Immunodominant influenza epitope GILGFVFTL engage common and divergent TCRs when presented as a 9-mer or a 15-mer peptide

Antigen-specific T-cells are a powerful modality for treating cancer and other life-threatening viral and bacterial diseases. Technologies to identify and expand antigen-specific T-cells rapidly can shorten the time, and lower the cost of treatment. In this regard, screening short overlapping peptides to identify antigen-specific T-cells in ex vivo T-cell activation assay is becoming routine. Screening assays typically use 15-mer peptides to stimulate patient-derived, or healthy peripheral blood mononuclear cells to activate T-cells and identify expanded TCRs by next generation sequencing. Previous studies comparing the kinetics of T-cell activation using a 9 and a 15-mer peptide versions of a CMV immunodominant epitope demonstrated that 15-mer peptides induced CD8 T-cell activation at a slower kinetics reaching a lower magnitude compared to 9-mer peptides. The fact that 9-mer peptides are an optimal fit for the MHC class-I binding groove could explain this difference, with the 15-mer peptide requiring additional proteolytic processing before binding to the deeper binding groove of class-I MHC. Alternatively, the delay in kinetics and magnitude can result from the activation of a wider diversity of TCRs engaging novel epitopes generated from the 15-mer peptide whose activation profile may be different from the profile of TCRs that normally respond to the 9-mer immunodominant epitope. We sought to address these two possibilities by comparing T-cell engagement to the HLA-2-restricted GILGFVFTL epitope presented as a 9-mer, or a 15-mer peptide and analyzing CDR3 expansion as a measure of T-cell engagement diversity. This approach also addressed an important question as to whether optimal TCRs could be missed using a 15-mer peptide used routinely in screening assays.