Morphogenesis and maintenance of the 3D thymic medulla and prevention of nude skin phenotype require FoxN1 in pre- and post-natal K14 epithelium

Expansion of thymic epithelial cysts represents disruption of an organized three-dimensional (3D) thymic epithelial cell (TEC) meshwork, which is crucial for Tlymphocyte development. Although the FoxN1-null mutant develops a rudimentary two-dimensional (2D) cystic thymus, 2D thymic cyst lining resulting from a dGUO culture was reported to be FoxN1-independent; thus, it is unclear whether loss of FoxN1 facilitates cyst formation and whether FoxN1 regulates the morphogenesis and maintenance of the 3D thymic microstructure. Using the loxPfloxed- FoxN1 mouse model, we demonstrated that specific deletion of FoxN1 in keratin (K)-14 promoter-driven TECs induced the loss of 3D thymic medullary structure by producing a large number of morphologic pulmonary alveolar-like 2D epithelial cysts, which increased with age. The cystic lining was positive for differential polarized keratins and had strong claudin-3,4, but reduced MHC-II, expression. However, an increased percentage of claudin-3,4⁺ TECs, which are presumptive precursors of UEA-1⁺ and Aire⁺ mature medullary TECs, failed to promote the development of these mature descendants. Meanwhile, the K14Cremediated FoxN1 deletion alone was sufficient to induce a complete hair follicle defect, causing a nude phenotype in the skin, but was not sufficient to cause a complete loss of the thymus. All these changes to occur require deletion of FoxN1 in both prenatal (Cre-recombinase from parents during fertilization) and postnatal (Cre-recombinase from offspring themselves after fertilization) life. These findings provide new insights into FoxN1 regulation of 3D thymic epithelial morphogenesis and maintenance, the distinct impacts of FoxN1 in the K14 epithelial subset of the thymus and skin, and its postnatal requirement.