title: Alternative translational reading frames as a novel source of epitopes for an expanded CD8 T-cell repertoire: use of a retroviral system to assess the translational requirements for CTL recognition and lysis.
abstract: CD8 T-cell responses constitute a key host defense mechanism against tumor cells and a variety of viral infections, including retroviral infections that lead to acquired immunodeficiency. However, both for tumor cells and for many viral infections, there can be a relative paucity of immunodominant protective CD8 T-cell responses. For retroviruses, their rapid and error-prone replication, coupled with initial CD8 T-cell immunoselection of epitope-variant, retroviral quasi-species, are major impediments to sustaining a protective CD8 T-cell response. To approach this limitation of functional CD8 T-cell epitopes, here we further characterize an underappreciated source of additional T-cell epitopes: cryptic determinants, in particular those encoded in unconventional, alternative reading frames (ARFs). By use of the CD8 T-cell epitope, SYNTGRFPPL, which we have defined as encoded by the +1NT ARF of the gag gene of the LP-BM5 retrovirus that causes murine AIDS, we further characterize the regulation of ARF-epitope expression. Specifically, we examine the translation initiation requirements for production of sufficient epitope for effector CD8 T-cell recognition. Such translation must arise from rare frame-shifting events, making it crucial to understand any other constraints on epitope production, and therefore on the ability of the anti-Kd/SYNTGRFPPL CD8 T cells to protect from LP-BM5 pathogenesis and retroviral load, as we have previously shown. The data herein demonstrate that ARF epitope production depends entirely on conventional AUG-initiated translation, and that the more proximal in-frame ARF AUG is most important. However, maximal epitope production for protective CD8 T-cell lytic function also requires synergy of this initiation codon with a counterpart conventional AUG codon upstream in the same ARF (ORF 2), and with the classic ORF 1 AUG that initiates conventional gag polyprotein translation. These results have implications for the design of ARF-epitope-based vaccines, both to counter retroviral pathogenesis, as well as potentially more broadly, including in tumor systems.
authors.affiliation: Department of Microbiology and Immunology and the Norris Cotton Cancer Center, Dartmouth Medical School, Lebanon, New Hampshire 03756, USA.
authors.forename: Timothy L
authors.initials: TL
authors.lastname: Carlson
authors.forename: Kathy A
authors.initials: KA
authors.lastname: Green
authors.forename: William R
authors.initials: WR
authors.lastname: Green
mesh_terms: D000818:Animals; D017951:Antigen Presentation; D002460:Cell Line; D018387:Codon, Initiator; D003602:Cytotoxicity, Immunologic; D018984:Epitopes, T-Lymphocyte / Q000235:genetics* / Q000276:immunology; D016368:Frameshift Mutation; D018965:Frameshifting, Ribosomal; D008297:Male; D051379:Mice; D016366:Open Reading Frames; D014176:Protein Biosynthesis; D012190:Retroviridae / Q000235:genetics* / Q000276:immunology; D013602:T-Lymphocytes, Cytotoxic / Q000276:immunology*
chemical_list: D018387:Codon, Initiator; D018984:Epitopes, T-Lymphocyte