Graft rejection after unrelated donor hematopoietic stem cell transplantation for thalassemia is associated with nonpermissive HLA-DPB1 disparity in host-versus-graft direction

Dual α-globin and truncated EPO receptor knockin restores hemoglobin production in α-thalassemia-derived red blood cells

Alpha-thalassemia is an autosomal recessive disease with increasing worldwide prevalence. The molecular basis is due to mutation or deletion of one or more duplicated α-globin genes, and disease severity is directly related to the number of allelic copies compromised. The most severe form, α-thalassemia major (αTM), results from loss of all four copies of α-globin and has historically resulted in fatality in utero. However, in utero transfusions now enable survival to birth. Postnatally, patients face challenges similar to β-thalassemia, including severe anemia and erythrotoxicity due to imbalance of β-globin and α-globin chains. While curative, hematopoietic stem cell transplantation (HSCT) is limited by donor availability and potential transplant-related complications. Despite progress in genome editing treatments for β-thalassemia, there is no analogous curative option for patients suffering from α-thalassemia. To address this, we designed a novel Cas9/AAV6-mediated genome editing strategy that integrates a functional α-globin gene into the β-globin locus in αTM patient-derived hematopoietic stem and progenitor cells (HSPCs). Incorporation of a truncated erythropoietin receptor transgene into the α-globin integration cassette dramatically increased erythropoietic output from edited HSPCs and led to the most robust production of α-globin, and consequently normal hemoglobin. By directing edited HSPCs toward increased production of clinically relevant RBCs instead of other divergent cell types, this approach has the potential to mitigate the limitations of traditional HSCT for the hemoglobinopathies, including low genome editing and low engraftment rates. These findings support development of a definitive ex vivo autologous genome editing strategy that may be curative for α-thalassemia.

Hematopoietic Stem Cell Transplantation in Thalassemia

Allogeneic hematopoietic stem cell transplantation (allo-HSCT) is the only consolidated, potentially curative treatment for patients with transfusion-dependent thalassemia major. In the past few decades, several new approaches have reduced the toxicity of conditioning regimens and decreased the incidence of graft-versus-host disease, improving patients' outcomes and quality of life. In addition, the progressive availability of alternative stem cell sources from unrelated or haploidentical donors or umbilical cord blood has made HSCT a feasible option for an increasing number of subjects lacking an human leukocyte antigen (HLA)-identical sibling. This review provides an overview of allogeneic hematopoietic stem cell transplantation in thalassemia, reassesses current clinical results, and discusses future perspectives.

Graft failure in allogeneic hematopoietic stem cell recipients: diagnosis and treatment

Graft failure is a group of complications after allogeneic hematopoietic stem cell transplantation, which occurs according to different data up to 30%. The group of complications includes primary and secondary graft failure, primary, secondary and transient poor graft function and graft rejection. Diagnostic difficulties consist in the lack of unified diagnostic criteria accepted in the transplantation community and in the dual interpretation of these complications according to the foreign literature. The purpose of this literature review was to identify the most common criteria of different types of graft failure and determine the tactics of diagnosis and treatment. In this review we analyzed data from various literature sources, gave definitions of graft failure and poor graft function. We analyzed the literature data on the methods used to treat these conditions.

Promiscuity of Peptides Presented in HLA-DP Molecules from Different Immunogenicity Groups Is Associated With T-Cell Cross-Reactivity

In the context of HLA-DP-mismatched allogeneic stem cell transplantation, mismatched HLA-DP alleles can provoke profound allo-HLA-DP-specific immune responses from the donor T-cell repertoire leading to graft-versus-leukemia effect and/or graft-versus-host disease in the patient. The magnitude of allo-HLA-DP-specific immune responses has been shown to depend on the specific HLA-DP disparity between donor and patient and the immunogenicity of the mismatched HLA-DP allele(s). HLA-DP peptidome clustering (DPC) was developed to classify the HLA-DP molecules based on similarities and differences in their peptide-binding motifs. To investigate a possible categorization of HLA-DP molecules based on overlap of presented peptides, we identified and compared the peptidomes of the thirteen most frequently expressed HLA-DP molecules. Our categorization based on shared peptides was in line with the DPC classification. We found that the HLA-DP molecules within the previously defined groups DPC-1 or DPC-3 shared the largest numbers of presented peptides. However, the HLA-DP molecules in DPC-2 segregated into two subgroups based on the overlap in presented peptides. Besides overlap in presented peptides within the DPC groups, a substantial number of peptides was also found to be shared between HLA-DP molecules from different DPC groups, especially for groups DPC-1 and -2. The functional relevance of these findings was illustrated by demonstration of cross-reactivity of allo-HLA-DP-reactive T-cell clones not only against HLA-DP molecules within one DPC group, but also across different DPC groups. The promiscuity of peptides presented in various HLA-DP molecules and the cross-reactivity against different HLA-DP molecules demonstrate that these molecules cannot be strictly categorized in immunogenicity groups.

Conditioning Regimens in Patients with β-Thalassemia Who Underwent Hematopoietic Stem Cell Transplantation: A Scoping Review

The success of transplant procedures in patients with beta-thalassemia major (β-thalassemia) goes hand-in-hand with improvements in disease knowledge, better supportive care, discoveries in immunogenetics, increase in stem cell sources, and enhancement of conditioning regimens. The aim of this scoping review was to report the evolution of conditioning regimes for β-thalassemia hematopoietic stem cell transplantation. We performed a systematic search for all relevant articles published before July 2021, using the following Medical Subject Headings: "bone marrow transplantation", "stem cell transplantation", "allogeneic", "thalassemia", "β-thalassemia", and "thalassemia major". The final analysis included 52 studies, published between 1988 and 2021, out of 3877 records. The most common conditioning regimen was a combination of busulfan and cyclophosphamide, with successive dose adjustments or remodulation based on patient characteristics. Pre-transplant treatments, reductions in cyclophosphamide dosage, or the adoption of novel agents such as treosulphan all improved overall survival and thalassemia-free survival in transplant-related mortality high-risk patients. Conditioning regimes were modulated for those without a suitable fully matched sibling or unrelated donor, with encouraging results. Hematopoietic stem cell transplantation with haploidentical donors is currently available to virtually all patients with β-thalassemia. However, disparities in outcome are still present around the world. In developing and limited-resource countries, where most diagnoses are focused, transplants are not always available. Therefore, more efforts are needed to close this treatment gap.

Tools for optimizing risk assessment in hematopoietic cell transplant - What can we get away with?

Unrelated allogeneic hematopoietic cell transplant (HCT) is a critical modality to treat hematologic malignancies. The current objective of donor selection is to match donor and recipient at the HLA (human leukocyte antigen) peptide-binding region which should lower the risk of graft-versus-host disease. However, depending on the patient's ethnicity/race, finding a matched donor is challenging, especially for HLA-DPB1 which is due to the weak linkage disequilibrium between HLA-DPB1 and the other HLA class II loci. Recent evidence, on the molecular level, has shown that certain HLA mismatches carry lower clinical risk. More specifically, there is an increasing understanding of polymorphisms of the innate and adaptive immune systems and their impact on transplant outcomes, allowing us to expand our "toolkit" for optimization of donor selection in HCT. Therefore, in this review we discuss matching strategies based on comparing donor and recipient polymorphisms that may influence innate and adaptive immune response genes in allorecognition and the role of single nucleotide polymorphisms in non-HLA genes that have the potential for providing additional tools to refine risk stratification.

Curative therapy for hemoglobinopathies: an International Society for Cell & Gene Therapy Stem Cell Engineering Committee review comparing outcomes, accessibility and cost of ex vivo stem cell gene therapy versus allogeneic hematopoietic stem cell transplantation

Thalassemia and sickle cell disease (SCD) are the most common monogenic diseases in the world and represent a growing global health burden. Management is limited by a paucity of disease-modifying therapies; however, allogeneic hematopoietic stem cell transplantation (HSCT) and autologous HSCT after genetic modification offer patients a curative option. Allogeneic HSCT is limited by donor selection, morbidity and mortality from transplant conditioning, graft-versus-host disease and graft rejection, whereas significant concerns regarding long-term safety, efficacy and cost limit the broad applicability of gene therapy. Here the authors review current outcomes in allogeneic and autologous HSCT for transfusion-dependent thalassemia and SCD and provide our perspective on issues surrounding accessibility and costs as barriers to offering curative therapy to patients with hereditary hemoglobinopathies.

Repeated CD45RA-depleted DLI successfully increases donor chimerism in a patient with beta-thalassemia major after haploidentical stem cell transplant

Allogeneic hematopoietic stem cell transplantation is curative for transfusion-dependent thalassemia, but mixed chimerism (MC) may herald graft rejection. We report a child who failed bone marrow transplant (BMT) from matched unrelated donor (MUD) successfully salvaged with haploidentical peripheral blood stem cell transplant (PBSCT), but had MC in T-lymphocyte compartment despite near-complete donor chimerism in myeloid compartment. MC was successfully improved by repeated CD45RA-depleted donor lymphocyte infusion (DLI). A 2-year-old Chinese girl with beta-thalassemia major underwent 12/12-MUD BMT with HU/AZA/Cy/Flu/Bu/TT conditioning resulted in graft rejection. As donor refused second donation, rescue haploidentical PBSCT was performed with alemtuzumab/fludarabine/treosulfan conditioning. Harvest product was CD3/CD45RA depleted with extra products cryopreserved. Split cell chimerism performed 1-month after haplo-transplant showed 97% mother, 3% MUD, and 0% host for granulocytes but 38% mother, 62% MUD, and 0% host for CD3 + T cells. In view of low haploidentical donor chimerism in T-lymphocyte compartment, CD45RA-depleted DLI using cryopreserved product was performed on day + 38, after thymoglobulin 3 mg/kg given as T-cell depletion 3 days beforehand. T-cell chimerism improved to 51% mother and 49% MUD post-DLI. Second cryopreserved CD45RA-depleted DLI was given 17 days after the first DLI (day + 55), and 100% full chimerism of mother's T cells was gradually established without significant graft-versus-host disease (GVHD) or viral reactivation. To conclude, split lineage chimerism determination is beneficial to guide management strategy. For MC in T-cell compartment, CD45RA-depleted DLI is a potential alternative to unselected T cells as it carries lower risk of GVHD and infection.

Use of DPB1 T-cell epitope algorithm among italian transplant centers: A survey on behalf of Associazione Italiana di Immunogenetica e Biologia dei Trapianti

The HLA-DPB1 locus has been demonstrated to have a significant role on patients' outcome after allogeneic HSCT, and the so-called T-cell epitope (TCE) algorithm has been incorporated in international guidelines for the selection of unrelated donors. The purpose of the present study is to measure, through a national survey conducted on behalf of the Associazione Italiana di Immunogenetica e Biologia dei Trapianti (AIBT), the extent of awareness and use of HLA-DPB1 TCE-based algorithms during the donor search. 89% of the HLA laboratories answered to a short questionnaire and the results showed a progressive increase of the laboratories typing DPB1 in patients and their potential donors during the search (from 44% to 79% during the 2010-2019 period) as well as the application of a TCE-based algorithm for the donor choice whenever possible (from 24% to 65% during the same period). The DP-permissiveness status is detailed in the official HLA typing report by 12%, 32% and 50% of laboratories in 2010, 2015 and 2019, respectively. The present data indicate an encouraging raise in the awareness of the HLA-DPB1 role in unrelated donor selection; noteworthy, mentioning the TCE-based permissiveness status in the HLA typing report of each potential unrelated donor represents a notable mean to raise awareness among transplant physicians and to support them in their task of choosing the best donor. Nonetheless, despite the compelling evidence of the predictive ability of TCE-based algorithms, further efforts are still needed to extend its application to all transplant centers in Italy.

Selecting β-thalassemia Patients for Gene Therapy: A Decision-making Algorithm

This expert opinion originally developed by a panel of the Italian Society of Thalassemias and Hemoglobinopathies (SITE), reviewed and adopted by the European Hematology Association (EHA) through the EHA Scientific Working Group on Red Cells and Iron, has been developed as priority decision-making algorithm on evidence and consensus with the aim to identify which patients with transfusion-dependent beta-thalassemia (TDT) could benefit from a gene therapy (GT) approach. Even if the wide utilized and high successful allogeneic hematopoietic stem-cell transplantation provides the possibility to cure several patients a new scenario has been opened by GT. Therefore, it is important to establish the patients setting for whom it is priority indicated, particularly in the early phase of the diffuse use outside experimental trials conducted in high selected centers. Moreover, actual price, limited availability, and resources disposal constitute a further indication to a rational and progressive approach to this innovative treatment. To elaborate this algorithm, the experience with allogeneic transplantation has been used has a predictive model. In this large worldwide experience, it has been clearly demonstrated that key for the optimal transplant outcome is optimal transfusion and chelation therapy in the years before the procedure and consequently optimal patient’s clinical condition. In the document, different clinical scenarios have been considered and analyzed for the possible impact on treatment outcome. According to the European Medicine Agency (EMA) for the GT product, this expert opinion must be considered as a dynamic, updatable, priority-based indications for physicians taking care of TDT patients.

The role of interferon-gamma and its signaling pathway in pediatric hematological disorders

Interferon-gamma (IFN-γ) plays a key role in the pathophysiology of hemophagocytic lymphohistiocytosis (HLH), and available evidence also points to a role in other conditions, including aplastic anemia (AA) and graft failure following allogeneic hematopoietic stem cell transplantation. Recently, the therapeutic potential of IFN-γ inhibition has been documented; emapalumab, an anti-IFN-γ monoclonal antibody, has been approved in the United States for treatment of primary HLH that is refractory, recurrent or progressive, or in patients with intolerance to conventional therapy. Moreover, ruxolitinib, an inhibitor of JAK/STAT intracellular signaling, is currently being investigated for treating HLH. In AA, IFN-γ inhibits hematopoiesis by disrupting the interaction between thrombopoietin and its receptor, c-MPL. Eltrombopag, a small-molecule agonist of c-MPL, acts at a different binding site to IFN-γ and is thus able to circumvent its inhibitory effects. Ongoing trials will elucidate the role of IFN-γ neutralization in secondary HLH and future studies could explore this strategy in controlling hyperinflammation due to CAR T cells.

Gene replacement of α-globin with β-globin restores hemoglobin balance in β-thalassemia-derived hematopoietic stem and progenitor cells

β-Thalassemia pathology is due not only to loss of β-globin (HBB), but also to erythrotoxic accumulation and aggregation of the β-globin-binding partner, α-globin (HBA1/2). Here we describe a Cas9/AAV6-mediated genome editing strategy that can replace the entire HBA1 gene with a full-length HBB transgene in β-thalassemia-derived hematopoietic stem and progenitor cells (HSPCs), which is sufficient to normalize β-globin:α-globin messenger RNA and protein ratios and restore functional adult hemoglobin tetramers in patient-derived red blood cells. Edited HSPCs were capable of long-term and bilineage hematopoietic reconstitution in mice, establishing proof of concept for replacement of HBA1 with HBB as a novel therapeutic strategy for curing β-thalassemia.

Identification of New Soluble Factors Correlated With the Development of Graft Failure After Haploidentical Hematopoietic Stem Cell Transplantation

Graft failure is a severe complication of allogeneic hematopoietic stem cell transplantation (HSCT). The mechanisms involved in this phenomenon are still not completely understood; data available suggest that recipient T lymphocytes surviving the conditioning regimen are the main mediators of immune-mediated graft failure. So far, no predictive marker or early detection method is available. In order to identify a non-invasive and efficient strategy to diagnose this complication, as well as to find possible targets to prevent/treat it, we performed a detailed analysis of serum of eight patients experiencing graft failure after T-cell depleted HLA-haploidentical HSCT. In this study, we confirm data describing graft failure to be a complex phenomenon involving different components of the immune system, mainly driven by the IFNγ pathway. We observed a significant modulation of IL7, IL8, IL18, IL27, CCL2, CCL5 (Rantes), CCL7, CCL20 (MIP3a), CCL24 (Eotaxin2), and CXCL11 in patients experiencing graft failure, as compared to matched patients not developing this complication. For some of these factors, the difference was already present at the time of infusion of the graft, thus allowing early risk stratification. Moreover, these cytokines/chemokines could represent possible targets, providing the rationale for exploring new therapeutic/preventive strategies.

Haploidentical Hematopoietic Stem Cell Transplantation for XIAP Deficiency: a Single-Center Report

PURPOSE

X-linked inhibitor of apoptosis (XIAP) deficiency caused by mutations in the XIAP/BIRC4 gene is a rare inherited primary immunodeficiency also known as X-linked lymphoproliferative syndrome type 2 (XLP2). Hematopoietic stem cell transplantation (HSCT) is currently the only curative strategy available. However, few studies of haploidentical HSCT have been published regarding the outcomes in patients with this syndrome.

METHODS

We evaluated the XIAP gene analysis and clinical characteristics of four Chinese patients with XIAP who underwent haploidentical HSCT.

RESULTS

The mutations in the two of four patients had not yet been reported in the literature. All of the patients had recurrent hemophagocytic lymphohistiocytosis but did not have a good matched donor and underwent haploidentical HSCT at BCH in China between September 2016 and December 2018. All four patients received antithymocyte globulin with fludarabine-based regimens. Two patients underwent reduced intensity conditioning (RIC), and the other two received modified myeloablative conditioning (MAC) regimens. Three of the four patients survived. Three patients experienced complications with mixed chimerism. One of the four patients who underwent RIC had early graft loss and then developed grade IV acute graft-versus-host disease (GVHD) after donor lymphocyte infusion with bone marrow. The two patients who received MAC survived with no or mild GVHD, even though one of them developed hepatic veno-occlusive disease in the early stage of transplantation.

CONCLUSIONS

Haploidentical HSCT may be a treatment option for patients with XIAP deficiency who lack a good matched donor. More studies are needed to determine whether modified MAC with reduced toxicity is more suitable for haploidentical transplantation.

Role of HLA-DP Expression in Graft-Versus-Host Disease After Unrelated Donor Transplantation

PURPOSE

The main aim of this study was to evaluate the significance of HLA-DPB1 expression in acute graft-versus-host disease (GVHD) after hematopoietic cell transplantation (HCT) from HLA-A, -B, -C, -DRB1, -DQB1-matched and -mismatched unrelated donors.

PATIENTS AND METHODS

Between January 1, 2017, and January 10, 2019, we assessed 19,136 patients who received HCT from an HLA-A, -B, -C, -DRB1, -DQB1-matched or -mismatched unrelated donor performed in Australia, the European Union, Japan, North America, and the United Kingdom between 1988 and 2016. Among transplant recipients with one HLA-DPB1 mismatch, the patient's mismatched HLA-DPB1 allotype was defined as low or high expression. Multivariable regression models were used to assess risks of GVHD associated with high expression relative to low expression HLA-DPB1 mismatches. The effect of increasing numbers of HLA-DPB1 mismatches on clinical outcome was assessed in HLA-mismatched transplant recipients.

RESULTS

In HLA-A, -B, -C, -DRB1,-DQB1-matched transplant recipients, donor mismatching against one high-expression patient HLA-DPB1 increased moderate (odds ratio [OR], 1.36; P = .001) and severe acute GVHD (OR, 1.32; P = .0016) relative to low-expression patient mismatches, regardless of the expression level of the donor's mismatched HLA-DPB1. Among transplant recipients with one HLA-A, -B, -C, -DRB1, or -DQB1 mismatch, the odds of acute GVHD increased with increasing numbers of HLA-DPB1 mismatches (OR, 1.23 for one; OR, 1.40 for two mismatches relative to zero mismatches for moderate GVHD; OR, 1.19 for one; OR, 1.40 for two mismatches relative to zero for severe GVHD), but not with the level of expression of the patient's mismatched HLA-DPB1 allotype.

CONCLUSION

The level of expression of patient HLA-DPB1 mismatches informs the risk of GVHD after HLA-A, -B, -C, -DRB1, -DQB1-matched unrelated HCT, and the total number of HLA-DPB1 mismatches informs the risk of GVHD after HLA-mismatched unrelated HCT. Prospective consideration of HLA-DPB1 may help to lower GVHD risks after transplantation.

HLA-DPB1 Reactive T Cell Receptors for Adoptive Immunotherapy in Allogeneic Stem Cell Transplantation

HLA-DPB1 antigens are mismatched in about 80% of allogeneic hematopoietic stem cell transplantations from HLA 10/10 matched unrelated donors and were shown to be associated with a decreased risk of leukemia relapse. We recently developed a reliable in vitro method to generate HLA-DPB1 mismatch-reactive CD4 T-cell clones from allogeneic donors. Here, we isolated HLA-DPB1 specific T cell receptors (TCR DP) and used them either as wild-type or genetically optimized receptors to analyze in detail the reactivity of transduced CD4 and CD8 T cells toward primary AML blasts. While both CD4 and CD8 T cells showed strong AML reactivity in vitro, only CD4 T cells were able to effectively eliminate leukemia blasts in AML engrafted NOD/SCID/IL2Rγc^−/− (NSG) mice. Further analysis showed that optimized TCR DP and under some conditions wild-type TCR DP also mediated reactivity to non-hematopoietic cells like fibroblasts or tumor cell lines after HLA-DP upregulation. In conclusion, T cells engineered with selected allo-HLA-DPB1 specific TCRs might be powerful off-the-shelf reagents in allogeneic T-cell therapy of leukemia. However, because of frequent (common) cross-reactivity to non-hematopoietic cells with optimized TCR DP T cells, safety mechanisms are mandatory.

Priming of Allo-HLA-DP-Specific Reactivity from the Naïve T Cell Compartment Is Not Exclusively Mediated by Professional Antigen-Presenting Cells

Allogeneic (allo) stem cell transplantation is applied to patients suffering from hematologic malignancies to replace the diseased hematopoietic system with cells derived from a donor stem cell graft. The majority of 10/10-matched unrelated donors are HLA-DP-mismatched, and this may result in varying degrees of the graft-versus-leukemia (GVL) effect with or without the occurrence of graft-versus-host disease (GVHD). Allo-HLA-reactive T cells are commonly present in the donor T cell repertoire, and thus a very profound alloreactive immune response can be provoked in the HLA-DP-mismatched setting. The magnitude and the diversity of the allo-HLA-DP-specific immune response likely dictates the balance between the occurrence of GVL and/or GVHD after transplantation. To understand the nature of the allo-HLA-DP-specific immune response provoked under different stimulatory conditions, immune responses were induced from both the naïve and memory T cell compartments using either HLA-DP-mismatched professional antigen-presenting cells (APCs) (monocyte-derived dendritic cells [allo-DCs]) or HLA-DP-mismatched nonprofessional APCs (skin-derived fibroblasts [allo-fibroblasts]) as stimulator cells. In this study, we observed that allo-HLA-DP-reactive T cells could be provoked from both the naïve and memory compartments by both types of APCs. However, the magnitude of the allo-HLA-DP-specific immune response was greater when stimulation was performed with allo-DCs. Moreover, we found that the frequency of allo-HLA-DP-reactive T cells was greater in the naïve T cell compartment compared with the memory T cell compartment, but we observed a comparable lineage specificity of these allo-HLA-DP-specific reactivities. Overall, the data from this study illustrate that the presence of professional APCs of recipient origin will mostly dictate the magnitude of the allo-HLA-DP-specific immune response derived from both the naïve and memory T cell compartments, but does not exclusively mediate the induction of these immune responses.

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.

[Graft failure after allogeneic hematopoietic stem cell transplantation: Definition and risk factors]

Allogeneic hematopoietic stem cell transplantation is the only curative therapy for numerous malignant and non-malignant haematological diseases. A sustained engraftment of the donor stem cells is essential for transplant success and overall outcome. Graft failure is a rare but severe event after allogeneic hematopoietic stem cell transplantation. While different risk factors such as underlying disease, graft source or HLA matching have been found to be consistently associated with graft failure, other factors such as ABO mismatch graft-versus-host disease prophylaxis or infections, particularly viral reactivations, are more controversial. In this article, we review the different factors associated with graft failure.

Indications for haematopoietic stem cell transplantation for haematological diseases, solid tumours and immune disorders: current practice in Europe, 2019

This is the seventh special EBMT report on the indications for haematopoietic stem cell transplantation for haematological diseases, solid tumours and immune disorders. Our aim is to provide general guidance on transplant indications according to prevailing clinical practice in EBMT countries and centres. In order to inform patient decisions, these recommendations must be considered together with the risk of the disease, the risk of the transplant procedure and the results of non-transplant strategies. In over two decades since the first report, the EBMT indications manuscripts have incorporated changes in transplant practice coming from scientific and technical developments in the field. In this same period, the establishment of JACIE accreditation has promoted high quality and led to improved outcomes of patient and donor care and laboratory performance in transplantation and cellular therapy. An updated report with operating definitions, revised indications and an additional set of data with overall survival at 1 year and non-relapse mortality at day 100 after transplant in the commonest standard-of-care indications is presented. Additional efforts are currently underway to enable EBMT member centres to benchmark their risk-adapted outcomes as part of the Registry upgrade Project 2020 against national and/or international outcome data.

Hematopoietic Stem Cell Transplantation in Thalassemia

Although recent advances in gene therapy are expected to increase the chance of disease cure in thalassemia major, at present hematopoietic stem cell transplantation (HSCT) remains the only consolidated curative approach for this disorder. The widest experience has been obtained in the HLA-matched family donor (MFD) setting, with probabilities of overall and thalassemia-free survival exceeding 90% and 85%, respectively. As for most patients a suitable MFD is not available, alternative donors (HLA-matched unrelated donor, unrelated cord blood, HLA-haploidentical relative) have been increasingly explored, translating into the expansion of the number of patients treatable with HSCT.

The allogeneic HLA-DP-restricted T-cell repertoire provoked by allogeneic dendritic cells contains T cells that show restricted recognition of hematopoietic cells including primary malignant cells

Stem cell grafts from 10/10 HLA-matched unrelated donors are often mismatched for HLA-DP. In some patients, donor T-cell responses targeting the mismatched HLA-DP allele(s) have been found to induce a specific graft-versus-leukemia effect without coinciding graft-versus-host disease, whereas in other cases significant graft-versus-host disease occurred. Cell-lineage-specific recognition patterns within the allogeneic HLA-DP-specific donor T-cell repertoire could explain the differential clinical effects mediated by donor T cells after HLA-DP-mismatched allogeneic stem cell transplantation. To unravel the composition of the HLA-DP T-cell repertoire, donor T-cell responses were provoked by in vitro stimulation with allogeneic HLA-DP-mismatched monocyte-derived dendritic cells. A strategy including depletion of reactivity against autologous dendritic cells allowed efficient identification and enrichment of allo-reactive T cells upon stimulation with HLA-DP-mismatched dendritic cells. In this study we elucidated that the allogeneic HLA-DP-restricted T-cell repertoire contained T cells with differential cell-lineage-specific recognition profiles. As expected, some of the allogeneic HLA-DP-restricted T cells showed broad recognition of a variety of hematopoietic and non-hematopoietic cell types expressing the targeted mismatched HLA-DP allele. However, a significant proportion of the allogeneic HLA-DP-restricted T cells showed restricted recognition of hematopoietic cells, including primary malignant cells, or even restricted recognition of only myeloid cells, including dendritic cells and primary acute myeloid leukemia samples, but not of other hematopoietic and non-hematopoietic cell types. These data demonstrate that the allogeneic HLA-DP-specific T-cell repertoire contains T cells that show restricted recognition of hematopoietic cells, which may contribute to the specific graft-versus-leukemia effect without coinciding graft-versus-host disease.

Unrelated Donor Transplantation in Children with Thalassemia using Reduced-Intensity Conditioning: The URTH Trial

Allogeneic hematopoietic stem cell transplantation (HSCT) can cure transfusion-dependent thalassemia (TDT). In a multicenter trial we investigated the efficacy of reduced-intensity conditioning (RIC) before unrelated donor (URD) HSCT in children with TDT. Thirty-three children, ages 1 to 17 years, received bone marrow (BM) or umbilical cord blood (UCB) allografts. Median time to neutrophil engraftment was 13 days (range, 10 to 25) and 24 days (range, 18 to 49) and platelet engraftment 23 days (range, 12 to 46) and 50 days (range, 31 to 234) after BM and UCB allografts, respectively. With a median follow-up of 58 months (range, 7 to 79), overall and thalassemia-free survival was 82% (95% CI, .64% to .92%) and 79% (95% CI, .6% to .9%), respectively. The cumulative incidence of grades II to IV acute graft-versus-host disease (GVHD) after BM and UCB allografts was 24% and 44%; the 2-year cumulative incidence of chronic extensive GVHD was 29% and 21%, respectively; 71% of BM and 91% of UCB recipients discontinued systemic immunosuppression by 2 years. Six patients who had Pesaro risk class 2 (n = 5) and class 3 (n = 1) died of GVHD (n = 3), viral pneumonitis (n = 2) and pulmonary hemorrhage (n = 1). Outcomes after this RIC compared favorably with URD HSCT outcomes for TDT and supported engraftment in 32 of 33 patients. Efforts to reduce GVHD and infectious complications are being pursued further.

Hidden genomic MHC disparity between HLA-matched sibling pairs in hematopoietic stem cell transplantation

Matching classical HLA alleles between donor and recipient is an important factor in avoiding adverse immunological effects in HSCT. Siblings with no differences in HLA alleles, either due to identical-by-state or identical-by-descent status, are considered to be optimal donors. We carried out a retrospective genomic sequence and SNP analysis of 336 fully HLA-A, -B, -DRB1 matched and 14 partially HLA-matched sibling HSCT pairs to determine the level of undetected mismatching within the MHC segment as well as to map their recombination sites. The genomic sequence of 34 genes locating in the MHC region revealed allelic mismatching at 1 to 8 additional genes in partially HLA-matched pairs. Also, fully matched pairs were found to have mismatching either at HLA-DPB1 or at non-HLA region within the MHC segment. Altogether, 3.9% of fully HLA-matched HSCT pairs had large genomic mismatching in the MHC segment. Recombination sites mapped to certain restricted locations. The number of mismatched nucleotides correlated with the risk of GvHD supporting the central role of full HLA matching in HSCT. High-density genome analysis revealed that fully HLA-matched siblings may not have identical MHC segments and even single allelic mismatching at any classical HLA gene often implies larger genomic differences along MHC.

Long-term survival of beta thalassemia major patients treated with hematopoietic stem cell transplantation compared with survival with conventional treatment

Allogeneic hematopoietic stem cell transplantation (HSCT) in thalassemia remains a challenge. We reported a single-centre case-control study of a large cohort of 516 children and adult patients treated with HSCT or blood transfusion support and iron chelation therapy; 258 patients (median age 12, range 1-45) underwent sibling (67%) or unrelated (33%) HSCT; 97 patients were adults (age ≥ 16 years). The median follow-up after HSCT was 11 years (range 1-30). The conditioning regimen was busulfan (80.6%) or treosulfan-based (19.4%). A cohort of 258 age-sex matched conventionally treated (CT) patients was randomly selected. In transplanted patients the 30-year overall survival (OS) and thalassemia-free survival (TFS) were 82.6 ± 2.7% and 77.8 ± 2.9%, compared to the OS of 85.3 ± 2.7% in CT patients (P = NS); The incidence of grade II-IV acute and chronic graft versus host disease (GvHD) was 23.6% and 12.9% respectively. The probability of rejection was 6.9%. Transplant-related mortality (TRM) (13.8%) was similar to the probability of dying of cardiovascular events in CT patients (12.2%). High-risk Pesaro score (class 3) was associated with lower OS (OR = 1.99, 95% C.I.=1.31-3.03) and TFS (OR = 1.54, 95% C.I.=1.12-2.12). In adult patients, the 23-years OS and TFS after HSCT were 70 ± 5% and 67.3 ± 5%, compared to 71.2 ± 5% of OS in CT (P = NS). Finally, treosulfan was associated with lower risk of acute GvHD (P = .004; OR = 0.28, 95% C.I.=0.12-0.67). In conclusion, the 30-year survival rate of ex-thalassemia patients after HSCT was similar to that expected in CT thalassemia patients, with the vast majority of HSCT survivors cured from thalassemia.

What compatibility in 2017 for the haematopoietic stem cell transplantation?

The diversification of potential donors to perform stem cell allografts now enables to propose a compatible graft cell source adapted to the different clinical situations. Transplants with a geno-identical sibling donor, otherwise with the most HLA-compatible unrelated donor, remain the first-line solutions. Alternative transplants allow to graft patients having no donors in international registries, owing to the rarity of their HLA typing. They are carried out with fairly incompatible grafts and are therefore limited by the existence in the recipient of preformed anti-HLA antibodies which predispose to their rejection. The simple prevention of acute Graft-versus-host disease in haplo-identical transplants, as well as the availability of donors, explain why they have very often replaced placental stem cell transplants. These latter remain useful for pediatric patients or in the absence of family donors.

HLA-DP in unrelated hematopoietic cell transplantation revisited: challenges and opportunities

When considering HLA-matched hematopoietic cell transplantation (HCT), sibling and unrelated donors (UDs) are biologically different because UD-HCT is typically performed across HLA-DP disparities absent in sibling HCT. Mismatched HLA-DP is targeted by direct alloreactive T cell responses with important implications for graft-versus-host disease and graft-versus-leukemia. This concise review details special features of HLA-DP as model antigens for clinically permissive mismatches mediating limited T-cell alloreactivity with minimal toxicity, and describes future avenues for their exploitation in cellular immunotherapy of malignant blood disorders.

Hematopoietic Stem Cell Transplantation for XIAP Deficiency in Japan

Background

X-linked inhibitor of apoptosis protein (XIAP) deficiency is a rare immunodeficiency that is characterized by recurrent hemophagocytic lymphohistiocytosis (HLH) and splenomegaly and sometimes associated with refractory inflammatory bowel disease (IBD). Although hematopoietic stem cell transplantation (HSCT) is the only curative therapy, the outcomes of HSCT for XIAP deficiency remain unsatisfactory compared with those for SLAM-associated protein deficiency and familial HLH.

Aim

To investigate the outcomes and adverse events of HSCT for patients with XIAP deficiency, a national survey was conducted.

Methods

A spreadsheet questionnaire was sent to physicians who had provided HSCT treatment for patients with XIAP deficiency in Japan.

Results

Up to the end of September 2016, 10 patients with XIAP deficiency had undergone HSCT in Japan, 9 of whom (90%) had survived. All surviving patients had received a fludarabine-based reduced intensity conditioning (RIC) regimen. Although 5 patients developed post-HSCT HLH, 4 of them survived after etoposide administration. In addition, the IBD associated with XIAP deficiency improved remarkably after HSCT in all affected cases.

Conclusion

The RIC regimen and HLH control might be important factors for successful HSCT outcomes, with improved IBD, in patients with XIAP deficiency.

Electronic supplementary material

The online version of this article (doi:10.1007/s10875-016-0348-4) contains supplementary material, which is available to authorized users.

What Unrelated Hematopoietic Stem Cell Transplantation in Thalassemia Taught us about Transplant Immunogenetics

Although the past few decades have shown an improvement in the survival and complication-free survival rates in patients with beta-thalassemia major and gene therapy is already at an advanced stage of experimentation, hematopoietic stem cell transplantation (HSCT) continues to be the only effective and realistic approach to the cure of this chronic non-malignant disease. Historically, human leukocyte antigen (HLA)-matched siblings have been the preferred source of donor cells owing to superior outcomes compared with HSCT from other sources. Nowadays, the availability of an international network of voluntary stem cell donor registries and cord blood banks has significantly increased the odds of finding a suitable HLA matched donor. Stringent immunogenetic criteria for donor selection have made it possible to achieve overall survival (OS) and thalassemia-free survival (TFS) rates comparable to those of sibling transplants. However, acute and chronic graft-versus-host disease (GVHD) remains the most important complication in unrelated HSCT in thalassemia, leading to significant rates of morbidity and mortality for a chronic non-malignant disease. A careful immunogenetic assessment of donors and recipients makes it possible to individualize appropriate strategies for its prevention and management. This review provides an overview of recent insights about immunogenetic factors involved in GVHD, which seem to have a potential role in the outcome of transplantation for thalassemia.

A trial of unrelated donor marrow transplantation for children with severe sickle cell disease

Children with sickle cell disease experience organ damage, impaired quality of life, and premature mortality. Allogeneic bone marrow transplant from an HLA-matched sibling can halt disease progression but is limited by donor availability. A Blood and Marrow Transplant Clinical Trials Network (BMT CTN) phase 2 trial conducted from 2008 to 2014 enrolled 30 children aged 4 to 19 years; 29 were eligible for evaluation. The primary objective was 1-year event-free survival (EFS) after HLA allele-matched (at HLA-A, -B, -C, and -DRB1 loci) unrelated donor transplant. The conditioning regimen included alemtuzumab, fludarabine, and melphalan. Graft-versus-host disease (GVHD) prophylaxis included calcineurin inhibitor, short-course methotrexate, and methylprednisolone. Transplant indications included stroke (n = 12), transcranial Doppler velocity >200 cm/s (n = 2), ≥3 vaso-occlusive pain crises per year (n = 12), or ≥2 acute chest syndrome episodes (n = 4) in the 2 years preceding enrollment. Median follow-up was 26 months (range, 12-62 months); graft rejection was 10%. The 1- and 2-year EFS rates were 76% and 69%, respectively. The corresponding rates for overall survival were 86% and 79%. The day 100 incidence rate of grade II-IV acute GVHD was 28%, and the 1-year incidence rate of chronic GVHD was 62%; 38% classified as extensive. There were 7 GVHD-related deaths. A 34% incidence of posterior reversible encephalopathy syndrome was noted in the first 6 months. Although the 1-year EFS met the prespecified target of ≥75%, this regimen cannot be considered sufficiently safe for widespread adoption without modifications to achieve more effective GVHD prophylaxis. The BMT CTN #0601 trial was registered at www.clinicaltrials.gov as #NCT00745420.

Alternative donor allogeneic hematopoietic cell transplantation for hemoglobinopathies

Allogeneic hematopoietic stem cell transplantation (HSCT) offers a curative therapy for patients with hemoglobinopathies, mainly severe sickle cell disease (SCD) and thalassemia (TM). However, the applicability of HSCT has been limited mainly by donor availability, with a less than 25%-30% of eligible patients having human leukocyte antigen (HLA)-matched sibling donors. Previous outcomes using alternate donor options have been markedly inferior due to increased regimen-related toxicity, transplant-related mortality, graft failure, and graft-versus-host disease (GVHD). Advances in transplant technology, including high-resolution HLA typing, improved GVHD prophylactic approaches with tolerance induction, and better supportive care over the last decade, are addressing these historical challenges, resulting in increasing donor options. Herein, we review alternate donor HSCT approaches for severe SCD and TM using unrelated donors, umbilical cord blood units, or related haploidentical donors. Though this is an emerging field, early results are promising and in selected patients, this may be the preferred option to mitigate against the age-related morbidity and early mortality associated with these disorders.

Long-term outcomes of fludarabine, melphalan and antithymocyte globulin as reduced-intensity conditioning regimen for allogeneic hematopoietic stem cell transplantation in children with primary immunodeficiency disorders: a prospective single center study

Reduced-intensity conditioning (RIC) has offered many primary immunodeficiency disorder (PID) patients who are ineligible for myeloablative regimens a chance of cure. However, the beneficial role of RIC was questioned following reports suggesting higher chance of rejection and lower symptom resolution rate in mixed chimerism settings. Forty-five children affected by PIDs with a median age of 21 months underwent allogeneic hematopoietic stem cell transplantation in our institute from 2007 to 2013. All patients received an identical RIC regimen. Forty-one patients had successful primary engraftment (91%). Of the successful engraftments, 80% (n=33) had stable full donor chimerism at last contact. Overall, eleven transplant-related mortalities were reported including five patients due to sepsis, three children due to grade IV acute GvHD, two due to chronic GvHD and one patient due to sepsis after primary graft failure. The median post-transplantation follow-up of deceased patients was 55 days. Five-year overall survival and disease-free survival was 75.6% and 68.89%, respectively. All surviving patients with successful engraftment became disease free, regardless of having full or mixed chimerism. Our study suggests that RIC regimen provides satisfactory rates of successful engraftment and full chimerism. Furthermore, patients with mixed chimerism were stable in long-term follow-up and this chimerism status offered the potential to resolve symptoms of immunodeficiency.

Hematopoietic stem cell transplantation in thalassemia major and sickle cell disease: indications and management recommendations from an international expert panel

Thalassemia major and sickle cell disease are the two most widely disseminated hereditary hemoglobinopathies in the world. The outlook for affected individuals has improved in recent years due to advances in medical management in the prevention and treatment of complications. However, hematopoietic stem cell transplantation is still the only available curative option. The use of hematopoietic stem cell transplantation has been increasing, and outcomes today have substantially improved compared with the past three decades. Current experience world-wide is that more than 90% of patients now survive hematopoietic stem cell transplantation and disease-free survival is around 80%. However, only a few controlled trials have been reported, and decisions on patient selection for hematopoietic stem cell transplantation and transplant management remain principally dependent on data from retrospective analyses and on the clinical experience of the transplant centers. This consensus document from the European Blood and Marrow Transplantation Inborn Error Working Party and the Paediatric Diseases Working Party aims to report new data and provide consensus-based recommendations on indications for hematopoietic stem cell transplantation and transplant management.

Indications for allo- and auto-SCT for haematological diseases, solid tumours and immune disorders: current practice in Europe, 2015

This is the sixth special report that the European Society for Blood and Marrow Transplantation regularly publishes on the current practice and indications for haematopoietic SCT for haematological diseases, solid tumours and immune disorders in Europe. Major changes have occurred in the field of haematopoietic SCT over the last years. Cord blood units as well as haploidentical donors have been increasingly used as stem cell sources for allo-SCT, thus, augmenting the possibility of finding a suitable donor for a patient. Continuous refinement of conditioning strategies has also expanded not only the number of potential indications but also has permitted consideration of older patients or those with co-morbidity for a transplant. There is accumulating evidence of the role of haematopoietic SCT in non-haematological disorders such as autoimmune diseases. On the other hand, the advent of new drugs and very effective targeted therapy has challenged the role of SCT in some instances or at least, modified its position in the treatment armamentarium of a given patient. An updated report with revised tables and operating definitions is presented.

Primary graft failure after myeloablative allogeneic hematopoietic cell transplantation for hematologic malignancies

Clinical outcomes after primary graft failure (PGF) remain poor. Here we present a large retrospective analysis (n=23,272) which investigates means to prevent PGF and early detection of patients at high risk. In patients with hematologic malignancies, who underwent their first myeloablative allogeneic hematopoietic cell transplantation, PGF was reported in 1,278 (5.5%), and there was a marked difference in PGFs using peripheral blood stem cell compared to bone marrow grafts (2.5 vs. 7.3%; P<0.001). A 4-fold increase of PGF was observed in myeloproliferative disorders compared to acute leukemia (P<0.001). Other risk factors for PGF included recipient age below 30, HLA-mismatch, male recipients of female donor grafts, ABO-incompatibility, busulfan/cyclophosphamide conditioning, and cryopreservation. In bone marrow transplants, total nucleated cell doses ≤2.4 × 10⁸/kg were associated with PGF (OR 1.39; P<0.001). The use of tacrolimus-based immunosuppression and granulocyte colony-stimulating factor were associated with decreased PGF risk. These data, allow clinicians to do more informed choices with respect to graft source, donor selection, conditioning and immunosuppressive regimens to reduce the risk of PGF. Moreover, a novel risk score determined on day 21 post-transplant may provide the rationale for an early request for additional hematopoietic stem cells.

Gene therapy for hemoglobinopathies: the state of the field and the future

After nearly two decades of struggle, gene therapy for hemoglobinopathies using vectors carrying β or γ-globin gene has finally reached the clinical doorsteps. This was made possible by advances made in our understanding of critical regulatory elements required for high level of globin gene expression and improved gene transfer vectors and methodologies. Development of gene editing technologies and reprogramming somatic cells for regenerative medicine holds the promise of genetic correction of hemoglobinopathies in the future. This article will review the state of the field and the upcoming technologies that will allow genetic therapeutic correction of hemoglobinopathies.

Allogeneic stem cell transplantation for thalassemia major

Allogeneic stem cell transplant remains the only curative option for β-thalassemia major. In patients with good risk features it is reasonable to anticipate a greater than 90% chance of a successful transplant outcome. The conventional risk stratification system has limitations and alternative systems are being explored to better identify subsets that require innovative approaches. Several novel regimens have been evaluated to reduce treatment-related morbidity and mortality. There remain challenges in improving the clinical outcome of high-risk patients. There are limited data on the role of splenectomy before transplantation or optimal posttransplant chelation and care of these patients.

Soluble HLA technology as a strategy to evaluate the impact of HLA mismatches

HLA class I incompatibilities still remain one of the main barriers for unrelated bone marrow transplantation (BMT); hence the molecular understanding of how to mismatch patients and donors and still have successful clinical outcomes will guide towards the future of unrelated BMT. One way to estimate the magnitude of polymorphisms within the PBR is to determine which peptides can be selected by individual HLA alleles and subsequently presented for recognition by T cells. The features (structure, length, and sequence) of different peptides each confer an individual pHLA landscape and thus directly shape the individual immune response. The elution and sequencing of peptides by mass spectrometric analysis enable determining the bona fide repertoire of presented peptides for a given allele. This is an effective and simple way to compare the functions of allelic variants and make a first assessment of their degree of permissivity. We describe the methodology used for peptide sequencing and the limitations of peptide prediction tools compared to experimental methods. We highlight the altered peptide features that are observed between allelic variants and the need to discover the altered peptide repertoire in situations of "artificial" graft versus host disease (GvHD) that occur in HLA-specific hypersensitive immune responses to drugs.

Hematopoietic stem cell transplantation donor sources in the 21st century: choosing the ideal donor when a perfect match does not exist

Most patients who require allogeneic stem cell transplantation do not have a matched sibling donor, and many patients do not have a matched unrelated donor. In an effort to increase the applicability of transplantation, alternative donors such as mismatched adult unrelated donors, haploidentical related donors, and umbilical cord blood stem cell products are frequently used when a well matched donor is unavailable. We do not yet have the benefit of randomized trials comparing alternative donor stem cell sources to inform the choice of donor; however, the existing data allow some inferences to be made on the basis of existing observational and phase 2 studies. All 3 alternative donor sources can provide effective lymphohematopoietic reconstitution, but time to engraftment, graft failure rate, graft-versus-host disease, transplant-related mortality, and relapse risk vary by donor source. These factors all contribute to survival outcomes and an understanding of them should help guide clinicians when choosing among alternative donor sources when a matched related or matched unrelated donor is not available.

Unrelated Umbilical Cord Blood Transplant for Children with β-Thalassemia Major

Beta thalassemia major, one of the most prevalent hemoglobinopathy throughout the word, can be cured by allogenic stem cell transplantation (SCT) (Bone Marrow Transplant 36:971-975, 2005). Many patients, however, lack a suitably matched related sibling donor. Unrelated umbilical cord blood (UCB) can be used as an alternative stem cell source for these patients. This report describes SCT for nine children with beta-thalassemia major using partially HLA-matched unrelated UCB. Conditioning included oral busulfan 16 mg/kg (day -10 to -7), cyclophosphamide (Cy) 200 mg/kg (day -5 to -2), fludarabine 90 mg/kg (day -13 to -11), and antithymocyte globulin (rabbit) 7.5 mg/kg (day -3 to -1). The infused cell dose was 10.71 × 10(7)/kg total nucleated cells (TNC) (range 6.5-17 × 10(7)/kg TNC). The patients ranged in age from 1.5 to 7 years, in weight from 10.5 to 17 kg. A second transplant with two unrelated cord blood units was attempted in two patients who had primary graft failure. The retransplant recipients were preconditioned with i.v Cy 120 mg/kg (day -3 to -2). Five of the nine patients engrafted promptly with 50-100 % donor chimerism (56 %). They engrafted at a median of 17 days (range 12-19). One patient is transfusion free for 36 months; a second patient is transfusion free for 18 months and a third is transfusion free for 9 months. There was no transplant related mortality. Four of the nine children had autologous recovery without engraftment. Primary graft rejection is the major complication. Post transplant complications were mild hepatic veno-occlusive disease, acute GVHD grade II, and CMV interstitial pneumonia. The chronic GVHD was limited and could be controlled by Methylprednisolone combined with Mycophenolate. The lack of a marrow donor registry in India makes UCBT from related and unrelated donors a good alternative. Transplant should be delayed until the child is at least 18 months of age. The dose of UCB stem cells is the most important factor for engraftment. UCB has the advantages of rapid availability and low risk of severe GVHD despite donor-recipient HLA disparity (Transplant Proc 37:2667-2669, 2005). We demonstrate the feasibility of this procedure in the setting of a developing country.

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.