Ovine TNFSF15 (Tumor Necrosis Factor Superfamily Member 15, also known as TL1A or VEGI—vascular endothelial growth inhibitor) is a cytokine belonging to the tumor necrosis factor (TNF) superfamily, which includes related ligands such as TNF-α, Fas ligand (FasL), CD40 ligand (CD40L), TRAIL, and RANKL that regulate immune signaling, apoptosis, and inflammatory responses. In sheep (Ovis aries), TNFSF15 is produced primarily by endothelial cells, macrophages, dendritic cells, and other immune cells in response to infection or inflammatory stimuli. TNFSF15 exerts its biological effects mainly through binding to death receptor 3 (DR3, TNFRSF25) on T lymphocytes and other immune cells, activating intracellular signaling pathways such as NF-κB and MAPK that regulate T cell activation, cytokine production, and inflammatory responses. In ovine physiology, TNFSF15 is also associated with regulation of vascular endothelial cell growth and angiogenesis, consistent with its designation as vascular endothelial growth inhibitor (VEGI). Through these functions, TNFSF15 contributes to immune regulation, inflammatory responses, and vascular homeostasis in sheep. Because cytokines of the TNF superfamily are key mediators of host defense and inflammatory signaling, ovine TNFSF15 is of interest in veterinary immunology and infectious disease research, particularly for understanding immune responses to bacterial, viral, and parasitic infections that affect sheep health and productivity.