– Research Science & Technical Lead, Stanford Blood Center, United States
Aim: HLA antigen is defined as a group of HLA proteins that exhibit the same serological specificity. We proposed novel antigens corresponding to the worldwide common HLA alleles; this update has been approved by the WHO Nomenclature Committee. Solid Phase Single Antigen Bead (SP-SAB) is a sensitive assay that detects anti-HLA antibodies directed to epitopes present in native proteins as well as cryptic epitopes displayed only in denatured proteins. Using the new nomenclature system, we aim to establish guidelines to identify clinically significant allo HLA antibodies while eliminating spurious reactivity.
Methods: We performed correlation analyses of SABs, serum adsorptions including lymphocytes and SAB to reduce the complexity of anti-HLA antibody reactivity and flow crossmatches (FXMs).
Results: We observed high correlations in reactivity between SABs belonging to the same antigen, e.g., A*02:01 vs. A*02:06 and lower correlations with distinct reactivities between closely related SABs belonging to different antigens, e.g., A*66:01 vs. A*66:02. In contrast, we confirmed that high MFI reactivity with the SABs A*02:07, A*11:02, B*13:02, B*44:02, B*45:01, DRB1*01:02 and DQA1*01:01~DQB1*05:01 separate from negative reactivity with SABs corresponding to the same antigen, A*02:01, A*11:01, B*13:01, B*44:03, B*50:02, DRB1*01:01 and DQA1*01:01~DQB1*05:03, respectively, were spurious as defined by FXM negative results. We identified antigens that were not previously defined with certainty and displayed inter-locus crossreactivities, e.g., A32, A74, B-3512, B-5504, B-5603, B57, Cw9, Cw10 etc.; the reactivity was explained by an epitope containing 103V~109L. The cell-based assays were informative to confirm native residues determining epitopes allowing us to assign a total of 305 HLA antigens for 11 HLA loci identified for the 606 common alleles. Of these 591 were assigned to FULL and 15 were assigned to SEROTYPE.
Conclusion: We assigned all available SABs used for clinical testing to one of the updated antigens. The new definition allows developing rationale for assessing immunologic risk via pretransplant virtual crossmatches and post-transplant DSA. The use of imminently expanded SAB panels representing all serotypes together with HLA typing at a resolution to the level of novel serotypes will maximize the antigenic information provided by the novel HLA antigens, UASA.
