EBF1 regulates the blood-thymus barrier and ETP influx through Claudin-5 in thymic endothelial cells

The blood-thymus-barrier (BTB) is essential for regulating the entry of thymic-seeding progenitors into the thymus, an organ lacking self-renewing T cell progenitors. Here, we identify Early B-cell factor 1 (Ebf1), a transcription factor with established roles in hematopoiesis, as specifically expressed in thymic endothelial cells. Tie2-Cre-mediated deletion of Ebf1 results in a significant reduction of early thymic progenitors (ETPs), despite unaltered bone marrow lymphoid progenitor populations, unaltered thymic stromal subset frequencies, and intact downstream thymocyte differentiation. Transcriptome analysis of Ebf1-deficient thymic endothelial cells reveals increased expression of tight-junction-associated genes, in particular Cldn5, a key tight-junction component that restricts the BTB. Consistent with reduced BTB permeability, ETP subset analysis demonstrates a proportional reduction in the ETP1 fraction, the progenitor population that resides in closest proximity to thymic endothelial cells at the cortico-medullary junction. Together, our study implicates Ebf1 in the regulation of the BTB permeability, through modulation of tight-junction programs, thereby enabling thymus-seeding progenitor entry.