Cynomolgus Monkey (Macaca fascicularis) CTLA-4 (Cytotoxic T-Lymphocyte–Associated Protein 4, also known as CD152) is an immune checkpoint receptor belonging to the immunoglobulin superfamily that functions as a key negative regulator of T-cell activation. CTLA-4 is primarily expressed on activated CD4⁺ and CD8⁺ T lymphocytes and on regulatory T cells (Tregs) following antigen stimulation. It binds to the costimulatory ligands CD80 (B7-1) and CD86 (B7-2) on antigen-presenting cells such as dendritic cells, macrophages, and B cells, competing with the activating receptor CD28 for these ligands. Engagement of CTLA-4 transmits inhibitory signals that suppress T-cell proliferation, cytokine production, and immune activation, thereby maintaining immune homeostasis and peripheral tolerance. Because cynomolgus monkeys share strong genetic and immunological similarity with humans, CTLA-4 signaling in this species closely mirrors human immune checkpoint regulation. As a result, cynomolgus monkeys are widely used as preclinical animal models for immune checkpoint therapies, including studies evaluating anti-CTLA-4 monoclonal antibodies such as ipilimumab, where the model helps assess immune activation, toxicity, and antitumor responses before clinical use in humans. These studies make cynomolgus monkey CTLA-4 important in translational immunology, cancer immunotherapy development, and immune checkpoint research.