Methodological aspects of anti-human leukocyte antigen antibody analysis in solid organ transplantation

HLA antibody affinity determination: From HLA-specific monoclonal antibodies to donor HLA specific antibodies (DSA) in patient serum

Organs transplanted across donor-specific HLA antibodies (DSA) are associated with a variety of clinical outcomes, including a high risk of acute kidney graft rejection. Unfortunately, the currently available assays to determine DSA characteristics are insufficient to clearly discriminate between potentially harmless and harmful DSA. To further explore the hazard potential of DSA, their concentration and binding strength to their natural target, using soluble HLA, may be informative. There are currently a number of biophysical technologies available that allow the assessment of antibody binding strength. However, these methods require prior knowledge of antibody concentrations. Our objective within this study was to develop a novel approach that combines the determination of DSA-affinity as well as DSA-concentration for patient sample evaluation within one assay. We initially tested the reproducibility of previously reported affinities of human HLA-specific monoclonal antibodies and assessed the technology-specific precision of the obtained results on multiple platforms, including surface plasmon resonance (SPR), bio-layer interferometry (BLI), Luminex (single antigen beads; SAB), and flow-induced dispersion analysis (FIDA). While the first three (solid-phase) technologies revealed comparable high binding-strengths, suggesting measurement of avidity, the latter (in-solution) approach revealed slightly lower binding-strengths, presumably indicating measurement of affinity. We believe that our newly developed in-solution FIDA-assay is particularly suitable to provide useful clinical information by not just measuring DSA-affinities in patient serum samples but simultaneously delivering a particular DSA-concentration. Here, we investigated DSA from 20 pre-transplant patients, all of whom showed negative CDC-crossmatch results with donor cells and SAB signals ranging between 571 and 14899 mean fluorescence intensity (MFI). DSA-concentrations were found in the range between 11.2 and 1223 nM (median 81.1 nM), and their measured affinities fall between 0.055 and 24.7 nM (median 5.34 nM; 449-fold difference). In 13 of 20 sera (65%), DSA accounted for more than 0.1% of total serum antibodies, and 4/20 sera (20%) revealed a proportion of DSA even higher than 1%. To conclude, this study strengthens the presumption that pre-transplant patient DSA consists of various concentrations and different net affinities. Validation of these results in a larger patient cohort with clinical outcomes will be essential in a further step to assess the clinical relevance of DSA-concentration and DSA-affinity.

Correlation of Luminex-Based Single Antigen Based Results With Complement-Dependent Cytotoxicity Crossmatch and Flow Cytometry Crossmatch Results: A Single-Center Experience From Istanbul

BACKGROUND

This study aimed to retrospectively investigate the correlation of mean Class I donor-specific antibody (DSA) intensity values detected in Luminex-based techniques with the results of complement-dependent cytotoxicity crossmatch (CDC-XM) and flow cytometry crossmatch (FC-XM) results.

METHODS

A total of 335 patients with kidney failure and their living donors whose CDC-XM, FC-XM, and single antigen based (SAB) tests were studied between 2018 and 2020 for transplant preparation from living donor candidates were included in the study. Patients were divided into 4 groups according to their mean fluorescence intensity (MFI) values of SAB assay.

RESULTS

Anti-HLA antibodies (class I and/or class II) were detected using SAB in 91.6% patients included in the study (MFI >1000). Class I DSA was positive in 34.8% of patients with anti-HLA antibodies. When CDC-XM and FC-XM results were evaluated in the 4 groups separated according to MFI values, 3 patients with DSA MFI <1000 had negative CDC-XM and T-B-FC-XM results. Of 32 patients with DSA-MFI between 1000 and 3000, 93.75% (n = 30) had T-B-FC-XM or CDC-XM-negative results, and 6.25% (n = 2) had B-FC-XM-positive results. The CDC-XM, T, and B-FC-XM were negative in all 17 patients with DSA-MFI between 3000 and 5000. Our results showed that MFI >5834 DSA values were significantly correlated with positive T-FC-XM (P < .001), and MFI >6016 values were significantly correlated with positive CDC-XM (P = .002). In addition, MFI values >5000 were associated with both CDC-XM and FC-XM in our study.

CONCLUSIONS

The MFI values >5000 correlated with both CDC-XM and FC-XM.

Effects of different sensitization events on HLA alloimmunization in renal transplant cases; a retrospective observation in 1066 cases

BACKGROUND

Patients awaiting solid organ transplantation may develop anti-HLA antibodies after sensitization events such as transfusions, pregnancies, or previous transplantations. However, the effects of a particular sensitization event on HLA alloimmunization have not been well studied in parallel using cell-based assays and solid-phase assays. In this study, we evaluated and compare how different sensitization events affect the HLA antibody screening (HLA-Ab) and donor specific antibody (DSA) status in solid renal organ transplantation patients.

METHODS

HLA antibody (HLA-Ab) screening tests like complement-dependent cytotoxicity crossmatch (CDC-XM), flow cytometry crossmatch (FC-XM) & Luminex panel-reactive antibody (L-PRA) were performed in all 1066 patients (635 males and 431 females). If any of these tests turned out to be positive, a Luminex single antigen bead (L-SAB) assay was performed for DSA identification. Test positive rates and antibody strengths were analyzed according to the different sensitization events and gender.

RESULTS

In this study, HLA-Ab screening tests positive rates (L-PRA, FC-XM and CDC-XM) were significantly higher in patients with previous transplantation (73.91%, 86.95% and 56.52% p < 0.001), previous pregnancy (57.46%, 70.14% and 18.85% p < 0.001) or blood transfusion (27.33%, 35.55% and 7.33% p < 0.001) compared with patients without a sensitizing event (6.17%, 13.58% & 1.09). In this study, re-transplantation group showed significantly stronger antibody strength (DSA) than non sensitized group (class I & II MFI 11418.04, 17,837.78 vs class I & II MFI 2659, 3329; P < 0.001) and those with single sensitization events of transfusion (class I & II MFI 11418.04, 17,837.78 vs class I & II MFI 5598.82, 5436.12; P < 0.001) or pregnancy (class I & II MFI 11418, 17,837 vs class I & II MFI 8580.12, 7556.08; P < 0.001).

CONCLUSIONS

Pregnancy and blood transfused had high allo-immunization rate for class I HLA antigens. While re-transplantation patients had high allo-immunization rate for both the HLA classes (HLA- class I & HLA- class II). Re-transplantation group showed significantly stronger antibody strength, followed by pregnancy and then transfusion.

Allo-Specific Humoral Responses: New Methods for Screening Donor-Specific Antibody and Characterization of HLA-Specific Memory B Cells

Antibody-mediated allograft rejection (AMR) causes more kidney transplant failure than any other single cause. AMR is mediated by antibodies recognizing antigens expressed by the graft, and antibodies generated against major histocompatibility complex (MHC) mismatches are especially problematic. Most research directed towards the management of clinical AMR has focused on identifying and characterizing circulating donor-specific HLA antibody (DSA) and optimizing therapies that reduce B-cell activation and/or block antibody secretion by inhibiting plasmacyte survival. Here we describe a novel set of reagents and techniques to allow more specific measurements of MHC sensitization across different animal transplant models. Additionally, we have used these approaches to isolate and clone individual HLA-specific B cells from patients sensitized by pregnancy or transplantation. We have identified and characterized the phenotypes of individual HLA-specific B cells, determined the V(D)J rearrangements of their paired H and L chains, and generated recombinant antibodies to determine affinity and specificity. Knowledge of the BCR genes of individual HLA-specific B cells will allow identification of clonally related B cells by high-throughput sequence analysis of peripheral blood mononuclear cells and permit us to re-construct the origins of HLA-specific B cells and follow their somatic evolution by mutation and selection.

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

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

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.

Complement-dependent cytotoxicity and Luminex technology for human leucocyte antigen antibody detection in kidney transplant candidates exposed to different sensitizing events

Background

The aim of this study was to determine the frequency of exposure to different sensitizing events (SEs) and to assess their effects on human leucocyte antigen (HLA) alloimmunization in transplant candidates using two different HLA antibody screening techniques: complement-dependent cytotoxicity (CDC) and Luminex.

Methods

This retrospective study included HLA antibody screening results for 163 patients on the kidney transplant waiting list (WL) tested from March 2012 until the end of December 2015 at the Tissue Typing Laboratory, Rijeka, Croatia. All sera samples were tested using the CDC and Luminex techniques in parallel.

Results

Two-thirds of the patients [114 (70%)] on the WL were exposed to transfusions, pregnancies and/or kidney transplant. The pre-transplant sera of 104 (63.80%) patients were negative for antibodies. In the sera of 23 (14.11%) patients, HLA antibodies were detected by CDC and Luminex and in the sera of 36 (22.09%) patients by Luminex only.

Conclusion

In patients on kidney WL, previous organ transplantation represents the strongest immunogenic stimulus, followed by blood transfusions (the most frequent SE) and pregnancies. Although Luminex is more sensitive than CDC in HLA antibody detection, the decision on unacceptable HLA antigens in WL patients has to be based on the results of both assays and the patient's immunization history.

Comparative analysis of Luminex-based donor-specific antibody mean fluorescence intensity values with complement-dependent cytotoxicity & flow crossmatch results in live donor renal transplantation

Background & objectives:

Antibodies specific to donor human leucocyte antigen (HLA) play a critical role in graft rejection and graft loss. In recent years, techniques for their detection have evolved significantly providing an ever-increasing degree of sensitivity and specificity, from the conventional cell-based assays to the advanced solid-phase system based on the Luminex platform. Consensus is still evolving on the routine employment of all these methods, either stand alone or in combination. The objective of this study was to explore the near-accurate mean fluorescence intensity (MFI) cut-off values detected on Luminex platform predicting the strength of cell-based crossmatch results.

Methods:

Serum samples from 116 primary renal transplant recipients awaiting transplantation were tested for the presence of antidonor antibodies by the complement-dependent cytotoxicity (CDC) and flow crossmatch (FCXM) methods with their corresponding donors as well as for HLA-donor-specific antibodies (DSA) detection using a sensitive single antigen bead (SAB) assay.

Results:

None of the patients having HLA Class I DSA with MFI values <1000 showed positivity for T-cell FCXM or CDC crossmatch, while in the group having MFI values between 1000 and 3000, 54 per cent showed positivity for the FCXM but none by the CDC method. However, in the group having MFI values >3000, 95 per cent of cases were positive for FCXM. Further, those groups with MFI values between 3000 and 5000, only 36 per cent were positive for CDC crossmatch, while 90 per cent showed positivity in the group with MFI >7000.

Interpretation & conclusions:

A cut-off MFI value of 3000 for Luminex SAB-based assay was found to significantly correlate with the FCXM positivity while a MFI value of 7000 and above predicted a positive CDC crossmatch. MFI cut-off value obtained as a surrogate marker for CDC and FCXM tests will help in resolving the limitations of different cell-based techniques.

Pretransplantation monitoring of HLA antibodies by complement dependent cytotoxicity and Luminex-based assays

BACKGROUND

The introduction of more sensitive techniques, such as Luminex® for HLA antibody screening of patients awaiting organ transplantation has resulted in a better understanding of transplantation immunology and improvements in clinical practice.

OBJECTIVE

The interpretation of the results obtained only by Luminex® can lead to inaccurate evaluation of a patient's antibody status and unjustified rejection of a potential organ donor. The aim of this study was to demonstrate the benefits of performing HLA antibody screening in the sera of patients on the waiting list for organ transplantation by two different assays, complement dependent cytotoxicity (CDC) and Luminex®.

METHODS

A retrospective analysis was performed on 563 pretransplant serum samples from 141 patients on the kidney transplantation waiting list in Rijeka, tested from March 2012 until March 2015. All samples were tested in parallel by the CDC assay and the Luminex®-based assay.

RESULTS

Out of the 563 samples tested 302 (53.7%) tested negative for HLA antibodies and 88 (15.6%) positive by both assays. From 173 (30.7%) samples with discordant results 149 (26.5%) were CDC negative and Luminex® positive, while 24 (4.3%) were CDC positive and Luminex® negative. Among the Luminex positive patients seven did not experience any immunizing events.

CONCLUSION

Evaluation of the HLA antibody status in patients on a waiting list for organ transplantation should be based on the results of the both CDC and Luminex® (or other sensitive) assays in accordance to information about patient's clinical status and exposure to immunizing events.

[Immunological monitoring in kidney transplantation: 13 years experience of a Moroccan histocompatibility laboratory]

PURPOSE OF STUDY

The quality of the immunological monitoring is crucial because it determines the success of the kidney transplantation. The scope of this work is to describe the experience of the department of immunological unity of the Ibn Sina university hospital in Rabat regarding the immunological monitoring of patients transplanted between 2001 and 2014.

PATIENTS AND METHODS

Patient samples were collected from nephrology services of different public and private hospitals of Morocco. The tests conducted in the context of immunological monitoring are ABO typing, HLA-A, B, DR, DQ typing, anti-HLA antibodies detection and identification and cross-match.

RESULTS

One hundred and fourteen benefited from a pre- and post-transplant immunological monitoring in our laboratory. The percentage of recipients having between 2 and 5 stored sera is 60.5 before transplantation and 56.1 after transplantation. Immunized patients account for 22.8% before the transplant and 17.6% after transplantation. Ninety-seven patients still have a functional graft, while 4 of them had DSA of low intensity before transplantation. Five immunological rejections were reported while the cross-match were negative and no DSA was identified before transplantation. Patient survival and graft at 1 year was 98.2% and 92.7% respectively.

CONCLUSION

Conducting regular immunological monitoring is sometimes difficult in our context, however, the results are satisfactory in terms of graft and patients survival.

Antibody-reactive class I epitopes defined by pairs of mismatched eplets and self-eplets

The identification of human leukocyte antigen (HLA) antibodies in the sera of candidates awaiting organ transplantation has evolved over time. This has been possible because of the introduction of more sensitive techniques and to the increasing focus on the structural aspects of the HLA epitopes. The use of the HLAMatchmaker algorithm in the analysis of positive sera and the verification of HLA ABC epitopes in the HLA Epitope Registry website provide new stimuli on the interpretation of antibody reactivity. The epitopes defined by eplet pairs often involve a nonself-eplet and a self-eplet (nonself-self paradigm), suggesting that the antibody response to an HLA mismatch must have an auto-reactive component. Here, we report an application of the nonself-self paradigm that provides a basis for better knowledge and interpretation of HLA-antibody reactivity in Luminex assays with single alleles.