PDCD1 and neuroblastoma: Currently, to enhance CAR T cell activity against solid tumors, there are active clinical trials investigating combinatorial approaches, such as oncolytic adenovirus combined with HER2-targted CAR T for multiple types of solid tumors (NCT03740256); pembrolizumab (PD-1 inhibitor) combined with EGFRvIII-targeted CAR T for glioblastoma (NCT03726515); PD-1 knockout engineered T cells combined with MUC1-targeted CAR T for advanced esophageal cancer (NCT03706326); varicella zoster virus combined with GD2-targted CAR T cells for advanced osteosarcoma and neuroblastoma (NCT01953900).