The role of the transcription factor Sox9 for thymic epithelial cell differentation and function

The thymus provides throughout life the specialized stromal microenvironment required for the lifelong generation of naïve T cells. This vital function is dependent on the regular composition and organization of its microenvironment, where specialized stromal cells promote thymocytes development and selection to functionally mature T cells. The thymic stroma is heterogeneous and mainly composed of thymic epithelial cells (TEC) which can be broadly subdivided according to their anatomical location, morphology, specific antigenic properties and function into two specialized subsets, i.e. cortical (c) and medullary (m) TEC. Although it is well established that appropriate Wnt signalling activity is required for normal TEC development and maintenance, the cellular and molecular mechanisms responsible for their differentiation and homeostasis remain largely undefined. The canonical Wnt signalling engaging via β–catenin directly regulates Foxn1 expression in TEC and a precise regulation of intracellular β-catenin protein levels is required for normal thymus development and function. Transcription factor Sox9 has been reported to physically interact with β-catenin to regulate Wnt/β-catenin transcriptional activity in chondrocytes. While the role of Sox9-Wnt interactions in development and maintenance of various organs has been acknowledged, no such association has yet been demonstrated in thymic epithelia and Sox9 target genes in TEC have not yet been identified.
In the present study, we demonstrate that Sox9, which is differentially expressed in the thymus by non-hematopoietic stromal cells, negatively regulates TEC proliferation and inversely correlates with the expression of the transcription factor Foxn1. The TEC-targeted loss of Sox9 disturbs thymus genesis and results in a hypoplastic thymus with a phenotypically altered epithelial compartment. Mice with a TEC-targeted Sox9 deficiency display subset-specific changes in TEC composition and proliferation, a phenotype which correlates with an upregulation of Foxn1 expression. Despite these alterations, thymopoiesis remains unaffected indicating that Sox9 expression is not required for the TEC`s capacity to support T cell development. In vitro studies revealed that Sox9 regulates Foxn1 transcription indirectly by binding and regulating the amount of intracellular β-catenin protein. This correlation suggests a crosstalk between Sox9 and the canonical Wnt pathway to occur in thymic epithelia.
To our best of knowledge, this study provides the first functional evidence that Sox9 controls TEC proliferation and differentiation in a dose-sensitive and subset-specific manner and negatively regulates Foxn1 expression in TEC. Although, the loss of Sox9 expression in TEC is not sufficient to jeopardize T cell development, differential Sox9 expression is critical for the establishment and maintenance of a regular thymic microenvironment.