Date of Award

12-17-2015

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Biology

First Advisor

Charlie Benson

Abstract

Radiation is a common therapeutic modality for cancer however it fails to control advanced and malignant disease. As a result, novel approaches that aim to stimulate immune attack of tumors are currently being investigated and approved by the US FDA. It is clear, however, that no single agent will be responsible for achieving long-term control and treatment of cancer and that combination therapies will be required. Previous studies indicate that colorectal cancer cells that survive radiation up-regulate surface expression of cytotoxic T lymphocyte (CTL) relevant proteins including death receptors, cell adhesion molecules and tumor-associated antigens (TAA). The aim of this dissertation was to investigate the effect of non-cytolytic doses of ionizing radiation (IR) on co-stimulatory molecule expression on tumor cells and evaluate the impact of their modulation on effector T-cell biology and tumoricidal activity. Here, several human tumor cell lines were exposed to various doses of radiation (0-10Gy) and TAA-specific T-cell tumoricidal activity and expression of effector CTL co-stimulatory molecules were evaluated. I found OX40L and 41BBL to be the most consistently upregulated proteins post-IR by flow cytometry. Furthermore, I saw enhanced survival and activation of human CD8+ T-cells exposed to irradiated tumor cells. Importantly, enhanced killing of irradiated tumor cells by TAA-specific CTLs in cytotoxicity assays was reported. Blocking OX40L and 41BBL reversed radiation-enhanced T-cell killing. My data also indicate that expression of 41BBL and OX40L can be epigenetically regulated, as inhibition of histone deacetylases (HDAC) and of DNA methyltransferases (DNMT) resulted in increased OX40L and 41BBL mRNA and protein expression. Furthermore, chromatin immunoprecipitation experiments revealed increased histone H3 acetylation specifically at the 41BBL promoter following irradiation. Last, I began exploring the ability of IR to reverse immune suppression by evaluating the impact of radiation on regulatory T cells (TREGS) that can suppress the function of effector CTLs. I found that RT could reduce TREG numbers likely by altering their phenotype. Overall, this dissertation demonstrates that radiation can be used to make human tumors more immunogenic through epigenetic modulation of genes stimulatory to effector T-cells and that it may also reverse immune suppression by phenotypically altering TREG cells.

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