Extending options for highly sensitized patients to receive a suitable kidney graft

Chimeric HLA antibody receptor T cells to target HLA-specific B cells in solid organ transplantation

HLA-sensitized patients on the transplant waiting list harbor antibodies and memory B cells directed against allogeneic HLA molecules, which decreases the chance to receive a compatible donor organ. Current desensitization strategies non-specifically target circulating antibodies and B cells, warranting the development of therapies that specifically affect HLA-directed humoral immune responses. We developed Chimeric HLA Antibody Receptor (CHAR) constructs comprising the extracellular part of HLA-A2 or HLA-A3 coupled to CD28-CD3ζ domains. CHAR-transduced cells expressing reporter constructs encoding T-cell activation markers, and CHAR-transduced CD8+ T cells from healthy donors were stimulated with HLA-specific monoclonal antibody-coated microbeads, and HLA-specific B cell hybridomas. CHAR T cell activation was measured by upregulation of T cell activation markers and IFNγ secretion, whereas CHAR T cell killing of B cell hybridomas was assessed in chromium release assays and by IgG ELISpot. HLA-A2- and HLA-A3-CHAR expressing cells were specifically activated by HLA-A2- and HLA-A3-specific monoclonal antibodies, either soluble or coated on microbeads, as shown by CHAR-induced transcription factors. HLA-A2 and HLA-A3 CHAR T cells efficiently produced IFNγ with exquisite specificity and were capable of specifically lysing hybridoma cells expressing HLA-A2- or HLA-A3-specific B-cell receptors, respectively. Finally, we mutated the α3 domain of the CHAR molecules to minimize any alloreactive T-cell reactivity against CHAR T cells, while retaining CHAR activity. These data show proof of principle for CHAR T cells to serve as precision immunotherapy to specifically desensitize (highly) sensitized solid organ transplant candidates and to treat antibody-mediated rejection after solid organ transplantation.

Transplantation in highly sensitized patients: challenges and recommendations

INTRODUCTION

Highly sensitized patients awaiting a kidney transplant accrue on the transplant waiting list. The breadth of HLA antibodies in this subpopulation of patients precludes receiving a compatible organ offer through regular allocation within an acceptable time-frame. Areas covered: Several alternative options to receive a transplant exist for these patients, including additional priority in regular allocation, special programs based on allocation through acceptable antigens, kidney paired donation programs, desensitization protocols, or a combination of the latter two. In this review, these options and their outcomes are discussed as well as some possibilities to further enhance transplantation of this disadvantaged group of patients. Expert commentary: For highly sensitized patients, special attention is required, for which several strategies may apply. A step-wise approach may be the optimal strategy to facilitate successful transplantation of highly sensitized patients.

Are We Ready for Epitope-Based HLA Matching in Clinical Organ Transplantation?

This overview describes recent developments demonstrating the significance of epitopes in HLA antibody responses and matching for organ transplantation. HLA epitopes are defined by molecular modeling and amino acid comparisons between HLA alleles and the HLAMatchmaker algorithm considers eplets as essential components. Each allele represents a distinct string of eplets and matching is done by aligning donor and recipient strings. Evidence is summarized how mismatched eplet loads affect antibody responses and transplant outcomes. Epitope-based matching has been applied not only to identify acceptable mismatches for sensitized transplant candidates but also to identify more suitably mismatched donors for nonsensitized patients. Three recently proposed theories will further our understanding of the immunogenicity of individual HLA eplets.It has become apparent that epitope-based matching is superior to antigen matching; we should be ready soon to apply this principle in the clinical transplant setting very soon.

HLA Mismatching Strategies for Solid Organ Transplantation - A Balancing Act

HLA matching provides numerous benefits in organ transplantation including better graft function, fewer rejection episodes, longer graft survival, and the possibility of reduced immunosuppression. Mismatches are attended by more frequent rejection episodes that require increased immunosuppression that, in turn, can increase the risk of infection and malignancy. HLA mismatches also incur the risk of sensitization, which can reduce the opportunity and increase waiting time for a subsequent transplant. However, other factors such as donor age, donor type, and immunosuppression protocol, can affect the benefit derived from matching. Furthermore, finding a well-matched donor may not be possible for all patients and usually prolongs waiting time. Strategies to optimize transplantation for patients without a well-matched donor should take into account the immunologic barrier represented by different mismatches: what are the least immunogenic mismatches considering the patient’s HLA phenotype; should repeated mismatches be avoided; is the patient sensitized to HLA and, if so, what are the strengths of the patient’s antibodies? This information can then be used to define the HLA type of an immunologically optimal donor and the probability of such a donor occurring. A probability that is considered to be too low may require expanding the donor population through paired donation or modifying what is acceptable, which may require employing treatment to overcome immunologic barriers such as increased immunosuppression or desensitization. Thus, transplantation must strike a balance between the risk associated with waiting for the optimal donor and the risk associated with a less than optimal donor.

Reflections on HLA Epitope-Based Matching for Transplantation

HLA antibodies are primary causes of transplant rejection; they recognize epitopes that can be structurally defined by eplets. There are many reviews about HLA epitope-based matching in transplantation. This article describes some personal reflections about epitopes including a historical perspective of HLA typing at the antigen and allele levels, the repertoires of antibody-verified HLA epitopes, the use of HLAMatchmaker in determining the specificities of antibodies tested in different assays, and, finally, possible strategies to control HLA antibody responses.

Should epitope-based HLA compatibility be used in the kidney allocation system?

The new kidney allocation system (KAS) still applies donor-recipient HLA compatibility mostly at the antigen level and although some four-digit alleles have been included. This system is used to record unacceptable mismatches for sensitized transplant candidates with serum HLA antibodies. Since the reactivities of such antibodies are specifically associated with epitopes rather than HLA antigens, a more scientifically accurate assessment of mismatch acceptability could be based on epitopes. HLA class I and class II epitope specificity analyses can now be readily performed with serum antibody assays with single allele panels. This report describes an epitope-based HLA compatibility system for KAS and involves recipient and donor HLA typing at the four-digit allele level. It focuses on sensitized patients who have serum antibodies specific for HLA epitopes that can be entered as unacceptable mismatches in the transplant candidate database. Newly developed software programs could readily identify compatible HLA types.

HLA Matching at the Eplet Level Protects Against Chronic Lung Allograft Dysfunction

Donor selection in lung transplantation (LTx) is historically based upon clinical urgency, ABO compatibility, and donor size. HLA matching is not routinely considered; however, the presence or later development of anti-HLA antibodies is associated with poorer outcomes, particularly chronic lung allograft dysfunction (CLAD). Using eplet mismatches, we aimed to determine whether donor/recipient HLA incompatibility was a significant predictor of CLAD. One hundred seventy-five LTx undertaken at the Alfred Hospital between 2008 and 2012 met criteria. Post-LTx monitoring was continued for at least 12 months, or until patient death. HLA typing was performed by sequence-based typing and Luminex sequence-specific oligonucleotide. Using HLAMatchmaker, eplet mismatches between each donor/recipient pairing were analyzed and correlated against incidences of CLAD. HLA-DRB1/3/4/5+DQA/B eplet mismatch was a significant predictor of CLAD (hazard ratio [HR] 3.77, 95% confidence interval [CI]: 1.71-8.29 p < 0.001). When bronchiolitis obliterans syndrome (BOS) and restrictive allograft syndrome (RAS) were analyzed independently, HLA-DRB1/3/4/5 + DQA/B eplet mismatch was shown to significantly predict RAS (HR 8.3, 95% CI: 2.46-27.97 p < 0.001) but not BOS (HR 1.92, 95% CI: 0.64-5.72, p = 0.237). HLA-A/B eplet mismatch was shown not to be a significant predictor when analyzed independently but did provide additional stratification of results. This study illustrates the importance of epitope immunogenicity in defining donor-recipient immune compatibility in LTx.

Single antigen flow beads for identification of human leukocyte antigen antibody specificities in hypersensitized patients with chronic renal failure

Aims of this study

Aims of this study were to identify class I and class II antibodies in highly sensitized patients by flow cytometry single antigen bead (FC-SAB) assay and to evaluate according to donor HLA type in order to increase their kidney transplantation chance.

Material and methods

We analyzed 60 hypersensitive patients of 351 individuals, who applied to our laboratory for PRA test in November 2013-December 2014. Flow cytometric PRA screening and single antigen bead commercial kits were used for these analyses.

Results

In our study group, 19 (31.7%) of these patients were male while 41 (68.3%) patients were female. The most common acceptable antigens were A*02 (10.11%), HLA-A*23 (10.11%), HLA-B*38 (8.79%) and HLA-DRB1*03 (7.83%) in hypersensitive patients. The highest antibody reactivity on SAB was observed against HLA-A*25, HLA-B*45, HLA-DRB1*04 and HLA-DRB1*08 antigens.

Conclusions

The determination of these acceptable and unacceptable antigens may increase their transplantation chance. Pre-transplant HLA antibody identifications provide prognostic information with respect to the determination of patients who are at increased risk of graft loss.

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 applications of next-generation sequencing in histocompatibility and transplantation

PURPOSE OF REVIEW

Next-generation sequencing (NGS) can overcome traditional methodological barriers to facilitate detailed studies of large genomes. Here, we summarize recent NGS-based developments in histocompatibility and transplantation, and highlight the dynamic range of clinical applications achievable on this platform.

RECENT FINDINGS

Multiple NGS-based protocols have been established to achieve unambiguous human leukocyte antigen genotyping. These methods are presently engaged to serve the high-throughput demand of large bone marrow registries; however, the scalable nature of NGS makes it an equally attractive technology for select applications within solid organ transplantation. Recently, the exquisite sensitivity of NGS has been leveraged to perform noninvasive allograft monitoring by tracking the dynamics of donor-derived cell-free DNA. Further, NGS-based T-cell receptor and immunoglobulin heavy chain repertoire profiling appear to be useful in clarifying disease-specific diagnoses in certain complex allograft pathology; detecting/quantifying minimal residual disease following allogeneic stem cell transplantation; and tracking donor-reactive T cells to understand the mechanism of tolerance in kidney transplant recipients.

SUMMARY

NGS is superior to classical Sanger sequencing in its throughput, sensitivity, and the ability to provide phase-defined sequence data. These unique properties allow its broad application to diverse areas in clinical transplantation.

Shared molecular eplet stimulates acute antibody-mediated rejection in a kidney transplant recipient with low-level donor-specific antibodies: a case report

HLA antibodies usually recognize epitopes rather than antigens. This case report reveals that acute antibody-mediated rejection (AMR) that occurred in a kidney transplant recipient with low-level donor-specific antibodies (DSAs) could be explained by shared epitope. A 39-year-old woman received a first kidney transplant from a deceased donor (HLA-DRB1 11:06, 12:02, DRB3 02:02, 03:01). She developed acute AMR confirmed by kidney biopsy on day 4 after transplantation. Antibody testing with pretransplant serum showed anti-DR11 DSA below cutoff level (mean fluorescence intensity [MFI], 702; cutoff >1,000). However, high-level DSAs were detected on day 5 after transplantation (anti-DR11 MFI, 8,531; anti-DR12 MFI, 3,146). We hypothesized that the sharp rise in DSA levels was a result of anamnestic response with donor-antigen sensitization that occurred during pregnancy. High-resolution HLA-DR typing of her husband showed HLA-DRB1 03:01, 15:02:01, DRB3 02:02, DRB5 01:02. No sharing between donor HLAs eliciting reactive antibodies and her husband's HLAs was detected. Nevertheless, we speculated that shared epitope, not antigen, was the cause of allosensitization. To identify the shared epitope recognized by patient's antibodies, we used HLAmatchmaker, a computer algorithm that considers small configurations of polymorphic residues referred to as eplets as essential components of HLA epitopes for analysis. The results showed that 149H, which was the eplet shared by HLA-DRB1 03:01 (from her husband) and DRB1 11:06, DRB1 12:02, DRB3 03:01 (from donor), was the most prevalent eplet on DRB1 reactive alleles in Luminex assay. In conclusion, pretransplant low-level DSAs can induce AMR early after transplantation as a result of shared epitopes with a previous immunizer.

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.

B-cell tolerance in transplantation: is repertoire remodeling the answer?

T lymphocytes are the primary targets of immunotherapy in clinical transplantation; however, B lymphocytes and their secreted alloantibodies are also highly detrimental to the allograft. Therefore, the achievement of sustained organ transplant survival will likely require the induction of B-lymphocyte tolerance. During development, acquisition of B-cell tolerance to self-antigens relies on clonal deletion in the early stages of B-cell compartment ontogeny. We contend that this mechanism should be recapitulated in the setting of alloantigens and organ transplantation to eliminate the alloreactive B-cell subset from the recipient. Clinically feasible targets of B-cell-directed immunotherapy, such as CD20 and B-lymphocyte stimulator (BLyS), should drive upcoming clinical trials aimed at remodeling the recipient B-cell repertoire.

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.

Antibody-reactive epitope determination with HLAMatchmaker and its clinical applications

Antibodies against allogeneic human leukocyte antigen (HLA) molecules are important impediments to the success of different clinical procedures including transplantation and platelet transfusion. In these settings, characterization of the repertoire of immunogenic epitopes is important for permissible mismatch determination and the identification of acceptable mismatches for sensitized patients. HLAMatchmaker is a computer algorithm that considers small configurations of polymorphic residues referred to as eplets as essential components of HLA epitopes. This review critically elaborates the concepts underlying the HLAMatchmaker and describes the usefulness of HLAMatchmaker in the clinical setting. Recent developments have increased our understanding of structural basis of HLA antigenicity (i.e. reactivity with specific antibody) and immunogenicity (i.e. its ability to induce an antibody response).

Preexisting donor-specific HLA antibodies predict outcome in kidney transplantation

The clinical importance of preexisting HLA antibodies at the time of transplantation, identified by contemporary techniques, is not well understood. We conducted an observational study analyzing the association between preexisting donor-specific HLA antibodies (HLA-DSA) and incidence of acute antibody-mediated rejection (AMR) and survival of patients and grafts among 402 consecutive deceased-donor kidney transplant recipients. We detected HLA-DSA using Luminex single-antigen assays on the peak reactive and current sera. All patients had a negative lymphocytotoxic cross-match test on the day of transplantation. We found that 8-year graft survival was significantly worse (61%) among patients with preexisting HLA-DSA compared with both sensitized patients without HLA-DSA (93%) and nonsensitized patients (84%). Peak HLA-DSA Luminex mean fluorescence intensity (MFI) predicted AMR better than current HLA-DSA MFI (P = 0.028). As MFI of the highest ranked HLA-DSA detected on peak serum increased, graft survival decreased and the relative risk for AMR increased: Patients with MFI >6000 had >100-fold higher risk for AMR than patients with MFI <465 (relative risk 113; 95% confidence interval 31 to 414). The presence of HLA-DSA did not associate with patient survival. In conclusion, the risk for both AMR and graft loss directly correlates with peak HLA-DSA strength. Quantification of HLA antibodies allows stratification of immunologic risk, which should help guide selection of acceptable grafts for sensitized patients.

B-lymphocyte homeostasis and BLyS-directed immunotherapy in transplantation

Current strategies for immunotherapy after transplantation are primarily T-lymphocyte directed and effectively abrogate acute rejection. However, the reality of chronic allograft rejection attests to the fact that transplantation tolerance remains an elusive goal. Donor-specific antibodies are considered the primary cause of chronic rejection. When naive, alloreactive B-cells encounter alloantigen and are activated, a resilient "sensitized" state, characterized by the presence of high-affinity antibody, is established. Here, we will delineate findings that support transient B-lymphocyte depletion therapy at the time of transplantation to preempt sensitization by eliminating alloreactive specificities from the recipient B-cell pool (ie, "repertoire remodeling"). Recent advances in our understanding of B-lymphocyte homeostasis provide novel targets for immunomodulation in transplantation. Specifically, the tumor necrosis factor-related cytokine BLyS is the dominant survival factor for "tolerance-susceptible" transitional and "preimmune" mature follicular B-cells. The transitional phenotype is the intermediate through which all newly formed B-cells pass before maturing into the follicular subset, which is responsible for mounting an alloantigen-specific antibody response. Systemic BLyS levels dictate the stringency of negative selection during peripheral B-cell repertoire development. Thus, targeting BLyS will likely provide an opportunity for repertoire-directed therapy to eliminate alloreactive B-cell specificities in transplant recipients, a requirement for the achievement of humoral tolerance and prevention of chronic rejection. In this review, the fundamentals of preimmune B-cell selection, homeostasis, and activation will be described. Furthermore, new and current B-lymphocyte-directed therapy for antibody-mediated rejection and the highly sensitized state will be discussed. Overall, our objective is to propose a rational approach for induction of humoral transplantation tolerance by remodeling the primary B-cell repertoire of the allograft recipient.

Successful expansion of the living donor pool by alternative living donation programs

Between January 2000 and December 2007, 786 potential recipients and 1059 potential donors attended our pretransplant unit with the request for a living-donor renal transplant procedure. The recipients brought one potential donor in 77.2% and two or more donors in 22.8% of cases. In the regular living donor program, a compatible donor was found for 467 recipients. Without considering alternative donation, 579 donors would have been refused. Alternative living donation programs led to 114 compatible combinations: kidney-exchange program (35), ABO-incompatible donation (25), anonymous donation (37) and domino-paired anonymous donation (17). Together, the 114 alternative program donations and the 467 regular living donations led to 581 living donor transplantations (24.4% increase). Eventually for 54.9% (581/1059) of our donors, a compatible combination was found. Donor-recipient incompatibility comprised 19.4% (89/458) in the final refused population, which is 8.8% of the potential donor-recipient couples. Without considering alternative donation, 30.1% (174/579) of the refused donors would have been refused on incompatibility and 6.4% (37/579) because they were anonymous. This is 20% of the potential donor population (211/1059). The implementation of alternative living donation programs led to a significant increase in the number of transplantations, while transplantations via the direct donation program steadily increased.

Correlations between Terasaki's HLA class I epitopes and HLAMatchmaker-defined eplets on HLA-A, -B and -C antigens

Although the determination of human leukocyte antigen (HLA) antibody specificity has traditionally been directed toward HLA antigens, there is now increasing attention to structurally defined HLA epitopes. An understanding of the HLA epitope repertoire is important to acceptable mismatching for sensitized patients and to a new epitope-based matching algorithm aimed to reduce antibody-mediated rejection. There are two strategies to determine the HLA epitope repertoire. Terasaki's group has used an empirical method to analyze the reactivity of single allele Luminex panels with mouse monoclonal antibodies (mAbs) and absorbed/eluted alloantibodies with a computer program based on shared residues in the amino acid sequences of reactive alleles. HLAMatchmaker is a theoretical algorithm that predicts HLA epitopes on the HLA molecular surface from stereochemical modeling of epitope-paratope interfaces of antigen-antibody complexes. Our epitope repertoire is based on so-called 'eplets' representing 3-A patches of at least one polymorphic residue on the molecular surface. A comparative analysis has shown that 81/103 Terasaki's HLA class I epitopes are equivalent to individual eplets (n = 50) or pairs of eplets (n = 31) separated far enough to serve as potential contact sites for two complementarity-determining regions of antibody. An additional 12 Terasaki's epitopes (TerEps) correspond to eplets with permissible residue combinations that do not seem to affect epitope specificity. We could not identify corresponding eplets for the remaining 10 TerEps, including 8 that might be considered xeno-epitopes defined by mouse mAbs. Conversely, HLAMatchmaker has 38 additional eplets in well-exposed surface positions that do not have equivalent TerEps, and for many of them, we have found specific antibodies. These findings strengthen the concept that eplets are essential basic units of HLA epitopes and that they provide a better understanding of HLA immunogenicity (i.e. ability to induce an antibody response) and antigenicity (i.e. reactivity with specific antibody).

Organ procurement and transplantation network/united network for organ sharing histocompatibility committee collaborative study to evaluate prediction of crossmatch results in highly sensitized patients

BACKGROUND

The requirement for a prospective crossmatch limits some organ allocation to local areas. The delay necessitated by the crossmatch restricts the distance across which offers can be made without unduly increasing the ischemia time. A collaborative study involving 14 transplant centers was undertaken by the Organ Procurement and Transplantation Network/United Network for Organ Sharing (OPTN/UNOS) Histocompatibility Committee to evaluate the accuracy with which the detection of unacceptable human leukocyte antigen (HLA) antigens by most advanced solid phase immunoassays can predict crossmatch results. In addition, using actual patients' unacceptable HLA antigens, the number of compatible donors that would have been available from the OPTN deceased kidney donors during 2002 to 2004 were investigated.

METHODS

Panel reactive antibodies were performed by conventional or solid phase assays, and crossmatches were performed by cytotoxicity or flow cytometry. Analyses were stratified for T and B cell and by method of identifying unacceptable HLA antigens and crossmatch techniques.

RESULTS

Combination of solid phase immunoassays and flow cytometry crossmatches resulted in a higher prediction rates of positive T cell (86.1%-93.5%) and B-cell crossmatches (91%-97.8%). Prediction of negative crossmatches based on different combination of panel reactive antibodies and crossmatch techniques varied from 14.3% to 57.1%. Furthermore, numerous potential compatible donors were identified for each patient, regardless of their ethnicity, in the OPTN database, when predicted incompatible ones were excluded.

CONCLUSIONS

The above results showed that with the advent of solid phase immunoassays, HLA antibodies can now be accurately detected resulting in prediction of crossmatch outcome. This should facilitate organ allocation and prevents shipment of organs to distant incompatible recipients.

HLA antibody testing: a tool to facilitate not to prevent organ transplantation

The introduction of very sensitive HLA antibody screening assays has destroyed the old dogma that pre-existence of donor specific HLA antibodies in the patient is a contra-indication for transplantation. The challenge is now to reach consensus on the parameters which predict the clinical relevance of donor specific HLA antibodies. Antibody screening assays should not only be used to prevent transplantation of patients with donor specific antibodies but also to facilitate transplantation of highly sensitized patients, both by defining acceptable HLA mismatches and non-detrimental donor specific HLA antibodies.

[Transplantation strategies in immunized and hyperimmunized patients]

The rate of access to transplantation of hyperimmunized patients has improved thanks to a modification of the organ allocation rule in 2002 and the initiation of the Acceptable Mismatch program. Transplantation of immunized patients must be now preceded by a fine description of the characteristics of pretransplantation immunization using sensitive techniques and by an indispensable clinical and biological discussion to choose the best transplantation strategy, taking into account the risk/benefit balance for the patient of each strategy. Whichever transplantation strategy is adopted, immunosuppression should be adapted to the immunological status of the patient, often using the new therapeutic strategies (IVIg, rituximab, plasma exchange) with a specific follow-up, notably for anti-HLA antibodies, checking their evolution post treatment.

Structural epitope matching for HLA-alloimmunized thrombocytopenic patients: a new strategy to provide more effective platelet transfusion support?

BACKGROUND

During the past three decades, HLA matching for platelet (PLT) transfusion of refractory thrombocytopenic patients has been based on serologic cross-reactivity between HLA-A and HLA-B antigens. Although many blood banks are using this matching strategy, the general experience is that such matched PLT transfusions are often ineffective.

STUDY DESIGN AND METHODS

This report describes a new HLA matching algorithm that considers structurally defined epitopes recognized by antibodies. HLAMatchmaker is a computer program that determines histocompatibility at the amino acid level initially designed as triplets (i.e., linear sequences of three residues in molecular surface-exposed positions) but now updated as eplets representing patches of antibody-accessible polymorphic residues surrounded by residues within a 3-A radius. The eplet version of HLAMatchmaker is also useful in the analysis of HLA antibody reactivity patterns of alloimmunized patients so that acceptable mismatches can be identified.

CONCLUSION

An HLA epitope-based matching protocol is proposed that may permit a more effective PLT transfusion management of refractory patients. This protocol includes high-resolution HLA-A, -B, and -C typing of patients and donors, serum screening to identify acceptable mismatches, and the identification of suitable donors in a donor database that incorporates HLAMatchmaker as a search engine. HLAMatchmaker programs can be downloaded from the Web site http://tpis.upmc.edu/tpis/HLAMatchmaker/.

14th International HLA and Immunogenetics Workshop: report on the structural basis of HLA compatibility

Under auspices of the 14th International Human Leukocyte Antigen (HLA) and Immunogenetics Workshop, multilaboratory collaborative studies were organized addressing the following projects: (1) identification of structurally defined class I and class II HLA epitopes by human monoclonal antibodies, (2) assessment of the relative immunogenicity of structurally defined donor HLA mismatches in kidney transplantation, and (3) evaluation of the usefulness of structurally based HLA matching in platelet transfusion support of alloimmunized thrombocytopenic patients. This progress report describes a preliminary HLAMatchmaker-based data analysis in each project. The findings support the clinical usefulness of structural HLA matching.

Identification of acceptable HLA mismatches in immunized patients using single-antigen-expressing cell lines

Sera of highly sensitized patients (HSP) contain complex human leukocyte antigen (HLA) antibodies, minimizing the chance to identify crossmatch-negative donors. Expression of 3-6 HLA class I antigens on lymphocytes hampers identification of acceptable mismatches (AMs) by conventional screening (C-SCR). The single-antigen-expressing cell line (SAL) concept circumvents this problem. As a proof of principle, 26 sera of sensitized patients were tested by flow cytometry for immunoglobulin G antibodies against 16 HLA-A and -B SALs. Results were compared with C-SCR. Mostly, SAL reactions confirmed presence/absence of HLA antibodies. While C-SCR sometimes failed to provide unambiguous antibody specificity, we defined 24 new HLA antibody specificities with SALs and proposed 33 new AM by non-reactivity with SALs. Thus, the SAL concept is useful for confirmation/identification of AM and will enhance transplantation of HSP.

Intravenous immunoglobulin, HLA allele typing and HLAMatchmaker facilitate successful transplantation in highly sensitized pediatric renal allograft recipients

The use of intravenous immunoglobulin (IVIG) in sensitized transplant candidates has resulted in reduced HLA antibody levels and shorter transplant wait times. In addition, the HLAMatchmaker program has been used to identify acceptable mismatches to permit transplantation in highly sensitized patients. We used IVIG desensitization in conjunction with high resolution HLA allele typing and HLAMatchmaker grading of donor offers to facilitate successful transplantation in two highly sensitized children who were awaiting second renal transplants. Both patients lost their initial transplant in <10 days to accelerated acute rejection, and were on dialysis for an average of 50 months with high panel reactive antibody (PRA) levels. They were started on monthly IVIG infusions (2 g/kg/dose). Within one wk following their third and fifth IVIG doses, both patients received a crossmatch compatible, deceased donor renal transplant selected by HLAMatchmaker as a suitable donor offer. Both patients remain rejection free with excellent renal function 19 and 15 months post-transplant, respectively. In conclusion, combining IVIG therapy and donor selection by HLA humoral epitope matching permitted successful transplantation of two highly sensitized children. Further studies in larger numbers of patients with longer follow-up are needed to determine the individual role played by, and relative importance of each component of this combined strategy.

HLAMatchmaker: a molecularly based algorithm for histocompatibility determination. V. Eplet matching for HLA-DR, HLA-DQ, and HLA-DP

This report describes the design of the eplet version of HLAMatchmaker to determine class II compatibility at the structural level. This matching algorithm is based on the hypothesis, developed from molecular modeling of crystallized antigen-antibody complexes, that functional epitopes are represented by patches of surface-exposed nonself-amino acid residues surrounded by residues within a 3-A radius. Patch determinations with a molecular viewer of crystalline structural models downloaded from the Entrez Molecular Modeling Database Web site led to the identification of 44 DRB, 33DQB, 29 DQA, 20 DPB, and 9 DPA unique combinations of polymorphic positions. The residue compositions of these patches were then determined from amino acid sequences. This analysis resulted in a repertoire of 146 DRB, 74 DQB, 58 DQA, 45 DPB, and 19 DPA eplets. In many eplets, the residues are in short linear sequences, but many other eplets have discontinuous sequences of residues that cluster on or near the molecular surface. This analysis has also shown that all serologically defined DR and DQ antigens detectable by monospecific antibodies have unique eplets. Other eplets are present in groups of class II antigens, many of which appear as cross-reacting. The eplet version of HLAMatchmaker should be considered as a hypothetical model for the structural assessment of donor-recipient compatibility and the determination of mismatch acceptability for sensitized patients. This computer algorithm can be downloaded from the HLA Matchmaker Webside at http://tpis.upmc.edu.

Successful combined partial auxiliary liver and kidney transplantation in highly sensitized cross-match positive recipients

Combined liver and renal transplantations can be performed against a positive cross-match, indicating that the liver protects the kidney from the harmful HLA antibodies. This led us to the hypothesis that a partial auxiliary liver graft may have a similar protective effect when performed together with the kidney in highly sensitized patients. Seven patients, with broadly reacting HLA antibodies and positive crossmatches, were transplanted with a partial liver and a kidney from the same donor. In one of the cases a living donor was used. We performed lymphocytotoxic and flow cross-matches before and after the transplantation. Cross-matches turned negative after grafting in five of seven cases. The kidney function was excellent, without rejections, during the follow-up (24-60 months) in these patients. In two cases the cross-match remained positive after transplantation, one with a never-functioning renal graft and the other with an early graft failure, probably due to humoral rejection. A simultaneous transplantation of a partial auxiliary liver graft from the same donor, with the sole purpose of protecting the kidney from harmful lymphocytotoxic antibodies, can be performed successfully despite a positive cross-match and may thus be a new option of treatment for highly sensitized patients waiting for a kidney transplant.

A structurally based approach to determine HLA compatibility at the humoral immune level

HLAMatchmaker is a structurally based matching program. Each HLA antigen is viewed as a string of epitopes represented by short sequences (triplets) involving polymorphic amino acid residues in antibody-accessible positions. HLAMatchmaker determines which triplets are different between donor and recipient, and this algorithm is clinically useful in determining HLA mismatch acceptability. Triplets provide however an incomplete description of the HLA epitope repertoire and expanded criteria must be used including longer sequences and polymorphic residues in discontinuous positions. Such criteria should consider the structural basis of antibody-antigen interactions including contact areas and binding energy, the essence of antigenicity. This report describes the development of a structurally defined HLA epitope repertoire based on stereochemical modeling of crystallized complexes of antibodies and different protein antigens. This analysis considered also data in the literature about contributions of amino acid residues to antigen-antibody binding energy. The results have led to the concept that HLA antigens like other antigenic proteins have structural epitopes consisting of 15-22 residues that constitute the binding face with alloantibody. Each structural epitope has a functional epitope of about 2-5 residues that dominate the strength and specificity of binding with antibody. The remaining residues of a structural epitope provide supplementary interactions that increase the stability of the antigen-antibody complex. Each functional epitope has one or more non-self residues and the term "eplet" is used to describe polymorphic HLA residues within 3.0-3.5 A of a given sequence position on the molecular surface. Many eplets represent short linear sequences identical to those referred to as triplets but others have residues in discontinuous sequence positions that cluster together on the molecular surface. Serologically defined HLA determinants correspond well to eplets. The eplet version of HLAMatchmaker represents therefore a more complete repertoire of structurally defined HLA epitopes and provides a more detailed assessment of HLA compatibility.

Transplanting the highly sensitized patient: The emory algorithm

Renal transplant patients sensitized to HLA antigens comprise nearly one-third of the UNOS wait-list and receive 14% of deceased donor (DD) transplants, a rate half that of unsensitized patients. Between 1999 and 2003, we performed 492 adult renal transplants from DD; 120 patients (approximately 25%) had a panel reactive antibody (PRA) of >30%, with nearly half (n = 58) having a PRA of >80%. Our approach is based upon high-resolution solid-phase HLA antibody analysis to identify class I/II antibodies and a 'virtual crossmatch' to predict compatible donor/recipient combinations. Recipients are excluded from the United Network for Organ Sharing match run if donors possess unacceptable antigens. Thus, when sensitized patients appear on the match run, they have a high probability of a negative final crossmatch. Here, we describe our 5-year experience with this approach. Five-year graft survival ranged from 66% to 70% among unsensitized (n = 272), moderately sensitized (PRA < 30%, n = 100) and highly sensitized (>30% PRA; n = 120) patients, equal to the average national graft survival (65.7%). The application of this approach (the Emory Algorithm) provides a logical and systematic approach to improve the access of sensitized patients to DD organs and promote more equitable allocation to a highly disadvantaged group of patients awaiting renal transplantation.