Change in T Cell Signalling During Ageing

T cell differentiation is a highly complex process controlled not only by costimulation but also by the strength and duration of T cell receptor (TCR) signalling [34]. Nearly all TCR signalling pathways have been found altered during ageing (Fig. 2.1), with several studies suggesting aberrancies in early TCR mediated signalling events, such as changes to protein tyrosine phosphorylation, calcium mobilization and the translocation of protein kinase C to the plasma membrane [35-37]. In addition, there is also a decline in proximal and intermediate signalling events leading to decreased transcription factor activity, notably NF-kB and NF-AT [38]. Furthermore recent findings have shown the Lck pathway to be an important factor controlling T cell signalling [39]. Lck activity is regulated by two phospho- tyrosine residues, Tyr394 which stabilises an open conformation and promotes kinase activity, and Tyr505 which results in a closed conformation decreasing activity. The lack of phosphorylation in either site can result in partial activation of lck [40]. The balance among these three pools of differentially phosphorylated lck is thought to determine the general level of enzymatic activity of lck in T cells [40]. Age-related changes in both Tyr394 and Tyr505 have been reported for CD4+ T cells, both showing increased phosphorylation [36], confounding the authors as phosphorylation at these sites have opposing effects. However recent data suggests that the local concentration of active lck molecules is more important than the

T cell receptor signalling pathways

Fig. 2.1 T cell receptor signalling pathways. This scheme highlights the most investigated signalling pathways in the context of ageing research overall phosphorylated state of lck [41]. Therefore it remains to be determined whether highly differentiated T cells exhibit changes in either the location or movement of lck molecules in the membrane.

The immune microenvironment also plays a crucial role in shaping lineage commitment and ultimately the function of T cells (Fig. 2.1). There is now much evidence that mTOR (mammalian target of rapamycin), plays a central role controlling T cell function through its ability to connect immune signalling to metabolism [42]. The kinase mTOR belongs to the phosphatidylinositol 3-kinase (PI3K)-related kinase (PIKK) family of proteins that act as regulators of cellular growth and metabolism [43] . mTOR is the catalytic subunit of two distinct signalling complexes, mTOR complex 1 (mTORC1) and mTOR complex 2 (mTORC2), the activity of which are differentially regulated by distinct accessory proteins [42, 43]. Increased mTOR activity has been linked to senescence and ageing [44-47], however there is growing evidence to suggest that this increase is not universally observed in every cell type nor is it evident in older humans [27, 48-50]. Highly differentiated CD4+ (unpublished observations) and CD8+ T cells are unable to phosphorylate either the mTOCR1 [27] or the mTORC2 complex [51].

mTORC1 activity is a requirement for the generation of effector molecules [52], and despite highly differentiated CD8+ T cells apparent lack of mTOR activity they are highly potent effectors [26, 27]. mTORC1 has also been shown to control transcriptional programs that determine CD8+ effector fate via a HIF1- dependent mechanism [53]. However HIF1-null CD8+ T cells were shown to have many characteristics of effector CD8+ T cells such as high levels of IFNy production, they lacked perforin and granzyme expression. A situation which is mirrored in highly differentiated CD8+ T cells isolated from old individuals, where these cells display high levels of TNFa and IFNy but lower levels of perforin and granzyme [25]. Furthermore when the mTORC1 inhibitor rapamycin was incubated with highly differentiated CD8+ T cells it had no effect on IFNy production [54]. Thus corroborating with the idea that mTOR may not play a role in T cell senescence.

An increasingly recognized pathway for modulating T cell signalling is via reactive oxygen species (ROS; Fig. 2.1) [55]. ROS influences the balance between protein tyrosine kinase and phosphatase activities through redox- dependent regulation of signalling [38]. During TCR stimulation there is a transient increase in ROS, which inactivates SHP-1 facilitating TCR signalling, however in continued ROS SHP-1 regulation is further altered, leading to the negative regulation of TCR function. High levels of ROS have been found in highly differentiated CD8+ T cells [27] and during ageing [56], generated in part by impaired mitochondrial function [27]. Indeed the requirement for robust mitochondria in antigen-specific T cell expansion has been demonstrated using mice with T cell-specific alterations to complex III [57]. Thus, ageing increased oxidative stress, together with changes in tyrosine kinase and phosphatase activities all contribute to the altered T cell signalling observed during T cell differentiation and ageing.

 
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