Keratinocyte proinflammatory responses to adherent and nonadherent group A streptococci

Constitutive secretion of pro-IL-18 allows keratinocytes to initiate inflammation during bacterial infection

Group A Streptococcus (GAS, Streptococcus pyogenes) is a professional human pathogen that commonly infects the skin. Keratinocytes are one of the first cells to contact GAS, and by inducing inflammation, they can initiate the earliest immune responses to pathogen invasion. Here, we characterized the proinflammatory cytokine repertoire produced by primary human keratinocytes and surrogate cell lines commonly used in vitro. Infection induces several cytokines and chemokines, but keratinocytes constitutively secrete IL-18 in a form that is inert (pro-IL-18) and lacks proinflammatory activity. Canonically, IL-18 activation and secretion are coupled through a single proteolytic event that is regulated intracellularly by the inflammasome protease caspase-1 in myeloid cells. The pool of extracellular pro-IL-18 generated by keratinocytes is poised to sense extracellular proteases. It is directly processed into a mature active form by SpeB, a secreted GAS protease that is a critical virulent factor during skin infection. This mechanism contributes to the proinflammatory response against GAS, resulting in T cell activation and the secretion of IFN-γ. Under these conditions, isolates of several other major bacterial pathogens and microbiota of the skin were found to not have significant IL-18-maturing ability. These results suggest keratinocyte-secreted IL-18 is a sentinel that sounds an early alarm that is highly sensitive to GAS, yet tolerant to non-invasive members of the microbiota.

Impact of Phosphorylcholine Expression on the Adherence and Invasion of Streptococcus pyogenes to Epithelial Cells

Phosphorylcholine (PC) is a structural component of various pathogens and is involved in bacterial adhesion via the platelet-activating factor receptor (PAF-R). In this study, we investigated how PC expression affects cell adhesion and invasion of Streptococcus pyogenes (S. pyogenes). Eight clinical strains of S. pyogenes were cultured, and PC expression was measured using fluorescence-activated cell sorting. Bacterial adherence and invasion were examined using Detroit 562 cells. An anti-PC-specific monoclonal antibody (TEPC-15) was used to inhibit bacterial PC, and a PAF-R antagonist (ABT-491) was used to inhibit cellular PAF-R. The emm gene was amplified by the polymerase chain reaction with the standard primers. The level of PC expressed on the S. pyogenes surfaces differed in each strain and differed even in the same emm genotype. Adherence assay experiments showed a significant negative correlation between TEPC-15 and ABT-491 inhibitory effects and PC expression in S. pyogenes. Similarly, intracellular invasion assay experiments showed a significant negative correlation between TEPC-15 and ABT-491 inhibitory effects and PC expression in S. pyogenes. This study suggests that S. pyogenes is involved in cell adhesion and invasion by PC.

An Engineered Infected Epidermis Model for In Vitro Study of the Skin's Pro-Inflammatory Response

Wound infection is a major clinical challenge that can significantly delay the healing process, can create pain, and requires prolonged hospital stays. Pre-clinical research to evaluate new drugs normally involves animals. However, ethical concerns, cost, and the challenges associated with interspecies variation remain major obstacles. Tissue engineering enables the development of in vitro human skin models for drug testing. However, existing engineered skin models are representative of healthy human skin and its normal functions. This paper presents a functional infected epidermis model that consists of a multilayer epidermis structure formed at an air-liquid interface on a hydrogel matrix and a three-dimensionally (3D) printed vascular-like network. The function of the engineered epidermis is evaluated by the expression of the terminal differentiation marker, filaggrin, and the barrier function of the epidermis model using the electrical resistance and permeability across the epidermal layer. The results showed that the multilayer structure enhances the electrical resistance by 40% and decreased the drug permeation by 16.9% in the epidermis model compared to the monolayer cell culture on gelatin. We infect the model with Escherichia coli to study the inflammatory response of keratinocytes by measuring the expression level of pro-inflammatory cytokines (interleukin 1 beta and tumor necrosis factor alpha). After 24 h of exposure to Escherichia coli, the level of IL-1β and TNF-α in control samples were 125 ± 78 and 920 ± 187 pg/mL respectively, while in infected samples, they were 1429 ± 101 and 2155.5 ± 279 pg/mL respectively. However, in ciprofloxacin-treated samples the levels of IL-1β and TNF-α without significant difference with respect to the control reached to 246 ± 87 and 1141.5 ± 97 pg/mL respectively. The robust fabrication procedure and functionality of this model suggest that the model has great potential for modeling wound infections and drug testing.

Group A Streptococcus-Mediated Host Cell Signaling

In the past decade, the field of the cellular microbiology of group A Streptococcus (S. pyogenes) infection has made tremendous advances and touched upon several important aspects of pathogenesis, including receptor biology, invasive and evasive phenomena, inflammasome activation, strain-specific autophagic bacterial killing, and virulence factor-mediated programmed cell death. The noteworthy aspect of S. pyogenes-mediated cell signaling is the recognition of the role of M protein in a variety of signaling events, starting with the targeting of specific receptors on the cell surface and on through the induction and evasion of NETosis, inflammasome, and autophagy/xenophagy to pyroptosis and apoptosis. Variations in reports on S. pyogenes-mediated signaling events highlight the complex mechanism of pathogenesis and underscore the importance of the host cell and S. pyogenes strain specificity, as well as in vitro/in vivo experimental parameters. The severity of S. pyogenes infection is, therefore, dependent on the virulence gene expression repertoire in the host environment and on host-specific dynamic signaling events in response to infection. Commonly known as an extracellular pathogen, S. pyogenes finds host macrophages as safe havens wherein it survives and even multiplies. The fact that endothelial cells are inherently deficient in autophagic machinery compared to epithelial cells and macrophages underscores the invasive nature of S. pyogenes and its ability to cause severe systemic diseases. S. pyogenes is still one of the top 10 causes of infectious mortality. Understanding the orchestration of dynamic host signaling networks will provide a better understanding of the increasingly complex mechanism of S. pyogenes diseases and novel ways of therapeutically intervening to thwart severe and often fatal infections.

Group A Streptococcus M1T1 Intracellular Infection of Primary Tonsil Epithelial Cells Dampens Levels of Secreted IL-8 Through the Action of SpyCEP

Streptococcus pyogenes (Group A Streptococcus; GAS) commonly causes pharyngitis in children and adults, with severe invasive disease and immune sequelae being an infrequent consequence. The ability of GAS to invade the host and establish infection likely involves subversion of host immune defenses. However, the signaling pathways and innate immune responses of epithelial cells to GAS are not well-understood. In this study, we utilized RNAseq to characterize the inflammatory responses of primary human tonsil epithelial (TEpi) cells to infection with the laboratory-adapted M6 strain JRS4 and the M1T1 clinical isolate 5448. Both strains induced the expression of genes encoding a wide range of inflammatory mediators, including IL-8. Pathway analysis revealed differentially expressed genes between mock and JRS4- or 5448-infected TEpi cells were enriched in transcription factor networks that regulate IL-8 expression, such as AP-1, ATF-2, and NFAT. While JRS4 infection resulted in high levels of secreted IL-8, 5448 infection did not, suggesting that 5448 may post-transcriptionally dampen IL-8 production. Infection with 5448ΔcepA, an isogenic mutant lacking the IL-8 protease SpyCEP, resulted in IL-8 secretion levels comparable to JRS4 infection. Complementation of 5448ΔcepA and JRS4 with a plasmid encoding 5448-derived SpyCEP significantly reduced IL-8 secretion by TEpi cells. Our results suggest that intracellular infection with the pathogenic GAS M1T1 clone induces a strong pro-inflammatory response in primary tonsil epithelial cells, but modulates this host response by selectively degrading the neutrophil-recruiting chemokine IL-8 to benefit infection.

Superoxide anions produced by Streptococcus pyogenes group A-stimulated keratinocytes are responsible for cellular necrosis and bacterial growth inhibition

Gram-positive Streptococcus pyogenes (group A Streptococcus or GAS) is a major skin pathogen and interacts with keratinocytes in cutaneous tissues. GAS can cause diverse suppurative and inflammatory infections, such as cellulitis, a common acute bacterial dermo-hypodermitis with a high morbidity. Bacterial isolation yields from the lesions are low despite the strong local inflammation observed, raising numerous questions about the pathogenesis of the infection. Using an in vitro model of GAS-infected keratinocytes, we show that the major ROS produced is the superoxide anion ([Formula: see text]), and that its production is time- and dose-dependent. Using specific modulators of ROS production, we show that [Formula: see text] is mainly synthesized by the cytoplasmic NADPH oxidase. Superoxide anion production leads to keratinocyte necrosis but incomplete inhibition of GAS growth, suggesting that GAS may be partially resistant to the oxidative burst. In conclusion, GAS-stimulated keratinocytes are able to develop an innate immune response based on the production of ROS. This local immune response limits GAS development and induces keratinocyte cell death, resulting in the skin lesions observed in patients with cellulitis.

Vigilant keratinocytes trigger pathogen-associated molecular pattern signaling in response to streptococcal M1 protein

The human skin exerts many functions in order to maintain its barrier integrity and protect the host from invading microorganisms. One such pathogen is Streptococcus pyogenes, which can cause a variety of superficial skin wounds that may eventually progress into invasive deep soft tissue infections. Here we show that keratinocytes recognize soluble M1 protein, a streptococcal virulence factor, as a pathogen-associated molecular pattern to release alarming inflammatory responses. We found that this interaction initiates an inflammatory intracellular signaling cascade involving the activation of the mitogen-activated protein kinases extracellular signal-regulated kinase (ERK), p38, and Jun N-terminal protein kinase and the subsequent induction and mobilization of the transcription factors NF-κB and AP-1. We also determined the imprint of the inflammatory mediators released, such as interleukin-8 (IL-8), growth-related oncogene alpha, migration inhibitory factor, extracellular matrix metalloproteinase inducer, IL-1α, IL-1 receptor a, and ST2, in response to streptococcal M1 protein. The expression of IL-8 is dependent on Toll-like receptor 2 activity and subsequent activation of the mitogen-activated protein kinases ERK and p38. Notably, this signaling seems to be distinct for IL-8 release, and it is not shared with the other inflammatory mediators. We conclude that keratinocytes participate in a proinflammatory manner in streptococcal pattern recognition and that expression of the chemoattractant IL-8 by keratinocytes constitutes an important protective mechanism against streptococcal M1 protein.

Genetic susceptibility to non-necrotizing erysipelas/cellulitis

BACKGROUND

Bacterial non-necrotizing erysipelas and cellulitis are often recurring, diffusely spreading infections of the skin and subcutaneous tissues caused most commonly by streptococci. Host genetic factors influence infection susceptibility but no extensive studies on the genetic determinants of human erysipelas exist.

METHODS

We performed genome-wide linkage with the 10,000 variant Human Mapping Array (HMA10K) array on 52 Finnish families with multiple erysipelas cases followed by microsatellite fine mapping of suggestive linkage peaks. A scan with the HMA250K array was subsequently performed with a subset of cases and controls.

RESULTS

Significant linkage was found at 9q34 (nonparametric multipoint linkage score (NPL(all)) 3.84, p=0.026), which is syntenic to a quantitative trait locus for susceptibility to group A streptococci infections on chromosome 2 in mouse. Sequencing of candidate genes in the 9q34 region did not conclusively associate any to erysipelas/cellulitis susceptibility. Suggestive linkage (NPL(all)>3.0) was found at three loci: 3q22-24, 21q22, and 22q13. A subsequent denser genome scan with the HMA250K array supported the 3q22 locus, in which several SNPs in the promoter of AGTR1 (Angiotensin II receptor type I) suggestively associated with erysipelas/cellulitis susceptibility.

CONCLUSIONS

Specific host genetic factors may cause erysipelas/cellulitis susceptibility in humans.

Cyclooxygenase-2 enhances antimicrobial peptide expression and killing of Staphylococcus aureus

Antimicrobial peptides such as human β-defensins (hBDs) and cathelicidins are critical for protection against infection and can be induced by activation of TLRs, a pathway that also activates cyclooxygenase(Cox)-2 expression. We hypothesized that Cox-2 is induced by TLR activation and is necessary for optimal AMP production, and that inhibitors of Cox-2 may therefore inhibit antimicrobial action. Normal human keratinocytes (NHEKs) stimulated with a TLR2/6 ligand, macrophage-activating lipopeptide-2, or a TLR3 ligand, polyinosinic-polycytidylic acid, increased Cox-2 mRNA and protein and increased PGE(2), a product of Cox-2. Treatment with a Cox-2 selective inhibitor (SC-58125) or Cox-2 small interfering RNA attenuated hBD2 and hBD3 production in NHEKs when stimulated with macrophage-activating lipopeptide-2, polyinosinic-polycytidylic acid, or UVB (15 mJ/cm(2)), but it did not attenuate vitamin D3-induced cathelicidin. SC-58125 also inhibited TLR-dependent NF-κB activation. Conversely, treatment with Cox-derived prostanoids PGD(2) or 15-deoxy-Δ(12,14)-PGJ(2) induced hBD3 or hBD2 and hBD3, respectively. The functional significance of these observations was seen in NHEKs that showed reduced anti-staphylococcal activity when treated with a Cox-2 inhibitor. These findings demonstrate a critical role for Cox-2 in hBD production and suggest that the use of Cox-2 inhibitors may adversely influence the risk for bacterial infection.

Expression of Toll-like Receptor 2 in Cultured Human Keratinocytes: The Effect of Bacterial Antigens, Cytokines and Calcium Concentration

BACKGROUND

Toll-like receptors (TLRs) are expressed by human epidermal keratinocytes and are involved in immune responses.

OBJECTIVE

The goal of this was to investigate the expression of TLR2 in response to bacterial antigens, cytokines, and different calcium concentrations.

METHODS

The expression of TLR2 was assessed after stimulation by lipoteichoic acid (LTA) and streptolysin O (SLO). In addition, TLR2 expression was evaluated after treatment with IFN-gamma and TNF-alpha, and different concentrations of calcium. The expression levels of TLR2 mRNA and protein were studied using RT-PCR and Western blot analysis.

RESULTS

Cultured human epidermal keratinocytes constitutively expressed TLR2 and the expression was stimulated by LTA and SLO; in addition, IFN-gamma and TNF-alpha upregulated TLR2 expression. However, the changes in TLR2 expression associated with the calcium concentrations were insignificant.

CONCLUSION

TLR2 expression increased with the concentration and duration of bacterial pathogens and this increase was amplified by several cytokines, from activated keratinocytes and other cells.

Streptolysin S inhibits neutrophil recruitment during the early stages of Streptococcus pyogenes infection

In contrast to infection of superficial tissues, Streptococcus pyogenes infection of deeper tissue can be associated with a significantly diminished inflammatory response, suggesting that this bacterium has the ability to both promote and suppress inflammation. To examine this, we analyzed the behavior of an S. pyogenes mutant deficient in expression of the cytolytic toxin streptolysin S (SLS-) and evaluated events that occur during the first few hours of infection by using several models including injection of zebrafish (adults, larvae, and embryos), a transepithelial polymorphonuclear leukocyte (PMN) migration assay, and two-photon microscopy of mice in vivo. In contrast to wild-type S. pyogenes, the SLS- mutant was associated with the robust recruitment of neutrophils and significantly reduced lethal myositis in adult zebrafish. Similarly, the mutant was attenuated in embryos in its ability to cause lethality. Infection of larva muscle allowed an analysis of inflammation in real time, which revealed that the mutant had recruited PMNs to the infection site. Analysis of transepithelial migration in vitro suggested that SLS inhibited the host cells' production of signals chemotactic for neutrophils, which contrasted with the proinflammatory effect of an unrelated cytolytic toxin, streptolysin O. Using two-photon microscopy of mice in vivo, we showed that the extravasation of neutrophils during infection with SLS- mutant bacteria was significantly accelerated compared to infection with wild-type S. pyogenes. Taken together, these data support a role for SLS in the inhibition of neutrophil recruitment during the early stages of S. pyogenes infection.

Adherence and internalization of Streptococcus gordonii by epithelial cells involves beta1 integrin recognition by SspA and SspB (antigen I/II family) polypeptides

Streptococcus gordonii is a commensal bacterium that colonizes the hard and soft tissues present in the human mouth and nasopharynx. The cell wall-anchored polypeptides SspA and SspB expressed by S. gordonii mediate a wide range of interactions with host proteins and other bacteria. In this article we have determined the role of SspA and SspB proteins, which are members of the streptococcal antigen I/II (AgI/II) adhesin family, in S. gordonii adherence and internalization by epithelial cells. Wild-type S. gordonii DL1 expressing AgI/II polypeptides attached to and was internalized by HEp-2 cells, whereas an isogenic AgI/II- mutant was reduced in adherence and was not internalized. Association of S. gordonii DL1 with HEp-2 cells triggered protein tyrosine phosphorylation but no significant actin rearrangement. By contrast, Streptococcus pyogenes A40 showed 50-fold higher levels of internalization and this was associated with actin polymerization and interleukin-8 upregulation. Adherence and internalization of S. gordonii by HEp-2 cells involved beta1 integrin recognition but was not fibronectin-dependent. Recombinant SspA and SspB polypeptides bound to purified human alpha5beta1 integrin through sequences present within the NAV (N-terminal) region of AgI/II polypeptide. AgI/II polypeptides blocked interactions of S. gordonii and S. pyogenes with HEp-2 cells, and S. gordonii DL1 cells expressing AgI/II proteins inhibited adherence and internalization of S. pyogenes by HEp-2 cells. Conversely, S. gordonii AgI/II- mutant cells did not inhibit internalization of S. pyogenes. The results suggest that AgI/II proteins not only promote integrin-mediated internalization of oral commensal streptococci by host cells, but also potentially influence susceptibility of host tissues to more pathogenic bacteria.

Role of NADase in virulence in experimental invasive group A streptococcal infection

Group A streptococci (GAS) produce several exoproteins that are thought to contribute to the pathogenesis of human infection. Two such proteins, streptolysin O (SLO) and NAD(+)-glycohydrolase (NADase), have been shown to interact functionally as a compound signaling toxin. When GAS are bound to the surface of epithelial cells in vitro, SLO forms pores in the cell membrane and delivers NADase to the epithelial cell cytoplasm. In vitro, intoxication of keratinocytes with NADase is associated with cytotoxic effects and induction of apoptosis; however, the importance of NADase during infection of an animal host has not been established. We employed isogenic GAS mutants to assess the contribution of NADase activity to GAS virulence in vivo using mouse models of invasive soft-tissue infection and septicemia. In both models, mutant GAS that lacked NADase activity were significantly attenuated for virulence compared with the isogenic wild-type parent, confirming an important role for NADase in the infection of a host animal. A double mutant lacking SLO and NADase activity had an intermediate virulence phenotype, consistent with the hypothesis that SLO evokes a protective innate immune response. We conclude that NADase and SLO together enhance GAS virulence in vivo.

Specificity of streptolysin O in cytolysin-mediated translocation

Cytolysin-mediated translocation (CMT) is a recently described process in the Gram-positive pathogen Streptococcus pyogenes that translocates an effector protein of streptococcal origin into the cytoplasm of a host cell. At least two proteins participate in CMT, the pore-forming molecule streptolysin O (SLO) and an effector protein with the characteristics of a signal transduction protein, the Streptococcus pyogenes NAD-glycohydrolase (SPN). In order to begin to elucidate the molecular details of the translocation process, we examined whether perfringolysin O (PFO), a pore-forming protein related to SLO, could substitute for SLO in the translocation of SPN. When expressed by S. pyogenes, PFO, like SLO, had the ability to form functional pores in keratinocyte membranes. However, unlike SLO, PFO was not competent for translocation of SPN across the host cell membrane. Thus, pore formation by itself was not sufficient to promote CMT, suggesting that an additional feature of SLO was required. This conclusion was supported by the construction of a series of mutations in SLO that uncoupled pore formation and competence for CMT. These mutations defined a domain in SLO that was dispensable for pore formation, but was essential for CMT. However, introduction of this domain into PFO did not render PFO competent for CMT, implying that an additional domain of SLO is also critical for translocation. Taken together, these data indicate that SLO plays an active role in the translocation process that extends beyond that of a passive pore.

Syntheses of prostaglandin E2 and E-cadherin and gene expression of beta-defensin-2 by human gingival epithelial cells in response to Actinobacillus actinomycetemcomitans

The interaction between epithelial cells and microorganisms is the most important step in bacterial infections. Actinobacillus actinomycetemcomitans was suggested to play a significant role in the initiation of periodontitis because of its bacteriological characteristics. Prostaglandins (PG) mediate the inflammatory response. Human beta-defensin-2 (hBD-2) is an antimicrobial peptide and contributes to innate immunity. E-cadherin is responsible for an epithelial intercellular junction. In this study, we investigated the syntheses of PGE2 and E-cadherin and the expression of hBD-2 in human gingival epithelial cells (HGEC) following exposure to A. actinomycetemcomitans. The levels of PGE2 and cyclooxygenase-2, which are responsible for an increase in PGE2, were increased depending on bacteria exposure time. hBD-2 mRNA was induced by A. actinomycetemcomitans, while HGEC exposed to A. actinomycetemcomitans showed a decrease in E-cadherin levels. Etodolac, a selective cyclooxygenase-2 inhibitor reinforced the increase in hBD-2 mRNA levels by A. actinomycetemcomitans. Furthermore, the etodolac suppressed the decrease in E-cadherin levels. Thus, endogenous PGE2 is involved in the hBD-2 and E-cadherin responses of HGEC to A. actinomycetemcomitans. These findings suggest that the inflammatory and antimicrobial response of gingival epithelial cells to A. actinomycetemcomitans is involved in the initiation of periodontal inflammation. A. actinomycetemcomitans may destroy the mechanical epithelial barrier by destroying E-cadherin.

Proinflammatory cytokine production by human keratinocytes stimulated with Propionibacterium acnes and P. acnes GroEL

BACKGROUND

Keratinocytes form the first line of defence in the skin and alert the host to danger by the production of a number of cytokines and chemokines. However, the interaction of commensal microorganisms with keratinocytes has not been well studied.

OBJECTIVES

To investigate the effect of viable and nonviable cells of Propionibacterium acnes in both exponential and stationary growth phases, and of P. acnes GroEL on cytokine production by human primary keratinocytes.

METHODS

Actively proliferating or contact-inhibited keratinocytes were cocultured with viable or formaldehyde-killed P. acnes cells in either the exponential or stationary phase of growth. Culture supernatants were assayed by enzyme-linked immunosorbent assay for the cytokines interleukin (IL)-1alpha, tumour necrosis factor (TNF)-alpha and granulocyte/macrophage colony-stimulating factor (GM-CSF). Keratinocytes were also stimulated with different concentrations of P. acnes GroEL and supernatants assayed for cytokines.

RESULTS

Viable P. acnes in the stationary phase of growth stimulated keratinocyte monolayers to produce significantly higher amounts of IL-1alpha, TNF-alpha and GM-CSF than unstimulated keratinocytes. Viable exponential-phase bacteria stimulated production of significantly higher amounts of TNF-alpha and GM-CSF but these levels were significantly lower than those for stimulation with stationary-phase bacteria. Nonviable P. acnes from either growth phase was not able to stimulate cytokine production. P. acnes GroEL at concentrations in the range 0.05-1.0 micro g mL(-1) was able to induce increased production of cytokines by keratinocytes in a dose-dependent manner. This was analogous to stimulation with Escherichia coli GroEL.

CONCLUSIONS

Stimulation of cytokine production by P. acnes and P. acnes GroEL may be important in the pathogenesis of inflammatory acne vulgaris and may have wider implications for the immunomodulation of the human immune system by commensal skin microorganisms.

Long-term effect of heat shock protein 60 from Actinobacillus actinomycetemcomitans on epithelial cell viability and mitogen-activated protein kinases

Our previous studies showed that bacterial heat shock protein 60 (hsp60) induces cultured epithelial cell proliferation within 24 h. Here we investigated the long-term effects of heat shock protein 60 isolated from Actinobacillus actinomycetemcomitans on skin keratinocyte (HaCaT cell line) viability and the cell signaling involved. Prolonged incubation in the presence of hsp60 increased the rate of epithelial cell death. The number of viable cells in hsp60-treated culture was 37% higher than the number in the control at 24 h but 27% lower at 144 h. A kinetics study of the effect of hsp60 on the phosphorylation of mitogen-activated protein kinases (MAPKs) involving Western blotting with phospho-specific antibodies showed that in addition to a transient early increase in p38 levels, a second peak appeared in keratinocytes 24 h after the addition of hsp60. In contrast, prolonged incubation with hsp60 caused a decrease in the level of phosphorylated extracellular signal-regulated kinase 1/2 (ERK1/2) compared with that in the controls, possibly as a result of protein phosphatase activity. We found that hsp60 increased the levels of several phosphatases, including MAP-2, which strongly dephosphorylates ERK1/2. Moreover, hsp60 increased the level of tumor necrosis factor alpha (TNF-alpha) in culture medium in a dose-dependent manner. TNF-alpha added to culture showed a cytotoxic effect on epithelial cells, particularly with longer incubation periods. TNF-alpha also induced the phosphorylation of p38. Finally, our results show that bacterial hsp60 inhibited stress-induced synthesis of cellular hsp60. Therefore, several cell behavior changes caused by long-term exposure to bacterial hsp60 may lead to impaired epithelial cell viability.

Normal keratinocytes express Toll-like receptors (TLRs) 1, 2 and 5: modulation of TLR expression in chronic plaque psoriasis

BACKGROUND

Toll-like receptors (TLRs) are part of the innate immune system involved in the response to microbial pathogens. TLR2 recognizes various ligands expressed by Gram-positive bacteria, while TLR3, TLR4 and TLR5 are specific for double-stranded RNA, Gram-negative lipopolysaccharides and bacterial flagellin, respectively.

OBJECTIVES

To determine, firstly, whether epidermal keratinocytes of normal skin express TLRs and, secondly, whether modulation of TLR expression occurs in psoriasis, an inflammatory skin disease associated with certain microorganisms such as streptococci, staphylococci and yeasts.

METHODS

Eight samples of normal, and 15 samples of lesional and nonlesional psoriatic skin were stained with polyclonal antibodies specific for TLR1-5 using an avidin-biotin-peroxidase technique.

RESULTS

Epidermal keratinocytes in normal skin constitutively expressed TLR1, TLR2 and TLR5, while TLR3 and TLR4 were, in most cases, barely detectable. Cytoplasmic TLR1 and TLR2 were expressed throughout the epidermis, with higher staining of the latter on basal keratinocytes, while TLR5 expression was concentrated in the basal layer. In contrast, in lesional epidermis from patients with psoriasis, TLR2 was more highly expressed on the keratinocytes of the upper epidermis than on the basal layer, while TLR5 was downregulated in basal keratinocytes compared with corresponding nonlesional psoriatic epidermis. In addition, nuclear TLR1 staining was observed in the upper layers of both nonlesional and lesional psoriatic epidermis, but not in that of normal skin.

CONCLUSIONS

These findings suggest that TLRs expressed by epidermal keratinocytes constitute part of the innate immune system of the skin. The relevance of altered keratinocyte TLR expression in psoriasis remains to be determined.

Infection by Streptococcus pyogenes induces the receptor activator of NF-kappaB ligand expression in mouse osteoblastic cells

Group A Streptococcus pyogenes is known to induce nongonococcal septic arthritis in addition to pharyngitis, scarlet fever, and poststreptococcal sequelae. However, little is known about the interaction between S. pyogenes and bone cells. We report here that S. pyogenes strain JRS4 (M6) attached to and invaded mouse primary osteoblasts. Reverse transcription-PCR demonstrated that S. pyogenes infection of osteoblasts stimulated expression of mRNA for the receptor activator of NF-kappaB ligand (RANKL). Western blot analysis followed by ligand precipitation with the receptor activator of NF-kappaB receptor showed that there was an increase in RANKL protein in infected osteoblasts. Production of interleukin-6 was also stimulated, but no production of interleukin-1beta or tumor necrosis factor alpha was observed. Stimulation of RANKL production was not observed in osteoblasts stimulated with heat-inactivated S. pyogenes, suggesting that an active interaction of S. pyogenes with osteoblasts is essential for this phenomenon. A Western blot analysis performed with antibodies specific for phosphorylated signal transduction proteins demonstrated that S. pyogenes infection induces phosphorylation of p38 mitogen-activated protein kinase. A specific inhibitor of this kinase, SB203580, inhibited RANKL production by infected osteoblasts. These results suggest that infection of osteoblasts by S. pyogenes stimulates RANKL production and may trigger bone destruction in infected bone tissue.

Interleukin-8 and tumor necrosis factor alpha production in human epidermal keratinocytes induced by Trichophyton mentagrophytes

Production of interleukin-8 (IL-8) and tumor necrosis factor alpha (TNF-alpha) was confirmed by enzyme-linked immunosorbent assay in a medium where human epidermal keratinocytes were cocultured with Trichophyton mentagrophytes for 1 to 12 h. IL-8 and TNF-alpha mRNAs were also detected in the keratinocytes cocultured with T. mentagrophytes.

Expression of IL-1 beta and IL-8 by human gingival epithelial cells in response to Actinobacillus actinomycetemcomitans

The interaction between epithelial cells and microorganisms is the most important step in bacterial infections. Epithelial cells in response to exposure to pathogenic bacteria produce cytokines that initiate inflammation. However, little is known about the cytokine response of gingival epithelial cells to periodontopathogenic bacteria. Actinobacillus actinomycetemcomitans is thought to play a significant role in the initiation of periodontitis because of its bacteriological characteristics. In the present study, we investigated the cytokine induction by human gingival epithelial cells (HGEC) following exposure to A. actinomycetemcomitans in comparison with human gingival fibroblasts (HGF) in culture. Northern blot analysis showed that mRNAs of interleukin 1beta (IL-1beta) and IL-8, but not IL-6, in HGEC were induced in response to A. actinomycetemcomitans. Secretion of IL-8 by HGEC was also increased following A. actinomycetemcomitans challenge, whereas production of IL-1beta could not be detected. The levels of IL-8 and its mRNA were increased depending on the concentration of A. actinomycetemcomitans. The co-culture with HGF and A. actinomycetemcomitans resulted in an increase in the levels of IL-6 and IL-8 mRNA in HGF. However, HGF exposed to A. actinomycetemcomitans, showed no expression of IL-1beta mRNA. These findings demonstrated that HGEC and HGF stimulated with A. actinomycetemcomitans have different profiles in cytokine mRNA expression. Furthermore, A. actinomycetemcomitans may play an important role in amplifying the local immune response and in initiating inflammatory reaction through release of IL-8 from gingival epithelial cells.

Virulence of group A streptococci in fertile hens eggs is mainly effected by M protein and streptolysin O

In this study we have investigated whether streptolysin O contributes to the virulence of group A streptococci. For this purpose we generated M-negative and SLO-negative mutants by insertion mutagenesis into the chromosome of an M type 1 strain. The inactivation of M1 protein expression was achieved by the construction of the integrative plasmid pSFABS, which contains the internal fragment abs of the emm1 gene. Integration of pSFABS by homologous recombination into the chromosome of strain 38 541 resulted in the generation of mutant EMM1. Inactivation of slo with plasmid pFWSLOD resulted in two different mutant forms. The homologous recombination with plasmid pFWSLOD carrying the two slo fragments slo1 (899 base pairs in the 5' region) and slo2 (709 base pairs in the downstream part) resulted in mutants SLO3, SLO4 and SLO17. In SLO17 a double crossover event took place with insertion of the spectinomycin resistance gene aad9 between the slo fragments 1 and 2. In mutants SLO3 and SLO4 the homologous recombination with the same plasmid led to the integration of the whole plasmid construct into the chromosome of strain 38 541. Both forms of mutation failed to express SLO. In mutant SLO4 additionally M1 protein expression was significantly decreased. The mutants EMM1 (M-, SLO+) and SLO4 (M decreased, SLO-) showed a reduced binding to collagen-coated surfaces. In contrast the mutants SLO3 and SLO17 (both M+, SLO-) and the wild-type strain 38 541 (M+, SLO+) showed an affinity to collagen similar to purified M1 protein. All mutants were less virulent for chicken embryos compared to the wild-type strain after infection by intravenous injection as well as by application onto the chorioallantoic membrane. The results show that besides M protein SLO can also influence virulence of group A streptococci. Moreover, it became obvious that streptococci need more than one tool to fully develop their infectious potential.

Cytolysin-mediated translocation (CMT): a functional equivalent of type III secretion in gram-positive bacteria

Type III secretion for injection of effector proteins into host cells has not been described for Gram-positive bacteria despite their importance in disease. Here, we describe an injection pathway for the Gram-positive pathogen Streptococcus pyogenes that utilizes streptolysin O (SLO), a cholesterol-dependent cytolysin. The data support a model in which an effector is translocated through the SLO pore by a polarized process. The effector, SPN (S. pyogenes NAD-glycohydrolase), is capable of producing the potent second messenger cyclic ADP-ribose, and SLO and SPN act synergistically to trigger cytotoxicity. These data provide a novel paradigm for the function of the cholesterol-dependent cytolysin family and its wide distribution suggests that cytolysin-mediated translocation (CMT) may be the equivalent of type III secretion for Gram-positive pathogens.

Pathogenesis of group A streptococcal infections

Group A streptococci are model extracellular gram-positive pathogens responsible for pharyngitis, impetigo, rheumatic fever, and acute glomerulonephritis. A resurgence of invasive streptococcal diseases and rheumatic fever has appeared in outbreaks over the past 10 years, with a predominant M1 serotype as well as others identified with the outbreaks. emm (M protein) gene sequencing has changed serotyping, and new virulence genes and new virulence regulatory networks have been defined. The emm gene superfamily has expanded to include antiphagocytic molecules and immunoglobulin-binding proteins with common structural features. At least nine superantigens have been characterized, all of which may contribute to toxic streptococcal syndrome. An emerging theme is the dichotomy between skin and throat strains in their epidemiology and genetic makeup. Eleven adhesins have been reported, and surface plasmin-binding proteins have been defined. The strong resistance of the group A streptococcus to phagocytosis is related to factor H and fibrinogen binding by M protein and to disarming complement component C5a by the C5a peptidase. Molecular mimicry appears to play a role in autoimmune mechanisms involved in rheumatic fever, while nephritis strain-associated proteins may lead to immune-mediated acute glomerulonephritis. Vaccine strategies have focused on recombinant M protein and C5a peptidase vaccines, and mucosal vaccine delivery systems are under investigation.

Humanized in vivo model for streptococcal impetigo

An in vivo model for group A streptococcal (GAS) impetigo was developed, whereby human neonatal foreskin engrafted onto SCID mice was superficially damaged and bacteria were topically applied. Severe infection, indicated by a purulent exudate, could be induced with as few as 1,000 CFU of a virulent strain. Early findings (48 h) showed a loss of stratum corneum and adherence of short chains of gram-positive cocci to the external surface of granular keratinocytes. This was followed by an increasing infiltration of polymorphonuclear leukocytes (neutrophils) of mouse origin, until a thick layer of pus covered an intact epidermis, with massive clumps of cocci accumulated at the outer rim of the pus layer. By 7 days postinoculation, the epidermis was heavily eroded; in some instances, the dermis contained pockets (ulcers) filled with cocci, similar to that observed for ecthyma. Importantly, virulent GAS underwent reproduction, resulting in a net increase in CFU of 20- to 14,000-fold. The majority of emm pattern D strains had a higher gross pathology score than emm pattern A, B, or C (A-C) strains, consistent with epidemiological findings that pattern D strains have a strong tendency to cause impetigo, whereas pattern A-C strains are more likely to cause pharyngitis.

Apoptosis of human keratinocytes after bacterial invasion

In this study, we examined the invasive capacity of Staphylococcus aureus and Salmonella typhi in human keratinocytes and monitored the number of viable intracellular bacteria at different post-infection times. The strains tested entered keratinocytes; both S. typhi and S. aureus were internalized within 30 min to 2 h after infection. No intracellular multiplication was observed, but S. typhi and S. aureus remained viable 72 h after infection. We also demonstrated that keratinocyte death following S. typhi and S. aureus invasion occurs by apoptosis as shown by DNA fragmentation. After 24 h of infection with S. typhi, the number of cells undergoing apoptosis were higher compared to infection with S. aureus. For prolonged infection times (48 h, 72 h) with both bacteria, there was no significant change in the number of cells undergoing apoptosis. The results demonstrated that viable intracellular S. typhi and S. aureus induced apoptosis in keratinocyte cells.

Examination of early interactions between Haemophilus ducreyi and host cells by using cocultured HaCaT keratinocytes and foreskin fibroblasts

Haemophilus ducreyi is the etiologic agent of chancroid, a sexually transmitted genital ulcer disease. Keratinocytes are likely the first cell type encountered by H. ducreyi upon infection of human skin; thus, the interaction between H. ducreyi and keratinocytes is probably important for the ability of H. ducreyi to establish infection. We have used the HaCaT keratinocyte cell line grown in monolayers and in cocultures with HS27 fibroblasts to investigate H. ducreyi interactions with keratinocytes and the host-cell response to H. ducreyi infection. Using quantitative adherence and gentamicin protection assays, we determined that approximately 13% of H. ducreyi adhered to HaCaT cell monolayers, while only a small proportion (0.0052%) was intracellular. By transmission electron microscopy, we observed numerous H. ducreyi organisms adherent to but rarely within HaCaT cells cocultured with fibroblasts. Both live H. ducreyi and purified H. ducreyi lipooligosaccharide (LOS) induced significant interleukin 8 (IL-8) expression from HaCaT cell-HS27 cell cocultures. However, the level of IL-8 expression in response to LOS alone was not as pronounced. H. ducreyi LOS was a more potent inducer of IL-8 from cocultures than Escherichia coli lipopolysaccharide (LPS) at the same concentration, suggesting a unique effect of H. ducreyi LOS on cocultures. Neither live H. ducreyi nor purified H. ducreyi LOS or E. coli LPS induced tumor necrosis factor alpha expression from cocultures. H. ducreyi induced drastically different cytokine profiles from cocultures than from HS27 or HaCaT cells cultured separately. IL-8 expression by skin cells in response to H. ducreyi infection in vivo may be responsible for the massive influx of polymorphonuclear leukocytes and other inflammatory cells to the site of infection. This influx of inflammatory cells may be partly responsible for the tissue destruction characteristic of chancroid.

Surface proteins of gram-positive bacteria and mechanisms of their targeting to the cell wall envelope

The cell wall envelope of gram-positive bacteria is a macromolecular, exoskeletal organelle that is assembled and turned over at designated sites. The cell wall also functions as a surface organelle that allows gram-positive pathogens to interact with their environment, in particular the tissues of the infected host. All of these functions require that surface proteins and enzymes be properly targeted to the cell wall envelope. Two basic mechanisms, cell wall sorting and targeting, have been identified. Cell well sorting is the covalent attachment of surface proteins to the peptidoglycan via a C-terminal sorting signal that contains a consensus LPXTG sequence. More than 100 proteins that possess cell wall-sorting signals, including the M proteins of Streptococcus pyogenes, protein A of Staphylococcus aureus, and several internalins of Listeria monocytogenes, have been identified. Cell wall targeting involves the noncovalent attachment of proteins to the cell surface via specialized binding domains. Several of these wall-binding domains appear to interact with secondary wall polymers that are associated with the peptidoglycan, for example teichoic acids and polysaccharides. Proteins that are targeted to the cell surface include muralytic enzymes such as autolysins, lysostaphin, and phage lytic enzymes. Other examples for targeted proteins are the surface S-layer proteins of bacilli and clostridia, as well as virulence factors required for the pathogenesis of L. monocytogenes (internalin B) and Streptococcus pneumoniae (PspA) infections. In this review we describe the mechanisms for both sorting and targeting of proteins to the envelope of gram-positive bacteria and review the functions of known surface proteins.

Streptolysin O and adherence synergistically modulate proinflammatory responses of keratinocytes to group A streptococci

In contrast to a mutant adhesin-deficient Streptococcus pyogenes (group A streptococcus), its isogenic parental strain binds to human keratinocytes and promotes a vigorous proinflammatory response, characterized by enhanced expression of several cytokines, a more rapid release of prostaglandin E2 (PGE2) and damage to keratinocyte membranes. However, adherence alone is not sufficient to induce these responses. In this study, we have begun to examine the contribution of other streptococcal products in interactions with keratinocytes by the construction and evaluation of mutants deficient in expression of the secreted pore-forming haemolysin, streptolysin O (SLO). Inactivation of SLO did not prevent the streptococci from adhering to cultured HaCaT keratinocytes or from expressing an unrelated second streptococcal haemolysin, streptolysin S, during infection of keratinocytes. As measured by a quantitative reverse transcriptase polymerase chain reaction (PCR) assay, inactivation of SLO also did not have a marked effect on the expression of interleukin 1alpha (IL-1alpha) during infection. However, the lack of the ability to produce SLO was associated with a considerable reduction in expression of IL-1beta, IL-6 and IL-8 by infected keratinocytes. Measurement of the release of PGE2 by an enzyme-linked immunosorbent assay demonstrated that the SLO-deficient mutants were also not capable of promoting the rapid high level of PGE2 release characteristic of the adherent SLO-producing parental strain. Finally, analyses using the fluorescent probe ethidium homodimer-1 and measurements of release of keratinocyte lactate dehydrogenase indicated that the failure of the SLO-deficient mutants to induce responses was associated with the failure of these mutants to damage the integrity of the keratinocyte membrane. These data implicate SLO as a factor that acts synergistically with an adhesin to modulate the signalling responses of keratinocytes during infection.