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Figure 4. Representation of T cell activation in antitumor response and deactivation through immune checkpoints. Antigen-presenting cells (APCs) process and package tumor antigens into peptides to be displayed on its surface with major histocompatibility complex (MHC) molecules to T cells, accompanied by costimulatory signal binding of CD28 and CD80/86 to promote T cell activation. In addition, the presentation of tumor surface antigens via MHC activates T cell response and expansion. The binding of PD-L1/2 to PD-1 and CD80/86 to CTLA4 suppresses the antitumor immune response, allowing immune evasion by tumor cells. Blockade of these immune checkpoints through ICI restores T cell activity and reactivates the antitumor response. ICI: Immune checkpoint inhibitor.