Predictions in the face of clinical reality: HistoCheck versus high-risk HLA allele mismatch combinations responsible for severe acute graft-versus-host disease

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.

Exploratory Study of Predicted Indirectly ReCognizable HLA Epitopes in Mismatched Hematopoietic Cell Transplantations

HLA-mismatches in hematopoietic stem-cell transplantation are associated with an impaired overall survival (OS). The aim of this study is to explore whether the Predicted Indirectly ReCognizable HLA-Epitopes (PIRCHE) algorithm can be used to identify HLA-mismatches that are related to an impaired transplant outcome. PIRCHE are computationally predicted peptides derived from the patient's mismatched-HLA molecules that can be presented by donor-patient shared HLA. We retrospectively scored PIRCHE numbers either presented on HLA class-I (PIRCHE-I) or class-II (PIRCHE-II) for a Dutch multicenter cohort of 103 patients who received a single HLA-mismatched (9/10) unrelated donor transplant in an early phase of their disease. These patients were divided into low and high PIRCHE-I and PIRCHE-II groups, based on their PIRCHE scores, and compared using multivariate statistical analysis methods. The high PIRCHE-II group had a significantly impaired OS compared to the low PIRCHE-II group and the 10/10 reference group (HR: 1.86, 95%-CI: 1.02–3.40; and HR: 2.65, 95%-CI: 1.53–4.60, respectively). Overall, PIRCHE-II seem to have a more prominent effect on OS than PIRCHE-I. This impaired OS is probably due to an increased risk for severe acute graft-vs.-host disease. These data suggest that high PIRCHE-II scores may be used to identify non-permissible HLA mismatches within single HLA-mismatched hematopoietic stem-cell transplantations.

In silico prediction of nonpermissive HLA-DPB1 mismatches in unrelated HCT by functional distance

In silico prediction of high-risk donor-recipient HLA mismatches after unrelated donor (UD) hematopoietic cell transplantation (HCT) is an attractive, yet elusive, objective. Nonpermissive T-cell epitope (TCE) group mismatches were defined by alloreactive T-cell cross-reactivity for 52/80 HLA-DPB1 alleles (TCE-X). More recently, a numerical functional distance (FD) scoring system for in silico prediction of TCE groups based on the median impact of exon 2-encoded amino acid polymorphism on T-cell alloreactivity was developed for all DPB1 alleles (TCE-FD), including the 28/80 common alleles not assigned by TCE-X. We compared clinical outcome associations of nonpermissive DPB1 mismatches defined by TCE-X or TCE-FD in 8/8 HLA-matched UD-HCT for acute leukemia, myelodysplastic syndrome, and chronic myelogenous leukemia between 1999 and 2011 (N = 2730). Concordance between the 2 models was 92.3%, with most differences arising from DPB1*06:01 and DPB1*19:01 being differently assigned by TCE-X and TCE-FD. In both models, nonpermissive mismatches were associated with reduced overall survival (hazard ratio [HR], 1.15, P < .006 and HR, 1.12, P < .03), increased transplant-related mortality (HR, 1.31, P < .001 and HR, 1.26, P < .001) as well as acute (HR, 1.16, P < .02 and HR, 1.22, P < .001) and chronic (HR, 1.20, P < .003 and HR, 1.22, P < .001) graft-versus-host disease (GVHD). We show that in silico prediction of nonpermissive DPB1 mismatches significantly associated with major transplant outcomes is feasible for any DPB1 allele with known exon 2 sequence based on experimentally elaborated FD scores. This proof-of-principle observation opens new avenues for developing HLA risk-prediction models in HCT and has practical implications for UD searches.

Evaluation of HistoCheck as a Predictor of Clinical Outcomes after Haploidentical Hematopoietic Stem Cell Transplantation

Haploidentical hematopoietic stem cell transplantation (haplo-HSCT) is available for nearly all patients without matching HLA-related or -unrelated donors. There was not a valid evaluation system to predict the most proper donor. HistoCheck was based on the functional similarity of amino acids to estimate the allogenicity of HLA mismatches with a sequence similarity matching (SSM) score. We investigated whether HistoCheck could predict clinical outcomes in 500 patients with acute leukemia or myelodysplastic syndrome receiving haplo-HSCT. The total SSM score of the 5 loci (HLA-A, -B, -C, -DRB1, and -DQB1) had no association with clinical outcomes. HLA-C SSM score was significantly associated with transplant-related mortality (TRM) (hazard ratio [HR], .in691; 95% confidence interval [CI], .520 to .917; P = .011), disease-free survival (DFS) (HR, .714; 95% CI, .586 to .869; P = .001), and overall survival (OS) (HR, .711; 95% CI, .574 to .881; P = .002) by multivariate analysis. No significant associations were observed between other single-locus SSM score and clinical outcomes. In summary, our data demonstrate that a high HLA-C HistoCheck SSM score may lead to lower TRM and improved DFS and OS after haplo-HSCT and inclusion of HLA-C HistoCheck in donor selection criteria may need to be further confirmed in prospective studies.

Predicting alloreactivity in transplantation

Human leukocyte Antigen (HLA) mismatching leads to severe complications after solid-organ transplantation and hematopoietic stem-cell transplantation. The alloreactive responses underlying the posttransplantation complications include both direct recognition of allogeneic HLA by HLA-specific alloantibodies and T cells and indirect T-cell recognition. However, the immunogenicity of HLA mismatches is highly variable; some HLA mismatches lead to severe clinical B-cell- and T-cell-mediated alloreactivity, whereas others are well tolerated. Definition of the permissibility of HLA mismatches prior to transplantation allows selection of donor-recipient combinations that will have a reduced chance to develop deleterious host-versus-graft responses after solid-organ transplantation and graft-versus-host responses after hematopoietic stem-cell transplantation. Therefore, several methods have been developed to predict permissible HLA-mismatch combinations. In this review we aim to give a comprehensive overview about the current knowledge regarding HLA-directed alloreactivity and several developed in vitro and in silico tools that aim to predict direct and indirect alloreactivity.

Identification of a permissible HLA mismatch in hematopoietic stem cell transplantation

In subjects mismatched in the HLA alleles C*03:03/C*03:04 no allogeneic cytotoxic T-lymphocyte responses are detected in vitro. Hematopoietic stem cell transplantation (HSCT) with unrelated donors (UDs) showed no association between the HLA-C allele mismatches (CAMMs) and adverse outcomes; antigen mismatches at this and mismatches other HLA loci are deleterious. The absence of effect of the CAMM may have resulted from the predominance of the mismatch C*03:03/C*03:04. Patients with hematologic malignancies receiving UD HSCT matched in 8/8 and 7/8 HLA alleles were examined. Transplants mismatched in HLA-C antigens or mismatched in HLA-A, -B, or -DRB1 presented significant differences (P < .0001) in mortality (hazard ratio [HR] = 1.37, 1.30), disease-free survival (HR = 1.33, 1.27), treatment-related mortality (HR = 1.54, 1.54), and grade 3-4 acute graft-versus-host disease (HR = 1.49, 1.77) compared with the 8/8 group; transplants mismatched in other CAMMs had similar outcomes with HR ranging from 1.34 to 172 for these endpoints. The C*03:03/C*03:04 mismatched and the 8/8 matched groups had identical outcomes (HR ranging from 0.96-1.05). The previous finding that CAMMs do not associate with adverse outcomes is explained by the predominance (69%) of the mismatch C*03:03/03:04 in this group that is better tolerated than other HLA mismatches.

A perspective on the selection of unrelated donors and cord blood units for transplantation

Selection of a suitable graft for allogeneic hematopoietic stem cell transplantation involves consideration of both donor and recipient characteristics. Of primary importance is sufficient donor-recipient HLA matching to ensure engraftment and acceptable rates of GVHD. In this Perspective, the National Marrow Donor Program and the Center for International Blood and Marrow Transplant Research provide guidelines, based on large studies correlating graft characteristics with clinical transplantation outcomes, on appropriate typing strategies and matching criteria for unrelated adult donor and cord blood graft selection.

Scoring HLA class I mismatches by HistoCheck does not predict clinical outcome in unrelated hematopoietic stem cell transplantation

Currently, there is no well-accepted rating system for reliably predicting which HLA-mismatched (MM) unrelated donor should be selected for a patient without an HLA allele-matched donor. We evaluated the ability of an MM ranking system, HistoCheck, to predict the risk associated with HLA class I disparity in a population of 744 single allele or antigen HLA-A, -B, or -C MM myeloablative unrelated donor hematopoietic stem cell transplantation recipients with acute myelogenous leukemia, acute lymphoblastic leukemia, chronic myelogenous leukemia, or myelodysplastic syndrome, facilitated through the National Marrow Donor Program between 1988 and 2003. Multivariate models were used to adjust for other significant clinical risk factors. HLA MMs were scored using the HistoCheck Web-based tool, and the patients were divided into 4 quartiles: dissimilarity score (DSS) 1.04-2.84 (allele MM), DSS >2.84-13.75 (allele and antigen MM), DSS >13.75-19.39 (antigen MM), and DSS >19.39-36.62 (antigen MM). Using the lowest scoring quartile as the reference, the DSS groups were evaluated for associations with relapse, treatment-related mortality, acute and chronic graft-versus-host disease, leukemia-free survival, and overall survival in the entire cohort and also in subset analyses by disease and disease stage. No significant associations were found between DSS and any outcomes in the overall cohort using the quartile categories or treating DSS as a continuous variable. Higher DSS scores were associated with decreased engraftment in early-stage disease (P = .0003), but not in other disease stages. In summary, DSS does not correlate with transplantation outcomes, and the HistoCheck scoring system does not provide an effective technique for ranking HLA class I MM. The dataset used in this study is available to evaluate new algorithms proposed for donor selection.