Towards Regenerative Therapy for Thymic Insufficiency after Hematopoietic Stem Cell Transplantation: Generation of MTS24 Positive Definitive Endoderm from Murine Embryonic Stem Cells.
Authors: Vroegindeweij Erik , Rombouts Wilhelmus J. , Ropela Joanna A. , Nishikawa Shin-Ichi , Cupedo Tom , Cornelissen Jan J. 
Published: April 2020

Abstract
Following hematopoietic stem cell transplantation (HSCT), longterm T-cell reconstitution should be established by thymus-dependent de-novo generation of naïve T-cells (thymopoiesis), which is especially important for generating a naïve T-cell pool with a broad T-cell receptor (TCR) repertoire. However, while erythroid and myeloid hematopoietic cell lineages recover rapidly following HSCT, T-cell development may severely lag behind due to thymic insufficiency. Recent studies in fetal mice have identified common thymic epithelial progenitor cells (TEPC) that were capable to re-establish a thymus in-vivo upon transplantation into a-thymic nude mice. These TEPC are characterized by expression of the transcription factor Foxn1 and by cell surface expression of MTS24. These TEPC arise exclusively from progenitors originating from the anterior foregut endoderm during embryogenesis. Therefore, we hypothesized that common TEPC may be generated in-vitro from embryonic stem (ES) cells that have differentiated towards definitive endoderm. Currently, the mechanisms underlying commitment of definitive endoderm towards a thymic fate are unknown. In order to differentiate murine ES cells towards definitive endoderm and TEPC and to identify the factors involved in the commitment of endoderm towards a thymic fate we investigated the expression of MTS24 and of genes associated with thymic differentiation in ES-cell derived endoderm using a Gcs–GFP/Sox17–huCD25 reporter ES cell line. Culture of these GscgfpSox17huCD25 ES cells in the presence of Activin A resulted in a rapid induction of mesendodermal differentiation. After 6 days of culture the majority of cells differentiated towards mesoderm (Gsc+Sox17−, 60%) and definitive endoderm (GSC+Sox17+, 35%). Apart from the addition of Activin A, the use of low passage number ES-cells and a seeding density between 200–300 cells/cm2 were the most important factors determining efficient differentiation towards definitive endoderm. Addition of insulin or WNT-3a had no significant effect on differentiation, while usage of a high passage number of ES-cells and/or a high seeding density mainly promoted development of visceral endoderm. Real-time quantitative PCR of the definitive endoderm fraction of these cultures not only showed expression of genes associated with definitive endoderm and gut tube formation (i.e. Sox17, Foxa2, Hnf4a and TCF2) but also of genes associated with anterior foregut endoderm (i.e. Hhex, Pax9) and a low, but significant, expression of Foxn1. Analysis of MTS24 expression within these cultures showed the presence of this antigen on all three cell types. The percentage of cells expressing MTS24 was highest in visceral endoderm (30–50%) and lowest in mesoderm (5–10%). The expression was approximately 12% in definitive endoderm. We conclude that murine ES cells cultured in the presence of Activin A can efficiently differentiate towards gut-tube like endoderm, including anterior forgut endoderm, and that a fraction of the generated endoderm also expresses the surface marker MTS24, suggesting the generation of epithelial progenitors with phenotypic characteristics of TEPC.