Naturally occurring interference in Luminex assays for HLA-specific antibodies: characteristics and resolution

Modes of assay interference and the effectiveness of serum pretreatment approaches in detection of anti-HLA antibodies

AIMS

Several modes of assay interference common to immunoassays affect solid-phase single-antigen bead-based immunoassays (SAB) used to detect antibodies against human leucocyte antigens (HLA). Best practice recommendations include methods to address assay interference, though the clinical impact and optimal approaches are undefined. We sought to evaluate assay interference in HLA SAB to identify an efficient approach for avoiding erroneous results.

METHODS

Retrospective analysis of 14 059 patient samples tested for anti-HLA antibodies was performed. This included 4685 samples tested prior to implementation of serum pretreatment with EDTA and 4982 samples tested with routine EDTA treatment using the same testing algorithm. An algorithm for efficiently identifying and processing samples with suspected interference was evaluated in a separate cohort of 4392 EDTA-treated samples.

RESULTS

EDTA serum pretreatment reduced assay interference, but did not eliminate all modes of interference. A protocol for identification and testing of samples with suspected interference facilitated efficient detection of interference while reducing the amount of additional testing required.

CONCLUSIONS

Our data indicate that a single-method approach is insufficient to address all sources of interference in HLA SAB. A multimodal approach with a proactive screening is a more effective way to minimise risk of erroneous results.

Impact of COVID-19 pandemic on transplant laboratories: How to mitigate?

A positive flow cytometry crossmatch (FCXM) due to donor specific antibodies (DSA) constitutes a risk for kidney transplantation; such a finding may indicates an unacceptable donor for this patient. However, positive FCXM in the absence of DSA is considered discordant and need further investigations. During COVID-19 pandemic, we observed 22% discordant results out of 445 FCXM performed during eight months period in our laboratory and another 7% were invalid due to high background negative control (NC). No study has addressed the impact of COVID-19 pandemic on FCXM and the overall pre-kidney transplant workups or described a solution to deal with these non-specific reactivities. Herein, we analyzed all FCXM results in SARS-CoV-2 seropositive patients and addressed how this pandemic affected significantly the pre-kidney transplant workups, highlighting both technical and financial implications. We also shared our modified FCXM procedures using dithiotheritol (DTT) sera treatment or blocking donor cells with negative control human serum (NCS) which we found to be successful to abrogate 98% of all discordant FCXM results and to validate all invalid results due to high background NC. In conclusion, COVID-19 pandemic has affected our HLA laboratory significantly by creating many false positive or invalid crossmatch results. Transplant laboratories must consider this before test interpretations and immune risk assessments. We recommend the use of DTT serum treatment to remove nonspecific bindings in the sera of kidney transplant candidates and the use of NCS-blocked donor cells to correct high background when performing FCXM in transplant candidates or donors with recent history of SARS-CoV-2 immunization respectively.

Predictors of graft failure after first detection of de novo donor-specific HLA antibodies in kidney transplant recipients

BACKGROUND

De novo donor-specific antibodies (dnDSAs) may cause antibody-mediated rejection and graft dysfunction. Little is known about the clinical course after first detection of dnDSAs during screening in asymptomatic patients. We aimed to assess the value of estimated glomerular filtration rate (eGFR) and proteinuria to predict graft failure in patients with dnDSAs and their potential utility as surrogate endpoints.

METHODS

All 400 kidney transplant recipients with dnDSAs at our centre (1 March 2000-31 May 2021) were included in this retrospective study. The dates of graft loss, rejection, doubling of creatinine, ≥30% eGFR decline, proteinuria ≥500 mg/g and ≥1000 mg/g were registered from the first dnDSA appearance.

RESULTS

During 8.3 years of follow-up, graft failure occurred in 33.3% of patients. Baseline eGFR and proteinuria correlated with 5-year graft loss (area under the receiver operating characteristics curve 0.75 and 0.80, P < .001). Creatinine doubled after a median of 2.8 years [interquartile range (IQR) 1.5-5.0] from dnDSA and the time from doubling creatinine to graft failure was 1.0 year (IQR 0.4-2.9). Analysing eGFR reduction ≥30% as a surrogate endpoint (148/400), the time from dnDSA to this event was 2.0 years (IQR 0.6-4.2), with a positive predictive value (PPV) of 45.9% to predict graft loss, which occurred after 2.0 years (IQR 0.8-3.2). The median time from proteinuria ≥500 mg/g and ≥1000 mg/g to graft failure was identical, 1.8 years, with a PPV of 43.8% and 49.0%, respectively. Composite endpoints did not improve PPV. Multivariable analysis showed that rejection was the most important independent risk factor for all renal endpoints and graft loss.

CONCLUSIONS

Renal function, proteinuria and rejection are strongly associated with graft failure in patients with dnDSA and may serve as surrogate endpoints.

Complement-activating donor-specific anti-HLA antibodies in solid organ transplantation: systematic review, meta-analysis, and critical appraisal

Introduction

Several studies have investigated the impact of circulating complement-activating anti-human leukocyte antigen donor-specific antibodies (anti-HLA DSAs) on organ transplant outcomes. However, a critical appraisal of these studies and a demonstration of the prognostic value of complement-activating status over anti-HLA DSA mean fluorescence intensity (MFI) level are lacking.

Methods

We conducted a systematic review, meta-analysis and critical appraisal evaluating the role of complement-activating anti-HLA DSAs on allograft outcomes in different solid organ transplants. We included studies through Medline, Cochrane, Scopus, and Embase since inception of databases till May 05, 2023. We evaluated allograft loss as the primary outcome, and allograft rejection as the secondary outcome. We used the Newcastle-Ottawa Scale and funnel plots to assess risk of bias and used bias adjustment methods when appropriate. We performed multiple subgroup analyses to account for sources of heterogeneity and studied the added value of complement assays over anti-HLA DSA MFI level.

Results

In total, 52 studies were included in the final meta-analysis (11,035 patients). Complement-activating anti-HLA DSAs were associated with an increased risk of allograft loss (HR 2.77; 95% CI 2.33-3.29, p<0.001; I²=46.2%), and allograft rejection (HR 4.98; 95% CI 2.96-8.36, p<0.01; I²=70.9%). These results remained significant after adjustment for potential sources of bias and across multiple subgroup analyses. After adjusting on pan-IgG anti-HLA DSA defined by the MFI levels, complement-activating anti-HLA DSAs were significantly and independently associated with an increased risk of allograft loss.

Discussion

We demonstrated in this systematic review, meta-analysis and critical appraisal the significant deleterious impact and the independent prognostic value of circulating complement-activating anti-HLA DSAs on solid organ transplant risk of allograft loss and rejection.

Detection and management of HLA sensitization in candidates for adult heart transplantation

Heart transplantation (HT) remains the preferred therapy for patients with advanced heart failure. However, for sensitized HT candidates who have antibodies to human leukocyte antigens , finding a suitable donor can be challenging and can lead to adverse waitlist outcomes. In recent years, the number of sensitized patients awaiting HT has increased likely due to the use of durable and mechanical circulatory support as well as increasing number of candidates with underlying congenital heart disease. This State-of-the-Art review discusses the assessment of human leukocyte antigens antibodies, potential desensitization strategies including mechanisms of action and specific protocols, the approach to a potential donor including the use of complement-dependent cytotoxicity, flow cytometry, and virtual crossmatches, and peritransplant induction management.

The natural history of de novo donor-specific HLA antibodies after kidney transplantation

Background

De novo donor-specific HLA antibodies (dnDSA) are key factors in the diagnosis of antibody-mediated rejection (ABMR) and related to graft loss.

Methods

This retrospective study was designed to evaluate the natural course of dnDSA in graft function and kidney allograft survival and to assess the impact of mean fluorescence intensity (MFI) evolution as detected by annual Luminex^® screening. All 400 kidney transplant recipients with 731 dnDSA against the last graft (01/03/2000-31/05/2021) were included.

Results

During 8.3 years of follow-up, ABMR occurred in 24.8% and graft loss in 33.3% of the cases, especially in patients with class I and II dnDSA, and those with multiple dnDSA. We observed frequent changes in MFI with 5-year allograft survivals post-dnDSA of 74.0% in patients with MFI reduction ≥ 50%, 62.4% with fluctuating MFI (MFI reduction ≥ 50% and doubling), and 52.7% with doubling MFI (log-rank p < 0.001). Interestingly, dnDSA in 168 (24.3%) cases became negative at some point during follow-up, and 38/400 (9.5%) patients became stable negative, which was associated with better graft survival. Multivariable analysis revealed the importance of MFI evolution and rejection, while class and number of dnDSA were not contributors in this model.

Conclusion

In summary, we provide an in-depth analysis of the natural course of dnDSA after kidney transplantation, first evidence for the impact of MFI evolution on graft outcomes, and describe a relevant number of patients with a stable disappearance of dnDSA, related to better allograft survival.

European Guideline for the Management of Kidney Transplant Patients With HLA Antibodies: By the European Society for Organ Transplantation Working Group

This guideline, from a European Society of Organ Transplantation (ESOT) working group, concerns the management of kidney transplant patients with HLA antibodies. Sensitization should be defined using a virtual parameter such as calculated Reaction Frequency (cRF), which assesses HLA antibodies derived from the actual organ donor population. Highly sensitized patients should be prioritized in kidney allocation schemes and linking allocation schemes may increase opportunities. The use of the ENGAGE 5 ((Bestard et al., Transpl Int, 2021, 34: 1005–1018) system and online calculators for assessing risk is recommended. The Eurotransplant Acceptable Mismatch program should be extended. If strategies for finding a compatible kidney are very unlikely to yield a transplant, desensitization may be considered and should be performed with plasma exchange or immunoadsorption, supplemented with IViG and/or anti-CD20 antibody. Newer therapies, such as imlifidase, may offer alternatives. Few studies compare HLA incompatible transplantation with remaining on the waiting list, and comparisons of morbidity or quality of life do not exist. Kidney paired exchange programs (KEP) should be more widely used and should include unspecified and deceased donors, as well as compatible living donor pairs. The use of a KEP is preferred to desensitization, but highly sensitized patients should not be left on a KEP list indefinitely if the option of a direct incompatible transplant exists.

Optimization, validation and initial clinical implications of a Luminex-based immunoassay for the quantification of Fragile X Protein from dried blood spots

Fragile X Syndrome (FXS) is caused by a trinucleotide expansion leading to silencing of the FMR1 gene and lack of expression of Fragile X Protein (FXP, formerly known as Fragile X Mental Retardation Protein, FMRP). Phenotypic presentation of FXS is highly variable, and the lack of reproducible, sensitive assays to detect FXP makes evaluation of peripheral FXP as a source of clinical variability challenging. We optimized a Luminex-based assay to detect FXP in dried blot spots for increased reproducibility and sensitivity by improving reagent concentrations and buffer conditions. The optimized assay was used to quantify FXP in 187 individuals. We show that the optimized assay is highly reproducible and detects a wide range of FXP levels. Mosaic individuals had, on average, higher FXP levels than fully methylated individuals, and trace amounts of FXP were consistently detectable in a subset of individuals with full mutation FXS. IQ scores were positively correlated with FXP levels in males and females with full mutation FXS demonstrating the clinical utility of this method. Our data suggest trace amounts of FXP detectable in dried blood spots of individuals with FXS could be clinically relevant and may be used to stratify individuals with FXS for optimized treatment.

Estimating alloantibody levels in highly sensitized renal allograft candidates: Using serial dilutions to demonstrate a treatment effect in clinical trials

Small reductions in calculated panel-reactive antibody (cPRA) are associated with increased kidney transplantation in 100% cPRA patients. However, the high level of antibody in these patients is such that desensitization may reduce antibody but not cPRA, thus the cPRA change on undiluted serum with desensitization is an insensitive measure of effectiveness. We evaluated cPRA reduction, calculated per antibody titer, as a desensitization trial endpoint. To accomplish this, two serum samples from 20 kidney transplant candidates with cPRA ≥99.9% (100%) were obtained and serially diluted in triplicate to determine the titer of individual human leukocyte antigen (HLA) antibody specificities. CPRA was computed per dilution to identify the titer at which cPRA drops below 98%. Inter- and intra-assay variability and changes overtime were determined. The dilution needed to reach a cPRA <98% was within 1 titer for replicates from the same sample, with 90% (36/40) concordance. This indicates that only changes >2 titers can be deemed clinically meaningful. The median (IQR) titer difference was 0 (0-1) from baseline to follow-up within 12 months. The cPRA per titer also risk-stratified candidates for trial inclusion. In conclusion, determining the cPRA per titer is a reliable approach to simplify complex antibody data and an ideal endpoint for desensitization trials.

Parvovirus B19 in kidney transplantation: key points and essential pitfalls to know

Parvovirus B19 infection is s new viral threat in post-kidney transplantation. It is a viral infection often acquired from the donor, occurring in young transplant patients during the first post-kidney transplantation months, and with a non-specific clinico-biological picture. The hallmark symptom is regenerative anaemia which may be severe, requiring blood transfusion. The C3 and C4 complement fractions are reduced and constitute an early and inexpensive diagnostic marker. Diagnosis is often delayed due to the non-specific clinico-biological picture. However, severe anaemia and hypocomplementemia are early and suggestive signs of parvovirus B19 infection. Levels of parvovirus B19 DNA, as determined by real time-polymerase chain reaction (RT-PCR), are often very high and tend to decrease slowly over several months. Treatment is based on adaptive reduction of immunosuppression, adequate in the forms with few symptoms, discontinuation of antiproliferative agents, or a switch to other molecules associated with intravenous immunoglobulins in the severe and highly symptomatic forms. Screening for other concomitant viral infections, particularly for cytomegalovirus, Epstein Barr virus, and BK virus is systematic. Relapses are quite frequent during the first-year post-transplantation. Clinical-biological follow-up aims to detect any recurrence of the parvovirus B19 infection, the occurrence of parvovirus B19-related glomerulopathy, and acute rejection. Parvovirus B19 infection is a new viral threat in post-kidney transplantation and requires broader and/or randomised studies to better establish the diagnostic and therapeutic approach.

Detection of antibodies to denatured human leucocyte antigen molecules by single antigen Luminex

The anti-HLA antibody detection has been improved in sensitivity and specificity with solid-phase antigen bead (SAB) assays based on Luminex. However, false positive results due to denatured HLA (dHLA) may arise after single antigen test. The aim of this study was to compare the performance of the two Luminex technology-based anti-HLA detection kits available in the market in showing undesired anti-HLA antibody results. A prospective cohort was assessed for anti-HLA antibodies with single antigen A manufacturer (AM) kit and a comparison cohort with single antigen B manufacturer (BM) kit. A total of 11 out of 90 patients in a prospective cohort presented monospecific HLA-I antibodies with AM, and 5 out of 11 confirmed monospecific reaction with BM. Despite the confirmation of monospecific reaction with both manufacturers, 80% were assigned as dHLA reaction by specific crossmatch. Further comparative cohorts detected four out of six monospecific reactions with BM that were confirmed as possible dHLA reactions. A positive SAB test should rule out a reaction against a dHLA molecule, thus avoidance of prolonged waitlist periods or misattribution of anti-HLA reactions after transplantation.

Outcomes of cPRA 100% deceased donor kidney transplant recipients under the new Kidney Allocation System: A single-center cohort study

In light of changes in donor/recipient case-mix and increased cold ischemia times under the Kidney Allocation System (KAS), there is some concern that cPRA 100% recipients might be doing poorly under KAS. We used granular, single-center data on 109 cPRA 100% deceased donor kidney transplant (DDKT) recipients to study post-KAS posttransplant outcomes not readily available in national registry data. We found that 3-year patient (96.4%) and death-censored graft survival (96.8%) was excellent. We also found that cPRA 100% recipients had a relatively low incidence of T cell-mediated rejection (9.2%) and antibody-mediated rejection (AMR) (13.8%). T cell-mediated rejection episodes tended to be relatively mild-50% (5 episodes) were grade 1, 50% (5 episodes) were grade 2, and none were grade 3. Only 1 episode was associated with graft loss, but this was in the context of a mixed rejection. Although only 15 recipients (13.8%) developed an AMR episode, 2 of these were associated with a graft loss. Despite the rejection episodes, the vast majority of recipients had excellent graft function 3 years posttransplant (median serum creatinine 1.5 mg/dL). In conclusion, cPRA 100% DDKT recipients are doing well under KAS, although every effort should be made to prevent AMR to ensure long-term outcomes remain excellent.

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.

The effect of serum pretreatment regimens for the detection of HLA class I antibodies in platelet-refractory patients

BACKGROUND

Single antigen bead (SAB) assays are used to identify human leukocyte antigen (HLA) antibodies in patients with platelet refractoriness due to HLA Class I alloimmunization. Some laboratories use serum pretreatment regimens to eliminate interference from immunoglobulin M antibodies and complement. These modifications may contribute to interlaboratory variability, which is a recognized problem with the SAB assay.

STUDY DESIGN AND METHODS

Five patients' sera were overnight shipped to 12 laboratories in the United States and internationally. Recipients used their lab's SAB procedure to identify HLA Class I antibodies. The resultant mean fluorescence intensity (MFI) data were compared by instrumentation, bead lot, and pretreatment regimens. Laboratory-specific cutoffs for positive antibodies were applied to the results.

RESULTS

Interlaboratory variability for MFI values appears to be associated with different pretreatment regimens. The coefficient of variation (CV) of MFI from samples pretreated with ethylenediaminetetraacetic acid, dithiothreitol, or heat inactivation (EDHI) were similar, ranging from 14% to 56% (mean, 22%). For samples with no pretreatment, the CVs were significantly higher than EDHI-treated samples, ranging from 25% to 74% (mean, 39%; 95% confidence interval, 12.10-21.90; p < 0.0001). An intralaboratory comparison of pretreatment regimens confirmed these findings. Some positive antibody specificities present in EDHI-treated samples were negative in corresponding samples with no pretreatment when laboratory-specific cutoffs for positive antibodies were applied.

CONCLUSION

Our results show that greater interlaboratory precision can be achieved when samples are pretreated with EDHI as opposed to no pretreatment, likely because these pretreatments eliminate interference from inhibitors. Inhibitors may mask antibodies, leading to missed (or uncalled) specificities when no pretreatment is used.

Eplet mismatch analysis and allograft outcome across racially diverse groups in a pediatric transplant cohort: a single-center analysis

HLA eplet mismatch load has been suggested as an improvement to HLA antigen mismatch determination for organ selection. Given that eplet mismatches are determined based on amino acid sequence difference among HLA alleles, and that the frequency of HLA alleles varies between racial groups, we investigated the correlation between eplet mismatch load and allograft outcomes in 110 pediatric kidney transplant recipients who received their first organ from a donor of the same race (SRT) versus a donor of a different race (DRT). Adjusted modified Poisson regression was used to assess the interaction between eplet mismatch load and race mismatch and its effect on outcome. Caucasians and living donor recipients had lower eplet mismatched loads against their donors compared with non-Caucasian and deceased donor recipients. Overall, for the entire population, the risk of de novo HLA-DSA development was significantly increased with higher eplet loads (p < 0.001). Compared with the SRT group, the DRT group had higher eplet loads when compared with their donor, for HLA class I but not HLA class II molecules; however, there was no significant difference in the incidence of de novo HLA-DSA between the 2 groups. The risk of rejection increased significantly for DRT compared with SRT, only when class I eplet load was ≥ 70 (p = 0.04). Together this data show that eplet mismatch load analysis is an effective tool for alloimmune risk assessment. If considered for donor selection, acceptable eplet mismatch loads determined from studies in homogenous populations may restrict transplantation across racially diverse donor and patient groups with no evidence of poor outcome. Therefore, an acceptable eplet mismatch load threshold must consider the heterogeneity of the transplant population.

HLA-Mediated Platelet Refractoriness

OBJECTIVES

To provide an overview of the complexities associated with the human leukocyte antigen (HLA)-mediated platelet refractoriness. HLA antibody detection technologies and limitations associated with methodologies are discussed.

METHODS

A case scenario and review of relevant literature describing platelet refractoriness are presented, followed by a discussion of HLA antibody testing.

RESULTS

Following diagnosis of HLA-mediated refractoriness, a decision is made regarding the approach to obtain the appropriate platelets. The panel reactive antibodies (PRA) % of the patient, HLA typing, and limitations of the HLA testing should be taken into account when deciding which type of product would be the best option for a given patient.

CONCLUSIONS

Following confirmation and review of HLA antibody testing, platelets are ordered based upon the PRA% and approach employed, HLA-matched platelets, antigen restricted platelets, or cross-matched platelets. The platelets are transfused and a posttransfusion increment count is monitored to determine transfusion success.

Prognostic tools to assess candidacy for and efficacy of antibody-removal therapy

Currently, the ability to predict or monitor the efficacy of HLA antibody-removal therapies is deficient. We previously reported that titration studies are a consistent and accurate means of assessing antibody strength. To test whether titration studies can also predict which patients are better candidates for desensitization, we studied 38 patients from 3 centers (29 receiving plasmapheresis/low-dose intravenous immunoglobulin [IVIg]; 9 patients receiving high-dose IVIg). For patients undergoing plasmapheresis/low-dose IVIg, antibody titer reduction correlated with number of treatment cycles for both class I and II antibodies but only up to approximately 4 cycles. Reduction in titer slowed with additional cycles, suggesting a limit to the efficacy of this approach. Furthermore, initial titer (predesensitization) can guide the selection of candidates for successful antibody-removal treatment. In our experience, patients with antibodies at an initial titer >1:512 could not be reduced to the goal of a negative lymphocyte crossmatch, corresponding to a 1:16 titer, despite a significant increase in the number of treatment cycles. Change in mean fluorescence intensity (MFI) value did not correlate with success of treatment if initial MFI values were >10 000, likely due to single antigen bead saturation. Overall, we present a potential prognostic tool to predict candidacy and a monitoring tool to assess efficacy of desensitization treatment.

Virtual Crossmatch: Reality of Perception

Results of 773 actual flow crossmatches (aFXMs) and virtual flow crossmatches (vFXMs) performed for living and deceased donor kidney transplantation in our center were analyzed retrospectively and evaluated for their concordance. Prediction of vFXMs was based on antibody identification using single antigen bead assay and locally established mean fluorescence intensity cutoff point compared with donor HLA antigens. The vast majority of aFXMs were in concordance with vFXMs with an overall concordance of 97%. Twenty-three predicted to be negative showed positive aFXMs; 12 of them had 0% calculated panel-reactive antibody, and 11 were found in patients with multiple non-donor-specific HLA antibodies. Three predicted positive vFXMs yielded negative aFXMs; 2 of them had allele-specific antibodies. CONCLUSIONS: vFXMs based on precise characterization of antibody specificities detected by single antigen bead assay using our cutoff point accurately predicted FXMs in the majority of patients and can be used safely to allocate kidney offers without performing physical crossmatches in selected patients.

Discrepant Antibody Testing Results: Which One to Believe?

The impact of solid-phase immunoassay for HLA antibody detection on the field of transplantation has been extremely significant by providing the most sensitive and precise method for characterization of HLA antibodies. However, despite all the benefits, technical limitations and inherent artifacts represent significant challenges, particularly with Luminex-based single-antigen bead (SAB) assay. Discordant results between antibody detection (screening assay) and identification (SAB) is not uncommon. Positive SAB assay in the context of negative screening testing is well documented and attributed to altered tertiary structure of HLA molecules exposing new epitopes or detection of naturally occurring antibodies. However, there are few reports that addressed the opposite scenario when negative SAB appeared in the context of positive screening assay. In such discrepant results, unmissed HLA antibody has to be excluded with certainty by other tests; however, with the availability of variable assays it may be difficult to choose the best combinations that clarify discrepancy without adding more confusion. Here we describe the results of correlation between 2 antibody screening solid-phase immunoassays (LABScreen Mixed using Luminex and FlowPRA Screen) on conventional flow cytometry and compare their outcomes with SAB and crossmatch results.

Microvascular inflammation in renal allograft biopsies assessed by endothelial and leukocyte co-immunostain: a retrospective study on reproducibility and clinical/prognostic correlates

The most prominent histologic lesion in antibody-mediated rejection is microvascular inflammation (MVI); however, its recognition and scoring can be challenging and poorly reproducible between pathologists. We developed a dual immunohistochemical (IHC)-stain (anti-CD34/anti-CD45 for endothelium/leukocytes) as ancillary tool to improve on the semi-quantitative Banff scores and allow quantification of MVI. We examined the relationship between CD34-CD45 IHC-based quantitative MVI score (the inflamed peritubular capillary ratio, iptcr) and renal-graft failure or donor-specific antibodies (DSA) strength at the time of biopsy. Quantitative iptcr score was significantly associated with renal graft failure (hazard ratio 1.81, per 1 SD-unit [0.13 points] of iptcr-increase; P = 0.026) and predicted the presence and strength of DSA (ordinal odds ratio: 2.42; P = 0.005; 75 biopsies/60 kidney transplant recipients; 30 HLA- and/or ABO-incompatible). Next, we assessed inter-pathologist agreement for ptc score and ptc extent (focal/diffuse) using CD34-CD45 IHC as compared to conventional stain. Compared to conventional stain, CD34-CD45 IHC significantly increased inter-pathologist agreement on ptc score severity and extent (κ-coefficient from 0.52-0.80 and 0.46-0.68, respectively, P < 0.001). Our findings show that CD34-CD45 IHC improves reproducibility of MVI scoring and facilitates MVI quantification and introduction of a dual anti-CD34/CD45 has the potential to improve recognition of MVI ahead of DSA results.

Maintaining the Health of the Renal Allograft: Laboratory and Histologic Monitoring After Kidney Transplantation

Advances in posttransplant care, including new immunosuppressive medications have led to excellent short-term renal allograft survival. However, there is a small therapeutic window within which the patient and the clinician must balance the risk of rejection, with side effects such as infection, malignancy, and toxicity. Laboratory testing plays a key role in this ongoing monitoring, which includes relatively simple tests, such as serum creatinine, to complex tests, such as solid-phase assays, used to monitor for donor-specific antibody and surveillance allograft biopsies. This article reviews the role of the laboratory tests and surveillance biopsies in posttransplant monitoring.

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.

Pre-transplant Screening for Non-HLA Antibodies: Who should be Tested?

Retrospective studies of angiotensin II type 1 receptor antibodies (AT1R-Ab) and anti-endothelial cell antibodies (AECA) have linked these antibodies to allograft injury. Because rising healthcare costs dictate judicious use of laboratory testing, we sought to define characteristics of kidney transplant recipients who may benefit from screening for non-HLA antibodies. Kidney recipients transplanted between 2011 and 2016 at Johns Hopkins, were evaluated for AT1R-Ab and AECA. Pre-transplant antibody levels were compared to clinical and biopsy indications of graft dysfunction. Biopsies were graded using the Banff' 2009-2013 criteria. AT1R-Ab and AECA were detected using ELISA and endothelial cell crossmatches, respectively. AT1R-Ab levels were higher in patients who were positive for AECAs. Re-transplanted patients (p < 0.0001), males (p = 0.008) and those with FSGS (p = 0.04) and younger (p = 0.04) at time of transplantation were more likely to be positive for AT1R-Ab prior to transplantation. Recipients who were positive for AT1R-Ab prior to transplantation had increases in serum creatinine within 3 months post-transplantation (p < 0.0001) and developed abnormal biopsies earlier than did AT1R-Ab negative patients (126 days versus 368 days respectively; p = 0.02). Defining a clinical protocol to identify and preemptively treat patients at risk for acute rejection with detectable non-HLA antibodies is an important objective for the transplant community.

Photocleavage-based affinity purification of biomarkers from serum: Application to multiplex allergy testing

Multiplex serological immunoassays, such as implemented on microarray or microsphere-based platforms, provide greater information content and higher throughput, while lowering the cost and blood volume required. These features are particularly attractive in pediatric food allergy testing to facilitate high throughput multi-allergen analysis from finger- or heel-stick collected blood. However, the miniaturization and microfluidics necessary for creating multiplex assays make them highly susceptible to the “matrix effect” caused by interference from non-target agents in serum and other biofluids. Such interference can result in lower sensitivity, specificity, reproducibility and quantitative accuracy. These problems have in large part prevented wide-spread implementation of multiplex immunoassays in clinical laboratories. We report the development of a novel method to eliminate the matrix effect by utilizing photocleavable capture antibodies to purify and concentrate blood-based biomarkers (a process termed PC-PURE) prior to detection in a multiplex immunoassay. To evaluate this approach, it was applied to blood-based allergy testing. Patient total IgE was purified and enriched using PC-PURE followed by multiplex microsphere-based detection of allergen-specific IgEs (termed the AllerBead assay). AllerBead was formatted to detect the eight most common pediatric food allergens: milk, soy, wheat, egg, peanuts, tree nuts, fin fish and shellfish, which account for >90% of all pediatric food allergies. 205 serum samples obtained from Boston Children’s Hospital were evaluated. When PC-PURE was employed with AllerBead, excellent agreement was obtained with the standard, non-multiplex, ImmunoCAP^® assay (average sensitivity above published negative predictive cutoffs = 96% and average Pearson r = 0.90; average specificity = 97%). In contrast, poor ImmunoCAP^®-correlation was observed when PC-PURE was not utilized (average sensitivity above published negative predictive cutoffs = 59% and average Pearson r = 0.61; average specificity = 97%). This approach should be adaptable to improve a wide range of multiplex immunoassays such as in cancer, infectious disease and autoimmune disease.

Clinically relevant interpretation of solid phase assays for HLA antibody

Purpose of review

Accurate and timely detection and characterization of human leukocyte antigen (HLA) antibodies are critical for pre-transplant and post-transplant immunological risk assessment. Solid phase immunoassays have provided increased sensitivity and specificity, but test interpretation is not always straightforward. This review will discuss the result interpretation considering technical limitations; assessment of relative antibody strength; and the integration of data for risk stratification from complementary testing and the patient's immunological history.

Recent findings

Laboratory and clinical studies have provided insight into causes of test failures – false positive reactions because of antibodies to denatured HLA antigens and false negative reactions resulting from test interference and/or loss of native epitopes. Test modifications permit detection of complement-binding antibodies and determination of the IgG subclasses. The high degree of specificity of single antigen solid phase immunoassays has revealed the complexity and clinical relevance of antibodies to HLA-C, HLA-DQ, and HLA-DP antigens. Determination of antibody specificity for HLA epitopes enables identification of incompatible antigens not included in test kits.

Summary

Detection and characterization of HLA antibodies with solid phase immunoassays has led to increased understanding of the role of those antibodies in graft rejection, improved treatment of antibody-mediated rejection, and increased opportunities for transplantation. However, realization of these benefits requires careful and accurate interpretation of test results.

Comparative Assessment of Anti-HLA Antibodies Using Two Commercially Available Luminex-Based Assays

BACKGROUND

Allospecific anti-HLA antibodies (Abs) are associated with rejection of solid organ grafts. The 2 main kits to detect anti-HLA Ab in patient serum are commercialized by Immucor and One Lambda/ThermoFisher. We sought to compare the performance of both platforms.

METHODS

Background-adjusted mean fluorescence intensity (MFI) values were used from both platforms to compare sera collected from 125 pretransplant and posttransplant heart and lung transplant recipients.

RESULTS

Most HLA class I (94.5%) and HLA class II (89%) Abs with moderate to high MFI titer (≥4000) were detected by both assays. A modest correlation was observed between MFI values obtained from the 2 assays for both class I (r = 0.3, r2 = 0.09, P < 0.0001) and class II Ab (r = 0.707, r2 = 0.5, P < 0.0001). Both assays detected anti-class I and II Ab that the other did not; however, no specific HLA allele was detected preferentially by either of the 2 assays. For a limited number of discrepant sera, dilution resulted in comparable reactivity profiles between the 2 platforms.

CONCLUSIONS

Immucor and One Lambda/ThermoFisher assays have a similar, albeit nonidentical, ability to detect anti-HLA Ab. Although the correlation between the assays was present, significant variances exist, some of which can be explained by a dilution-sensitive "prozone" effect.

Technical Limitations of the C1q Single-Antigen Bead Assay to Detect Complement Binding HLA-Specific Antibodies

BACKGROUND

Solid-phase assays to distinguish complement binding from noncomplement binding HLA-specific antibodies have been introduced, but technical limitations may compromise their interpretation. We have examined the extent to which C1q-binding to HLA-class I single-antigen beads (SAB) is influenced by denatured HLA on SAB, antibody titre, and complement interference that causes a misleading low assessment of HLA-specific antibody levels.

METHODS

Sera from 25 highly sensitized patients were tested using Luminex IgG-SAB and C1q-SAB assays. Sera were tested undiluted, at 1:20 dilution to detect high-level IgG, and after ethylene diamine tetraacetic acid treatment to obviate complement interference. Conformational HLA and denatured HLA protein levels on SAB were determined using W6/32 and HC-10 monoclonal antibodies, respectively. Denatured HLA was expressed as HC-10 binding to untreated SAB as a percentage of maximal binding to acid-treated SAB.

RESULTS

For undiluted sera, Luminex mean fluorescence intensity (MFI) values for IgG-SAB and C1q-SAB correlated poorly (r = 0.42). ethylene diamine tetraacetic acid and serum dilution improved the correlation (r = 0.57 and 0.77, respectively). Increasing levels of denatured HLA interfered with the detection of C1q binding. Consequently, the correlation between IgG-SAB MFI and C1q-SAB MFI was lowest using undiluted sera and SAB with greater than 30% denatured HLA (r = 0.40) and highest using diluted sera and SAB with 30% or less denatured HLA (r = 0.86).

CONCLUSIONS

Antibody level, complement interference, and denatured HLA class I on SAB may all affect the clinical interpretation of the C1q-SAB assay. The C1q-SAB assay represents a substantial additional cost for routine clinical use, and we question its justification given the potential uncertainty about its interpretation.

Histocompatibility and management of the highly sensitized kidney transplant candidate

PURPOSE OF REVIEW

Increasing national participation in kidney paired donation and implementation of new sharing policies via the current kidney allocation system have brought about greater opportunities for the most highly sensitized patients awaiting a kidney transplant. The purpose of this review is to discuss the application of histocompatibility data in the context of the clinical practice of kidney transplantation as pertains to the sensitized candidate.

RECENT FINDINGS

With desensitization techniques, transplantation across virtually any antibody barrier is technically feasible, but long-term outcomes after transplantation are improved when the immunologic match between donor and recipient is optimized. Solid-phase immunoassays have changed the landscape of histocompatibility testing. These sensitive and specific assays for identifying donor-specific antibody not only help determine feasibility of transplantation but have enabled outcomes studies aimed at understanding the spectrum of risk posed by different antibody profiles. This, in turn, has helped guide decision-making in donor selection, in particular for sensitized patients.

SUMMARY

Careful evaluation of donor-specific antibody profiles with individualized, patient-specific determination of unacceptable antigens is necessary to ensure that highly sensitized patients receive every opportunity for transplantation.

Quantitative Evaluation of the Impact of Ethylenediaminetetraacetic Acid Pretreatment on Single-Antigen Bead Assay

Background

Ethylenediaminetetraacetic acid (EDTA) pretreatment has been shown to overcome complement interference in the single-antigen bead (SAB) assay. However, a quantitative evaluation of its impact on the assay for preemptive application to diverse clinical samples is still lacking.

Methods

Serum samples from 95 renal transplant candidates were tested with and without EDTA-pretreatment in parallel. Changes in mean fluorescence intensity (MFI) values were analyzed to determine the impact of EDTA-pretreatment and the characteristics of complement interference.

Results

MFI values from EDTA-treated and untreated sera showed good correlations (r = 0.99) and were linear after excluding outliers (slopes, 1; intercepts, −63.7 and −24.2 for class I and II, respectively). Using an assay cutoff of 2000 MFI, positive/negative assignments were concordant for 99% of the 9215 class I beads and 9025 class II beads tested. As defined by an MFI increment above 4000 after EDTA pretreatment, complement interference affected 172 class I beads in 12 samples (12.6%) and 60 class II beads in 7 samples (7.4%), and the findings were supported in 83% and 86% of these samples by dilution studies. In a case study, EDTA pretreatment prevented falsely low MFI values and facilitated the interpretation of titration curves. Finally, EDTA pretreatment reduced the coefficient of variance (CV) by 2.1% and 2.4% for class I and II beads respectively (P < 0.0001).

Conclusions

It is safe to preemptively treat all clinical samples with EDTA before SAB assay to prevent false negative results or falsely low MFI values. EDTA pretreatment has the added benefit of improved assay precision.

Reason and Resolution of High Negative Control Beads in Solid-Phase Immunoassay

OBJECTIVES

The Luminex technology is the most sensitive diagnostic method for HLA antibody detection and identification. However, the interpretation of immunoassays is commonly affected by the artifact, and non-specific background. Sera from some patients show high negative control bead (NC) value, which makes assessing and interpretation of HLA antibodies difficult. In this study, we evaluated the effect of Adsorb Out reagent, dithiothreitol (DTT), and Ethylenediaminetetraacetic acid (EDTA) on the NC median fluorescence intensity value by comparing treated versus untreated patient sera. In addition, we wanted to identify whether kidney disease and administered medication influenced high NC median fluorescence intensity values by comparing patient versus control results.

MATERIALS AND METHODS

HLA antibody screening was performed on 3500 serum samples. Sera were analyzed using the standard protocol for Luminex antibody screening. Sera with high NC values were preincubated with Adsorb Out, DTT, and EDTA. Screening of these sera was then performed.

RESULTS

We found that 4% of samples showed high NC values. Adsorb Out, DTT, and EDTA decreased the NC values at 723.5 (299.25-1443) versus 85 (34-218; P < .001), at 723.5 (299.25-1443) versus 184 (106-597; P < .001), and at 723.5 (299.25-1443) versus 455 (131-1177; P = .004). These succeeded in bringing back NC values to normal range in 69.2%, 43%, and 30% of treated sera, respectively. In addition, the differences of corticoids, immunosuppressive, and heparin drugs between patients and controls were statistically significant (P < .001, < .001, and = .043). However, presence of kidney disease was not significant between these groups.

CONCLUSIONS

All pretreatments had an important effect in decreasing negative control values, with Adsorb Out having highest efficiency. Serum-specific components could contribute to high negative control bead median fluorescence intensity values. Further studies are needed to determine the adequate pretreatment of patient sera.

Luminex Solid-Phase Crossmatch for De Novo Donor-Specific Antibodies in Living-Donor Related Transplants

OBJECTIVES

There are no reports of de novo donor-specific antibody monitoring by a low-cost solid-phase crossmatch assay using donor lysate after renal transplant.

MATERIALS AND METHODS

We prospectively evaluated 121 complement-dependant cytotoxicity crossmatch-negative living-donor kidney transplant recipients for development of de novo donor-specific antibodies (class I and II HLA) by solid-phase crossmatch Luminex assay after transplant.

RESULTS

Of 121 recipients in our study group, 26 (21.5%) developed de novo donor-specific antibody within 3 months after transplant. Fifteen (58%) of these 26 recipients developed class II de novo donor-specific antibody, 8 patients (30%) developed class I, and 3 (12%) developed both class I and class II. Of the remaining 95 patients (79%) who did not develop de novo donor-specific antibody, 6 (33.3%) had antibody-mediated rejection with glomerulitis (2 with C4d-positive disease). Donor-specific antibody was detected by Luminex solid-phase crossmatch in 18 patients (5 with class I, 11 with class II, and 2 with both class I and II), all with no evidence of clinical rejection. Development of de novo donor-specific antibody detected by solid-phase crossmatch was associated with more acute rejection (31% in de novo donor-specific antibody-positive group versus 19% in the negative group). The positive group had more antibody-mediated rejection (75% of acute rejections), whereas only 33.3% of acute rejections in the negative group were antibody-mediated rejection. Of 12 patients with antibody-mediated rejection, 9 were C4d negative (75%) and were diagnosed by donor-specific antibody positivity detected by solid-phase cros?match testing and histologic findings. The use of donor lysate in solid-phase crossmatch assays is more economical than the single-antigen bead Luminex assay (per test cost of US $45.20 vs $403.20).

The Humoral Theory of Transplantation: Epitope Analysis and the Pathogenicity of HLA Antibodies

Central to the humoral theory of transplantation is production of antibodies by the recipient against mismatched HLA antigens in the donor organ. Not all mismatches result in antibody production, however, and not all antibodies are pathogenic. Serologic HLA matching has been the standard for solid organ allocation algorithms in current use. Antibodies do not recognize whole HLA molecules but rather polymorphic residues on the surface, called epitopes, which may be shared by multiple serologic HLA antigens. Data are accumulating that epitope analysis may be a better way to determine organ compatibility as well as the potential immunogenicity of given HLA mismatches. Determination of the pathogenicity of alloantibodies is evolving. Potential features include antibody strength (as assessed by antibody titer or, more commonly and inappropriately, mean fluorescence intensity) and ability to fix complement (in vitro by C1q or C3d assay or by IgG subclass analysis). Technical issues with the use of solid phase assays are also of prime importance, such as denaturation of HLA antigens and manufacturing and laboratory variability. Questions and controversies remain, and here we review new relevant data.