TCE group classification of DPB1 alleles by TCE-X or TCE-FD. (A) TCE classification of DPB1 alleles based on in vitro cross-reactivity patterns of alloreactive T cells nominally directed against DPB1*09:01.²⁶  Alleles that were reproducibly cross-recognized by a panel of 5 DPB1*09:01-specific alloreactive T-cell effectors were grouped together as TCE group 1 (red), whereas DPB1 alleles recognized by only a part or none of these T cells were classified into TCE groups 2 (orange) and 3 (green), respectively.²⁶  In total, 72 of the 975 DPB1 alleles known to date were experimentally tested for TCE-X (see Supplemental Table 1 for a comprehensive list of TCE group assignment for these alleles). These 72 DPB1 alleles classifiable by TCE-X represent only 7.4% of all known DPB1 alleles and account for only 65% of the CWD alleles for this locus.^33,34  Shown here are 19 DPB1 alleles reported with a frequency ≥ 0.5% in in a large population of European descent,⁶⁸  which together make up 98% of the total DPB1 allele frequency. Two alleles (DPB1*06:01 and *19:01) that changed classification from TCE3 in TCE-X to TCE2 in TCE-FD are indicated in bold. (B) TCE classification of any known DPB1 allele based on in silico prediction by FD scores. The latter were experimentally obtained by generating a panel of 12 site-directed single amino acid mutants of DPB1*09:01 and assessing the median strength of recognition for each mutant by alloreactive T cells relative to wild-type DPB1*09:01.²⁸  Based on this, the FD amino acid (FD_aa) scores were calculated for each mutant, with high scores representing a high functional impact and low scores representing a low functional impact of the mutation. For each DPB1 allele, the sum of FD_aa scores of each of the 12 polymorphic residues analyzed was used to calculate the FD_allele score. Experimental TCE classification by TCE-X was found to reflect the range of FD_allele scores as follows: TCE1, FD_allele < 0.6; TCE2, 0.6 < FD_allele < 2.0; and TCE3, FD_allele > 2.0.²⁸  TCE classification of the 19 DPB1 from panel A is indicated with TCE1, TCE2, and TCE3 in red, orange, and green, respectively. Two alleles (DPB1*06:01 and *19:01) that changed classification from TCE3 in TCE-X to TCE2 in TCE-FD are indicated in bold.