Immune tolerance is key to the maintenance of the integrity of organisms against foreign invaders with respect to self-constituents. Deregulation of this mechanism promotes the occurrence of life-threatening autoimmune diseases that affect 5-10% of the general population. Previously, the transcriptional activator (Aire) was shown to play a key role in immune tolerance. Indeed, Aire induces medullary epithelial cells in the thymus (MECs) to synthesize and present a large repertoire of peripheral self-antigens (a “self-shadow”), leading to the clonal deletion of self-reactive maturing T cells and thereby protecting against autoimmune manifestations. In addition to activating transcription, preliminary results indicate that Aire induces 3’UTR shortening of its sensitive transcripts in MECs, and that these cells show an accumulation of miRNAs. The goal of our project is to describe a post-transcriptional control of the Aire-driven expression in the thymus that leads to higher levels of Aire-dependent self-antigens, and to identify key molecular players involved in this mechanism. We first propose to confirm that Aire-induced transcripts have shorter 3’UTR ends by mRNA high-throughput sequencing in primary MECs, and that these transcripts escape a miRNA-mediated post-transcriptional repression in a MEC cell line. We will also seek factors involved in Aire-triggered 3’UTR shortening by performing a lentivirus-based RNAi screen of a set of RNA-binding proteins in an in vitro model that we will set up. Finally we will validate some of the best candidates in vivo by generating knock-down mice using a high-speed lentigenic approach based on oocytes infection with lentiviruses delivering targeting short-hairpin RNAs. Altogether the expected results should uncover an important layer of control of self-antigen expression in the thymus mediating immune tolerance, and provide new potential targets for therapeutic intervention in autoimmune diseases.