Development of Novel Prime-Boost Strategies Based on a Tri-Gene Fusion Recombinant <i>L. tarentolae</i> Vaccine against Experimental Murine Visceral Leishmaniasis

<div><p>Visceral leishmaniasis (VL) is a vector-borne disease affecting humans and domestic animals that constitutes a serious public health problem in many countries. Although many antigens have been examined so far as protein- or DNA-based vaccines, none of them conferred complete long-term protection. The use of the lizard non-pathogenic to humans <i>Leishmania (L.) tarentolae</i> species as a live vaccine vector to deliver specific <i>Leishmania</i> antigens is a recent approach that needs to be explored further. In this study, we evaluated the effectiveness of live vaccination in protecting BALB/c mice against <i>L. infantum</i> infection using prime-boost regimens, namely Live/Live and DNA/Live. As a live vaccine, we used recombinant <i>L. tarentolae</i> expressing the <i>L. donovani</i> A2 antigen along with cysteine proteinases (CPA and CPB without its unusual C-terminal extension (CPB^-CTE)) as a tri-fusion gene. For DNA priming, the tri-fusion gene was encoded in pcDNA formulated with cationic solid lipid nanoparticles (cSLN) acting as an adjuvant. At different time points post-challenge, parasite burden and histopathological changes as well as humoral and cellular immune responses were assessed. Our results showed that immunization with both prime-boost A2-CPA-CPB^-CTE-recombinant <i>L. tarentolae</i> protects BALB/c mice against <i>L. infantum</i> challenge. This protective immunity is associated with a Th1-type immune response due to high levels of IFN-γ production prior and after challenge and with lower levels of IL-10 production after challenge, leading to a significantly higher IFN-γ/IL-10 ratio compared to the control groups. Moreover, this immunization elicited high IgG1 and IgG2a humoral immune responses. Protection in mice was also correlated with a high nitric oxide production and low parasite burden. Altogether, these results indicate the promise of the A2-CPA-CPB^-CTE-recombinant <i>L. tarentolae</i> as a safe live vaccine candidate against VL.</p>