Radiation Dose Escalation Is Crucial in Anti-CTLA-4 Antibody Therapy to Enhance Local and Distant Antitumor Effect in Murine Osteosarcoma

Abstract

We previously reported that a combination of 10 Gy of X-ray irradiation and dual immune checkpoint blockade with anti-CTLA-4 (C4) and anti-PD-L1 antibodies produced a significant shrinkage of irradiated and unirradiated tumors (abscopal effect) and prolonged overall survival. However, the optimal radiation delivery regimen combined with single immune checkpoint blockade of C4 for inducing a maximum systemic antitumor response still remains unclear, particularly for patients with osteosarcoma. We used syngeneic C3H mice that were subcutaneously injected with LM8 osteosarcoma cells into both legs. C4 was administered three times, and one side of the tumor was irradiated by X-ray beams. The optimal radiation dose required to induce the abscopal effect was explored with a focus on the induction of the type-I interferon pathway. Radiation delivered in a single fraction of 10 Gy, 4.5 Gy × 3 fractions (fx), and 2 Gy × 8 fx with C4 failed to produce significant inhibition of unirradiated tumor growth compared with monotherapy with C4. Dose escalation to 16 Gy in a single fraction, or the equivalent hypofractionated dose of 8 Gy × 3 fx, which significantly increased secretion of IFN-β in vitro, produced a dramatic regression of both irradiated and unirradiated tumors and prolonged overall survival in combination with C4. Furthermore, irradiation at 16 Gy in both a single fraction and 8 Gy × 3 fx diminished regulatory T cells in the unirradiated tumor microenvironment. These results suggest that total dose escalation of radiation is crucial in C4 therapy to enhance the antitumor response in both local and distant tumors and prolonged overall survival regardless of fractionation for osteosarcoma.

Comparative Analysis of the Antitumor Immune Profiles of Paired Radiotherapy-naive and Radiotherapy-treated Cervical Cancer Tissues

“…Importantly, Tregs surviving 10 Gy irradiation had a weakened ability to suppress CD8 + T-cells. Similarly, Takenaka et al reported that a single dose of 10 Gy X-rays, or 24 Gy delivered in three fractions, reduced Treg numbers in tumor tissue in a tumor transplant mouse model (28). These studies suggest that our results may be due to a reduction of CTLA-4 expressing Tregs in response to X-rays.…”

The Immune Landscape of Osteosarcoma: Implications for Prognosis and Treatment Response

“…Using the same OS mouse model, Takenada and colleagues showed that the combination of high-dose local radiation and anti-CTLA4 antibody administration is able to halt tumor growth not only of the treated lesion, but also of a contralateral, untreated tumor (abscopal effect). Also in this work, treatment was able to severely reduce the number of FOXP3 + Tregs in both treated and untreated lesion [ 42 ]. Another T cell sub-population that has been implicated in tumor progression, although with controversial data, is the T helper 17 (Th17) subset [ 43 ].…”

Radiation therapy enhances systemic antitumor efficacy in PD-L1 therapy regardless of sequence of radiation in murine osteosarcoma

“…Further analysis revealed that both P1+pre-Rad and P1+conc-Rad regimens successfully suppressed tumor growth not only in local, but also distant tumors. However, only 2 (15%) and 1 (5.9%) mice in P1+pre-Rad and P1+conc-Rad groups, respectively, survived for more than 50 days after the initial treatment, whereas our previous studies demonstrated that 3 of 7 (42.9%) mice in P1C4 with radiation therapy at 10 Gy [ 20 ] and more than 30% in C4 with radiation at 16 Gy or 8 Gy × 3 fraction groups [ 39 ] survived for more than 50 days after the initial treatment. Therefore, further improvement in therapeutic efficacies may be achieved by using escalated dose to a local tumor.…”