MMP9 and chronic obstructive pulmonary disease: Additionally, higher concentrations of mRNA for tumor necrosis factor (TNF)-α in BAL macrophages were positively correlated with BAL cadmium concentrations, as were proinflammatory cytokines interleukin-6 (IL-6), interleukin-8 (IL-8) and matrix metallopeptidase 9 (MMP-9, an extracellular matrix degrading protease thought to be involved in tissue remodeling during development and progression of chronic obstructive pulmonary disease) in cell-free sputum [79].