A major challenge for healthcare in the 21st century is the increasing levels of resistance to antimicrobial compounds of S. aureus
. While the search for novel antibacterial drugs continues, isolates that are resistant even to novel antibiotics, such as linazolid, are quickly found (1
). An effective vaccine to prevent the often life-threatening infections is urgently needed. S. aureus
strains possess many antigens on their surfaces that have been investigated for their potential as either therapeutic or prophylactic vaccines. A vaccine to capsular polysaccharide conjugate is currently being tested in clinics and was found to reduce infection by 56% over a 10-month period in hemodialysis patients (35
). This vaccine targets two S. aureus
capsule serotypes that represent approximately 70% of S. aureus
clinical isolates (29
). There are also ongoing studies to investigate S. aureus
surface proteins, including clumping factor A (ClfA) (10
), fibronectin binding proteins (34
), fibrinogen binding proteins (20
), collagen binding protein (Cna) (27
), and secreted toxins (12
). Some of these antigens have been tested in murine sepsis models similar to the one used in this study. Cna (27
), toxic shock syndrome toxin (12
), and enterotoxin have been tested in active immunization studies, and ClfA (10
) has been tested in passive immunization studies using a ClfA-specific monoclonal antibody. The overall survival rates for vaccinated groups for which positive data were reported ranged from 20 to 87%. In our hands the survival rates for IsdB-immunized animals in which significant levels of protection were observed ranged from 20 to 80%. The differences in survival between vaccine and sham groups ranged from 15 to 74%, compared to 20 to 40% for IsdB-vaccinated animals. The reduced efficacy window and the lack of 100% efficacy for all of these antigens in the murine sepsis model are believed to be artifacts of this model. Extremely high challenge doses of S. aureus
cells) are required to overcome the natural innate immunity of mice, even with naïve animals. The S. aureus
vaccine candidates studied to date all play important roles in vivo; however, none is essential, and they may not be expressed in all phases of infection and may have variable distribution among S. aureus
clinical isolates. The Cna antigen showed excellent potential with the murine sepsis model, and the survival rates were 64 and 74% greater than the control survival rates (27
); however, epidemiology studies have shown that this antigen is present in only a small proportion of clinical isolates (33
). In S. aureus
there is a high level of redundancy in the virulence protein repertoire, so loss of a specific protein may not be fatal. This built-in redundancy makes the search for successful vaccines a challenge, and for current investigational targets it is clear that multiple approaches are needed to combat disease in the long term. We demonstrated that although IsdB is not an essential protein for S. aureus
in vitro, loss of this protein results in a reduction in virulence in vivo, which makes it an attractive vaccine candidate.
We demonstrated that IsdB, when formulated with AAHSA, is highly immunogenic. The induction of IsdB-specific antibody responses correlated with reproducible and significant protection in a mouse model of infection with broad coverage against different S. aureus clinical isolates, including methicillin-resistant strains. Specifically, we demonstrated that mice immunized with IsdB and challenged with a strain which had isdB harA deletions were not protected from death, thus identifying the specificity of protection provided by an immune response targeted against surface-expressed IsdB.
IsdB is conserved among diverse S. aureus clinical isolates, both methicillin resistant and methicillin sensitive, and it is expressed on the surface of all isolates tested. It is interesting that the bacteria used to challenge animals were routinely prepared from plates containing TSA, a medium which represses IsdB expression. S. aureus Becker does not have detectable levels of IsdB on its surface under these growth conditions. However, the ability to detect surface expression of the IsdB protein on bacteria grown in vivo, as well as the protection data, suggest that the protein is expressed very quickly during infection.
Finally, we demonstrated that the IsdB-immunized mice which survived the lethal challenge had higher antibody responses than the mice that succumbed to the infection. This underscores the role of IsdB antibody responses in protection in the sepsis model. One question that remains to be answered is the mechanism of action for the protection mediated by this vaccine. We demonstrated that there is a protective effect mediated by expression of this protein by S. aureus
cells and that the vaccine induces high antibody responses to the protein. Human clearance of bacterial infections is expected to be via opsonic killing which is mediated after phagocyte uptake (40
). There are many convincing examples for this from studies using gram-positive polysaccharide antigens, such as Streptococcus pneumoniae
capsular polysaccharide (3
) and S. aureus
capsular polysaccharide. The S. aureus
capsular polysaccharide vaccine that is currently undergoing clinical trials has demonstrated that there is a correlation between opsonophagocytic killing and human postimmune capsule titers (9
). There is not such clarity for gram-positive protein antigens. Uptake by phagocytes has been observed (26
), but direct killing has been harder to demonstrate. Despite these results, monoclonal antibodies to proteins have been shown to confer protection against S. aureus
challenge in animal models of infection (31
); however, mechanisms other than opsonophagocytic killing may account for the protection observed.
One of the most striking observations in this study was the brisk immune response to IsdB in rhesus macaques, which gave us reason to hypothesize that the vaccine described here may generate an anamnestic response in humans, who, like monkeys, have preexisting titers to the antigen, thus providing rapid protection against the vaccine. This suggests that it may be possible to develop a truly nosocomial vaccine that can prevent S. aureus infection in the hospital setting without a lengthy immunization schedule. Humans are constantly exposed to S. aureus, a natural colonizer of the skin and nares, which may account for the presence of preexisting titers. We found that in a murine sepsis model, survival is correlated with the magnitude of the titer to IsdB. We believe that an IsdB-based vaccine should be an effective antigen for the prevention of S. aureus infection that boosts preexisting antibody titers to this protein to a threshold that can provide protection from infection.