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 choriomenin gitis virus (LCMV) infection and are thus a determining force in the select ion of viral variants. To examine how escape mutations affect the presentat ion and recognition of overlapping T-cell epitopes, we isolated an LCMV var iant that is not recognized by T-cell receptor (TCR)-transgenic H-2D(b)-res tricted LCMV GP33-41-specific cytotoxic T lymphocytes (CTL). The variant vi rus carried a single-amino-acid substitution (valine to alanine) at positio n 35 of the viral glycoprotein. This region of the LCMV glycoprotein encode s both the D-b-restricted GP33-43 epitope and a second epitope (GP34-42) pr esented by the K-b molecule. We determined that the V-to-A CTL escape mutan t failed to induce a D-b GP33-43-specific CTL response and that D-b-restric ted GP33-43-specific CTL induced by the wild-type LCMV strain were unable t o 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 s ubstitution severely impaired peptide binding to D-b but not to K-b molecul es. Strikingly, the V-to-A mutation did not change any of the anchor residu es, 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 GP3 3-43-specific response but still elicits the K-b-restricted response. These findings also illustrate that mutations within regions encoding overlappin g T-cell epitopes can differentially affect the presentation and recognitio n of individual epitopes.