Mechanisms of cytotoxic immune evasion by disseminated tumor cells in breast cancer metastasis

Abstract

Metastasis is the major cause of death for patients with solid cancer. Within these malignancies, Triple Negative Breast Cancer (TNBC) is one of the most aggressive and lethal diseases for women worldwide. Metastases arise from cancer cells that disseminate from the original tumors, survive systemic immune surveillance, and colonize new organs. Current therapies often fail to eradicate these Disseminated Tumor Cells (DTCs), leading to metastatic recurrence. Targeted clearance of DTCs, therefore, presents an opportunity to prevent metastasis. Immunotherapy holds immense promise as a strategy to sustain a systemic anti-tumor immune response against metastasizing cells. However, current immunotherapies yield low response rates and metastatic recurrence remains frequent. Increasing evidence suggests that this is because DTCs develop distinct tactics to evade immune responses that differ from those employed in a primary tumor. As such, leveraging the immune system to target DTCs requires precise understanding of how tumor cells overcome anti-tumor immunity upon dissemination. Studies in this context to date remain limited, largely due to the challenge of studying the interactions between surveillant immune cells and the rare surviving DTCs that resist their attack. Developing effective immunotherapies to eliminate DTCs requires a stronger understanding of immune evasion during the initial stage of metastatic seeding. To address this question, we leveraged a visible antigen and cognate CD8+ T cells in a TNBC model of lung metastasis to profile resistant DTCs and identify their acquired mechanisms of immune escape. These studies uncovered a program of Glucocorticoid Receptor (GR) activation in disseminated tumor cells that evade immune response. We found that loss of GR in cancer cells reduced initial metastatic seeding by sensitizing DTCs to both CD8+ T cell and Natural Killer (NK) cell elimination, proving that GR activation drives metastasis by providing DTCs with broad resistance to cytotoxic lymphocytes. We engineered a niche labeling system to profile lymphocytes that interact with DTCs and discovered Fas-Fas Ligand (FasL) as a key cytotoxic pathway in DTCs for early metastatic survival. GR activation represses the death receptor Fas on tumor cells, which blocks FasL-mediated killing by cytotoxic lymphocytes. Pharmacologic inhibition of GR in combination with immunotherapy drastically reduced DTC metastatic survival and extended mouse lifespan. Thus, we uncovered the GR-Fas axis as a metastasis-specific mechanism of immune evasion that operates specifically in DTCs and protects from pan-cytotoxic immunity. These findings illustrate the unique features of cancer-immune interactions during different stages of disease, and highlight the distinct adaptations developed by DTCs for immune resistance at this bottleneck stage of the metastatic cascade. Therapeutic strategies to prevent metastasis are lacking, and our findings suggest that there are opportunities to precisely eliminate DTCs separately from treatments aimed at primary tumors. This work proposes GR as one promising combinatorial target to attack DTCs and reduce metastatic recurrence.