SIGNEPI4TOL · Cell signaling pathways and epigenetic mechanisms controlling the differentiation of medullary thymic epithelial cells, key mediators in the induction of central self-tolerance

Cell signaling pathways and epigenetic mechanisms controlling the differentiation of medullary thymic epithelial cells, key mediators in the induction of central self-tolerance

The establishment of the immune system tolerance is critical in the host defense against pathogens and malignant cells while being tolerant towards our own tissues. Medullary thymic epithelial cells (mTECs) are essential cellular actors in the generation of a self-tolerant T cell repertoire. Through their fascinating ability to express a wide range of tissue-restricted self-antigens (TRAs), this cell type plays a key role in the elimination of potentially hazardous autoreactive T cells and in the induction of Foxp3+ regulatory T cells that control autoreactive T cells that have escaped the thymic selection processes. This promiscuous gene expression program in mTECs is known to be regulated so far by the two transcription factors, Aire (Autoimmune Regulator) and Fezf2 (Fez family zinc finger 2). While mTECs control the selection of developing T cells, reciprocally thymocytes control the differentiation of mTECs, a process referred to as “thymic crosstalk”. In this context, my previous works have shown that mTEC organization and Aire+ cell differentiation are controlled by antigen-specific interactions with CD4+ thymocytes. Although these cell interactions are of pivotal importance for the generation of Aire+ mTECs, the underlying mechanisms remain to be fully determined. In particular, the transcriptional and epigenetic properties of mTECs regulated by this cellular crosstalk are unknown. The objective of this EU-funded SIGnEPI4Tol project was to decipher the impact of antigen-specific interactions with CD4+ T cells at the transcriptional and epigenetic level on mTEC biology. To achieve this objective, we used high-throughput RNA-sequencing (RNA-seq) and chromatin immunoprecipitation (ChIP-seq) on mTECs purified from unique transgenic mouse models, in which antigen presentation to CD4+ thymocytes is selectively abrogated in mTECs. The combination of these state-of the-art approaches with the use of these powerful mouse models allowed us to identify the set of genes as well as the histone marks regulated in mTECs by crosstalk with CD4+ thymocytes. The first main finding of this project is that antigen-specific interactions with CD4+ thymocytes control several unsuspected facets of mTEC biology that are pivotal for the induction of central tolerance. This crosstalk upregulates the expression of not only several transcriptional regulators that control mTEC differentiation and function but also of (1) numerous TRAs involved in T-cell repertoire selection, (2) chemokines, involved in the trafficking of developing T cells and dendritic cells and (iii) adhesion molecules involved in mTEC-T cell interactions. In line with these findings, we demonstrated that mTEC/CD4+ thymocyte crosstalk regulates the expression of CCR2 ligands (CCL2, CCL8 and CCL12) in mTECs through the lymphotoxin alpha axis. Importantly, we found that this chemokine regulation controls the thymic entry of antigen-presenting cells, which fine-tunes T cell clonal deletion (Nature Communications, 2018). This project also revealed that antigen-specific interactions with CD4+ thymocytes enhance the level of active histone marks, indicative of an underlying epigenetic regulation of the transcriptional programs induced. Lastly, we demonstrated that this cellular crosstalk in the thymus prevents the development of autoimmunity. Altogether, these results provide important insights into the mechanisms that drive the generation of a functional mTEC compartment capable to ensure T-cell tolerance induction. Thus, this project greatly improves our knowledge on mechanisms that sustain the establishment immunological self-tolerance in the thymus. We anticipate that the results obtained during the course of this project should pave the way to develop new therapeutic interventions in the field of autoimmune disorders. Website: http://cvscience.aviesan.fr/cv/1387/magali-irla Contact: Dr. Magali Irla; PhD, HDR Inserm CR1 Centre d'Immunologie de Marseille-Luminy (CIML) Campus de Luminy case 906 13288 Marseille Cedex 09 France E-mail: Magali.Irla@inserm.fr

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