Bystander CD8+T cells are Important Antiviral Effectors in the Brain

Abstract The central nervous system (CNS) presents a unique immunological environment with restricted infiltration of immune cells from the periphery. Since much is unknown about CNS-infiltrating immune cells during infection, we utilized the attenuated Venezuelan equine encephalitis virus vaccine strain TC83 (VEEV-TC83) to study T cell responses to this viral infection. VEEV-TC83 intracranial (IC) infection of wild-type mice produces high viral titers in the brain without lethality, enabling the study of the complete antiviral immune response in the CNS. Many T cells rapidly (5dpi) infiltrate the CNS, in a seemingly antigen-independent manner, upon IC VEEV-TC83 infection. During infection, CD8+ T cells (~40% of CNS-infiltrating T cells) in the brain express high levels of innate-memory markers. These bystander CD8+ T cells can produce IFNγ when treated with IL-12 and IL-18, cytokines known to induce innate signaling. IL-12, IL-18, and IL-15 (another innate-memory T cell supporter) are upregulated upon IC VEEV-TC83 infection. IC infection of IL-15 knockout (KO) mice results in decreased CNS infiltration by bystander CD8+ T cells. CD8+ T cells that do infiltrate the brains of IL-15 KO mice express fewer activation markers and cytokines, and are therefore likely not fully functional. Additionally, microglia express innate-memory cytokines during infection and likely support the recruitment and function of bystander CD8+ T cells in the CNS during infection. Data deposited in GEO indicate that infection with many neurotropic viruses induces Il15, Il12, and Il18 expression in CNS cells and tissues. Bystander CD8+ T cells are likely an important first line of defense during viral infection of the CNS. Supported by grants from the NIH (Training grant T32 AI07405 and RO1 NS123431)