Development of secretory leukocyte protease inhibitor (SLPI) variants resistant to degradation by neutrophil elastase

Cystic fibrosis (CF) is characterised by repeated cycles of lung infection and inflammation, causing a gradual destruction of lung tissue and eventually the death of over 95% of patients due to respiratory failure. Neutrophil elastase (NE), a serine protease released by neutrophils as part of the innate immune response, plays a key role in this lung tissue degradation process. Secretory leukocyte protease inhibitor (SLPI) is a small cationic antiprotease displaying potent NE inhibition but also anti-inflammatory and antibacterial properties. SLPI has previously shown promising therapeutic activity in the treatment of CF lung disease, however its potential benefits are compromised by its rapid cleavage due to excessively high levels of NE in the Pseudomonas aeruginosa infected lung. In this study, two SLPI variants (SLPI-A16G and SLPI-S15G-A16G) were engineered by site-directed mutagenesis for resistance to cleavage by NE, in an attempt to improve the efficacy of this antiprotease. The variants showed enhanced resistance to degradation in the presence of excess NE as well as CF patient sputum when compared to wild-type SLPI. Both proteins retained strong NE inhibitory activity. The abilities of each variant to bind bacterial LPS and to interact with NF-κB DNA binding sites, two properties important for SLPI antibacterial and anti-inflammatory activity, were also preserved. These SLPI variants may therefore have increased therapeutic potential in the treatment of CF.