Regulation of Staphylococcus aureus MntC expression and its role in response to oxidative stress

Bacterial single-cell RNA sequencing captures biofilm transcriptional heterogeneity and differential responses to immune pressure

Biofilm formation is an important mechanism of survival and persistence for many bacterial pathogens. These multicellular communities contain subpopulations of cells that display metabolic and transcriptional diversity along with recalcitrance to antibiotics and host immune defenses. Here, we present an optimized bacterial single-cell RNA sequencing method, BaSSSh-seq, to study Staphylococcus aureus diversity during biofilm growth and transcriptional adaptations following immune cell exposure. BaSSSh-seq captures extensive transcriptional heterogeneity during biofilm compared to planktonic growth. We quantify and visualize transcriptional regulatory networks across heterogeneous biofilm subpopulations and identify gene sets that are associated with a trajectory from planktonic to biofilm growth. BaSSSh-seq also detects alterations in biofilm metabolism, stress response, and virulence induced by distinct immune cell populations. This work facilitates the exploration of biofilm dynamics at single-cell resolution, unlocking the potential for identifying biofilm adaptations to environmental signals and immune pressure.

Bacterial single-cell RNA sequencing captures biofilm transcriptional heterogeneity and differential responses to immune pressure

Biofilm formation is an important mechanism of survival and persistence for many bacterial pathogens. These multicellular communities contain subpopulations of cells that display vast metabolic and transcriptional diversity along with high recalcitrance to antibiotics and host immune defenses. Investigating the complex heterogeneity within biofilm has been hindered by the lack of a sensitive and high-throughput method to assess stochastic transcriptional activity and regulation between bacterial subpopulations, which requires single-cell resolution. We have developed an optimized bacterial single-cell RNA sequencing method, BaSSSh-seq, to study Staphylococcus aureus diversity during biofilm growth and transcriptional adaptations following immune cell exposure. We validated the ability of BaSSSh-seq to capture extensive transcriptional heterogeneity during biofilm compared to planktonic growth. Application of new computational tools revealed transcriptional regulatory networks across the heterogeneous biofilm subpopulations and identification of gene sets that were associated with a trajectory from planktonic to biofilm growth. BaSSSh-seq also detected alterations in biofilm metabolism, stress response, and virulence that were tailored to distinct immune cell populations. This work provides an innovative platform to explore biofilm dynamics at single-cell resolution, unlocking the potential for identifying biofilm adaptations to environmental signals and immune pressure.

Analysis of the Manganese and MntR Regulon in Corynebacterium diphtheriae

Corynebacterium diphtheriae is the causative agent of a severe respiratory disease in humans. The bacterial systems required for infection are poorly understood, but the acquisition of metals such as manganese (Mn) is likely critical for host colonization. MntR is a Mn-dependent transcriptional regulator in C. diphtheriae that represses the expression of the mntABCD genes, which encode a putative ABC metal transporter. However, other targets of Mn and MntR regulation in C. diphtheriae have not been identified. In this study, we use comparisons between the gene expression profiles of wild-type C. diphtheriae strain 1737 grown without or with Mn supplementation and comparisons of gene expression between wild-type and an mntR deletion mutant to characterize the C. diphtheriae Mn and MntR regulon. MntR was observed to both repress and induce various target genes in a Mn-dependent manner. Genes induced by MntR include the Mn-superoxide dismutase, sodA, and the putative ABC transporter locus, iutABCD. DNA binding studies showed that MntR interacts with the promoter regions for several genes identified in the expression study, and a 17-bp consensus MntR DNA binding site was identified. We found that an mntR mutant displayed increased sensitivity to Mn and cadmium that could be alleviated by the additional deletion of the mntABCD transport locus, providing evidence that the MntABCD transporter functions as a Mn uptake system in C. diphtheriae. The findings in this study further our understanding of metal uptake systems and global metal regulatory networks in this important human pathogen. Importance Mechanisms for metal scavenging are critical to the survival and success of bacterial pathogens, including Corynebacterium diphtheriae. Metal import systems in pathogenic bacteria have been studied as possible vaccine components due to high conservation, critical functionality, and surface localization. In this study, we expand our understanding of the genes controlled by the global manganese regulator, MntR. We determined a role for the MntABCD transporter in manganese import using evidence from manganese and cadmium toxicity assays. Understanding the nutritional requirements of C. diphtheriae and the tools used to acquire essential metals will aid in the development of future vaccines.

Performance of a Four-Antigen Staphylococcus aureus Vaccine in Preclinical Models of Invasive S. aureus Disease

A Staphylococcus aureus four-antigen vaccine (SA4Ag) was designed for the prevention of invasive disease in surgical patients. The vaccine is composed of capsular polysaccharide type 5 and type 8 CRM₁₉₇ conjugates, a clumping factor A mutant (Y338A-ClfA) and manganese transporter subunit C (MntC). S. aureus pathogenicity is characterized by an ability to rapidly adapt to the host environment during infection, which can progress from a local infection to sepsis and invasion of distant organs. To test the protective capacity of the SA4Ag vaccine against progressive disease stages of an invasive S. aureus infection, a deep tissue infection mouse model, a bacteremia mouse model, a pyelonephritis model, and a rat model of infectious endocarditis were utilized. SA4Ag vaccination significantly reduced the bacterial burden in deep tissue infection, in bacteremia, and in the pyelonephritis model. Complete prevention of infection was demonstrated in a clinically relevant endocarditis model. Unfortunately, these positive preclinical findings with SA4Ag did not prove the clinical utility of SA4Ag in the prevention of surgery-associated invasive S. aureus infection.

Staphylococcus aureus lacking a functional MntABC manganese import system has increased resistance to copper

S. aureus USA300 isolates utilize the copBL and copAZ gene products to prevent Cu intoxication. We created and examined a ΔcopAZ ΔcopBL mutant strain (cop-). The cop- strain was sensitive to Cu and accumulated intracellular Cu. We screened a transposon (Tn) mutant library in the cop- background and isolated strains with Tn insertions in the mntABC operon that permitted growth in the presence of Cu. The mutations were in mntA and they were recessive. Under the growth conditions utilized, MntABC functioned in manganese (Mn) import. When cultured with Cu, strains containing a mntA::Tn accumulated less Cu than the parent strain. Mn(II) supplementation improved growth when cop- was cultured with Cu and this phenotype was dependent upon the presence of MntR, which is a repressor of mntABC transcription. A ΔmntR strain had an increased Cu load and decreased growth in the presence of Cu, which was abrogated by the introduction of mntA::Tn. Over-expression of mntABC increased cellular Cu load and sensitivity to Cu. The presence of a mntA::Tn mutation protected iron-sulfur (FeS) enzymes from inactivation by Cu. The data presented are consistent with a model wherein defective MntABC results in decreased cellular Cu accumulation and protection to FeS enzymes from Cu poisoning.

Persistence of Immune Responses Through 36 Months in Healthy Adults After Vaccination With a Novel Staphylococcus aureus 4-Antigen Vaccine (SA4Ag)

Background

Staphylococcus aureus causes serious health care- and community-associated disease, requiring improved preventive measures such as vaccines. The investigational S. aureus 4-antigen vaccine (SA4Ag), comprising capsular polysaccharide serotypes 5 and 8 (CP5 and CP8) conjugated to CRM₁₉₇, recombinant mutant clumping factor A (rmClfA), and recombinant manganese transporter protein C (rP305A or rMntC), was well tolerated, inducing robust functional immune responses to all 4 antigens through 12 months postvaccination. This is a serological extension study through 36 months postvaccination.

Methods

In 2 previous studies, healthy adults received SA4Ag, SA3Ag (without rMntC), or placebo; serology was also assessed at ~24 and ~36 months postvaccination. Functional immune responses (antibody responses that facilitate killing of S. aureus or neutralize S. aureus virulence mechanisms) were assessed with opsonophagocytic activity killing assays (CP5 or CP8) and a fibrinogen-binding inhibition assay (ClfA). A competitive Luminex immunoassay assessed ClfA and rMntC responses. Adverse events within 48 hours of blood draw were recorded.

Results

Four hundred forty subjects (18-64 years old, 255; 65-85 years old, 185) were enrolled. At 24 and 36 months postvaccination, subjects receiving SA4Ag had substantially higher geometric mean titers (GMTs) for CP5, CP8, and ClfA vs baseline; geometric mean fold rises (GMFRs) from baseline to month 36 were 2.7-8.1. For rMntC, 36-month GMTs declined from peak levels but remained above baseline for all SA4Ag groups; GMFRs from baseline to month 36 were 1.8 and 1.5 in the younger and older cohorts, respectively.

Conclusions

Persistent functional immune responses to S. aureus antigens were observed through 36 months in healthy adults.

ClinicalTrialsgov

NCT01643941 and NCT01364571.

RsaC sRNA modulates the oxidative stress response of Staphylococcus aureus during manganese starvation

The human opportunistic pathogen Staphylococcus aureus produces numerous small regulatory RNAs (sRNAs) for which functions are still poorly understood. Here, we focused on an atypical and large sRNA called RsaC. Its length varies between different isolates due to the presence of repeated sequences at the 5′ end while its 3′ part is structurally independent and highly conserved. Using MS2-affinity purification coupled with RNA sequencing (MAPS) and quantitative differential proteomics, sodA mRNA was identified as a primary target of RsaC sRNA. SodA is a Mn-dependent superoxide dismutase involved in oxidative stress response. Remarkably, rsaC gene is co-transcribed with the major manganese ABC transporter MntABC and, consequently, RsaC is mainly produced in response to Mn starvation. This 3′UTR-derived sRNA is released from mntABC-RsaC precursor after cleavage by RNase III. The mature and stable form of RsaC inhibits the synthesis of the Mn-containing enzyme SodA synthesis and favors the oxidative stress response mediated by SodM, an alternative SOD enzyme using either Mn or Fe as co-factor. In addition, other putative targets of RsaC are involved in oxidative stress (ROS and NOS) and metal homeostasis (Fe and Zn). Consequently, RsaC may balance two interconnected defensive responses, i.e. oxidative stress and metal-dependent nutritional immunity.

Associations of pathogen-specific and host-specific characteristics with disease outcome in patients with Staphylococcus aureus bacteremic pneumonia

Objective

To understand the relationships of Staphylococcus aureus (SA) bacteremic pneumonia (SABP) outcome with patient‐specific and SA‐specific variables.

Methods

We analysed SA bloodstream isolates and matching sera in SABP patients by sequencing SA isolates (n = 50) and measuring in vitro AT production, haemolytic activity and expression of ClfA and ClfB. Controls were sera from gram‐negative bacteremia patients with or without pneumonia and uninfected subjects. Levels of IgGs, IgMs and neutralizing antibodies (NAbs) against SA antigens were quantified and analysed by one‐way ANOVA. Associations of patient outcomes with patient variables, antibody levels and isolate characteristics were evaluated by univariate and multivariate logistic regression analyses.

Results

SABP patients had higher levels of IgGs against eight virulence factors and anti‐alpha toxin (AT) NAbs than uninfected controls. Levels of IgG against AT and IgMs against ClfA, FnbpA and SdrC were higher in clinically cured SABP patients than in clinical failures. Anti‐LukAB NAb levels were elevated in all cohorts. Increased odds of cure correlated with higher haemolytic activity of SA strains, longer time between surgery and bacteremia (> 30 days), longer duration of antibiotic therapy, lower acute physiology and total APACHE II scores, lack of persistent fever for > 72 h and higher levels of antibodies against AT (IgG), ClfA (IgM), FnbpA (IgM) and SdrC (IgM).

Discussion

Limitations included the cross‐sectional observational nature of the study, small sample size and inability to measure antibody levels against all SA virulence factors.

Conclusion

Our results suggest that SABP patients may benefit from immunotherapy targeting multiple SA antigens.

Molecular epidemiology and expression of capsular polysaccharides in Staphylococcus aureus clinical isolates in the United States

Staphylococcus aureus capsular polysaccharides (CP) are important virulence factors under evaluation as vaccine antigens. Clinical S. aureus isolates have the biosynthetic capability to express either CP5 or CP8 and an understanding of the relationship between CP genotype/phenotype and S. aureus epidemiology is valuable. Using whole genome sequencing, the clonal relatedness and CP genotype were evaluated for disease-associated S. aureus isolates selected from the Tigecycline Evaluation and Surveillance Trial (T.E.S.T) to represent different geographic regions in the United States (US) during 2004 and 2009–10. Thirteen prominent clonal complexes (CC) were identified, with CC5, 8, 30 and 45 representing >80% of disease isolates. CC5 and CC8 isolates were CP type 5 and, CC30 and CC45 isolates were CP type 8. Representative isolates from prevalent CC were susceptible to in vitro opsonophagocytic killing elicited by anti-CP antibodies, demonstrating that susceptibility to opsonic killing is not linked to the genetic lineage. However, as not all S. aureus isolates may express CP, isolates representing the diversity of disease isolates were assessed for CP production. While approximately 35% of isolates (primarily CC8) did not express CP in vitro, CP expression could be clearly demonstrated in vivo for 77% of a subset of these isolates (n = 20) despite the presence of mutations within the capsule operon. CP expression in vivo was also confirmed indirectly by measuring an increase in CP specific antibodies in mice infected with CP5 or CP8 isolates. Detection of antigen expression in vivo in relevant disease states is important to support the inclusion of these antigens in vaccines. Our findings confirm the validity of CP as vaccine targets and the potential of CP-based vaccines to contribute to S. aureus disease prevention.

Synergy between Nutritional Immunity and Independent Host Defenses Contributes to the Importance of the MntABC Manganese Transporter during Staphylococcus aureus Infection

During infection, the host utilizes a diverse array of processes to combat invaders, including the restriction of availability of essential nutrients such as manganese. Similarly to many other pathogens, Staphylococcus aureus possesses two manganese importers, MntH and MntABC. Several infection models have revealed a critical role for MntABC during staphylococcal infection. However, culture-based studies have suggested parity between the two transporters when cells are resisting manganese starvation imposed by the manganese binding immune effector calprotectin. In this investigation, initial elemental analysis revealed that MntABC is the primary transporter responsible for obtaining manganese in culture in the presence of calprotectin. MntABC was also necessary to maintain wild-type levels of manganese-dependent superoxide dismutase activity in the presence of calprotectin. Building on this framework, we investigated if MntABC enabled S. aureus to resist the synergistic actions of nutritional immunity and other host defenses. This analysis revealed that MntABC critically contributes to staphylococcal growth when S. aureus is subjected to manganese limitations and exposed to oxidative stress. This transporter was also important for growth in manganese-limited environments when S. aureus was forced to consume glucose as an energy source, which occurs when it encounters nitric oxide. MntABC also expanded the pH range conducive for S. aureus growth under conditions of manganese scarcity. Collectively, the data presented in this work provide a robust molecular basis for the crucial role of MntABC in staphylococcal virulence. Further, this work highlights the importance of synergy between host defenses and the necessity of evaluating the contribution of virulence factors to pathogenesis in the presence of multiple stressors.

MntC-Dependent Manganese Transport Is Essential for Staphylococcus aureus Oxidative Stress Resistance and Virulence

Work outlined in this report demonstrated that MntC-dependent manganese transport is required for S. aureus virulence. These study results support the model that MntC-specific antibodies elicited by a vaccine have the potential to disrupt S. aureus manganese transport and thus abrogate to its virulence.

Staphylococcus aureus is a human pathogen that has developed several approaches to evade the immune system, including a strategy to resist oxidative killing by phagocytes. This resistance is mediated by production of superoxide dismutase (SOD) enzymes which use manganese as a cofactor. S. aureus encodes two manganese ion transporters, MntABC and MntH, and a possible Nramp family manganese transporter, exemplified by S. aureus N315 SA1432. Their relative contributions to manganese transport have not been well defined in clinically relevant isolates. For this purpose, insertional inactivation mutations were introduced into mntC, mntH, and SA1432 individually and in combination. mntC was necessary for full resistance to methyl viologen, a compound that generates intracellular free radicals. In contrast, strains with an intact mntH gene had a minimal increase in resistance that was revealed only in mntC strains, and no change was observed upon mutation of SA1432 in strains lacking both mntC and mntH. Similarly, MntC alone was required for high cellular SOD activity. In addition, mntC strains were attenuated in a murine sepsis model. To further link these observations to manganese transport, an S. aureus MntC protein lacking manganese binding activity was designed, expressed, and purified. While circular dichroism experiments demonstrated that the secondary and tertiary structures of this protein were unaltered, a defect in manganese binding was confirmed by isothermal titration calorimetry. Unlike complementation with wild-type mntC, introduction of the manganese-binding defective allele into the chromosome of an mntC strain did not restore resistance to oxidative stress or virulence. Collectively, these results underscore the importance of MntC-dependent manganese transport in S. aureus oxidative stress resistance and virulence.

IMPORTANCE Work outlined in this report demonstrated that MntC-dependent manganese transport is required for S. aureus virulence. These study results support the model that MntC-specific antibodies elicited by a vaccine have the potential to disrupt S. aureus manganese transport and thus abrogate to its virulence.

Iron and Zinc Regulate Expression of a Putative ABC Metal Transporter in Corynebacterium diphtheriae

Corynebacterium diphtheriae, a Gram-positive, aerobic bacterium, is the causative agent of diphtheria and cutaneous infections. While mechanisms required for heme iron acquisition are well known in C. diphtheriae, systems involved in the acquisition of other metals such as zinc and manganese remain poorly characterized. In this study, we identified a genetic region that encodes an ABC-type transporter (iutBCD) and that is flanked by two genes (iutA and iutE) encoding putative substrate binding proteins of the cluster 9 family, a related group of transporters associated primarily with the import of Mn and Zn. We showed that IutA and IutE are both membrane proteins with comparable Mn and Zn binding abilities. We demonstrated that the iutABCD genes are cotranscribed and repressed in response to iron by the iron-responsive repressor DtxR. Transcription of iutE was positively regulated in response to iron availability in a DtxR-dependent manner and was repressed in response to Zn by the Zn-dependent repressor Zur. Electrophoretic mobility shift assays showed that DtxR does not bind to the iutE upstream region, which indicates that DtxR regulation of iutE is indirect and that other regulatory factors controlled by DtxR are likely responsible for the iron-responsive regulation. Analysis of the iutE promoter region identified a 50-bp sequence at the 3' end of the iutD gene that is required for the DtxR-dependent and iron-responsive activation of the iutE gene. These findings indicate that transcription of iutE is controlled by a complex mechanism that involves multiple regulatory factors whose activity is impacted by both Zn and Fe.IMPORTANCE Vaccination against diphtheria prevents toxin-related symptoms but does not inhibit bacterial colonization of the human host by the bacterium. Thus, Corynebacterium diphtheriae remains an important human pathogen that poses a significant health risk to unvaccinated individuals. The ability to acquire iron, zinc, and manganese is critical to the pathogenesis of many disease-causing organisms. Here, we describe a gene cluster in C. diphtheriae that encodes a metal importer that is homologous to broadly distributed metal transport systems, some with important roles in virulence in other bacterial pathogens. Two metal binding components of the gene cluster encode surface exposed proteins, and studies of such proteins may guide the development of second-generation vaccines for C. diphtheriae.

Targeting fundamental pathways to disrupt Staphylococcus aureus survival: clinical implications of recent discoveries

The emergence of community-associated methicillin-resistant Staphylococcus aureus during the past decade along with an impending shortage of effective antistaphylococcal antibiotics have fueled impressive advances in our understanding of how S. aureus overcomes the host environment to establish infection. Backed by recent technologic advances, studies have uncovered elaborate metabolic, nutritional, and virulence strategies deployed by S. aureus to survive the restrictive and hostile environment imposed by the host, leading to a plethora of promising antimicrobial approaches that have potential to remedy the antibiotic resistance crisis. In this Review, we highlight some of the critical and recently elucidated bacterial strategies that are potentially amenable to intervention, discuss their relevance to human diseases, and address the translational challenges posed by current animal models.

Protective humoral and CD4+ T cellular immune responses of Staphylococcus aureus vaccine MntC in a murine peritonitis model

Staphylococcus aureus can cause different types of diseases from mild skin infections to life-threatening sepsis worldwide. Owing to the emergence and transmission of multidrug-resistant strains, developing an impactful immunotherapy especially vaccine control approach against S. aureus infections is increasingly encouraged and supported. S. aureus manganese transport protein C (MntC), which is a highly-conserved cell surface protein, can elicit protective immunity against S. aureus and Staphylococcus epidermidis. In this study, we evaluated the humoral immune response and CD4⁺ T cell-mediated immune responses in a mouse peritonitis model. The results showed that MntC-specific antibodies conferred an essential protection for mice to reduce invasion of S. aureus, which was corroborated via the opsonophagocytic killing assay and passive immunization experiment in mice, and moreover MntC-induced Th17 played a remarkable part in preventing S. aureus infection since the MntC-induced protective immunity decreased after neutralization of IL-17 by antibody in vivo and the Th17 adoptive transferred-mice could partly resist S. aureus challenge. In conclusion, we considered that the MntC-specific antibodies and MntC-specific Th17 cells play cooperative roles in the prevention of S. aureus infection.

Safety of Staphylococcus aureus four-antigen and three-antigen vaccines in healthy adults: A meta-analysis of randomized controlled trials

PURPOSE

Two new Staphylococcus aureus vaccines, S. aureus four-antigen (SA4Ag) and three-antigen (SA3Ag) vaccines, have good immunogenicity and tolerance. However, the safety of these vaccines is worth exploring. Here, we performed a meta-analysis to investigate the safety of SA3Ag and SA4Ag by evaluating systemic and local adverse events.

METHODS

The Medline, EMBASE, and Cochrane databases were searched for randomized clinical trials confirming the safety of SA4Ag and SA3Ag. Two investigators independently selected suitable trials, assessed trial quality, and extracted data.

RESULTS

Three studies comprising a total of 1,148 participants were included in this review. The two S. aureus vaccines did not increase systemic adverse events (relative ratio 1.1 [95% confidence interval 0.98, 1.24]), but increased the incidence of local adverse events (2.89 [2.15, 3.90]). However, the incidence of severe local adverse events (4.06 [0.78, 21.24]) did not rise significantly.

CONCLUSIONS

SA4Ag and SA3Ag have acceptable safety in adults.

Identification and characterization of CD4+ T cell epitopes on manganese transport protein C of Staphylococcus aureus

Manganese transport protein C (MntC) of Staphylococcus aureus represents an excellent vaccine-candidate antigen. The important role of CD4⁺ T cells in effective immunity against S. aureus infection was shown; however, CD4⁺ T cell-specific epitopes on S. aureus MntC have not been well identified. Here, we used bioinformatics prediction algorithms to evaluate and identify nine candidate epitopes within MntC. Our results showed that peptide M8 emulsified in Freund's adjuvant induced a much higher cell-proliferation rate as compared with controls. Additionally, CD4⁺ T cells stimulated with peptide M8 secreted significantly higher levels of interferon-γ and interleukin-17A. These results suggested that peptide M8 represented an H-2d (I-E)-restricted Th17-specific epitope.

Vaccine development to prevent Staphylococcus aureus surgical-site infections

BACKGROUND

Staphylococcus aureus surgical-site infections (SSIs) are a major cause of poor health outcomes, including mortality, across surgical specialties. Despite current advances as a result of preventive interventions, the disease burden of S. aureus SSI remains high, and increasing antibiotic resistance continues to be a concern. Prophylactic S. aureus vaccines may represent an opportunity to prevent SSI.

METHODS

A review of SSI pathophysiology was undertaken in the context of evaluating new approaches to developing a prophylactic vaccine to prevent S. aureus SSI.

RESULTS

A prophylactic vaccine ideally would provide protective immunity at the time of the surgical incision to prevent initiation and progression of infection. Although the pathogenicity of S. aureus is attributed to many virulence factors, previous attempts to develop S. aureus vaccines targeted only a single virulence mechanism. The field has now moved towards multiple-antigen vaccine strategies, and promising results have been observed in early-phase clinical studies that supported the recent initiation of an efficacy trial to prevent SSI.

CONCLUSION

There is an unmet medical need for novel S. aureus SSI prevention measures. Advances in understanding of S. aureus SSI pathophysiology could lead to the development of effective and safe prophylactic multiple-antigen vaccines to prevent S. aureus SSI.

Safety, tolerability, and immunogenicity of a 4-antigen Staphylococcus aureus vaccine (SA4Ag): Results from a first-in-human randomised, placebo-controlled phase 1/2 study

BACKGROUND

A prophylactic Staphylococcus aureus four-antigen vaccine (SA4Ag) is under development for prevention of invasive S. aureus disease. A preliminary S. aureus three-antigen vaccine (SA3Ag) was reformulated to include a novel manganese transporter protein (MntC or rP305A). This study describes the first-in-human dose-finding, safety, and immunogenicity results for SA4Ag.

METHODS

In this double-blind, sponsor-unblind, placebo-controlled, phase 1/2 study, 454 healthy adults aged 18-64years were randomised to receive a single dose of one of three formulations of SA4Ag with escalating dose levels of rP305A or placebo. Functional immune responses were measured using opsonophagocytic activity (OPA) killing and fibrinogen-binding inhibition (FBI) assays; antigen-specific immunogenicity was assessed using a four-plex competitive Luminex® immunoassay (cLIA).

RESULTS

A high proportion of SA4Ag recipients met the pre-defined antibody thresholds for each antigen at Day 29. A substantial and dose-level dependent immune response was observed for rP305A, with up to 18-fold rises in cLIA titres at Day 29. Robust functional responses were demonstrated, with >80-fold and >20-fold rises in OPA assay titres at Day 29 using S. aureus strains expressing capsular polysaccharide serotypes 5 and 8, respectively. Durable antibody responses were observed through month 12, gradually waning from peak levels achieved by days 11-15. SA4Ag was well tolerated, and no vaccine-related serious adverse events were reported.

CONCLUSIONS

Single-dose vaccination of SA4Ag in healthy adults aged 18-64years safely induced rapid and robust functional immune responses that were durable through month 12, supporting further development of this vaccine.

TRIAL REGISTRATION NUMBER

NCT01364571.

High Resolution Mapping of Bactericidal Monoclonal Antibody Binding Epitopes on Staphylococcus aureus Antigen MntC

The Staphylococcus aureus manganese transporter protein MntC is under investigation as a component of a prophylactic S.aureus vaccine. Passive immunization with monoclonal antibodies mAB 305-78-7 and mAB 305-101-8 produced using MntC was shown to significantly reduce S. aureus burden in an infant rat model of infection. Earlier interference mapping suggested that a total of 23 monoclonal antibodies generated against MntC could be subdivided into three interference groups, representing three independent immunogenic regions. In the current work binding epitopes for selected representatives of each of these interference groups (mAB 305-72-5 – group 1, mAB 305-78-7 – group 2, and mAB 305-101-8 – group 3) were mapped using Hydrogen-Deuterium Exchange Mass Spectrometry (DXMS). All of the identified epitopes are discontinuous, with binding surface formed by structural elements that are separated within the primary sequence of the protein but adjacent in the context of the three-dimensional structure. The approach was validated by co-crystallizing the Fab fragment of one of the antibodies (mAB 305-78-7) with MntC and solving the three-dimensional structure of the complex. X-ray results themselves and localization of the mAB 305-78-7 epitope were further validated using antibody binding experiments with MntC variants containing substitutions of key amino acid residues. These results provided insight into the antigenic properties of MntC and how these properties may play a role in protecting the hostagainst S. aureus infection by preventing the capture and transport of Mn²⁺, a key element that the pathogen uses to evade host immunity.

Staphylococcus aureus protein MntC is a metal-binding protein of the ABC-type transporter involved in the acquisition of an essential nutrient, Mn²⁺, by the pathogen. An earlier study demonstrated that use of MntC as an antigen in experimental vaccine can provide protection against staphylococcal infections in animals and identified three groups of protective monoclonal antibodies induced by the protein. In the current work we employed Deuterium-Hydrogen Exchange Mass Spectrometry (DXMS) to determine binding sites of selected representatives from each of those three groups. DXMS results were further validated using X-ray crystallography, site-directed mutagenesis and functional studies. Locations of the binding sites and results of the functional studies were used to draw conclusion on molecular mechanisms of protection afforded by MntC: antibodies belonging to two of the groups are predicted to interfere with Mn²⁺ transfer from the protein to the transmembrane channel pore, while the third group of the antibodies is expected to interfere with Mn²⁺ binding to MntC itself. The net result in both cases is impaired Mn²⁺ transport across the bacterial membrane and increased susceptibility of the bacterium to the oxidative stress, likely due to the reduced activity of superoxide dismutase which requires Mn²⁺ as an essential co-factor for activity.

Safety and immunogenicity of a booster dose of a 3-antigen Staphylococcus aureus vaccine (SA3Ag) in healthy adults: A randomized phase 1 study

OBJECTIVE

A 2-stage, phase 1, randomized, placebo-controlled study in healthy adults to assess immunogenicity and safety of a booster dose at three dose levels of a 3-antigen Staphylococcus aureus vaccine (SA3Ag) containing recombinant clumping factor A (ClfA) and capsular polysaccharides 5 and 8 (CP5 and CP8) conjugated to a diphtheria toxoid.

METHODS

Six months after initial single vaccination, in Stage 2, SA3Ag recipients were randomized (1:1) to booster vaccination or placebo, while Stage 1 placebo recipients received placebo again. Pre- and post-vaccination blood samples were analyzed.

RESULTS

In Stage 2 (n = 345), pre-booster CP5 and CP8 titers remained high with no increase post-booster. ClfA titers remained high after initial vaccination and increased post-booster, approaching the peak response to the initial dose. Post-booster local reactions were more frequent and of greater severity than reported after the initial vaccination, particularly for the high-dose level recipients. Post hoc analysis showed no dose-response pattern and no obvious association between diphtheria toxoid titers and local reactions after initial or booster vaccination.

CONCLUSION

Immune responses after the initial vaccination persisted for the 12 months studied, with little additional response after the booster dose at 6 months. Post-booster injection site reactions were more frequent and more severe but self-limiting. CLINICALTRIALS.

GOV IDENTIFIER

NCT01018641.

Staphylococcus aureus: the current state of disease, pathophysiology and strategies for prevention

Staphylococcus aureus is both a commensal organism and also an important opportunistic human pathogen, causing a variety of community and hospital-associated pathologies, such as bacteremia-sepsis, endocarditis, pneumonia, osteomyelitis, arthritis and skin diseases. The resurgence of S. aureus during the last decade in many settings has been facilitated not only by bacterial antibiotic resistance mechanisms but also by the emergence of new S. aureus clonal types with increased expression of virulence factors and the capacity to neutralize the host immune response. Prevention of the spread of S. aureus infection relies on the use of contact precautions and adequate procedures for infection control that so far have not been fully effective. Prevention using a prophylactic vaccine would complement these processes, having the potential to bring additional, significant progress toward decreasing invasive disease due to S. aureus.

Vaccines for Staphylococcus aureus and Target Populations

Staphylococcus aureus is a leading pathogen in surgical site, intensive care unit, and skin infections, as well as healthcare-associated pneumonias. These infections are associated with an enormous burden of morbidity, mortality, and increase of hospital length of stay and patient cost. S. aureus is impressively fast in acquiring antibiotic resistance, and multidrug-resistant strains are a serious threat to human health. Due to resistance or insufficient effectiveness, antibiotics and bundle measures leave a tremendous unmet medical need worldwide. There are no licensed vaccines on the market despite the significant efforts done by public and private initiatives. Indeed, vaccines tested in clinical trials in the last two decades have failed to show efficacy. However, they targeted single antigens and contained no adjuvants and efficacy trials were performed in severely ill subjects. Herein, we provide a comprehensive evaluation of potential target populations for efficacy trials taking into account key factors such as population size, incidence of S. aureus infection, disease outcome, primary endpoints, as well as practical advantages and disadvantages. We describe the whole-blood assay as a potential surrogate of protection, and we show the link between phase III clinical trial data of failed vaccines with their preclinical observations. Finally, we give our perspective on how new vaccine formulations and clinical development approaches may lead to successful S. aureus vaccines.

Structural analysis of bacterial ABC transporter inhibition by an antibody fragment

Bacterial ATP-binding cassette (ABC) importers play critical roles in nutrient acquisition and are potential antibacterial targets. However, structural bases for their inhibition are poorly defined. These pathways typically rely on substrate binding proteins (SBPs), which are essential for substrate recognition, delivery, and transporter function. We report the crystal structure of a Staphylococcus aureus SBP for Mn(II), termed MntC, in complex with FabC1, a potent antibody inhibitor of the MntABC pathway. This pathway is essential and highly expressed during S. aureus infection and facilitates the import of Mn(II), a critical cofactor for enzymes that detoxify reactive oxygen species (ROS). Structure-based functional studies indicate that FabC1 sterically blocks a structurally conserved surface of MntC, preventing its interaction with the MntB membrane importer and increasing wild-type S. aureus sensitivity to oxidative stress by more than 10-fold. The results define an SBP blocking mechanism as the basis for ABC importer inhibition by an engineered antibody fragment.

Evaluation of approaches to monitor Staphylococcus aureus virulence factor expression during human disease

Staphylococcus aureus is a versatile pathogen of medical significance, using multiple virulence factors to cause disease. A prophylactic S. aureus 4-antigen (SA4Ag) vaccine comprising capsular polysaccharide (types 5 and 8) conjugates, clumping factor A (ClfA) and manganese transporter C (MntC) is under development. This study was designed to characterize S. aureus isolates recovered from infected patients and also to investigate approaches for examining expression of S. aureus vaccine candidates and the host response during human infection. Confirmation of antigen expression in different disease states is important to support the inclusion of these antigens in a prophylactic vaccine. Hospitalized patients with diagnosed S. aureus wound (27) or bloodstream (24) infections were enrolled. Invasive and nasal carriage S. aureus isolates were recovered and characterized for genotypic diversity. S. aureus antigen expression was evaluated directly by real-time, quantitative, reverse-transcriptase PCR (qRT-PCR) analysis and indirectly by serology using a competitive Luminex immunoassay. Study isolates were genotypically diverse and all had the genes encoding the antigens present in the SA4Ag vaccine. S. aureus nasal carriage was detected in 55% of patients, and in those subjects 64% of the carriage isolates matched the invasive strain. In swab samples with detectable S. aureus triosephosphate isomerase housekeeping gene expression, RNA transcripts encoding the S. aureus virulence factors ClfA, MntC, and capsule polysaccharide were detected by qRT-PCR. Antigen expression was indirectly confirmed by increases in antibody titer during the course of infection from acute to convalescent phase. Demonstration of bacterial transcript expression together with immunological response to the SA4Ag antigens in a clinically relevant patient population provides support for inclusion of these antigens in a prophylactic vaccine.

A randomized phase I study of the safety and immunogenicity of three ascending dose levels of a 3-antigen Staphylococcus aureus vaccine (SA3Ag) in healthy adults

BACKGROUND

Staphylococcus aureus is a common cause of healthcare-acquired morbidity and mortality and increased healthcare resource utilization. A prophylactic vaccine is being developed that may reduce this disease burden.

METHODS

Volunteers in good general health aged 50-85 (n=312) and 18-24 (n=96) years were randomized to receive a single intramuscular dose of one of three dose levels of a non-adjuvanted, 3-antigen S. aureus vaccine (SA3Ag) or placebo. SA3Ag antigens included capsular polysaccharides 5 and 8 (CP5 and CP8), each conjugated to cross-reactive material 197 (CRM197), and recombinant clumping factor A (ClfA). Safety, tolerability, and immunogenicity were evaluated.

RESULTS

At day 29 post-vaccination, robust immune responses were observed in both age cohorts at all three SA3Ag dose levels. In the primary analysis population, the 50- to 85-year age stratum, geometric mean-fold-rises in competitive Luminex(®) immunoassay antibody titers from baseline ranged from 29.2 to 83.7 (CP5), 14.1 to 31.0 (CP8), and 37.1 to 42.9 (ClfA), all (P<0.001) exceeding the pre-defined two-fold rise criteria. Similar rises in opsonophagocytic activity assay titers demonstrated functionality of the immune response. Most injection-site reactions were mild in severity and there were no substantial differences (SA3Ag vs. placebo) with regard to systemic or adverse events.

CONCLUSIONS

In this study of healthy adults aged 50-85 and 18-24 years, SA3Ag elicited a rapid and robust immune response and was well tolerated, with no notable safety concerns.

Staphylococcus aureus manganese transport protein C (MntC) is an extracellular matrix- and plasminogen-binding protein

Infections caused by Staphylococcus aureus – particularly nosocomial infections - represent a great concern. Usually, the early stage of pathogenesis consists on asymptomatic nasopharynx colonization, which could result in dissemination to other mucosal niches or invasion of sterile sites, such as blood. This pathogenic route depends on scavenging of nutrients as well as binding to and disrupting extracellular matrix (ECM). Manganese transport protein C (MntC), a conserved manganese-binding protein, takes part in this infectious scenario as an ion-scavenging factor and surprisingly as an ECM and coagulation cascade binding protein, as revealed in this work. This study showed a marked ability of MntC to bind to several ECM and coagulation cascade components, including laminin, collagen type IV, cellular and plasma fibronectin, plasminogen and fibrinogen by ELISA. The MntC binding to plasminogen appears to be related to the presence of surface-exposed lysines, since previous incubation with an analogue of lysine residue, ε-aminocaproic acid, or increasing ionic strength affected the interaction between MntC and plasminogen. MntC-bound plasminogen was converted to active plasmin in the presence of urokinase plasminogen activator (uPA). The newly released plasmin, in turn, acted in the cleavage of the α and β chains of fibrinogen. In conclusion, we describe a novel function for MntC that may help staphylococcal mucosal colonization and establishment of invasive disease, through the interaction with ECM and coagulation cascade host proteins. These data suggest that this potential virulence factor could be an adequate candidate to compose an anti-staphylococcal human vaccine formulation.

Covering all the Bases: Preclinical Development of an Effective Staphylococcus aureus Vaccine

A key aspect of the pathogenesis of the Gram positive bacterium Staphylococcus aureus is its ability to rapidly adapt to the host environment during the course of an infection. To successfully establish infection, the organism deploys a variety of survival and immune evasion strategies, ranging from the acquisition of essential nutrients and expression of adhesins, which promote colonization and survival, to the elaboration of virulence factors such as capsule, which aids host immune evasion. The ability of S. aureus to deploy different virulence factors must be taken into account for S. aureus vaccine design. Here, we present a strategy for designing an effective vaccine against S. aureus disease by evaluating vaccine candidate performance in multiple in vivo models targeted to mimic aspects of human disease, and by co-development of functional in vitro immunoassays that measure the neutralization of relevant S. aureus virulence factors.