acase@tulane.edu / 1 / Taylor Foreman

"January 2002" Bibliography: leaves 204-230.

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.

"January 2002" / Bibliography: leaves 204-230. / vii, 230, [31] leaves : ill. (chiefly col.), plates (some col.) ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Thesis (Ph.D.)--University of Adelaide, Dept. of Molecular Biosciences, 2002

The immune system has been identified as a very sensitive target for the toxic effects of 2,3,7,S-tetrachlorodibenzo-p-dioxin (TCDD). Exposure to TCDD has been shown to disrupt the generation of both cell-mediated and humoral T cell-dependent immunity in laboratory animals; however, the mechanism remains unknown. In this dissertation, the hypothesis is tested that TCDD exposure alters T cell activation and differentiation either directly or by inhibiting the activation of antigen presenting cells (APC). Previous studies from our laboratory using the PSI5 tumor allograft model suggest that TCDD inhibited T cell activation by suppressing the induction of the costimulatory molecule CDS6 on B220+ and Mac-1+ cells. To address the effects of TCDD on APC, we further characterized the activation of splenic APC in the PSI5 model and found that TCDD suppressed the induction of the accessory molecules CDS6, CD54 and MHC II on APC as well as their production of IL-12. Although it was determined that the induction of these costimulatory molecules following PSI5 immunization was CD40independent, their in vivo expression could be enhanced by administering an agonistic antibody to CD40 to mice. APC from anti-CD40 treated mice expressed significantly higher levels of these accessory molecules and IL-12, and this enhanced APC activation was largely unaffected by TCDD. However, TCDD-treated mice receiving both P815 and anti-CD40 were unable to generate T cell-dependent allograft immunity suggesting that suppression of APC activation may not be underlying TCDD immunosuppression. To address the direct effects of TCDD on T cell activation, we adoptively-transferred DO11.10 TCR transgenic T cells into syngeneic recipients and monitored their activation in vivo following exposure to antigen. Although treatment of adoptively-transferred mice had no effect on the expansion or activation of the OVA-specific CD4+ T cells, the production of the T cell-derived cytokines IL-2, IFN-��, IL-4 and IL-10 was suppressed. These data suggest that TCDD may suppress the differentiation of OVA-specific T cells into effector T helper cells which are capable of driving T cell-dependent immune responses. / Graduation date: 2000

Graduation date: 1998

Progressive obesity and its associated metabolic syndromes represent a globally growing challenge, yet mechanistic understanding and current therapeutics are unsatisfactory. We discovered that CD4+ T-lymphocytes, resident in visceral adipose tissue (VAT), control insulin-resistance in diet-induced obese (DIO) mice and likely humans. DIO VAT-associated T cells display biased TCR-Valpha/betarepertoires suggesting antigen-specific expansion. CD4+ T-lymphocyte control of glucose homeostasis is compromised in DIO when VAT accumulates pathogenic IFNgamma-secreting Th1 cells, overwhelming static numbers of Th2 (CD4+GATA-3+) and regulatory Foxp3+ T cells. CD4+ T cell transfer into DIO, lymphocyte-free RAGnull mice reversed weight gain and insulin resistance predominately through Th2 cells. Brief systemic treatment with anti-CD3 antibody or its F(ab’)2 fragment, restores the Th1/Foxp3+ balance and reverses insulin resistance for months, despite continuing high-fat diet. CD4+ T cells impact the progression of obesity-associated metabolic abnormalities and can be manipulated by immunotherapy.

Progressive obesity and its associated metabolic syndromes represent a globally growing challenge, yet mechanistic understanding and current therapeutics are unsatisfactory. We discovered that CD4+ T-lymphocytes, resident in visceral adipose tissue (VAT), control insulin-resistance in diet-induced obese (DIO) mice and likely humans. DIO VAT-associated T cells display biased TCR-Valpha/betarepertoires suggesting antigen-specific expansion. CD4+ T-lymphocyte control of glucose homeostasis is compromised in DIO when VAT accumulates pathogenic IFNgamma-secreting Th1 cells, overwhelming static numbers of Th2 (CD4+GATA-3+) and regulatory Foxp3+ T cells. CD4+ T cell transfer into DIO, lymphocyte-free RAGnull mice reversed weight gain and insulin resistance predominately through Th2 cells. Brief systemic treatment with anti-CD3 antibody or its F(ab’)2 fragment, restores the Th1/Foxp3+ balance and reverses insulin resistance for months, despite continuing high-fat diet. CD4+ T cells impact the progression of obesity-associated metabolic abnormalities and can be manipulated by immunotherapy.

Thesis (Ph. D.)--University of Chicago, Committee on Immunology, 2002. / Includes bibliographical references. Also available on the Internet.

published_or_final_version / Paediatrics and Adolescent Medicine / Doctoral / Doctor of Philosophy