Specificity, strength, and evolution of pretransplant donor-specific HLA antibodies determine outcome after kidney transplantation

Pathophysiology of rejection in kidney transplantation

INTRODUCTION

Rejection remains a major obstacle to successful kidney transplantation. The complex pathophysiology of rejection depends on a fine-tuned interplay between the innate and adaptive immune systems.

AREAS COVERED

This review provides a comprehensive analysis of the pathophysiology of rejection of kidney grafts, performed through careful selection of most relevant papers available on the topic in the PubMed database. The two types of rejection usually observed at the kidney biopsy, i.e. cellular and humoral rejection, are described with an accurate outline of the biological processes that lead to their development.

EXPERT OPINION

The incidence of T-cell-mediated rejection is decreasing, and most cases promptly respond to appropriate immunosuppression. However, late diagnosis or incomplete response to treatment may have deleterious consequences in the long term. The main issue is represented by antibody-mediated rejection, which unsatisfactorily responds to aggressive immunosuppression, especially when diagnosed late. Prevention of acute ABMR rests on HLA-specific antibody detection prior to transplantation, adequate immunosuppression, and optimal patients' compliance. Late diagnosis and poor response to treatment inevitably lead to chronic ABMR, for which no therapies are currently available.

Comparative outcomes of DSA positive, crossmatch negative living donor kidney transplants versus remaining on the waitlist for an HLA compatible deceased donor

INTRODUCTION

The clinical relevance of preformed donor specific antibodies in the setting of a negative crossmatch (DSA + XM-) remains controversial. In this study we investigate the outcomes of patients with a DSA + XM- living donor (LDi) who proceeded with an HLA-incompatible (HLAi) transplant compared with those who waited for an HLA-compatible deceased donor (DDc).

MATERIALS AND METHODS

We investigated 359 patients on the transplant waiting list who had at least one potential HLAi living donor, from which 203 DSA + XM- pairs were identified and outcomes analysed.

RESULTS

Out of 203 patients, 96 (47.3%) received a LD transplant: 52/96 (54.2%) a LDi, and 44/96 (45.8%) an alternative compatible LD. In addition, 107 patients out of 203(52.7%) waited for a DDc, of which 47(43.9%) were subsequently transplanted. Our adjusted analysis showed that the LDi transplantation did not offer a superior patient survival over waiting for a DDc transplant. For those transplanted, there was no difference in patient (p = 0.065) or death censored allograft survival (p = 0.37) between DDc and LDi recipients. However, there was a higher incidence of acute allograft rejection (p = 0.043) and antibody-mediated rejection (p = 0.005) in the LDi group. Having a high pre-transplant calculated reaction frequency and preformed DSA to both class I and class II antigens were associated with inferior outcomes in the LDi transplants.

CONCLUSIONS

Given the lack of difference in allograft survival between LDi and DDc transplants, in the absence of an alternative compatible living donor, proceeding with a LDi should be supported despite a higher rejection risk, providing individual risk assessment and shared decision making is undertaken.

Anti-HLA Class II Antibodies Are the Most Resistant to Desensitization in Crossmatch-positive Living-donor Kidney Transplantations: A Patient Series

Background

In HLA-incompatible kidney transplantation, the efficacy of desensitization in terms of anti-HLA antibody kinetics is not well characterized. We present an overview of the course of anti-HLA antibodies throughout plasma exchange (PE) desensitization in a series of crossmatch-positive patients.

Methods

All consecutive candidates in the Dutch HLA-incompatible kidney transplantation program between November 2012 and January 2022 were included. The eligibility criteria were a positive crossmatch with a living kidney donor and no options for compatible transplantation. Desensitization consisted of 5-10 PE with low-dose IVIg.

Results

A total of 16 patient-donor pairs were included. Patients had median virtual panel-reactive antibody of 99.58%. Cumulative donor-specific anti-HLA antibody (cumDSA) mean fluorescence intensity (MFI) was 31 399 median, and immunodominant DSA (iDSA) MFI was 18 677 for class I and 21 893 for class II. Median anti-HLA antibody MFI response to desensitization was worse in class II as compared with class I (P < 0.001), particularly for HLA-DQ. Class I cumDSA MFI decreased 68% after 4 PE versus 53% in class II. The decrease between the fifth and the 10th PE sessions was modest with 21% in class I versus 9% in class II. Antibody-mediated rejection occurred in 85% of patients, with the iDSA directed to the same mismatched HLA as before desensitization, except for 3 patients, of whom 2 had vigorous rebound of antibodies to repeated mismatches (RMMs). Rebound was highest (86%) in RMM-DSA with prior grafts removed (transplantectomy n = 7), lower (39%) in non-RMM-DSA (n = 30), and lowest (11%) for RMM-DSA with in situ grafts (n = 5; P = 0.018 for RMM-DSA transplantectomy versus RMM-DSA graft in situ). With a median follow-up of 59 mo, 1 patient had died resulting in a death-censored graft survival of 73%.

Conclusions

Patients with class II DSA, and particularly those directed against HLA-DQ locus, were difficult to desensitize.

Highly Sensitized Kidney Transplant Outcomes After the 2014 Kidney Allocation System Change

Introduction: Kidney Allocation System (KAS) was implemented by United Network for Organ Sharing in 2014 to reduce allocation disparities. Research Questions: Outcomes of highly sensitized patients (calculated panel reactive antibody (cPRA) ≥ 97%) before and after KAS were compared to low-risk recipients (cPRA <10%) in the post-KAS era were examined. The impact on racial disparities was determined. Design: This was a retrospective study of national registry data. Two cohorts of adult candidates waitlisted for deceased donor transplantation during 3-year periods before and after KAS were identified. Results: Highly sensitized patients (N = 1238 and 4687) received a deceased donor kidney transplant between January 1, 2011 and December 31, 2013 and between January 1, 2015 and December, 31, 2017. Racial disparity for highly sensitized patients improved, yet remained significant (P < 0.001), with Black patients comprising 40% and 41% of the highly sensitized candidates and 28% and 34% of the recipients pre- and post-KAS. While posttransplant death-censored graft failure for highly sensitized recipients was similar overall, post-KAS was associated with improved graft survival in the first year after transplant (HR 0.56, 95% CI 0.40-0.78). When compared to contemporaneous lowrisk recipients, both death-censored and all-cause graft failure were similar for highly sensitized recipients and was associated with increased risk for death-censored graft failure beyond the first year (HR 1.39, 95% CI 1.11-1.73). Conclusion: The allocation system led to an increase in transplantation in highly sensitized candidates without compromising outcomes. Although KAS has led to more balanced transplant rates between highly sensitized Black and White patients, racial inequalities persist.

Chronic Rejection After Kidney Transplantation

In kidney transplantation, ongoing alloimmune processes-commonly triggered by HLA incompatibilities-can trigger chronic transplant rejection, affecting the microcirculation and the tubulointerstitium. Continuous inflammation may lead to progressive, irreversible graft injury, culminating in graft dysfunction and accelerated transplant failure. Numerous experimental and translational studies have delineated a complex interplay of different immune mechanisms driving rejection, with antibody-mediated rejection (AMR) being an extensively studied rejection variant. In microvascular inflammation, a hallmark lesion of AMR, natural killer (NK) cells have emerged as pivotal effector cells. Their essential role is supported by immunohistologic evidence, bulk and spatial transcriptomics, and functional genetics. Despite significant research efforts, a substantial unmet need for approved rejection therapies persists, with many trials yielding negative outcomes. However, several promising therapies are currently under investigation, including felzartamab, a monoclonal antibody targeting the surface molecule CD38, which is highly expressed in NK cells and antibody-producing plasma cells. In an exploratory phase 2 trial in late AMR, this compound has demonstrated potential in resolving molecular and morphologic rejection activity and injury, predominantly by targeting NK cell effector function. These findings inspire hope for effective treatments and emphasize the necessity of further pivotal trials focusing on chronic transplant rejection.

A simple predictor for donor-specific anti-HLA antibody desensitisation in haploidentical haematopoietic stem cell transplantation

Donor-specific HLA antibody (DSA) has been recognised as an independent risk factor for graft failure in patients undergoing haploidentical haematopoietic stem cell transplantation (HID HSCT). Therapeutic plasma exchange (TPE), as a first-line strategy for DSA desensitisation, can promptly reduce serum DSA levels. This study aimed to investigate DSA characteristics and identify a biomarker predicting the efficacy of DSA desensitisation in patients proceeding to HID HSCT. We retrospectively enrolled 32 patients with DSA from April 2021 to January 2024, and analysed the mean fluorescence intensity (MFI) value of DSA at the different time points of desensitisation treatment. Compared with baseline DSA level before TPE, the median MFI of HLA class I DSA was reduced from 8178.6 to 795.3 (p < 0.001), and HLA class II DSA decreased from 6210.9 to 808.8 (p < 0.001) after TPE. The DSA level in 1:16 diluted pre-TPE serum correlated well with DSA value in post-TPE serum (class I, r = 0.85, p < 0.0001; class II, r = 0.94, p < 0.0001), predicting TPE efficacy in 84.4% of patients. Based on the degree of DSA reduction after TPE, patients were divided into complete responders (decreased by >70%), partial responders (decreased by 30 to 70%) and non-responders (decreased by <30%) and the percentages were 43.8%, 25% and 31.2%, respectively. Non-responders receiving aggressive immunotherapy had longer overall survival compared to those receiving standard strategies (p < 0.05). The 1:16 diluted pre-TPE serum may predict the efficacy of TPE and allow for more rational immunotherapy strategy for patients with DSA proceeding to HID HSCT.

Relevance of Proteinuria in Kidney Transplant Recipients and Allograft Outcomes

BACKGROUND

The onset of proteinuria in renal allograft recipients is frequently associated with an increased risk for both graft failure and mortality. We investigated the risk associated with post-transplant proteinuria and its time-dynamics in a select group treated for biopsy proven antibody-mediated rejection (ABMR).

METHODS

Eighty-five patients who underwent transplantation were enrolled in our study and followed up from transplantation until October 31, 2020, death, or the date of the return to dialysis. We created two main groups: the ABMR group (n = 19) and an ABMR-negative control group with stable kidney function (n = 52) without donor-specific antibodies (DSA) and a subgroup with DSAs but stable graft function (n = 14) without ABMR. Differences in patient, donor, and transplant graft characteristics between the groups were assessed by Fisher's exact test for categorical variables. Death-censored graft loss was evaluated with the help of Kaplan-Meier analysis using log risk statistics.

RESULTS

Proteinuria decreased after treatment in the ABMR group (P < .0009). Pre-treatment every 10 mg/mmol increase in proteinuria was associated with a 7% increase in the risk for graft failure in the ABMR group. The estimated 3-year graft survival was 87.5% in the ABMR group, compared to 93% in the group without ABMR but with pre-formed DSA, and 100% in the DSA negative subgroup (log-rank probe P < .0666).

CONCLUSION

Proteinuria is an independent predictor for graft failure, can be lowered by treatment for ABMR but ABMR is associated with lower graft survival in our study population.

Blood Gene Signature as a Biomarker for Subclinical Kidney Allograft Rejection: Where Are We?

The observation decades ago that inflammatory injuries because of an alloimmune response might be present even in the absence of concomitant clinical impairment in allograft function conduced to the later definition of subclinical rejection. Many studies have investigated the different subclinical rejections defined according to the Banff classification (subclinical T cell-mediated rejection and antibody-mediated rejection), overall concluding that these episodes worsened long-term allograft function and survival. These observations led several transplant teams to perform systematic protocolar biopsies to anticipate treatment of rejection episodes and possibly prevent allograft loss. Paradoxically, the invasive characteristics and associated logistics of such procedures paved the way to investigate noninvasive biomarkers (urine and blood) of subclinical rejection. Among them, several research teams proposed a blood gene signature developed from cohort studies, most of which achieved excellent predictive values for the occurrence of subclinical rejection, mainly antibody-mediated rejection. Interestingly, although all identified genes relate to immune subsets and pathways involved in rejection pathophysiology, very few transcripts are shared among these sets of genes, highlighting the heterogenicity of such episodes and the difficult but mandatory need for external validation of such tools. Beyond this, their application and value in clinical practice remain to be definitively demonstrated in both biopsy avoidance and prevention of clinical rejection episodes. Their combination with other biomarkers, either epidemiological or biological, could contribute to a more accurate picture of a patient's risk of rejection and guide clinicians in the follow-up of kidney transplant recipients.

A personalised delisting strategy enables successful kidney transplantation in highly sensitised patients with preformed donor-specific anti HLA antibodies

This study investigates kidney transplant outcomes in highly sensitised patients after implementing a delisting strategy aimed at enabling transplantation despite preformed donor-specific antibodies (preDSA), with the goal of reducing acute antibody-mediated rejection (aAMR) risk. Fifty-three sensitised recipients underwent kidney transplant after delisting prohibited HLA antigens, focusing initially in low MFI antibodies (<5000), except for anti-HLA-DQ. If insufficient, higher MFI antibodies were permitted, especially for those without an immunogenic eplet pattern assigned. Delisting of Complement-fixing antibodies (C1q+) was consistently avoided. Comparison cohorts included 53 sensitised recipients without DSA (SwoDSA) and 53 non-sensitised (NS). The average waiting time prior to delisting was 4.4 ± 1.8 years, with a reduction in cPRA from 99.7 ± 0.5 to 98.1 ± 0.7, followed by transplantation within 7.2 ± 8.0 months (analysed in 34 patients). Rejection rates were similar among preDSA, SwoDSA, and NS groups (16%, 8%, and 11%, respectively; p = 0.46). However, aAMR was higher in the preDSA group (12%, 4%, and 2%, respectively; p = 0.073), only presented in recipients with DSA of MFI >5000. The highest MFI DSA were against HLA-DP (Median: 10796 MFI), with 50% of preDSA aAMR cases due to anti-DP antibodies (n = 3). Graft survival rates at 1 and 5 years in preDSA group were 94%, and 67%, comparable to SwoDSA (94%, and 70%; p = 0.69), being significantly higher in the NS group (p = 0.002). The five-year recipient survival rate was 89%, comparable to SwoDSA and NS groups (p = 0.79). A delisting strategy enables safe kidney transplant in highly sensitised patients with preDSA, with a slight increase in aAMR and comparable graft and patient survivals to non-DSA cohorts.

Sensitive HLA antibody testing and the risk of antibody-mediated rejection and graft failure

Solid phase detection and identification of HLA antibodies in kidney transplantation currently relies on single antigen bead (Luminex®) assays, which is more sensitive than the previously used enzyme-linked immunosorbent assays (ELISA). To evaluate the impact of more sensitive HLA testing on antibody-mediated rejection (AMR) occurrence and allograft survival, we analysed 1818 renal allograft recipients transplanted between March 2004 and May 2021. In 2008, solid phase testing switched from ELISA to Luminex. We included 393 (21.6%) transplantations before and 1425 (78.4%) transplantations after transition from ELISA- to Luminex-based testing. For this study, bio-banked ELISA era samples were tested retrospectively with Luminex. Significantly less pretransplant DSA were found in patients transplanted with pre-existing HLA antibodies in the Luminex (109/387) versus the ELISA period (43/90) (28% vs. 48%, p < 0.01). Throughout histological follow-up, 169 of 1818 (9.3%) patients developed AMR. After implementing Luminex-based testing, the rate of AMR significantly decreased (p = 0.003). However, incidence of graft failure did not significantly differ between both eras. In conclusion, less patients with pretransplant DSA were transplanted since the implementation of Luminex HLA testing. Transition from ELISA- to Luminex-based HLA testing was associated with a significant decrease in AMR occurrence post-transplantation. Since the decline of AMR did not translate into improved graft survival, Luminex-based testing has the added value of preventing low-risk AMR cases. Therefore, Luminex' high sensitivity must be balanced against waiting time for a suitable organ.

Current approaches for risk assessment of intestinal transplant patients: A view from the histocompatibility laboratory

Despite its recent decline in volumes, intestinal transplantation remains an important option for patients with irreversible intestinal failures. The long-term outcome of an intestinal transplant has stagnated. The major cause of graft loss is rejection, resulting from mismatches in human leukocyte antigens (HLA) and the presence of antibodies to mismatched donor-specific HLA antigens (DSA). Literature has reported that DSAs, either preformed before transplantation or developed de novo after transplantation, are harmful to intestinal grafts, especially for those without combined liver grafts. A comprehensive assessment of DSA by the histocompatibility laboratory is critical for successful intestinal transplantation and its long-term survival. This paper briefly reviews the history and current status of different methods for detecting DSA and their clinical applications in intestinal transplantation. The focus is on applying different antibody assays to manage immunologically challenging intestinal transplant patients before and after transplantation. A clinical case is presented to illustrate the complexity of HLA tests and the necessity of multiple assays. The review of risk assessment by the histocompatibility laboratory also highlights the need for close interaction between the laboratory and the intestinal transplant program.

Automated Urinary Chemokine Assays for Noninvasive Detection of Kidney Transplant Rejection: A Prospective Cohort Study

RATIONALE & OBJECTIVE

Prior studies have demonstrated the diagnostic potential of urinary chemokines C-X-C motif ligand 9 (CXCL9) and CXCL10 for kidney transplant rejection. However, their benefit in addition to clinical information has not been demonstrated. We evaluated the diagnostic performance for detecting acute rejection of urinary CXCL9 and CXCL10 when integrated with clinical information.

STUDY DESIGN

Single-center prospective cohort study.

SETTING & PARTICIPANTS

We analyzed 1,559 biopsy-paired urinary samples from 622 kidney transplants performed between April 2013 and July 2019 at a single transplant center in Belgium. External validation was performed in 986 biopsy-paired urinary samples.

TESTS COMPARED

We quantified urinary CXCL9 (uCXCL9) and CXCL10 (uCXCL10) using an automated immunoassay platform and normalized the values to urinary creatinine. Urinary chemokines were incorporated into a multivariable model with routine clinical markers (estimated glomerular filtration rate, donor-specific antibodies, and polyoma viremia) (integrated model). This model was then compared with the tissue diagnosis according to the Banff classification for acute rejection.

OUTCOME

Acute rejection detected on kidney biopsy using the Banff classification.

RESULTS

Chemokines integrated with routine clinical markers had high diagnostic value for detection of acute rejection (n=150) (receiver operating characteristic area under the curve 81.3% [95% CI, 77.6-85.0]). The integrated model would help avoid 59 protocol biopsies per 100 patients when the risk for rejection is predicted to be below 10%. The performance of the integrated model was similar in the external validation cohort.

LIMITATIONS

The cross-sectional nature obviates investigating the evolution over time and prediction of future rejection.

CONCLUSIONS

The use of an integrated model of urinary chemokines and clinical markers for noninvasive monitoring of rejection could enable a reduction in the number of biopsies. Urinary chemokines may be useful noninvasive biomarkers whose use should be further studied in prospective randomized trials to clarify their role in guiding clinical care and the use of biopsies to detect rejection after kidney transplantation.

PLAIN-LANGUAGE SUMMARY

Urinary chemokines CXCL9 and CXCL10 have been suggested to be good noninvasive biomarkers of kidney transplant rejection. However, defining a context of use and integration with clinical information is necessary before clinical implementation can begin. In this study, we demonstrated that urinary chemokines CXCL9 and CXCL10, together with clinical information, have substantial diagnostic accuracy for the detection of acute kidney transplant rejection. Application of urinary chemokines together with clinical information may guide biopsy practices following kidney transplantation and potentially reduce the need for kidney transplant biopsies.

The impact of pretransplant suspected HLA antibody on the long-term outcome of the graft kidney: A retrospective cohort study

INTRODUCTION

The preoperative examination of kidney transplantation includes HLA antibody screening to initially determine the presence of preexisting donor-specific antibody (DSA) that mediates hyperacute rejection. Recipients with positive HLA antibodies require further HLA specificity analysis to type the antigen and determine the antigen mismatches between the donor and recipient. However, recipients with suspected antibodies would have no further HLA specificity analysis. It is unclear whether suspected HLA antibodies would affect renal graft function. This study aimed to explore the impact of pretransplant suspected HLA antibody on the long-term outcome of the graft kidney and thus determine the necessity of routinely performing the HLA specificity analysis in recipients with suspected HLA antibodies preoperatively.

METHODS

This is a single-center retrospective cohort study. 179 kidney transplant recipients (KTRs) were included and further divided into HLA antibody-negative group (Group 1) and HLA antibody-suspected groups (Group 2) based on the result of the pretransplant HLA antibody screen test. And the antibody-suspected group was further divided into a low-mismatched group (Group A) and a high-mismatched group (Group B) according to the HLA specificity analysis. We tracked the renal function indexes, biochemical indexes, and posttransplant adverse events within 5 years after transplantation and explored the necessity of further HLA specificity analysis in recipients with pretransplant suspected HLA antibodies.

RESULTS

There was no statistically significant difference in demographics between HLA antibody-negative group and HLA antibody-suspected groups. At 5 years of follow-up, the KTRs in HLA antibody-negative group had significantly higher eGFR levels, lower serum creatinine levels, and less urinary protein compared to those in antibody-suspected group. Meanwhile, the KTRs in low-mismatched group also had significantly higher eGFR levels, lower serum creatinine levels, and less proteinuria compared to those in high-mismatched group. Correlation analysis showed that the age of KTRs, urinary protein levels and the load capacity of HLA mismatches were associated with eGFR levels of KTRs at 5 year posttransplant.

CONCLUSION

KTRs with suspected HLA antibodies before kidney transplantation have worse graft function than the preoperative HLA antibody-negative recipients in the long-term posttransplant follow-up. The specific load capacity of HLA mismatches, the age of the recipient and the urinary protein was found to be negatively correlated with long-term posttransplant renal outcomes. It is necessary to undergo further HLA specificity analysis for recipients with suspected HLA antibodies in HLA antibody screen test to explicit HLA mismatches and improve long-term posttransplant outcomes.

HLA class I peptide polymorphisms contribute to class II DQβ0603:DQα0103 antibody specificity

Antibodies reactive to human leukocyte antigens (HLA) represent a barrier for patients awaiting transplantation. Based on reactivity patterns in single-antigen bead (SAB) assays, various epitope matching algorithms have been proposed to improve transplant outcomes. However, some antibody reactivities cannot be explained by amino acid motifs, leading to uncertainty about their clinical relevance. Antibodies against the HLA class II molecule, DQβ0603:DQα0103, present in some candidates, represent one such example. Here, we show that peptides derived from amino acids 119-148 of the HLA class I heavy chain are bound to DQβ0603:DQα0103 proteins and contribute to antibody reactivity through an HLA-DM-dependent process. Moreover, antibody reactivity is impacted by the specific amino acid sequence presented. In summary, we demonstrate that polymorphic HLA class I peptides, bound to HLA class II proteins, can directly or indirectly be part of the antibody binding epitope. Our findings have potential important implications for the field of transplant immunology and for our understanding of adaptive immunity.

Determination of the clinical relevance of donor epitope-specific HLA-antibodies in kidney transplantation

In kidney transplantation, survival rates are still partly impaired due to the deleterious effects of donor specific HLA antibodies (DSA). However, not all luminex-defined DSA appear to be clinically relevant. Further analysis of DSA recognizing polymorphic amino acid configurations, called eplets or functional epitopes, might improve the discrimination between clinically relevant vs. irrelevant HLA antibodies. To evaluate which donor epitope-specific HLA antibodies (DESAs) are clinically important in kidney graft survival, relevant and irrelevant DESAs were discerned in a Dutch cohort of 4690 patients using Kaplan-Meier analysis and tested in a cox proportional hazard (CPH) model including nonimmunological variables. Pre-transplant DESAs were detected in 439 patients (9.4%). The presence of certain clinically relevant DESAs was significantly associated with increased risk on graft loss in deceased donor transplantations (p < 0.0001). The antibodies recognized six epitopes of HLA Class I, 3 of HLA-DR, and 1 of HLA-DQ, and most antibodies were directed to HLA-B (47%). Fifty-three patients (69.7%) had DESA against one donor epitope (range 1-5). Long-term graft survival rate in patients with clinically relevant DESA was 32%, rendering DESA a superior parameter to classical DSA (60%). In the CPH model, the hazard ratio (95% CI) of clinically relevant DESAs was 2.45 (1.84-3.25) in deceased donation, and 2.22 (1.25-3.95) in living donation. In conclusion, the developed model shows the deleterious effect of clinically relevant DESAs on graft outcome which outperformed traditional DSA-based risk analysis on antigen level.

Plasma protein signatures reflect systemic immunity and allograft function in kidney transplantation

Kidney transplantation causes large perturbations of the immune system. While many studies focus on the allograft, insights into systemic effects are largely missing. Here, we analyzed the systemic immune response in 3 cohorts of kidney transplanted patients. Using serum proteomics, laboratory values, mass cytometry, histological and clinical parameters, inter-patient heterogeneity was leveraged for multi-omic co-variation analysis. We identified circulating immune modules (CIM) that describe extra-renal signatures of co-regulated plasma proteins. CIM are present in nontransplanted controls, in transplant conditions and during rejection. They are enriched in pathways linked to kidney function, extracellular matrix, signaling, and cellular activation. A complex leukocyte response in the blood during allograft quiescence and rejection is associated with CIM activity and CIM-specific cytokines. CIM activity correlates with kidney function including a 2-month prediction. Together, the data suggest a systemic and multi-layered response of transplant immunity that might be insightful for understanding allograft dysfunction and developing translational biomarkers.

BSHI and BTS UK guideline on the detection of alloantibodies in solid organ (and islet) transplantation

Solid organ transplantation represents the best (and in many cases only) treatment option for patients with end-stage organ failure. The effectiveness and functioning life of these transplants has improved each decade due to surgical and clinical advances, and accurate histocompatibility assessment. Patient exposure to alloantigen from another individual is a common occurrence and takes place through pregnancies, blood transfusions or previous transplantation. Such exposure to alloantigen's can lead to the formation of circulating alloreactive antibodies which can be deleterious to solid organ transplant outcome. The purpose of these guidelines is to update to the previous BSHI/BTS guidelines 2016 on the relevance, assessment, and management of alloantibodies within solid organ transplantation.

How to measure human leukocyte antigen-specific B cells

PURPOSE OF REVIEW

The implementation of highly sensitive immune assays measuring anti-human leukocyte antigen (HLA) antibodies has modified alloimmune risk stratification and diagnosis of rejection. Nonetheless, anti-HLA antibodies represent the downstream effector mechanism of the B-cell response. Better characterizing the cellular components of the humoral immune response (including memory B cells (mBCs) and long-lived plasma cells) could help to further stratify the alloimmune risk stratification and enable discovery of new therapeutic targets. Several tests that characterize HLA-specific mBCs, either functionally or phenotypically, have been developed in the last years, showing promising applications as well as some limitations.

RECENT FINDINGS

Functional assays involving ex vivo polyclonal activation of mBC have been refined to allow the detection of HLA-specific mBC capable of producing anti-HLA Abs, using different and complementary detection platforms such as multiplex Fluorospot and single antigen bead assay on culture supernatants. Detection of circulating HLA-specific B cells by flow cytometry remains hindered by the very low frequency of HLA-specific mBC.

SUMMARY

Technological refinements have allowed the development of tests detecting HLA-specific mBC. Further evaluation of these assays in clinical trials, both for immune risk stratification and to assess treatment efficacy (desensitization strategies, rescue therapies for ABMR) are now urgently needed.

The Clinical Utility of Post-Transplant Monitoring of Donor-Specific Antibodies in Stable Renal Transplant Recipients: A Consensus Report With Guideline Statements for Clinical Practice

Solid phase immunoassays improved the detection and determination of the antigen-specificity of donor-specific antibodies (DSA) to human leukocyte antigens (HLA). The widespread use of SPI in kidney transplantation also introduced new clinical dilemmas, such as whether patients should be monitored for DSA pre- or post-transplantation. Pretransplant screening through SPI has become standard practice and DSA are readily determined in case of suspected rejection. However, DSA monitoring in recipients with stable graft function has not been universally established as standard of care. This may be related to uncertainty regarding the clinical utility of DSA monitoring as a screening tool. This consensus report aims to appraise the clinical utility of DSA monitoring in recipients without overt signs of graft dysfunction, using the Wilson & Junger criteria for assessing the validity of a screening practice. To assess the evidence on DSA monitoring, the European Society for Organ Transplantation (ESOT) convened a dedicated workgroup, comprised of experts in transplantation nephrology and immunology, to review relevant literature. Guidelines and statements were developed during a consensus conference by Delphi methodology that took place in person in November 2022 in Prague. The findings and recommendations of the workgroup on subclinical DSA monitoring are presented in this article.

Clinical Outcome of Kidney Transplant Recipients with C1q-Binding De Novo Donor Specific Antibodies: A Single-Center Experience

Complement activation by HLA antibodies is a key component of immune-mediated graft injury. We examined the clinical outcomes of kidney transplant recipients with complement-fixing de novo donor-specific antibodies (dnDSA) who were followed in our center. The C1q-binding ability was retrospectively assessed in 69 patients with dnDSA and mean fluorescence intensity (MFI) values > 2000 out of the 1325 kidney transplant recipients who were screened for DSA between 2015 and 2019. Luminex IgG single antigen beads (SAB)and C1q-SAB assays (One Lambda) were used. C1q-binding dnDSA was identified in 32/69 (46.4%) of the patients. Significantly higher MFI values were observed in C1q-positive DSA (18,978 versus 5840, p < 0.001). Renal graft biopsies were performed in 43 of the kidney transplant recipients (62.3%) with allograft dysfunction. Antibody-mediated rejection (ABMR) was detected in 29/43 (67.4%) of the patients. The incidence of ABMR was similar among patients with C1q-binding and non-C1q-binding DSA (51.7% vs. 48.3%, p = 0.523). Graft loss occurred in 30/69 (43.5%) of the patients at a median time of 82.5 months (IQR 45-135) from DSA detection. C1q-binding DSA was present in more patients who experienced graft loss (53.1% vs. 35.1%, p = 0.152). Higher MFI values and inferior clinical outcomes occurred in most of the kidney transplant recipients with C1q-binding dnDSA.

Natural Antibodies Are Associated With Rejection and Long-term Renal Allograft Loss in a Multicenter International Cohort

BACKGROUND

Potentially harmful nonhuman leukocyte antigen antibodies have been identified in renal transplantation, including natural immunoglobulin G antibodies (Nabs) reactive to varied antigenic structures, including apoptotic cells.

METHODS

In this retrospective, multicenter study, we assessed Nabs by reactivity to apoptotic cells in sera collected from 980 kidney transplant recipients across 4 centers to determine their association with graft outcomes.

RESULTS

Elevated pretransplant Nabs were associated with graft loss (hazard ratio [HR] 2.71; 95% confidence interval [CI], 1.15-6.39; P   =  0.0232), the composite endpoint of graft loss or severe graft dysfunction (HR 2.40; 95% CI, 1.13-5.10; P  = 0.0232), and T cell-mediated rejection (odds ratio [OR] 1.77; 95% CI, 1.07-3.02; P  = 0.0310). High pretransplant Nabs together with donor-specific antibodies (DSAs) were associated with increased risk of composite outcomes (HR 6.31; 95% CI, 1.81-22.0; P  = 0.0039). In patients with high pretransplant Nabs, the subsequent development of posttransplant Nabs was associated with both T cell-mediated rejection (OR 3.64; 95% CI, 1.61-8.36; P  = 0.0021) and mixed rejection (OR 3.10; 95% CI, 1.02-9.75; P  = 0.0473). Finally, elevated pre- and posttransplant Nabs combined with DSAs were associated with increased risk of composite outcomes (HR 3.97; 95% CI, 1.51-10.43; P  = 0.0052) and T cell-mediated rejection (OR 7.28; 95% CI, 2.16-25.96; P  = 0.0016).

CONCLUSIONS

The presence of pre- and posttransplant Nabs, together with DSAs, was associated with increased risk of poor graft outcomes and rejection after renal transplantation.

Imlifidase for Kidney Transplantation of Highly Sensitized Patients With a Positive Crossmatch: The French Consensus Guidelines

Imlifidase recently received early access authorization for highly sensitized adult kidney transplant candidates with a positive crossmatch against an ABO-compatible deceased donor. These French consensus guidelines have been generated by an expert working group, in order to homogenize patient selection, associated treatments and follow-up. This initiative is part of an international effort to analyze properly the benefits and tolerance of this new costly treatment in real-life. Eligible patients must meet the following screening criteria: cPRA ≥ 98%, ≤ 65-year of age, ≥ 3 years on the waiting list, and a low risk of biopsy-related complications. The final decision to use Imlifidase will be based on the two following criteria. First, the results of a virtual crossmatch on recent serum, which shall show a MFI for the immunodominant donor-specific antibodies (DSA) > 6,000 but the value of which does not exceed 5,000 after 1:10 dilution. Second, the post-Imlifidase complement-dependent cytotoxicity crossmatch must be negative. Patients treated with Imlifidase will receive an immunosuppressive regimen based on steroids, rATG, high dose IVIg, rituximab, tacrolimus and mycophenolic acid. Frequent post-transplant testing for DSA and systematic surveillance kidney biopsies are highly recommended to monitor post-transplant DSA rebound and subclinical rejection.

Impact of Resolved Preformed, Persistent Preformed, and De Novo Anti-HLA Donor-Specific Antibodies in Kidney Transplant Recipients on Long-Term Renal Graft Outcomes

The post-transplant evolution of antihuman leukocyte antigen donor-specific antibodies (anti-HLA DSAs) includes three clinical patterns: resolved preformed DSAs, persistent preformed DSAs, and de novo DSAs. The aim of this retrospective study was to analyze the impact of resolved preformed, persistent preformed, and de novo anti-HLA-A, -B, and -DR DSAs in kidney transplant recipients on long-term renal allograft outcomes. This is a post hoc analysis of the study conducted in our transplant center. One hundred eight kidney transplant recipients were included in the study. Patients were followed for a minimum of 24 months after allograft biopsy, which was performed 3 to 24 months after kidney transplantation. The identification of persistent preformed DSAs at the time of biopsy was the most significant predictor of the combined endpoint of the study (>30% decline in estimated glomerular filtration rate or death-censored graft loss; HR = 5.96, 95% CI 2.041-17.431, p = 0.0011), followed by the occurrence of de novo DSAs (HR = 4.48, 95% CI 1.483-13.520, p = 0.0079). No increased risk was observed in patients with resolved preformed DSAs (HR = 1.10, 95% CI 0.139-8.676, p = 0.9305). Patients with resolved preformed DSAs have similar graft prognoses as patients without DSAs, therefore, the persistence of preformed DSAs and development of de novo DSAs are associated with inferior long-term allograft outcomes.

Effect of blood transfusion post kidney transplantation on de novo human leukocytes antigen donor-specific antibody development and clinical outcomes in kidney transplant recipients: A systematic review and meta-analysis

The relationship between blood transfusion following kidney transplantation (KT) and the development of de novo donor-specific antibodies (dnDSA) is controversial. This was investigated by conducting a meta-analysis of studies on patients who underwent KT with or without blood transfusion, and by evaluating the effect of post-KT blood transfusion on clinical outcomes of kidney transplant recipients. Relevant studies in the PubMed, EMBASE, and Cochrane Library databases were identified from inception to July 1, 2022. Two reviewers independently extracted data from the selected articles and estimated study quality. A fixed effects or random effects model was used to pool data according to the heterogeneity among studies. Data included in the meta-analysis were derived from 11 studies with a total of 19,543 patients including 6191 with and 13,352 without blood transfusion post-KT. We assessed the pooled associations between blood transfusion and occurrence of dnDSA and clinical outcomes of transplant recipients. Blood transfusion was strongly correlated with the development of dnDSA (relative risk [RR] = 1.40, 95% confidence interval [CI]: 1.17-1.67; P < 0.05). Patients with blood transfusion had a higher risk of developing anti-human leukocyte antigen (HLA) class I dnDSA than non-transfused patients (RR = 1.75, 95% CI: 1.14-2.69; P < 0.05) as well as significantly higher rates of antibody-mediated rejection (AMR) (RR = 1.41, 95% CI: 1.21-2.35; P < 0.05) and graft loss (RR = 1.75, 95% CI: 1.30-2.35; P < 0.05). There were no statistically significant differences between the two groups in the development of anti-HLA antibodies, anti-HLA class II dnDSA, and anti-HLA class I and II dnDSA; delayed graft function; T cell-mediated rejection; acute rejection; borderline rejection; or patient death. Our results suggest that blood transfusion was associated with dnDSA development in KT recipients. The findings of this systematic review also suggest that post-KT blood transfusion recipients have a higher risk of AMR, and graft loss compared with non-transfused patients. Evidence from this meta-analysis indicates that the use of blood transfusion post-KT is associated with a significantly higher risk of immunological sensitization. More and higher quality results from large randomized controlled trials are still needed to inform clinical practice.

Alloimmune risk assessment for antibody-mediated rejection in kidney transplantation: A practical proposal

Kidney transplantation is the treatment of choice for patients with end-stage renal disease. Although an improvement in graft survival has been observed in the last decades with the use of different immunosuppressive drugs, this is still limited in time with antibody-mediated rejection being a main cause of graft-loss. Immune monitoring and risk assessment of antibody-mediated rejection before and after kidney transplantation with useful biomarkers is key to tailoring treatments to achieve the best outcomes. Here, we provide a review of the rationale and several accessible tools for immune monitoring, from the most classic to the modern ones. Finally, we end up discussing a practical proposal for alloimmune risk assessment in kidney transplantation, including histocompatibility leukocyte antigen (HLA) and non-HLA antibodies, HLA molecular mismatch analysis and characterization of peripheral blood immune cells.

Seeking Standardized Definitions for HLA-incompatible Kidney Transplants: A Systematic Review

BACKGROUND

There is no standard definition for "HLA incompatible" transplants. For the first time, we systematically assessed how HLA incompatibility was defined in contemporary peer-reviewed publications and its prognostic implication to transplant outcomes.

METHODS

We combined 2 independent searches of MEDLINE, EMBASE, and the Cochrane Library from 2015 to 2019. Content-expert reviewers screened for original research on outcomes of HLA-incompatible transplants (defined as allele or molecular mismatch and solid-phase or cell-based assays). We ascertained the completeness of reporting on a predefined set of variables assessing HLA incompatibility, therapies, and outcomes. Given significant heterogeneity, we conducted narrative synthesis and assessed risk of bias in studies examining the association between death-censored graft failure and HLA incompatibility.

RESULTS

Of 6656 screened articles, 163 evaluated transplant outcomes by HLA incompatibility. Most articles reported on cytotoxic/flow T-cell crossmatches (n = 98). Molecular genotypes were reported for selected loci at the allele-group level. Sixteen articles reported on epitope compatibility. Pretransplant donor-specific HLA antibodies were often considered (n = 143); yet there was heterogeneity in sample handling, assay procedure, and incomplete reporting on donor-specific HLA antibodies assignment. Induction (n = 129) and maintenance immunosuppression (n = 140) were frequently mentioned but less so rejection treatment (n = 72) and desensitization (n = 70). Studies assessing death-censored graft failure risk by HLA incompatibility were vulnerable to bias in the participant, predictor, and analysis domains.

CONCLUSIONS

Optimization of transplant outcomes and personalized care depends on accurate HLA compatibility assessment. Reporting on a standard set of variables will help assess generalizability of research, allow knowledge synthesis, and facilitate international collaboration in clinical trials.

Clinical recommendations for posttransplant assessment of anti-HLA (Human Leukocyte Antigen) donor-specific antibodies: A Sensitization in Transplantation: Assessment of Risk consensus document

Although anti-HLA (Human Leukocyte Antigen) donor-specific antibodies (DSAs) are commonly measured in clinical practice and their relationship with transplant outcome is well established, clinical recommendations for anti-HLA antibody assessment are sparse. Supported by a careful and critical review of the current literature performed by the Sensitization in Transplantation: Assessment of Risk 2022 working group, this consensus report provides clinical practice recommendations in kidney, heart, lung, and liver transplantation based on expert assessment of quality and strength of evidence. The recommendations address 3 major clinical problems in transplantation and include guidance regarding posttransplant DSA assessment and application to diagnostics, prognostics, and therapeutics: (1) the clinical implications of positive posttransplant DSA detection according to DSA status (ie, preformed or de novo), (2) the relevance of posttransplant DSA assessment for precision diagnosis of antibody-mediated rejection and for treatment management, and (3) the relevance of posttransplant DSA for allograft prognosis and risk stratification. This consensus report also highlights gaps in current knowledge and provides directions for clinical investigations and trials in the future that will further refine the clinical utility of posttransplant DSA assessment, leading to improved transplant management and patient care.

Extended genomic HLA typing identifies previously unrecognized mismatches in living kidney transplantation

Introduction

Antibody mediated rejection (ABMR) is the most common cause of long-term allograft loss in kidney transplantation (KT). Therefore, a low human leukocyte antigen (HLA) mismatch (MM) load is favorable for KT outcomes. Hitherto, serological or low-resolution molecular HLA typing have been adapted in parallel. Here, we aimed to identify previously missed HLA mismatches and corresponding antibodies by high resolution HLA genotyping in a living-donor KT cohort.

Methods

103 donor/recipient pairs transplanted at the University of Leipzig Medical Center between 1998 and 2018 were re-typed using next generation sequencing (NGS) of the HLA loci -A, -B, -C, -DRB1, -DRB345, -DQA1, -DQB1, -DPA1, and -DPB1. Based on these data, we compiled HLA MM counts for each pair and comparatively evaluated genomic HLA-typing with pre-transplant obtained serological/low-resolution HLA (=one-field) typing results. NGS HLA typing (=two-field) data was further used for reclassification of de novo HLA antibodies as "donor-specific".

Results

By two-field HLA re-typing, we were able to identify additional MM in 64.1% (n=66) of cases for HLA loci -A, -B, -C, -DRB1 and -DQB1 that were not observed by one-field HLA typing. In patients with biopsy proven ABMR, two-field calculated MM count was significantly higher than by one-field HLA typing. For additional typed HLA loci -DRB345, -DQA1, -DPA1, and -DPB1 we observed 2, 26, 3, and 23 MM, respectively. In total, 37.3% (69/185) of de novo donor specific antibodies (DSA) formation was directed against these loci (DRB345 ➔ n=33, DQA1 ➔ n=33, DPA1 ➔ n=1, DPB1 ➔ n=10).

Conclusion

Our results indicate that two-field HLA typing is feasible and provides significantly more sensitive HLA MM recognition in living-donor KT. Furthermore, accurate HLA typing plays an important role in graft management as it can improve discrimination between donor and non-donor HLA directed cellular and humoral alloreactivity in the long range. The inclusion of additional HLA loci against which antibodies can be readily detected, HLA-DRB345, -DQA1, -DQB1, -DPA1, and -DPB1, will allow a more precise virtual crossmatch and better prediction of potential DSA. Furthermore, in living KT, two-field HLA typing could contribute to the selection of the immunologically most suitable donors.

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

RATIONALE & OBJECTIVE

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.

STUDY DESIGN

Retrospective cohort study.

SETTING & PARTICIPANTS

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).

EXPOSURE

Scores calculated by the HLA matching algorithm PIRCHE-II and HLA eplet mismatches.

OUTCOME

TCMR, borderline changes suggestive of TCMR, and allograft failure.

ANALYTICAL APPROACH

Multivariable cause-specific hazards models were fit to characterize the association between HLA epitopes targets and study outcomes.

RESULTS

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.

LIMITATIONS

Observational clinical data and residual confounding.

CONCLUSIONS

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.

PLAIN-LANGUAGE SUMMARY

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.

Posttransplant de novo DSA and NDSA affect GvHD, OS, and DFS after haplo-HSCT in patients without pre-existing HLA antibodies of hematological malignancies

To examine the production time, type, and MFI of post-transplantation de novo HLA antibodies, and their effects on haplo-HSCT outcomes, we retrospectively included 116 patients who were negative for pre-existing HLA antibodies. In total, 322 serum samples from pre-transplantation to post-transplantation were dynamically tested by Luminex and single-antigen bead reagents. Patients were divided into: HLA antibody persistently negative group (group 1), the de novo HLA antibody transiently positive group (group 2), the de novo HLA antibody non-persistently positive group (group 3), and the de novo HLA antibody persistently positive group (group 4). Group 4 included DSA+non-DSA (NDSA) (group 4a) and NDSA (group 4b) groups. The detection rate of de novo HLA antibodies was 75.9% (88/116). The median MFI for de novo HLA antibodies was 2439 (1033-20162). The incidence of II-IV aGvHD was higher in group 2 than in group 1 (52.6% vs 17.9%, P < 0.01); in group 4a than in group 1 (87.5% vs 17.9%, P < 0.001); and in group 4a than in group 4b (87.5% vs 40.0%, P = 0.001). The DFS (37.5% vs 85.7%, P < 0.01) and OS (37.5% vs 85.7%, P < 0.01) of group 4a were lower than those of group 1. The DFS (48.0% vs 85.7%, P < 0.01) and OS (56.0% vs 85.7%, P = 0.03) of group 4b were lower than those of group 1. Multivariate analysis showed that de novo HLA antibody being transiently positive (HR: 5.30; 95% CI: 1.71-16.42, P = 0.01) and persistently positive (HR: 5.67; 95% CI: 2.00-16.08, P < 0.01) were both associated with a higher incidence of II-IV aGvHD. Persistently positive de novo HLA antibodies were a risk factor for reduced DFS (HR: 6.57; 95% CI: 2.08-20.70, P < 0.01) and OS (HR: 5.51; 95% CI: 1.73-17.53, P < 0.01). DSA and NDSA can be detected since 15 days after haplo-HSCT in patients without pre-existing HLA antibodies, and affect aGvHD, DFS, and OS. Haplo-HSCT patients must be monitored for HLA antibodies changes for appropriate preventive clinical management, and we recommend that 1-month post-transplantation is the best test time point.

Impact of Sensitization on Waiting Time Prior to Kidney Transplantation in Germany

BACKGROUND

Assignment of unacceptable HLA mismatches (UAMs) prevents transplantation of incompatible grafts but potentially prolongs waiting time. Whether this is true in the Eurotransplant Kidney Allocation System (ETKAS) and the Eurotransplant Senior Program in Germany is highly debated and relevant for UAM policies.

METHODS

Donor pool restriction due to UAM was expressed as percent virtual panel-reactive antibodies (vPRAs). Kaplan-Meier estimates and multivariable Cox regression models were used to analyze the impact of vPRA levels on waiting time and transplant probability during a period of 2 y in all patients eligible for a kidney graft unter standard circumstances in Germany on February 1, 2019 (n = 6533). Utility of the mismatch probability score to compensate for sensitization in ETKAS was also investigated.

RESULTS

In ETKAS, donor pool restriction resulted in significant prolongation of waiting time and reduction in transplant probability only in patients with vPRA levels above 85%. This was most evident in patients with vPRA levels above 95%, whereas patients in the acceptable mismatch program had significantly shorter waiting times and higher chances for transplantation than nonsensitized patients. In the Eurotransplant Senior Program, vPRA levels above 50% resulted in significantly longer waiting times and markedly reduced the chance for transplantation. Compensation for sensitization by the mismatch probability score was insufficient.

CONCLUSIONS

Donor pool restriction had no significant impact on waiting time in most sensitized patients. However, despite the existence of the acceptable mismatch program, the majority of highly sensitized patients is currently disadvantaged and would benefit from better compensation mechanisms.

Rationale for the IMAGINE study for chronic active antibody-mediated rejection (caAMR) in kidney transplantation

Chronic active antibody-mediated rejection (caAMR) in kidney transplantation is a major cause of late graft loss and despite all efforts to date, there is no proven effective therapy. Indeed, the Transplant Society (TTS) consensus opinion called for a conservative approach optimizing baseline immunosuppression and supportive care focused on blood pressure, blood glucose, and lipid control. This review provides the rationale and early evidence in kidney transplant recipients with caAMR that supported the design of the IMAGINE study whose goal is to evaluate the potential impact of targeting the IL6/IL6R pathway.

DSA in solid organ transplantation: is it a matter of specificity, amount, or functional characteristics?

PURPOSE OF REVIEW

The present review describes the clinical relevance of human leukocyte antigen (HLA) donor-specific antibodies (HLA-DSAs) as biomarkers of alloimmunity and summarizes recent improvements in their characterization that provide insights into immune risk assessment, precision diagnosis, and prognostication in transplantation.

RECENT FINDINGS

Recent studies have addressed the clinical utility of HLA-DSAs as biomarkers for immune risk assessment in pretransplant and peritransplant, diagnosis and treatment evaluation of antibody-mediated rejection, immune monitoring posttransplant, and risk stratification.

SUMMARY

HLA-DSAs have proved to be the most advanced immune biomarkers in solid organ transplantation in terms of analytical validity, clinical validity and clinical utility. Recent studies are integrating multiple HLA-DSA characteristics including antibody specificity, HLA class, quantity, immunoglobulin G subclass, and complement-binding capacity to improve risk assessment peritransplant, diagnosis and treatment evaluation of antibody-mediated rejection, immune monitoring posttransplant, and transplant prognosis evaluation. In addition, integration of HLA-DSAs to clinical, functional and histological transplant parameters has further consolidated the utility of HLA-DSAs as robust biomarkers and allows to build new tools for monitoring, precision diagnosis, and risk stratification for individual patients. However, prospective and randomized-controlled studies addressing the clinical benefit and cost-effectiveness of HLA-DSA-based monitoring and patient management strategies are required to demonstrate that the use of HLA-DSAs as biomarkers can improve current clinical practice and transplant outcomes.

Antibody-mediated rejection: prevention, monitoring and treatment dilemmas

PURPOSE OF REVIEW

Antibody-mediated rejection (AMR) has emerged as the leading cause of late graft loss in kidney transplant recipients. Donor-specific antibodies are an independent risk factor for AMR and graft loss. However, not all donor-specific antibodies are pathogenic. AMR treatment is heterogeneous due to the lack of robust trials to support clinical decisions. This review provides an overview and comments on practical but relevant dilemmas physicians experience in managing kidney transplant recipients with AMR.

RECENT FINDINGS

Active AMR with donor-specific antibodies may be treated with plasmapheresis, intravenous immunoglobulin and corticosteroids with additional therapies considered on a case-by-case basis. On the contrary, no treatment has been shown to be effective against chronic active AMR. Various biomarkers and prediction models to assess the individual risk of graft failure and response to rejection treatment show promise.

SUMMARY

The ability to personalize management for a given kidney transplant recipient and identify treatments that will improve their long-term outcome remains a critical unmet need. Earlier identification of AMR with noninvasive biomarkers and prediction models to assess the individual risk of graft failure should be considered. Enrolling patients with AMR in clinical trials to assess novel therapeutic agents is highly encouraged.

The impact of pre-transplant donor specific antibodies on the outcome of kidney transplantation – Data from the Swiss transplant cohort study

Background

Pre-transplant donor specific antibodies (DSA), directed at non-self human leukocyte antigen (HLA) protein variants present in the donor organ, have been associated with worse outcomes in kidney transplantation. The impact of the mean fluorescence intensity (MFI) and the target HLA antigen of the detected DSA has, however, not been conclusively studied in a large cohort with a complete virtual cross-match (vXM).

Methods

We investigated the effect of pre-transplant DSA on the risk of antibody-mediated rejection (ABMR), graft loss, and the rate of eGFR decline in 411 DSA positive transplants and 1804 DSA negative controls.

Results

Pre-transplant DSA were associated with a significantly increased risk of ABMR, graft loss, and accelerated eGFR decline. DSA directed at Class I and Class II HLA antigens were strongly associated with increased risk of ABMR, but only DSA directed at Class II associated with graft loss. DSA MFI markedly affected outcome, and Class II DSA were associated with ABMR already at 500-1000 MFI, whereas Class I DSA did not affect outcome at similar low MFI values. Furthermore, isolated DSA against HLA-DP carried comparable risks for ABMR, accelerated eGFR decline, and graft loss as DSA against HLA-DR.

Conclusion

Our results have important implications for the construction and optimization of vXM algorithms used within organ allocation systems. Our data suggest that both the HLA antigen target of the detected DSA as well as the cumulative MFI should be considered and that different MFI cut-offs could be considered for Class I and Class II directed DSA.

Clinical significance of low pre-transplant donor specific antibodies (DSA) in living donor kidney recipients with negative complement-dependent cytotoxicity crossmatches (CDCXM), and negative flow cytometry crossmatches (FLXM) - A single-center experience

BACKGROUND

It is controversial whether all donor-specific antibodies (DSA) detected by the solid-phase single antigen bead (SAB) assay negatively affect kidney transplantation outcomes. The study aimed to evaluate the possible clinical significance of low pre-transplant DSA in living donor kidney recipients. We analyzed a group of patients with HLA-A, B, and -DR DSA reactivities below a virtual crossmatch (VXM) value of 5000 MFI but with all VXM DSA reactivities at HLA-DQ, -DP, and -Cw, which were not typed routinely for donors prior to transplantation. We also investigated the incidence of persistent and de novo DSAs in available posttransplant SAB assays.

METHODS

From the historical cohort of living donor recipients transplanted between 2014 and 2018 at our center (n = 82), 55 patients met the inclusion criteria, namely: these patients were > 18 years old with non-HLA identical sibling donors, who were not desensitized, who had available pre-transplant SAB results, and who had negative both complement-dependent cytotoxicity crossmatch (CDCXM) and flow cytometry crossmatch (FLXM) results. An additional donor HLA typing, performed for all 55 recipients, identified donor additional HLA-DQ, -DP, and -Cw DSA reactivities. These patients were then divided by SAB reactivity into three groups: 1) those with DSA-positive reactivities; 2) those with non-donor-specific anti-HLA reactivities (NDSA); and, 3) those who were anti-HLA-negative. All these recipients were followed for three years and checked for their de novo or persistent DSA.

RESULTS

In the studied cohort, DSA-positive, NDSA reactive, and anti-HLA negative recipients constituted 33%, 36%, and 31% of 55 patients, respectively. Non-routinely considered pre-transplant HLA-DQ, -DP, and -Cw DSA-positive reactivities were shown in as many as 78% of DSA-positive cases (group 1) with the lowest MFI value of 319 to DP4 and the highest MFI of 5767 to DQ2. Of the pre-transplant HLA-A, B, and -DR DSA reactivities, only -DR52 DSA reactivity reached the highest MFI value of 2191. These detected DSAs did not reduce the mean estimated glomerular filtration rate (eGFR) values and did not increase the incidence of proteinuria in recipients. While the 3-year graft survival was lower in the DSA-positive group (94.4%) with one recipient who lost kidney transplant, the difference was not significantly different (p = 0.7) from the NDSA (100%) and negative (100%) groups. In terms of the incidence of de novo acute antibody-mediated rejection (AMR) at three years after transplantation, no case has been reported in the cohort. This may suggest that low DSA-positive recipients do not experience higher rejection rate. However, DSA-positive recipients had a tendency for a higher frequency of C4d deposits in peritubular capillaries (PTC) and de novo DSA.

CONCLUSION

Our 3-year follow-up of patients with low pre-transplant DSA found no association with a deterioration in graft function and worse graft survival. Furthermore, we did not observe an increase in AMR in our patients with low DSA. A larger cohort and a longer follow-up period may be needed to evaluate the tendency of low DSA-positive recipients towards the higher incidence of C4d deposits in PTC and/or de novo DSA.

Proteomics for Biomarker Discovery for Diagnosis and Prognosis of Kidney Transplantation Rejection

Renal transplantation is currently the treatment of choice for end-stage kidney disease, enabling a quality of life superior to dialysis. Despite this, all transplanted patients are at risk of allograft rejection processes. The gold-standard diagnosis of graft rejection, based on histological analysis of kidney biopsy, is prone to sampling errors and carries high costs and risks associated with such invasive procedures. Furthermore, the routine clinical monitoring, based on urine volume, proteinuria, and serum creatinine, usually only detects alterations after graft histologic damage and does not differentiate between the diverse etiologies. Therefore, there is an urgent need for new biomarkers enabling to predict, with high sensitivity and specificity, the rejection processes and the underlying mechanisms obtained from minimally invasive procedures to be implemented in routine clinical surveillance. These new biomarkers should also detect the rejection processes as early as possible, ideally before the 78 clinical outputs, while enabling balanced immunotherapy in order to minimize rejections and reducing the high toxicities associated with these drugs. Proteomics of biofluids, collected through non-invasive or minimally invasive analysis, e.g., blood or urine, present inherent characteristics that may provide biomarker candidates. The current manuscript reviews biofluids proteomics toward biomarkers discovery that specifically identify subclinical, acute, and chronic immune rejection processes while allowing for the discrimination between cell-mediated or antibody-mediated processes. In time, these biomarkers will lead to patient risk stratification, monitoring, and personalized and more efficient immunotherapies toward higher graft survival and patient quality of life.

Dynamic Behaviour of Donor Specific Antibodies in the Early Period Following HLA Incompatible Kidney Transplantation

In HLA-incompatible kidney transplantation, monitoring donor-specific antibodies (DSA) plays a crucial role in providing appropriate treatment and increases kidney survival times. This work aimed to determine if early post-transplant DSA dynamics inform graft outcome over and above other predictive factors. Eighty-eight cases were classified by unsupervised machine learning into five distinct DSA response groups: no response, fast modulation, slow modulation, rise to sustained and sustained. Fast modulation dynamics gave an 80% rate for early acute rejection, whereas the sustained group was associated with the lowest rejection rates (19%). In complete contrast, the five-year graft failure was lowest in the modulation groups (4–7%) and highest in the sustained groups (25–31%). Multivariable analysis showed that a higher pre-treatment DSA level, male gender and absence of early acute rejection were strongly associated with a sustained DSA response. The modulation group had excellent five-year outcomes despite higher rates of early rejection episodes. This work further develops an understanding of post-transplant DSA dynamics and their influence on graft survival following HLA-incompatible kidney transplantation.

Circulating Donor-Specific Anti-HLA Antibodies Associate With Immune Activation Independent of Kidney Transplant Histopathological Findings

Despite the critical role of cytokines in allograft rejection, the relation of peripheral blood cytokine profiles to clinical kidney transplant rejection has not been fully elucidated. We assessed 28 cytokines through multiplex assay in 293 blood samples from kidney transplant recipients at time of graft dysfunction. Unsupervised hierarchical clustering identified a subset of patients with increased pro-inflammatory cytokine levels. This patient subset was hallmarked by a high prevalence (75%) of donor-specific anti-human leukocyte antigen antibodies (HLA-DSA) and histological rejection (70%) and had worse graft survival compared to the group with low cytokine levels (HLA-DSA in 1.7% and rejection in 33.7%). Thirty percent of patients with high pro-inflammatory cytokine levels and HLA-DSA did not have histological rejection. Exploring the cellular origin of these cytokines, we found a corresponding expression in endothelial cells, monocytes, and natural killer cells in single-cell RNASeq data from kidney transplant biopsies. Finally, we confirmed secretion of these cytokines in HLA-DSA-mediated cross talk between endothelial cells, NK cells, and monocytes. In conclusion, blood pro-inflammatory cytokines are increased in kidney transplant patients with HLA-DSA, even in the absence of histology of rejection. These observations challenge the concept that histology is the gold standard for identification of ongoing allo-immune activation after transplantation.

Microvascular Inflammation of the Renal Allograft: A Reappraisal of the Underlying Mechanisms

Antibody-mediated rejection (ABMR) is associated with poor transplant outcomes and was identified as a leading cause of graft failure after kidney transplantation. Although the hallmark histological features of ABMR (ABMRh), i.e., microvascular inflammation (MVI), usually correlate with the presence of anti-human leukocyte antigen donor-specific antibodies (HLA-DSAs), it is increasingly recognized that kidney transplant recipients can develop ABMRh in the absence of HLA-DSAs. In fact, 40-60% of patients with overt MVI have no circulating HLA-DSAs, suggesting that other mechanisms could be involved. In this review, we provide an update on the current understanding of the different pathogenic processes underpinning MVI. These processes include both antibody-independent and antibody-dependent mechanisms of endothelial injury and ensuing MVI. Specific emphasis is placed on non-HLA antibodies, for which we discuss the ontogeny, putative targets, and mechanisms underlying endothelial toxicity in connection with their clinical impact. A better understanding of these emerging mechanisms of allograft injury and all the effector cells involved in these processes may provide important insights that pave the way for innovative diagnostic tools and highly tailored therapeutic strategies.

Stability of Anti-HLA Sensitization Profiles in Highly Sensitized Kidney Transplantation Candidates: Toward a Rational Serological Testing Strategy

BACKGROUND

Highly sensitized (HS) anti-HLA patients awaiting kidney transplantation benefit from specific allocation programs. Serological monitoring at 3-mo intervals is recommended to prevent unexpected positive crossmatch (XM), but this strategy is not evidence-based. Therefore, we assessed its relevance when using single-antigen flow bead (SAFB) and screening flow bead (SFB) assays.

METHODS

We included 166 HS patients awaiting a transplant and assessed their SAFB profile during the year preceding their inclusion. Anti-HLA antibodies were evaluated by SAFB assay and compared within patients as serum pairs at 3, 6, and 9 mo. We assessed the performance of SFB for detecting changes in SAFB profiles with 35 serum pairs.

RESULTS

On comparing 354, 218, and 107 serum pairs at 3, 6, and 9 mo, respectively, only 0.6%, 0.7%, and 1% of all antigens tested exceeded for the first time the unacceptable antigen threshold (mean fluorescence intensity ≥2000) in the most recent sample. Irrespective of the follow-up period, the calculated panel-reactive antibodies increased by a mean of 1%, and there was no significant increase in the proportion of donors at risk for positivity of flow- or complement-dependent cytotoxicity XM. The SFB did not accurately detect the variations of SAFB profiles.

CONCLUSIONS

Changes in HS patient profiles are anecdotal and show little association with transplant access or risk for positive XM. Less-frequent monitoring in HS patients should be considered to improve cost-effectiveness without affecting transplant safety.

Allorecognition and the spectrum of kidney transplant rejection

Detection of mismatched human leukocyte antigens by adaptive immune cells is considered as the main cause of transplant rejection, leading to either T-cell mediated rejection or antibody-mediated rejection. This canonical view guided the successful development of immunosuppressive therapies and shaped the diagnostic Banff classification for kidney transplant rejection that is used in clinics worldwide. However, several observations have recently emerged that question this dichotomization between T-cell mediated rejection and antibody-mediated rejection, related to heterogeneity in the serology, histology, and prognosis of the rejection phenotypes. In parallel, novel insights were obtained concerning the dynamics of donor-specific anti-human leukocyte antigen antibodies, the immunogenicity of donor-recipient non-human leukocyte antigen mismatches, and the autoreactivity against self-antigens. Moreover, the potential of innate allorecognition was uncovered, as exemplified by natural killer cell-mediated microvascular inflammation through missing self, and by the emerging evidence on monocyte-driven allorecognition. In this review, we highlight the gaps in the current classification of rejection, provide an overview of the expanding insights into the mechanisms of allorecognition, and critically appraise how these could improve our understanding and clinical approach to kidney transplant rejection. We argue that consideration of the complex interplay of various allorecognition mechanisms can foster a more integrated view of kidney transplant rejection and can lead to improved risk stratification, targeted therapies, and better outcome after kidney transplantation.

Increased Levels of sCD30 Have No Impact on the Incidence of Early ABMR and Long-Term Outcome in Intermediate-Risk Renal Transplant Patients With Preformed DSA

Background: It is still incompletely understood why some patients with preformed donor-specific anti-HLA antibodies (DSA) have reduced kidney allograft survival secondary to antibody-mediated rejection (ABMR), whereas many DSA-positive patients have favorable long-term outcomes. Elevated levels of soluble CD30 (sCD30) have emerged as a promising biomarker indicating deleterious T-cell help in conjunction with DSA in immunologically high-risk patients. We hypothesized that this would also be true in intermediate-risk patients.

Methods: We retrospectively analyzed pre-transplant sera from 287 CDC-crossmatch negative patients treated with basiliximab induction and tacrolimus-based maintenance therapy for the presence of DSA and sCD30. The incidence of ABMR according to the Banff 2019 classification and death-censored allograft survival were determined.

Results: During a median follow-up of 7.4 years, allograft survival was significantly lower in DSA-positive as compared to DSA-negative patients (p < 0.001). In DSA-positive patients, most pronounced in those with strong DSA (MFI > 5,000), increased levels of sCD30 were associated with accelerated graft loss compared to patients with low sCD30 (3-year allograft survival 75 vs. 95%). Long-term survival, however, was comparable in DSA-positive patients irrespective of sCD30 status. Likewise, the incidence of early ABMR and lesion score characteristics were comparable between sCD30-positive and sCD30-negative patients with DSA. Finally, increased sCD30 levels were not predictive for early persistence of DSA.

Conclusion: Preformed DSA are associated with an increased risk for ABMR and long-term graft loss independent of sCD30 levels in intermediate-risk kidney transplant patients.

Noninvasive Assessment of the Alloimmune Response in Kidney Transplantation

Transplantation remains the optimal mode of kidney replacement therapy, but unfortunately long-term graft survival after 1 year remains suboptimal. The main mechanism of chronic allograft injury is alloimmune, and current clinical monitoring of kidney transplants includes measuring serum creatinine, proteinuria, and immunosuppressive drug levels. The most important biomarker routinely monitored is human leukocyte antigen (HLA) donor-specific antibodies (DSAs) with the frequency based on underlying immunologic risk. HLA-DSA should be measured if there is graft dysfunction, immunosuppression minimization, or nonadherence. Antibody strength is semiquantitatively estimated as mean fluorescence intensity, with titration studies for equivocal cases and for following response to treatment. Determination of in vitro C1q or C3d positivity or HLA-DSA IgG subclass analysis remains of uncertain significance, but we do not recommend these for routine use. Current evidence does not support routine monitoring of non-HLA antibodies except anti-angiotensin II type 1 receptor antibodies when the phenotype is appropriate. The monitoring of both donor-derived cell-free DNA in blood or gene expression profiling of serum and/or urine may detect subclinical rejection, although mainly as a supplement and not as a replacement for biopsy. The optimal frequency and cost-effectiveness of using these noninvasive assays remain to be determined. We review the available literature and make recommendations.

The Problem of Subclinical Antibody-mediated Rejection in Kidney Transplantation

Defined as histologic evidence of rejection on a protocol biopsy in the absence of kidney dysfunction, subclinical rejection has garnered attention since the 1990s. The major focus of much of this research, however, has been subclinical T cell-mediated rejection (TCMR). Herein, we review the literature on subclinical antibody-mediated rejection (AMR), which may occur with either preexisting donor-specific antibodies (DSA) or upon the development of de novo DSA (dnDSA). In both situations, subsequent kidney function and graft survival are compromised. Thus, we recommend protocol biopsy routinely within the first year with preexisting DSA and at the initial detection of dnDSA. In those with positive biopsies, baseline immunosuppression should be maximized, any associated TCMR treated, and adherence stressed, but it remains uncertain if antibody-reduction treatment should be initiated. Less invasive testing of blood for donor DNA or gene profiling may have a role in follow-up of those with negative initial biopsies. If a protocol biopsy is positive in the absence of detectable HLA-DSA, it also remains to be determined whether non-HLA-DSA should be screened for either in particular or on a genome-wide basis and how these patients should be treated. Randomized controlled trials are clearly needed.

Significance of HLA-DQ in kidney transplantation: time to reevaluate human leukocyte antigen matching priorities to improve transplant outcomes? An expert review and recommendations

The weight of human leukocyte antigen (HLA) matching in kidney allocation algorithms, especially in the United States, has been devalued in a stepwise manner, supported by the introduction of modern immunosuppression. The intent was further to reduce the observed ethnic/racial disparity, as data emerged associating HLA matching with decreased access to transplantation for African American patients. In recent years, it has been increasingly recognized that a leading cause of graft loss is chronic antibody-mediated rejection, attributed to the development of de novo antibodies against mismatched donor HLA expressed on the graft. These antibodies are most frequently against donor HLA-DQ molecules. Beyond their impact on graft survival, generation of de novo donor-specific HLA antibodies also leads to increased sensitization, as measured by panel-reactive antibody metrics. Consequently, access to transplantation for patients returning to the waitlist in need of a second transplant is compromised. Herein, we address the implications of reduced HLA matching policies in kidney allocation. We highlight the observed diminished outcome data, the significant financial burden, the long-term health consequences, and, more important, the unintended consequences. We further provide recommendations to examine the impact of donor-recipient HLA class II and specifically HLA-DQα₁β₁ mismatching, focusing on collection of appropriate data, application of creative simulation approaches, and reconsideration of best practices to reduce inequalities while optimizing patient outcomes.

The evolution of histological changes suggestive of antibody-mediated injury, in the presence and absence of donor-specific anti-HLA antibodies

The interplay between donor-specific anti-HLA antibodies (HLA-DSA), histology of active antibody-mediated rejection (aABMR_h ), transplant glomerulopathy (cg) and graft failure in kidney transplantation remains insufficiently understood. We performed a single-center cohort study (n=1000) including 2761 protocol and 833 indication biopsies. Patients with pre-transplant HLA-DSA were more prone to develop aABMRh (OR 22.7, 95% CI, 11.8 - 43.7, p<0.001), cg (OR 5.76, 95% CI, 1.67 - 19.8, p=0.006) and aABMRh/cg (OR 19.5, 95% CI, 10.6 - 35.9, p<0.001). The negative impact of pre-transplant HLA-DSA on graft survival (HR 2.12, 95% CI, 1.41 - 3.20, p<0.001) was partially mediated through aABMRh and cg occurrence. When adjusted for time-dependent HLA-DSA (HR 4.03, 95% CI, 2.21 - 7.15, p=0.002), graft failure was only affected by aABMR_h when cg was evident. In HLA-DSA negative patients, aABMR_h was associated with impaired graft outcome only when evolving to cg (HR 1.32, 95% CI, 1.07 - 1.61, p=0.008). We conclude that the kinetics of HLA-DSA are important to estimate the rate of graft failure, and that histological follow-up is necessary to discover, often subclinical, ABMR and cg. In the absence of HLA-DSA, patients experience similar histological lesions and the evolution to transplant glomerulopathy associates with impaired graft outcome.

Revisiting the changes in the Banff classification for antibody-mediated rejection after kidney transplantation

The Banff classification for antibody-mediated rejection (ABMR) has undergone important changes, mainly by inclusion of C4d-negative ABMR in Banff'13 and elimination of suspicious ABMR (sABMR) with the use of C4d as surrogate for HLA-DSA in Banff'17. We aimed to evaluate the numerical and prognostic repercussions of these changes in a single-center cohort study of 949 single kidney transplantations, comprising 3662 biopsies that were classified according to the different versions of the Banff classification. Overall, the number of ABMR and sABMR cases increased from Banff'01 to Banff'13. In Banff'17, 248 of 292 sABMR biopsies were reclassified to No ABMR, and 44 of 292 to ABMR. However, reclassified sABMR biopsies had worse and better outcome than No ABMR and ABMR, which was mainly driven by the presence of microvascular inflammation and absence of HLA-DSA, respectively. Consequently, the discriminative performance for allograft failure was lowest in Banff'17, and highest in Banff'13. Our data suggest that the clinical and histological heterogeneity of ABMR is inadequately represented in a binary classification system. This study provides a framework to evaluate the updates of the Banff classification and assess the impact of proposed changes on the number of cases and risk stratification. Two alternative classifications introducing an intermediate category are explored.