Date on Master's Thesis/Doctoral Dissertation

5-2004

Document Type

Doctoral Dissertation

Degree Name

Ph. D.

Department

Biochemistry and Molecular Biology

Committee Chair

Grimes, Harry Leighton III

Author's Keywords

Health and environmental sciences; Biological sciences; Growth factor independence-1B; T lymphopoiesis; T cells; Thymocytes

Subject

Lymphoid Tissue--Immunology

Abstract

T lymphopoiesis has been an intense focus of immunological research since the discovery of the major histocompatibility complex (MHC) and T cell mediated transplant rejection. Additionally, researchers have long appreciated the role of T cells, and their development, in autoimmune disorders. Interest in this field has increased upon the realization that many leukemic oncoproteins are the very factors that control normal T cell development. Thus, the transcriptional networks that drive the function and development of T lymphocytes are closely linked with disease states.Growth Factor Independence 1 (GFI1) and GFI1B are two very similar transcriptional repressor oncoproteins that are encoded by two different genes. Though nearly identical in their DNA binding and repressor domains, GFI1 and GFI1B are differentially expressed in normal tissues and in tumors of lymphoid lineage. GFI1 is frequently activated in mouse T cell leukemias, whereas GFI1B has not been found in T cell tumors. The work described in this dissertation provides insight into this phenomenon by delineating functional differences for GFI1 and GFI1B in T cells.The first set of experiments compares the phenotypes engendered by transgenic expression of either GFI1 or GFI1B in developing and mature T lymphocytes. These analyses revealed that GFI1 enhances the response to T cell activation, whereas GFI1B decreases this response. Furthermore, transgenic GFI1B causes defects in thymocyte development, some of which result from a lack of survival signals. These defects can be corrected by transgenic expression of either BCL2, an inhibitor of apoptosis, or GFI1, suggesting that GFI1 and GFI1B play opposing roles in T cell survival.At least part of the effect of transgenic GFI1B results from GFI1B-mediated repression of "Gfi1" transcription. We show that the transcription of "Gfi1" is repressed in T cells by both GFI1 and GFI1B and that this is the result of direct binding to evolutionarily conserved GFI1/GFI1B recognition sequences in the "Gfi1" promoter. Furthermore, we provide evidence that endogenous GFI1 regulates its own promoter in T cells, but not in a myeloid lineage cell line.Finally, a more detailed analysis of the effect of GFI1B in T lymphopoiesis confirms a role for GFI1B in the survival and differentiation of thymocytes. This analysis revealed that transgenic expression of GFI1B results in altered expression of several members of T cell receptor (TCR) signaling pathways that are largely responsible for the survival and differentiation of thymocytes. The activity of the downstream effectors of these pathways appears to be decreased, providing mechanistic insight in to the function of GFI1B in T cells.Our work is the first to describe a role for GFIB in T cells. We demonstrate that GFI1B negatively regulates the cellular response to activation through the TCR complex and provide preliminary evidence of a mechanism by which GFI1B mediates these effects. Furthermore, we delineate functional differences for GFI1 and GFI1B in T cells, providing insight into the differential expression of these two transcription factors.

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