Foxn1 Regulates Lineage Progression in Cortical and Medullary Thymic Epithelial Cells But Is Dispensable for Medullary Sublineage Divergence

Abstract

The forkhead transcription factor Foxn1 is indispensable for thymus development, but the mechanisms by which it mediates thymic epithelial cell (TEC) development are poorly understood. To examine the cellular and molecular basis of Foxn1 function, we generated a novel and revertible hypomorphic allele of Foxn1. By varying levels of its expression, we identified a number of features of the Foxn1 system. Here we show that Foxn1 is a powerful regulator of TEC differentiation that is required at multiple intermediate stages of TE lineage development in the fetal and adult thymus. We find no evidence for a role for Foxn1 in TEC fate-choice. Rather, we show it is required for stable entry into both the cortical and medullary TEC differentiation programmes and subsequently is needed at increasing dosage for progression through successive differentiation states in both cortical and medullary TEC. We further demonstrate regulation by Foxn1 of a suite of genes with diverse roles in thymus development and/or function, suggesting it acts as a master regulator of the core thymic epithelial programme rather than regulating a particular aspect of TEC biology. Overall, our data establish a genetics-based model of cellular hierarchies in the TE lineage and provide mechanistic insight relating titration of a single transcription factor to control of lineage progression. Our novel revertible hypomorph system may be similarly applied to analyzing other regulators of development.

Developmental expression of chicken FOXN1 and putative target genes during feather development

“…2B). This is consistent with the proposed function of mouse FOXN1 in regulation of keratin gene expression, which is present in medullary and cortical thymus epithelial cells during early thymus development (Rodewald, 2008;Nowell et al, 2011). As chick embryonic development progresses (stage ED13) the thymus lobes migrate laterally and expand; expression of FOXN1 persisted ( Fig.…”

FOXN1 recombinant protein enhances T‐cell regeneration after hematopoietic stem cell transplantation in mice

“…Several investigators have also shown that, in addition to maintain existing TECs, FOXN1 also regulates TEC progenitors [15,16,18,27,31], which may also contribute to the increased number of TECs. The number of ETPs was also significantly increased after rFOXN1 treatment.…”

“…It is generally acknowledged that FOXN1 is a pivotal regulator for TEC development [11][12][13][14][15][16][17][18][19]. Mice homozygous for loss-of-function mutation in FOXN1 display the 'nude' phenotype (FOXN1 nu/nu ), which is characterized by congenital athymia and hairlessness.…”

“…FOXN1 is required not only for TEC development in fetal thymus, but also for maintenance of the postnatal thymus [15,16,23,24,26]. The reduced expression or induced deletion of FOXN1 resulted in reduced TEC number and a disrupted thymic microenvironment [16,18,22,24,27]. Conversely, an intrathymic injection of FOXN1 cDNA-bearing plasmid into the thymus of aged mice or FOXN1 transgenic mice resulted in an increased number of TECs, leading to enhanced thymopoiesis and an increased naïve T-cell output [23,25].…”

“…These results suggest that FOXN1 induced mTEC proliferation is due, at least in part, to the enhanced expression of cyclin D1. Whether cyclin D1 and p53 are the directly or indirectly downstream target genes remains to be determined.In addition to an increased number of mature TECs, rFOXN1 treatment also increased the number of MHC II lo immature TECs.Several investigators have also shown that, in addition to maintain existing TECs, FOXN1 also regulates TEC progenitors [15,16,18,27,31], which may also contribute to the increased number of TECs. The number of ETPs was also significantly increased after rFOXN1 treatment.…”