cPRA Increases With DQA,DPA, and DPB Unacceptable Antigens in the Canadian cPRA Calculator

A calculated panel reactive antibody (cPRA) estimates the percentage of donors with unacceptable antigens (UA) for a recipient. cPRA may be underestimated in transplant candidates with UA to DQA, DPA, and DPB if these are not included in the calculation program. To serve the National Canadian Transplant Programs, a cPRA calculator was developed with complete molecular typing for all donors at HLA‐A, B, C, DRB1, DRB3/4/5, DQA1, DQB1, DPA1, and DPB1, all resolved to serologic equivalents. The prevalence of UA at DQA, DPA and DPB was evaluated in a sensitized regional population. The impact of adding these additional UA to cPRA was calculated alone and in combination, and compared to the baseline cPRA for UA at A, B, C, DR, DR51/52/53, and DQ. Of 740 sensitized transplant candidates, 18% of total and 32% with cPRA≥95% had DQA UA. Twenty‐seven percent of total and 54% with cPRA≥95% had DPB UA. Of 280/740 subjects with these UA, 36/280 (13%) had cPRA increase of >20% when they were included, 7% increased cPRA to ≥80% and 6% to ≥95%. Inclusion of DQA, DPA, and DPB UA in Canadian cPRA calculations improves the accuracy of cPRA where these are relevant in allocation.     

Should Pediatric Patients Wait for HLA‐DR‐Matched Renal Transplants?

Graft survival rates from deceased donors aged 35 years or less among all primary pediatric kidney transplant recipients in the United States between 1996 and 2004 were retrospectively examined to determine the effect of HLA‐DR mismatches on graft survival. Zero HLA‐DR‐mismatched kidneys had statistically comparable 5‐year graft survival (71%), to 1‐DR‐mismatched kidneys (69%) and 2‐DR‐mismatched kidneys (71%). When compared to donors less than 35 years of age, the relative rate of allograft failure was 1.32 (p = 0.0326) for donor age greater than or equal to age 35. There was no statistical increase in the odds of developing a panel‐reactive antibody (PRA) greater than 30% at the time of second waitlisting, based upon the degree of HLA‐A, ‐B or ‐DR mismatch of the first transplant, nor was there a ‘dose effect’ when more HLA antigens were mismatched between the donor and recipient. Therefore, pediatric transplant programs should utilize the recently implemented Organ Procurement and Transplantation Network's (OPTN) allocation policy, which prioritizes pediatric recipients to receive kidneys from deceased donors less than 35 years of age, and should not turn down such kidney offers to wait for a better HLA‐DR‐matched kidney.     

Unintended Consequences of the New National Kidney Allocation Policy in the United States

The new national Kidney Allocation System of the Organ Procurement and Transplantation Network (OPTN), effective as of December 4, 2014, was designed to improve the chances of transplanting the most highly sensitized patients on the waitlist, those with calculated panel reactive antibody values of 98%, 99% and 100%. Recently, it was suggested that these highly sensitized patients will experience inequitable access, given the reported high prevalence of antibodies to HLA‐DP, and the fact that only about 1/3 of deceased donors are typed for HLA‐DP antigens. Here we report that 320/2948 flow cytometric crossmatches performed for the Northwestern transplant program over the past 28 months were positive solely due to HLA‐DP donor‐specific antibodies (11%; 16.5% of patients with HLA antibodies—sensitized patients). We further show that 58/207 (12%) HLA‐DR serologically matched donor‐recipient pairs had a positive B cell flow crossmatch due to donor‐specific HLA class II antibodies, and 2/34 (6%) serologic zero‐HLA‐A‐B‐DR mismatch had a positive flow crossmatch due to HLA‐DSA. We therefore provide information regarding the necessity and importance of complete donor HLA typing including both chains of the HLA‐DP antigen (encoded by HLA‐DPA1 and HLA‐DPB1) at the time of organ offer.     

The impact of belatacept on third‐party HLA alloantibodies in highly sensitized kidney transplant recipients

Recent evidence suggests that belatacept reduces the durability of preexisting antibodies to class I and class II human leukocyte antigens (HLAs). In this case series of 163 highly sensitized kidney transplant candidates whose calculated panel‐reactive antibody (cPRA) activity was ≥98% to 100%, the impact of belatacept on preexisting HLA antibodies was assessed. Of the 163 candidates, 72 underwent transplantation between December 4, 2014 and April 15, 2017; 60 of these transplanted patients remained on belatacept consecutively for at least 6 months. We observed a decrease in the breadth and/or strength of HLA class I antibodies as assessed by FlowPRA in belatacept‐treated patients compared to controls who did not receive belatacept. Specifically, significant HLA antibody reduction was evident for class I (P < .0009). Posttransplant belatacept‐treated patients also had a clinically significant reduction in their cPRA compared to controls (P < .01). Collectively, these findings suggest belatacept can reduce HLA class I antibodies in a significant proportion of highly sensitized recipients and could be an option to improve pretransplant compatibility with organ donors.     

Alloantibody Responses After Renal Transplant Failure Can Be Better Predicted by Donor–Recipient HLA Amino Acid Sequence and Physicochemical Disparities Than Conventional HLA Matching

We have assessed whether HLA immunogenicity as defined by differences in donor–recipient HLA amino‐acid sequence (amino‐acid mismatch score, AMS; and eplet mismatch score, EpMS) and physicochemical properties (electrostatic mismatch score, EMS) enables prediction of allosensitization to HLA, and also prediction of the risk of an individual donor–recipient HLA mismatch to induce donor‐specific antibody (DSA). HLA antibody screening was undertaken using single‐antigen beads in 131 kidney transplant recipients returning to the transplant waiting list following first graft failure. The effect of AMS, EpMS, and EMS on the development of allosensitization (calculated reaction frequency [cRF]) and DSA was determined. Multivariate analyses, adjusting for time on the waiting list, maintenance on immunosuppression after transplant failure, and graft nephrectomy, showed that AMS (odds ratio [OR]: 1.44 per 10 units, 95% CI: 1.02–2.10, p = 0.04) and EMS (OR: 1.27 per 10 units, 95% CI: 1.02–1.62, p = 0.04) were independently associated with the risk of developing sensitization to HLA (cRF > 15%). AMS, EpMS, and EMS were independently associated with the development of HLA‐DR and HLA‐DQ DSA, but only EMS correlated with the risk of HLA‐A and ‐B DSA development. Differences in donor–recipient HLA amino‐acid sequence and physicochemical properties enable better assessment of the risk of HLA‐specific sensitization than conventional HLA matching.     

Calculated PRA: Initial Results Show Benefits for Sensitized Patients and a Reduction in Positive Crossmatches

The calculated panel reactive antibody (CPRA), which is based upon unacceptable HLA antigens listed on the waitlist form for renal transplant candidates, replaced PRA as the measure of sensitization among US renal transplant candidates on October 1, 2009. An analysis of the impact of this change 6 months after its implementation shows an 83% reduction in the number of kidney offers declined nationwide because of a positive crossmatch. The increasing acceptance and utilization of unacceptable HLA antigens to avoid offers of predictably crossmatch‐positive donor kidneys has increased the efficiency of kidney allocation, resulting in a significant increase in the percentage of transplants to broadly sensitized (80+% PRA/CPRA) patients from 7.3% during the period 07/01/2001–6/30/2002 to 15.8% of transplants between 10/1/09–3/31/10. The transplant rates per 1000 active patient‐years on the waitlist also increased significantly for broadly sensitized patients after October 1, 2009. These preliminary results suggest that ‘virtual’ positive crossmatch prediction based on contemporary tools for identifying antibodies directed against HLA antigens is effective, increases allocation efficiency and improves access to transplants for sensitized patients awaiting kidney transplantation.     

HLA‐DR/DQ molecular mismatch: A prognostic biomarker for primary alloimmunity

Alloimmune risk stratification in renal transplantation has lacked the necessary prognostic biomarkers to personalize recipient care or optimize clinical trials. HLA molecular mismatch improves precision compared to traditional antigen mismatch but has not been studied in detail at the individual molecule level. This study evaluated 664 renal transplant recipients and correlated HLA‐DR/DQ single molecule eplet mismatch with serologic, histologic, and clinical outcomes. Compared to traditional HLA‐DR/DQ whole antigen mismatch, HLA‐DR/DQ single molecule eplet mismatch improved the correlation with de novo donor‐specific antibody development (area under the curve 0.54 vs 0.84) and allowed recipients to be stratified into low, intermediate, and high alloimmune risk categories. These risk categories were significantly correlated with primary alloimmune events including Banff ≥1A T cell–mediated rejection (P = .0006), HLA‐DR/DQ de novo donor‐specific antibody development (P < .0001), antibody‐mediated rejection (P < .0001), as well as all‐cause graft loss (P = .0012) and each of these correlations persisted in multivariate models. Thus, HLA‐DR/DQ single molecule eplet mismatch may represent a precise, reproducible, and widely available prognostic biomarker that can be applied to tailor immunosuppression or design clinical trials based on individual patient risk.     

DQ molecules are the principal stimulators of de novo donor‐specific antibodies in nonsensitized pediatric recipients receiving a first kidney transplant

Data on the different HLA‐antibody (Ab) categories in pediatric kidney recipients developing de novo donor‐specific Abs (DSA) after transplantation are scarce. We retrospectively evaluated 82 consecutive nonsensitized pediatric recipients of a first kidney graft for de novo HLA Ab occurrence and antigen specificity. At a median follow‐up of 6 years, 29% of patients developed de novo DSA, while 45% had de novo non‐DSA. DSA appeared at 25‐month median time post‐transplant and were mostly directed toward HLA‐DQ antigens. Considering each HLA antigen, the estimated rate of DQ DSA (7.55 per 100 person‐years) was much higher than the rates observed for non‐DQ DSA. The HLA‐DQ Ab recognized determinants of the DQβ chain in 70% of cases, α chain in 25% of cases, and both chains in one patient. Non‐DSA peaked earlier than DSA, and were largely directed against HLA class I specificities that belonged to HLA‐A‐ and HLA‐B‐related cross‐reacting epitope groups (CREG) in 56% of cases. Our results indicate a need for evaluating HLA‐DQ compatibilities in kidney allocation, in order to minimize post‐transplant development of de novo DSA, known to be responsible for antibody‐mediated rejection and graft loss.     

The 6‐year clinical outcomes for patients registered in a multiregional United States Kidney Paired Donation program ‐ a retrospective study

We examined what happened during a 6‐year period to 1121 end‐stage renal disease patients who registered with their willing/incompatible living donors for kidney exchanges with the Alliance for Paired Donation (APD). Of all patients, 65% were transplanted: 37% in kidney paired donation (APD‐KPD, APD‐other‐KPD); 10% with compatible live donors (APD‐LD); and 18% with deceased donors (APD‐DD). The remaining patients were withdrawn (sick/died/others; 15%), or were still waiting (20%). For those patients with a cPRA 0–94%, 72% received a transplant. In contrast, only 49% of very highly sensitized (VHS; cPRA 95–100%) were transplanted. Of the VHS patients, 50% were transplanted by KPD/APD‐LD while 50% benefited through prioritization of deceased donors in the modified kidney allocation system (KAS introduced in 2014). All APD transplanted groups had similar death‐censored 4‐year graft survivals as their relevant Organ Procurement and Transplantation Network (OPTN) groups. It is noteworthy that VHS graft and patient survival results were comparable to less sensitized and nonsensitized patients. All patients should be encouraged to search for compatible donors through different options. Expanding the donor pool through KPD and the new KAS of the OPTN increases the likelihood of transplantation for VHS patients.     

HLA Epitopes as Viewed by Antibodies: What Is it All About?

The need for new approaches to define HLA antibodies, in the context of organ transplantation, is intensely debated among HLA professionals. In this review, we sought to provide background and perspective to current understanding of the immunogenicity of HLA mismatches with respect to the humoral alloimmune response and the definition of B cell epitopes. Initial data suggest that epitope matching not only assists in defining better matches for the current transplant, but also minimizes the risk of developing de novo HLA‐donor‐specific‐antibodies posttransplant. In other words, other than lowering the risk of current graft rejection, epitope matching is likely to lower overall future sensitization levels and thus increases the likelihood of finding a compatible donor when the need for a retransplantation arises. More detailed knowledge of epitopes makes it possible to investigate what constitutes permissible versus non‐permissible HLA mismatches. The currently available evidence suggest that epitope matching is the most rational way to decrease the risk of HLA‐linked transplant rejection. This review is aimed at stimulating further and more intense collaborative effort in this field.     

Corrected Panel-Reactive Antibody Positivity Rates for Hypersensitized Patients in Turkish Population With Calculated Panel-Reactive Antibody Software

Introduction

High rates of panel-reactive antibody (PRA) may decrease the chance of kidney transplantation and may result in long waiting periods before transplantation. The calculated PRA (cPRA) is performed based on unacceptable HLA antigens. These antigens are identified by a program that was created based on the antibodies that developed against the HLA antigens circulating in serum and on the risk of binding of these antibodies to antigens. The antigen profile of the population and antigen frequencies can be measured, and more realistic cPRA positivity rates may be obtained using this method.

The quest to decipher HLA immunogenicity: Telling friend from foe

Molecular mismatch load analysis was recently introduced as a means for performing risk stratification following organ transplantation. However, although good correlation was demonstrated between molecular mismatch load and generation of de novo donor‐specific HLA antibody (DSA), quite a few exceptions exist, and the underlying factors that define HLA immunogenicity remain unclear. Herein, we present a new paradigm to interrogate differences between molecular mismatches that lead to the generation of de novo DSA and those that do not (the 2MM1DSA cohort). Specifically, patients transplanted across 2 HLA‐DQ mismatches, who formed de novo DSA only to one mismatch (foe) but not the other (friend), provide a unique environment in which patient‐specific factors that affect the immune response other than immunogenicity, such as infection and immunosuppression, can be controlled for. It further permits focusing on mismatches uniquely exhibited by the de novo DSA allele, rather than mismatches shared by both DSA and non‐DSA alleles. This concept paper illustrates several examples, highlights the need for center‐specific or population‐specific cutoff values for posttransplant risk stratification, and mostly argues that if there is no direct correlation between molecular mismatch load and immunogenicity, then molecular mismatch load must not be adopted as an approach for equitable organ allocation.     

Assessing the utilization of high‐resolution 2‐field HLA typing in solid organ transplantation

HLA typing in solid organ transplantation (SOT) is necessary for determining HLA‐matching status between donor‐recipient pairs and assessing patients’ anti‐HLA antibody profiles. Histocompatibility has traditionally been evaluated based on serologically defined HLA antigens. The evolution of HLA typing and antibody identification technologies, however, has revealed many limitations with using serologic equivalents for assessing compatibility in SOT. The significant improvements to HLA typing introduced by next‐generation sequencing (NGS) require an assessment of the impact of this technology on SOT. We have assessed the role of high‐resolution 2‐field HLA typing (HR‐2F) in SOT by retrospectively evaluating NGS‐typed pre‐ and post‐SOT cases. HR‐2F typing was highly instructive or necessary in 41% (156/385) of the cases. Several pre‐ and posttransplant scenarios were identified as being better served by HR‐2F typing. Five different categories are presented with specific case examples. The experience of another center (Temple University Hospital) is also included, whereby 21% of the cases required HR‐2F typing by Sanger sequencing, as supported by other legacy methods, to properly address posttransplant anti‐HLA antibody issues.     

Donor Specificity but Not Broadness of Sensitization Is Associated With Antibody‐Mediated Rejection and Graft Loss in Renal Allograft Recipients

Panel‐reactive antibodies are widely regarded as an important immunological risk factor for rejection and graft loss. The broadness of sensitization against HLA is most appropriately measured by the “calculated population‐reactive antibodies” (cPRA) value. In this study, we investigated whether cPRA represent an immunological risk in times of sensitive and accurate determination of pretransplantation donor‐specific HLA antibodies (DSA). Five hundred twenty‐seven consecutive transplantations were divided into four groups: cPRA 0% (n = 250), cPRA 1–50% (n = 129), cPRA 51–100% (n = 43), and DSA (n = 105). Patients without DSA were considered as normal risk and received standard immunosuppression without T cell–depleting induction. Patients with DSA received an enhanced induction therapy and maintenance immunosuppression. Surveillance biopsies were performed at 3 and 6 months. Median follow‐up was 5.7 years. Among the three cPRA groups, there were no differences regarding the 1‐year incidence of ABMR (p = 0.16) and TCMR (p = 0.75). The 5‐year allograft survival rates were similar and around 87% (p = 0.28). The estimated glomerular filtration rate at last follow‐up was 50–53 mL/min (p = 0.45). On multivariable Cox proportional hazard analysis, the strongest independent predictor for ABMR and (death‐censored) graft survival was pretransplantation DSA. cPRA were not predictive for ABMR, TCMR, or (death‐censored) graft survival. We conclude that with current DSA assignment, the broadness of sensitization measured by cPRA does not imply an immunological risk.     

Coronavirus disease 2019: Utilizing an ethical framework for rationing absolutely scarce health‐care resources in transplant allocation decisions

The novel coronavirus disease 2019 (COVID‐19) is impacting transplant programs around the world, and, as the center of the pandemic shifts to the United States, we have to prepare to make decisions about which patients to transplant during times of constrained resources. In this paper, we discuss how to transition from the traditional justice versus utility consideration in organ allocation to a more nuanced allocation scheme based on ethical values that drive decisions in times of absolute scarcity. We recognize that many decisions are made based on the practical limitations that transplant programs face, especially at the extremes. As programs make the transition from a standard approach to a resource‐constrained approach to transplantation, we utilize a framework for ethical decisions in settings of absolutely scarce resources to help guide programs in deciding which patients to transplant, which donors to accept, how to minimize risk, and how to ensure the best utilization of transplant team members.     

The Synergistic Effect of Class II HLA Epitope‐Mismatch and Nonadherence on Acute Rejection and Graft Survival

Predicting long‐term outcomes in renal transplant recipients is essential to optimize medical therapy and determine the frequency of posttransplant histologic and serologic monitoring. Nonadherence and human leukocyte antigen (HLA) mismatch are risk factors that have been associated with poor long‐term outcomes and may help individualize care. In the present study, class II HLA mismatches were determined at the HLA epitope level in 195 renal transplant recipients in whom medication adherence was prospectively measured using electronic monitors in medication vial caps. Recipients were grouped by medication adherence and high (≥10 HLA‐DR, ≥17 HLA‐DQ) or low epitope‐mismatch load. We found that the combination of higher epitope mismatch and poor adherence acted synergistically to determine the risk of rejection or graft loss. Nonadherent recipients with HLA‐DR epitope mismatch ≥10 had increased graft loss (35% vs. 8%, p < 0.01) compared to adherent recipients with low epitope mismatch. At the HLA‐DQ locus nonadherent recipients with HLA‐DQ epitope mismatch ≥17 had increased graft loss (33% vs. 10%, p < 0.01) compared to adherent recipients with low epitope mismatch. Subclinical nonadherence early posttransplant combined with HLA class II epitope mismatch may help identify recipients that could benefit from increased clinical, histologic, and serologic monitoring.     

Outcomes in the highest panel reactive antibody recipients of deceased donor kidneys under the new kidney allocation system

Since the institution of the new kidney allocation system in December 2014, kidney transplant candidates with the highest calculated panel reactive antibodies (cPRA) of 99‐100 have been transplanted at much higher rates. However, concerns have been raised that outcomes in these patients might be impaired due to higher immunological risk and longer cold ischemia times resulting from long‐distance sharing of kidneys. Here, we compare outcomes at the University of Wisconsin between study patients with cPRA 99‐100 and all other recipients of deceased donor kidneys transplanted between 12/04/2014 and 12/31/2015. All patients had at least 6 months post‐transplant follow‐up. The mean follow‐up was 13.9±3 months in cPRA ≥99% and 12.3±3.5 months in cPRA ≤98%. There was a total of 152 transplants, 25 study patients, and 127 controls. No statistically significant differences were found between the two groups in delayed graft function, rejection, kidney function, graft and patient survival, or infections. We conclude that transplanting the most highly sensitized patients with kidneys shared outside their local donation service areas is associated with excellent short‐term outcomes that are comparable to controls.     

Mastering the Risk of HLA Antibodies in Kidney Transplantation: An Algorithm Based on Pretransplant Single‐Antigen Flow Bead Techniques

The utilization of sensitive techniques of detection of HLA antibodies to define and measure sensitization has greatly evolved in recent years. We present here an approach to minimize the risk of HLA antibodies in kidney transplantation based on the evaluation of graft accessibility of sensitized patients by calculated PRA (cPRA) and estimation of potential matched donors (PMD) using a national simulation software program. This study included all registered patients on our waiting list (WL) for deceased donor (DD) kidney transplants. All patients were screened by single‐antigen flow bead (SAFB) techniques. Of the 502 registered patients, 174 (34.7%) were sensitized. Among these, 48.3% (84 pts) had a cPRA>85%. For 75.3% of sensitized patients (90 pts with cPRA≤85% and 41 pts with cPRA>85%), the flow of PMD was considered sufficient to allow a transplant avoiding all unacceptable antigens. The 41 patients with a cPRA>85% (48.8%) had a satisfactory donor flow in the framework of the national prioritization program for highly sensitized patients. Finally, 43 sensitized patients (24.7%) were deemed eligible for a strategy of higher immunological risk through desensitization protocols or transplantation against HLA‐DSAs. This approach provides a logical and systematic strategy to rationalize the access of sensitized patients to kidney transplantation minimizing the risk of HLA antibodies.     

Evidence for the alloimmune basis and prognostic significance of Borderline T cell–mediated rejection

Prognostic biomarkers of T cell–mediated rejection (TCMR) have not been adequately studied in the modern era. We evaluated 803 renal transplant recipients and correlated HLA‐DR/DQ molecular mismatch alloimmune risk categories (low, intermediate, high) with the severity, frequency, and persistence of TCMR. Allograft survival was reduced in recipients with Banff Borderline (hazard ratio [HR] 2.4, P = .003) and Banff ≥ IA TCMR (HR 4.3, P < .0001) including a subset who never developed de novo donor‐specific antibodies (P = .002). HLA‐DR/DQ molecular mismatch alloimmune risk categories were multivariate correlates of Banff Borderline and Banff ≥ IA TCMR and correlated with the severity and frequency of rejection episodes. Recipient age, HLA‐DR/DQ molecular mismatch category, and cyclosporin vs tacrolimus immunosuppression were independent correlates of Banff Borderline and Banff ≥ IA TCMR. In the subset treated with tacrolimus (720/803) recipient age, HLA‐DR/DQ molecular mismatch category, and tacrolimus coefficient of variation were independent correlates of TCMR. The correlation of HLA‐DR/DQ molecular mismatch category with TCMR, including Borderline, provides evidence for their alloimmune basis. HLA‐DR/DQ molecular mismatch may represent a precise prognostic biomarker that can be applied to tailor immunosuppression or design clinical trials based on individual patient risk.