Neutrophil serine proteases in antibacterial defense
Introduction
Neutrophils are the most abundant circulating leukocytes [1] and are the first cells of the innate immune system to migrate to an infection site [1]. Neutrophils can rapidly kill bacteria using three mechanisms that all depend on their antimicrobial granular components (Figure 1) [2]. First, neutrophils can engulf bacteria (phagocytosis) and subsequently kill them inside the phagocytic vacuole after fusion with granules. Second, they can release their granular content into the extracellular milieu via exocytosis (degranulation) [1]. Third, they can release neutrophil extracellular traps (NETosis), which contain the antimicrobial granule proteins, to entrap and kill bacteria [3]. It is now evident that neutrophil serine proteases (NSPs) play key roles in each of these antibacterial responses.
This protease family consists of neutrophil elastase (NE), proteinase 3 (PR3), cathepsin G (CG) and the recently discovered neutrophil serine protease-4 (NSP4) [4]. NSPs are stored within the acidic granules tightly bound to proteoglycans that inactivate them [5]. They only become active after their release into the phagocytic vacuole [2, 6] where their concentrations are believed to reach as high as 50 mg/ml (based on calculations for MPO [5, 7, 8]). In addition to their intracellular role, NSPs are also important components of neutrophil ‘degranulation fluid’ and NETs [9]. NSPs belong to the chymotrypsin family of serine proteases, in which a charge-relay system of His-Asp-Ser forms the catalytic site (for excellent reviews on NSP biochemistry please read [10] and [11]). Despite their similar sequences (35–56% identical) and tertiary structures, however, they display different substrate specificities. Together they have the ability to cleave a wide variety of substrates. This broad substrate specificity, and the fact that they act at multiple locations (intracellular and extracellular), often complicates detailed understanding of NSP contributions to anti-bacterial host defense. Here we discuss recent insights into how NSPs contribute to the defense against bacteria and illustrate how bacteria can effectively antagonize NSP activity.
Section snippets
NSP functions in antibacterial defense
Although NSPs can also indirectly modulate the immune response, for instance by functioning as chemoattractants or cleaving chemokines (see [12, 13, 14] for recent reviews), we will here focus on the more direct interactions of NSPs with bacteria (Figure 2).
Bacterial mechanisms to block NSPs
Increasing evidence now shows that bacterial pathogens have evolved strategies to counteract human NSPs. The mechanisms identified thus far range from protecting bacterial substrates against proteolytic cleavage to the production of protease inhibitors that directly block NSPs (Figure 3).
Conclusions
Recent advances in understanding the molecular interplay between NSPs and bacteria now indicate that the role of NSPs in antibacterial host defense is much more diverse than simply directly killing of bacteria. Considering the few species it has been proven for, a directly bactericidal activity of NSPs seems very much overstated. On the contrary, a large body of evidence shows that these proteases can diminish bacterial virulence in many ways and their specific activities may even differ
References and recommended reading
Papers of particular interest, published within the period of review, have been highlighted as:
• of special interest
•• of outstanding interest
Acknowledgements
The authors were supported by grants of the Netherlands Organization for Scientific Research (NWO-Vidi # 91711379, to S.H.M.R.) and the U.S. National Institutes of Health (NIH # AI071028, to B.V.G.).
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