (ARS) represents a complex clinical situation, consisting of the hematopoietic syndrome, the gastrointestinal syndrome and (only clinically relevant for doses exceeding 10 Gy) the cerebrovascular syndrome (50). ARS regularly occurs after radiation exposure of the whole body with doses exceeding 0.5-1 Gy over a short time period. Currently, the standard of care for ARS consists of mainly symptomatic measures, e.g. intravenous hydration, antiemetic and analgesics medication, antibiotic treatment, blood transfusions, application of hematopoietic growth factors and rarely stem cell transplantation. Depending on the extent of medical interventions, the LD_50/60 (dose that kills 50% of the population within the first 60 days) ranges between 2.5 and 5 Gy. There are several scenarios in which a particle radiation-induced ARS may occur, e.g. during radiation accidents, nuclear terrorism or warfare and in deep space. The rising number of centers providing particle radiotherapy as well as the increasing usage of nuclear technology for medical purposes, industrial procedures and military functions emphasizes the importance of effective medical countermeasures to treat particle radiation-induced ARS.

Animal studies have shown beneficial effects of MSC-based treatments (including both bone marrow MSCs and MSC-derived exosomes) on the hematopoietic system in lethally irradiated mice (51–54). There are also several case reports showing both the feasibility and efficacy of MSC-based therapies for the treatment of tissue damage caused by radiation accidents (55–58). However, no studies have yet been reported that described the impact of MSC-based therapies on particle irradiation-induced ARS. MSCs from different tissues of origin (bone marrow and umbilical cord) have successfully been investigated for ARS treatment (51, 59). Besides its relevance for unintended radiation accidents, stem cell-based treatments including both MSCs and hematopoietic stem cells are discussed also in the case of nuclear terrorism (60). In this context, the United States Army Medical Research Institute of Chemical Defense (USAMRICD) has established production methods in order to have MSC-based treatments available for ARS (61).

Work by our group demonstrated a relatively radioresistant phenotype of bone marrow-derived MSCs after ¹²C particle radiation ( Table 1 ). The relative biological effectiveness (RBE) calculated at 10% clonogenic survival in bone marrow-derived MSCs ranged between 2.0 and 3.1 compared to photon radiation, underpinning the known heterogeneity of MSCs (69). In line with a radioresistant phenotype after particle radiation, apoptosis rates in bone marrow-MSCs remained low even after exposure to 4 Gy of ¹²C irradiation. However, no data are available for the influence of protracted courses of low-dose ¹²C radiotherapy on MSCs that may be more relevant in the space environment and also during radiation exposure in the time of cancer treatment. The different biological effects between photon and ⁵⁶Fe irradiation on human bone marrow-MSCs have also been thoroughly characterized in vitro (68). While it was reported that neither photon nor ⁵⁶Fe ions impaired the osteogenic differentiation capability of MSCs, ⁵⁶Fe irradiation with 1 Gy resulted in a G2/M-phase arrest, which was more pronounced than after physically equivalent doses of photon irradiation. Microarray analyses showed that several genes playing a role in cell cycle progression including cyclin B1 and cyclin E2 were significantly downregulated after 0.1 Gy ⁵⁶Fe ions, while only a marginal response was observed after photon radiation. Additionally, a more pronounced activation of p53 was found after ⁵⁶Fe ion radiation than after photon treatment.

While these analyses were based on 2D systems, some groups also investigated the effects of particle irradiation on adipose-derived stem cells in 3D sphere cultures based on agar coating (70). Both after photon and carbon ion radiation, radiation sensitivity was higher in 2D culture than in 3D culture, which could to some extent be related to the development of radioprotective tissue hypoxia inside of the 3D spheres. In this regard, one should take into consideration that particle irradiation can partly overcome the radioresistance caused by tissue hypoxia (71).

The influence of particle radiation on the defining stem cell characteristics has been investigated both for ¹²C and ⁵⁶Fe ions. After ¹²C irradiation, bone marrow-derived MSCs maintained their adhesive and cellular motility abilities independently of the MSC donor as well as their multi-lineage differentiation capability, which are all pre-requisites for the cells’ regenerative effects (69). Kurpinski et al. could show that the osteogenic differentiation potential of human bone marrow-derived MSCs was maintained after exposure to 1 Gy ⁵⁶Fe ions. While undirected cellular motility as assessed by time-lapse microscopy remained unaffected after ¹²C irradiation, directed migration towards particle-irradiated hematopoietic cells has not been examined so far. For lighter alpha particles, the preservation of MSC functions seems to depend on the dose, and only higher doses (2 Gy) have been shown to inhibit the stemness capacity of bone marrow MSCs (67).

As particle irradiation induces clustered and more complex DNA lesions than photon irradiation, many cell lines have been found to exhibit increased numbers of initial and residual DNA lesions after physically equivalent doses of particle radiation (72–74). However, γH2AX foci analyses in human bone marrow-derived MSCs revealed an effective DNA double strand-break repair after 4 Gy ¹²C irradiation as evident by low numbers of residual double strand-breaks and only temporary activation of the DNA double-strand break signaling pathways (69). This effective DNA double strand-break repair may also contribute to the reported low apoptosis rates after ¹²C irradiation in this study (69). However, in the study of Alessio, residual γH2AX levels were significantly higher after 2 Gy alpha particle irradiation than after 2 Gy photon irradiation in bone marrow-isolated MSCs (67). In line with these findings, the authors also observed increased pATM expression at 48 hours after 2 Gy alpha particle irradiation when compared to baseline levels, which was not observed after 2 Gy photon irradiation. Lower alpha particle doses of 0.04 Gy were found to result in an upregulation of pATM at 1 hour after exposure and a decline at later timepoints; additionally, there was no significant difference regarding the number of γH2AX-positive cells at 48 hours after 0.04 Gy alpha particle irradiation compared to unirradiated controls. In the study of Kurpinski and colleagues, pathway and network analyses of transcriptomic profiles revealed differential effects of ⁵⁶Fe and photons on DNA replication, DNA strand elongation and DNA binding/transferase activity of human bone marrow-MSCs with a more pronounced effect of ⁵⁶Fe ions on these pathways. Furthermore 1 Gy⁵⁶Fe ions resulted in a stronger activation of p53 than 1 Gy photons.

Galactic cosmic rays (GCR) and solar cosmic radiation (SCR) constitute the main components of space irradiation. With an proportion of about 90%, protons form by far the largest component of both GCR and SCR, followed by helium ions (about 10%), electrons and heavier ions (75). Solar particle events (SPEs) as part of SCR, are unpredictable events that occur when protons are accelerated during a flare or during a coronal mass ejection. Besides highly energetic protons and helium ions, SPEs include heavier charged (HZE) particles such as carbon, oxygen and iron ions. Long-term missions to the Mars that may last more than 2 years harbor the risk for considerable radiation exposures, as the shielding provided by the Earth’s magnetic field is absent. Therefore, the National Aeronautics and Space Administration (NASA) classifies the ARS caused by SPEs as a major obstacle to long-term manned space expeditions (76).

Besides higher doses encountered by SPEs, cumulative annual GCR particle doses inside a space craft amount to 176 ± 29 mGy based on measurements in the Mars Science Laboratory (77). In vivo data demonstrated that even low and protracted oxygen ion (¹⁶O) radiation doses of 0.1 Gy impair hematopoiesis (78), and the effective proton dose to reduce the whole blood cell count (WBC) by half was shown to be approximately 1 Gy (79).

A major step towards the usage of MSCs for space irradiation-induced toxicities was performed recently by Huang and colleagues, who demonstrated the feasibility to grow human bone marrow-MSC aboard the International Space Station (ISS) (80). The MSCs’ phenotype was observed unaltered during proliferation in space, and MSCs maintained their proliferative characteristics aboard ISS. Interestingly, space-expanded MSCs seem to exhibit pronounced immunosuppressive effects compared to MSCs grown on the Earth. At least in short-term space cultures, there were no signs for tumorigenic transformation and genomic instability in MSCs.

Recently, a Chinese study reported transcriptional changes of murine bone marrow cells after whole-body ¹²C irradiation with 2 Gy (81), that would mimic the acute bone marrow exposure during a severe SPE. ¹²C irradiation resulted in increased reactive oxygen species production, γH2AX foci and apoptosis levels in bone marrow cells. Significant alterations in genes belonging to the immune response, DNA damage repair, MAPK, TNF signaling and apoptosis pathways were observed in murine bone marrow cells after 2 Gy ¹²C irradiation.

A high-quality study simulating the combined effects of GCR and SPE on the interaction between human bone marrow MSCs and hematopoietic progenitor cells was conducted by Almeida-Porada (66). Interestingly, ⁵⁶Fe ions upregulated many cytokines involved in hematopoiesis and immune cell development, whereas proton irradiation produced the opposite effect and downregulated key cytokines involved in these functions. Also very importantly, the transcriptional changes after proton and ⁵⁶Fe ion irradiation were long-lasting and persisted over several passages.

It is important to consider that low-dose irradiation (≤ 0.1 Gy) has shown to have significantly different effects on MSCs and adipose-derived stem cells than higher radiation doses (24, 82, 83). Low dose irradiation was found to enhance proliferation and to increase the secretion of stem-cell factor (SCF) and GM-CSF, which may favor hematopoiesis (83). In the study of Yang et al., the pro-survival effects of low-dose irradiation on human bone marrow MSCs were mediated via several proteins involved in cell cycle control such as Rb, cyclin E, CDK1, and CDC25B (83). Following these findings, there are considerations to use low-dose irradiation to support large scale expansion as well as therapeutic effects of MSCs (82, 84). Unfortunately, to the best of our knowledge, no studies have reported effects of low-dose particle irradiation, especially protons, on the proliferation rate of MSCs. Additionally, protracted low-dose irradiation over several weeks to mimic GCR are complicated by the difficulty to long-term culture MSCs due to premature senescence and therefore limited passage numbers (85). In the case of prolonged MSC culturing, which may be necessary to expand autologous MSC of astronauts prior to space missions, one should be aware that the DNA repair capacity is lowered leading to more spontaneous and radiation-induced micronuclei (86).

In the space environment, the more convenient way would be the usage of commercially available allogenic off-the-shelf MSCs. Due to the low immunogenicity and immune-privileged behavior, allogenic MSC treatments are generally feasible without prior immune suppression, as demonstrated in large clinical trials (10, 87). In this regard, effective shielding of these off-the-shelf MSCs would be desirable to avoid damage prior to application in a case of SPE.