Smad4 and Transforming Growth Factor β Regulate the CD8 T Cell Response to Influenza Virus Infection

Transforming growth factor β (TGFβ) is an immunosuppressive cytokine with multiple functions in the immune system, including regulation of cytotoxic T lymphocytes. This thesis will examine how TGFβ, and the downstream signaling molecule Smad4, alter the CD8 T cell response to infection with different strains of influenza A virus (IAV). Chapter III shows how TGFβ and Smad4 regulate the phenotype, localization and function of antigen-specific CD8 T cells in the lungs. Our data reveal a previously unappreciated role of Smad4 in regulating effector and memory CD8 T cell differentiation independently of TGFβRII. Specifically we find that Smad4 promotes terminal differentiation of KLRG1+ T effector (TEFF) cells, a subset that is inhibited by TGFβ. Smad4 and TGFβ also play reciprocal roles in regulation of CD103+ tissue resident memory T (TRM) cells and CD62L+ central memory T (TCM) cells. Altered expression of multiple homing receptors changes the distribution of antigen-specific CD8 T cells in vivo, and therefore impacts viral clearance during primary viral infection and heterosubtypic immunity. Chapter IV explores the mechanism by which Smad4 regulates the CD8 T cell response, by examining the global transcription profile, cellular metabolism and signaling pathways of mutant and wild type CTLs. Our data suggests that Smad4 is activated by a novel pathway to promote terminal differentiation of effector T cells possibly through enhancing IL-12 signaling and T-bet expression, and inhibiting OXPHOS activity. Also, by enhancing T-bet and Eomesodermin, Smad4 may act as a suppressor of CD103 and later promotes re-expression of CD62L. This novel pathway of Smad4 ligand is counter-regulated by TGFβ, possibly by competing for Smad4 with phosphorylated Smad2/3. Our study sheds new light on the signals that regulate the differentiation and distribution of virus-specific CD8 T cells during IAV infection, which can be utilized to optimize cell-mediated immunity during the design of IAV vaccine