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.
Highlights
Several reports showed that radiotherapy and antitumor immunity are closely associated
CD8(+) cell depletion using an anti-CD8 antibody significantly decreased the therapeutic efficacy of irradiation (TGD, 8.7 days; median survival time (MST), 49 days)
We examined whether T cell modulation affected the efficacy of radiotherapy
Summary
Several reports showed that radiotherapy and antitumor immunity are closely associated. The abscopal effect is a well-known but rare phenomenon in which local radiotherapy is associated with the regression of a metastatic tumor located at a distance from the irradiated site. This effect is thought to be mediated by activation of anti-tumor immunity. Postow et al reported a case of the abscopal effect in a patient with melanoma treated with radiotherapy and ipilimumab, an antagonistic antibody against cytotoxic T-lymphocyte-associated antigen 4 (CTLA-4). Demaria et al used a mouse syngeneic mammary carcinoma model to show that abscopal effects result from irradiation-activated anti-tumor immunity [3] Taken together, these observations indicate that local radiotherapy can induce systemic tumor-specific immune responses
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