Antibody orientation at bacterial surfaces is related to invasive infection

Development of Phage-Based Antibody Fragment Reagents for Affinity Enrichment of Bacterial Immunoglobulin G Binding Proteins

Disease and death caused by bacterial infections are global health problems. Effective bacterial strategies are required to promote survival and proliferation within a human host, and it is important to explore how this adaption occurs. However, the detection and quantification of bacterial virulence factors in complex biological samples are technically demanding challenges. These can be addressed by combining targeted affinity enrichment of antibodies with the sensitivity of liquid chromatography-selected reaction monitoring mass spectrometry (LC-SRM MS). However, many virulence factors have evolved properties that make specific detection by conventional antibodies difficult. We here present an antibody format that is particularly well suited for detection and analysis of immunoglobulin G (IgG)-binding virulence factors. As proof of concept, we have generated single chain fragment variable (scFv) antibodies that specifically target the IgG-binding surface proteins M1 and H of Streptococcus pyogenes. The binding ability of the developed scFv is demonstrated against both recombinant soluble protein M1 and H as well as the intact surface proteins on a wild-type S. pyogenes strain. Additionally, the capacity of the developed scFv antibodies to enrich their target proteins from both simple and complex backgrounds, thereby allowing for detection and quantification with LC-SRM MS, was demonstrated. We have established a workflow that allows for affinity enrichment of bacterial virulence factors.

Localization-triggered bacterial pathogenesis

Bacterial infections are becoming an increasing problem worldwide and there is a need for a deeper understanding of how bacteria turn pathogenic. Here, we suggest that one answer may be found by taking into account the localization of the bacteria, both at an anatomical level and at a microenvironment level. Both commensals and traditional pathogens alter their interaction with the human host depending on the local surroundings--turning either more or less virulent. These localization effects could derive from the characteristics of different anatomical sites but also from local differences within a microenvironment. In order to understand the adaptive functions of bacterial virulence factors, we need to study the bacteria in the environments where they have evolved.

Comparative genomics of Streptococcus pyogenes M1 isolates differing in virulence and propensity to cause systemic infection in mice

Streptococcus pyogenes serotype M1 is a frequent cause of severe infections in humans. Some M1 isolates are pathogenic in mice and used in studies on infection pathogenesis. We observed marked differences in murine infections caused by M1 strain SF370, 5448, 5448AP or AP1 which prompted us to sequence the whole genome of isolates 5448 and AP1 for comparative analysis. Strain 5448 is known to acquire inactivating mutations in the CovRS two-component system during mouse infection, producing hypervirulent progeny such as 5448AP. Isolates AP1 and 5448AP, more than 5448, caused disseminating infections that became systemic and lethal. SF370 was not pathogenic. Phages caused gross genetic differences and increased the gene content of AP1 by 8% as compared to 5448 and SF370. Each of six examined M1 genomes contained two CRISPR-Cas systems. Phage insertion destroyed a type II CRISPR-Cas system in AP1 and other strains of serotypes M1, M3, M6 and M24, but not in M1 strains 5448, SF370, MGAS5005, A20 or M1 476. A resulting impaired defence against invading genetic elements could have led to the wealth of phages in AP1. AP1 lacks genetic features of the MGAS5005-like clonal complex including the streptodornase that drives selection for hypervirulent clones with inactivated CovRS system. Still, inactivating mutations in covS were a common genetic feature of AP1 and the MGAS5005-like isolate 5448AP. Abolished expression of the cysteine proteinase SpeB, due to CovRS inactivation could be a common cause for hypervirulence of the two isolates. Moreover, an additional protein H-coding gene and a mutation in the regulator gene rofA distinguished AP1 form other M1 isolates. In conclusion, hypervirulence of S. pyogenes M1 in mice is not limited to the MGAS5005-like genotype.

The immunoglobulin M-degrading enzyme of Streptococcus suis, IdeSsuis, is involved in complement evasion

Streptococcus (S.) suis is one of the most important pathogens in pigs causing meningitis, arthritis, endocarditis and serositis. Furthermore, it is also an emerging zoonotic agent. In our previous work we identified a highly specific IgM protease in S. suis, designated Ide(Ssuis) . The objective of this study was to characterize the function of Ide(Ssuis) in the host-pathogen interaction. Edman-sequencing revealed that Ide(Ssuis) cleaves the heavy chain of the IgM molecule between constant domain 2 and 3. As the C1q binding motif is located in the C3 domain, we hypothesized that Ide(Ssuis) is involved in complement evasion. Complement-mediated hemolysis induced by porcine hyperimmune sera containing erythrocyte-specific IgM was abrogated by treatment of these sera with recombinant Ide(Ssuis) . Furthermore, expression of Ide(Ssuis) reduced IgM-triggered complement deposition on the bacterial surface. An infection experiment of prime-vaccinated growing piglets suggested attenuation in the virulence of the mutant 10Δide(Ssuis). Bactericidal assays confirmed a positive effect of Ide(Ssuis) expression on bacterial survival in porcine blood in the presence of high titers of specific IgM. In conclusion, this study demonstrates that Ide(Ssuis) is a novel complement evasion factor, which is important for bacterial survival in porcine blood during the early adaptive (IgM-dominated) immune response.

Platelet interaction with bacterial toxins and secreted products

Bacteria that enter the bloodstream will encounter components of the cellular and soluble immune response. Platelets contribute to this response and have emerged as an important target for bacterial pathogens. Bacteria produce diverse extracellular proteins and toxins that have been reported to modulate platelet function. These interactions can result in complete or incomplete platelet activation or inhibition of platelet activation, depending on the bacteria and bacterial product. The nature of the platelet response may be highly relevant to disease pathogenesis.

Bromine, bear-claw scratch fasciotomies, and the Eagle effect: management of group A streptococcal necrotising fasciitis and its association with trauma

Necrotising fasciitis is a rare, but potentially fatal, soft-tissue infection. Historical depictions of the disease have been described since classical times and were mainly recorded in wartime reports of battle injuries. Although several different species of bacteria can cause necrotising fasciitis, perhaps the most widely known is group A streptococcus (GAS). Infection control, early surgical debridement, and antibiotic therapy are now the central tenets of the clinical management of necrotising fasciitis; these treatment approaches all originate from those used in wars in the past 150 years. We review reports from the 19th century, early 20th century, and mid-20th century onwards to show how the management of necrotising fasciitis has progressed in parallel with prevailing scientific thought and medical practice. Historically, necrotising fasciitis has often, but not exclusively, been associated with penetrating trauma. However, along with a worldwide increase in invasive GAS disease, recent reports have cited cases of necrotising fasciitis following non-combat-related injuries or in the absence of antecedent events. We also investigate the specific association between GAS necrotising fasciitis and trauma. In the 21st century, molecular biology has improved our understanding of GAS pathogenesis, but has not yet affected attributable mortality.

Protein A is released into the Staphylococcus aureus culture supernatant with an unprocessed sorting signal

The immunoglobulin binding protein A (SpA) of Staphylococcus aureus is synthesized as a precursor with a C-terminal sorting signal. The sortase A enzyme mediates covalent attachment to peptidoglycan so that SpA is displayed on the surface of the bacterium. Protein A is also found in the extracellular medium, but the processes involved in its release are not fully understood. Here, we show that a portion of SpA is released into the supernatant with an intact sorting signal, indicating that it has not been processed by sortase A. Release of SpA was reduced when the native sorting signal of SpA was replaced with the corresponding region of another sortase-anchored protein (SdrE). Similarly, a reporter protein fused to the sorting signal of SpA was released to a greater extent than the same polypeptide fused to the SdrE sorting signal. Released SpA protected bacteria from killing in human blood, indicating that it contributes to immune evasion.

AFFIRM--a multiplexed immunoaffinity platform that combines recombinant antibody fragments and LC-SRM analysis

Targeted measurements of low abundance proteins in complex mixtures are in high demand in many areas, not the least in clinical applications measuring biomarkers. We here present the novel platform AFFIRM (AFFInity sRM) that utilizes the power of antibody fragments (scFv) to efficiently enrich for target proteins from a complex background and the exquisite specificity of SRM-MS based detection. To demonstrate the ability of AFFIRM, three target proteins of interest were measured in a serum background in single-plexed and multiplexed experiments in a concentration range of 5-1000 ng/mL. Linear responses were demonstrated down to low ng/mL concentrations with high reproducibility. The platform allows for high throughput measurements in 96-well format, and all steps are amendable to automation and scale-up. We believe the use of recombinant antibody technology in combination with SRM MS analysis provides a powerful way to reach sensitivity, specificity, and reproducibility as well as the opportunity to build resources for fast on-demand implementation of novel assays.

The arginine decarboxylase pathways of host and pathogen interact to impact inflammatory pathways in the lung

The arginine decarboxylase pathway, which converts arginine to agmatine, is present in both humans and most bacterial pathogens. In humans agmatine is a neurotransmitter with affinities towards α2-adrenoreceptors, serotonin receptors, and may inhibit nitric oxide synthase. In bacteria agmatine serves as a precursor to polyamine synthesis and was recently shown to enhance biofilm development in some strains of the respiratory pathogen Pseudomonas aeruginosa. We determined agmatine is at the center of a competing metabolism in the human lung during airways infections and is influenced by the metabolic phenotypes of the infecting pathogens. Ultra performance liquid chromatography with mass spectrometry detection was used to measure agmatine in human sputum samples from patients with cystic fibrosis, spent supernatant from clinical sputum isolates, and from bronchoalvelolar lavage fluid from mice infected with P. aeruginosa agmatine mutants. Agmatine in human sputum peaks during illness, decreased with treatment and is positively correlated with inflammatory cytokines. Analysis of the agmatine metabolic phenotype in clinical sputum isolates revealed most deplete agmatine when grown in its presence; however a minority appeared to generate large amounts of agmatine presumably driving sputum agmatine to high levels. Agmatine exposure to inflammatory cells and in mice demonstrated its role as a direct immune activator with effects on TNF-α production, likely through NF-κB activation. P. aeruginosa mutants for agmatine detection and metabolism were constructed and show the real-time evolution of host-derived agmatine in the airways during acute lung infection. These experiments also demonstrated pathogen agmatine production can upregulate the inflammatory response. As some clinical isolates have adapted to hypersecrete agmatine, these combined data would suggest agmatine is a novel target for immune modulation in the host-pathogen dynamic.

Streptococcus pneumoniae phosphoglycerate kinase is a novel complement inhibitor affecting the membrane attack complex formation

The Gram-positive bacterium Streptococcus pneumoniae is a major human pathogen that causes infections ranging from acute otitis media to life-threatening invasive disease. Pneumococci have evolved several strategies to circumvent the host immune response, in particular the complement attack. The pneumococcal glycolytic enzyme phosphoglycerate kinase (PGK) is both secreted and bound to the bacterial surface and simultaneously binds plasminogen and its tissue plasminogen activator tPA. In the present study we demonstrate that PGK has an additional role in modulating the complement attack. PGK interacted with the membrane attack complex (MAC) components C5, C7, and C9, thereby blocking the assembly and membrane insertion of MAC resulting in significant inhibition of the hemolytic activity of human serum. Recombinant PGK interacted in a dose-dependent manner with these terminal pathway proteins, and the interactions were ionic in nature. In addition, PGK inhibited C9 polymerization both in the fluid phase and on the surface of sheep erythrocytes. Interestingly, PGK bound several MAC proteins simultaneously. Although C5 and C7 had partially overlapping binding sites on PGK, C9 did not compete with either one for PGK binding. Moreover, PGK significantly inhibited MAC deposition via both the classical and alternative pathway at the pneumococcal surface. Additionally, upon activation plasmin(ogen) bound to PGK cleaved the central complement protein C3b thereby further modifying the complement attack. In conclusion, our data demonstrate for the first time to our knowledge a novel pneumococcal inhibitor of the terminal complement cascade aiding complement evasion by this important pathogen.

Current views of haemolytic streptococcal pathogenesis

PURPOSE OF REVIEW

Increasing disease caused by beta-haemolytic streptococci indicates the need for improved understanding of pathogenesis.

RECENT FINDINGS

Streptococcus pyogenes, or group A Streptococcus (GAS), causes significant disease worldwide. The closely related Streptococcus dysgalactiae subspecies equisimilis (SDSE) is increasingly recognized as causing a similar disease spectrum. Whole-genome sequencing applied to the study of outbreaks may reveal factors that contribute to pathogenesis and changes in epidemiology. The role of quorum sensing in biofilm formation, and interspecies communication with other streptococci, is discussed. GAS has evolved multiple mechanisms to evade the humoral arm of innate immunity, including complement, which is well known in protecting the host from bacteria, and the coagulation-fibrinolytic system, which is increasingly recognized as an innate immune effector.

SUMMARY

Molecular biology has enhanced our understanding of the intricate balance of host-pathogen interactions that result in clearance or establishment of invasive streptococcal infection. Although the skin and oropharynx remain the usual ecological niche of GAS and SDSE, occasionally the bacteria find themselves within deeper tissues and blood. Recent research has armed us with better knowledge of bacterial adaptations to this alternative environment. However, the challenge is to translate this knowledge into clinical practice, through the development of novel therapeutic options and ultimately a vaccine against GAS.

Neutrophil-Mediated Phagocytosis of Staphylococcus aureus

Initial elimination of invading Staphylococcus aureus from the body is mediated by professional phagocytes. The neutrophil is the major phagocyte of the innate immunity and plays a key role in the host defense against staphylococcal infections. Opsonization of the bacteria with immunoglobulins and complement factors enables efficient recognition by the neutrophil that subsequently leads to intracellular compartmentalization and killing. Here, we provide a review of the key processes evolved in neutrophil-mediated phagocytosis of S. aureus and briefly describe killing. As S. aureus is not helpless against the professional phagocytes, we will also highlight its immune evasion arsenal related to phagocytosis.

Vaccination against the M protein of Streptococcus pyogenes prevents death after influenza virus: S. pyogenes super-infection

Influenza virus infections are associated with a significant number of illnesses and deaths on an annual basis. Many of the deaths are due to complications from secondary bacterial invaders, including Streptococcus pneumoniae, Staphylococcus aureus, Haemophilus influenzae, and Streptococcus pyogenes. The β-hemolytic bacteria S. pyogenes colonizes both skin and respiratory surfaces, and frequently presents clinically as strep throat or impetigo. However, when these bacteria gain access to normally sterile sites, they can cause deadly diseases including sepsis, necrotizing fasciitis, and pneumonia. We previously developed a model of influenza virus:S. pyogenes super-infection, which we used to demonstrate that vaccination against influenza virus can limit deaths associated with a secondary bacterial infection, but this protection was not complete. In the current study, we evaluated the efficacy of a vaccine that targets the M protein of S. pyogenes to determine whether immunity toward the bacteria alone would allow the host to survive an influenza virus:S. pyogenes super-infection. Our data demonstrate that vaccination against the M protein induces IgG antibodies, in particular those of the IgG1 and IgG2a isotypes, and that these antibodies can interact with macrophages. Ultimately, this vaccine-induced immunity eliminated death within our influenza virus:S. pyogenes super-infection model, despite the fact that all M protein-vaccinated mice showed signs of illness following influenza virus inoculation. These findings identify immunity against bacteria as an important component of protection against influenza virus:bacteria super-infection.

Subterfuge and sabotage: evasion of host innate defenses by invasive gram-positive bacterial pathogens

The development of a severe invasive bacterial infection in an otherwise healthy individual is one of the most striking and fascinating aspects of human medicine. A small cadre of gram-positive pathogens of the genera Streptococcus and Staphylococcus stand out for their unique invasive disease potential and sophisticated ability to counteract the multifaceted components of human innate defense. This review illustrates how these leading human disease agents evade host complement deposition and activation, impede phagocyte recruitment and activation, resist the microbicidal activities of host antimicrobial peptides and reactive oxygen species, escape neutrophil extracellular traps, and promote and accelerate phagocyte cell death through the action of pore-forming cytolysins. Understanding the molecular basis of bacterial innate immune resistance can open new avenues for therapeutic intervention geared to disabling specific virulence factors and resensitizing the pathogen to host innate immune clearance.

Functional and structural properties of a novel protein and virulence factor (Protein sHIP) in Streptococcus pyogenes

Streptococcus pyogenes is a significant bacterial pathogen in the human population. The importance of virulence factors for the survival and colonization of S. pyogenes is well established, and many of these factors are exposed to the extracellular environment, enabling bacterial interactions with the host. In the present study, we quantitatively analyzed and compared S. pyogenes proteins in the growth medium of a strain that is virulent to mice with a non-virulent strain. Particularly, one of these proteins was present at significantly higher levels in stationary growth medium from the virulent strain. We determined the three-dimensional structure of the protein that showed a unique tetrameric organization composed of four helix-loop-helix motifs. Affinity pull-down mass spectrometry analysis in human plasma demonstrated that the protein interacts with histidine-rich glycoprotein (HRG), and the name sHIP (streptococcal histidine-rich glycoprotein-interacting protein) is therefore proposed. HRG has antibacterial activity, and when challenged by HRG, sHIP was found to rescue S. pyogenes bacteria. This and the finding that patients with invasive S. pyogenes infection respond with antibody production against sHIP suggest a role for the protein in S. pyogenes pathogenesis.

A comprehensive analysis of the Streptococcus pyogenes and human plasma protein interaction network

Streptococcus pyogenes is a major human bacterial pathogen responsible for severe and invasive disease associated with high mortality rates. The bacterium interacts with several human blood plasma proteins and clarifying these interactions and their biological consequences will help to explain the progression from mild to severe infections. In this study, we used a combination of mass spectrometry (MS) based techniques to comprehensively quantify the components of the S. pyogenes-plasma protein interaction network. From an initial list of 181 interacting human plasma proteins defined using liquid chromatography (LC)-MS/MS analysis we further subdivided the interacting protein list using selected reaction monitoring (SRM) depending on the level of enrichment and protein concentration on the bacterial surface. The combination of MS methods revealed several previously characterized interactions between the S. pyogenes surface and human plasma along with many more, so far uncharacterised, possible plasma protein interactions with S. pyogenes. In follow-up experiments, the combination of MS techniques was applied to study differences in protein binding to a S. pyogenes wild type strain and an isogenic mutant lacking several important virulence factors, and a unique pair of invasive and non-invasive S. pyogenes isolates from the same patient. Comparing the plasma protein-binding properties of the wild type and the mutant and the invasive and non-invasive S. pyogenes bacteria revealed considerable differences, underlining the significance of these protein interactions. The results also demonstrate the power of the developed mass spectrometry method to investigate host-microbial relationships with a large proteomics depth and high quantitative accuracy.

Quantitative assessment of neutrophil phagocytosis using flow cytometry

Neutrophils have an incredible ability to find and eradicate intruders such as bacteria and fungi. They do this largely through the process of phagocytosis, where the target is internalized into a phagosome, and eventually destroyed by the hostile phagosomal environment. It is important to study phagocytosis in order to understand how neutrophils interact with various pathogens and how they respond to different stimuli. Here, I describe a method to study neutrophil phagocytosis of bacteria using flow cytometry. The bacteria are fluorescently labeled before being introduced to neutrophils. After phagocytosis, both any remaining extracellular bacteria and neutrophils are labeled using one-step staining before three-color analysis. To assess phagocytosis, first the average time it takes for the neutrophils to internalize all bound bacteria is determined. Experiments are then performed using that time point while varying the bacteria-to-neutrophil ratio for full control of the analysis. Due to the ease with which multiple samples can be analyzed, and the quantitative nature of flow cytometry, this approach is both reproducible and sensitive.

Phagocytosis escape by a Staphylococcus aureus protein that connects complement and coagulation proteins at the bacterial surface

Upon contact with human plasma, bacteria are rapidly recognized by the complement system that labels their surface for uptake and clearance by phagocytic cells. Staphylococcus aureus secretes the 16 kD Extracellular fibrinogen binding protein (Efb) that binds two different plasma proteins using separate domains: the Efb N-terminus binds to fibrinogen, while the C-terminus binds complement C3. In this study, we show that Efb blocks phagocytosis of S. aureus by human neutrophils. In vitro, we demonstrate that Efb blocks phagocytosis in plasma and in human whole blood. Using a mouse peritonitis model we show that Efb effectively blocks phagocytosis in vivo, either as a purified protein or when produced endogenously by S. aureus. Mutational analysis revealed that Efb requires both its fibrinogen and complement binding residues for phagocytic escape. Using confocal and transmission electron microscopy we show that Efb attracts fibrinogen to the surface of complement-labeled S. aureus generating a ‘capsule’-like shield. This thick layer of fibrinogen shields both surface-bound C3b and antibodies from recognition by phagocytic receptors. This information is critical for future vaccination attempts, since opsonizing antibodies may not function in the presence of Efb. Altogether we discover that Efb from S. aureus uniquely escapes phagocytosis by forming a bridge between a complement and coagulation protein.

Staphylococcus aureus is a leading cause of severe bacterial infections in both hospital and community settings. Due to its increasing resistance to antibiotics, development of additional therapeutic strategies like vaccination is required to control this pathogen. Vaccination attempts against S. aureus have not been successful so far and an important reason may be the pathogen's elaborate repertoire of molecules that dampen the immune response. These evasion molecules not only suppress natural immunity but also hamper the current attempts to create effective vaccines. In this paper, we describe a novel mechanism by which S. aureus can prevent uptake by phagocytic immune cells. We discover that the secreted S. aureus protein Extracellular fibrinogen binding protein (Efb) generates a ‘capsule’-like shield around the bacterial surface through a dual interaction with the plasma proteins complement C3b and fibrinogen. The Efb-dependent fibrinogen shield masks important opsonic molecules like C3b and antibodies from binding to phagocyte receptors. This information is critical for future vaccination attempts, since opsonizing antibodies may not function in the presence of this anti-phagocytic shield.

Non-immune binding of human IgG to M-related proteins confers resistance to phagocytosis of group A streptococci in blood

The non-immune binding of immunoglobulins by bacteria is thought to contribute to the pathogenesis of infections. M-related proteins (Mrp) are group A streptococcal (GAS) receptors for immunoglobulins, but it is not known if this binding has any impact on virulence. To further investigate the binding of immunoglobulins to Mrp, we engineered mutants of an M type 4 strain of GAS by inactivating the genes for mrp, emm, enn, sof, and sfbX and tested these mutants in IgG-binding assays. Inactivation of mrp dramatically decreased the binding of human IgG, whereas inactivation of emm, enn, sof, and sfbx had only minor effects, indicating that Mrp is a major IgG-binding protein. Binding of human immunoglobulins to a purified, recombinant form of Mrp indicated that it selectively binds to the Fc domain of human IgG, but not IgA or IgM and that it preferentially bound subclasses IgG₁>IgG₄>IgG₂>IgG₃. Recombinant proteins encompassing different regions of Mrp were engineered and used to map its IgG-binding domain to its A-repeat region and a recombinant protein with 3 A-repeats was a better inhibitor of IgG binding than one with a single A-repeat. A GAS mutant expressing Mrp with an in-frame deletion of DNA encoding the A-repeats had a dramatically reduced ability to bind human IgG and to grow in human blood. Mrp exhibited host specificity in binding IgG; human IgG was the best inhibitor of the binding of IgG followed by pig, horse, monkey, and rabbit IgG. IgG from goat, mouse, rat, cow, donkey, chicken, and guinea pig were poor inhibitors of binding. These findings indicate that Mrp preferentially binds human IgG and that this binding contributes to the ability of GAS to resist phagocytosis and may be a factor in the restriction of GAS infections to the human host.

Distinct localization of the complement C5b-9 complex on Gram-positive bacteria

The plasma proteins of the complement system fulfil important immune defence functions, including opsonization of bacteria for phagocytosis, generation of chemo-attractants and direct bacterial killing via the Membrane Attack Complex (MAC or C5b-9). The MAC is comprised of C5b, C6, C7, C8, and multiple copies of C9 that generate lytic pores in cellular membranes. Gram-positive bacteria are protected from MAC-dependent lysis by their thick peptidoglycan layer. Paradoxically, several Gram-positive pathogens secrete small proteins that inhibit C5b-9 formation. In this study, we found that complement activation on Gram-positive bacteria in serum results in specific surface deposition of C5b-9 complexes. Immunoblotting revealed that C9 occurs in both monomeric and polymeric (SDS-stable) forms, indicating the presence of ring-structured C5b-9. Surprisingly, confocal microscopy demonstrated that C5b-9 deposition occurs at specialized regions on the bacterial cell. On Streptococcus pyogenes, C5b-9 deposits near the division septum whereas on Bacillus subtilis the complex is located at the poles. This is in contrast to C3b deposition, which occurs randomly on the bacterial surface. Altogether, these results show a previously unrecognized interaction between the C5b-9 complex and Gram-positive bacteria, which might ultimately lead to a new model of MAC assembly and functioning.

IgG protease Mac/IdeS is not essential for phagocyte resistance or mouse virulence of M1T1 group A Streptococcus

The Mac/IdeS protein of group A Streptococcus (GAS) is a secreted cysteine protease with cleavage specificity for IgG and is highly expressed in the GAS serotype M1T1 clone, which is the serotype most frequently isolated from patients with life-threatening invasive infections. While studies of Mac/IdeS with recombinant protein have shown that the protein can potentially prevent opsonophagocytosis of GAS by neutrophils, the role of the protein in immune evasion as physiologically produced by the living organism has not been studied. Here we examined the contribution of Mac/IdeS to invasive GAS disease by generating a mutant lacking Mac/IdeS in the hyperinvasive M1T1 background. While Mac/IdeS was highly expressed and proteolytically active in the hyperinvasive strain, elimination of the bacterial protease did not significantly influence GAS phagocytic uptake, oxidative-burst induction, cathelicidin sensitivity, resistance to neutrophil or macrophage killing, or pathogenicity in pre- or postimmune mouse infectious challenges. We conclude that in the highly virulent M1T1 background, Mac/IdeS is not essential for either phagocyte resistance or virulence. Given the conservation of Mac/IdeS and homologues across GAS strains, it is possible that Mac/IdeS serves another important function in GAS ecology or contributes to virulence in other strain backgrounds.

Group A Streptococcus (GAS) causes human infections ranging from strep throat to life-threatening conditions such as flesh-eating disease and toxic shock syndrome. Common disease-associated clones of GAS can cause both mild and severe infections because of a characteristic mutation and subsequent change in the expression of several genes that develops under host immune selection. One of these genes encodes Mac/IdeS, a protease that has been shown to cleave antibodies important to the immune defense system. In this study, we found that while Mac/IdeS is highly expressed in hypervirulent GAS, it does not significantly contribute to the ability of the bacteria to survive white blood cell killing or produce invasive infection in the mouse. These data underscore the importance of correlating studies on virulence factor function with physiologic expression levels and the complexity of streptococcal pathogenesis and contribute to our overall understanding of how GAS causes disease.

Immune complex binding Streptococcus pyogenes type M12/emm12 in experimental glomerulonephritis

In a rabbit model, we have previously reported evidence for a pathogenic role of streptococcal IgG Fc-binding proteins (IgGFcBP) in poststreptococcal glomerulonephritis (PSGN). These proteins, of the M protein family, were shown to trigger anti-IgG production and enhance renal deposition of IgG and/or immune complexes (ICs), with resulting activation of complement and cytokine cascades. In the present study, type M12/emm12, group A streptococci (GAS) were found often to bind artificial ICs, viz. peroxidase-anti-peroxidase rabbit IgG (PAP) or tetanus toxoid-anti-tetanus human IgG (TAT), rather than monomeric IgG. Animals injected with each of four IC binding clinical isolates (from patients with scarlet fever or PSGN) showed pronounced inflammatory and degenerative glomerular changes, morphologically similar to human PSGN, with membrane thickening and IgG and complement C3 deposition, as well as secretion of IL-6 and TNF-α by mesangial and endothelial cells. In contrast, non-binding strains (two from asymptomatic carriers and one from a PSGN case) failed to trigger any renal changes. Only the IC binding strains induced elevated titres of anti-IgG. Though the streptococcal binding component(s) has not been demonstrated, the selective binding of ICs by type M12/emm12 strains appears important for the well-known, marked nephritogenic potential of this GAS type.

A new and robust method of tethering IgG surrogate antigens on lipid bilayer membranes to facilitate the TIRFM based live cell and single molecule imaging experiments

Our understanding of cell-cell interactions has been significantly improved in the past years with the help of Total Internal Reflection Fluorescence Microscope (TIRFM) in combination with an antigen presenting system supported by planar lipid bilayer (PLB) membranes, which are used to mimic the extensive receptor and ligand interactions within cell-cell contact interface. In TIRFM experiments, it is a challenge to uniformly present ligand molecules in monomeric format on the surface of PLB membranes. Here, we introduce a new and robust method of tethering IgG surrogate antigen ligands on the surface of Ni²⁺-containing PLB membranes. In this method, we use a modified D domain from staphylococcal protein A molecule that is fused with an N-terminus polyhistidine tag (H12-D-domain) to tether IgG surrogate antigens on Ni²⁺-containing PLB membranes. We systematically assessed the specificity and capability of H12-D-domain construct to capture IgG molecules from different species through live cell and single molecule TIRFM imaging. We find that these IgG surrogate antigens tethered by H12-D-domain show better lateral mobility and are more uniformly distributed on PLB membranes than the ones tethered by streptavidin. Neither IgM molecules, nor Fab or F(ab’)₂ fragments of IgG molecules can be tethered on PLB membranes by H12-D-domain construct. These tethered IgG surrogate antigens strongly induce the formation and accumulation of signaling active antigen receptor microclusters within the immunological synapse in B or T lymphocyte cells. Thus our method provides a new and robust method to tether IgG surrogate antigens or other molecules fused with IgG Fc portion on PLB membranes for TIRFM based molecule imaging experiments.