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 molecules on the surface of antigen-presenting cells. Naturally processed antigenic fragments have been isolated from the class II molecules and 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 cleft of the class II molecules. Thus, the longer peptides observed bound 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 antigenicity of peptides of increasing length beyond the minimally defined T cell antigenic peptide. Here we show that the full functional activities of peptides representing the major antigenic determinant of the protein antigen, cytochrome c, minimally require that the peptides be 23 amino acids long. The long peptides do not require processing and are presented by purified class II molecules incorporated into synthetic 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, containing a 29-residue cytochrome c peptide and a "promiscuous" peptide of tetanus toxoid, is more antigenic than the 23-residue peptide alone and significantly, does not require processing. Thus, the additional peptide length, although not predicted to bind in the peptide-binding groove of the MHC class II molecule, has a significant impact on the ability of the peptides to stimulate T cell responses maximally.