Identification of molecular mechanisms used by Finegoldia magna to penetrate and colonize human skin

Finegoldia magna is a Gram-positive anaerobic commensal of the human skin microbiota, but also known to act as an opportunistic pathogen. Two primary virulence factors of F. magna are the subtilisin-like extracellular serine protease SufA and the adhesive protein FAF. This study examines the molecular mechanisms F. magna uses when colonizing or establishing an infection in the skin. FAF was found to be essential in the initial adherence of F. magna to human skin biopsies. In the upper layers of the epidermis FAF mediates adhesion through binding to galectin-7 - a keratinocyte cell marker. Once the bacteria moved deeper into the skin to the basement membrane layer, SufA was found to degrade collagen IV which forms the backbone structure of the basement membrane. It also degraded collagen V, whereby F. magna could reach deeper dermal tissue sites. In the dermis, FAF interacts with collagen V and fibrillin, which presumably helps the bacteria to establish infection in this area. The findings of this study paint a clear picture of how F. magna interacts with human skin and explain how it is such a successful opportunistic pathogen in chronic wounds and ulcers.

Expression of MIG/CXCL9 in cystic fibrosis and modulation of its activities by elastase of Pseudomonas aeruginosa

In cystic fibrosis (CF), colonization of the airways with Pseudomonas aeruginosa is associated with disease deterioration. The mechanism behind the disease progression is not fully understood. The present work shows that the antibacterial chemokine MIG/CXCL9 is present in the airways and in sputum of CF patients. MIG/CXCL9 showed high bactericidal activity against. P. aeruginosa, including some strains from the airways of CF patients. Full-length MIG/CXCL9 was detected in sputum from healthy controls and CF patients colonized with P. aeruginosa. However, degraded MIG/CXCL9 was only found in CF sputum. In vitro, elastase of P. aeruginosa cleaved off a fragment of similar size and two additional fragments from MIG/CXCL9. The fragments showed less bactericidal activity against P. aeruginosa compared with the full-length protein. The fragments did not activate the MIG/CXCL9 receptor CXCR3 (expressed e.g. by NK cells, mast cells, and activated T cells) but instead displayed noncompetitive inhibition. In vitro, a decrease in CXCR3-bearing cells was found within and in the proximity of the bronchial epithelium of CF lung tissue compared with controls. Taken together, both bactericidal and cell-recruiting activities of MIG/CXCL9 are corrupted by P. aeruginosa through release of elastase, and this may contribute to impaired airway host defense in CF.

Treatment with p33 curtails morbidity and mortality in a histone-induced murine shock model

Collateral damage caused by extracellular histones has an immediate impact on morbidity and mortality in many disease models. A significant increase in the levels of extracellular histones is seen in critically ill patients with trauma and sepsis. We showed that histones are released from necrotic cells in patients with invasive skin infections. Under in vitro conditions, endogenous p33, an endothelial surface protein also known as the gC1q receptor, interacts with histones released from damaged endothelial cells. Functional analyses have revealed that recombinantly expressed p33 completely neutralizes the harmful features of histones, i.e. hemolysis of erythrocytes, lysis of endothelial cells and platelet aggregation. We also noted that mice treated with a sublethal dose of histones developed severe signs of hemolysis, thrombocytopenia and lung tissue damage already 10 min after inoculation. These complications were fully counteracted when p33 was administered together with the histones. Moreover, application of p33 significantly improved survival in mice receiving an otherwise lethal dose of histones. Together, our data suggest that treatment with p33 is a promising therapeutic approach in severe infectious diseases.

Pathogenic mechanisms in lupus nephritis: Nucleosomes bind aberrant laminin β1 with high affinity and colocalize in the electron-dense deposits

OBJECTIVE

Apoptotic nucleosomes are structurally and immunologically involved in lupus nephritis. The purpose of this study was to examine the expression and function of laminins and their interactions with nucleosomes in the kidneys of patients with lupus nephritis, using surface plasmon resonance (SPR) analysis.

METHODS

SPR interaction analysis was used to quantify the strength of laminin-nucleosome interactions. Electron microscopy techniques were used to determine in vivo colocalization of IgG, chromatin, and laminin β1, as well as to characterize nucleosome-laminin interactions in vitro.

RESULTS

Nucleosomes were found to possess high affinity for laminin β1-containing laminins and to have the potential to form stable molecular complexes with these structures. In vivo, laminin β1 was aberrantly expressed in the glomerular basement membrane (GMB) of lupus nephritis patients, and in situ, it acted as a nucleosome ligand, selectively colocalizing with nucleosomes within electron-dense deposits (EDDs). Normal adult laminin 11, which contains laminin β2, did not bind nucleosomes, and it did not colocalize in vivo with the nucleosomes in the nephritic GBM. In addition, TGFβ1 was expressed by the glomerular mesangium, glomerular endothelial cells, and by podocytes in patients with lupus nephritis. It was trapped in situ within EDDs by an as-yet-unknown ligand. As was recently described in a transgenic mouse model, paracrine kidney glomerular TGFβ1 may thereby contribute to the development of glomerulopathy via the induction of laminin β1 incorporation in the GBM, whereas systemic blood vessel-derived TGFβ1 could be trapped during filtration.

CONCLUSION

Our findings of the specific high-affinity binding of nucleosomes to aberrantly expressed laminin β1 in the GBM and their colocalization in the GBM may explain important features of the initial steps in the pathogenesis of lupus nephritis, the planted antigen hypothesis.

The combined role of galactose-deficient IgA1 and streptococcal IgA-binding M Protein in inducing IL-6 and C3 secretion from human mesangial cells: implications for IgA nephropathy

IgA nephropathy (IgAN) is characterized by mesangial cell proliferation and extracellular matrix expansion associated with immune deposits consisting of galactose-deficient polymeric IgA1 and C3. We have previously shown that IgA-binding regions of streptococcal M proteins colocalize with IgA in mesangial immune deposits in patients with IgAN. In the present study, the IgA-binding M4 protein from group A Streptococcus was found to bind to galactose-deficient polymeric IgA1 with higher affinity than to other forms of IgA1, as shown by surface plasmon resonance and solid-phase immunoassay. The M4 protein was demonstrated to bind to mesangial cells not via the IgA-binding region but rather via the C-terminal region, as demonstrated by flow cytometry. IgA1 enhanced binding of M4 to mesangial cells, but not vice versa. Costimulation of human mesangial cells with M4 and galactose-deficient polymeric IgA1 resulted in a significant increase in IL-6 secretion compared with each stimulant alone. Galactose-deficient polymeric IgA1 alone, but not M4, induced C3 secretion from the cells, and costimulation enhanced this effect. Additionally, costimulation enhanced mesangial cell proliferation compared with each stimulant alone. These results indicate that IgA-binding M4 protein binds preferentially to galactose-deficient polymeric IgA1 and that these proteins together induce excessive proinflammatory responses and proliferation of human mesangial cells. Thus, tissue deposition of streptococcal IgA-binding M proteins may contribute to the pathogenesis of IgAN.

Oral iodinated activated charcoal improves lung function in patients with COPD

The effect of 8 weeks treatment with oral iodinated activated charcoal (IAC) on lung function of patients with moderate chronic obstructive pulmonary disease (COPD) was examined in a double blind randomized placebo controlled parallel group study with 40 patients. In the IAC group, patients showed a statistically significant improvement of FEV1 baseline by 130 ml compared to placebo, corresponding to 8.2% improvement (p = 0.031*). Correlation statistics revealed that the improvement of FEV1 baseline was significantly correlated both to FEV1 post-bronchodilator (p = 0.0020**) and FEV1 post-exercise (0.033*) values. This demonstrates that the improved baseline lung function by IAC did not inhibit a further beta2-adrenoceptor relaxation, and thus that patients did not reach a limit for maximal improvement of the lung function after IAC treatment. Eight patients in the IAC group developed abnormal thyroid hormone levels transiently during the treatment. This side effect was not correlated to improvement of lung function (p = 0.82). No serious adverse effects directly related to the treatment were recorded. In summary, this study demonstrates that iodinated activated charcoal surprisingly and significantly improved lung function of patients with moderate COPD. The underlying mechanism of action is unclear, but is likely to be different from the drugs used today. The immediate conclusion is that further studies are now justified in order to determine clinical efficacy of IAC in COPD and explore possible mechanisms of action.

Midkine is expressed and differentially processed during chronic obstructive pulmonary disease exacerbations and ventilator-associated pneumonia associated with Staphylococcus aureus infection

Staphylococcus aureus is sometimes isolated from the airways during acute exacerbations of chronic obstructive pulmonary disease (COPD) but more commonly recognized as a cause of ventilator-associated pneumonia (VAP). Antimicrobial proteins, among them midkine (MK), are an important part of innate immunity in the airways. In this study, the levels and possible processing of MK in relation to S. aureus infection of the airways were investigated, comparing COPD and VAP, thus comparing a state of disease with preceding chronic inflammation and remodeling (COPD) with acute inflammation (that is, VAP). MK was detected in the small airways and alveoli of COPD lung tissue but less so in normal lung tissue. MK at below micromolar concentrations killed S. aureus in vitro. Proteolytic processing of MK by the staphylococcal metalloprotease aureolysin (AL), but not cysteine protease staphopain A (SA), resulted in impaired bactericidal activity. Degradation was seen foremost in the COOH-terminal portion of the molecule that harbors high bactericidal activity. In addition, MK was detected in sputum from patients suffering from VAP caused by S. aureus but less so in sputum from COPD exacerbations associated with the same bacterium. Recombinant MK was degraded more rapidly in sputum from the COPD patients than from the VAP patients and a greater proteolytic activity in COPD sputum was confirmed by zymography. Taken together, proteases of both bacteria and the host contribute to degradation of the antibacterial protein MK, resulting in an impaired defense of the airways, in particular, in COPD where the state of chronic inflammation could be of importance.

A novel role for pro-coagulant microvesicles in the early host defense against streptococcus pyogenes

Previous studies have shown that stimulation of whole blood or peripheral blood mononuclear cells with bacterial virulence factors results in the sequestration of pro-coagulant microvesicles (MVs). These particles explore their clotting activity via the extrinsic and intrinsic pathway of coagulation; however, their pathophysiological role in infectious diseases remains enigmatic. Here we describe that the interaction of pro-coagulant MVs with bacteria of the species Streptococcus pyogenes is part of the early immune response to the invading pathogen. As shown by negative staining electron microscopy and clotting assays, pro-coagulant MVs bind in the presence of plasma to the bacterial surface. Fibrinogen was identified as a linker that, through binding to the M1 protein of S. pyogenes, allows the opsonization of the bacteria by MVs. Surface plasmon resonance analysis revealed a strong interaction between pro-coagulant MVs and fibrinogen with a K_D value in the nanomolar range. When performing a mass-spectrometry-based strategy to determine the protein quantity, a significant up-regulation of the fibrinogen-binding integrins CD18 and CD11b on pro-coagulant MVs was recorded. Finally we show that plasma clots induced by pro-coagulant MVs are able to prevent bacterial dissemination and possess antimicrobial activity. These findings were confirmed by in vivo experiments, as local treatment with pro-coagulant MVs dampens bacterial spreading to other organs and improved survival in an invasive streptococcal mouse model of infection. Taken together, our data implicate that pro-coagulant MVs play an important role in the early response of the innate immune system in infectious diseases.

The coagulation system is much more than a passive bystander in our defense against exogenous microorganisms. Over the last years there has been a growing body of evidence pointing to an integral part of coagulation in innate immunity and a special focus has been on bacterial entrapment in a fibrin network. However, thus far, pro-coagulant MVs have not been discussed in this context, though it is known that their numbers can dramatically increase in many pathological conditions, including severe infectious diseases. In the present study we see a significant increase of pro-coagulant MVs in an invasive streptococcal mouse model, suggesting that their release is an immune response to the infection. We find that pro-coagulant MVs bind to Streptococcus pyogenes and promote clotting, entrapment, and killing of the bacteria in a fibrin network. As a proof of concept pro-coagulant MVs were applied as local treatment in the streptococcal infection model, and it was demonstrated that this led to a significantly improved survival in mice.

Antimicrobial activity of fibrinogen and fibrinogen-derived peptides--a novel link between coagulation and innate immunity

Fibrinogen is a key player in the blood coagulation system, and is upon activation with thrombin converted into fibrin that subsequently forms a fibrin clot. In the present study, we investigated the role of fibrinogen in the early innate immune response. Here we show that the viability of fibrinogen-binding bacteria is affected in human plasma activated with thrombin. Moreover, we found that the peptide fragment GHR28 released from the β-chain of fibrinogen has antimicrobial activity against bacteria that bind fibrinogen to their surface, whereas non-binding strains are unaffected. Notably, bacterial killing was detected in Group A Streptococcus bacteria entrapped in a fibrin clot, suggesting that fibrinogen and coagulation is involved in the early innate immune system to quickly wall off and neutralise invading pathogens.

Characterization of released polypeptides during an interferon-γ-dependent antibacterial response in airway epithelial cells

When pathogenic bacteria breach the epithelial lining at mucosal surfaces, rapidly available innate immune mechanisms are critical to halt the infection. In the present study, we characterized the production of antibacterial polypeptides released by epithelial cells. IFN-γ, but neither TNF nor IL-1β alone, induced release of antibacterial activity to a cell culture medium, causing a lytic appearance of killed bacteria as revealed by electron microscopy. Addition of the protein streptococcal inhibitor of complement, derived from Streptococcus pyogenes, known for its ability to neutralize antimicrobial polypeptides (AMPs), reduced the antibacterial activity of the medium. Characterization of the antibacterial incubation medium using mass spectrometric approaches and ELISAs, displayed presence of several classical AMPs, antibacterial chemokines, as well as complement factors and proteases that may interfere with bacterial killing. Many were constitutively produced, that is, being released by cells incubated in a medium alone. While a combination of IFN-γ and TNF did not increase bacterial killing, the presence of TNF boosted the amounts and detectable number of AMPs, including antibacterial chemokines. However, the methods applied in the study failed to single out certain AMPs as critical mediators, but rather demonstrate the broad range of molecules involved. Since many AMPs are highly amphiphatic in nature (i.e., cationic and hydrophobic), it is possible that difficulties in optimizing recovery present limitations in the context investigated. The findings demonstrate that epithelial cells have a constitutive production of AMPs and that IFN-γ is an important inducer of an antibacterial response in which is likely to be a critical part of the innate host defense against pathogenic bacteria at mucosal surfaces.

Streptococcal surface proteins activate the contact system and control its antibacterial activity

Group G streptococci (GGS) are important bacterial pathogens in humans. Here, we investigated the interactions between GGS and the contact system, a procoagulant and proinflammatory proteolytic cascade that, upon activation, also generates antibacterial peptides. Two surface proteins of GGS, protein FOG and protein G (PG), were found to bind contact system proteins. Experiments utilizing contact protein-deficient human plasma and isogenic GGS mutant strains lacking FOG or PG showed that FOG and PG both activate the procoagulant branch of the contact system. In contrast, only FOG induced cleavage of high molecular weight kininogen, generating the proinflammatory bradykinin peptide and additional high molecular weight kininogen fragments containing the antimicrobial peptide NAT-26. On the other hand, PG protected the bacteria against the antibacterial effect of NAT-26. These findings underline the significance of the contact system in innate immunity and demonstrate that GGS have evolved surface proteins to exploit and modulate its effects.

Thymic stromal lymphopoietin exerts antimicrobial activities

Thymic stromal lymphopoietin (TSLP) is an interleukin-7-like cytokine expressed by epithelial cells and reported to be involved in allergic diseases and atopic eczema. The presence of several predicted α-helical regions in TSPL, a structure characterizing many classical antimicrobial peptides (AMPs), prompted us to investigate whether TSLP exerts antimicrobial activities. Recombinant human TSLP exerted antimicrobial activity, particularly against Gram-negative bacteria. Using synthetic overlapping peptide 20-mers of TSLP, it was demonstrated that the antimicrobial effect is primarily mediated by the C-terminal region of the protein. MKK34 (MKKRRKRKVTTNKCLEQVSQLQGLWRRFNRPLLK), a peptide spanning a C-terminal α-helical region in TSLP, showed potent antimicrobial activities, in physiological salt conditions and in the presence of human plasma. Fluorescent studies of peptide-treated bacteria, electron microscopy and liposome leakage models showed that MKK34 exerted membrane-disrupting effects comparable to those of the classical AMP LL-37. Moreover, TSLP was degraded into multiple fragments by staphylococcal V8 proteinase. One major antimicrobial degradation fragment was found to encompass the C-terminal antimicrobial region defined by the MKK34 peptide. We here describe a novel antimicrobial role for TSLP. The antimicrobial activity is primarily mediated by the C-terminal part of the protein. In combination with the previously known cytokine function of TSLP, our result indicates dual functions of the molecule and a previously unknown role in host defense.

A novel C3 mutation causing increased formation of the C3 convertase in familial atypical hemolytic uremic syndrome

Atypical hemolytic uremic syndrome has been associated with dysregulation of the alternative complement pathway. In this study, a novel heterozygous C3 mutation was identified in a factor B-binding region in exon 41, V1636A (4973 T > C). The mutation was found in three family members affected with late-onset atypical hemolytic uremic syndrome and symptoms of glomerulonephritis. All three patients exhibited increased complement activation detected by decreased C3 levels and glomerular C3 deposits. Platelets from two of the patients had C3 and C9 deposits on the cell surface. Patient sera exhibited more C3 cleavage and higher levels of C3a. The C3 mutation resulted in increased C3 binding to factor B and increased net formation of the C3 convertase, even after decay induced by decay-accelerating factor and factor H, as assayed by surface plasmon resonance. Patient sera incubated with washed human platelets induced more C3 and C9 deposition on the cell surface in comparison with normal sera. More C3a was released into serum over time when washed platelets were exposed to patient sera. Results regarding C3 and C9 deposition on washed platelets were confirmed using purified patient C3 in C3-depleted serum. The results indicated enhanced convertase formation leading to increased complement activation on cell surfaces. Previously described C3 mutations showed loss of function with regard to C3 binding to complement regulators. To our knowledge, this study presents the first known C3 mutation inducing increased formation of the C3 convertase, thus explaining enhanced activation of the alternative pathway of complement.

Peptidylarginine deiminases present in the airways during tobacco smoking and inflammation can citrullinate the host defense peptide LL-37, resulting in altered activities

Bacterial colonization of the lower respiratory tract is frequently seen in chronic obstructive pulmonary disease (COPD), and may cause exacerbations leading to disease progression. Antimicrobial peptides comprise an important part of innate lung immunity, and not least the cathelicidin human cationic antimicrobial protein-18/LL-37. Peptidylarginine deiminases (PADIs) post-translationally modify proteins by converting cationic peptidylarginine residues to neutral peptidylcitrulline. An increased presence of PADI2 and citrullinated proteins was demonstrated in the lungs of smokers. In this study, preformed PADI4, stored in granulocytes and extracellularly in the lumina of bronchi, was found in lung tissue of individuals suffering from COPD. In vitro, recombinant human PADI2 and PADI4 both caused a time- and dose-dependent citrullination of LL-37. The citrullination resulted in impaired antibacterial activity against Staphylococcus aureus, Streptococcus pneumoniae, and nontypable Haemophilus influenzae, but less so against Pseudomonas aeruginosa. Using artificial lipid bilayers, we observed discrete differences when comparing the disrupting activity of native and citrullinated LL-37, suggesting that differences in cell wall composition are important during interactions with whole bacteria. Furthermore, citrullinated LL-37 showed higher chemotactic activity against mononuclear leukocytes than did native LL-37, but was less efficient at neutralizing lipolysaccharide, and also in converting apoptotic neutrophils into a state of secondary necrosis. In addition, citrullinated LL-37 was more prone to degradation by proteases, whereas the V8 endopetidase of S. aureus cleaved the modified peptide at additional sites, compared with native LL-37. Together, these findings demonstrate novel mechanisms whereby the inflammation-dependent deiminases PADI2 and PADI4 can alter the activites of antibacterial polypeptides, affecting the course of inflammatory disorders such as COPD.

Constitutive and inflammation-dependent antimicrobial peptides produced by epithelium are differentially processed and inactivated by the commensal Finegoldia magna and the pathogen Streptococcus pyogenes

Epithelial linings serve as physical barriers and produce antimicrobial peptides (AMPs) to maintain host integrity. Examples are the bactericidal proteins midkine (MK) and BRAK/CXCL14 that are constitutively produced in the skin epidermal layer, where the anaerobic Gram-positive coccoid commensal Finegoldia magna resides. Consequently, this bacterium is likely to encounter both MK and BRAK/CXCL14, making these molecules possible threats to its habitat. In this study, we show that MK expression is upregulated during inflammation, concomitant with a strong downregulation of BRAK/CXCL14, resulting in changed antibacterial conditions. MK, BRAK/CXCL14, and the inflammation-dependent antimicrobial β-defensins human β-defensin (hBD)-2 and hBD-3 all showed bactericidal activity against both F. magna and the virulent pathogen Streptococcus pyogenes at similar concentrations. SufA, a released protease of F. magna, degraded MK and BRAK/CXCL14 but not hBD-2 nor hBD-3. Cleavage was seen at lysine and arginine residues, amino acids characteristic of AMPs. Intermediate SufA-degraded fragments of MK and BRAK/CXCL14 showed stronger bactericidal activity against S. pyogenes than F. magna, thus promoting survival of the latter. In contrast, the cysteine-protease SpeB of S. pyogenes rapidly degraded all AMPs investigated. The proteins FAF and SIC, released by F. magna and S. pyogenes, respectively, neutralized the antibacterial activity of MK and BRAK/CXCL14, protein FAF being the most efficient. Quantitation and colocalization by immunoelectron microscopy demonstrated significant levels and interactions of the molecules in in vivo and ex vivo samples. The findings reflect strategies used by a permanently residing commensal and a virulent pathogen, the latter operating during the limited time course of invasive disease.

Human Siglec-5 inhibitory receptor and immunoglobulin A (IgA) have separate binding sites in streptococcal beta protein

Sialic acid-binding immunoglobulin-like lectins (Siglecs) are receptors believed to be important for regulation of cellular activation and inflammation. Several pathogenic microbes bind specific Siglecs via sialic acid-containing structures at the microbial surface, interactions that may result in modulation of host responses. Recently, it was shown that the group B Streptococcus (GBS) binds to human Siglec-5 (hSiglec-5), an inhibitory receptor expressed on macrophages and neutrophils, via the IgA-binding surface β protein, providing the first example of a protein/protein interaction between a pathogenic microbe and a Siglec. Here we show that the hSiglec-5-binding part of β resides in the N-terminal half of the protein, which also harbors the previously determined IgA-binding region. We constructed bacterial mutants expressing variants of the β protein with non-overlapping deletions in the N-terminal half of the protein. Using these mutants and recombinant β fragments, we showed that the hSiglec-5-binding site is located in the most N-terminal part of β (B6N region; amino acids 1–152) and that the hSiglec-5- and IgA-binding domains in β are completely separate. We showed with BIAcore^TM analysis that tandem variants of the hSiglec-5- and IgA-binding domains bind to their respective ligands with high affinity. Finally, we showed that the B6N region, but not the IgA-binding region of β, triggers recruitment of the tyrosine phosphatase SHP-2 to hSiglec-5 in U937 monocytes. Taken together, we have identified and isolated the first microbial non-sialic acid Siglec-binding region that can be used as a tool in studies of the β/hSiglec-5 interaction.

The membrane bound LRR lipoprotein Slr, and the cell wall-anchored M1 protein from Streptococcus pyogenes both interact with type I collagen

Streptococcus pyogenes is an important human pathogen and surface structures allow it to adhere to, colonize and invade the human host. Proteins containing leucine rich repeats (LRR) have been indentified in mammals, viruses, archaea and several bacterial species. The LRRs are often involved in protein-protein interaction, are typically 20–30 amino acids long and the defining feature of the LRR motif is an 11-residue sequence LxxLxLxxNxL (x being any amino acid). The streptococcal leucine rich (Slr) protein is a hypothetical lipoprotein that has been shown to be involved in virulence, but at present no ligands for Slr have been identified. We could establish that Slr is a membrane attached horseshoe shaped lipoprotein by homology modeling, signal peptidase II inhibition, electron microscopy (of bacteria and purified protein) and immunoblotting. Based on our previous knowledge of LRR proteins we hypothesized that Slr could mediate binding to collagen. We could show by surface plasmon resonance that recombinant Slr and purified M1 protein bind with high affinity to collagen I. Isogenic slr mutant strain (MB1) and emm1 mutant strain (MC25) had reduced binding to collagen type I as shown by slot blot and surface plasmon resonance. Electron microscopy using gold labeled Slr showed multiple binding sites to collagen I, both to the monomeric and the fibrillar structure, and most binding occurred in the overlap region of the collagen I fibril. In conclusion, we show that Slr is an abundant membrane bound lipoprotein that is co-expressed on the surface with M1, and that both these proteins are involved in recruiting collagen type I to the bacterial surface. This underlines the importance of S. pyogenes interaction with extracellular matrix molecules, especially since both Slr and M1 have been shown to be virulence factors.

Binding of albumin promotes bacterial survival at the epithelial surface

Human serum albumin (HSA) is the dominating protein in human plasma. Many bacterial species, especially streptococci, express surface proteins that bind HSA with high specificity and affinity, but the biological consequences of these protein-protein interactions are poorly understood. Group G streptococci (GGS), carrying the HSA-binding protein G, colonize the skin and the mucosa of the upper respiratory tract, mostly without causing disease. In the case of bacterial invasion, pro-inflammatory cytokines are released that activate the epithelium to produce antibacterial peptides, in particular the chemokine MIG/CXCL9. In addition, the inflammation causes capillary leakage and extravasation of HSA and other plasma proteins, environmental changes at the epithelial surface to which the bacteria need to respond. In this study, we found that GGS adsorbed HSA from both saliva and plasma via binding to protein G and that HSA bound to protein G bound and inactivated the antibacterial MIG/CXCL9 peptide. Another surface protein of GGS, FOG, was found to mediate adherence of the bacteria to pharyngeal epithelial cells through interaction with glycosaminoglycans. This adherence was not affected by activation of the epithelium with a combination of IFN-γ and TNF-α, leading to the production of MIG/CXCL9. However, at the activated epithelial surface, adherent GGS were protected against killing by MIG/CXCL9 through protein G-dependent HSA coating. The findings identify a previously unknown bacterial survival strategy that helps to explain the evolution of HSA-binding proteins among bacterial species of the normal human microbiota.

Midkine and pleiotrophin have bactericidal properties: preserved antibacterial activity in a family of heparin-binding growth factors during evolution

Antibacterial peptides of the innate immune system combat pathogenic microbes, but often have additional roles in promoting inflammation and as growth factors during tissue repair. Midkine (MK) and pleiotrophin (PTN) are the only two members of a family of heparin-binding growth factors. They show restricted expression during embryogenesis and are up-regulated in neoplasia. In addition, MK shows constitutive and inflammation-dependent expression in some non-transformed tissues of the adult. In the present study, we show that both MK and PTN display strong antibacterial activity, present at physiological salt concentrations. Electron microscopy of bacteria and experiments using artificial lipid bilayers suggest that MK and PTN exert their antibacterial action via a membrane disruption mechanism. The predicted structure of PTN, employing the previously solved MK structure as a template, indicates that both molecules consist of two domains, each containing three antiparallel beta-sheets. The antibacterial activity was mapped to the unordered C-terminal tails of both molecules and the last beta-sheets of the N-terminals. Analysis of the highly conserved MK and PTN orthologues from the amphibian Xenopus laevis and the fish Danio rerio suggests that they also harbor antibacterial activity in the corresponding domains. In support of an evolutionary conserved function it was found that the more distant orthologue, insect Miple2 from Drosophila melanogaster, also displays strong antibacterial activity. Taken together, the findings suggest that MK and PTN, in addition to their earlier described activities, may have previously unrealized important roles as innate antibiotics.

SufA of the opportunistic pathogen finegoldia magna modulates actions of the antibacterial chemokine MIG/CXCL9, promoting bacterial survival during epithelial inflammation

The anaerobic bacterium Finegoldia magna is part of the human commensal microbiota, but is also an important opportunistic pathogen. This bacterium expresses a subtilisin-like serine proteinase, SufA, which partially degrade the antibacterial chemokine MIG/CXCL9. Here, we show that MIG/CXCL9 is produced by human keratinocytes in response to inflammatory stimuli. In contrast to the virulent human pathogen Streptococcus pyogenes, the presence of F. magna had no enhancing effect on the MIG/CXCL9 expression by keratinocytes, suggesting poor detection of the latter by pathogen-recognition receptors. When MIG/CXCL9 was exposed to SufA-expressing F. magna, the molecule was processed into several smaller fragments. Analysis by mass spectrometry showed that SufA cleaves MIG/CXCL9 at several sites in the COOH-terminal region of the molecule. At equimolar concentrations, SufA-generated MIG/CXCL9 fragments were not bactericidal against F. magna, but retained their ability to kill S. pyogenes. Moreover, the SufA-generated MIG/CXCL9 fragments were capable of activating the angiostasis-mediating CXCR3 receptor, which is expressed on endothelial cells, in an order of magnitude similar to that of intact MIG/CXCL9. F. magna expresses a surface protein called FAF that is released from the bacterial surface by SufA. Soluble FAF was found to bind and inactivate the antibacterial activity of MIG/CXCL9, thereby further potentially promoting the survival of F. magna. The findings suggest that SufA modulation of the inflammatory response could be a mechanism playing an important role in creating an ecologic niche for F. magna, decreasing antibacterial activity and suppressing angiogenesis, thus providing advantage in survival for this anaerobic opportunist compared with competing pathogens during inflammation.

SpeB of Streptococcus pyogenes differentially modulates antibacterial and receptor activating properties of human chemokines

Background

CXC chemokines are induced by inflammatory stimuli in epithelial cells and some, like MIG/CXCL9, IP–10/CXCL10 and I–TAC/CXCL11, are antibacterial for Streptococcus pyogenes.

Methodology/Principal Findings

SpeB from S. pyogenes degrades a wide range of chemokines (i.e. IP10/CXCL10, I-TAC/CXCL11, PF4/CXCL4, GROα/CXCL1, GROβ/CXCL2, GROγ/CXCL3, ENA78/CXCL5, GCP-2/CXCL6, NAP-2/CXCL7, SDF-1/CXCL12, BCA-1/CXCL13, BRAK/CXCL14, SRPSOX/CXCL16, MIP-3α/CCL20, Lymphotactin/XCL1, and Fractalkine/CX3CL1), has no activity on IL-8/CXCL8 and RANTES/CCL5, partly degrades SRPSOX/CXCL16 and MIP-3α/CCL20, and releases a 6 kDa CXCL9 fragment. CXCL10 and CXCL11 loose receptor activating and antibacterial activities, while the CXCL9 fragment does not activate the receptor CXCR3 but retains its antibacterial activity.

Conclusions/Significance

SpeB destroys most of the signaling and antibacterial properties of chemokines expressed by an inflamed epithelium. The exception is CXCL9 that preserves its antibacterial activity after hydrolysis, emphasizing its role as a major antimicrobial on inflamed epithelium.

Identification of a novel protein promoting the colonization and survival of Finegoldia magna, a bacterial commensal and opportunistic pathogen

Anaerobic bacteria dominate the human normal microbiota, but strikingly little is known about these commensals. Finegoldia magna is a Gram-positive anaerobe found in the skin and at other non-sterile body surfaces, but it is also an opportunistic pathogen. This study describes a novel protein designated FAF (F. magna adhesion factor) and expressed by more than 90% of F. magna isolates. The protein is present in substantial quantities at the F. magna surface but is also released from the surface. FAF forms large protein aggregates in solution and surface-associated FAF causes bacterial clumping. In skin F. magna bacteria were localized to the epidermis, where they adhere to basement membranes. FAF was found to mediate this adhesion via interactions with BM-40, a basement membrane protein. The biological significance of FAF is further underlined by the observation that it blocks the activity of LL-37, a major human antibacterial peptide. Altogether, the data demonstrate that FAF plays an important role in colonization and survival of F. magna in the human host.

Antibodies against a surface protein of Streptococcus pyogenes promote a pathological inflammatory response

Streptococcal toxic shock syndrome (STSS) caused by Streptococcus pyogenes is a clinical condition with a high mortality rate despite modern intensive care. A key feature of STSS is excessive plasma leakage leading to hypovolemic hypotension, disturbed microcirculation and multiorgan failure. Previous work has identified a virulence mechanism in STSS where M1 protein of S. pyogenes forms complexes with fibrinogen that activate neutrophils to release heparin-binding protein (HBP), an inducer of vascular leakage. Here, we report a marked inter-individual difference in the response to M1 protein-induced HBP release, a difference found to be related to IgG antibodies directed against the central region of the M1 protein. To elicit massive HBP release, such antibodies need to be part of the M1 protein-fibrinogen complexes. The data add a novel aspect to bacterial pathogenesis where antibodies contribute to the severity of disease by promoting a pathologic inflammatory response.

Activation of TAFI on the surface of Streptococcus pyogenes evokes inflammatory reactions by modulating the kallikrein/kinin system

Bacteria-controlled regulation of host responses to infection is an important virulence mechanism that has been demonstrated to contribute to disease progression. Here we report that the human pathogen Streptococcus pyogenes employs the procarboxypeptidase TAFI (thrombin-activatable fibrinolysis inhibitor) to modulate the kallikrein/kinin system. To this end, bacteria initiate a chain of events starting with the recruitment and activation of TAFI. This is followed by the assembly and induction of the contact system at the streptococcal surface, eventually triggering the release of bradykinin (BK). BK is then carboxyterminally truncated by activated TAFI, which converts the peptide from a kinin B(2) receptor ligand to a kinin B(1) receptor (B1R) agonist. Finally, we show that streptococcal supernatants indirectly amplify the B1R response as they act on peripheral blood mononuclear cells to secrete inflammatory cytokines that in turn stimulate upregulation of the B1R on human fibroblasts. Taken together our findings implicate a critical and novel role for streptococci-bound TAFI, as it processes BK to a B1R agonist at the bacterial surface and thereby may redirect inflammation from a transient to a chronic state.

The proinflammatory CXC-chemokines GRO-alpha/CXCL1 and MIG/CXCL9 are concomitantly expressed in ulcerative colitis and decrease during treatment with topical corticosteroids

BACKGROUND

Ulcerative colitis is characterized by relapsing mucosal inflammation where the lesions include tissue-damaging granulocytes. In addition, T cells and natural killer (NK) cells play important pathophysiologic roles. Chemokines are a large family of peptides that play key roles in the regulation of inflammation. The CXC-chemokines, growth-related oncogene (GRO)-alpha/CXCL1 and interleukin (IL)-8/CXCL8, both recruit neutrophils and possess mitogenic properties, whereas the interferon-dependent CXC-chemokines monokine induced by gamma-interferon (MIG)/CXCL9, interferon-gamma inducible protein of 10 kD/CXCL10, and IFN-inducible T cell alpha chemoattractant/CXCL11 recruit and activate T cells and NK cells.

MATERIALS AND METHODS

The expression of CXC-chemokines was studied in eight controls and in 11 patients suffering from ulcerative colitis in the distal part of the colon, before and during topical treatment with corticosteroids. Perfusates (obtained before, after 7 days, and after 28 days of treatment) and pinch biopsies (obtained before and after 28 days of treatment) were collected by colonoscopy. The rectal release of GRO-alpha and MIG was determined by enzyme-linked immunosorbent assay (ELISA), and tissue expression of the chemokines was detected in colonic tissue by immunohistochemistry.

RESULTS

In perfusates, high levels of GRO-alpha, IL-8, and MIG were detected compared with controls (p=0.02, 0.005, and p=0.03, respectively). During treatment with corticosteroids, both GRO-alpha and MIG decreased. In clinical nonresponders, characterized by sustained inflammation, the levels of GRO-alpha and MIG remained elevated. Both epithelial cells and granulocytes, present in the submucosa, expressed GRO-alpha and MIG as detected by immunohistochemistry.

CONCLUSIONS

CXC-chemokines are likely to be important in the pathophysiology of ulcerative colitis and may become targets for novel treatment strategies. In addition, GRO-alpha may serve as a marker of disease activity.

Soluble M1 protein of Streptococcus pyogenes triggers potent T cell activation

Streptococcus pyogenes of the M1 serotype is commonly associated with large outbreaks of invasive streptococcal infections and development of streptococcal toxic shock syndrome (STSS). The pathogenesis behind these infections is believed to involve bacterial superantigens that induce potent inflammatory responses, but the reason why strains of the M1 serotype are over-represented in STSS is still not understood. In the present investigation, we show that a highly purified soluble form of the M1 protein from S. pyogenes, which lacks the membrane-spanning region, is a potent inducer of T cell proliferation and release of Th1 type cytokines. M1 protein-evoked T cell proliferation was HLA class II-dependent but not MHC-restricted, did not require intracellular processing and was Vbeta-restricted. Extensive mass spectrometry studies indicated that there were no other detectable proteins in the preparation. Taken together, our data demonstrate that soluble M1 protein is a novel streptococcal superantigen, which likely contributes to the excessive T cell activation and hyperinflammatory response seen in severe invasive streptococcal infections.

The CXC chemokine MIG/CXCL9 is important in innate immunity against Streptococcus pyogenes

Pharyngitis caused by Streptococcus pyogenes is one of the most common bacterial infections in humans and is also a starting point for invasive S. pyogenes infection. Here, we describe that tonsil fluid from patients with streptococcal pharyngitis contains high amounts of the interferon (IFN)-dependent CXC chemokine known as monokine induced by IFN- gamma (MIG)/CXCL9. Also in vitro, inflamed pharyngeal epithelium produced large amounts of MIG/CXCL9 in the presence of bacteria. The CXC chemokines MIG/CXCL9, IFN-inducible protein-10/CXCL10, and IFN-inducible T cell alpha -chemoattractant/CXCL11 all showed antibacterial activity against S. pyogenes, and inhibition of MIG/CXCL9 expression reduced the antibacterial activity at the surface of inflamed pharyngeal cells. S. pyogenes of the clinically important M1 serotype secrets the protein streptococcal inhibitor of complement (SIC), which inhibited the antibacterial activity of the chemokines. As exemplified by S. pyogenes pharyngitis, the data identify a novel innate defense mechanism against invasive bacteria on epithelial surfaces.

Fibulin-2 is present in murine vascular lesions and is important for smooth muscle cell migration

OBJECTIVE

The vascular extracellular matrix (ECM) can affect smooth muscle cell (SMC) adhesion, migration and proliferation-events that are important during the atherosclerotic process. Fibulin-2 is a member of the ECM protein family of fibulins and has been found to cross-link versican/hyaluronan complexes, an ECM network that has been suggested to be important during tissue repair. In this study we have analysed the presence of fibulin-2 in two different models of murine vascular lesions. We have also examined how the fibulin-2/versican network influences SMC migration.

METHODS

Presence of fibulin-2 was analysed by immunohistochemistry in atherosclerotic aortas and in mechanically injured carotid arteries from mice. Fibulin-2 protein levels were also studied by Western blotting during rat aortic SMC phenotypic modulation in vitro. The importance of a fibulin-2/versican interaction for SMC migration was studied in the presence of two inhibiting peptides (FN III 3-5 and aggrecan C-type lectin-like domain).

RESULTS

Fibulin-2 is expressed in SMC rich regions of atherosclerotic lesions where it colocalises with versican and hyaluronan. It is also present in injury-induced vascular lesions and is upregulated during SMC phenotypic modulation in cell culture. Moreover, treatments with peptides that block the interaction between versican and fibulin-2 inhibit SMC migration in vitro.

CONCLUSIONS

Fibulin-2 can be produced by SMC as a response to injury and may participate in the ECM organisation that regulates SMC migration during vessel wall repair.

Structural basis for interactions between tenascins and lectican C-type lectin domains: evidence for a crosslinking role for tenascins

The C-terminal G3 domains of lecticans mediate crosslinking to diverse extracellular matrix (ECM) proteins during ECM assembly, through their C-type lectin (CLD) subdomains. The structure of the rat aggrecan CLD in a Ca(2+)-dependent complex with fibronectin type III repeats 3-5 of rat tenascin-R provides detailed support for such crosslinking. The CLD loops bind Ca2+ like other CLDs, but no carbohydrate binding is observed or possible. This is thus the first example of a direct Ca(2+)-dependent protein-protein interaction of a CLD. Surprisingly, tenascin-R does not coordinate the Ca2+ ions directly. Electron microscopy confirms that full-length tenascin-R and tenascin-C crosslink hyaluronan-aggrecan complexes. The results are significant for the binding of all lectican CLDs to tenascin-R and tenascin-C. Comparison of the protein interaction surface with that of P-selectin in complex with the PGSL-1 peptide suggests that direct protein-protein interactions of Ca(2+)-binding CLDs may be more widespread than previously appreciated.

Alternative splicing in the aggrecan G3 domain influences binding interactions with tenascin-C and other extracellular matrix proteins

The proteoglycans aggrecan, versican, neurocan, and brevican bind hyaluronan through their N-terminal G1 domains, and other extracellular matrix proteins through the C-type lectin repeat in their C-terminal G3 domains. Here we identify tenascin-C as a ligand for the lectins of all these proteoglycans and map the binding site on the tenascin molecule to fibronectin type III repeats, which corresponds to the proteoglycan lectin-binding site on tenascin-R. In the G3 domain, the C-type lectin is flanked by epidermal growth factor (EGF) repeats and a complement regulatory protein-like motif. In aggrecan, these are subject to alternative splicing. To investigate if these flanking modules affect the C-type lectin ligand interactions, we produced recombinant proteins corresponding to aggrecan G3 splice variants. The G3 variant proteins containing the C-type lectin showed different affinities for various ligands, including tenascin-C, tenascin-R, fibulin-1, and fibulin-2. The presence of an EGF motif enhanced the affinity of interaction, and in particular the splice variant containing both EGF motifs had significantly higher affinity for ligands, such as tenascin-R and fibulin-2. The mRNA for this splice variant was shown by reverse transcriptase-PCR to be expressed in human chondrocytes. Our findings suggest that alternative splicing in the aggrecan G3 domain may be a mechanism for modulating interactions and extracellular matrix assembly.

The proteoglycans aggrecan and Versican form networks with fibulin-2 through their lectin domain binding

Aggrecan, versican, neurocan, and brevican are important components of the extracellular matrix in various tissues. Their amino-terminal globular domains bind to hyaluronan, but the function of their carboxyl-terminal globular domains has long remained elusive. A picture is now emerging where the C-type lectin motif of this domain mediates binding to other extracellular matrix proteins. We here demonstrate that aggrecan, versican, and brevican lectin domains bind fibulin-2, whereas neurocan does not. As expected for a C-type lectin, the interactions are calcium-dependent, with K(D) values in the nanomolar range as measured by surface plasmon resonance. Solid phase competition assays with previously identified ligands demonstrated that fibulin-2 and tenascin-R bind the same site on the proteoglycan lectin domains. Fibulin-1 has affinity for the common site on versican but may bind to a different site on the aggrecan lectin domain. By using deletion mutants, the interaction sites for aggrecan and versican lectin domains were mapped to epidermal growth factor-like repeats in domain II of fibulin-2. Affinity chromatography and solid phase assays confirmed that also native full-length aggrecan and versican bind the lectin domain ligands. Electron microscopy confirmed the mapping and demonstrated that hyaluronan-aggrecan complexes can be cross-linked by the fibulins.