Molecular Prerequisites for Neutrophil Extracellular Trap Formation and Evasion Mechanisms of Staphylococcus aureus

Abstract

NETosis is a multi-facetted cellular process that promotes the formation of neutrophil extracellular traps (NETs). NETs as web-like structures consist of DNA fibers armed with granular proteins, histones, and microbicidal peptides, thereby exhibiting pathogen-immobilizing and antimicrobial attributes that maximize innate immune defenses against invading microbes. However, clinically relevant pathogens often tolerate entrapment and even take advantage of the remnants of NETs to cause persistent infections in mammalian hosts. Here, we briefly summarize how Staphylococcus aureus, a high-priority pathogen and causative agent of fatal diseases in humans as well as animals, catalyzes and concurrently exploits NETs during pathogenesis and recurrent infections. Specifically, we focus on toxigenic and immunomodulatory effector molecules produced by staphylococci that prime NET formation, and further highlight the molecular and underlying principles of suicidal NETosis compared to vital NET-formation by viable neutrophils in response to these stimuli. We also discuss the inflammatory potential of NET-controlled microenvironments, as excessive expulsion of NETs from activated neutrophils provokes local tissue injury and may therefore amplify staphylococcal disease severity in hospitalized or chronically ill patients. Combined with an overview of adaptation and counteracting strategies evolved by S. aureus to impede NET-mediated killing, these insights may stimulate biomedical research activities to uncover novel aspects of NET biology at the host-microbe interface.

Keywords: NETosis; Staphylococcus aureus; immune evasion; neutrophil extracelluar traps; pathogenesis.

Figure 1

NET formation pathways in response to S. aureus. NET formation in response to live staphylococci and their exoproducts may occur via two predominant signaling pathways. While suicidal NETosis leads to neutrophil cell death (left panel), rapidly occurring vital NET formation retains the ability of PMNs to migrate and phagocytose bacterial invaders (right panel). Numbers 1-6 in each panel indicate the order of events during NET formation. S. aureus readily escapes from NET-mediated entrapment and killing by secreting multiple virulence determinants (e.g. thermonuclease (Nuc)), thereby boosting staphylococcal persistence and dissemination of disease. Characteristic features and key host signaling molecules including toll-like receptor 2 (TLR2), myeloperoxidase (MPO), neutrophil elastase (NE), protein-arginine deiminase type 4 (PAD4), cathelicidin LL-37, reactive or mitochondrial reactive oxygen species (ROS; mtROS), protein kinase C (PKC), Raf–MEK–ERK cascade (Raf; MEK; ERK), membrane-bound NADPH oxidase, processed gasdermin D (GSDM-D) and associated pores are highlighted.

Figure 2

Fluorescence microscopy images of human blood-derived neutrophils forming NETs during incubation with S. aureus. Human blood-derived neutrophils were stimulated with FITC-labelled S. aureus Newman for 90 min (green). Next, the formation of NETs was visualized using confocal fluorescence microscopy with antibodies against DNA-histone-complexes (red) as previously described (48). Nuclei were stained with DAPI (blue). The main image shows staphylococci (green) entrapped by NETs (red).