A universally conserved OMV-YidR conjugate nanovaccine confers broad protection against hypervirulent and carbapenem-resistant Klebsiella pneumoniae.

The escalating crisis of multi-drug resistant (MDR) Klebsiella pneumoniae (KP) renders standard antibiotic treatments increasingly ineffective, necessitating a shift toward alternative approaches like vaccination. However, the development of a broadly protective vaccine remains hindered by the pathogen's extensive serotype diversity. To overcome this limitation, we targeted YidR-a highly conserved protein with potential as a universal antigen-and engineered a semi-synthetic conjugate vaccine by covalently linking it to KP-derived outer membrane vesicles (OMVs). The OMV-YidR conjugate successfully overcame the poor intrinsic immunogenicity of soluble YidR, eliciting robust antigen-specific IgG titers >20,000-fold higher than the unconjugated protein. Furthermore, the conjugate vaccine drove a shift from a weak Th2 bias to a potent, protective Th1/Th17 cellular profile. Crucially, this enhanced immunogenicity translated into broad-spectrum efficacy: while unconjugated OMVs protected only against the homologous strain, the OMV-YidR conjugate conferred complete (100%) survival against lethal challenges with diverse clinical isolates, including hypervirulent and carbapenem-resistant lineages. Mechanistic and comparative analyses revealed that covalent linkage-rather than simple physical co-administration-was essential for this superior efficacy, which was correlated with the preferential trafficking and uptake of the antigen by B cells in the draining lymph nodes. These findings establish the OMV-YidR conjugate as a highly potent, universal vaccine candidate capable of bypassing serotype limitations to prevent untreatable MDR K. pneumoniae infections.