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+ 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.
Keywords: MHC class I antigen presentation; immune tolerance; mRNA translation.