Of mites and men: trypsin-like proteases in the lungs

It is no surprise to anyone who has tried to memorize clotting and complement cascades that there are a great many trypsin-like proteases-too many, it may seem. This fecund enzyme family encompasses such a range of biological roles that other important members and functions are sure to await discovery. Of known trypsin-like proteases, few, if any, are lung-specific. Nonetheless, many contribute in ways critical to lung function, such as fighting microbial invaders, regulating the formation and removal of polymerized fibrin, and rejecting tumors or transplanted tissues. Some of these enzymes, such as plasminogen activators and mast cell tryptases, are native to the lung and live where they work; identical enzymes live and work in other tissues. Other enzymes, such as most complement and hemostatic proteases, are migrants, typically born elsewhere (e.g., the liver). They pass through the lung, looking for something to do; if nothing is found, they pass on, circulating, perhaps to return later. Still others are unwanted, uninvited intruders, such as dust mite proteases. This minireview provides a selective glimpse of the lives of some of the trypsin-like enzymes at work in the lung and airways.