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Knowledge of an individual's human leukocyte antigen (HLA) genotype is essential for modern medical genetics, and is crucial for hematopoietic stem cell and solid-organ transplantation. However, the high levels of polymorphism known for the HLA genes make it difficult to generate an HLA genotype that unambiguously identifies the alleles that are present at a given HLA locus in an individual. For the last 20 years, the histocompatibility and immunogenetics community has recorded this HLA genotyping ambiguity using allele codes developed by the National Marrow Donor Program (NMDP). While these allele codes may have been effective for recording an HLA genotyping result when initially developed, their use today results in increased ambiguity in an HLA genotype, and they are no longer suitable in the era of rapid allele discovery and ultra-high allele polymorphism. Here, we present a text string format capable of fully representing HLA genotyping results. This Genotype List (GL) String format is an extension of a proposed standard for reporting killer-cell immunoglobulin-like receptor (KIR) genotype data that can be applied to any genetic data that use a standard nomenclature for identifying variants. The GL String format uses a hierarchical set of operators to describe the relationships between alleles, lists of possible alleles, phased alleles, genotypes, lists of possible genotypes, and multilocus unphased genotypes, without losing typing information or increasing typing ambiguity. When used in concert with appropriate tools to create, exchange, and parse these strings, we anticipate that GL Strings will replace NMDP allele codes for reporting HLA genotypes.

We have updated the catalogue of common and well-documented (CWD) human leukocyte antigen (HLA) alleles to reflect current understanding of the prevalence of specific allele sequences. The original CWD catalogue designated 721 alleles at the HLA-A, -B, -C, -DRB1, -DRB3/4/5, -DQA1, -DQB1, and -DPB1 loci in IMGT (IMmunoGeneTics)/HLA Database release 2.15.0 as being CWD. The updated CWD catalogue designates 1122 alleles at the HLA-A, -B, -C, -DRB1, -DRB3/4/5, -DQA1, -DQB1, -DPA1 and -DPB1 loci as being CWD, and represents 14.3% of the HLA alleles in IMGT/HLA Database release 3.9.0. In particular, we identified 415 of these alleles as being 'common' (having known frequencies) and 707 as being 'well-documented' on the basis of ~140,000 sequence-based typing observations and available HLA haplotype data. Using these allele prevalence data, we have also assigned CWD status to specific G and P designations. We identified 147/151 G groups and 290/415 P groups as being CWD. The CWD catalogue will be updated on a regular basis moving forward, and will incorporate changes to the IMGT/HLA Database as well as empirical data from the histocompatibility and immunogenetics community. This version 2.0.0 of the CWD catalogue is available online at cwd.immunogenomics.org, and will be integrated into the Allele Frequencies Net Database, the IMGT/HLA Database and National Marrow Donor Program's bioinformatics web pages.

The IPD-MHC Database project (http://www.ebi.ac.uk/ipd/mhc/) serves as a comprehensive and expertly curated repository for major histocompatibility complex (MHC) sequences from non-human species, providing the necessary infrastructure and tools to study the function and evolution of this highly polymorphic genomic region. In its latest version, the IPD-MHC database has expanded both in content and in the tools for data visualization and comparison. The database now hosts over 18 000 MHC alleles from 125 species, organized into eleven taxonomic groups, all manually curated and named by the Comparative MHC Nomenclature Committee. A cetacean section has recently been included, offering researchers valuable data to study the immune system of whales, dolphins, and porpoises, as well establishing the official nomenclature platform for the Cetacea Leukocyte Antigens (CeLA). In response to user demand and reflecting broader trends in bioinformatics and immunogenetics, IPD-MHC now includes the predicted tertiary structure of over 8000 alleles and allows comparison and visualisation of allele variation within and between species at single residue resolution. These latest developments maintain the critically important link between official nomenclature of curated alleles and the ability to analyse this complex polymorphism using the most up to date methods within a single repository., (© The Author(s) 2024. Published by Oxford University Press on behalf of Nucleic Acids Research.)

Eight novel HLA class I alleles have been identified using full gene sequencing., (© 2024 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd.)

HLA antigens were historically defined according to the unique reactivity pattern of cells expressing HLA molecules with distinctive clusters of allo-antisera and/or monoclonal antibodies. Subsequently, amino acid residues determining epitopes (DEP) in the HLA molecule were correlated with reactivity patterns. In current clinical practice, the presence of allo-antibodies is assessed using Luminex-based solid phase single antigen bead (SAB) assays for transplantation. Recently, novel antigens were proposed for HLA molecules with DEP patterns that do not match any serologically defined antigens recognised by the WHO Nomenclature Committee. To validate the antigens, mean fluorescence intensity values of SABs tested on >13,000 patients' sera were extracted from clinical databases and analysed by scatter plots using a linear regression model. We found that when two proteins were considered as the same antigen in the original study, for example, HLA-A*02:01 and -A*02:06, their correlation ranked among the highest values at each locus. In contrast, discrete asymmetric outliers were observed when there were different antigens, for example, HLA-A*30:01 and -A*30:02, allowing validation and confirmation of 20 novel antigens for HLA-A, -B, -C and -DR. The outliers were confirmed to be true or false by flow cytometric crossmatches. In addition to the previously defined residues for antigen assignments, findings suggest that further distinction should be made for common antigens by including the substitutions at residue 67 of HLA-B, 67 and 74 of -DR. These serologic analyses can be applied systematically to identify and confirm novel antigens. These developments will lead to designing optimal SAB panels and further improving virtual donor-specific antibodies assessment., (© 2024 The Author(s). HLA: Immune Response Genetics published by John Wiley & Sons Ltd.)

Strategies to mobilise natural killer (NK) cells against cancer include tumour-targeting antibodies, NK cell engagers (NKCEs) and the adoptive transfer of ex vivo expanded healthy donor-derived NK cells. Genetic and functional studies have revealed that expression of the activating killer immunoglobulin-like receptor KIR2DS2 is associated with enhanced function in NK cells from healthy donors and improved outcome in several different malignancies. The optimal strategy to leverage KIR2DS2+ NK cells therapeutically is however currently unclear. In this study, we therefore evaluated the response of KIR2DS2-expressing NK cells to activation against cancer with clinically relevant tumour-targeting antibodies and following ex vivo expansion. We identified that KIR2DS2 high NK cells from patients with chronic lymphocytic leukaemia and hepatocellular carcinoma had enhanced activation in response to tumour-targeting antibodies compared to KIR2DS2- NK cells. However, the superior function of healthy donor derived KIR2DS2 high NK cells was lost following ex vivo expansion which is required for adoptive transfer-based therapeutic strategies. These data provide evidence that targeting KIR2DS2 directly in cancer patients may allow for the utilisation of their enhanced effector function, however such activity may be lost following their ex vivo expansion., Competing Interests: MB and SK have applied for a patent for peptide mediated NK cell activation. MB has received research funding from Karyopharm Therapeutics. MC is a retained consultant for BioInvent International and has performed educational and advisory roles for Baxalta and Boehringer Ingleheim. He has consulted for GSK, Radiant, iTeos Therapeutics, Surrozen, Hanall and Mestag and received research funding from BioInvent, Surrozen, GSK, UCB and iTeos. FF has received research support from Abbvie and has performed consultancies or educational activities for AstraZeneca, AbbVie, Janssen-Cilag, BC-Platform and Beigene. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2024 Graham, Fisher, Doyle, Sale, Del Rio, French, Mayor, Turner, Marsh, Cragg, Forconi, Khakoo and Blunt.)

Twenty-five years ago, in 1998, the HLA Informatics Group of the Anthony Nolan Research Institute released the IMGT/HLA Database. Since this time, this online resource has acted as the repository for the numerous variant sequences of HLA alleles named by the WHO Nomenclature Committee for Factors of the HLA System. The IPD-IMGT/HLA Database has provided a stable, highly accessible, user-friendly repository for this work. During this time, the technology underlying HLA typing has undergone significant changes. Next generation sequencing (NGS) has superseded previous methodologies of HLA typing and can generate large amounts of high-resolution sequencing data. This has resulted in a drastic increase in the number and complexity of sequences submitted to the database. The challenge for the IPD-IMGT/HLA Database has been to maintain the highest standards of curation, while supporting the core set of tools and functionality to our users with increased numbers of submissions and sequences. Traditional methods of accessing and presenting data have been challenged and new methods utilising new computing technologies have had to be developed to keep pace and support a shifting user demographic., (© 2024 The Author(s). HLA: Immune Response Genetics published by John Wiley & Sons Ltd.)

The importance of the HLA gene system in haematopoietic cell transplant outcomes was established early on and advances in both fields have led to ever increasing success of this clinical therapy. In large part, improvements in the understanding of HLA have been driven by the advancement in typing technologies. Each iteration of typing technology has improved the resolution of HLA typing, and often enabled the identification of polymorphism within the HLA loci. The discovery of the enormous amount of variation in the HLA genes, and the need to be able to characterise this for clinical HLA typing, has often resulted in a move away from one typing method to another more suited to typing of this complexity. Today, the gold standard for HLA typing are methods that can produce definitive HLA typing results., (Copyright © 2024 Elsevier Ltd. All rights reserved.)

Optimizing natural killer (NK) cell alloreactivity could further improve outcome after allogeneic hematopoietic cell transplantation (alloHCT). The donor's Killer-cell Immunoglobulin-like Receptor (KIR) genotype may provide important information in this regard. In the past decade, different models have been proposed aiming at maximizing NK cell activation by activating KIR-ligand interactions or minimizing inhibitory KIR-ligand interactions. Alternative classifications intended predicting outcome after alloHCT by donor KIR-haplotypes. In the present study, we aimed at validating proposed models and exploring more classification approaches. To this end, we analyzed samples stored at the Collaborative Biobank from HLA-compatible unrelated stem cell donors who had donated for patients with acute myeloid leukemia (AML) or myelodysplastic neoplasm (MDS) and whose outcome data had been reported to EBMT or CIBMTR. The donor KIR genotype was determined by high resolution amplicon-based next generation sequencing. We analyzed data from 5,017 transplants. The median patient age at alloHCT was 56 years. Patients were transplanted for AML between 2013 and 2018. Donor-recipient pairs were matched for HLA-A, -B, -C, -DRB1, and -DQB1 (79%) or had single HLA mismatches. Myeloablative conditioning was given to 56% of patients. Fifty-two percent of patients received anti-thymocyte-globulin-based graft-versus-host disease prophylaxis, 32% calcineurin-inhibitor-based prophylaxis, and 7% post-transplant cyclophosphamide-based prophylaxis. We tested several previously reported classifications in multivariable regression analyses but could not confirm outcome associations. Exploratory analyses in 1,939 patients (39%) who were transplanted from donors with homozygous centromeric (cen) or telomeric (tel) A or B motifs, showed that the donor cen B/B -tel A/A diplotype was associated with a trend to better event-free survival (HR 0.84, p=.08) and reduced risk of non-relapse mortality (NRM) (HR 0.65, p=.01). When we further dissected the contribution of B subtypes, we found that only the cen B01/B01 - telA/A diplotype was associated with a reduced risk of relapse (HR 0.40, p=.04) while all subtype combinations contributed to a reduced risk of NRM. This exploratory finding has to be validated in an independent data set. In summary, the existing body of evidence is not (yet) consistent enough to recommend use of donor KIR genotype information for donor selection in routine clinical practice., Competing Interests: All authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. The reviewer TT declared shared affiliation with the author’s JSc and FH to the handling editor at the time of review., (Copyright © 2024 Schetelig, Baldauf, Heidenreich, Hoogenboom, Spellman, Kulagin, Schroeder, Sengeloev, Dreger, Forcade, Vydra, Wagner-Drouet, Choi, Paneesha, Miranda, Tanase, de Wreede, Lange, Schmidt, Sauter, Fein, Bolon, He, Marsh, Gadalla, Paczesny, Ruggeri, Chabannon and Fleischhauer.)

Two novel alleles, HLA-G*01:04:09 and HLA-DPB1*04:01:01:136, were identified in a single healthy individual., (© 2024 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd.)

HLA-DPB1 is the classical HLA class II genes with the least recorded variation on the IPD-IMGT/HLA Database, suggesting the full extent of its diversity is perhaps yet to be characterized. Here, a full-gene typing strategy was employed to genotype a UK cohort of 1470 HCT recipients (n = 744) and donors (n = 726). In total, 2940 full-length HLA-DPB1 sequences were generated, comprising 193 distinct alleles. Of these, 107 sequences contained novel variation, totaling 49 unique intronic HLA-DPB1 alleles, and one coding variant (HLA-DPB1*1188:01). Full-gene sequencing resulted in zygosity changes for 129 individuals by identifying two distinct intronic variants of the same coding allele. We verified the existence of nine unconfirmed alleles and extended the sequence of two existing alleles on the IPD-IMGT/HLA Database., (© 2023 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd.)

Two novel non-classical HLA class I alleles have been characterized, HLA-F*01:16 and -F*01:17., (© 2023 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd.)

The Genotype List (GL) String grammar for reporting HLA and Killer-cell Immunoglobulin-like Receptor (KIR) genotypes in a text string was described in 2013. Since this initial description, GL Strings have been used to describe HLA and KIR genotypes for more than 40 million subjects, allowing these data to be recorded, stored and transmitted in an easily parsed, text-based format. After a decade of working with HLA and KIR data in GL String format, with advances in HLA and KIR genotyping technologies that have fostered the generation of full-gene sequence data, the need for an extension of the GL String system has become clear. Here, we introduce the new GL String delimiter "?," which addresses the need to describe ambiguity in assigning a gene sequence to gene paralogs. GL Strings that do not include a "?" delimiter continue to be interpreted as originally described. This extension represents version 1.1 of the GL String grammar., (© 2023 The Authors. HLA: Immune Response Genetics published by John Wiley & Sons Ltd.)

The guidelines for the implementation and reporting of HLA nomenclature for the World Marrow Donor Association have served as a reliable standard for communication of HLA data in the hematopoietic cell transplantation process. Wider use of next-generation sequencing made a special provision of the guidelines increasingly pertinent: how to communicate novel HLA alleles. Novel alleles need to be recognized by the WHO Nomenclature Committee for Factors of the HLA system to obtain official allele designations. Until then they have to be handled according to the specific rules. Leaving the actual rules basically unchanged we give some advice on how to communicate novel alleles to best facilitate the search process for cases where novel alleles are identified on donor or patient side., (© 2023 The Authors. HLA: Immune Response Genetics published by John Wiley & Sons Ltd.)

Mutation-bearing peptide ligands from mutated nucleophosmin-1 (NPM1) protein have been empirically found to be presented by HLA class I in acute myeloid leukemia (AML). We hypothesized that HLA genotype may impact allogeneic hematopoietic stem cell transplantation (allo-HCT) outcomes in NPM1-mutated AML owing to differences in antigen presentation. We evaluated the effect of the variable of predicted strong binding to mutated NPM1 peptides using HLA class I genotypes from matched donor-recipient pairs on transplant recipients' overall survival (OS) and disease-free survival (DFS) as part of the primary objectives and cumulative incidence of relapse and nonrelapse mortality (NRM) as part of secondary objectives. Baseline and outcome data reported to the Center for International Blood and Marrow Transplant Research from a study cohort of adult patients (n = 1020) with NPM1-mutated de novo AML in first (71%) or second (29%) complete remission undergoing 8/8 matched related (18%) or matched unrelated (82%) allo-HCT were analyzed retrospectively. Class I alleles from donor-recipient pairs were analyzed for predicted strong HLA binding to mutated NPM1 using netMHCpan 4.0. A total of 429 (42%) donor-recipient pairs were classified as having predicted strong-binding HLA alleles (SBHAs) to mutated NPM1. In multivariable analyses adjusting for clinical covariates, the presence of predicted SBHAs was associated with a lower risk of relapse (hazard ratio [HR], .72; 95% confidence interval [CI], .55 to .94; P = .015). OS (HR, .81; 95% CI, .67 to .98; P = .028) and DFS (HR, .84; 95% CI, .69 to 1.01; P = .070) showed a suggestion of better outcomes if predicted SBHAs were present but did not meet the prespecified P value of <.025. NRM did not differ (HR, 1.04; P = .740). These hypothesis-generating data support further exploration of HLA genotype-neoantigen interactions in the allo-HCT context., (Copyright © 2023 The American Society for Transplantation and Cellular Therapy. All rights reserved.)

Purpose: Immunopeptidome divergence between mismatched HLA-DP is a determinant of T-cell alloreactivity and clinical tolerability after fully HLA-A, -B, -C, -DRB1, -DQB1 matched unrelated donor hematopoietic cell transplantation (UD-HCT). Here, we tested this concept in HLA-A, -B, and -C disparities after single class I HLA-mismatched UD-HCT., Patients and Methods: We studied 2,391 single class I HLA-mismatched and 14,426 fully HLA-matched UD-HCT performed between 2008 and 2018 for acute leukemia or myelodysplastic syndromes. Hierarchical clustering of experimentally determined peptide-binding motifs (PBM) was used as a proxy for immunopeptidome divergence of HLA-A, -B, or -C disparities, allowing us to classify 1,629/2,391 (68.1%) of the HLA-mismatched UD-HCT as PBM-matched or PBM-mismatched. Risks associated with PBM-matching status were assessed by Cox proportional hazards models, with overall survival (OS) as the primary end point., Results: Relative to full matches, bidirectional or unidirectional PBM mismatches in graft-versus-host (GVH) direction (PBM-GVH mismatches, 60.7%) were associated with significantly lower OS (hazard ratio [HR], 1.48; P < .0001), while unidirectional PBM mismatches in host-versus-graft direction or PBM matches (PBM-GVH matches, 39.3%) were not (HR, 1.13; P = .1017). PBM-GVH mismatches also had significantly lower OS than PBM-GVH matches in direct comparison (HR, 1.32; P = .0036). The hazards for transplant-related mortality and acute and chronic graft-versus-host disease but not relapse increased stepwise from full HLA matches to single PBM-GVH matches, and single PBM-GVH mismatches. A webtool for PBM-matching of single class I HLA-mismatched donor-recipient pairs was developed., Conclusion: PBM-GVH mismatches inform mortality risks after single class I HLA-mismatched UD-HCT, suggesting that prospective consideration of directional PBM-matching status might improve outcome. These findings highlight immunopeptidome divergence between mismatched HLA as a driver of clinical tolerability in UD-HCT.

When using post-transplantation cyclophosphamide (PTCy) graft-versus-host disease (GVHD) prophylaxis for lymphoma patients, it is currently unknown whether a matched unrelated donor (MUD) or a haploidentical related donor is preferable if both are available. In this study we wanted to test whether using a haploidentical donor has the same results of a MUD. A total of 2140 adults (34% Center for International Blood and Marrow Transplant Research, 66% European Society for Blood and Marrow Transplantation registry) aged ≥18 years who received their first haploidentical hematopoietic cell transplantation (haplo-HCT) or MUD-HCT (8/8 match at HLA-loci A, B, C, and DRB1) for lymphoma using PTCy-based GVHD prophylaxis from 2010 to 2019 were retrospectively analyzed. The majority of both MUD and haploidentical HCTs received reduced intensity/nonmyeloablative conditioning (74% and 77%, respectively) and used a peripheral blood stem cell graft (91% and 60%, respectively) and a 3-drug GVHD prophylaxis (PTCy + calcineurin inhibitor + MMF in 54% and 90%, respectively). Haploidentical HCT has less favorable results versus MUD cohort in terms of overall mortality (hazard ratio [HR= = 1.69; 95% confidence interval [CI], 1.30-2.27; P < .001), progression-free survival (HR=1.39; 95% CI, 1.10-1.79; P = .008), nonrelapse mortality (HR = 1.93; 95% CI, 1.21-3.07; P = .006), platelet engraftment (HR = 0.69; 95% CI, 0.59-0.80; P < .001), acute grade 2-4 GVHD incidence (HR = 1.65; 95% CI, 1.28-2.14; P < .001), and chronic GVHD (HR = 1.79; 95% CI, 1.30-2.48, P < .001). No significant differences were observed in terms of relapse and neutrophil engraftment. Adjusting for propensity score yielded similar results. Whenever MUD is available in a timely manner, it should be preferred over a haploidentical donor when using PTCy-based GVHD prophylaxis for patients with lymphoma., (Copyright © 2022 The American Society for Transplantation and Cellular Therapy. All rights reserved.)

It is 24 years since the IPD-IMGT/HLA Database, http://www.ebi.ac.uk/ipd/imgt/hla/, was first released, providing the HLA community with a searchable repository of highly curated HLA sequences. The database now contains over 35 000 alleles of the human Major Histocompatibility Complex (MHC) named by the WHO Nomenclature Committee for Factors of the HLA System. This complex contains the most polymorphic genes in the human genome and is now considered hyperpolymorphic. The IPD-IMGT/HLA Database provides a stable and user-friendly repository for this information. Uptake of Next Generation Sequencing technology in recent years has driven an increase in the number of alleles and the length of sequences submitted. As the size of the database has grown the traditional methods of accessing and presenting this data have been challenged, in response, we have developed a suite of tools providing an enhanced user experience to our traditional web-based users while creating new programmatic access for our bioinformatics user base. This suite of tools is powered by the IPD-API, an Application Programming Interface (API), providing scalable and flexible access to the database. The IPD-API provides a stable platform for our future development allowing us to meet the future challenges of the HLA field and needs of the community., (© The Author(s) 2022. Published by Oxford University Press on behalf of Nucleic Acids Research.)

Until recently the number of alleles of the nonclassical HLA class I gene HLA-E documented in the IPD-IMGT/HLA Database was small and as a result, the gene was often not considered to be notably polymorphic. Here, we describe our work in identifying and submitting 86 novel HLA-E alleles after full-gene single-molecule real-time (SMRT) DNA sequencing of 6227 DNA samples. These samples were comprised of 2468 patients undergoing hematopoietic cell transplantation and 3759 unrelated potential donors. A total of 111 unique HLA-E alleles were detected in this cohort. The majority of novel alleles (79.1%) contained polymorphisms in intronic regions, highlighting the significant undiscovered variation present in the noncoding regions of the HLA-E gene., (© 2022 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd.)

HLA serological specificities were defined by the reactivity of HLA molecules with sets of sera and monoclonal antibodies. Many recently identified alleles defined by molecular typing lack their serotype assignment. We surveyed the literature describing the correlation of the reactivity of serologic reagents with AA residues. 20 - 25 AA residues determining epitopes (DEP) that correlated with 82 WHO serologic specificities were identified for HLA class I loci. Thirteen DEP each located in the beta-1 domains that correlated with 24 WHO serologic specificities were identified for HLA-DRB1 and -DQB1 loci. The designation of possible HLA-DPB1, -DQA1, -DPA1, and additional serological specificities that result from epitopes defined by residues located at both -DQA1 and -DQB1 subunits were also examined. HATS software was developed for automated serotype assignments to HLA alleles in one of the three hierarchical matching criteria: (1) all DEP (FULL); (2) selected DEP specific to each serological specificity (SEROTYPE); (3) one AA mismatch with one or more SEROTYPES (INCOMPLETE). Results were validated by evaluating the alleles whose serotypes do not correspond to the first field of the allele name listed in the HLA dictionary. Additional 85 and 21 DEP patterns that do not correspond to any WHO serologic specificities for common HLA class I and DRB1 alleles were identified, respectively. A comprehensive antibody identification panel would allow for accurate unacceptable antigen listing and compatibility predictions in solid organ transplantation. We propose that antibody-screening panels should include all serologic specificities identified in this study., (© 2022 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd.)

Patients with severe aplastic anemia (SAA) can have an unrecognized inherited bone marrow failure syndrome (IBMFS) because of phenotypic heterogeneity. We curated germline genetic variants in 104 IBMFS-associated genes from exome sequencing performed on 732 patients who underwent hematopoietic cell transplant (HCT) between 1989 and 2015 for acquired SAA. Patients with pathogenic or likely pathogenic (P/LP) variants fitting known disease zygosity patterns were deemed unrecognized IBMFS. Carriers were defined as patients with a single P/LP variant in an autosomal recessive gene or females with an X-linked recessive P/LP variant. Cox proportional hazard models were used for survival analysis with follow-up until 2017. We identified 113 P/LP single-nucleotide variants or small insertions/deletions and 10 copy number variants across 42 genes in 121 patients. Ninety-one patients had 105 in silico predicted deleterious variants of uncertain significance (dVUS). Forty-eight patients (6.6%) had an unrecognized IBMFS (33% adults), and 73 (10%) were carriers. No survival difference between dVUS and acquired SAA was noted. Compared with acquired SAA (no P/LP variants), patients with unrecognized IBMFS, but not carriers, had worse survival after HCT (IBMFS hazard ratio [HR], 2.13; 95% confidence interval[CI], 1.40-3.24; P = .0004; carriers HR, 0.96; 95% CI, 0.62-1.50; P = .86). Results were similar in analyses restricted to patients receiving reduced-intensity conditioning (n = 448; HR IBMFS = 2.39; P = .01). The excess mortality risk in unrecognized IBMFS attributed to death from organ failure (HR = 4.88; P < .0001). Genetic testing should be part of the diagnostic evaluation for all patients with SAA to tailor therapeutic regimens. Carriers of a pathogenic variant in an IBMFS gene can follow HCT regimens for acquired SAA., (© 2022 by The American Society of Hematology.)