Cytolytic CD8+ T Cells Directed against a Cryptic Epitope Derived from a Retroviral Alternative Reading Frame Confer Disease Protection

Abstract

Cytolytic CD8+ T cells (CTL) are key to the immune response that controls virus infections and mediates disease protection. The ability of CTL to induce apoptosis of infected cells and/or limit viral replication is determined by recognition of processed viral peptide epitopes on the surface of the target cell. An understudied source of MHC class I-presented peptides is the aptly named “cryptic epitopes,” defined by their nontraditional methods of generation, including derivation from alternative reading frames (ARFs). Although ARF-encoded epitopes have now been documented in a few systems, their potential functional relevance in vivo has been debated. In this study, we demonstrate the physiological significance of an ARF-derived CTL epitope in a retrovirus-induced disease model. We show that disease-susceptible CD8-deficient mice reconstituted with CTL specific for the retroviral ARF-derived SYNTGRFPPL epitope controlled an infection by the LP-BM5 retrovirus isolate, evidently at the level of viral clearance, resulting in protection of these mice from disease. These data indicate that ARF-derived epitopes are indeed relevant inducers of the immune system and demonstrate the importance of atypically generated peptides as functional Ag with a physiologic role in disease protection.

The Programmed Death-1 and Interleukin-10 Pathways Play a Down-Modulatory Role in LP-BM5 Retrovirus-Induced Murine Immunodeficiency Syndrome

“…In disease-resistant BALB/c mice, however, we have shown that CD8-deficient (chronic in vivo anti-CD8 MAb treated or CD8 KO) mice convert to disease susceptibility (27,45) and that WT BALB/c mice generate a strong antiviral CD8 ϩ T-cell response (63). Indeed, the BALB/c CD8 ϩ T effector cells are highly lytic, secrete IFN-␥, and are protective against LP-BM5 virus challenge in vivo, in terms of both inhibition of disease and the ultimate viral load, after their adoptive transfer into BALB/c.…”

“…Indeed, the BALB/c CD8 ϩ T effector cells are highly lytic, secrete IFN-␥, and are protective against LP-BM5 virus challenge in vivo, in terms of both inhibition of disease and the ultimate viral load, after their adoptive transfer into BALB/c. CD8 KO recipients (27,44). Of note, both the polyclonal and clonal CD8 ϩ T-cell populations that were protective were specific for a unique K d -presented immunodominant epitope, SYNTGRFPPL, which derives from a previously unrecognized alternative (ϩ1 nucleotide) gag translational open reading frame of the LP-BM5 retrovirus (44,45).…”

Alternative Translational Products and Cryptic T Cell Epitopes: Expecting the Unexpected

“…Such epitopes positioned in an area of overlapping reading frames might make excellent candidates for conserved targets that the virus cannot vary to escape from immune detection. Thus, despite their presumably low abundance, ARF epitopes can be expressed at functional levels in vivo at the priming stage and at the target level without the use of overexpression systems (9), and can provide in vivo targets for protective immune responses (29,48).…”

“…1) (9). By adoptive transfer studies using disease-susceptible CD8-deficient BALB/c recipient mice, we have reported recently a physiologically relevant role for this ARF-encoded ORF2 epitope by demonstrating CTL-mediated full protection against LP-BM5-induced disease in vivo (29).…”

“…Translation of the ARF (ORF2) may initiate at either of two start sites, ORF2a and ORF2b, via a ϩ1 frameshift of the primary ORF (ORF1). The cryptic K d -restricted immunodominant CTL peptide SYNTGRFPPL (highlighted), processed from the larger precursor proteins, induces a protective CTL response against MAIDS disease (29). Conversely, the ORF2a and ORF2b protein products, individually or in combination, appear to be necessary for MAIDS pathogenesis (59).…”