Historical Evolution of Unrelated Donor (URD) Recruitment of the Murcia Region Registry and Conditioning Social Events: Assessment of Quality, Effective URD Improvement, and Molecular HLA Characterization by NGS

The Murcia Region unrelated donor (URD) registry was established in 1992 as part of the Spanish Bone Marrow Donor Registry (REDMO). This study aimed to comprehensively characterize the long-term evolution of URD recruitment, human leukocyte antigen (HLA) genetic diversity, allele and haplotype frequencies, and the spectrum of novel HLA alleles detected over the 34 years from 1992 to 2025. Recruitment trends, URD demographic characteristics, HLA allele and haplotype frequencies, and newly detected HLA alleles were analyzed retrospectively. HLA typing was performed using serological methods (1992–1996); PCR-SSO, PCR-SSP, and PCR-SSO Luminex (1997–2018); sequence-based typing (SBT, 2018–2021); and next-generation sequencing (NGS, 2021–2025). NGS was used to analyze a subset of 650 URDs to determine high-resolution HLA allele frequencies, Hardy–Weinberg equilibrium, expected heterozygosity, and extended HLA haplotypes. Newly detected HLA alleles were further characterized by the affected HLA locus and the localization of sequence variants within exons, introns, and the 3′-untranslated region (3′UTR). Between 1992 and 2025, 33.271 URDs were recruited, of whom 27.107 remained active in REDMO at the time of analysis. URD recruitment increased substantially following the implementation of the Spanish National Bone Marrow Plan in 2012 and reached a historical high in 2017. Additional recruitment peaks were associated with major regional public awareness campaigns. High-resolution HLA analysis of the NGS cohort demonstrated extensive immunogenetic diversity across all analyzed HLA loci, with a mean expected heterozygosity of 0.914. Ten frequent extended HLA haplotypes were identified, the most prevalent being HLA-A*29:02-C*16:01-B*44:03-DRB1*07:01-DQB1*02:02. Thirty-nine novel HLA alleles were characterized, exhibiting heterogeneous distributions among HLA loci and across coding and non-coding genomic regions. Exonic variants were predominantly detected in HLA-C, whereas intronic variants were mainly observed in HLA-C, HLA-DQB1, and HLA-A. This study integrates 34 years of URD recruitment with high-resolution immunogenetic characterization of the Murcia Region URD registry, providing new insights into the evolution of donor recruitment, HLA diversity, and the discovery of novel HLA alleles. The implementation of NGS enabled precise molecular characterization of HLA diversity and substantially expanded the detection and characterization of novel HLA alleles. Together, these findings provide a robust immunogenetic framework for the Murcia URD population, supporting donor recruitment strategies, optimization of URD searches, registry management, and future research in population genetics and hematopoietic stem cell transplantation.