Type 1 cytokine/chemokine production by mouse NK cells following activation of their TLR/MyD88-mediated pathways

It is well established that IL-18R- and toll-like receptor (TLR)-mediated signalings share a common signal pathway mediated by signal adaptor, MyD88, and that IL-18 synergizes with IL-12 for IFN-γ production by NK cells. Here, we investigated whether TLR agonists can replace IL-18 for production of IFN-γ by NK cells. Freshly isolated NK cells possessed functional LPS receptor composed of TLR4/MD2 complex and of CD14, and also expressed other various tlrs. Hepatic CD3−DX5+ NK cells produced IFN-γ in response to TLR2 or TLR7 agonists only when co-stimulated with IL-12, indicating that TLR agonists synergize with IL-12 for IFN-γ. The tlr2−/− or tlr7−/− NK cells could not produce IFN-γ in response to IL-12 plus TLR2 or TLR7 ligands, respectively, indicating requirement of the corresponding TLRs. Furthermore, upon stimulation with these combinations, wild-type NK cells produced type 1 chemokines, such as CCL3, CCL4 and CCL5 as well. NK cells from bacterium (e.g. Propionibacterium acnes)-inoculated rag2−/− mice, when compared with those from naive mice, exhibited significantly enhanced capacity to produce these CC chemokines and IFN-γ, suggesting that microbial infection enhances responsiveness of NK cells to TLR agonists. These results indicate that upon microbial infection, macrophages produce IL-12 that renders NK cells highly responsive to TLR agonists to produce IFN-γ and chemokines, which might in turn recruit and fully activate macrophages, leading to the development of inflammatory foci presumably necessary for efficient microbial eradication. Thus, NK cells, like T cells, induce orchestrated immune responses in collaboration with macrophages to show potent host defense effects during early infectious phase.