Aim1. Phenotypic characterisation of thymic stroma ex vivo and in vitro to define functional cortex/medulla specification and functional T cells
We performed comprehensive analyses of mature postnatal thymic tissue and collected human thymic foetal samples to study changes in epithelial cells during thymic maturation. We identified novel subtypes of cortical and medullary epithelial cells, characterized their transcriptional features, and isolated them in vitro. We explored the appearance of Hassall’s Bodies structures during foetal thymic maturation and observed a significantly higher medullary content in foetal thymi compared to postnatal counterparts. Furthermore, we isolated and expanded thymic epithelial stem cells across different ages, as well as RNA-seq and epigenetic profiling to understand transcriptional changes during development. Single-cell RNA-seq was performed on postnatal thymocytes and integrated with human foetal thymocytes datasets to understand the trajectory of progenitors undergoing T cell maturation. The study concludes by defining the properties of thymic epithelial cells in supporting T cell function and identifying epithelial stem cells in the human thymus.
Aim2. Recapitulate human foetal T cell development within a 3D natural matrix in vitro by modulating the introduction of stromal components
We next investigated the interaction between thymic epithelial cells and thymocytes across different human developmental stages. Initially, we established working conditions using postnatal cells. We utilized a rat thymic natural matrix scaffold injected with thymic stem cells and other stromal components, which reorganized along the three-dimensional extracellular matrix structure. Thymocyte precursors were injected into the scaffolds to evaluate their functionality in supporting T cell development. The resulting cells mirrored the ratio found in the native human thymus. The study confirmed the differentiation of thymic epithelial stem cells and their re-expression of functional molecules. This ex vivo system is now utilized to analyse T cell maturation upon injection of thymic epithelial cells from different developmental ages and anatomical compartments. T cell output is assessed to understand various aspects of human T cell development, including the establishment of T cell subsets and the role of human-specific genes expressed by stromal cells in driving the immune maturation process.
Aim3. Genetic ablation of stromal factors during foetal development and evaluation of its impact on T cell
maturation.
After extensive analysis of thymic epithelial compartment, we found a transcription factor to be highly expressed in several medullary cell subtypes, suggesting its importance in thymic function. Knocking out this transcription regulator in human thymic epithelial cells has led to a decrease in medullary markers, prompting further investigation into its molecular mechanisms in thymic function and across organ development.
Exploitation and dissemination:
-Thymic epithelial stem cell properties across development have now been published and the bioinformatic datasets of the study are publicly available: doi: 10.1016/j.devcel.2023.08.017.
-Aim1-2 results have been disseminated in presentations for teaching activities at University College London (UCL) and in two conferences: International Society for Stem Cell Research (ISSCR) in 2023 and London Stem Cell Symposium in 2021 and workshop in 2024
-Human foetal epithelial cells data generated under this project are currently under revision in the following manuscript:
“Advanced three-dimensional X-ray imaging unravels structural development of the human thymus compartments”. Authors: Savvas Savvidis, Roberta Ragazzini, Valeria Conde de Rafael, J. Ciaran Hutchinson, Lorenzo Massimi, Fabio A. Vittoria, Sara Campinoti, Olumide K. Ogunbiyi, Alessia Atzeni, Neil J. Sebire, Paolo De Coppi, Alberto Mittone, Alberto Bravin, Paola Bonfanti and Alessandro Olivo.
-Spatial transcriptomics data about thymic stem cell niche during development will be made publicly available upon publication.
