MMP9 and chronic obstructive pulmonary disease: COPD is characterized by the development of a complex inflammatory response in the lungs due to chronic exposure to irritants such as CS [5], consisting of the infiltration of inflammatory cells (e.g., macrophages, neutrophils, and T-lymphocytes) and increased expression of proinflammatory mediators (e.g., TNF-α, IL-6, and IL-1β) and matrix metalloproteinases (e.g., MMP-9 and MMP-12) [6].