The Study Of T Cell Receptor Signaling Pathway And Function Of Co-receptor

Part I Regulation of Akt by PKC-a downstream of TCRAkt, also known as protein kinase B (PKB), is a serine/threonine protein kinase belonging to AGC kinase family. It consists of three isoforms, they're Aktl/PKBa, Akt2/PKBβand Akt3/PKBy, which are encoded by different genes and share high homology. Akt is conserved from primitive metazoan to humans and play important roles in a plethora of cellular functions such as cell survival/apoptosis, proliferation, differentiation, glucose metabolism, growth transcription, cell-cycle progression, motility, transformation and angeogenesis.Since its discovery as a target of PI3K, it has been intensively studied during the past 15 years. Plenty of studies report that PI3K/Akt pathway is involved in the development of a number of human cancers, and numerous chemicals targeting members in this pathway have been or are being developed as anti-tumor drugs. Recently, people became interested in what kind of role the PI3K/Akt pathway might play in immunology especially in T cells. It's reported that PI3K/Akt pathway could be activated by signals transduced from T cell receptor (TCR) and/or costimulator CD28; PI3K/Akt pathway is very important to T cell development as well as thymocyte survival; PI3K/Akt pathway also participates in T cell migration (mainly refers to distribution among tissues here). More and more studies show that inhibition of PI3K/Akt/mTOR pathway can increase the generation of inducible regulatory T cells (iTreg) whom were hotly studied recently.Till now, the mechanism of Akt activation upon TCR stimulation is still unknown. The structure of Akt is highly conserved, it consists of three domains: 1)the N-terminal regulatory domain; 2) a central kinase domain which contains a threonine residue (Thr308) which can be phosphorylated by the kinase named phosphoinositide-dependent kinase 1 (PDK1); 3) a C-terminal region which contains the hydrophobic motif (HM) that can be phosphorylated at a serine residue (Ser473) by the kinase namely PDK2. Only when Akt is phosphorylated at both sites, it gets its full activity. For a long time, PDK1 is well accepted for phosphorylating Akt at Thr308, but the PDK2 responsible for Akt Ser473 phosphorylation is still elusive. There're at least 10 PDK2 candidates, including mitogen-activated protein (MAP) kinase-activated protein kinase-2 (MK2), integrin-linked kinase (ILK), p38 MAP kinase, protein kinase C-a (PKC-α), PKC-b, the NIMA-related kinase-6 (NEK6), the mammalian target of rapamycin complex 2 (mTORC2), the double-stranded DNA-dependent protein kinase (DNK-PK). It's possible that phosphorylation of Akt at the Ser473 could be cell type, signaling pathway, and substrate specific.It's reported that PKC-βcan phosphorylates Akt at Ser473 but not Thr308. Then we wondered whether PKC-βor other PKC isotype(s) are involved in the regulation of Akt.Using biochemical and genetic approaches, we first demonstrated that loss of PKCq didn't affect the phosphorylation of Akt downstream of TCR. Inhibitor experiments indicated that conventional PKC was involved in the regulation of Akt, while further in vitro kinase assay revealed that only PKCa but not PKCβfrom T cells could phosphorylate Akt at Ser473 and such regulation was dependent on TCR signaling. Consistent with this, absence of PKCa reduced phosphorylation of Akt and its downstream p70S6K and FOXOl/3a, while phosphorylation of GSK3 was not affected. At last, we found that the regulation of Akt by PKCa wasn't through mTORC2. Hence our study demonstrated that PKCa acted as a PDK2 in T cells upon TCR stimulation for the first timePartⅡCTLA-4-B7 Interaction Suppresses Th17 Cell DifferentiationTh17 cells are a distinct subset of Th cells that produce IL-17 implicated in several autoimmune diseases. Although CD28-B7 interaction has been shown to be required in Th17 differentiation in vitro, the role of CTLA-4-B7 interaction in controlling Th17 development is completely unknown. We report here that using hCTLA-4Ig or anti-CTLA-4 antibody which can block the CTLA-4-B7 interaction potentiates IL-17 production and Th17 cell differentiation in vitro and in vivo. Furthermore, blocking CTLA-4-B7 interaction in CD28-/- mice increases its susceptibility to experimental autoimmune myocarditis (EAM), which is mediated by heightened Th17 responses. With these results, we are the first to demonstrate that CTLA-4-B7 interaction inhibits Th17 differentiation then suppresses Th17-mediated autoimmunity.