To validate the specificity of PD-L1-dependent PD-1 activation, WT HEK 293 T cell or human PD-L1-overexpressing HEK 293 T cells (Fig. 2b) and breast cancer cell lines MCF7 (PD-L1 (−)) or MDAMB231 (PD-L1 (+)) (Fig. 2c,e,f) were used. Here, PDCD1 is linked to breast cancer.