High-resolution HLA Matching Research Articles

Occurrence and Impact of Minor Histocompatibility Antigens' Disparities on Outcomes of Hematopoietic Stem Cell Transplantation from HLA-Matched Sibling Donors

We have examined the alleles of eleven minor histocompatibility antigens (MiHAs) and investigated the occurrence of immunogenic MiHA disparities in 62 recipients of allogeneic hematopoietic cell transplantation (allo-HCT) with myeloablative conditioning performed between 2000 and 2008 and in their HLA-matched sibling donors. Immunogenic MiHA mismatches were detected in 42 donor-recipient pairs: in 29% MiHA was mismatched in HVG direction, in another 29% in GVH direction; bidirectional MiHA disparity was detected in 10% and no MiHA mismatches in 32%. Patients with GVH-directed HY mismatches had lower both overall survival and disease-free survival at 3 years than patients with compatible HY; also higher incidence of both severe acute GvHD and extensive chronic GVHD was observed in patients with GVH-directed HY mismatch. On contrary, GVH-directed mismatches of autosomally encoded MiHAs had no negative effect on overall survival. Results of our study help to understand why posttransplant courses of allo-HCT from siblings may vary despite the complete high-resolution HLA matching of a donor and a recipient.

Improved survival in matched unrelated donor transplant for childhood ALL since the introduction of high-resolution matching at HLA class I and II

We present the first detailed study analysing OS in BMT for paediatric ALL following the introduction of high-resolution (HR) HLA matching. A total of 356 consecutive paediatric ALL stem cell transplants performed between 1988 and 2007 were reviewed; 80 of them were performed following the introduction of HR HLA class I and class II matching to the transplant programme in 2002. Comparisons of matched unrelated donor (MUD) transplant outcomes before and after this period were made. Matching at the HR level for HLA-A, -B, -C, -DRB1 and -DQB1 (HR-MUD) correlated with a greater than 25% improvement in 2- and 5-year OS in paediatric ALL patients transplanted with MUDs (P=0.009, P=0.005, respectively). Two-year OS for contemporaneous HLA-matched sibling transplants (80.8%) and HR-MUD transplants (78.8%) was equivalent. At 6%, non-relapse mortality (NRM) in MUD transplants since 2002 was significantly reduced compared with previous epochs. Changes in treatment and epoch-dependent improvements in outcome were reviewed for possible confounders to the influence of HR typing using univariate and multivariate analysis.

Graft-Versus-Host Disease (GVHD) After Double-Unit Cord Blood Transplantation (DCBT) Is Associated with Unique Clinical Features Including a Higher Incidence of Grade III-IV Acute GVHD in Children

Abstract 3044While the GVHD incidence after unrelated donor CBT is lower than expected for the degree of human leukocyte antigen (HLA)-mismatch, GVHD can be a serious complication and at our center has been the second most common cause of transplant-related mortality after DCBT. However, relatively little is known about DCBT GVHD manifestations, treatment response, and risk factors. Therefore, we evaluated 108 DCBT recipients (median 37 years, range 0.9–69) transplanted for hematologic malignancies. The majority had acute leukemia and high-risk disease. Patients received either myeloablative (n = 81) or non-myeloablative (n = 27) conditioning and 4–6/6 HLA-matched grafts. GVHD prophylaxis consisted of a calcineurin-inhibitor with mycophenolate mofetil, and no patient received anti-thymocyte globulin (ATG). With a median follow-up of 28 months (range 9–64), the cumulative incidences of day 180 grade II-IV and III-IV acute GVHD (aGVHD) were 52% (95%CI :42–62) and 24% (95%CI :15–32), respectively. The median onset was 40 days (range 14–161); the gut was most commonly affected (43/54, 80%) followed by skin (35/54, 65%). Twenty-five patients with mainly grade II gut aGVHD were treated with budesonide alone, 26 patients with predominantly grade III-IV aGVHD received systemic corticosteroids, and complete or partial treatment response was achieved in over 80% by day 56 of therapy. However, 41 patients had active GVHD after day 100 with the majority (25/41, 61%) having aGVHD (persistent, recurrent or late onset), particularly of the gut. Overlap syndrome and classical chronic GVHD were uncommon. Only 1 patient had oral ulceration, and no patient had moderate or severe ocular or sclerotic skin involvement, joint, or pulmonary GVHD manifestations. Univariate analysis of the association between patient/ graft characteristics and grade III-IV aGVHD showed the only significant factor associated with a higher severe aGVHD incidence was age 0–15 years (Figure). Diagnosis, patient ancestry, cytomegalovirus seropositivity, conditioning intensity, and infused cell doses/kg (total graft and engrafting-unit nucleated cell, CD34+ and CD3+) were not significant. A higher engrafting unit-recipient HLA-match of 8–9/10 was associated with a lower incidence of severe aGVHD, and a better unit-unit HLA-match of 6–10/10 was associated with a higher incidence of severe aGVHD, although these differences were not significant (p = 0.128 and 0.266, respectively). To further investigate these findings multivariate Cox regression analysis was performed (Table). Younger age was independently associated with a higher incidence of severe aGVHD (p = 0.042) whereas better engrafting unit-recipient match at 8–9/10 HLA-alleles was protective (p = 0.053). There was a trend toward better unit-unit HLA-match being associated with a higher incidence of grade III-IV aGVHD, but, surprisingly, total infused TNC/kg had no relationship. The 2-year PFS of 72% (95%CI :51–94) in children was higher than the 56% (95%CI :45–66) in adults despite their greater incidence of severe aGVHD. Nine patients (all adult) have died of GVHD including 5 patients initially treated with systemic corticosteroids and 4 with budesonide. We conclude that aGVHD after DCBT is common in the absence of ATG, predominantly affects the gut, and has a high rate of treatment response. Furthermore, GVHD after day 100 frequently has acute features. While the GVHD incidence does not preclude a high rate of survival, improved prophylaxis and treatment are needed. Notably, in contrast to single-unit CBT and adult hematopoietic stem cell transplantation, children receiving DCBT are at a higher risk for severe disease. A possible approach to reduce aGVHD in pediatric DCBT recipients with adequate CB units doses would be to prioritize high resolution HLA-match. Moreover, our data does not currently support an upper limit of infused TNC/kg in DCBT recipients. Further investigation of the biology underlying these unique observations (including the role of specific cellular subsets) should be a major priority. [Display omitted] [Display omitted] Disclosures:Off Label Use: Mycophenolate Mofetil as GvHD prophylaxis.

Predictive impact of allele-matching and EBMT risk score for outcome after T-cell depleted unrelated donor transplantation in poor-risk acute leukemia and myelodysplasia

Many parameters predict for outcome after unrelated donor (URD) allogeneic hematopoietic stem cell transplantation (alloSCT). High-resolution HLA-matching significantly impacts outcome and also the European Group of Blood and Marrow Transplantation (EBMT) risk score, based on patient age, disease stage, donor type, time from diagnosis to SCT and gender combination, may predict for non-relapse mortality and overall survival (OS). We evaluated the individual and combined effects of allele-matching and the EBMT risk score in 327 patients with poor-risk acute leukemia or myelodysplasia, who received a T-cell depleted URD alloSCT. Matching for HLA-A, -B, -C and -DRB1 alleles (8/8 match) was associated with a 5-year OS of 40% compared with 30% for mismatched (≤7/8) pairs (P=0.02). Patients with EBMT risk scores of 1-2, 3, 4 and 5-7 had 5-year OS estimates of 53, 43, 30 and 20%, respectively (P<0.001). The favorable prognostic impact of an 8/8 donor was most pronounced if the EBMT risk score was low (1-2). Five-year OS was 74±8% vs 39±11% for fully matched patients with a low-risk EBMT score as compared with EBMT low-risk patients with ≤7/8 donors. These data underscore the importance of incorporating both the EBMT risk score and the degree of high-resolution HLA-matching in the risk assessment prior to URD alloSCT.

Scoring HLA Mismatches by HistoCheck Does Not Predict Clinical Outcome in HCT

High resolution HLA matching at HLA-A, B, C and DRB1 has proven critical for optimal outcomes following unrelated donor (URD) hematopoietic stem cell transplantation (HCT). Approximately 30% of URD HCT facilitated through by the National Marrow Donor Program (NMDP) are mismatched (MM) at one or more loci. Currently, no rating system exists to reliably predict which HLA MM URD should be selected for patients who do not have an HLA allele matched donor. In 2002, Elsner and Blasczyk suggested that a rating system, HistoCheck, might be used to identify acceptable MM.

8/8 High-Resolution HLA Match Rate: The Impact of Race

8/8 High-Resolution HLA Match Rate: The Impact of Race

HLA-C Antigen Mismatch Is Associated with Worse Outcome in Unrelated Donor Peripheral Blood Stem Cell Transplantation

The association between HLA matching and outcome in unrelated-donor peripheral blood stem cell (PBSC) transplantation has not yet been established. In the present study, a total of 1933 unrelated donor-recipient pairs who underwent PBSC transplantation between 1999 and 2006 for acute myelogenous leukemia, acute lymphoblastic leukemia, myelodysplastic syndrome, or chronic myelogenous leukemia and received high-resolution HLA typing for HLA-A, -B, -C, -DRB1, -DQA1, and -DQB1 were included in the analysis. Outcomes were compared between HLA-matched and HLA-mismatched pairs, adjusting for patient and transplant characteristics. Matching for HLA-A, -B, -C, and -DRB1 alleles (8/8 match) was associated with better survival at 1 year compared with 7/8 HLA-matched pairs (56% vs 47%). Using 8/8 HLA-matched patients as the baseline (n = 1243), HLA-C antigen mismatches (n = 189) were statistically significantly associated with lower leukemia-free survival (relative risk [RR], 1.36; 95% confidence interval [CI], 1.13-1.64; P = .0010), and increased risk for mortality (RR, 1.41; 95% CI, 1.16-1.70; P = .0005), treatment-related mortality (RR, 1.61; 95% CI, 1.25-2.08; P = .0002), and grade III-IV graft-versus-host disease (RR, 1.98; 95% CI, 1.50-2.62; P < .0001). HLA-B antigen or allele mismatching was associated with an increased risk for acute GVHD grade III-IV. No statistically significant differences in outcome were observed for HLA-C allele (n = 61), HLA-A antigen/allele (n = 136), HLA-DRB1 allele (n = 39), or HLA-DQ antigen/allele (n = 114) mismatches compared with 8/8 HLA-matched pairs. HLA mismatch was not associated with relapse or chronic GVHD. HLA-C antigen-mismatched unrelated PBSC donors were associated with worse outcomes compared with 8/8 HLA-matched donors. The study's limited power due to small sample size precludesconclusions about other mismatches.

108-P: 8/8 High-Resolution (HR) HLA Match Rate: Caucasian (CAU) and African American (AFA) Patients

108-P: 8/8 High-Resolution (HR) HLA Match Rate: Caucasian (CAU) and African American (AFA) Patients

The impact of HLA genotyping on survival following unrelated donor haematopoietic stem cell transplantation

One of the major factors that have contributed to improving the outcomes of Stem Cell Transplantation is progress made in the field of human leucocyte antigen(s) (HLA). This is evident not only in developing techniques for rapid and accurate tissue typing, but also in the greatly improved understanding of the HLA system and the impact of HLA matching on transplant complications. It is now accepted that high-resolution HLA matching for transplant recipients and unrelated donors is associated with the best clinical outcomes. The most important HLA determinants are the six 'classical' polymorphic HLA loci: HLA-A, -B, -C, -DRB1,-DQB1, -DPB1. For several years, based on the outcome of numerous studies, a 10/10 matched donor (HLA-A, -B, -C, -DRB1, -DQB1) was considered the ideal. The impact of HLA-DPB1 has been less clear, in view of reduced likelihood of patient/donor matching for this locus. More recently, several large studies have questioned the importance of HLA-DQB1 matching on outcome. Based on the findings of recent studies, the current gold standard unrelated donor is believed to be one matched for 8/8 alleles at high resolution i.e. matched for HLA-A, -B, -C, -DRB1, however, in certain circumstances, mismatches may be tolerated and/or permissive.

Poor-Risk Acute Leukemia Patients with An EBMT Low-Risk Score and An 8/8 Matched Unrelated Donor Show Excellent Survival After Hematopoietic Stem Cell Transplantation.

Abstract 1196Poster Board I-218 Introduction:Allogeneic hematopoietic stem cell transplantation (alloSCT) from volunteer unrelated donors (URD) may be associated with a higher non-relapse mortality (NRM) and worse outcome as compared to alloSCT using HLA-identical sibling donors. However, many parameters next to donor type define NRM. The impact on outcome of allele-matching for HLA-A, -B, -C and -DRB1 between donor and recipient has clearly been demonstrated. The prognostic impact of the EBMT risk score, that takes into account age, stage of disease, time from diagnosis to transplantation, donor type and donor-recipient gender combination, has recently been validated in a variety of hematological malignancies including acute leukemia and myelodysplastic syndrome (MDS). We evaluated the relative prognostic value of high-resolution HLA matching and the EBMT risk score for patients with poor-risk acute leukemia and MDS who received an URD transplant. Patients and methods:Between 1987 and 2006, 327 patients (≥16y) with poor-risk acute leukemia and MDS underwent URD alloSCT in the Netherlands. Patients were in 1st complete remission (CR1, n=129), 2nd CR (CR2, n=91), beyond CR2 or not in remission (n=107). The leukemia-risk was considered to be poor if patients had adverse cytogenetics or were not in CR1. The majority of the grafts was T-cell depleted (94%). High-resolution typing of HLA-A, -B, -C, and -DRB1 alleles was available for analysis in 270 donor-recipient pairs and had in part been performed retrospectively. Results:We evaluated the impact of high-resolution matching for HLA-A, -B, -C and -DRB1 on progression free survival (PFS) and overall survival (OS). Patients who were fully matched (8/8) with their donors (n=170) hadsignificantly superior PFS (40+/-4% vs 26+/-5%, hazard ratio (HR)=0.68; 95%CI 0.50–0.92, p=0.01) and OS (39+/-4% vs 29+/-5%, HR=0.70; 95%CI 0.51-0.96, p=0.03), compared to patients with mismatched (≤7/8) donors (n=100). Superior OS in the 8/8 group appeared to be due to a lower NRM (24+/-4% vs 39+/-5%, HR=0.54; 95%CI 0.35-0.85, p=0.008), while the relapse mortality rate was identical in both groups (37+/-4% vs 32+/-5%). Patients with EBMT risk scores of 1-2 (n=71), 3 (n=77), 4 (n=76) and 5-7 (n=103) had a predicted 5 year OS of 52%, 41% (HR=1.57; 95%CI 0.98-2.52), 29% (HR=2.07; 95%CI 1.32-3.26) and 19% (HR=2.69; 95%CI 1.76-4.11), respectively (p<0.001). Relapse mortality rate and NRM increased with increasing EBMT risk score. As shown in the table, the impact of allele-matching on OS was most evident in the EBMT low-risk group. EBMT low-risk (1-2) patients with 8/8 donors showed excellent 5 year OS compared to EBMT low-risk patients with ≤7/8 donors (73+/-8% vs 35+/-12%). The favorable impact of a fully matched donor was absent in patients with higher EBMT risk scores. Conclusions:Both the EBMT risk score and the degree of allele-matching independently predicted outcome after URD alloSCT. The predictive value of allele-matching was especially evident in EBMT low-risk patients, while patients with the highest EBMT risk scores (>4) had a dismal outcome, despite allele-matching. These results emphasize the importance of incorporating age, disease stage, donor-recipient gender combination and time interval from diagnosis to transplantation (EBMT risk score parameters) as well as high-resolution HLA-typing in the risk assessment prior to URD alloSCT. As excellent OS was noted in well matched EBMT low-risk patients, our data underscore the importance of an immediate search for an unrelated donor in poor-risk leukemia patients in CR1 below the age of 40, who should then receive their alloSCT as early consolidation therapy following induction chemotherapy.5-year overall survival (OS) rates with 95% CI, and HR with 95% CI by EBMT risk group and matchingEBMT risk score8/8 donors≤ 7/8 donorsN5-year OS [%] (95% CI)N5-year OS [%] (95% CI)HR* (95% CI)0.72)1-23873 (54-85)2035 (14-58)0.30 (0.13-0.72)34545 (28-61)2534 (16-53)0.68 (0.35-1.30)43731 (16-48)2234 (14-54)0.86 (0.44-1.68)5-7509 (2-24)3318 (7-34)1.02 (0.61-1.69)*HR of 8/8 donors (n=170) compared to ≤ 7/8 donors (n=100) Disclosures:No relevant conflicts of interest to declare.

21-P: High-resolution donor-recipient HLA matching and mismatching frequencies effect on choosing donor in unrelated Allo-HSCT

21-P: High-resolution donor-recipient HLA matching and mismatching frequencies effect on choosing donor in unrelated Allo-HSCT

An Approach to Predicting Hematopoietic Stem Cell Transplantation Outcome Using HLA-Mismatch Information Mapped on Protein Structure Data

In hematopoietic stem cell transplantation (HSCT), the outcome is predicted using HLA-matching procedures, which are very time-consuming. There exists substantial evidence of the importance of early donor acceptance in HSCT outcome. In cases when the donor cannot be perfectly matched, it often is unclear which mismatch is less harmful and thus has a greater likelihood of acceptance. We modeled and analyzed interactions between the protein products of different HLA alleles of the transplant recipient and natural killer and T lymphocyte cell receptors of the donor's immune system. Reactions between these 2 systems often lead to graft-versus-host disease (GVHD). Sequence polymorphisms that define HLA I and II alleles predict not only GVHD, but also host-versus-graft and graft-versus-leukemia effects, all of which influence the overall transplantation outcome. Although complete high-resolution HLA matching of the donor-recipient pair seems to be associated with optimal post-HSCT survival, recent reports suggest that not every HLA disparity is functionally relevant. We performed interaction energy calculations for selected pairs of donor-recipient HLA alleles. Based on the results, we conclude that the energy of contact between the T lymphocyte cell receptor (TCR) and HLA residues can help predict the future development of an immune reaction and, consequently, the outcome of allogeneic HSCT.

High‐resolution HLA matching in double‐umbilical‐cord‐blood reduced‐intensity transplantation in adults

Double-cord-blood transplantation (DCBT) offers an option for patients receiving reduced-intensity transplants. These unique transplants have two donors, both of whom are usually HLA mismatched at one to two loci. Fifty-three patients were recipients of a reduced-intensity DCBT. Cords were at least 4/6 allele-level HLA-A, -B, and -DR match with the patient and each other with a minimum combined cell dose of more than 3.7 x 10(7) total nucleated cells per kg. Twenty-one patients received cyclosporine/mycophenolate mofetil and 32 patients received sirolimus/tacrolimus (SIR/TAC) for graft-versus-host disease prophylaxis. The effect of allele level HLA typing on clinical endpoints of overall survival (OS), disease-free survival (DFS), engraftment, and acute graft-versus-host disease (aGVHD) were assessed. Neutrophil (p = 0.001) engraftment and platelet engraftment (p = 0.027) were significantly faster in patients who have closer Class I (HLA-A, -B, -C) matching. Neutrophil engraftment was faster in patients who had closer HLA-B matching to their combined cords (p = 0.007). There was a low incidence of aGVHD overall, especially in the SIR/TAC group. Class I HLA matching had no effect on aGVHD. HLA-DR and -DQ had no effect on engraftment or aGVHD. Class I allele matching, and HLA-B matching specifically, were associated with faster neutrophil engraftment. High-resolution HLA matching did not affect OS or DFS.

Unrelated stem cell transplantation for severe acquired aplastic anemia: improved outcome in the era of high-resolution HLA matching between donor and recipient

Severe acquired aplastic anemia (SAA) is a potentially fatal bone marrow failure syndrome occurring mainly in children and young adults. Immunosuppressive regimens and hematopoietic stem cell transplantation (HSCT) are the only two available curative treatments. Patients who lack an HLA-identical sibling donor may receive HSCT from an unrelated donor, a strategy historically associated with high mortality rates. Thus, for patients refractory to immunosuppressive regimens, the decision to transplant stem cells from unrelated donors is weighed against supportive care and often represents a dilemma for physicians. We aimed to determine whether outcome after unrelated HSCT has improved in recent years and, if so, to determine the factors responsible for the improvement. We analyzed the outcome of 89 patients (median age 17 years, range 0-52) with acquired SAA undergoing HSCT from an unrelated donor between 1989 and 2004. Cases were consecutively reported to the French Registry (SFGM-TC) by 25 centers. Patients transplanted during two successive time-periods (1989-1998 and 1999-2004) had different 5-year survival probabilities (+/-95% confidence interval): 29%+/-7% and 50%+/-7%, respectively (p<0.01). The main difference between the two cohorts concerned HLA matching between donors and recipients at the allelic level for the ten HLA-A, -B, -C, -DRB1 and -DQB1 antigens, which was more frequent in 1999-2004 than in the former period (p=0.0004). In multivariate analysis, the only two factors affecting survival were HLA allelic matching (p<0.01) and younger age of recipient (17 pounds sterling years, p<0.0001). Survival reached 78%+/-11% at 5 years for the younger, fully HLA-matched patients. Survival after unrelated HSCT for SAA has improved significantly over the past 15 years, mainly due to better HLA matching. Results for young patients who are fully HLA-matched at the allelic level with their donor are comparable to those observed after HSCT from a related donor.

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.

Impact of high-resolution HLA matching on outcomes of unrelated donor hematopoietic stem cell transplantation

Impact of high-resolution HLA matching on outcomes of unrelated donor hematopoietic stem cell transplantation

Genomic Polymorphism and Allogeneic Hematopoietic Transplantation Outcome

Polymorphisms in genes that encode antigen-presenting molecules, antigen receptors, and immune mediators are crucial determinants of the risk of complications after allogeneic hematopoietic cell transplantation (allo-HCT). Matching for HLA alleles remains the cornerstone of donor selection, and recent studies are refining our understanding and use of HLA typing in allo-HCT. High-resolution allelic HLA matching generally improves transplant outcome but may limit the donor pool and delay transplantation. Allelic mismatches may be permissible in certain circumstances without compromising outcome. There is growing interest in the role of natural killer (NK) cell–mediated immunity in allo-HCT. NK cells express an array of activating and inhibitory killer cell immunoglobulin-like receptors (KIR), and NK cell activation is negatively regulated by KIR interaction with HLA class I molecules. In haploidentical transplants, NK cell alloreactivity in the graft-versus-host direction can be predicted by the HLA class I and KIR genotypes of donor and recipient and has been associated with potent graft-versus-leukemic effects and low rates of graft-versus-host disease. KIR genotype and expression may influence transplantation outcomes in both HLA-matched and -mismatched transplants. Graft-versus-host disease and major infection remain problematic despite HLA matching, and there is mounting evidence that polymorphisms in non-HLA immune mediators and host defense genes influence the risk of these complications. The importance of non-HLA genomics in nonmyeloablative transplants is poorly understood and is under investigation. These findings suggest that tissue typing for allo-HCT is entering an exciting era in which both HLA and non-HLA genomic data may be used in a more sophisticated fashion to select donors, stratify risk, identify novel therapeutic targets, and ultimately improve outcome for allo-HCT recipients.

P441 - High resolution HLA matching associated with decreased mortality after unrelated bone marrow transplantation

As compared with related HLA-identical sibling donors, bone marrow transplantation (BMT) with phenotypically HLA ABDR-compatible unrelated donors is associated with increased mortalii. This may be due to hidden HLA incompatibiliiies not detected by conventional typing. We have analyzed 44 unrelated patient-donor pairs who were matched for HLA-A, -B, and -DR by routine tissue typing. Our comprehensive HLA typing approach consisted of serology, cytotoxic T-cell precursor (CTLp) tests, T-cell cloning, oligotyping, and DNA sequencing. Using these techniques, we identified numerous HLA allele mismatches not detected by the previously applied routine typing. Twenty-four patientdonor pairs were highly matched and had a low CTLp frequency, whereas the remaining 20 pairs were allele-mis-