Abstract
Thymic epithelial cells (TEC) are essential for thymocyte differentiation and repertoire selection. Despite their indispensable role in generating functional T cells, the molecular mechanisms that orchestrate TEC development from endodermal progenitors in the third pharyngeal pouch (3rd PP) are not fully understood. We recently reported that the T-box transcription factor TBX1 negatively regulates TEC development. Although initially expressed throughout the 3rd PP, Tbx1 becomes downregulated in thymus-fated progenitors and when ectopically expressed impairs TEC progenitor proliferation and differentiation. Here we show that ectopic Tbx1 expression in thymus fated endoderm increases expression of Polycomb repressive complex 2 (PRC2) target genes in TEC. PRC2 is an epigenetic modifier that represses gene expression by catalyzing trimethylation of lysine 27 on histone H3. The increased expression of PRC2 target genes suggests that ectopic Tbx1 interferes with PRC2 activity and implicates PRC2 as an important regulator of TEC development. To test this hypothesis, we used Foxn1Cre to delete Eed, a PRC2 component required for complex stability and function in thymus fated 3rd PP endoderm. Proliferation and differentiation of fetal and newborn TEC were disrupted in the conditional knockout (EedCKO) mutants leading to severely dysplastic adult thymi. Consistent with PRC2-mediated transcriptional silencing, the majority of differentially expressed genes (DEG) were upregulated in EedCKO TEC. Moreover, a high frequency of EedCKO DEG overlapped with DEG in TEC that ectopically expressed Tbx1. These findings demonstrate that PRC2 plays a critical role in TEC development and suggest that Tbx1 expression must be downregulated in thymus fated 3rd PP endoderm to ensure optimal PRC2 function.
Highlights
T cell development occurs in the thymus where a unique three-dimensional network of thymic epithelial cells (TEC) provides indispensable signals for thymocyte growth, differentiation and T cell receptor repertoire selection
We found increased expression of Polycomb repressive complex 2 (PRC2) target genes in Foxn1Cre;R26iTbx Thymic epithelial cells (TEC). This observation suggested that inappropriate expression of Tbx[1] in TEC progenitors interferes with PRC2-mediated transcriptional repression, which in turn may contribute to an aberrant thymus phenotype
We investigated the molecular basis for this phenotype, by performing genome-wide transcriptome analysis on TEC isolated from E15.5 Foxn1Cre/+;R26iTbx/+ mutant and control Foxn1Cre/+;R26+/+ thymi
Summary
T cell development occurs in the thymus where a unique three-dimensional network of thymic epithelial cells (TEC) provides indispensable signals for thymocyte growth, differentiation and T cell receptor repertoire selection. RNA-seq analysis revealed that the majority of differentially expressed genes (DEG) in Foxn1Cre;Eedfl/fl TEC were upregulated, consistent with the canonical gene silencing function of PRC2. Consistent with our previous report showing that Foxn[1] expression is decreased in iTbx[1] TEC7, the RNA-seq data revealed that 159 of 450 high confidence Foxn[1] target genes[21] were included in iTbx[1] DEG (Fig. 1a).
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