The impact of radiation therapy on the antitumor immunity: local effects and systemic consequences

The main antitumor efficacy of irradiation relies in its direct cytotoxic effect. Increasing evidence indicates a systemic effect of radiation though, mediated mainly by the immune system. In this review we wish to focus on the radiotherapy induced modifications of the soluble and cellular mediators of the antitumor immune response and summarize some of the mechanisms by which radiation driven local and systemic bystander effects can influence tumor immunogenicity. In different tumor types due to the intrinsic immunogenicity of the tumor cells and the immunological characteristics of the tumor microenvironment, different radiation induced immune modulatory mechanisms are predominant. Radiation most probably can only amplify or augment a pro-immunogenic phenotype and can hardly change by itself a net immune suppressing environment into an immune stimulating one. This immune modulatory potential of radiotherapy could be exploited in tumor treatment by developing combined radiotherapeutic and immunotherapeutic approaches. The last few years showed a dramatic increase in the knowledge of radiation induced out-of field and systemic effects, which foresees a rapid progress in the development and clinical application of these new, combined therapies for cancer cure.