Chemokines secreted by solid tumors are usually not suitable for T cell infiltration (Bertrand et al., 2014; Wang et al., 2017); 2) Activation obstacle: Immune checkpoints such as PD-1, TIM-3, and CTLA-4 will be upregulated once CAR-T cells bind to tumor cells, thereby inhibiting immune responses (Long et al., 2015; Chmielewski and Abken, 2017; Adachi et al., 2018; Ma et al., 2020). Here, PDCD1 is linked to neoplasm.