PDCD1 and breast cancer: In the model we established, to provide preclinical data for both tumor microenvironment and immunotherapy studies related to the interaction between breast cancer and immune cells, T cell activation was initially achieved using PHA-M (10 μg/mL) and recombinant human PD-1 protein (1 μg/mL) for 72 h, thus increasing PD-1 expression levels (Supplementary Figures S1–S3 and Table S1) (p < 0.001).