These days it has been increasingly recognized that mast cells (MCs) are critical components of host defense against pathogens. In this study, we have provided the first evidence that MCs can kill bacteria by entrapping them in extracellular structures similar to the extracellular traps described for neutrophils (NETs). We took advantage of the ability of MCs to kill the human pathogen Streptococcus pyogenes by a phagocytosisindependent mechanism in order to characterize the extracellular antimicrobial activity of MCs. Close contact of bacteria and MCs was required for full antimicrobial activity. Immunofluorescence and electron microscopy revealed that S pyogenes was entrapped by extracellular structures produced by MCs ( MCs derived from bone-marrow progenitor cells circulate in the peripheral blood and migrate into vascularized tissue before undergoing final maturation under the influence of local factors. Maturated MCs are commonly found in tissues that interface with the external environment such as the skin and mucosa of the respiratory and gastrointestinal tract (reviewed by Mekori and Metcalf 2 ). Because these sites are also common portals of infection, MCs are likely to be among the first inflammatory cells to interact with invading pathogens.Several recent reports in the literature indicate that MCs can mediate a variety of antimicrobial activities following activation upon contact with pathogens. First, MCs have been shown to release preformed and newly synthesized inflammatory mediators, proteases, cytokines, and chemokines that recruit neutrophils to the site of infection. 3,4 They are the only cell type known to prestore TNF-␣ in their secretory granules, which can be released immediately upon activation by pathogens to initiate the early phase of the inflammatory response. 5,6 Secondly, there is increasing experimental evidence that MCs themselves can directly kill various Gramnegative and Gram-positive bacteria. [7][8][9][10] Finally, experiments using MC-deficient mice have clearly demonstrated that MCs are essential for mounting an effective immune response against bacterial infections such as Citrobacter rodentium, 9 Pseudomonas aeruginosa, 11 Klebsiella pneumoniae, 6 or enteropathogenic Escherichia coli. 12-15 Based on these observations, it has been proposed that MCs play a central role in the host defense against infectious pathogens (recently reviewed by Dawicki and Marshall 16 ).Regarding the direct antimicrobial activity of MCs, several studies have shown that MCs are capable of bacterial recognition and intracellular uptake. Bacteria endocytosed after opsoninmediated binding are internalized via a route involving the endosome-lysosome pathway, in which the bacteria are killed through a combination of oxidative and nonoxidative killing systems (reviewed in Féger et al 8 ). These observations suggest that MCs are able to eliminate bacteria through an intracellular bactericidal mechanism similar to that of professional phagocytes.However, recent investigations have reported that various p...

Neutrophils are key effectors of the host innate immune response against bacterial infection. Staphylococcus aureus is a preeminent human pathogen, with an ability to produce systemic infections even in previously healthy individuals, thereby reflecting a resistance to effective neutrophil clearance. The recent discovery of neutrophil extracellular traps (NETs) has opened a novel dimension in our understanding of how these specialized leukocytes kill pathogens. NETs consist of a nuclear DNA backbone associated with antimicrobial peptides, histones and proteases that provide a matrix to entrap and kill various microbes. Here, we used targeted mutagenesis to examine a potential role of S. aureus nuclease in NET degradation and virulence in a murine respiratory tract infection model. In vitro assays using fluorescence microscopy showed the isogenic nuclease-deficient (nuc-deficient) mutant to be significantly impaired in its ability to degrade NETs compared with the wild-type parent strain USA 300 LAC. Consequently, the nuc-deficient mutant strain was significantly more susceptible to extracellular killing by activated neutrophils. Moreover, S. aureus nuclease production was associated with delayed bacterial clearance in the lung and increased mortality after intranasal infection. In conclusion, this study shows that S. aureus nuclease promotes resistance against NET-mediated antimicrobial activity of neutrophils and contributes to disease pathogenesis in vivo.

Since the discovery and definition of neutrophil extracellular traps (NETs) 14 years ago, numerous characteristics and physiological functions of NETs have been uncovered. Nowadays, the field continues to expand and novel mechanisms that orchestrate formation of NETs, their previously unknown properties, and novel implications in disease continue to emerge. The abundance of available data has also led to some confusion in the NET research community due to contradictory results and divergent scientific concepts, such as pro-and anti-inflammatory roles in pathologic conditions, demarcation from other forms of cell death, or the origin of the DNA that forms the NET scaffold. Here, we present prevailing concepts and state of the science in NET-related research and elaborate on open questions and areas of dispute.

SUMMARY Statins are inhibitors of 3-hydroxy 3-methylglutaryl coenzyme A (HMG-CoA) reductase, the rate-limiting enzyme in cholesterol biosynthesis. Several recent clinico-epidemiologic studies have revealed that patients receiving statin therapy have reduced mortality associated with severe bacterial infection. Here we study pharmacological effect of statins on the innate immune capacity of phagocytic cells, focusing on the leading human bacterial pathogen Staphylococcus aureus. These studies revealed a beneficial effect of statins on S. aureus clearance using in vivo, ex vivo and in vitro models of phagocyte function, although paradoxically both phagocytosis and oxidative burst were inhibited. Probing instead for an extracellular mechanism of killing, we found statins boosted the production of anti-bacterial DNA-based extracellular traps (ETs) by human and murine neutrophils and also monocyte/macrophages. The effect of statins to induce phagocyte ETs was linked to sterol pathway inhibition by RNA interference and specific pharmacologic inhibitors. We conclude that a drug therapy taken chronically by tens of millions of individuals alters the functional behavior of phagocytic cells, which could have ramifications for susceptibility and response to bacterial infections in these patients.

The importance of tumor associated neutrophils (TANs) in cancer development is in the meantime well established. Numerous of clinical data document the adverse prognostic effects of neutrophil infiltration in solid tumors. However, certain tumor therapies need functional neutrophils to be effective, suggesting altered neutrophil polarization associated with different outcomes for cancer patients. Therefore, modulation of neutrophilic phenotypes represents a potent therapeutic option, but factors mediating neutrophil polarization are still poorly defined. In this manuscript we provide evidence that type I IFNs alter neutrophilic phenotype into anti‐tumor, both in mice and human. In the absence of IFN‐β, pro‐tumor properties, such as reduced tumor cytotoxicity with low neutrophil extracellular traps (NETs) expression, low ICAM1 and TNF‐α expression, dominated neutrophil phenotypes in primary lesion and premetastatic lung. Interestingly, such neutrophils have significantly prolonged life‐span. Notably, interferon therapy in mice altered TAN polarization towards anti‐tumor N1. Similar changes in neutrophil activation could be observed in melanoma patients undergoing type I IFN therapy. Altogether, these data highlight the therapeutic potential of interferons, suggesting optimization of its clinical use as potent anti‐tumor agent.