MMP9 and chronic obstructive pulmonary disease: Overexpression of MMP9 in alveolar macrophages, bronchial tissues, sputum, and serum, has been previously associated with chronic obstructive pulmonary disease (COPD), emphysema, and asthma [46] and upregulation of MMP9 in the blood has been associated with severe COVID-19 [7,47].