Bacterial outer membrane lipoproteins represent potent immunogens for the design of recombinant subunit vaccines. However, recombinant lipoprotein production and purification could be a challenge notably in terms of expression yield, protein solubility, and post-translational acylation. Together with the cost effectiveness, facilitated production, and purification as well as good stability, DNA-based vaccines encoding lipoproteins could become an alternative strategy for antibacterial vaccinations. Although the immunogenicity and the efficacy of DNA-based vaccines can be demonstrated in small rodents, such vaccine candidates could request concrete optimization as they are weak immunogens in primates and humans and particularly when administered by conventional injection. Therefore, the goal of the present study was to optimize the immunogenicity of a DNA vaccine encoding an outer membrane lipoprotein. LipL32, the major outer membrane protein from pathogenic Leptospira, was selected as a model antigen. We evaluated the influence of antigen secretion, the in vivo DNA delivery by electroporation, the adjuvant co-administration, as well as the heterologous prime-boost regimen on the induction of anti-LipL32 specific immune responses. Our results clearly showed that, following transfections, a DNA construct based on the authentic full-length LipL32 gene (containing leader sequence and the N-terminus cysteine residue involved in the protein anchoring) drives antigen secretion with the same efficiency as a plasmid-encoding anchor-less LipL32 and for which the bacterial leader sequence was replaced with a viral signal peptide. The in vivo DNA delivery by electroporation drastically enhanced the production of strong Th1 responses characterized by specific IgG2a antibodies and the IFNγ secretion in a restimulation assay, regardless of the DNA constructs used. In comparison with the heterologous prime-boost regimen, the homologous prime-boost vaccinations with DNA co-administrated with polyinosinic-polycytidylic acid (poly I:C) generated the highest specific IgG and IgG2a titers as well as the greatest IFNγ production. Taken together, these data suggest that optimization of outer membrane lipoprotein secretion is not critical for the induction of antigen-specific responses through DNA vaccination. Moreover, the potent antibody response induced by DNA plasmid encoding lipoprotein formulated with poly I:C and delivered through electroporation provides the rationale for the design of new prophylactic vaccines against pathogenic bacteria.
