Memory CD8⁺ T Cells Provide Innate Immune Protection against Listeria monocytogenes in the Absence of Cognate Antigen

Interferon (IFN)-γ plays an important role in the innate immune response against intracellular bacterial pathogens. It is commonly thought that natural killer cells are the primary source of this cytokine that is involved in activating antibacterial effects in infected cells and polarizing CD4⁺ T cells toward the Th1 subset. However, here we show that both effector and memory CD8⁺ T cells have the potential to secrete IFN-γ in response to interleukin (IL)-12 and IL-18 in the absence of cognate antigen. We demonstrate that memory CD8⁺ T cells specific for the ovalbumin protein secrete IFN-γ rapidly after infection with wild-type Listeria monocytogenes (LM). Furthermore, small numbers of ovalbumin-specific, memory CD8⁺ T cells can reduce spleen and liver bacterial counts in IFN-γ–deficient mice 3 d after LM infection. Up-regulation of the receptors for IL-12 and IL-18 provides a mechanism for the ability of memory CD8⁺ T cells to respond in this antigen nonspecific manner. Thus, CD8⁺ T cells play an important role in the innate immune response against intracellular pathogens by rapidly secreting IFN-γ in response to IL-12 and IL-18.