Distinct Gene Circuits Control the Differentiation of Innate Versus Adaptive IL-17 Producing T Cells: A Dissertation

Abstract

T lymphocytes are distinguished by the expression of αβ TCR or γδ TCR on their cell surface. The kinetic differences in the effector functions classifies γδ T cells as innate-like lymphocytes and αβ T cells as adaptive lymphocytes. Although distinct, αβ and γδ T cell lineages produce a common array of cytokines to mount an effective immune response against a pathogen. The production of cytokine IL-17 is a shared characteristic between the γδ T (Tγδ17) cells and the CD4 T (Th17) cells. γδ T cells develop into Tγδ17 cells in the thymus whereas CD4 T cells differentiate into Th17 cells in response to antigens in the peripheral lymphoid tissues. γδ T cells exported from the thymus, as pre-made effectors, are the early IL-17 producers compared with the late IL-17 producing Th17 cells. In this thesis we describe how TGFβ-SMAD2 dependent pathway selectively regulates Th17 cell differentiation but not Tγδ17 cells generation. We further illustrate the requirement of WNT-HMG box transcription factor (TF) signaling for the thymic programming of Tγδ17 cells. Cytokine TGFβ in co-operation with IL-6 induces the differentiation of Th17 cells. Conversely, TGFβ signaling also regulates the differentiation and maintenance of CD4+FOXP3+ regulatory T cells. The mechanism by which TGFβ signals synergize with IL-6 to generate inflammatory versus immunosuppressive T cell subsets is unclear. TGFβ signaling activates receptor SMADs, SMAD2 and SMAD3, which associate with a variety of nuclear factors to regulate gene transcription. Defining relative contributions of distinct SMAD molecules for CD4 T cell differentiation is critical for mapping the versatile intracellular TGFβ signaling pathways that tailor TGFβ activities to the state of host interaction with pathogens. We show here that SMAD2 is essential for Th17 cell differentiation and that it acts in part by modulating the expression of IL-6R on T cells. While mice lacking SMAD2 specifically in T cells do not develop spontaneous lymphoproliferative autoimmunity, Smad2-/- T cells are impaired in their response to TGFβ in vitro and in vivo and they are more pathogenic than controls when transferred into lymphopenic mice. These results demonstrate that SMAD2 is essential for TGFβ signaling in CD4+ T effector cell differentiation and that it possesses functional capabilities distinct from SMAD3. Although SMAD2 is essential for the differentiation of Th17 cells, TGFβ signaling via SMAD2 is not required for the thymic programming of innate Tγδ17 cells. Among different γδ T cells, Vγ2+ (V2) γδ T cells are the major IL-17 producing subsets. We demonstrate that Sry-high mobility group (HMG) box TFs regulate the development of V2 Tγδ17 cells. We show that the HMG box TF, SOX13 functions in a positive loop for the intrathymic generation of V2 Tγδ17 cells. SOX13 regulates the programming of Tγδ17 cells by controlling the expression of B-lymphoid kinase (BLK) in developing immature V2 γδ T cells. BLK is an Src-family kinase expressed by all Tγδ17 cells. Furthermore, we show another HMG box TF, TCF1, the nuclear effector of canonical WNT signaling, is the primary negative regulator of IL-17 production by all γδ T cells. We propose that the antagonism of SOX13 and TCF1 determines the generation of IL-17 producing γδ T cells. We also show that extrinsic cues from αβ T cells do not affect the generation of IL-17 producing γδ T cells. Using OP9-DL1 culture system, we demonstrate that the progenitors of V2 Tγδ17 cells are the c-Kit+ early thymic precursors.