Pre-Clinical Development of Two DNA Vaccines Encoding MrkA of Klebsiella pneumoniae: Effect of Plasmid Backbone on Immunogenicity and Protective Efficacy Against Sepsis.

Development of vaccines against Klebsiella pneumoniae has potential to prevent infections in both community and hospital settings and reduce associated mortality and morbidity worldwide. DNA vaccines represent a promising approach given their stability, safety, design flexibility and cost-effectiveness for rapid production. In this study, two plasmid backbones, namely the commercial pVAX1 and the newly designed PLASMIVAX (containing additional CpG motifs), were tested to develop two DNA vaccines targeting the MrkA fimbrial subunit from K. pneumoniae. MrkA is highly conserved among K. pneumoniae and was expressed and secreted in vitro, fused to immunostimulatory elements, by both DNA vaccines following transfection of HEK-293T cells. Immunogenicity was assessed in mice by measuring antibody levels (IgM, IgG, IgG1, IgG2a and IgG3) after intramuscular immunization with three doses of 50 μg of each DNA vaccine. Efficacy of protection was established in a sepsis infection model. The pVAX1-MrkA DNA vaccine elicited the most robust antibody response and protection against K. pneumoniae infection (80%, p = 0.0373), while the PLASMIVAX-MrkA DNA vaccine protected at a lower rate (30%, p = 0.0163), consistent with lower post-immunization antibody titers elicited by this vaccine. This study reveals that DNA plasmid properties strongly influence immunogenicity and vaccine efficacy and highlights the potential role of MrkA as a vaccine antigen and DNA platforms as a valuable tool for developing vaccines against multidrug-resistant bacteria.