MMP9 and bronchiolitis obliterans syndrome: It has been demonstrated that levels of matrix metalloproteinase-2 (MMP-2), matrix metalloproteinase-9 (MMP-9), IL-6, and TGF-β were significantly elevated, along with an increase in the numbers of leukocytes, neutrophils, and lymphocytes in the lung tissues and BALF of mice with BO.