Exogenous IL-33 restores dendritic cell activation and maturation in established cancer

The role of IL-33, particularly in tumor growth and tumor immunity, remains ill-defined. We show that exogenous IL-33 can induce robust antitumor effect through a CD8⁺ T cell-dependent mechanism. Systemic administration of rIL-33 alone was sufficient to inhibit growth of established tumors in transplant and de novo melanoma tumorigenesis models. Notably, in addition to a direct action on CD8⁺ T cell expansion and IFN-γ production, rIL-33 therapy activated myeloid dendritic cells (mDCs) in tumor-bearing mice, restored antitumor T cell activity, and increased Ag cross-presentation within the tumor microenvironment. Furthermore, combination therapy consisting of rIL-33 and agonistic anti-CD40 Abs demonstrated synergistic antitumor activity. Specifically, MyD88, an essential component of the IL-33 signaling pathway, was required for the IL-33-mediated increase in mDC number and upregulation in expression of costimulatory molecules. Importantly, we identified that the IL-33 receptor ST2, MyD88, and STAT1 cooperate to induce costimulatory molecule expression on mDCs in response to rIL-33. Thus, our study revealed a novel IL-33-ST2-MyD88-STAT1 axis that restores mDC activation and maturation in established cancer and, thereby, the magnitude of antitumor immune responses, suggesting a potential use of rIL-33 as a new immunotherapy option to treat established cancer.