Semester

Spring

Date of Graduation

2022

Document Type

Dissertation

Degree Type

PhD

College

School of Medicine

Department

Microbiology, Immunology, and Cell Biology

Committee Chair

Edwin Wan

Committee Co-Chair

Christopher Cuff

Committee Member

Christopher Cuff

Committee Member

Rosana Schafer

Committee Member

Timothy Eubank

Committee Member

Gordon Meares

Committee Member

Emidio Pistilli

Abstract

Signal transducers and activators of transcription 5 (STAT5A and STAT5B) are members of the STAT protein family. STAT5 plays a critical role in mediating cellular responses following cytokine stimulation. Critical STAT5 signaling is achieved following functional dimerization, which occurs via SH2 domain interactions. The binding of two STAT5 dimers to tandemly linked TTCN3GAA γ-interferon-activated sequence (GAS) motifs and N-terminal domain interactions, facilitates functional tetramerization. The biological role of STAT5 tetramers has been investigated using the Stat5a-Stat5b N-domain double knock-in (DKI) mouse strain, in which STAT5 tetramers cannot be formed due to substitutions in critical amino acid residues in the N-domain (I28A, F81A), but STAT5A and STAT5B dimers remain functional. Previous studies have demonstrated that STAT5 tetramers control the immunomodulatory function of regulatory T cells (Tregs), the expansion of CD8+ T cells, and the terminal differentiation of natural killer (NK) cells. However, the biological role of STAT5 tetramers in the context of autoimmune-mediated inflammation has not been investigated. The cytokine granulocyte-macrophage colony-stimulating factor (GM-CSF) plays a crucial role in activating JAK2-STAT5 signaling. Additionally, GM-CSF is known to play a central role in several chronic inflammatory autoimmune diseases, including both multiple sclerosis (MS) and inflammatory bowel disease (IBD). Thus, the goal of this dissertation is to define the biological role of STAT5 tetramers in the context of autoimmune-mediated inflammation. Using the DKI mouse strain, we determined that STAT5 tetramers promote the pathogenesis of experimental autoimmune encephalomyelitis (EAE), a murine model of MS. We established that STAT5 tetramerization in both CD4+ T cells and monocyte-derived cells (MDCs) was important for promoting EAE pathogenesis. The decrease in EAE severity in the DKI mice correlated with a decrease in T-helper 17 (Th17) cell extravasation and decreased interactions with MDCs within the spinal cord meninges. Moreover, we determined that GM-CSF-mediated STAT5 tetramers regulate to expression of the chemokine CCL17 by MDCs. We ascertained that CCL17 acts on the Th17 cells to promote their extravasation capacity and pathogenicity, and this is dependent on very late antigen 4 (VLA-4). Thus, we established a GM-CSF-STAT5 tetramer-CCL17 signaling axis that promotes autoimmune neuroinflammation. We sought to further investigate whether STAT5 tetramers could regulate inflammation associated with other autoimmune diseases. GM-CSF and STAT5 signaling are known to confer protection against intestinal inflammation. Therefore, we posited that GM-CSF-mediated STAT5 tetramerization may play a role in controlling colitis pathogenesis. Using the DKI mouse strain, we identified a central role of STAT5 tetramers in controlling colonic inflammation. We utilized the dextran sulfate sodium (DSS) model of colitis to demonstrate that the inhibition of functional STAT5 tetramerization promotes the expression of arginase I by the DKI monocytes, and this was dependent on GM-CSF signaling. The expression of arginase I by the DKI monocytes increased their pathogenicity. Thus, we established that STAT5 tetramers can delay colitis severity via the regulation of L-arginine metabolism in monocytes. Taken together, we have established essential and opposing roles of STAT5 tetramers in autoimmune-associated inflammation. STAT5 tetramers promote neuroinflammation but confer protection against colitis.

Embargo Reason

Publication Pending

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