Differential epitope recognition in the immunodominant staphylococcal antigen A of Staphylococcus aureus by mouse versus human IgG antibodies

Metabolic Profile of the Genome-Reduced Bacillus subtilis Strain IIG-Bs-27-39: An Attractive Chassis for Recombinant Protein Production

The Gram-positive bacterium Bacillus subtilis is extensively used in the industry for the secretory production of proteins with commercial value. To further improve its performance, this microbe has been the subject of extensive genome engineering efforts, especially the removal of large genomic regions that are dispensable or even counterproductive. Here, we present the genome-reduced B. subtilis strain IIG-Bs-27-39, which was obtained through systematic deletion of mobile genetic elements, as well as genes for extracellular proteases, sporulation, flagella formation, and antibiotic production. Different from previously characterized genome-reduced B. subtilis strains, the IIG-Bs-27-39 strain was still able to grow on minimal media. We used this feature to benchmark strain IIG-Bs-27-39 against its parental strain 168 with respect to heterologous protein production and metabolic parameters during bioreactor cultivation. The IIG-Bs-27-39 strain presented superior secretion of difficult-to-produce staphylococcal antigens, as well as higher specific growth rates and biomass yields. At the metabolic level, changes in byproduct formation and internal amino acid pools were observed, whereas energetic parameters such as the ATP yield, ATP/ADP levels, and adenylate energy charge were comparable between the two strains. Intriguingly, we observed a significant increase in the total cellular NADPH level during all tested conditions and increases in the NAD+ and NADP(H) pools during protein production. This indicates that the IIG-Bs-27-39 strain has more energy available for anabolic processes and protein production, thereby providing a link between strain physiology and production performance. On this basis, we conclude that the genome-reduced strain IIG-Bs-27-39 represents an attractive chassis for future biotechnological applications.

Molecular Targets for Antibody-Based Anti-Biofilm Therapy in Infective Endocarditis

Infective endocarditis (IE) is a heart disease caused by the infection of heart valves, majorly caused by Staphilococcus aureus. IE is initiated by bacteria entering the blood circulation in favouring conditions (e.g., during invasive procedures). So far, the conventional antimicrobial strategies based on the usage of antibiotics remain the major intervention for treating IE. Nevertheless, the therapeutic efficacy of antibiotics in IE is limited not only by the bacterial drug resistance, but also by the formation of biofilms, which resist the penetration of antibiotics into bacterial cells. To overcome these drawbacks, the development of anti-biofilm treatments that can expose bacteria and make them more susceptible to the action of antibiotics, therefore resulting in reduced antimicrobial resistance, is urgently required. A series of anti-biofilm strategies have been developed, and this review will focus in particular on the development of anti-biofilm antibodies. Based on the results previously reported in the literature, several potential anti-biofilm targets are discussed, such as bacterial adhesins, biofilm matrix and bacterial toxins, covering their antigenic properties (with the identification of potential promising epitopes), functional mechanisms, as well as the antibodies already developed against these targets and, where feasible, their clinical translation.

Investigation of novel putative immunogenic targets against Staphylococcus aureus using a reverse vaccinology strategy

BACKGROUND

The emergence of methicillin-resistant Staphylococcus aureus (MRSA) and vancomycin-resistant S. aureus (VRSA) strains is a significant public health concern. Considering the high morbidity and mortality of invasive S. aureus infections and multi-drug resistant strains, there is an urgent need for non-antibiotic immune-based approaches to cure these infections. Despite all efforts, vaccine candidates targeting S. aureus failed in human clinical trials, and no approved vaccine is available against this pathogen. Therefore, this study aimed to introduce suitable candidates for immunization against S. aureus using a comprehensive reverse vaccinology approach.

METHODS

In this study, we retrieved putative immunogenic targets from three different levels (literature review, automated reverse vaccinology, and manual reverse vaccinology) and evaluated them using several immunoinformatics analyses including antigenicity, allergenicity, PSI-BLAST to human proteome, physiochemical properties, B-cell, and T-cell epitopes. In the next step, the quartile method scoring was used to the shortlisted proteins. Finally, the molecular docking and immune simulation of immunogenic targets were performed.

RESULTS

This study presents 12 vaccine candidates, including three enzymatic proteins (WP_000222271.1, WP_001170274, and WP_000827736.1), three cell wall-associated proteins (WP_001125631.1, WP_000731642, and WP_000751265.1), two hemolysins (WP_000594517.1, and WP_000916697.1), one secretion involved protein (WP_000725226.1), one heme‑iron binding protein (WP_001041573.1), one superantigen like protein (WP_000668994.1) and one hypothetical proteins (WP_000737711.1).

CONCLUSION

Through quartile scoring method, immune simulation and molecular docking, four promising targets including lytic transglycosylase IsaA, HlgA, secretory antigen precursor SsaA, and heme uptake protein IsdB were selected as the shortlisted proteins. It seems that a polarized immunization (Th1/Th17) response is needed for protection against this bacterium. An optimized formulation based on these putative immunogenic proteins and a wisely adjuvant selection may drive the immune system toward a full protection.

Differential binding of human and murine IgGs to catalytic and cell wall binding domains of Staphylococcus aureus peptidoglycan hydrolases

Staphylococcus aureus is an opportunistic pathogen causing high morbidity and mortality. Since multi-drug resistant S. aureus lineages are nowadays omnipresent, alternative tools for preventive or therapeutic interventions, like immunotherapy, are urgently needed. However, there are currently no vaccines against S. aureus. Surface-exposed and secreted proteins are regarded as potential targets for immunization against S. aureus infections. Yet, many potential staphylococcal antigens of this category do not elicit protective immune responses. To obtain a better understanding of this problem, we compared the binding of serum IgGs from healthy human volunteers, highly S. aureus-colonized patients with the genetic blistering disease epidermolysis bullosa (EB), or immunized mice to the purified S. aureus peptidoglycan hydrolases Sle1, Aly and LytM and their different domains. The results show that the most abundant serum IgGs from humans and immunized mice target the cell wall-binding domain of Sle1, and the catalytic domains of Aly and LytM. Interestingly, in a murine infection model, these particular IgGs were not protective against S. aureus bacteremia. In contrast, relatively less abundant IgGs against the catalytic domain of Sle1 and the N-terminal domains of Aly and LytM were almost exclusively detected in sera from EB patients and healthy volunteers. These latter IgGs may contribute to the protection against staphylococcal infections, as previous studies suggest that serum IgGs protect EB patients against severe S. aureus infection. Together, these observations focus attention on the use of particular protein domains for vaccination to direct potentially protective immune responses towards the most promising epitopes within staphylococcal antigens.

Bactericidal fully human single-chain fragment variable antibodies protect mice against methicillin-resistant Staphylococcus aureus bacteraemia

OBJECTIVES

The increasing prevalence of antibiotic-resistant Staphylococcus aureus, besides the inadequate numbers of effective antibiotics, emphasises the need to find new therapeutic agents against this lethal pathogen.

METHODS

In this study, to obtain antibody fragments against S. aureus, a human single-chain fragment variable (scFv) library was enriched against living methicillin-resistant S. aureus (MRSA) cells, grown in three different conditions, that is human peripheral blood mononuclear cells with plasma, whole blood and biofilm. The antibacterial activity of scFvs was evaluated by the growth inhibition assay in vitro. Furthermore, the therapeutic efficacy of anti-S. aureus scFvs was appraised in a mouse model of bacteraemia.

RESULTS

Three scFv antibodies, that is MEH63, MEH158 and MEH183, with unique sequences, were found, which exhibited significant binding to S. aureus and reduced the viability of S. aureus in in vitro inhibition assays. Based on the results, MEH63, MEH158 and MEH183, in addition to their combination, could prolong the survival rate, reduce the bacterial burden in the blood and prevent inflammation and tissue destruction in the kidneys and spleen of mice with MRSA bacteraemia compared with the vehicle group (treated with normal saline).

CONCLUSION

The combination therapy with anti-S. aureus scFvs and conventional antibiotics might shed light on the treatment of patients with S. aureus infections.

Evasion of Immunological Memory by S. aureus Infection: Implications for Vaccine Design

Recurrent S. aureus infections are common, suggesting that natural immune responses are not protective. All candidate vaccines tested thus far have failed to protect against S. aureus infections, highlighting an urgent need to better understand the mechanisms by which the bacterium interacts with the host immune system to evade or prevent protective immunity. Although there is evidence in murine models that both cellular and humoral immune responses are important for protection against S. aureus, human studies suggest that T cells are critical in determining susceptibility to infection. This review will use an “anatomic” approach to systematically outline the steps necessary in generating a T cell-mediated immune response against S. aureus. Through the processes of bacterial uptake by antigen presenting cells, processing and presentation of antigens to T cells, and differentiation and proliferation of memory and effector T cell subsets, the ability of S. aureus to evade or inhibit each step of the T-cell mediated response will be reviewed. We hypothesize that these interactions result in the redirection of immune responses away from protective antigens, thereby precluding the establishment of “natural” memory and potentially inhibiting the efficacy of vaccination. It is anticipated that this approach will reveal important implications for future design of vaccines to prevent these infections.

Fighting Staphylococcus aureus infections with light and photoimmunoconjugates

Infections caused by multidrug-resistant Staphylococcus aureus, especially methicillin-resistant S. aureus (MRSA), are responsible for high mortality and morbidity worldwide. Resistant lineages were previously confined to hospitals but are now also causing infections among healthy individuals in the community. It is therefore imperative to explore therapeutic avenues that are less prone to raise drug resistance compared with today’s antibiotics. An opportunity to achieve this ambitious goal could be provided by targeted antimicrobial photodynamic therapy (aPDT), which relies on the combination of a bacteria-specific targeting agent and light-induced generation of ROS by an appropriate photosensitizer. Here, we conjugated the near-infrared photosensitizer IRDye700DX to a fully human mAb, specific for the invariantly expressed staphylococcal antigen immunodominant staphylococcal antigen A (IsaA). The resulting immunoconjugate 1D9-700DX was characterized biochemically and in preclinical infection models. As demonstrated in vitro, in vivo, and in a human postmortem orthopedic implant infection model, targeted aPDT with 1D9-700DX is highly effective. Importantly, combined with the nontoxic aPDT-enhancing agent potassium iodide, 1D9-700DX overcomes the antioxidant properties of human plasma and fully eradicates high titers of MRSA. We show that the developed immunoconjugate 1D9-700DX targets MRSA and kills it upon illumination with red light, without causing collateral damage to human cells.

An immunoconjugate for targeted photodynamic therapy of Staphylococcus aureus infections kills MRSA with high efficacy upon illumination with red light.

Influenza hemagglutinin antigenic distance measures capture trends in HAI differences and infection outcomes, but are not suitable predictive tools

Vaccination is the most effective method to combat influenza. Vaccine effectiveness is influenced by the antigenic distance between the vaccine strain and the actual circulating virus. Amino acid sequence based methods of quantifying the antigenic distance were designed to predict influenza vaccine effectiveness in humans. The use of these antigenic distance measures has been proposed as an additive method for seasonal vaccine selection. In this report, several antigenic distance measures were evaluated as predictors of hemagglutination inhibition titer differences and clinical outcomes following influenza vaccination or infection in mice or ferrets. The antigenic distance measures described the increasing trend in the change of HAI titer, lung viral titer and percent weight loss in mice and ferrets. However, the variability of outcome variables produced wide prediction intervals for any given antigenic distance value. The amino acid substitution based antigenic distance measures were no better predictors of viral load and weight loss than HAI titer differences, the current predictive measure of immunological correlate of protection for clinical signs after challenge.

A Kayvirus Distant Homolog of Staphylococcal Virulence Determinants and VISA Biomarker Is a Phage Lytic Enzyme

Staphylococcal bacteriophages of the Kayvirus genus are candidates for therapeutic applications. One of their proteins, Tgl, is slightly similar to two staphylococcal virulence factors, secreted autolysins of lytic transglycosylase motifs IsaA and SceD. We show that Tgl is a lytic enzyme secreted by the bacterial transport system and localizes to cell peripheries like IsaA and SceD. It causes lysis of E. coli cells expressing the cloned tgl gene, but could be overproduced when depleted of signal peptide. S. aureus cells producing Tgl lysed in the presence of nisin, which mimics the action of phage holin. In vitro, Tgl protein was able to destroy S. aureus cell walls. The production of Tgl decreased S. aureus tolerance to vancomycin, unlike the production of SceD, which is associated with decreased sensitivity to vancomycin. In the genomes of kayviruses, the tgl gene is located a few genes away from the lysK gene, encoding the major endolysin. While lysK is a late phage gene, tgl can be transcribed by a host RNA polymerase, like phage early genes. Taken together, our data indicate that tgl belongs to the kayvirus lytic module and encodes an additional endolysin that can act in concert with LysK in cell lysis.

Exoproteome Heterogeneity among Closely Related Staphylococcus aureus t437 Isolates and Possible Implications for Virulence

Staphylococcus aureus with spa-type t437 has been identified as a predominant community-associated methicillin-resistant S. aureus clone from Asia, which is also encountered in Europe. Molecular typing has previously shown that t437 isolates are highly similar regardless of geographical regions or host environments. The present study was aimed at assessing to what extent this high similarity is actually reflected in the production of secreted virulence factors. We therefore profiled the extracellular proteome, representing the main reservoir of virulence factors, of 20 representative clinical isolates by mass spectrometry. The results show that these isolates can be divided into three groups and nine subgroups based on exoproteome abundance signatures. This implies that S. aureus t437 isolates show substantial exoproteome heterogeneity. Nonetheless, 30 highly conserved extracellular proteins, of which about 50% have a predicted role in pathogenesis, were dominantly identified. To approximate the virulence of the 20 investigated isolates, we employed infection models based on Galleria mellonella and HeLa cells. The results show that the grouping of clinical isolates based on their exoproteome profile can be related to virulence. We consider this outcome important as our approach provides a tool to pinpoint differences in virulence among seemingly highly similar clinical isolates of S. aureus.

Antibody-Based Agents in the Management of Antibiotic-Resistant Staphylococcus aureus Diseases

Staphylococcus aureus is a human pathogen that can cause a wide spectrum of diseases, including sepsis, pneumonia, arthritis, and endocarditis. Ineffective treatment of a number of staphylococcal infections with antibiotics is due to the development and spread of antibiotic-resistant strains following decades of antibiotic usage. This has generated renewed interest within the scientific community in alternative therapeutic agents, such as anti-S. aureus antibodies. Although the role of antibodies in the management of S. aureus diseases is controversial, the success of this pathogen in neutralizing humoral immunity clearly indicates that antibodies offer the host extensive protection. In this review, we report an update on efforts to develop antibody-based agents, particularly monoclonal antibodies, and their therapeutic potential in the passive immunization approach to the treatment and prevention of S. aureus infections.

Human antibody responses against non-covalently cell wall-bound Staphylococcus aureus proteins

Human antibody responses to pathogens, like Staphylococcus aureus, are important indicators for in vivo expression and immunogenicity of particular bacterial components. Accordingly, comparing the antibody responses to S. aureus components may serve to predict their potential applicability as antigens for vaccination. The present study was aimed at assessing immunoglobulin G (IgG) responses elicited by non-covalently cell surface-bound proteins of S. aureus, which thus far received relatively little attention. To this end, we applied plasma samples from patients with the genetic blistering disease epidermolysis bullosa (EB) and healthy S. aureus carriers. Of note, wounds of EB patients are highly colonized with S. aureus and accordingly these patients are more seriously exposed to staphylococcal antigens than healthy individuals. Ten non-covalently cell surface-bound proteins of S. aureus, namely Atl, Eap, Efb, EMP, IsaA, LukG, LukH, SA0710, Sle1 and SsaA2, were selected by bioinformatics and biochemical approaches. These antigens were recombinantly expressed, purified and tested for specific IgG responses using human plasma. We show that high exposure of EB patients to S. aureus is mirrored by elevated IgG levels against all tested non-covalently cell wall-bound staphylococcal antigens. This implies that these S. aureus cell surface proteins are prime targets for the human immune system.