SIR2-Deficient Leishmania infantam induces a defined IFN-γ/IL-10 pattern that correlates with protection

The ability to manipulate the Leishmania genome to create genetically modified parasites by introducing or eliminating genes is considered a powerful alternative for developing a new generation vaccine against leishmaniasis. Previously, we showed that the deletion of one allele of the Leishmania infantum silent information regulatory 2 (LiSIR2) locus was sufficient to dramatically affect amastigote axenic proliferation. Furthermore, LiSIR2 single knockout (US1112(+/+)) amastigotes were unable to replicate in vitro inside macrophages. Because this L. infuntunt mutant persisted in BALB/c mice for up to 6 wk but failed to establish an infection, we tested its ability to provide protection toward a virulent L. infuntum challenge. Strikingly, vaccination with a single i.p. injection of LiSIR2(+/+) single knockout elicits complete protection. Thus, vaccinated BALB/c mice showed a reversal of T cell anergy with specific anti-Leishinania cytotoxic activity and high levels of NO production. Moreover, vaccinated mice simultaneously generated specific anti-Leishmania IgG Ab subclasses suggestive of both type I and type 2 responses. A strong correlation was found between the elimination of the parasites and an increased Leishmania-specific IFN-,gamma/IL-10 ratio. Therefore, we propose that the polarization to a high IFN-,gamma/low IL-10 ratio after challenge is a clear indicator of vaccine success. Furthermore these mutants, which presented attenuated virulence, represent a good model to understand the correlatives of protection in visceral leishmaniasis.