PDCD1 and neoplasm: In addition to the PD-L1/PD-1 axis, other immune checkpoint molecules, including CTLA-4, lymphocyte activation gene 3 (LAG-3), TIM-3, B and T lymphocyte attenuator (BTLA), T cell immunoreceptor with Ig and ITIM domains (TIGIT) or V-domain Ig suppressor of T cell activation (VISTA), engage with specific molecules expressed at the surface of the tumor cells and inhibit T cell proliferation, cytokine production and cytolytic function [48].