Staphylococcus aureus Clumping Factor A Remains a Viable Vaccine Target for Prevention of S. aureus Infection

Antibodies to coagulase of Staphylococcus aureus crossreact to Efb and reveal different binding of shared fibrinogen binding repeats

Staphylococcus aureus pathology is caused by a plethora of virulence factors able to combat multiple host defence mechanisms. Fibrinogen (Fg), a critical component in the host coagulation cascade, plays an important role in the pathogenesis of this bacterium, as it is the target of numerous staphylococcal virulence proteins. Amongst its secreted virulence factors, coagulase (Coa) and Extracellular fibrinogen-binding protein (Efb) share common Fg binding motives and have been described to form a Fg shield around staphylococcal cells, thereby allowing efficient bacterial spreading, phagocytosis escape and evasion of host immune system responses. Targeting these proteins with monoclonal antibodies thus represents a new therapeutic option against S. aureus. To this end, here we report the selection and characterization of fully human, sequence-defined, monoclonal antibodies selected against the C-terminal of coagulase. Given the functional homology between Coa and Efb, we also investigated if the generated antibodies bound the two virulence factors. Thirteen unique antibodies were isolated from naïve antibodies gene libraries by antibody phage display. As anticipated, most of the selected antibodies showed cross-recognition of these two proteins and among them, four were able to block the interaction between Coa/Efb and Fg. Furthermore, our monoclonal antibodies could interact with the two main Fg binding repeats present at the C-terminal of Coa and distinguish them, suggesting the presence of two functionally different Fg-binding epitopes.

Multi-Antigen Outer Membrane Vesicle Engineering to Develop Polyvalent Vaccines: The Staphylococcus aureus Case

Modification of surface antigens and differential expression of virulence factors are frequent strategies pathogens adopt to escape the host immune system. These escape mechanisms make pathogens a "moving target" for our immune system and represent a challenge for the development of vaccines, which require more than one antigen to be efficacious. Therefore, the availability of strategies, which simplify vaccine design, is highly desirable. Bacterial Outer Membrane Vesicles (OMVs) are a promising vaccine platform for their built-in adjuvanticity, ease of purification and flexibility to be engineered with foreign proteins. However, data on if and how OMVs can be engineered with multiple antigens is limited. In this work, we report a multi-antigen expression strategy based on the co-expression of two chimeras, each constituted by head-to-tail fusions of immunogenic proteins, in the same OMV-producing strain. We tested the strategy to develop a vaccine against Staphylococcus aureus, a Gram-positive human pathogen responsible for a large number of community and hospital-acquired diseases. Here we describe an OMV-based vaccine in which four S. aureus virulent factors, ClfA_Y338A, LukE, SpA_KKAA and Hla_H35L have been co-expressed in the same OMVs (CLSH-OMVs_Δ60). The vaccine elicited antigen-specific antibodies with functional activity, as judged by their capacity to promote opsonophagocytosis and to inhibit Hla-mediated hemolysis, LukED-mediated leukocyte killing, and ClfA-mediated S. aureus binding to fibrinogen. Mice vaccinated with CLSH-OMVs_Δ60 were robustly protected from S. aureus challenge in the skin, sepsis and kidney abscess models. This study not only describes a generalized approach to develop easy-to-produce and inexpensive multi-component vaccines, but also proposes a new tetravalent vaccine candidate ready to move to development.

Commensal bacteria augment Staphylococcus aureus infection by inactivation of phagocyte-derived reactive oxygen species

Staphylococcus aureus is a human commensal organism and opportunist pathogen, causing potentially fatal disease. The presence of non-pathogenic microflora or their components, at the point of infection, dramatically increases S. aureus pathogenicity, a process termed augmentation. Augmentation is associated with macrophage interaction but by a hitherto unknown mechanism. Here, we demonstrate a breadth of cross-kingdom microorganisms can augment S. aureus disease and that pathogenesis of Enterococcus faecalis can also be augmented. Co-administration of augmenting material also forms an efficacious vaccine model for S. aureus. In vitro, augmenting material protects S. aureus directly from reactive oxygen species (ROS), which correlates with in vivo studies where augmentation restores full virulence to the ROS-susceptible, attenuated mutant katA ahpC. At the cellular level, augmentation increases bacterial survival within macrophages via amelioration of ROS, leading to proliferation and escape. We have defined the molecular basis for augmentation that represents an important aspect of the initiation of infection.

S. aureus is a commensal inhabitant of the human skin and nares. However, it can cause serious diseases if it is able to breach our protective barriers such as the skin, often via wounds or surgery. If infection occurs via a wound, this initial inoculum contains both the pathogen, other members of the microflora and also wider environmental microbes. We have previously described “augmentation”, whereby this other non-pathogenic material can enhance the ability of S. aureus to lead to a serious disease outcome. Here we have determined the breadth of augmenting material and elucidated the cellular and molecular basis for its activity. Augmentation occurs via shielding of S. aureus from the direct bactericidal effects of reactive oxygen species produced by macrophages. This initial protection enables the effective establishment of S. aureus infection. Understanding augmentation not only explains an important facet of the interaction of S. aureus with our innate immune system, but also provides a platform for the development of novel prophylaxis approaches.

Intrapulmonary Vaccination Induces Long-lasting and Effective Pulmonary Immunity against Staphylococcus aureus Pneumonia

Background

Staphylococcus aureus causes community- and hospital-acquired pneumonia linked to a high mortality rate. The emergence and rapid transmission of multidrug-resistant S. aureus strains has become a serious health concern, highlighting the challenges associated with the development of a vaccine to combat S. aureus pneumonia.

Methods

This study evaluated the effects of intrapulmonary immunization on the immune response and protection against S. aureus lung infection in a respiratory mouse model using a subunit vaccine.

Results

Compared with the intranasal immunized mice, the intrapulmonarily immunized mice had lower levels of pulmonary bacterial colonization and lethality, accompanied by alleviated lung inflammation with reduced proinflammatory cytokines and increased levels of interleukin-10 and antimicrobial peptide following intrapulmonary challenge. Optimal protection was associated with increased pulmonary antibodies and resident memory T cells. Moreover, intrapulmonary immunization provided long-lasting pulmonary protection for at least 6 months, with persistent cellular and humoral immunity in the lungs.

Conclusions

Vaccine reaching the deep lung by intrapulmonary immunization plays a significant role in the induction of efficacious and long-lasting immunity against S. aureus in the lung parenchyma. Hence, intrapulmonary immunization can be a strategy for the development of a vaccine against S. aureus pneumonia.

Immunization through the intrapulmonary route with a subunit of S. aureus vaccine elicited tissue resident memory T cells and antigen-specific antibodies in the lungs, and provided optimal and long-term protection against S. aureus pneumonia.

A synergistic association between adhesion-related genes and multidrug resistance patterns of Staphylococcus aureus isolates from different patients and healthy individuals

OBJECTIVES

Biofilm -forming capacity of Staphylococcus aureus (S. aureus) as a commensal opportunistic bacterial species induce a growth in antibiotic resistance in chronic diseases. Since expression of biofilm- related genes and antibiotic resistance function are interdependent, the present study was an attempt to inquire biofilm formation and its relationship with antibiotic resistance in clinical isolates.

METHODS

208 S. aureus clinical isolates from four major provinces of Iran were investigated in terms of presence of adhesion genes (icaA, icaD, icaB, icaC, fnbpA, fnbpB, clfA, clfB, cna, sasC, sasG and bap) using PCR. In addition, microtiter plate (Mtp) assay was performed to examine quantitative biofilm formation of the isolates and their antibiotic resistance patterns against 16 antibiotics determined upon CLSI criteria.

RESULTS

The results revealed high prevalence rate (almost 100%) of icaADBC and MSCRAMMs genes in the isolates. Moreover, bap gene was not detected in any of the tested clinical isolates. Based on phenotypic method 169 isolates (81.25%) were also found to have biofilm formation ability. Among 208 isolates, 98 (47.12%) isolates were multidrug resistant (MDR). Vancomycin, linezolid, nitrofurantoin and quinupristin/dalfopristin were the most effective drugs against MDR strains. Furthermore, the findings demonstrated a significant relationship between MDR and biofilm forming capacity.

CONCLUSION

Prevalence rate of adhesion- related genes was high in S. aureus from isolates in Iran ;so these genes might be expressed under certain conditions and cause emergence of MDR strains. Therefore, further investigations are necessary to prevent initial attachment based on new candidate adhesion genes for vaccine design.

Whole genome sequencing of a ST2594 MRSA strain causing non-mucosal preoperative colonization and low-grade postoperative infection

We present a post-operative infection caused by a methicillin-resistant Staphylococcus aureus strain, previously isolated in the preoperative screening, in a patient submitted to femoral osteosynthesis, successfully treated with oral ciprofloxacin. The isolate exhibited in vitro resistance to ciprofloxacin, Staphylococcal Cassette Chromosome mec type IV, it was negative for the lukS-PV Panton-Valentine leucocidin gene and belonged to ST2594 in multilocus sequence typing analysis. Whole genome sequencing revealed a genome size of 2,818,289 base pairs. The annotated genomes of ST2594 and N315 strains were compared, looking for genes related to virulence and resistance. The lack of the tst, sec, sel genes, associated with a mutation in the clfA gene, may partially explain the low morbity in this case.

Safety, tolerability, and immunogenicity of a single dose 4-antigen or 3-antigen Staphylococcus aureus vaccine in healthy older adults: Results of a randomised trial

BACKGROUND

The decline in immune function with age is a challenge to vaccine development. Following an initial study in adults aged 18-64years, this study evaluated the safety and immunogenicity of Staphylococcus aureus (S. aureus) 4-antigen (SA4Ag) and 3-antigen (SA3Ag) vaccine in older adults. SA3Ag included capsular polysaccharide serotypes 5 and 8 (CP5 and CP8) conjugated to the nontoxic mutant form of diphtheria toxin (CRM₁₉₇) and a recombinant version of clumping factor A (ClfA). SA4Ag included these antigens, with the addition of a recombinant manganese transporter C (rP305A or MntC). Both vaccines were unadjuvanted.

METHODS

In this double-blind, sponsor-unblinded, placebo-controlled, phase 1/2 study, 284 healthy adults (aged 65-85years) were randomised to receive a single dose of one of three formulations of SA4Ag with escalating dose levels of rP305A, SA3Ag, or placebo. Functional immune responses were measured using opsonophagocytic activity (OPA) killing and fibrinogen-binding inhibition (FBI) assays; immunogenicity was also assessed using a competitive Luminex® immunoassay (cLIA). T-cell responses were measured in a small subgroup of subjects using intracellular cytokine staining (ICS) assays.

RESULTS

The results demonstrated rapid and robust functional immune responses to all antigens in healthy older adults. A high proportion of active vaccine recipients met the pre-defined antibody thresholds for each antigen at Day 29. SA4Ag elicited a dose-level response to rP305A with up to a 13-fold rise in cLIA titres at Day 29. Opsonophagocytic activity (OPA) assays showed >50- and >20-fold rises in functional titres using S. aureus strains expressing CP5 and CP8, respectively, at Day 29. T-cell cytokine responses were not substantially above background levels. There were no safety concerns in this study population and no increases in adverse events with higher rP305A dose levels.

CONCLUSIONS

Single-dose vaccination of SA4Ag and SA3Ag in healthy adults aged 65-85years safely induced rapid and robust functional immune responses, supporting further development of SA4Ag for the prevention of S. aureus disease in adults up to age 85years.

TRIAL REGISTRATION NUMBER

NCT01643941.