Session Item

Localized radiotherapy (RT) can cause a T cell-mediated abscopal effect on non-irradiated tumor lesions, especially in combination with immune checkpoint blockade (ICB). However, this effect is still clinically rare and improvements are highly desirable. We investigated whether triple combination with a low dose of clinically approved liposomal doxorubicin (Doxil) could augment abscopal responses compared with RT plus ICB and Doxil plus ICB.

We used Doxil in combination with RT and αPD-1 in two tumor models (B16-CD133 melanoma and MC38 colon carcinoma) with mice bearing two tumors, only one of which was irradiated.

We show in these two models that triple therapy with RT, αPD-1, and single low-dose Doxil strongly enhanced the RT-induced abscopal effect. Complete cures of non-irradiated tumors were mainly observed in triple-treated mice. Triple therapy induced more cross-presenting dendritic cells (DCs) and more tumor-specific CD8+ T cells than RT/αPD-1 and Doxil/αPD-1, particularly in non-irradiated tumors. CD8+ T cell depletion or implantation of STING-deficient tumor cells abolished the abscopal effect. cGAS/STING detects cytosolic DNA, triggering type I IFN expression, which drives antitumor CD8+ T cell responses through cross-presenting DCs. To date, it is not fully understood how doxorubicin/Doxil induces type I IFN and which nucleic acid species is critical for this. By using inhibitors and knockout cells, we show that doxorubicin/Doxil-induced IFNβ1 markedly depended on the cGAS/STING pathway. In mitochondrial DNA (mtDNA)-depleted tumor cells, doxorubicin/Doxil induced less secretion of IFNβ1, the related T cell-recruiting chemokine CXCL10, and ATP; coincubation with mtDNA-depleted tumor cells strongly reduced IFNβ1 secretion by DCs. Implantation of mtDNA-depleted tumor cells, particularly at the non-irradiated/abscopal site, substantially diminished the Doxil-enhanced abscopal effect and tumor infiltration by tumor-specific CD8+ T cells.

These data show that single low-dose Doxil can substantially enhance the RT-induced abscopal effect, with a strong increase in cross-presenting DCs and CD8+ tumor-specific T cells particularly in abscopal tumors compared with RT/αPD-1 and Doxil/αPD-1. Moreover, they indicate that the mtDNA/cGAS/STING/IFN-I axis is important for the immunogenic doxorubicin effects. Our findings may be helpful for the planning of clinical radiochemoimmunotherapy trials in (oligo)metastatic patients.