Session Item

Localized radiotherapy (RT) can cause T cell-mediated abscopal effects on non-irradiated metastases, particularly in combination with immune checkpoint blockade (ICB). However, results of prospective clinical trials have not met the expectations. We study whether chemotherapeutics can enhance the abscopal effect. Oxaliplatin (Oxa) has been considered as immunogenic, but cisplatin (Cis) and carboplatin (Carbo) have not. We compared these three platinum derivatives in two different abscopal mouse models.

In mice bearing bilateral tumors, the primary tumor was irradiated with 2 × 12 Gy (B16 melanoma model) or 2 × 8 Gy (C51 colon carcinoma model); Cis, Carbo, or Oxa were given once together with RT; anti-PD1 was given weekly. Tumor growth and survival of mice were determined (5–15 mice per group). The dependence of the therapeutic effects on CD8+ T cells and on extracellular ATP (eATP) was determined by using T cell-depleting antibodies and PPADS (a P2 purinergic antagonist), respectively. Frequencies and functionality (differentiation and exhaustion state) of tumor-specific CD8+ T cells were determined by FACS using MHC tetramers and various antibodies. In vitro, chemosensitivity of the tumor cell lines and their production of extracellular ATP were determined by CellTiter-Glo^® 2.0. Ethidium bromide and 2′,3′-dideoxycytidine incubation was used to deplete mitochondrial DNA (mtDNA) of tumor cells.

The tumor control compared to RT/αPD-1 was as follows: RT/αPD-1/Cis (p < 0.01) > RT/αPD-1/Oxa (p < 0.01) >> RT/αPD-1/Carbo (p > 0.05) (B16 melanoma); RT/αPD-1/Cis (p < 0.001) ≈ RT/αPD-1/Carbo (p < 0.01) >> RT/αPD-1/Oxa (p > 0.05) (C51 model). Triple therapy with Cis resulted in complete abscopal regression in 7/15 and 8/9 mice bearing B16 or C51 tumors, respectively. Triple therapy with Cis was also significantly better than the Cis/αPD-1 double combination in both tumor models (B16 model, p < 0.01; C51 model, p < 0.05). In vitro, the chemosensitivity was as follows: Cis ≈ Oxa > Carbo (B16 cells); Cis ≈ Carbo > Oxa (C51 cells). In the C51 model, Cis induced more eATP and more cytosolic mtDNA leakage than Oxa. Cis-induced IFNb1 secretion was decreased in mtDNA-depleted compared to wild-type C51 cells. In the C51 model, triple therapy with Cis induced more mature dendritic cells (p < 0.05), more Ki67+ CD8+ T cells (p < 0.05), and more tumor-specific T cells (p < 0.01) compared to triple therapy with Oxa. The enhanced abscopal effect was abrogated when CD8⁺ T cells were depleted, extracellular ATP signaling was blocked, or mtDNA-depleted C51 cells were injected as primary (irradiated) tumor.

The chemosensitivity of tumor cells as well as the potency of chemotherapeutics to induce eATP and mtDNA leakage are crucial for the platinum-mediated enhancement of the abscopal effect. Our findings may be important for the planning of clinical trials of triple combinations of RT, chemotherapy, and ICB in metastatic patients.