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Antigen-specific T cell turnover and expansion in vivo during chronic immune stimulation

Ladell, Kristin Ingrid 2013. Antigen-specific T cell turnover and expansion in vivo during chronic immune stimulation. PhD Thesis, Cardiff University.
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Effective immunity is fundamental to life on a dirty planet. Appropriate immune responses control infections and protect against cancer. Inappropriate immune responses lead to autoimmunity and allergy. A fine balance between aggression and tolerance is therefore central to effective immune function at the system level. This is a particular problem for T cells, which recognize peptide antigens bound to host major histocompatibility complex (MHC) molecules. Faced with a composite antigenic structure, the distinction between “foreign and dangerous” and “self and harmless” becomes both difficult and imperative, especially when the antigen persists. In this thesis, antigen-specific T cell responses were investigated under conditions of chronic antigenic stimulation to inform our understanding of this process. In T cell receptor transgenic mice, continuous antigenic stimulation without adjuvant lead to increased in vivo turnover of antigen-specific CD4+ T cells but “aborted” immune activation, characterized by depletion of these cells from the circulation and spleen. Full immune activation and expansion of antigen-specific memory/effector CD4+ T cells required the presence of adjuvant, in this case IL-1β, which induces an inflammatory environment. Further isotope labelling studies in human immunodeficiency virus-infected subjects suggested that the surface marker CD57 demarcates a “steady state” within the CD8+ T cell memory compartment, whereby CD57+ cells have lower in vivo turnover rates compared to their CD57- counterparts. These observations provide a potential mechanistic explanation for the preferential accumulation of CD57+CD8+ cells under conditions of chronic antigenic stimulation. Another persistent pathogen, cytomegalovirus (CMV), expresses a viral interleukin (IL)-10 homologue. Memory T cell inflation and antiviral cytokine production in murine CMV(MCMV)-infected mice were suppressed by IL-10. Conversely, IL-10 blockade or deficiency lead to the inflation of certain antigenspecific T cell populations and reduced viral load, most likely as a consequence of the enhanced immune response. Reactivation of human CMV was also apparent in subjects with dasatinib-associated large granular lymphocyte expansions. Consistent with a causative association, the expanded T cell and NK cell populations in these subjects were oligoclonal and exhibited a late differentiated (CD27-CD57+) phenotype, indicative of chronic antigenic stimulation. In addition, CD8high and CD8low T cells were observed within both the total and CMV-specific CD8+ T cell compartments, consistent with CMVdriven activation. In summary, these data show that antigen alone is not sufficient to induce full immune activation, even under conditions of chronic stimulation. Additional signals, such as those provided by an inflammatory environment, are required to trigger full T cell activation and expansion. Persistent viruses attempt to undermine this process, for example by the expression of homologues that mimic host immune regulators. Even in the presence of viral reactivation and immune system perturbations, however, the T cell compartment can demonstrate remarkable resilience in its ability to generate fully differentiated and functional expansions. The persistence of certain memory T cell subsets under such conditions appears to play an important role in the immune response to chronic “dangerous” antigens.

Item Type: Thesis (PhD)
Status: Unpublished
Schools: Medicine
Subjects: Q Science > QR Microbiology > QR180 Immunology
R Medicine > R Medicine (General)
Date of First Compliant Deposit: 30 March 2016
Last Modified: 04 Jun 2017 04:58

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