Interrogation of transcriptional, regulatory and signalling networks in fetal thymic epithelial cell development via in silico analyses

Abstract

The thymus is the primary lymphoid organ responsible for the development and maturation of T lymphocytes (aka T-cells) in vertebrates. The complex architecture of the thymic microenvironment orchestrates the formation of a diverse and self-tolerant T-cell repertoire capable of supporting the development and maintenance of a functional immune system. The main component of this microenvironment, the thymic epithelium, is crucially required to direct thymus organogenesis and homeostasis, and to mediate T-cell repertoire development and selection. The thymic epithelial progenitor cells (TEPCs) from which the mature thymus develops originate from the endoderm of the 3rd pharyngeal pouch by embryonic day 9 in mouse development (or early week 6 in human embryos). Expression of the transcription factor FOXN1 is required to drive TEPCs differentiation in each thymic epithelial lineage (TEC), while the absence of functional FOXN1 causes athymia. Moreover, forced expression of Foxn1 in mouse embryonic fibroblasts (MEFs) converts these MEFs into TECs that can support the development of a normal thymic system. Despite the great therapeutic potential that TEPCs present in regenerative medicine, there is currently no detailed model describing regulation of the TEPC state and its differentiation into cortical (c) and medullary (m) TECs, or explaining the dominant role of FOXN1 in the thymic epithelial system. Comparative transcriptomics analysis in conjunction with pathway enrichment analysis of the developing TEPCs could reveal the signalling pathways that regulate the early TEPC state and progression into differentiation. Additionally, integrative bioinformatics analysis of transcriptomics and genomics datasets could identify the functional networks that are directly regulated by FOXN1 during early TEC progression. In this thesis I provide, for the first time, an in silico model explaining fetal TEPC differentiation into the functionally distinct TEC lineages, in the cellular, molecular and signalling contexts of thymus early development. Furthermore, I present evidence which suggests that FOXN1 could be a pioneer factor, capable of fully establishing the transcriptional programme that underpins thymic epithelial cell identity and function. Finally, in this thesis, I introduce the development of an interactive thymic-specific database that provides a platform for easy access, analysis and integration of curated bioinformatics datasets.