Bedside to Bench

The pathogen Staphylococcus aureus, a leading cause of human bacterial infections worldwide, is capable of causing a diversity of syndromes, from mild and common skin and soft tissue infections to severe, necrotizing and highly invasive disease. The epidemiology of S. aureus disease is strongly influenced by rapid acquisition of antibiotic resistance, as some strains become resistant to nearly all front-line antibiotics1. For example, S. aureus acquired resistance to β-lactam antibiotics such as penicillin and methicillin shortly after they were introduced for treatment of human infections1.

MRSA is a leading cause of hospital-associated bloodstream infections2 and the most common cause of community-associated bacterial infections in the US1,3. The outlook for therapeutic options is confounded by a general lack of new classes of antimicrobial agents in the drug discovery pipeline4 and the absence of a licensed vaccine for prevention of S. aureus disease.

Although there is a clearly defined need for an effective S. aureus vaccine, previous efforts to develop a vaccine have been largely unsuccessful5,6. The lack of success is probably owing to the use of conventional vaccine strategies that are directed to enhance opsonophagocytosis, a process in which specific antibodies and serum complement bind bacterial surface antigens and thereby promote ingestion of the microbe by phagocytes (Fig. 1). But this approach is insufficient, as virtually all humans possess antibodies against S. aureus, and opsonization of prominent MRSA strains, such as USA300, with normal human serum promotes efficient uptake by human neutrophils7. In addition, there can be considerable survival of ingested S. aureus after phagocytosis and consequent destruction of neutrophils7.

Figure 1: Vaccine approaches against S. aureus, including resistant strains.
figure 1

Katie Vicari

Left, a vaccine directed to promote or enhance phagocytosis—a traditional bacterial vaccine approach—has the caveat that almost all humans have preexisting antibodies and serum factors that promote efficient phagocytosis of S. aureus. Ingested S. aureus may also survive after phagocytosis, causing neutrophil lysis. Middle, disease-contributing cytolytic toxins such as α-toxin produced by S. aureus can be neutralized with specific antibodies (anti-toxin) to prevent or moderate disease (for example, severe S. aureus pneumonia or necrotic skin infections). Right, S. aureus secretes coagulases that promote abscess formation and these molecules can be targeted with specific antibodies (anti-coagulase) to inhibit this process.

The forecast for new agents to treat or prevent MRSA has been relatively grim; however, a series of recent studies serves as a springboard for the next generation of S. aureus vaccines that do not aim to enhance opsonophagocytic clearance directly8,9,10 (Fig. 1). These studies are based largely on the principle that vaccine antigens should be crucial for establishment of disease. In particular, a recent study by Cheng et al.9 identified two S. aureus blood-clotting factors as essential molecules for the formation of S. aureus kidney abscesses in a mouse infection model and showed how immunization against these molecules protects against disease in mice.

S. aureus abscesses are walled-off cavities comprised of fibrin (as a major component of the abscess wall), bacteria, inflammatory cells such as neutrophils (pus) and necrotic tissue. They can form in virtually any tissue of the body, particularly the skin. Abscesses are one of the most common pathological manifestations of S. aureus infection and form initially as a host mechanism to prevent the spread of bacteria. But, when left untreated, they can rupture to release bacteria or enlarge to inhibit function of vital organs.

The S. aureus clotting factors, or coagulases, known as coagulase (Coa) and von Willebrand factor binding protein (vWbp), activate prothrombin, which then cleaves fibrinogen to fibrin. Cheng et al.9 found fibrinogen and fibrin distributed diffusely throughout S. aureus abscesses in the mouse kidney and more concentrated protein levels at the periphery of the lesion. Coa and vWbp also colocalized with prothrombin, fibrinogen and fibrin at specific sites in these abscesses. Using S. aureus strains with deletion of the genes encoding Coa and vWbp, both singly and in combination, they evaluated the ability of these strains to cause disease in mouse infections models9. A S. aureus strain with deletion of both coagulases caused a reduction in lethal bacteremia and fewer kidney abscesses, demonstrating that Coa and vWbp are crucial for pathogenesis.

An early study already suggested that coagulases could have a role in the development of S. aureus lesions11. At the time, coagulases were largely considered as nonantigenic and thus not a good target for immune protection against staphylococci. Yet subsequent studies showed that antibodies specific for S. aureus coagulase are present in sera from the majority of healthy or infected individuals tested12,13.

Cheng et al.9 generated rabbit antibodies to neutralize Coa and vWbp function and performed passive immunization experiments in the mouse infection models. These antibodies significantly reduced mortality in mice infected with the community-associated MRSA (CA-MRSA) strain USA300 and largely eliminated staphylococcal abscess formation. Mice vaccinated with purified Coa or vWbp were also protected against formation of USA300 abscesses, suggesting that immunization with these coagulases may be used to protect humans against severe MRSA infections.

Coagulases are attractive candidate vaccine antigens, as most S. aureus produce these factors and the mechanism of protection is not based upon generating antibodies that enhance opsonophagocytosis. Rather, the antibodies inhibit coagulase function and the cascade of events leading to accumulation of fibrin and the formation of an abscess wall at the site of infection (Fig. 1).

Previously, this research group used a similar approach to identify additional putative S. aureus vaccine antigens, such as the cytolytic toxin α-hemolysin (α-toxin)14,15 and IsdA and IsdB10, which are surface-associated molecules involved in heme uptake by S. aureus. Vaccination against α-toxin protected mice against lethal S. aureus pneumonia14 and severe USA300 skin infection15, and passive immunization with antibodies specific for IsdA and IsdB protected mice against abscess formation and fatal bacteremia by inhibiting heme uptake and utilization10. A protective mechanism involving vaccine targeting of bacterial heme uptake has the potential to be used broadly for developing new therapies or preventative measures to control antibiotic resistant pathogens.

A second study showed that another protein—S. aureus protein A (SpA)—suppresses the humoral immune response, limiting the antibody responses against S. aureus and the protection against severe disease8. Vaccination of mice with a nonfunctional SpA mutant protein protected mice from MRSA infections by targeting SpA, allowing them to mount normal antibody responses to staphylococcal antigens and preventing or resolving infections more readily.

Collectively, these studies represent a fresh approach to S. aureus vaccine design. Inasmuch as these vaccine antigens were effective in preventing or moderating disease in mouse models of S. aureus infection, they may be considered further as candidates for a human S. aureus vaccine. Indeed, an IsdB-based vaccine known as V710 (Merck) was recently evaluated for safety and immunogenicity in humans in a multicenter phase 1 study16, showing positive immune responses in the majority of individuals tested with the vaccine and no serious adverse events16. Another possible maneuver could combine these recently described antigens with other S. aureus molecules relevant for infection into a single vaccine formulation—an approach currently under consideration.

The question of whether it is possible to develop an effective vaccine against S. aureus, a human commensal microbe, remains open. In an era of high antibiotic use and increasing resistance among bacterial pathogens, it will be necessary to continue moving forward with the development of new vaccine approaches designed to prevent or lessen the severity of MRSA infections.