Caveats of HLA antibody detection by solid‐phase assays

Anti-HLA antibodies may be a subset of polyreactive immunoglobulins generated after viral superinfection

Chronic rejection remains an obstacle to long-term allograft survival. Donor-specific anti-HLA antibodies (DSA) play a significant role in causing chronic antibody-mediated allograft rejection. Exposure to mismatched HLA antigens via transfusion, pregnancy, or transplanted tissue has been described in the literature as an immunogenic stimulus of anti-HLA antibodies. Yet anti-HLA antibodies also develop in the absence of traditional sensitization events and molecular mimicry has been postulated as a stimulus for these naturally occurring alloantibodies. While heterologous reactivity has been documented between virus components and allogeneic T cells, there is insufficient evidence to support the development of anti-HLA antibodies from viral components. We hypothesized that anti-HLA antibodies may develop following viral coinfection or superinfection. The objectives of this investigation included: 1) developing an in-silico algorithm to identify viral peptide components that exhibit HLA-specific homology, and 2) identifying cellular changes that take place during ischemia/reperfusion injury which could facilitate the generation of novel anti-HLA antibodies from viral sources. We developed the neoepitope transplant rejection and autoimmune disease (NETRAD) algorithm to identify amino acid sequence homology between viral envelope proteins and HLA. The algorithm integrates post-translational protein modifications that are consistent with ischemia/reperfusion injury. Seventy-two HLA-specific epitopes were demarcated as examples using this approach. In conclusion, we present in-silico evidence which supports the identification of anti-HLA antibodies as a subset of polyreactive antibodies generated from stress-modified viral envelope proteins. Remarkably, each targeted HLA epitope associated with a distinct anti-HLA antibody could be consistently attributed to a major envelope glycoprotein component of Epstein Barr virus. Transplant Immunology manuscript # TRIM-D-24-00351. Dryad data repository:https://doi.org/10.5061/dryad.qjq2bvqpq.

Antigen quantity is responsible for the discrepancy between phenotype and single antigen beads for the detection of human leukocyte antigen DQ antibody: Potential clinical implications

De novo donor-specific human leukocyte antigen DQ antibodies detected by single antigen beads (SAB) are significantly associated with chronic antibody-mediated rejection and lower overall graft survival. However, some DQ antibodies identified by SAB cannot be confirmed by phenotype antigen-bearing class II bead assays, raising concerns about the validity of SAB data. The inability to detect these antibodies on phenotype antigen beads could be due to a lower quantity of DQ antigens present on the surface of the beads compared to DR antigens. In this study, we demonstrate that DQ-enriched phenotype antigens exhibit the same reactivity with DQ antibodies detected by SAB, confirming the hypothesis that it is antigen quantity and not structural differences that account for discrepancies in human leukocyte antigen DQ antibody detection between phenotype antigen beads and SAB. In addition, we show that the expression of individual DQ antigens in heterozygous cells can vary significantly, further confounding correlation studies. Therefore, the common clinical practice of using the phenotype antigen beads as a screening assay, reflexing to SAB testing only when positive, may inadvertently fail to detect DQ-specific antibodies. Such errors could impact organ acceptance practices, immunosuppression treatment decisions, and/or the need for additional diagnostic testing to rule out antibody-mediated rejection.

Guidelines From the French-Speaking Society of Histocompatibility and Immunogenetics for Virtual Crossmatching for Deceased Donor Kidney Transplantation and the Use of Wet Crossmatch in the Decision-Making Process

The systematic use of Single Antigen Flow Beads assays and the implementation of high-resolution HLA typing for donors and kidney transplant recipients allow a precise identification of anti-HLA donor-specific antibodies. In France, the availability of detailed molecular biology HLA typing for deceased donors in the national organ allocation software enables anticipation of wet crossmatch results and estimation of the immunological risk for a recipient/donor pair. This key process, named virtual crossmatching, involves a thorough analysis of the recipient's anti-HLA sensitisation records. Its main goal is to reduce cold ischaemia time in order to extend graft survival. In this article, we present the guidelines for virtual crossmatching developed by a working group from the French-speaking Society of Histocompatibility and Immunogenetics. The guidelines address several considerations regarding HLA typing, anti-HLA antibody testing, and sensitisation event history, which are required to perform virtual crossmatching. We also propose a decision-making process, which situates prospective or retrospective wet crossmatch depending on virtual crossmatch results. The guidelines specifically emphasise the need for a strong clinical-biological agreement to standardise practices and provide a framework for omission of wet crossmatch for both non-sensitised and sensitised recipients.

Prevalence of anti-HLA antibodies among live related renal transplant recipients: A retrospective observational study from a tertiary healthcare Center in India

AIM

Detection of anti-HLA antibodies is crucial for pre-transplant histocompatibility testing, donor selection, and graft survival. The aim of this study was to evaluate the spectrum of anti-HLA antibodies among live related renal transplant recipients from one of the largest transplant centers in north India.

METHODS

In this study, retrospective data of transplant workup done in past four years were analyzed using GraphPad Prism 9 Version 9.2.0 (La Jolla, CA, USA). All samples received for pre-transplant work-up if showed positive screening results underwent Luminex single antigen bead (L-SAB) assay. Antibodies identified on L-SAB were evaluated for their specificity and the strength of their mean florescence intensity (MFI).

RESULTS

A total of 1250 renal transplant samples were included for analysis. Out of these, 458 (36.64 %) samples were found positive in screening tests algorithm. All 458 samples were further analyzed for both class I and class II HLA antibodies by Luminex single antigen bead (L-SAB) assay. In this study, we observed that anti-A*24:01, anti-B*15:01 and anti-C*07*01 were the three most prevalent anti-HLA antibodies identified against HLA-class I antigens. However, anti-DRB1*11:01, anti-DQA1*05:01-DQB1*03:01 and anti-DPA1*02:01-DPB1*17:01 were the most common anti-HLA antibodies identified against HLA-class II antigen. Furthermore, our study found a significant association between anti-HLA class I antibody and the history of pregnancy. However, in re-transplant cases, we observed the presence of antibodies both against HLA class I and II antigens.

CONCLUSION

For a transplant center, it is of utmost importance to have comprehensive knowledge about the prevalence of HLA antibodies, their MFI, and their association with various sensitization events. This study may immensely help transplant communities in selecting appropriate prospective organ donors, planning desensitization regimes, managing recipients' care and predicting transplant outcomes in live related renal transplantation.

From hero to zero: a single center retrospective review of the utility of routine physical crossmatching

Pretransplantation human leukocyte antigen antibody testing is necessary to assess compatibility between donor and recipient pairs. Over the past decade, the virtual crossmatch (VXM) has replaced the physical crossmatch (PXM) as the main assessment of pretransplant histocompatibility. At our center, most transplants have proceeded based solely on the VXM, followed by a retrospective PXM. In this study, we sought to determine whether VXMs alone are sufficient and if routine retrospective PXMs can be discontinued. A review of PXMs for single solid organ transplant cases between May 19, 2020, and December 9, 2022, was performed to examine the concordance between the PXM and VXM (including predictions of the PXM made in the VXM report). The relationship between PXM results and donor-specific antibody (DSA) mean fluorescence intensity values was also examined. The overall PXM prediction/PXM result concordance rate was 99.7% based on PXMs attributed to human leukocyte antigen antibody. Furthermore, DSA mean fluorescence intensity values can guide clinical decisions on whether to perform a PXM. The data confirm that, in the era of the VXM, routine PXMs performed for all transplants are not warranted. However, a PXM may still be informative in select cases where low-to-intermediate strength DSAs are present or when the virtual assessment is ambiguous.

Combined Molecular Mismatch Approaches to Predict Immunological Events Within the First Year After Renal Transplantation

Several molecular mismatch assessment approaches exist, but data on their combined use are limited. In this study, we aimed to define distinct risk groups for rejection based on the combination of three molecular mismatch assessment approaches (i.e., eplet mismatch count, the number of highly immunogenic eplets and PIRCHE-II score) in 439 consecutive immunological standard risk transplantations. For each molecular mismatch assessment approach, ROC analyses were used to define cut-offs for prediction of (sub) clinical rejection according to Banff 2019 classification within the first year post-transplant as a reference. If all three scores were below the cut-off, the patient was assigned to the low-risk group (19% of patients); if all three scores were above the cut-off, the patient was assigned to the high-risk group (21% of patients). The one-year incidence of (sub) clinical rejection was 12% in the low-risk group and 33% in the high-risk group (p = 0.003). Internal validation of the assigned risk groups for prediction of other outcomes revealed a high consistency: clinical rejection (6% vs. 24%; p = 0.004), ATG-treated rejection (1% vs. 16%; p < 0.001) and development of de novo HLA-DSA at 5 years post-transplant (6% vs. 25%; p = 0.003). The molecular mismatch risk group was an independent predictor for (sub) clinical rejection (high-risk vs. low-risk: hazard ratio 3.11 [95%-CI 1.50-6.45]; p = 0.002). We conclude that combining molecular mismatch approaches allows us to distinguish low- and high-risk groups among standard renal allograft recipients. Independent validation in other patient populations and different ethnicities is required.

Functionalized magnetic nanoparticles remove donor-specific antibodies (DSA) from patient blood in a first ex vivo proof of principle study

The presence of donor-specific antibodies (DSA) such as antibodies directed against donor class I human leucocyte antigen (e.g., HLA-A) is a major barrier to kidney transplant success. As a proof of concept, functionalized magnetic nanoparticles have been designed to eliminate DSA from saline, blood and plasma of healthy donors and sensitized patients. Specific HLA-A1 protein was covalently bound to functionalized cobalt nanoparticles (fNP), human serum albumin (HSA) as control. fNP were added to anti-HLA class I-spiked saline, spiked volunteers' whole blood, and to whole blood and plasma of sensitized patients ex vivo. Anti-HLA-A1 antibody levels were determined with Luminex technology. Antibodies' median fluorescent intensity (MFI) was defined as the primary outcome. Furthermore, the impact of fNP treatment on blood coagulation and cellular uptake was determined. Treatment with fNP reduced MFI by 97 ± 2% and by 94 ± 4% (p < 0.001 and p = 0.001) in spiked saline and whole blood, respectively. In six known sensitized anti-HLA-A1 positive patients, a reduction of 65 ± 26% (p = 0.002) in plasma and 65 ± 33% (p = 0.012) in whole blood was achieved. No impact on coagulation was observed. A minimal number of nanoparticles was detected in peripheral mononuclear blood cells. The study demonstrates-in a first step-the feasibility of anti-HLA antibody removal using fNP. These pilot data might pave the way for a new personalized DSA removal technology in the future.

Advancing precision in histocompatibility and immunogenetics: a comprehensive review of the UCLA exchange program

Precise typing of human leukocyte antigens (HLA) is crucial for clinical hematopoietic stem cell and solid organ transplantations, transfusion medicine, HLA-related disease association, and drug hypersensitivity analysis. The UCLA Cell Exchange program has played a vital role in providing educational and proficiency testing surveys to HLA laboratories worldwide for the past 5 decades. This article highlights the significant contribution of the UCLA Cell and DNA Exchange Programs in advancing HLA antibody testing, genotyping, crossmatches, and, more recently, virtual crossmatches. Additionally, we discuss future directions of the UCLA Cell Exchange program to support histocompatibility testing to adapt to the fast-evolving field of immunotherapy, tolerance and xenotransplantation.

HLA class I peptide polymorphisms contribute to class II DQβ0603:DQα0103 antibody specificity

Antibodies reactive to human leukocyte antigens (HLA) represent a barrier for patients awaiting transplantation. Based on reactivity patterns in single-antigen bead (SAB) assays, various epitope matching algorithms have been proposed to improve transplant outcomes. However, some antibody reactivities cannot be explained by amino acid motifs, leading to uncertainty about their clinical relevance. Antibodies against the HLA class II molecule, DQβ0603:DQα0103, present in some candidates, represent one such example. Here, we show that peptides derived from amino acids 119-148 of the HLA class I heavy chain are bound to DQβ0603:DQα0103 proteins and contribute to antibody reactivity through an HLA-DM-dependent process. Moreover, antibody reactivity is impacted by the specific amino acid sequence presented. In summary, we demonstrate that polymorphic HLA class I peptides, bound to HLA class II proteins, can directly or indirectly be part of the antibody binding epitope. Our findings have potential important implications for the field of transplant immunology and for our understanding of adaptive immunity.

Modern advances in heart transplantation

Heart transplantation (HTx) is the only definitive therapy for patients with end stage heart disease. With the increasing global prevalence of heart failure, the demand for HTx has continued to grow and outpace supply. In this paper, we will review advances in the field of HTx along the clinical journey of a HTx recipient. Starting with the sensitized patient, we discuss current methods to define sensitization, and assays to help identify clinically relevant anti-HLA antibodies. Desensitization strategies targeting all levels of the adaptive immune system are discussed with emphasis on novel techniques such as anti-CD 38 blockade and use of the Immunoglobulin G-Degrading Enzyme of Streptococcus Pyogenes. We next discuss donor procurement and the resurgence of donation after circulatory death as a viable strategy to significantly and safely increase the donor pool. Post-transplant, we evaluate non-invasive surveillance techniques including gene expression profiling and donor-derived cell-free DNA. Last, we discuss the ground-breaking developments in the field of xenotransplantation.

Role of Complement-dependent Cytotoxicity Crossmatch and HLA Typing in Solid Organ Transplant

BACKGROUND

Solid organ transplantation is a life-saving medical operation that has progressed greatly because of developments in diagnostic tools and histocompatibility tests. Crossmatching for complement-dependent cytotoxicity (CDC) and human leukocyte antigen (HLA) typing are two important methods for checking graft compatibility and reducing the risk of graft rejection. HLA typing and CDC crossmatching are critical in kidney, heart, lung, liver, pancreas, intestine, and multi-organ transplantation.

METHODS

A systematic literature search was conducted on the internet, using PubMed, Scopus, and Google Scholar databases, to identify peer-reviewed publications about solid organ transplants, HLA typing, and CDC crossmatching.

CONCLUSION

Recent advances in HLA typing have allowed for high-resolution evaluation, epitope matching, and personalized therapy methods. Genomic profiling, next-generation sequencing, and artificial intelligence have improved HLA typing precision, resulting in better patient outcomes. Artificial intelligence (AI) driven virtual crossmatching and predictive algorithms have eliminated the requirement for physical crossmatching in the context of CDC crossmatching, boosting organ allocation and transplant efficiency. This review elaborates on the importance of HLA typing and CDC crossmatching in solid organ transplantation.

De Novo Donor-Specific Antibodies after Heart Transplantation: A Comprehensive Guide for Clinicians

Antibodies directed against donor-specific human leukocyte antigens (HLAs) can be detected de novo after heart transplantation and play a key role in long-term survival. De novo donor-specific antibodies (dnDSAs) have been associated with cardiac allograft vasculopathy, antibody-mediated rejection, and mortality. Advances in detection methods and international guideline recommendations have encouraged the adoption of screening protocols among heart transplant units. However, there is still a lack of consensus about the correct course of action after dnDSA detection. Treatment is usually started when antibody-mediated rejection is present; however, some dnDSAs appear years before graft failure is detected, and at this point, damage may be irreversible. In particular, class II, anti-HLA-DQ, complement binding, and persistent dnDSAs have been associated with worse outcomes. Growing evidence points towards a more aggressive management of dnDSA. For that purpose, better diagnostic tools are needed in order to identify subclinical graft injury. Cardiac magnetic resonance, strain techniques, or coronary physiology parameters could provide valuable information to identify patients at risk. Treatment of dnDSA usually involves plasmapheresis, intravenous immunoglobulin, immunoadsorption, and ritxumab, but the benefit of these therapies is still controversial. Future efforts should focus on establishing effective treatment protocols in order to improve long-term survival of heart transplant recipients.

A modeling approach for mean fluorescence intensity value harmonization and cutoff prediction for luminex single antigen bead assays of two different vendors

Luminex single antigen bead (SAB) kits from One Lambda (OL) and Lifecodes (LC) are widely used for HLA antibody detection but have substantial differences in design and assay protocol resulting in different mean fluorescence intensity (MFI) values. Here, we present a non-linear modeling approach to accurately convert MFI values between two vendors and to establish user-independent MFI cutoffs when analyzing big datasets. HLA antibody data from a total of 47 EDTA-treated sera tested using both OL and LC SAB kits were analyzed. MFI comparisons were made for the common 84 HLA class I and 63 class II beads. In the exploration set (n = 24), a non-linear hyperbola model on raw MFI corrected by locus-specific highest self MFI subtraction yielded the highest correlation (class I r2 : 0.946, class II r2 : 0.898). Performance of the model was verified in an independent validation set (n = 12) (class I r2 : 0.952, class II r2 : 0.911). Furthermore, in an independent cohort of post-transplant serum samples (n = 11) using the vendor-specific MFI cutoffs dictated by the current model, we found 94% accuracy in bead-specific reactivity assignments by the two vendors. We recommend using the non-linear hyperbola modeling approach with self HLA correction and locus-specific analyzes to harmonize MFI values between two vendors in particular research datasets. As there are considerable variations between the two assays, using MFI conversion for individual patient samples is not recommended.

BSHI and BTS UK guideline on the detection of alloantibodies in solid organ (and islet) transplantation

Solid organ transplantation represents the best (and in many cases only) treatment option for patients with end-stage organ failure. The effectiveness and functioning life of these transplants has improved each decade due to surgical and clinical advances, and accurate histocompatibility assessment. Patient exposure to alloantigen from another individual is a common occurrence and takes place through pregnancies, blood transfusions or previous transplantation. Such exposure to alloantigen's can lead to the formation of circulating alloreactive antibodies which can be deleterious to solid organ transplant outcome. The purpose of these guidelines is to update to the previous BSHI/BTS guidelines 2016 on the relevance, assessment, and management of alloantibodies within solid organ transplantation.

Utilizing principal component analysis in the identification of clinically relevant changes in patient HLA single antigen bead solid phase testing patterns

BACKGROUND

HLA antibody testing is essential for successful solid-organ allocation, patient monitoring post-transplant, and risk assessment for both solid-organ and hematopoietic transplant patients. Luminex solid-phase testing is the most common method for identifying HLA antibody specificities, making it one of the most complex immunoassays as each panel contains over 90 specificities for both HLA class I and HLA class II with most of the analysis being performed manually in the vendor-provided software. Principal component analysis (PCA), used in machine learning, is a feature extraction method often utilized to assess data with many variables.

METHODS & FINDINGS

In our study, solid organ transplant patients who exhibited HLA donor-specific antibodies (DSAs) were used to characterize the utility of PCA-derived analysis when compared to a control group of post-transplant and pre-transplant patients. ROC analysis was utilized to determine a potential threshold for the PCA-derived analysis that would indicate a significant change in a patient's single antigen bead pattern. To evaluate if the algorithm could identify differences in patterns on HLA class I and HLA class II single antigen bead results using the optimized threshold, HLA antibody test results were analyzed using PCA-derived analysis and compared to the clinical results for each patient sample. The PCA-derived algorithm had a sensitivity of 100% (95% CI, 73.54%-100%), a specificity of 75% (95% CI, 56.30%-92.54%), with a PPV of 65% (95% CI, 52.50%-83.90%) and an NPV of 100%, in identifying new reactivity that differed from the patients historic HLA antibody pattern. Additionally, PCA-derived analysis was utilized to assess the potential over-reactivity of single antigen beads for both HLA class I and HLA class II antibody panels. This assessment of antibody results identified several beads in both the HLA class I and HLA class II antibody panel which exhibit over reactivity from 2018 to the present time.

CONCLUSIONS

PCA-derived analysis would be ideal to help automatically identify patient samples that have an HLA antibody pattern of reactivity consistent with their history and those which exhibit changes in their antibody patterns which could include donor-specific antibodies, de novo HLA antibodies, and assay interference. A similar method could also be applied to evaluate the over-reactivity of beads in the HLA solid phase assays which would be beneficial for lot comparisons and instructive for transplant centers to better understand which beads are more prone to exhibiting over-reactivity and impact patient care.

Virtual and Reality: An Analysis of the UCLA Virtual Crossmatch Exchanges

The "virtual" crossmatch (VXM) has become a critical tool to predict the compatibility between an organ donor and a potential recipient. Yet, nonstandardized laboratory practice can lead to variability in VXM interpretation. Therefore, UCLA's VXM Exchange survey was designed to understand factors that influence the variability of VXM prediction in the presence of HLA donor-specific antibody (DSA). Thirty-six donor blood samples and 72 HLA reference sera were sent to 35 participating laboratories to perform HLA antibody testing, flow crossmatch (FXM), and VXM from 2014 to 2019, consisting of 144 T/B-cell FXM pairs and 112 T/B-cell VXM pairs. In the FXM survey, 86% T-cell FXM and 84% B-cell FXM achieved >80% concordance among laboratories. In the VXM survey, 81% T-cell VXM and 80% VXM achieved >80% concordance. The concordance between FXM and VXM was 79% for T cell and 87% for B cell. The consensus between VXM and FXM was high with strong DSA. However, significant variability was observed in sera with (1) very high titer antibodies that exit prozone effect; (2) weak-to-moderate DSA, particularly in the presence of multiple weak DSAs; and (3) DSA against lowly expressed antigens. With the increasing use the VXM, standardization and continuous learning via exchange surveys will provide better understanding and quality controls for VXM to improve accuracy across all centers.

Donor-specific HLA antibodies are associated with graft failure and delayed hematologic recovery after unrelated donor hematopoietic cell transplantation

BACKGROUND

Graft failure (GF) is one of the major concerns after allogeneic hematopoietic cell transplantation (allo-HCT) and remains a significant cause of morbidity and mortality. Although earlier reports have associated the presence of donor-specific HLA antibodies (DSAs) with increased risk of GF after unrelated donor allo-HCT, recent studies have failed to confirm this association.

OBJECTIVE

We sought to validate the presence of DSAs as a risk factor for GF and hematologic recovery in the unrelated donor allo-HCT setting.

STUDY DESIGN

We retrospectively evaluated 303 consecutive patients who underwent their first unrelated donor allo-HCT at our institution from January 2008 to December 2017. DSA evaluation was performed using 2 Single Antigen Beads (SAB) assays, DSA titration with 1:2, 1:8, and 1:32 dilutions, C1q-binding assay, and absorption/elution protocol to assess possible false-positive DSA reactivity. The primary endpoints were neutrophil and platelet recovery and GF, whereas the secondary endpoint was overall survival. Multivariable analyses were performed using Fine-Gray competing risks regression or Cox proportional hazards regression models.

RESULTS

The median patient age was 14 years (range, 0-61 years), 56.1% were male, and 52.5% were transplanted for nonmalignant diseases. Eleven patients (3.63%) were DSA-positive. Of them, 10 had preexisting DSAs, and one showed post-transplant de novo DSA. Nine patients had 1 DSA, 1 had 2 DSAs, and 1 had 3 DSAs, with a median MFI of 4334 (range, 588-20,456) and 3581 (range, 227-12,266) in LABScreen and LIFECODES SAB assays, respectively. Overall, 21 patients experienced GF. Of them, 12 had primary graft rejection, 8 had secondary graft rejection, and 1 had primary poor graft function. The cumulative incidences of GF at 28, 100, and 365 days were 4.0% (95% CI, 2.2%-6.6%), 6.6% (95% CI, 4.2%-9.8%), and 6.9% (95% CI, 4.4%-10.2%), respectively. In the multivariable analyses, DSA-positive patients had significantly delayed neutrophil (subdistribution hazard ratio [SHR] = 0.48; 95% CI, 0.29-0.81; P = .006) and platelet recovery (SHR = 0.51; 95% CI, 0.35-0.74; P = .0003) than patients without DSAs. In addition, only DSAs were significant predictors of primary GF at 28 days (SHR = 2.78; 95% CI, 1.65-4.68; P = .0001). The Fine-Gray regression also demonstrated that the presence of DSAs was strongly associated with a higher incidence of overall GF (SHR = 7.60; 95%CI, 2.61-22.14; P = .0002). DSA-positive patients with GF had significantly higher median MFI values than DSA-positive patients who achieved engraftment in LIFECODES SAB assay using neat serum (10,334 vs. 1250; P = .006) and in LABScreen SAB at 1:32 dilution (1627 vs. 61; P = .006). All 3 patients with C1q-positive DSAs failed to engraft. DSAs were not predictive of inferior survival (hazard ratio = 0.50; 95% CI, 0.20-1.26, P = .14).

CONCLUSIONS

Our results validate the presence of DSAs as a significant risk factor for GF and poor hematologic recovery after unrelated donor allo-HCT. Thus, careful pre-transplant DSA evaluation may optimize unrelated donor selection and improve allo-HCT outcomes.

High-Dose immunoglobulin Intervention as an effective and simple strategy for donor specific Anti-HLA antibody desensitization in haploidentical transplant

Donor-specific anti-HLA antibody (DSA) is a significant obstacle to successful haploidentical hematopoietic stem cell transplantation (haplo-HSCT) and is associated with poor engraftment rates. DSA strongly positive patients with a mean fluorescence intensity (MFI) over 5000 have a primary poor graft function (PGF) rate of over 60%. Currently, there is no consensus on the desensitization of DSA, and existing strategies are complex and have limited effectiveness. To address this issue, we conducted a retrospective study on 19 patients with strongly positive DSA (MFI over 5000) who underwent haplo-HSCT and were treated with intravenous immunoglobulin (IVIg)-based therapy. We also included 38 baseline-matched patients with DSA-negative as controls. Our findings revealed that the cumulative incidence of engraftment, PGF, graft-versus-host disease (GVHD), virus infection, overall survival (OS), disease-free survival (DFS), relapse, and non-relapse mortality (NRM) in the DSA strongly positive group after desensitization were comparable to those in the DSA negative group (P > 0.05). Our multivariable analysis showed that disease remission was a protective factor against PGF (P = 0.005, OR = 0.019, 95% CI 0.001-0.312). Subgroup analysis revealed that the desensitization efficacy was equal regardless of DSA type against HLA-I or II, and MFI value over 5000 or not. In conclusion, we propose a simple and effective DSA desensitization strategy based on immunoglobulin to ensure successful engraftment and improve patient prognosis.

Harnessing the B Cell Response in Kidney Transplantation - Current State and Future Directions

Despite dramatic improvement in kidney transplantation outcomes over the last decades due to advent of modern immunosuppressive agents, long-term outcomes remain poor. Antibody-mediated rejection (ABMR), a B cell driven process, accounts for the majority of chronic graft failures. There are currently no FDA-approved regimens for ABMR; however, several clinical trials are currently on-going. In this review, we present current mechanisms of B cell response in kidney transplantation, the clinical impact of sensitization and ABMR, the B cell response under current immunosuppressive regimens, and ongoing clinical trials for ABMR and desensitization treatment.

The impact of HLA donor-specific antibodies on engraftment and the evolving desensitization strategies

The majority of contemporary allogeneic hematopoietic stem cell transplantation (HCT) procedures utilize partially HLA-mismatched stem cell grafts. Donor-specific anti-HLA antibodies (DSA) are associated with primary graft failure independent of the graft source, conditioning intensity and other patient and donor factors. Here we provide an update on testing and monitoring of DSA, review the impact of DSA on stem cell engraftment, and present promising desensitization modalities. Ultimately, we attempt to provide practical recommendations for DSA screening and mitigation strategies.

Highly sensitised patients awaiting deceased donor renal transplants are disadvantaged by the presence of denatured HLA antibody detected in routine HLA antibody testing

Luminex Single Antigen Bead (SAB) assays used to detect HLA antibodies may artificially increase sensitisation in highly sensitised patients. The presence of denatured HLA (dHLA) within the assay enables antibodies specific to cryptic HLA epitopes to bind, such antibodies are not clinically relevant. We sought to exclude dHLA reactivity in a cohort of very highly sensitised patients (HSP), calculated reaction frequency (cRF) 95-100%, and determine the effect upon sensitisation. Such patients have limited access to suitable donors and small changes in their HLA antibody profile, particularly where their cRF is 100%, can increase their opportunity of a transplant. We determined the presence of dHLA by aligning antibody reactivity which did not correspond to known HLA class I epitope mismatches with the results of assays modified to detect class I dHLA. 130 class I dHLA reactions were identified within 11 HSP, all of whom had clear sensitising events. cRF was corrected for dHLA, mean cRF 98.2% (93-100) pre and 95.5% (87-100) post correction (P = 0.0156). An increase in the number of predicted compatible donors (p = 0.0078) after dHLA correction was demonstrated. Two manufacturers SAB assays were used. A reduction of patients with 100% cRF was observed for both manufactures. dHLA is contributing to sensitisation in HSP and is detrimental to their chances of receiving a compatible transplant. The observed dHLA reactivity varied according to kit manufacturers (P = 0.0001), this is potentially a useful finding for laboratories wishing to discriminate between nHLA and dHLA, but without the resources required to regularly perform dHLA assay and epitope analyses. This article is protected by copyright. All rights reserved.

Determination of unacceptable HLA antigen mismatches in kidney transplant recipients

With the introduction of the virtual allocation crossmatch in the Eurotransplant (ET) region in 2023, the determination of unacceptable antigen mismatches (UAM) in kidney transplant recipients is of utmost importance for histocompatibility laboratories and transplant centers. Therefore, a joined working group of members from the German Society for Immunogenetics (Deutsche Gesellschaft für Immungenetik, DGI) and the German Transplantation Society (Deutsche Transplantationsgesellschaft, DTG) revised and updated the previous recommendations from 2015 in light of recently published evidence. Like in the previous version, a wide range of topics is covered from technical issues to clinical risk factors. This review summarizes the evidence about the prognostic value of contemporary methods for HLA antibody detection and identification, as well as the impact of UAM on waiting time, on which these recommendations are based. As no clear criteria could be determined to differentiate potentially harmful from harmless HLA antibodies, the general recommendation is to assign all HLA against which plausible antibodies are found as UAM. There is, however, a need for individualized solutions for highly immunized patients. These revised recommendations provide a list of aspects that need to be considered when assigning UAM to enable a fair and comprehensible procedure and to harmonize risk stratification prior to kidney transplantation between transplant centers. This article is protected by copyright. All rights reserved.

Frequency, reactivity and evolution of human leukocyte antigen and human platelet antigen antibodies in the setting of hematopoietic cell transplantation

BACKGROUND AND OBJECTIVES

Antibodies (Ab) against HLA and HPA antigens play an important role in HCT. In this prospective study we evaluated prevalence and kinetics of HLA- and HPA-Ab after HCT, including a possible donor-recipient transfer and their clinical relevance in respect to platelet transfusion refractoriness (PTR).

MATERIALS AND METHODS

Patients were consecutively recruited. Ab were determined by microbead assay technique and a mean fluorescence intensity cut-off of 1,000.

RESULTS

At baseline, 21 donors (42 %) and 27 patients (54 %) had HLA-Ab with a mean panel reactivity (cPRA) of 34.9 ± 29.4 % and 46.1 ± 36.5 %, respectively. We observed a significant higher number of HLA-Ab specificities in female donors and patients and a predominance of HLA-class I Ab. At day 0 we detected an increase of HLA-Ab (from 526 to 673) and cPRA (55.2 ± 31.9 %). Thirty-six patients (72 %) developed new HLA-Ab, mainly 3 weeks after HCT. In 7 patients an HLA-Ab with the same specificity as detected in the corresponding donor emerged, suggesting a possible transfer from the donor to the recipient. Overall, MFI showed a high variation. Type and number of transfusions were not associated with number and intensity of HLA-Ab (ρ: -0.05 - 0.02). Number of HLA-Ab, cPRA and intensity were not associated with PTR, which occurred in 9 patients (18 %) and none had bleeding WHO > 2.

CONCLUSIONS

Although a considerable number of patients have and develop HLA-Ab before and early after HCT, we found no association with PTR and bleeding and management should be individualized.

Transfusions in Aplastic Anemia Patients Cause HLA Alloimmunization: Comparisons of Current and Past Cohorts Demonstrate Progress

Transfusions are the mainstay of supportive therapy in patients with aplastic anemia (AA) and may lead to anti- HLA alloimmunization, thereby also increasing the risk for donor-specific antibodies in the setting of HLA-mismatched transplantation. Historically, AA patients were thought to be at particularly high risk for HLA alloimmunization. In past decades, blood product manufacturing (leukoreduction) and HLA antibody testing have improved significantly by single antigen bead (SAB) technology. It is currently unknown how those developments have impacted HLA alloimmunization and treatment outcome in patients with AA. We retrospectively investigated 54 AA patients treated by immunosuppressive therapy or allogeneic hematopoietic cell transplantation after the introduction of the SAB assay at our center. We compared the HLA antibody results to a historical AA cohort (n = 26), treated before introduction of leukoreduced blood products from 1975 to 1995. HLA alloimmunization was detected in 43 of 54 (80%) recently treated patients. Past pregnancy, female gender, disease severity, age, and a history of other transfusions were significantly associated with a larger number or higher intensity (mean fluorescence intensity) of HLA antibodies. Treatment outcome including bleeding episodes, response to treatment, engraftment, graft-versus-host disease, and overall survival was not associated with HLA alloimmunization. In the historical cohort a significantly higher number of HLA antibodies (P < .01) with a higher mean fluorescent intensity (P < .01) was observed. HLA alloimmunization remains frequent in AA tested by current techniques, but it has significantly decreased since prior decades and does not affect treatment outcome. © 2021 American Society for Blood and Marrow Transplantation. Published by Elsevier Inc. All rights reserved.

Measuring human leukocyte antigen alloantibodies: beyond a binary decision

PURPOSE OF REVIEW

Accurate measurement of human leukocyte antigen antibodies is critical for making clinical decisions treating patients awaiting transplantation or monitoring them post transplantation. Single antigen bead assay results are given as Mean Fluorescence Intensity, falling short of providing the required quantitative measure.

RECENT FINDINGS

Titration studies were shown to circumvent the limitation of target-saturation that affect interpretation of single antigen bead assays especially in highly sensitized patients with strong antibodies. In fact, titration information can serve to measure efficacy of antibody removal during pretransplant desensitization using plasmapheresis/intravenous immunoglobulin (PP/IVIg) approaches. Moreover, recent studies indicate that knowing the donor-specific antibody titer has prognostic value that can guide PP/IVIg desensitization treatments. Newer data demonstrates an additional layer of information obtained by titration studies allowing to stratify patients with very high cPRA (>99%) based on the strength of the antibodies present, rather than the breadth. This data can thereby identify patients that are more likely to benefit from desensitization approaches on the transplant wait-list.

SUMMARY

Titration studies have a prognostic value with regards to quantifying antibody strength. Obtaining this information does not require performing the complete set of dilutions. In fact, performing two to three specific dilutions can provide relevant information while maintaining practical cost.

The impact of donor-specific anti-human leukocyte antigen antibodies in salvage haploidentical hematopoietic cell transplantation with posttransplant cyclophosphamide in patients with nonmalignant disorders

The presence of donor-specific anti-human leukocyte antigen (HLA) antibodies (DSAs) has been recognized as a major risk factor for graft failure (GF) after haploidentical hematopoietic cell transplantation with posttransplant cyclophosphamide (haplo-PTCy). However, the role of DSAs in salvage haplo-PTCy for rescuing patients with nonmalignant disorders (NMDs) has not yet been reported. The present study retrospectively analyzed 22 patients with NMDs who underwent salvage haplo-PTCy from January 2008 to December 2017. The median age at the time of the rescue haplo-PTCy was 9 years (range, 1-26 years). Median time from the first transplant to second haplo-PTCy was 56 days (range, 37-591 days). Among all patients, six (27.3%) had DSAs, with a median DSA strength (mean fluorescence intensity [MFI]) of 5201 (range, 1412-11,543) in the first DSA testing. In addition, the median DSA MFI was 2672 (range, 832-10,498) before the bone marrow infusion. Overall, GF occurred in 5 (25%) of the 20 assessable patients. Three of four (75%) patients with DSAs experienced GF versus 2 of 16 (12.5%) DSA-negative patients (P = 0.032). The median DSA MFI for patients with GF was 6437 (range, 1412-10,498) versus 1845 (range, 832-2672) for those who engrafted or had early death (P = 0.030). One-year event-free survival was significantly lower in DSA-positive patients than in those without DSAs (16.7% vs. 62.5%, P = 0.002). DSA-negative patients had an acceptable 1-year survival of 62.5%. In conclusion, this study suggests that DSAs may be associated with deleterious outcomes after salvage haplo-PTCy in patients with NMDs.

False-positive reactivity of anti-human leukocyte antigen antibodies detected using the single-antigen bead assay

The single-antigen bead assay (SABA) demonstrates high sensitivity and specificity for detecting anti-human leukocyte antigen (HLA) antibodies. However, SABA may produce false-positive results for anti-HLA antibodies. Herein, we analyzed the data of patients with complement-dependent cytotoxic crossmatch^-/flow cytometric crossmatch^-/SABA^+/- results to determine false-positive results for anti-HLA antibodies. We also determined the prevalence of false-positive results by comparing false-positive data from our laboratory and national allele frequency data obtained with high-resolution HLA typing. For HLA-A, -B, -C, and -DR, a ratio of positive frequency to allele frequency of ≥3 in our laboratory was considered a false-positive result. For HLA-DQA1/DQB1 and HLA-DPA1/DPB1, we considered the positive frequency of ≥3 as a false positive result due to lack of haplotype frequency data. SABA results from 284 patients (78.0%) demonstrated false reactivity. The antibody against HLA-C*17:01 displayed the highest frequency ratio (298.3). If false-positive reactivity is suspected, results should be confirmed using different methods. If confirmation tests are unfeasible, comparing the allele frequency with the positive rate of detected anti-HLA antibodies and using a ratio ≥3 may facilitate the interpretation of SABA results. The positive rate of anti-HLA antibodies can be validated using the HLA allele frequency of the population to determine false-positive results.

Complex Linkage Disequilibrium Effects in HLA-DPB1 Expression and Molecular Mismatch Analyses of Transplantation Outcomes

BACKGROUND

HLA molecular mismatch (MM) is a risk factor for de novo donor-specific antibody (dnDSA) development in solid organ transplantation. HLA expression differences have also been associated with adverse outcomes in hematopoietic cell transplantation. We sought to study both MM and expression in assessing dnDSA risk.

METHODS

One hundred three HLA-DP-mismatched solid organ transplantation pairs were retrospectively analyzed. MM was computed using amino acids (aa), eplets, and, supplementarily, Grantham/Epstein scores. DPB1 alleles were classified as rs9277534-A (low-expression) or rs9277534-G (high-expression) linked. To determine the associations between risk factors and dnDSA, logistic regression, linkage disequilibrium (LD), and population-based analyses were performed.

RESULTS

A high-risk AA:GX (recipient:donor) expression combination (X = A or G) demonstrated strong association with HLA-DP dnDSA (P = 0.001). MM was also associated with HLA-DP dnDSA when evaluated by itself (eplet P = 0.007, aa P = 0.003, Grantham P = 0.005, Epstein P = 0.004). When attempting to determine the relative individual effects of the risk factors in multivariable analysis, only AA:GX expression status retained a strong association (relative risk = 18.6, P = 0.007 with eplet; relative risk = 15.8, P = 0.02 with aa), while MM was no longer significant (eplet P = 0.56, aa P = 0.51). Importantly, these risk factors are correlated, due to LD between the expression-tagging single-nucleotide polymorphism and polymorphisms along HLA-DPB1.

CONCLUSIONS

The MM and expression risk factors each appear to be strong predictors of HLA-DP dnDSA and to possess clinical utility; however, these two risk factors are closely correlated. These metrics may represent distinct ways of characterizing a common overlapping dnDSA risk profile, but they are not independent. Further, we demonstrate the importance and detailed implications of LD effects in dnDSA risk assessment and possibly transplantation overall.

Screening strategy for de novo donor-specific HLA antibodies beyond the first year after kidney transplantation: Personalized or "one size fits all"?

Screening for de novo donor-specific HLA antibodies (DSAs) after kidney transplantation is widely recommended. The aim of this single-center, cross-sectional study was to investigate the frequency of therapeutic interventions triggered by de novo DSA screening. We included 464 patients screened for de novo DSA at annual visits after a median of 5 years post-transplant (range 1 to 19 years). Overall, de novo DSAs were detected in 55/464 patients (11.9%) with a stepwise increase of the prevalence from 4.9% at 1 year post-transplant to 18.9% at >10 years post-transplant. Subsequent allograft biopsies were performed in 24/55 patients (44%). The main reasons to omit biopsies were good/stable allograft function and anticipated lack of clinical consequences (eg, relevant comorbidities). Rejection processes were detected in 16/24 biopsies (67%). Therapeutic interventions were made in 18/464 screened patients (3.9%) with a significantly higher rate in the youngest quartile of patients (≤48 years; 7.9%) compared to the middle 50% (49-67 years; 3%) and the oldest quartile (≥68 years; 1.7%) (P = .03). Our study suggests that the frequency of therapeutic interventions triggered by de novo DSA screening after kidney transplantation is overall low, but significantly higher in younger patients, arguing for a personalized, age-adapted de novo DSA screening strategy.

Investigating the relationship between class I HLA-specific immunoglobulin-G subclasses, Pan-IgG single antigen bead assays and complement mediated interference in sera from renal transplant recipients

INTRODUCTION

Antibody mediated rejection is the leading cause of kidney transplant failure. Not all antibodies are harmful and some may be protective. Immunoglulin Gs, of which there are four subtypes, are detected by single antigen bead testing. The aims of this study were to characterise the IgG subclass profiles for class I HLA-specific antibodies in an uncensored post-transplant population and to determine the underlying relationship between reactivity patterns and MFI cut-offs with the pan-IgG assay.

METHODS

Patients were recruited to the study who were transplanted in our centre between 2009 and 2014. Prospectively stored post-transplant serum initially underwent a Labscreen Mixed assay and those positive for class I HLA-specific antibody underwent standard SAB testing, EDTA, 1 in 10 dilution and IgG subclass modifications using the Luminex platform. A total of 4947 bead reactions from 51 patients were analysed.

RESULTS

A 1 in 10 dilution was used as a comparator pan-IgG assay for summed subclass and individual subclass linear regression analyses. Using a dilution to standard assay ratio we characterised all reactions for prozone potential i.e. how likely there is to be inhibition related to complement complex formation. We stratified samples into degrees of association and were able to determine suggested MFI thresholds of Log 5.35 for the dilution assay and Log 5.05 for the summed subclass assay when considering a Log MFI of 6.9 (1000) in the standard assay. Using individual subclass dominant reactions (>70%) we were able to determine linear relationships between the 1 in 10 dilution pan-IgG assay and the individual subclass assays (excluding prozone potential reactions for IgG1/3) enabling us to suggest Log MFI thresholds of 5.03, 3.58, 4.3 and 4.05 respectively for IgG1-4.

DISCUSSION

We recommend a 1 in 10 dilution as the optimum pan-IgG comparator assay for a subclass analysis. We advocate the utilisation of the summed subclass assay to determine overall relationships and potential subclass failures. Following others, we recommend serum pre-treatment of the subclass assays to mitigate prozone. We suggest cut-offs for each IgG subclass which should be used with caution given the many inhibitory influences which may include competitive inhibition for bead binding, IgM and IgA interference and under-representation of specific subclasses on the bead panel.

HLA-specific memory B-cell detection in kidney transplantation: Insights and future challenges

Humoral alloimmunity mediated by anti‐human leucocyte antigen (HLA) antibodies is a major challenge in kidney transplantation and impairs the longevity of the transplanted organ. The immunological risk of an individual patient is currently mainly assessed by detection of HLA antibodies in the serum, which are produced by long‐lived bone marrow‐residing plasma cells. However, humoral alloimmunity is complex, and alloreactive memory B cells constitute an additional factor in the interplay of immune cells. These recirculating “silent” cells are responsible for the immunological recall response by differentiating into antibody‐producing cells upon antigen re‐encounter. Historically, due to the lack of appropriate and routinely applicable assays to determine the presence and HLA specificity of alloreactive memory B cells, their contribution to the humoral alloimmune response has clinically often been suspected but could not be determined. In this review, we give an overview of recent advances in techniques to detect alloreactive memory B cells and discuss their strengths and limitations. Furthermore, we summarize experiences with these techniques in alloimmunized individuals and transplant recipients, thereby emphasizing unmet needs to be addressed in future studies.