Bovine Programmed Death-Ligand 1 (PD-L1) (also known as CD274 or B7-H1) is an immune checkpoint protein that plays an important role in regulating T cell activation and immune tolerance in cattle (Bos taurus). PD-L1 is expressed on antigen-presenting cells such as macrophages and dendritic cells, as well as epithelial cells, endothelial cells, and some tumor or infected cells, and interacts with its receptor PD-1 (Programmed Death-1, CD279) on activated T lymphocytes. Engagement of PD-L1 with PD-1 delivers inhibitory signals that suppress T cell proliferation, cytokine production, and cytotoxic activity, helping to maintain immune homeostasis and prevent excessive inflammatory responses. In cattle, PD-L1 signaling has been associated with immune regulation during chronic infectious diseases, including bovine leukemia virus (BLV) infection, bovine tuberculosis caused by Mycobacterium bovis, and Johne’s disease caused by Mycobacterium avium subsp. paratuberculosis, where increased PD-L1 expression can contribute to T cell exhaustion and reduced pathogen clearance. PD-L1 is also involved in controlling inflammatory responses during infection and tissue damage. Because immune checkpoint pathways influence host immunity and pathogen persistence, bovine PD-L1 is an important target of study in veterinary immunology, chronic infection research, and development of immunotherapeutic or vaccine strategies aimed at improving disease resistance in cattle.