IL6/STAT3 signaling orchestrates premetastatic niche formation and immunosuppressive traits in lung

Cancer cells that succeed in forming metastasis need to be reprogrammed to evade immune surveillance and survive in a new microenvironment. This is facilitated by metastatic niches that are either postformed through reciprocal signaling between tumor cells and local stromal cells or preformed as premetastatic niches before tumor cell arrival. IL6/STAT3 signaling is aberrantly activated in lung tumorigenesis and metastasis, however, the roles and mechanisms of action of IL6 remain controversial. Here, we showed that blockade of intrinsic STAT3 signaling in lung tumor cells suppressed lung metastasis in immune-competent syngeneic mice, but not in immune-deficient nude mice. Consistently, repression of STAT3 signaling in tumor cells made themsusceptible to T-cell-mediated cytotoxicity. Thus, STAT3- mediated immunosuppression is crucial for metastasis. Noticeably, lung metastasis was greatly increased in Gprc5a-knockout (ko; 5a^-/-) mice compared with wild-type mice, which correlated with upregulated IL6 in the tumor microenvironment. Depletion of IL6 via combined deletion of Il6 and Gprc5a genes almost completely eliminated lung metastasis in Gprc5a-ko/Il6-ko (5a^-/-Il6^-/- mice. Mechanistically, dysregulated IL6 reprogrammed the STAT3 pathway in metastatic tumor cells, and induced recruitment of myeloid-derived suppressor cells and polarized macrophages to evade host immunity. Consistently, IHC staining showed that activated STAT3 correlated with repressed infiltration of CD8⁺ T cells in non-small cell lung cancer. Therefore, IL6/STAT3 signaling is crucial for orchestrating premetastatic niche formation and immunosuppression in lung. Significance: IL6 plays important roles not only in cell autonomous propensity for metastasis, but also in establishing the metastatic niche.