PDCD1 and neoplasm: However, many tumors, including neuroblastoma and other pediatric tumors, do not respond or have limited response to CTLA-4 and PD-1/Programmed death-ligand 1 (PD-L1) blockade (32, 33), and studies have found mechanisms of resistance, including poor infiltration of T cells, tumor cells that lack response to interferon-γ (IFN-γ), and immunosuppressive cell populations in the tumor microenvironment (61).