Relative antibacterial functions of complement and NETs: NETs trap and complement effectively kills bacteria. (May 2018)

• Record Type: Journal Article
• Authors: Azzouz, Louiza; Cherry, Ahmed; Riedl, Magdalena; Khan, Meraj; Pluthero, Fred G.; Kahr, Walter H.A.; Palaniyar, Nades; Licht, Christoph
• Is Part Of: Molecular immunology. Volume 97(2018:May)
• Page Start: 71
• Page End: 81
• Publication Date: 2018-05
• Subjects: AP alternative pathway -- CD Cytochalasin D -- CFU Colony Forming Units -- CP classical pathway -- Er erythrocyte -- HIS Heat-Inactivated Serum -- LP lectin pathway -- MAC membrane attack complex (C5b-9) -- MOI multiplicity of infection -- NETs neutrophil extracellular traps -- NHS normal human serum -- OD Optical Density -- PAKgfp Pseudomonas aeruginosa (GFP Labeled) -- PI propidium iodide -- PMA phorbol 12-myristate 13-acetate -- SEM Standard Error of Mean -- TP terminal pathway; Neutrophils -- Neutrophil extracellular traps -- Complement -- Pseudomonas aeruginosa, Staphylococcus aureus; Immunochemistry -- Periodicals; Molecular biology -- Periodicals; Immunochemistry -- Periodicals; Allergy and Immunology -- Periodicals; Molecular Biology -- Periodicals; Immunochimie -- Périodiques; Biologie moléculaire -- Périodiques; Immunochemistry; Molecular biology; Periodicals; Electronic journals; 571.96
• DOI: 10.1016/j.molimm.2018.02.019
• Languages: English
• ISSNs: 0161-5890

Abstract

Highlights: Classical and lectin complement pathways are most efficient in killing bacteria while NETs conditions are not. Removal of NETs allows for complement mediated-killing of bacteria to resume. NETs lead to depletion of complement C3 protein from normal human serum and reduce complement's hemolytic ability. Abstract: Neutrophil extracellular traps (NETs) are web-like DNA structures released by activated neutrophils. These structures are decorated with antimicrobial proteins, and considered to trap and kill bacteria extracellularly. However, the exact functions of NETs remain elusive, and contradictory observations have been made with NETs functioning as an antimicrobial or a pathogentrapping mechanism. There is a disconnect in the interpretation of the involvement of other major immune mechanisms, such as the complement system, as effectors of the function of NETs. We have recently shown that NETs activate complement. In this study, we aimed to elucidate the relative antimicrobial roles of NETs in the absence and presence of complement. Using primary human neutrophils, human serum (normal, heat inactivated, and C5-depleted), P. aeruginosa (at multiplicity of infection, MOI, of 1 or 10), S. aureus (MOI of 1), colony-counting assays and confocal microscopy, we demonstrate that most bacteria trapped by NETs remain viable, indicating that NETs have limited bactericidal properties. By contrast, complement effectively killed bacteria, but NETs decreased the bactericidal ability of complement and degrading NETs by DNases restored complement-mediated killing. Experiments with conditions allowing for specific pathway activation showed that the complement classical and lectin, but not the alternative, pathway lead to bacterial killing. NETs under static conditions showed limited killing of bacteria while NETs under dynamic conditions showed enhanced bacteria trapping and reduced killing. Furthermore, NETs incubated with normal human serum depleted complement and reduced the hemolytic capacity of the serum. This report, for the first time, clarifies the relative bactericidal contributions of NETs and complement. We propose that – while NETs can ensnare bacteria such as P. aeruginosa – complement is necessary for efficient bacterial killing.