Memory CD8⁺ T cells provide an early source of IFN-γ

During the non-Ag-specific early phase of infection, IFN-γ is believed to be primarily provided by NK and NKT cells in response to pathogen-derived inflammatory mediators. To test whether other cell types were involved in early IFN-γ release, IFN-γ-producing cells were visualized in spleens and lymph nodes of LPS-injected mice. In addition to NK and NKT cells, IFN-γ was also detected in a significant fraction of CD8⁺ T cells. CD8⁺ T cells represented the second major population of IFN-γ-producing cells in the spleen (∼30%) and the majority of IFN-γ⁺ cells in the lymph nodes (∼70%). LPS-induced IFN-γ production by CD8⁺ T cells was MHC class I independent and was restricted to CD44^high (memory phenotype) cells. Experiments performed with C3H/HeJ (LPS-nonresponder) mice suggested that CD8⁺ T cells responded to LPS indirectly through macrophage/dendritic cell-derived IFN-α/β, IL-12, and IL-18. IFN-γ was also detected in memory CD8⁺ T cells from mice injected with type I IFN or with poly(I:C), a synthetic dsRNA that mimics early activation by RNA viruses. Taken together, these results suggest that in response to bacterial and viral products, memory T cells may contribute to innate immunity by providing an early non-Ag-specific source of IFN-γ.