PO-386 Dissecting the role of regulatory T cells in metastatic breast cancer

IntroductionBreast cancer is the most frequent malignancy among women worldwide. More than 90% of breast cancer-related deaths are due to metastatic disease. Despite these facts, breast cancer metastasis remains incurable. A key player in metastasis is the immune system. Cancer-induced immunosuppression contributes to a tumour’s ability to evade immune destruction. A major cell type in this process is the regulatory CD4+ T cell (Treg). It has been reported that Tregs are found in the microenvironment of primary tumours and metastases, and that their depletion reduces metastatic burden; however, the underlying mechanisms remain poorly understood. Using state-of-the-art breast cancer mouse models that closely recapitulate primary human breast cancer and metastatic disease, our goal is to elucidate the impact of Tregs on breast cancer metastasis, focusing on differences between (pre-)metastatic tissues and different steps in the metastatic cascade.Material and methodsTo study primary spontaneous mammary tumours and the pre-metastatic niche, we primarily use the FVB K14cre;Cdh1F/F;Trp53F/F (KEP) conditional mouse model for invasive lobular carcinoma. For metastatic disease, KEP tumour fragments are orthotopically transplanted into in wild-type syngeneic FVB mice. Following tumour outgrowth and mastectomy, widespread metastatic disease is present in lungs, lymph nodes and other distant organs, providing an accurate representation of the different steps of the metastatic cascade.Results and discussionsWe observed systemically elevated levels of Tregs prior to metastatic disease. These tumor-educated Tregs display a distinct phenotype and specifically accumulate in the lung and lymph node metastases that arise in our metastasis models, perhaps indicating their possible importance for metastasis formation and progression.In addition to these systemic changes, within primary tumours and metastases a large population of PD1high Tregs is found. Preliminary data suggest that tumor-associated myeloid cells influence this population.ConclusionWe are currently setting out to dissect the mechanism behind this interplay of intra-tumoral immune cells and the role of distinct Tregs found prior to metastatic disease.