Access Type

Open Access Dissertation

Date of Award

January 2011

Degree Type

Dissertation

Degree Name

Ph.D.

Department

Immunology and Microbiology

First Advisor

Henry K. Wong

Second Advisor

Wei-Zen Wei

Abstract

Cytotoxic T-lymphocyte antigen 4 (CTLA-4) is a T cell surface protein that is homologous to CD28 and binds to the B7 family of ligands. Unlike CD28, CTLA-4 interaction transmits a negative signal in T cells, leading to suppression of proliferation. CTLA-4 is constitutively expressed on regulatory T cells (Tregs) but is also inducible in effector T cells. The mechanisms driving transcriptional regulation of CTLA-4 are poorly understood. Our previous work identified a bona fide NFAT1 binding site in the proximal promoter for effector T cells. In addition, we found histone acetylation occurred after stimulation.

As a result of its role in suppressing T cell proliferation, CTLA-4 is important for regulation of T cell responses. CTLA-4-immunoglobulin fusion proteins have shown efficacy to quell the overactive immune system in various types of autoimmune diseases. Alternatively, blocking antibodies to CTLA-4 have been used in cancer therapies to boost the anti-tumor immune response. Malignant cells of cutaneous T cell lymphoma (CTCL) express elevated levels of CTLA-4, which may contribute to reduced

tumor immunity as the disease progresses. The objective of this work was to identify mechanisms of transcriptional regulation of CTLA-4 to better understand how this gene can be modulated to potentially cater the immune response for a variety of immune-mediated diseases.

The major findings of this work include a mechanism by which proteasome inhibition augments CTLA-4 transcription in normal primary human CD4 T cells. The Th2 associated transcription factor GATA3 is both elevated and activated by phosphorylation after treatment with the proteasome inhibitor bortezomib, which in turn leads to CTLA-4 transcriptional activation in primary CD4 T cells. This finding may in part explain the increase in CTLA-4 found in CTCL, where GATA3 is also more abundant, particularly in its phosphorylated state. The increased CTLA-4 in our primary cell model is capable of suppressing T cell proliferation, demonstrating the potency of transcriptional modulation of this gene. Additionally we explored epigenetic and topological modifications that occur for CTLA-4 activation. We found the histone acetyltransferases p300 is responsible for histone 3 acetylation at the CTLA-4 promoter, and its activity is required for CTLA-4 transcription. We also discovered a previously undiscovered role for topoisomerase I in expression of a variety of induced genes, including CTLA-4. These results define novel mechanisms governing transcriptional activation of CTLA-4 in human effector CD4 T cells.

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