H2-M3 presents a nonformylated viral epitope to CTLs generated in vitro

Most CTL responses to epitopes from influenza virus are restricted by MHC class Ia molecules. However, a synthetic peptide corresponding to residues 173 to 190 of influenza A/JAP/305/57 hemagglutinin (HA) can induce, in vitro, a CTL response to peptide presented by a mouse class Ib molecule encoded by a gene telomeric to H2-Q. Here, we identify the molecule as H2-M3 and show that the last five residues of HA173-190, MLIIW, is the minimal epitope for CTL recognition. Cells that express M3wt, from C57BL/6 or BALB/c mice, are sensitized by both MLIIW and the longer peptide HA173-190, whereas cells that express M3f, from A.CA or B10.M mice, are sensitized only by MLIIW; a single amino acid change at residue 31 (V-->M) of M3 accounts for this difference. Although M3-restricted CTLs preferably recognize N-formylated epitopes, i.e., those of mitochondrial or prokaryotic origin, our findings show that M3-restricted primary CTL responses can be generated in vitro against nonformylated epitopes.