Visualisation and characterisation of mononuclear phagocytes in the chicken respiratory tract using CSF1R-transgenic chickens

The respiratory tract is a key organ for many avian pathogens infections as well as a major route for vaccination in the poultry industry. To improve immune responses after vaccination of chickens through increased uptake of vaccines and targeting to antigen presenting cells, a better understanding of the avian respiratory immune system is required. We used transgenic MacReporter birds expressing a reporter gene (eGFP or mApple) under the control of the CSF1R promoter and enhancer in cells of the mononuclear phagocyte (MNP) lineage to visualize the ontogeny of the lymphoid tissue, macrophages and dendritic cells, in the trachea, lung and airsac of birds from embryonic day 18 to 63 weeks of age. Small aggregates of CSF1R-transgene+ cells start to form at the openings of the secondary bronchi at 1 week of age, indicative of the early development of the organized bronchus-associated lymphoid tissue. Immunohistochemical staining revealed subpopulations of MNPs in the lung, based on expression of CSF1R-transgene, CD11, TIM4, LAMP1, and MHC II. Specialised epithelial cells or M cells covering the bronchus-associated lymphoid tissue expressed CSF1R-transgene and type II pneumocytes expressed LAMP1 suggesting that these epithelial cells are phagocytic and transcytose antigen. Highly organised lymphoid tissue was seen in the trachea from 4 weeks onwards. Throughout the airsacs at all ages, CSF1R-transgene+ cells were scattered and at later stages CSF1R-transgene+ cells lined the capillaries. The results presented will serve as a base for further functional characterization of macrophages and dendritic cells and their role in respiratory diseases and vaccine responses.