Immunocytokines
The immunosuppressive microenvironment is the target of cytokine-based cancer immunotherapies. Allowing the T cells to invade the tumour, polarising them into an antitumour Th1 phenotype, and prompting their clonal expansion and cytolytic activity might be achieved by targeting cytokines to the tumour [36, 37]. Interleukin (IL)-2 has been used successfully in advanced melanoma over recent decades [38, 39]. The major drawback is that, depending on the exact concentration, IL-2 might either foster clonal expansion of antitumour T cells or might even reinforce immunosuppression by enhancing the number and function of regulatory T (Treg) cells [40]. IL-12 is more specific for prompting Th1 responses and has also been tested in clinical trials showing good antitumour effects, yet the toxicities, including patient deaths, stopped further development [41]. Immunocytokines are fusion proteins of cytokines attached to (a part) of tumour-targeting antibodies. These constructs have been tested preclinically and clinically in lymphomas and other solid tumours [42, 43].
Other Immunotherapy Strategies
Among various other immunotherapy strategies, the following have been chosen exemplarily as they have already been introduced to clinical trials in combination with radiotherapy. As TGFβ is known to be immunosuppressive and is induced in irradiated tissue (mostly studied in the context of radiation-induced fibrosis [44]), inhibition of TGFβ is an additional strategy to overcome intratumoural immunosuppression. Blocking of TGFβ by fresolimumab has shown initial signs of antitumour activity in a phase I study [45]. In analogy to immune checkpoint inhibition blocking negative regulators of T cell activation, the targeting of immunostimulatory molecules on T cells might enhance effector functions. CD40, 4–1BB, and OX40 are part of a class of molecules called the tumour necrosis factor receptor family. As reviewed by Moran et al. [46], they are targets for immunostimulatory drugs to enhance antitumour immunity. In addition to targeting T cells, strategies have been developed to enhance antigen presentation by dendritic cells such as toll-like receptor agonists [47].
Challenges in Immunotherapy Development
The development of cancer immunotherapies poses different challenges. Most in vivo chemotherapy and radiotherapy research has been performed with human xenografts in immunodeficient animals or in syngeneic models of mouse tumours. For species-specific immunotherapies, these models are not suitable for preclinical evaluation. In addition, mechanisms of the combination of radiotherapy and immunotherapy might be different to the mechanisms of radiosensitisation through chemotherapy. Th1 cytokines, for example, have been described to induce senescence in cancer cells as a mechanism of response to therapy [48]. In the clinical setting, responses of immunotherapy show slower kinetics than responses to conventional cancer treatments. Pseudoprogression has been observed shortly after the start of treatment [49]. Clinical use of checkpoint inhibitors has revealed a novel spectrum of side effects attributed to autoimmune phenomena, including hypophysitis and colitis [50].
Immunological Effects of Radiation, Rationale for Combination Therapies
Local tumour irradiation itself alters the tumour microenvironment and may support antitumour immunity, especially in combination with immunotherapy [51]. The direct cell kill by irradiation exposes a variety of tumour-associated antigens and leads to a release of danger-associated molecular patterns, or DAMPs, activating the innate immunity and thus paving the way for effective tumour eradication [52–55]. Surviving tumour cells show upregulation of MHC-I and NKG2D [56] and become more vulnerable to attack by T cells. In addition, the release of proinflammatory cytokines triggered by tissue damage through irradiation may polarise the immune response to a Th1 type, which also helps to support antitumour immunity. Primed and activated T cells will enter irradiated tumours more easily than nonirradiated tumours, for example due to increased expression of VCAM-1 in the tumour vasculature and the altered cytokine milieu [57, 58]. In in vivo models with more than 1 tumour in 1 animal, the combination of radiation and immunotherapy has been described to elicit “abscopal responses,” with tumour shrinkage not only in the irradiated tumour but also in non-irradiated lesions in the same animal [59, 60]. This effect has also been observed in the clinic [61–63]. However, irradiation alone only rarely leads to antitumour immune effects in patients, and abscopal effects have only been described in a few case reports. This might be explained by the upregulation of immunosuppressive mechanisms, like the induction of Treg cells and a possible Th2 polarisation after irradiation [64, 65]. Tumours might also be able to upregulate PD-L1 after irradiation [66].
Combination Concepts
Combination of Irradiation and Immune Checkpoint Inhibition
As immune checkpoint inhibition is the most advanced modality for cancer immunotherapy, the combination with irradiation has been evaluated in a number of preclinical and clinical studies. CTLA-4 blockade showed enhanced local and distant control in combination with irradiation in in vivo models of breast cancer, colorectal cancer, and glioma [67–69]. Similar results have been reported for the combination of radiation with PD-1 blockade [70, 71]. However, the first clinical