Antigenic peptides derived from introns are presented on major histocompatibility (MHC) class I molecules, but how these peptides are produced is poorly understood. Here, we show that an MHC class I epitope (SL8) sequence inserted in the second intron of the β-globin gene in a C57BL/6 mouse (HBB) generates immune tolerance. Introduction of SL8-specific CD8
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T cells derived from OT-1 transgenic mice resulted in a threefold increase in OT-1 T cell proliferation in HBB animals, as compared to wild-type animals. The growth of MCA sarcoma cells expressing the intron-derived SL8 epitope was suppressed in wild-type animals compared to HBB mice. The β-globin pre-mRNA was detected in the light polysomal fraction, and introducing stop codons identified a non-AUG initiation site between +228 and +255 nts upstream of the SL8. Isolation of ribosome footprints confirmed translation initiation within this 27 nt sequence. Furthermore, treatment with splicing inhibitor shifts the translation of the pre-mRNA to monosomal fractions and results in an increase of intron-derived peptide substrate as shown by polysome profiling and cell imaging. These results show that non-AUG-initiated translation of pre-mRNAs generates peptides for MHC class I immune tolerance and helps explain why alternative tissue-specific splicing is tolerated by the immune system.
Cellular invasion is a complex process that requires several interdependent biological mechanisms, which are initiated by changes in adhesion that establish a morphology favorable for migration. Hence, the regulation of adhesion potential is a rate-limiting step in metastasis. Our previous work revealed that de novo translation is necessary to regulate the adhesion of mesenchymal-like cells; however, the underlying translational regulatory mechanism and the identity of newly synthesized proteins needed for the adhesion process remain unidentified. Here, we describe a translational regulatory mechanism that modulates mesenchymal cell adhesion. We observed a drastic decrease in translation during the initial phase of adhesion, followed by a reprogramming of the translatome, characterized by an orchestrated wave of mRNA translation that increases the expression of specific proteins involved in adhesion. We observed that phosphorylation of eukaryotic translation initiation factor 2 subunit alpha (eIF2α), which inhibits general translation initiation, was drastically increased at the beginning of cell adhesion. As adhesion progressed, the selective increase in the translation of adhesion-related mRNAs intensified as eIF2α phosphorylation gradually decreased over time in mensenchymal cells, but not in epithelial cells. Taken together, we have identified a translational regulatory mechanism specifically affecting the adhesion process of mesenchymal cells, as well as metastatic cells that have undergone epithelial-to-mesenchymal transition.One sentence summaryTranslation regulates mesenchymal cell adhesion
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