It’s about time: The development and validation of a rapid optimized single antigen bead (ROB) assay protocol for LABScreen

Predicting flow cytometry crossmatch results from single-antigen bead testing

The aim of this study was to devise an algorithm that would predict flow cytometry crossmatch (FCXM) results using single-antigen bead (SAB) mean fluorescent intensity (MFI) levels using samples received through the National External Quality Assurance Scheme (NEQAS) 2B external proficiency testing scheme between 2019 and 2023. A total of 159 serum samples were retrospectively screened using LABScreen Single Antigen Class I and II (SAB), and 40 peripheral blood samples were human leucocyte antigen (HLA) typed with LABType SSO. Donor-specific antibodies were identified for each cell-serum combination tested, and cumulative MFI values were calculated for each test before correlating the screening result with the consensus crossmatch results for this scheme. HLA Class I MFIs were combined to predict the T cell crossmatch. For the B cell crossmatch prediction, two options were considered: (i) HLA Class II MFI values alone and (ii) HLA Class I + Class II MFIs. Receiver operating characteristic analysis was carried out to identify the combined MFI threshold that predicted NEQAS consensus results with the greatest sensitivity and specificity. HLA Class I combined MFI >5000 predicted T cell crossmatch results with 96% sensitivity, 100% specificity, 100% positive predictive value (PPV) and 92% negative predictive value (NPV). For B cell results, HLA Class I + Class II combined MFIs >11,000 gave the best model, showing 97% sensitivity, 82% specificity, 96% PPV and 85% NPV. However, for samples with only HLA Class II sensitization, combined MFIs >13,000 improved the B cell crossmatch predictions: 92% sensitivity, 95% specificity, 96% PPV and 91% NPV. Using this model, combined MFI can be used to predict the immunological risk posed by donor-specific antibodies when it is not possible to carry out an FCXM.

Complement-dependent cytotoxicity crossmatch in solid organ transplantation: The gold standard or golden history?

Complement-dependent cytotoxicity crossmatch (CDC-XM) has been considered for many years the standard of practice for determining compatibility in solid organ transplantation (SOT). However, as this method is laborious, time intensive and lacks sensitivity and specificity, it has been replaced in many laboratories worldwide by flow cytometry crossmatch (FCXM) and/or virtual crossmatch (vXM). With this study we intend to show the relevance of performing CDC-XM in the era of virtual crossmatching. We retrospectively analyzed 1,007 consecutive T and B cell deceased donor (DD) CDC-XMs performed in parallel using non-treated and dithiothreitol (DTT) treated sera between May 2022 and January 2023 in waitlisted patients with no donor specific antibodies (DSA) against HLA-A, B and/or DR antigens. Thirty five of 1,007 (3.5%) T cell crossmatches and 132 of 1,007 (13.1%) B cell crossmatches were positive with non-treated sera. Correlation with the vXM demonstrated no DSA in any of the positive T cell crossmatches. DSA were also absent in 126/132 positive B cell crossmatches, indicating a high rate of false positive CDC-XM. Indeed, only 4/35 T cell and 13/132 B cell CDC-XM remained positive after treatment with DTT, confirming that false positive reactivity with non-treated sera is high. Class I HLA DSA against C locus antigens were present in 17/1,007 T cell crossmatches and none were detected by CDC-XM (sensitivity = 0%). Similarly, only 6/77 B cell crossmatches with DSA targeting HLA-C, DQ and/or DP antigens were CDC-XM positive (sensitivity = 7.8%). Furthermore, only 4/6 positive B cell CDC-XM were confirmed to have complement binding potential using the C1q assay, suggesting additional false positive reactivity in 2/6 of the positive CDC-XM. Our study demonstrates that CDC-XM exhibits poor sensitivity, high false positive reactivity (especially without DTT treatment) and does not meaningfully contribute to pre-transplant compatibility testing in the context of vXM based allocation. Furthermore, the use of CDC-XM can unnecessarily delay or even prevent safe and appropriate transplant allocation.

Cutting through the weeds: Evaluation of a novel adsorption with crossmatch cells and elution protocol to sharpen HLA antibody identification by the single antigen bead assay

The single antigen bead (SAB) assay is the most used test for the identification of HLA specific antibodies pre- and post-transplant. Nevertheless, detection of spurious reactivities remains a recognized assay limitation. In addition, the presence of weak reactivity patterns can complicate unacceptable antigen assignment. This work presents the evaluation of the adsorption with crossmatch cells and elution (AXE) technique, which was designed to help differentiate weak HLA specific antibodies targeting native antigens from spurious and background SAB assay reactivity. The AXE protocol uses selected donor cells to adsorb HLA specific antibodies from sera of interest. Bound antibodies are then eluted off washed cells and identified using the SAB assay. Only antibodies targeting native HLA are adsorbed. Assay evaluation was performed using five cell donors and pooled positive control serum. AXE efficiency was determined by comparing SAB reactivity of adsorbed/eluted antibody to that of the antibodies in unadsorbed sera. A robust efficiency was seen across a wide range of original MFI for donor specific antibodies (DSA). A higher absorption/elution recovery was observed for HLA class I antigens vs. class II. Locus-specific variation was also observed, with high-expression HLA loci (HLA-A/B/DR) providing the best recovery. Importantly, negligible reactivity was detected in the last wash control, confirming that AXE eluates were not contaminated with HLA antibody carry-over. Donor cells incubated with autologous and DSA-containing allogeneic sera showed that AXE selectively adsorbed HLA antibodies in a donor antigen-specific manner. Importantly, antibodies targeting denatured epitopes or other non-HLA antigens were not detected by AXE. AXE was particularly effective at distinguishing weak HLA antibodies from background reactivity. When combined with epitope analysis, AXE enhanced precise identification of antibody-targeted eplets and even facilitated the characterization of a potential novel eplet. Comparison of AXE to flow cytometric crossmatching further revealed that AXE was a more sensitive technique in the detection of weak DSA. Spurious reactivities on the current SAB assay have a deleterious impact on the assignment of clinically relevant HLA specificities. The AXE protocol is a novel test that enables users to interrogate reactive patterns of interest and discriminate HLA specific antibodies from spurious reactivity.

An early evaluation of the HISTO SPOT® AB ID Class I & II test in cardiothoracic transplant patients

The HISTO SPOT^® AB ID assay (BAG Diagnostics GmbH) is a novel single antigen HLA Class I & II antibody definition test used with the MR.SPOT^® processor. We compared this assay with Luminex^® -based assays to assess its potential application in defining unacceptable antigens for transplantation in patients awaiting transplants with cardiothoracic organs. A cohort of 40 sensitized cardiothoracic patients were identified, and one sample was selected from each patient. The required screening was based on the patients' antibody profiles (Class I, n = 17, Class II, n = 11, Class I & II, n = 12). Samples were screened with LABScreen™ Single Antigen (SAg), LIFECODES^® LSA™, HISTO SPOT^® AB ID, and an acid modified LABScreen™ SAg test for detecting antibodies against denatured HLA. Results indicated that HISTO SPOT^® AB ID had reduced sensitivity (68% for Class I; 69% for Class II). When compared to LABScreen™ and LIFECODES^® , HISTO SPOT^® AB ID failed to detect Luminex^® -defined antibodies with median fluorescence intensity (MFI) ranging from 1114 to 24,489. The HISTO SPOT^® AB ID panel used in the study had reduced antigen representation compared with Luminex^® -based assays which further compromised its capacity for antibody detection and definition. Further work is needed to evaluate the clinical relevance of these differences between the performance of HISTO SPOT^® and Luminex^® -based methods.

Deciphering the role of the conjugate's phycoerythrin label in complement-mediated interference occurring in HLA single antigen Luminex bead assays

Complement-mediated interference is a well described phenomenon in single antigen bead (SAB) Luminex assay that leads to falsely low or negative results for anti-HLA antibody (Ab). In a context of high amount of Ab, the enrichment of the Ab around the bead can lead to complement cascade activation and deposition, thereafter impairing Ab detection. EDTA is now routinely used to circumvent this interference. In this report, we attempted to decipher the role of the phycoerythrin (PE) label conjugated to the secondary Ab in this interference. Indeed, PE is a huge molecule (240 kDa) that could participate to limiting access of the conjugate to its Ab target on the bead. To this purpose, 22 sera displaying complement interference without pre-treatment with EDTA were compared on SAB assay with three detection strategies: the recommended PE-conjugated secondary Ab (IgGPE), an Alexa Fluor 532-conjugated Ab (IgGAF) bearing a tiny 724 Da fluorochrome, and a biotinylated Ab followed by PE-conjugated streptavidin (IgGBiot). Complement interference occurred with the three detection methods, but its depth, defined by the percentage of MFI loss with neat serum, was the highest for IgGPE. Our study highlighted the partial role of the PE fluorochrome in complement interference in SAB assays.

Human leukocyte antigen epitope mismatch loads and the development of de novo donor-specific antibodies in cardiothoracic organ transplantation

De novo donor-specific human leucocyte antigen (HLA) antibodies (dnDSA) are associated with increased risk of rejection and mortality in solid organ transplantation. Such dnDSA is produced in some recipients upon allorecognition of mismatched HLA post-transplant. HLA matching is not currently considered in the allocation of deceased donor hearts and lungs and pre-transplant immunological risk stratification is based entirely on the mean fluorescence intensity (MFI) of circulating donor-directed HLA antibodies. HLA epitope-based matching tools predict B-cell or T-cell HLA epitopes that are present in the donor's HLA but absent in the recipient's HLA. We hypothesized that patients with higher epitope mismatch loads would be at increased risk of dnDSA development. We retrospectively analysed 73 heart and/or lung transplant recipients who were tested for DSA between 2015 and 2020. HLAMatchmaker, PIRCHE-II and HLA epitope mismatch algorithm (HLA-EMMA) were used to calculate eplet mismatch (EpMM) loads, T-cell epitope mismatch (TEpMM) loads and solvent accessible amino acid mismatch (SAMM) loads, respectively. Multivariate analyses showed that HLA-EMMA was the only tool with a significant association between the total score for all HLA loci and dnDSA production [odds ratio (OR) 1.021, 95% confidence interval (CI) 1.003-1.042, p = .0225] though this increased risk was marginal. The majority of dnDSA were directed against HLA-DQ and patients with higher HLA-DQ TEpMM loads (OR = 1.008, CI = 1.002-1.014, p = .007), and HLA-DR+DQ SAMM loads (OR = 1.035, CI = 1.010-1.064, p = .0077) were most at risk of producing dnDSA. We also showed that patients with a risk epitope within the HLA molecule encoded for by HLA-DQA1*05 + HLA-DQB1*02/03:01 were significantly more likely to produce dnDSA. The use of HLA epitope-based matching tools could be used for cardiothoracic transplant risk stratification to enable early intervention and monitoring of patients at increased risk of producing dnDSA.

Serum dilutions as a predictive biomarker for peri-operative desensitization: An exploratory approach to transplanting sensitized heart candidates

Antibody-mediated rejection (AMR) of cardiac allografts mediated by anti-HLA Donor Specific Antibodies (DSA) is one of the major barriers to successful transplantation for the treatment of end-stage heart failure. Therapeutic plasma exchange (TPE) is a first-line treatment for pre-transplant desensitization. However, indications for treatment regimens and treatment end-points have not been well established. In this study, we investigated how sera dilutions could guide TPE regimens for effective peri-operative desensitization and early AMR treatment. Our data show that 1:16 dilutions of EDTA-treated sera and 1.5 volume TPE reduce anti-HLA class I and class II antibody levels in the same manner and, therefore, allows to predict which antibodies would respond to peri-operative TPE. We successfully applied this approach to transplanting three highly sensitized cardiac recipients (CPRA 85-93%) with peri-operative desensitization based on a virtual crossmatch performed on 1:16 diluted serum. Furthermore, we have used sera dilutions to guide DSA treatment post-transplant. Although these findings have to be confirmed in a larger prospective study, our data suggest that serum dilutions can serve as a predictive biomarker to guide peri-operative desensitization and post-transplant immunologic management.

Patients with immunological diseases or on peritoneal dialysis are prone to false positive flow cytometry crossmatch

Despite implementation of virtual crossmatches, flow cytometry crossmatches (FCXM) are still used by many transplant centers to determine immunological risk before kidney transplantation. To determine if common profiles of patients prone to false positive FCXM exist, we examined the demographics and native diseases of kidney patients tested with autologous FCXM (n = 480). Improvements to FCXM and cell isolation methods significantly reduced the positive rate from 15.1% to 5.3%. Patients with native diseases considered 'immunological' (vasculitis, lupus, IgA nephropathy) had more positive autologous FCXM (OR = 3.36, p = 0.003) vs. patients with all other diseases. Patients who were tested using our updated method (n = 321) still showed that these immunological diseases were a significant predictor for positive autologous FCXM (OR = 4.79, p = 0.006). Interestingly, patients on peritoneal dialysis (PD) also had significantly more positive autologous FCXM than patients on hemodialysis or waiting for pre-emptive kidney transplants (OR = 3.27, p = 0.02). These findings were confirmed in patients who had false positive allogeneic FCXM. Twenty of 24 (83.3%) patients with false positive allogeneic FCXM tested with updated method either had immunological diseases originally or were on PD. Our findings are helpful when interpreting an unexpected positive FCXM, especially for transplantation from deceased donors.