M2 macrophages are more resistant than M1 macrophages following radiation therapy in the context of glioblastoma

In some highly inflammatory tumors, such as glioblastoma (GB), macrophages (MΦ) represent the most abundant population of reactive cells. MΦ, initially denoted as M0 MΦ, can be polarized into two further phenotypes: the antitumor M1 MΦ, and the protumor M2 MΦ. The three phenotypes can reside simultaneously in the tumor mass and various external factors may influence MΦ polarization. Radiotherapy is a common modality of cancer treatment aiming to target tumor cells. However, the specific effects of X-ray radiation on the inflammatory cells are, so far, controversial and not fully understood. In the present investigation, we have first analyzed, in vivo, the effect of X-ray radiation on MΦ present in GB tumors. We have observed a decrease in MΦ number paralleled by an increase in the proportion of M2 MΦ. To understand this phenomenon, we then evaluated, in vitro, the effects of X-rays on the MΦ phenotypes and survival. We have found that X-ray radiation failed to modify the phenotype of the different MΦ. However, M1 MΦ were more sensitive to ionizing radiation than M2 MΦ, both in normoxia and in hypoxia, which could explain the in vivo observations. To conclude, M2 MΦ are more radioresistant than M0 and M1 MΦ and the present study allows us to propose that X-ray radiotherapy could contribute, along with other phenomena, to the increased density in the protumor M2 MΦ in GB.

Abstract A30: Hypoxia induces macrophage polarization and re-education towards an M2 phenotype in glioblastoma

Introduction: Hypoxia, resulting in a decrease in tissue oxygen tension, is a main characteristic of glioblastoma (GBM) and is associated to a poor life expectancy of GBM patients (Spence et al., 2008). In the same time, it is now known that GBM are also inflammatory tumors and that macrophages (MΦ) are the most abundant immune cells found in these tumors (Badie et al., 2000). MΦ present in tissue can be polarized into at least two distinct phenotypes: the M1 phenotype (with antitumoral activity) and the M2 phenotype (with protumoral activity) (Mantovani et al., 2002), the most often phenotype found in patients with GBM (Lu-Emerson et al., 2013). However, factors responsible for their migration and phenotype acquisition in the context of GBM are not yet identified. The aim of this study was to evaluate, in vivo, the impact of hypoxia on MΦ tropism and polarization in a preclinical model of human GBM that progressively evolves towards severe hypoxia and, in vitro, the effect of hypoxia on MΦ polarization and re-education.

Method: Animal models consisted of injection of human GBM U251 cells into the brain of immunodeficient rats. Hypoxia has been evaluated by pimonidazole and CAIX staining. MΦ have been detected on brain slices through a CD68 staining and polarization toward M1 or M2 phenotype with iNOS and Arginase1 staining, respectively. In parallel, murine primary cultures of MΦ have been freshly prepared from bone marrow. M0, M1 and M2 MΦ have been exposed to moderate (1% oxygen) or severe (0.2% oxygen) hypoxia for 24h and their phenotypes have been characterized by RT-qPCR and enzymatic assays.

Results: In the U251 model, we have shown that MΦ migrated at the tumor site only when hypoxia takes place. We also demonstrated that the acquisition of the M2 phenotype was clearly a dynamic process with hypoxia as the major trigger for this transition. Furthermore, MΦ present at the shell of the tumor express M1 markers unlike MΦ found in hypoxic zones which exhibit M2 markers. In support of these in vivo findings, M0 but also M1 MΦ cultured in 1% or 0.2% oxygen displayed a phenotype close to that of M2 MΦ whose this phenotype was further reinforced by severe hypoxia.

Conclusion: In this study, using a human preclinical GBM model which progressively develops severe hypoxia, we showed that the presence of MΦ in the tumor depends on presence or not of hypoxia. We have also shown that hypoxia polarizes and re-educates MΦ towards M2 MΦ but also that it fine-tunes the M2 phenotype. These results highlight the effect of hypoxia in the aggressiveness of GBM and may help to improve therapies for these tumors by taking in account that hypoxia drives MΦ towards a protumoral phenotype.

Acknowledgements: Conseil Régional de Basse-Normandie, Université de Caen Basse-Normandie, Archade, CNRS, the European Regional Development Fund of Basse-Normandie (FEDER), “l'Europe s'engage en Basse Normandie” and the French Ministère de l'Enseignement Supérieur et de la Recherche (MESR).

Citation Format: Marine M. Leblond, Aurélie N. Gérault, Aurélien Corroyer-Dulmont, Edwige Petit, Myriam Bernaudin, Samuel Valable. Hypoxia induces macrophage polarization and re-education towards an M2 phenotype in glioblastoma. [abstract]. In: Proceedings of the AACR Special Conference: Advances in Brain Cancer Research; May 27-30, 2015; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2015;75(23 Suppl):Abstract nr A30.

Multimodal imaging based on MRI and PET reveals [(18)F]FLT PET as a specific and early indicator of treatment efficacy in a preclinical model of recurrent glioblastoma

PURPOSE

The primary objective of this study was to compare the ability of PET and MRI biomarkers to predict treatment efficacy in a preclinical model of recurrent glioblastoma multiforme.

METHODS

MRI (anatomical, diffusion, vasculature and oxygenation) and PET ([(18)F]FDG and [(18)F]FLT) parameters were obtained 3 days after the end of treatment and compared with late tumour growth and survival.

RESULTS

Early after tumour recurrence, no effect of treatment with temozolomide combined with bevacizumab was observed on tumour volume as assessed by T2-W MRI. At later times, the treatment decreased tumour volume and increased survival. Interestingly, at the earlier time, temozolomide + bevacizumab decreased [(18)F]FLT uptake, cerebral blood volume and oedema. [(18)F]FLT uptake, oedema and cerebral blood volume were correlated with overall survival but [(18)F]FLT uptake had the highest specificity and sensitivity for the early prediction of treatment efficacy.

CONCLUSION

The present investigation in a preclinical model of glioblastoma recurrence underscores the importance of multimodal imaging in the assessment of oedema, tumour vascular status and cell proliferation. Finally, [(18)F]FLT holds the greatest promise for the early assessment of treatment efficacy. These findings may translate clinically in that individualized treatment for recurrent glioma could be prescribed for patients selected after PET/MRI examinations.

Hypoxia induces macrophage polarization and re-education toward an M2 phenotype in U87 and U251 glioblastoma models

Hypoxia is a common feature of solid tumors, particularly in glioblastoma (GBM), and known to be a poor prognosis factor in GBM patients. The growth of GBM is also associated with a marked inflammation partially characterized by an accumulation of macrophage (MΦ) of the M2 phenotype. However, the transition between M1 MΦ (antitumoral) and M2 MΦ (protumoral) phenotypes is a dynamic process. We made the assumption that oxygen (O2) availability could be a major regulator of this transition and that the intratumoral O2 gradient is of importance. We evaluated, in vivo, the impact of hypoxia on MΦ tropism and polarization in two models of human GBM, well differentiated by their degree of hypoxia. MΦ migration in the tumor was more pronounced in the more hypoxic tumor of the two GBM models. In the more hypoxic of the models, we have shown that MΦ migrated at the tumor site only when hypoxia takes place. We also demonstrated that the acquisition of the M2 phenotype was clearly an evolving phenomenon with hypoxia as the major trigger for this transition. In support of these in vivo finding, M0 but also M1 MΦ cultured in moderate or severe hypoxia displayed a phenotype close to that of M2 MΦ whose phenotype was further reinforced by severe hypoxia. These results highlight the role of hypoxia in the aggressiveness of GBM, in part, by transforming MΦ such that a protumoral activity is expressed.