The Capsular Polysaccharide of Acinetobacter baumannii Is an Obstacle for Therapeutic Passive Immunization Strategies

Abstract

Acinetobacter baumannii has become an important concern for human health due to rapid development and wide spread of antimicrobial-resistant strains and high mortality associated with the infection. Passive immunizations with antisera targeting outer membrane proteins (OMPs) have shown encouraging results in protecting mice from A. baumannii infection, but monoclonal anti-OMP antibodies have not been developed, and their potential therapeutic properties have not been explored. The goal of this report is to evaluate the antibacterial activity of monoclonal antibodies (MAbs) targeting outer membrane protein A (OmpA) of A. baumannii. Five anti-OmpA MAbs were developed using hybridoma technology and showed strong binding to strain ATCC 19606. However, low antibody binding was observed when they were tested against six clinical isolates, which included extensively drug-resistant strains. In contrast, high binding to an isogenic K1 capsulenegative mutant (AB307.30) was shown, suggesting that capsular polysaccharide mediated the inhibition of MAb binding to OmpA. Anti-OmpA MAbs increased the macrophage-mediated bactericidal activity of AB307.30 but failed to increase phagocytic killing of capsule-positive strains. Capsular polysaccharide was also protective against complement-mediated bactericidal activity in human ascites in the presence and absence of opsonization. Lastly, passive immunization with antiOmpA MAbs did not confer protection against challenge with AB307-0294, the encapsulated parent strain of AB307.30, in a mouse sepsis infection model. These results reveal the important role of capsule polysaccharide in shielding OmpA and thereby inhibiting anti-OmpA MAb binding to clinical isolates. This property of capsule hindered the therapeutic utility of anti-OmpA MAbs, and it may apply to other conserved epitopes in A. baumannii.

Antibody Dependent Enhancement ofAcinetobacter baumanniiInfection in a Mouse Pneumonia Model

“…An mIgG3 (k) mAb, 8E3, with specificity for the K2 capsular polysaccharide of A. baumannii, was developed via immunizing mice with isolated K2 capsular polysaccharides. The binding specificity of 8E3 to K2 capsules was assessed via Western blot analysis of isolated K2 capsules and whole-cell lysates of A. baumannii capsule types K1, K2, K4, K11, and K16 (Wang-Lin et al, 2017). Positive binding to isolated K2 capsule and K2 whole-cell lysates but not in cell lysates of other capsule types confirmed the specificity to the K2 capsule ( Fig.…”

“…Anti-K2 Capsule mAb 8E3 Augmented Phagocytic Killing of A. baumannii In Vitro. An anti-K1 capsule IgM, 13D6, developed previously in the laboratory of T.A.R., has been demonstrated to enhance neutrophil-mediated bactericidal activity when tested against K1 capsule type A. baumannii strains in vitro (Russo et al, 2013;Wang-Lin et al, 2017). Thus, we hypothesized that opsonization with anti-K2 capsule mAb 8E3 would increase macrophagemediated killing of AB899.…”

Bacteriophages targetingAcinetobacter baumanniicapsule induce antimicrobial resensitization

“…The trade-offs of impaired capsule production and resensitization to antimicrobials could affect A. baumannii ’s ability to invade and survive in mammalian hosts (40, 41). We therefore sought to observe the in vivo fitness of our phage-resistant mutants.…”

Microevolution in the major outer membrane protein OmpA ofAcinetobacter baumannii

“…In fact, natural transformation with highly fragmented (≥ 20 bp) and even partially damaged DNA has been shown to be much more efficient than spontaneous mutation to generate adjacent nucleotide polymorphism in the Acinetobacter genome (46). The OmpA periplasmic domain contributes to the stability of the cell envelope by interacting non-covalently with the peptidoglycan (25), and its deletion in A. baumannii resulted in an increased cell susceptibility to different antimicrobials including cell wall synthesis inhibitors (50). This region of OmpA has been found to associate with different OM and periplasmic proteins including an acquired OXA-23 β-lactamase in the carbapenem-resistant A. baumannii strain AB5075 (36).…”

“…This preservation supports the notion that these two OmpA variants play pivotal roles in A. baumannii pathogenicity (3, 6, 18, 19, 24, 28–36). Their escape from host defenses could result, at least in part, from the recently disclosed shielding by capsule polysaccharide production during the course of A. baumannii infection (50). Thus, the observation that particular ompA variants, variant alleles, or even ompA gene sectors can be effectively exchanged by either assortative or fine-scale recombination not only among different A. baumannii lineages but also between different ACB species (Fig.…”