Six unrelated HLA-DR-matched adults recognize identical CD4+ T cell epitopes from influenza A haemagglutinin that are not simply peptides with high HLA-DR binding affinities.

Influenza A/Beijing/32/92 (H3N2) haemagglutinin (HA)-specific short-term CD4+ T cell lines were generated from six unrelated HLA-DR0701, 1501 positive adults (aged 27-60 years) 3 months following administration of an influenza subunit vaccine containing HA A/Beijing/32/92. Epitope recognition was examined using 118 HA A/Beijing/32/92-specific 16mer peptides which overlapped by 11 residues and which spanned the entire molecule. Following influenza vaccination the donors recognized identical HA peptides. The selected peptides represented HA regions which have been free from extensive drift mutation since the emergence of human H3N2 influenza A strains. Using DAP DR7.0701 cells (a murine cell line expressing HLA-DR0701) as antigen-presenting cells the majority of CD4+ T cell responses were shown to be HLA-DR0701 restricted. The relationship between HA peptide recognition and relative strength of HA peptide-HLA-DR0701 binding was then explored in a competition assay with biotinylated CLIP peptide. Although peptides representing dominant HA epitopes bound to DR0701, the relationship between relative strength of binding and immunodominance was complex, and many strongly binding peptides, particularly those with glycosylation sites and showing inter-strain variation, were not recognized. These results illustrate the control HLA class II exerts over CD4+ T cell HA epitope selection in unrelated adult humans. Immunodominance appears not to be directly related to the relative strength of HA peptide-HLA class II binding, and thus reflects complex interactions between antigen processing, intracellular competition for HLA binding, TCR repertoires and repeated exposure to different strains of influenza A viruses.