CTLA-4 function and the impact of immune checkpoint blockade. (A) In health, Tregs express CTLA-4, which binds CD80 and CD86 expressed on APCs. CTLA-4 binds to CD80 and CD86 with higher affinity and avidity than does CD28, preventing conventional T cell (Tcon) stimulation through CD80/CD86 interaction with CD28. Removal of CD80/CD86 ligands by transendocytosis results in impaired costimulation of T cells via CD28, resulting in immune regulation. (B) When the immune system is stimulated in the presence of inflammatory/innate immune stimuli, APCs upregulate the expression of CD80/CD86, overcoming their control by Treg, enabling costimulation and the proliferation of T cells. (C) In the tumor microenvironment, CD80/CD86 is controlled by Treg, and the abundance of Treg leads to the suppression of immune responses. (D) Anti–CTLA-4 antibodies bind to CTLA-4 molecules with high affinity, leading to Treg depletion or functional blockade, resulting in enhanced T-cell activation and immunological responses to cancer. The impacts of CTLA-4 blockade can be mediated by a variety of mechanisms: prevention of transendocytosis increasing CD80/CD86 levels on APCs, direct Treg cytotoxicity, and antibody-dependent cellular cytotoxicity mediated by FcR-IV expressing intratumoral macrophages. Note: Only CD80 is shown here for clarity. ADCC, antibody-dependent cellular cytotoxicity; MHC, major histocompatibility complex.