The BALT region in CSF1R-transgenic birds. The BALT of the chicken lung is constitutive, consisting of several GC (A) with tightly packed CSF1R-eGFP+ cells in the light zone and a characteristic halo of MPN surrounding the GC (B). In the GC, CSF1R-transgene+ cells express CD11 (C) and lack MHC II (D). TIM4+ CSF1R-transgene− cells are present in the GC known as tingible body macrophages. Outside the GC occasional TIM4+ CSF1R-transgene+ cells were observed (E). CSF1R-eGFP+ cells can be found in regions of the epithelial overlaying the BALT (FAE), called M cells (F–H). Lung M cells lack MHC II (F) and TIM4 (G) expression. The M cells express high levels of F-actin (H) similar to IFE cells (I). M cells lack jacalin binding in contrast to the IFE (J). LAMP1 is expressed in CSF1R-transgene+ (yellow arrow, K) and CSF1R-transgene− cells (white arrow, L). LAMP1 is also highly expressed by epithelial cells (L). GC are indicated by white dashed lines with halo of cells identified with a yellow dashed circle, epithelial layer is indicated by yellow dashed lines.