PEPTIDES OF 23 RESIDUES OR GREATER ARE REQUIRED TO STIMULATE A HIGH-AFFINITY CLASS II-RESTRICTED T-CELL RESPONSE

Helper T cells recognize fragments of antigen bound to the class II mo lecules on the surface of antigen-presenting cells. Naturally processe d antigenic fragments have been isolated from the class II molecules a nd shown to be heterogeneous in length, ranging from 13 to 25 residues , and to vary at both the N and C termini. A 15-residue peptide in an extended conformation is predicted to fit in an open peptide-binding c left of the class II molecules. Thus, the longer peptides observed bou nd to class II presumably have regions which reside outside the cleft. It is not known if the additional length contributes significantly to T cell activation. We have carried out a systematic analysis of the a ntigenicity of peptides of increasing length beyond the minimally defi ned T cell antigenic peptide. Here we show that the full functional ac tivities of peptides representing the major antigenic determinant of t he protein antigen, cytochrome c, minimally require that the peptides be 23 amino acids long. The long peptides do not require processing an d are presented by purified class II molecules incorporated into synth etic membranes, indicating that such peptides associate directly with class II and require no additional cellular machinery for presentation . We also show that a hybrid peptide, 51 residues in length, containin g a 29-residue cytochrome c peptide and a ''promiscuous'' peptide of t etanus toxoid, is more antigenic than the 23-residue peptide alone and significantly, does not require processing. Thus, the additional pept ide length, although not predicted to bind in the peptide-binding groo ve of the MHC class II molecule, has a significant impact on the abili ty of the peptides to stimulate T cell responses maximally.