Radiotherapy-Induced Anti-Tumor Immunity Contributes to the Therapeutic Efficacy of Irradiation and Can Be Augmented by CTLA-4 Blockade in a Mouse Model

Abstract

PurposeThere is growing evidence that tumor-specific immune responses play an important role in anti-cancer therapy, including radiotherapy. Using mouse tumor models we demonstrate that irradiation-induced anti-tumor immunity is essential for the therapeutic efficacy of irradiation and can be augmented by modulation of cytotoxic T lymphocyte (CTL) activity.Methods and MaterialsC57BL/6 mice, syngeneic EL4 lymphoma cells, and Lewis lung carcinoma (LL/C) cells were used. Cells were injected into the right femurs of mice. Ten days after inoculation, tumors were treated with 30 Gy of local X-ray irradiation and their growth was subsequently measured. The effect of irradiation on tumor growth delay (TGD) was defined as the time (in days) for tumors to grow to 500 mm3 in the treated group minus that of the untreated group. Cytokine production and serum antibodies were measured by ELISA and flow cytometry.ResultsIn the EL4 tumor model, tumors were locally controlled by X-ray irradiation and re-introduced EL4 cells were completely rejected. Mouse EL4-specific systemic immunity was confirmed by splenocyte cytokine production and detection of tumor-specific IgG1 antibodies. In the LL/C tumor model, X-ray irradiation also significantly delayed tumor growth (TGD: 15.4 days) and prolonged median survival time (MST) to 59 days (versus 28 days in the non-irradiated group). CD8(+) cell depletion using an anti-CD8 antibody significantly decreased the therapeutic efficacy of irradiation (TGD, 8.7 days; MST, 49 days). Next, we examined whether T cell modulation affected the efficacy of radiotherapy. An anti-CTLA-4 antibody significantly increased the anti-tumor activity of radiotherapy (TGD was prolonged from 13.1 to 19.5 days), while anti-FR4 and anti-GITR antibodies did not affect efficacy.ConclusionsOur results indicate that tumor-specific immune responses play an important role in the therapeutic efficacy of irradiation. Immunomodulation, including CTLA-4 blockade, may be a promising treatment in combination with radiotherapy.

CD70: An emerging target in cancer immunotherapy

“…Moreover, combinations of radiotherapy with immunomodulatory antibodies such as CTLA-4 or PD-1 blocking antibodies have already shown clear synergistic potential (Yoshimoto et al, 2014). As reviewed by Shababi et al, radiotherapy can make tumors more immunogenic by enhancing antigen presentation, antigen uptake and the secretion of immunostimulatory factors (Shahabi et al, 2015).…”

Immunotherapy Combined with Large Fractions of Radiotherapy: Stereotactic Radiosurgery for Brain Metastases—Implications for Intraoperative Radiotherapy after Resection

“…Indeed, some evidence suggests that immunogenic cell death contributes to the efficacy of hypofractionated or single-dose radiotherapy (37, 69, 70). However, data regarding the influence of dose and fractionation are conflicting, thus warranting a critical review of the dose dependence of immune activation.…”

Strategies for Combining Immunotherapy with Radiation for Anticancer Therapy

“…In another series of experiments involving mice implanted with Lewis lung carcinoma cells, Yoshimoto and colleagues found that radiation (30 Gy) combined with CTLA4 blockade extended both tumor growth delay and median survival time (56 days) compared with radiation alone (46 days) or anti-CTLA4 IgG alone (<40 days). They also noted that administering anti-CD8 antibody with radiation further extended median survival time and tumor growth inhibition [25]. In other studies using tumor-infiltrating lymphocytes after anti-CTLA4 plus radiation in a melanoma mouse model, Victor and colleagues noted that anti-CTLA4 antibody decreased the number of Tregs, whereas radiation increased T-cell receptor (TCR) diversity.…”