PDCD1 and neoplasm: Interestingly, the xenograft tumor model treated with a low dose of anti-PD-1 antibody failed to inhibit tumor growth or enhance the antitumor efficacy of CAR-T cells even though the amount of circulating antibody (~0.7 mg/mL) was still 15-fold higher than the amount detected in the anti-PD-1 antibody secreting CAR-T cell treatment group.