Two Roads Diverged: Interferon α/β– and Interleukin 12–mediated Pathways in Promoting T Cell Interferon γ Responses during Viral Infection

Viral infections induce CD8 T cell expansion and interferon (IFN)-γ production for defense, but the innate cytokines shaping these responses have not been identified. Although interleukin (IL)-12 has the potential to contribute, IL-12–dependent T cell IFN-γ has not been detected during viral infections. Moreover, certain viruses fail to induce IL-12, and elicit high levels of IFN-α/β to negatively regulate it. The endogenous factors promoting virus-induced T cell IFN-γ production were defined in studies evaluating CD8 T cell responses during lymphocytic choriomeningitis virus infections of mice. Two divergent supporting pathways were characterized. Under normal conditions of infections, the CD8 T cell IFN-γ response was dependent on endogenous IFN-α/β effects, but was IL-12 independent. In contrast, in the absence of IFN-α/β functions, an IL-12 response was revealed and substituted an alternative pathway to IFN-γ. IFN-α/β–mediated effects resulted in enhanced, but the alternative pathway also promoted, resistance to infection. These observations define uniquely important IFN-α/β–controlled pathways shaping T cell responses during viral infections, and demonstrate plasticity of immune responses in accessing divergent innate mechanisms to achieve similar ultimate goals.