Ionizing radiation modulates human macrophages towards a pro-inflammatory phenotype preserving their pro-invasive and pro-angiogenic capacities

1 Scientific RepoRts | 6:18765 | DOI: 10.1038/srep18765 www.nature.com/scientificreports Ionizing radiation modulates human macrophages towards a pro-inflammatory phenotype preserving their pro-invasive and pro-angiogenic capacities Ana Teresa Pinto1,2,3, Marta Laranjeiro Pinto1,2,4, Ana Patrícia Cardoso1,2,3, Cátia Monteiro1,2, Marta Teixeira Pinto1,5, André Filipe Maia1,6, Patrícia Castro1,5, Rita Figueira7, Armanda Monteiro7, Margarida Marques7, Marc Mareel8, Susana Gomes dos Santos1,2,4, Raquel Seruca1,5,9, Mário Adolfo Barbosa1,2,4, Sónia Rocha10 & Maria José Oliveira1,2,9 In order to improve the efficacy of conventional radiotherapy, attention has been paid to immune cells, which not only modulate cancer cell response to therapy but are also highly recruited to tumours after irradiation. Particularly, the effect of ionizing radiation on macrophages, using therapeutically relevant doses, is not well understood. To evaluate how radiotherapy affects macrophage behaviour and macrophage-mediated cancer cell activity, human monocyte derived-macrophages were subjected, for a week, to cumulative ionizing radiation doses, as used during cancer treatment (2 Gy/fraction/day). Irradiated macrophages remained viable and metabolically active, despite DNA damage. NF-kappaB transcription activation and increased Bcl-xL expression evidenced the promotion of pro-survival activity. A significant increase of pro-inflammatory macrophage markers CD80, CD86 and HLA-DR, but not CCR7, TNF and IL1B was observed after 10 Gy cumulative doses, while anti-inflammatory markers CD163, MRC1, VCAN and IL-10 expression decreased, suggesting the modulation towards a more proinflammatory phenotype. Moreover, ionizing radiation induced macrophage morphological alterations and increased their phagocytic rate, without affecting matrix metalloproteases (MMP)2 and MMP9 activity. Importantly, irradiated macrophages promoted cancer cell-invasion and cancer cell-induced angiogenesis. Our work highlights macrophage ability to sustain cancer cell activities as a major concern that needs to be addressed to improve radiotherapy efficacy. Radiation therapy is a widely used and highly cost effective cancer treatment modality 1,2 . The biological principle of its application relies mainly on the direct effect on cancer cells, as they usually exhibit higher proliferative rates and impairment in DNA repair mechanisms, when compared to host cells3. Although physics and technological evolution in the field of radiotherapy, involving new imaging modalities and more advanced software/equipment, have largely contributed to improve local control, it is necessary to improve radiotherapy targeting, control disease progression and predict treatment outcome4,5. To achieve the desired effectiveness, new therapeutic strategies, 1I3S-Instituto de Investigação e Inovação em Saúde, Universidade do Porto, Porto, 4200-135, Portugal. 2INEBInstitute of Biomedical Engineering, University of Porto, Porto, 4200-465, Portugal. 3FEUP-Faculty of Engineering, University of Porto, Porto, 4200-465, Portugal. 4ICBAS-Institute of Biomedical Sciences Abel Salazar, University of Porto, Porto, 4050-313, Portugal. 5IPATIMUP-Institute of Molecular Pathology and Immunology, University of Porto, Porto, 4200-465, Portugal. 6IBMC-Institute for Molecular and Cell Biology, University of Porto, Porto, 4200465, Portugal. 7Radiotherapy Service, Centro Hospitalar S. João, EPE, Porto, 4200–319, Portugal. 8Department of Radiation Oncology and Experimental Cancer Research, Ghent University Hospital, Ghent, B-9000, Belgium. 9Department of Pathology and Oncology, Faculty of Medicine, University of Porto, Porto, 4200–319, Portugal. 10Centre for Gene Regulation and Expression, College of Life Sciences, University of Dundee, Dundee, DD1 5EH, UK. Correspondence and requests for materials should be addressed to M.J.O. (email: [email protected]) received: 24 June 2015 accepted: 25 November 2015 Published: 06 January 2016 OPEN www.nature.com/scientificreports/ 2 Scientific RepoRts | 6:18765 | DOI: 10.1038/srep18765 administered concomitantly, before or after radiotherapy, are required6. The key may rely on a better understanding of the effect of ionizing radiation on tumour stromal cells, as they are crucial for disease progression and treatment outcome, and are also irradiated7–10. Ionizing radiation-induced cancer cell death releases death-signals, which lead to the recruitment of more immune cells, including monocytes which differentiate into macrophages at the injured region11. Within the different immune cells present at the tumour microenvironment, macrophages are particularly relevant, as they constitute, in many tumours, the major inflammatory stromal component and are also known as obligate partners for cancer cell migration, invasion and metastasis12,13. Due to their sophisticated phagocytic ability, macrophages perform a crucial role in clearing dying cells, contributing to the induction of tolerance, or stimulation of adaptive antitumour immunity11,14. Nevertheless, it is not well understood how ionizing radiation, namely clinically relevant doses, directly affects macrophage behaviour as well as macrophage regulation of cancer progression. Till now a clear limitation on determining the clinical effect of ionizing radiation on elements of the tumour microenvironment are the model systems analysed and the dose of radiation used. Ionizing radiation for cancer treatment is usually delivered in a multi-fractionated regimen, with daily doses of typically 2 Gy (5× /week), often delivered during several weeks15. However, the majority of the studies aiming to reveal the effect of ionizing radiation on macrophages are performed in mouse models and make use of single, low (< 0.1 Gy) or moderate (0.1 Gy–1 Gy) doses 16–20 , thus not relevant in a human clinical context. The present work aims to reveal the effect of fractionated ionizing radiation on human macrophages, mimicking a week of cancer patients’ treatment. In order to achieve this, human monocyte-derived macrophages were differentiated in the presence of M-CSF, a factor involved in the recruitment of monocytes to the tissues, and exposed to cumulative ionizing radiation doses of 2 Gy each, up to 10 Gy. A plethora of functions in macrophages was then characterized. We demonstrate that irradiated macrophages are viable and metabolically active, activate NF-κ B, exhibit a reduced anti-inflammatory profile, increased phagocytosis and unaltered MMP-2 and 9-mediated proteolysis. We also evidenced that irradiated macrophages are still able to promote tumour cell invasion and angiogenesis. Overall, our work adds novelty to the current literature and reinforces the idea of targeting macrophage differentiation and/or their molecular targets as a complementary strategy to improve radiotherapy efficacy. Results Irradiated macrophages are viable and metabolically active, despite DNA damage. To confirm DNA damage induced by the fractionated irradiation protocol, 10 Gy irradiated macrophages were fixed and immunostained for phosphorylated H2AX (Ser139) (γ H2AX), a sensitive marker of DNA double-strand breaks (DSBs)21. Immunofluorescence images and respective quantification indicated that a significantly (P < 0.01) higher percentage (87.17 ± 7.99%) of irradiated macrophages presented γ H2AX foci, in comparison to non-irradiated ones (6.85 ± 1.03%) (Fig.1A). Upon ionizing radiation exposure, DNA damage signals are generally sensed and propagated through a kinase cascade, including Ataxia Telangiectasia Mutated Kinase (ATM) and also Checkpoint kinase 2 (Chk2)22. Therefore, Chk2 expression and phosphorylation status were evaluated by western blot analysis. Chk2 activation was consistently observed in all donors at 1, 6 and 24 h after cumulative ionizing radiation doses (2, 6 and 10 Gy) (Fig.1B). This data confirms that the irradiation protocol applied induced macrophage DNA damage, from single to several cumulative doses, and that the triggered signalling response involves Chk2 activation. Depending on the level of radiation-induced DNA damage, cell death may or not occur 22 . Apoptosis is known as the major cell death modality induced by ionizing radiation in cells from the myeloid lineage, which is composed by progenitor and mature effector myeloid cells, such as macrophages, dendritic cells, erythrocytes and platelets 23–25 . In the last years, a heterogeneous population of immature myeloid cells with immunosuppressive properties, termed myeloid-derived suppressor cells (MDSCs), has been defined 26 . Together with macrophages, this population is frequently recruited into tumours after ionizing radiation exposure and less radiosensitive than other lymphocyte subsets27–30. Complementary approaches to detect cell apoptosis may include evaluation of several cellular aspects, such as caspase activation, cell morphology and mitochondrial status31. If apoptosis was occurring in irradiated macrophages, effector caspases, those responsible for the apoptosis execution phase, such as caspase-7, should be functionally activated, this is proteolytically cleaved 31 . Therefore, caspase-7 expression was evaluated by western blot analysis, using an antibody which detects both the full length caspase-7 (35 kDa) and the large fragment of cleaved caspase-7 (20 kDa) (Fig.1C). We demonstrated that there was no caspase-7 cleavage in irradiated macrophages. Additionally and focusing on the strongest cumulative dose (10 Gy) (6 h time point) (Supplementary Fig. S1), we confirmed, by using a positive control, that caspase-7 was not being cleaved in irradiated macrophages. At the same time, we also evaluated the cleavage of caspase-3, which is is a critical executioner of apoptosis31, and the cleavage of PARP, a substrate of several cell death proteases 32 , including caspase-3 (Supplementary Fig. S1). The antibody which detects the full length caspase-3 (35 kDa) also recognizes the large fragment of cleaved caspase-3 (17 kDa). Positive controls for caspase-3 and -7 activation as well as for PARP cleavage are presented in this western blot panel. As demonstrated by membrane overexposure, no cleavage of caspase-3/– 7 nor even of PARP were observed in irradiated macrophages, suggesting that apoptosis was not occurring. In order to identify apoptosis-related morphological features in macrophages after exposure to cumulative ionizing radiation doses, cells were observed and photographed after each irradiation dose. No apoptotic signs, such as cell shrinkage, pyknosis or loss of membrane integrity31, were observed in 10 Gy irradiated and non-irradiated macrophages, suggesting that both populations were similarly viable (Fig.1D). To complement previous observations, macrophage mitochondrial function was evaluated 20 h after exposure to 2, 6 and 10 Gy cumulative doses, using the resazurin reduction assay (Fig.1E). Results evidenced that, independently of the dose, ionizing radiation did not affect macrophage metabolic activity. www.nature.com/scientificreports/ 3 Scientific RepoRts | 6:18765 | DOI: 10.1038/srep18765 Figure 1. Irradiated human monocyte-derived macrophages are viable and metabolically active, despite DNA damage. (A) Radiation-induced macrophage DNA damage is demonstrated by immunocytochemistry for Ser139-phosphorylated H2AX (γ H2AX) (red), while nuclei were counterstained with DAPI (blue). Scale bar represents 20 μ m. Graph indicates the percentage of macrophages (n = 3 and 400 cells/donor counted) exhibiting γ H2AX foci. Data was analysed with paired t-test. **P < 0.01. (B) Western blot analysis of total and phosphorylated Chk2 (Thr68) expression on non-irradiated (− ) or irradiated (2, 6 and 10 Gy) (+ ) macrophages (n = 3), upon 1, 6 and 24 h. (C) Western blot analysis of caspase-7 expression on non-irradiated (− ) or irradiated (2, 6 and 10 Gy) (+ ) macrophages (n = 3), upon 1, 6 and 24 h. In all Western blots β -actin was used as loading control. (D) Brightfield microscopic images of non-irradiated (0 Gy) and irradiated macrophages (10 Gy). Scale bar represents 100 μ m. (E) Quantification of the metabolic activity of irradiated macrophages (2, 6 or 10 Gy) (n = 8), normalized to the activity of non-irradiated ones, and expressed as percentage. One-sample t-test was performed. www.nature.com/scientificreports/ 4 Scientific RepoRts | 6:18765 | DOI: 10.1038/srep18765 Overall, we concluded that irradiated macrophages are viable and metabolically active, and do not activate apoptosis, despite radiation-induced DNA damage. Ionizing radiation activates macrophage pro-survival and NF-κB signalling pathway. Considering macrophage survival following ionizing radiation exposure, a typical radiation-induced survival pathway mediated by NF-κ B33, was next investigated. Western blot analysis for the five NF-κ B family subunits, RelA, RelB, cRel, p52/p100 and p50/p105 (Fig.2A) as well as for Iκ Bα (Supplementary Fig. S2), a NF-κ B inhibitor, were performed. Densitometry analysis (Supplementary Fig. S3) confirmed that ionizing radiation increased the expression of RelB at every time points in every donors and tend to slightly increase cRel expression (Supplementary Fig. S3A). Additionally, ionizing radiation also tend to slightly increase p100 processing, as suggested by a reduced p52/p100 ratio at 2 and 6 Gy. No major alterations were found in phRel/RelA and p105/p50 levels. Moreover, alterations in RelB suggested that ionizing radiation upregulates macrophage NF-κ B, mainly through the non-canonical pathway. As RelB was the most consistently upregulated subunit in response to radiation, we next confirmed RelB nuclear translocation, by evaluating its expression, in nuclear and cytoplasmic extracts, 6 h after 10 Gy cumulative dose (Fig.2B). In fact, ionizing radiation increased RelB nuclear expression (Supplementary Fig. S3B), suggesting its nuclear translocation and subsequent activation. As NF-κ B can induce the expression of anti-apoptotic proteins, such as Bcl2 and Bcl-xL34, we evaluated the expression of these targets after macrophage irradiation (2, 6 and 10 Gy) (Fig.2C). The most relevant alteration was the Bcl-xL increased expression, particularly at 10 Gy (Supplementary Fig. S3C). The increase was visible at 1 h and sustained after 6 h, suggesting induction of macrophage pro-survival activity. Since Bcl-xL maximum expression tends to occur after 10 Gy, all subsequent experiments were performed with macrophages submitted to this cumulative ionizing radiation dose. Ionizing radiation induces a reduction in anti-inflammatory macrophage phenotype. To evaluate the effect of ionizing radiation on macrophage polarization profile, we characterized the pattern of expression of proor anti-inflammatory cytokines/chemokines and cell surface receptors, by a combination of quantitative Figure 2. Ionizing radiation induces macrophage NF-κB activation and increases Bcl-xL expression. (A) Evaluation of RelA phosphorylation (Ser536) and RelB, cRel, p100/p52 and p105/p50 subunit expression, 1 and 6 h after irradiation (2, 6 and 10 Gy). (B) RelB nuclear translocation 6 h after macrophage irradiation (10 Gy). Histone deacetylase 1 (HDAC1) and β -actin were used as loading controls for nuclear and cytoplasmic fractions, respectively. (C) Evaluation of Bcl2 and Bcl-xL expression after macrophage irradiation. Western blot images are representative of protein expression/phosphorylation status in distinct donors (at least n = 4), evaluated in two independent experiments. www.nature.com/scientificreports/ 5 Scientific RepoRts | 6:18765 | DOI: 10.1038/srep18765 real-time PCR, flow cytometry and ELISA. Ionizing radiation induced a significant increase (P < 0.05) of CD80 expression, but not of other pro-inflammatory gene markers. In fact, IL6 and CCL2 were significantly downregulated (P < 0.01) after macrophage irradiation (Fig.3A). Furthermore, all anti-inflammatory gene markers tested, CD163, MRC1 and VCAN were significantly downregulated (P < 0.05 or P < 0.01) upon irradiation (Fig.3A). To investigate how these changes could translate into cell surface receptor expression, flow cytometry was then performed. Results evidenced that ionizing radiation significantly increased HLA-DR (P < 0.01) and CD86 (P < 0.05) (macrophage pro-inflammatory markers), and tend to decrease CD163 (macrophage anti-inflammatory marker), but did not alter the expression of the monocyte/macrophage lineage marker CD14 (Fig.3B). To further complete the polarization profile analysis, levels of pro-(IL-6 and IL-12/IL-23(p40)) and anti-inflammatory (TGF-β 1 and IL-10) cytokines were evaluated by ELISA, using conditioned medium (CM) from non-irradiated (0 Gy) and irradiated (10 Gy) macrophages, normalized to protein concentration (Fig.3C). Interestingly, only IL-10 levels were significantly downregulated (P < 0.01) by ionizing radiation. IFN-γ and TNF-α levels were also investigated, but Figure 3. Irradiated macrophages present a reduced anti-inflammatory phenotype. (A) mRNA expression profile of pro-(CXCL8, CD80, IL1B, TNF, CCR7, CCL2, IL6) and anti-inflammatory (CD163, MRC1, VCAN) macrophage markers, by real-time PCR, 20 h after 10 Gy. Graphs represent mRNA expression of irradiated macrophages compared to non-irradiated ones (dotted line) (at least n = 7 per marker). β -actin was used as housekeeping gene. Wilcoxon signed rank test was used to compare the median of each dataset against a hypothetical median value of 1. (B) Expression of a monocyte/macrophage lineage (CD14), pro-(HLA-DR and CD86) and anti-inflammatory (CD163) macrophage markers was determined, by flow cytometry, 20 h after irradiation (at least n = 6 per marker). Paired t-test was used for statistical analysis. (C) Levels of pro-(IL-6, IL-12/IL-23(p40)) and anti-inflammatory (TGF-β 1 and IL-10) cytokines were determined in macrophage CM (n = 9) by ELISA, 20 h after irradiation. Data was normalized to protein concentration. Wilcoxon matched pair test was used for statistical analysis. *P < 0.05, **P < 0.01. Median is represented by the horizontal line inside the box plots, while the average is indicated by a “+ ”. In IL-6 and IL-10 graphics outliers are also indicated. www.nature.com/scientificreports/ 6 Scientific RepoRts | 6:18765 | DOI: 10.1038/srep18765 were undetectable for both macrophage populations. Altogether, this data suggests that ionizing radiation directs macrophages towards a reduced anti-inflammatory phenotype, without achieving however a complete classical pro-inflammatory profile. Upon exogenous stimulation, irradiated macrophages polarize towards proor anti-inflammatory phenotypes. Unstimulated M-CSF differentiated macrophages typically exhibit a more anti-inflammatory-like phenotype, but they are able to polarize towards a proor an anti-inflammatory phenotype, after proper exogenous stimulation35,36. The functional response to external stimuli, termed plasticity, is one of the major hallmarks of the mononuclear phagocyte system37. To assess how ionizing radiation affects macrophage plasticity, we evaluated whether irradiated macrophages were still able to polarize towards a proor an anti-inflammatory phenotype, upon proper exogenous stimulation. Therefore, upon 10 Gy cumulative dose exposure, macrophages were stimulated for 20 h with LPS (100 ng/mL) and IFN-γ (20 ng/mL), towards a pro-inflammatory phenotype, or with M-CSF (10 ng/mL) and IL-10 (20 ng/mL), towards an anti-inflammatory one. The expression of proand anti-inflammatory cell surface markers and cytokines/chemokines was evaluated by flow cytometry (Fig.4A) and ELISA (Fig.4B), respectively. Comparison between exogenously stimulated (LPS/IFN-γ or M-CSF/IL-10) non-irradiated (0 Gy) and irradiated (10 Gy) macrophages indicated that ionizing radiation did not affect macrophage ability to polarize towards a proor an anti-inflammatory phenotype, upon proper exogenous stimuli. LPS/IFN-γ -stimulated macrophages (both non-irradiated and irradiated) present higher levels of pro-inflammatory markers such as HLA-DR, CD86, TNF-α , IL-6 and IL-12/IL-23 (p40), being the last four significantly different (from P < 0.05 to P < 0.001), when compared to M-CSF/IL-10-stimulated macrophages (Fig.4A,B). On the other side, LPS/IFN-γ -stimulated macrophages tend to present lower levels of the anti-inflammatory marker CD163, comparing to M-CSF/IL-10-stimulated macrophages (Fig.4A). Our results evidenced that, contrary to expectations, the pro-inflammatory stimuli (LPS/IFN-γ ) also induced higher levels of immunosuppressive IL-10 cytokine than the anti-inflammatory ones (M-CSF/IL-10). However, this phenomenon was also previously reported in M-CSF-conditioned dendritic cell precursors, which exhibit a rapid IL-10 release upon LPS stimulation and may therefore participate in the modulation of inflammation and immune response38. Figure 4. Upon exogenous stimulation, irradiated macrophages polarize towards a proor an antiinflammatory phenotype. Non-irradiated and 10 Gy irradiated macrophages (n = 6) were stimulated, for 20 h, with LPS (100 ng/ml) and IFN-γ (20 ng/ml) towards a pro-inflammatory, or with M-CSF (10 ng/ml) and IL-10 (20 ng/ml) towards an anti-inflammatory phenotype. (A) Expression of monocyte/macrophage lineage (CD14), pro-(HLA-DR and CD86) and anti-inflammatory (CD163) macrophage markers was determined by flow cytometry. (B) Macrophage CM levels of pro-(TNF-α , IL-6, IL-12/IL-23(p40)) and anti-inflammatory (TGF-β 1 and IL-10) cytokines were determined by ELISA. Data was normalized to protein concentration. Paired t-test and one-way ANOVA were used for statistical analysis. *P < 0.05, **P < 0.01, ***P < 0.001. Median is represented by the horizontal line inside the box plots, while the average is indicated by a “+ ”. www.nature.com/scientificreports/ 7 Scientific RepoRts | 6:18765 | DOI: 10.1038/srep18765 Interestingly, irradiated macrophages stimulated with LPS/IFN-γ presented significantly (P < 0.05) higher levels of HLA-DR, when compared to their non-irradiated counterparts (Fig.4A). On the other hand, under stimulation with M-CSF/IL-10, irradiated macrophages tend to exhibit lower CD163 and higher HLA-DR levels, when compared to non-irradiated macrophages (Fig.4A,B). Also, irradiated macrophages polarized towards a proor an anti-inflammatory phenotype presented a significant decrease (P < 0.05) in IL-10 levels, when compared to non-irradiated and also exogenous stimulated ones (Fig.4B). Altogether, these results demonstrate that, upon exogenous stimulation, irradiated macrophages are still able to polarize towards a proor an anti-inflammatory phenotype. However, ionizing radiation by itself tends to promote the pro-inflammatory phenotype. Ionizing radiation induces macrophage morphological alterations and increases their phagocytic rate, without altering MMP-2 and MMP-9 activities. In order to characterize morphological changes, macrophage area and aspect ratio were determined 1 h after exposure to 10 Gy (Fig.5A). Results indicated that irradiated macrophages presented a significantly higher (P < 0.05) cell area (2,471 ± 513.3 μ m2) and tend to increase cell aspect ratio (quotient between major and minor cell axes) (2.38 ± 0.63), when compared to non-irradiated ones (area 2,047 ± 616.3 μ m2 and aspect ratio 1.72 ± 0.24). Besides functional and morphological plasticity, other features like phagocytic ability and proteolysis are also characteristic of macrophages39. To evaluate the effect of ionizing radiation on macrophage phagocytic ability, non-irradiated and 10 Gy irradiated macrophages were incubated, for 1 h, with FITC-labelled and killed Staphylococcus (S.) aureus particles. Results revealed that ionizing radiation significantly increased (P < 0.01) the percentage of macrophages able to phagocyte S. aureus particles (Fig.5B). The impact of ionizing radiation on the activity of MMP-2 and MMP-9, two matrix metalloproteases involved in the promotion of cancer cell invasion and angiogenesis 40 , was determined by gelatin zymography. Using 1 μ g of protein from macrophage CM, no differences in pro-MMP-9 proteolytic activity were found, between non-irradiated and 10 Gy irradiated macrophages (Fig.5C). The same conclusion was extended to MMP-9 and pro-MMP-2 pro - teolytic bands, when 15 μ g of macrophage CM protein were loaded. Our results evidence that ionizing radiation induces macrophage morphologic alterations and increases their phagocytic rate, without affecting macrophage MMP-2 and MMP-9 proteolytic activities. Irradiated macrophages promote cancer cell invasion and angiogenesis. To investigate the effect of ionizing radiation on macrophage pro-invasive cancer cell activity, non-irradiated or 10 Gy irradiated macrophages and irradiated colorectal cancer cells were confronted on Matrigel invasion assays (Fig.6A). Irradiated as non-irradiated macrophages significantly promoted (P < 0.001 or P < 0.01) the invasion of irradiated RKO Figure 5. Ionizing radiation increases macrophage area, aspect ratio and phagocytic rate, but does not alter MMP-2 and -9 activities. (A) Actin (green) and tubulin (red) stainings of non-irradiated and 10 Gy irradiated macrophages (n = 4). Quantification of cell area and aspect ratio was performed using Fiji software. Scale bar indicates 20 μ m. (B) Phagocytic ability of non-irradiated and 10 Gy-irradiated macrophages (n = 5) was determined after 1 h incubation with FITC-labelled (green) Staphylococcus aureus particles. F-actin was stained with rhodamine phalloidin (red). The percentage of macrophages able to phagocyte S.aureus particles was quantified using Fiji software. Scale bar indicates 20 μ m. (C) MMP-2 and -9 activity was evaluated by gelatin zymography, using 1 and 15 μ g of protein from CM of non-irradiated and 10 Gy-irradiated macrophages (n = 10). White bands of proteolytic activity were revealed on a Coomassie Blue-stained gelatin gel. All data was analysed with paired t-test. **P < 0.01, ***P < 0.001. Median is represented by the horizontal line inside the box plots, while the average is indicated by a “+ ”. www.nature.com/scientificreports/ 8 Scientific RepoRts | 6:18765 | DOI: 10.1038/srep18765 cells. To clarify if macrophage pro-invasive ability was dependent on the presence of irradiated colorectal cancer cells, we used non-irradiated RKO cells. Herein, we verified the same effect previously observed indicating that irradiated macrophages do not lack the ability to promote cancer cell invasion. Moreover, we also observed that irradiation of RKO cells alone significantly decrease (P < 0.05) its invasive potential. We could indeed predict that RKO cell invasive potential was going to be highly affected by radiation exposure due to the RKO intrinsic radiosensitivity, previously reported in the literature. An accumulation of about 70% of RKO cells was described in G2/M-phase 16 ± 24 h after 12 Gy irradiation in pH 7.5 medium41. RKO cells were also included in a group of radiosensitive cells, upon evaluation of the clonogenic survival of 27 human tumour cell lines after ionizing radiation exposure42. In order to investigate the effect of irradiated macrophages on cancer cell-induced angiogenesis, a chick embryo chorioallantoic membrane (CAM) assay was performed. CM from both non-irradiated and 10 Gy irradiated macrophages significantly promoted (P < 0.05) RKO-induced angiogenesis (Fig.6B). No differences were observed between RKO cells treated with CM derived from non-irradiated and irradiated macrophages. Altogether, Matrigel invasion and CAM-based assays demonstrated that, in the present experimental context, ionizing radiation per se was not able to restrain macrophages’ endogenous ability to promote cancer cell invasion and cancer cell–induced angiogenesis, which constitute two main hallmarks of cancer3. Discussion The present work aims to understand the effect of ionizing radiation on human macrophages, as they are important components of the tumour microenvironment, and also highly recruited into tumours during radiation therapy 43 . The recurrent use of mouse models and lack of clinically relevant doses in other studies have not allowed a full understanding of this effect16–20. In the present study, we characterized, for the first time, the response of human primary macrophages to cumulative ionizing radiation doses, using the same fractionated scheme as used during Figure 6. Ionizing radiation does not affect macrophage ability to promote cancer cell invasion and cancer cell-mediated angiogenesis. (A) Matrigel invasion assays were established confronting RKO cells (upper compartment) and macrophages (lower compartment) after being separately exposed or not to 10 Gy cumulative ionizing radiation dose. The six possible combinations are represented in the scheme. Invasive cells were counterstained with DAPI and counted on the microscope. (B) RKO cells were inoculated, for 72 h, with CM from non-irradiated or 10 Gy irradiated macrophages, in rings (inoculation areas), on top of CAM. The comparison of RKO+ CM Mac 0 Gy versus RKO+ CM Mac 10 Gy was evaluated in two rings within the same fertilized egg (n = 18), while the condition RKO+ RPMI was performed in a single distinct egg (n = 16). Analysis of RKO-induced angiogenesis was performed through quantification of the number of new vessels in control and experimental conditions. ANOVA analysis demonstrated a significant difference between groups. *P < 0.05, **P < 0.01, ***P < 0.001. The median is represented by the horizontal line inside the box plots, while the average is indicated by a “+ ”. www.nature.com/scientificreports/ 9 Scientific RepoRts | 6:18765 | DOI: 10.1038/srep18765 cancer patients’ treatment (2 Gy/fraction/day), up to 10 Gy cumulative dose. As a model, we used M-CSF-cultured macrophages, differentiated from peripheral-blood monocytes, as it is considered the predominant in vitro system to study human tissue macrophages44. M-CSF is a growth factor involved in the recruitment of monocytes/ macrophages to tissues and also in the regulation of macrophage function within tumours45. Taking into account that the cellular radiation response is a complex process 46 , several features like DNA damage, NF-κ B signalling pathway, polarization profile, plasticity, phagocytosis, proteolysis and the ability to promote cancer cell activities, were evaluated in irradiated human macrophages. Macrophage resistance to ionizing radiation was first reported in mouse models some decades ago47,48. Nowadays, it is recognized that human macrophages, similarly to regulatory T cells (Tregs), dendritic cells, Natural Killer (NK) cells and thrombocytes, as well as MDSCs, display a more radiation resistant phenotype than other immune cell populations, such as monocytes 27,29,30,46 . However, most of the studies were performed in mouse macrophages irradiated with X-ray doses, which barely mimic the fractionated scheme used in cancer patients’ treatment. Our results demonstrated that irradiated macrophages exhibited higher DNA damage, confirmed through increased H2AX phosphorylation, than non-irradiated ones. DNA damage induced by ionizing radiation is known to lead to Chk2-specific phosphorylation (Thr68) at sites of DSBs 49 . In agreement, we demonstrated that Chk2 phosphorylation increased along time and according to the exposure doses. Despite DNA damage, irradiated macrophages remain viable and metabolically active, leading to understand which survival pathways were activated. We focused on NF-κ B signalling, which is known to induce a pro-survival response in cells exposed to single 2 Gy doses 33,50 . Our results demonstrated that RelB expression was consistently increased after macrophage exposure to 2, 6 and 10 Gy cumulative doses. RelB nuclear translocation suggested the involvement of the non-canonical NF-κ B pathway in radiation-induced macrophage response. Our data also demonstrated an increased expression of the anti-apoptotic Bcl-xL protein, which also contributes to the promotion of pro-survival activity. Interestingly, a recent study demonstrated that ionizing radiation induces RelB to activate Bcl-xL in cancer cells51. NF-κ B transcription factors are not only important for cell survival upon irradiation, but are also considered major regulators of inflammation processes and, particularly in macrophages, their activation is required for the antito pro-inflammatory phenotype transition52–54. Radiation-induced NF-κ B alterations in macrophages led to the characterization of macrophage inflammatory status upon irradiation, which data was summarized in the scheme of Fig.7. In the present study, 10 Gy cumulative ionizing radiation dose significantly decreased both anti-inflammatory gene markers (CD163, MRC1, VCAN) and the immunosuppressive cytokine IL-10, and increased HLA-DR and CD86 expression of M-CSF differentiated macrophages (Fig.7). Additionally, we also demonstrated that bacterial phagocytosis, a classical feature of pro-inflammatory macrophages, was found to be significantly increased in irradiated macrophages. In fact, mouse macrophages subjected to 8 Gy single dose were also described to slightly enhance phagocytosis of inert latex beads55. Together, our data supports the hypothesis that ionizing radiation may drive macrophages towards a pro-inflammatory phenotype. Although irradiated macrophages exhibited increased CD80 expression, other classical pro-inflammatory markers, like IL1B, TNF and IL6 were unaltered, downregulated, or undetectable at the cytokine level. This suggests that irradiated macrophages did not reach a classical pro-inflammatory phenotype, despite the observed reduction of their original anti-inflammatory-like phenotype. The mechanism behind this shift in macrophage phenotype may indeed rely on radiation-induced NF-κ B alterations, particularly in RelB subunit as its expression is increased in irradiated macrophages. In fact, very recently, a switch of anti-inflammatory to pro-inflammatory macrophages was found to be directly mediated by RelB induction in M-CSF and TNF-stimulated osteoclast precursors56. However, further experiments are still required to confirm RelB involvement in macrophage anti-inflammatory phenotype reduction, suggested to occur after macrophage ionizing radiation exposure. To simplify the understanding of our hypothesis, we represented the typical proand anti-inflammatory macrophage profiles as two extremes of a continuous polarization spectrum (generally represented as a line). However, macrophage polarization has been revealed as a very complex and dynamic system and new representations are emerging as long as new knowledge in this area is coming out. Recently, Ruffell and Coussens summarized the macrophage polarization system as a circle, which does not consider proand anti-inflammatory macrophage profiles, but rather emphasize that macrophage functional roles (angiogenesis, cytotoxicity, stimulation, suppression or chemotaxis) (included in an inner circle) are dictated by the integration of multiple stimuli (represented in an outer circle) 57 . Therefore, we may speculate that ionizing radiation exposure could also constitute one of the distinct stimuli capable of polarizing macrophages into a different phenotype, which does not completely corresponds to a proor an anti-inflammatory one, but could rather include features from both phenotypes, as we demonstrated in this work, or even from other phenotypes still to be defined. Overall, our study defines, for the first time, a molecular profile for human macrophages subjected to cumulative ionizing radiation doses, emphasizing the important role of fractionated ionizing radiation doses, as used in radiotherapy, to direct macrophages towards a pro-inflammatory phenotype, which is recognized to be tumour cytotoxic37. According to the literature, the relation between ionizing radiation exposure and inflammatory response seems to be dependent not only on the cell type analysed and radiation quality, but mainly on the delivered dose58. Low doses (maximum of 12 Gy at ≤ 1.0 Gy/fraction), usually applied in non-malignant disorders or received by normal tissues outside the tumour target volume, induce an anti-inflammatory phenotype, while higher doses (single doses ≥ 2 Gy, total doses ≥ 40 Gy) are reported to have a pro-inflammatory effect59,60. Most of the studies, aiming to reveal the role of irradiation on macrophage inflammatory status, are performed using in vitro or ex vivo mouse macrophages. Although mouse models have widely contributed to our understanding of ionizing radiation-induced effect on macrophages, it is also well recognized that mouse and human macrophages present many distinct features 61 . Particularly, mouse macrophages are high producers of nitric oxide (NO) and L-citrulline from L-arginine, via inducible nitric oxide synthase (iNOS) activation, while NOS and arginase activities in human macrophages are quite debatable62,63. These considerations should be taken into account when extrapolating data from one species to another.