Characterizing the Effects of Developmental Activation of the Aryl Hydrocarbon Receptor on CD4+ T Cell Responses Later in Life

Thesis (Ph.D.)--University of Rochester. School of Medicine & Dentistry. Dept. of Microbiology and Immunology, 2015.Developmental exposures have been shown to alter immune-mediated processes later in life, yet the mechanism by which this occurs is unknown. In fact, in most cases it is unclear which cell types and molecular machinery are affected. Many studies have examined early life exposures to chemicals that activate the aryl hydrocarbon receptor (AhR). Cells of the immune system express this receptor, and AhR activation in the fully mature immune system (i.e., non-developmental exposure) alters CD4+ T cell-dependent immune responses. However, little is known about how activation of the AhR during development changes the function of CD4+ T cells in adult offspring. Therefore, using mouse models of human disease, we determined whether developmental exposure to the prototypical AhR ligand 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) affects CD4+ T cell responses in adult offspring. For the majority of work in this project, we used influenza A virus infection to initiate an immune response. Developmentally exposed adult mice infected with influenza virus had fewer conventional effector CD4+ T cells in draining lymph nodes of the lung, yet an increase in regulatory CD4+ T cells (Tregs). In contrast, the lungs of these mice had an increase in all CD4+ T cell subsets. Using a combination of tools, including adoptive transfers and the cre-loxP system, we determined that the altered CD4+ T cell response to infection in developmentally exposed mice reflects a combination of intrinsic and extrinsic changes in CD4+ T cells, which are revealed in a tissue-specific manner. While the extrinsic factors triggered by developmental exposure that influence CD4+ T cell function later in life remain to be elucidated, we determined that these changes require live, replicating virus in the lung, suggesting a novel effect of developmental exposure on the structural cells of the lung. The implications of this work extend beyond influenza virus infection, as we observe changes in disease progression in developmentally exposed offspring in a CD4+ T cell dependent model of systemic autoimmunity. Together, this work establishes that CD4+ T cells are changed by activation of the AhR during development, and that these modifications have the potential to mediate the persistent alterations in immune responses observed in exposed human populations

Conditional deletion of Ahr alters gene expression profiles in hematopoietic stem cells.

The aryl hydrocarbon receptor (AHR) is a ligand activated bHLH transcription factor that belongs to the Per-Arnt-Sim (PAS) superfamily of proteins involved in mediating responses to cellular environment regulating normal physiological and developmental pathways. The AHR binds a broad range of naturally derived and synthetic compounds, and plays a major role in mediating effects of certain environmental chemicals. Although our understanding of the physiological roles of the AHR in the immune system is evolving, there is little known about its role in hematopoiesis and hematopoietic diseases. Prior studies demonstrated that AHR null (AHR-KO) mice have impaired hematopoietic stem cell (HSC) function; they develop myeloproliferative changes in peripheral blood cells, and alterations in hematopoietic stem and progenitor cell populations in the bone marrow. We hypothesized mice lacking AHR expression only within hematopoietic cells (AHRVav1 mice) would develop similar changes. However, we did not observe a complete phenocopy of AHR-KO and AHRVav1 animals at 2 or 18 months of age. To illuminate the signaling mechanisms underlying the alterations in hematopoiesis observed in these mice, we sorted a population of cells highly enriched for HSC function (LSK cells: CD34-CD48-CD150+) and performed microarray analyses. Ingenuity Pathway and Gene Set Enrichment Analyses revealed that that loss of AHR within HSCs alters several gene and signaling networks important for HSC function. Differences in gene expression networks among HSCs from AHR-KO and AHRVav1 mice suggest that AHR in bone marrow stromal cells also contributes to HSC function. In addition, numerous studies have suggested a role for AHR in both regulation of hematopoietic cells, and in the development of blood diseases. More work is needed to define what these signals are, and how they act upon HSCs