Radioimmunotherapy with carbon ions plus CTLA4 blockage elicits potent local and abscopal immune effects in a preclinical HER2+ breast cancer model. Mus musculus domesticus breed:C57BL6

Carbon ion radiotherapy has shown superior biophysical characteristics over conventional photon irradiation. However, immunological effects and ideal checkpoint inhibitors for radioimmunotherapy remain elusive. Here, we demonstrate that aCTLA4- but not aPD-L1-based radioimmunotherapy induces complete rejection in a murine breast cancer model (EO771). Intriguingly, efficient local tumor control led to an effective in-situ immunization and consequently eradication of non-irradiated distant tumors. Moreover, cured mice were protected against rechallenge with EO771 cells indicating long-lasting protective immunological memory. Deconvolution of immunological effects at single cell resolution revealed i) predominating myeloid cells in irradiated tumors developing into distinct tumor-associated macrophage clusters with upregulated expression of TNF and IL1 responsive genes; ii) activated NK cells in irradiated and iii) activated T cells in non-irradiated tumors. Therefore, in addition to well-known high precision of particle therapy with carbon ions, this therapy could be well combined with aCTLA4 for induction of potent and enduring systemic anti-tumor immunity