Nancy Bigley (Committee Member), Katherine Excoffon (Committee Member), Barbara Hull (Committee Member), Courtney Sulentic (Advisor)
Master of Science (MS)
The aryl hydrocarbon receptor (AhR) is a ligand-activated cytosolic transcription factor that regulates xenobiotic-metabolizing enzymes. It mediates the toxicity of various environmental chemicals such as 2,3,7,8-tetracholorodibenzo-p-dioxin (TCDD). TCDD inhibits the differentiation of B cells into antibody-secreting cells and inhibits immunoglobulin (Ig) expression in various animal models. We have previously determined that TCDD-induced inhibition of the mouse Ig heavy chain gene (mo-Igh) is AhR-dependent. This inhibition may be mediated by binding of the AhR to dioxin response elements (DREs) within the 3'Igh regulatory region (3'IghRR) and inhibition of 3'IghRR activity, a significant transcriptional regulator of Ig expression. However, there are structural differences between the mouse and human 3'IghRR. The mouse contains four enhancers (hs3A; hs1,2; hs3B; and hs4), whereas the human contains three (hs3; hs1,2; and hs4). In addition, the human hs1,2 is known to be highly polymorphic and has been associated with several autoimmune diseases. The current study focuses on elucidating the role of the AhR in human Ig expression and class switch recombination (CSR). We disrupted the AhR signaling pathway in a human B-cell line (CL-01) using two different shRNA constructs or with the chemical AhR antagonist (CH-223191). Although the CL-01 AhR has three heterozygous single nucleotide polymorphisms (SNPs) that results in loss of CYP1A1 gene induction, TCDD significantly inhibits IgG expression, whereas IgM expression has very low sensitivity to TCDD. Interestingly, decreased AhR protein levels results in low IgG expression, while there was no change in IgM expression. In contrast, the AhR antagonist induced greater IgG secretion in stimulated B cells, which was not replicated by the AhR knockdown suggesting a mechanistic difference between the chemical antagonist and AhR knockdown. Reduced AhR levels caused an isotype-specific inhibition of the CSR to IgG1, but not to IgG2, IgA or IgE. These results demonstrate that there are different mechanisms regulating different Ig isotypes. With the growing number of human immune-related disorders correlating with the polymorphic hs1,2 enhancer, understanding the role of the AhR in 3'IGHRR activity and Ig expression could provide insight into potential therapeutic interventions.
Department or Program
Microbiology and Immunology
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