Browsing by Subject "Microbiology, Immunology and Cancer Biology."

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    Analysis of the pre-immune T cell repertoire.
    (2009-11) Chu, Hon Man Hamlet
    Cell-mediated immune responses are initiated when a population of pre-immune (or naïve) T cells recognizes their cognate ligands in the form of a specific peptide bound to a self-major histocompatibility complex molecule (pMHC). This recognition is made possible by highly specific Tcell receptors (TCR) on individual T cell clones specific for a given pMHC complex. The pre-immune T cell repertoire is comprised of populations specific for at least 100,000 different pMHC, each containing multiple clones. It is important to understand the composition of this repertoire because it is the repository of the host’s potential for future cell-mediated immune responses to microbes and tumors, and in some cases its own tissues. However, the study of individual pre-immune pMHC-specific T cell populations within such a diverse repertoire has been extremely difficult because of their small size. A novel soluble pMHC-based magnetic enrichment technique was therefore developed to analyze naive T cell populations in mice and humans. Using this procedure, different pMHCII-specific naive CD4+ T cell populations were identified and enumerated. The size of these populations was found to vary depending on pMHC specificity. Additionally, these differences were directly proportional to the magnitude of the primary CD4+ T cell response after immunization with the relevant peptide.Thus, variation in naive T cell frequencies can explain why some peptides give rise to greater T cell responses than others. We explored this issue by enumerating pMHCII-specific CD4+ T cell populations that normally number 20 or 200 cells per mouse. Thymic positive or negative selection was required for optimizing the absolute size of each population but did not alter the 10-fold ratio between the two populations. Large naïve population size was related to the presence of certain amino acids at TCR contact sites within the peptide. These results suggest that certain MHCII-bound peptides are immunodominant because they contain amino acids with chemical properties that foster binding to many TCRs.
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    Maintenance of immune fitness during reconstitution from T cell lymphopenia by CD4-positive, CD25-positive, and Foxp3-positive regulatory T cells.
    (2009-07) Winstead, Colleen Jean
    Work presented in this doctoral thesis focuses on the role of regulatory T cells (Tregs) in controlling T cell homeostasis and emergence of autoimmunity during immune reconstitution from lymphopenia. It is recognized that lymphopenia may be a common trigger of many autoimmune diseases due to oligoclonal expansion of self-reactive T cells with an effector phenotype. It is also a clinical fact that autoimmunity is often associated with immune deficiency and poor responsiveness to vaccines and infections. Research supporting work presented in chapter 2 of this thesis was based on the hypothesis that poor Treg function may play a central role in these phenomena. This work, published in June of 2008 in the Journal of Immunology, clearly demonstrated that Tregs selectively restrain one specific form of lymphopenia-induced proliferation characterized by burst-like cell cycle activity and effector T cell differentiation (spontaneous proliferation). The spontaneous form of lymphopenia-induced proliferation is the likely source of oligoclonal expansion of self-reactive T cells that drive autoimmunity. Work presented in chapter 3 of this thesis addresses the hypothesis that such oligoclonal expansion by a few T cell clones consumes resources away from the rest of the T cell population, which ultimately results in loss of T cell diversity. Using the technique of T cell adoptive transfer, we measured immune responses to infection with the gram negative bacteria Listeria monocytogenes in lymphopenic mice reconstituted in the presence or absence of Tregs by analysis of T cell receptor (TCR) Vbeta chain usage and repertoire sampling using Vbeta-Jbeta chain TCR spectratyping and magnetic bead enrichment with specific major histocompatibility (MHC) class I and II tetramers. Experimental results suggest that the presence of Tregs during immune reconstitution preserves TCR structural diversity and allows for more accumulation of pathogen-associated antigen-specific T cells in secondary lymphoid tissues following clearance of the infection. This result may otherwise seem paradoxical as Tregs are typically thought of as generalized suppressors of the immune system. Regardless, we believe this unappreciated ability to maintain T cell homeostasis through preservation of peripheral diversity will shed considerable insight into the role of Tregs in the immune system, vaccine responsiveness, pathogenesis of autoimmunity, and immune senescence.