We describe 2127 new human leukocyte antigen (HLA) class I alleles found in registered stem cell donors. These alleles represent 28.9% of the currently known class I alleles. Comparing new allele sequences to homologous sequences, we found 68.1% nonsynonymous nucleotide substitutions, 28.9% silent mutations and 3.0% nonsense mutations. Many substitutions occurred at positions that have not been known to be polymorphic before. A large number of HLA alleles and nucleotide variations underline the extreme diversity of the HLA system. Strikingly, 156 new alleles were found not only multiple times, but also in carriers of various parentage, suggesting that some new alleles are not necessarily rare. Moreover, new alleles were found especially often in minority donors. This emphasizes the benefits of specifically recruiting such groups of individuals.

Donor safety is of utmost importance in the setting of hematopoietic stem cell donation. Follow-up is indicated to detect potential long-term risks for donors. We sent a follow-up questionnaire to 15,445 donors of peripheral blood stem cells (PBSCs) or bone marrow (BM) within a retrospective study design. The return rate was 91.3%, resulting in 37,287 observation years for PBSC donors and 25,656 for BM donors. Most donors assessed their health conditions as very good or good and had not been hospitalized or received long-term medical treatment including prescribed medication for more than 4 weeks since donation. Although there were no differences in the frequency of reported health events, BM donors more often rated their general health as very good or good. Ninety-five percent of donors after BM or PBSC donation would consider a second stem cell donation. In total, 93 malignancies were reported. The standardized incidence ratio (SIR) for a diagnosis of any type of cancer after PBSC donation was .94 (95% CI, .70 to 1.24) with a SIR below 1 indicating a lower risk than in the age- and sex-matched population. The SIR for a diagnosis of leukemia was 0 (95% CI, 0 to 1.88). In summary, we found no evidence that either PBSC or BM donation are associated with increased risks of malignancies or other severe health problems.

We characterized 549 new human leukocyte antigen (HLA) class I and class II alleles found in newly registered stem cell donors as a result of high‐throughput HLA typing. New alleles include 101 HLA‐A, 132 HLA‐B, 105 HLA‐C, 2 HLA‐DRB1, 89 HLA‐DQB1 and 120 HLA‐DPB1 alleles. Mainly, new alleles comprised single nucleotide variations when compared with homologous sequences. We identified nonsynonymous nucleotide mutations in 70.7% of all new alleles, synonymous variations in 26.4% and nonsense substitutions in 2.9% (null alleles). Some new alleles (55, 10.0%) were found multiple times, HLA‐DPB1 alleles being the most frequent among these. Furthermore, as several new alleles were identified in individuals from ethnic minority groups, the relevance of recruiting donors belonging to such groups and the importance of ethnicity data collection in donor centers and registries is highlighted.

We have characterized 372 novel human leukocyte antigen (HLA) class II alleles identified in newly registered stem cell donors, this includes 281 HLA-DRB1 alleles, 89 HLA-DQB1 alleles and 2 HLA-DPB1 alleles. Most novel alleles were single nucleotide variants when compared to their respective most homologous alleles. In 66.4% of all novel alleles non-synonymous nucleotide variations were identified, in 30.4% synonymous substitutions and in 3.2% nonsense mutations. Ninty-three (25.0%) novel alleles were found in several individuals; most often these were novel HLA-DRB1 alleles. Lastly, we underline the importance of recruiting ethnic minority donors in countries such as Germany and the United States, as novel alleles were frequently found among these groups.

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