Immunization with glutamine synthetase-deficient live attenuated parasites induces the differentiation of memory T cells to protect hosts from virulent infection of Leishmania donovani.

Visceral leishmaniasis (VL) is a major threat to economically challenged sectors of the world, and there are only limited options in vaccines available at present. Vaccination using live attenuated strains is preferred for their retention of true antigens, however, the major concern about these vaccines is raised by their ability to protect from virulent challenge at the early time point and also bearing the risk of a potential outbreak of their own. In this study, we reported a metabolically deficient glutamine synthetase double knock-out live attenuated strain of Leishmania donovani (LdGS-/-) that, with a very low inoculum single dose, is highly effective in generating an IFN-γ+ Th1-mediated immunity during virulent challenge as early as 2 weeks post-challenge time point. Additionally, at this early time point, immunization with these mutant parasites also led to the differentiation of CD4+ T effector and T memory cells in the spleens of BALB/c mice, providing long-term memory cell based protection from virulent LdGS+/+ (wild type) infection. The levels of expression of pro-inflammatory and T memory inducing and maintenance cytokines were also found to be higher in animals immunized with LdGS-/- parasites and challenged with the wild type parasites at even as high as the 8th week post-challenge. In contrast, there was no significant increase in these cytokines when immunized with the same number of LdGS+/+ and challenged with wild type parasites. Hence, our findings present that LdGS-/- parasites are novel and valuable for designing a potential live attenuated vaccine candidate against the virulent infection of L. donovani.