Every year about 2 million of the estimated 40 million people admitted to hospitals in the U.S. will develop a nosocomial infection (Anonyomous 1997). With a mortality rate of approximately 4.4%, nosocomial infections contribute to 88,000 deaths per year. The cost of hospital-acquired infections in the U.S. has been estimated at $4.5 billion per year (Weinstein 1998). These estimates do not include infections occurring in the 31 million outpatient surgeries performed each year (National Center for Health Statistics' website), the 1.5 million nursing home residents, the extended care facilities, or among those receiving ambulatory care procedures.
Staphylococcus aureus and coagulase-negative staphylococci (CoNS), particularly S. epidermidis, are Gram-positive opportunistic nosocomial pathogens that are responsible for the majority of nosocomial infections. Staphylococcal infections account for nearly 25% (approximately 500,000) of all nosocomial infections (Haley, Culver et al. 1985) (Boyce 1997). Up to 1% of all admissions in some hospitals result in S. aureus infections (Storch and Rajagopalan 1986). Staphylococci (S. aureus and S. epidermidis) account for about 47% of the nosocomial bloodstream infections, 24% of the surgical site infections (SSI), and 17% of hospital-acquired pneumonia (Anonyomous 1997). The mortality rate of patients with nosocomial S. aureus and CoNS infections varies considerably, ranging from 5% to 68% (Nada, Ichiyama et al. 1996); (Thylefors, Harbarth et al. 1998).
Staphylococcal infections are diverse in scope, ranging from cutaneous infections, such as impetigo, boils, wound infections and infections from implanted devices, to severe life-threatening infections, such as osteomyelitis, endocarditis and bacteremia with metastatic complications. This diversity makes the design of an efficacious immunogenic composition against staphylococci a true challenge. A sharp increase in the appearance of drug-resistant nosocomial bacteria makes such a design even more difficult. Methicillin-resistant S. aureus causes approximately 40% of the deaths attributed to nosocomial infections (Boyce 1997). The recent emergence of vancomycin intermediate-resistant S. aureus (VISA) has raised even greater concern over its spread. Thus, there is a strong and rapidly growing need for an efficacious immunogenic composition against nosocomial infections.
Capsular Polysaccharides
The involvement of capsular polysaccharides (CP) in the virulence of many bacterial pathogens, including Haemophilus influenzae, Streptococcus pneumoniae and group B streptococci, is well established. Encapsulated bacteria are resistant to phagocytosis by leukocytes, and thus can infect the blood and tissues. Because antibodies to capsular polysaccharides neutralize the anti-phagocytic properties of the bacterial capsule (Karakawa, Sutton et al. 1988; Thakker, Park et al. 1998), the staphylococcal capsule has been a major target in the development of immunogenic compositions to prevent staphylococcal infection in humans.
Of the 12 known capsular serotypes of S. aureus, serotype 5 (CP5) and serotype 8 (CP8) account for approximately 85-90% of all clinical isolates (Arbeit, Karakawa et al. 1984; Karakawa, Fournier et al. 1985; Essawi, Na'was et al. 1998; Na'was, Hawwari et al. 1998). Most methicillin-resistant S. aureus isolates express CP5 (Sompolinsky, Samra et al. 1985). Antibodies to CP5 and CP8 induce type-specific opsonophagocytic killing by human polymorphonuclear neutrophils in vitro and confer protection in animals (Karakawa, Sutton et al. 1988; Fattom, Sarwar et al. 1996).
Most bacterial capsular polysaccharides are poor immunogens in animals and humans. However, if the purified polysaccharides are conjugated to protein carrier molecules, they acquire immunogenicity and T-cell dependency. Several laboratories have synthesized immunogenic conjugates consisting of CP5 and CP8 covalently linked to protein. These conjugates are highly immunogenic in mice and humans and induce antibodies that opsonize microencapsulated S. aureus for phagocytosis (Fattom, Schneerson et al. 1993; Gilbert et al. 1994; Reynaud-Rondier et al. 1991). Monovalent immunogenic compositions containing CP5 conjugated to Pseudomonas aeruginosa recombinant exotoxin A are immunogenic and well tolerated in healthy adults and in patients with end-stage renal disease (Welch et al. 1996). In a double-blind trial involving patients with end-stage renal disease who were receiving hemodialysis, a bivalent conjugate vaccine composed of CP5 and CP8 covalently bound to Pseudomonas aeruginosa recombinant exotoxin A conferred partial immunity against S. aureus bacteremia for approximately 40 weeks, after which protection decreased as antibody levels decreased (Shinefield et al. 2002). The outcome of this trial indicates a need for an improved immunogenic composition that could contribute to more complete protection.
Another type of extracellular polysaccharide, referred to as polysaccharide adhesin (PS/A; (Tojo, Yamashita et al. 1988)), poly-N-succinyl β-1-6 glucosamine (PNSG; (McKenney, Pouliot et al. 1999)), poly-N-acetylglucosamine surface polysaccharide (PNAG; (Maira-Litran, Kropec et al. 2002)), or polysaccharide intercellular adhesin (PIA (Mack, Fischer et al. 1996)) is expressed by both S. aureus and S. epidermidis. PIA or PS/A is a linear β-1,6-linked glucosaminoglycan. Immunization of mice with PS/A (PNSG, PNAG) reduces colonization of kidneys and protects mice from death after challenge with S. aureus strains that produced little PS/A (PNSG, PNAG) in vitro (McKenney, Pouliot et al. 1999). PIA plays an important role in the pathogenesis of intravascular catheter-associated infections (Rupp, Ulphani et al. 1999; Rupp, Ulphani et al. 1999; Rupp and Fey 2001; Rupp, Fey et al. 2001). In addition to promoting adhesion between individual S. epidermidis cells, PIA binds to erythrocytes and acts as a hemagglutinin (Fey, Ulphani et al. 1999).
Staphylococcal Surface Adhesins
Staphylococci express multiple surface adhesins (termed microbial surface components recognizing adhesive matrix molecules) which include, for example, fibronectin-binding protein, fibrinogen-binding protein, collagen-binding protein and vitronectin-binding protein. These adhesins specifically recognize and bind to extracellular matrix (ECM) components, such as, for example, fibronectin, fibrinogen, collagen and vitronectin. The redundancy and multitude of adhesion factors expressed by S. aureus contribute to its pathogenicity by providing alternate methods for adherence to, and infection of, a variety of tissues. Antibodies to staphylococcal adhesins may reduce disease by preventing bacteria from invading mammalian host tissues or by promoting opsonophagocytosis. Rats immunized with a portion of the S. aureus fibronectin-binding protein A (provided as a fusion protein) endowed the rats with a modest degree of protection from experimental endocarditis. A similar immunogenic composition designed to elicit antibodies to fibronectin-binding protein A was tested in a mouse model of S. aureus mastitis. Immunized mice showed fewer cases of severe mastitis than the control mice and fewer bacteria were recovered from the mammary glands of immunized mice than of control mice. Mice immunized with fibrinogen-binding proteins of 19 and 87 kDa showed a reduced incidence of mastitis compared with nonimmunized controls, whereas immunization with collagen-binding protein was not protective (Lee, Pier 1997).
However, despite these and other efforts to conjugate polysaccharide antigens to a variety of protein carriers, there currently is no efficacious immunogenic composition for treating or preventing nosocomial infections.