Relative contributions of NK and CD8 T cells to IFN-γ mediated innate immune protection against Listeria monocytogenes

During the innate immune response to Listeria monocytogenes (LM), the secretion of IFN-γ is crucial in controlling bacterial numbers. We have shown recently that CD8 T cells have the ability to rapidly secrete IFN-γ independent of Ag, in response to IL-12 and IL-18, during a LM infection. In the current study, we compared the relative abilities of NK and CD8 T cells to provide innate immune protection. Upon transfer of either NK or memory OT-I T cells (specific for the OVA protein) into IFN-γ-deficient hosts that were infected subsequently with wild-type LM, both cell types were found in the spleen and had the ability to secrete IFN-γ. However, the OT-I T cells were more effective at providing innate immune protection as determined by spleen and liver LM burdens. We used immunocytochemistry to demonstrate that upon infection with LM, marginal zone macrophages were localized to the T cell area of the splenic follicle. Transferred memory OT-I T cells were also found in the T cell area of the spleen, colocalizing with the LM and macrophages. In sharp contrast, NK cells were found predominantly in the red pulp region of the spleen. In addition, memory OT-I T cells were also found to be associated with LM lesions in the liver. These results highlight the importance of CD8 T cells in innate immune responses to LM and suggest that their increased protective ability compared with NK cells is the result of their colocalization with LM and macrophages.