anscriptomic census reveals that Rbfox contributes to a broad but selective recapitulation of peripheral tissue splicing patterns in the thymus

Abstract Thymic epithelial cells (TEC) control the selection of a T-cell repertoire reactive to pathogens but tolerant of self. While this process is known to involve the promiscuous expression of virtually the entire protein-coding gene repertoire the extent to which TEC recapitulate peripheral isoforms, and the mechanisms by which they do so, have remained largely unknown. We performed the first assembly-based transcriptomic census of transcript structures and splicing factor (SF) expression in mouse medullary TEC (mTEC) and 21 peripheral tissues. Mature mTEC expressed 60.1% of all protein-coding transcripts, more than was detected in any of the peripheral tissues. However, for genes with tissue-restricted expression, we found that mTEC produced fewer isoforms than did the relevant peripheral tissues. Analysis of exon inclusion revealed an absence of brain-specific micro-exons in mTEC. We did not find unusual numbers of novel transcripts in TEC and show that Aire , the facilitator of promiscuous gene expression, promotes usage of long transcripts but has only a limited impact on alternative splicing in mTEC. Comprehensive assessment of SF expression in mTEC identified a small set of non-promiscuously expressed SF genes amongst which we confirmed RBFOX to be present with AIRE in mTEC nuclei. Using a conditional loss of function approach, we show that Rbfox2 promotes mTEC development and regulates the alternative splicing of promiscuously expressed genes. These data indicate that TEC recommission a small number of peripheral SFs, including members of the Rbfox family, to generate a broad but selective representation of the peripheral splice isoform repertoire.