Peptide-MHC class I interaction measured by an enzyme-linked immunosorbent assay Peptide-MHC-I interaction was also measured in a modified version of a previously described enzyme-linked immunosorbent assay (ELISA) (Sylvester-Hvid et al. 2002) . Briefly, denatured biotinylated recombinant MHC-I heavy chains were diluted into a renaturation buffer containing β 2 m and graded concentrations of the peptide to be tested and incubated at 18°C for 48 h allowing equilibrium to be reached. We have previously demonstrated that denatured MHC molecules can de novo fold efficiently, however, only in the presence of appropriate peptide. The concentration of peptide-MHC complexes generated was measured in a quantitative sandwich ELISA (using streptavidin as capture layer and the monoclonal anti-β 2 m antibody, BBM1, as detection layer) and plotted against the concentration of peptide offered (Sylvester-Hvid et al. 2002) . A prefolded, biotinylated FLPSDYFPSV/ HLA-A*02:01 (Kast et al. 1994 ) complex was used as standard. Because the effective concentration of MHC (3-5 nM) used in these assays is below the equilibrium dissociation constant (K D ) of most high-affinity peptide-MHC interactions, the peptide concentration, ED 50 , leading to half-saturation of the MHC is a reasonable approximation of the affinity of the interaction.