Carbon Ion Irradiated Neural Injury Induced the Peripheral Immune Effects in Vitro or in Vivo

Siwu decoction mitigates radiation-induced immune senescence by attenuating hematopoietic damage

BACKGROUND

To investigate the long term effects of ionizing radiation (IR) on hematopoietic stem/progenitor cells (HSPCs), immune tissues and cells, and the effects of Siwu decoction (SWD) on immune senescence mice.

METHODS

C57BL/6 J mice were exposed to 6.0 Gy 60Co γ irradiation. After 8-weeks of IR, SWD (5, 10, 20 g/kg/d) was administered for 30 days. The changes of HSPCs in bone marrow (BM) and T, B type lymphocyte and natural killer (NK) cells in spleen were detected by flow cytometry. The changes of peripheral blood cells were also examined. Hematoxylin-eosin staining were used to detect the pathological lesions of hippocampus, spleen and thymus tissues. Absolute mouse telomere length quantification qPCR assay kit was used to measure the telomere length of BM cells. The expression of factors associated with inflammation and aging such as p16, β-galactosidase in spleen, thymus and BM was determined.

RESULTS

Administration of SWD could increase the proportion of LSK (Lin-, Sca-1 + , c-Kit-), multipotent progenitor cells and multipotent progenitor cells and decrease the proportion of common myeloid progenitors and granulocyte-macrophage progenitors in BM. The proportion of B cells and NK cells in spleen and the content of white blood cells, red blood cells, hemoglobin, lymphocytes and eosinophils in peripheral blood were increased, at the same time, the proportion of neutrophils and monocytes was reduced by SWD. The pathological lesions of hippocampus, spleen and thymus were improved. The expression of p16 and β-galactosidase in spleen, thymus and BM was reduced and shortening of the telomere of BM cells was inhibited after administration. In addition, SWD could reduce the content of Janus activated kinase (JAK) 1, JAK2 and signal transducer and activator of transcription 3 (STAT3) in BM and spleen.

CONCLUSIONS

SWD could slow down IR-induced immune senescence by improving hematopoietic and immunologic injury. SWD might reduce the inflammation level of BM hematopoietic microenvironment by acting on JAK/STAT signaling pathway, while the immune damage of mice was improved by affecting the differentiation of HSPCs. The remission of hematopoietic and immunologic senescence was further demonstrated at the overall level.

Corrigendum: Altered expression of inflammation-associated molecules in striatum: an implication for sensitivity to heavy ion radiations

[This corrects the article DOI: 10.3389/fncel.2023.1252958.].

Effects of X-ray cranial irradiation on metabolomics and intestinal flora in mice

Cranial radiotherapy is an important treatment for intracranial and head and neck tumors. To investigate the effects of cranial irradiation (C-irradiation) on gut microbiota and metabolomic profile, the feces, plasma and cerebral cortex were isolated after exposing mice to cranial X-ray irradiation at a dose rate of 2.33 Gy/min (5 Gy/d for 4 d consecutively). The gut microorganisms and metabolites were detected by 16 S rRNA gene sequencing method and LC-MS method, respectively. We found that compared with sham group, the gut microbiota composition changed at 2 W and 4 W after C-irradiation at the genus level. The fecal metabolomics showed that compared with Sham group, 44 and 66 differential metabolites were found to be annotated into metabolism pathways at 2 W and 4 W after C-irradiation, which were significantly enriched in the arginine and proline metabolism. Metabolome analysis of serum and cerebral cortex showed that, at 4 W after C-irradiation, the expression pattern of metabolites in serum samples of mice was similar to that of sham group, and the cerebral cortex metabolites of the two groups were completely separated. KEGG functional analysis showed that serum and brain tissue differential metabolites were respectively enriched in tryptophan metabolism, and arginine proline metabolism. The correlation analysis showed that the changes of gut microbiota genera were significantly correlated with the changes of metabolism, especially Helicobacter, which was significantly correlated with many different metabolites at 4 W after C-irradiation. These data suggested that C-irradiation could affect the gut microbiota and metabolism profile, even at relatively long times after C-irradiation.

Altered expression of inflammation-associated molecules in striatum: an implication for sensitivity to heavy ion radiations

Background and objective

Heavy ion radiation is one of the major hazards astronauts face during space expeditions, adversely affecting the central nervous system. Radiation causes severe damage to sensitive brain regions, especially the striatum, resulting in cognitive impairment and other physiological issues in astronauts. However, the intensity of brain damage and associated underlying molecular pathological mechanisms mediated by heavy ion radiation are still unknown. The present study is aimed to identify the damaging effect of heavy ion radiation on the striatum and associated underlying pathological mechanisms.

Materials and methods

Two parallel cohorts of rats were exposed to radiation in multiple doses and times. Cohort I was exposed to 15 Gy of 12C6+ ions radiation, whereas cohort II was exposed to 3.4 Gy and 8 Gy with 56Fe26+ ions irradiation. Physiological and behavioural tests were performed, followed by 18F-FDG-PET scans, transcriptomics analysis of the striatum, and in-vitro studies to verify the interconnection between immune cells and neurons.

Results

Both cohorts revealed more persistent striatum dysfunction than other brain regions under heavy ion radiation at multiple doses and time, exposed by physiological, behavioural, and 18F-FDG-PET scans. Transcriptomic analysis revealed that striatum dysfunction is linked with an abnormal immune system. In vitro studies demonstrated that radiation mediated diversified effects on different immune cells and sustained monocyte viability but inhibited its differentiation and migration, leading to chronic neuroinflammation in the striatum and might affect other associated brain regions.

Conclusion

Our findings suggest that striatum dysfunction under heavy ion radiation activates abnormal immune systems, leading to chronic neuroinflammation and neuronal injury.

Advances in the study of the molecular biological mechanisms of radiation-induced brain injury

Radiation therapy is one of the most commonly used treatments for head and neck cancers, but it often leads to radiation-induced brain injury. Patients with radiation-induced brain injury have a poorer quality of life, and no effective treatments are available. The pathogenesis of this condition is unknown. This review summarizes the molecular biological mechanism of radiation-induced brain injury and provides research directions for future studies. The molecular mechanisms of radiation-induced brain injury are diverse and complex. Radiation-induced chronic neuroinflammation, destruction of the blood-brain barrier, oxidative stress, neuronal damage, and physiopathological responses caused by specific exosome secretion lead to radiation-induced brain injury.

The Biological Implication of Semicarbazide-Sensitive Amine Oxidase (SSAO) Upregulation in Rat Systemic Inflammatory Response under Simulated Aerospace Environment

The progress of space science and technology has ushered in a new era for humanity's exploration of outer space. Recent studies have indicated that the aerospace special environment including microgravity and space radiation poses a significant risk to the health of astronauts, which involves multiple pathophysiological effects on the human body as well on tissues and organs. It has been an important research topic to study the molecular mechanism of body damage and further explore countermeasures against the physiological and pathological changes caused by the space environment. In this study, we used the rat model to study the biological effects of the tissue damage and related molecular pathway under either simulated microgravity or heavy ion radiation or combined stimulation. Our study disclosed that ureaplasma-sensitive amino oxidase (SSAO) upregulation is closely related to the systematic inflammatory response (IL-6, TNF-α) in rats under a simulated aerospace environment. In particular, the space environment leads to significant changes in the level of inflammatory genes in heart tissues, thus altering the expression and activity of SSAO and causing inflammatory responses. The detailed molecular mechanisms have been further validated in the genetic engineering cell line model. Overall, this work clearly shows the biological implication of SSAO upregulation in microgravity and radiation-mediated inflammatory response, providing a scientific basis or potential target for further in-depth investigation of the pathological damage and protection strategy under a space environment.

The Abscopal Effects of Cranial Irradiation Induce Testicular Damage in Mice

To investigate whether the abscopal effects of cranial irradiation (C-irradiation) cause testicular damage in mice, male C57BL/6 mice (9weeks of age) were randomly divided into a sham irradiation group, a shielded group and a C-irradiation group and administered sham/shielded irradiation or C-irradiation at a dose rate of 2.33Gy/min (5Gy/d for 4 d consecutively). All mice were sacrificed at 4weeks after C-irradiation. We calculated the testis index, observed testicular histology by haematoxylin-eosin (HE) staining and observed testicular ultrastructure by transmission electron microscopy. Western blotting was used to determine the protein levels of Bax, Bcl-2, Cleaved caspase 3, glial cell line-derived neurotrophic factor (GDNF) and stem cell factor (SCF) in the testes of mice. Immunofluorescence staining was performed to detect the expression of Cleaved caspase 3 and 3β hydroxysteroid dehydrogenase (3βHSD), and a TUNEL assay was used to confirm the location of apoptotic cells. The levels of testosterone (T), GDNF and SCF were measured by ELISA. We also evaluated the sperm quality in the cauda epididymides by measuring the sperm count, abnormality, survival rate and apoptosis rate. The results showed that there was no significant difference in testicular histology, ultrastructure or sperm quality between the shielded group and sham group. Compared with the sham/shielded group, the C-irradiation group exhibited a lower testis index and severely damaged testicular histology and ultrastructure at 4weeks after C-irradiation. The levels of apoptosis in the testes increased markedly in the C-irradiation group, especially in spermatogonial stem cells. The levels of serum T and testicular 3βHSD did not obviously differ between the sham group and the C-irradiation group, but the levels of GDNF and SCF in the testes increased in the C-irradiation group, compared with the sham group. In addition, the sperm count and survival rate decreased in the C-irradiation group, while the abnormality and apoptosis rate increased. Under these experimental conditions, the abscopal effects of C-irradiation induced testicular damage with regard to both structure and function and ultimately decreased sperm quality in mice. These findings provide novel insights into prevention and treatment targets for male reproductive damage induced by C-irradiation.

Effects of Chronic Low-Dose Internal Radiation on Immune-Stimulatory Responses in Mice

The Linear-No-Threshold (LNT) model predicts a dose-dependent linear increase in cancer risk. This has been supported by biological and epidemiological studies at high-dose exposures. However, at low-doses (LDR ≤ 0.1 Gy), the effects are more elusive and demonstrate a deviation from linearity. In this study, the effects of LDR on the development and progression of mammary cancer in FVB/N-Tg(MMTVneu)202Mul/J mice were investigated. Animals were chronically exposed to total doses of 10, 100, and 2000 mGy via tritiated drinking water, and were assessed at 3.5, 6, and 8 months of age. Results indicated an increased proportion of NK cells in various organs of LDR exposed mice. LDR significantly influenced NK and T cell function and activation, despite diminishing cell proliferation. Notably, the expression of NKG2D receptor on NK cells was dramatically reduced at 3.5 months but was upregulated at later time-points, while the expression of NKG2D ligand followed the opposite trend, with an increase at 3.5 months and a decrease thereafter. No noticeable impact was observed on mammary cancer development, as measured by tumor load. Our results demonstrated that LDR significantly influenced the proportion, proliferation, activation, and function of immune cells. Importantly, to the best of our knowledge, this is the first report demonstrating that LDR modulates the cross-talk between the NKG2D receptor and its ligands.

Exosomes and exosomal microRNA in non-targeted radiation bystander and abscopal effects in the central nervous system

Localized cranial radiotherapy is a dominant treatment for brain cancers. After being subjected to radiation, the central nervous system (CNS) exhibits targeted effects as well as non-targeted radiation bystander effects (RIBE) and abscopal effects (RIAE). Radiation-induced targeted effects in the CNS include autophagy and various changes in tumor cells due to radiation sensitivity, which can be regulated by microRNAs. Non-targeted radiation effects are mainly induced by gap junctional communication between cells, exosomes containing microRNAs can be transduced by intracellular endocytosis to regulate RIBE and RIAE. In this review, we discuss the involvement of microRNAs in radiation-induced targeted effects, as well as exosomes and/or exosomal microRNAs in non-targeted radiation effects in the CNS. As a target pathway, we also discuss the Akt pathway which is regulated by microRNAs, exosomes, and/or exosomal microRNAs in radiation-induced targeted effects and RIBE in CNS tumor cells. As the CNS-derived exosomes can cross the blood-brain-barrier (BBB) into the bloodstream and be isolated from peripheral blood, exosomes and exosomal microRNAs can emerge as promising minimally invasive biomarkers and therapeutic targets for radiation-induced targeted and non-targeted effects in the CNS.

Effect of Heavy Ion 12C6+ Radiation on Lipid Constitution in the Rat Brain

Heavy ions refer to charged particles with a mass greater than four (i.e., alpha particles). The heavy ion irradiation used in radiotherapy or that astronauts suffer in space flight missions induces toxicity in normal tissue and leads to short-term and long-term damage in both the structure and function of the brain. However, the underlying molecular alterations caused by heavy ion radiation have yet to be completely elucidated. Herein, untargeted and targeted lipidomic profiling of the whole brain tissue and blood plasma 7 days after the administration of the 15 Gy (260 MeV, low linear energy (LET) = 13.9 KeV/μm) plateau irradiation of disposable ¹²C⁶⁺ heavy ions on the whole heads of rats was explored to study the lipid damage induced by heavy ion radiation in the rat brain using ultra performance liquid chromatography-mass spectrometry (UPLC–MS) technology. Combined with multivariate variables and univariate data analysis methods, our results indicated that an orthogonal partial least squares discriminant analysis (OPLS–DA) could clearly distinguish lipid metabolites between the irradiated and control groups. Through the combination of variable weight value (VIP), variation multiple (FC), and differential (p) analyses, the significant differential lipids diacylglycerols (DAGs) were screened out. Further quantitative targeted lipidomic analyses of these DAGs in the rat brain tissue and plasma supported the notion that DAG 47:1 could be used as a potential biomarker to study brain injury induced by heavy ion irradiation.

Effects of low dose radiation on immune cells subsets and cytokines in mice

Whole-body exposure to low-dose radiation due to diagnostic imaging procedures, occupational hazards and radiation accidents is a source of concern. In this study, we analyzed the effects of single and long-term low-dose irradiation on the immune system. Male Balb/c mice received a single whole-body dose of irradiation (0.01, 0.05, 0.2, 0.5 or 1 Gy). For long-term irradiation, mice were irradiated 10 times (total dose of 0.2, 0.5 or 1 Gy) over a period of 6 weeks. Two days after single or long-term irradiation, the numbers of splenic macrophages, natural killer cells and dendritic cells were reduced, and the spleen organ coefficient was decreased. At 2 Days after long-term low-dose irradiation, the number of white blood cells in the peripheral blood of the mice decreased. Between 7 and 14 Days after long-term low-dose irradiation, the number of immune cells in the thymus and spleen began to increase and then stabilized. Th1/Th2 cytokines and reactive oxygen species-related proteins first decreased and then increased to a plateau. Our results show a significant difference in the effects of single and long-term low-dose irradiation on the immune system.

Hadrontherapy Interactions in Molecular and Cellular Biology

The resistance of cancer cells to radiotherapy is a major issue in the curative treatment of cancer patients. This resistance can be intrinsic or acquired after irradiation and has various definitions, depending on the endpoint that is chosen in assessing the response to radiation. This phenomenon might be strengthened by the radiosensitivity of surrounding healthy tissues. Sensitive organs near the tumor that is to be treated can be affected by direct irradiation or experience nontargeted reactions, leading to early or late effects that disrupt the quality of life of patients. For several decades, new modalities of irradiation that involve accelerated particles have been available, such as proton therapy and carbon therapy, raising the possibility of specifically targeting the tumor volume. The goal of this review is to examine the up-to-date radiobiological and clinical aspects of hadrontherapy, a discipline that is maturing, with promising applications. We first describe the physical and biological advantages of particles and their application in cancer treatment. The contribution of the microenvironment and surrounding healthy tissues to tumor radioresistance is then discussed, in relation to imaging and accurate visualization of potentially resistant hypoxic areas using dedicated markers, to identify patients and tumors that could benefit from hadrontherapy over conventional irradiation. Finally, we consider combined treatment strategies to improve the particle therapy of radioresistant cancers.

The Effect of Gamma-Ray-Induced Central Nervous System Injury on Peripheral Immune Response: An In Vitro and In Vivo Study

Radiotherapy to treat brain tumors can potentially harm the central nervous system (CNS). The radiation stimulates a series of immune responses in both the CNS as well as peripheral immune system. To date, studies have mostly focused on the changes occurring in the immune response within the CNS. In this study, we investigated the effect of γ-ray-induced CNS injury on the peripheral immune response using a cell co-culture model and a whole-brain irradiation (WBI) rat model. Nerve cells (SH-SY5Y and U87 MG cells) were γ-ray irradiated, then culture media of the irradiated cells (conditioned media) was used to culture immune cells (THP-1 cells or Jurkat cells). Analyses were performed based on the response of immune cells in conditioned media. Sprague-Dawley rats received WBI at different doses, and were fed for one week to one month postirradiation. Spleen and peripheral blood were then isolated and analyzed. We observed that the number of monocytes in peripheral blood, and the level of NK cells and NKT cells in spleen increased after CNS injury. However, the level of T cells in spleen did not change and the level of B cells in the spleen decreased after γ-ray-induced CNS injury. These findings indicate that CNS injury caused by ionizing radiation induces a series of changes in the peripheral immune system.

Dosimetry of the brain and hypothalamus predicting acute lymphopenia and the survival of glioma patients with postoperative radiotherapy

Background

The aim of this study was to investigate dosimetric factors for predicting acute lymphopenia and the survival of glioma patients with postoperative intensity‐modulated radiotherapy (IMRT).

Methods

A total of 148 glioma patients were reviewed. Acute lymphopenia was defined as a peripheral lymphocyte count (PLC) lower than 1.0 × 10⁹/L during radiotherapy with a normal level at pretreatment. PLCs with the corresponding dates and dose volume histogram parameters were collected. Univariate and multivariate Cox regression analyses were constructed to assess the significance of risk factors associated with lymphopenia and overall survival (OS).

Results

Sixty‐nine (46.6%) patients developed lymphopenia during radiotherapy. Multivariate analyses revealed that the risk increased with the maximal dose of the hypothalamus (HT Dmax) ≥56 Gy (58.9% vs 28.5%, P = 0.002), minimal dose of the whole brain (WB Dmin) ≥2 Gy (54.3% vs 33.9%, P = 0.006), or mean dose of the WB (WB Dmean) ≥34 Gy (56.0% vs 37.0%, P = 0.022). Patients with older age, high‐grade glioma, development of lymphopenia, high HT Dmax, WB Dmin, and WB Dmean had significantly inferior OS in the multivariate analyses.

Conclusions

HT Dmax, WB Dmin, and WB Dmean are promising indicators of lymphopenia and the survival of glioma patients undergoing postoperative IMRT. The necessity and feasibility of dosimetric constraints for HT and WB is warranted with further investigation.

Exogenous melatonin modulates carbon ion radiation-induced immune dysfunction in mice

In spite of carbon ion radiotherapy is a talented modality for malignant tumor patients, the radiation damage of normal tissues adjacent to tumor and the dysfunction of immune system limits therapeutic gain. Protecting immune system against carbon ion radiation-caused damage has the possibility to improve cancer treatment, but it is uncertain whether conventional radioprotective agents play a role in carbon ion radiation. To certify carbon ion caused immune dysfunction and assess the radioprotective effect of melatonin on immune system, animal experiments were performed in radiosensitive BALB/C mice. Here, we observed the bodyweight loss, death and apoptosis, abnormal T-cell distributions in immune system in carbon ion radiated mice. Pretreatment with melatonin could increase the index of thymus and spleen, reduce cell apoptosis in thymus and spleen, and attenuate the carbon ion radiation-caused imbalance of T lymphocytes and disorder of cytokines. These results suggest that melatonin can act as an effective protector against carbon ion radiation-caused immune dysfunction. Furthermore, we also found melatonin restored the activity of the antioxidant enzymes and reduced the level of lipid peroxidation in serum. These data have provided baseline information both for radiation workers and cancer patients to use melatonin as a radioprotector during the carbon ion radiation treatment.

Whole-Body 12C Irradiation Transiently Decreases Mouse Hippocampal Dentate Gyrus Proliferation and Immature Neuron Number, but Does Not Change New Neuron Survival Rate

High-charge and -energy (HZE) particles comprise space radiation and they pose a challenge to astronauts on deep space missions. While exposure to most HZE particles decreases neurogenesis in the hippocampus—a brain structure important in memory—prior work suggests that ¹²C does not. However, much about ¹²C’s influence on neurogenesis remains unknown, including the time course of its impact on neurogenesis. To address this knowledge gap, male mice (9–11 weeks of age) were exposed to whole-body ¹²C irradiation 100 cGy (IRR; 1000 MeV/n; 8 kEV/µm) or Sham treatment. To birthdate dividing cells, mice received BrdU i.p. 22 h post-irradiation and brains were harvested 2 h (Short-Term) or three months (Long-Term) later for stereological analysis indices of dentate gyrus neurogenesis. For the Short-Term time point, IRR mice had fewer Ki67, BrdU, and doublecortin (DCX) immunoreactive (+) cells versus Sham mice, indicating decreased proliferation (Ki67, BrdU) and immature neurons (DCX). For the Long-Term time point, IRR and Sham mice had similar Ki67+ and DCX+ cell numbers, suggesting restoration of proliferation and immature neurons 3 months post-¹²C irradiation. IRR mice had fewer surviving BrdU+ cells versus Sham mice, suggesting decreased cell survival, but there was no difference in BrdU+ cell survival rate when compared within treatment and across time point. These data underscore the ability of neurogenesis in the mouse brain to recover from the detrimental effect of ¹²C exposure.

Effects of 12C6+ Heavy Ion Radiation on Dendritic Cells Function

Background

Carbon ion radiotherapy has been shown to be more effective in cancer radiotherapy than photon irradiation. Influence of carbon ion radiation on cancer microenvironment is very important for the outcomes of radiotherapy. Tumor-infiltrating dendritic cells (DCs) play critical roles in cancer antigen processing and antitumor immunity. However, there is scant literature covering the effects of carbon ion radiation on DCs. In this study, we aimed to uncover the impact of carbon ion irradiation on bone marrow derived DCs.

Material/Methods

Bone marrow cells were co-cultured with GM-CSF and IL-4 for seven days, and the population of DCs was confirmed with flow cytometry. We used an Annexin V and PI staining method to detect cell apoptosis. Endocytosis assay of DCs was determined by using a flow cytometry method. DCs migration capacity was tested by a Transwell method. We also used ELISA assay and western blotting assay to examine the cytokines and protein expression, respectively.

Results

Our data showed that carbon ion radiation induced apoptosis in both immature and mature DCs. After irradiation, the endocytosis and migration capacity of DCs was also impaired. Interestingly, carbon irradiation triggered a burst of IFN-γ and IL-12 in LPS or CpG treated DCs, which provide novel insights into the combination of immunotherapy and carbon ion radiotherapy. Finally, we found that carbon ion irradiation induced apoptosis and migration suppression was p38 dependent.

Conclusions

Our present study demonstrated that carbon ion irradiation induced apoptosis in DCs, and impaired DCs function mainly through the p38 signaling pathway. Carbon ion irradiation also triggered anti-tumor cytokines secretion. This work provides novel information of carbon ion radiotherapy in DCs, and also provides new insights on the combination of immune adjuvant and carbon ion radiotherapy.

RBE and related modeling in carbon-ion therapy

Carbon ion therapy is a promising evolving modality in radiotherapy to treat tumors that are radioresistant against photon treatments. As carbon ions are more effective in normal and tumor tissue, the relative biological effectiveness (RBE) has to be calculated by bio-mathematical models and has to be considered in the dose prescription. This review (i) introduces the concept of the RBE and its most important determinants, (ii) describes the physical and biological causes of the increased RBE for carbon ions, (iii) summarizes available RBE measurements in vitro and in vivo, and (iv) describes the concepts of the clinically applied RBE models (mixed beam model, local effect model, and microdosimetric-kinetic model), and (v) the way they are introduced into clinical application as well as (vi) their status of experimental and clinical validation, and finally (vii) summarizes the current status of the use of the RBE concept in carbon ion therapy and points out clinically relevant conclusions as well as open questions. The RBE concept has proven to be a valuable concept for dose prescription in carbon ion radiotherapy, however, different centers use different RBE models and therefore care has to be taken when transferring results from one center to another. Experimental studies significantly improve the understanding of the dependencies and limitations of RBE models in clinical application. For the future, further studies investigating quantitatively the differential effects between normal tissues and tumors are needed accompanied by clinical studies on effectiveness and toxicity.

Intercellular Communication of Tumor Cells and Immune Cells after Exposure to Different Ionizing Radiation Qualities

Ionizing radiation can affect the immune system in many ways. Depending on the situation, the whole body or parts of the body can be acutely or chronically exposed to different radiation qualities. In tumor radiotherapy, a fractionated exposure of the tumor (and surrounding tissues) is applied to kill the tumor cells. Currently, mostly photons, and also electrons, neutrons, protons, and heavier particles such as carbon ions, are used in radiotherapy. Tumor elimination can be supported by an effective immune response. In recent years, much progress has been achieved in the understanding of basic interactions between the irradiated tumor and the immune system. Here, direct and indirect effects of radiation on immune cells have to be considered. Lymphocytes for example are known to be highly radiosensitive. One important factor in indirect interactions is the radiation-induced bystander effect which can be initiated in unexposed cells by expression of cytokines of the irradiated cells and by direct exchange of molecules via gap junctions. In this review, we summarize the current knowledge about the indirect effects observed after exposure to different radiation qualities. The different immune cell populations important for the tumor immune response are natural killer cells, dendritic cells, and CD8+ cytotoxic T-cells. In vitro and in vivo studies have revealed the modulation of their functions due to ionizing radiation exposure of tumor cells. After radiation exposure, cytokines are produced by exposed tumor and immune cells and a modulated expression profile has also been observed in bystander immune cells. Release of damage-associated molecular patterns by irradiated tumor cells is another factor in immune activation. In conclusion, both immune-activating and -suppressing effects can occur. Enhancing or inhibiting these effects, respectively, could contribute to modified tumor cell killing after radiotherapy.