Abstract 2697: Simultaneous activation of innate immune suppressive components impairs antitumor efficacy of immunotherapy against the innate immune system

Abstract Cancer immunotherapy has shown noticeable clinical efficacy in many cancer types, while clinical benefits are only observed in 20-40% of treated patients. Therefore, combination cancer immunotherapy targeting multiple factors that limit antitumor immunity has been tested in the clinic. Accumulating evidence reveals that cancers that harbor limited immune cell infiltration (so-called immunologically cold tumors) respond poorly to current immunotherapy. Preclinical studies using animal models have shown that activation of innate immunity by TLR agonists or STING agonists can successfully remodel the TME to effectively induce antitumor immunity in those immunologically cold tumors. However, unexpectedly, all TLR agonists and STING agonists tested have not shown clinical efficacy in combination with immune checkpoint blockade therapy, including PD-1/PD-L1 blockade therapy. Hence, we addressed the mechanism (s) why stimulators of innate immunity failed to elicit effective antitumor immunity in cancer patients. We then employed OK-432, which reportedly stimulates TLR-2, TLR-4, and/or TLR-9, as OK-432 is widely used in Japan in the clinic. In vitro OK-432 treatment accelerated the maturation DCs, and efficiently induced CD8+ T cells in humans. However, the combination treatment with anti-PD-1 mAb and OK-432 failed to augment antitumor immunity in animal models using anti-PD-1 mAb monotherapy-resistant tumors; the data mirrors the failure of clinical trials of combination therapy with immune checkpoint blockade and TLR agonists or STING agonists in humans. Therefore, using the animal models, we explored the acquired resistance mechanism developed after combination treatment. While OK-432 treatment effectively stimulated DCs, innate immune suppressive cells, such as MDSCs, were concurrently accumulated in the TME. The accumulation of PMN-MDSCs after OK-432 treatment was induced in a CXCL1-CXCR2-dependent fashion. Accordingly, triple combination treatment with a PMN-MDSC inhibitor along with OK-432 and anti-PD-1 mAb attenuated the accumulation of PMN-MDSCs in the TME and improved the antitumor effect of the combination treatment. Taken together, we propose that successful combination treatment stimulating innate immunity against immunogenically cold cancers requires controlling the unwanted co-activation of innate immune suppressive cells. This study clearly demonstrates the mechanism of acquired resistance after administration of reagents that activate innate immunity, such as TLR agonists or STING agonists, and provides the basis for future successful combination therapy. Citation Format: Hitomi Nishinakamura, Sayoko Shinya, Takuma Irie, Shugo Sakihama, Tadaaki Ioroi, Takeo Naito, Keisuke Watanabe, Daisuke Sugiyama, Motohiro Tamiya, Tatsuya Yoshida, Tetsunari Hase, Takao Yoshida, Kennosuke Karube, Yuka Maeda, Shohei Koyama, Hiroyoshi Nishikawa. Simultaneous activation of innate immune suppressive components impairs antitumor efficacy of immunotherapy against the innate immune system abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts) ; 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84 (6Suppl): Abstract nr 2697.