Tnj. Bullock et Lc. Eisenlohr, RIBOSOMAL SCANNING PAST THE PRIMARY INITIATION CODON AS A MECHANISM FOR EXPRESSION OF CTL EPITOPES ENCODED IN ALTERNATIVE READING FRAMES, The Journal of experimental medicine, 184(4), 1996, pp. 1319-1329
An increasing amount of evidence has shown that epitopes restricted to
MHC class I molecules and recognized by CTL need not be encoded in a
primary open reading frame (ORF). Such epitopes have been demonstrated
after stop codons, in alternative reading frames (RF) and within intr
ons. We have used a series of frameshifts (FS) introduced into the Inf
luenza A/PR/8/34 nucleoprotein (NP) gene to confirm the previous in vi
tro observations of cryptic epitope expression, and show that they are
sufficiently expressed to prime immune responses in vivo. This presen
tation is not due to sub-dominant epitopes, transcription from cryptic
promoters beyond the point of the FS, or internal initiation of trans
lation. By introducing additional mutations to the construct exhibitin
g the most potent presentation, we have identified initiation codon re
adthrough (termed scanthrough here, where the scanning ribosome bypass
es the conventional initiation codon, initiating translation further d
ownstream) as the likely mechanism of epitope production. Further muta
tional analysis demonstrated that, while it should operate during the
expression of wild-type (WT) protein, scanthrough does not provide a m
ajor source of processing substrate in our system. These findings sugg
est (i) that the full array of self- and pathogen-derived epitopes ava
ilable during thymic selection and infection has not been fully apprec
iated and (ii) that cryptic epitope expression should be considered wh
en the specificity of a CTL response cannot be identified or in therap
eutic situations when conventional CTL targets are limited, as may be
the case with latent viral infections and transformed cells. Finally,
initiation codon readthrough provides a plausible explanation for the
presentation of exocytic proteins by MHC class I molecules.