Ingrid L. Scully

Vaccine review: "Staphyloccocus aureus vaccines: Problems and prospects"

Staphylococcus aureus is a leading cause of both healthcare- and community-associated infections globally. S. aureus exhibits diverse clinical presentations, ranging from benign carriage and superficial skin and soft tissue infections to deep wound and organ/space infections, biofilm-related prosthesis infections, life-threatening bacteremia and sepsis. This broad clinical spectrum, together with the high incidence of these disease manifestations and magnitude of the diverse populations at risk, presents a high unmet medical need and a substantial burden to the healthcare system. With the increasing propensity of S. aureus to develop resistance to essentially all classes of antibiotics, alternative strategies, such as prophylactic vaccination to prevent S. aureus infections, are actively being pursued in healthcare settings. Within the last decade, the S. aureus vaccine field has witnessed two major vaccine failures in phase 3 clinical trials designed to prevent S. aureus infections in either patients undergoing cardiothoracic surgery or patients with end-stage renal disease undergoing hemodialysis. This review summarizes the potential underlying reasons why these two approaches may have failed, and proposes avenues that may provide successful vaccine approaches to prevent S. aureus disease in the future.

Staphylococcus aureus Manganese Transport Protein C Is a Highly Conserved Cell Surface Protein That Elicits Protective Immunity Against S. aureus and Staphylococcus epidermidis

Staphylococcus aureus and other staphylococci cause severe human disease, and there are currently no vaccines available. We evaluated whether manganese transport protein C (MntC), which is conserved across the staphylococcal species group, could confer protection against S. aureus and Staphylococcus epidermidis. In vivo analysis of S. aureus MntC expression revealed that expression occurs very early during the infectious cycle. Active immunization with MntC was effective at reducing the bacterial load associated with S. aureus and S. epidermidis infection in an acute murine bacteremia model. Anti-MntC monoclonal antibodies have been identified that can bind S. aureus and S. epidermidis cells and are protective in an infant rat passive protection model and induce neutrophil respiratory burst activity. This is the first description of a protein that has the potential to provide protection across the staphylococcal species group.

Development of a multicomponent Staphylococcus aureus vaccine designed to counter multiple bacterial virulence factors

Staphylococcus aureus is a major cause of healthcare-associated infections and is responsible for a substantial burden of disease in hospitalized patients. Despite increasingly rigorous infection control guidelines, the prevalence and corresponding negative impact of S. aureus infections remain considerable. Difficulties in controlling S. aureus infections as well as the associated treatment costs are exacerbated by increasing rates of resistance to available antibiotics. Despite ongoing efforts over the past 20 years, no licensed S. aureus vaccine is currently available. However, learnings from past clinical failures of vaccine candidates and a better understanding of the immunopathology of S. aureus colonization and infection have aided in the design of new vaccine candidates based on multiple important bacterial pathogenesis mechanisms. This review outlines important considerations in designing a vaccine for the prevention of S. aureus disease in healthcare settings.

Capsular polysaccharides are an important immune evasion mechanism for Staphylococcus aureus

Staphylococcus aureus can cause severe life threatening invasive diseases. The principal immune effector mechanism by which humans are protected from Gram positive bacteria such as S. aureus is antigen specific antibody- and complement-dependent opsonophagocytosis. This process can be measured in vitro using the opsonophagocytic antibody assay (OPA), which is a complex assay composed of live S. aureus bacteria, a complement source, phagocytic effector cells such as differentiated HL-60 cells, and test serum. In this report, we investigated the impact on the OPA of S. aureus surface antigens capsular polysaccharides (CP) and protein A (SpA). We demonstrated that higher CP expression renders bacteria more resistant to non-specific opsonophagocytic killing than increased SpA expression, suggesting that the expression of capsular polysaccharides may be the more important immune evasion strategy for S. aureus. Bacteria that were not fully encapsulated were highly susceptible to non-specific killing in the assay in the absence of immune serum. This non-specific killing was prevented by growing the bacteria under conditions that increased capsular polysaccharide levels on the surface of the bacteria. In contrast, the level of SpA expression had no detectable effect on non-specific killing in OPA. Using anti-CP antibodies we demonstrated type-specific killing in OPA of both MRSA and MSSA clinical isolates. SpA expression on the cell surface did not interfere with OPA activity, providing evidence that despite the role of SpA in sequestering antibodies by their Fc region, killing is easily accomplished in the presence of high titered anti-capsular polysaccharide antibodies. This highlights the role of CP as an important immune evasion mechanism and supports the inclusion of capsular polysaccharide antigens in the formulation of multi-component prophylactic vaccines against S. aureus.

A Recombinant Clumping Factor A-Containing Vaccine Induces Functional Antibodies to Staphylococcus aureus That Are Not Observed after Natural Exposure

ABSTRACT Staphylococcus aureus is a Gram-positive pathogen that causes devastating disease and whose pathogenesis is dependent on interactions with host cell factors. Staphylococcal clumping factor A (ClfA) is a highly conserved fibrinogen (Fg)-binding protein and virulence factor that contributes to host tissue adhesion and initiation of infection. ClfA is being investigated as a possible component of a staphylococcal vaccine. We report the development of an Fg-binding assay that is specific for ClfA-mediated binding. Using the assay, we show that despite the presence of anti-ClfA antibodies, human sera from unvaccinated subjects are unable to prevent the binding of S. aureus to an Fg-coated surface. In contrast, antibodies elicited by a recombinant ClfA-containing vaccine were capable of blocking the ClfA-dependent binding of a diverse and clinically relevant collection of staphylococcal strains to Fg. These functional antibodies were also able to displace S. aureus already bound to Fg, suggesting that the ligand-binding activity of ClfA can be effectively neutralized through vaccination.

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.

Demonstration of the preclinical correlate of protection for Staphylococcus aureus clumping factor A in a murine model of infection

The Staphylococcus aureus virulence factor clumping factor A (ClfA) is a component of an investigational S. aureus prophylactic vaccine. ClfA enables S. aureus to bind to fibrinogen and platelets during the initial stages of invasive disease. Here we demonstrate that ectopic expression of ClfA is sufficient to render nonpathogenic Lactococcus lactis lethal in a murine model of systemic infection. In contrast, L. lactis expressing ClfAY338A, which cannot bind fibrinogen, did not cause death in the mice. Pathogenicity was also prevented by immunization with ClfA. This model was then used to define a preclinical correlate of protection by measuring functional antibody in a S. aureus fibrinogen binding inhibition assay (FBI) and correlating that titer with protective outcomes. Although many humans have pre-existing antibodies that bind to ClfA, only sera with a threshold functional titer in the FBI were protective in this preclinical model. This confirms that fibrinogen binding is critical for ClfA-mediated pathogenesis and demonstrates that functional antibodies against ClfA are sufficient to protect against ClfA-mediated pathogenesis in vivo, enabling the definition of a preclinical correlate of protection for ClfA-containing vaccines based on FBI titer.

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

ABSTRACT 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.

Anti-infective vaccination in the 21st century — new horizons for personal and public health

The 21st century has seen the licensure of new anti-infective vaccines that have demonstrated their benefit for both individual and population (herd) protection. Despite this there are still many human pathogens for which no vaccine is available. As we learn more about these pathogens, and as technologies advance, more opportunities for vaccine development have become available. This review will address these advances and highlight the paradigm shift from vaccines that are used on a population basis, to others which will have an individual benefit, if successfully licensed, but are not expected to have widespread population based use. The development of the latter vaccines has resulted in a paradigm shift toward vaccinating individuals at specific risk for infection from bacteria such as Staphylococcus aureus or Clostridium difficile, which are members of normal human flora but can cause severe disease under certain circumstances. Increasing levels of antibiotic resistance in such bacteria such as S. aureus have also driven the urgency for the identification of alternative methods of protection that do not rely on treatment or prophylaxis with antibiotics.

Immunofluorescence Microscopy for the Detection of Surface Antigens in Methicillin-Resistant Staphylococcus aureus (MRSA)

Immunofluorescence microscopy is a widely used laboratory method which allows detection and visualization of specific antigens. The method employs the specificity of antibodies to deliver fluorophore to a specific target and then visualize it with a microscope. The power of the technique is that it requires relatively little manipulation and relatively few bacterial cells, enabling the detection of antigen expression where other methods cannot, such as during an actual infection in an animal. Here, we apply the method to follow antigen expression on the surface of MRSA cells over time in in vivo infection models.