Age-related development and functional maturation of bronchus-associated lymphoid tissue (BALT) in broiler chickens

Around the world, broiler chickens are an essential source of affordable animal protein. But their intensive farming practices make them particularly vulnerable to respiratory infections, which cause significant economic losses. To control these respiratory infections and to establish therapeutic targets against diseases, a deep understanding of the respiratory immune system is essential. Bronchus-associated lymphoid tissue (BALT) is the main entry point of pathogens for chickens, but its development and cellular constituents in broiler chickens under physiological conditions remain poorly understood. Our present study aimed to investigate the age-related development, cellular composition, and role of BALT in the lungs of healthy broiler chickens. BALT was absent in day-old chicken, but small BALT appeared at 15 days of age, which later fully developed at 32 days of age. Cellular analysis revealed that fully developed BALT is composed of B-cells, T-cells, and macrophages. It also contains a well-organized vascular network, including high endothelial venules and lymphatic vessels. BALT shows the presence of macrophage and MHCII⁺ molecules, which indicate that antigen presentation occurs in BALT. In addition, germinal centers are present in mature BALT, indicating the maturation of B-cells and production of plasma cells. Therefore, BALT offers a new immunological gateway for respiratory-related therapeutic research and vaccine development.