Epitopic analysis suggests a new conformational DPαβ epitope in a patient searching for an unrelated hematopoietic stem cell donor

Abstract

The best available unrelated donor (URD) for a patient with acute lymphoblastic leukemia in need of hematopoietic stem cell transplant was HLA 11/12 with a DPB1*04:01 mismatch. Search for donor-specific HLA antibodies (DSA) by solid phase assay with isolated HLA molecules (SAB) on a first panel (manufacturer 1) identified a DSA against DPB1*04:01 (MFI = 3090). Epitope analysis using HLA Fusion and HLAMatchmaker was not conclusive due to an atypical 85GPM epitope pattern, showing positive reactions with DPB1*04:01 (MFI = 3090), DP2 (MFI = 1655), DPB1*04:02 (MFI = 4571), DP23 (MFI = 1549) and DP28 (MFI = 1409, 1324, 1256), but lack of reactivity with DP15 and DP18, which also express 85GPM. The DP α 31 M + β 85GPM + β 96 R configuration, obtained with the HLA Epitopia Map, justified the observed pattern because it is present in the DP4, DP2, DP23 and DP28 heterodimers that reacted positively, but absent in DP15 ( α 31 Q + β 85GPM + β 96 K ) and DP18 ( α 31 Q + β 85GPM + β 96 K and α 31 M + β 85GPM + β 96 K ) whose reactions were negative. To validate this α / β epitope predicted in silico , the serum was tested with an SAB panel which included five distinct DP4 heterodimers (manufacturer 2) composed of DPB1*04:01 combined to different alpha chains. The result showed that only heterodimers with α 31 M + β 85GPM + β 96 R configuration were reactive: DPA1*01:03/DPB1*04:01 (MFI = 3609), DPA1*03:01/DPB1*04:01 (MFI = 3778) and DPA1*04:01/DPB1*04:01 (MFI = 3750). The α 31 Q + β 85GPM + β 96 R heterodimers showed negative reactions (DPA1*02:01/DPB1*04:01, MFI = 298; DPA1*02:02/DPB1*04:01, MFI = 94) (Table). Patient was transplanted with this URD without previous desensitization due to the weak reactivity of this antibody. This DP αβ DSA did not show clinical relevance in the rejection vector, since there was hematological engraftment on day +26 and chimerism with 100% donor cells on day +30. This report suggests the presence of a new conformational DP αβ epitope and shows that HLA Epitopia Map tool contributes to the elucidation of atypical reactivity patterns in SAB solid phase assays.