The development of functionally competent, self-tolerant T cells in the thymus is an essential prerequisite for the formation of adaptive immune responses against foreign pathogens. Thymic epithelial cells (TECs) form specialized stromal microenvironments in the cortex (cTEC) and medulla (mTEC) of the thymus that support all stages of T cell development, from the entry of thymocyte progenitors to the exit of mature naive T cells. Conversely, TECs require crosstalk with developing thymocytes and surrounding mesenchymal stromal cells (TMCs) for their own differentiation and maturation. However, the signals governing TEC homeostasis and functions remain incompletely understood.
The aim of the work presented in this thesis was to understand the role of the vitamin A metabolite retinoic acid (RA), a known regulator of immune responses and epithelial homeostasis, in TEC functionality and T cell development (Paper I) as well as for the generation of cytotoxic CD8+ T cell responses (Paper II). Additionally, we set out to investigate the poorly characterized heterogeneity within the TMC compartment and to better define the ontogeny and developmental origin of mesenchymal stromal cell subsets in lymphoid organs (Paper III).
In Paper I, we show that in vivo RA signaling in TECs regulates their homeostasis and controls gene expression associated with distinct biological pathways in cTEC and mTEC. Consequently, inability to respond to RA due to expression of a dominant-negative RA receptor (dnRAR) resulted in increased cTEC proliferation and the accumulation of cTECs expressing an immature precursor-like phenotype, while mTEC numbers were reduced during the early postnatal phase of TEC expansion. Additionally, RA signaling in TECs impacted on thymopoiesis with impaired generation of CD4 single-positive (SP) and CD8SP thymocytes.
In Paper II, we addressed the role of RA signaling in CD8+ T cell development and function directly, by inducing expression of the dnRAR in thymocytes and peripheral T cells. Using this model, we showed that absence of RA signaling in developing thymocytes severely perturbs thymopoiesis, leading to an accumulation of phenotypically mature CD8SP cells displaying alterations in T cell receptor expression levels and repertoire. Moreover, naive peripheral CD8+ T cells were skewed towards an activated phenotype in the absence of RA signaling and while these cells displayed enhanced survival and expansion upon TCR stimulation, they were impaired in mediating cytotoxic effector functions and displayed reduced expression of various RA-controlled effector genes.
In Paper III, we performed comparative analysis of mesenchymal stromal cells from thymus and lymph nodes and identified a population of CD34+ adventitial cells as a conserved component of the vascular niche in lymphoid tissues. Furthermore, we demonstrated that adult CD34+ cells contain progenitors that have the potential to give rise to lymphoid stromal populations in a context-dependent manner in in vivo grafting experiments.
Collectively, the work included in this thesis identified RA signaling as an important regulator of thymic epithelium and T cell functionality and broadens our understanding of mesenchymal stromal cell functions and development in lymphoid organs.