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Abstract: Allogeneic hematopoietic stem cell transplantation (HSCT) with mismatched unrelated donors (MMUD) is associated with inferior outcome compared with matched unrelated donors (MUDs). We aimed to identify permissible mismatches using HLA epitope mismatch algorithm, which determines permissibility by analyzing amino acid sequences, in a single-center cohort of 70 pediatric 9/10 MMUD HSCTs and 157 10/10 MUDs for comparison. Amino acid matching was evaluated for the whole HLA protein, the α-helices, and the β-sheets, in both host vs graft (HvG) and graft vs host (GvH) direction. Superior event-free survival (EFS) was found in 13 patients permissibly mismatched in the HvG direction (totalHvG, 92% vs 58% at 1 year; P = .009) and in 21 patients matched on the α-helices (αHvG, 90% vs 53%; P = .002). These rates were similar to EFS rates in patients with 10/10 MUDs (90% vs 80%; P = .60). EFS was not related to β-sheet amino acid matching, nor to matching in the GvH direction. Overall survival (OS) rates trended similarly to those of EFS for amino acid mismatches (totalHvG, 92% vs 74%; P = .075; αHvG, 90% vs 71%; P = .072). These findings were reproduced in an EBMT Registry inborn errors cohort of 271 pediatric 9/10 MMUD HSCTs and 929 10/10 MUD HSCTs, showing a significant effect of αHvG matching on both OS and EFS and similar OS and EFS between αHvG matched MMUDs and 10/10 MUDs. In summary, HvG amino acid matching on the α-helices identifies 9/10 MMUDs with permissible mismatches, which are correlated with favorable transplant outcomes similar to those of matched donors., (© 2024 by The American Society of Hematology. Licensed under Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0), permitting only noncommercial, nonderivative use with attribution. All other rights reserved.)

On 24 January 2023, Eurotransplant has introduced the virtual crossmatch for kidney and pancreas allocation as a better alternative for the physical Complement Dependent Cytotoxicity (CDC) crossmatches at the donor centre, which were associated with a longer cold ischaemia time and false positive reactions. For the time being, the physical CDC crossmatch at the recipient centre will remain in place as the final histocompatibility check. While Eurotransplant is certainly not the first organ allocation organisation to introduce virtual crossmatching, several novel aspects have been introduced, such as calculation of the virtual panel reactive antibody (vPRA) on 11 loci at the second-field level in addition to the serological broad and split level, electronic HLA typing data transmission using Histoimmunogenetics Markup Language (HML) file format, and the actual virtual crossmatch based on ambiguous, second-field HLA typing of the donor on all 11 loci. This short communication will focus on these novel aspects of the virtual crossmatch in Eurotransplant., (© 2024 The Author(s). HLA: Immune Response Genetics published by John Wiley & Sons Ltd.)

Female recipients of a spousal donor kidney transplant are at greater risk of donor-specific pre-immunization, which may increase the risk of acute antibody-mediated rejection (ABMR). We assessed the incidence of early ABMR (within two weeks after transplantation), risk factors for ABMR and graft function in 352 complement-dependent cytotoxicity test-negative LURD transplant recipients, transplanted between 1997 and 2014 at the Leiden University Medical Center in The Netherlands. Risk factors for immunization were retrieved from the health records. As methods to screen for preformed donor-specific antibodies (pDSA) have developed through time, we retrospectively screened those with ABMR for pDSA using pooled-antigen bead (PAB) and single-antigen bead (SAB) assays. The cumulative incidence of rejection in the first six months after transplantation was 18% (TCMR 15%; early ABMR 3%). Early ABMR resulted in inferior graft survival and was more common in women who received a kidney from their spouse (10%) than in other women (2%) and men (<1%). The SAB assay retrospectively identified pDSA in seven of nine cases of early ABMR (78%), while the PAB detected pDSA in only three cases (33%). Seeing that early ABMR occurred in 10% of women who received a kidney from their spouse, a SAB assay should be included in the pre-transplant assessment of this group of women, regardless of the result of the PAB assay., (Copyright © 2021. Published by Elsevier B.V.)

Highly sensitized kidney patients accrue on the transplant waiting list due to their broad immunization against non-self Human Leucocyte Antigens (HLA). Although challenging, the best option for highly sensitized patients is transplantation with a crossmatch negative donor without any additional therapeutic intervention. The Eurotransplant Acceptable Mismatch (AM) program was initiated more than 30 years ago with the intention to increase the chance for highly sensitized patients to be transplanted with such a compatible donor. The AM program allows for enhanced transplantation to this difficult to transplant patient group by allocating deceased donor kidneys on the basis of a match with the recipient's own HLA antigens in combination with predefined acceptable antigens. Acceptable antigens are those HLA antigens towards which the patients has never formed antibodies, as determined by extensive laboratory testing. By using this extended HLA phenotype for allocation and giving priority whenever a compatible donor organ becomes available, organ offers are made for roughly 80% of patients in this program. Up till now, more than 1700 highly sensitized patients have been transplanted through the AM program. Recent studies have shown that the concept of acceptable mismatches being truly immunologically acceptable holds true for both rejection rates and long-term graft survival. Patients that were transplanted through the AM program had a similar rejection incidence and long-term graft survival rates identical to non-sensitized patients transplanted through regular allocation. However, a subset of patients included in the AM program does not receive an organ offer within a reasonable time frame. As these are often patients with a rare HLA phenotype in comparison to the Eurotransplant donor population, extension of the donor pool for these specific patients through further European collaboration would significantly increase their chances of being transplanted. For those patients that will not benefit from such strategy, desensitization is the ultimate solution., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2021 Heidt, Haasnoot, van der Linden-van Oevelen and Claas.)

Recent data suggest that HLA epitope matching is beneficial for the prevention of de novo donor specific antibody (DSA) formation after transplantation. In this review, different approaches to predict the immunogenicity of an HLA mismatch will be discussed. The parameters used in these models are often called epitopes but the actual antibody epitope is far more complex. Exact knowledge of the antibody epitope is crucial if epitope matching is also used as a tool to select compatible donors for (highly) sensitized patients. Evidence is provided that it is not always possible to give an exact definition of an antibody epitope. We conclude that HLA "epitope" matching is superior over HLA antigen matching with respect to the prevention of de novo DSA formation and will enhance the prediction of acceptable HLA mismatches for sensitized patients. However, epitope matching at our current level of knowledge will not solve all histocompatibility problems as unexpected antibody reactivity still may occur., (© 2018 The Authors. Transplant International published by John Wiley & Sons Ltd on behalf of Steunstichting ESOT.)

The best treatment for patients with end-stage renal disease is kidney transplantation. Although graft survival rates have improved in the last decades, patients still may lose their grafts partly due to the detrimental effects of donor-specific antibodies (DSA) against human leukocyte antigens (HLA) and to a lesser extent also by antibodies directed against non-HLA antigens expressed on the donor endothelium. Assays to detect anti-HLA antibodies are already in use for many years and have been proven useful for transplant risk stratification. Currently, there is a need for assays to additionally detect multiple non-HLA antibodies simultaneously in order to study their clinical relevance in solid organ transplantation. This study describes the development, technical details and validation of a high-throughput multiplex assay for the detection of antibodies against 14 non-HLA antigens coupled directly to MagPlex microspheres or indirectly via a HaloTag. The non-HLA antigens have been selected based on a literature search in patients with kidney disease or following transplantation. Due to the flexibility of the assay, this approach can be used to include alternative antigens and can also be used for screening of other organ transplant recipients, such as heart and lung.

Histocompatibility testing, and HLA antibody screening in particular, varies in practice among laboratories. Currently, standards are lacking regarding the reporting of testing methods in publications. It is essential that scientific methods are rigorously and transparently described upon publication, so that results can be accurately interpreted and independently corroborated. Additionally, this would allow work groups to compile diverse data to achieve clinically significant conclusions from meta-analyses. These efforts are hindered when there is a paucity of method descriptions and where variability in serum treatment, protocol modifications, and assay thresholds affecting assay interpretation are known to exist. Thus, the ASHI Science and Technology Initiatives Committee (ASHI STIC) undertook the task of formulating recommendations for reporting HLA antibody testing by solid phase assays, and the associated HLA typing required for interpretation, in scientific publications. Herein we put forth standards for minimum information about HLA antibody testing methods reported in histocompatibility publications., (Copyright © 2018. Published by Elsevier Inc.)

Luminex single antigen bead assays revolutionized human leukocyte antigen (HLA) antibody detection owing to their superior sensitivity compared to conventional methods. Nevertheless, the advent of higher sensitivity came at the expense of difficulty in clinical decision-making, since not all luminex detectable antibodies are clinically relevant. Therefore, new tools such as C1q/C3d assays and IgG subclass analysis emerged with the aim to discriminate the inert antibodies from the deleterious ones. Here, we provide an overview on the technical challenges related to these different HLA antibody detection systems and briefly refer to the recent literature regarding the clinical relevance of these assays, mainly in the field of kidney transplantation., (© 2018 Steunstichting ESOT.)

Purpose of Review: Highly sensitized patients awaiting a renal transplant have a low chance of receiving an organ offer. Defining acceptable antigens and using this information for allocation purposes can vastly enhance transplantation of this subgroup of patients, which is the essence of the Eurotransplant Acceptable Mismatch program. Acceptable antigens can be determined by extensive laboratory testing, as well as on basis of human leukocyte antigen (HLA) epitope analyses., Recent Findings: Within the Acceptable Mismatch program, there is no effect of HLA mismatches on long-term graft survival. Furthermore, patients transplanted through the Acceptable Mismatch program have similar long-term graft survival to nonsensitized patients transplanted through regular allocation. Although HLA epitope analysis is already being used for defining acceptable HLA antigens for highly sensitized patients in the Acceptable Mismatch program, increasing knowledge on HLA antibody - epitope interactions will pave the way toward the definition of acceptable epitopes for highly sensitized patients in the future., Summary: Allocation based on acceptable antigens can facilitate transplantation of highly sensitized patients with excellent long-term graft survival.

Individual HLA mismatches may differentially impact graft survival after kidney transplantation. Therefore, there is a need for a reliable tool to define permissible HLA mismatches in kidney transplantation. We previously demonstrated that donor-derived Predicted Indirectly ReCognizable HLA Epitopes presented by recipient HLA class II (PIRCHE-II) play a role in de novo donor-specific HLA antibodies formation after kidney transplantation. In the present Dutch multi-center study, we evaluated the possible association between PIRCHE-II and kidney graft failure in 2,918 donor-recipient couples that were transplanted between 1995 and 2005. For these donors-recipients couples, PIRCHE-II numbers were related to graft survival in univariate and multivariable analyses. Adjusted for confounders, the natural logarithm of PIRCHE-II was associated with a higher risk for graft failure [hazard ratio (HR): 1.13, 95% CI: 1.04-1.23, p  = 0.003]. When analyzing a subgroup of patients who had their first transplantation, the HR of graft failure for ln(PIRCHE-II) was higher compared with the overall cohort (HR: 1.22, 95% CI: 1.10-1.34, p  < 0.001). PIRCHE-II demonstrated both early and late effects on graft failure in this subgroup. These data suggest that the PIRCHE-II may impact graft survival after kidney transplantation. Inclusion of PIRCHE-II in donor-selection criteria may eventually lead to an improved kidney graft survival.

This review describes the recent developments in the applicability and clinical significance of epitope matching in kidney transplantation. As incremental human leukocyte antigen (HLA) mismatches are associated with increased risk of rejection and allograft loss, HLA-matching remains one of the standard immunological triage tests to determine transplant suitability. Advancements in tissue-typing techniques have led to innovative concepts of HLA-matching at the epitope level. Epitopes are configurations of polymorphic amino acid residues that are recognized by B cells, and antibodies reactive with these epitopes lead to rejection and/or premature allograft loss. Even though there is substantial advancement in the characterization and understanding of epitopes, evidence supporting the added clinical benefit of epitope matching over traditional antigen matching and the ability to identify clinically relevant immunogenic epitopes remains poorly defined. We present an overview of the evidence surrounding the immunogenicity of mismatched donor epitopes and the applicability of HLA epitope matching in kidney transplantation. A better understanding of the immunogenicity and structural characteristics of HLA epitopes will guide clinicians to integrate epitope matching as an important parameter for donor selection in kidney transplantation., (Copyright © 2017 International Society of Nephrology. Published by Elsevier Inc. All rights reserved.)

Highly sensitized renal transplant candidates accumulate on transplant waiting lists since they produce antibodies to many HLA antigens, which in this way become unacceptable. Organ allocation to these patients is usually based on avoiding transplantation of organs bearing these unacceptable antigens. In contrast, allocation through the Eurotransplant Acceptable Mismatch (AM) program is based on extension of the patient's own HLA type with so-called acceptable HLA antigens to which strictly no antibodies are formed, as shown by extensive laboratory testing. We questioned which type of allocation results in the best long-term graft survival. Therefore, we selected 58,727 cadaveric single renal transplant recipients transplanted within Eurotransplant between 1996 and 2015 and determined factors influencing graft survival for patients transplanted through the AM program. Next, we compared ten-year graft survival of patients with various sensitization grades who received a renal transplant through regular allocation to that of highly sensitized patients transplanted through the AM program. Unlike regular allocation, no effect for HLA mismatches existed for AM patients, while factors that did affect graft survival were similar to those of the general kidney transplant population. AM patients had significantly superior ten-year graft survival compared to highly sensitized patients transplanted on the basis of avoidance of unacceptable mismatches. Strikingly, graft survival of AM patients receiving a repeat transplant was similar to that of nonsensitized repeat transplant recipients. Thus, allocation of kidneys to highly sensitized patients based on proven acceptable antigens results in a significantly better graft survival compared to mere avoidance of unacceptable mismatches., (Copyright © 2017 International Society of Nephrology. Published by Elsevier Inc. All rights reserved.)

Humoral responses against mismatched donor HLA are routinely measured as serum HLA antibodies, which are mainly produced by bone marrow-residing plasma cells. Individuals with a history of alloimmunization but lacking serum antibodies may harbor circulating dormant memory B cells, which may rapidly become plasma cells on antigen reencounter. Currently available methods to detect HLA-specific memory B cells are scarce and insufficient in quantifying the complete donor-specific memory B cell response due to their dependence on synthetic HLA molecules. We present a highly sensitive and specific tool for quantifying donor-specific memory B cells in peripheral blood of individuals using cell lysates covering the complete HLA class I and class II repertoire of an individual. Using this enzyme-linked immunospot (ELISpot) assay, we found a median frequency of 31 HLA class I and 89 HLA class II-specific memory B cells per million IgG-producing cells directed at paternal HLA in peripheral blood samples from women (n = 22) with a history of pregnancy, using cell lysates from spouses. The donor-specific memory B cell ELISpot can be used in HLA diagnostic laboratories as a cross-match assay to quantify donor-specific memory B cells in patients with a history of sensitizing events., (© 2017 The American Society of Transplantation and the American Society of Transplant Surgeons.)

The historical observation of Jon van Rood that pregnancy can lead to the induction of HLA antibodies (1) was the start of a long history of research on immunogenetics and immunology of transplantation in Leiden. Some of the current research topics are presented in this mini-review. These include amongst others the differential immunogenicity of HLA mismatches in clinical transplantation, a special strategy to transplant highly sensitized patients, new tools to monitor donor specific memory B cells, new parameters for risk assessment in transplant recipients and the importance of the innate immune response in transplantation., (Copyright © 2014. Published by Elsevier B.V.)