The clinical application of T cells engineered with chimeric antigen receptors (CARs) for solid tumors is challenging. A major reason for this involves tumor immune evasion mechanisms,including the high expression of immune checkpoint molecules,such as the programmed death 1 (PD-1) ligands PD-L1 and PD-L2. The inducible expression of PD-L1 in tumors has been observed after CAR-T-cell infusion,even in tumors natively not expressing PD-L1. Furthermore,numerous types of pediatric cancer do not have suitable targets for CAR-T-cell therapy. Therefore,the present study aimed to develop novel CAR-T cells that target PD-L1 and PD-L2,and to evaluate their efficacy against pediatric solid tumors. A novel CAR harboring the immunoglobulin V-set domain of the human PD-1 receptor as an antigen binding site (PD-1 CAR-T) was developed without using a single-chain variable fragment. PD-1 CAR-T cells were successfully manufactured by adding an anti-PD-1 antibody,nivolumab,to the ex vivo expansion culture to prevent fratricide during the manufacturing process due to the inducible expression of PD-L1 in activated human T cells. The expression of PD-L1 (and PD-L2 to a lesser extent) was revealed to be highly upregulated in various pediatric solid tumor cells,which displayed no or very low expression initially,on in vitro exposure to interferon-γ and/or tumor necrosis factor-α,which are cytokines secreted by tumor-infiltrating T cells. Furthermore,PD-1 CAR-T cells exhibited strong cytotoxic activity against pediatric solid tumor cells expressing PD-L1 and PD-L2. Conversely,the effect of PD-1 CAR-T cells was significantly attenuated against PD-L1-positive cells coexpressing CD80,suggesting that the toxicity of PD-1 CAR-T cells to normal immune cells,including antigen presenting cells,can be minimized. In conclusion,PD-1 ligands are promising therapeutic targets for pediatric solid tumors. PD-1 CAR-T cells,either alone or in combination with CAR-T cells with other targets,represent a potential treatment option for solid tumors.
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