C4B and chronic obstructive pulmonary disease: When comparing MMP12 substrates from murine peritonitis and joint inflammation model to the sputum of COPD patients, multiple identical substrates were identified including alpha-2-HS-glycoprotein, complement C3 (C3), complement C4-B (C4b), hemopexin, antithrombin III (SERPINC1), but also new substrates were identified such as transmembrane protease serine 7 (TMPRSS7) and DEP domain-containing mTOR-interacting protein (DEPTOR) [7,71].