While immune-based therapeutic interventions have led to remarkable treatment responses in many patients, the majority of patients either fail to respond or acquire resistance to immunotherapy. Downregulation of MHC class I is a common feature of human cancers, confers resistance to T cell mediated killing, and can cause insensitivity to conventional immunotherapy, such as anti-PD-1 or anti-CTLA-4 antibodies. Though decreased MHC class I expression triggers NK-mediated cytotoxicity, NK cells readily become exhausted when encountering MHC Class I deficient tumor cells. We observed that among intratumoral lymphocytes, NK cells display high levels of expression of the IL-18 receptor. We thus hypothesized that IL-18 pathway agonism could represent a potential strategy to reverse NK cell exhaustion and activate NK-mediated immunity in the setting of MHC class I loss. However, we and others have found that tumors frequently express high levels of the ultra-high affinity inhibitory decoy receptor of IL-18, IL-18BP. We therefore reasoned that IL-18BP could pose a major barrier to effective IL-18 therapy. To address this challenge, we used directed evolution with yeast surface display to engineer a “decoy-resistant” variant of IL-18 (DR-18), which is fully capable of activating the IL-18 receptor, but is impervious to inhibition by IL-18BP. To assess the activity of DR-18 on MHC class I deficient tumors, we generated B2m-deficient variants of both MC38 and YUMMER1.7 tumor models and observed that, compared to treatment with WT IL-18 or combined anti-CTLA4/anti-PD-1 therapy, only DR-18 treatment elicited tumor regression and a significant survival benefit. Similarly, we observed strong anti-tumor efficacy of DR-18 in additional NK cell sensitive tumor models including B16F10 melanoma and RMA-S lymphoma. Flow cytometric and sc-RNAseq analysis revealed DR-18 treatment promoted the functional maturation of highly proliferative NK cells with the polyfunctional capacity to produce effector molecules such as IFN-γ, Gzmb, and TRAIL. Finally, we observed that ablation of NK cells or neutralization of IFN-γ abrogated DR-18 mediated responses. Collectively our results highlight the ability of IL-18 pathway agonism to promote NK cell responses against checkpoint immunotherapy -refractory tumors that have lost MHC class I expression, which poses a major resistance mechanism that presently is not addressed by currently approved immunotherapeutic regimens.

Citation Format: Orr-El Weizman, Ting Zhou, William Damsky, Meaghan McGeary, Marcus W. Bosenberg, Aaron M. Ring. IL-18 immunotherapy is efficacious against checkpoint-immunotherapy refractory tumors by promoting the maturation of highly proliferative, polyfunctional NK cells [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 3424.