Immune checkpoint blockade (ICB) immunotherapy has revolutionized cancer treatment by prolonging overall survival of patients with cancer. Despite advances in the clinical setting, the immune cellular network in the tumor microenvironment (TME) that mediates such therapy is not well understood. IL33 is highly expressed in normal epithelial cells but downregulated in tumor cells in advanced carcinoma. Here, we showed that IL33 was induced in tumor cells after treatment with ICB such as CTL antigen-4 (CTLA-4) and programmed death-1 (PD-1) mAbs. ST2 signaling in nontumor cells, particularly CD8+ T cells, was critical for the antitumor efficacy of ICB immunotherapy. We demonstrated that tumor-derived IL33 was crucial for the antitumor efficacy of checkpoint inhibitors. Mechanistically, IL33 increased the accumulation and effector function of tumor-resident CD103+CD8+ T cells, and CD103 expression on CD8+ T cells was required for the antitumor efficacy of IL33. In addition, IL33 also increased the numbers of CD103+ dendritic cells (DC) in the TME and CD103+ DC were required for the antitumor effect of IL33 and accumulation of tumor-infiltrating CD8+ T cells. Combination of IL33 with CTLA-4 and PD-1 ICB further prolonged survival of tumor-bearing mice. Our study established that the “danger signal” IL33 was crucial for mediating ICB cancer therapy by promoting tumor-resident adaptive immune responses.

The immune checkpoint blockade (ICB) immunotherapy has prolonged overall survival for cancer patients but the response rates are low. The resistance to ICB is likely due to compensatory upregulation of additional immune inhibitory molecules. In this study, we first systematically examined Tim-3 expression in immune cells in mouse tumors and found that Tim-3 was specifically up-regulated in a large number of Treg, conventional CD4 + , CD8 + T cells, dendritic cell 1 (DC1), and macrophage 1 (M1) in the tumor microenvironment (TME). Interestingly, Tim-3 + T cells in the TME were phenotypically effector but not "exhausted" T cells because Tim-3 + PD-1 + CD8 + T cells had a higher number of mitochondria, greater levels of glycolysis, and higher tumorspecific cytolytic activities compared to Tim-3 − PD-1 − CD8 + T cells. The combination treatment with Tim-3 and PD-1 mAbs resulted in a synergistic antitumor activity but also increased the expression of Lag-3 and GITR in TIL, demonstrating cross-regulation between multiple checkpoint molecules. Furthermore, we found that the antitumor efficacy with triple combination of Tim-3, PD-1, and Lag3 mAbs was much greater than any two antibodies. Mechanistically, we demonstrated that simultaneous targeting of Tim-3, PD-1, and Lag-3 cooperatively increased the levels of granzyme B and tumor-specific cytolytic activities of CD8 + TIL. Our data indicate that multiple checkpoint molecules are coordinately upregulated to inhibit the function of hyperactivated T cells in the TME and requirement for the simultaneous blockade of PD-1, Tim-3 and Lag3 for cancer treatment.

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