Doctor of Philosophy / The experiments described in this thesis document the development of two in vivo models, to investigate the effect of competition for peptide-MHC and factors independent of MHC on T cell proliferation, differentiation, generation of memory cells and affinity maturation. The first model made use of 3 strains of T cell receptor (TCR) transgenic (tg) mice of varying specificity for antigen-MHC class II. To determine the effect of antigen specific and non-specific competition on the early stages of the T cell response, the efficiency with which naïve antigen-specific CD4+ T cells were recruited into an ongoing immune response was investigated. Recruitment into cell division and cytokine production was shown to decrease with an increasing time delay between two cell cohorts of the same specificity, leading to a significant drop in recruitment with a delay of only 24 hours. Injection of additional antigen could partially compensate for this decrease, suggesting that lack of available antigen limited recruitment of specific cells trafficking to the node after the initiation of the response. A role for antigen non-specific factors such as access to APCs, costimulatory signals or cytokines was ruled out by showing that the response to a second, independent antigen was unaffected by an ongoing response, even when the same APCs were presenting both antigens. The second system modelled a situation in which a clone of uniformly high affinity T cells competed against a polyclonal population containing mixture of affinities. This situation would arise during a normal response to a single epitope, and would mimic the process of competition that drives affinity maturation of the CD4+ T cell response. By substituting a high affinity response to a different antigen, a more complex reaction to multiple antigens, of different affinities was modelled. To avoid any possible effect of the two antigens competing for access to processing machinery, or binding to the same MHC class II allele, the two antigens were provided as synthetic peptides that bind to different MHC molecules. The data indicated that CD4+ T cell competition for peptide-MHC is far more potent than competition between CD4+ T cell responses of different specificity. Antigen-specific competition reduced the level of T cell stimulation detected as early as day 3 of the response. In the face of high affinity antigen-specific competition, the representation of mixed affinity T cells within the effector and effector memory cells (TEM) population declined progressively throughout the primary and secondary responses, suggesting that continued access to peptide-MHC is required to maintain maximum numbers of effector and TEM cells. In contrast, the contribution of central memory (TCM) was stable from day 7 onwards. Competition by CD4+ cells of an unrelated antigenic specificity led to a minor reduction in peak cell number and cytokine production in the primary response, without altering the number or potency of memory cells. Together these two models demonstrated a mechanism whereby the immune system exerts tight control over the size and kinetics of each individual antigen specific response without affecting the ability to respond to secondary infections or late-phase lytic antigens. Overall the results demonstrate a continued requirement for TCR stimulation for the generation of effector cells and the maintenance of a population of cytokine producing memory cells. However the generation of a stable population of central memory cells was unaffected by conditions of reduced T cell stimulation, ensuring that long-term memory can be maintained in the absence of antigen.
| Identifer | oai:union.ndltd.org:ADTP/283303 |
| Date | January 2006 |
| Creators | Spencer, Alexandra Jane |
| Publisher | University of Sydney. |
| Source Sets | Australiasian Digital Theses Program |
| Language | en_AU |
| Detected Language | English |
| Rights | The author retains copyright of this thesis., http://www.library.usyd.edu.au/copyright.html |
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