Staphylococcus aureus manganese transport protein C is a highly conserved cell surface protein that elicits protective immunity against S. aureus and Staphylococcus epidermidis

Abstract

Staphylococcus aureus and other staphylococci cause severe human disease, and there are currently no vaccines available. We evaluated whether manganese transport protein C (MntC), which is conserved across the staphylococcal species group, could confer protection against S. aureus and Staphylococcus epidermidis. In vivo analysis of S. aureus MntC expression revealed that expression occurs very early during the infectious cycle. Active immunization with MntC was effective at reducing the bacterial load associated with S. aureus and S. epidermidis infection in an acute murine bacteremia model. Anti-MntC monoclonal antibodies have been identified that can bind S. aureus and S. epidermidis cells and are protective in an infant rat passive protection model and induce neutrophil respiratory burst activity. This is the first description of a protein that has the potential to provide protection across the staphylococcal species group.

Figure 1.

Visualization of antigen expression by Staphylococcus aureus PFESA0155 during bacteremia. In this strain, manganese transport protein C (MntC) is expressed rapidly after infection and remains expressed throughout the experiment. In contrast, IsdA expression is delayed and not continuous throughout the experiment. The expression profiles for additional clinical isolates are given in Table 1. Three groups of 3 mice each were infected by intraperitoneal injection of 5 × 10⁸ colony-forming units of the S. aureus clinical isolate PFESA0155. At 1, 4, and 6 hours after infection, one group was sacrificed, blood from the animals was pooled, and the bacteria were isolated. Bacteria at the time of challenge (T0) and bacteria isolated from the bloodstream (T1, T4, and T6) were stained with affinity-purified rabbit anti-MntC, anti-IsdA, or control immunoglobulin G and visualized with a fluorescent microscope. A, Differential interference contrast (DIC) images of anti-MntC smears. B, Fluorescence microscopy images of MntC expression. C, DIC images of anti-IsdA smears. D, Fluorescence microscopy images of IsdA expression.

Figure 2.

Characterization of anti–manganese transport protein C (MntC) monoclonal antibodies (mAbs). Reactivity patterns of anti-MntC mAbs (A) and binding interference patterns detected by BIAcore (B). A, The 23 mAbs generated were grouped into 4 distinct patterns, by reactivity to MntC and SitC protein in enzyme-linked immunosorbant assays, and to MntC on the cell surface of Staphyloccocus aureus, by flow cytometry. B, The overlapping ellipses display interference by BIACore analysis. Pattern numbers of the mAbs are indicated within the ellipses, which represent antibody interference space and are arbitrary in size. The 3 interference groups are identified.

Figure 3.

Anti–manganese transport protein C (MntC) antibody 305-78-7 induces neutrophil respiratory burst activity against both Staphyloccocus aureus and Staphyloccocus epidermidis. HL-60 neutrophil-like cells were incubated with bacteria and a source of complement alone, or with bacteria, complement, and anti-MntC antibodies (closed symbols). Respiratory burst activity was measured by chemiluminescence in a kinetic manner, with reads every 5 minutes. A, The anti-MntC antibody 305-78-7 was able to elicit a robust respiratory burst against S. aureus (closed symbols), which was abrogated by preincubation with MntC protein (open symbols). B, An irrelevant isotype controlled antibody was not able to elicit a robust respiratory burst against S. aureus (closed symbols), and preincubation with MntC protein demonstrated only a minor reduction of respiratory burst activity (open symbols). C, The anti-MntC monoclonal antibody (mAb) 305-78-7 was also able to elicit a respiratory burst against S. epidermidis, compared with an irrelevant control antibody. Abbreviation: RLU, relative light units.