BPIFA1 and chronic obstructive pulmonary disease: Our current study demonstrates that: (1) HNE directly reduced human airway epithelial defense against NTHi; (2) HNE degraded human SPLUNC1 protein; (3) Recombinant SPLUNC1 protein reduced NTHi levels in HNE-treated human airway epithelial cells; (4) NTHi levels in lungs of SPLUNC1 knockout mice were increased compared to wild-type mice; and (5) SPLUNC1 was reduced in lungs of COPD patients.