Ubiquitin conjugation of open reading frame F DNA vaccine leads to enhanced cell-mediated immune response and induces protection against both antimony-susceptible and -resistant strains of Leishmania donovani

Abstract

Resistance of Leishmania donovani to sodium antimony gluconate has become a critical issue in the current, prolonged epidemic in India. Hence, there is an urgent need for a vaccine that is protective against both antimony-susceptible and -resistant strains of L. donovani. The multigene LD1 locus located on chromosome 35 of Leishmania is amplified in ~15% of the isolates examined. The open reading frame F (ORFF), a potential vaccine candidate against visceral leishmaniasis, is part of the multigene LD1 locus. ORFF was expressed as a chimeric conjugate of ubiquitin to elicit an Ag-specific cell-mediated immune response. Analysis of the cellular immune responses of ubiquitin-conjugated ORFF (UBQ-ORFF) DNA-immunized, uninfected BALB/c mice demonstrated that the vaccine induced enhanced IFN-γ -producing CD4⁺ and CD8⁺ T cells compared with nonubiquitinated ORFF DNA vaccine. Higher levels of IL-12 and IFN-γ and the low levels of IL-4 and IL-10 further indicated that the immune responses with UBQ-ORFF were mediated toward the Th1 rather than Th2 type. Infection of immunized mice with either the antimony-susceptible (AG83) or -resistant (GE1F8R) L. donovani strain showed that UBQ-ORFF DNA vaccine induced higher protection when compared with ORFF DNA. UBQ-ORFF DNA-immunized and -infected mice showed a significant increase in IL-12 and IFN-γ and significant down-regulation of IL-10. High levels of production of nitrite and superoxide, two macrophage-derived oxidants that are critical in controlling Leishmania infection, were observed in protected mice. The feasibility of using ubiquitinated-conjugated ORFF DNA vaccine as a promising immune enhancer for vaccination against both antimony-susceptible and -resistant strains of L. donovani is reported.