Abstract 6134: Identifying the mechanism of acquired resistance against cancer immunotherapy targeting innate immunity

Abstract Immune checkpoint blockade (ICB) efficiently reactivates effector T cells and induces antitumor efficacy. However, more than half of patients, particularly patients with the immunologically cold tumor microenvironment (TME), failed to experience clinical benefits, even in combination with other drugs. To augment the antitumor efficacy by ICB therapy, various attempts to modify the immunologically cold TME are under investigation. The activation of innate immune cells, particularly antigen-presenting cells (APCs), through pattern recognition receptors, such as Toll-like receptors (TLRs), is essential for the subsequent activation of acquired immunity, including CD8+ T cells. However, several approaches employing ICB and reagents that can activate innate immunity have not shown antitumor efficacy in clinical trials, suggesting the existence of a resistance mechanism(s) against the combination treatment. Here we examine the potential of the combination treatment of OK-432 (Picibanil: a penicillin-inactivated and lyophilized preparation of a low-virulence strain of Streptococcus pyogenes), an agonist for multiple TLRs, including TLR2, TLR4, and/or TLR9, with PD-1 blockade against tumors with immunologically cold TME. Although OK-432 treatment effectively stimulated APCs in vitro, many innate immune suppressive cells, such as polymorphonuclear-myeloid-derived suppressor cells (PMN-MDSCs), were co-activated and accumulated in the TME upon OK-432 treatment in a CXCL1-CXCR2-dependent manner (acquired resistance). Accordingly, CXCR2 neutralizing antibody or anti-Ly6G antibody reduced the recruitment of PMN-MDSCs to the TME, consequently the triple combination with CXCR2 neutralizing antibody/anti-Ly6G antibody, OK-432, and PD-1 blockade exhibited a far stronger anti-tumor effect. On the other hand, anti-Ly6G antibody combined with PD-1 blockade sometimes inhibited tumor growth in certain tumor models inherently harboring the TME with abundant PMN-MDSCs (adaptive resistance). To further address the difference, tumor-infiltrating immune cells were comprehensively examined in 10 cancer cell lines; a PMN-MDSC-to-CD8 ratio and an infiltrating CD45+ cells count became biomarkers for the necessity of the triple combination treatment. Thus, we argue that stimulators of innate immunity, such as TLR agonists including OK-432, are a double-edged sword. A stimulator of innate immunity may become an effective cancer immunotherapy by controlling the acquired resistance mechanism, namely, the activation of innate immune suppressive cells in the TME. Citation Format: Hitomi Nishinakamura, Sayoko Shinya, Takuma Irie, Shugo Sakihama, Takeo Naito, Keisuke Watanabe, Daisuke Sugiyama, Motohiro Tamiya, Tatsuya Yoshida, Tetsunari Hase, Takao Yoshida, Kennosuke Karube, Yuka Maeda, Shohei Koyama, Hiroyoshi Nishikawa. Identifying the mechanism of acquired resistance against cancer immunotherapy targeting innate immunity [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 6134.