A TonB-dependent cobalamin receptor as a vaccine target against Acinetobacter baumannii: Structural prediction and murine protection studies

Acinetobacter baumannii has become a leading cause of severe hospital-acquired infections, underscoring the urgency of deciphering its pathogenic mechanisms and developing effective vaccines. So, in this study, the immunostructure of a putative TonB-dependent vitamin B12 receptor was predicted and evaluated, and its vaccine potential was examined by in vivo analysis. Structural prediction of the putative B12 receptor confirmed a 22-stranded beta-barrel core enclosing an immunologically shielded internal plug domain. To design multi-epitope chimeric and cocktail vaccines, three highly conserved, surface-exposed extracellular loops (L3, L7, and L8) were selected, and subsequently, the survival rate of active and passively immunized mice that were intranasally challenged with lethal doses of A. baumannii was evaluated. Active and passive immunizations with the whole B12 protein, its loops, chimeric construct, and Cocktail induced high antigen-specific IgG titers, conferring 60%-100% survival against lethal challenges. Immunized cohorts showed reduced pro-inflammatory cytokines (TNF-α, IFN-γ, and IL-6) and enhanced bacterial clearance, robustly halting systemic dissemination. Conversely, the internal plug domain showed no protective efficacy. These findings demonstrate that the extracellular loops of the putative vitamin B12 receptor are primary immunoprotective determinants, establishing the chimeric construct and Cocktail formulations as highly promising candidates to combat A. baumannii infections.