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...

BackgroundGroup A streptococcal severe soft tissue infections, such as necrotizing fasciitis, are rapidly progressive infections associated with high mortality. Group A streptococcus is typically considered an extracellular pathogen, but has been shown to reside intracellularly in host cells.Methods and FindingsWe characterized in vivo interactions between group A streptococci (GAS) and cells involved in innate immune responses, using human biopsies (n = 70) collected from 17 patients with soft tissue infections. Immunostaining and in situ image analysis revealed high amounts of bacteria in the biopsies, even in those collected after prolonged antibiotic therapy. Viability of the streptococci was assessed by use of a bacterial viability stain, which demonstrated viable bacteria in 74% of the biopsies. GAS were present both extracellularly and intracellularly within phagocytic cells, primarily within macrophages. Intracellular GAS were predominantly noted in biopsies from newly involved tissue characterized by lower inflammation and bacterial load, whereas purely extracellular GAS or a combination of intra- and extracellular GAS dominated in severely inflamed tissue. The latter tissue was also associated with a significantly increased amount of the cysteine protease streptococcal pyrogenic exotoxin SpeB. In vitro studies confirmed that macrophages serve as reservoirs for viable GAS, and infection with a speB-deletion mutant produced significantly lower frequencies of cells with viable GAS following infection as compared to the wild-type bacteria.ConclusionsThis is the first study to demonstrate that GAS survive intracellularly in macrophages during acute invasive infections. This intracellular presence may have evolved as a mechanism to avoid antibiotic eradication, which may explain our finding that high bacterial load is present even in tissue collected after prolonged intravenous antibiotic therapy. This new insight into the pathogenesis of streptococcal soft tissue infections highlights a need for alternative therapeutic strategies.

Streptococcus pyogenes can cause a variety of diseases in immunocompetent individuals, from pharyngotonsillitis to life-threatening invasive diseases, such as streptococcal toxic shock syndrome, and rapidly progressing deep-tissue infections, such as necrotizing fasciitis. Necrotizing fasciitis is often seen in combination with streptococcal toxic shock syndrome, which further increases morbidity and mortality. We review here the host-pathogen interactions in the tissue milieu and discuss the use of intravenous immunoglobulin as potential adjunctive therapy in these life-threatening infections.