The role of the immune system in triple-negative breast cancer

The interaction between tumour and immune cells in tumour stroma is very important for the formation, development and progression of cancer. The most evaluated interactions are tumour lymphocyte infiltration (TIL) and programmed death ligand 1 (PD-L1) expression. Higher TIL density is an independent prognostic factor for the achievement of complete pathologic remission (pCR) after neoadjuvant systemic treatment in HER2-positive and triple-negative breast cancer. In triple-negative breast cancer, TIL is also prognostic for overall survival. Many studies with immunotherapy (checkpoint inhibitors) are ongoing in triple-negative breast cancer. In routine clinical practice, patients can be treated with atezolizumab+Nab-paclitaxel in primary metastatic breast cancer and those with progession after more than 12 months after the end of adjuvant treatment if their tumours express PD-L1. Many, studies addressing the mechanisms of the immune system and biomarkers are ongoing to reveal why only a subset of patients profit from immunotherapy.