The Gram-positive anaerobic commensal Finegoldia magna colonizes the skin and other non-sterile body surfaces, and is an important opportunistic pathogen. Here we analyzed the effect of F. magna on human primary neutrophils. F. magna strains ALB8 (expressing protein FAF), 312 (expressing protein L) and 505 (naturally lacking both protein FAF and L) as well as their associated proteins activate neutrophils to release reactive oxygen species, an indication for neutrophil oxidative burst. Co-incubation of neutrophils with the bacteria leads to a strong increase of CD66b surface expression, another indicator for neutrophil activation. Furthermore, all tested stimuli triggered the release of NETs from the activated neutrophils, pointing to a host defense mechanism in response to the tested stimuli. This phenotype is dependent on actin rearrangement, NADPH oxidases and the ERK1/2 pathway. Proteins FAF and L also induced the secretion of several pro-inflammatory neutrophil proteins; HBP, IL-8 and INFγ. This study shows for the first time a direct interaction of F. magna with human neutrophils and suggests that the activation of neutrophils plays a role in F. magna pathogenesis.

The discovery, in 2004, of extracellular traps released by neutrophils has extended our understanding of the mode of action of various innate immune cells. This fascinating discovery demonstrated the extracellular trapping and killing of various pathogens by neutrophils. During the last decade, evidence has accumulated showing that extracellular traps play a crucial role in the defence mechanisms of various cell types present in vertebrates, invertebrates, and plants. The aim of this review is to summarise the relevant literature on the evolutionary history of extracellular traps used as a weapon in various kingdoms of life.

Extracellular vesicles (EVs) play a crucial role in intercellular communication by transferring bioactive molecules from donor to recipient cells. As a result, EV fusion leads to the modulation of cellular functions and has an impact on both physiological and pathological processes in the recipient cell. This study explores the impact of EV fusion on cellular responses to inflammatory signaling. Our findings reveal that fusion renders non-responsive cells susceptible to inflammatory signaling, as evidenced by increased NF-κB activation and the release of inflammatory mediators. Syntaxin-binding protein 1 is essential for the merge and activation of intracellular signaling. Subsequent analysis show that EVs transfer their functionally active receptors to target cells, making them prone to an otherwise unresponsive state. EVs in complex with their agonist, require no further stimulation of the target cells to trigger mobilization of NF-κB. While receptor antagonists were unable to inhibit NF-κB activation, blocking of the fusion between EVs and their target cells with heparin mitigated inflammation in mice challenged with EVs.

The circulating zymogen form of Factor VII activating protease (FSAP) can be activated by histones and nucleosomes in vivo. These cell-death-associated nuclear factors are also actively extruded into the extracellular space by neutrophils through a process called neutrophil extracellular trap (NET) formation (NETosis). NETs are thought to be involved in host defense, inflammation as well as thrombosis. We have investigated the bidirectional interactions of FSAP and NETs. Phorbol ester-mediated NET formation was marginally stimulated by FSAP. Plasma-derived FSAP as well as exogenous FSAP bound to NETs. There was co-localization of FSAP and NETs in coronary thrombi from patients with acute myocardial infarction. Contrary to our expectations no activation of pro-FSAP by NETs was evident. However, after disintegration of NETs with DNase, a robust activation of pro-FSAP, due to release of histones from nucleosomes, was detected. The released histones were in turn degraded by FSAP. Histone cytotoxicity towards endothelial cells was neutralized by FSAP more potently than by activated protein C (APC). One more consequence of histone degradation was a decrease in nucleosome release from apoptotic neutrophils. Taken together, NETs bind to FSAP, but do not activate pro-FSAP unless histones are released from NETs by DNAse. This activation of FSAP is likely to be important in diminishing the cytotoxic effect of histones, thus limiting the damaging effect of NETosis.

The formation of neutrophil extracellular traps (NETs) is a host defence mechanism, known to facilitate the entrapment and growth inhibition of many bacterial pathogens. It has been implicated that the translocation of myeloperoxidase (MPO) from neutrophilic granules to the nucleus is crucial to this process. Under disease conditions, however, excessive NET formation can trigger self-destructive complications by releasing pathologic levels of danger-associated molecular pattern molecules (DAMPs). To counteract such devastating immune reactions, the host has to rely on precautions that help circumvent these deleterious effects. Though the induction of DAMP responses has been intensively studied, the mechanisms that are used by the host to down-regulate them are still not understood. In this study, we show that p33 is an endothelial-derived protein that has the ability to annul NET formation. We found that the expression of human p33 is up-regulated in endothelial cells upon infections with Streptococcus pyogenes bacteria. Using tissue biopsies from a patient with streptococcal necrotising fasciitis, we monitored co-localisation of p33 with MPO. Further in vitro studies revealed that p33 is able to block the formation of DAMP-induced NET formation by inhibiting the enzymatic activity of MPO. Additionally, mice challenged with S. pyogenes bacteria demonstrated diminished MPO activity when treated with p33. Together, our results demonstrate that host-derived p33 has an important immunomodulating function that helps to counterbalance an overwhelming DAMP response.

Streptococcus pyogenes is a major bacterial pathogen in the human population and isolates of the clinically important M1 serotype secrete protein Streptococcal inhibitor of complement (SIC) known to interfere with human innate immunity. Here we find that SIC from M1 bacteria interacts with TLR2 and CD14 on monocytes leading to the activation of the NF-κB and p38 MAPK pathways and the release of several pro-inflammatory cytokines (e.g. TNFα and INFγ). In human plasma, SIC binds clusterin and histidine-rich glycoprotein, and whole plasma, and these two purified plasma proteins enhanced the activation of monocytes by SIC. Isolates of the M55 serotype secrete an SIC homolog, but this protein did not activate monocytes. M1 isolates are common in cases of invasive S. pyogenes infections characterized by massive inflammation, and the results of this study indicate that the pro-inflammatory property of SIC contributes to the pathology of these severe clinical conditions.