ABSTRACT
Immune surveillance by CD8 T cells requires that peptides derived from intracellular proteins be presented by MHC class I molecules on the target cell surface. Interestingly, MHC molecules can also present peptides encoded in alternate translational reading frames, some even without conventional AUG initiation codons. Using T cells to measure expression of MHC bound peptides, we identified the non-AUG translation initiation codons and established that their activity was at the level of translational rather than DNA replication or transcription mechanisms. This translation mechanism decoded the CUG initiation codon not as the canonical methionine but as the leucine residue, and its activity was independent of upstream translation initiation events. Naturally processed peptide/MHC complexes can thus arise from "noncoding" mRNAs via a novel translation initiation mechanism.
Subject(s)
Antigen Presentation/immunology , H-2 Antigens/immunology , Peptides/genetics , Peptides/immunology , Protein Biosynthesis/immunology , Reading Frames/immunology , Amino Acid Sequence , Animals , Antigen Presentation/genetics , Base Sequence , COS Cells , Cell Line , Codon, Initiator/genetics , DNA Replication/genetics , H-2 Antigens/chemistry , Leucine/genetics , Methionine/genetics , Mice , Molecular Sequence Data , Peptide Biosynthesis/genetics , Peptides/analysis , Reading Frames/genetics , Recombinant Fusion Proteins/biosynthesis , TransfectionABSTRACT
Minor histocompatibility (H) Ags elicit T cell responses and thereby cause chronic graft rejection and graft-vs-host disease among MHC identical individuals. Although numerous independent H loci exist in mice of a given MHC haplotype, certain H Ags dominate the immune response and are thus of considerable conceptual and therapeutic importance. To identify these H Ags and their genes, lacZ-inducible CD8+ T cell hybrids were generated by immunizing C57BL/6 (B6) mice with MHC identical BALB.B spleen cells. The cDNA clones encoding the precursor for the antigenic peptide/Kb MHC class I complex were isolated by expression cloning using the BCZ39.84 T cell as a probe. The cDNAs defined a new H locus (termed H60), located on mouse chromosome 10, and encoded a novel protein that contains the naturally processed octapeptide LTFNYRNL (LYL8) presented by the Kb MHC molecule. Southern blot analysis revealed that the H60 locus was polymorphic among the BALB and the B6 strains. However, none of the H60 transcripts expressed in the donor BALB spleen were detected in the host B6 strain. The expression and immunogenicity of the LYL8/Kb complex in BALB.B and CXB recombinant inbred strains strongly suggested that the H60 locus may account for one of the previously described antigenic activity among these strains. The results establish the source of an immunodominant autosomal minor H Ag that, by its differential transcription in the donor vs the host strains, provides a novel peptide/MHC target for host CD8+ T cells.
