Lysosomal cysteine proteases in the lung: role in protein processing and immunoregulation

Lysosomal cysteine proteases are a family comprising > 10 enzymes. For many years it was believed that these enzymes catalyse protein breakdown unselectively, are highly redundant in their substrate specificity and are also expressed ubiquitously. This view has changed dramatically since a number of new lysosomal cysteine proteases with restricted expression and outstanding enzymatic activity have been described. In addition, knockout mice and selective protease inhibitors have been used to characterise specific functions of single proteases. In this review, some of these functions are discussed in relation to the lungs, especially the role of lysosomal cysteine proteases in matrix remodelling, immunoregulation and surfactant protein processing.