Low-dose radiation treatment enhances systemic antitumor immune responses by overcoming the inhibitory stroma

Background

Despite some successes with checkpoint inhibitors for treating cancer, most patients remain refractory to treatment, possibly due to the inhibitory nature of the tumor stroma that impedes the function and entry of effector cells. We devised a new technique of combining immunotherapy with radiotherapy (XRT), more specifically low-dose XRT, to overcome the stroma and maximize systemic outcomes.

Methods

We bilaterally established 344SQ lung adenocarcinoma tumors in 129Sv/Ev mice. Primary and secondary tumors were irradiated with either high-dose or low-dose of XRT with systemic anti-programmed cell death protein 1 and anti-cytotoxic T-lymphocyte associated protein 4 administration. Survival and tumor growth were monitored for the various groups, and secondary tumors were phenotyped by flow cytometry for immune populations. Tumor growth factor-beta (TGF-β) cytokine levels were assessed locally after low-dose XRT, and specific immune-cell depletion experiments were conducted to identify the major contributors to the observed systemic antitumor effect.

Results

Through our preclinical and clinical studies, we observed that when tumor burden was high, there was a necessity of combining high-dose XRT to ‘prime’ T cells at the primary tumor site, with low-dose XRT directed to secondary (metastatic) tumors to ‘modulate the stroma’. Low-dose XRT improved the antitumor outcomes of checkpoint inhibitors by favoring M1 macrophage polarization, enhancing natural killer (NK) cell infiltration, and reducing TGF-β levels. Depletion of CD4⁺ T cells and NK cells abrogated the observed antitumor effect.

Conclusion

Our data extend the benefits of low-dose XRT to reprogram the tumor environment and improve the infiltration and function of effector immune cells into secondary tumors.

Triple Therapy with MerTK and PD1 Inhibition Plus Radiotherapy Promotes Abscopal Antitumor Immune Responses

PURPOSE

Radiotherapy (RT) traditionally has been used for local tumor control in the treatment of cancer. The recent discovery that radiotherapy can have anticancer effects on the immune system has led to recognition of its ability to sensitize the tumor microenvironment to immunotherapy. However, radiation can also prompt adverse immunosuppressive effects that block aspects of systemic response at other tumor sites. Our hypothesis was that inhibition of the MER proto-oncogene tyrosine kinase (MerTK) in combination with anti-programmed cell death-1 (α-PD1) checkpoint blockade will enhance immune-mediated responses to radiotherapy.

EXPERIMENTAL DESIGN

We tested the efficacy of this triple therapy (Radiation + α-PD1 + α-MerTK mAbs) in 129Sv/Ev mice with bilateral lung adenocarcinoma xenografts. Primary tumors were treated with stereotactic radiotherapy (36 Gy in 3 12-Gy fractions), and tumors were monitored for response.

RESULTS

The triple therapy significantly delayed abscopal tumor growth, improved survival rates, and reduced numbers of lung metastases. We further found that the triple therapy increased the activated CD8⁺ and NK cells populations measured by granzyme B expression with upregulation of CD8⁺CD103⁺ tissue-resident memory cells (T_RM) within the abscopal tumor microenvironment relative to radiation only.

CONCLUSIONS

The addition of α-PD1 + α-MerTK mAbs to radiotherapy could alter the cell death to be more immunogenic and generate adaptive immune response via increasing the retention of T_RM cells in the tumor islets of the abscopal tumors which was proven to play a major role in survival of non-small cell lung cancer patients.

Anti-glucocorticoid-induced Tumor Necrosis Factor-Related Protein (GITR) Therapy Overcomes Radiation-Induced Treg Immunosuppression and Drives Abscopal Effects

Despite the potential to cure metastatic disease, immunotherapy on its own often fails outright or early on due to tumor immune evasion. To address this obstacle, we investigated combinations of anti-GITR, anti-PD1 and radiation therapy (XRT) in our previously developed anti-PD1 resistant 344SQ non-small cell lung adenocarcinoma preclinical tumor model. We hypothesized that targeting multiple mechanisms of immune evasion with this triple therapy would lead to an enhanced tumor-specific immune response and improve survival more so than any mono- or dual therapy. In a two tumor 344SQR murine model, treatment with anti-GITR, anti-PD1, and XRT led to significantly improved survival and an abscopal response, with half of the mice becoming tumor free. These mice showed durable response and increased CD4+ and CD8+ effector memory on tumor rechallenge. Regulatory T cells (Tregs) expressed the highest level of GITR at the tumor site and anti-GITR therapy drastically diminished Tregs at the tumor site. Anti-tumor effects were largely dependent on CD4+ T cells and partially dependent on CD8+ T cells. Anti-GITR IgG2a demonstrated superior efficacy to anti-GITR IgG1 in driving antitumor effects. Collectively, these results suggest that combinatorial strategies targeting multiple points of tumor immune evasion may lead to a robust and lasting antitumor response.