PDCD1 and neoplasm: found that programmed cell death 1 (PD-1), which is widely expressed in tumor-infiltrating lymphocytes of triple-negative breast cancer (TNBC) and is significantly associated with poor prognosis of TNBC (33, 34), can be secreted by activated T cells on the surface of exosomes, interacting remotely with PD-L1 on the cell surface or exosomes, and restoring tumor surveillance by attenuating PD-L1-induced suppression of tumor-specific cytotoxic T cell activity.