Visualising early signaling events during TCR activation.
Doctoral thesis, UCL (University College London).
T cell activation, differentiation and expansion are induced by signals generated upon engagement of TCR by agonist peptide:MHC. At the site of T cell interaction with an antigen presenting cell, TCR, accessory proteins such as CD28 and LFA-1, and the Src family kinases (SFK) Lck and Fyn, accumulate and form the immunological synapse. Lck is the key signaling molecule, initiating TCR signals by phosphorylating the ITAM in the CD3/ subunits resulting in the recruitment and activation of Zap-70 and further propagation of the signaling cascade. Fyn is a positive mediator of T cell signaling, sharing specific substrates in common with Lck, and also acts as a negative regulator by phosphorylating the adaptor protein, PAG. Phosphorylated PAG binds Csk and its recently identified binding partner, the protein phosphatase PEP, recruiting them to the plasma membrane where they can inhibit the SFK. The differential localisation of the kinases, with Fyn compartmentalised to lipid rafts in mature T cells and the majority of Lck being non-lipid raft associated could explain the unique functions attributed to each kinase. In order to examine the interactions between SFK and their negative regulators in T cell activation, CD8+ T cells from mice expressing the class I MHC-restricted TCR, F5, were investigated. In naïve T cells Lck, Fyn, Csk and PEP were shown to co-localise to the site of activation-induced tyrosine phosphorylation following early TCR crosslinking. In contrast, in memory T cells Csk, PEP and Fyn not only co-localised with the site of TCR activation but also distributed at the distal end of the cell, suggesting differential redistribution of key negative regulators away from the site of TCR engagement in these cells. In the absence of Fyn there was no change in Csk localisation despite the loss of PAG phosphorylation, suggesting another adaptor protein can recruit Csk to the plasma membrane. Caveolin-1, a cholesterol-rich membrane protein was investigated as a possible candidate for recruiting Csk and was shown to be present in CD8+ T cells. Moreover, caveolin-1 was shown to be phosphorylated by Lck, Fyn and Abl kinases upon TCR engagement and to redistribute and polarise to the cSMAC, co-localising with Csk. In summary we have shown that there are alterations in the distribution of the negative regulators Csk, PEP and Fyn, once T cells have encountered antigen, and that this could be associated to the more efficient responses of memory cells upon re-exposure to antigen. Furthermore, its appears there is redundancy of both signaling molecules and adaptor proteins, which may contribute to the fine tuning of the TCR signaling pathway, such that signals derived from the TCR can still modulate an appropriate immune response even in the absence of key regulators.
|Title:||Visualising early signaling events during TCR activation|
|Open access status:||An open access version is available from UCL Discovery|
|UCL classification:||UCL > School of Life and Medical Sciences > Faculty of Medical Sciences > Infection and Immunity (Division of)|
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