Validation of an Algorithm to Predict the Likelihood of an 8/8 HLA-Matched Unrelated Donor at Search Initiation

Racial/ethnic disparities in availability of volunteer unrelated donors for allogeneic transplantation

ABSTRACT

Despite the global unrelated donor (URD) registry size, the degree to which URD availability is a transplant barrier is not established. We evaluated the availability of 3,843 URDs requested for 455 diverse adult patients (predominantly with acute leukemia). URDs for non-Europeans were more likely to be domestic and had markedly lower Donor Readiness scores. Of URDs requested for confirmatory HLA-typing (CT) alone (ie, without simultaneous workup), 1,894 of 3,529 (54%) were available. Availability of domestic URDs was 45%. Donor Readiness score was highly predictive of CT availability. More non-European patients (n = 120) than Europeans (n = 335) had >10 URDs requested and <5 available. Of workup requests (after CT or CT-workup), <70% (604/889 [68%]) were available. More non-Europeans had <2 URDs available. URD availability for CT was markedly worse for non-Europeans, with availabilities for African, non-Black Hispanic, and Asian patients being 150/458 (33%), 120/258 (47%), and 119/270 (44%), respectively, with further decrements in URD workup availability. Our data suggest the functional size of the URD pool is much smaller than appreciated, mandating major operational changes for transplant centers and donor registries. Likelihood of donor availability should have a high priority in donor selection. Considering patient ancestry and URD Donor Readiness scores, centers should pursue, and registries permit, simultaneous pursuit of many URDs and abandon futile searches. Patients should be informed about their likelihood of donor availability and alternative options. Finally, although registries should address high URD attrition and speed procurement, use of all HLA-disparate graft types is needed to facilitate timely transplant for all.

Existence of HLA-Mismatched Unrelated Donors Closes the Gap in Donor Availability Regardless of Recipient Ancestry

In patients without a matched sibling donor (MSD) or well-matched unrelated donor (MUD), hematopoietic cell transplantation (HCT) can still be successful when using an HLA-mismatched unrelated donor (MMUD) in combination with post-transplantation cyclophosphamide (PTCy), abatacept, or other novel approaches. This may allow clinicians to choose a suitable donor from a wide range of donor options while optimizing other donor selection characteristics, including donor age. We hypothesized that allowing for a 5/8 HLA match level considering high-resolution matching at HLA-A, -B, -C and -DRB1, there is a potential to close the donor availability gap for all patients regardless of their race/ethnicity. In this work, we estimate the likelihood of matching for all racial/ethnic groups at different HLA match thresholds. Our study aimed to assess the potential for identifying an available MUD or MMUD in the National Marrow Donor Program (NMDP)/Be The Match (BTM) donor registry for 21 detailed and 5 broad racial/ethnic groups, using high-resolution HLA matching for HLA-A, -B, -C, and -DRB1 at various levels (8/8, 7/8, 6/8, and 5/8). We used donor registry population data from the NMDP/BTM in 2020 and redistributed the donor registry data according to existing population ratios, accounting for demonstrated donor availability. Finally, we used a genetic model at the population level to estimate the match likelihood for detailed and broad racial/ethnic groups. Likelihood of 8/8 HLA match ranging from 16% to 74% were obtained for various detailed racial/ethnic groups with available donors age ≤35 years. When considering more mismatches in the HLA loci, registry coverage became >99% with a 5/8 HLA match level for donors of all ages or those age ≤35 years, with HLA-DPB1 T cell epitope permissive matching, or when searching for donors outside of their racial/ethnic group. Our registry models demonstrate the potential for using MMUDs at various HLA match levels to study whether this will expand access to HCT across racial/ethnic groups. Expanded donor options may erase the donor availability gap for all patients while allowing for selection of MMUDs with favorable characteristics, such as younger age.

Analysis of disparities in time to allogeneic transplantation in adults with acute myelogenous leukemia

Although alternative donors extend transplant access, whether recipient ancestry affects the time to allogeneic transplant is not established. We analyzed the likelihood of clinically significant delays to allograft by patient ancestry in 313 adult patients with acute myelogenous leukemia (AML) who underwent transplantation. Non-European ancestry patients (n = 99) were more likely than Europeans (n = 214) to receive HLA-mismatched donor allografts (45% vs 24%). Overall, the median time from transplant indication to allograft was 127 days (range, 57-1683). In multivariable analysis, non-Europeans had an increased risk of prolonged indication to transplant time >180 days owing to significant delays in indication to consult >90 days and consult to transplant >120 days. Compared with recipients of HLA-matched unrelated donors (URDs), HLA-mismatched adult donor recipients were at an increased risk of delayed indication to transplant, whereas HLA-identical sibling and cord blood recipients were at a lower risk. Subanalysis showed more indication to transplant delays >180 days in non-European (44%) vs European (19%) 8/8 URD recipients. Finally, the pandemic further exacerbated delays for non-Europeans. In summary, although non-European patients with AML are less likely to receive 8/8 URDs as expected, if they do, their transplants are delayed. HLA-identical siblings and cord blood facilitate the fastest transplants regardless of patient ancestry, whereas other adult donor transplants are delayed. Strategies to mitigate referral barriers, hasten donor evaluation, and use all alternative donor sources are critical to ensure timely transplantation for patients with AML.

An Optimized Search Prognosis Tool to Predict 8/8 HLA Allele-Matched Unrelated Donor Procurement

For patients in need of allogeneic transplantation who lack an HLA-identical sibling, an 8/8 HLA allele-matched unrelated donor (URD) is a standard alternative. However, delays in URD procurement can adversely impact patient care. Recipient genotype and search assessment (MSKv1.0)-based tools can predict search prognosis for many, but both tools have lower performance in non-European ancestry patients. Using the MSKv1.0 tool, we analyzed searches from 1530 potential allograft recipients (including 863 who underwent transplantation) with the aim of creating an optimized MSKv2.0 search prognosis tool that can classify a URD search as either Good or Poor with a high level of accuracy while also limiting an ambiguous Fair search prognosis regardless of patient ancestry. By MSKv2.0, the 8/8 URD search prognosis distribution was 57% Good, 21% Fair, and 22% Poor in Europeans and 15% Good, 21% Fair, and 63% Poor in non-Europeans. Importantly, compared to MSKv1.0, the likelihood of Fair categorization was reduced to <25% with comparable Fair rates (P = .847) in both European and non-European groups. Moreover, all patients with an MSKv2.0 Good prognosis had an 8/8 URD identified, and almost all of those who underwent transplantation had an 8/8 URD (Europeans, 99%; non-Europeans, 98%; P = .504). The MSKv2.0 tool also was highly accurate at classifying Poor searches, with <10% identifying an 8/8 URD, and almost all patients who underwent transplantation (Europeans, 95%; non-Europeans, 96%) receiving an alternative donor. Using preliminary search results, MSKv2.0 accurately classifies patients by likelihood of 8/8 URD procurement, greatly facilitating triage to 8/8 URD (Good prognosis) or alternative donor (Poor prognosis) transplantations.

Development and evaluation of checklists to support the recruitment of committed hematopoietic stem cell donors

BACKGROUND

Checklists are memory recall tools used across healthcare to improve outcomes. Here, we describe the development and evaluation of checklists to support recruitment of committed allogeneic hematopoietic stem cell donors.

STUDY DESIGN AND METHODS

Checklists were developed with the following objectives: (1) improve best-practice adherence; (2) reduce errors; and (3) support standardization at stem cell drives. Topics included: recruiting needed donors; securing informed consent; maintaining good-documentation practices; and supervising registration and tissue sample collection. Checklists were iteratively revised with input from stakeholders. We evaluated the checklists by examining recruitment outcomes and errors (i.e., preventing registrants from being listed as donors) pre- (11/2011-8/2016) and post- (9/2016-11/2019) implementation by the Canadian donor recruitment organization Stem Cell Club. Quantitative and qualitative methods were employed to analyze recruiters' perspectives on the checklists.

RESULTS

The checklists supported recruitment of donors from needed demographic groups as Stem Cell Club expanded its recruitment effort from 4118 registrants (60% male, 58% non-European) pre-implementation to 10,621 (52% male, 56% non-European) post-implementation. Checklist implementation was associated with a marked reduction in errors (from 13.2% to 1.9%) and a three-fold increase in the match rate of recruited donors (from 0.024% to 0.075%). Qualitative and quantitative analysis of recruiter feedback supported that the checklists' objectives were realized from the recruiter perspective.

DISCUSSION

We developed checklists to support donor recruitment and showed that their implementation was valued by recruiters and associated with both reduced errors and improved donor recruitment outcomes. The checklists are relevant to donor recruitment organizations worldwide.

Likelihood of Proceeding to Allogeneic Hematopoietic Cell Transplantation in the United States after Search Activation in the National Registry: Impact of Patient Age, Disease, and Search Prognosis

The National Marrow Donor Program (NMDP) operates the Be The Match Registry to serve patients who require an allogeneic hematopoietic cell transplant (alloHCT). The factors that result in progression of an active donor search (ie, request for tissue typing or stem cell donation) to alloHCT are poorly understood. Some factors, such as differences in access by ethnic group, are known; however, deeper understanding of other patient and search factors is needed. Our study sought to identify the likelihood of patient progression from initiation of an active search for an unrelated adult donor/umbilical cord blood to transplant and to evaluate factors associated with proceeding to transplantation within 6 months. A retrospective cohort of US donor searches (ie, transplant center's first request of donor/cord blood unit testing; N = 8816) of the Be The Match Registry from January to December 2016 was analyzed. An adult unrelated donor search prognosis score, which categorizes the prognosis of the donor search as good, fair, or poor based on the patient HLA type and race/ethnic group, was included. At 6 months, 3744 (42%) patients had received a transplant. White patients were more likely to receive a transplant (n = 2590 of 5687, 45%) compared to black/African American patients (n = 187 of 700, 27%; P < .001). In multivariate analysis, the adult unrelated donor search prognosis score was associated with proceeding to adult donor or cord blood transplant within 6 months across all patient populations. A poor search prognosis score had an odds ratio (OR) of 0.32 (95% confidence interval [CI], 0.26 to 0.39, P < .001), 0.22 (95% CI, 0.09 to 0.54, P = .001), 0.39 (95% CI, 0.23 to 0.65, P < .001), and 0.26 (95% CI, 0.14 to 0.45, P < .001) for adults with malignant disease, adults with nonmalignant disease, children with malignant disease, and children with nonmalignant disease, respectively. This study identified important factors in the likelihood of a patient proceeding to HCT and suggests areas for future intervention to reduce the barriers to transplant.

Racial disparities in access to HLA-matched unrelated donor transplants: a prospective 1312-patient analysis

Availability of 8/8 HLA-allele matched unrelated donors (URDs) is a barrier for ethnic and racial minorities. We prospectively evaluated receipt of 8/8 HLA-allele matched URD or either 7/8 URD or cord blood (CB) transplants by patient ancestry from 2005 to 2017. Matched URDs were given priority if they were available. Of 1312 patients, 723 (55%) received 8/8 URD, 219 (17%) 7/8 URD, 319 (24%) CB, and 51 (4%) had no 7/8 or 8/8 URD or CB graft. Europeans were more likely to receive an 8/8 URD transplant than non-Europeans (67% vs 33%) and less likely to have no URD or CB graft (1% vs 9%). Southern Europeans received 8/8 URD transplants (41%) at rates similar to those of Asians (34%) and white Hispanics (35%); Africans were the least likely (18%) to undergo 8/8 URD transplantation. CB and 7/8 URDs extended transplant access to all groups. In 742 recent patients, marked racial disparity in 8/8 URD access between groups observed in earlier years persisted with only a modest increase in the percentage of 8/8 URD transplants. Of 78 recent African patients, 46% received a CB transplant and 14% had no 7/8 or 8/8 URD or CB graft. Increasing registry size has not resolved the racial disparity in URD access, which emphasizes the importance of alternative graft sources.

Post-transplantation cyclophosphamide GvHD prophylaxis after hematopoietic stem cell transplantation from 9/10 or 10/10 HLA-matched unrelated donors for acute leukemia

HLA-matching largely contributes to unrelated donor hematopoietic cell transplantation (UD-HCT) success but, due to the selective deletion of alloreactive T-cells, post-transplantation cyclophosphamide (PTCy) could modulate its negative impact on outcomes. We retrospectively compared acute leukemia patients receiving 10/10 or 9/10 HLA allele-matched UD-HCT with PTCy-GvHD prophylaxis between 2010 and 2017, reported to EBMT registry. The 100-day incidence of grade ≥2 and grade ≥3 aGvHD were comparable for 10/10 and 9/10 UD (28% versus 28%, p = 0.8 and 10% versus 8%, p = 0.5, respectively). The 2-year cGvHD and extensive cGvHD were similar between 10/10 and 9/10 UD (35% versus 44%, p = 0.2 and 21% versus 20%, p = 0.6, respectively). The 2-year nonrelapse mortality was 20% after 10/10 and 16% after 9/10 UD-HCT (p = 0.1). Relapse incidence at 2-year was 24% for 10/10 and 28% for 9/10 UD-HCT (p = 0.4). Leukemia-free survival at 2-year was the same for 10/10 and 9/10 UD (56 and 56%, p = 0.6, respectively), with comparable overall survival (62 and 59%, p = 0.9, respectively). Multivariate analysis showed no effect of HLA-matching on outcomes. An advanced disease status and patient disability remained the most important factors portending a worse survival. PTCy could alleviate the detrimental effect of HLA-allele mismatching in UD-HCT, potentially expanding the donor pool for acute leukemia patients.

Selection of matched unrelated donors moving forward: from HLA allele counting to functional matching

Matched unrelated donors (URD) are the most frequent source of stem cells for allogeneic hematopoietic cell transplantation (HCT) to date, with HCT performed mainly under conventional immunosuppression by methotrexate and cyclosporine. In this setting, every single allelic donor-recipient mismatch for HLA-A, -B, -C, -DRB1 (8/8), but not for HLA-DQB1, -DPB1, has a significant negative effect on overall survival (OS). When several 8/8 HLA-matched URD are available, donor age is the most important factor impacting OS. Moving forward from the traditional way of counting the number of donor-recipient HLA allele mismatches to biology-driven algorithms for functional matching has led to the unraveling of an association between permissive, low-risk HLA-DPB1 mismatches and improved outcome after URD HCT for malignant disease but not for nonmalignant disease. Functional HLA matching might prove to have increasing importance for URD selection in the era of new immunosuppressive regimens that have the potential to substantially reshuffle the role of HLA mismatches in URD HCT.

Urgent Time to Allogeneic Hematopoietic Cell Transplantation: A National Survey of Transplant Physicians and Unrelated Donor Search Coordinators Facilitated by the Histocompatibility Advisory Group to the National Marrow Donor Program

To characterize donor search and selection practices, the National Marrow Donor Program (NMDP) Histocompatibility Advisory Group developed a survey of allogeneic hematopoietic cell transplant (HCT) physicians and search coordinators. The objectives were to describe search practices, understand practices surrounding urgent time to HCT, and characterize strategies used when identifying a matched unrelated donor is unlikely. Participants included US physician members of the American Society for Transplantation and Cellular Therapy and donor search coordinators within the NMDP network. The web-based survey was conducted from February to May 2018. Three hundred seventeen of 858 physicians (37%) and 225 of 327 coordinators (69%) responded, of which 263 and 194, respectively, were eligible and included in the analysis. Most centers, 142 (95%), were represented; 108 (72%) had at least 1 physician and 128 (85%) had at least 1 coordinator respondent. Most (68% physicians, 61% coordinators) indicated donor selection decisions were made by individual physicians. Urgent time to HCT was most commonly (90% and 87% of physicians and coordinators, respectively) defined as HCT within 4 to 6 weeks of search initiation. Higher HCT urgency was associated with a higher disease risk index. For urgent cases with low probability of an 8/8 matched unrelated donor , 75% and 80% of physicians and coordinators endorsed a short (1 to 2 weeks) unrelated donor search before proceeding to an alternative donor source. NMDP-provided solutions to expedite donor identification were strongly endorsed. This survey clarified current donor selection practices in the United States and defined urgent time to HCT. These data provide insight to NMDP on potential solutions to support the path to transplant, such as highlighting futile searches and providing alternative donor options at the time of search initiation.

Outcomes of Related and Unrelated Donor Searches Among Patients with Primary Immunodeficiency Diseases Referred for Allogeneic Hematopoietic Cell Transplantation

Patients with primary immunodeficiencies (PIDs) are potentially cured by allogeneic hematopoietic cell transplantation (HCT). The spectrum of PIDs has expanded greatly beyond those that present in infancy or are diagnosed on newborn screening and require urgent, preemptive HCT. Many PID diagnoses are now made later in life, and the role of HCT is only considered for severe disease manifestations; in these cases, the kinetics and goals of a donor search may be different than for severe combined immunodeficiency. Across all PIDs, related donor searches have the additional selection factor of the inherited disease, and such searches may yield more limited options than searches for patients with hematologic malignancies; thus, unrelated donor options often become more critical in these patients. We retrospectively evaluated the outcomes of donor searches among patents with PIDs referred for HCT at the National Institutes of Health, where the minimum patient age for evaluation is 3 years and where donor options include matched sibling donors or matched related donors, HLA-haploidentical (haplo), or 7-8/8 HLA matched unrelated donors (mMUDs/MUDs). Patient (n = 161) and donor demographics, MUD search results, HLA typing, pedigrees, mutation testing, and donor selection data were collected. The National Marrow Donor Program HapLogic 8/8 HLA match algorithm was used to predict the likelihood of a successful MUD search and categorized as very good, good, fair, poor, very poor, or futile per the Memorial Sloan Kettering Cancer Center (MSKCC) Search Prognosis method. There were significant differences by PID mode of inheritance in patient age, disposition (receipt of HCT or not), donor source, and donor relatedness. A related or unrelated donor option could be identified for 94% of patients. Of living first-degree relatives (median, 3; range, 0 to 12 per patient), a median of 1 donor remained for autosomal dominant and X-linked (XL) diseases after HLA typing, mutation testing, and other exclusions, and a median of 2 donors remained for autosomal recessive (AR) diseases. Among patients with a PID of known mode of inheritance (n = 142), the best related donor was haplo for 99 (70%) patients, with 56 (39%) haplos age 40 years or older and 5 (4%) second-degree haplos; 13 (9%) had no family donor options. The best related donor was a heterozygote/asymptomatic carrier of the PID mutation in 36 (49%) patients with AR or XL disease (n = 73). Among patients with MUD search performed (n = 139), 53 (38%) had very poor/futile 8/8 MUD searches, including 6 (32%) of those with unknown PID mutation and therefore no family donor options. The MSKCC Search Prognosis was less favorable for those of non-European ancestry compared with European ancestry (P = .002). Most patients of Hispanic or African ancestry had very poor/futile MUD searches, 71% and 63%, respectively. No HCT recipients with very poor/futile MUD searches (n = 38) received 8/8 MUD grafts. Alternative donor options, including haplo and unrelated donors, are critical to enable HCT for patients with PIDs. MUD search success remains low for those of non-European ancestry, and this is of particular concern for patients with PIDs caused by an unknown genetic defect. Among patients with PIDs, related donor options are reduced and haplos age 40 years and older and/or mutation carriers are often the best family option.

Selection of unrelated donors and cord blood units for hematopoietic cell transplantation: guidelines from the NMDP/CIBMTR

Allogeneic hematopoietic cell transplantation involves consideration of both donor and recipient characteristics to guide the selection of a suitable graft. Sufficient high-resolution donor-recipient HLA match is of primary importance in transplantation with adult unrelated donors, using conventional graft-versus-host disease prophylaxis. In cord blood transplantation, optimal unit selection requires consideration of unit quality, cell dose and HLA-match. In this summary, the National Marrow Donor Program (NMDP) and the Center for International Blood and Marrow Transplant Research, jointly with the NMDP Histocompatibility Advisory Group, provide evidence-based guidelines for optimal selection of unrelated donors and cord blood units.