Immunotherapy has shown remarkable success in some types of cancer, especially melanoma. While a modest proportion of patients with triple-negative breast cancer (TNBC) respond well to a form of immunotherapy, called anti-PD-L1 therapy, the majority do not respond - probably because they have too few or inactive lymphocytes (cancer cell-killing immune cells) in the tumour. We have evidence that neutrophil-like cells that normally fight bacterial infections can turn into another type of cell (G-MDSC) in a cancer patient, which in turn helps the cancer grow and spread. By targeting these G-MDSC, we hope to harness the benefits of immunotherapy for many more patients with TNBC. To achieve this goal, we will test three new therapies for their ability to inhibit the G-MDSC population without affecting the actual neutrophils needed to fight infection. The outcome will be a combination therapy targeting G-MDSC cells and PD-L1 that will increase the reach and response to immunotherapy in TNBC patients.