IL-18BP is a secreted immune checkpoint and barrier to effective IL-18 immunotherapy

Recombinant cytokines were the first modern immunotherapies to produce durable cures in metastatic cancer, but their application has been hampered by only modest efficacy and limited tolerability. Next-generation cytokine therapies are therefore under development to overcome the biological limitations of native cytokines. By analyzing single-cell transcriptomic data from tumor infiltrating lymphocytes (TIL), we found that components of the Interleukin-18 (IL-18) pathway are upregulated on activated and dysfunctional TIL, suggesting that IL-18 therapy could potentially restore anti-tumor immunity by stimulating these key effector cells. However, recombinant IL-18 therapy has consistently failed to demonstrate anti-tumor efficacy in clinical trials. Here we show that the secreted, high-affinity decoy receptor IL-18BP is frequently upregulated in the tumor microenvironment of diverse human cancers and syngeneic murine tumor models. Using directed evolution, we engineered a 'decoy-resistant' IL-18 (DR-18), which maintains signaling potential, but is impervious to binding and inhibition by IL-18BP. In contrast to wild-type IL-18, DR-18 exhibits potent anti-tumor efficacy as monotherapy and in combination with anti-PD-1 checkpoint immunotherapy in multiple tumor models. Mechanistically, DR-18 drives the development of poly-functional effector CD8+ T cells, decreases the prevalence of exhausted CD8+ T cells expressing the transcription factor TOX, and expands the pool of TCF1+ precursor CD8+ T cells. DR-18 also enhances NK cell activity and maturation to effectively treat anti-PD-1 resistant tumors that have lost MHC class I surface expression. Together, these results highlight the IL-18 pathway as a powerful target for immunotherapeutic intervention and implicate the secreted immune checkpoint IL-18BP as an obstacle to effective IL-18 immunotherapy. Overall design: Droplet-based 3' end single cell RNA sequencing was performed on pooled cells from 3 tumors per condition, consisting of 50% CD45+CD3e+, 25% CD45+CD3e-, 25% CD45- after FACS. Droplets and libraries were prepared using Chromium Single Cell 3' Reagent Kits v2 according to manufacturer's protocol (10X genomics). Generated libraries were sequenced on Illumina HiSeq 4000.