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Mutation Research 741– 742 (2013) 51– 56

Contents lists available at SciVerse ScienceDirect

Mutation Research/Fundamental and MolecularMechanisms of Mutagenesis

jo ur n al hom ep a ge: www.elsev ier .com/ locate /molmutC om mun i ty a ddress : www.elsev ier .com/ locate /mutres

ose response of micronuclei induced by combination radiation of �-particlesnd �-rays in human lymphoblast cells

uiping Ren1, Mingyuan He, Chen Dong, Yuexia Xie, Shuang Ye, Dexiao Yuan, Chunlin Shao ∗

nstitute of Radiation Medicine, Fudan University, No. 2094 Xie-Tu Road, Shanghai 200032, China

r t i c l e i n f o

rticle history:eceived 4 September 2012eceived in revised form1 November 2012ccepted 28 December 2012vailable online 8 January 2013

eywords:-Particles

a b s t r a c t

Combination radiation is a real situation of both nuclear accident exposure and space radiation environ-ment, but its biological dosimetry is still not established. This study investigated the dose–response ofmicronuclei (MN) induction in lymphocyte by irradiating HMy2.CIR lymphoblast cells with �-particles,�-rays, and their combinations. Results showed that the dose–response of MN induced by �-rays waswell-fitted with the linear-quadratic model. But for �-particle irradiation, the MN induction had a biphasicphenomenon containing a low dose hypersensitivity characteristic and its dose response could be well-stimulated with a state vector model where radiation-induced bystander effect (RIBE) was involved. Forthe combination exposure, the dose response of MN was similar to that of �-irradiation. However, the

-Raysombination radiationystander effectynergistic effectdaptive response

yield of MN was closely related to the sequence of irradiations. When the cells were irradiated with �-particles at first and then �-rays, a synergistic effect of MN induction was observed. But when the cellswere irradiated with �-rays followed by �-particles, an antagonistic effect of MN was observed in thelow dose range although this combination radiation also yielded a synergistic effect at high doses. Whenthe interval between two irradiations was extended to 4 h, a cross-adaptive response against the other

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. Introduction

It is well known that radiation, especially high linear energyransfer (LET) radiation, is an important environment carcinogenicvent. For example, �-particle has became a prominent publicealth concern because of its presence in the residential environ-ent (i.e., radon (222Rn) gas) and occupational environment (i.e.edical imaging and cancer therapy) [1,2]. Recent studies have

emonstrated a positive correlation between the occurrence ofung cancer and exposure to 222Rn gas [3,4]. DNA damage inducedy high LET radiation is much more serious than low LET radiation,or instance, low dose rate exposures to alpha emitters were 15–20imes more damaging than exposures to beta or gamma irradia-ion [5]. It has been known that the relative biological effect (RBE)f 210Po alpha-particle could vary from 1.6 to 21, depending on

he endpoint: about 21 for cell viability, 13 for decrease in live cellumber, 5.3 for LDH release, but only 1.6 for clonogenic survivalue to X-ray hypersensitivity of endothelial cells at low doses [6],

Abbreviations: LET, linear energy transfer; MN, micronuclei; RIBE, radiationnduced bystander effect; SVM, state vector model.∗ Corresponding author. Tel.: +86 21 64048677; fax: +86 21 64048677.

E-mail address: clshao@shmu.edu.cn (C. Shao).1 Current address: Chemoradiotherapy Center of Oncology, Yinzhou Poeple’s Hos-ital, Ningbo, China.

027-5107/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.mrfmmm.2012.12.007

w dose of �-rays but not �-particles.© 2013 Elsevier B.V. All rights reserved.

and DNA repair rate in �-particle irradiated cells is much slowerthan that of a low LET irradiation such as X-rays [7].

With chromosome aberration and micronuclei (MN) formationas checkpoints, the biological dosimetry of �-rays has been well-established and widely applied. But the biological dosimetries of�-particles and related combination radiations are largely not clear.Complex radiation may happen in many situations. For example,during a nuclear reactor accident, radioactive isotopes including239Pu, 137Cs, and 60Co can be released simultaneously. 239Pu canemit �-particles whereas 137Cs and 60Co yields �-rays, the victimsof a nuclear accident will suffer from their combination irradia-tion. On the other hand, radiation risk for the astronaut in spaceflight has been paid great attention since human activity in space.Space radiation includes X-rays, high energy protons, helium ions,and high-energy heavy particles. Even though being protected, theastronaut could be inevitably irradiated with multi-sorts of parti-cles in space, which may result in significant risks of carcinogenesisand degenerative diseases at a long time of space flight. It has beenreported that different irradiations of low- and high-LET may havecross-adaptive or synergic effect on cell survival [8]. Brooks et al.demonstrated that both cell killing and the induction of MN wereincreased by the combined exposures of alpha-particles and X-rays

compared with that predicted for separate exposures [9]. Althoughunderstanding the biological dosimetry of complex radiation inspace is a big challenge in the near future, it may give us essentialinformation in evaluating cancer risk of space flight.

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phoblast cells had a very low background frequency of MN, only5 × 10−4. The yield of MN increased slightly when the �-irradiationdose was lower than 0.25 Gy but it increased steeply afterwards.

0 1 2 3 4 5

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Fig. 1. Dose responses of MN in HMy2.CIR lymphoblast cells irradiated by different

2 R. Ren et al. / Mutation R

At present, cancer risks at doses lower than those for whichirect epidemiological observations available are obtained by a lin-ar extrapolation from the higher doses [10]. There are a numberf low dose phenomena that might modulate the biological effectsuch that a linear extrapolation might not truly reflect low dose risk.hese include radiation induced bystander effect (RIBE), adaptiveesponse, and potential radiation sensitive subgroups in the humanopulation. They might impact on the dose response of DNA dam-ge at low doses of ionizing radiation [11,12]. In this study, MNnduction in lymphocyte is employed to analyze the dose responsef DNA damage induced by �-particles, �-rays, and their combina-ions. It was found that a bystander effect was involved in the doseesponse of �-particle irradiation and either synergistic or adaptiveesponse could be observed in different combination exposures.

. Materials and methods

.1. Cell culture

Human B lymphoblast cells (HMy2.CIR) were purchased from Shanghai Cell Bankf China and maintained in IMDM medium (HyClone, Beijing, China) containingenicillin (100 U/ml), streptomycin (100 U/ml) and 10% fetal bovine serum (Gibco

nvitrogen, Grand Island, NY, USA) in humidified atmosphere of 5% CO2 in air at7 ◦C.

For �-particle irradiation, cells were grown on a 2.5 �m thickness Mylar filmased dish to allow the penetration of �-particles from the bottom of the dish.he Mylar film was coated with 150–300 kD polylysine (Sigma) overnight then theog-phase HMy2.CIR cells were seeded on the dish and cultured for 4 h so that theymphoblast cells well attached on the film at the time of �-particle exposure.

.2. Cell irradiation

A 241Am �-particle plate source (Atom HighTech Co., Ltd., Beijing, China) and137Cs �-ray irradiator (Model the Gamma-cell 40, Nordion Company of Canada)ere applied for the cell irradiation in random. The energy of �-particles emitted

rom 241Am isotope is 5.48 MeV. It was detected that, after passing through 3 layersf Mylar-film and possible air layer between films, the energy of �-particles arrivingt cells was 4.4 MeV. It was calculated by a TRIM program that the LET of 4.4 MeV-particle in water-equivalent tissue was 100 keV/�m and thus the dose rate of �-articles was calculated to be 0.244 Gy/min. The dose rate of �-rays was 0.8 Gy/min.he irradiation doses were chosen as 0.1, 0.25, 0.5, 0.75, 1, 2, 3, 4, 5 Gy for �-rays and.01, 0.025, 0.05, 0.1, 0.15, 0.2, 0.5, 0.75, 1 Gy for �-particles. For the combinationadiation, the HMy2.CIR cells were firstly irradiated with 0.025, 0.1, 0.2, or 0.5 Gy-particles and then immediately (with an interval less then 5 min) exposed to 0.25,.75, 1, 2, 3 or 4 Gy �-rays; or the cells were firstly irradiated with 0.25, 0.75, 1, or

Gy �-rays and then given 0.025, 0.1, 0.2, or 0.5 Gy �-particles immediately.On the other hand, to investigate the possible adaptive response between dif-

erent irradiations, an interval of 4 h was given between �- and �-irradiation. It haseen known that this interval time was ideal for the induction of adaptive response.riefly, the cells were irradiated with 0.025 or 0.1 Gy of �-particles, after 4 h, theyere challenged with 2 Gy of �-rays; or the cells were triggered with 0.1 Gy of �-rays,

fter 4 h, the cells were challenged with 0.2 and 0.5 Gy of �-particles.

.3. Micronucleus assay

After irradiation, cell damage was evaluated by MN assay using the cytokinesis-lock technique. Briefly, after irradiation, all cells were collected and then treatedith 3 �g/ml cytochalasin-B (Sigma) for 30 h for low dose irradiated cells. Our pilot

xperiments showed that a high dose of �-particle irradiation could cause cell cyclerrest, delayed nuclear division, and reduced MN formation. Therefore, when theells were irradiated with �-particles of a dose higher than 0.1 Gy, the CB treatmentime was prolonged to 48 h so that the cells could overcome cycle arrest as far asossible and yielded enough MN. After the CB treatment, the cells were treated withypotonic solution of 75 �M KCl for 20 min and fixed in methanol with acetic acid9:1), dropped onto glass slide, stained with Giemsa solution (Sigma), and observednder a microscope (Olympus, Tokyo, Japan). MN were scored in at least 500 bin-cleated cells and the MN yield, YMN, was calculated as the ratio of the number ofN to the scored number of binucleated cells.

.4. Statistical analysis

Data were obtained from 3 to 5 independent experiments with three replicatesn each case and were presented as mean ± SE. Statistical significance was acceptablet the level of P < 0.05. Data analysis was performed using the software SPSS11.5SPSS Inc., Chicago, IL, USA) and the dose–response curves were fitted with the Originoftware (OriginLab Co., Northampton, MA, USA).

h 741– 742 (2013) 51– 56

3. Results

3.1. Dose response of MN induced by �-rays

Fig. 1 illustrates the dose–responses of the yield of MN inHMy2.CIR lymphoblast cells irradiated by �-particles and �-rays. Asshown in Fig. 1A, the yield of MN induced by �-rays was well fittedwith a linear–quadratic model represented by YMN = c + �D + �D2,where D is radiation dose, c is the frequency of backgroundMN without irradiation, and ̨ and ̌ are linear and quadraticcoefficients, respectively. It can be seen that the HMy2.CIR lym-

radiations. (A) Dose response of MN induced by �-rays. The curve was fitted by alinear-quadratic model with an equation YMN = c + ˛D + ˇD2. (B) Dose response of MNinduced by �-particles. The solid curve was fitted by a state vector model with anequation of YMN = c + ˛D + �(1 − exp(−ıD)) exp(−ˇD). The dash line presented thedose response of the yield of MN induced by �-particles assuming no bystandereffect. (C) Dose response of MN induced by �-particle radiation where the cellswere pretreated with DMSO (�) and c-PTIO (�) before irradiation.

esearch 741– 742 (2013) 51– 56 53

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Fig. 2. Dose response of MN in HMy2.CIR lymphoblast cells irradiated with �-particles followed by �-rays immediately (less than 5 min). (A) Response of the MNyield against the dose of �-rays. (B) Response of the MN yield against the dose of�-particles, and the data were redrawn from plot A.

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R. Ren et al. / Mutation R

.2. Dose response of MN induced by ˛-particles

Fig. 1B illustrates the dose–response of �-particle induced MNn HMy2.CIR cells. The MN yield corresponding to �-particle wasignificantly higher than that of �-rays at the same dose (P < 0.01).owever, the dose response curve of MN induced by different irra-iations was totally different. For �-irradiation, the MN yield almost

inearly increased with dose when it was lower than 0.25 Gy, andhe MN yield increased continually but the its curve bended downhen the dose further increased. This dose response curve is quite

imilar to that of helium-particle induced MN in glioma cells [13]nd �-particle induced CD59-mutation frequency in AL cells [14]here RIBE was involved in the radiation damage. It can be specu-

ated that RIBE may also contribute to this �-particle induced MNn HMy2.CIR cells especially in the low dose range. If RIBE wereot involved in the MN induction, the dose response of high-LET

rradiation-induced MN should be linear as the dash-line in Fig. 1B.A state vector model (SVM) has been established for the induc-

ion of chromosome aberration throughout direct and bystanderechanisms with an equation of YMN = ˛D + �(1 − exp(−ıD))

xp(−ˇD) [15]. Since MN are generated from chromo-ome damage, here we applied this model to fit the doseesponse of �-particle induced MN with a modified equationMN =c + ˛D + �(1 − exp(−ıD))exp(−ˇD), where c = 5 × 10−4 is theackground level of MN. A well-fitted solid curve was shown

n Fig. 1B, the gap of MN yield between the solid curve and theash-line should be derived from the contribution of RIBE.

Our previous studies disclosed that reactive oxygen speciesROS) and nitric oxide (NO) were involved in the bystander effect

ediated through cell culture medium [16,17]. To further clarifyhe contribution of RIBE in the �-particle induced MN formation,efore �-particle irradiation, we pretreated the cells with DMSOnd c-PTIO, the scavenger of ROS and NO free radical, respectively.ince RIBE occurred mainly at low doses (see Section 4), we onlypplied free radical scavenger when the cells were irradiated with

dose lower than 0.5 Gy. Under both drug conditions, the yieldsf MN were lower than that without chemical treatment (Fig. 1C).uch importantly, the MN yields had linear dose responses andere close to the values predicted in Fig. 1B assuming no bystander

ffect occurred in the low dose range, indicating that both ROS andO play a role of bystander signals and contribute to the radiation-

nduced MN formation. In addition, the MN was much effectivelyeduced by c-PTIO than DMSO, indicating that NO has much con-ribution than ROS in radiation-induced MN formation.

.3. Dose response of MN induced by combination radiation

When the cells were irradiated with �-particles followed by �-ays, the dose response of MN had a biphasic curve (Fig. 2). Whenhe dose of �-particle was lower than 0.1 Gy, the MN yield of com-ination radiation increased with the dose of �-rays and had a doseesponse curve similar to that of �-irradiation. But when the dosef �-particles was higher than 0.2 Gy, the MN yield of combinationadiation increased with the dose of �-rays and had a dose responseurve similar to that of �-irradiation (Fig. 2A). To better understand-ng this dose response, we redrew the data in Fig. 2A where the MNield was corresponded to the dose of �-particles. Clearly, 0.2 Gy of-particles was a turning-point of the dose response curve under

his combination radiation.For the combination radiation of �-rays in the first and then �-

article, the response of MN yield against �-irradiation dose had

bending curve, especially at high dose of �-particles, which isimilar to that of �-irradiation alone (Fig. 3A). Alternatively, Fig. 3Bhowed that, against the dose of �-particle, the dose response curvef MN under the combination radiation was similar to that in Fig. 2B.

Fig. 3. Dose response of MN in HMy2.CIR lymphoblast cells irradiated with �-raysfollowed by �-particles immediately (less than 5 min). (A) Response of the MN yieldagainst the dose of �-rays. (B) Response of the MN yield against the dose of �-particles, and the data were redrawn from plot A.

54 R. Ren et al. / Mutation Research 741– 742 (2013) 51– 56

Table 1The yield of MN in HMy2.CIR lymphoblast cells under different radiation conditions.

�-Particles(Gy) �-Rays(Gy) The sum of MN yield under �-and �-irradiation alone

MN under combination radiation of�-particles followed by �-rays

MN under combination radiation of�-rays followed by �-particles

0.025

0.25 0.0135 ± 0.0028 0.0160 ± 0.0028 0.0100 ± 0.00160.75 0.0295 ± 0.0027 0.0220 ± 0.0059 0.0160 ± 0.00431 0.0305 ± 0.0028 0.0480 ± 0.0043 0.0245 ± 0.00252 0.0685 ± 0.0111 0.0665 ± 0.0055 0.0450 ± 0.01144 0.2585 ± 0.0184 0.3080 ± 0.0040 –

0.1

0.25 0.0350 ± 0.0033 0.0340 ± 0.0040 0.0185 ± 0.00520.75 0.0510 ± 0.0032 0.0567 ± 0.0081 0.0250 ± 0.00681 0.0520 ± 0.0033 0.0580 ± 0.0049 0.0310 ± 0.00262 0.0900 ± 0.0112 0.0773 ± 0.0110 0.0550 ± 0.00144 0.2800 ± 0.0185 0.3191 ± 0.0423 –

0.2

0.25 0.0635 ± 0.0079 0.1089 ± 0.0102 0.0535 ± 0.00440.75 0.0795 ± 0.0079 0.1720 ± 0.0295 0.0800 ± 0.01121 0.0805 ± 0.0079 0.2080 ± 0.0087 0.0940 ± 0.01912 0.1185 ± 0.0133 0.2430 ± 0.0109 0.1385 ± 0.01943 0.2195 ± 0.0102 0.2895 ± 0.0180 –

0.25 0.1020 ± 0.0155 0.2115 ± 0.0365 0.1050 ± 0.01330.75 0.1180 ± 0.0154 0.2285 ± 0.0192 0.1405 ± 0.0102

0.2460 ± 0.0053 0.1686 ± 0.02260.2715 ± 0.0233 0.2015 ± 0.01320.3520 ± 0.0728 –

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Fig. 4. Radiation induced cross-adaptive response in HMy2.CIR lymphoblast cells.(A) MN induction in the cells irradiated by �-particles with an indicated dose and

0.5 1 0.1190 ± 0.0155

2 0.1570 ± 0.0188

3 0.2580 ± 0.0167

Comparing the data in Fig. 2 and Fig. 3, it can be seen that theN yield under the situation of �-irradiation at first followed by

-irradiation was higher than that �-irradiation at first and then-irradiation, indicating that the exposure sequence of irradiationith different LET is important for cell damage response. To betternderstanding the radiation damage of the combination exposure,e listed the MN yields under different combination radiations and

alculated the sum of MN yields under �- and �-irradiation alonen Table 1.

In general, the yields of MN in the cells irradiated by �-particlesith a dosage from 0.025 to 0.5 Gy and the irradiated in subse-

uent with �-rays from 0.25 to 4 Gy were significantly higher thanhe sum of the yields of MN induced by corresponding �-particlesnd �-rays alone, indicating that this combination radiation had

synergistic effect on cell damage. With respect to the combina-ion radiation of �-rays at first and then followed by �-particles, theields of MN had a complex relationship to the sum of the MN yieldsnder corresponding �- and �-irradiation alone, which was relatedo the dose of �-particles. When the dose of �-particles was equalr higher than 0.2 Gy, the yields of MN under combination radiationere higher than those sum of MN yields. But when the dose of �-article was lower than 0.2 Gy, the yields of MN under combinationadiation were lower than the above sum of MN yields. It seems that

low dose of �-rays irradiation could induce an adaptive responsegainst the following �-particle irradiation.

.4. Adaptive response of the combination radiation

To determine whether cells pretreated with a low dose of �-articles or �-rays have an adaptive response to the subsequenthallenge irradiation, we prolonged the interval between two dif-erent irradiations to 4 h. This interval time has been known to bedeal for the induction of adaptive response. It was found that whenhe cells were irradiated with 0.025 or 0.1 Gy �-particles and 4 hater irradiated with 2 Gy �-rays, the yield of MN of this combina-ion radiation was not lower than that of 2 Gy �-rays alone (Fig. 4A),hich means that these lower doses of �-irradiation could not

rigger cellular adaptive response to the subsequent challenge of

-irradiation. However, when the cells were irradiated with 0.1 Gy-rays and then 4 h later irradiated with 0.2 or 0.5 Gy �-particles,he yields of MN of this combination radiation were significantlyower than that of corresponding �-irradiation alone (P < 0.01),

then challenged with 2 Gy �-rays. (B) MN induction in the cells irradiated with0.1 Gy �-rays and then challenged with �-particles. *, P < 0.01 between the indicatedgroups. The interval between two irradiations were 0 and 4 h, respectively.

indicating that the priming irradiation of 0.1 Gy �-rays triggeredan adaptive response against the subsequent �-irradiation.

4. Discussion

The present study found that the yield of MN in lymphoblastcells irradiated by �-rays followed a well-known linear-quadraticdose response but the yield of �-particle induced MN showed a

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iphasic dose response that had an upbowing curve with a tur-ing point at 0.2 Gy. Our previous study has disclosed that the MN

nduction in glioma cell population irradiated with counted heliumarticles also had a biphasic response to the number of targetedells i.e. the total radiation dose [13]. This kind of biphasic doseesponse curve was also found for �-particle induced chromosomeberration [18], gene mutation [14], and cellular transformation19]. Previous studies demonstrated that RIBE was involved in thisiphasic dose–response curve where the bystander effect enhancedadiation damage in the low dose range, consistent with the reporthat cell damage at 0.05 Gy or less of protons or iron ions may beominated by bystander response [20].

At a low dose irradiation, only a part of cells in the populationould be actually traversed by particles [21]. Here if the aver-ge nuclear cross section of HMy2.CIR lymphoblast cells is about0 �m2, one nuclear traversal of single �-particle of 100 keV/�meposits 0.2 Gy each nucleus. The fraction of cells traversed by at

east one particle can therefore be calculated using Poisson statis-ics with an equation 1 − exp(−D/0.2), where D is the irradiationose. Hence, at 0.05, 0.2 and 0.5 Gy of �-particles, 22%, 63% and 91%f cells were hit by at least one particle, respectively, which furtherupports the current deduction that the RIBE contributes to the cellamage at low doses.

Evidence showed that the signaling factors released from irradi-ted cells played an important role in bystander effect by attackingonirradiated cells [22,23]. Our recent investigation confirmed thatO and ROS are two important mediators of RIBE [16], which islso applicable to the current finding that the pretreatment ofells with ROS and NO scavenger, DMSO and c-PTIO, diminished-particle induced DNA damage especially at low dose range so

hat the dose response curve of MN became linear. On the otherords, the bystander signals may allow cells to be more sensitive

o �-particles, in accord with our previous report that a low-doseypersensitive effect on MN induction was induced by carbon-ion

rradiation but was eliminated by DMSO [21].The high LET �-particle is much more effective in inducing DNA

amage than �-rays. It can be calculated from the data in Fig. 1 that,o generate an equivalent yield of MN, the dose of �-rays was 3- to5-fold of �-particles. This RBE depends on the irradiation dose: the

ower of the dose, the higher of this fold value, and the low doseypersensitivity and/or bystander effect of �-particle irradiationay contribute to this phenomenon. Moreover, with respect to the

ombination radiation of �-particles and �-rays, its dose responseurve of MN is similar to that of �-irradiation alone, indicating thathe high LET �-particle induced DNA damage plays dominationole in the combination effect of MN formation. Importantly, theose response of combination radiation depends on the sequencef irradiation i.e., �-irradiation followed by �-irradiation generates

synergistic effect on DNA damage, but �-irradiation followed by-irradiation generates an antagonistic effect in low dose range and

t gives a synergistic effect when the dose of �-rays and �-particlesere higher than 0.75 Gy and 0.2 Gy, respectively. The reason of

his special phenomenon may due to the repair capacity of DNAamage. It is well known that a high-LET irradiation could makeore DNA fragments rather than single strand breaks and base

amage and hence may also impair DNA repair system. Therefore,-particle induced DNA damage is difficult to be repaired, which

eads to severe cellular damage and weakens cell defense ability.nder this radiation status, any another irradiation could amplifyellular damage and results in a synergistic effect.

On the other side, amount evidence has shown that a lowose of �-rays could trigger cellular adaptive response against

ubsequent challenging irradiation in vitro and in vivo [24,25] byising some cell defense proteins such as NF-�B [26]. Adaptiveesponse could be efficiently induced by a dose within 0.005–0.2 Gy27]. Our previous study found that ATM protein could be

[

h 741– 742 (2013) 51– 56 55

activated immediately after a stress [28]. It was believed that highLET radiation could not induce adaptive response but adapted cellscould repair high LET-induced chromosome damage [29]. The cur-rent work also demonstrated that the priming irradiation of 0.1 Gy�-rays could trigger an adaptive response against challenging �-particles but the priming �-particle irradiation did not induceadaptive response (see Fig. 4). The adaptive response and someunknown quick cellular responses may contribute to the antagonis-tic effect on DNA damage after the combination radiation of �-raysfollowed by �-particle. However, it is noticed that the antagonis-tic effect occurred not only for low dose radiation but also at highdoses such as 1 Gy �-rays followed by 0.1 Gy �-particle, its reasonstill need further investigation.

Understanding the combination effect of low- and high-LETirradiation induced MN in human lymphocyte cells has specialimplications to evaluate the risk of space radiation since MN inlymphocyte has been commonly applied as a radiation dosimeter.Space radiation is unique and complex and composes of galacticcosmic rays and solar particles [30]. Galactic �-rays and very lowfluence of high-LET particle events are full of the space radiationenvironment so that an organism cell in this environment may havea large opportunity to be irradiated by �-rays but irradiated by high-LET particles only by chance. Accordingly, the MN dose response ofthe combination radiation of �-rays followed by �-particles maysomehow reveal the regularity of space radiation dosimetry whereboth adaptive response and bystander response should be borne inmind.

Conflict of interest

The authors declare that there are no conflicts of interests.

Acknowledgments

This study was supported by the National Nature Science Foun-dation of China (Grant Nos. 11179002 and 31070758).

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