Antibodies to Staphylococcus aureus Serotype 8 Capsular Polysaccharide React with and Protect against Serotype 5 and 8 Isolates

Abstract

bMost Staphylococcus aureus isolates produce either a serotype 5 (CP5) or 8 (CP8) capsular polysaccharide, and the CP antigens are targets for vaccine development. Since CP5 and CP8 have similar trisaccharide repeating units, it is important to identify an epitope shared by both CP5 and CP8. To characterize cross-reactivity between CP5 and CP8, the immunogenicity of CP5 and CP8 conjugate vaccines in mice and rabbits was evaluated by serological assays. Immune sera were also tested for functional activity by in vitro opsonophagocytic-killing assays and a murine bacteremia model. Antibodies to the CP5-cross-reactive material 197 (CRM197) conjugate vaccine bound only to purified CP5. In contrast, antibodies to the CP8-CRM conjugate vaccine reacted with CP8 and (to a lesser extent) CP5. De-O-acetylation of CP5 increased its reactivity with CP8 antibodies. Moreover, CP8 antibodies bound to Pseudomonas aeruginosa O11 lipopolysaccharide, which has a trisaccharide repeating unit similar to that of the S. aureus CPs. CP8-CRM antibodies mediated in vitro opsonophagocytic killing of S. aureus expressing CP5 or CP8, whereas CP5-CRM antibodies were serotype specific. Passive immunization with antiserum to CP5-CRM or CP8-CRM protected mice against bacteremia induced by a serotype 5 S. aureus isolate, suggesting that CP8-CRM elicits antibodies cross-reactive to CP5. The identification of epitopes shared by CP5 and CP8 may inform the rational design of a vaccine to protect against infections caused by CP5-or CP8-producing strains of S. aureus.

Antibodies toStaphylococcus aureuscapsular polysaccharides 5 and 8 perform similarlyin vitrobut are functionally distinctin vivo

“…Mice given CP5 conjugated to cross-reactive material 197 (CRM197), a genetically detoxified form of diphtheria toxin, showed a serotype-specific antibody response to CP5. As we reported previously, 26 serum antibodies from mice immunized with CP8-CRM197 showed not only good binding to purified CP8, but also showed some crossreactive binding to microtiter plates coated with CP5 (Fig. S1A).…”

“…[27][28][29][30] To determine the specificity of the mAbs, we tested their binding by ELISA to purified native or chemically O-deacetylated CP5 or CP8. Unlike polyclonal sera, 26 none of the mAbs tested showed crossreactivity between CP5 and CP8 (Fig. 1).…”

“…25,26 Therefore, the CP5 backbone specific mAb 4C2 was evaluated for its ability to protect mice against bacteremia in passive immunization experiments. As shown in Fig.…”

“…[24][25][26] To determine whether the CP5-and CP8-specific mAbs were functionally opsonic, we performed in vitro OPK assays with agar-grown S. aureus and serial dilutions of either a purified backbone-specific or O-acetyl-specific mAb. Opsonic activity was compared by determining the mAb concentration that yielded 50% killing of the bacterial inoculum.…”

“…aureus clearance by the host is dependent upon opsonic antibodies that promote uptake and killing of this microbe by host neutrophils. Antibodies to S. aureus CP5 and CP8 are particularly effective in promoting opsonophagocytic killing of S. aureus [24][25][26] and reducing bacteremia in a murine infection model., 25,26 In this report we evaluate mAbs against S. aureus CP5 and CP8 for their specificity in binding to a wide variety of encapsulated clinical isolates and their ability to mediate in vitro opsonophagocytic bacterial killing. We assessed protection against bacteremia provoked by both serotype 5 and 8 S. aureus, and we discovered that serotype 8 S. aureus clinical isolates release soluble CP8 during culture and infection.…”