An OmpX mRNA-LNP vaccine induces protective humoral and cellular immunity against Klebsiella pneumoniae in a C57BL/6 mouse model

Klebsiella pneumoniae (KP) poses a significant threat to global public health, and effective preventive measures are urgently needed. The widespread antibiotic resistance of KP underscores the critical need for vaccine-based strategies. Although numerous vaccine candidates against KP have been investigated, none has yet been licensed. mRNA-LNP vaccines against KP have only recently begun to be explored, and their ability to deliver conserved outer membrane protein antigens remains largely untested. In this study, we constructed an mRNA vaccine encoding the highly conserved outer membrane protein X (OmpX) of KP-the first mRNA-LNP vaccine targeting this antigen-and evaluated its immunogenicity and protective efficacy in mice. Immunoinformatics analyses predicted concordance between the in silico and in vivo outcomes: bioinformatics tools were used to evaluate the antigenicity, immunogenicity, physicochemical parameters, and secondary and tertiary structures of OmpX, and the C-ImmSim server was used to simulate the vaccine-elicited immune responses. We then performed mouse immunization and challenge experiments to characterize the humoral and cellular immune responses induced by the OmpX vaccine. The mRNA vaccine was immunogenic in mice, eliciting high titers of OmpX-specific antibodies. OmpX-Fc mRNA-LNPs also induced robust cell-mediated immunity, with significantly elevated IFN-γ and IL-4 levels indicative of combined Th1/Th2 cellular responses. Serum from OmpX-immunized mice promoted significantly greater complement-mediated KP cell lysis than serum from the alum or luciferase mRNA-LNP control groups. Notably, OmpX-Fc mRNA-LNP immunization conferred complete protection against lethal KP challenge, with 100% survival and a significant reduction in visceral bacterial burden. These data establish a promising mRNA vaccine strategy for inducing broad immune responses and promoting serotype-independent immunity against KP.