The metastatic spread of cancer from the location of the primary tumour to other tissues is associated with an estimated 90% of cancer-related deaths, highlighting the critical need for novel effective therapeutic strategies to treat and prevent metastatic disease. Recent studies have suggested that the primary tumour of cancer patients secretes factors that cause host immune suppressor cells to accumulate in other distant tissues. These cells locally repress the anti-tumour response of the host immune system, providing a new favourable location for metastatic cancer cells to develop. Clinical evidence has shown that the immune suppressor cells that accumulate in distant tissues of breast cancer patients are comparable to those found in mice bearing metastatic mammary tumours. We have found that these immune suppressor cells accumulate in the lung tissue of mouse models of metastatic breast cancer and persist even after the primary tumour has been removed. The goal of my research is to find unique cell surface markers that allow this resistant subset of immune suppressor cells to be used to identify patients that are at higher risk of developing metastatic disease after primary tumour resection. I then intend to design a method to systematically screen various compounds to identify specific inhibitors of these immune suppressor cells based on their unique cell surface markers. These data will have important clinical implications for breast cancer patients, as inhibiting this subset of immune suppressor cells could assist in the reduction of secondary metastatic tumour growth after primary tumour removal in the treatment of breast cancer.