Coactivation of innate immune suppressive cells induces acquired resistance against cancer immunotherapy targeting the innate immune system [TIICs RNA-seq]

Immune checkpoint blockade therapy has been successfully applied in clinical settings as a standard therapy for many cancer types, but its clinical efficacy is restricted to patients with immunologically hot tumors. Various strategies to modify the tumor microenvironment (TME), such as Toll-like receptor (TLR) agonists, have been explored but have not been successful. Here, we identified a mechanism of acquired resistance to combination treatment consisting of an agonist for multiple TLRs, OK-432 (Picibanil), and PD-1 blockade. Adding the TLR agonist failed to convert the TME from immunogenically cold to hot. The combination treatment did not augment antitumor immunity, particularly CD8+ T cell responses, in multiple animal models. The failure was attributed to the coactivation of innate suppressive cells, such as CD11b+ Gr1+ Ly6C- Ly6G+ myeloid-derived suppressor cells (MDSCs) expressing CXCR2, through high CXCL1 production by macrophages in the TME upon OK-432 treatment. Thus, a triple combination treatment with OK-432, PD-1 blockade, and a CXCR2 neutralizing antibody overcame the resistance induced by MDSCs, resulting in a far stronger antitumor effect than that of any dual combination or single treatment. The accumulation of MDSCs was similarly observed in the pleural effusion of lung cancer patients after OK-432 administration. We propose that successful combination cancer immunotherapy stimulating innate immunity against immunologically cold tumors requires modulation of unwanted activation of innate immune suppressive cells, including MDSCs. To investigate cancer types suitable for OK-432 treatment, we performed a comprehensive analysis of CD45+ tumor-infiltrating immune cells across 10 different cancer cell lines using bulk RNA-seq.