1. Field of the Invention
The present invention relates to vaccines effective against methicillin-resistant Staphylococcus aureus (MRSA) and methicillin-sensitive Staphylococcus aureus (MSSA), and to methods of using the vaccines in the treatment, prevention and prophylaxis of MRSA and MSSA in subjects.
2. Related Art
While once only a hospital acquired pathogen, methicillin-resistant Staphylococcus aureus (MRSA) infection has spread to the community and is now reaching epidemic proportions. A recent study has found that nearly 19,000 people per year die from MRSA infections in the United States, a death toll higher than that of AIDS (1). In addition, up to 20% of patients who undergo surgery acquire at least one nosocomial infection (2), which is estimated to add $5-10 billion in costs to the US healthcare system (3, 4). S. aureus is one of the most common etiologic agents of these infections (5, 6). These numbers of deaths, as well as the associated healthcare costs, do not even take into account the morbidity and mortality caused by methicillin sensitive S. aureus (MSSA) strains that still cause the majority of staphylococcal infections. Therefore, the generation of a vaccine that is protective against S. aureus would have the potential to significantly reduce the morbidity and mortality associated with these infections. One of the major ways that S. aureus is able to persist is through growth as a biofilm, which is recalcitrant to clearance by antimicrobials, further limiting the efficacy of presently-available antimicrobial agents. With fewer appropriate means of treating the illnesses caused by this bacterium, prevention of disease is essential.
There have been several approaches to designing an effective S. aureus vaccine. Whole live or killed S. aureus vaccines have proved to be largely ineffective in animal models (7, 8). Thus, research has focused on using purified forms of either polysaccharide or protein from the bacterial surface. Much research has centered on the capsular polysaccharide types 5 and 8 (9). One such vaccine, StaphVAX™ demonstrated protective efficacy in animal models of infection; IgG produced as a result of vaccination showed high levels of opsonophagocytosis in vitro (10) and in a Phase III clinical trial. However, protection waned with time and by one year post-vaccination, it was less than 30% (11). Active or passive immunization with polysaccharide intracellular adhesin (PIA), the principal exopolysaccharide component of S. aureus and S. epidermidis biofilms, has been shown to be protective against S. aureus infection in a kidney infection model (12). However, recent research has illustrated that only one component of PIA is immunogenic and responses to this antigen are variable (13). Deacetylation of PNAG, as well as conjugation to diphtheria toxin as a carrier protein, does help increase protection levels (14). However, not all clinical isolates of either S. aureus or S. epidermidis produce PIA (15-18), making the relevance of such a vaccine questionable.
Protein-based vaccines have focused mainly on the ‘microbial surface components recognizing adhesive matrix molecules’ (MSCRAMM) subset of cell wall-associated proteins. Individual component vaccines consisting of clumping factor A (ClfA), ClfB, iron-regulated surface determinant B (IsdB), and fibronectin-binding protein (FnBBP) have all been tested. Recombinant ClfA was shown to be only partially protective when used in an animal model of septic arthritis (19). ClfA is also being investigated as a DNA vaccine candidate in mice and cattle. However, while injection of plasmid containing clfA increased clearance in a mastitis model, protection was not generated against infection in an intraperitoneal challenge (20). Immunization with rClfB led to lessened colonization of murine nares by S. aureus (21). Vaccination with IsdB led to increased survival rates of 20-40% in a murine sepsis model (22). A fusion protein consisting of the binding regions of Cna (collagen binding protein) and FnBP showed some protection in a mouse intraperitoneal model (23).
The vaccines discussed above have several limitations, including incomplete protection and the differential expression of the component proteins amongst S. aureus isolates (24, 25). Use of a multi-component vaccine has shown promise in promoting significant protection against S. aureus infection. When IsdA and B, as well as SdrD and SdrE, were combined into a single vaccine, complete protection was afforded in a mouse renal abscess model, with bacterial levels being reduced below levels of detection and a lack of clinical signs of disease (26).
Even with the advancements being made in this field, the vast majority of research focuses on protection from acute, planktonic-associated S. aureus infection. Also, the studies discussed above all make use of non-biofilm animal models of infection. A number of groups (27-30) have shown that gene expression and protein production between the two states of biofilm and planktonic modes of growth differ greatly. Therefore, the vaccine candidates that prevent infection in acute, planktonic-associated models (for example, sepsis, intraperitoneal infection, and renal abscess models) may not be effective against biofilm infections like osteomyelitis, endocarditis, or prosthetic implant infections.
Previous work identified several cell wall-associated proteins that are immunogenic during S. aureus biofilm infection and whose genes are up-regulated during biofilm growth (27). Vaccination with a recombinant form of one of these proteins (autolysin) led to significant decreases in biofilm disease severity and symptoms in the same biofilm infection model but no decrease in bacterial levels. Since vaccination was directed against a biofilm up-regulated antigen, the inability of the host to clear the infection may have been due to the persistence of planktonic populations. Therefore, the administration of antimicrobial agents, while not effective against biofilm communities, may be required for clearance of the remaining planktonic staphylococci. In addition, the tested antigen, as well as other biofilm up-regulated antigens, may not be homogenously produced throughout the biofilm, making it impossible for the host to appropriately respond to the entire infectious microbial community.
Accordingly, novel vaccines that are effective in the prevention and treatment of MRSA and MSSA, along with methods of prevention and treatment that utilize such vaccines, are required.