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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.

T-cell responses to minor histocompatibility antigens (mHAs) mediate graft-versus-leukemia (GVL) effects and graft-versus-host disease (GVHD) in allogeneic hematopoietic cell transplantation. Therapies that boost T-cell responses improve allogeneic hematopoietic cell transplant (alloHCT) efficacy but are limited by concurrent increases in the incidence and severity of GVHD. mHAs with expression restricted to hematopoietic tissue (GVL mHAs) are attractive targets for driving GVL without causing GVHD. Prior work to identify mHAs has focused on a small set of mHAs or population-level single-nucleotide polymorphism–association studies. We report the discovery of a large set of novel GVL mHAs based on predicted immunogenicity, tissue expression, and degree of sharing among donor-recipient pairs (DRPs) in the DISCOVeRY-BMT data set of 3231 alloHCT DRPs. The total number of predicted mHAs varied by HLA allele, and the total number and number of each class of mHA significantly differed by recipient genomic ancestry group. From the pool of predicted mHAs, we identified the smallest sets of GVL mHAs needed to cover 100% of DRPs with a given HLA allele. We used mass spectrometry to search for high-population frequency mHAs for 3 common HLA alleles. We validated 24 predicted novel GVL mHAs that are found cumulatively within 98.8%, 60.7%, and 78.9% of DRPs within DISCOVeRY-BMT that express HLA-A∗02:01, HLA-B∗35:01, and HLA-C∗07:02, respectively. We confirmed the immunogenicity of an example novel mHA via T-cell coculture with peptide-pulsed dendritic cells. This work demonstrates that the identification of shared mHAs is a feasible and promising technique for expanding mHA-targeting immunotherapeutics.

Post-transplantation cyclophosphamide (PTCy) has been shown to effectively control graft-versus-host disease (GvHD) in haploidentical (Haplo) transplantations. In this retrospective registry study, we compared GvHD organ distribution, severity, and outcomes in patients with GvHD occurring after Haplo transplantation with PTCy GvHD prophylaxis (Haplo/PTCy) versus HLA-matched unrelated donor transplantation with conventional prophylaxis (MUD/conventional). We evaluated 2 cohorts: patients with grade 2 to 4 acute GvHD (aGvHD) including 264 and 1163 recipients of Haplo and MUD transplants; and patients with any chronic GvHD (cGvHD) including 206 and 1018 recipients of Haplo and MUD transplants, respectively. In comparison with MUD/conventional transplantation ± antithymocyte globulin (ATG), grade 3-4 aGvHD (28% versus 39%, P = .001), stage 3-4 lower gastrointestinal (GI) tract aGvHD (14% versus 21%, P = .01), and chronic GI GvHD (21% versus 31%, P = .006) were less common after Haplo/PTCy transplantation. In patients with grade 2-4 aGvHD, cGcHD rate after Haplo/PTCY was also lower (hazard ratio [HR] = .4, P < .001) in comparison with MUD/conventional transplantation without ATG in the nonmyeloablative conditioning setting. Irrespective of the use of ATG, non-relapse mortality rate was lower (HR = .6, P = .01) after Haplo/PTCy transplantation, except for transplants that were from a female donor into a male recipient. In patients with cGvHD, irrespective of ATG use, Haplo/PTCy transplantation had lower non-relapse mortality rates (HR = .6, P = .04). Mortality rate was higher (HR = 1.6, P = .03) within, but not after (HR = .9, P = .6) the first 6 months after cGvHD diagnosis. Our results suggest that PTCy-based GvHD prophylaxis mitigates the development of GI GvHD and may translate into lower GvHD-related non-relapse mortality rate.

At the onset of the COVID-19 pandemic, the National Marrow Donor Program mandated the cryopreservation of hematopoietic cell grafts from volunteer unrelated donors due to numerous patient and donor safety concerns and logistical hurdles. Using the Center for International Blood and Marrow Transplant Research (CIBMTR) outcomes database, we report the impact of cryopreservation on overall survival (OS) and other outcomes within one year following hematopoietic cell transplantation (HCT). We analyzed 1,543 recipients of cryopreserved allografts receiving HCT at US centers during the first 6-months of the pandemic and compared them to 2,499 recipients of fresh allografts during the same 6-month period in 2019. On multivariable regression analysis, we observed no difference in OS (HR, 1.12; 95% CI: 0.98-1.28; P=0.09), non-relapse mortality (HR, 1.01; 95% CI: 0.84-1.22; P=0.89), graft-versus-host disease (GVHD), or GVHD-free, relapse-free survival (HR 1.03; 95% CI 0.93-1.14; P=0.58) in recipients of cryopreserved versus fresh allografts. Disease-free survival (DFS) was lower in the cryopreserved group (HR, 1.18; 95% CI: 1.05-1.33; P=0.006) due to a higher risk of relapse (HR, 1.21; 95% CI: 1.04-1.41; P=0.01). Primary graft failure was higher with cryopreservation (OR 1.48; 95% CI: 1.10-2.00; P=0.01) and the risk of chronic GVHD was lower (HR, 0.65; 95% CI: 0.50-0.84; P=0.001). In conclusion, while there was no negative impact of cryopreservation on OS, relapse was higher and DFS was lower. Fresh grafts are recommended as the pandemic related logistical hurdles resolve. Cryopreservation should be considered an option for patients when fresh grafts are not feasible.

Supplemental Table 1. Multivariate analysis of late relapse, transplant-related mortality, overall survival after HCT for each disease (AML, ALL, CML, MDS)

Purpose: Malignancy relapse remains a major obstacle for successful allogeneic hematopoietic cell transplantation (HCT). Chronic graft-versus-host disease (cGVHD) is associated with fewer relapses. However, when studying effects of cGVHD on relapse, it is difficult to separate from acute GVHD effects as most cases of cGVHD occur within the first year after transplant at the time when acute GVHD is still active.Experimental Design: This study based on CIBMTR registry data investigated cGVHD and its association with the incidence of late relapse and survival in 7,489 patients with acute myelogenous leukemia (AML), acute lymphoblastic leukemia (ALL), chronic myelogenous leukemia (CML), and myelodysplastic syndromes (MDS), who were leukemia free at 12 months after myeloablative allogeneic HCT.Results: Forty-seven percent of the study population was diagnosed with cGVHD at 12 months after transplant. The protective effect of cGVHD on late relapse was present only in patients with CML [RR, 0.47; 95% confidence interval (CI), 0.37–0.59; P < 0.0001). cGVHD was significantly associated with higher risk of treatment-related mortality (TRM; RR, 2.43; 95% CI, 2.09–2.82; P < 0.0001) and inferior overall survival (RR, 1.56; 95% CI, 1.41–1.73; P < 0.0001) for all diseases. In patients with CML, all organ sites and presentation types of cGVHD were equally associated with lower risk of late relapse.Conclusions: These results indicate that clinically relevant antileukemia effects of cGVHD on late relapses are present only in CML but not in AML, ALL, or MDS. Chronic GVHD in patients who are 1-year survivors after myeloablative allogeneic HCT is primarily associated with higher TRM and inferior survival. Clin Cancer Res; 21(9); 2020–8. ©2014 AACR.See related commentary by Gill, p. 1981

Supplemental Table 4. Multivariate analysis showing impact of cGVHD characteristics on treatment related mortality, disease free survival, overall survival in patients with AML, ALL, MDS who developed cGVHD

Supplemental Table 2. Multivariate analysis showing impact of cGVHD characteristics in CML patients on relapse

Supplemental Table 3. Multivariate analysis showing impact of cGVHD characteristics on treatment related mortality, disease free survival and overall survival in patients with CML who developed cGVHD

Human cytomegalovirus (CMV)-induced adaptive natural killer (NK) cells display distinct phenotypic and functional characteristics, including properties of immune memory. We hypothesized that these cells may be more resistant to suppression mediated by immunoregulatory cell subsets, making them attractive for use in cancer therapy. Here we report that relative to conventional NK cells, adaptive NK cells express lower levels of the inhibitory receptor T-cell Ig and ITIM domain (TIGIT), which results in resistance to immune suppression mediated by myeloid-derived suppressor cells (MDSC), as derived from cytokine induction in normal blood or patients with myelodysplastic syndrome. In contrast, conventional NK cells were potently suppressed by MDSCs, an effect abrogated completely by TIGIT blockade. Mechanistically, TIGIT signaling in NK cells after MDSC coculture led to a decrease in the phosphorylation of ZAP70/Syk and ERK1/2. These effects were reversed by blocking TIGIT on NK cells or by inhibiting production of reactive oxygen species (ROS) by MDSCs, the latter of which upregulated the TIGIT ligand CD155 on MDSCs. Accordingly, the blunted cytotoxicity of NK cells cocultured with MDSCs against tumor cells could be reversed by blocking TIGIT or ROS production. Overall, our results show how adaptive NK cells arising in response to CMV infection can escape MDSC-mediated suppression, and defined TIGIT antagonists as a novel type of checkpoint inhibitor to enhance NK-cell–mediated responses against cancer and infection. Cancer Res; 76(19); 5696–706. ©2016 AACR.

Supplemental figure. 1: Gating strategy of conventional and adaptive NK cells in healthy donors and MDS patients. Supplemental figure. 2: Gating strategy for MDSC and monocytes, CD16 expression on conventional and adaptive NK cells, expression of different NK cell receptors following co-culture with monocytes or MDSC, and differential production of IFNg on TIGIT high/low NK cells. Supplemental figure. 3: Evaluation of TIGIT blocking antibodies. IFNg production following DNAM-1 and TIGIT co-blocking. NK cell TNFa production in conventional and adaptive NK cells cultured with monocytes or MDSC. Supplemental Table. 1: All antibodies used throughout the study including clone, manufactory, fluorochrome, and marker.

Supplemental Figure 1. Cumulative incidence of cGVHD, relapse and death in patients with AML, ALL, CML, MDS

T cell responses to minor histocompatibility antigens (mHAs) mediate graft versus leukemia (GvL) effects and graft versus host disease (GvHD) in allogeneic hematopoietic cell transplant (alloHCT). Therapies that boost T cell responses improve the efficacy of alloHCT; however, these have been limited by concurrent increases in the incidence and severity of GvHD. mHAs with expression restricted to hematopoietic tissue (GvL mHAs) are attractive targets for driving GvL without causing GvHD. Prior work to identify mHAs has focused on a small set of mHAs or population-level SNP association studies. We report here the discovery of a large set of novel GvL mHAs based on predicted peptide immunogenicity, restriction of expression to hematopoietic tissue or GvHD target organs, and degree of sharing among donor-recipient pairs (DRPs) in the DISCOVeRY-BMT dataset of 3231 alloHCT DRPs. The total number of predicted mHAs and count within each class of predicted mHAs significantly differed by recipient genomic ancestry group, with European American>Hispanic>African American for each. The number of mHAs also differed markedly by HLA allele, even among alleles of the same gene. From the pool of predicted mHAs, we identified the smallest sets of GvL mHAs needed to cover 100% of DRPs with a given HLA allele. We then used mass spectrometry to search for high population frequency mHAs for three common HLA alleles. We validated a total of 24 novel predicted GvL mHAs that cumulatively are found within 98.8%, 60.7%, and 78.9% of DRPs within DISCOVeRY-BMT that express HLA-A*02:01, HLA-B*35:01, and HLA-C*07:02 respectively. We also confirmed in vivo immunogenicity of one example novel mHA via coculture of healthy human CD8 T cells with mHA-pulsed dendritic cells. This work demonstrates that identification of shared mHAs is a feasible and promising technique for expanding mHA-targeting immunotherapeutics to larger numbers of patients.

HLA-DQ heterodimers increase the susceptibility to autoimmune diseases, but their role in hematopoietic cell transplantation is unknown. We tested the hypothesis that outcome after HLA-matched and HLA-DQ–mismatched hematopoietic cell transplantation is influenced by HLA-DQ heterodimers. Heterodimers were defined in 5164 HLA-matched and 520 HLA-DQ–mismatched patients and their transplant donors according to well-established crystallographic criteria. Group 1 (G1) heterodimers are any DQA1*02/03/04/05/06α paired with any DQB1*02/03/04β. Group 2 (G2) heterodimers are DQA1*01α paired with any DQB1*05/06β. Multivariable models identified significantly higher relapse risk in G1G2 and G2G2 compared with G1G1 HLA-matched patients with malignant disease; risk increased with an increasing number of G2 molecules. In HLA-DQ–mismatched transplantation for malignant diseases, matching or mismatching for G2 increased relapse risk. G2 lowered disease-free survival after both HLA-matched and HLA-DQ–mismatched transplantation. A paradigm based on HLA-DQ heterodimers provides a functional definition of the hematopoietic cell transplantation barrier and a means to lower risks for future patients.

The effect of chronic graft-versus-host disease (cGVHD) on the risk of nonrelapse mortality (NRM) and relapse has not been specifically studied in older adults, who are increasingly undergoing allogeneic hematopoietic cell transplantation (alloHCT) and surviving long-term to develop cGVHD. In this Center for International Blood and Marrow Transplant Research (CIBMTR) analysis, we tested our hypothesis that the risk of NRM was higher with the development of cGVHD, particularly among older adults (age ≥60 years). We included 4429 adults age ≥40 years who underwent a first HLA-matched peripheral blood stem cell alloHCT for acute myelogenous leukemia or myelodysplastic syndrome between 2008 and 2017. We compared outcomes of 4 groups—older adults (≥60 years) and younger adults (40 to 59 years) with cGVHD and older and younger adults without cGVHD—to determine the effect of older age and cGVHD on various outcomes. We used Cox proportional hazard models to determine the risk of NRM, relapse, and overall survival (OS). We treated cGVHD as a time-dependent covariate. The severity of cGVHD was based on the CIBMTR clinical definitions. cGVHD was significantly associated with a higher risk of NRM and lower risk of relapse regardless of age. The risk of NRM was higher for older adults versus younger adults. Adults who developed cGVHD as a group had longer OS compared with age-matched cohorts without cGVHD. Older adults had worse OS regardless of cGVHD. Among adults with cGVHD, clinically moderate or severe cGVHD was associated with a significantly higher risk of NRM and lower risk of relapse; severe cGVHD was associated with shorter OS, whereas mild to moderate cGVHD was associated with longer OS. Among both younger and older adults, the development of cGVHD was associated with a higher risk of NRM, lower risk of relapse, and longer OS. Older adults had a higher risk of NRM, but the increased risk of NRM associated with cGVHD did not differ based on age. The development of mild to moderate cGVHD offered the most favorable balance between minimizing NRM and decreasing the risk of relapse. The relapse risk was lowest for adults with severe cGVHD, but high NRM resulted in shorter OS. Developing strategies to avoid clinically severe cGVHD is critically important. © 2021 American Society for Transplantation and Cellular Therapy. Published by Elsevier Inc.

The role of minor histocompatibility antigens (mHAs) in mediating graft versus leukemia (GvL) and graft versus host disease (GvHD) following allogeneic hematopoietic cell transplantation (alloHCT) is recognized but not well-characterized. By implementing improved methods for mHA prediction in two large patient cohorts, this study aimed to comprehensively explore the role of mHAs in alloHCT by analyzing whether (1) the number of predicted mHAs, or (2) individual mHAs are associated with clinical outcomes using multi-variate survival models corrected for multiple testing. Cox proportional hazard results showed that patients with a class I mHA count greater than the population median had an increased hazard of GvHD mortality (HR=1.39, 95%CI 1.01, 1.77, P=0.046). Competing risk analyses identified the class I mHAs DLRCKYISL (gene GSTP), WEHGPTSLL (CRISPLD2) and STSPTTNVL (SERPINF2) were associated with increased GVHD death (HR=2.84, 95%CI 1.52, 5.31, P=0.01), decreased leukemia-free survival (LFS) (HR=1.94,95%CI 1.27, 2.95, P=0.044), and increased disease-related mortality (DRM) (HR=2.32, 95%CI 1.5, 3.6, P=0.008), respectively. One class II mHA YQEIAAIPSAGRERQ (TACC2) was associated with increased risk of treatment-related mortality (TRM) (HR=3.05, 95%CI 1.75, 5.31, P=0.02). WEHGPTSLL and STSPTTNVL were present in conjunction within HLA haplotype B*40:01-C*03:04 and showed a positive dose-response relationship with increased all-cause mortality and DRM and decreased LFS, indicating these two mHAs contribute to risk of mortality in an additive manner. Our study reports the first large scale investigation of the associations of predicted class I and class II mHA peptides with clinical outcomes following alloHCT.Graphical Abstract

In the treatment of acute myelogenous leukemia with allogeneic hematopoietic cell transplantation, we previously demonstrated that there is a greater protection from relapse of leukemia when the hematopoietic cell transplantation donor has either the Cen B/B KIR genotype or a genotype having two or more KIR B gene segments. In those earlier analyses, KIR genotyping could only be assessed at the low resolution of gene presence or absence. To give the analysis greater depth, we developed high-resolution KIR sequence-based typing that defines all the KIR alleles and distinguishes the expressed alleles from those that are not expressed. We now describe and analyze high-resolution KIR genotypes for 890 donors of this human transplant cohort. Cen B01 and Cen B02 are the common CenB haplotypes, with Cen B02 having evolved from Cen B01 by deletion of the KIR2DL5, 2DS3/5, 2DP1, and 2DL1 genes. We observed a consistent trend for Cen B02 to provide stronger protection against relapse than Cen B01. This correlation indicates that protection depends on the donor having inhibitory KIR2DL2 and/or activating KIR2DS2, and is enhanced by the donor lacking inhibitory KIR2DL1, 2DL3, and 3DL1. High-resolution KIR typing has allowed us to compare the strength of the interactions between the recipient’s HLA class I and the KIR expressed by the donor-derived NK cells and T cells, but no clinically significant interactions were observed. The trend observed between donor Cen B02 and reduced relapse of leukemia points to the value of studying ever larger transplant cohorts.

Telomere length (TL) is a marker of biological aging associated with several health outcomes. High throughput reproducible TL measurements are needed for large epidemiological studies. We compared the novel DNA methylation-based estimator (DNAmTL) with the high-throughput quantitative PCR (qPCR) and the highly accurate flow cytometry with fluorescent in situ hybridization (flow FISH) methods using blood samples from healthy adults. We used Pearson’s correlation coefficient, Bland Altman plots and linear regression models for statistical analysis. Shorter DNAmTL was associated with older age, male sex, white race, and cytomegalovirus seropositivity (p70% of TL categorized above or below the median, but the accuracy dropped with increasing TL categories. The ability of DNAmTL to detect associations with age and other TL-related factors in the absence of strong correlation with measured TL may indicate its capture of aspects of telomere maintenance mechanisms and not necessarily TL. The inaccuracy of DNAmTL prediction should be considered during data interpretation and across-study comparisons.

The COVID-19 pandemic has resulted in the increased use of cryopreserved grafts for allogeneic hematopoietic cell transplantation (HCT). However, information about the effect of cryopreservation on outcomes for patients receiving allogeneic donor grafts is limited. We evaluated outcomes of HCT recipients who received either fresh or cryopreserved allogeneic bone marrow (BM) or peripheral blood stem cell (PBSC) grafts reported to the Center for International Blood and Marrow Transplant Research. A total of 7397 patients were included in the analysis. Recipients of cryopreserved graft were divided into 3 cohorts based on graft source: HLA-matched related PBSC donors (n = 1051), matched unrelated PBSC donors (n = 678), and matched related or unrelated BM donors (n = 154). These patients were propensity score matched with 5514 patients who received fresh allografts. The primary endpoint was engraftment. Multivariate analyses showed no significant increased risk of delayed engraftment, relapse, nonrelapse mortality (NRM), or survival with cryopreservation of BM grafts. In contrast, cryopreservation of related donor PBSC grafts was associated with decreased platelet recovery (hazard ratio [HR], 0.73; 95% confidence interval [CI], 0.68 to 0.78; P < .001) and an increased risk of grade II-IV (HR, 1.27; 95% CI, 1.09 to 1.48; P = .002) and grade III-IV (HR, 1.48; 95% CI, 1.19 to 1.84; P < .001) acute graft-versus-host disease. Cryopreservation of unrelated PBSC grafts was associated with delayed engraftment of neutrophils (HR, 0.77; 95% CI, 0.71 to 0.84; P < .001) and platelets (HR, 0.61; 95% CI, 0.56 to 0.66; P < .001) as well as an increased risk of NRM (HR, 1.4; 95% CI, 1.18 to 1.66; P < .001) and relapse (HR, 1.32; 95% CI, 1.11 to 1.58; P = .002) and decreased progression-free survival (HR, 1.36; 95% CI, 1.20 to 1.55; P < .001) and overall survival (OS) (HR, 1.38; 95% CI, 1.22 to 1.58; P < .001). Reasons for cryopreservation were not routinely collected; however, in a subset of unrelated donor HCT recipients, the reason was typically a change in patient condition. Products cryopreserved for patient reasons were significantly associated with inferior OS in multivariate analysis (HR, 0.65; 95% CI, 0.44 to 0.96; P = .029). We conclude that cryopreservation is associated with slower engraftment of PBSC grafts, which may be associated with inferior transplantation outcomes in some patient populations. However, the small numbers in the cryopreserved BM cohort and the lack of information on the reason for cryopreservation in all patients suggests that these data should be interpreted with caution, particularly in the context of the risks associated with unexpected loss of a graft during the pandemic. Future analyses addressing outcomes when cryopreservation is universally applied are urgently required.

The use of Post-Transplant Cyclophosphamide (PTCy) as Graft Versus Host Disease (GVHD) prophylaxis has resulted in reductions in GVHD and improved outcomes in allogeneic hematopoietic cell transplant (HCT) using HLA-mismatched related donors. We report the 3-year outcomes of the first multi-center prospective clinical trial using PTCy in the setting of mismatched unrelated donor (MMUD) bone marrow HCT. The study enrolled 80 patients (Either myeloablative (MAC) (N=40) or reduced intensity conditioning (RIC) (N=40)) with the primary endpoint of 1-year overall survival (OS). The median follow-up for this report is 34 months (range 12-46) in RIC and 36 months (range 18-49) in MAC. Three-year OS and non-relapse mortality (NRM) were 70% and 15%, and 62% and 10% in the RIC and MAC strata, respectively. No GVHD was reported after 1 year. Relapse incidence was 29% and 51% in RIC and MAC strata. OS did not differ based on HLA match grade (63% in the 7/8 strata and 71% in the 4-6/8 strata). These encouraging outcomes, sustained 3 years post-HCT, support the continued exploration of MMUD HCT using a PTCy platform. Important future areas to address include relapse reduction and furthering our understanding of optimal donor selection based on HLA and non-HLA factors.

Allogeneic hematopoietic stem cell transplantation is the only potentially curative treatment for patients with myelodysplastic syndrome (MDS), but long-term survival is limited by the risk of transplant-related complications. Short telomere length, mediated by inherited or acquired factors, impairs cellular response to genotoxic and replicative stress and could identify patients at higher risk for toxicity after transplantation. We measured relative telomere length in pretransplant recipient blood samples in 1514 MDS patients and evaluated the association of telomere length with MDS disease characteristics and transplantation outcomes. Shorter telomere length was significantly associated with older age, male sex, somatic mutations that impair the DNA damage response, and more severe pretransplant cytopenias, but not with bone marrow blast count, MDS treatment history, or history of prior cancer therapy. Among 1267 patients ≥40 years old, telomere length in the shortest quartile was associated with inferior survival (P < .001) because of a high risk of nonrelapse mortality (NRM; P = .001) after adjusting for significant clinical and genetic variables. The adverse impact of shorter telomeres on NRM was independent of recipient comorbidities and was observed selectively among patients receiving more intensive conditioning, including myeloablative regimens and higher dose melphalan-based reduced-intensity regimens. The effect of shorter telomeres on NRM was prominent among patients who developed severe acute graft-versus-host disease, suggesting that short telomere length may limit regenerative potential of mucosal tissues after acute injury. MDS patients with shorter telomere length, who have inferior survival driven by excess toxicity, could be considered for strategies focused on minimizing toxic effects of transplantation.