Defense Date

2012

Document Type

Dissertation

Degree Name

Doctor of Philosophy

Department

Microbiology & Immunology

First Advisor

Ronald B. Smeltz

Abstract

The human pathogen Trypanosoma cruzi is an intracellular parasite and the etiological agent of Chagas disease. Protective immune responses to T. cruzi are highly dependent on T helper 1 and CD8+ T cells which produce interferon-gamma. A deficiency in these responses has severe consequences on the ability to control infection. Our investigation into the role of the Th1 transcription factor, T-bet, during murine T. cruzi infection revealed that T-bet is required for resistance. Contrary to our expectations, T-bet was not required for the development of Th1 immunity during infection, as T-bet-deficient mice still developed interferon-gamma-producing T cells. However, T-bet was required to suppress the differentiation of Th17 cells and for the expansion of T. cruzi-specific CD8+ T cells. We first sought to determine the cause of reduced numbers of T. cruzi-specific CD8+ T cells in infected T-bet-deficient mice. First, we found that impaired migration or survival did not contribute to the low number of T. cruzi-specific CD8+ T cells. Secondly, we determined that reduced numbers of CD8+ T cells was not secondary to a defect in antigen-presenting cell activation or priming of CD8+ T cells. A recapitulation of defective expansion in mice with normal T-bet-expressing antigen-presenting cells demonstrated that T-bet expression in T cells was required. Thus, we determined that T-bet regulates the expansion of antigen-specific CD8+ T cells during T. cruzi infection in a T cell-intrinsic manner. Although it was evident T-bet had an integral role in suppressing the development of Th17 cells in response to infection with T. cruzi, several issues remained unclear. The first was the apparent lack of a negative regulatory effect of IFN-g/IFN-g-signaling on Th17 cells, which contradicted published reports. To clarify the role of IFN-g, we investigated the effect of IFN-g- or Stat-1-deficiency during T. cruzi infection. Surprisingly, IFN-g did not have a major role in up-regulating T-bet or for suppressing the development of Th17 responses, whereas Stat-1 was necessary for both. This was unexpected as Stat-1 is an IFN-g-inducible transcription factor, and its activation leads to T-bet induction. Thus, the T-bet-mediated inhibition of Th17 responses during T. cruzi infection is dependent on Stat-1, but not IFN-g. The final aim of this project was to identify the cytokines that negatively regulate Th17 differentiation in response to T. cruzi. We focused on the IL-12-family cytokines, IL-12 and IL-27, which are known to regulate T cell responses. Indeed, IL-12-deficient mice infected with T. cruzi developed a significant increase in Th17 cells similar to that observed in T-bet-deficient mice. Surprisingly, and in contrast to published results in other models, IL-27-deficient mice did not exhibit an increase in Th17 development. Our results demonstrate that IL-12, but not IL-27, is necessary for optimal T-bet expression and regulation of Th17 responses during T. cruzi infection.

Rights

© The Author

Is Part Of

VCU University Archives

Is Part Of

VCU Theses and Dissertations

Date of Submission

June 2012

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