Abscopal Responses to Radiation Combined with Anti-PD-1 and CD122-Directed IL-2/Anti-IL-2 Complexes

Abstract

<h3>Purpose/Objective(s)</h3> Early clinical trials have provided evidence for RT-induced systemic effects in conjunction with αPD-1 or IL-2 in metastatic patients, but strong abscopal responses are clinically rare. Dual combinations of αPD-1 with more effective and less toxic IL-2 derivatives, e.g., CD122-directed pegylated IL-2, are also currently under investigation. Whether a combination of RT, αPD-1, and CD122-directed IL-2/anti-IL-2 complexes (IL-2c) can increase abscopal effects against established non-irradiated tumors is unknown. Also, in-depth analyses of the differentiation of tumor-specific CD8⁺ T cells have not yet been reported for αPD-1/IL-2c in combination with RT. We investigated how adding IL-2c to hRT/αPD1 affects tumor-specific CD8⁺ T cell differentiation in mouse tumor models and the potential of this triple combination to enhance the abscopal effect compared to the respective dual treatments. <h3>Materials/Methods</h3> Mice bearing bilateral tumors were treated with two fractions of 8 Gy (C51 colon carcinoma model) or 12 Gy (B16 melanoma model); αPD1 was given weekly; IL-2c was given for five consecutive days. CD8 T cell-depleting and CXCR3-blocking antibodies were used to determine if the therapeutic effects depend on CD8⁺ and CXCR3⁺ T cells. Differentiation stages of tumor-specific CD8⁺ T cells in tumor-draining lymph nodes, spleen, blood, and tumors were determined by flow cytometry using MHC-I tetramers and various antibodies. <h3>Results</h3> The abscopal effect was significantly stronger in triple-treated mice compared to mice treated with RT/αPD1 (C51 model: p < 0.01; B16 model: p < 0.05), RT/IL-2c (C51 model: p < 0.01; B16 model: p < 0.001) or αPD1/IL-2c (C51 model: p < 0.0001, B16 model: p < 0.01). Moreover, triple therapy improved survival and resulted in complete cures of 3/12 mice in the C51 model and 2/12 mice in the B16 model. These anti-tumor effects were associated with dramatic expansion of tumor-specific CD8⁺ T cells. Undifferentiated stem-like and effector-like but not terminally differentiated exhausted cells particularly strongly increased. Moreover, IL-2c induced CXCR3 expression on tumor-specific CD8⁺ T cells. Both CD8⁺ (C51 model: p < 0.0001; B16 model: p < 0.01) and CXCR3⁺ (C51 model: p < 0.0001) T cells were crucial for the RT-induced abscopal effect upon RT/αPD-1/IL-2c treatment. <h3>Conclusion</h3> RT/αPD-1/IL-2c triple treatment resulted in superior local and systemic expansion of tumor-specific CD8⁺ T cells with stem- and effector-like phenotypes. Also, IL-2c strongly increased CXCR3⁺ CD8⁺ T cells that were associated with pronounced abscopal responses in models with an established non-irradiated tumor resistant to αPD-1/IL-2c and only transiently responding to RT/αPD-1 or RT/IL-2c. Therefore, such triple combinations appear promising for clinical evaluation in metastatic patients.