Activation of Invariant NKT Cells Exacerbates Experimental Visceral Leishmaniasis.

We report that natural killer T (NKT) cells play only a minor physiological role in protection from Leishmania donovani infection in C57BL/6 mice. Furthermore, attempts at therapeutic activation of invariant NKT (iNKT) cells with agalactosylceramide (α-GalCer) during L. donovani infection exacerbated, rather than ameliorated, experimental visceral leishmaniasis. The inability of (α-GalCer to promote anti-parasitic immunity did not result from inefficient antigen presentation caused by infection because (α-GalCer-loaded bone marrow-derived dendritic cells were also unable to improve disease resolution. The immune-dampening affect of (α-GalCer correlated with a bias towards increased IL-4 production by iNKT cells following (α-GalCer stimulation in infected mice compared to naïve controls. However, studies in IL- 4-deficient mice, and IL-4 neutralisation in cytokine-sufficient mice revealed that (α-GalCer-induced IL-4 production during infection had only a minor role in impaired parasite control. Analysis of liver cell composition following (α-GalCer stimulation during an established L. donovani infection revealed important differences, predominantly a decrease in IFNγ⁺ CD8⁺ T cells, compared with control-treated mice. Our data clearly illustrate the double-edged sword of NKT cell-based therapy, showing that in some circumstances, such as when sub-clinical or chronic infections exist, iNKT cell activation can have adverse outcomes. [ABSTRACT FROM AUTHOR]