OR5 Donor specific antibody (DSA) assessments using HLA P groups: The devil is in the details

Aim Use of HLA P groups for reporting purposes and DSA assessments for potential donors has been steadily increasing. P groups for HLA class II encompass exon2 sequences and capture the polymorphic rich peptide binding domain that serves as the primary target for alloantibody. Nevertheless, we have observed multiple sera exhibiting HLA-DQA1 or HLA-DPB1 reactivity to polymorphic sites outside of exon 2 and confirmed this reactivity using cell adsorptions. Methods HLA antibodies were assessed using Lifecodes single antigen; Immucor and LabScreen Single Antigen; One Lambda. Sera were pretreated to remove interference using Melon Columns™ (ThermoFisher). HLA typing was performed using rSSO LabType, One Lambda and confirmed using NGS TruSight HLA, Illumina. 10th international workshop B cell lines were used for adsorptions. Results We present a representative example in Fig. 1. Patient #1 typed as a DQA1∗05:05/DQB1∗03:19 yet her sera reacted with the DQA1∗05:03/DQB1∗03:01 bead with a median fluorescence intensity of 11,000. Sequence alignments illustrate an amino acid difference between DQA1∗05:03 and DQA1∗05:05 at position 159. There is also one amino acid difference between DQB1∗03:01 and DQB1∗03:19 but the DQA1∗05:05/DQB∗03:01 bead reactivity was negative, negating the possibility that the antibody was directed at HLA-DQB1. Serum adsorptions using a DQA1∗05:03/DQB1∗03:01 cell line showed an 84% reduction in the DQA1∗05:03/DQB1∗03:01 bead reactivity and a concomitant 3% reduction in reactivity with the DQA1∗03:01/DQB1∗03:01 bead. These data show that the serum reactivity with the DQA1∗05:03/DQB1∗03:01 bead was directed toward the HLA-DQA1 protein, which differed from the recipient by a single polymorphic difference outside of exon 2. Conclusions We show evidence of sensitization to a polymorphism outside of exon 2 implying the potential danger of using P group designations when performing virtual crossmatch assessments or unrelated stem cell donor searches for patients with complex HLA sensitization. A.M. Jackson: 3. Speaker’s Bureau; Company/Organization; Thermo Fisher. M. Bettinotti: 3. Speaker’s Bureau; Company/Organization; ThermoFisher.