Leveraging radiotherapy to improve immunotherapy outcomes: rationale, progress and research priorities

The most successful immunotherapies for solid malignancies to date, immune checkpoint inhibitors, target the essential role of T cells in antitumor immunity. However, T-cell dysfunction presents a major hindrance to treatment efficacy, warranting research into combined treatment strategies for improving outcomes. The use of radiotherapy for this purpose has garnered much interest. Preclinical study has established that radiotherapy activates various immune mechanisms to improve T-cell activation, localisation and function within tumors, which improves response to immune checkpoint inhibitors. However, so far, these strategies have not been successfully translated into the clinic. Here, we briefly reflect on the development of immune checkpoint inhibitors and the mechanistic insights revealed by an evolving understanding of T-cell dysfunction in cancer, before providing an overview of the immunomodulatory effects of radiotherapy in the context of the T-cell-mediated antitumor immune response. We discuss the mixed results of clinical trials, comment on various factors that may preclude immuno-radiotherapy responses in the clinic, and highlight priorities for preclinical and clinical study. Finally, we discuss the role of emerging combinations of radiotherapy and immunotherapy to potentially provide additional treatment options and improve outcomes for patients.