Cyclin D1 T Unlike D2/D3 cyclins, cyclin D1 is not up-regulated following primary T cell stimulation.
Cyclin D3 B Cyclin D3, but not D l, compensates cyclin D2 deficiency following antigen receptor stimulation.
Cyclin A/cdk2 T,B Inhibition of Rag-2 accumulation and V(D)J recombination.
Cdk2/4/6 T Anti-CD3 activation of quiescent cells results in cdk up regulation.
pl5/16 T Accumulate in senescent cells.
pl8 T,B pi 8'^' mice develop lymphoproliferative disorders
p27 T STAT6 controls IL-4-dependent proliferation by p27 regulation.
p27 T CD28 co-stimulation leads to down regulation.
p27 T Elevated in anergic cells.
p21/27 T Decreased by IL-2.
p21/27 T Ageing-related unresponsiveness associated with elevated levels.
p21 B CD40 ligation increases expression.
p21 M(() Mediates IFN-y-dependent cell cycle arrest.
p21 T Deletion results in T cell hyper-proliferation, accumulation of m emory T cells, loss o f tolerance a nd systemic lupus erythematosus
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1.4 Signalling mediated by yc cytokines
It has already been stated that cytokines that signal via the common y receptor of the IL-2 receptor (yc cytokines) are capable of promoting the survival of T cells from CDD, These cytokines, which include IL-2, IL-4, IL-7, IL-9 and IL-15 all activate specific members of the STAT family (Fig. 1.9.). STATs are latent cytoplasmic transcription factors that become activated once a specific tyrosine residue is phosphorylated. This phosphorylation leads to dimérisation of STAT molecules. STAT dimers migrate to the nucleus to regulate the transcription of many different genes
( 1 3 3 ) .
The IL-2 receptor is composed of 3 chains: the a chain (IL-2Ra or CD25), p chain (IL-2RP or CD 122) and the y chain (IL-2Ry). IL-2Ry does not possess intrinsic tyrosine kinase activity and therefore cannot activate latent STAT molecules by itself. For this reason IL-2Ry is non-covalently linked to a member of the Janus kinase family that becomes activated upon cytokine signalling and is able to activate STAT molecules (134). Binding of a yc cytokine to its receptor results in dimérisation of receptor chains bringing together two Janus activated kinases (JAK) that are activated by transphosphorylation. Once activated, JAK kinases can activate STAT molecules by phosphorylating tyrosine residues (134). JAKl phosphorylates STATl, 3 and 5 while JAK3 is capable of phosphorylating STAT3 and 5. STATS has been associated with IL- 2 mediated signalling (135) and has been found to be involved in the induction of genes required for cell proliferation and survival.
Phosphorylation of the IL-2Rp results in the activation of two different intracellular pathways. Firstly, ligation of the IL-2Rp results in activation of the mitogen activated protein kinase (MAPK) pathway. Once activated, MAPK phosphorylates cdk2 allowing it to associate with cyclin E. Cyclin E/cdk2 complex phosphorylates pRb thereby releasing the transcription factor E2F, initiating Gi to S- phase transition. Activation of the MAPK pathway is also shown to be linked with up regulation of Bcl-2 and B c 1 -x l. Secondly, phosphorylation of IL-2RP has been shown to lead to the recruitment of phosphatidylinositol 3-kinase (PI3K). Activation of the PI3K pathway has been shown to result in the inhibition of apoptosis by regulating members of the Bcl-2 family (97).
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In summary, yc cytokines promote T cell survival by inducing cell cycle progression through initiation of the JAK/STAT pathway, and also by activating both the MAPK and P13K pathways leading to up regulation o f anti-apoptotic members of the Bcl-2 family.
yc Cytokine
lL-2Ry
JAK
JAK
STAT molecule
Dimérisation
Activation of transcription
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1.5 Type I IFN
IFN was initially discovered as a substance able to induce an antiviral state in cells (136). IFNs are divided into two categories, type I IFN and type II IFN that share a large degree of homology and have overlapping functions (137,138). The type I IFN family is comprised of IFN-a, IFN-P, IFN-®, IFN-t (139) and the more recently described Limitin (140).
Type I IFN are capable of mediating antiviral (141), anti-proliferative (114), immunomodulatory (142-144) and anti-apoptotic effects (106,145) on lymphocytes and other cell types.
In humans, at 1 east 14 n onallelic genes e ncode structurally d ifferent forms of IFN-a (137,146,147) whereas only one gene encodes for IFN-p (146). All human type I IFN genes are clustered on the short arm of chromosome 9 (148). IFN-a and IFN-P proteins are 15-21kDa and 22kDa in size respectively (139). IFN-a and IFN-ro are produced by leucocytes and primary dendritic cells (79,149,150) while IFN-p is produced b y fibroblasts and other stromal cells (139). In conjunction with those cell types already mentioned, specialised leucocytes termed natural IFN-producing cells (IPC) have also been described (i.e. plasmacytoid cells / type 2 dendritic cells [DC2]) (151). IPCs express CD4 and MHC class II but lack haematopoietic-lineage markers (79). It has recently been demonstrated that an average of 180 units/ml of type I IFN was produced by peripheral blood mononuclear cells (PBMC) stimulated by herpes simplex virus (HSV), whereas the amount produced by purified precursor DC2 was in the order of 20,000 to 638,000 units/ml (79).
1.5.1 IFN signal transduction
There is one receptor for all type I IFN and another for IFN-y. The type I IFN receptor is composed of two subunits, IFN receptor (IFNAR) 1 and IFNAR2 that possess extracellular, transmembrane and cytoplasmic domains. IFNAR 1 and IFNAR2 associate in the cell membrane when either unit binds type I IFN (146). Interaction between two protein tyrosine kinases, Tyk2 on IFNRAl and JAKl on IFNRA2, results in a chain reaction leading to the phosphorylation of STATS (Fig. 1.10.). STATl and 2 are bound to the cytoplasmic tail of IFNAR2 (152) and may form homodimer
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(STATl:STATl) or heterodimer (STATl:STAT2) complexes once phosphorylated (153). These activated STAT complexes form what is termed the ISGF3 complex by binding to an intracellular protein p48 (146). The ISGF3 complex migrates to the nucleus where it binds to a DNA element present in the promoter of all IFN-inducible genes (146). This chain of events is termed the classical pathway. Limitin, a recently described molecule with a degree of homology to type I IFN (32% to IFN-a and 26% to IFN-p), has also been shown to activate the classical pathway by tyrosine phosphorylation of JAKl, Tyk2, STATl and STATS but not JAK2, JAK3 or STAT3 (139).
Alternative pathways have also been demonstrated (Fig. 1.10.). IFN-p has been shown to be involved with components o f the MAP kinase pathway by inducing the phosphorylation o f ERK2 ( 154). Furthermore, IFN-a has been shown to activate the TCR-associated proteins CD45 and Ick, by inducing their phosphorylation (131).
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