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Extracellular High Mobility Group Box 1 (HMGB1) has been associated with acute and chronic inflammatory conditions. However, little is known about HMGB1 in necrotizing bacterial infections. We hypothesized that the local HMGB1 response is excessive in severe soft tissue infections (STIs), which are characterized by necrosis and hyperinflammation. To explore this, tissue biopsies were collected from patients with varying severity of Streptococcus pyogenes skin and STIs, including erysipelas, cellulitis, and necrotizing fasciitis. Tissue sections were immunostained for HMGB1, S. pyogenes, and inflammatory cell infiltrates and results quantified by acquired computerized image analysis (ACIA). HMGB1 expression increased in parallel to disease severity and was significantly higher in necrotizing fasciitis than in erysipelas (p = 0.0023). Confocal microscopy of sections co-stained for HMGB1 and cell markers revealed both extracellular and cytoplasmic HMGB1, the latter of which was found predominantly in macrophages. To further verify macrophages as main source of activation triggered HMGB1 release, human macrophages were infected with clinical S. pyogenes isolates. The results demonstrated infection triggered release of HMGB1. Dual staining's visualized HMGB1 in areas close to, but not overlapping, with neutrophils, indicating a potential chemotactic role. In vitro transmigration experiments showed a chemotactic effect of HMGB1 on neutrophils. The data furthermore provided in vivo support that HGMB1 may form immunostimulatory complexes with IL-1β. Taken together, the findings provide the first in vivo evidence that HMGB1 is abundant at the local site of severe bacterial STIs and its levels correlated to severity of infections; hence, indicating its potential value as a biomarker for tissue pathology.

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.

Bacterial skin infections, such as erysipelas or cellulitis, are characterized by fever and a painful erythematous rash. Despite the high prevalence of these infections, little is known about the underlying pathogenic mechanisms. This is partly due to the fact that a bacterial diagnosis is often difficult to attain. To gain insight into the pathogenesis of erysipelas, we investigated the samples obtained from infected and noninfected areas of skin from 12 patients with erysipelas. Bacterial cultures, detection of specific streptococcal antibodies in convalescent sera, and immunohistochemical analyses of biopsies indicated group A streptococcal etiology in 11 of the 12 patients. Also, electron micrographs of erythematous skin confirmed the presence of group A streptococcal cells and showed a limited solubilization of the surface-attached M protein. Degradation of high-molecular-weight kininogen and upregulation of the bradykinin-1 receptor in inflamed tissues indicated activation of the contact system in 11 patients. Analyses of release of the vasoactive heparin-binding protein (HBP) showed increased levels in the infected as compared with the noninfected areas. The results suggest that group A streptococci induce contact activation and HBP release during skin infection, which likely contribute to the symptoms seen in erysipelas: fever, pain, erythema, and edema.

Severe soft tissue infections, such as necrotizing fasciitis and severe cellulitis, caused by group A streptococci (GAS) are rapidly progressing life-threatening infections characterized by massive bacterial loads in the tissue even late after the onset of infection. Antimicrobial peptides are important components of the innate host defense, and cathelicidins have been shown to protect against murine necrotic skin infections caused by GAS. However, it has been demonstrated that the streptococcal cysteine protease SpeB proteolytically inactivates the human cathelicidin LL-37 in vitro. Here we have investigated the expression of LL-37 and its interaction with GAS and SpeB during acute severe soft tissue infections by analyses of patient tissue biopsy specimens. The results showed large amounts of LL-37, both the proform (hCAP18) and the mature peptide, in the tissue. Confocal microscopy identified neutrophils as the main source of the peptide. A distinct colocalization between the bacteria and LL-37 could be noted, and bacterial loads showed positive correlation to the LL-37 levels. Areas with high LL-37 levels coincided with areas with large amounts of SpeB. Confocal microscopy confirmed strong colocalization of GAS, SpeB, and LL-37 at the bacterial surface. Taken together, the findings of this study provide in vivo support of the hypothesis that SpeB-mediated inactivation of LL-37 at the streptococcal surface represents a bacterial resistance mechanism at the infected tissue site in patients with severe GAS tissue infections.

Background: Group 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 Findings: We 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., Conclusions: This 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.

The amount and time course of SpeA release from group A streptococci (GAS) was studied at different starting inoculate after exposure to different doses of penicillin, clindamycin or a combination of the 2. The release was related to the bacterial concentration and killing rate. A clinical GAS strain was exposed to benzylpenicillin, 2 and 1000 x MIC, clindamycin, 2 and 32 x MIC, or combinations of the 2. Samples for viable counts and SpeA analyses were drawn before and after the addition of antibiotics and at 3, 6 and 24 h. The SpeA release was higher at low than at high concentrations of penicillin and the combination (both, p<0.05). The addition of clindamycin to penicillin reduced SpeA production at both concentrations (p<0.01). Most SpeA was released before 3 h, and for penicillin and the combination, the amount correlated to the number of killed bacteria during this period (r=0.50; p<0.05). A positive correlation was found between the inoculum size and the SpeA concentration at time zero (r=0.54; p<0.05). The SpeA concentration was dependent on the initial number of bacteria, the class of antibiotic, the dose of penicillin and the killing rate.

Intravenous polyspecific immunoglobulin G (IVIG) has been reported to be efficacious as adjunctive therapy in patients with toxic shock syndrome caused by a group A streptococci (GAS). GAS is also an important cause of necrotizing fasciitis, for which an early and extensive surgical intervention is currently advocated. Here we report on the use of an aggressive medical regimen including high-dose IVIG together with a conservative surgical approach in severe GAS soft tissue infection. We describe 7 patients with severe soft tissue infection caused by GAS, who all were treated with effective antimicrobials and high-dose IVIG. Surgery was either not performed or only limited exploration was carried out. Six of the patients had toxic shock syndrome. All patients survived. Immunostaining of tissue biopsies from 2 of the patients revealed high levels of GAS, superantigen and pro-inflammatory cytokines initially, which were dramatically reduced in a repeat biopsy of the initial operative site collected from 1 of the patients 66 h post-IVIG administration. The study suggests that the use of a medical regimen including IVIG in patients with severe GAS soft tissue infections may allow an initial non-operative or minimally invasive approach, which can limit the need to perform immediate wide debridements and amputations in unstable patients.