Abstract
Metastases are responsible for the vast majority of cancer-related deaths. Although tumor cells can become invasive early during cancer progression, metastases formation typically occurs as a late event. How the immune response to primary tumors may dictate this outcome remains poorly understood, which hampers our capacity to manipulate it therapeutically. Here, we used a two-step experimental model, based on the highly aggressive B16F10 melanoma, that temporally segregates the establishment of primary tumors (subcutaneously) and the formation of lung metastases (from intravenous injection). This allowed us to identify a protective innate immune response induced by primary tumors that inhibits experimental metastasis. We found that in the presence of primary tumors, increased numbers of natural killer (NK) cells with enhanced IFNγ, granzyme B, and perforin production were recruited to the lung upon metastasis induction. These changes were mirrored by a local accumulation of patrolling monocytes and macrophages with high expression of MHC class II and NOS2. Critically, the protective effect on metastasis was lost upon patrolling monocyte or NK cell depletion, IL15 neutralization, or IFNγ ablation. The combined analysis of these approaches allowed us to establish a hierarchy in which patrolling monocytes, making IL15 in response to primary tumors, activate NK cells and IFNγ production that then inhibit lung metastasis formation. This work identifies an innate cell network and the molecular determinants responsible for "metastasis immunosurveillance," providing support for using the key molecular mediator, IL15, to improve immunotherapeutic outcomes. Cancer Immunol Res; 5(9); 812-20. ©2017 AACR.