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

Abstract

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, learning 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.

Keywords: Staphylococcus aureusvaccine; clinical trial; immune evasion; vaccine development; virulence factor.

Figure 1. Expression of capsule during infection. Immunofluorescence analysis was performed on the laboratory S. aureus strain Reynolds and the USA300 strain CDC3 to determine the timing and degree of capsule expression in a murine infection model of bacteremia. CD1 mice (n = 3) were infected by intraperitoneal injection of approximately 2 × 10⁸ colony-forming units. Immunofluorescence was performed on bacteria harvested at various time points using primary rabbit antibodies and an ALEXA488-conjugated goat-α-rabbit antibody. Both strains express high levels of capsule after four hours of infection (reproduced with permission from (107). Note this is in contrast to several reports that suggest USA300 strains are acapsular due to the lack of observed expression in vitro.^,

Figure 2. Reduction of S. aureus Bacteremia in Infant Rats By Passive Administration of an Anti-MntC Monoclonal Antibody [adapted with permission from Table 4 in (106)]. Groups of Sprague-Dawley infant rats were immunized intraperitoneally with 0.4 mg of either test mAb or isotype control mAb. Sixteen hours after immunization, rats were challenged intraperitoneally with 1 × 10⁸ colony forming unites of a S. aureus clinical isolate, PFESA0140. Four hours later, blood was collected, and serial dilutions were plated to enumerate recovered bacteria. Statistical significance was determined via the Student t-test, and a P value of ≤ 0.05 was considered significant.

Figure 3. Immune sera from CP-conjugates induce opsonophagocytosis. Sera from three different cynomolgous monkeys vaccinated with either CP8 or CP5 conjugates (each shown with a different symbol) were evaluated for their ability to promote opsonophagocytic killing as described in (82). Preimmune sera is shown in dashed lines and post-immune sera, harvested at six weeks post dose, is shown in solid lines. The titer is defined as the reciprocal serum dilution that kills 50% of the test bacteria. The CFU associated with 50% bacterial killing is indicated by the dotted line.

Figure 4. Schematic representation of the S. aureus Fibrinogen Binding Inhibition Assay. Microplate wells were coated with fibrinogen (Fg), incubated with blocking solution to prevent non-specific binding and rinsed. Live S. aureus cells (1 × 10⁶ colony forming units) anti-sera were mixed and added to the plate. After 30 min incubation at 37°C adherent cells were washed and quantified using the Luciferase-based BacTiter-Glo^® reagent, which measures ATP of live bacteria and associated luminescence.

Figure 5.S. aureus FBI assay is Specific for Anti-ClfA Functional Monoclonal Antibodies. Serial dilutions of monoclonal antibodies (mAb) were pre-incubated with S. aureus strain PFESA0237 and their binding to immobilized fibrinogen was assessed as described in the legend to Figure 4. Despite similar binding affinities,only the functional mAb 12–9 shows binding inhibition in a dilution-dependent manner.

Figure 6. Human Immune Sera from the Clinical Tri-antigen Vaccine Study (NCT01018641) Prevents S. aureus (strain PFESA0237) from Binding Fibrinogen. Serial dilutions of serum taken from human subjects either before or four weeks after immunization with either a placebo (A) or the trivalent vaccine (B) were tested for activity in the FBI assay as described in the legend to Figure 4.