Foxo3 Regulates the Homeostasis and Function of Thymic Epithelial Cells.

Abstract Within the thymus, thymic epithelial cells (TECs) create dedicated microenvironments for T-cell development and selection. Considering that TECs are sensitive to distinct pathophysiological conditions, uncovering the molecular elements that coordinate their thymopoietic role has important fundamental and clinical implications. Particularly, medullary thymic epithelial cells (mTECs) play a crucial role in central tolerance. Our previous studies, along with others, suggest that mTECs depend on molecular factors linked to genome-protecting pathways, but the precise mechanisms underlying their function remain unknown. These observations led us to examine the role of Foxo3, as it is expressed in TECs and involved in DNA damage response. Our findings show that mice with TEC-specific deletion of Foxo3 (Foxo3cKO) displayed a disrupted mTEC compartment, with a more profound impact on the numbers of CCL21+ and thymic tuft mTEClo subsets. Notably, the combined deficiency in Foxo3 and Aire revealed a central role of Foxo3 in regulating Aire-dependent and Aire-independent mTEC lineages. At the molecular level, Foxo3 controls distinct functional modules in the transcriptome of cTEC and mTEC under normal conditions, which includes the regulation of ribosomal biogenesis and DNA damage response, respectively. These changes in the TEC compartment resulted in a reduced total thymocyte cellularity and specific changes in regulatory T cell and iNKT cell development in the Foxo3cKO thymus. Lastly, the thymic defects observed in adulthood extended to the periphery, as aged Foxo3cKO mice presented moderate signs of altered peripheral immunotolerance, which were aggravated in the Aire-deficient autoimmune background. Our findings highlight the importance of Foxo3 in preserving the homeostasis of TECs and in supporting their role in T-cell development and tolerance.