Carbon ion and photon radiotherapy show enhanced anti-tumoral therapeutic efficacy with neoantigen RNA-LPX vaccines in preclinical colon carcinoma models

Personalized liposome-formulated mRNA vaccines (RNA-LPX) are a new powerful tool in cancer immunotherapy. In preclinical tumor models, RNA-LPX are known to achieve potent results when combined together with conventional X-ray radiotherapy (XRT). Densely ionizing radiation used in carbon-ion radiotherapy (CIRT) may induce distinct effects in combination with immunotherapy, compared to sparsely ionizing X-rays. Within this study, we investigate the potential of CIRT and isoeffective doses of XRT to mediate tumor growth inhibition and survival in murine colon adenocarcinoma models in conjunction with neoantigen (neoAg)-specific RNA-LPX vaccines, encoding both MHC class I and II-restricted tumor-specific neoantigens. We characterize tumor immune infiltrates and antigen-specific T cell responses by flow cytometry and IFNγ ELISpot respectively. RNA-LPX vaccines significantly potentiate radiotherapy-mediated tumor growth inhibition. CIRT and XRT alone marginally prime neoAg-specific T cell responses in the tumor, but not in the blood or spleen of the mice. The infiltration and cytotoxicity of neoAg-specific T cells is strongly driven by RNA-LPX vaccines and accompanied by reduced expression of the inhibitory markers PD-1 and Tim-3 on these cells. NeoAg RNA-LPX vaccines shows similar overall therapeutic efficacy in combination with both CIRT and XRT, even if the physical radiation dose is lower for carbon ions than X-rays. We hence conclude that the combination of CIRT and neoAg RNA-LPX is a promising strategy for treatment of radioresistant tumors.