With approximately 70,000 sufferers and 8 million carriers worldwide, cystic fibrosis (CF) is one of the most common lethal genetic diseases. CF is caused by mutations in the gene encoding the cystic fibrosis transmembrane regulator (CFTR). These mutations result in altered anion (Cl- and HCO3-) secretion across airway epithelia which in combination with Na+ hyperabsorption via the epithelial sodium channel (ENaC)(1), results in reduced airway surface liquid (ASL) volume, impaired mucociliary clearance, chronic inflammation and bacterial colonization. As such, CF airways become obstructed due to the presence of excessive mucopurulent material (MM) which contains numerous proteases. We have previously reported that the secreted protein, SPLUNC1, regulates ENaC activity and hence, ASL hydration levels and that SPLUNC1 regulation is defective in CF airways(2). We hypothesize that proteases present within the MM can cleave SPLUNC1 and may alter its ability to regulate ENaC. Post-mortem MM was harvested from Pseudomonas aeruginosa- and Staphyloccocus aureus- positive CF lungs. MM was incubated with SPLUNC1 at 37°C and SPLUNC1 levels were determined by western blot. Significant breakdown of 10 µM SPLUNC1 was observed after incubation with MM diluted 1:2 with PBS (t 1/2 = 1.17 hours; N = 3). To determine which enzymes were responsible for SPLUNC1’s cleavage, MM was pre-incubated with protease inhibitors including aprotinin, EDTA, E64 and leupeptinin. These compounds failed to inhibit SPLUNC1 cleavage (all N = 3). Sivelestat, a neutrophil elastase (NE) inhibitor, significantly prevented SPLUNC1 cleavage (10µM **** p < 0.0001, N = 4). Given that SPLUNC1 regulates ENaC activity and SPLUNC1 is cleaved by MM, the effects of MM and SPLUNC1, alone and in combination, on ENaC protein levels in normal human bronchial epithelial cells (HBECs) were investigated. Our preliminary data (N = 3) indicated that MM proteolytically cleaved γENaC and increased levels of cleaved γENaC subunit expression were observed. Data from normal HBECs incubated with SPLUNC1 alone or SPLUNC1 and SMM in combination suggests decreased γENaC protein levels, consistent with ENaC degradation and reduced ENaC activity. The effect of SPLUNC1 and MM in CF HBECs remains to be determined. In conclusion, we have shown that MM readily cleaves SPLUNC1, and that cleavage can be prevented by sivelestat. MM cleavage of SPLUNC1 may alter SPLUNC1’s ability to regulate innate defense in CF airways.