Molecular basis for preferential protective efficacy of antibodies directed to the poorly acetylated form of staphylococcal poly-N-acetyl-beta-(1-6)-glucosamine.

Poly-N-acetyl-glucosamine (PNAG) is a staphylococcal surface polysaccharide influencing biofilm formation that is also under investigation for its vaccine potential. Antibodies that bind to PNAG with either low (<15%) or high (>90%) levels of acetate are superior at opsonic and protective activity compared with antibodies that bind to PNAG with only high levels (>70%) of acetate. PNAG is synthesized by four proteins encoded within the intercellular adhesin (ica) locus icaADBC. In Staphylococcus epidermidis, icaB encodes a deacetylase needed for the surface retention of PNAG and optimal biofilm formation. In this study, we confirmed that icaB plays a similar role in Staphylococcus aureus and found that an icaB mutant of S. aureus expressed significantly less surface-associated PNAG, was highly susceptible to antibody-independent opsonic killing that could not be enhanced with antibody raised against deacetylated PNAG (dPNAG), and had reduced survival capacity in a murine model of bacteremia. In contrast, an icaB-overexpressing strain produced primarily surface-associated PNAG, was more susceptible to opsonophagocytosis with antibody to dPNAG, and had increased survival in a murine bacteremia model. The highly acetylated secreted PNAG was more effective at blocking opsonic killing mediated by a human monoclonal antibody (mAb) to native PNAG than it was at blocking killing mediated by a human mAb to dPNAG, which by itself was a more effective opsonin. Retention of dPNAG on the surface of S. aureus is key to increased survival during bacteremia and also provides a molecular mechanism explaining the superior opsonic and protective activity of antibody to dPNAG.

Characterization of the opsonic and protective activity against Staphylococcus aureus of fully human monoclonal antibodies specific for the bacterial surface polysaccharide poly-N-acetylglucosamine.

Carbohydrate antigens are important targets of the immune system in clearing bacterial pathogens. Although the immune system almost exclusively uses antibodies in response to foreign carbohydrates, there is still much to learn about the role of different epitopes on the carbohydrate as targets of protective immunity. We examined the role of acetyl group-dependent and -independent epitopes on the staphylococcal surface of polysaccharide poly-N-acetylated glucosamine (PNAG) by use of human monoclonal antibodies (MAbs) specific for such epitopes. We utilized hybridoma technology to produce fully human immunoglobulin G2 (IgG2) MAbs from B cells of an individual post-Staphylococcus aureus infection and cloned the antibody variable regions to produce an IgG1 form of each original MAb. Specificity and functionality of the purified MAbs were tested in vitro using enzyme-linked immunosorbent assays, complement deposition, and opsonophagocytic assays. We found that a MAb (MAb F598) that bound the best to nonacetylated or backbone epitopes on PNAG had superior complement deposition and opsonophagocytic activity compared to two MAbs that bound optimally to PNAG that was expressed with a native level (>90%) of N-acetyl groups (MAbs F628 and F630). Protection of mice against lethality due to S. aureus strains Mn8 and Reynolds further showed that the backbone-specific MAb had optimal protective efficacy compared with the acetate-specific MAbs. These results provide evidence for the importance of epitope specificity in inducing the optimal protective antibody response to PNAG and indicate that MAbs to the deacetylated form of PNAG could be immunotherapeutic agents for preventing or treating staphylococcal infections.

The role of epitope specificity in the human opsonic antibody response to the staphylococcal surface polysaccharide poly N-acetyl glucosamine.

BACKGROUND: The staphylococcal surface polysaccharide poly N-acetyl glucosamine (PNAG) is a target for killing and protective antibody in animals. We investigated the human antibody response and specificity of binding and opsonic antibodies for different epitopes on PNAG in serum samples from patients with cystic fibrosis (CF) colonized and not colonized with Staphylococcus aureus. METHODS: Serum samples from patients with CF colonized and not colonized with S. aureus were used to compare levels and specificities of binding and opsonic antibodies to native PNAG (>95% acetylation) and deacetylated PNAG (dPNAG, approximately 15% acetylation). RESULTS: Colonized patients had higher killing activity mediated by opsonic antibody than did noncolonized patients in a PNAG-specific opsonophagocytic assay (P<.0001) but no difference in average levels of antibody to either PNAG or dPNAG by enzyme-linked immunosorbent assay. Killing activity in serum samples of the colonized patients correlated with the level of IgG specific to dPNAG more than to native PNAG. dPNAG and PNAG shared expression of the epitopes binding opsonic antibody, as evidenced by comparable inhibition of opsonophagocytic killing by both antigens. Affinity-purified antibodies specific to dPNAG were superior in mediating opsonic killing. CONCLUSION: Human antibodies to PNAG that mediate opsonic killing bind primarily to the nonacetylated epitopes of this antigen, which indicates that these antigenic determinants are the dominant targets of the functional human antibody response to staphylococcal PNAG.