Clinical usefulness of HLAMatchmaker in HLA epitope matching for organ transplantation

Implications of MHC-restricted immunopeptidome in transplantation

The peptide presentation by donor and recipient major histocompatibility complex (MHC) molecules is the major driver of T-cell responses in transplantation. In this review, we address an emerging area of interest, the application of immunopeptidome in transplantation, and describe the potential opportunities that exist to use peptides for targeting alloreactive T cells. The immunopeptidome, the set of peptides presented on an individual's MHC, plays a key role in immune surveillance. In transplantation, the immunopeptidome is heavily influenced by MHC-derived peptides, delineating a key subset of the diverse peptide repertoire implicated in alloreactivity. A better understanding of the immunopeptidome in transplantation has the potential to open up new approaches to identify, characterize, longitudinally quantify, and therapeutically target donor-specific T cells and ultimately support more personalized immunotherapies to prevent rejection and promote allograft tolerance.

Qualitative, rather than quantitative, differences between HLA-DQ alleles affect HLA-DQ immunogenicity in organ transplantation

Prolonging the lifespan of transplanted organs is critical to combat the shortage of this life-saving resource. Chronic rejection, with irreversible demise of the allograft, is often caused by the development of donor-specific HLA antibodies. Currently, enumerating molecular (amino acid) mismatches between recipient and donor is promoted to identify patients at higher risk of developing HLA antibodies, for use in organ allocation, and immunosuppression-minimization strategies. We have counseled against the incorporation of such approaches into clinical use and hypothesized that not all molecular mismatches equally contribute to generation of donor-specific immune responses. Herein, we document statistical shortcomings in previous study design: for example, use of individuals who lack the ability to generate donor-specific-antibodies (HLA identical) as part of the negative cohort. We provide experimental evidence, using CRISPR-Cas9-edited cells, to rebut the claim that the HLAMatchmaker eplets represent "functional epitopes." We further used unique sub-cohorts of patients, those receiving an allograft with two HLA-DQ mismatches yet developing antibodies only to one mismatch (2MM1DSA), to interrogate differential immunogenicity. Our results demonstrate that mismatches of DQα05-heterodimers exhibit the highest immunogenicity. Additionally, we demonstrate that the DQα chain critically contributes to the overall qualities of DQ molecules. Lastly, our data proposes that an augmented risk to develop donor-specific HLA-DQ antibodies is dependent on qualitative (evolutionary and functional) divergence between recipient and donor, rather than the mere number of molecular mismatches. Overall, we propose an immunological mechanistic rationale to explain differential HLA-DQ immunogenicity, with potential ramifications for other pathological processes such as autoimmunity and infections.

HLA (emphasis on DQ) compatibility for longer allograft survival in pediatric transplantation: Modern evidence and challenges

Kidney transplantation is the treatment of choice for children with end-stage kidney failure, yet suboptimal outcomes, the need for long-term immunosuppression, and the dependency on consecutive transplants pose significant barriers to success. Providing better HLA-matched organs to pediatric patients seems to be the most logical approach to improve graft and patient outcomes and to reduce risk of anti-HLA sensitization after graft failure. We here review recent literature on HLA matching in pediatric kidney transplantation. We further review newer approaches attempting to improve matching by using molecular mismatch load analysis. Our main focus is on the role of HLA-DQ compatibility between recipient and donor. We further emphasize the need to develop creative approaches that will support HLA (and DQ) matching utilization in organ allocation schemes, at least in those geared specifically for pediatric patients.

The impact of pretransplant suspected HLA antibody on the long-term outcome of the graft kidney: A retrospective cohort study

INTRODUCTION

The preoperative examination of kidney transplantation includes HLA antibody screening to initially determine the presence of preexisting donor-specific antibody (DSA) that mediates hyperacute rejection. Recipients with positive HLA antibodies require further HLA specificity analysis to type the antigen and determine the antigen mismatches between the donor and recipient. However, recipients with suspected antibodies would have no further HLA specificity analysis. It is unclear whether suspected HLA antibodies would affect renal graft function. This study aimed to explore the impact of pretransplant suspected HLA antibody on the long-term outcome of the graft kidney and thus determine the necessity of routinely performing the HLA specificity analysis in recipients with suspected HLA antibodies preoperatively.

METHODS

This is a single-center retrospective cohort study. 179 kidney transplant recipients (KTRs) were included and further divided into HLA antibody-negative group (Group 1) and HLA antibody-suspected groups (Group 2) based on the result of the pretransplant HLA antibody screen test. And the antibody-suspected group was further divided into a low-mismatched group (Group A) and a high-mismatched group (Group B) according to the HLA specificity analysis. We tracked the renal function indexes, biochemical indexes, and posttransplant adverse events within 5 years after transplantation and explored the necessity of further HLA specificity analysis in recipients with pretransplant suspected HLA antibodies.

RESULTS

There was no statistically significant difference in demographics between HLA antibody-negative group and HLA antibody-suspected groups. At 5 years of follow-up, the KTRs in HLA antibody-negative group had significantly higher eGFR levels, lower serum creatinine levels, and less urinary protein compared to those in antibody-suspected group. Meanwhile, the KTRs in low-mismatched group also had significantly higher eGFR levels, lower serum creatinine levels, and less proteinuria compared to those in high-mismatched group. Correlation analysis showed that the age of KTRs, urinary protein levels and the load capacity of HLA mismatches were associated with eGFR levels of KTRs at 5 year posttransplant.

CONCLUSION

KTRs with suspected HLA antibodies before kidney transplantation have worse graft function than the preoperative HLA antibody-negative recipients in the long-term posttransplant follow-up. The specific load capacity of HLA mismatches, the age of the recipient and the urinary protein was found to be negatively correlated with long-term posttransplant renal outcomes. It is necessary to undergo further HLA specificity analysis for recipients with suspected HLA antibodies in HLA antibody screen test to explicit HLA mismatches and improve long-term posttransplant outcomes.

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.

Can We Use Eplets (or Molecular) Mismatch Load Analysis to Improve Organ Allocation? The Hope and the Hype

In recent years, there have been calls for implementation of "epitope matching" in deceased-donor organ allocation policies (later changed to "eplet matching"). Emerging data indeed support the use of molecular mismatch load analysis in specific patient groups, with the objective of posttransplant stratification into different treatment arms. For this purpose, the expectation is to statistically categorize patients as low- or high-immune-risk. Importantly, these patients will continue to be monitored' and their risk category, as well as their management, can be adjusted according to on-going findings. However, when discussing deceased donor organ allocation and matching algorithms, where the decision is not modifiable and has lasting impact on outcomes, the situation is fundamentally different. The goal of changing allocation schemes is to achieve the best possible HLA compatibility between donor and recipient. Immunologically speaking, this is a very different objective. For this purpose, the specific interplay of immunogenicity between the donor and any potential recipient must be understood. In seeking compatibility, the aim is not to redefine matching but to identify those mismatches that are "permissible" or' in other words, less immunogenic. In our eagerness to improve transplant outcome, unfortunately, we have conflated the hype with the hope. Terminology is used improperly, and new terms are created in the process with no sufficient support. Here, we call for a cautious evaluation of baseline assumptions and a critical review of the evidence to minimize unintended consequences.

The impact of human leukocyte antigen mismatch on recipient outcomes in living-donor liver transplantation

Donor-recipient human leukocyte antigen (HLA) compatibility has not been considered to significantly affect liver transplantation (LT) outcomes; however, its significance in living-donor LT (LDLT), which is mostly performed between blood relatives, remains unclear. This retrospective cohort study included 1954 LDLTs at our institution (1990-2020). The primary and secondary endpoints were recipient survival and the incidence of T cell-mediated rejection (TCMR) after LDLT, respectively, according to the number of HLA mismatches at all five loci: HLA-A, HLA-B, HLA-C, HLA-DR, and HLA-DQ. Subgroup analyses were also performed in between-siblings that characteristically have widely distributed 0-10 HLA mismatches. A total of 1304 cases of primary LDLTs were finally enrolled, including 631 adults (recipient age at LT ≥18 years) and 673 children (<18 years). In adult-to-adult LDLT, the more HLA mismatches at each locus, the significantly worse the recipient survival was (p = 0.03, 0.01, 0.03, 0.001, and <0.001 for HLA-A, HLA-B, HLA-C, HLA-DR, and HLA-DQ, respectively). This trend was more pronounced when multiple loci were combined (all p < 0.001 for A + B + DR, A + B + C, DR + DQ, and A + B + C + DR + DQ). Notably, a total of three or more HLA-B + DR mismatches was an independent risk factor for both TCMR (hazard ratio [HR] 2.66, 95% confidence interval [CI] 1.21-5.87; p = 0.02) and recipient survival (HR 2.44, 95% CI 1.11-5.35; p = 0.03) in between-siblings. By contrast, HLA mismatch did not affect pediatric LDLT outcomes at any locus or in any combinations; however, it should be noted that all donor-recipient relationships are parent-to-child that characteristically possesses one or less HLA mismatch at each locus and maximally five or less mismatches in total. In conclusion, HLA mismatch significantly affects not only TCMR development but also recipient survival in adult LDLT, but not in children.

Assessment of human leukocyte antigen matching algorithm PIRCHE-II on liver transplantation outcomes

For liver transplantations, human leukocyte antigen (HLA) matching is not routinely performed because observed effects have been inconsistent. Nevertheless, long-term liver transplantation outcomes remain suboptimal. The availability of a more precise HLA-matching algorithm, Predicted Indirectly Recognizable HLA Epitopes II (PIRCHE-II), now enables robust assessment of the association between HLA matching and liver transplantation outcomes. We performed a single-center retrospective cohort study of 736 liver transplantation patients. Associations between PIRCHE-II and HLAMatchmaker scores and mortality, graft loss, acute and chronic rejection, ischemic cholangiopathy, and disease recurrence were evaluated with Cox proportional hazards models. Associations between PIRCHE-II with 1-year, 2-year, and 5-year outcomes and severity of acute rejection were assessed with logistic and linear regression analyses, respectively. Subgroup analyses were performed for autoimmune and nonautoimmune indications, and patients aged 30 years and younger, and older than 30 years. PIRCHE-II and HLAMatchmaker scores were not associated with any of the outcomes. However, patients who received transplants for autoimmune disease showed more acute rejection and graft loss, and these risks negatively associated with age. Rhesus mismatch more than doubled the risk of disease recurrence. Moreover, PIRCHE-II was inversely associated with graft loss in the subgroup of patients aged 30 years and younger with autoimmune indications. The absence of associations between PIRCHE-II and HLAMatchmaker scores and the studied outcomes refutes the need for HLA matching for liver (stem cell) transplantations for nonautoimmune disease. For autoimmune disease, the activated immune system seems to increase risks of acute rejection and graft loss. Our results may suggest the benefits of transplantations with rhesus matched but PIRCHE-II mismatched donor livers.

Human leukocyte antigen eplet mismatching is associated with increased risk of graft loss and rejection after pediatric heart transplant

BACKGROUND

While mismatching between donor and recipient human leukocyte antigen (HLA) alleles has been associated with increased graft loss in pediatric heart recipients, it is actually the surface amino acid structures, termed eplets, which determine the antigenicity of each HLA molecule. We hypothesized that HLA eplet mismatch analysis is a better predictor of adverse outcomes after pediatric heart transplant than conventional allele mismatch comparison.

METHODS

A retrospective review of the Pediatric Heart Transplant Society database identified pediatric heart recipients (<18 years at listing) with complete donor and recipient HLA typing (A, B, and DR). Imputed high-resolution HLA genotypes were entered into HLAMatchmaker software which then calculated the number of eplet mismatches between each donor-recipient pair. Multivariable Cox regression analysis was used to examine associations between allele or eplet mismatching and adverse outcomes.

RESULTS

Compared to those with <20 HLA class I eplet mismatches, recipients with 20 or more HLA class I eplet mismatches had an increased risk of graft loss (HR 1.46 [1.01-2.12], p = .049). HLA class I eplet mismatching was also associated with rejection (>20 mismatches: HR 1.30 [1.03-1.65], p = .030), while HLA class II eplet mismatching was associated with specified antibody-mediated rejection (10-20 mismatches: HR 1.57 [1.06-2.34], p = .025; >20 mismatches: HR 3.14 [1.72-5.71], p < .001). Neither HLA class I nor class II allele mismatching was significantly associated with graft loss or rejection.

CONCLUSION

Eplet mismatch analysis was more predictive of adverse post-transplant outcomes (including graft loss and rejection) than allele mismatch comparison. Further study, including prospective high-resolution HLA typing, is warranted.

Noninvasive Assessment of the Alloimmune Response in Kidney Transplantation

Transplantation remains the optimal mode of kidney replacement therapy, but unfortunately long-term graft survival after 1 year remains suboptimal. The main mechanism of chronic allograft injury is alloimmune, and current clinical monitoring of kidney transplants includes measuring serum creatinine, proteinuria, and immunosuppressive drug levels. The most important biomarker routinely monitored is human leukocyte antigen (HLA) donor-specific antibodies (DSAs) with the frequency based on underlying immunologic risk. HLA-DSA should be measured if there is graft dysfunction, immunosuppression minimization, or nonadherence. Antibody strength is semiquantitatively estimated as mean fluorescence intensity, with titration studies for equivocal cases and for following response to treatment. Determination of in vitro C1q or C3d positivity or HLA-DSA IgG subclass analysis remains of uncertain significance, but we do not recommend these for routine use. Current evidence does not support routine monitoring of non-HLA antibodies except anti-angiotensin II type 1 receptor antibodies when the phenotype is appropriate. The monitoring of both donor-derived cell-free DNA in blood or gene expression profiling of serum and/or urine may detect subclinical rejection, although mainly as a supplement and not as a replacement for biopsy. The optimal frequency and cost-effectiveness of using these noninvasive assays remain to be determined. We review the available literature and make recommendations.

Preformed T cell alloimmunity and HLA eplet mismatch to guide immunosuppression minimization with tacrolimus monotherapy in kidney transplantation: Results of the CELLIMIN trial

Personalizing immunosuppression is a major objective in transplantation. Transplant recipients are heterogeneous regarding their immunological memory and primary alloimmune susceptibility. This biomarker-guided trial investigated whether in low immunological-risk kidney transplants without pretransplant DSA and donor-specific T cells assessed by a standardized IFN-γ ELISPOT, low immunosuppression (LI) with tacrolimus monotherapy would be non-inferior regarding 6-month BPAR than tacrolimus-based standard of care (SOC). Due to low recruitment rates, the trial was terminated when 167 patients were enrolled. ELISPOT negatives (E-) were randomized to LI (n = 48) or SOC (n = 53), E+ received the same SOC. Six- and 12-month BPAR rates were higher among LI than SOC/E- (4/35 [13%] vs. 1/43 [2%], p = .15 and 12/48 [25%] vs. 6/53 [11.3%], p = .073, respectively). E+ patients showed similarly high BPAR rates than LI at 6 and 12 months (12/55 [22%] and 13/66 [20%], respectively). These differences were stronger in per-protocol analyses. Post-hoc analysis revealed that poor class-II eplet matching, especially DQ, discriminated E- patients, notably E-/LI, developing BPAR (4/28 [14%] low risk vs. 8/20 [40%] high risk, p = .043). Eplet mismatch also predicted anti-class-I (p = .05) and anti-DQ (p < .001) de novo DSA. Adverse events were similar, but E-/LI developed fewer viral infections, particularly polyoma-virus-associated nephropathy (p = .021). Preformed T cell alloreactivity and HLA eplet mismatch assessment may refine current baseline immune-risk stratification and guide immunosuppression decision-making in kidney transplantation.

Kidney transplantation in highly sensitized recipients

In kidney transplantation (KT), overcoming donor shortage is particularly challenging in patients with preexisting donor-specific antibodies (DSAs) against human leukocyte antigen (HLA), called HLA-incompatible KT (HLAi KT), carrying the risk of rejection and allograft loss. Thus, it is necessary to accurately evaluate the degree of sensitization before HLAi KT, and undertake appropriate pretreatment strategies. To determine the degree of sensitization, complement-dependent cytotoxicity has been the only method employed; the development of a method using flow cytometry further improved the test sensitivity. However, these tests present disadvantages, including the need for living cells, with a solid-phase assay developed to resolve this problem. Currently, the method using Luminex (Luminex Corp.) is widely used in clinical practice. As this method measures DSAs using single antigen beads, it is possible to classify immunological risks by measuring the type and amount of DSAs. Furthermore, there have been major advances in methods that involve DSA removal before HLAi KT. In the early stages of desensitization, plasmapheresis and intravenous immunoglobulins were the main treatment methods employed; however, the introduction of CD20 monoclonal antibody and proteasome inhibitors further increased the success rate of desensitization. Currently, HLAi KT has been established as an important transplant method, but an understanding of DSAs and a novel desensitization treatment are warranted.

Sensitization in transplantation: Assessment of risk (STAR) 2019 Working Group Meeting Report

The purpose of the STAR 2019 Working Group was to build on findings from the initial STAR report to further clarify the expectations, limitations, perceptions, and utility of alloimmune assays that are currently in use or in development for risk assessment in the setting of organ transplantation. The goal was to determine the precision and clinical feasibility/utility of such assays in evaluating both memory and primary alloimmune risks. The process included a critical review of biologically driven, state-of-the-art, clinical diagnostics literature by experts in the field and an open public forum in a face-to-face meeting to promote broader engagement of the American Society of Transplantation and American Society of Histocompatibility and Immunogenetics membership. This report summarizes the literature review and the workshop discussions. Specifically, it highlights (1) available assays to evaluate the attributes of HLA antibodies and their utility both as clinical diagnostics and as research tools to evaluate the effector mechanisms driving rejection; (2) potential assays to assess the presence of alloimmune T and B cell memory; and (3) progress in the development of HLA molecular mismatch computational scores as a potential prognostic biomarker for primary alloimmunity and its application in research trial design.

Eplet incompatibility in pediatric renal transplantation

Eplet incompatibility appears to be a better predictor of the de novo appearance of DSA post-Tx than HLA antigen matching in adults. We evaluated the HLA Matchmaker® software (version 2.1) in our pediatric cohort to predict the appearance of DSA post-Tx. We included 70 pediatric patients (26 girls, 10 living donors, mean age 11.2 ± 3.9 years) after a first R-Tx (January 2010-August 2016), without prior immunization, having complete HLA typing (A, B, C, DRB1 and DQB1) and DSA follow-up for at least one year. The mean of HLA and eplet incompatibilities was 4.7 ± 1.3 and 15.5 ± 6.1, respectively, with a correlation coefficient r² between these two variables of 0.34 (P < .001). The eplet load was 12.8 ± 5.0 in living donors vs 15.9 ± 6.2 in deceased donors (P = NS), 12.6 ± 6.1 in preemptive R-Tx (n = 14) vs 16.3 ± 5.9 for non-preemptive R-Tx (P = .04). Seven patients (10%) developed DSA during the 3.5 ± 1.2 years post-Tx. The eplet load was 13.7 ± 5.5 for those who developed DSA vs 15.7 ± 6.1 for the others (P = NS). In our single-center series of pediatric R-Tx with good HLA matching and lower eplet load than previously published series, eplet incompatibilities do not predict the development of DSA. The question of the HLA matching requirement and the daily interest of the HLA Matchmaker® software to help select the grafts remain open.

PIRCHE-II: an algorithm to predict indirectly recognizable HLA epitopes in solid organ transplantation

Human leukocyte antigen (HLA) mismatches between donors and recipients may lead to alloreactivity after solid organ transplantation. Over the last few decades, our knowledge of the complexity of the HLA system has dramatically increased, as numerous new HLA alleles have been identified. As a result, the likelihood of alloreactive responses towards HLA mismatches after solid organ transplantation cannot easily be assessed. Algorithms are promising solutions to estimate the risk for alloreactivity after solid organ transplantation. In this review, we show that the recently developed PIRCHE-II (Predicted Indirectly ReCognizable HLA Epitopes) algorithm can be used to minimize alloreactivity towards HLA mismatches. Together with the use of other algorithms and simulation approaches, the PIRCHE-II algorithm aims for a better estimated alloreactive risk for individual patients and eventually an improved graft survival after solid organ transplantation.

Epitope load identifies kidney transplant recipients at risk of allosensitization following minimization of immunosuppression

Human leukocyte antigen (HLA) mismatching and minimization of immunosuppression are two major risk factors for the development of de novo donor-specific antibodies, which are associated with reduced kidney graft survival. Antibodies do not recognize whole HLA antigens but rather individual epitopes, which are short sequences of amino acids in accessible positions. However, compatibility is still assessed by the simple count of mismatched HLA antigens. We hypothesized that the number of mismatched epitopes, or ("epitope load") would identify patients at the highest risk of developing donor specific antibodies following minimization of immunosuppression. We determined epitope load in 89 clinical trial participants who converted from cyclosporine to everolimus 3 months after kidney transplantation. Twenty-nine participants (32.6%) developed de novo donor specific antibodies. Compared to the number of HLA mismatches, epitope load was more strongly associated with the development of donor specific antibodies. Participants with an epitope load greater than 27 had a 12-fold relative risk of developing donor-specific antibodies compared to those with an epitope load below that threshold. Using that threshold, epitope load would have missed only one participant who subsequently developed donor specific antibodies, compared to 8 missed cases based on a 6-antigen mismatch. DQ7 was the most frequent antigenic target of donor specific antibodies in our population, and some DQ7 epitopes appeared to be more frequently involved than others. Assessing epitope load before minimizing immunosuppression may be a more efficient tool to identify patients at the highest risk of allosensitization.

Applicability of an instrument to identify human leukocyte antigen-compatible donors for platelet transfusions

Background

The selection of compatible human leukocyte antigen platelets has been associated with improved platelet increments. Therefore, an effective strategy would be the selection of donors who are genetically compatible according to the human leukocyte antigen system. Nonetheless, this is costly as it concerns a highly polymorphic system, which requires a large bank of genotyped donors.

Methods

This study evaluated the feasibility of virtual crossmatching using EpVix software, which simplifies the identification of compatible donors or donors with acceptable incompatibilities.

Results

Forty-three oncohematological patients were evaluated, in 96 platelet transfusion episodes with 16.3% of the patients being found to be refractory to platelet transfusions. Eight alloimmunized, multitransfused patients were selected to evaluate human leukocyte antigen compatibility against a bank of 336 platelet donors. At least partially compatible donors were found for all patients. The number of compatible donors was found to be inversely proportional to the human leukocyte antigen-panel reactive antibody score of each patient. It was noted that five patients with scores of 15% or less had at least 190 compatible donors; four fully compatible donors were found for two other patients with scores greater than 80% and only one patient (score of 93%) did not have a fully compatible donor. However, for this last patient, 40 donors were partially compatible according to the software.

Conclusion

The results showed the effectiveness of the use of the EpVix tool to identify potential platelet donors for multitransfused and/or alloimmunized patients, even with a small number of human leukocyte antigen genotyped donors available.

Terasaki Epitope Mismatch Burden Predicts the Development of De Novo DQ Donor-Specific Antibodies and are Associated With Adverse Allograft Outcomes

BACKGROUND

De novo DQ donor-specific antibody (DSA) are associated with antibody-mediated rejection and allograft loss. Given the lack of effective treatment of de novo DQ DSA, their prevention is vital if there is to be an improvement of long-term allograft survival. Using the HLA Matchmaker program, DQ epitope matching has been shown to be superior to HLA antigen mismatching in predicting de novo DQ DSA development. Whether DQ epitopes determined by Terasaki may more accurately predict de novo DQ development over HLA antigen matching is not known.

METHODS

We retrospectively analyzed the immunogenicity of the different HLA antigens, DQB1 alleles and DQB1 Teraskai epitopes (TerEp) in a large cohort of renal transplant recipients, by comparing patient mismatches with de novo DSA development.

RESULTS

Patients mismatched at a DQB1 allele were at significantly higher risk of developing de novo DSA compared with other mismatched HLA antigens. Patients mismatched at the DQ7 allele appear to be at specific risk. For patients mismatched at a single DQB1 allele, the risk of de novo DQ DSA development increases with the number of TerEp mismatches. However, the immunogenicity of the different DQ TerEps does not appear to be equal. Patients who develop antibodies against TerEps are at increased risk of adverse allograft outcomes, specifically antibody-mediated rejection.

CONCLUSIONS

Epitope mismatch burden, determined by TerEps, helps predict risk of de novo DQ DSA development and offers an alternative approach to predict an alloimmune response.

Inaccuracies in epitope repertoire estimations when using Multi-Locus Allele-Level hla genotype imputation tools

BACKGROUND

Limited availability of allele-level HLA genotypes prompts their imputation from allele-group genotypes to estimate epitope mismatches. We evaluated the accuracy of epitope load and repertoire assignment when imputing allele-level HLA genotypes.

METHODS

Analyses were conducted on 175 hematopoietic stem cell (HSC) donors from the Héma-Québec registry (HQR) and 57 HSC donor-recipient pairs from McGill University Health Centre (MUHC), Québec, Canada, genotyped for HLA-A, -B, -C, -DRB1 and -DQB1. Multi-locus allele-level imputation was performed using HaploStats. Disagreement in B- and T-cell epitope assignment and epitope mismatches were ascertained for imputed vs. measured allele-level HLA genotypes in HSC donors and donor-recipient pairs, respectively.

RESULTS

Imputation resulted in no differences in overall eplet mismatches and PIRCHE-II for HLA-A, -B, and -C in 83.4% and 93.7% of HQR donors and 87.7% and 87.7% of MUHC donors, respectively. HLA-DRB1- and -DQB1-derived eplet mismatches and PIRCHE-II were correctly assigned in 72.0% and 85.1% of HQR donors and 70.2% and 71.9% of MUHC donors, respectively. No discrepancies in eplet load or PIRCHE-II were observed in 96.5% and 86.0% of HSC donor-recipient pairs and in 70.2% and 70.1% of pairs for HLA-A, -B and -C and -DRB1 and -DQB1, respectively. Kappa statistics of 0.9708 and 0.9725, 0.8724 and 0.8177, 0.9827 and 0.9022, 0.5644 and 0.4939, 0.5085 and 0.6361 were demonstrated when assessing agreement between eplet mismatches and PIRCHE-II of imputed vs. measured HLA-A, -B, -C, -DRB1 and -DQB1 types, respectively.

CONCLUSIONS

To avoid inaccuracies in epitope compatibility estimation, mainly for class II HLA, multi-locus allele-level genotype measurement is recommended. This article is protected by copyright. All rights reserved.

Kidney Transplantation: The Challenge of Human Leukocyte Antigen and Its Therapeutic Strategies

Kidney transplantation remains the treatment of choice for end-stage renal failure. When the immune system of the recipient recognizes the transplanted kidney as a foreign object, graft rejection occurs. As part of the host immune defense mechanism, human leukocyte antigen (HLA) is a major challenge for graft rejection in transplantation therapy. The impact of HLA mismatches between the donor and the potential recipient prolongs the time for renal transplantation therapy, tethered to dialysis, latter reduces graft survival, and increases mortality. The formation of pretransplant alloantibodies against HLA class I and II molecules can be sensitized through exposures to blood transfusions, prior transplants, and pregnancy. These preformed HLA antibodies are associated with rejection in kidney transplantation. On the other hand, the development of de novo antibodies may increase the risk for acute and chronic rejections. Allograft rejection results from a complex interplay involving both the innate and the adaptive immune systems. Thus, further insights into the mechanisms of tissue rejection and the risk of HLA sensitization is crucial in developing new therapies that may blunt the immune system against transplanted organs. Therefore, the purpose of this review is to highlight facts about HLA and its sensitization, various mechanisms of allograft rejection, the current immunosuppressive approaches, and the directions for future therapy.

Human leukocyte antigen, allele, and eplet mismatches in liver transplantation; observations from a small, single center cohort

BACKGROUND

The impact of human leukocyte antigen (HLA) matching on outcomes in liver transplantation is controversial. Varying levels of HLA matching resolutions were examined in a uniform patient population with no pre-transplant DSA from a small, single center cohort.

METHODS

Retrospective chart review from a single center yielded 131 patients, 67 of which were confirmed to be DSA negative, all of which received induction immunotherapy and post-operative immunosuppression. HLA typing was achieved by sequence specific oligonucleotide probe (SSOP) method using LABType® kits. Eplet mismatch analysis was conducted using HLAMatchMaker software.

RESULTS

The mean number of HLA-A antigen mismatches was significantly higher in patients experiencing acute rejection (1.8 vs 1.6, p = 0.006). Rejection patients more frequently possessed two HLA-A mismatches compared to their non-rejection counterparts (77% vs 43%, p = 0.071). Patient survival was found to be non-significantly decreased in patients with a higher eplet mismatch load at the HLA-A locus (p = 0.155). No other loci were found to be predictive.

CONCLUSION

In conclusion, HLA mismatches were found to increase acute rejection and be associated with decreased patient survival. The outcomes of this study suggest an involvement of HLA-A locus mismatches in predicting liver transplant rejection and patient survival.

Providing Better-Matched Donors for HLA Mismatched Compatible Pairs Through Kidney Paired Donation

BACKGROUND

Participation of compatible pairs (CP) in kidney paired donation (KPD) could be attractive to CPs who have a high degree of HLA mismatch, if the CP recipient will gain a better HLA match. Because KPD programs were not designed to help CP, it is important to define allocation metrics that enable CP to receive a better-matched kidney, without disadvantage to incompatible pairs (ICP).

METHODS

Simulations using 46 ICPs and 11 fully HLA-mismatched CPs were undertaken using the Australian KPD matching algorithm. Allocations were preformed adding 1 CP at a time or all 11 CPs at once, and with and without exclusion of unacceptable antigens selected to give a virtual calculated panel-reactive antibody ranging 70% to 80% to improve HLA matching in CP recipients.

RESULTS

On average, most CP recipients could be matched and had a lower eplet mismatch (EpMM) with the matched donor (57 ± 15) than with their own donor (78 ± 19, P < 0.02). However, only recipients who had an EpMM to own donor greater than 65 achieved a significant reduction in the EpMM with the matched donor. The gain in EpMM was larger when CPs were listed with unacceptable antigens. Furthermore, inclusion of 1 CP at a time increased matching in ICP by up to 33%, and inclusion of all 11 CPs at once increased ICP matching by 50%.

CONCLUSIONS

Compatible pair participation in KPD can increase match rates in ICP and can provide a better immunological profile in CP recipients who have a high EpMM to their own donor when using allocation based on virtual crossmatch.

Usefulness of the ElliPro epitope predictor program in defining the repertoire of HLA-ABC eplets

HLA matching at the epitope level offers new opportunities to identify suitable donors for transplant patients. The International HLA Epitope Registry (www.Epregistry.com.br) describes for the various HLA loci, repertoires of eplets including those that correspond to epitopes experimentally verified with specific antibodies. There are also many eplets which have remained as theoretical entities because no informative antibodies have been found. Which of them have immunogenic potential or conversely, might be considered as non-epitopes that cannot elicit specific antibody responses? This question is important for the application of epitope-based HLA matching in clinical transplantation. Correct predictions of B-cell epitopes on antigenic proteins are essential to the effective design of microbial vaccines and the development of specific antibodies used in immunotherapy and immunodiagnostics but prediction programs based on structural and physiochemical properties of amino acid residues are generally ineffective. Recent prediction programs based on three-dimensional structures of antigen-antibody complexes are more promising. One such program is called ElliPro developed by Ponomarenko. This report describes studies demonstrating that ElliPro can predict alloantibody responses to HLA-ABC eplets. Antibody-verified eplets have amino acid residues with much higher ElliPro scores than eplets for which no specific antibodies have been found. The latter group includes residues with very low ElliPro scores; they appear to represent eplets that might be classified as non-epitopes. In conclusion, ElliPro offers a new approach to characterize epitope repertoires that are clinically relevant in HLA matching.

Computational Approaches to Facilitate Epitope-Based HLA Matching in Solid Organ Transplantation

Epitope-based HLA matching has been emerged over the last few years as an improved method for HLA matching in solid organ transplantation. The epitope-based matching concept has been incorporated in both the PIRCHE-II and the HLAMatchmaker algorithm to find the most suitable donor for a recipient. For these algorithms, high-resolution HLA genotype data of both donor and recipient is required. Since high-resolution HLA genotype data is often not available, we developed a computational method which allows epitope-based HLA matching from serological split level HLA typing relying on HLA haplotype frequencies. To validate this method, we simulated a donor-recipient population for which PIRCHE-II and eplet values were calculated when using both high-resolution HLA genotype data and serological split level HLA typing. The majority of the serological split level HLA-determined ln(PIRCHE-II)/ln(eplet) values did not or only slightly deviate from the reference group of high-resolution HLA-determined ln(PIRCHE-II)/ln(eplet) values. This deviation was slightly increased when HLA-C or HLA-DQ was omitted from the input and was substantially decreased when using two-field resolution HLA genotype data of the recipient and serological split level HLA typing of the donor. Thus, our data suggest that our computational approach is a powerful tool to estimate PIRCHE-II/eplet values when high-resolution HLA genotype data is not available.

[New aspects of HLA compatibility in organ transplantation]

In organ transplantation, HLA compatibility between a donor and a recipient is currently assessed through the comparison of their HLA antigens and the sole count of incompatible HLA antigens. Similarly, antibodies were originally described as antigenic-specific. With solid phase assays detection of anti-HLA antibodies and crystallographic studies, it is now recognized that anti-HLA antibodies are not specific for antigens, but for epitopes, i.e. short polymorphic sequences of amino acids that are more often in positions accessible to allo-antibodies. These epitopes, due to the distribution of HLA molecules polymorphism, may be shared by different HLA antigens. This explains why an immunization towards a given HLA antigen can lead to synthesis of antibodies reactive towards other antigens sharing one or more epitopes. Similarly, structural comparison of the HLA molecules of a recipient and his donor defines the epitope load, i.e. the number of incompatible epitopes. This epitope load is correlated with the risk of developing antibodies specific for the donor after transplantation. New tools, such as the HLAMatchmaker software, allow to determine the epitopic load and to analyze the epitopic specificity of alloantibodies. These tools will possibly lead to rethink the method of graft allocation, or at least to take differently into account HLA compatibility in allocation algorythms.

Using Genetic Variation to Predict and Extend Long-term Kidney Transplant Function

Renal transplantation has transformed the life of patients with end-stage renal disease and other chronic kidney disorders by returning endogenous kidney function and enabling patients to cease dialysis. Several clinical indicators of graft outcome and long-term function have been established. Although rising creatinine levels and graft biopsy can be used to determine graft loss, identifying early predictors of graft function will not only improve our ability to predict long-term graft outcome but importantly provide a window of opportunity to therapeutically intervene to preserve graft function before graft failure has occurred. Since understanding the importance of matching genetic variation at the HLA region between donors and recipients and translating this into clinical practise to improve transplant outcome, much focus has been placed on trying to identify additional genetic predictors of transplant outcome/function. This review will focus on how candidate gene studies have identified variants within immunosuppression, immune response, fibrotic pathways, and specific ethnic groups, which correlate with graft outcome. We will also discuss the challenges faced by candidate gene studies, such as differences in donor and recipient selection criteria and use of small data sets, which have led to many genes failing to be consistently associated with transplant outcome. This review will also look at how recent advances in our understanding of and ability to screen the genome are starting to provide new insights into the mechanisms behind long-term graft loss and with it the opportunity to target these pathways therapeutically to ultimately increase graft lifespan and the associated benefits to patients.

Clinical relevance of circulating antibodies and B lymphocyte markers in allograft rejection

The main challenge in solid organ transplantation remains to tackle antibody-mediated rejection. Our understanding of the antibody-mediated response and the capacity to detect it has improved in the last decade. However, the sensitivity and specificity of the current clinical tools to monitor B cell activation are perfectible. New strategies, including the refinement in the characterization of HLA and non-HLA antibodies, as well as a better understanding of the circulating B cell phenotype will hopefully help to non-invasively identify patients at risk or undergoing antibody-mediated allograft damage. The current review discusses the current knowledge of the B cell biomarkers in solid organ transplantation, with a focus on circulating antibodies and peripheral B cells.

Human leukocyte antigen epitope antigenicity and immunogenicity

PURPOSE OF REVIEW

Human leukocyte antigen (HLA) antibodies are now recognized as being specific for epitopes which can be defined structurally with amino acid differences between HLA alleles. This article addresses two general perspectives of HLA epitopes namely antigenicity, that is their reactivity with antibody and immunogenicity, that is their ability of eliciting an antibody response.

RECENT FINDINGS

Single-antigen bead assays have shown that HLA antibodies recognize epitopes that are equivalent to eplets or corresponding to eplets paired with other residue configurations. There is now a website-based Registry of Antibody-Defined HLA Epitopes (http://www.epregistry.com.br). Residue differences within eplet-defined structural epitopes may also explain technique-dependent variations in antibody reactivity determined in Ig-binding, C1q-binding and lymphocytotoxicity assays.HLA antibody responses correlate with the numbers of eplets on mismatched HLA antigens, and the recently proposed nonself-self paradigm of epitope immunogenicity may explain the production of epitope-specific antibodies.

SUMMARY

These findings support the usefulness of HLA matching at the epitope level, including the identification of acceptable mismatches for sensitized patients and permissible mismatching for nonsensitized patients aimed to reduce HLA antibody responses.

HLA molecular epitope mismatching and long-term graft loss in pediatric heart transplant recipients

BACKGROUND

Although evidence links HLA allele mismatching to worse outcomes in pediatric heart transplantation, no studies to our knowledge have applied the quantification of structural HLA differences between donor and recipient to risk evaluation. We examine the association between molecular-level HLA mismatching and long-term graft loss in pediatric recipients of heart transplants.

METHODS

HLA Matchmaker was used to quantify the number of mismatched class-specific HLA eplets among 4,851 heart transplant recipients ≤18 years of age in the Scientific Registry of Transplant Recipients (1987-2012). Survival analysis was used to compare long-term probabilities of graft loss by number of eplet mismatches and allele mismatches stratified by eplet mismatches.

RESULTS

Recipients with 10 to 20 or >20 class I (HLA-A and HLA-B) eplet mismatches experienced increased long-term graft loss compared with recipients with <10 class I eplet mismatches (adjusted hazard ratio = 1.23 [95% confidence interval = 1.06-1.42], adjusted hazard ratio = 1.27 [95% confidence interval = 1.08-1.50], respectively). Recipients with 2 to 4 class I allele mismatches had increased long-term graft loss compared with recipients with 0 to 1 class I allele mismatches. Neither class II (HLA-DR) eplet mismatching nor class II allele mismatching was associated with graft loss. On stratification by allele and structural eplet mismatching, only recipients with 2 to 4 class I allele mismatches and ≥10 class I eplet mismatches had an increased probability of graft loss compared with recipients with 0 to 1 class I allele mismatches (adjusted hazard ratio = 1.42 [95% confidence interval = 1.09-1.57]).

CONCLUSIONS

Molecular-level HLA mismatching may aid in identifying recipients at increased risk of long-term graft loss who could benefit from intensified post-transplant surveillance and management.

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.

HLA epitope based matching for transplantation

As important risk factors for transplant rejection and failure, HLA antibodies are now recognized as being specific for epitopes which can be defined structurally with amino acid differences between HLA alleles. Donor-recipient compatibility should therefore be assessed at the epitope rather than the antigen level. HLAMatchmaker is a computer algorithm that considers each HLA antigen as a series of small configurations of polymorphic residues referred to as eplets as essential components of HLA epitopes. It includes epitopes on antigens encoded by all HLA-A, B, C, DR, DQ and DP loci as well as MICA. HLA epitopes have two characteristics namely antigenicity, i.e. the reactivity with antibody and immunogenicity, i.e. the ability of eliciting an antibody response. This article addresses the relevance of determining epitope-specificities of HLA antibodies, the effect of epitope structure on technique-dependent antibody reactivity and the identification of acceptable mismatches for sensitized patients considered for transplantation. Permissible mismatching for non-sensitized patients aimed to prevent or reduce HLA antibody responses could consider epitope loads of mismatched antigens and the recently developed nonself-self paradigm of epitope immunogenicity.

Clinical relevance of the de novo production of anti-HLA antibodies following intestinal and multivisceral transplantation

Despite a negative pretransplant cross-match, intestinal transplant recipients can mount humoral immune responses soon after transplantation. Moreover, the development of donor-specific anti-HLA antibodies (DSAs) is associated with severe graft injury. Between June 2000 and August 2011, 30 patients (median age 37.6±9.8 years) received isolated intestinal transplantations (ITX, n=18) or multivisceral transplantations (MVTXs, n=12) at our center. We screened for human leukocyte antigen (HLA) antibodies pre- and post-transplant. If patients produced DSAs, treatment with plasmapheresis and intravenous immunoglobulin (IVIG) was initiated. In the event of DSA persistence and/or treatment-refractory rejection, rituximab and/or bortezomib were added. Ten patients developed DSAs and simultaneously showed significant signs of rejection. These patients received plasmapheresis and IVIG. Eight patients additionally received rituximab, and two patients were treated with bortezomib. DSA values decreased upon antirejection therapy in 8 of the 10 patients. The development of DSAs following ITX is often associated with acute rejection. We observed that the number of mismatched antigens and epitopes correlates with the probability of developing de novo DSAs. Early diagnosis and therapy, including B-cell depletion and plasma cell inhibition, are crucial to preventing further graft injury.

Structural aspects of HLA class I epitopes reacting with human monoclonal antibodies in Ig-binding, C1q-binding and lymphocytotoxicity assays

This study addresses the reactivity patterns of human cytotoxic HLA class I epitope-specific monoclonal antibodies in Ig-binding and complement component C1q-binding Luminex assays in comparison with complement-dependent lymphocytotoxicity data reported at the 13th International HLA Workshop. Some monoclonal antibodies reacted similarly with epitope-carrying alleles in all three assays but others showed different reactivity patterns. These reactivity differences were analyzed with HLAMatchmaker and we incorporated the concept that eplets are essential parts of structural epitopes which can contact the six Complementarity Determining Regions (CDRs) of antibody. The data show that technique-dependent reactivity patterns are associated with distinct differences between polymorphic amino acid configurations on eplet-defined structural epitopes. The findings have been viewed in context of antigen-antibody complex formation that results in the release of free energy necessary to stabilize binding and to induce conformational changes in the antibody molecule to expose the C1q binding site, the first step of complement activation. Moreover the amount of free energy should be sufficient to induce a conformational change of C1q thereby initiating the first stages of the classical complement cascade leading to lymphocytotoxicity. The complement-fixing properties of HLA antibodies require not only specific recognition of eplets but also depend on interactions of other CDRs with critical amino acid configurations within the structural epitope. Eplet-carrying alleles that lack such configurations may only bind with antibody. This concept is important to our understanding whether or not complement-fixing donor-specific HLA antibodies can initiate antibody-mediated rejection.

16th IHIW: a website for antibody-defined HLA epitope Registry

The concept that HLA antibodies are specific for epitopes rather than HLA antigens is important not only for the determination of mismatch acceptability for sensitized patients but also for a better understanding of the antibody response to an HLA mismatch. Numerous publications describe epitope-specific antibodies, but there is no standardized information about the repertoire of clinically relevant HLA epitopes. Under auspices of the 16th IHIW, we have developed a website-based registry of antibody-verified HLA epitopes. Epitope notations are based on HLA molecular modelling of amino acid residues in polymorphic sequence positions. Informative epitope-specific antibodies had been induced by a transplant, transfusion or pregnancy and were monoclonal antibodies or eluates of sera absorbed with single HLA alleles. Antibody reactivity was determined in binding assays with single-allele panels. Antibody producer/immunizer HLA types enhanced the characterization of specific epitopes. The Registry also includes epitopes described in original research publications. Based on the extent of antibody reactivity information, we assigned epitope status as confirmed (well documented) or provisional (more data are needed). At present, the Registry has 69 HLA-ABC, 53 DRB1/3/4/5, 17 DQ, 8 DP and 22 MICA antibody-verified epitopes and will be updated on a quarterly basis. Laboratories worldwide continue to submit data about previously unreported antibody-specific epitopes. For each epitope, the website shows its amino acid composition and HLA alleles that share the epitope. Links show antibody reactivity patterns, sensitization information and references. Other links show molecular modelling of corresponding structural epitopes and polymorphic residue information for epitope-carrying alleles. The website will also have a link to epitope frequency information in different populations. Search functions will list mismatched epitopes on mismatched alleles for selected HLA types. The HLA Epitope Registry will become a valuable resource for researchers interested in HLA compatibility at the epitope level and investigating antibody responses to HLA mismatches.

The immune-modulator AS101 reduces anti-HLA antibodies in sera of sensitized patients: a structural approach

BACKGROUND

Significant efforts are dedicated to identification of agents that eliminate anti-HLA antibodies (Ab) from sera of transplant candidates. Antibody titer following in vitro incubation of sera with desensitizing agents has shown to reflect the probability that a patient would benefit from clinical de-sensitization protocols. AS101 is a non-toxic, synthetic, organic tellurium compound. The aim of this research was to assess the ability of AS101 to reduce anti-HLA Abs and to identify patients likely to benefit from this effect.

METHODS

Sera of sensitized patients awaiting transplant were incubated in the presence of AS101. Measured mean fluorescence intensity (MFI) represents reactivity of anti HLA Abs in the serum, as detected by the Luminex platform. The repertoire of HLA antigen epitopes was recognized using HLA Matchmaker software.

RESULTS

AS101 Incubation caused a significant Ab titer decrease in approximately two thirds of the samples. The median Class I and II MFI decrease among the responding samples was 16.7% and 14%, respectively (p<0.05). HLA Matchmaker analysis of the patients' class I epitope sequences revealed apparent amino-acid differences between the patterns of the responding and non-responding patients.

CONCLUSION

In vitro incubation of sera in the presence of AS101 causes a decrease in the anti-HLA Ab's reactivity in several patient samples. Sera most likely to demonstrate this effect are characterized by a moderate MFI level and a distinct antibody reactivity pattern specific for defined HLA antigen epitopes. These results support further investigation of AS101 as a potential agent for desensitization of humoral reactivity prior to transplantation.

New Cell-Based Therapy Paradigm: Induction of Bone Marrow-Derived Multipotent Mesenchymal Stromal Cells into Pro-Inflammatory MSC1 and Anti-inflammatory MSC2 Phenotypes

Cell-based therapies (CBTs) are quickly taking hold as a revolutionary new approach to treat many human diseases. Among the cells used in these treatments, multipotent mesenchymal stromal cells, also often and imprecisely termed mesenchymal stem cells (MSC), are widely used because they are considered clinically safe, unique in their immune-modulating capabilities, easily obtained from adult tissues, and quickly expanded as well as stored. However, despite these established advantages, there are limiting factors to employing MSCs in these therapeutic strategies. Foremost is the lack of a general consensus on a definition of these cells, marring efforts to prepare homogeneous lots and more importantly complicating their in vitro and in vivo investigation. Furthermore, although one of the most profound clinical effects of MSC intravenous administration is the modulation of host immune responses, no adequate ex vivo assays exist to consistently predict the therapeutic effect of each MSC lot in the treated patient. Until these issues are addressed, this very promising and safe new therapeutic approach cannot be used to its full advantage. However, these confounding issues do present exciting opportunities. The first is an opportunity to discover unknown aspects of host immune responses because the unique effect driven by MSC infusion on a patient's immunity has not yet been identified. In addition, there is an opportunity to develop methods, tests, and tools to better define MSCs and MSC-based therapy and provide consistency in preparation and effect. To this end, my laboratory recently developed a new approach to induce uniform pro-inflammatory MSC1 and anti-inflammatory MSC2 phenotypes from bone marrow-derived MSC preparations. I anticipate that MSC1 and MSC2 provide convenient tools with which to address some of these limitations and will help advance safe and effective CBTs for human disease.

Advances in direct T-cell alloreactivity: function, avidity, biophysics and structure

Although T-cell-based adaptive immunity plays a crucial role in protection against infectious pathogens and uncontrolled outgrowth of malignant cells, a large portion of these T cells are also capable of responding to allogeneic HLA molecules, violating the paradigm of self-major histocompatibility complex (MHC) restriction. Recent studies have provided insights into the mechanisms by which these T cells recognize allogeneic targets. The role of antiviral T cells in direct alloreactivity through peptide-dependent molecular mimicry and alternate peptide-MHC docking modes has emerged as major models for the human alloresponse. Here, we review in depth recent advances in this field and discuss how molecular interactions between T cells and HLA molecules drive the activation of these effector cells and its potential implications for alloreactivity in human transplantation.

Structural aspects of human leukocyte antigen class I epitopes detected by human monoclonal antibodies

This study addresses the concept that human leukocyte antigen (HLA) class I-specific alloantibodies are specific for epitopes that correspond to HLAMatchmaker-defined eplets. Eplets are essential parts of so-called structural epitopes that make contact with the 6 complementarity determining regions of an antibody. From published molecular models of crystallized protein antigen-antibody complexes, we have calculated that contact residues on structural HLA epitopes should reside within a 15-Å radius of a mismatched eplet. This study addresses the structural basis of high-frequency HLA class I epitopes reacting with human monoclonal antibodies (mAbs) derived from women sensitized during pregnancy. All mAbs were tested in Luminex assays with single HLA allele panels. The HLAMatchmaker algorithm was used to determine their specificity in context with eplet sharing between the immunizing allele and antibody-reactive alleles. To assess the autoreactive B cell origin of these antibodies, we have applied the recently developed nonself-self paradigm of epitope immunogenicity to analyze residue differences between the immunizer and the alleles of the antibody producer. A total of 9 mAbs were specific for epitopes associated with the 41T, 80NRG, 163LW, 69AA, or 80ERILR eplets. In each case, the immunizing allele had within 15 Å of the mismatched eplet, no residue differences with 1 of the alleles of the antibody producer. This observation is consistent with the concept that these mAbs originated from B cells with self HLA immunoglobulin receptors. Eplet-carrying alleles exhibited different levels of reactivity, which, when compared with the immunizing allele, ranged from high to intermediate to very low. In many cases, lower reactivities were associated with differences from self to nonself residues in surface locations within 15 Å of the specific eplet. Apparently, such locations may serve as critical contact sites for the antibody. In other cases, other residue differences did not appear to affect binding with the antibody, suggesting that these locations do not play a major role in antibody binding. For these mAbs we did not obtain convincing evidence that residue differences in hidden positions below the molecular surface had significant effects on antibody binding. These findings have increased our understanding of the structural basis of the immunogenicity and antigenicity of HLA class I epitopes and provide a basis for interpreting HLA antibody reactivity patterns in Luminex assays with single alleles.

Humoral alloimmunity in transplantation: relevance of HLA epitope antigenicity and immunogenicity

HLA mismatching is an important risk factor for antibody-mediated rejection and transplant failure. With the realization HLA antibodies recognize epitopes rather than antigens, it has become apparent that donor-recipient compatibility should be assessed at the epitope level. Recent developments have increased our understanding of the structural basis of HLA antigenicity, i.e., the reactivity with specific antibody and, immunogenicity, i.e., the ability to induce an antibody response. HLAMatchmaker is a computer algorithm that considers each HLA antigen as a series of small configurations of polymorphic residues referred to as eplets as essential components of HLA epitopes. This article addresses the relevance of determining epitope-specificities of HLA antibodies in the identification of acceptable mismatches for sensitized patients considered for transplantation. Permissible mismatching for non-sensitized patients aimed to prevent or reduce HLA antibody responses could consider epitope loads of mismatched antigens and the recently developed non-self–self paradigm of epitope immunogenicity.

Human leukocyte antigen epitope analysis to assess complement- and non-complement-binding donor-specific antibody repertoire in a pediatric heart transplant recipient

This case report summarizes the spectrum of anti-human leukocyte antigen (HLA) antibody reactivity determined by single-allele Luminex immunoglobulin G and C1q binding assays before transplant, during an episode of antibody-mediated rejection (AMR), and following treatment in a sensitized pediatric heart transplant (Tx) recipient. We were able to discriminate between complement- and non-complement-binding epitope-specific antibodies present against a single donor antigen (HLA-A2) during the progression of AMR and its resolution. Our findings illustrate the usefulness of determining antibody specificities against epitopes using various Luminex-based assays.