Recognition of out‐of‐frame major histocompatibility complex class I‐restricted epitopes in vivo

In the course of constructing a recombinant vaccinia virus encoding the influenza A nucleoprotein (NP) gene preceeded by the hemagglutinin leader sequence, we isolated a single base‐pair deletion mutant which gave rise to L+NP(1–159) in which only the first 159 amino acids were in frame. Despite this, when we infected target cells, we found that the point mutant was able to sensitize them for lysis not only by cytotoxic T cells recognizing residues 50–58 (the in‐frame portion), but also by CTL to epitopes which are downstream of the mutation (366–374 and 378–386). Furthermore, normal C57BL/6 mice can be primed with the frameshift NP to recognize the immunodominant Db‐restricted epitope 366–374 (which is out of frame). Experiments in which the mutant gene product was processed in the endoplasmic reticulum of target cells suggested that the apparent suppression occured during polypeptide extension.