The structural requirements for class II (I-Ad)-restricted T cell recognition of influenza hemagglutinin: B cell epitopes define T cell epitopes

A majority of I-Ad-restricted CD4 clones elicited by influenza X31 (H3N2) virus infection, recognize a synthetic peptide of hemagglutinin (HA) corresponding to an antibody binding region of the HA1 subunit (site B: HA1 177–199). The structural requirements for class II-restricted T cell recognition were investigated by determining the proliferative responses of representative CD4 clones to truncated HA1 peptides and synthetic peptide analogues. Two distinct T cell epitopes were identified and CD4 clones, specific for either determinant, were sensitive to the same single amino acid substitutions in synthetic peptides at HA1 193 SN or HA1 198E, that had featured in antigenic drift and abrogated antibody binding to native HA. Competitive inhibition studies, between stimulatory HA1 peptides and non-stimulatory analogue peptides, for antigen presentation to CD4 clones established that the 193 S N and 198 A E substitutions could affect either interaction with the T cell receptor or class II molecule, according to the specificity of the CD4 clone examined. The structural requirements for class II-restricted T cell recognition of the linear sequence determinants of HA are, therefore, integrally linked to conformationdependent antibody recognition of the native molecule.