Clarification of vasculature-associated lymphoid tissue as a key player in avian respiratory immunity

Broiler chickens are intensively reared to meet the growing meat demand. A relatively high growth rate makes them susceptible to various infectious diseases, among which respiratory infections are particularly detrimental. Importantly, immune cells from lymphoid tissue play a cardinal role against these respiratory pathogens. In our previous study, we identified a unique type of vasculature-associated lymphoid tissue (VALT) within the kidney that regulated the mobilization of immune cells in the kidney and the renal outcome. However, VALT has not been fully elucidated in chicken lungs. In this study, for the first time, we characterized its structural and cellular features in chicken lungs. Lung samples from 20 birds in each age group (1, 15, and 28 days) were examined via hematoxylin and eosin staining, picrosirius red staining and immunofluorescence. VALTs appeared in the lungs of day-old chicks as scattered immune cells. These cells began to be diffusely organized at 15 days and were fully developed and structurally distinct at 28 days. Cellular characterization at 28 days revealed that VALT consists of B cells, T cells, and macrophages. VALT contained blood vessels, lymphatic vessels, high endothelial venules, and a well-organized connective tissue network, which supports, organizes, and mobilizes the immune cells. VALT also contained macrophages and MHCII⁺ molecules, indicating potential for antigen presentation. Therefore, VALT in chicken lungs may provide a structural basis for future investigations into respiratory immunology, including potential implications for vaccine development and therapeutic strategies.