Search for an unrelated HLA-compatible stem cell donor

How to select the best available related or unrelated donor of hematopoietic stem cells?

Recognition of HLA incompatibilities by the immune system represents a major barrier to allogeneic hematopoietic stem cell transplantation. HLA genotypically identical sibling donors are, therefore, the gold standard for transplantation purposes, but only 30% patients have such a donor. For the remaining 70% patients alternative sources of stem cells are a matched unrelated adult volunteer donor, a haploidentical donor or a cord blood unit. The definition of 'HLA matching' depends on the level of resolution and on which loci are tested. The development of HLA molecular typing technologies and the availability of more than 27 million donors in the international database has greatly facilitated unrelated donor searches. The gold standard is high resolution typing at the HLA-A, -B, -C, -DRB1, and -DQB1 loci (10/10 match). Single disparities for HLA-A, -B, - C, or -DRB1 are associated with increased risk of post-transplant complications, but less so in patients with advanced disease, and in those undergoing T-cell-depleted allografting. HLA-DQB1 mismatches seem to be better tolerated and some HLA-C, -DRB1 and -DPB1 disparities are potentially less immunogenic. HLA typing by next-generation sequencing methods is likely to change matching algorithms by providing full sequence information on all HLA loci in a single step. In most European populations a 10/10 matched donor can be found for at least 50% of patients and an additional 20-30% patients may have a 9/10 matched donor. Genetic factors that help in identifying donors with less immunogenic mismatches are discussed. Haploidentical donors are increasingly used as an alternative source of stem cells for those patients lacking a matched unrelated donor.

HLA-C Incompatibilities in Allogeneic Unrelated Hematopoietic Stem Cell Transplantation

An increasingly larger fraction of patients with hematological diseases are treated by hematopoietic stem cells transplantation (HSCT) from HLA matched unrelated donors. Polymorphisms of HLA genes represent a major barrier to HSCT because HLA-A, -B, -C and DRB1 incompatibilities confer a higher risk of acute graft-versus-host disease (aGVHD) and mortality. Although >22 million volunteer HLA-typed donors are available worldwide, still a significant number of patients do not find a highly matched HSC donor. Because of the large haplotypic diversity in HLA-B–C associations, incompatibilities occur most frequently at HLA-C, so that unrelated donors with a single HLA-C mismatch often represent the only possible choice. The ratio of HLA-C-mismatched HSCT over the total number of transplants varies from 15 to 30%, as determined in 12 multicenter studies. Six multicenter studies involving >1800 patients have reported a 21–43% increase in mortality risk. By using in vitro cellular assays, a large heterogeneity in T-cell allorecognition has been observed. Yet the permissiveness of individual HLA-C mismatches remains poorly defined. It could be linked to the position and nature of the mismatched residues on HLA-C molecules, but also to variability in the expression levels of the mismatched alleles. The permissive C*03:03–03:04 mismatch is characterized by full compatibility at residues 9, 97, 99, 116, 152, 156, and 163 reported to be key positions influencing T-cell allorecognition. With a single difference among these seven key residues the C*07:01–07:02 mismatch might also be considered by analogy as permissive. High variability of HLA-C expression as determined by quantitative RT-PCR has been observed within individual allotypes and shows some correlation with A–B–C–DRB1 haplotypes. Thus in addition to the position of mismatched amino acid residues, expression level of patient’s mismatched HLA-C allotype might influence T-cell allorecognition, with patients low expression-C alleles representing possible permissive mismatches.

Unrelated hematopoietic stem cell donor matching probability and search algorithm

In transplantation of hematopoietic stem cells (HSCs) from unrelated donors a high HLA compatibility level decreases the risk of acute graft-versus-host disease and mortality. The diversity of the HLA system at the allelic and haplotypic level and the heterogeneity of HLA typing data of the registered donors render the search process a complex task. This paper summarizes our experience with a search algorithm that includes at the start of the search a probability estimate (high/intermediate/low) to identify a HLA-A, B, C, DRB1, DQB1-compatible donor (a 10/10 match). Based on 2002–2011 searches about 30% of patients have a high, 30% an intermediate, and 40% a low probability search. Search success rate and duration are presented and discussed in light of the experience of other centers. Overall a 9-10/10 matched HSC donor can now be identified for 60–80% of patients of European descent. For high probability searches donors can be selected on the basis of DPB1-matching with an estimated success rate of >40%. For low probability searches there is no consensus on which HLA incompatibilities are more permissive, although HLA-DQB1 mismatches are generally considered as acceptable. Models for the discrimination of more detrimental mismatches based on specific amino acid residues rather than specific HLA alleles are presented.

HLA-B51 and haplotypic diversity of B-Cw associations: implications for matching in unrelated hematopoietic stem cell transplantation

In unrelated hematopoietic stem cell transplantation (HSCT), human leukocyte antigen (HLA)-C locus incompatibilities occur frequently and are associated with increased risk of posttransplant complications. Because HLA-B51 is associated with a high rate of Cw disparities, we performed a comprehensive four-digit typing analysis of 140 ABCDRB1 B51 genotypes proven by pedigree analysis and 311 unrelated donors selected for 75 B51-positive patients. In addition, 145 A1/Ax-B8/B51-DR3/DRx donors were HLA typed at a high-resolution level and tested for three microsatellite (Msat) polymorphisms located in the HLA class I and III regions. Based on these data sets, 182 different ABCDR haplotypes with 14 different B-Cw associations were detected. Rates of Cw mismatches were shown to be highly correlated with the ABDRB1 haplotypes. We have computed 21 B51 haplotypes that disclose a high probability of HLA-C allele matching and 30 haplotypes with a low (<25%) probability. The HLA-C allele frequency profiles were quite different in these two groups, with a more heterogeneous distribution in the low matching probability group. HLA-Cw*1502 was inversely correlated with the likelihood to identify a Cw-mismatched donor: it was present in 61% of the high vs 18% of the low probability group (P < 0.0001). The analysis of three Msats in the class I and III regions showed a higher allelic diversity in B51-positive haplotypes compared with the conserved A1-B8-DR3 haplotype. HLA-B51 haplotypes therefore exhibit a high diversity at the level of B-Cw associations and of non-HLA polymorphisms in the class I and III regions. Such heterogeneity negatively impacts on overall matching in HSCT.

Impact of high-resolution matching in allogeneic unrelated donor stem cell transplantation in Switzerland

It is currently unknown what degree of human leukocyte antigen (HLA)-mismatching is acceptable in unrelated donor hematopoietic stem cell transplantation (UD-HSCT). Mismatches at some loci may be more permissive than others. We have analyzed the effect of high-resolution HLA-matching on outcome of all 214 consecutive recipients of UD-HSCT carried out in Switzerland. All typing was by the Swiss reference laboratory. Donor-recipient pairs were HLA-10/10 matched (n=130) or mismatched for either HLA-A/-B/-DRB1/multiple loci (n=33; (HLA-A/-B=10); (-DRB1=8); (multiple=15)); HLA-C (n=29) or HLA-DQ/-DRB3 (n=22; (DQ=16); (-DRB1=6)). The median follow-up was 32 months. Survival probabilities (+/-95% confidence interval) at 3 years were 57 (+/-10)% for recipients of HLA 10/10-matched transplants, 53 (+/-22)% for recipients of HLA-DQ/-DRB3-mismatched transplants, 44 (+/-20)% for recipients of HLA-C-mismatched transplants and 0% for recipients of transplants mismatched at HLA-A/-B/-DRB1/multiple loci (P<0.0001). In multivariate analyses, HLA compatibility was the variable most significantly associated with survival and treatment-related mortality. We found important differences in survival in recipients of UD-HSCT with best results for transplants from 10/10 matched donors. Single mismatches at HLA-DQ/-DRB3 were well tolerated, mismatches at HLA-C had intermediate results and mismatches at HLA-A/-B/-DRB1/multiple loci resulted in poor survival.

Isolated HLA-C mismatches in unrelated donor transplantation for CML

HLA-incompatibility is a major factor associated with outcome of allogeneic stem cell transplantation, but little is known on the impact of isolated HLA-C mismatches. We analyzed the outcome of 114 CML patients transplanted with marrow from unrelated donors of whom 24 were mismatched for HLA-C only (9/10 match). Univariate estimates of 5-year survival (SRV) (median follow-up: 47 months) in the HLA-matched group were 68+/-12 vs 42+/-20% (P=0.03) for the patients mismatched for HLA-C only and 33+/-33% in the mismatched group (non-HLA-C single mismatches and multiple mismatches) (P=0.0004). Disease stage, GVHD-prophylaxis (T-cell depletion), CMV-status and HLA-incompatibility were the risk factors associated (all P< or =0.005) with poor outcome. In the multivariate analysis, patients mismatched for loci other than HLA-C were at high risk of an adverse outcome (death: RR, 2.9; CI, 1.6-5.4, P=0.008, transplant-related mortality (TRM): RR, 3; CI, 1.5-5.9, P=0.0015). For patients mismatched for HLA-C only, the increased risk was of borderline significance (death: RR, 1.9; CI, 1-3.9, P=0.06, TRM: RR, 2.1; CI, 1-4.5, P=0.07). In spite of their lower expression, HLA-C antigens still represent relevant transplantation barriers that should be considered when searching for an unrelated donor.

Selection of unrelated bone marrow donors by serology, molecular typing and cellular assays

As compared to hematopoietic stem cell transplantation (HSCT) with HLA genotypically identical donors, phenotypically matched unrelated HSCT is associated with lower survival. Serologically undisclosed HLA disparities account for the increased rate of post-transplant complications. With more than 1300 alleles currently identified, high-resolution molecular typing techniques have to be applied to distinguish the extensive degree of allelic polymorphism of the HLA system. Whereas a HLA-ABDR-serologically identical donor can be identified in the International Registry for >90% of the patients, only half of them can benefit of a highly compatible donor if donor selection is based on allele level matching for HLA-A/B/Cw/DRB1/B3/B5/DQB1 loci. During the last 10 years, we identified only approximately 20% of all known HLA alleles in the searches for our mainly Caucasoid patients. Rare alleles (i.e. alleles that represent <1% of a given serotype) do not have a major impact in patient/donor matching. Most of the incompatibilities are clustered in a limited number of serotypes that can be targeted first during the searches. However, due to linkage disequilibrium (e.g. B-Cw or DRB1-DQB1), incompatibilities at a given locus are often associated with disparities at adjacent loci. In vitro cellular assays such as the cytotoxic T-lymphocyte precursor frequency (CTLpf) analysis may contribute in discriminating functionally relevant HLA class I disparities, as well as minor antigen mismatches in case of sensitized donors. When a rare variant or an uncommon association in the patient's HLA haplotype has been found, the tissue typing laboratory may recommend considering a mismatched donor early in the search procedure instead of continuing a search with a low probability of success.

Molecular basis of HLA polymorphism: implications in clinical transplantation

Polymorphism of the human leukocyte antigens (HLA) represents a major barrier to organ and hematopoietic stem cell (HSC) transplantation. The cloning and sequencing of HLA class I and II genes has not only provided a clear picture of the molecular basis of allelic polymorphism, but also allowed the development of a variety of PCR-based DNA typing techniques. Such methods are now progressively replacing serological typing for assessing donor/recipient HLA compatibility in clinical transplantation. The 100 serological HLA-A,B,Cw,DR,DQ,DP specificities now comprise more than 1300 alleles defined at the DNA sequence level. Most of the serotypes are subdivided into numerous allelic subtypes in worldwide populations (up to 50 alleles in some cases), although a limited number of alleles are detected in a given population group. In organ transplantation application of HLA molecular typing allowed to improve typing quality, leading to a more precise matching assessment with better clinical results. Knowledge of the molecular basis of class I gene polymorphisms also led to the development of new matching algorithms such as HLA-Matchmaker, based on immunogenic amino acid triplets localized on antibody-accessible external domains of class I antigens. The most impressive impact of novel DNA typing methods concerns matching for allogeneic HSC transplantation because subtle serologically silent sequence differences between allelic subtypes are efficiently recognized by alloreactive T-cells with potentially serious consequences for graft outcome. High resolution HLA class I and II matching has contributed to improve patients survival after unrelated HSC transplantation, although the relative importance of individual loci remains to be elucidated. Donor matching criteria should take into account parameters such as the time frame allowed by the patient's disease and the probability to identify a well matched donor based on the patient's HLA phenotype.

Bone marrow transplantation with unrelated donors: what is the probability of identifying an HLA-A/B/Cw/DRB1/B3/B5/DQB1-matched donor?

Patients transplanted with marrow from an HLA-ABDR serologically matched unrelated donor suffer from more post-transplant complications than those who are transplanted with marrow from an HLA-identical sibling. This is most likely due to either HLA-ABDR incompatibilities not resolved by standard techniques and/or HLA polymorphisms not tested for by routine tissue typing (HLA-Cw,-DQ). By resolving these incompatibilities by molecular techniques combined with the in vitro cytotoxic T lymphocyte precursor frequency (CTLpf) test, we have shown that a high degree of HLA compatibility is associated with increased patient survival. However, higher requirements for HLA matching decrease the number of available donors. We have estimated the probability of finding an HLA-A/B/Cw/DRB1/DRB3/DRB5/DQB1 compatible donor based on 104 consecutive unrelated bone marrow donor searches initiated between January 1995 and December 1997, with December 1998 as the endpoint. For 96 patients (92.3%), one or more ABDR-identical donors were listed in the Bone Marrow Donor Worldwide Registry (BMDW). After contacting the registries, we obtained at least one (mean, 5.36; range, 1-20; total, 461) blood sample for 86 patients. A highly compatible donor was identified for 33/86 patients (38.4%), after testing an average number of 4.5 donors/patients (range, 1-13). However, by accepting an HLA-DRB3 or -DQB1 or -Cw incompatibility, this number would be as high as 68.6%. Approximately half of the patients (n = 40) for whom a search had been initiated have been transplanted: 22 patients with a perfectly matched donor, 15 patients with an HLA-DRB3 or -DQB1 or -Cw mismatch and three with other mismatches. The average time needed to identify the most compatible donor was 4 months. Extremely long searches seemed to be less useful, because after testing the first seven, a more compatible donor was seldom found. These results show that even when requirements for compatibility are high, the chances of finding a donor remain considerably low.