High‐affinity human leucocyte antigen class I binding variola‐derived peptides induce CD4+ T cell responses more than 30 years post‐vaccinia virus vaccination

Interferon‐γ secreting T lymphocytes against pox virus‐derived synthetic 9‐mer peptides were tested by enzyme‐linked immunospot in peripheral blood of individuals vaccinated with vaccinia virus more than 30 years ago. The peptides were characterized biochemically as high‐affinity human leucocyte antigen (HLA) class I binders (K_D ≤ 5 nM). However, five of the individuals tested did not show typical CD8⁺ T cell‐mediated HLA class I‐restricted responses. Instead, these donors showed CD4⁺ T cell‐dependent responses against four of a total of eight antigenic 9‐mer peptides discovered recently by our group. These latter responses were blocked specifically in the presence of anti‐HLA class II antibody. We conclude that long‐lived memory responses against pox virus‐derived 9‐mer peptides, with high binding affinity for HLA class I molecules, are mediated in some cases by CD4⁺ T cells and apparently restricted by HLA class II molecules.