Monoclonal antibodies (MAbs) are gaining significant momentum as novel therapeutics for infections caused by antibiotic-resistant bacteria. We evaluated the mechanism by which anti-bacterial MAb therapy protects against Acinetobacter baumannii infections. Anti-capsular MAb enhanced macrophage opsonophagocytosis and rescued mice from lethal infections by harnessing complement, macrophages, and neutrophils, yet the degree of bacterial burden did not correlate with survival. Furthermore, MAb therapy reduced pro-inflammatory (IL-1β, IL-6, TNFα) and anti-inflammatory (IL-10) cytokines, which correlated inversely with survival. Although disrupting IL-10 abrogated the survival advantage conferred by the MAb, IL-10-knockout mice treated with MAb could still survive if TNFα production was suppressed directly (via anti-TNFα neutralizing antibody) or indirectly (via macrophage depletion). Thus, even for a MAb that enhances microbial clearance via opsonophagocytosis, clinical efficacy required modulation of pro- and anti-inflammatory cytokines. These findings may inform future MAb development targeting bacteria that trigger the sepsis cascade.

Background: Extremely drug-resistant (XDR) Acinetobacter baumannii is a notorious and frequently encountered pathogen demanding novel therapeutic interventions. An initial monoclonal antibody (MAb), C8, raised against A. baumannii capsule proved a highly effective treatment against a minority of clinical isolates. To overcome this limitation, we broadened coverage by developing a second antibody for use in a combination regimen. Methods: We sought to develop an additional anti- A. baumannii MAb through hybridoma technology by immunizing mice with sublethal inocula of virulent, XDR clinical isolates not bound by MAb C8. Results: We identified a new antibacterial MAb, 65, which bound to strains in a pattern distinct from and complementary to MAb C8. MAb 65 enhanced macrophage opsonophagocytosis of targeted strains and markedly improved survival in lethal bacteremic sepsis and aspiration pneumonia murine models of A. baumannii infection. MAb 65 was also synergistic with colistin, substantially enhancing protection compared to monotherapy. Treatment with MAb 65 significantly reduced blood bacterial density, ameliorated cytokine production (IL-1β, IL-6, IL-10, and TNF), and sepsis biomarkers. Conclusions: We describe a novel MAb targeting A. baumannii that broadens immunotherapeutic strain coverage, is highly potent and effective, and synergistically improves outcomes in combination with antibiotics.

Background We previously reported developing two anti-capsular monoclonal antibodies (MAbs) as a novel therapy for Acinetobacter baumannii infections. We sought to determine whether a bispecific MAb (BsAb) could improve avidity and efficacy while maximizing strain coverage in one molecule. Methods Humanized MAb 65 was cloned into a single-chain variable fragment and attached to humanized MAb C8, combining their paratopes into a single BsAb (C73). We tested BsAb C73’s strain coverage, binding affinity, ex vivo opsonic activity, and in vivo efficacy compared to each MAb alone and combined. Results The BsAb demonstrated strain coverage, binding affinity, opsonization, and in vivo efficacy superior to either original MAb alone or combined. Conclusions A humanized BsAb targeting distinct A. baumannii capsule moieties enabled potent and effective coverage of disparate A. baumannii clinical isolates. The BsAb enhances feasibility of development by minimizing the number of components of a promising novel therapeutic for these difficult-to-treat infections.

No abstract

Acinetobacter baumannii is an extremely drug-resistant pathogen necessitating the development of new therapies. We seek to generate a cocktail of monoclonal antibodies (MAbs) that can target the full diversity of A. baumannii isolates.