ESTRO 2024 - Abstract Book

S5183

Radiobiology - Immuno-radiobiology

ESTRO 2024

1 University of Oslo, Department of Physics, Oslo, Norway. 2 Aarhus University, Department of experimental Clinical Oncology, Aarhus, Denmark. 3 Arhus University Hospital, Danish Centre for Particle Therapy, Aarhus, Denmark. 4 University of Oslo, Institute of Oral Biology, Oslo, Norway

Purpose/Objective:

Immune checkpoint inhibitors have emerged as an efficient cancer treatment for some patients, but the effect appears to depend on individual cancer specific immunogenic responses. One way to induce such an immunogenic response is irradiation. Due to the difference in energy deposition, our hypothesis is that proton irradiation is superior to X-rays in inducing an immunogenic response. However, radiation can also cause immunosuppression. In this study, we used two different mouse oral carcinoma cells; the immunogenic MOC1 and the moderately immunogenic MOC2. The two cell lines were characterized in vitro for radiosensitivity as well as immunogenic (calreticulin, HMBG1) and immunosuppressive (PD-L1, PGE2) signalling. The combination of immune checkpoint inhibitor (ICI) anti-PDL1 and proton/X-ray irradiation were then investigated in syngeneic mouse models.

Material/Methods:

In vitro experiments included the colony assay for radiosensitivity, flow cytometry for calreticulin and PD-L1 expression, ELISA for PGE2 and HMBG1.

MOC1/MOC2 cells were inoculated in the leg of C57Bl6/Jrj mice. The mice were irradiated with different doses of protons or X-rays and treated with anti-PDL1 or PBS on days 1, 4, 8, and 11 after irradiation. Tumour volume was measured all weekdays until termination. Tumor tissues were harvested for immunohistochemistry. A mathematical model was fitted to tumor volume measurements up to day 19 for MOC1 and day 7 for MOC2: , where V is tumor volume on day t, V0 is tumor volume on day 0, and D is dose. γ and η are the fitted parameters. γ is a measure of the growth rate and η estimates the dose effect on top of the PBS or anti-PD-L1 effect as growth delay per dose. RBE, w/wo anti-PD-L1, and synergy for each radiation type, were calculated as: , . For MOC1 tumors binary treatment effect on day 45 were analyzed using logistic regression and subsequent estimate of the tumor dose giving 50 % effect (TD50), which were used to estimate RBE and synergy for treatment effect (TE) similarly to the tumor growth parameters. In vitro experiments showed higher initial levels of calreticulin and HMGB1 (immunogenic signals) in MOC1 cells and PD-L1 and PGE2 (immunosuppressive signals) in MOC2. X-irradiation increased the immunogenic signals in MOC2 and the immunosuppressive signals in MOC1. Proton irradiation increased all signals in both cell lines. In vivo, MOC2 tumors grew faster than MOC1 tumors, and all mice with MOC2 tumors developed metastases. While the radiosensitivity of the two cell lines in vitro was very similar, MOC2 tumors were more radioresistant than MOC1 tumors. ICI alone reduced the growth rate for MOC1 with 15±2% compared to controls, while no reduction was seen for MOC2. For MOC1 tumors, there was no significant benefit in using protons over X-rays with or without ICI, but the combination of either radiation type and ICI synergistically reduced tumor growth and improved cure rates. For MOC2 tumors, a clear benefit of using protons over X-rays was seen with and without ICI. A synergistic effect was seen for MOC2 tumors for the combination of ICI with protons, but not with X-rays. The estimated values for RBE and synergy are given in the table. Results: