Specific absorption of human serum albumin, immunoglobulin A, and immunoglobulin G with selected strains of group A and G streptococci

Exploring affinity chromatography in proteomics: A comprehensive review

Over the past decades, the proteomics field has undergone rapid growth. Progress in mass spectrometry and bioinformatics, together with separation methods, has brought many innovative approaches to the study of the molecular biology of the cell. The potential of affinity chromatography was recognized immediately after its first application in proteomics, and since that time, it has become one of the cornerstones of many proteomic protocols. Indeed, this chromatographic technique exploiting the specific binding between two molecules has been employed for numerous purposes, from selective removal of interfering (over)abundant proteins or enrichment of scarce biomarkers in complex biological samples to mapping the post-translational modifications and protein interactions with other proteins, nucleic acids or biologically active small molecules. This review presents a comprehensive survey of this versatile analytical tool in current proteomics. To navigate the reader, the haphazard space of affinity separations is classified according to the experiment's aims and the separated molecule's nature. Different types of available ligands and experimental strategies are discussed in further detail for each of the mentioned procedures.

Quantification of Adaptive Immune Responses Against Protein-Binding Interfaces in the Streptococcal M1 Protein

Bacterial or viral antigens can contain subdominant protein regions that elicit weak antibody responses upon vaccination or infection although there is accumulating evidence that antibody responses against subdominant regions can enhance the protective immune response. One proposed mechanism for subdominant protein regions is the binding of host proteins that prevent antibody production against epitopes hidden within the protein binding interfaces. Here, we used affinity purification combined with quantitative mass spectrometry (AP-MS) to examine the level of competition between antigen-specific antibodies and host-pathogen protein interaction networks using the M1 protein from Streptococcus pyogenes as a model system. As most humans have circulating antibodies against the M1 protein, we first used AP-MS to show that the M1 protein interspecies protein network formed with human plasma proteins is largely conserved in naïve mice. Immunizing mice with the M1 protein generated a time-dependent increase of anti-M1 antibodies. AP-MS analysis comparing the composition of the M1-plasma protein network from naïve and immunized mice showed significant enrichment of 292 IgG peptides associated with 56 IgG chains in the immune mice. Despite the significant increase of bound IgGs, the levels of interacting plasma proteins were not significantly reduced in the immune mice. The results indicate that the antigen-specific polyclonal IgG against the M1 protein primarily targets epitopes outside the other plasma protein binding interfaces. In conclusion, this study demonstrates that AP-MS is a promising strategy to determine the relationship between antigen-specific antibodies and host-pathogen interaction networks that could be used to define subdominant protein regions of relevance for vaccine development.

Syphilis vaccine: challenges, controversies and opportunities

Syphilis is a sexually or vertically (mother to fetus) transmitted disease caused by the infection of Treponema pallidum subspecie pallidum (TPA). The incidence of syphilis has increased over the past years despite the fact that this bacterium is an obligate human pathogen, the infection route is well known, and the disease can be successfully treated with penicillin. As complementary measures to preventive campaigns and early treatment of infected individuals, development of a syphilis vaccine may be crucial for controlling disease spread and/or severity, particularly in countries where the effectiveness of the aforementioned measures is limited. In the last century, several vaccine prototypes have been tested in preclinical studies, mainly in rabbits. While none of them provided protection against infection, some prototypes prevented bacteria from disseminating to distal organs, attenuated lesion development, and accelerated their healing. In spite of these promising results, there is still some controversy regarding the identification of vaccine candidates and the characteristics of a syphilis-protective immune response. In this review, we describe what is known about TPA immune response, and the main mechanisms used by this pathogen to evade it. Moreover, we emphasize the importance of integrating this knowledge, in conjunction with the characterization of outer membrane proteins (OMPs), to expedite the development of a syphilis vaccine that can protect against TPA infection.

Streptococcus pyogenes Forms Serotype- and Local Environment-Dependent Interspecies Protein Complexes

Streptococcus pyogenes is known to cause both mucosal and systemic infections in humans. In this study, we used a combination of quantitative and structural mass spectrometry techniques to determine the composition and structure of the interaction network formed between human plasma proteins and the surfaces of different S. pyogenes serotypes. Quantitative network analysis revealed that S. pyogenes forms serotype-specific interaction networks that are highly dependent on the domain arrangement of the surface-attached M protein. Subsequent structural mass spectrometry analysis and computational modeling of one of the M proteins, M28, revealed that the network structure changes across different host microenvironments. We report that M28 binds secretory IgA via two separate binding sites with high affinity in saliva. During vascular leakage mimicked by increasing plasma concentrations in saliva, the binding of secretory IgA was replaced by the binding of monomeric IgA and C4b-binding protein (C4BP). This indicates that an upsurge of C4BP in the local microenvironment due to damage to the mucosal membrane drives the binding of C4BP and monomeric IgA to M28. These results suggest that S. pyogenes has evolved to form microenvironment-dependent host-pathogen protein complexes to combat human immune surveillance during both mucosal and systemic infections.

IMPORTANCEStreptococcus pyogenes (group A Streptococcus [GAS]), is a human-specific Gram-positive bacterium. Each year, the bacterium affects 700 million people globally, leading to 160,000 deaths. The clinical manifestations of S. pyogenes are diverse, ranging from mild and common infections like tonsillitis and impetigo to life-threatening systemic conditions such as sepsis and necrotizing fasciitis. S. pyogenes expresses multiple virulence factors on its surface to localize and initiate infections in humans. Among all these expressed virulence factors, the M protein is the most important antigen. In this study, we perform an in-depth characterization of the human protein interactions formed around one of the foremost human pathogens. This strategy allowed us to decipher the protein interaction networks around different S. pyogenes strains on a global scale and to compare and visualize how such interactions are mediated by M proteins.

Extended plasma half-life of albumin-binding domain fused human IgA upon pH-dependent albumin engagement of human FcRn in vitro and in vivo

Albumin has a serum half-life of 3 weeks in humans. This feature can be used to improve the pharmacokinetics of shorter-lived biologics. For instance, an albumin-binding domain (ABD) can be used to recruit albumin. A prerequisite for such design is that the ABD-albumin interaction does not interfere with pH-dependent binding of albumin to the human neonatal Fc receptor (FcRn), as FcRn acts as the principal regulator of the half-life of albumin. Thus, there is a need to know how ABDs act in the context of fusion partners and human FcRn. Here, we studied the binding and transport properties of human immunoglobulin A1 (IgA1), fused to a Streptococcus protein G-derived engineered ABD, in in vitro and in vivo systems harboring human FcRn. IgA has great potential as a therapeutic protein, but its short half-life is a major drawback. We demonstrate that ABD-fused IgA1 binds human FcRn pH-dependently and is rescued from cellular degradation in a receptor-specific manner in the presence of albumin. This occurs when ABD is fused to either the light or the heavy chain. In human FcRn transgenic mice, IgA1-ABD in complex with human albumin, gave 4-6-fold extended half-life compared to unmodified IgA1, where the light chain fusion showed the longest half-life. When the heavy chain-fused protein was pre-incubated with an engineered human albumin with improved FcRn binding, cellular rescue and half-life was further enhanced. Our study reveals how an ABD, which does not interfere with albumin binding to human FcRn, may be used to extend the half-life of IgA.

Abbreviations: ABD - Albumin binding domain, ADA – anti-drug-antibodies, ADCC - Antibody-dependent cellular cytotoxicity, ELISA - Enzyme-linked Immunosorbent assay, FcαRI - Fcα receptor, FcγR - Fcγ receptor, FcRn - The neonatal Fc receptor, GST - Glutathione S-transferase, HC - Heavy chain, HERA - Human endothelial cell-based recycling assay, Her2 - Human epidermal growth factor 2, HMEC - Human microvascular endothelial cells, IgG - Immunoglobulin G, IgA - Immunoglobulin A, LC - Light chain, QMP - E505Q/T527M/K573P, WT - Wild type

Human Serum Albumin Binds Streptolysin O (SLO) Toxin Produced by Group A Streptococcus and Inhibits Its Cytotoxic and Hemolytic Effects

The pathogenicity of group A Streptococcus (GAS) is mediated by direct bacterial invasivity and toxin-associated damage. Among the extracellular products, the exotoxin streptolysin O (SLO) is produced by almost all GAS strains. SLO is a pore forming toxin (PFT) hemolitically active and extremely toxic in vivo. Recent evidence suggests that human serum albumin (HSA), the most abundant protein in plasma, is a player in the innate immunity "orchestra." We previously demonstrated that HSA acts as a physiological buffer, partially neutralizing Clostridioides difficile toxins that reach the bloodstream after being produced in the colon. Here, we report the in vitro and ex vivo capability of HSA to neutralize the cytotoxic and hemolytic effects of SLO. HSA binds SLO with high affinity at a non-conventional site located in domain II, which was previously reported to interact also with C. difficile toxins. HSA:SLO recognition protects HEp-2 and A549 cells from cytotoxic effects and cell membrane permeabilization induced by SLO. Moreover, HSA inhibits the SLO-dependent hemolytic effect in red blood cells isolated from healthy human donors. The recognition of SLO by HSA may have a significant protective role in human serum and sustains the emerging hypothesis that HSA is an important constituent of the innate immunity system.

Subdominance in Antibody Responses: Implications for Vaccine Development

Vaccines work primarily by eliciting antibodies, even when recovery from natural infection depends on cellular immunity. Large efforts have therefore been made to identify microbial antigens that elicit protective antibodies, but these endeavors have encountered major difficulties, as witnessed by the lack of vaccines against many pathogens. This review summarizes accumulating evidence that subdominant protein regions, i.e., surface-exposed regions that elicit relatively weak antibody responses, are of particular interest for vaccine development. This concept may seem counterintuitive, but subdominance may represent an immune evasion mechanism, implying that the corresponding region potentially is a key target for protective immunity. Following a presentation of the concepts of immunodominance and subdominance, the review will present work on subdominant regions in several major human pathogens: the protozoan Plasmodium falciparum, two species of pathogenic streptococci, and the dengue and influenza viruses. Later sections are devoted to the molecular basis of subdominance, its potential role in immune evasion, and general implications for vaccine development. Special emphasis will be placed on the fact that a whole surface-exposed protein domain can be subdominant, as demonstrated for all of the pathogens described here. Overall, the available data indicate that subdominant protein regions are of much interest for vaccine development, not least in bacterial and protozoal systems, for which antibody subdominance remains largely unexplored.

Human IgG Increases Virulence of Streptococcus pyogenes through Complement Evasion

Streptococcus pyogenes is an exclusively human pathogen that can provoke mild skin and throat infections but can also cause fatal septicemia. This gram-positive bacterium has developed several strategies to evade the human immune system, enabling S. pyogenes to survive in the host. These strategies include recruiting several human plasma proteins, such as the complement inhibitor, C4b-binding protein (C4BP), and human (hu)-IgG through its Fc region to the bacterial surface to evade immune recognition. We identified a novel virulence mechanism whereby IgG-enhanced binding of C4BP to five of 12 tested S. pyogenes strains expressed diverse M proteins that are important surface-expressed virulence factors. Importantly, all strains that bound C4BP in the absence of IgG bound more C4BP when IgG was present. Further studies with an M1 strain that additionally expressed protein H, also a member of the M protein family, revealed that binding of hu-IgG Fc to protein H increased the affinity of protein H for C4BP. Increased C4BP binding accentuated complement downregulation, resulting in diminished bacterial killing. Accordingly, mortality from S. pyogenes infection in hu-C4BP transgenic mice was increased when hu-IgG or its Fc portion alone was administered concomitantly. Electron microscopy analysis of human tissue samples with necrotizing fasciitis confirmed increased C4BP binding to S. pyogenes when IgG was present. Our findings provide evidence of a paradoxical function of hu-IgG bound through Fc to diverse S. pyogenes isolates that increases their virulence and may counteract the beneficial effects of IgG opsonization.

Orientation and density control of proteins on solid matters by outer membrane coating: Analytical and diagnostic applications

Autodisplay is an expression system for the display of recombinant proteins on the outer membrane (OM) of gram negative bacteria and has been developed for translocation studies, whole cell biocatalysis, bioremediation, inhibitor screening, and enzyme refolding. Recently, affinity proteins such as IgG-binding Z-domains and biotin-binding streptavidin have been autodisplayed on the OM of Escherichia coli for analytical and biomedical applications. The secretion mechanism of the autodisplay system was used and orientation and density control of these affinity proteins were determined. Affinity protein-autodisplaying E. coli cells have been used to coat solid supports in immunoassays. For this purpose, the OM of autodisplayed E. coli cells was separated and isolated by the aid of detergents. The structure of the resulting OM liposomes as well as their physico-chemical parameters, were analyzed. OM liposomes were used subsequently for coating various solid matters including microplates and biosensor transducer surfaces and the formation of OM layers were monitored. OM layer formation on solid matters was shown to increase the sensitivity of immunoassays and biosensors. In this review, analytical and diagnostic applications are described in particular concerning orientation and density control of autodisplayed affinity proteins.

Targeted Proteomics and Absolute Protein Quantification for the Construction of a Stoichiometric Host-Pathogen Surface Density Model

Sepsis is a systemic immune response responsible for considerable morbidity and mortality. Molecular modeling of host-pathogen interactions in the disease state represents a promising strategy to define molecular events of importance for the transition from superficial to invasive infectious diseases. Here we used the Gram-positive bacterium Streptococcus pyogenes as a model system to establish a mass spectrometry based workflow for the construction of a stoichiometric surface density model between the S. pyogenes surface, the surface virulence factor M-protein, and adhered human blood plasma proteins. The workflow relies on stable isotope labeled reference peptides and selected reaction monitoring mass spectrometry analysis of a wild-type strain and an M-protein deficient mutant strain, to generate absolutely quantified protein stoichiometry ratios between S. pyogenes and interacting plasma proteins. The stoichiometry ratios in combination with a novel targeted mass spectrometry method to measure cell numbers enabled the construction of a stoichiometric surface density model using protein structures available from the protein data bank. The model outlines the topology and density of the host-pathogen protein interaction network on the S. pyogenes bacterial surface, revealing a dense and highly organized protein interaction network. Removal of the M-protein from S. pyogenes introduces a drastic change in the network topology, validated by electron microscopy. We propose that the stoichiometric surface density model of S. pyogenes in human blood plasma represents a scalable framework that can continuously be refined with the emergence of new results. Future integration of new results will improve the understanding of protein-protein interactions and their importance for bacterial virulence. Furthermore, we anticipate that the general properties of the developed workflow will facilitate the production of stoichiometric surface density models for other types of host-pathogen interactions.

Analysis of Bacterial Surface Interactions with Mass Spectrometry-Based Proteomics

Host-pathogen protein-protein interaction networks are highly complex and dynamic. In this experimental protocol we describe a method to isolate host proteins attached to the bacterial surface followed by quantitative mass spectrometry based proteomics analysis. This technique provides an overview of the host-pathogen interaction network, which can be used to guide directed perturbations of the system, and to select target of specific interest for further studies.

Hepatocytes respond differently to major dietary trans fatty acid isomers, elaidic acid and trans-vaccenic acid

BACKGROUND

It has been discussed if the adverse health effect associated with the ingestion of trans fatty acids correlates with the food source, as the composition of the isomers varies in different foods. We have investigated the hepatocellular responses to the predominant trans fatty acid isomers in industrially produced partially hydrogenated vegetable oils (elaidic acid) and products of ruminant origin (trans-vaccenic acid).

RESULTS

The responses of HepG2-SF cells exposed to 100 μM fatty acids during 7 days were examined. Elaidic acid decreased the cellular proliferation rate while trans-vaccenic acid had no effect. Analysis of cellular triacylglycerol fractions showed, that both trans fatty acids were metabolized by HepG2-SF cells, although elaidic acid, to a higher degree than trans-vaccenic, accumulated in the triacylglycerol fraction. Proteome analysis revealed that the overlap of differentially regulated proteins only contained four proteins, suggesting that the two trans fatty acid isomers affect the cells in different ways. The data are available via ProteomeXchange with identifier PXD000760.

CONCLUSIONS

Our investigations revealed that the hepatocellular response to the two most abundant dietary positional C18:1 trans fatty acid isomers differ substantially. In addition, the results suggest that trans-vaccenic acid does not affect cholesterol metabolism adversely compared to elaidic acid.

Unconditioned commercial embryo culture media contain a large variety of non-declared proteins: a comprehensive proteomics analysis

STUDY QUESTION

Which non-declared proteins (proteins not listed on the composition list of the product data sheet) are present in unconditioned commercial embryo culture media?

SUMMARY ANSWER

A total of 110 non-declared proteins were identified in unconditioned media and between 6 and 8 of these were quantifiable and therefore represent the majority of the total protein in the media samples.

WHAT IS KNOWN ALREADY

There are no data in the literature on what non-declared proteins are present in unconditioned (fresh media in which no embryos have been cultured) commercial embryo media.

STUDY DESIGN, SIZE, DURATION

The following eight commercial embryo culture media were included in this study: G-1 PLUS and G-2 PLUS G5 Series from Vitrolife, Sydney IVF Cleavage Medium and Sydney IVF Blastocyst Medium from Cook Medical and EmbryoAssist, BlastAssist, Sequential Cleav and Sequential Blast from ORIGIO. Two batches were analyzed from each of the Sydney IVF media and one batch from each of the other media. All embryo culture media are supplemented by the manufacturers with purified human serum albumin (HSA 5 mg/ml). The purified HSA (HSA-solution from Vitrolife) and the recombinant human albumin supplement (G-MM from Vitrolife) were also analyzed.

PARTICIPANTS/MATERIALS, SETTING, METHODS

For protein quantification, media samples were in-solution digested with trypsin and analyzed by liquid chromatography-tandem mass spectrometry (LC-MS/MS). For in-depth protein identification, media were albumin depleted, dialyzed and concentrated before sodium dodecyl sulfate polyacrylamide gel electrophoresis. The gel was cut into 14 slices followed by in-gel trypsin digestion, and analysis by LC-MS/MS. Proteins were further investigated using gene ontology (GO) terms analysis.

MAIN RESULTS AND THE ROLE OF CHANCE

Using advanced mass spectrometry and high confidence criteria for accepting proteins (P < 0.01), a total of 110 proteins other than HSA were identified. The average HSA content was found to be 94% (92-97%) of total protein. Other individual proteins accounted for up to 4.7% of the total protein. Analysis of purified HSA strongly suggests that these non-declared proteins are introduced to the media when the albumin is added. GO analysis showed that many of these proteins have roles in defence pathways, for example 18 were associated with the innate immune response and 17 with inflammatory responses. Eight proteins have been reported previously as secreted embryo proteins.

LIMITATIONS, REASONS FOR CAUTION

For six of the commercial embryo culture media only one batch was analyzed. However, this does not affect the overall conclusions.

WIDER IMPLICATIONS OF THE FINDINGS

The results showed that the HSA added to IVF media contained many other proteins and that the amount varies from batch to batch. These variations in protein profiles are problematic when attempting to identify proteins derived from the embryos. Therefore, when studying the embryo secretome and analyzing conditioned media with the aim of finding potential biomarkers that can distinguish normal and abnormal embryo development, it is important that the medium used in the experimental and control groups is from the same batch. Furthermore, the proteins present in unconditioned media could potentially influence embryonic development, gestation age, birthweight and perhaps have subsequent effects on health of the offspring.

STUDY FUNDING/COMPETING INTERESTS

The study was supported by the Danish Agency for Science, Technology and Innovation. Research at the Fertility Clinic, Aarhus University Hospital is supported by an unrestricted grant from Merck Sharp & Dohme Corp and Ferring. The authors declare no conflicts of interest.

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.

Coagulation factor XIIIa substrates in human plasma: identification and incorporation into the clot

Coagulation factor XIII (FXIII) is a transglutaminase with a well defined role in the final stages of blood coagulation. Active FXIII (FXIIIa) catalyzes the formation of ε-(γ-glutamyl)lysine isopeptide bonds between specific Gln and Lys residues. The primary physiological outcome of this catalytic activity is stabilization of the fibrin clot during coagulation. The stabilization is achieved through the introduction of cross-links between fibrin monomers and through cross-linking of proteins with anti-fibrinolytic activity to fibrin. FXIIIa additionally cross-links several proteins with other functionalities to the clot. Cross-linking of proteins to the clot is generally believed to modify clot characteristics such as proteolytic susceptibility and hereby affect the outcome of tissue damage. In the present study, we use a proteomic approach in combination with transglutaminase-specific labeling to identify FXIIIa plasma protein substrates and their reactive residues. The results revealed a total of 147 FXIIIa substrates, of which 132 have not previously been described. We confirm that 48 of the FXIIIa substrates were indeed incorporated into the insoluble fibrin clot during the coagulation of plasma. The identified substrates are involved in, among other activities, complement activation, coagulation, inflammatory and immune responses, and extracellular matrix organization.

Effects of elaidic acid on lipid metabolism in HepG2 cells, investigated by an integrated approach of lipidomics, transcriptomics and proteomics

Trans fatty acid consumption in the human diet can cause adverse health effects, such as cardiovascular disease, which is associated with higher total cholesterol, a higher low density lipoprotein-cholesterol level and a decreased high density lipoprotein-cholesterol level. The aim of the study was to elucidate the hepatic response to the most abundant trans fatty acid in the human diet, elaidic acid, to help explain clinical findings on the relationship between trans fatty acids and cardiovascular disease. The human HepG2 cell line was used as a model to investigate the hepatic response to elaidic acid in a combined proteomic, transcriptomic and lipidomic approach. We found many of the proteins responsible for cholesterol synthesis up-regulated together with several proteins involved in the esterification and hepatic import/export of cholesterol. Furthermore, a profound remodeling of the cellular membrane occurred at the phospholipid level. Our findings contribute to the explanation on how trans fatty acids from the diet can cause modifications in plasma cholesterol levels by inducing abundance changes in several hepatic proteins and the hepatic membrane composition.

The albumin-binding domain as a scaffold for protein engineering

The albumin-binding domain is a small, three-helical protein domain found in various surface proteins expressed by gram-positive bacteria. Albumin binding is important in bacterial pathogenesis and several homologous domains have been identified. Such albumin-binding regions have been used for protein purification or immobilization. Moreover, improvement of the pharmacokinetics, through the non-covalent association to albumin, by fusing such domains to therapeutic proteins has been shown to be successful. Domains derived from streptococcal protein G and protein PAB from Finegoldia magna, which share a common origin and therefore represent an interesting evolutionary system, have been thoroughly studied structurally and functionally. Their albumin-binding sites have been mapped and these domains form the basis for a wide range of protein engineering approaches. By substitution-mutagenesis they have been engineered to achieve a broader specificity, an increased stability or an improved binding affinity, respectively. Furthermore, novel binding sites have been incorporated either by replacing the original albumin-binding surface, or by complementing it with a novel interaction interface. Combinatorial protein libraries, where several residues have been randomized simultaneously, have generated a large number of new variants with desired binding characteristics. The albumin-binding domain has also been utilized to explore the relationship between three-dimensional structure and amino acid sequence. Proteins with latent structural information built into their sequence, where a single amino acid substitution shifts the equilibrium in favor of a different fold with a new function, have been designed. Altogether, these examples illustrate the versatility of the albumin-binding domain as a scaffold for protein engineering.

Streptococcus pyogenes in human plasma: adaptive mechanisms analyzed by mass spectrometry-based proteomics

Streptococcus pyogenes is a major bacterial pathogen and a potent inducer of inflammation causing plasma leakage at the site of infection. A combination of label-free quantitative mass spectrometry-based proteomics strategies were used to measure how the intracellular proteome homeostasis of S. pyogenes is influenced by the presence of human plasma, identifying and quantifying 842 proteins. In plasma the bacterium modifies its production of 213 proteins, and the most pronounced change was the complete down-regulation of proteins required for fatty acid biosynthesis. Fatty acids are transported by albumin (HSA) in plasma. S. pyogenes expresses HSA-binding surface proteins, and HSA carrying fatty acids reduced the amount of fatty acid biosynthesis proteins to the same extent as plasma. The results clarify the function of HSA-binding proteins in S. pyogenes and underline the power of the quantitative mass spectrometry strategy used here to investigate bacterial adaptation to a given environment.

Mass spectrometry of peptides and proteins from human blood

It is difficult to convey the accelerating rate and growing importance of mass spectrometry applications to human blood proteins and peptides. Mass spectrometry can rapidly detect and identify the ionizable peptides from the proteins in a simple mixture and reveal many of their post-translational modifications. However, blood is a complex mixture that may contain many proteins first expressed in cells and tissues. The complete analysis of blood proteins is a daunting task that will rely on a wide range of disciplines from physics, chemistry, biochemistry, genetics, electromagnetic instrumentation, mathematics and computation. Therefore the comprehensive discovery and analysis of blood proteins will rank among the great technical challenges and require the cumulative sum of many of mankind's scientific achievements together. A variety of methods have been used to fractionate, analyze and identify proteins from blood, each yielding a small piece of the whole and throwing the great size of the task into sharp relief. The approaches attempted to date clearly indicate that enumerating the proteins and peptides of blood can be accomplished. There is no doubt that the mass spectrometry of blood will be crucial to the discovery and analysis of proteins, enzyme activities, and post-translational processes that underlay the mechanisms of disease. At present both discovery and quantification of proteins from blood are commonly reaching sensitivities of ∼1 ng/mL.

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.

Scavenger receptor gp340 aggregates group A streptococci by binding pili

Group A streptococci (GAS) are the most frequent cause of bacterial pharyngitis. The first obstacle to GAS colonization of the pharynx is saliva. As well as forming a physical barrier, saliva contains components of innate and acquired immunity. Previous work has shown that saliva induces bacterial aggregation, which may serve as a clearance mechanism. As the aggregation of some oral streptococci in saliva is mediated by long proteinaceous appendages, we hypothesized that pili of GAS might behave similarly. Wild-type GAS M1 strain SF370 aggregated in saliva, while pilus-defective mutants did not. Similarly, heterologous expression of diverse GAS pili on the surface of Lactococcus lactis induced aggregation in saliva, while control strains were unaffected. Further studies revealed that aggregating bacteria bound salivary component gp340. Purified gp340 aggregated wild-type GAS and L. lactis expressing GAS pili, but not control strains. GAS pilus-defective mutants were abrogated in gp340 binding and aggregation. Furthermore, gp340-mediated aggregation reduced bacterial adhesion to human epithelial cells, suggesting a role in host defence.

Induction of secretory immunity and memory at mucosal surfaces

Mucosal epithelia comprise an extensive vulnerable barrier which is reinforced by numerous innate defence mechanisms cooperating intimately with adaptive immunity. Local generation of secretory IgA (SIgA) constitutes the largest humoral immune system of the body. Secretory antibodies function both by performing antigen exclusion at mucosal surfaces and by virus and endotoxin neutralization within epithelial cells without causing tissue damage. SIgA is thus persistently containing commensal bacteria outside the epithelial barrier but can also target invasion of pathogens and penetration of harmful antigens. Resistance to toxin-producing bacteria such as Vibrio cholerae and enterotoxigenic Escherichia coli appears to depend largely on SIgA, and so does herd protection against horizontal faecal-oral spread of enteric pathogens under naïve or immunized conditions--with a substantial innate impact both on cross-reactivity and memory. Like natural infections, live mucosal vaccines or adequate combinations of non-replicating vaccines and mucosal adjuvants, give rise not only to SIgA antibodies but also to longstanding serum IgG and IgA responses. However, there is considerably disparity with regard to migration of memory/effector cells from mucosal inductive sites to secretory effector sites and systemic immune organs. Also, although immunological memory is generated after mucosal priming, this may be masked by a self-limiting response protecting the inductive lymphoid tissue in the gut. The intranasal route of vaccine application targeting nasopharynx-associated lymphoid tissue may be more advantageous for certain infections, but only if successful stimulation is achieved without the use of toxic adjuvants that might reach the central nervous system. The degree of protection obtained after mucosal vaccination ranges from reduction of symptoms to complete inhibition of re-infection. In this scenario, it is often difficult to determine the relative importance of SIgA versus serum antibodies, but infection models in knockout mice strongly support the notion that SIgA exerts a decisive role in protection and cross-protection against a variety of infectious agents. Nevertheless, relatively few mucosal vaccines have been approved for human use, and more basic work is needed in vaccine and adjuvant design, including particulate or live-vectored combinations.

Binding of human plasma proteins to Streptococcus pyogenes M protein determines the location of opsonic and non-opsonic epitopes

Antibodies directed against a pathogenic microorganism may recognize either protective or non-protective epitopes. Because antibodies elicited by a vaccine must be directed against protective epitopes, it is essential to understand the molecular properties that distinguish the two types of epitope. Here we analyse this problem for the antiphagocytic M protein of Streptococcus pyogenes, using the opsonizing capacity of antibodies to estimate their ability to confer protection in vivo. Our studies were focused on the M5 protein, which has three surface-exposed regions: the amino-terminal hypervariable region (HVR) and the B- and C-repeat regions. We first analysed the role of different M5 regions in phagocytosis resistance under non-immune conditions, employing chromosomal mutants expressing M5 proteins with internal deletions, and demonstrate that only the B-repeat region is essential for phagocytosis resistance. However, only antibodies to the HVR were opsonic. This apparent paradox could be explained by the ability of fibrinogen and albumin to specifically bind to the B- and C-repeats, respectively, causing inhibition of antibody binding under physiological conditions, while antibodies to the HVR could bind and promote deposition of complement. These data indicate that binding of human plasma proteins plays an important role in determining the location of opsonic and non-opsonic epitopes in streptococcal M protein.

Crystal Structure and Biological Implications of a Bacterial Albumin Binding Module in Complex with Human Serum Albumin

Many bactericide species express surface proteins that interact with human serum albumin (HSA). Protein PAB from the anaerobic bacterium Finegoldia magna (formerly Peptostreptococcus magnus) represents one of these proteins. Protein PAB contains a domain of 53 amino acid residues known as the GA module. GA homologs are also found in protein G of group C and G streptococci. Here we report the crystal structure of HSA in complex with the GA module of protein PAB. The model of the complex was refined to a resolution of 2.7 A and reveals a novel binding epitope located in domain II of the albumin molecule. The GA module is composed of a left-handed three-helix bundle, and residues from the second helix and the loops surrounding it were found to be involved in HSA binding. Furthermore, the presence of HSA-bound fatty acids seems to influence HSA-GA complex formation. F. magna has a much more restricted host specificity compared with C and G streptococci, which is also reflected in the binding of different animal albumins by proteins PAB and G. The structure of the HSA-GA complex offers a molecular explanation to this unusually clear example of bacterial adaptation.

Uptake and intracellular transportation of a bacterial surface protein in lymphoid cells

Some strains of the human pathogen Streptococcus pyogenes express a surface protein called protein H, which is released from the streptococcal surface by a cysteine proteinase produced by the bacteria. Here, we find that soluble protein H binds to the surface of lymphocytes and granulocytes, and that the molecule is taken up by lymphocytes and transported to the perinuclear region. The translocation over the cell membrane is rapid, and the uptake and intracellular transportation is not dependent on actin polymerization. Protein H could be immunoprecipitated from cell extracts and nuclear preparations of lymphocytes, and analysis of molecular interactions between protein H and proteins of different cellular compartments demonstrated a binding to nucleophosmin/ B23, a protein known to shuttle between the cytoplasm and the nucleus, and to the nuclear proteins SET and hnRNP A2/B1. Nucleophosmin/B23 was co-immunoprecipitated with protein H from cell and nuclear extracts, and binding experiments, including kinetic analyses, suggest that protein H dissociating from nucleophosmin/B23 complexes in the perinuclear region or in the nucleus binds to proteins SET and hnRNP A2/B1. Finally, the uptake and intracellular transportation of protein H was found to result in a cytostatic effect on B and T lymphocytes.

Mutational analysis of the interaction between albumin-binding domain from streptococcal protein G and human serum albumin

Streptococcal protein G (SpG) is a bacterial cell surface receptor exhibiting affinity to both human immunoglobulin (IgG) and human serum albumin (HSA). Interestingly, the serum albumin and immunoglobulin-binding activities have been shown to reside at functionally and structurally separated receptor domains. The binding domain of the HSA-binding part has been shown to be a 46-residue triple alpha-helical structure, but the binding site to HSA has not yet been determined. Here, we have investigated the precise binding region of this bacterial receptor by protein engineering applying an alanine-scanning procedure followed by binding studies by surface plasmon resonance (SPR). The secondary structure as well as the HSA binding of the resulting albumin-binding domain (ABD) variants were analyzed using circular dichroism (CD) and affinity blotting. The analysis shows that the HSA binding involves residues mainly in the second alpha-helix.

Pathogenic mechanism of acute post-streptococcal glomerulonephritis

Considerable knowledge has been accumulated regarding the characteristics of acute post-streptococcal glomerulonephritis (APSGN), and many attempts have been made to identify a streptococcal factor or factors responsible for triggering this disease. However, the pathogenic mechanism behind APSGN remains largely unknown. As glomerular deposition of C3 is generally demonstrated before that of IgG in the disease process, it is likely that the inflammatory response is initiated by renal deposition of a streptococcal product, rather than by deposition of antibodies or pre-formed immune complexes. During recent years, a number of streptococcal products have been suggested to be involved in the pathogenic process. In this review, possible roles of these factors are discussed in the context of the clinical and renal findings most often demonstrated in patients with APSGN. Streptokinase was observed to be required in order to induce signs of APSGN in mice, and a number of findings suggest that the initiation of the disease may occur as a result of renal binding by certain nephritis-associated variants of this protein. However, additional factors may be required for the development of the disease.

Streptococcal protein H forms soluble complement-activating complexes with IgG, but inhibits complement activation by IgG-coated targets

Protein H, a surface protein of Streptococcus pyogenes interacting with the constant Fc region of IgG, is known to be released from the streptococcal surface by a cysteine proteinase produced by the bacteria. Poststreptococcal glomerulonephritis and rheumatic fever are conditions in which immune complexes and autoimmune mechanisms have been suggested to play pathogenetic roles. The present study demonstrates that addition of protein H to human serum produces complement activation with dose-dependent cleavage of C3. The activation was IgG-dependent and the result of complexes formed between IgG and protein H. These complexes were size heterogeneous with molecular masses of 400 kDa to 1.4 MDa. Using complement-depleted serum reconstituted with complement proteins, the activation by protein H was found to be dependent of the classical, but independent of the alternative pathway of complement. In contrast to results of experiments based on soluble protein H.IgG complexes, complement activation was inhibited by protein H when IgG was immobilized on a surface. The interaction between C1q and immunoglobulins represents the first step in the activation of the classical pathway, and protein H efficiently inhibited the binding of C1q to IgG immobilized on polyacrylamide beads. Protein H reduced C3 deposition on the IgG-coated beads and inhibited immune hemolysis of IgG-sensitized erythrocytes. Finally, significantly less C3 was deposited on the surface of protein H-expressing wild-type streptococci than on the surface of isogenic mutant bacteria devoid of protein H. The results demonstrate that protein H.IgG complexes released from the streptococcal surface can produce complement breakdown at the sites of infection, whereas complement activation on bacterial surfaces is inhibited. This should have important implications for host-parasite relationships. In addition, soluble protein H.IgG complexes might contribute to immunological complications of streptococcal infections.

Five cases of measles secondary vaccine failure with confirmed seroconversion after live measles vaccination

We report 5 patients with secondary vaccine failure (SVF) who were infected with natural measles 2, 5, 5, 7 and 12 years, respectively, after vaccination with further attenuated live measles vaccine during infancy. Their seroconversion had been confirmed after vaccination. Three of the 5 patients had mild (modified) measles, while the remaining 2 patients had typical measles. The hemagglutination inhibition antibody titers to measles virus in paired acute and convalescent sera showed a secondary response pattern in 4/5 patients, and a primary response pattern was present in the remaining patient. Measles IgM antibodies were present in all patients during the convalescent stage. The patient with the primary response pattern may have had a decrease in the B cell memory during the 5-year period between vaccination and infection. This may be the first SVF case report that confirms the existence of completely waning immunity in recipients of the further attenuated live measles vaccines.

Biological properties of a Streptococcus pyogenes mutant generated by Tn916 insertion in mga

The mga regulon of Streptococcus pyogenes contains genes which contribute to the pathogenicity and virulence of this significant human pathogen. Transposon insertional inactivation of the regulatory mga gene in a S. pyogenes strain of the clinically important M1 serotype, blocked the expression of four genes located downstream of mga. These genes encode the M1 protein, the IgG-binding protein H, protein SIC which is an extracellular inhibitor of complement, and the C5a peptidase which interferes with granulocyte migration. The wild-type strain is resistant to phagocytosis and adheres to human skin tissue sections; properties that were lost in the transposon mutant. Moreover, the mutant was less virulent to mice but more cytolytic to human lymphocytes, the latter due to an increased activity of streptolysin S, whereas the production of streptolysin O, another toxin of S. pyogenes, was not affected. The mga mutation was complemented in trans with an intact mga gene which restored the phenotype of the wild-type strain.

Streptococcal protein MAG--a protein with broad albumin binding specificity

Protein MAG is a cell surface protein from Streptococcus dysgalactiae which binds alpha 2-macroglobulin (alpha 2M), serum albumin and immunoglobulin G (IgG). In this work protein MAG was expressed in Escherichia coli, purified and analysed for its albumin-binding specificity. The binding of protein MAG to serum albumins of different species origin was studied in a dot-blot assay and compared with the binding of streptococcal protein G, so far the best studied bacterial albumin receptor. The albumin-binding of protein MAG was also characterized using real-time biospecific interaction analysis (BIA), and the ka, kd and the Kaff values for different albumins were determined. Amino acid sequence alignment revealed homology between the albumin-binding domain of protein MAG and earlier described streptococcal albumin receptors including protein G. However, the MAG protein was reactive with serum albumin from bovine, dog, goat, horse, human, mouse, pig, rat and sheep origin and therefore displays a broader albumin-binding profile than protein G concerning the albumins tested in this work. Comparison of the albumin-binding spectrum of protein MAG with the earlier described albumin receptors of various S. dysgalactiae strains and other streptococci, suggests that protein MAG is a new type of albumin receptor.

Streptokinase activity among group A streptococci in relation to streptokinase genotype, plasminogen binding, and disease manifestations

Certain genotypic variants of streptokinase (ska) of beta-hemolytic streptococci group A have been associated with acute post-streptococcal glomerulonephritis (APSGN). In our earlier studies on strains isolated from Ethiopian children with various streptococcal disease manifestation, we reported an even distribution of streptokinase genotypes with no association to disease patterns. Considering the possibility that strains could differ in their ability to secrete the protein, levels of streptokinase activity in culture supernatants of these strains were determined by a plasminogen activation assay using a synthetic tripeptide, H-D-valyl-leucyl-lysin-p-nitroaniline, as a substrate. Of the 53 streptococcal group A strains, ten (19%), which belonged to genotype ska4 and ska8, did not activate human plasminogen. These strains did not activate bovine, sheep, horse, rabbit or porcine plasminogens either. They represented at least five M protein and non-typeable serotypes, and were characterized by high human plasminogen binding activity. Six of the 53 strains (11%) harbouring genotype ska3 and ska7 showed low levels of human plasminogen activation. Strains of ska1 and ska2, 37/53, activated human plasminogen at a higher level (p < 0.005). Levels of plasminogen activation were not significantly different among the ska1 and ska2 strains associated with various streptococcal disease manifestations. Antibody levels against streptokinase were higher (p < 0.05) in convalescent sera from acute rheumatic fever and APSGN patients in comparison with sera from other patient categories and healthy controls. Streptokinase genotype and in vitro streptokinase production do not correlate directly to streptococcal disease manifestation, indicating a probable significance of additional streptococcal and/or host factors in the initiation of APSGN.

Streptococcus pyogenes type M12 protein shows selective binding to some human immunoglobulin G3 myeloma proteins

Purified, recombinant M12 protein from Streptococcus pyogenes CS24 has recently been demonstrated to bind human immunoglobulin G3 (IgG3). The binding site for IgG has been localized to an internal peptide encoded by a PvuII fragment of the gene emm12. We have investigated the ability of an isolated recombinant M12 protein consisting of the peptide encoded by the PvuII fragment to bind various monoclonal human IgG3 myeloma proteins representing a number of both Caucasian and Oriental IgG3 Gm(allotypic) phenotypes. Of nine Caucasian IgG3 myeloma proteins, only two bound strongly to the recombinant M12 protein in enzyme-linked immunosorbent assays. The allotypic phenotypes of the reactive proteins were IgG3m(b+)(g-) and IgG3m(b-)(g+). No binding was seen for seven IgG3 myeloma proteins of Oriental origin with G3m(st+)(u-)(b+)(g-), G3m(st-)(u+)(b+)(g-), G3m(st-)(u+)(b-)(g+), and G3m(st-)(u-)(b-)(g+) phenotypes. The binding of human IgG3 to M12 protein seems to be related to features other than its Gm allotypic markers. Selective reactivity of IgG3 myeloma proteins with M12 protein may provide another way to subclassify human IgG3 molecules. The biological significance of the selective reactivity is not known.

M1 protein and protein H: IgGFc- and albumin-binding streptococcal surface proteins encoded by adjacent genes

M1 protein and Protein H are surface proteins simultaneously present at the surface of certain strains of Streptococcus pyogenes, important pathogenic bacteria in humans. The present study concerns the structure, protein-binding properties and relationship between these two molecules. The gene encoding M1 protein (emm1) was found immediately upstream of the Protein H gene (sph). Both genes were preceded by a promoter region. Comparison of the sequences revealed a high degree of similarity in the signal peptides, the C repeats located in the central parts of the molecules and in the C-terminal cell-wall-attached regions, whereas the N-terminal sequences showed no significant similarity. Protein H has affinity for the Fc region of IgG antibodies. Also M1 protein, isolated from streptococcal culture supernatants or from Escherichia coli expressing emm1, was found to bind human IgGFc. When tested against polyclonal IgG from eight other mammalian species, M1 protein and Protein H both showed affinity for baboon, rabbit and pig IgG. M1 protein also reacted with guinea-pig IgG, whereas both streptococcal proteins were negative in binding experiments with rat, mouse, bovine and horse IgG. The two proteins were also tested against other members of the immunoglobulin super family: human IgM, IgA, IgD, IgE, beta 2-microglobulin, and major histocompatibility complex (MHC) class-I and class-II antigens. M1 protein showed no affinity for any of these molecules whereas Protein H reacted with MHC class-II antigens. M1 protein is known to bind albumin and fibrinogen also. The binding sites for these two plasma proteins and for IgGFc were mapped to different sites on M1 protein. Thus albumin bound to the C repeats and IgGFc to a region (S) immediately N-terminal of the C repeats. Finally, fibrinogen bound further towards the N-terminus but close to the IgGFc-binding site. On the fibrinogen molecule, fragment D was found to mediate binding to M1 protein. The IgGFc-binding region of M1 protein showed no similarity to that of Protein H. Still, competitive binding experiments demonstrated that the two streptococcal proteins bound to overlapping sites on IgGFc.

Studies on binding of glycosaminoglycans to Streptococcus pyogenes by using 125I-heparan sulphate as a probe

Binding of 125I-heparan sulphate to the cell surface of Streptococcus pyogenes is mediated by proteins, that could be released from the streptococcal cell wall by using alkaline buffer. SDS-electrophoresis revealed two bands with molecular weights of 63 and 58 kDa. Binding of the 125I-labelled heparan sulphate probe to streptococci seems to be due to charge interactions, as the same probe was displaced by unlabelled heparan sulphate, other negatively charged molecules such as heparin, dextran sulphate, dermatan sulphate or by high ionic strength. The interaction was also strongly influenced by pH. The binding constant at pH 7.2 was estimated to be 9.8 x 10(6) mol/l, suggesting a moderate affinity. The presence of collagen of different types enhanced binding of 125I-labelled heparan sulphate to streptococci, whereas fibronectin and vitronectin had an inhibitory effect. The cooperation between heparan sulphate and collagen could be important for the adhesion of streptococci to connective tissue.

Distribution of presumptive pathogenicity factors among beta-hemolytic streptococci isolated from Ethiopia

Beta-hemolytic streptococci are known to bind several mammalian proteins, which are presumed to be important in pathogenicity. The distribution of such binding structures was examined for mouse albumin, human serum IgA, human IgG, human fibrinogen, and human plasminogen. A total of 218 group A beta-hemolytic streptococci (GAS) were studied: 5 isolates from children with acute rheumatic fever (ARF), 18 from acute post-streptococcal glomerulonephritis (APSGN), 57 from tonsillitis, 52 from skin infections, and 86 from healthy carriers. Sixty-eight Streptococcus equisimilis and 20 group G streptococci were also included. Most of the S. equisimilis (60/68) and group G (14/20) were obtained from apparently healthy carriers. The results were evaluated with respect to T type, serum opacity reaction (SOR), site of isolation, and disease type. No direct correlation was detected between the protein-binding structures studied. There was no apparent correlation between any particular protein-binding structure and specific T type. Albumin-binding and IgA-binding activities were inversely correlated among skin and nephritis GAS isolates. A strong correlation was demonstrated between IgA-binding activity and SOR production, while albumin-binding activity correlated with SOR-negative strains. Albumin-binding levels in isolates from ARF, APSGN and tonsillitis were significantly higher than in isolates from healthy carriers (P < 0.001). A higher albumin-binding capacity was shown in skin isolates from APSGN than in isolates from impetigo (P < 0.001).

Detection of rubella virus-specific polymeric immunoglobulin A by enzyme-linked immunosorbent assay in combination with streptococcal pretreatment of serum

An enzyme-linked immunosorbent assay combined with streptococcal treatment of serum was assessed for its ability to detect serum polymeric immunoglobulin A. This technique detects rubella virus-specific polymeric immunoglobulin A antibody, which appears for only a short time after infection, and it is useful for serodiagnosis of recent rubella virus infection.

Two types of receptors for human plasminogen on group G streptococci

To investigate the nature of plasminogen binding to streptococci, strains selected for high reactivity with human plasminogen were examined for binding pattern against a panel of plasminogen fragments. The strains included human isolates of groups A, C and G as well as bovine isolates of group G. All strains reacted substantially with the plasminogen fragment kringle 1-3. Using the miniplasminogen fragment (kringle 5 and the B chain) a small but reproducible uptake was detected for human group G strains but not for group A or C strains. The group G strains of bovine origin on the other hand demonstrated high uptake of miniplasminogen, suggesting the possibility of an alternative plasminogen receptor for this species. This interpretation was supported by blocking experiments with the lysine analogue EACA where low concentrations (1 mM) completely blocked plasminogen binding to human streptococci, whereas a 100-fold higher concentration was needed for bovine group G strains. Scatchard plots with human isolates resulted in straight lines and Kd values were generally in the range of 20-80 nM. The number of receptors was estimated to be 45,000 for a selected group A strain and about 10,000 for the selected group C and G strains. Scatchard analysis with bovine group G isolates on the other hand revealed a two phase interaction, supporting the assumption of two different receptor structures on these strains. Kd for the first phase was estimated to be about 20 nM (10,000-20,000 receptors per bacterium), which was similar to the human strains, whereas the second phase was in the range of 400-500 nM (50,000 and 150,000 receptors per bacterium with two selected strains). Scatchard plots with the miniplasminogen fragment as ligand mimicked the phase two reaction with plasminogen, supporting the concept that this reaction represents a new and not previously described receptor. Both the receptor reacting with the kringle 1-3 portion and the one reacting with the miniplasminogen portion bound plasmin and plasminogen with similar affinity.

Aggregation of group A streptococci by human saliva and effect of saliva on streptococcal adherence to host cells

The aggregation of group A streptococci by whole, stimulated human saliva (WHS) and the effect of saliva on streptococcal adherence to host cells was investigated. WHS samples from 11 individuals were found to aggregate both M+ and M- group A streptococci to various degrees. The aggregating activity was sensitive to heat, EDTA, EGTA [ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid], sodium dodecyl sulfate, and lipoteichoic acid. None of the simple sugars tested, mercaptoethanol, albumin, or nonionic detergents had any effect on aggregation. The aggregating activity of EDTA-treated saliva was restored by 0.1 mM Ca2+ and 1.0 mM Mn2+ but not by up to 5 mM Mg2+. Only streptococci from the stationary phase were aggregated. Hyaluronidase treatment of streptococci from the exponential phase of growth restored their ability to be aggregated, suggesting that the hyaluronic acid capsule interferes with agglutination. Adsorption of WHS by one strain of Streptococcus pyogenes removed aggregating activity for other strains of S. pyogenes and Streptococcus sanguis but not agglutinins for Escherichia coli, suggesting that the agglutinin is specific for certain gram-positive bacteria. Molecular sieve chromatography of WHS and identification of streptococcus-binding components of saliva suggest that either a glycoprotein of approximately 360 kDa or a mucin of saliva of greater than 1,000 kDa mediates aggregation of streptococci. WHS also inhibited adherence of S. pyogenes to buccal epithelial cells.

New receptor for human plasminogen on gram positive cocci

180 bacterial strains representing 17 different species of gram positive cocci were tested for the ability to interact with human plasminogen. Receptors for plasminogen could be detected on 23/24 strains of S. pyogenes, 15/15 strains of S. equisimilis, 14/16 strains of human group G streptococci and 14/14 strains of S. pneumoniae. Eight of nineteen strains representing five species of alpha-hemolytic streptococci were also positive. S. equisimilis demonstrated the highest uptake with a median value of 58 per cent (20%-67%). On the other hand, all strains of S. agalactiae, the majority of S. faecalis and all S. aureus, S. epidermidis and S. saprophyticus strains tested were negative. The concentration of unlabelled plasminogen causing a 50 per cent reduction of bound tracer was between 50 and 150 mM. These estimates of the dissociation constant confirmed the specific nature of the interaction. Binding of plasminogen could be blocked by addition of plasmin-aprotinin complex, suggesting that plasminogen and plasmin bind to the same receptor. Binding was also blocked by the plasminogen fragment kringle 1-3, but not by miniplasminogen, a fragment containing kringle 5 and the B-chain region. As streptokinase interacts mainly with the B-chain of plasmin it is clear that the bacterial receptor for plasminogen is not identical to streptokinase.

Immunological applications of type III Fc binding proteins. Comparison of different sources of protein G

Protein G, a type III bacterial IgG Fc receptor isolated from certain group C or G streptococci, shows a wider range of species and subclass immunoglobulin reactivity than staphylococcal protein A and has been shown to be more useful than protein A for many immunochemical applications. Recently, two forms of wild type protein G and three forms of recombinant protein G have become commercially available. Each form of protein G was tested for reactivity with a variety of species of immunoglobulin and albumin. Additionally, one form of wild type protein G and two forms of the recombinant protein G were examined for their ability to stimulate in vitro proliferation of human peripheral blood leukocytes (PBL). Similar IgG species reactivity was observed for all forms of unlabeled protein G. By contrast, considerable variability in the relative IgG binding potentials of different protein G preparations was observed following radioiodination. Binding to human serum albumin was observed with one of the wild type protein G samples, however, the IgG binding activity of this protein was not affected by the presence of excess human serum albumin. In the human PBL proliferation assays, wild type protein G was weakly mitogenic and one form of recombinant protein G was shown to be a potent mitogen, while another form of recombinant protein G displayed no mitogenic potential. Differences in both functional and biological reactivities were observed among the various sources of protein G. These differences may lead to confusion if investigators consider all sources of protein G as a single entity with common properties.

Ultrastructural localization of the fibrinogen-binding domain of streptococcal M protein

Binding of fibrinogen to the M protein located on the surface fibrillae of group A streptococci impedes deposition of complement and thus contributes to the virulence of these organisms. We investigated this binding by electron microscopy using postembedding immunogold labeling. Both fibrinogen and its D fragment formed a distinct dense layer in the surface fibrillae, separated by 10 nm from the compact part of the cell wall. Labeling the sections with anti-fibrinogen or anti-fragment D showed that the fibrinogen-binding region lay within a 25-nm segment of the fibrillae beginning approximately 30 nm from the inner surface of the cell wall. The outer surface of the fibrinogen layer could be labeled with antibody to the amino-terminal half of type 24 M protein, indicating that the fibrillar tips remained exposed after fibrinogen binding. The degree of labeling with anti-fibrinogen, determined by gold particle counting, was the same whether the bacterial cells had been incubated with purified fibrinogen or whole plasma. These results indicate that the fibrinogen-binding region lies in the distal (amino-terminal) half of the M protein molecule but excludes the most distal portion, which is the site of epitopes that interact with opsonic anti-M antibody, and that plasma proteins other than fibrinogen, a number of which are known to bind to group A streptococci, do not interfere with fibrinogen binding.

Solubilization of IgG-binding proteins from group A and G streptococci

The release of IgG-binding proteins from the cell surface of streptococcal strains AR-1 and G148 with various proteolytic enzymes, acid, alkali or SDS was investigated. The IgG-binding proteins were purified by affinity chromatography using IgG-Sepharose Fast Flow. After SDS-polyacrylamide gel electrophoresis and immuno-electroblotting the major proteins identified varied in relative molecular mass from 15,000 to 65,000 depending on the solubilizing agent used. The results showed that solubilization with trypsin gave the highest yield of IgG-binding proteins, that strain G148 yielded about twice the amount of protein as strain AR-1, and that elastase released an IgG-binding protein of high relative molecular mass of 65,000.

Receptor for IgA in group A streptococci: cloning of the gene and characterization of the protein expressed in Escherichia coli

The gene for an IgA-binding protein from a group A streptococcal strain was cloned and expressed in Escherichia coli. The IgA-binding protein, called protein Arp, was purified on IgA-Sepharose, allowing complete purification in a single step. Analysis of protein Arp by Western immunoblotting demonstrated a major IgA-binding band, with an apparent molecular weight of 42 kD. The purified protein was shown to bind serum IgA and secretory IgA, as well as monoclonal IgA of both subclasses. There was no binding to IgM, IgD or IgE, but a weak binding to IgG. Inhibition experiments with whole bacteria indicated that IgA and IgG bind at separate sites. Experiments with immunoglobulin fragments showed that protein Arp binds to the Fc region of both IgA and IgG. The equilibrium constant of the reaction between protein Arp and polyclonal human IgA was determined to be 5.6 x 10(8) M-1. Amino acid sequencing of protein Arp demonstrated a direct repeat of 7 amino acids in the NH2-terminal region, a feature previously found in several streptococcal M proteins. This suggests that protein Arp, like M proteins, may be a streptococcal virulence factor.

New aspects in the pathogenesis and prevention of polymer-associated foreign-body infections caused by coagulase-negative staphylococci

The significance of polymer-associated infections caused by coagulase-negative staphylococci is discussed. The aspects of bacterial adhesion to polymeric materials as the first important pathogenetic step in the development of such infections are treated. The role of extracellular slime substance (ESS) produced by the bacteria in the pathogenesis is elucidated and newer results concerning the interference of ESS with host defense mechanisms and antibiotic therapy are presented. As an approach to the prevention of polymer-associated foreign-body infections, the modification of the polymeric materials is introduced. Results of recent studies to achieve antiadhesive materials by radiation modification of polymers as well as the development of antimicrobial surfaces by incorporating or bonding antibiotics to polymers are presented.

Isolation and characterization of a specific receptor for human albumin on a group L Streptococcus

Certain group L streptococci demonstrate surface receptors for human albumin. Binding of 125I-albumin to group L streptococci could be inhibited by unlabelled albumin preparations from humans, dogs, mice and bovines, but not by albumin from rabbits. The albumin-binding proteins (ABP) could be solubilized from the streptococcal surface by hot acid treatment of the bacteria and isolated by affinity chromatography on human-albumin sepharose. ABP and specific antisera produced against ABP inhibited 125I-albumin binding to group L streptococci. The molecular weight of ABP determined by SDS-PAGE and Western blotting, was approximately 48,000 Dalton. ABP preparations of group G streptococci isolated from bovines and humans demonstrated cross reactivity with antiserum produced against group L streptococcal ABP.

Heparin-inhibitable basement membrane-binding protein of Streptococcus pyogenes

Solubilized surface proteins of Streptococcus pyogenes serotype M6 were found by indirect immunofluorescence assays to bind selectively to proteoglycan-containing regions of basement membranes of kidney and cardiac muscle in vitro. Epithelial, endothelial, and interstitial cells were unstained. Binding of streptococcal protein to basement membranes was competitively inhibited by heparin and, to a lesser extent, by heparan sulfate. Weak inhibition was also observed with other glycosaminoglycans, including dermatan sulfate, chondroitin sulfate, and hyaluronic acid. Type IV collagen, gelatin, serum fibronectin, glucuronic acid, and a selection of monosaccharides had no significant effects on binding. The heparin-inhibitable basement membrane-binding protein was purified by affinity chromatography on heparin-Sepharose 6-B. Polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate and urea dissociated the affinity-purified protein into two polypeptides of 9,000 and 15,000 mrs. Chemical analyses revealed that the purified protein was devoid of cysteine, amino and neutral sugars, and phosphate. Thus, the polypeptides are not glycosylated or complexed with trace amounts of lipoteichoic acid or polysaccharide. Binding of purified protein to tissue was determined by direct radioassay and indirect immunofluorescence and was inhibitable by heparin. Although the in vivo effects of this streptococcal component remain to be determined, its deposition on basement membranes in vitro supports the hypothesis that it contributes to the pathogenesis of poststreptococcal glomerulonephritis or acute rheumatic fever.

Analysis and use of the serum albumin binding domains of streptococcal protein G

Streptococcal protein G is an IgG-binding receptor with a molecular weight of 63 kDa as predicted from the sequence of the corresponding gene. Here we show that a truncated recombinant protein of 23 kDa still has IgG-binding capacity and also interacts specifically with human serum albumin (HSA). This demonstrates that protein G is a bifunctional receptor. To investigate the structures needed for IgG- and albumin-binding, different parts of the receptor molecule were produced in E. coli using a coupled expression/secretion system. Affinity chromatography, using IgG or HSA immobilized on Sepharose, showed that the two binding activities are structurally separated. From these experiments, it was concluded that a region of 64 amino acid residues is sufficient for albumin-binding. The structure of this part of the protein suggests either a divalent or a trivalent binding capacity. The specific interaction to albumin was used to purify a heterologous protein by affinity chromatography to yield a pure fusion protein in a one-step procedure. The implication of this novel affinity system as a tool to facilitate protein immobilization and purification is discussed.