I-Ad restricted T cell recognition of influenza hemagglutinin. Synthetic peptides identify multiple epitopes corresponding to antibody-binding regions of the HA1 subunit.

In a recent study, we reported extensive diversity in the Iak-restricted T cell repertoire for the hemagglutinin molecule (HA) of influenza A viruses (H3 subtype). Synthetic peptides identified six nonoverlapping epitopes on the HA1 subunit, and CD4+ T cell clones, specific for these regions, discriminated between natural variant viruses that had accumulated amino acid substitutions during antigenic drift. Here, we demonstrate similar specificity and diversity for the Iad haplotype and have identified multiple T cell epitopes within the sequences HA1 56-76, 71-91, 81-97, 177-199, 186-205, and 206-227. These also include recognition sites for neutralizing antibodies and correlations can be made between antigenic drift substitutions in H3 subtype viruses and the specificity of individual CD4+ clones for mutant HA. Moreover, these peptides appear not to exhibit structural homology and fail to compete for Ag presentation, indicating heterogeneity in peptide-Ia interaction. To explain the observation that CD4+ T cells, from two major haplotypes, recognize antibody binding regions of the HA molecule, we propose that surface Ig receptors of the Ag-specific B memory cell exert a direct effect on the processing of HA peptides and subsequent selection of the class II-restricted T cell memory repertoire after natural infection.