IL‐18, but not IL‐15, contributes to the IL‐12‐dependent induction of NK‐cell effector functions by Leishmania infantum in vivo

Activation of NK cells is a hallmark of infections with intracellular pathogens. We previously showed that the protozoan parasite Leishmania infantum triggered a rapid NK-cell response in mice that required TLR9-positive myeloid DC and IL-12, but no IFN-alpha/beta. Here, we investigated whether IL-15 or IL-18 mediate the activity of IL-12 or function as independent activators of NK cells. In contrast to earlier studies that described IL-15 as crucial for NK-cell priming in response to TLR ligands, the expression of IFN-gamma, FasL, perforin and granzyme B by NK cells in L. infantum-infected mice was completely preserved in the absence of IL-15, whereas the proliferative capacity of NK cells was lower than in WT mice. IFN-gamma secretion, cytotoxicity and FasL expression of NK cells from infected IL-18(-/-) mice were significantly reduced compared with controls, but, unlike IL-12, IL-18 was not essential for NK-cell effector functions. Part of the NK-cell-stimulatory effect of IL-12 was dependent on IL-18. We conclude that IL-15 is not functioning as a universal NK-cell priming signal and that IL-18 contributes to the NK-cell response in visceral leishmaniasis. The cytokine requirements for NK-cell activation appear to differ contingent upon the infectious pathogen.