Radiotherapy and Cisplatin Increase Immunotherapy Efficacy by Enabling Local and Systemic Intratumoral T-cell Activity

To increase cancer immunotherapy success, PD-1 blockade must be combined with rationally selected treatments. Here, we examined, in a poorly immunogenic mouse breast cancer model, the potential of antibody-based immunomodulation and conventional anticancer treatments to collaborate with anti-PD-1 treatment. One requirement to improve anti-PD-1-mediated tumor control was to promote tumorspecific cytotoxic T-cell (CTL) priming, which was achieved by stimulating the CD137 costimulatory receptor. A second requirement was to overrule PD-1-unrelated mechanisms of CTL suppression in the tumor microenvironment (TME). This was achieved by radiotherapy and cisplatin treatment. In the context of CD137/PD-1-targeting immunotherapy, radiotherapy allowed for tumor elimination by altering the TME, rather than intrinsic CTL functionality. Combining this radioimmunotherapy regimen with low-dose cisplatin improved CTL-dependent regression of a contralateral tumor outside the radiation field. Thus, systemic tumor control may be achieved by combining immunotherapy protocols that promote T-cell priming with (chemo) radiation protocols that permit CTL activity in both the irradiated tumor and (occult) metastases.