Resolving Ambiguities in Deceased Donor HLA Typing: Next Generation Sequencing’s Role in DPB1 Allele Discrimination and Immunological Implications

Abstract HLA typing is crucial for assessing immunological compatibility between donors and recipients before transplantation. Deceased donor (DD) typing, typically via real-time PCR (rtPCR), provides rapid results vital for organ allocation. rtPCR primarily targets exons encoding the highly polymorphic peptide binding region (PBR) of HLA molecules. rtPCR cannot target all polymorphic sequences. Consequently, several possibilities of allele combinations are provided, leading to ambiguous typing results. As recent evidence suggests antibody responses can target regions in exons outside the PBR, notably in Exon 3 of the DPB1 locus, typing ambiguities can lead to the inability to assess histocompatibility between a donor and recipient. Herein, the aim was to investigate the utility of Next Generation Sequencing (NGS) in resolving DPB1 ambiguities in DD typing. A retrospective analysis of DD typing from February 2022 to January 2024 examined instances where NGS was used to resolve ambiguities reported via rtPCR. Each case was reviewed to understand the reason for resorting to NGS typing. The study focused on DPB1 ambiguities and assessed whether NGS effectively reduced ambiguity and differentiated alleles with polymorphisms outside the PBR. In total, 29 DD cases were found to have NGS testing results. In 14 cases, NGS was specifically performed to resolve DPB1 ambiguities. The ambiguities were significantly reduced from hundreds detected in rtPCR to 4 or fewer possible allele combinations with NGS. Eleven cases demonstrated polymorphism in exons outside the PBRs. Of these, only 2 cases showed no typing ambiguity with NGS. In the remaining 9 cases, though polymorphisms could be resolved in the PBR, ambiguities remained in exon 3, specifically at codon 96. In these cases, NGS could not distinguish lysine (K) from arginine (R). For example, DPB1*835:01 with the 96R motif could not be distinguished from DPB1*34:01with the 96K motif. The current investigation found that NGS cannot resolve all ambiguities at position 96, likely due, at least in part, to persistent cis-trans ambiguities. Our results suggest that although NGS drastically limited the number of ambiguities in 9 of 11 cases, it still could not adequately distinguish all immunologically relevant alleles. Inability to resolve the 96R/K epitope, which has been shown to be immunogenic, could impact solid organ transplantation, potentially leading to rejection in mismatched recipient-donor pairs at this position. Sequencing methods that generate longer reads, such as nanopore, may help resolve some of these cases if they are indeed due to cis-trans ambiguities.