Editorial. Non-DNA targeted effects

Non-targeted effects of radiation: a personal perspective on the role of exosomes in an evolving paradigm

PURPOSE

Radiation-induced non-targeted effects (NTE) have implications in a variety of areas relevant to radiation biology. Here we evaluate the various cargo associated with exosomal signalling and how they work synergistically to initiate and propagate the non-targeted effects including Genomic Instability and Bystander Effects.

CONCLUSIONS

Extra cellular vesicles, in particular exosomes, have been shown to carry bystander signals. Exosome cargo may contain nucleic acids, both DNA and RNA, as well as proteins, lipids and metabolites. These cargo molecules have all been considered as potential mediators of NTE. A review of current literature shows mounting evidence of a role for ionizing radiation in modulating both the numbers of exosomes released from affected cells as well as the content of their cargo, and that these exosomes can instigate functional changes in recipient cells. However, there are significant gaps in our understanding, particularly regarding modified exosome cargo after radiation exposure and the functional changes induced in recipient cells.

Disproportionate Impacts of Radiation Exposure on Women, Children, and Pregnancy: Taking Back our Narrative

Narratives surrounding ionizing radiation have often minimized radioactivity's impact on the health of human and non-human animals and the natural environment. Many Cold War research policies, practices, and interpretations drove nuclear technology forward by institutionally obscuring empirical evidence of radiation's disproportionate and low-dose harm-a legacy we still confront. Women, children, and pregnancy development are particularly sensitive to exposure from radioactivity, suffering more damage per dose than adult males, even down to small doses, making low doses a cornerstone of concern. Evidence of compounding generational damage could indicate increased sensitivity through heritable impact. This essay examines the existing empirical evidence demonstrating these sensitivities, and how research institutions and regulatory authorities have devalued them, willingly sacrificing health in the service of maintaining and expanding nuclear technology (Nadesan 2019). Radiation's disproportionate impacts should now be the research and policy focus, as society is poised to make crucial and long-lasting decisions regarding climate change mitigation and future energy sources (Brown 2019b).

Immune-modulating properties of ionizing radiation: rationale for the treatment of cancer by combination radiotherapy and immune checkpoint inhibitors

Radiotherapy (RT) utilizes the DNA-damaging properties of ionizing radiation to control tumor growth and ultimately kill tumor cells. By modifying the tumor cell phenotype and the tumor microenvironment, it may also modulate the immune system. However, out-of-field reactions of RT mostly assume further immune activation. Here, the sequence of the applications of RT and immunotherapy is crucial, just as the dose and fractionation may be. Lower single doses may impact on tumor vascularization and immune cell infiltration in particular, while higher doses may impact on intratumoral induction and production of type I interferons. The induction of immunogenic cancer cell death seems in turn to be a common mechanism for most RT schemes. Dendritic cells (DCs) are activated by the released danger signals and by taking up tumor peptides derived from irradiated cells. DCs subsequently activate T cells, a process that has to be tightly controlled to ensure tolerance. Inhibitory pathways known as immune checkpoints exist for this purpose and are exploited by tumors to inhibit immune responses. Cytotoxic T lymphocyte antigen 4 (CTLA-4) and programmed cell death protein 1 (PD-1) on T cells are two major checkpoints. The biological concepts behind the findings that RT in combination with anti-CTLA-4 and/or anti-PD-L1 blockade stimulates CD8+ T cell-mediated anti-tumor immunity are reviewed in detail. On this basis, we suggest clinically significant combinations and sequences of RT and immune checkpoint inhibition. We conclude that RT and immune therapies complement one another.

Non-targeted effects of ionising radiation--implications for low dose risk

Non-DNA targeted effects of ionising radiation, which include genomic instability, and a variety of bystander effects including abscopal effects and bystander mediated adaptive response, have raised concerns about the magnitude of low-dose radiation risk. Genomic instability, bystander effects and adaptive responses are powered by fundamental, but not clearly understood systems that maintain tissue homeostasis. Despite excellent research in this field by various groups, there are still gaps in our understanding of the likely mechanisms associated with non-DNA targeted effects, particularly with respect to systemic (human health) consequences at low and intermediate doses of ionising radiation. Other outstanding questions include links between the different non-targeted responses and the variations in response observed between individuals and cell lines, possibly a function of genetic background. Furthermore, it is still not known what the initial target and early interactions in cells are that give rise to non-targeted responses in neighbouring or descendant cells. This paper provides a commentary on the current state of the field as a result of the non-targeted effects of ionising radiation (NOTE) Integrated Project funded by the European Union. Here we critically examine the evidence for non-targeted effects, discuss apparently contradictory results and consider implications for low-dose radiation health effects.

How does ionizing irradiation contribute to the induction of anti-tumor immunity?

Radiotherapy (RT) with ionizing irradiation is commonly used to locally attack tumors. It induces a stop of cancer cell proliferation and finally leads to tumor cell death. During the last years it has become more and more evident that besides a timely and locally restricted radiation-induced immune suppression, a specific immune activation against the tumor and its metastases is achievable by rendering the tumor cells visible for immune attack. The immune system is involved in tumor control and we here outline how RT induces anti-inflammation when applied in low doses and contributes in higher doses to the induction of anti-tumor immunity. We especially focus on how local irradiation induces abscopal effects. The latter are partly mediated by a systemic activation of the immune system against the individual tumor cells. Dendritic cells are the key players in the initiation and regulation of adaptive anti-tumor immune responses. They have to take up tumor antigens and consecutively present tumor peptides in the presence of appropriate co-stimulation. We review how combinations of RT with further immune stimulators such as AnnexinA5 and hyperthermia foster the dendritic cell-mediated induction of anti-tumor immune responses and present reasonable combination schemes of standard tumor therapies with immune therapies. It can be concluded that RT leads to targeted killing of the tumor cells and additionally induces non-targeted systemic immune effects. Multimodal tumor treatments should therefore tend to induce immunogenic tumor cell death forms within a tumor microenvironment that stimulates immune cells.

Combined treatment of human colorectal tumor cell lines with chemotherapeutic agents and ionizing irradiation can in vitro induce tumor cell death forms with immunogenic potential

Chemotherapeutic agents (CT) and ionizing radiation (X-ray) induce DNA damage and primarily aim to stop the proliferation of tumor cells. However, multimodal anti-cancer therapies should finally result in tumor cell death and, best, in the induction of systemic anti-tumor immunity. Since distinct therapy-induced tumor cell death forms may create an immune activating tumor microenvironment, this study examined whether sole treatment with CT that are used in the therapy for colorectal cancer or in combination with X-ray result in colorectal tumor cell death with immunogenic potential. 5-Fluorouracil (5-FU), Oxaliplatin (Oxp), or Irinotecan (Irino) in combination with X-ray were all potent inhibitors of colorectal tumor cell colony formation. This study then examined the forms of cell death with AnnexinA5-FITC/Propidium Iodide staining. Necrosis was the prominent form of cell death induced by CT and/or X-ray. While only a combination of Irino with X-ray leads to death induction already 1 day after treatment, also the combinations of Oxp or 5-FU with X-ray and X-ray alone resulted in high necrosis rates at later time points after treatment. Inhibition of apoptosis increased the amount of necrotic tumor cells, suggesting that a programmed form of necrosis can be induced by CT + X-ray. 5-FU and Oxp alone or in combination with X-ray and Irino plus X-ray were most effective in increasing the expression of RIP, IRF-5, and p53, proteins involved in necrotic and apoptotic cell death pathways. All treatments further resulted in the release of the immune activating danger signals high-mobility group box 1 (HMGB1) and heat shock protein 70 (HSP70). The supernatants of the treated tumor cells induced maturation of dendritic cells. It is, therefore, concluded that combination of CT with X-ray is capable of inducing in vitro cell death forms of colorectal tumors with immunogenic potential.

Low dose ionising radiation leads to a NF-κB dependent decreased secretion of active IL-1β by activated macrophages with a discontinuous dose-dependency

PURPOSE

Therapy with low doses of ionising radiation (X-rays) exerts anti-inflammatory effects. Little is known about whether and how low doses of X-ray treatment modulate the inflammatory phenotype of macrophages, especially the secretion of Interleukin-1beta (IL-1β).

MATERIALS AND METHODS

Macrophages were differentiated from human THP-1 monocytes, activated with lipopolysaccharide (LPS), treated with distinct low doses of X-rays, and co-activated with monosodium urate crystals (MSU) to induce inflammasome activation. Secretion of IL-1β was analysed by an enzyme-linked immunosorbent assay (ELISA) and Western blot. Furthermore, we analysed the intracellular amounts of the serine/threonine protein kinase B (named: Akt), mitogen-activated protein kinase p38 (p38), the v-rel reticuloendotheliosis viral oncogene homolog A (RelA), and pro- and cleaved IL-1β.

RESULTS

Low dose X-rays led to decreased secretion of active IL-1β in a manner discontinuous with dose which was most pronounced after 0.5 or 0.7 Gy. Passive release of lactate dehydrogenase (LDH) was not influenced by X-rays. The decreased secretion of IL-1β correlated with reduced translocation of RelA, being part of the nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) complex, into the nucleus. After 0.5 or 0.7 Gy of X-rays, the intracellular protein amounts of up (p38) and downstream molecules (Akt) of NF-κB were reduced in activated macrophages, as were the pro- and cleaved forms of IL-1β.

CONCLUSIONS

Distinct low doses of X-rays induce an anti-inflammatory phenotype of activated macrophages by lowering the amount of secreted IL-1β in a NF-κB dependent manner.

Analysis of the common deletions in the mitochondrial DNA is a sensitive biomarker detecting direct and non-targeted cellular effects of low dose ionizing radiation

One of the key issues of current radiation research is the biological effect of low doses. Unfortunately, low dose science is hampered by the unavailability of easily performable, reliable and sensitive quantitative biomarkers suitable detecting low frequency alterations in irradiated cells. We applied a quantitative real time polymerase chain reaction (qRT-PCR) based protocol detecting common deletions (CD) in the mitochondrial genome to assess direct and non-targeted effects of radiation in human fibroblasts. In directly irradiated (IR) cells CD increased with dose and was higher in radiosensitive cells. Investigating conditioned medium-mediated bystander effects we demonstrated that low and high (0.1 and 2Gy) doses induced similar levels of bystander responses and found individual differences in human fibroblasts. The bystander response was not related to the radiosensitivity of the cells. The importance of signal sending donor and signal receiving target cells was investigated by placing conditioned medium from a bystander response positive cell line (F11-hTERT) to bystander negative cells (S1-hTERT) and vice versa. The data indicated that signal sending cells are more important in the medium-mediated bystander effect than recipients. Finally, we followed long term effects in immortalized radiation sensitive (S1-hTERT) and normal (F11-hTERT) fibroblasts up to 63 days after IR. In F11-hTERT cells CD level was increased until 35 days after IR then reduced back to control level by day 49. In S1-hTERT cells the increased CD level was also normalized by day 42, however a second wave of increased CD incidence appeared by day 49 which was maintained up to day 63 after IR. This second CD wave might be the indication of radiation-induced instability in the mitochondrial genome of S1-hTERT cells. The data demonstrated that measuring CD in mtDNA by qRT-PCR is a reliable and sensitive biomarker to estimate radiation-induced direct and non-targeted effects.