Chicken 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 chickens (Gallus gallus), TNFSF15 is primarily produced by endothelial cells, macrophages, dendritic cells, and other immune cells following infection or inflammatory stimulation. 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 including NF-κB and MAPK, which regulate T cell activation, cytokine production, and inflammatory responses. In chickens, TNFSF15 is also associated with regulation of vascular endothelial cell growth and angiogenesis, reflecting its alternative designation as VEGI. The cytokine contributes to immune responses against bacterial and viral pathogens, and it may influence T cell–mediated immunity and inflammatory regulation in avian species. Because chickens serve as important models for avian immunology and host–pathogen interactions, characterization of chicken TNFSF15 supports research on immune regulation, cytokine signaling, and inflammatory responses relevant to poultry health and disease resistance.