MMP9 and cystic fibrosis: Biochemical studies showed reciprocal processing of endogenous inhibitors of MMP-9 and NE, where MMP-9 was able to inactivate the NE inhibitor α1-antitrypsin and NE, in turn, inactivated TIMP1 [19,20], suggesting that MMP-9 and NE may reciprocally increase their activities in CF airways.