Activation of naive NK cells in response to Listeria monocytogenes requires IL-18 and contact with infected dendritic cells.

The mechanisms for NK cell activation during infection by intracellular bacterial pathogens are not clearly defined. To dissect how Listeria monocytogenes infection elicits NK cell activation, we evaluated the requirements for activation of naive splenic NK cells by infected bone marrow-derived dendritic cells (BMDCs). We found that NK cell activation in this setting required infection of BMDCs by live wild type bacteria. NK cells were not activated when BMDCs were infected with a live hemolysin deficient (Deltahly) strain. Neutralization of IL-12, TNF-alpha, or caspase-1 each dramatically reduced NK cell IFN-gamma production in response to live wt L. monocytogenes infection. Addition of recombinant IL-18, but not IL-1beta, reversed the effects of caspase-1 inhibition. Recombinant IL-18 also restored NK cell activation by BMDCs infected with Deltahly L. monocytogenes, which produced IL-12 but not IL-18. IL-18 acted on NK cells because MyD88 expression was required in responding NK cells, but not infected BMDC. However, secreted cytokines were not sufficient for activation of naive NK cells by infected BMDCs. Rather, NK cell activation additionally required contact between infected BMDCs and NK cells. These data suggest that the activation of NK cells during L. monocytogenes infection requires both secreted cytokines and ligation of NK activating receptors during direct contact with infected DCs.