Neutrophil Serine Proteases in Health and Disease

Abstract

Neutrophils are filled with many antimicrobial agents, including the neutrophil serine proteases (NSPs); a group of proteases including Elastase, Proteinase 3, and Cathepsin G. Synthesized as inactive proforms, the NSPs are activated through proteolytic processing by Cathepsin C (CTSC). The NSPs have been demonstrated to degrade microbes in vitro. Therefore, NSPs have been described as crucial for microbial killing and are also believed to be critical for NETosis, a neutrophil-specific type of cell death capable of ensnaring extracellular microbes. Periodontitis is a destructive inflammation of the tooth supporting tissues initiated by colonizing bacteria. Neutrophils are abundantly present in the gingival crevice and more are recruited during periodontitis. Certain defects in neutrophil functions, like the Papillon-Lefèvre Syndrome (PLS), are associated with severe forms of periodontitis. PLS is a rare autosomal recessive loss-of-function mutation in the CTSC gene. The subsequent absence of CTSC activity in PLS neutrophils, results in absence of NSP activity. Interestingly, patients with PLS do not typically display increased susceptibility to opportunistic infections. The cardinal symptom of PLS is instead a rapidly progressing periodontitis with prepubertal onset. By studying PLS neutrophils, this thesis shows that NSP activity is indeed important for certain but not all types of NETosis (paper I). The work also demonstrates that PLS neutrophils from two families with distinct CTSC mutations, are indeed devoid of CTSC as well as NSP activities (paper II). Despite this, PLS neutrophils appeared to function normally with the exception of NETosis. To explain how lack of NSP activity results in periodontitis, it was hypothesised that NSPs may regulate inflammation by the cleavage of cytokines. In vitro, it was shown that whereas normal neutrophils were capable of degrading certain cytokines, PLS neutrophils were unable to do so (paper III). Most notably, IL-17A, IL-23, CCL20, and RANKL, all of potential importance for driving periodontal pathology, were quite susceptible to NSP-mediated cleavage. Other cytokines, e.g., IL-17F and CCL2, were resistant to NSP mediated cleavage indicating that degradation was not indiscriminate. In conclusion, this thesis shows that NSP activity is important for certain types of NET formation, but otherwise dispensable for basic neutrophil function. It also demonstrates that NSP activity may be able to regulate inflammatory processes and could help to explain the aggressive periodontal pathology seen in patients with PLS.