Neutrophil serine proteases in antibacterial defense

Abstract

Neutrophil serine proteases (NSPs) are critical for the effective functioning of neutrophils and greatly contribute to immune protection against bacterial infections. Thanks to their broad substrate specificity, these chymotrypsin-like proteases trigger multiple reactions that are detrimental to bacterial survival such as direct bacterial killing, generation of antimicrobial peptides, inactivation of bacterial virulence factors and formation of neutrophil extracellular traps. Recently, the importance of NSPs in antibacterial defenses has been further underscored by discoveries of unique bacterial evasion strategies to combat these proteases. Bacteria can indirectly disarm NSPs by protecting bacterial substrates against NSP cleavage, but also produce inhibitory molecules that potently block NSPs. Here we review recent insights in the functional contribution of NSPs in host protection against bacterial infections and the elegant strategies that bacteria use to counteract these responses.

Figure 1. Locations where bacteria encounter NSPs

(a) After neutrophils ingest opsonized bacteria, the granules fuse with the phagocytic vacuole to release NSPs and antimicrobial components. (b) During degranulation, neutrophils exocytose their granule contents into the extracellular space. (c) NSPs can translocate to the nucleus and induce NET formation. NSPs are embedded within NETs that capture bacteria.

Figure 2. Antimicrobial functions of NSPs

(a) NSPs can directly kill bacteria by attacking membrane associated (E. coli) or capsule proteins (S. pneumonia), which leads to loss of membrane integrity. (b) NSPs can cleave host immune proteins to generate antimicrobial peptides that can directly kill bacteria. (c) NSPs can target and inactivate bacterial virulence factors, resulting in attenuated bacteria.

Figure 3. Bacterial mechanisms to block NSPs

(a) Modification of bacterial substrates. (1) Glycosyltransferases SdgA and SdgB modify staphylococcal virulence factors with N-acetylglucosamine (GlcNAc) to prevent them being cleaved by CG. (2) LptA modifies Neisserial lipid A to prevent killing by CG. (b) Bacterial inhibitors of chymotrypsin-like serine proteases. (1) Gram-negative bacteria express ecotin, or the bMG proteins ECAM (E. coli) or MagD (P. aeruginosa) in their periplasmic space to prevent activity of a broad range of serine proteases. (2) S. aureus secretes the Eap family of proteins, consisting of Eap, EapH1, and EapH2, that specifically inhibits NSPs. Full-length Eap consists of multiple domains (resembling EapH1 and EapH2) that each bind one NSP molecule. (c) Model of NE, CG, and PR3 binding to different Eap proteins (EapH1, EapH2, and the second domain of Eap (Eap-D2)), inferred from the co-crystal structure of NE with EapH1 (PDB code 4NZL).