The IMPACT of Amino Acid Variability Defines a Functional Distance Predictive of Permissive HLA-DPB1 Mismatches in Hematopoietic CELL Transplantation

A major challenge in unrelated hematopoietic cell transplantation (HCT) is the prediction of permissive HLA mismatches, i.e. those associated with lower clinical risks compared to their non-permissive counterparts. For HLA-DPB1, a clinically prognostic model has been shown to be matching for T-cell epitope (TCE) groups assigned by crossreactivity of T-cells alloreactive to HLA-DPB1*09:01, however the molecular basis of this observation is not fully understood. Here we have mutated amino acids (aa) in 10 positions of HLA-DPB1*09:01 to other naturally occurring variants, expressed them by lentiviral vectors in B cell lines and quantitatively measured allorecognition by 17 CD4+ T-cell effectors from 6 unrelated individuals. A significant impact on the median alloresponse was observed for peptide contact positions 9, 11, 35, 55, 69, 76 and 84, but not for positions 8, 56 and 57 pointing away from the groove. A score for the “functional distance” (FD) from HLA-DPB1*09:01 was defined as the sum of the median impact of polymorphic aa in a given HLA-DPB1 allele on T-cell alloreactivity. Established TCE group assignment of 23 alleles correlated with FD scores of ≤0.5, 0.6-1.9 and ≥2 for TCE groups 1, 2 and 3, respectively. Based on this, prediction of TCE group assignment will be possible for any given HLA-DPB1 allele. In silico TCE group classification was performed for currently 328 HLA-DPB1 alleles encoding distinct proteins for which T-cell crossreactivity patterns are unknown, with subsequent functional confirmation for 7/7 of them. These findings have practical implications for the applicability of TCE group matching in unrelated HCT, and provide new insights into the molecular mechanisms underlying this model. The innovative concept of FD opens new potential avenues for risk prediction in unrelated HCT.