MHCI-peptide length preference reflects the length of the “index” peptide. Target cells (6 × 10⁴) expressing either HLA A*0201 or HLA B*3508 were pulsed in duplicate with the following “sizing scan” mixtures (1mM) for 2 hours at 37°C: X⁸, X⁹, X¹⁰, X¹¹, X¹², and X¹³ (where X is any of the 19 proteogenic L-amino acids excluding cysteine). Subsequently, 3 × 10⁴ ILA1, 1E6, SB14, SB10, or SB27 CD8⁺ T cells (Table 1) were added and incubated overnight. The supernatant was then harvested and assayed for MIP1β by ELISA. Values are expressed as a percentage of the highest response, where black represents 100% of the highest response and white represents 0% of the highest response. The raw data used to construct this figure can be viewed in supplemental Figure 1.