IL-2/αIL-2 Complexes Massively Expand Systemic Tumor-Specific T Cells and Enhance Abscopal Responses to Radiation and αPD-1

Abstract

Early clinical trials have provided evidence for RT-induced systemic effects in conjunction with αPD1 or IL-2 in metastatic patients, but strong abscopal responses are clinically rare. We investigated the effect of a triple combination of CD122-directed IL-2/αIL-2 complexes (IL-2c), hRT, and αPD1 on tumor-specific CD8+ T cell differentiation and the abscopal effect vs. the respective dual treatments. 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. Dependence of the therapeutic effect on CD8+ T cells and T cells expressing the chemokine receptor CXCR3 was assessed using depleting or blocking antibodies. Differentiation stages of tumor-specific CD8+ T cells in various compartments were determined flow cytometrically using MHC-I tetramers and appropriate antibodies. Anti-tumoral effects of blood-derived and tumor-derived T cells were assessed in adoptive T cell transfer experiments. 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). 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 (TCF1+TIM3-PD1+) and effector-like (CD101-TIM3+TCF1-PD1+) but not terminally differentiated (CD101+TIM3+TCF1-PD1+) exhausted cells particularly strongly increased. Moreover, IL-2c induced CXCR3 mainly on non-terminally differentiated 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/αPD1/IL-2c treatment. Finally, we found that peripheral blood from triple-treated mice is an effective source of T cells for adoptive T cell transfer. RT/αPD1/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 metastasis resistant to αPD1/IL-2c and only transiently responding to RT/αPD1 or RT/IL-2c. Therefore, such triple combinations appear promising for clinical evaluation in metastatic patients.