Poly-N-Acetyl-β-(1-6)-Glucosamine Is a Target for Protective Immunity against Acinetobacter baumannii Infections

Abstract

ABSTRACTAcinetobacter baumanniihas emerged as a highly troublesome, global pathogen. Treatment is complicated by high levels of antibiotic resistance, necessitating alternative means to prevent or treatA. baumanniiinfections. We evaluated an immunotherapeutic approach againstA. baumannii, focusing on the surface polysaccharide poly-N-acetyl-β-(1-6)-glucosamine (PNAG). We used a synthetic oligosaccharide of 9 monosaccharide units (9Glc-NH2) conjugated to tetanus toxoid (TT) to induce antibodies in rabbits. In the presence of complement and polymorphonuclear cells, antisera to 9Glc-NH2-TT mediated the killing ofA. baumanniiS1, a high-PNAG-producing strain, but not its isogenic PNAG-negative, in-frame deletion mutant strain, S1 Δpga. Complementing thepgaABCDlocus intransin the shuttle vector pBAD18kan-ori, plasmid Δpga-c, restored the high levels of killing mediated by antibody to PNAG observed with the wild-type S1 strain. No killing was observed when normal rabbit serum (NRS) or heat-inactivated complement was used. Antiserum to 9Glc-NH2-TT was highly opsonic against an additional four unrelated multidrug-resistant clinical isolates ofA. baumanniithat synthesize various levels of surface PNAG. Using two clinically relevant models ofA. baumanniiinfection in mice, pneumonia and bacteremia, antisera to 9Glc-NH2-TT significantly reduced levels ofA. baumanniiin the lungs or blood 2 and 24 h postinfection, respectively, compared to levels of control groups receiving NRS. This was true for all fourA. baumanniistrains tested. Overall, these results highlight the potential of PNAG as a vaccine component for active immunization or as a target for passive antibody immunotherapy.