Search

CD4\(^+\) and CD8\(^+\) T cell populations can be divided into subpopulations based on expression of surface markers CCR7 and CD45RA. The resulting populations are referred to as naive, central memory, effector memory and effector memory RA\(^+\) (EMRA). The aim of this study was to identify potential lineage relationships between these subpopulations for both CD4\(^+\) and CD8\(^+\) T cells through microarray analysis. The genes found to distinguish between these subpopulations include many molecules with known functions in T cell differentiation, including CCR7, CD45RA, granzymes, L-selectin and TNF receptors. Several genes from the tetraspanin family of proteins were found to be differentially expressed at mRNA and protein level; suggesting a possible role for these genes in CD4\(^+\) and CD8\(^+\) T cell activation, migration and lysosomal function. Other genes identified, such as LRRN3 and CXCR5 which were expressed highest on naive and CM T cells respectively, provide interesting gene targets to follow up on their function in these T cell populations. Microarray data was validated through Real Time PCR and suggests that both CD4\(^+\) and CD8\(^+\) T cells differentiate along a linear pathway of naive to central memory to effector memory. The transcriptional programmes responsible for these differentiation steps were distinct between CD4\(^+\) and CD8\(^+\) T cells, although additional elements were common to both subsets.

The nematode Caenorhabditis elegans has been a powerful experimental organism for almost half a century. Over the past ten years, researchers have begun to exploit the power of C. elegans to investigate the biology of a number of human pathogens. This work continues to uncover mechanisms of host immunity and pathogen virulence that are either analogous to those involved during pathogenesis in alternative animal hosts or mechanisms which are, thus far, unique to the worm. In this thesis, we present data that describes an immunological balance in C. elegans, whereby heightened tolerance to one pathogen, the enteric bacteria Salmonella Typhimurium, comes at the cost of increased susceptibility to another, the fatal fungal human pathogen Cryptococcus neoformans. We find that this susceptibility trade-off is mediated by the reciprocal activity of two immune genes: the lysozyme lys-7 and the tyrosine kinase abl-1. We suggest that ABL-1 controls two different DAF-16-dependent pathways to regulate this balance. Both pathways are necessary for wild type resistance to C. neoformans, whilst the activity of only one pathway is a requirement for the tolerance phenotype to S. Typhimurium. We infer from sequence data that LYS-7 has an atypical mode of action in C. elegans, which we hypothesise to be detrimental to the worm during S. Typhimurium pathogenesis and thus a contributing factor to the tolerance phenotype. Furthermore, we find that this tolerance has a Salmonella-dependency which we propose to be under the control of the alternative sigma factor, RpoS. Taken together, we describe an immunological balance in C. elegans for the first time, one that is mediated by both host and pathogen factors. We therefore suggest that the innate immune response of C. elegans has a higher level of immune complexity than previously believed, and that such trade-offs are evolutionarily ancient mechanisms.

Cellular senescence is a state of permanent cell cycle arrest in which a cell is unable to further divide in response to normal growth stimuli. Telomere shortening, believed to be a mechanism of cellular senescence, is a result of numerous rounds of cell division and can be accelerated by heightened levels of oxidative stress. An accumulation of senescent T lymphocytes in blood and tissues has been suggested to contribute to the general immunosuppression seen in the elderly. Obese individuals and athletes taking part in regular high intensity exercise are subjected to heightened states of oxidative stress which appears to result in immunosuppression and increased susceptibility to infection. It was hypothesised that there would be an accumulation of senescent T lymphocytes with ageing, with obesity and during six months training for an Iron man competition. Senescent CD4+ and CD8+ T peripheral blood lymphocytes were identified by 4 colour flow cytometry, based on their cell surface expression of CD57, the killer cell lectin-like receptor G1 (KLRG1) and the lack of cell surface expression of CD28. Where telomere length measurements were performed, CD4+ and CD8+ T lymphocyte subsets were isolated and telomere lengths were compared to the proportions of KLRG1+, CD57+ and CD28+ cells in each subset. Older subjects and obese subjects had significantly larger proportions of senescent cells (KLRG1+/CD57+) in the CD8+ but not the CD4+ peripheral blood T lymphocyte subset. Furthermore, during six months training for an Iron Man competition, compared to the first month, club level athletes showed a 92% increase in the frequency of senescent cells (KLRG1+/CD57+) in the CD4+ but not the CD8+ T lymphocyte subset. Telomere length was negatively correlated with the proportion of KLRG1+ and CD57+ cells and positively correlated with the proportion of CD28+ cells in the CD4+ but not the CD8+ T lymphocyte subset. It is concluded from this work that KLRG1+/CD57+ senescent T lymphocytes accumulate with age, obesity and regular high intensity training, which may contribute to the increased immunosuppression associated with these states.