Interleukin 12–dependent Interferon γ Production by CD8α⁺Lymphoid Dendritic Cells

We investigated the role of antigen-presenting cells in early interferon (IFN)-γ production in normal and recombinase activating gene 2–deficient (Rag-2^−/−) mice in response to Listeria monocytogenes (LM) infection and interleukin (IL)-12 administration. Levels of serum IFN-γ in Rag-2^−/− mice were comparable to those of normal mice upon either LM infection or IL-12 injection. Depletion of natural killer (NK) cells by administration of anti-asialoGM1 antibodies had little effect on IFN-γ levels in the sera of Rag-2^−/− mice after LM infection or IL-12 injection. Incubation of splenocytes from NK cell–depleted Rag-2^−/− mice with LM resulted in the production of IFN-γ that was completely blocked by addition of anti–IL-12 antibodies. Both dendritic cells (DCs) and monocytes purified from splenocytes were capable of producing IFN-γ when cultured in the presence of IL-12. Intracellular immunofluorescence analysis confirmed the IFN-γ production from DCs. It was further shown that IFN-γ was produced predominantly by CD8α⁺ lymphoid DCs rather than CD8α⁻ myeloid DCs. Collectively, our data indicated that DCs are potent in producing IFN-γ in response to IL-12 produced by bacterial infection and play an important role in innate immunity and subsequent T helper cell type 1 development in vivo.