Peritumoral IL-15 Promotes Regional NK Activation and DC Licensing to Augment Radiation-Induced Anti-tumor Immunity

Abstract

Radiotherapy (RT) can elicit anti-tumor T cells by multiple mechanisms but the induced immune response is limited by the immunosuppression in the tumor microenvironment. The common gamma-chain cytokines IL-15 promotes the activation of natural killer (NK) cells and the proliferation of activated CD8+ T cells and is under investigation for its ability to induce T cell response in cancer patients. Here we tested the hypothesis that IL-15 strengthens the pro-immunogenic effect of local RT to potentiate durable tumor-specific immunity. The poorly immunogenic mouse TSA breast cancer cells were implanted s.c. in syngeneic wild type (WT) or batf3-deficient BALB/c mice and when tumors reached 5mm in average diameter, were randomly assigned to one of 4 treatment groups: control, RT, IL-15 or RT+IL-15. Local RT was delivered consecutively in 8 Gy fractions on days 13, 14, and 15 post tumor implantation. IL-15 (2 μg/mouse) was administered s.c. peritumorally daily for 10 days starting on the first day of RT. Mice were followed for tumor growth and surviving mice subsequently re-challenged. Some animals were depleted of CD8 and NK cells (anti-asialo-GM1 Ab) using standard protocols. A parallel experiment was done to characterize tumor-infiltrating lymphocytes (TILs) and measure ex vivo IFNg production by tumor-specific CD8 T-cells in the draining lymph nodes at the end of treatment (day 22). IL-15 as monotherapy had no effect on tumor control but significantly improved the ability of RT to delay tumor growth in WT mice (P < 0.01, RT vs RT+IL-15). Combination treatment led to durable and robust memory responses in 50% of animals compared to 0% of mice treated with RT alone. RT enhanced intratumoral infiltration of DX5+NKp46+ NK cells and was further increased by the addition of IL-15 (22.86% of CD45 TILs in RT vs 32% in RT+IL-15, P<0.05). CD8+ T cells expressing the activation marker CD137 were significantly increased in tumors of RT+IL-15-treated mice while the increase was modest with each monotherapy (35.3% in RT+IL-15 vs 18.8% in RT, 20.7% in IL-15, 5.90% in control, P < 0.05). Systemic depletion of either CD8 T-cells or NK cells abrogated the therapeutic effect of the combination. RT+IL-15 elicited more priming of tumor-specific T-cells in response to tumor epitope AH-1-A5 (1238 pg/mL in RT+IL-15 vs 3.3 pg/mL in control vs 28.7 pg/mL in IL-15 RT vs 2.63 pg/mL in RT; P<0.05). Batf3-/- mice that CD103+ DCs showed reduce response to RT+IL-15 compared to WT mice. Here we show that local IL-15 administration improves priming of anti-tumor CD8+ T cell and leads to long-term memory responses by a mechanism that is dependent on NK cells and Batf3-dependent DCs. Data suggest that IL-15 activated NK cells kill tumor cells and license DCs leading to a long-term T cell-mediated memory response. Overall these results support the testing of IL-15 in combination with tumor-targeted radiotherapy.