Significance of evaluating tumor-infiltrating lymphocytes (TILs) and programmed cell death-ligand 1 (PD-L1) expression in breast cancer

The immune system affects all phases of tumor growth from initiation to progression and dissemination. Tumor-infiltrating lymphocytes (TILs) are mononuclear immune cells that infiltrate tumor tissue. Several retrospective studies have suggested the potential of TILs as a prognostic as well as predictive factor of chemotherapy in some breast cancers. On the other hand, programmed cell death protein-1 (PD-1) and programmed cell death-ligand 1 (PD-L1) eliminate T cell activation in various types of cancers. Prospective trials to evaluate the efficacy of antibody agents to PD-1 and PD-L1 are ongoing in patients with breast cancer. The findings of these studies appear to support the potential of immune checkpoint inhibitors targeting the PD-1/PD-L1 axis in triple negative breast cancer. Further studies are needed in order to confirm previous findings on TILs and promote the development of new immune therapy approaches for breast cancer patients. Furthermore, the search for TILs will soon be introduced into actual clinical practice, for which the standardization of evaluation methods and establishment of a simple evaluation method are expected.