Contributions of GPCRs CCR8 and GALR2 to thymic central tolerance and thymic epithelial cell homeostasis

Thyagarajan, Hiran Maye
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T cells are crucial components of the immune system that distinguish self-antigens from foreign pathogens, recognize and remove such infectious agents, and provide lasting immunological memory to previously encountered pathogens. T cells develop throughout life in the thymus, an organ that educates developing T cells to ensure generation of a self-restricted and non-autoreactive naïve T cell pool. The fate of developing T cells is determined largely by T cell receptor (TCR)-mediated recognition of self-peptide: major histocompatibility complex (MHC) molecules presented by thymic antigen presenting cells (APCs). Positive selection enables only T cells expressing functional TCRs with at least minimal self-reactivity to survive and differentiate. Thymocytes expressing TCRs with a strong affinity for self-peptide:MHC complexes, either undergo apoptosis, referred to as negative selection, or differentiate into regulatory T cells (Treg). Both outcomes are critical for avoiding autoimmunity. Central tolerance, the result of these processes, ensures that autoreactive thymocytes are either deleted or diverted to a Treg cell fate. Developing thymocytes must communicate with heterogeneous thymic stromal cells to undergo proper differentiation and selection. Cross-talk between stromal cells and thymocytes is bi-directional, as stromal cells also need signals from developing thymocytes to proliferate and differentiate. Distinct thymic stromal cell types are partitioned into two main compartments in the thymus: the outer cortex and the central medulla. Early stages of T cell maturation occur in the cortex, but later stages, including the induction of central tolerance occurs in the medulla. Thus, thymocytes must migrate into the medulla as they mature to ensure central tolerance induction. The signals governing the dynamic relocation of developing thymocytes through different thymic microenvironments is mediated to a large extent by chemokine signaling. Chemokine receptors are a subset of the GPCR super family that enable medullary entry, motility and interactions with medullary stromal cells. However, the cellular and molecular mechanisms that control medullary entry of post-positive selection thymocytes, their interactions with medullary APCs, and the induction of central tolerance are not completely resolved. In this thesis, I investigate the role of the chemokine receptor CCR8 in medullary entry and central tolerance induction. Although CCR8 contributes to accumulation of mature thymocytes in the medulla, CCR8 was not required for negative selection of polyclonal or monoclonal thymocytes. Nonetheless, serum autoantibodies were present in aged CCR8-deficient mice, perhaps reflecting a role for CCR8 in peripheral tolerance. I also investigate the role of the GPCR GALR2, in cross-talk between thymic epithelial cell (TEC) subsets. TECs can be subdivided into cortical (cTEC) and medullary (mTEC) subsets, which are thought to arise from a common TEC progenitor. These cells turn over throughout life, but very little is known about how the balance between cTEC and mTEC fates are regulated. Although cross-talk between thymocytes and TECs are known to regulate maturation of both cell types, molecular crosstalk between TEC subsets have not been elucidated. Our qRT-PCR data revealed that the GPCR Galr2 is expressed by the cTECs, while its ligand Galanin is expressed by the mTECs. Hence, we hypothesized that Galr2 could mediate crosstalk between cTEC and mTEC subsets. Galr2 deficiency did not have a major impact on steady-state TEC cellularity or organization. However, Galr2 deficiency did impact regeneration of cortical thymic epithelial cells following acute thymic injury. Altogether, my studies have clarified the role of CCR8 in thymocyte differentiation and have suggested an intriguing role for GALR2 in modulating interactions between epithelial cells during recovery from thymic damage.