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Selection of the T cell repertoire in the thymus is governed by the need to create a repertoire of peripheral T cells that can respond to any foreign antigen in the context of self-major histocompatibility complex (MHC), while enforcing central tolerance to self-antigens. Perturbations in signalling molecules, that reduce the affinity of thymic selection, can lead to the production of a peripheral repertoire with increased autoimmunity, as has been shown for mutations in the Zap-70 kinase. Upstream of Zap-70 is Lck, the most proximal tyrosine kinase required for T cell receptor (TCR) triggering upon TCR engagement by peptide:MHC. In order to study how Lck influences T cell activation, a transgenic mouse model (LckVA), in which Lck is expressed constitutively from a T cell specific transgene and mice have very low expression of Lck (~5% of WT) in both the thymus and periphery, was used. It has been shown that Lck is critical for successful T cell development, yet the results of this thesis show that even 5% of WT levels of Lck are sufficient for selection of thymic T cells on both polyclonal and F5 TCR transgenic backgrounds. Previous studies utilising mice expressing an inducible Lck transgene, which also had reduced Lck expression in the periphery, showed Lck to be critical in determining the activation threshold of T cells. In contrast, peripheral T cells in LckVA mice had similar activation thresholds to wild type T cells, as measured by in vitro upregulation of early activation markers. Further analysis of LckVA peripheral T cells revealed differential influences of low expression of Lck on downstream signalling pathways upon TCR engagement. For example, ERK signalling was impaired, while calcium flux and proliferation were enhanced in LckVA T cells. Finally, LckVA T cells were altered in their ability to differentiate, showing enhanced production of cytokines and retaining the capacity to form memory cells.

GITR and CD30 are T cell costimulatory members of the TNFR superfamily known to regulate T cell responses. Elucidating the mechanisms whereby these receptors modulate T cell responses is crucial for maximizing their potential for immunotherapy. In this thesis, I examine the role of GITR and CD30 on CD8 T cell responses to influenza virus. I show that CD8 T cell intrinsic GITR is required for both maximal primary and secondary CD8 T cell expansion to influenza, while in contrast, CD30 is dispensable for anti-influenza CD8 T cell responses. GITR does not impact on CD8 T cell proliferation or homing, however, it mediates CD8 T cell survival signaling. GITR induces TRAF2/TRAF5 dependent, but TRAF1 independent, NF-κB activation, resulting in the upregulation of the pro-survival molecule Bcl-xL. Furthermore, I show that GITR on CD8 T cells can augment viral clearance and confer protection from death upon severe influenza infection of mice. Similarly, CD30 also elicits protection from death upon severe influenza infection, although the cells responsible for this effect remain to be elucidated. In this thesis, I also show that in unimmunized mice GITR expression is upregulated to higher than basal levels on a population of CD8 memory phenotype cells in the bone marrow. In contrast, CD8 memory phenotype T cells in the spleen and LN have GITR levels similar to that on naïve T cells. The upregulation of GITR in the bone marrow is IL-15 dependent and therefore, GITR serves as a marker for cells that have recently received an IL-15 signal. Furthermore, GITR is required for the persistence, but not for the homeostatic proliferation of CD8 memory phenotype T cells in the bone marrow. Therefore, GITR plays a key role for CD8 T cell intrinsic responses to influenza, as well as for the persistence of CD8 memory phenotype T cells.

GITR and CD30 are T cell costimulatory members of the TNFR superfamily known to regulate T cell responses. Elucidating the mechanisms whereby these receptors modulate T cell responses is crucial for maximizing their potential for immunotherapy. In this thesis, I examine the role of GITR and CD30 on CD8 T cell responses to influenza virus. I show that CD8 T cell intrinsic GITR is required for both maximal primary and secondary CD8 T cell expansion to influenza, while in contrast, CD30 is dispensable for anti-influenza CD8 T cell responses. GITR does not impact on CD8 T cell proliferation or homing, however, it mediates CD8 T cell survival signaling. GITR induces TRAF2/TRAF5 dependent, but TRAF1 independent, NF-κB activation, resulting in the upregulation of the pro-survival molecule Bcl-xL. Furthermore, I show that GITR on CD8 T cells can augment viral clearance and confer protection from death upon severe influenza infection of mice. Similarly, CD30 also elicits protection from death upon severe influenza infection, although the cells responsible for this effect remain to be elucidated. In this thesis, I also show that in unimmunized mice GITR expression is upregulated to higher than basal levels on a population of CD8 memory phenotype cells in the bone marrow. In contrast, CD8 memory phenotype T cells in the spleen and LN have GITR levels similar to that on naïve T cells. The upregulation of GITR in the bone marrow is IL-15 dependent and therefore, GITR serves as a marker for cells that have recently received an IL-15 signal. Furthermore, GITR is required for the persistence, but not for the homeostatic proliferation of CD8 memory phenotype T cells in the bone marrow. Therefore, GITR plays a key role for CD8 T cell intrinsic responses to influenza, as well as for the persistence of CD8 memory phenotype T cells.

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The contributions of peptide antigen affinity for TCR in driving T cell memory is unclear. Effector CD4 T cells must recognize cognate antigen again at an effector checkpoint, 5-8 days post-infection, to generate an optimal memory population. In this thesis, we examined whether peptide affinity for the TCR of effectors impacts the extent of memory and degree of protection against rechallenge. We used an influenza A virus (IAV) nucleoprotein (NP)-specific TCR transgenic strain, FluNP, and generated NP- peptide variants that bind FluNP TCR with a broad range of avidity. Varying peptide avidity in vivo at the effector checkpoint revealed that higher affinity interactions yielded greater numbers of FluNP memory cells in the spleen and most dramatically in the lung and dLN. The major impact of avidity was on memory cell number, not cytokine production, and was already apparent within several days of transfer. These memory cells demonstrated enhanced protection against lethal IAV infection with a robust early day 5 secondary effector response in the lung. We previously showed that autocrine IL-2 production during the effector checkpoint prevented default effector apoptosis and supported memory formation. Here, peptide avidity determined the level of IL-2 produced by effectors while IL-2R expression was unaffected. However, IL-2Ra expression by APC drove more memory cell formation, suggesting that transpresentation of IL-2 by APC at this checkpoint enhanced CD4 memory generation. Secondary memory generation was also avidity-dependent. We propose this pathway selects CD4 effectors of highest affinity to progress to memory and can instruct future vaccine design.