Distribution of the CD15 epitope in the mammalian developing lung is opposite in mouse compared with human

The distribution of the expression of the CD15 epitope was characterized by immunohistochemistry in the developing mouse and human lung on embryonic days E9.5-E19 and gestational weeks GW7-GW25, respectively. In the earliest stages in the mouse, the tracheal epithelial cells expressed CD15 on their apical and lateral cell membranes and, in the more proximal regions, also showed a faint cytoplasmatic CD15 expression. Only very few epithelial cells in the bronchial bud regions expressed CD15 on their apical surfaces. In later stages (E12-E17), cells in the proximal parts of the bronchi and bronchioli expressed CD15 on their apical, but also on their lateral membranes, and increasing numbers of cells expressed CD15 cytoplasmatically. Cells in the distal, presumably proliferating, areas of the bud regions were CD15 negative. This distribution pattern of CD15 was consistent until the latest embryonic stages. These results are completely opposite to those found in human developing lung where up to GW20 bronchial and bronchiolar bud regions were CD15 positive, while in the proximal parts of the airways the vast majority of cells were CD15 negative. After GW20, CD15 immunoreactivity in the bud regions vanished and was completely absent on GW25. This difference between human and mouse adds further evidence to profound species differences in the expression of CD15 in various organs, e.g., in the cerebellum or the retina, and should be taken into account when considering functional roles of CD15 and also when relating results from a (transgenic) mouse model to the respective human organ system.