Figure 1. PD-1/PD-L1 and CTLA-4 pathways: the TCR of CD8⁺ T cell activates upon recognizing the tumor antigen presented on MHC class I and consequently induces the expression of PD-L1 on tumor cells. PD-L1 conjugates the elevated PD-1 on T cell surface, triggering inhibitory effect of PD-1/PD-L1 axis. In addition, CD28 and ligands B7 (CD80 and CD86) are exposed to the TME, T cells become unreactive or eliminated by programmed cell death. CTLA-4 competes with CD28 to bind the costimulatory B7 receptors on antigen presenting cells. CTLA-4 transmits an inhibitory signal to T cells, whereas CD28 transmits a stimulatory signal. APC: antigen-presenting cell; CTLA-4: cytotoxic T-lymphocyte-associated protein 4; MHC: major histocompatibility complex; TCR: T-cell receptor; PD-1: programmed death-1; PD-L1: programmed death-ligand 1; TME: tumor microenvironment. Created in BioRender. Dey, T. (2025) https://BioRender.com/b08y621