Eplet Mismatch Research Articles

Belatacept with time-limited tacrolimus coimmunosuppression modifies the 3-year risk of eplet mismatch in kidney transplantation

Solid organ transplant donor-recipient eplet mismatch has been correlated with donor-specific antibody (DSA) formation, antibody-mediated rejection, and overall rejection rates. However, studies have been predominantly in patients on tacrolimus-based immunosuppression regimens and have not fully explored differences in ethnically and racially diverse populations. Evidence indicates that patients on belatacept have lower rates of DSA formation, suggesting mediation of the immunogenicity of mismatched human leukocyte antigen polymorphisms. We performed a retrospective, single-center analysis of class II eplet disparity in a cohort of kidney transplant recipients treated using belatacept with tacrolimus induction (Bela/TacTL) or tacrolimus regimens between 2016 and 2019. Bela/TacTL (n = 294) and tacrolimus (n = 294) cohorts were propensity score-matched with standardized difference <0.15. Single-molecule eplet risk level was associated with immune event rates for both groups. In Cox regression analysis stratified by eplet risk level, Bela/TacTL immunosuppression was associated with a decreased rate of DSA (hazard ratio [HR] = 0.4), antibody-mediated rejection (HR = 0.2), and rejection (HR = 0.45). In the low-risk group, cumulative graft failure was lower for patients on Bela/TacTL (P < .02). Analysis of eplet mismatch burden may be a useful adjunct in identifying high-risk populations with increased immunosuppression requirements and should encourage the design of allocation rules to incentivize lower-risk pairings without negatively impacting equity in access.

O9 Impact of induction on acute rejection in kidney transplant recipients with high and low risk class II eplet mismatches

O9 Impact of induction on acute rejection in kidney transplant recipients with high and low risk class II eplet mismatches

P400 Extending a virtual crossmatch analysis and reporting workflow to include HLA eplet mismatch load with risk stratification for post transplant kidney clinical decision support

P400 Extending a virtual crossmatch analysis and reporting workflow to include HLA eplet mismatch load with risk stratification for post transplant kidney clinical decision support

Impact of HLA eplet mismatch load in immunological outcomes after living donor kidney transplantation

IntroductionHLA eplets mismatches (eMM) have been associated with negative kidney outcomes after transplantation, such as the development of de novo donor-specific antibody (dnDSA), antibody-mediated rejection (ABMR), and early graft loss. This study aimed to evaluate the clinical effects of the HLA eMM load on dnDSA development, ABMR, renal function, allograft survival and graft loss. Material and methodsThis retrospective study involved 159 living donor kidney transplant patients categorized into groups based on antigen HLA mismatches assessed traditionally and HLA eMM load. Patients had followed for at least one year. The EpViX online program was used to evaluate the HLA eMM load. Cox models were constructed to assess the risk of graft loss. Kaplan-Meier survival curves were carried out. The analyses had performed using the R program and p < 0.05 was considered significant. ResultsFrom all 159 patients, 28 (17.6%) lost their allografts. Rejection episodes occurred in 37.1% of patients, 13.6% of whom were ABMR. Patients with rejection episodes had higher HLA-AB (p = 0.032) and HLA-DR (p = 0.008) HLA eMM load, HLA-AB (p = 0.006) and HLA-DR (p = 0.009) antigens mismatches, and higher proportions of the following eMM in the HLA-DR locus: 70R eMM (p = 0.015), 70RE (p = 0.015), 74E (p = 0.015) and 48Q (p = 0.047). In multiple models, the presence of HLA-DR 70qq eMM (HR 3.75, 95% CI 1.47; 9.55) add an increase in creatinine levels at 1-year (HR 3.87, 95% CI 2.30, 6.53) were associated with the risk of graft loss. ConclusionThe HLA eMM load was related to episodes of rejection and allograft loss. The HLA-DR eMM was most strongly associated with a worse immunologic outcome than eMM mismatches for HLA-AB.

DQA1 Eplet Mismatch Load As an Independent Risk Factor of CLAD After Lung Transplantation.

Based on clinical data, 240 lung transplant recipients were eligible for CLAD, and HLA and eplet mismatch was analyzed using the HLAMatchmaker 3.1 software. A total of 92 (38.3%) lung transplant recipients developed CLAD. The time free-of-CLAD was significantly decreased in patients with presence of DQA1 eplet mismatches (P = 0.015). Furthermore, when other previously described CLAD risk factors were studied in a multivariate analysis, the presence of DQA1 eplet mismatches was found to be independently associated with the early onset of CLAD. The concept of epitope load has arisen as a new tool to better define donor-recipient immunologic compatibility. The presence of DQA1 eplet mismatches potentially would increase the likelihood of developing CLAD.

Biostatistical Analysis of Relation between HLA-A/B and HLA-DR Eplet Mismatches and Renal Graft Outcome

Abstract: Despite all the clinical interventions that happened during the last 2 decades, the unavailability of a potent donor for renal transplantation remains a concern in the field of transplantation immunology. Recent studies promise the establishment of an effective clinical method to find a potent renal transplant donor, in the near future. Combining the use of molecular techniques with biostatistical methods can be a great tool for clinicians in choosing a potent donor. The present study was conducted from a cohort of 1144 (n=572 pair) renal transplant recipients and their donors to analyse eplet matches and mismatches. Initial observation of increased serum creatinine level which ranged from 2.0 to 21.0 mg/dL in patients treated for end-stage renal disease implied a single event of graft rejection. The statistical correlation between eplet mismatches and graft survival was observed to be significant with P value 0.003. A value of -0.215 at 0.01 significance showed the positive linear correlation between the increased numbers of HLA DR eplet mismatches towards chronic renal graft rejection. The correlation coefficient of -0.888 observed between the number of HLA DR eplet mismatches and days of graft survival clearly implies the effect of increased HLA DR eplet mismatch delayed graft rejection. The study gives an idea on the minimization of class II HLA DR eplet mismatches to decreased frequency of graft rejection as indicated by increased survival rate.

Predictive value of molecular matching tools for the development of donor specific HLA-antibodies in patients undergoing lung transplantation.

Molecular matching is a new approach for virtual histocompatibility testing in organ transplantation. The aim of our study was to analyze whether the risk for de novo donor-specific HLA antibodies (dnDSA) after lung transplantation (LTX) can be predicted by molecular matching algorithms (MMA) and their combination. In this retrospective study we included 183 patients undergoing LTX at our center from 2012-2020. We monitored dnDSA development for 1 year. Eplet mismatches (epMM) using HLAMatchmaker were calculated and highly immunogenic eplets based on their ElliPro scores were identified. PIRCHE-II scores were calculated using PIRCHE-II algorithm (5- and 11-loci). We compared epMM and PIRCHE-II scores between patients with and without dnDSA using t-test and used ROC-curves to determine optimal cut-off values to categorize patients into four groups. We used logistic regression with AIC to compare the predictive value of PIRCHE-II, epMM, and their combination. In total 28.4% of patients developed dnDSA (n = 52), 12.5% class I dnDSA (n = 23), 24.6% class II dnDSA (n = 45), and 8.7% both class II and II dnDSA (n = 16). Mean epMMs (p-value = 0.005), mean highly immunogenic epMMs (p-value = 0.003), and PIRCHE-II (11-loci) (p = 0.01) were higher in patients with compared to without class II dnDSA. Patients with highly immunogenic epMMs above 30.5 and PIRCHE-II 11-loci above 560.0 were more likely to develop dnDSA (31.1% vs. 14.8%, p-value = 0.03). The logistic regression model including the grouping variable showed the best predictive value. MMA can support clinicians to identify patients at higher or lower risk for developing class II dnDSA and might be helpful tools for immunological risk assessment in LTX patients.

HLA antigen and HLA eplet mismatches - important factors for developing donor-specific antibodies after kidney transplantation

Kidney donor-recipient mismatches (MM) in human leukocyte antigen (HLA) system can have the impact on graft survival as de novo formation of donor-specific HLA antibodies (DSA) post-transplant can increase the risk of acute and chronic rejections. Eplets, the smallest functional units of epitopes, are in recent years also being considered in recipient-donor matching. We have retrospectively analysed the relationship between MM at HLA-A, -B, -DRB1 and -DQB1 loci and the development of DSA in 47 kidney transplant recipients that were negative for the presence of HLA antibodies pre-transplant. A total of 19 patients (40%) developed DSA (DSA+) post-transplant, revealing sensitization to HLA class II MM antigens as the most prevalent (84% of DSA+ patients). MM at all HLA loci contributed to the development of HLA locus-specific antibodies, with the prevalence of HLA-DQ sensitization, as 41% of recipients with HLA-DQ MM developed DSA. HLAMatchmaker analysis for HLA-DR/DQ total eplet MM showed statistically significant difference between DSA- (N=33) and DSA+ recipients (N=14) for a total number of eplet MM (300 in DSA- vs 248 in DSA+; P=0.0004). These results emphasize the importance of HLA Class II matching in kidney transplantation.

Estimation of Antibody-Verified Eplet Mismatch Load, 2-Field HLA Resolution vs Imputation in a Large Cohort of European Donors

The assessment of class II eplet mismatch load is useful to determine the risk of chronic rejection in solid organ transplantation. However, high-resolution (2-field) HLA typing is mandatory to accurately define eplet mismatches. The imputation of the most frequent allele has been used in retrospective studies. Here, we sought to investigate the concordant of antibody-verified (AbV) eplet load in different class II alelles between real 2-field HLA typing and HLA imputed by most frequent allele in a large White cohort. The allelic frequency of the different HLA class II loci was calculated using a database of high-resolution typing of 23,628 voluntary Spanish bone marrow donors obtained from the Spanish Registry of Bone Marrow Donors, managed by the Josep Carreras Foundation. The AbV eplet count in the different class II alleles was performed using the HLA-Matchmaker v3.1 algorithm. The probability of imputing the correct allele compared to the most frequent for DRB1 and DQB1 loci was 69.3% and 53.0%, respectively. However, studying the less frequent alleles, the same AbV eplet load was observed in 82.22% and 76.15%, respectively. Our data show that despite the potential error in the imputation of class II HLA typing, the number of AbV eplets is not significantly over- or underestimated in our population. Until high-resolution typing is widely established for solid organ transplantation, retrospective studies of AbV eplet mismatch load with imputed high-resolution types provide an acceptable outcome in White people.

HLA-DR/DQ Single Molecule Eplet Mismatch and Formation of De Novo Donor-Specific Anti-HLA Antibodies After Kidney Transplantation in Children

HLA-DR/DQ Single Molecule Eplet Mismatch and Formation of De Novo Donor-Specific Anti-HLA Antibodies After Kidney Transplantation in Children

Eplet matching in pediatric heart transplantation: The SickKids experience.

Epitope-based tissue matching may be superior to HLA antigen matching. We compared antigen to molecular-level HLA matching on outcomes following pediatric heart transplantation (HTx). This is a retrospective, single centre cohort study (2013-2020). HLA antigen and eplet mismatch analyses were performed in HTx patients <18 years old. Primary endpoint was graft loss; secondary endpoints were rejection and cardiac allograft vasculopathy (CAV). A multivariable Cox regression analysis was used to examine associations between eplet or antigen mismatching and outcomes. A logistic regression analysis was performed to examine associations between eplet or antigen mismatching and outcomes. Seventy-seven patients (40% males) were included, with a median age at HTx 4.3 years [range 0.05-18]. Median HLA class I and II eplet mismatches were 10 (1-22) and 11 (1-23). Median class I and II antigen mismatches were 5 (1-6) and 4 (0-6). 9 patients (11.7%) died [median time 4 months (range 0.1-46)]. Eight (10.4%) patients developed AMR [median time 22 days (IQR=168)]. Twenty-one patients (27.3%) had acute cellular rejection [median time 40 days (IQR=85.5)]. In univariate analysis, patients with HLA Class II DPB eplet mismatches above the median for this cohort had an increased risk of graft loss (OR 5.3 [95%CI: 1.03-27.5], p=0.039). HLA eplet mismatching was not associated with rejection; antigen mismatching was not associated with either graft loss or rejection. In multivariable analysis, patients with HLA Class II DPB eplet mismatches above the median had an increased risk of graft loss (HR 8.14 [95% CI: 1.26-49], p=0.02). HLA eplet mismatching was not associated with rejection; antigen mismatching was not associated with graft loss or rejection. A logistic regression analysis including 'number of HLA Class II DPB eplet mismatches' correctly predicted 95.8% of the outcomes. In our cohort of pediatric heart transplant recipients, the number of HLA Class II DPB eplet mismatches was associated with graft loss. Molecular-level HLA matching is an emerging tool for graft loss risk stratification, but further study is required.

Determining Clinical Thresholds for Donor HLA Eplet Compatibility to Predict Best Outcomes Following Lung Transplantation.

Background.Currently, the assessment of immunological risk in lung transplantation (LTx) does not completely consider HLA compatibility at the molecular level. We have previously demonstrated the association of HLA eplets in predicting chronic lung allograft dysfunction following LTx; however, the associations between HLA eplet mismatch (epMM) loads and overall survival are unknown.Methods.In this retrospective, single-center study, 277 LTx donor-recipient pairs were high resolution HLA typed and analyzed for HLA epMMs using HLAMatchmaker (version 3.1). LTx pairs were also assessed for the presence of the previously described risk epitope mismatches DQ2-DQA1*05 and DQ7-DQA1*05.Results.HLA class I epMMs were not associated with deleterious outcomes; however, lower HLA class II (≤19), DQA1 (≤2), and combined HLA class I and II (≤29) epMM demonstrated an association with increased time to chronic lung allograft dysfunction and improved overall survival. The presence of a risk epitope mismatch was not associated with worse clinical outcomes.Conclusions.HLA epMM can risk-stratify LTx recipients and potentially guide donor-recipient matching and immunosuppression strategies.

315.11: Tacrolimus Withdrawal After Mesenchymal Stromal Cell Therapy Is Associated With Donor-Specific Antibody Formation in Kidney Transplant Recipients

Introduction: Recently, the clinical phase II Triton study demonstrated that mesenchymal stromal cell (MSC) therapy is a safe method to facilitate tacrolimus withdrawal in kidney transplant recipients (KTRs). Here, we analyzed de novo donor-specific antibody (dnDSA) formation and HLA molecular mismatch in MSC-treated KTRs. Methods: All patients underwent a first, living-donor kidney transplantation and received alemtuzumab induction, steroids, tacrolimus and everolimus. Patients in the MSC arm of the study received two infusions of autologous MSC at week 6 and 7 post-transplantation. Subsequently, tacrolimus was tapered and completely withdrawn at week 8. Patient sera were tested by luminex single antigen bead (SAB) assays (Immucor) for dnDSA assignment; positivity was defined as MFI > 1000. DSA were further characterized using a modified SAB assay to detect IgG1, IgG2, IgG3 and IgG4 subclasses. Complement binding was assessed with a C3d assay (Immucor). Patients and donors were HLA typed on 11 loci by next-generation sequencing. Eplet mismatches were calculated using HLAMatchmaker 2.0. Results: In total, 3 out of 27 patients (11%) in the control group and 11 out of 29 patients (38%) in the MSC group developed dnDSA, of which the majority was directed against HLA-DQ. The majority of DSA were complement binding and had different IgG subclasses. Although patients in the MSC group had an increased rate of dnDSA formation, the degree of antigen mismatch was similar to the control group (Figure 1). Cox regression showed that both MSC treatment group (HR=4.247, P=0.032) and HLA class II antibody-verified eplet mismatch (HR=1.176 per single eplet mismatch, P=0.001) were independently associated with dnDSA development. Despite the increased dnDSA formation in MSC-treated patients, incidence of rejection or graft loss was not increased and eGFR was stable at 2 years follow-up (Figure 2). Conclusion: Tacrolimus withdrawal after MSC therapy is associated with an increased rate of dnDSA formation in KTRs. However, no increased risk of rejection or inferior graft function was found, raising the question on the pathogenicity of these DSA, despite the presence of complement fixing IgG subclasses. We found that patients with a high HLA class II eplet mismatch load were less likely to tolerate tacrolimus withdrawal without developing dnDSA, which is in line with previous findings that HLA eplet mismatch load modulates tacrolimus through levels required to prevent dnDSA formation. Further research is warranted to explore HLA molecular mismatch load as a biomarker to guide personalized immunosuppression in transplantation. Jon J van Rood Transplantatiefonds.

316.11: HLA-DPB1 Molecular Mismatches Are Associated With Acute Rejection and Long-term Graft Function in First Kidney Transplants

Introduction: It is generally accepted that HLA-DP mismatches (MM) do not impact the survival of first transplants but that they may exert a deleterious effect in retransplants. This study aimed to investigate the impact of HLA-DPB1 allele and molecular MM (mMM) on the occurrence of acute rejection (AR) and low 5-year graft function (5-yGF) in first kidney transplants (KT). Method: HLA-DPB1 allele and mMM were determined in 130 first KT from deceased donors, performed between 2014 and 2016. The following mMM models were investigated: expression MM, with the high expression G allele in the donor, T cell epitope (TCE) MM, classified as permissive and nonpermissive MM, epitope MM (EMM), considering all six HLA-DPB1 hypervariable regions (EMM-ABCDEF HVR) or only ABEF regions (EMM-ABEF HVR), Eplet MM (EpMM), considering all or only antibody-verified (AbVer) eplets, and solvent accessible amino acid MM (SAMM). EMM, EpletMM and SAMM loads were categorized using thresholds determined by ROC analysis for each outcome. The outcomes were AR and 5-yGF (estimated by CKD-EPI). Logistic and Cox regressions were used to investigate variables associated with 5-yGF and AR, respectively. Results: 119 (91.5%) KT were performed with one or two HLA-DPB1 allele MM. The overall incidence of AR was 17.7% and the median 5-yGF was 40.3 (27.0 - 51.7) mL/min/1.73m2. No association between HLA-DPB1 allele MM and 5-yGF or rejection was observed. Considering only recipients of standard criteria donor, five logistic models for 5-yGF were built including variables that reached a p-value <0.10 in the univariable analysis (age and donor sex, cause of brain death, ABDR MM and AR) and the mMM (TCE, SAMM, all and AbVer EpMM, EMM-ABCDEF and ABEF HVR). In the model 2, variables associated with 5-yGF≤ 40 were donor age (OR=1.08; CI95%=1.02-1.14; p=0.005) and SAMM≥7 (OR=4.40; CI95%=1.56-12.36, p<0.005), while in the model 4, donor age (OR= 1.09; CI95%=1.03-1.15; p=0.003) and EpMM(AbVer≥2 (OR=3.39; CI95%=1.2-9.45, p=0.02) were associated with low GF. The AUC-ROC for predicting 5-yGF was 0.83 (0.75-0.92) for the model 2, and 0.68 (0.57-0.79) for the model 4. Lastly, for the outcome AR in all recipients, beyond the final donor creatinine, the following mMM models were statistically significant: TCE (HR=3.01; CI95%=1.33-6.84; p=0.008), SAMM≥5 (HR=2.69; CI95%=1.10-6.56; p=0.03), AbVerEpMM ≥3 (HR=3.38; 95%CI=1.32-8.70; p=0.01), and EMM-ABCDEF HVR ≥ 6 (HR=5.08; 95%CI=2.19-11.82; p<0.001). Conclusion: To the best of our knowledge, this is the first study that shows that some mMM, in contrast to classical allele HLA-DPB1 MM, are associated with acute rejection and long-term graft function in a population of exclusively first kidney transplants.

Combined Analysis of HLA Class II Eplet Mismatch and Tacrolimus Levels for the Prediction of De Novo Donor Specific Antibody Development in Kidney Transplant Recipients.

We investigated whether HLA class II eplet mismatch was related to dnDSA development and analyzed its combined impact with tacrolimus levels for kidney transplantation outcomes. A total of 347 kidney transplants were included. HLA Matchmaker was used for the single molecular eplet, total eplet, antibody (Ab)-verified eplet mismatch analyses, and Ab-verified single molecular analysis to identify HLA-DR/DQ molecular thresholds for the risk of dnDSA development. A time-weighted tacrolimus trough level (TAC-C0) of 5 ng/mL and a TAC-C0 time-weighted coefficient variability (TWCV) of 20% were applied to find the combined effects on dnDSA development. A high level of mismatch for single molecular eplet (DQ ≥ 10), total eplet (DQ ≥ 12), Ab-verified eplet (DQ ≥ 4), and Ab-verified single molecular eplet (DQ ≥ 4) significantly correlated with HLA class II dnDSA development. Class II dnDSA developed mostly in patients with low TAC-C0 and high eplet mismatch. In the multivariable analyses, low TAC-C0 and high eplet mismatch showed the highest hazard ratio for the development of dnDSA. No significant combined effect was observed in dnDSA development according to TWCV. In conclusion, the determination of HLA class II eplet mismatch may improve the risk stratification for dnDSA development, especially in conjunction with tacrolimus trough levels.

Immunologic risk stratification of pediatric heart transplant patients by combining HLAMatchmaker and PIRCHE-II.

Molecular-level human leukocyte antigen (HLA) mismatch is a powerful biomarker of rejection; however, few studies have explored its use in heart transplant recipients, and none have attempted to use the results of separate algorithms synergistically. Here we tested the hypothesis that a combination of HLAMatchmaker and Predicted Indirectly Recognizable HLA Epitopes (PIRCHE-II) can be used to identify more patients at low risk of rejection. We studied 274 recipient/donor pairs enrolled in the Clinical Trials in Organ Transplantation in Children (CTOTC) performing class I and II HLA genotyping by next-generation sequencing to determine eplet mismatch (epMM) load and PIRCHE-II score. Correlation with clinical outcomes was performed on 131 cases. Of the 131 patients, 100 without pre-formed donor specific antibody (DSA) were used to identify cutoffs for the Class I, HLA-DR, and HLA-DQ epMM load and PIRCHE-II score for risk of developing post-transplant DSA (epMM: Class I/DR/DQ=9/9/6; PIRCHE-II: 141/116/111) and antibody-mediated rejection (ABMR) (epMM: 9/8/8; PIRCHE-II: 157/80/201). Patients with above cut-off epMM load appear to be less likely to develop DSA and ABMR if their PIRCHE-II score is below cut-off (high epMM/high PIRCHE-II: 12.3%-20.3% DSA and 9%-13.5% ABMR vs high epMM/low PIRCHE-II: 4%-10% DSA and 0%-2% ABMR). For the first time in a pediatric heart transplant cohort, immunologic risk cut-offs for DSA and ABMR have been established. When used together, epMM load and PIRCHE-II score allow us to reclassify a portion of cases with high epMM load as having a lower risk for developing DSA and ABMR.

The Accuracy of Sequence-Specific Oligonucleotide and Real-Time Polymerase Chain Reaction HLA Typing in Determining the Presence of Pre-Transplant Donor-Specific Anti-HLA Antibodies and Total Eplet Mismatches for Deceased Donor Kidney Transplantation

High resolution human leukocyte antigen (HLA) typing is important in establishing eplet compatibility and the specificity of donor-specific anti-HLA antibodies (DSA). In deceased donor kidney transplantation, high resolution donor HLA typing may not be immediately available, leading to inaccuracies during the organ allocation process. We aimed to determine the concordance and agreement of HLA-Class I and II eplet mismatches calculated using population frequency based allelic haplotype association (linkage disequilibrium, LD) from sequence-specific oligonucleotide (SSO) and real-time polymerase chain reaction (rtPCR) donor HLA typing (available at time of donor kidney allocation) compared to high-resolution Next Generation Sequencing (NGS) donor typing. NGS high resolution HLA typing were available for all recipients prior to donor kidney allocation. A cohort of 94 deceased donor-recipient pairs from a single Western Australian center were included (77 individual donors typed, 55 local and 22 interstate). The number of class I (HLA-A+B+C) and class II (HLA-DRB1+DRB3/4/5+DQB1+DQA1+DPB1+DPA1) eplet mismatches were calculated using HLAMatchmaker, comparing LD- and NGS-HLA typing. The accuracy in assigning pre-transplant DSA was compared between methods. The concordance correlation coefficient (95%CI) for HLA-class I and II eplet mismatches were 0.994 (0.992 to 0.996) and 0.991 (0.986 to 0.993), respectively. The 95% limits of agreement for class I were -1.3 (-1.6 to -1.1) to 1.4 (1.2 to 1.7) and -4.8 (-5.7 to -3.9) to 5.0 (4.1 to 5.9) for Class II. Disagreement between the two methods were present for 11 and 37 of the Class I and II donor/recipient pairs. Of which, 5 had a difference of ≥5 class II eplet mismatches. There were 34 (36%) recipients with potential pre-transplant DSA, of which 8 (24% of recipients with DSA) had indeterminate and ultimately false positive DSA assigned by donor LD-typing. While the concordance between NGS- and LD-typing was high, the limits of agreement suggest meaningful differences between these two techniques. The inaccurate assignment of DSA from donor LD-typing may result in associated HLA being considered unacceptable mismatches, inappropriately precluding candidates’ access to transplantation. Accurate imputation of two-field HLA alleles based on LD from SSO and rtPCR HLA typing remains a substantial challenge in clinical practice in-lieu of widely available, rapid, high-resolution methods.

Association of Predicted HLA T-Cell Epitope Targets and T-Cell–Mediated Rejection After Kidney Transplantation

The relationship between human leukocyte antigen (HLA) molecular mismatches and T-cell-mediated rejection (TCMR) is unknown. We investigated the associations between the different donor HLA-derived T-cell targets and the occurrence of TCMR and borderline histologic changes suggestive of TCMR after kidney transplantation. Retrospective cohort study. All kidney transplant recipients at a single center between 2004 and 2013 with available biopsy data and a DNA sample for high-resolution HLA donor/recipient typing (N = 893). Scores calculated by the HLA matching algorithm PIRCHE-II and HLA eplet mismatches. TCMR, borderline changes suggestive of TCMR, and allograft failure. Multivariable cause-specific hazards models were fit to characterize the association between HLA epitopes targets and study outcomes. We found 277 patients developed TCMR, and 134 developed only borderline changes suggestive of TCMR on at least 1 biopsy. In multivariable analyses, only the PIRCHE-II scores for HLA-DRB1 and HLA-DQB1 were independently associated with the occurrence of TCMR and with allograft failure; this was not the case for HLA class I molecules. If restricted to rejection episodes within the first 3 months after transplantation, only the T-cell epitope targets originating from the donor's HLA-DRB1 and HLA-DQB1, but not class I molecules, were associated with the early acute TCMR. Also, the median PIRCHE-II score for HLA class II was statistically different between the patients with TCMR compared to the patients without TCMR (129 [IQR, 60-240] vs 201 [IQR, 96-298], respectively; P< 0.0001). These differences were not observed for class I PIRCHE-II scores. Observational clinical data and residual confounding. In the absence of HLA-DSA, HLA class II but not class I mismatches are associated with early episodes of acute TCMR and allograft failure. This suggests that current immunosuppressive therapies are largely able to abort the most deleterious HLA class I-directed alloimmune processes; however, alloresponses against HLA-DRB1 and HLA-DQB1 molecular mismatches remain insufficiently suppressed. Genetic differences in the human leukocyte antigen (HLA) complex between kidney transplant donors and recipients play a central role in T-cell-mediated rejection (TCMR), which can lead to failure of the transplanted kidney. Evaluating this genetic disparity (mismatch) in the HLA complex at the molecular (epitope) level could contribute to better prediction of the immune response to the donor organ posttransplantation. We investigated the associations of the different donor HLA-derived T-cell epitope targets and scores obtained from virtual crossmatch algorithms with the occurrence of TCMR, borderline TCMR, and graft failure after kidney transplantation after taking into account the influence of donor-specific anti-HLA antibodies. This study illustrates the greater importance of the molecular mismatches in class II molecules compared to class I HLA molecules.

Eplet Matching in Pediatric Heart Transplantation

<h3>Purpose</h3> To compare antigen to molecular-level HLA matching in regards to post-heart transplant outcomes in a pediatric population. <h3>Methods</h3> Retrospective single centre cohort study including all heart transplant patients aged < 18 years between 2013-2020. HLA typing, antigen mismatch, and eplet mismatch analysis were performed on all donor - recipient pairs. The primary endpoint was graft loss (death/ re-transplantation) and the secondary enpoint was the development of allograft rejection. Multivariable Cox regression analysis was used to examine associations between antigen or eplet mismatching and adverse outcomes. <h3>Results</h3> 77 patients were included, 31 (40%) males, median age at transplant 4.3 years [range 0.05 - 18]. Median number of HLA class I and II donor-recipient eplet mismatches was 10 (range 1-22) and 11 (range 1-23), respectively. Median number of donor-recipient antigen mismatches was 5 (range 1-6) for HLA class I and 4 (range 0-6) for HLA class II. In multivariable analysis, recipients with a number of HLA Class II DPB eplet mismatches above the median for this cohort had an increased risk of graft loss (HR 8.14 [95% CI: 1.26 - 49], p = 0.02). Neither HLA class I nor class II eplet mismatching was significantly associated with rejection. Similarly, HLA class I and II antigen mismatching was not significantly associated with graft loss or rejection. A logistic regression analysis was performed, with the primary outcome as the dependent variable. After iteration, the most accurate prediction was obtained with prior heart surgery, prior ECMO, sensitization against HLA class II antigens, and number of HLA Class II DPB eplet mismatches as independent variables. The model accounted for 35% - 67% of variance in the primary outcome, with 95% of outcomes correctly predicted. <h3>Conclusion</h3> In our cohort of paediatric heart transplant patients, the number of HLA Class II DPB donor-recipient eplet mismatches was independently associated with graft loss. Molecular-level HLA mismatching may aid in identifying recipients at increased risk of long-term graft loss but further study is required.

Mismatch epitope load predicts de novo-DSA-free survival in pediatric liver transplantation.

Our knowledge of de novo anti-HLA donor-specific antibodies (dnDSA) in liver transplantation continues to be defined. We hypothesized that differences of HLA-DR/DQ mismatches can improve precision in alloimmune risk categorization and be applied to tailor immunosuppression. A retrospective chart review of 244 pediatric patients consecutively transplanted at our center between 2003 and 2019 was performed to identify patients tested for dnDSA. Records were queried for: demographics, pre-transplant diagnosis, biopsy-proven T-cell-mediated rejection (TCMR), radiology proven biliary complications, tacrolimus trough levels, dnDSA characteristics, and HLA typing. The eplet mismatch analyses were performed using HLAMatchmaker™ 3.1. All statistical analyses were conducted using R software version 3.40. There were 99 dnDSA-negative patients and 73 dnDSA-positive patients (n=70 against class II and n=3 against class I and II). ROC analysis identified optimal cutoff of eplet mismatch load for dnDSA and defined risk groups for an alloimmune outcome. Kaplan-Meier curves and log-rank tests showed high eplet mismatch load was associated with shorter dnDSA-free survival (log-rank p=.001). Multivariable Cox regression models showed that tacrolimus coefficient of variation and tacrolimus mean levels were significantly associated with dnDSA-free survival (p<.001 and p=.036). Fisher's exact test showed that dnDSA was associated with an increased likelihood of TCMR (OR 14.94; 95% CI 3.65 - 61.19; p<.001). Patients without TCMR were more likely to have dnDSA to HLA-DQ7 and less likely to have dnDSA to HLA-DQ2 (p=.03, p=.080). Mismatched epitope load predicts dnDSA-free survival in pediatric liver transplant, while dnDSA specificity may determine alloimmune outcome.