In vitro-irradiated cancer vaccine enhances anti-tumor efficacy of radiotherapy and PD-L1 blockade in a syngeneic murine breast cancer model

Abstract

Background and Purpose

Local radiotherapy (RT) exerts immunostimulatory effects by inducing immunogenic cell death. However, it remains unknown whether in vitro–irradiated tumor cells can elicit anti-tumor responses and enhance the efficacy of local RT and immune checkpoint inhibitors when injected in vivo.

Methods and Materials

We tested the “in vitro–irradiated cancer vaccine (ICV)”, wherein tumor cells killed by varying doses of irradiation and their supernatants are intravenously injected. We examined the efficacy of combining local RT (24 Gy in three fractions), PD-L1 blockade, and the ICV in a murine breast cancer model. The immune cell profiles were analyzed via flow cytometry and immunohistochemistry. The cytokine levels were measured by multiplex immunoassays.

Results

The ICV significantly increased the effector memory phenotype and interferon-γ production capacity in splenic CD8⁺ T cells. The in vitro–irradiated products contained immune response-related molecules. When combined with local RT and PD-L1 blockade, the ICV significantly delayed the growth of irradiated and non-irradiated tumors. The triple combination therapy increased the proportions of CD8⁺ T cells and effector memory CD8⁺ T cells while decreasing the proportion of CTLA-4⁺ exhausted CD8⁺ T cells within tumor microenvironment. Additionally, plasma level of interferon-γ and proliferation of effector T cells in the spleen and tumor-draining lymph nodes were significantly increased by the triple combination therapy.

Conclusions

The ICV enhanced the therapeutic efficacy of local RT and PD-L1 blockade by augmenting anti-tumor immune responses. Our findings suggest a therapeutic potential of in vitro–irradiation products of tumor cells.