In Vivo Selection of a Lymphocytic Choriomeningitis Virus Variant That Affects Recognition of the GP33-43 Epitope by H-2D(b) but Not H-2K(b)

CD8 T cells drive the protective immune response to lymphocytic choriomeningitis virus (LCMV) infection and are thus a determining force in the selection of viral variants. To examine how escape mutations affect the presentation and recognition of overlapping T-cell epitopes, we isolated an LCMV variant that is not recognized by T-cell receptor (TCR)-transgenic H-2D(b)-restricted LCMV GP33-41-specific cytotoxic T lymphocytes (CTL). The variant virus carried a single-amino-acid substitution (valine to alanine) at position 35 of the viral glycoprotein. This region of the LCMV glycoprotein encodes both the D(b)-restricted GP33-43 epitope and a second epitope (GP34-42) presented by the K(b) molecule. We determined that the V-to-A CTL escape mutant failed to induce a D(b) GP33-43-specific CTL response and that D(b)-restricted GP33-43-specific CTL induced by the wild-type LCMV strain were unable to kill target cells infected with the variant LCMV strain. In contrast, the K(b)-restricted response was much less affected. We found that the V-to-A substitution severely impaired peptide binding to D(b) but not to K(b) molecules. Strikingly, the V-to-A mutation did not change any of the anchor residues, and the dramatic effect on binding was therefore unexpected. The strong decrease in D(b) binding explains why the variant virus escapes the D(b) GP33-43-specific response but still elicits the K(b)-restricted response. These findings also illustrate that mutations within regions encoding overlapping T-cell epitopes can differentially affect the presentation and recognition of individual epitopes.