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.