SummaryA central goal of regenerative medicine is to generate transplantable organs from cells derived or expanded in vitro. Although numerous studies have demonstrated production of defined cell-types in vitro1, creation of a fully intact organ has not been reported. The transcription factor Forkhead box N1 (FOXN1) is critically required for development of thymic epithelial cells (TECs)2,3 a key cell-type of the thymic stroma4. Here, we show that enforced Foxn1 expression is sufficient to reprogramme fibroblasts into functional TECs, an unrelated cell-type across a germ-layer boundary. These Foxn1-induced TECs (iTECs) supported efficient development of both CD4+ and CD8+ T cells in vitro. Upon transplantation, iTEC established a complete, fully organized and functional thymus, that contained all of the TEC sub-types required to support T cell differentiation and populated the recipient immune system with T cells. iTEC thus demonstrate that cellular reprogramming approaches can be used to generate an entire organ, and open the possibility of widespread use of thymus transplantation to boost immune function in patients.

Data availability RNA sequencing data that support the findings of this study have been deposited in the ArrayExpress database at EMBL-EBI (www.ebi.ac.uk/arrayexpress) under accession number E-MTAB-7660. All other data supporting the findings of this study are available from the corresponding author on reasonable request. Code for the biophysical modelling is provided as a Supplementary file.

SummaryThymic epithelial cells (TECs) are critically required for T cell development, but the cellular mechanisms that maintain adult TECs are poorly understood. Here, we show that a previously unidentified subpopulation, EpCam+UEA1−Ly-51+PLET1+MHC class IIhi, which comprises <0.5% of adult TECs, contains bipotent TEC progenitors that can efficiently generate both cortical (c) TECs and medullary (m) TECs. No other adult TEC population tested in this study contains this activity. We demonstrate persistence of PLET1+Ly-51+ TEC-derived cells for 9 months in vivo, suggesting the presence of thymic epithelial stem cells. Additionally, we identify cTEC-restricted short-term progenitor activity but fail to detect high efficiency mTEC-restricted progenitors in the adult thymus. Our data provide a phenotypically defined adult thymic epithelial progenitor/stem cell that is able to generate both cTECs and mTECs, opening avenues for improving thymus function in patients.