Facts and challenges of immunotherapy in triple-negative breast cancer

Triple-negative breast cancer (TNBC) is an aggressive but common cancer subtype in clinical practice. Immune activation has been observed in a subgroup of TNBC, suggesting that immunotherapy may be a potential therapeutic option. With the widespread use of monotherapy, specific immune checkpoint inhibitors (ICIs) such as avelumab, pembrolizumab, and atezolizumab have made significant contributions to improving outcomes in both early and advanced TNBC. In addition, the expressions of immune regulators such as cytotoxic T-lymphocyte-associated protein 4, programmed cell death 1 (PD-1), and programmed cell death-ligand 1 (PD-L1), which are influenced by tumor-infiltrating lymphocytes (TILs), are also critical factors in determining the effect of immunotherapy in TNBC. This review focuses on the updates on the biological underpinnings of TNBC and the associated treatment advances. We present the current landscape of well-known immune regulators and widely used ICIs for TNBC and highlight the future directions that are significant for further improving the efficacy and effect of targeted therapeutic strategies to immunotherapy in TNBC and more reliable prognostic predictions for tailored therapy in the future.