Radiotherapy can act as an in situ
vaccine, activating preventive
tumor-specific immune responses in patients. Although
carbon ion radiotherapy has superior biophysical properties over conventional photon irradiation, the immunological effects induced by this radiation type are poorly understood. Multiple strategies combining
radiotherapy with
immune checkpoint inhibition (
radioimmunotherapy) to enhance antitumor immunity have been described; however, immune cell composition in
tumors following
radioimmunotherapy with
carbon ions remains poorly explored. We developed a bilateral
tumor model based on time-shifted
subcutaneous injection of murine Her2+ EO771
tumor cells into immune-competent mice followed by selective irradiation of the primary
tumor. αCTLA4-, but not αPD-L1-based
radioimmunotherapy, induced complete
tumor rejection and mediated the eradication of even non-irradiated, distant
tumors. Cured mice were protected against the EO771 rechallenge, indicating long-lasting,
tumor-specific immunological memory. Single-cell
RNA sequencing and flow cytometric analyses of irradiated
tumors revealed activation of NK cells and distinct tumor-associated macrophage clusters with upregulated expression of TNF and IL1 responsive genes. Distant
tumors in the irradiated mice showed higher frequencies of naïve T cells activated upon the combination with CTLA4 blockade. Thus,
radioimmunotherapy with
carbon ions plus CTLA4 inhibition reshapes the
tumor-infiltrating immune cell composition and can induce complete rejection even of non-irradiated
tumors. Our data suggest combining
radiotherapy approaches with CTLA4 blockade to achieve durable antitumor immunity. Evaluation of future
radioimmunotherapy approaches should not be restricted to immunological impact at the irradiation site but should also consider systemic immunological effects on non-irradiated
tumors.