Expression of cathepsins B, H, K, L, and S during human fetal lung development

Cathepsins are involved in lysosomal protein degradation, proenzyme activation, antigen processing, and hormone maturation. They are secreted by tumor cells and macrophages and catalyze the remodeling of extracellular matrix proteins. To gain insight into the expression pattern of cathepsins during fetal lung development, the expression of cathepsins B, H, K, L, and S at protein and mRNA levels were evaluated by using immunohistochemistry and in situ hybridization. Early expression of cathepsins B, H, and K was found in epithelial cells of the branching presumptive bronchi (<12th week of gestation). The most intense cathepsin K-specific immunoreactivity was found in developing airways with a lumen. Cathepsin K was found in epithelial cells only, whereas in contrast, cathepsins B and H were detected both in epithelial and interstitial cells. During fetal maturation, interstitial cells displayed cathepsin L immunoreactivity and, in the saccular phase (>26th week of gestation), both cathepsin L and S immunoreactivities. A continuous decline in the proportion of cathepsin H-positive interstitial CD68-positive cells was observed. These discrete temporal and spatial variations in cathepsin expression during organogenesis of the human lung indicate different physiological roles for the individual enzymes in different cell types and developmental stages.