A charged-particle microbeam: II. A single-particle micro-collimation and detection system

Combined ion beam irradiation platform and 3D fluorescence microscope for cellular cancer research

To improve particle radiotherapy, we need a better understanding of the biology of radiation effects, particularly in heavy ion radiation therapy, where global responses are observed despite energy deposition in only a subset of cells. Here, we integrated a high-speed swept confocally-aligned planar excitation (SCAPE) microscope into a focused ion beam irradiation platform to allow real-time 3D structural and functional imaging of living biological samples during and after irradiation. We demonstrate dynamic imaging of the acute effects of irradiation on 3D cultures of U87 human glioblastoma cells, revealing characteristic changes in cellular movement and intracellular calcium signaling following ionizing irradiation.

Radiation-induced bystander effect and its clinical implications

For many years, targeted DNA damage caused by radiation has been considered the main cause of various biological effects. Based on this paradigm, any small amount of radiation is harmful to the organism. Epidemiological studies of Japanese atomic bomb survivors have proposed the linear-non-threshold model as the dominant standard in the field of radiation protection. However, there is increasing evidence that the linear-non-threshold model is not fully applicable to the biological effects caused by low dose radiation, and theories related to low dose radiation require further investigation. In addition to the cell damage caused by direct exposure, non-targeted effects, which are sometimes referred to as bystander effects, abscopal effects, genetic instability, etc., are another kind of significant effect related to low dose radiation. An understanding of this phenomenon is crucial for both basic biomedical research and clinical application. This article reviews recent studies on the bystander effect and summarizes the key findings in the field. Additionally, it offers a cross-sectional comparison of bystander effects caused by various radiation sources in different cell types, as well as an in-depth analysis of studies on the potential biological mechanisms of bystander effects. This review aims to present valuable information and provide new insights on the bystander effect to enlighten both radiobiologists and clinical radiologists searching for new ways to improve clinical treatments.

Microbeam evolution: from single cell irradiation to pre-clinical studies

PURPOSE

This review follows the development of microbeam technology from the early days of single cell irradiations, to investigations of specific cellular mechanisms and to the development of new treatment modalities in vivo. A number of microbeam applications are discussed with a focus on pre-clinical modalities and translation towards clinical application.

CONCLUSIONS

The development of radiation microbeams has been a valuable tool for the exploration of fundamental radiobiological response mechanisms. The strength of micro-irradiation techniques lies in their ability to deliver precise doses of radiation to selected individual cells in vitro or even to target subcellular organelles. These abilities have led to the development of a range of microbeam facilities around the world allowing the delivery of precisely defined beams of charged particles, X-rays, or electrons. In addition, microbeams have acted as mechanistic probes to dissect the underlying molecular events of the DNA damage response following highly localized dose deposition. Further advances in very precise beam delivery have also enabled the transition towards new and exciting therapeutic modalities developed at synchrotrons to deliver radiotherapy using plane parallel microbeams, in Microbeam Radiotherapy (MRT).

Focus small to find big - the microbeam story

PURPOSE

Even though the first ultraviolet microbeam was described by S. Tschachotin back in 1912, the development of sophisticated micro-irradiation facilities only began to flourish in the late 1980s. In this article, we highlight significant microbeam experiments, describe the latest microbeam irradiator configurations and critical discoveries made by using the microbeam apparatus.

MATERIALS AND METHODS

Modern radiological microbeams facilities are capable of producing a beam size of a few micrometers, or even tens of nanometers in size, and can deposit radiation with high precision within a cellular target. In the past three decades, a variety of microbeams has been developed to deliver a range of radiations including charged particles, X-rays, and electrons. Despite the original intention for their development to measure the effects of a single radiation track, the ability to target radiation with microbeams at sub-cellular targets has been extensively used to investigate radiation-induced biological responses within cells.

RESULTS

Studies conducted using microbeams to target specific cells in a tissue have elucidated bystander responses, and further studies have shown reactive oxygen species (ROS) and reactive nitrogen species (RNS) play critical roles in the process. The radiation-induced abscopal effect, which has a profound impact on cancer radiotherapy, further reaffirmed the importance of bystander effects. Finally, by targeting sub-cellular compartments with a microbeam, we have reported cytoplasmic-specific biological responses. Despite the common dogma that nuclear DNA is the primary target for radiation-induced cell death and carcinogenesis, studies conducted using microbeam suggested that targeted cytoplasmic irradiation induces mitochondrial dysfunction, cellular stress, and genomic instability. A more recent development in microbeam technology includes application of mouse models to visualize in vivo DNA double-strand breaks.

CONCLUSIONS

Microbeams are making important contributions towards our understanding of radiation responses in cells and tissue models.

Single α-particle irradiation permits real-time visualization of RNF8 accumulation at DNA damaged sites

As well as being a significant source of environmental radiation exposure, α-particles are increasingly considered for use in targeted radiation therapy. A better understanding of α-particle induced damage at the DNA scale can be achieved by following their tracks in real-time in targeted living cells. Focused α-particle microbeams can facilitate this but, due to their low energy (up to a few MeV) and limited range, α-particles detection, delivery, and follow-up observations of radiation-induced damage remain difficult. In this study, we developed a thin Boron-doped Nano-Crystalline Diamond membrane that allows reliable single α-particles detection and single cell irradiation with negligible beam scattering. The radiation-induced responses of single 3 MeV α-particles delivered with focused microbeam are visualized in situ over thirty minutes after irradiation by the accumulation of the GFP-tagged RNF8 protein at DNA damaged sites.

Radioprotection of targeted and bystander cells by methylproamine

Introduction

Radioprotective agents are of interest for application in radiotherapy for cancer and in public health medicine in the context of accidental radiation exposure. Methylproamine is the lead compound of a class of radioprotectors which act as DNA binding anti-oxidants, enabling the repair of transient radiation-induced oxidative DNA lesions. This study tested methylproamine for the radioprotection of both directly targeted and bystander cells.

Methods

T98G glioma cells were treated with 15 μM methylproamine and exposed to ¹³⁷Cs γ-ray/X-ray irradiation and He²⁺ microbeam irradiation. Radioprotection of directly targeted cells and bystander cells was measured by clonogenic survival or γH2AX assay.

Results

Radioprotection of directly targeted T98G cells by methylproamine was observed for ¹³⁷Cs γ-rays and X-rays but not for He²⁺ charged particle irradiation. The effect of methylproamine on the bystander cell population was tested for both X-ray irradiation and He²⁺ ion microbeam irradiation. The X-ray bystander experiments were carried out by medium transfer from irradiated to non-irradiated cultures and three experimental designs were tested. Radioprotection was only observed when recipient cells were pretreated with the drug prior to exposure to the conditioned medium. In microbeam bystander experiments targeted and nontargeted cells were co-cultured with continuous methylproamine treatment during irradiation and postradiation incubation; radioprotection of bystander cells was observed.

Discussion and conclusion

Methylproamine protected targeted cells from DNA damage caused by γ-ray or X-ray radiation but not He²⁺ ion radiation. Protection of bystander cells was independent of the type of radiation which the donor population received.

A novel facility for 3D micro-irradiation of living cells in a controlled environment by MeV ions

We present a novel facility for micro-irradiation of living targets with ions from a 1.7 MV tandem accelerator. We show results using 1 MeV protons and 2 MeV He(2+). In contrast to common micro-irradiation facilities, which use electromagnetic or electrostatic focusing and specially designed vacuum windows, we employ a tapered glass capillary with a thin end window, made from polystyrene with a thickness of 1-2 μm, for ion focusing and extraction. The capillary is connected to a beamline tilted vertically by 45°, which allows for easy immersion of the extracted ions into liquid environment within a standard cell culture dish. An inverted microscope is used for simultaneously observing the samples as well as the capillary tip, while a stage-top incubator provides an appropriate environment for the samples. Furthermore, our setup allows to target volumes in cells within a μm(3) resolution, while monitoring the target in real time during and after irradiation.

SPICE-NIRS microbeam: a focused vertical system for proton irradiation of a single cell for radiobiological research

The Single Particle Irradiation system to Cell (SPICE) facility at the National Institute of Radiological Sciences (NIRS) is a focused vertical microbeam system designed to irradiate the nuclei of adhesive mammalian cells with a defined number of 3.4 MeV protons. The approximately 2-μm diameter proton beam is focused with a magnetic quadrupole triplet lens and traverses the cells contained in dishes from bottom to top. All procedures for irradiation, such as cell image capturing, cell recognition and position calculation, are automated. The most distinctive characteristic of the system is its stability and high throughput; i.e. 3000 cells in a 5 mm × 5 mm area in a single dish can be routinely irradiated by the 2-μm beam within 15 min (the maximum irradiation speed is 400 cells/min). The number of protons can be set as low as one, at a precision measured by CR-39 detectors to be 99.0%. A variety of targeting modes such as fractional population targeting mode, multi-position targeting mode for nucleus irradiation and cytoplasm targeting mode are available. As an example of multi-position targeting irradiation of mammalian cells, five fluorescent spots in a cell nucleus were demonstrated using the γ-H2AX immune-staining technique. The SPICE performance modes described in this paper are in routine use. SPICE is a joint-use research facility of NIRS and its beam times are distributed for collaborative research.

A focused scanning vertical beam for charged particle irradiation of living cells with single counted particles

The Surrey vertical beam is a new facility for targeted irradiation of cells in medium with singly counted ions. A duo-plasmatron ion source and a 2 MV Tandem™ accelerator supply a range of ions from protons to calcium for this beamline and microscope endstation, with energy ranges from 0.5 to 12 MeV. A magnetic quadrupole triplet lens is used to focus the beam of ions. We present the design of this beamline, and early results showing the capability to count single ions with 98% certainty on CR-39 track etch. We also show that the beam targeting accuracy is within 5 μm and selectively target human fibroblasts with a <5 μm carbon beam, using γ-H2AX immunofluorescence to demonstrate which cell nuclei were irradiated. We discuss future commissioning steps necessary to achieve submicron targeting accuracy with this beamline.

Bystander cell killing in normal human fibroblasts is induced by synchrotron X-ray microbeams

Abstract The radiation-induced bystander response is defined as a response in cells that have not been directly targeted by radiation but that are in the neighborhood of cells that have been directly exposed. In the work described here, it is shown that bystander cell killing of normal human fibroblast WI-38 cells was induced by synchrotron microbeam X radiation. Cell nuclei in confluent WI-38 cells were irradiated with the microbeam. All of the cells on the dish were harvested and plated 24 h after irradiation. It was found that the bystander cell killing effect showed a parabolic relationship to the radiation dose when five cells were irradiated. At doses above 1.9 Gy, the surviving fraction increased to approximately 1.0. This suggests that induction of bystander cell killing may require some type of activity in the targeted cells, because the dose resulting in 37% cell survival was about 2.0 Gy. Bystander cell killing was suppressed by a pretreatment with aminoguanidine [an inhibitor of inducible nitric oxide (NO) synthase] or carboxy-PTIO (a scavenger of NO). These results suggest that NO is the chief initiator/mediator of bystander cell killing induced by X-ray microbeams.

Lack of evidence for low-LET radiation induced bystander response in normal human fibroblasts and colon carcinoma cells

PURPOSE

To investigate radiation-induced bystander responses and to determine the role of gap junction intercellular communication and the radiation environment in propagating this response.

MATERIALS AND METHODS

We used medium transfer and targeted irradiation to examine radiation-induced bystander effects in primary human fibroblast (AG01522) and human colon carcinoma (RKO36) cells. We examined the effect of variables such as gap junction intercellular communication, linear energy transfer (LET), and the role of the radiation environment in non-targeted responses. Endpoints included clonogenic survival, micronucleus formation and foci formation at histone 2AX over doses ranging from 10-100 cGy.

RESULTS

The results showed no evidence of a low-LET radiation-induced bystander response for the endpoints of clonogenic survival and induction of DNA damage. Nor did we see evidence of a high-LET, Fe ion radiation (1 GeV/n) induced bystander effect. However, direct comparison for 3.2 MeV alpha-particle exposures showed a statistically significant medium transfer bystander effect for this high-LET radiation.

CONCLUSIONS

From our results, it is evident that there are many confounding factors influencing bystander responses as reported in the literature. Our observations reflect the inherent variability in biological systems and the difficulties in extrapolating from in vitro models to radiation risks in humans.

Radiation microbeams as spatial and temporal probes of subcellular and tissue response

Understanding the effects of ionizing radiations are key to determining their optimal use in therapy and assessing risks from exposure. The development of microbeams where radiations can be delivered in a highly temporal and spatially constrained manner has been a major advance. Several different types of radiation microbeams have been developed using X-rays, charged particles and electrons. For charged particles, beams can be targeted with sub-micron accuracy into biological samples and the lowest possible dose of a single particle track can be delivered with high reproducibility. Microbeams have provided powerful tools for understanding the kinetics of DNA damage and formation under conditions of physiological relevance and have significant advantages over other approaches for producing localized DNA damage, such as variable wavelength laser beam approaches. Recent studies have extended their use to probing for radiosensitive sites outside the cell nucleus, and testing for mechanisms underpinning bystander responses where irradiated and non-irradiated cells communicate with each other. Ongoing developments include the ability to locally target regions of 3D tissue models and ultimately to target localized regions in vivo. With future advances in radiation delivery and imaging microbeams will continue to be applied in a range of biological studies.

Apoptosis is initiated in human keratinocytes exposed to signalling factors from microbeam irradiated cells

PURPOSE

There is now no doubt that bystander signalling from irradiated cells occurs and causes a variety of responses in cells not targeted by the ionizing track. However, the mechanisms underlying these processes are unknown and the relevance to radiotherapy and risk assessment remains controversial. Previous research by our laboratory has shown bystander effects in a human keratinocyte cell line, HPV-G cells, exposed to medium from gamma irradiated HPV-G cells. The aim of this work was to investigate if similar mechanisms to those identified in medium transfer experiments occurred in these HPV-G cells when they are in the vicinity of microbeam irradiated cells. Demonstration of a commonality of mechanisms would support the idea that the process is not artifactual.

MATERIALS AND METHODS

HPV-G cells were plated as two separate populations on mylar dishes. One population was directly irradiated using a charged particle microbeam (1 - 10 protons). The other population was not irradiated. Bystander factor-induced apoptosis was investigated in both populations following treatment by monitoring the levels of reactive oxygen species and mitochondrial membrane potential using fluorescent probes. Expression of the anti-apoptotic protein, bcl-2, and cytochrome c were determined, as well as apoptosis levels.

RESULTS

Microbeam irradiation induced increases in reactive oxygen species and decreases in mitochondrial membrane potential at 6 h post-exposure, increased expression of bcl-2 and cytochrome c release at 6.5 h and increased apoptosis at 24 h.

CONCLUSION

This study shows that similar bystander signalling pathways leading to apoptosis are induced following microbeam irradiation and following medium transfer. This demonstrates that the mechanisms involved are common across different radiation qualities and conditions and indicates that they may be relevant in vivo.

Radiation response of primary human skin fibroblasts and their bystander cells after exposure to counted particles at low and high LET

PURPOSE

To investigate the dependence of bystander effects on linear energy transfer (LET) in the low dose region.

MATERIALS AND METHODS

The single-ion microbeam of the Physikalisch-Technische Bundesanstalt (PTB) was used to irradiate confluent primary human skin fibroblasts. Cells plated on a special irradiation dish were targeted with 10 MeV protons (LET 4.7 keV/microm) and 4.5 MeV a-particles (LET 100 keV/microm). During exposure, the cells were confluent allowing signal transfers through both gap junctions and diffusion.

RESULTS

For 10 MeV protons the clonogenic capability was significantly higher after exposure to 70 protons (0.31 Gy) compared with unirradiated cells. For higher doses the survival curve was exponential. Exposure of only 10% of all nuclei resulted in a similar radiation response in the low dose region. For higher doses up to 2.2 Gy no cell killing was observed. For 4.5 MeV alpha-particles an exponential survival curve was obtained. Irradiation of only 10% of all cell nuclei resulted in an survival curve as had been expected in the absence of any bystander effect.

CONCLUSION

The type and extent of bystander effects turned out to be dependent on the particles' LET and are likely to depend also on the cell line used and the techniques applied.

Ionizing radiation microbeam facilities for radiobiological studies in Europe

A growing body of experimental evidence gathered in the last 10-15 years with regard to targeted and non-targeted effects of low doses of ionizing radiation (hyper-radiosensitivity, induced radio-resistance, adaptive response, genomic instability, bystander effects) has pushed the radiobiology research towards a better understanding of the mechanisms underlying these phenomena, the extent to which they are active in-vivo, and how they are inter-related. In such a way factors could be obtained and included in the estimation of potential cancer risk to the human population of exposure to low levels of ionizing radiation. Different experimental approaches have been developed and employed to study such effects in-vitro (medium transfer experiments; broad-field irradiation at low doses also with insert or shielding systems...). In this regard, important contributions came from ionizing radiation microbeam facilities that turn to be powerful tools to perform selective irradiations of individual cells inside a population with an exact, defined and reproducible dose (i.e. number of particles, in case of charged particle microbeams). Over the last 20 years the use of microbeams for radiobiological applications increased substantially and a continuously growing number of such facilities, providing X-rays, electrons, light and heavy ions, has been developing all over the world. Nowadays, just in Europe there are 12 microbeam facilities fully-operational or under-development, out of more than 30 worldwide. An overview of the European microbeam facilities for radiobiological studies is presented and discussed in this paper.

Estrogen enhanced cell-cell signalling in breast cancer cells exposed to targeted irradiation

Background

Radiation-induced bystander responses, where cells respond to their neighbours being irradiated are being extensively studied. Although evidence shows that bystander responses can be induced in many types of cells, it is not known whether there is a radiation-induced bystander effect in breast cancer cells, where the radiosensitivity may be dependent on the role of the cellular estrogen receptor (ER). This study investigated radiation-induced bystander responses in estrogen receptor-positive MCF-7 and estrogen receptor-negative MDA-MB-231 breast cancer cells.

Methods

The influence of estrogen and anti-estrogen treatments on the bystander response was determined by individually irradiating a fraction of cells within the population with a precise number of helium-3 using a charged particle microbeam. Damage was scored as chromosomal damage measured as micronucleus formation.

Results

A bystander response measured as increased yield of micronucleated cells was triggered in both MCF-7 and MDA-MB-231 cells. The contribution of the bystander response to total cell damage in MCF-7 cells was higher than that in MDA-MB-231 cells although the radiosensitivity of MDA-MB-231 was higher than MCF-7. Treatment of cells with 17β-estradiol (E2) increased the radiosensitivity and the bystander response in MCF-7 cells, and the effect was diminished by anti-estrogen tamoxifen (TAM). E2 also increased the level of intracellular reactive oxygen species (ROS) in MCF-7 cells in the absence of radiation. In contrast, E2 and TAM had no influence on the bystander response and ROS levels in MDA-MB-231 cells. Moreover, the treatment of MCF-7 cells with antioxidants eliminated both the E2-induced ROS increase and E2-enhanced bystander response triggered by the microbeam irradiation, which indicates that ROS are involved in the E2-enhanced bystander micronuclei formation after microbeam irradiation.

Conclusion

The observation of bystander responses in breast tumour cells may offer new potential targets for radiation-based therapies in the treatment of breast cancer.

The use of microbeams to investigate radiation damage in living cells

The micro-irradiation technique continues to be highly relevant to a number of radiobiological studies in vitro. In particular, studies of the bystander effect show that direct damage to cells is not the only trigger for radiation-induced effects, but that unirradiated cells can also respond to signals from irradiated neighbours. Furthermore, the bystander response can be initiated even when no energy is deposited in the genomic DNA of the irradiated cell (i.e. by targeting just the cytoplasm).

Signaling factors for irradiated glioma cells induced bystander responses in fibroblasts

The aim of this study was to investigate the signaling factor and its pathway involved in the targeted irradiation-induced bystander response from glioblastoma cells to primary fibroblasts. After co-culturing with a glioblastoma T98G population where a fraction of cells had been individually irradiated with a precise number of helium particles, additional micronucleus (MN) were induced in the non-irradiated human fibroblasts AG01522 cells and its yield was independent of irradiation dose. This bystander MN induction was eliminated by treating the cells with either aminoguanidine (AG), an iNOS inhibitor, or anti-transforming growth factor-beta1 (anti-TGF-beta1). In addition, TGF-beta1 could be released from irradiated T98G cells but this release was inhibited by AG. In consistent, TGF-beta1 could also be induced from T98G cells treated with diethylamine nitric oxide (DEANO), a donor of nitric oxide (NO). Moreover, the effect of TGF-beta1 on bystander AG01522 cells was investigated. It was found that reactive oxygen species (ROS) and MN were induced in AG01522 cells after TGF-beta1 treatment. Our results indicate that, downstream of NO, TGF-beta1 plays an important role in the targeted T98G cells induced bystander response to AG0 cells by further causing DNA damage in vicinal fibroblasts through a ROS related pathway. This study may have implications for properly evaluating the secondary effects of radiotherapy.

Heavy-ion microbeam system at JAEA-Takasaki for microbeam biology

Research concerning cellular responses to low dose irradiation, radiation-induced bystander effects, and the biological track structure of charged particles has recently received particular attention in the field of radiation biology. Target irradiation employing a microbeam represents a useful means of advancing this research by obviating some of the disadvantages associated with the conventional irradiation strategies. The heavy-ion microbeam system at JAEA-Takasaki, which was planned in 1987 and started in the early 1990's, can provide target irradiation of heavy charged particles to biological material at atmospheric pressure using a minimum beam size 5 mum in diameter. A variety of biological material has been irradiated using this microbeam system including cultured mammalian and higher plant cells, isolated fibers of mouse skeletal muscle, silkworm (Bombyx mori) embryos and larvae, Arabidopsis thaliana roots, and the nematode Caenorhabditis elegans. The system can be applied to the investigation of mechanisms within biological organisms not only in the context of radiation biology, but also in the fields of general biology such as physiology, developmental biology and neurobiology, and should help to establish and contribute to the field of "microbeam biology".

Cytoplasmic irradiation induces mitochondrial-dependent 53BP1 protein relocalization in irradiated and bystander cells

The accepted paradigm for radiation effects is that direct DNA damage via energy deposition is required to trigger the downstream biological consequences. The radiation-induced bystander effect is the ability of directly irradiated cells to interact with their nonirradiated neighbors, which can then show responses similar to those of the targeted cells. p53 binding protein 1 (53BP1) forms foci at DNA double-strand break sites and is an important sensor of DNA damage. This study used an ionizing radiation microbeam approach that allowed us to irradiate specifically the nucleus or cytoplasm of a cell and quantify response in irradiated and bystander cells by studying ionizing radiation-induced foci (IRIF) formation of 53BP1 protein. Our results show that targeting only the cytoplasm of a cell is capable of eliciting 53BP1 foci in both hit and bystander cells, independently of the dose or the number of cells targeted. Therefore, direct DNA damage is not required to trigger 53BP1 IRIF. The use of common reactive oxygen species and reactive nitrogen species (RNS) inhibitors prevent the formation of 53BP1 foci in hit and bystander cells. Treatment with filipin to disrupt membrane-dependent signaling does not prevent the cytoplasmic irradiation-induced 53BP1 foci in the irradiated cells, but it does prevent signaling to bystander cells. Active mitochondrial function is required for these responses because pseudo-rho(0) cells, which lack mitochondrial DNA, could not produce a bystander signal, although they could respond to a signal from normal rho+ cells.

Role of TGF-beta1 and nitric oxide in the bystander response of irradiated glioma cells

The radiation-induced bystander effect (RIBE) increases the probability of cellular response and therefore has important implications for cancer risk assessment following low-dose irradiation and for the likelihood of secondary cancers after radiotherapy. However, our knowledge of bystander signaling factors, especially those having long half-lives, is still limited. The present study found that, when a fraction of cells within a glioblastoma population were individually irradiated with helium ions from a particle microbeam, the yield of micronuclei (MN) in the nontargeted cells was increased, but these bystander MN were eliminated by treating the cells with either aminoguanidine (an inhibitor of inducible nitric oxide (NO) synthase) or anti-transforming growth factor beta1 (anti-TGF-beta1), indicating that NO and TGF-beta1 are involved in the RIBE. Intracellular NO was detected in the bystander cells, and additional TGF-beta1 was detected in the medium from irradiated T98G cells, but it was diminished by aminoguanidine. Consistent with this, an NO donor, diethylamine nitric oxide (DEANO), induced TGF-beta1 generation in T98G cells. Conversely, treatment of cells with recombinant TGF-beta1 could also induce NO and MN in T98G cells. Treatment of T98G cells with anti-TGF-beta1 inhibited the NO production when only 1% of cells were targeted, but not when 100% of cells were targeted. Our results indicate that, downstream of radiation-induced NO, TGF-beta1 can be released from targeted T98G cells and plays a key role as a signaling factor in the RIBE by further inducing free radicals and DNA damage in the nontargeted bystander cells.

Radiation-induced bystander and adaptive responses in cell and tissue models

The use of microbeam approaches has been a major advance in probing the relevance of bystander and adaptive responses in cell and tissue models. Our own studies at the Gray Cancer Institute have used both a charged particle microbeam, producing protons and helium ions and a soft X-ray microprobe, delivering focused carbon-K, aluminium-K and titanium-K soft X-rays. Using these techniques we have been able to build up a comprehensive picture of the underlying differences between bystander responses and direct effects in cell and tissue-like models. What is now clear is that bystander dose-response relationships, the underlying mechanisms of action and the targets involved are not the same as those observed for direct irradiation of DNA in the nucleus. Our recent studies have shown bystander responses even when radiation is deposited away from the nucleus in cytoplasmic targets. Also the interaction between bystander and adaptive responses may be a complex one related to dose, number of cells targeted and time interval.

Calcium Fluxes Modulate the Radiation-Induced Bystander Responses in Targeted Glioma and Fibroblast Cells

Bystander responses have been reported to be a major determinant of the response of cells to radiation exposure at low doses, including those of relevance to therapy. This study investigated the role of changes in calcium levels in bystander responses leading to chromosomal damage in nonirradiated T98G glioma cells and AG01522 fibroblasts that had been either exposed to conditioned medium from irradiated cells or co-cultured with a population where a fraction of cells were individually targeted through the nucleus or cytoplasm with a precise number of microbeam helium-3 particles. After the recipient cells were treated with conditioned medium from T98G or AG01522 cells that had been irradiated through either nucleus or cytoplasm, rapid calcium fluxes were monitored in the nonirradiated recipient cells. Their characteristics were dependent on the source of the conditioned medium but had no dependence on radiation dose. When recipient cells were co-cultured with an irradiated population of either T98G or AG01522 cells, micronuclei were induced in the nonirradiated cells, but this response was eliminated by treating the cells with calcicludine (CaC), a potent blocker of Ca(2+) channels. Moreover, both the calcium fluxes and the bystander effect were inhibited when the irradiated T98G cells were treated with aminoguanidine, an inhibitor of nitric oxide synthase (NOS), and when the irradiated AG01522 cells were treated with DMSO, a scavenger of reactive oxygen species (ROS), which indicates that NO and ROS were involved in the bystander responses generated from irradiated T98G and AG01522 cells, respectively. Our findings indicate that calcium signaling may be an early response in radiation-induced bystander effects leading to chromosome damage.

ATR-dependent radiation-induced γH2AX foci in bystander primary human astrocytes and glioma cells

Radiotherapy is an important treatment for patients suffering from high-grade malignant gliomas. Non-targeted (bystander) effects may influence these cells' response to radiation and the investigation of these effects may therefore provide new insights into mechanisms of radiosensitivity and responses to radiotherapy as well as define new targets for therapeutic approaches. Normal primary human astrocytes (NHA) and T98G glioma cells were irradiated with helium ions using the Gray Cancer Institute microbeam facility targeting individual cells. Irradiated NHA and T98G glioma cells generated signals that induced gammaH2AX foci in neighbouring non-targeted bystander cells up to 48 h after irradiation. gammaH2AX bystander foci were also observed in co-cultures targeting either NHA or T98G cells and in medium transfer experiments. Dimethyl sulphoxide, Filipin and anti-transforming growth factor (TGF)-beta 1 could suppress gammaH2AX foci in bystander cells, confirming that reactive oxygen species (ROS) and membrane-mediated signals are involved in the bystander signalling pathways. Also, TGF-beta 1 induced gammaH2AX in an ROS-dependent manner similar to bystander foci. ROS and membrane signalling-dependent differences in bystander foci induction between T98G glioma cells and normal human astrocytes have been observed. Inhibition of ataxia telangiectasia mutated (ATM) protein and DNA-PK could not suppress the induction of bystander gammaH2AX foci whereas the mutation of ATM- and rad3-related (ATR) abrogated bystander foci induction. Furthermore, ATR-dependent bystander foci induction was restricted to S-phase cells. These observations may provide additional therapeutic targets for the exploitation of the bystander effect.

The average number of alpha-particle hits to the cell nucleus required to eradicate a tumour cell population

Alpha-particle emitters are currently being considered for the treatment of micrometastatic disease. Based on in vitro studies, it has been speculated that only a few alpha-particle hits to the cell nucleus are considered lethal. However, such estimates do not consider the stochastic variations in the number of alpha-particle hits, energy deposited, or in the cell survival process itself. Using a tumour control probability (TCP) model for alpha-particle emitters, we derive an estimate of the average number of hits to the cell nucleus required to provide a high probability of eradicating a tumour cell population. In simulation studies, our results demonstrate that the average number of hits required to achieve a 90% TCP for 10(4) clonogenic cells ranges from 18 to 108. Those cells that have large cell nuclei, high radiosensitivities and alpha-particle emissions occurring primarily in the nuclei tended to require more hits. As the clinical implementation of alpha-particle emitters is considered, this type of analysis may be useful in interpreting clinical results and in designing treatment strategies to achieve a favourable therapeutic outcome.

Bystander-induced differentiation: a major response to targeted irradiation of a urothelial explant model

A ureter primary explant technique, using porcine tissue sections was developed to study bystander effects under in vivo like conditions where dividing and differentiated cells are present. Targeted irradiations of ureter tissue fragments were performed with the Gray Cancer Institute charged particle microbeam at a single location (2 microm precision) with 10 3He2+ particles (5 MeV; LET 70 keV/microm). After irradiation the ureter tissue section was incubated for 7 days allowing explant outgrowth to be formed. Differentiation was estimated using antibodies to Uroplakin III, a specific marker of terminal urothelial differentiation. Even although only a single region of the tissue section was targeted, thousands of additional cells were found to undergo bystander-induced differentiation in the explant outgrowth. This resulted in an overall increase in the fraction of differentiated cells from 63.5+/-5.4% to 76.6+/-5.6%. These changes are much greater than that observed for the induction of damage in this model. One interpretation of these results is that in the tissue environment, differentiation is a much more significant response to targeted irradiation and potentially a protective mechanism.

An electron microbeam cell-irradiation system at KIRAMS: performance and preliminary experiments

An electron microbeam cell-irradiation (EMCI) system is now ready for routine operation in Korea. The system components include an electron gun operating at 1-100 keV, a beam transport chamber delivering a micron-sized beam, a cell image acquisition and positioning part and an automatic system control section. The present choice of source beam energy is 30 keV so that the radiation impact is conveyed to the targeted cells with a minimum spatial dispersion. The beam is available at 5 microm in diameter now, but can be changed in the range of 1-200 microm. The cellular dose is delivered with a standard deviation of 30% at 0.1 Gy, 10% at 1 Gy and 3% at 10 Gy. The cells are recognised by over 98% in a 1 mm x 1 mm area and the system is capable of irradiating up to 30,000 cells h(-1).

Experimental techniques for studying bystander effects in vitro by high and low-LET ionising radiation

Ionising radiation can induce responses within non-exposed neighbouring (bystander) cells which potentially have important implications on the estimates of risk from low dose or low dose rate exposures of ionising radiations. A range of strategies have been developed for investigating bystander effects in vitro for both high-LET alpha particles or low-LET ultrasoft X rays using either partial shielding (grids, half-shields and slits) or by using a co-culture system where two physically separated populations of cells can be cultured together, allowing one population of cells to be irradiated while the second population remains unirradiated. The techniques described provide a useful tool to study bystander effects and complement microbeam studies. Studies using these systems show significant increases in the unirradiated bystander cells for various end points including the induction of chromosomal instability in haemopoetic stem cells and transformation in CGL1 cells.

A comparative review of charged particle microbeam facilities

At the low doses (and low dose rates) relevant to environmental and occupational radiation exposure (0-50 mSv), which are of practical concern for radiation protection, very few cells in the human organism experience more than one traversal by densely ionising particles in their lifetime, the intervals between the tracks, if any, typically being months or years. The biological effects of exactly one particle are not well known and cannot be simulated in vitro by conventional broad-beam exposures, due to the random Poisson distribution of tracks. Charged particle microbeam facilities are a unique tool that allows targeting of single cells and analysis of the induced damage on a cell-by-cell basis. In the past few years, many charged particle microbeam facilities for radiobiology have come into operation or are under development worldwide. Different experimental designs have been adopted at various laboratories regarding the achievement of micrometre (or sub-micrometre) ion beam size, by mechanical collimation or focusing, particle detection, and cell recognition and positioning systems. The different approaches are reviewed and discussed in this paper.

A microcollimated ion beam facility for investigations of the effects of low-dose radiation

Charged-particle microbeams are unique tools to mimic low-dose exposure in vitro by delivering a defined number of particles to single mammalian cells down to only one particle per cell or group of cells. A horizontal single-ion microbeam facility has been built at the INFN-Laboratori Nazionali di Legnaro 7 MV Van de Graaff accelerator. Different light ions (1H+, 2H+, 3He2+, 4He2+) are available covering a wide range of LET from 7 to 150 keV/microm. Collimators of different geometries and materials have been tested, and beam spots 2-3 microm in diameter have been obtained using a tantalum disc. Cell visualization and recognition are performed with a phase-contrast optical microscope coupled with dedicated software. One unique characteristic of such a system is that neither cell staining nor UV light is used. Cells are automatically positioned on the beam spot through remotely controlled precision XY translation stages. A particle detector is positioned downstream of a specially designed petri dish to perform energy measurements and count particles crossing the cell. A particle counting rate of less than 1 ion/s can be reached. This feature, combined with a fast beam deflection system, ensures high reproducibility in administering a preset number of particles per cell.

A Variable-Energy Electron Microbeam: A Unique Modality for Targeted Low-LET Radiation

We have designed and constructed a low-cost, variable-energy low-LET electron microbeam that uses energetic electrons to mimic radiation damage produced by gamma and X rays. The microbeam can access lower regions of the LET spectrum, similar to conventional X-ray or 60Co gamma-ray sources. The device has two operating modes, as a conventional microbeam targeting single cells or subpopulations of cells or as a pseudo broad-beam source allowing for direct comparison with conventional sources. By varying the incident electron energy, the target cells can be selectively exposed to different parts of the energetic electron tracks, including the track ends.

New insights on cell death from radiation exposure

Ionising radiation has been an important part of cancer treatment for almost a century, being used in external-beam radiotherapy, brachytherapy, and targeted radionuclide therapy. At the molecular and cellular level, cell killing has been attributed to deposition of energy from the radiation in the DNA within the nucleus, with production of DNA double-strand breaks playing a central part. However, this DNA-centric model has been questioned because cell-death pathways, in which direct relations between cell killing and DNA damage diverge, have been reported. These pathways include membrane-dependent signalling pathways and bystander responses (when cells respond not to direct radiation exposure but to the irradiation of their neighbouring cells). New insights into mechanisms of these responses coupled with technological advances in targeting of cells in experimental systems with microbeams have led to a reassessment of the model of how cells are killed by ionising radiation. This review provides an update on these mechanisms.

Production and validation of CR-39-based dishes for alpha-particle radiobiological experiments

The study of radiobiological effects induced in vitro by low fluences of alpha particles would be significantly enhanced if the precise localization of each particle track in the cell monolayer was known. From this perspective, we developed a new method based on tailor-made UV-radiation-cured CR-39, the production of which is described. Its validation both as a petri dish and as solid-state nuclear track detectors is demonstrated. With respect to the demands on solid-state nuclear track detectors in such experiments, these biologically compatible detectors have a controlled micrometric thickness that allows them to be crossed by the alpha particles. In this study, we present a method for obtaining 10-mum-thick CR-39, its chemical characterization, and its properties as a solid-state nuclear track detector under the environmental conditions of radiobiological experiments. The experimental studies performed with 3.5 MeV alpha particles show that their transmitted energy is sufficient enough to cross the entire cellular volume. Under optimal conditions, etched tracks are clearly defined 2 h after etching. Moreover, the UV-radiation-cured CR-39 represents an essentially zero background that is due to the short time between the production and use of the polymer. Under a confocal microscope, this thin solid-state nuclear track detector allows the precise localization of the impact parameter at the subcellular level.

Irradiation of mammalian cultured cells with a collimated heavy-ion microbeam

As the first step for the analysis of the biological effect of heavy charged-particle radiation, we established a method for the irradiation of individual cells with a heavy-ion microbeam apparatus at JAERI-Takasaki. CHO-K1 cells attached on a thin film of an ion track detector, CR-39, were automatically detected under a fluorescence microscope and irradiated individually with an 40Ar13+ ion (11.5 MeV/nucleon, LET 1260 keV/microm) microbeam. Without killing the irradiated cells, trajectories of irradiated ions were visualized as etch pits by treatment of the CR-39 with an alkaline-ethanol solution at 37 degrees C. The exact positions of ion hits were determined by overlaying images of both cells and etch pits. The cells that were irradiated with argon ions showed a reduced growth in postirradiation observations. Moreover, a single hit of an argon ion to the cell nucleus resulted in strong growth inhibition. These results tell us that our verified irradiation method enables us to start a precise study of the effects of high-LET radiation on cells.

Bystander signaling between glioma cells and fibroblasts targeted with counted particles

Radiation-induced bystander effects may play an important role in cancer risks associated with environmental, occupational and medical exposures and they may also present a therapeutic opportunity to modulate the efficacy of radiotherapy. However, the mechanisms underpinning these responses between tumor and normal cells are poorly understood. Using a microbeam, we investigated interactions between T98G malignant glioma cells and AG01522 normal fibroblasts by targeting cells through their nuclei in one population, then detecting cellular responses in the other co-cultured non-irradiated population. It was found that when a fraction of cells was individually irradiated with exactly 1 or 5 helium particles ((3)He(2+)), the yield of micronuclei (MN) in the non-irradiated population was significantly increased. This increase was not related to the fraction of cells targeted or the number of particles delivered to those cells. Even when one cell was targeted with a single (3)He(2+), the induction of MN in the bystander non-irradiated population could be increased by 79% for AG01522 and 28% for T98G. Furthermore, studies showed that nitric oxide (NO) and reactive oxygen species (ROS) were involved in these bystander responses. Following nuclear irradiation in only 1% of cells, the NO level in the T98G population was increased by 31% and the ROS level in the AG0 population was increased by 18%. Treatment of cultures with 2-(4-carboxyphenyl)-4,4,5,5-tetramethyl-imidazoline-1-oxyl-3-oxide (c-PTIO), an NO scavenger, abolished the bystander MN induction in non-irradiated AG01522 cells but only partially in non-irradiated T98G cells, and this could be eliminated by treatment with either DMSO or antioxidants. Our findings indicate that differential mechanisms involving NO and ROS signaling factors play a role in bystander responses generated from targeted T98G glioma and AG0 fibroblasts, respectively. These bystander interactions suggest that a mechanistic control of the bystander effect could be of benefit to radiotherapy.

Microbeams of heavy charged particles

We have established a single cell irradiation system, which allows selected cells to be individually hit with defined number of heavy charged particles, using a collimated heavy-ion microbeam apparatus at JAERI-Takasaki. This system has been developed to study radiobiological processes in hit cells and bystander cells exposed to low dose and low dose-rate high-LET radiations, in ways that cannot be achieved using conventional broad-field exposures. Individual cultured cells grown in special dishes were irradiated in the atmosphere with a single or defined numbers of 18.3 MeV/amu 12C, 13.0 MeV/amu 20Ne, and 11.5 MeV/amu 40Ar ions. Targeting and irradiation of the cells were performed automatically at the on-line microscope of the microbeam apparatus according to the positional data of the target cells obtained at the off-line microscope before irradiation. The actual number of particle tracks that pass through cell nuclei was detected with prompt etching of the bottom of the cell dish made of ion track detector TNF-1 (modified CR-39), with alkaline-ethanol solution at 37 degrees C for 15-30 minutes. Using this system, separately inoculated Chinese hamster ovary cells, confluent normal human fibroblasts, and single plant cells (tobacco protoplasts) have been irradiated. These are the first studies in which single-ion direct hit effect and the bystander effect have been investigated using a high-LET heavy particle microbeam.

Targeted cytoplasmic irradiation induces bystander responses

The observation of radiation-induced bystander responses, in which cells respond to their neighbors being irradiated, has important implications for understanding mechanisms of radiation action particularly after low-dose exposure. Much of this questions the current dogma of direct DNA damage driving response in irradiated systems. In this study, we have used a charged-particle microbeam to target individual helium ions ((3)He(2+)) to individual cells within a population of radioresistant glioma cells cultured alone or in coculture with primary human fibroblasts. We found that even when a single cell within the glioma population was precisely traversed through its cytoplasm with one (3)He(2+) ion, bystander responses were induced in the neighboring nonirradiated glioma or fibroblasts so that the yield of micronuclei was increased by 36% for the glioma population and 78% for the bystander fibroblast population. Importantly, the yield of bystander-induced micronuclei was independent of whether the cytoplasm or nucleus of a cell was targeted. The bystander responses were fully eliminated when the populations were treated with 2-(4-carboxyphenyl)-4,4,5,5-tetramethyl-imidazoline-1-oxyl-3-oxide or filipin, which scavenge nitric oxide (NO) and disrupt membrane rafts, respectively. By using the probe 4-amino-5-methylamino-2',7'-difluorofluorescein, it was found that the NO level in the glioma population was increased by 15% after 1 or 10 cytoplasmic traversals, and this NO production was inhibited by filipin. This finding shows that direct DNA damage is not required for switching on of important cell-signaling mechanisms after low-dose irradiation and that, under these conditions, the whole cell should be considered a sensor of radiation exposure.

Heavy Charged Particles Produce a Bystander Effect via Cell-Cell Junctions

Radiation-induced damage to living cells results from either a direct hit to cellular DNA, or from indirect action which leads to DNA damage from radiation produced radicals. However, in recent years there is evidence that biological effects such as cell killing, mutation induction, chromosomal damage and modification of gene expression can occur in a cell population exposed to low doses of alpha particles. In fact these doses are so low that not all cells in the population will be hit directly by the radiation. Using a precision alpha-particle microbeam, it has been recently demonstrated that irradiated target cells can induce a bystander mutagenic response in neighboring "bystander" cells which were not directly hit by alpha particles. Furthermore, these results suggest that gap-junction mediated cell-to-cell communication plays a critical role in this bystander phenomenon. The purpose of this section is to describe recent studies on bystander biological effects. The recent work described here utilized heavy charged particles for irradiation, and investigated the role of gap-junction mediated cell-cell communication in this phenomenon.

Bystander responses induced by low LET radiation

Radiation-induced bystander responses are observed when cells respond to their neighbours being irradiated. Considerable evidence is now available regarding the importance of these responses in cell and tissue models. Most studies have utilized two approaches where either a media-transferable factor has been assessed or cells have been exposed to low fluences of charged particles, where only a few percent are exposed. The development of microbeams has allowed nontargeted responses such as bystander effects to be more carefully analysed. As well as charged particle microbeams, X-ray microprobes have been developed, and several groups are also developing electron microbeams. Using the Gray Cancer Institute soft X-ray microprobe, it has been possible to follow the response of individual cells to targeted low doses of carbon-characteristic soft X-rays. Studies in human fibroblasts have shown evidence of a significant radiation quality-dependent bystander effect, measured as chromosomal damage in the form of micronuclei which is radiation quality dependent. Other studies show that even under conditions when only a single cell is targeted with soft X-rays, significant bystander-mediated cell killing is observed. The observation of bystander responses with low LET radiation suggests that these may be important in understanding radiation risk from background levels of radiation, where cells observe only single electron track traversals. Also, the indirect evidence for these responses in vivo indicates that they may have a role to play in current radiotherapy approaches and future novel strategies involving modulating nontargeted responses.

Low-Dose Studies of Bystander Cell Killing with Targeted Soft X Rays

The Gray Cancer Institute ultrasoft X-ray microprobe was used to quantify the bystander response of individual V79 cells exposed to a focused carbon K-shell (278 eV) X-ray beam. The ultrasoft X-ray microprobe is designed to precisely assess the biological response of individual cells irradiated in vitro with a very fine beam of low-energy photons. Characteristic CK X rays are generated by a focused beam of 10 keV electrons striking a graphite target. Circular diffraction gratings (i.e. zone plates) are then employed to focus the X-ray beam into a spot with a radius of 0.25 microm at the sample position. Using this microbeam technology, the correlation between the irradiated cells and their nonirradiated neighbors can be examined critically. The survival response of V79 cells irradiated with a CK X-ray beam was measured in the 0-2-Gy dose range. The response when all cells were irradiated was compared to that obtained when only a single cell was exposed. The cell survival data exhibit a linear-quadratic response when all cells were targeted (with evidence for hypersensitivity at low doses). When only a single cell was targeted within the population, 10% cell killing was measured. In contrast to the binary bystander behavior reported by many other investigations, the effect detected was initially dependent on dose (<200 mGy) and then reached a plateau (>200 mGy). In the low-dose region (<200 mGy), the response after irradiation of a single cell was not significantly different from that when all cells were exposed to radiation. Damaged cells were distributed uniformly over the area of the dish scanned (approximately 25 mm2). However, critical analysis of the distance of the damaged, unirradiated cells from other damaged cells revealed the presence of clusters of damaged cells produced under bystander conditions.

Bystander effect induced by counted high-LET particles in confluent human fibroblasts: a mechanistic study

The possible mechanism of a radiation-induced bystander response was investigated by using a high-LET heavy particle microbeam, which allows selected cells to be individually hit with precise numbered particles. Even when only a single cell within the confluent culture was hit by one particle of 40Ar (approximately 1260 keV/microm) or 20Ne (approximately 380 keV/microm), a 1.4-fold increase of micronuclei (MN) was detected demonstrating a bystander response. When the number of targeted cells increased, the number of MN biphasically increased; however, the efficiency of MN induction per targeted cell markedly decreased. When 49 cells in the culture were individually hit by 1 to 4 particles, the production of MN in the irradiated cultures were approximately 2-fold higher than control levels but independent of the number and LET of the particles. MN induction in the irradiated-culture was partly reduced by treatment with DMSO, a scavenger of reactive oxygen species (ROS), and was almost fully suppressed by the mixture of DMSO and PMA, an inhibitor of gap junctional intercellular communication (GJIC). Accordingly, both ROS and GJIC contribute to the above-mentioned bystander response and GJIC may play an essential role by mediating the release of soluble biochemical factors from targeted cells.

Characterization of an alpha-particle irradiator for individual cell dosimetry measurements

A computer-controlled, alpha-particle irradiator is described that allows for the measurement of the number and location of alpha-particle hits to individual cell nuclei, and subsequent scoring of cell survival. Cells are grown on a track-etch material (LR 115) and images are obtained of the cells prior to irradiation. The cells are then irradiated from below by a planar, collimated Am-241 source. The exposure time is varied so that the average number of hits to cell nuclei ranges from 0 to 3. After cell survival has been scored, images of the etched material are obtained and spatially registered to the original cell images. The etched images and cellular images are superimposed allowing for the determination of the number and position of hits to individual cell nuclei. This paper characterizes the irradiator including the energy and fluence of the incident alpha particles. Additionally, we describe the sources of uncertainty associated with this experiment, including the cell dish repositioning and cell migration during scanning and irradiation.

A proliferation-dependent bystander effect in primary porcine and human urothelial explants in response to targeted irradiation

The aim of this study was to test whether radiation-induced bystander effects are involved in the response of multicellular systems to targeted irradiation. A primary explant technique was used that reconstructed the in vivo microarchitecture of normal urothelium with proliferating and differentiated cells present. Sections of human and porcine ureter were cultured as explants and irradiated on day 7 when the urothelial outgrowth formed a halo around the tissue fragment. The Gray Cancer Institute charge particle microbeam facility allowed the irradiation of individual cells within the explant outgrowth with a predetermined exact number of (3)He(2+) ions (which have very similar biological effectiveness to alpha-particles). A total of 10 individual cell nuclei were irradiated with 10 (3)He(2+) ions either on the periphery, where proliferating cells are located, or at the centre of the explant outgrowth, which consisted of terminally differentiated cells. Samples were fixed 3 days after irradiation, stained and scored. The fraction of apoptotic and micronucleated cells was measured and a significant bystander-induced damage was observed. Approximately 2000-6000 cells could be damaged by the irradiation of a few cells initially, suggesting a cascade mechanism of cell damage induction. However, the fraction of micronucleated and apoptotic cells did not exceed 1-2% of the total number of the cells within the explant outgrowth. It is concluded that the bystander-induced damage depends on the proliferation status of the cells and can be observed in an in vitro explant model.

Cellular communication and bystander effects: a critical review for modelling low-dose radiation action

Available data suggesting the occurrence of "bystander effects" (i.e. damage induction in cells not traversed by radiation) were collected and critically evaluated, in view of the development of low-dose risk models. Although the underlying mechanisms are largely unknown, cellular communication seems to play a key role. In this context, the main features of cellular communication were summarised and a few representative studies on bystander effects were reported and discussed. Three main approaches were identified: (1) conventional irradiation of cell cultures with very low doses of light ions; (2) irradiation of single cells with microbeam probes; (3) treatment with irradiated conditioned medium (ICM), i.e. feeding of unexposed cells with medium taken from irradiated cultures. Indication of different types of bystander damage (e.g. cell killing, gene mutations and modifications in gene expression) has been found in each of the three cases. The interpretations proposed by the investigators were discussed and possible biases introduced by specific experimental conditions were outlined. New arguments and experiments were suggested, with the main purpose of obtaining quantitative information to be included in models of low-dose radiation action. Implications in interpreting low-dose data and modelling low-dose effects at cellular and supra-cellular level, including cancer induction, were analysed. Possible synergism with other low-dose specific phenomena such as adaptive response (AR) (i.e. low-dose induced resistance to subsequent irradiation) was discussed.

Investigating the cellular effects of isolated radiation tracks using microbeam techniques

Studies of the effects of radiation at the cellular level have generally been carried out by exposing cells randomly to the charged-particle tracks of a radiation beam. Recently, a number of laboratories have developed techniques for microbeam irradiation of individual cells. These approaches are designed to remove much of the randomness of conventional methods and allow the nature of the targets and pathways involved in a range of radiation effects to be studied with greater selectivity. Another advantage is that the responses of individual cells can be followed in a time-lapse fashion and, for example, processes such as "bystander" effects can be studied clearly. The microbeam approach is of particular importance in mechanistic studies related to the risks associated with exposure to low fluences of charged particles. This is because it is now possible to determine the actions of strictly single particle tracks and thereby mimic, under in vitro conditions, exposures at low radiation dose that are significant for protection levels, especially those involving medium- to high-LET radiations. Overall, microbeam methods provide a new dimension in exploring mechanisms of radiation effect at the cellular level. Microbeam methods and their application to the study of the cellular effects of single charged-particle traversals are described.

A focused ultrasoft x-ray microbeam for targeting cells individually with submicrometer accuracy

The application of microbeams is providing new insights into the actions of radiation at the cell and tissue levels. So far, this has been achieved exclusively through the use of collimated charged particles. One alternative is to use ultrasoft X rays, focused by X-ray diffractive optics. We have developed a unique facility that uses 0.2-0.8-mm-diameter zone plates to focus ultrasoft X rays to a beam of less than 1 microm diameter. The zone plate images characteristic K-shell X rays of carbon or aluminum, generated by focusing a beam of 5-10 keV electrons onto the appropriate target. By reflecting the X rays off a grazing-incidence mirror, the contaminating bremsstrahlung radiation is reduced to 2%. The focused X rays are then aimed at selected subcellular targets using rapid automated cell-finding and alignment procedures; up to 3000 cells per hour can be irradiated individually using this arrangement.