IgG-binding proteins of bacteria

A Generic Antibody-Blocking Protein That Enables pH-Switchable Activation of Antibody Activity

Molecular strategies that allow for reversible control of antibody activity have drawn considerable interest for both therapeutic and diagnostic applications. Protein M is a generic antibody-binding protein that binds to the Fv domain of IgGs and, in doing so, blocks antigen binding. However, the dissociation of protein M is essentially irreversible, which has precluded its use as an antibody affinity reagent and molecular mask to control antibody activity. Here, we show that introduction of 8 histidine residues on the Fv binding interface of protein M results in a variant that shows pH-switchable IgG binding. This protein M-8his variant provides an attractive and universal affinity resin for the purification of IgGs, antibody fragments (Fab and single-chain variable fragments (scFv)), and antibody conjugates. Moreover, protein M-8his enables the pH-dependent blocking of therapeutic antibodies, allowing the selective targeting of cells at pH 6.0.

IdeS, a secreted proteinase of Streptococcus pyogenes, is bound to a nuclease at the bacterial surface where it inactivates opsonizing IgG antibodies

The important bacterial pathogen Streptococcus pyogenes secretes IdeS (immunoglobulin G-degrading enzyme of S. pyogenes), a proteinase that cleaves human immunoglobulin G (IgG) antibodies in the hinge region resulting in Fc (fragment crystallizable) and F(ab')2 (fragment antigen-binding) fragments and protects the bacteria against phagocytic killing. Experiments with radiolabeled IdeS and flow cytometry demonstrated that IdeS binds to the surface of S. pyogenes, and the interaction was most prominent in conditions resembling those in the pharynx (acidic pH and low salt), the habitat for S. pyogenes. SpnA (S. pyogenes nuclease A) is a cell wall-anchored DNase. A dose-dependent interaction between purified SpnA and IdeS was demonstrated in slot binding and surface plasmon resonance spectroscopy experiments. Gel filtration showed that IdeS forms proteolytically active complexes with SpnA in solution, and super-resolution fluorescence microscopy revealed the presence of SpnA-IdeS complexes at the surface of S. pyogenes. Finally, specific IgG antibodies binding to S. pyogenes surface antigens were efficiently cleaved by surface-associated IdeS. IdeS is secreted by all S. pyogenes isolates and cleaves IgG antibodies with a unique degree of specificity and efficiency. These properties and the finding here that the proteinase is present and fully active at the bacterial surface in complex with SpnA implicate an important role for IdeS in S. pyogenes biology and pathogenesis.

Interplay between inflammatory bowel disease therapeutics and the gut microbiome reveals opportunities for novel treatment approaches

Inflammatory bowel disease (IBD) is a complex heterogeneous disorder defined by recurring chronic inflammation of the gastrointestinal tract, attributed to a combination of factors including genetic susceptibility, altered immune response, a shift in microbial composition/microbial insults (infection/exposure), and environmental influences. Therapeutics generally used to treat IBD mainly focus on the immune response and include non-specific anti-inflammatory and immunosuppressive therapeutics and targeted therapeutics aimed at specific components of the immune system. Other therapies include exclusive enteral nutrition and emerging stem cell therapies. However, in recent years, scientists have begun to examine the interplay between these therapeutics and the gut microbiome, and we present this information here. Many of these therapeutics are associated with alterations to gut microbiome composition and functionality, often driving it toward a "healthier profile" and preclinical studies have revealed that such alterations can play an important role in therapeutic efficacy. The gut microbiome can also improve or hinder IBD therapeutic efficacy or generate undesirable metabolites. For certain IBD therapeutics, the microbiome composition, particularly before treatment, may serve as a biomarker of therapeutic efficacy. Utilising this information and manipulating the interactions between the gut microbiome and IBD therapeutics may enhance treatment outcomes in the future and bring about new opportunities for personalised, precision medicine.

Bovine Airway Models: Approaches for Investigating Bovine Respiratory Disease

Bovine respiratory disease (BRD) is a multifactorial condition where different genera of bacteria, such as Mannheimia haemolytica, Histophilus somni, Pasteurella multocida, and Mycoplasma bovis, and viruses, like bovine respiratory syncytial virus, bovine viral diarrhea virus, and bovine herpes virus-1, infect the lower respiratory tract of cattle. These pathogens can co-infect cells in the respiratory system, thereby making specific treatment very difficult. Currently, the most common models for studying BRD include a submerged tissue culture (STC), where monolayers of epithelial cells are typically covered either in cellular or spent biofilm culture medium. Another model is an air-liquid interface (ALI), where epithelial cells are exposed on their apical side and allowed to differentiate. However, limited work has been reported on the study of three-dimensional (3D) bovine models that incorporate multiple cell types to represent the architecture of the respiratory tract. The roles of different defense mechanisms in an infected bovine respiratory system, such as mucin production, tight junction barriers, and the production of antimicrobial peptides in in vitro cultures require further investigation in order to provide a comprehensive understanding of the disease pathogenesis. In this report, we describe the different aspects of BRD, including the most implicated pathogens and the respiratory tract, which are important to incorporate in disease models assembled in vitro. Although current advancements of bovine respiratory cultures have led to knowledge of the disease, 3D multicellular organoids that better recapitulate the in vivo environment exhibit potential for future investigations.

Role of FcγRIII in the nasal cavity of BALB/c mice in the primary amebic meningoencephalitis protection model

Different mechanisms of the host immune response against the primary amebic meningoencephalitis (PAM) in the mouse protection model have been described. It has been proposed that antibodies opsonize Naegleria fowleri trophozoites; subsequently, the polymorphonuclear cells (PMNs) surround the trophozoites to avoid the infection. FcγRs activate signaling pathways of adapter proteins such as Syk and Hck on PMNs to promote different effector cell functions which are induced by the Fc portion of the antibody-antigen complexes. In this work, we analyzed the activation of PMNs, epithelial cells, and nasal passage cells via the expression of Syk and Hck genes. Our results showed an increment of the FcγRIII and IgG subclasses in the nasal cavity from immunized mice as well as Syk and Hck expression was increased, whereas in the in vitro assay, we observed that when the trophozoites of N. fowleri were opsonized with IgG anti-N. fowleri and interacted with PMN, the expression of Syk and Hck was also increased. We suggest that PMNs are activated via their FcγRIII, which leads to the elimination of the trophozoites in vitro, while in the nasal cavity, the adhesion and consequently infection are avoided.

Toward Understanding How Staphylococcal Protein A Inhibits IgG-Mediated Phagocytosis

IgG molecules are crucial for the human immune response against bacterial infections. IgGs can trigger phagocytosis by innate immune cells, like neutrophils. To do so, IgGs should bind to the bacterial surface via their variable Fab regions and interact with Fcγ receptors and complement C1 via the constant Fc domain. C1 binding to IgG-labeled bacteria activates the complement cascade, which results in bacterial decoration with C3-derived molecules that are recognized by complement receptors on neutrophils. Next to FcγRs and complement receptors on the membrane, neutrophils also express the intracellular neonatal Fc receptor (FcRn). We previously reported that staphylococcal protein A (SpA), a key immune-evasion protein of Staphylococcus aureus, potently blocks IgG-mediated complement activation and killing of S. aureus by interfering with IgG hexamer formation. SpA is also known to block IgG-mediated phagocytosis in absence of complement, but the mechanism behind it remains unclear. In this study, we demonstrate that SpA blocks IgG-mediated phagocytosis and killing of S. aureus and that it inhibits the interaction of IgGs with FcγRs (FcγRIIa and FcγRIIIb, but not FcγRI) and FcRn. Furthermore, our data show that multiple SpA domains are needed to effectively block IgG1-mediated phagocytosis. This provides a rationale for the fact that SpA from S. aureus contains four to five repeats. Taken together, our study elucidates the molecular mechanism by which SpA blocks IgG-mediated phagocytosis and supports the idea that in addition to FcγRs, the intracellular FcRn is also prevented from binding IgG by SpA.

Microchip coupled with MALDI-TOF MS for the investigation of bacterial contamination of fish muscle products

Bacterial contamination is a significant concern in food safety. Traditional methods, though being a gold standard for bacterial detection, are time-consuming. In this work, we managed to establish a simple and versatile magnetic-assisted microfluidic method for rapid bacterial detection of fish muscle products, by manipulating anti-human IgG functionalized magnetic beads in a zig-zag shaped microfluidic channel, increasing the probability for bacteria capture. The captured bacteria were characterized by matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS). This method is capable of isolating Escherichia coli, Staphylococcus aureus and Klebsiella pneumoniae from 5 μL of sablefish sarcoplasmic protein sample, and detecting Escherichia coli in the range of 6.0 to 6.0×10⁴ CFU/mL with a detection limit of 6 CFU/mL. Bacterial growth on salmon sashimi during its period of storage was successfully monitored. The current protocol holds great potential for pathogen detection and microbial control in the food industry.

Natural Human Immunity Against Staphylococcal Protein A Relies on Effector Functions Triggered by IgG3

Staphylococcal protein A (SpA) is a multifunctional, highly conserved virulence factor of Staphylococcus aureus. By binding the Fc portion of all human IgG subclasses apart from IgG3, SpA interferes with antibody and complement deposition on the bacterial surface, impairing staphylococcal clearance by phagocytosis. Because of its anti-opsonic properties, SpA is not investigated as a surface antigen to mediate bacterial phagocytosis. Herein we investigate human sera for the presence of SpA-opsonizing antibodies. The screening revealed that sera containing IgG3 against SpA were able to correctly opsonize the target and drive Fcγ receptor-mediated interactions and phagocytosis. We demonstrated that IgG3 Fc is significantly more efficient in inducing phagocytosis of SpA-expressing S. aureus as compared to IgG1 Fc in an assay resembling physiological conditions. Furthermore, we show that the capacity of SpA antibodies to induce phagocytosis depends on the specific epitope recognized by the IgGs on SpA molecules. Overall, our results suggest that anti-SpA IgG3 antibodies could favor the anti-staphylococcal response in humans, paving the way towards the identification of a correlate of protection against staphylococcal infections.

Moonlighting in Rickettsiales: Expanding Virulence Landscape

The order Rickettsiales includes species that cause a range of human diseases such as human granulocytic anaplasmosis (Anaplasma phagocytophilum), human monocytic ehrlichiosis (Ehrlichia chaffeensis), scrub typhus (Orientia tsutsugamushi), epidemic typhus (Rickettsia prowazekii), murine typhus (R. typhi), Mediterranean spotted fever (R. conorii), or Rocky Mountain spotted fever (R. rickettsii). These diseases are gaining a new momentum given their resurgence patterns and geographical expansion due to the overall rise in temperature and other human-induced pressure, thereby remaining a major public health concern. As obligate intracellular bacteria, Rickettsiales are characterized by their small genome sizes due to reductive evolution. Many pathogens employ moonlighting/multitasking proteins as virulence factors to interfere with multiple cellular processes, in different compartments, at different times during infection, augmenting their virulence. The utilization of this multitasking phenomenon by Rickettsiales as a strategy to maximize the use of their reduced protein repertoire is an emerging theme. Here, we provide an overview of the role of various moonlighting proteins in the pathogenicity of these species. Despite the challenges that lie ahead to determine the multiple potential faces of every single protein in Rickettsiales, the available examples anticipate this multifunctionality as an essential and intrinsic feature of these obligates and should be integrated into available moonlighting repositories.

The Retropepsin-Type Protease APRc as a Novel Ig-Binding Protein and Moonlighting Immune Evasion Factor of Rickettsia

Rickettsiae are obligate intracellular Gram-negative bacteria transmitted by arthropod vectors. Despite their reduced genomes, the function(s) of the majority of rickettsial proteins remains to be uncovered. APRc is a highly conserved retropepsin-type protease, suggested to act as a modulator of other rickettsial surface proteins with a role in adhesion/invasion. However, APRc’s function(s) in bacterial pathogenesis and virulence remains unknown. This study demonstrates that APRc targets host serum components, combining nonimmune immunoglobulin (Ig)-binding activity with resistance to complement-mediated killing. We confirmed nonimmune human IgG binding in extracts of different rickettsial species and intact bacteria. Our results revealed that the soluble domain of APRc is capable of binding to human (h), mouse, and rabbit IgG and different classes of human Ig (IgG, IgM, and IgA) in a concentration-dependent manner. APRc-hIgG interaction was confirmed with total hIgG and normal human serum. APRc-hIgG displayed a binding affinity in the micromolar range. We provided evidence of interaction preferentially through the Fab region and confirmed that binding is independent of catalytic activity. Mapping the APRc region responsible for binding revealed the segment between amino acids 157 and 166 as one of the interacting regions. Furthermore, we demonstrated that expression of the full-length protease in Escherichia coli is sufficient to promote resistance to complement-mediated killing and that interaction with IgG contributes to serum resistance. Our findings position APRc as a novel Ig-binding protein and a novel moonlighting immune evasion factor of Rickettsia, contributing to the arsenal of virulence factors utilized by these intracellular pathogens to aid in host colonization.

Beware of Mycoplasma Anti-immunoglobulin Strategies

Mycoplasmas are small, genome-reduced bacteria. They are obligate parasites that can be found in a wide range of host species, including the majority of livestock animals and humans. Colonization of the host can result in a wide spectrum of outcomes. In many cases, these successful parasites are considered commensal, as they are found in the microbiota of asymptomatic carriers. Conversely, mycoplasmas can also be pathogenic, as they are associated with a range of both acute and chronic inflammatory diseases which are problematic in veterinary and human medicine. The chronicity of mycoplasma infections and the ability of these bacteria to infect even recently vaccinated individuals clearly indicate that they are able to successfully evade their host’s humoral immune response. Over the years, multiple strategies of immune evasion have been identified in mycoplasmas, with a number of them aimed at generating important antigenic diversity. More recently, mycoplasma-specific anti-immunoglobulin strategies have also been characterized. Through the expression of the immunoglobulin-binding proteins protein M or mycoplasma immunoglobulin binding (MIB), mycoplasmas have the ability to target the host’s antibodies and to prevent them from interacting with their cognate antigens. In this review, we discuss how these discoveries shed new light on the relationship between mycoplasmas and their host’s immune system. We also propose that these strategies should be taken into consideration for future studies, as they are key to our understanding of mycoplasma diseases' chronic and inflammatory nature and are probably a contributing factor to reduce vaccine efficacy.

Staphylococcal protein A inhibits complement activation by interfering with IgG hexamer formation

Antibodies are crucial for the immune response against bacteria. To drive bacterial killing, antibodies should bind to the bacterial cell and induce the complement reaction. This requires target-bound IgGs to form hexameric IgG platforms that are kept together by noncovalent Fc-Fc interactions. Interestingly, pathogenic bacteria produce IgG-binding molecules that bind specifically to the Fc region needed for hexamerization. Here we demonstrate that staphylococcal protein A (SpA) from Staphylococcus aureus specifically blocks formation of IgG hexamers and downstream activation of complement. Furthermore, we show that IgG3 antibodies (which are not recognized by SpA) have superior capacity to activate complement and induce killing of S. aureus by human phagocytes. These insights provide a crucial rationale for optimizing antibody therapies against S. aureus.

Immunoglobulin (Ig) G molecules are essential players in the human immune response against bacterial infections. An important effector of IgG-dependent immunity is the induction of complement activation, a reaction that triggers a variety of responses that help kill bacteria. Antibody-dependent complement activation is promoted by the organization of target-bound IgGs into hexamers that are held together via noncovalent Fc-Fc interactions. Here we show that staphylococcal protein A (SpA), an important virulence factor and vaccine candidate of Staphylococcus aureus, effectively blocks IgG hexamerization and subsequent complement activation. Using native mass spectrometry and high-speed atomic force microscopy, we demonstrate that SpA blocks IgG hexamerization through competitive binding to the Fc-Fc interaction interface on IgG monomers. In concordance, we show that SpA interferes with the formation of (IgG)₆:C1q complexes and prevents downstream complement activation on the surface of S. aureus. Finally, we demonstrate that IgG3 antibodies against S. aureus can potently induce complement activation and opsonophagocytic killing even in the presence of SpA. Together, our findings identify SpA as an immune evasion protein that specifically blocks IgG hexamerization.

The mycoplasma surface proteins MIB and MIP promote the dissociation of the antibody-antigen interaction

Mycoplasma immunoglobulin binding (MIB) and mycoplasma immunoglobulin protease (MIP) are surface proteins found in the majority of mycoplasma species, acting sequentially to capture antibodies and cleave off their V_H domains. Cryo-electron microscopy structures show how MIB and MIP bind to a Fab fragment in a "hug of death" mechanism. As a result, the orientation of the V_L and V_H domains is twisted out of alignment, disrupting the antigen binding site. We also show that MIB-MIP has the ability to promote the dissociation of the antibody-antigen complex. This system is functional in cells and protects mycoplasmas from antibody-mediated agglutination. These results highlight the key role of the MIB-MIP system in immunity evasion by mycoplasmas through an unprecedented mechanism, and open exciting perspectives to use these proteins as potential tools in the antibody field.

Does protein unfolding play a functional role in vivo?

Unfolding and refolding of multidomain proteins under force have yet to be recognized as a major mechanism of function for proteins in vivo. In this review, we discuss the inherent properties of multidomain proteins under a force vector from a structural and functional perspective. We then characterize three main systems where multidomain proteins could play major roles through mechanical unfolding: muscular contraction, cellular mechanotransduction, and bacterial adhesion. We analyze how key multidomain proteins for each system can produce a gain-of-function from the perspective of a fine-tuned quantized response, a molecular battery, delivery of mechanical work through refolding, elasticity tuning, protection and exposure of cryptic sites, and binding-induced mechanical changes. Understanding how mechanical unfolding and refolding affect function will have important implications in designing mechano-active drugs against conditions such as muscular dystrophy, cancer, or novel antibiotics.

C-Type Lectin Receptors in Host Defense Against Bacterial Pathogens

Antigen-presenting cells (APCs) are present throughout the human body—in tissues, at barrier sites and in the circulation. They are critical for processing external signals to instruct both local and systemic responses toward immune tolerance or immune defense. APCs express an extensive repertoire of pattern-recognition receptors (PRRs) to detect and transduce these signals. C-type lectin receptors (CLRs) comprise a subfamily of PRRs dedicated to sensing glycans, including those expressed by commensal and pathogenic bacteria. This review summarizes recent findings on the recognition of and responses to bacteria by membrane-expressed CLRs on different APC subsets, which are discussed according to the primary site of infection. Many CLR-bacterial interactions promote bacterial clearance, whereas other interactions are exploited by bacteria to enhance their pathogenic potential. The discrimination between protective and virulence-enhancing interactions is essential to understand which interactions to target with new prophylactic or treatment strategies. CLRs are also densely concentrated at APC dendrites that sample the environment across intact barrier sites. This suggests an–as yet–underappreciated role for CLR-mediated recognition of microbiota-produced glycans in maintaining tolerance at barrier sites. In addition to providing a concise overview of identified CLR-bacteria interactions, we discuss the main challenges and potential solutions for the identification of new CLR-bacterial interactions, including those with commensal bacteria, and for in-depth structure-function studies on CLR-bacterial glycan interactions. Finally, we highlight the necessity for more relevant tissue-specific in vitro, in vivo and ex vivo models to develop therapeutic applications in this area.

Characterization of an Immunoglobulin Binding Protein (IbpM) From Mycoplasma pneumoniae

Bacteria evolved many ways to invade, colonize and survive in the host tissue. Such complex infection strategies of other bacteria are not present in the cell-wall less Mycoplasmas. Due to their strongly reduced genomes, these bacteria have only a minimal metabolism. Mycoplasma pneumoniae is a pathogenic bacterium using its virulence repertoire very efficiently, infecting the human lung. M. pneumoniae can cause a variety of conditions including fever, inflammation, atypical pneumoniae, and even death. Due to its strongly reduced metabolism, M. pneumoniae is dependent on nutrients from the host and aims to persist as long as possible, resulting in chronic diseases. Mycoplasmas evolved strategies to subvert the host immune system which involve proteins fending off immunoglobulins (Igs). In this study, we investigated the role of MPN400 as the putative factor responsible for Ig-binding and host immune evasion. MPN400 is a cell-surface localized protein which binds strongly to human IgG, IgA, and IgM. We therefore named the protein MPN400 immunoglobulin binding protein of Mycoplasma (IbpM). A strain devoid of IbpM is slightly compromised in cytotoxicity. Taken together, our study indicates that M. pneumoniae uses a refined mechanism for immune evasion.

High-Affinity Antibody Detection with a Bivalent Circularized Peptide Containing Antibody-Binding Domains

Direct chemical labeling of antibody produces molecules with poorly defined modifications. Use of a small antibody-binding protein as an adapter can simplify antibody functionalization by forming a specific antibody-bound complex and introducing site-specific modifications. To stabilize a noncovalent antibody complex that may be used without chemical crosslinking, a bivalent antibody-binding protein is engineered with an improved affinity of interaction by joining two Z domains with a conformationally flexible linker. The linker is essential for the increase in affinity because it allows simultaneous binding of both domains. The molecule is further circularized using a split intein, creating a novel adapter protein ("lasso"), which binds human immunoglobulin G1 (IgG1) with K _D  = 0.53 n m and a dissociation rate that is 55- to 84-fold slower than Z. The lasso contains a unique cysteine for conjugation with a reporter and may be engineered to introduce other functional groups, including a biotin tag and protease recognition sequences. When used in enzyme-linked immunosorbent assay (ELISA), the lasso generates a stronger reporter signal compared to a secondary antibody and lowers the limit of detection by 12-fold. The small size of the lasso and a long half-life of dissociation make the peptide a useful tool in antibody detection and immobilization.

Fc-binding proteins enhance autoantibody-induced BP180 depletion in pemphigoid

Immunoglobulins (Igs) consist of two antigen-binding regions (Fab) and one constant region (Fc). Protein A and protein G are bacterial proteins used for the purification of IgG by virtue of their high affinities for the Fc fragment. Rheumatoid factors are autoantibodies against IgG Fc fragments, which are present in the body under physiological conditions. Little is known about the influence of Fc-binding proteins on the pathogenicity of antibody-induced autoimmune diseases. Pemphigoid diseases are a group of autoimmune subepidermal blistering disorders that includes bullous pemphigoid and mucous membrane pemphigoid. IgGs targeting the non-collagenous NC16A domain of the 180-kDa bullous pemphigoid antigen (BP180) are known to induce skin fragility in mice and the depletion of BP180 in keratinocytes. In this study, mAb against NC16A in combination with Fc-binding proteins was found to enhance BP180 depletion. Although mAb against the C-terminus of BP180 does not show pathogenicity in vivo or in vitro, mAb treatment with Fc-binding proteins clearly induced skin fragility in mice and BP180 depletion in keratinocytes. Anti-BP180 mAbs and Fc-binding proteins were colocalized in the cytoplasm and at the basement membrane zone. Cell adhesion strengths were decreased in parallel with BP180 amounts. Clinically, bullous pemphigoid patients had higher rheumatoid factor titers than controls. Anti-BP180 mAb in combination with high-titer rheumatoid factor serum was found to enhance BP180 depletion. Furthermore, saliva from mucous membrane pemphigoid patients contained larger quantities of bacteria and Fc-binding proteins than controls. Our results suggest that Fc-binding proteins (rheumatoid factor or protein G) may enhance the pathogenicity of autoantibodies in pemphigoid diseases. Copyright © 2018 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Surface Plasmon Resonance Investigations of Bioselective Element Based on the Recombinant Protein A for Immunoglobulin Detection

The developed surface plasmon resonance (SPR) biosensor based on the recombinant Staphylococcal protein A with an additional cysteine residue (SPA-Cys) used as a biorecognition component showed a good selectivity and sensitivity for the immunoglobulin detection. The developed biosensor with SPA-Cys-based bioselective element can also be used as a first step of immunosensor creation. The successful immobilization of SPA-Cys on the nanolayer gold sensor surface of the SPR spectrometer was performed. The efficiency of blocking nonspecific sorption sites on the sensor surface with milk proteins, gelatin, BSA, and HSA was studied, and a rather high efficiency of using gelatin was confirmed. The SPR biosensor selectively interacted with IgG and did not interact with the control proteins. The linear dependence of the sensor response on the IgG concentration in the range from 2 to 10 μg/ml was shown. Using the calibration curve, the IgG concentration was measured in the model samples. The determined concentrations are in good agreement (r ² = 0.97) with the given concentration of IgG.

Molecular Modeling of the Interaction of Protein L with Antibodies

Protein L (PpL) is a bacterial protein which is used in the affinity chromatography stage of the production of monoclonal antibodies because of its ability to form high affinity complexes with the light chains of immunoglobulins. In the present work, the binding interfaces between one domain of PpL and antigen-binding fragments (Fab) have been investigated adopting molecular dynamics with the aim of determining the binding contribution of the residues located at the Fab-PpL interface. Because it is known that PpL binds antibodies through two distinct binding sites with different affinities, simulations were performed for both sites to determine interaction free energies to assess the relative binding contribution of the two sites. Mutational studies were then performed only on the dominant binding site. The binding free energy was evaluated with the molecular mechanics Poisson-Boltzmann surface area (MMPBSA) and umbrella sampling/weighted histogram analysis methods. Key residues for the formation of the dominant binding site complex were identified by means of alanine scanning performed both for the Fab and PpL domains. Residues of the light chain of the antibody that contribute most to binding were found to be located between SER7 and VAL13. Four residues from PpL are important for the stability of the complex: PHE839, LYS840, GLU849, and TYR853. Three residues of PpL that do not contribute to the interaction were mutated to histidine (HIS), which changes its protonation state as a function of pH, to find whether this could allow us to control the binding interaction energy. This can be useful in the elution stage of the affinity chromatography purification of antibodies if PpL is used as a ligand. These residues are GLN835, THR836, and ALA837. Molecular dynamics simulations with both protonated and unprotonated HIS were performed to mimic how changing pH may reflect on protein-ligand interaction energies. The MMPBSA approach was used to evaluate the variation of the affinity of the mutated systems with reference to the wild type. Our results show that these mutations could help in disrupting the complex under acidic conditions without impairing the affinity of PpL for the light chains at higher pHs.

Mathematical Model of Serodiagnostic Immunochromatographic Assay

This article describes the mathematical model for an immunochromatographic assay for the detection of specific immunoglobulins against a target antigen (antibodies) in blood/serum (serodiagnosis). The model utilizes an analytical (non-numerical) approach and allows the calculation of the kinetics of immune complexes' formation in a continuous-flow system using commonly available software, such as Microsoft Excel. The developed model could identify the nature of the influence of immunochemical interaction constants and reagent concentrations on the kinetics of the formation of the detected target complex. On the basis of the model, recommendations are developed to decrease the detection limit for an immunochromatographic assay of specific immunoglobulins.

A rapid method for selecting suitable animal species for studying pathogen interactions with plasma protein ligands in vivo

Species tropism constitutes a serious problem for developing relevant animal models of infection. Human pathogens can express virulence factors that show specific selectivity to human proteins, while their affinity for orthologs from other species can vary significantly. Suitable animal species must be used to analyse whether virulence factors are potential targets for drug development. We developed an assay that rapidly predicts applicable animal species for studying virulence factors binding plasma proteins. We used two well‐characterized Staphylococcus aureus proteins, SSL7 and Efb, to develop an ELISA‐based inhibition assay using plasma from different animal species. The interaction between SSL7 and human C5 and the binding of Efb to human fibrinogen and human C3 was studied. Affinity experiments and Western blot analyses were used to validate the assay. Human, monkey and cat plasma interfered with binding of SSL7 to human C5. Binding of Efb to human fibrinogen was blocked in human, monkey, gerbil and pig plasma, while human, monkey, gerbil, rabbit, cat and guinea pig plasma inhibited the binding of Efb to human C3. These results emphasize the importance of choosing correct animal models, and thus, our approach is a rapid and cost‐effective method that can be used to prevent unnecessary animal experiments.

Resistance of Neisseria meningitidis to human serum depends on T and B cell stimulating protein B

The ability of the human bacterial pathogen Neisseria meningitidis to cause invasive disease depends on survival in the bloodstream via mechanisms to suppress complement activation. In this study, we show that prophage genes coding for T and B cell stimulating protein B (TspB), which is an immunoglobulin-binding protein, are essential for survival of N. meningitidis group B strain H44/76 in normal human serum (NHS). H44/76 carries three genes coding for TspB. Mutants having all tspB genes inactivated did not survive in >5% NHS or IgG-depleted NHS. TspB appeared to inhibit IgM-mediated activation of the classical complement pathway, since survival of the tspB triple knockout was the same as that of the parent strain or a complemented mutant when the classical pathway was inactivated by depleting NHS of C1q and was increased in IgM-depleted NHS. A mutant solely carrying tspB gene nmbh4476_0681 was as resistant as the parent strain, while mutants carrying only nmbh4476_0598 or nmbh4476_1698 were killed in ≥5% NHS. The phenotype associated with TspB is formation of a matrix containing TspB, IgG, and DNA that envelopes aggregates of bacteria. Recombinant proteins corresponding to particular subdomains of TspB were found to have human IgG Fcγ- and/or DNA-binding activity, but only TspB derivatives containing both domains formed large, biofilm-like aggregates when combined with purified IgG and DNA. Recognizing the role of TspB in serum resistance may lead to a better understanding of why strains that carry tspB genes are associated with invasive meningococcal disease.

Cloning and expression of a surface immunogenic protein in Streptococcus dysgalactiae isolated from fish and its application in enzyme-linked immunosorbent assays to diagnose S. dysgalactiae infections in fish

Lancefield group C Streptococcus dysgalactiae (GCSD) causes severe necrotic lesions in the caudal peduncle in the genus Seriola farmed in Japan. To develop a sero-diagnostic method for GCSD infection in farmed fish, we attempted to identify a surface immunogenic protein that induces an antibody after infection with GCSD by immunoblot analysis using sera collected from infected fish. A protein obtained from sodium dodecyl sulfate (SDS) extracts of GCSD was identified as S. dysgalactiae surface immunogenic protein (Sd-Sip). Sd-Sip exhibited more than 94% homology with a surface antigen or a hypothetical protein from S. dysgalactiae mammalian isolates at the nucleotide sequence level. Expression of the recombinant Sd-Sip (rSd-Sip) was confirmed by immunoblot analysis, that is, its reactivity to GCSD-infected sera. Antibody detection ELISA using rSd-Sip and their usefulness for diagnosis of GCSD infection were examined. GCSD-infected sera collected from farmed amberjack, Seriola dumerili (Risso), showed strong reaction with immobilized rSd-Sip. Meanwhile, sera immunized by other pathogenic bacteria of fish were showed ELISA values similar to those of non-infected sera. These results of this study suggest that the antibody detection ELISA using rSd-Sip is an effective diagnostic method for GCSD infection in fish.

Surface expression of protein A on magnetosomes and capture of pathogenic bacteria by magnetosome/antibody complexes

Magnetosomes are membrane-enclosed magnetite nanocrystals synthesized by magnetotactic bacteria (MTB). They display chemical purity, narrow size ranges, and species-specific crystal morphologies. Specific transmembrane proteins are sorted to the magnetosome membrane (MM). MamC is the most abundant MM protein of Magnetospirillum gryphiswaldense strain MSR-1. MamF is the second most abundant MM protein of MSR-1 and forms stable oligomers. We expressed staphylococcal protein A (SPA), an immunoglobulin-binding protein from the cell wall of Staphylococcus aureus, on MSR-1 magnetosomes by fusion with MamC or MamF. The resulting recombinant magnetosomes were capable of self-assembly with the Fc region of mammalian antibodies (Abs) and were therefore useful for functionalization of magnetosomes. Recombinant plasmids pBBR-mamC-spa and pBBR-mamF-spa were constructed by fusing spa (the gene that encodes SPA) with mamC and mamF, respectively. Recombinant magnetosomes with surface expression of SPA were generated by introduction of these fusion genes into wild-type MSR-1 or a mamF mutant strain. Studies with a Zeta Potential Analyzer showed that the recombinant magnetosomes had hydrated radii significantly smaller than those of WT magnetosomes and zeta potentials less than −30 mV, indicating that the magnetosome colloids were relatively stable. Observed conjugation efficiencies were as high as 71.24 μg Ab per mg recombinant magnetosomes, and the conjugated Abs retained most of their activity. Numbers of Vibrio parahaemolyticus (a common pathogenic bacterium in seafood) captured by recombinant magnetosome/Ab complexes were measured by real-time fluorescence-based quantitative PCR. One mg of complex was capable of capturing as many as 1.74 × 10⁷Vibrio cells. The surface expression system described here will be useful for design of functionalized magnetosomes from MSR-1 and other MTB.

Recombinant antibodies to histone post-translational modifications

Variability in the quality of antibodies to histone post-translational modifications (PTMs) presents widely recognized hindrance in epigenetics research. Here, by using antibody engineering technologies we produced recombinant antibodies directed to the trimethylated lysine residues of histone H3 with high specificity and affinity and no lot-to-lot variation. These recombinant antibodies performed well in common epigenetics applications, and their high specificity enabled us to identify positive and negative correlations among histone PTMs.

Plasma half-life extension of small recombinant antibodies by fusion to immunoglobulin-binding domains

Many therapeutic proteins possessing a small size are rapidly cleared from circulation. Half-life extension strategies have therefore become increasingly important to improve the pharmacokinetic and pharmacodynamic properties of protein therapeutics. Here, we performed a comparative analysis of the half-life extension properties of various bacterial immunoglobulin-binding domains (IgBDs) derived from Staphylococcus protein A (SpA), Streptococcus protein G (SpG), and Finegoldia (formerly Peptostreptococcus) protein L (PpL). These domains, composed of 50-60 amino acid residues, were fused to the C terminus of a single-chain Fv and a bispecific single-chain diabody, respectively. All fusion proteins were produced in mammalian cells and retained their antigen-binding properties. The half-lives of the antibody molecules were prolonged to varying extents for the different IgBDs. The strongest effects in mice were observed for domain C3 of SpG (SpG(C3)) followed by domains B and D of SpA, suggesting that SpG(C3) is particularly useful to extend the plasma half-life of small proteins.