Single Antigen Bead Testing Research Articles

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 and Non-Complement Binding Anti-HLA Antibodies Are Differentially Detected with Different Antigen Bead Assays in Renal Transplant Recipients.

Two semi-quantitative, Luminex-based, single-antigen bead (SAB) assays are available to detect anti-HLA antibodies and evaluate their reactivity with complement binding. Sera from 97 patients with positive panel reactive antibody tests (>5%) were analyzed with two SAB tests, Immucor (IC) and One-Lambda (OL), for anti-HLA antibody detection and the evaluation of their complement-binding capacity. IC detected 1608/8148 (mean fluorescent intensity (MFI) 4195 (1995-11,272)) and 1136/7275 (MFI 6706 (2647-13,184)) positive anti-HLA class I and II specificities, respectively. Accordingly, OL detected 1942/8148 (MFI 6185 (2855-12,099)) and 1247/7275 (MFI 9498 (3630-17,702)) positive anti-HLA class I and II specificities, respectively. For the IC assay, 428/1608 (MFI 13,900 (9540-17,999)) and 409/1136 (MFI 11,832 (7128-16,531)) positive class I and II specificities bound C3d, respectively. Similarly, OL detected 485/1942 (MFI 15,452 (9369-23,095)) and 298/1247 (MFI18,852 (14,415-24,707)) C1q-binding class I and II specificities. OL was more sensitive in detecting class I and II anti-HLA antibodies than IC was, although there was no significant difference in the number of class II specificities per case. MFI was higher for complement vs. non-complement-binding anti-HLA antibodies in both assays. Both methods were equal in detecting complement-binding anti-HLA class I antibodies, whereas the C3d assay was more sensitive in detecting complement-binding anti-HLA class II antibodies.

Alloantibody Levels, Immunoglobulin Degradation Products, and Platelet Transfusion Response over Successful Desensitization of Severe Alloimmune Platelet Refractoriness with a Novel IgG-Targeted Enzyme Therapeutic

INTRODUCTION Alloimmune platelet refractoriness (APR), an insufficient response to platelet transfusion due to anti-HLA alloantibodies, affects ~25% of acute myeloid leukemia (AML) patients. In APR, HLA- or crossmatch-compatible platelet units may not be identifiable, precluding transfusion and increasing hemorrhage risk. Single antigen bead testing (SAB) semi-quantitates antibody levels against cognate HLA antigens in units of mean fluorescence intensity (MFI). MFI >3,000 suggests an incompatible platelet antigen for transfusion. Imlifidase, an IgG-specific cysteine protease, cleaves IgG first into single-cleaved IgG (scIgG) with limited Fc-mediated function, then into inactive F(ab') fragments 1. We previously reported successful desensitization and provision of permissive platelet transfusions in an APR patient following desensitization treatment with imlifidase. Here we report pre- and post-desensitization alloantibody (PDAb) levels in correlation with IgG degradation products and response to single donor platelet transfusion. CASE As described previously 2, a 43-year-old highly sensitized (calculated PRA [cPRA] 99.9%) woman with AML and history of life-threatening thrombocytopenic hemorrhage secondary to APR presented for matched-unrelated donor stem cell transplantation. Nationwide, few compatible platelet units were identified to support her through the anticipated platelet nadir. Following conventional desensitization, cPRA did not change significantly. Following imlifidase-desensitization [0.25 mg/kg IV (D-1 and D1)], cPRA decreased significantly. 7 single-donor, antigen-avoidance platelet units produced corrected count increments (CCI) [median (interquartile range) of 6,000 (500-7500)]. The patient experienced no hemorrhage and was discharged stable, post-platelet engraftment on D+19. METHODS With IRB approval, patient data and samples (D-16; D-1 post-conventional desensitization with rituximab and therapeutic plasma exchange; D0 post-imlifidase dose 1; D2 post-imlifidase dose 2; and D7, D14 and D29 follow-up samples) were collected. Serial dilutions of patient D-1 serum were tested with SAB (LABScreen single antigen, One Lambda/ThermoFisher, West Hills, California). SDS-PAGE (gradient gel, non-reduced conditions), Western Blot (antiF(ab') detection), and SAB were performed and acceptable antigens (to molecular specificity, targeted by MFI ≤3,000) were compared over the clinical course. Summative MFIs targeting transfused single donor platelet units were compared with 1-hour CCI. Wilcoxon matched-pairs signed rank for matched, nonparametric values and Chi-squared or Fisher's exact for categorical variables were used. P <0.05 was significant. RESULTS/DISCUSSION The D-1 sample (post-conventional desensitization, pre-imlifidase) showed exquisite HLA sensitization. The highest Class A and B antibodies demonstrated an effective MFI of >4,000,000, each (Fig. A). PDAb and the number of unacceptable antigens decreased significantly in D0 (post-imlifidase) vs. D-1 (pre-imlifidase) samples; Intact IgG and scIgG were eliminated from the D0 and D2 samples (Fig. B). Of 7 single donor platelet transfusions, the 5 producing adequate response were each targeted by a summative MFI ≤4,000 (median 4,000 MFI, range 300-4,000). The 2 units producing inadequate CCI response were targeted by summative MFI >4,000 (5,050, 4,900-5,200), suggesting a threshold summative MFI precluding adequate response. CONCLUSIONS Here we show that the patient's pre-desensitization alloantibody levels significantly exceeded the analytical range of the SAB; PDAb levels declined significantly from baseline, correlating with lack of intact IgG and scIgG following imlifidase; and permissive transfusions (CCI >5,000) were seen to platelets targeted by ≤4,000 summative MFI. These results suggest imlifidase is capable of effective desensitization despite profound baseline sensitization, with rapid cleavage of intact IgG to F(ab'), resulting in the temporary reduction of unacceptable antigens. Larger scale investigation of imlifidase in desensitization of APR patients requiring significant transfusion over a limited period of time, along with investigation into transfusion strategies in desensitized APR, are warranted. REFERENCES Am J Transpl. 2022;22(3):691-697.Blood. 2022;140 (Suppl. 1):8562-3. .

P518 Ambiguous single antigen beads testing results: Impact on virtual crossmatch assessment and guide for resolution

P518 Ambiguous single antigen beads testing results: Impact on virtual crossmatch assessment and guide for resolution

The correlation of results of panel reactive antibody, identification, and single antigen beads in detection of anti-HLA antibodies: Istanbul Faculty of Medicine, tissue typing laboratory experience

BackgroundWe have performed a retrospective analysis of anti-HLA class I MHC and class II MHC antibodies measured using a single antigen bead (SAB) assay and a panel reactive antibody (PRA) assay. Material and methodsA group of 256 patients with end-stage renal disease (ESRD) was tested for anti-HLA antibodies in the tissue typing laboratory between 2017 and 2020. In the cohort, the serum samples of patients waiting for transplantation were tested. Both the PRA and SAB tests of these patients were analyzed using the Luminex (Immucor) method. The threshold of positivity was accepted as median fluorescence intensities (MFI) ≥1000 for PRA screening and MFI ≥750 for SAB screening. ResultsOverall, antibodies to HLA antigens were detected in 202 (78.9%) out of 256 patients in the PRA study. Antibodies against both class I/II antigens were detected only in 15.6% of these patients, whereas antibodies against only against class I HLA in 31.3% and only against class II HLA in 32.0%. By comparison, the SAB study found that 66.8% of patients were positive for HLA antigens. Furthermore, donor-specific antibodies (DSA) were detected in 52.0% of PRA-positive patients and 52.6% of SAB-positive patients. It was shown that 168 patients (83.2%) out of 202 PRA-positive patients were found to be SAB-positive. In addition, 51 patients negative in the SAB assay (94.4%) were also negative in the PRA assay. Statistical analysis established a significant correlation between the PRA and SAB positivity (p > 0.001).It was also shown that MFI ≥3000 PRA positivity for class I HLA antigens (p = 0.049) and MFI ≥5000 PRA positivity for class II antigens (p < 0.001) correlated with the SAB positivity in patients. ConclusionOur results showed the importance of both PRA and SAB assays to define the status of sensitization in patients.

Post-transplant donor-specific anti-HLA antibodies with a higher mean fluorescence intensity are associated with graft fibrosis in pediatric living donor liver transplantation.

The roles of post-transplant anti-HLA donor specific antibody (DSA) in pediatric liver transplantation (LT), including therapeutic strategies, remain controversial. This study aimed to identify the risks of post-transplant DSA for graft fibrosis progression in pediatric living donor LT (LDLT). We retrospectively evaluated 88 LDLT pediatric cases between December 1995 and November 2019. DSAs were assessed with single antigen bead test. Graft fibrosis was histopathologically scored with METAVIR and the centrilobular sinusoidal fibrosis system. Post-transplant DSAs were detected in 37 (52.9%) cases at 10.8 (1.3-26.9) years post-LDLT. The histopathological examination of 32 pediatric cases with post-transplant DSA revealed that 7 (21.9%) with a high DSA-MFI (≥9,378) showed graft fibrosis progression (≥F2). No graft fibrosis was observed in the subjects with a low DSA-MFI. The risk factors for developing graft fibrosis in pediatric cases with post-transplant DSA were an older graft age (>46.5 years old), lower platelet count (<10.7 × 104/ml) and higher Fib4 index (>0.7807, recipient age; >1.8952, donor age). Limited efficacy of additional immunosuppressants was observed in DSA positive pediatric cases. In conclusion, pediatric cases with a high DSA-MFI and risk factors should undergo a histological examination. The appropriate treatment for post-transplant DSA in pediatric LT needs to be determined.

RE: Imlifidase-generated Single-cleaved IgG: Implications for Transplantation.

With great interest, we read the article by Bockermann et al1 on crossmatch assays after the administration of the novel desensitization drug Imlifidase. Proteolysis by Imlifidase generates single-cleaved immunoglobulin G (IgG) in a rapid first reaction and subsequently 2 F(ab’)2 fragments and a fully separated Fc fragment in a longer second reaction.2 Bockermann et al1 demonstrated that Luminex single antigen bead (SAB) testing, and crossmatching with both flowcytometry (FCXM) and the anti-human IgG-amplified complement-dependent cytotoxicity crossmatch (CDCXM) do not fully distinguish single-cleaved IgG from intact IgG. Bockermann et al1 suggest that the nonamplified CDCXM is best capable of demonstrating relevant residual intact IgG. Our center performed the first Imlifidase reimbursed HLA-incompatible kidney transplantation in March 2022. Contrary to Bockermann et al,1 we had difficulties in the interpretation of nonamplified CDCXM after Imlifidase treatment. A 29-y-old female with 6-y dialysis vintage after 2 failed kidney transplants was running out of vascular access. Despite high-urgency status and priority allocation via the Eurotransplant Acceptable Mismatch program, she remained without kidney offers. This was caused by her virtual panel-reactive antibody status of 100%. Her historical SAB mean fluorescence intensity (MFI) levels were high with broad reactivity in B-cell memory, with current MFI levels in the lower ranges. To enable for a timely HLA-incompatible kidney transplant, we delisted all unacceptable antigens with corresponding MFI levels of <3000, provided that CDC screening was negative. By this delisting, we increased her donor frequency of 0.000%–3.95%. Eight days later, this resulted in an HLA 1-2-2 mismatched kidney offer, with 2 “delisted DR-unacceptables.” Imlifidase was administered based on a virtual crossmatch. Serum was sampled 2 and 4 h after Imlifidase administration. Nonamplified CDCXMs with donor spleen cells were performed with preblood simultaneously tested with post 2-h and post 4-h sampling. Pre-CDCXM without dithiothreitol (DTT) treatment (which dissolves the disulfide bridges of IgM tetramers) was positive, indicating autoreactivity. DTT treatment revealed a negative pre-Imlifidase CDCXM. To our surprise, both the 2- and 4-h Imlifidase CDCXMs with DTT were positive (value 35 of 100). We had to decide whether or not to proceed with transplantation based on the nonamplified CDCXMs alone, as additional FCXM and SAB testing were scheduled for the next day. Because our patient had not recently received blood transfusions, was without infectious episodes, and had a negative pre-Imlifidase CDCXM, we expected assay interference by Imlifidase and decided to proceed with transplantation. The retrospectively performed pre-Imlifidase FCXM was negative for T cells and was 1.94 MCS for B cells. Luminex SAB was positive on 2 donor DR loci with 2500 and 2200, and became negative after Imlifidase. We investigated CDCXM on frozen donor spleen cells retrospectively and noticed CDC-positivity 2 and 4 h after Imlifidase, with negative CDCXMs on day 3. The patient did experience a vascular rejection after 1 wk, without signs of AMR. SAB testing revealed HLA antibody rebound from day 4 onward; however, DSA remained negative until current follow-up of 4 mo. This case demonstrates that even in nonamplified CDCXM, an interference by Imlifidase may occur. We advise to combine different assays when interpreting desensitization results after Imlifidase treatment.

Clinical utility of serial serum dilutions for HLA antibody interpretation.

Luminex single antigen bead (SAB) testing has increased the sensitivity and specificity of accurately identifying HLA antibodies, in support of all organ transplantation. However, as described in manufacturers' recommendation, the output of the assay, using mean fluorescence intensity (MFI) units, is only semi-quantitative. Therefore, the ability to use MFI values to compare between different assays, to accurately guide clinical practice, or be used as an endpoint measure in clinical trials, is limited. To improve potential quantification, one must circumvent inherent limitations of SAB assays such as interference and saturation phenomena. In this review, we discuss how measurement of pre-transplant serum dilutions can be used to determine unacceptable antigens for wait-listing, determine the likelihood for successful HLA antibody reduction with desensitization, and compare degree of HLA (in)compatibility among various living donors. We also discuss how serum dilutions are optimal for measuring and comparing the efficacy of antibody depletion therapies for desensitization or antibody mediated rejection treatment post-transplant. Historically, one of the main criticisms for the use of serum dilutions and titer has been the potential labor and cost associated with additional testing. Here, we show how only one or two dilutions can add major value in most circumstances. In summary, the practical use of serum dilutions and titer determination are important methods that can be used before and after transplantation of all organs to quantify antibody accurately and reliably in routine practice and in clinical trials.

Misidentification of preformed anti-HLA-DP antibodies leads to antibody-mediated kidney transplant rejection: a case report

BackgroundPatients who are HLA-sensitized are at high risk for early antibody-mediated rejection (AMR) and worse outcomes. Therefore, it is crucial to detect the presence of donor-specific antibodies (DSAs) using pretransplant antibody identification and crossmatch assays. An error in antibody identification can lead to disastrous clinical outcomes. We present a case of acute AMR associated with preformed HLA-DPα and HLA-DPβ DSAs that were not identified before transplantation.Case presentationA 27-year-old woman received a second kidney transplant from a deceased donor. Her pretransplant panel-reactive antibody level was 94%. The complement-dependent cytotoxicity crossmatch was negative for T and B cells at the time of transplantation. She experienced early acute AMR proven by a kidney biopsy. Single antigen bead testing of the patient’s serum at the time of rejection as well as the pre-second transplant serum revealed strong antibodies against the DPA1*01:03 and DPB1*02:01 alleles in the second donor. These antibodies were not identified by phenotypic bead assay during the patient’s time on the waiting list. The patient was treated with plasmapheresis and anti-thymocyte globulin. However, she experienced abdominal pain on day 37 post-transplantation. Surgical exploration revealed a laceration on the transplanted kidney, which was then repaired. Subsequently, infected hematoma was suspected and the transplanted kidney was removed.ConclusionThe present case highlights the clinical significance of preformed HLA-DPα and HLA-DPβ DSAs. Accuracy in determination of HLA antibodies before transplantattion is critical for transplant outcome. HLA-DP typing and single antigen bead testing are recommended for a precise antibody interpretation, especially in highly sensitized patients. Careful interpretation of antibody testing results is essential for the success of organ transplantation.

Association between homograft tissue exposure and allosensitization prior to heart transplant in patients with congenital heart disease.

Surgical repair for patients with congenital heart disease (CHD) often incorporates homograft tissue or other foreign material that can lead to allosensitization. We sought to identify the relationship between pre-sensitization prior to heart transplant and exposure to homograft tissue in CHD patients. Retrospective chart review of all CHD patients who underwent heart transplant at a major pediatric transplant center between 1/1/2011-3/31/18. Operative records determined use of homograft tissue or foreign material. Panel reactive antibody (PRA) and LuminexTM single-antigen bead (SAB) testing results were reviewed. Statistical analysis determined odds of pre-sensitization in patients exposed to homograft tissue. Fifty-six CHD patients underwent transplant during the review period. Thirteen patients (23%) were pre-sensitized by PRA>10%. By SAB testing, 33 patients (59%) developed any anti-HLA antibody >0MFI, 30 patients (54%) >2000MFI, and 19 patients (34%) >6000MFI. Patients with homografts were more likely to be pre-sensitized by PRA (OR=7.31, p=.007), and to have developed any anti-HLA antibody at various levels, >0 (OR=4.52, p=.034), >2000 (OR=8.59, p=.003), and >6000 (OR=8.50, p=.004). Of patients with homografts, those pre-sensitized by PRA had longer exposure times (9.80 vs 4.96years, p=.025). There was no difference in exposure time with relation to pre-sensitization by SAB testing. Previous exposure to homograft tissue appears to increase the odds of pre-sensitization by either the PRA or SAB testing. Longer exposure time to homograft tissue prior to transplant is associated with increased pre-sensitization at transplant as determined by PRA, though not by SAB testing.

Spot Urine Protein Excretion in the First Year Following Kidney Transplantation Associates With Allograft Rejection Phenotype at 1-Year Surveillance Biopsies: An Observational National-Cohort Study.

Introduction: Urine protein excretion is routinely measured to assess kidney allograft injury, but the diagnostic value of this measurement for kidney transplant pathology remains unclear. Here we investigated whether spot urine protein excretion in the first year following transplantation associates with allograft rejection phenotype at 1-year surveillance biopsies and de-novo occurrence of donor-specific antibodies (DSA).Patients and Methods: This prospective, observational national-cohort study included 139 non-sensitized patients who received a deceased donor kidney transplant between December 2014 and 2018. All patients received basiliximab induction and tacrolimus-based immunosuppression. Estimated protein excretion rate (ePER) was calculated monthly from spot urine protein-to-creatinine ratios. At 1-year, all recipients underwent surveillance graft biopsy and were screened for de-novo DSA. Screening-positive sera were subjected to single antigen bead (SAB) testing. The occurrence of de-novo DSA was determined based on SAB reactivity patterns using a mean fluorescence intensity threshold >1,000.Results: Among the 139 study patients, 27 patients (19%) had histologic evidence of T cell-mediated rejection (TCMR), and 9 patients (7%) had histologic evidence of antibody-mediated rejection (AMR) at 1-year surveillance biopsy. One year after transplant, 19 patients (14%) developed de-novo DSA. Compared with patients without rejection and no de-novo DSA, mixed-effects linear regression analysis showed a significant difference in slope of ePER during the first year in patients with AMR and de-novo DSA at 1-year (46, 95% CI 25–68 mg/day/1.73 m2 per month and 34, 95% CI 20–49 mg/day/1.73 m2 per month, respectively). Patients with vascular TCMR also showed a significant difference in ePER slope over time compared with patients with non-rejection findings (31, 95% CI 9–52 mg/day/1.73 m2 per month). The discriminatory power of ePER for intragraft rejection processes was better in patients with AMR (AUC 0.95, 95% CI 0.90–0.99; P < 0.001) than in those with TCMR (AUC 0.68, 95% CI 0.59–0.79; P = 0.002), with 89% sensitivity and 93% specificity for proteinuria >550 mg/day/1.73m2.Conclusions: An increase in ePER in the first year following kidney transplantation associates with AMR, vascular TCMR and de-novo DSA at 1-year and may be used as a non-invasive clinical marker of intragraft endothelial cell injury.

Antibodies to human leukocyte antigens and their association with blood product exposures in pediatric patients undergoing cardiac transplantation

Previous blood product exposures may result in the development of antibodies to human leukocyte antigens (HLA). Pediatric heart transplant recipients who have these antibodies experience increased morbidity and mortality after transplantation. In this study, our aims were to confirm the association of previous allogeneic blood product exposures with the formation of anti-HLA antibodies, determine which blood components pose the greatest risk of developing antibodies, and assess differences in outcomes after transplantation between patients who had anti-HLA antibodies and those who did not. This retrospective investigation included all children who underwent cardiac transplantation at Children's Healthcare of Atlanta from January 1, 2015 through December 31, 2018. Chart reviews were performed to collect pertinent data. Anti-HLA antibodies were detected by single antigen bead testing. Antibody burden was tabulated using the calculated panel reactive antibody (cPRA) score immediately prior to transplantation. Statistical analyses were conducted to examine differences based on HLA antibody status and identify associations with outcomes of interest. Our results show a significant association between pretransplant blood product exposures and HLA antibody status. Children with a pretransplant blood product exposure had 7.98 times the odds of developing an anti-HLA antibody compared to those without a pretransplant blood product exposure (p=.01). We also found a significant association between a previous red blood cell (RBC) exposure and HLA antibody status (p=.01) which was not found for other blood component exposures. Patients who were HLA antibody positive were more likely to develop a donor-specific antibody (DSA) after transplantation (p=.04). Exposure to previous allogeneic blood products affects the development of anti-HLA antibodies in children presenting for heart transplantation. Previous RBC exposures resulted in HLA antibody positivity more than other blood component exposures. Importantly, the presence of HLA antibodies was associated with the development of DSAs post-transplantation. Developing transfusion strategies to reduce allogeneic blood product exposures in children who may need future cardiac transplantation should be a high priority.

Prevalence of clinically significant anti-HLA antibodies in renal transplant patients: Single-center report from north India

Introduction:Solid organ transplantation is the preferred therapeutic modality of treatment in patients affected by terminal organ failures. Human leukocyte antigens (HLAs) plays an important role in graft survival. In many of the cases of rejection, antibodies are directed against HLA antigens expressed on the cells of the transplanted organ. Pre-transplant compatibility testing involves the use of different methodologies for the determination of anti-HLA antibodies. Luminex single-antigen bead (SAB) assay demonstrates higher sensitivity and specificity in detecting anti-HLA antibodies. The aim of this study was to determine the prevalence of anti-HLA antibodies in pre-transplant work up recipients, planned for renal transplant at a tertiary care center in India.Methods:1640 patients visiting tertiary care hospital for pre-transplant compatibility testing were screened with complement-dependent micro-lymphocytotoxicity crossmatch (CDC-XM) and flow cytometric crossmatch (FC-XM). The patients positive for either or both screening tests were assayed with the Luminex SAB tests in order to establish defined antigen specificity of the alloantibodies and determining donor-specific antibody (DSA).Results:The two most frequent antibodies identified in each A, B, C locus of HLA class I were -A*24:03 (43.9%), A*25:01 (36.6%), B*57:01 (40.3%), B*15:12 (37.1%), C*17:01 (61.9%), C*07:01 (52.4%) and in DR, DQ DP locus in HLA class II were DRB1*09:01(40.0%), DRB1*14:04(37.6%), DQA1*04:01/DQB1*03:03 (58.4%), DQA1*05:01/DQB1*03:01 (55.1%), DPA1*02:01/DPB1*17:01 (55.0%), DPA1*02:01/DPB1*05:01 (45.0%).Conclusion:This study has found the prevalence and specificity of anti-HLA antibodies in north India.

The utility of second single antigen bead assay: Clearing the water or stirring up mud?

Though solid-phase single antigen bead (SAB) testing has provided major advances to the HLA community and organ allocation, it has not been without limitations. In particular, false-positive reactions lead to interpretative challenges and the potential to preclude a transplant if the corresponding antigens are deemed unacceptable. Two different vendor platforms are commercially available for SAB testing, one more recent than the other. The aim herein was to assess the benefit of using the newer SAB platform in situations where the primary platform yielded suspicious (specifically, false positive) reactions. Therefore, 42 serum samples with commonly encountered false-positive patterns observed in our laboratory were tested with the newer platform. Cases were classified as resolved, equivalent, or divergent based on whether the second platform produced no reactivity, the same pattern, or a distinctly different pattern compared to the primary platform, respectively. Approximately 33% of cases were resolved, 46% were equivalent, and 21% were divergent. The project revealed advantages of adding a second SAB platform to the laboratory’s test menu including resolving challenging samples and including broader coverage of different alleles and unique class II alpha/beta subunit combinations. However, the challenges of validating, maintaining, and billing for another test method in the laboratory may be barriers to routine use.

Investigating the relationship between class I HLA-specific immunoglobulin-G subclasses, Pan-IgG single antigen bead assays and complement mediated interference in sera from renal transplant recipients

IntroductionAntibody mediated rejection is the leading cause of kidney transplant failure. Not all antibodies are harmful and some may be protective. Immunoglulin Gs, of which there are four subtypes, are detected by single antigen bead testing. The aims of this study were to characterise the IgG subclass profiles for class I HLA-specific antibodies in an uncensored post-transplant population and to determine the underlying relationship between reactivity patterns and MFI cut-offs with the pan-IgG assay. MethodsPatients were recruited to the study who were transplanted in our centre between 2009 and 2014. Prospectively stored post-transplant serum initially underwent a Labscreen Mixed assay and those positive for class I HLA-specific antibody underwent standard SAB testing, EDTA, 1 in 10 dilution and IgG subclass modifications using the Luminex platform. A total of 4947 bead reactions from 51 patients were analysed. ResultsA 1 in 10 dilution was used as a comparator pan-IgG assay for summed subclass and individual subclass linear regression analyses. Using a dilution to standard assay ratio we characterised all reactions for prozone potential i.e. how likely there is to be inhibition related to complement complex formation. We stratified samples into degrees of association and were able to determine suggested MFI thresholds of Log 5.35 for the dilution assay and Log 5.05 for the summed subclass assay when considering a Log MFI of 6.9 (1000) in the standard assay. Using individual subclass dominant reactions (>70%) we were able to determine linear relationships between the 1 in 10 dilution pan-IgG assay and the individual subclass assays (excluding prozone potential reactions for IgG1/3) enabling us to suggest Log MFI thresholds of 5.03, 3.58, 4.3 and 4.05 respectively for IgG1–4. DiscussionWe recommend a 1 in 10 dilution as the optimum pan-IgG comparator assay for a subclass analysis. We advocate the utilisation of the summed subclass assay to determine overall relationships and potential subclass failures. Following others, we recommend serum pre-treatment of the subclass assays to mitigate prozone. We suggest cut-offs for each IgG subclass which should be used with caution given the many inhibitory influences which may include competitive inhibition for bead binding, IgM and IgA interference and under-representation of specific subclasses on the bead panel.

P1604SIGNIFICANT DETECTION OF UNEXPLAINED HLA ANTIBODIES WITH SINGLE-ANTIGEN BEADS IN PATIENTS ON THE WAITING-LIST WITHOUT PREVIOUS SENSITIZING EVENTS

Abstract Background and Aims The introduction of more sensitive and specific tools as solid phase immunoasssays has improved the ability to detect HLA antibodies. The techniques based on solid phase immunoassays in Luminex system can be performed either as screening assays (with pooled antigen panels that use bead populations coated in affinity-purified class I or class II HLA molecules) or to assess the specificity with single-antigen bead (SAB) assays (single cloned allelic HLA antigen).The sensitivity of each assay Is different, and their results may be discordant. Our objective was to evaluate the potential discrepancies between tests in a cohort of patients on our waiting list with no previous sensitizing events, for their potential influence in the access to transplantation. Method Observational study of 184 patients included in the kidney-transplant waiting list on March 31st, 2019. All possible sensitizing events (pregnancy, transfusion and previous transplants) were registered. HLA antibodies where analyzed both by screening and SAB tests. Results We limited the observations to males (64.7% of the cohort), as most women (95.4%) had a known sensitizing event, not being possible to discard possible pregnancies in the remaining. Of men, 46.2% had a sensitizing event and 63.8% did not. All 63 unsensitized patients showed a negative screening test, and 49 (77,78%) had at least one SAB done being positive in 73.4% (36/49). The positive SAB yielded a cPRA &amp;gt;10% in 30 cases (between 21-85%), mostly (27/30) due to HLA class II antibodies (88.8% with at least one DQ antibody) with a median MFI of 1571 (range:752-17650). A thorough review of specificities showed that 81% DQB1* and 89.6% DQA1* antibodies did not show consistency in all beads included for the same allele. Conclusion Screening and SAB assays show a significant discordancy in unsensitized men included in the waiting list for kidney transplantation. Automated detection of antibodies by SAB without a proper specialized interpretation may wrongly limit access to transplantation.

Comparison Between Phenotypic Bead Assay and Single Antigen Bead Assay for Determining Specificity of HLA Antibodies in Kidney Transplant Waiting List

ObjectivesThe determination of unacceptable antigens in patients on kidney transplant waiting list is a critical laboratory investigation in sensitized patients. The Luminex single antigen bead (SAB) assay has high sensitivity and accuracy. However, several countries have not yet implemented SAB testing for waitlisted patients because of limited financial resources. In Thailand, specificities of HLA antibodies are identified by using a phenotypic bead assay. The aim of this study was to evaluate the performance of the phenotypic bead assay for determining HLA antibody specificities when compared with the SAB assay. MethodsA total of 254 sera from patients awaiting kidney transplantation were included. Of 254 sera, 206 and 171 were positive for HLA class I and II antibodies, respectively. Antibody specificities of sera that were tested with both phenotypic and SAB assay were analyzed. The performances of the phenotypic bead assay were compared with those of the SAB assay as the gold standard by using estimation of pooling sensitivity, specificity, and accuracy. ResultsThe sensitivity, specificity, and accuracy of the phenotypic bead assay for determining HLA class I antibodies was 53.9%, 93.0%, and 78.1%, respectively. The sensitivity, specificity, and accuracy of the phenotypic bead assay for determining HLA class II antibodies were 57.3%, 94.9%, and 81.4% respectively. ConclusionIn waitlisted kidney transplant patients, the phenotypic bead assay had high specificity and moderate accuracy when compared with the SAB assay. However, the low sensitivity of the test suggests that the use of the phenotypic assay for determining HLA specificities should be applied with caution in sensitized patients.

Luminex screening first vs. direct single antigen bead assays: Different strategies for HLA antibody monitoring after kidney transplantation

Main problemLuminex panel and single antigen beads (SAB) are used for screening and DSA specificity determination respectively. The cost of SAB may limit its general use, so some labs perform SAB tests only after positive screening. MethodsWe compared both strategies: 1) SAB only if positive screening with kits from manufacturer A, and 2) direct SAB from manufacturer B, and correlate their sensitivity with histological findings. ResultsWe selected 118 kidney transplant recipients with a normal biopsy (n = 19), histological antibody-mediated damage (ABMR, n = 52) or interstitial fibrosis/tubular atrophy (IFTA, n = 47) following Banff 2015 and 2017 classification.Direct SAB detected DSA in 13 patients missed by screening. Strategy 1 detected DSA in 0% normal, 61.5% ABMR and 8.5% IFTA patients; percentages with strategy 2 were 5.2%, 78.8% and 14.8% (p=0.004). Strategy 2 identified DSA allowing full ABMR diagnosis in 17% cases missed by strategy 1. Thereafter, direct SAB from manufacturer A confirmed DSA in 46% DSA-positive cases with strategy 2 (55.5% ABMR cases). ConclusionsLuminex screening failed to identify clinically relevant HLA antibodies, hampering DSA detection in patients with possible ABMR. Direct SAB testing should be the chosen strategy for post-transplantation monitoring, albeit direct SAB from the two existing manufacturers may diverge in as much as 50% of cases.

Identification of a recurrent pattern of false-positivity by Luminex HLA MHC class I single antigen bead testing

Previously, a distinct MHC class II Luminex-single antigen bead (SAB) pattern was described and attributed to antibodies targeting denatured antigens. In this study, we describe a distinct MHC class I reactivity pattern observed in 1.8% (105/5992) of samples resulted in 2017. The pattern displays reactivity to the following Luminex-SABs: HLA-A*33:03, A*36:01, A*80:01, B*54:01, B*53:01, C*06:02, C*07:02, C*18:02, C*14:02, C*03:03, C*03:04, and C*15:02. This pattern was identified in patients with no sensitization history, negative FlowPRA results, and antibody to self-antigen(s). Epitope analysis failed to reveal a common determinant(s) to explain this pattern of reactivity. Additionally, we found this pattern to be prevalent in female patients (62%) and also those with systemic lupus erythematosus (62%). Given these findings, we speculate this pattern likely represents false-positive reactivity, possibly due to antibody targeting denatured antigens or a specific peptide, molecular mimicry, autoimmunity, or a combination thereof.

Design of a state of the art reporting system and process improvement for reporting of high complexity single antigen bead data for transplant patients to the electronic medical record

The UCLA Immunogenetics Center is an Immunogenetics and Histocompatibility laboratory that performs testing for multiple transplant programmes within and outside of UCLA. The single antigen bead (SAB) test is a...