MMP12 and pulmonary emphysema: The development of emphysema is thought to be caused by several factors, including proteolytic damage of alveolar septi by proteases released from neutrophils and macrophages in chronic airway inflammation, such as neutrophil elastase (NE) and macrophage elastase (MMP12), and potentially other processes related to alveolar septation during development and the persistent hyperinflation of the lungs causing air trapping [4].