PDCD1 and hepatocellular carcinoma: Moreover, MET-CAR.CD28ζ T cells demonstrated significantly higher PD-1 expression than MET-CAR.4-1BBζ T cells (HS: 49.6 ± 19.1 vs. 27.0 ± 16.5, HCC: 50.9 ± 18.9 vs. 31.8 ± 15.1, paired t test, p < 0.01) (Fig. 5E).