Novel insights into non-HLA alloimmunity in kidney transplantation

Non-HLA angiotensin-type-1 receptor autoantibodies mediate the long-term loss of grafted neurons in Parkinson's disease models

BACKGROUND

Clinical trials have provided evidence that transplants of dopaminergic precursors, which may be replaced by new in vitro stem cell sources, can integrate into the host tissue, and alleviate motor symptoms in Parkinson´s disease (PD). In some patients, deterioration of graft function occurred several months after observing a graft-derived functional improvement. Rejection of peripheral organs was initially related to HLA-specific antibodies. However, the role of non-HLA antibodies is now considered also relevant for rejection. Angiotensin-II type-1 receptor autoantibodies (AT1-AA) act as agonists of the AT1 receptors. AT1-AA are the non-HLA antibodies most widely associated with graft dysfunction or rejection after transplantation of different solid organs and hematopoietic stem cells. However, it is not known about the presence and possible functional effects of AT1-AA in dopaminergic grafts, and the effects of treatment with AT1 receptor blockers (ARBs) such as candesartan on graft survival.

METHODS

In a 6-hydroxydopamine PD rat model, we studied the short-term (10 days)- and long-term (3 months) effects of chronic treatment with the ARB candesartan on survival of grafted dopaminergic neurons and microglial graft infiltration, as well as the effects of dopaminergic denervation and grafting on serum and CSF AT1-AA levels. The expression of AT1 receptors in grafted neurons was determined by laser capture microdissection.

RESULTS

At the early period post-grafting, the number of grafted dopaminergic neurons that survived was not significantly different between treated and untreated hosts (i.e., control rats and rats treated with candesartan), probably because, just after grafting, other deleterious factors are predominant for dopaminergic cell death, such as mechanical trauma, lack of growth factors/nutrients and ischemia. However, several months post-grafting, we observed a significantly higher number of surviving dopaminergic neurons and a higher density of striatal dopaminergic terminals in the candesartan-treated group. For several months, grafted rats showed blood and cerebrospinal fluid levels of AT1-AA higher than normal controls, and also higher AT1-AA levels than non-grafted parkinsonian rats.

CONCLUSIONS

The results suggest the use of ARBs such as candesartan in PD patients, particularly before and after dopaminergic grafts, and the need to monitor AT1-AA levels in PD patients, particularly in those candidates for dopaminergic grafting.

Genetic and Epigenetic Associations with Post-Transplant Diabetes Mellitus

Post-transplant diabetes mellitus (PTDM) is a common complication of solid organ transplantation. PTDM prevalence varies due to different diabetes definitions. Consensus guidelines for the diagnosis of PTDM have been published based on random blood glucose levels, glycated hemoglobin (HbA1c), and oral glucose tolerance test (OGTT). The task of diagnosing PTDM continues to pose challenges, given the potential for diabetes to manifest at different time points after transplantation, thus demanding constant clinical vigilance and repeated testing. Interpreting HbA1c levels can be challenging after renal transplantation. Pre-transplant risk factors for PTDM include obesity, sedentary lifestyle, family history of diabetes, ethnicity (e.g., African-Caribbean or South Asian ancestry), and genetic risk factors. Risk factors for PTDM include immunosuppressive drugs, weight gain, hepatitis C, and cytomegalovirus infection. There is also emerging evidence that genetic and epigenetic variation in the organ transplant recipient may influence the risk of developing PTDM. This review outlines many known risk factors for PTDM and details some of the pathways, genetic variants, and epigenetic features associated with PTDM. Improved understanding of established and emerging risk factors may help identify people at risk of developing PTDM and may reduce the risk of developing PTDM or improve the management of this complication of organ transplantation.

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.

Efficiency of Platelet Transfusion in Patients with Moderate-to-Severe Chronic Kidney Disease and Thrombocytopenia

There have been relatively few studies revealing a decreased platelet count in chronic kidney disease (CKD). Although this hematological abnormality is not as well documented as renal anemia, platelet functions are altered in the uremic environment and there is an increased risk of bleeding. The aim of this study was to assess the effectiveness of the administration of platelet concentrate in CKD based on how patient prognosis was influenced by platelet transfusion therapy. The study monitored 104 patients with CKD and thrombocytopenia who received platelet transfusion during their hospitalization in the period from 2015 to 2021. The complete blood cell count, serum urea and creatinine, and inflammatory status were tested upon admission. The number of transfused platelet units were considered for each patient. A Kruskal-Wallis H test showed that for one transfused platelet unit, the distribution of the number of platelets (×103/µL) was the same across the categories of associated diagnoses, which was seen as possible risk factors for thrombocytopenia, including liver cirrhosis and urosepsis. With a single exception, all patients exceeded the critical threshold of 20 × 103/µL and 14 patients remained under 50 × 103/µL. Even though our patients exceeded the critical threshold of platelet numbers, in patients with multiple comorbidities, severe, uncontrolled hemorrhages could not be prevented in 4.83% of cases.

Autoantibodies against DNA topoisomerase I promote renal allograft rejection by increasing alloreactive T cell responses

Antibodies reactive to self-antigens are an important component of posttransplant immune responses. The generation requirements and functions of autoantibodies, as well as the mechanisms of their influence on alloimmune responses, still remain to be determined. Our study investigated the contribution of autoimmunity during rejection of renal allografts. We have previously characterized a mouse model in which the acute rejection of a life-supporting kidney allograft is mediated by antibodies. At rejection, recipient sera screening against >4000 potential autoantigens revealed DNA topoisomerase I peptide 205-219 (TI-I205-219) as the most prominent epitope. Subsequent analysis showed TI-I205-219-reactive autoantibodies are induced in nonsensitized recipients of major histocompatibility complex-mismatched kidney allografts in a T cell-dependent manner. Immunization with TI-I205-219 broke self-tolerance, elicited TI-I205-219 immunoglobin G autoantibodies, and resulted in acute rejection of allogeneic but not syngeneic renal transplants. The graft loss was associated with increased priming of donor-reactive T cells but not with donor-specific alloantibodies elevation. Similarly, passive transfer of anti-TI-I205-219 sera following transplantation increased donor-reactive T cell activation with minimal effects on donor-specific alloantibody levels. The results identify DNA topoisomerase I as a novel self-antigen in transplant settings and demonstrate that autoantibodies enhance activation of donor-reactive T cells following renal transplantation.

Novel insights in the clinical management of hyperimmune patients before and after transplantation

Despite improvements in anti-Human Leucocyte Antigens antibody detection, identification, and characterization offer a better in peri-operative management techniques, antibodies remain a serious cause of morbidity and mortality for patients both before and after organ transplantation. Hyperimmune patients are disadvantaged by having to wait longer to receive an organ from a suitably matched donor. They could benefit from desensitization protocols in both pre- and post-transplantation period. Clinical studies are underway to highlight which best desensitization strategies could be assure the best outcome in both heart and kidney transplantation. Although most clinical evidence about desensitization strategies by using anti-CD20 monoclonal antibodies, proteasome inhibitors, anti-CD38 monoclonal antibodies, interleukin-6 blockade, cysteine protease and complement inhibitors, comes from kidney transplantation studies, many of the debated novel concepts can be easily applied to desensitization also in heart transplantation. Here, we discuss the candidates and recipients' management by using most common standard of care and novel therapeutics, desensitization endpoints, and strategies for future studies.

Chronic Active T-Cell Mediated Kidney Rejection as a Clinically Significant Type of Allograft Loss?

The purpose of this article is to assess the present knowledge about chronic active (CA) T-cell mediated rejection (TCMR) of a kidney. In the research authors review current Banff diagnostic criteria used in kidney rejection, focus on their possible future evolution, and investigate the role of currently available molecular methods that could be implemented into the diagnostic scheme. Research also points out previously and currently available treatment methods applied to CA TCMR and takes into account possible side effects consequent upon the therapy. Moreover, attention is being paid to the CA TCMR coincidence with other kidney rejection types such as antibody-mediated rejection (ABMR) and its influence on the treatment approach. Authors also mark the possibility of non-HLA antibodies coexistence in patients with CA TCMR and describe its possible resonance on kidney allograft function. Nonetheless, it seems that current knowledge about CA TCMR is not sufficient and requires further investigation.

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.

The Impact of Donor and Recipient Genetic Variation on Outcomes After Solid Organ Transplantation: A Scoping Review and Future Perspectives

At the outset of solid organ transplantation, genetic variation between donors and recipients was recognized as a major player in mechanisms such as allograft tolerance and rejection. Genome-wide association studies have been very successful in identifying novel variant-trait associations, but have been difficult to perform in the field of solid organ transplantation due to complex covariates, era effects, and poor statistical power for detecting donor-recipient interactions. To overcome a lack of statistical power, consortia such as the International Genetics and Translational Research in Transplantation Network have been established. Studies have focused on the consequences of genetic dissimilarities between donors and recipients and have reported associations between polymorphisms in candidate genes or their regulatory regions with transplantation outcomes. However, knowledge on the exact influence of genetic variation is limited due to a lack of comprehensive characterization and harmonization of recipients' or donors' phenotypes and validation using an experimental approach. Causal research in genetics has evolved from agnostic discovery in genome-wide association studies to functional annotation and clarification of underlying molecular mechanisms in translational studies. In this overview, we summarize how the recent advances and progresses in the field of genetics and genomics have improved the understanding of outcomes after solid organ transplantation.

Association of HLA Mismatches and Histology Suggestive of Antibody-Mediated Injury in the Absence of Donor-Specific Anti-HLA Antibodies

BACKGROUND AND OBJECTIVES

The histology of antibody-mediated rejection after kidney transplantation is observed frequently in the absence of detectable donor-specific anti-HLA antibodies. Although there is an active interest in the role of non-HLA antibodies in this phenotype, it remains unknown whether HLA mismatches play an antibody-independent role in this phenotype of microcirculation inflammation.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

To study this, we used the tools HLAMatchmaker, three-dimensional electrostatic mismatch score, HLA solvent accessible amino acid mismatches, and mismatched donor HLA-derived T cell epitope targets to determine the degree of HLA molecular mismatches in 893 kidney transplant recipients with available biopsy follow-up. Multivariable Cox proportional hazards models were applied to quantify the cause-specific hazard ratios of the different types of HLA mismatch scores for developing antibody-mediated rejection or histology of antibody-mediated rejection in the absence of donor-specific anti-HLA antibodies. In all survival analyses, the patients were censored at the time of the last biopsy.

RESULTS

In total, 121 (14%) patients developed histology of antibody-mediated rejection in the absence of donor-specific anti-HLA antibodies, of which 44 (36%) patients had concomitant T cell-mediated rejection. In multivariable Cox analysis, all different calculations of the degree of HLA mismatch associated with developing histology of antibody-mediated rejection in the absence of donor-specific anti-HLA antibodies. This association was dependent neither on the presence of missing self (potentially related to natural killer cell activation) nor on the formation of de novo HLA antibodies. Also, glomerulitis and complement C4d deposition in peritubular capillaries associated with the degree of HLA mismatch in the absence of anti-HLA antibodies.

CONCLUSIONS

The histology of antibody-mediated rejection and its defining lesions are also observed in patients without circulating anti-HLA antibodies and relate to the degree of HLA mismatch.

The Role of Donor Sex in Females Undergoing Repeat Kidney Transplant: Does Prior Donor Sex Matter?

Female recipients of male donor kidneys are at increased risk for graft failure because of the HY antigen effect. However, whether prior transplant with a male donor impacts subsequent transplant outcomes is unknown. Therefore, the purpose of this study was to determine whether prior male-current male donor sex is associated with an increased risk of graft failure in female recipients.

Methods

We performed a cohort study of adult female recipients undergoing a second kidney transplant (2000-2017), identified using the Scientific Registry of Transplant Recipients. Using multivariable Cox models, we analyzed the risk of death-censored graft loss (DCGL) if the second transplant was from a male versus female kidney donor, conditional on donor sex at the time of the first transplant. In a secondary analysis, we stratified results by recipient age (>50 or ≤50 y) at the time of retransplant.

Results

Of 5594 repeat kidney transplants, 1397 (25.0%) developed DCGL. Overall, there was no association between first and second donor sex pairing and DCGL. A prior and current female donor (FD₁FD₂) posed a higher risk for DCGL in recipients aged >50 y at second transplant (hazard ratio,≤0.67, confidence interval 0.46-0.98, for all other donor combinations), but posed a lower risk if aged ≤50 y at retransplant (hazard ratio, ≥1.37, confidence interval 1.04-1.80, for all other donor combinations).

Conclusions

Overall, past-current donor sex pairing was not associated with DCGL in female recipients undergoing second kidney transplant; however, the risk with a past and current female donor was increased in older, and decreased in younger, female recipients at retransplant.

Non-HLA Antibodies in Kidney Transplantation: Immunity and Genetic Insights

The polymorphic human leukocyte antigen (HLA) system has been considered the main target for alloimmunity, but the non-HLA antibodies and autoimmunity have gained importance in kidney transplantation (KT). Apart from the endothelial injury, secondary self-antigen exposure and the presence of polymorphic alloantigens, respectively, auto- and allo- non-HLA antibodies shared common steps in their development, such as: antigen recognition via indirect pathway by recipient antigen presenting cells, autoreactive T cell activation, autoreactive B cell activation, T helper 17 cell differentiation, loss of self-tolerance and epitope spreading phenomena. Both alloimmunity and autoimmunity play a synergic role in the formation of non-HLA antibodies, and the emergence of transcriptomics and genome-wide evaluation techniques has led to important progress in understanding the mechanistic features. Among them, non-HLA mismatches between donors and recipients provide valuable information regarding the role of genetics in non-HLA antibody immunity and development.

Proposed Definitions of Antibody-Mediated Rejection for Use as a Clinical Trial Endpoint in Kidney Transplantation

Antibody-mediated rejection (AMR) is caused by antibodies that recognize donor human leukocyte antigen (HLA) or other targets. As knowledge of AMR pathophysiology has increased, a combination of factors is necessary to confirm the diagnosis and phenotype. However, frequent modifications to the AMR definition have made it difficult to compare data and evaluate associations between AMR and graft outcome. The present paper was developed following a Broad Scientific Advice request from the European Society for Organ Transplantation (ESOT) to the European Medicines Agency (EMA), which explored whether updating guidelines on clinical trial endpoints would encourage innovations in kidney transplantation research. ESOT considers that an AMR diagnosis must be based on a combination of histopathological factors and presence of donor-specific HLA antibodies in the recipient. Evidence for associations between individual features of AMR and impaired graft outcome is noted for microvascular inflammation scores ≥2 and glomerular basement membrane splitting of >10% of the entire tuft in the most severely affected glomerulus. Together, these should form the basis for AMR-related endpoints in clinical trials of kidney transplantation, although modifications and restrictions to the Banff diagnostic definition of AMR are proposed for this purpose. The EMA provided recommendations based on this Broad Scientific Advice request in December 2020; further discussion, and consensus on the restricted definition of the AMR endpoint, is required.

Renal Endothelial Cytotoxicity Assay to Diagnose and Monitor Renal Transplant Recipients for Anti-Endothelial Antibodies

Tissue-specific nonhuman leukocyte antigen (HLA) antigens can play crucial roles in allograft immunity and have been shown to trigger humoral responses leading to rejection of HLA-matched kidney allografts. Interest in the role of endothelial-specific antigens has grown over the past years, and several case reports have been described in which antibodies reacting with endothelial cells (ECs) are associated with rejection. Such antibodies escape the detection in conventional crossmatch tests as they do not react with lymphocytes. However, due to the heterogeneity of endothelial cells from different vascular beds, it remains difficult to draw organ-specific conclusions from studies describing endothelial crossmatch assays. We present a case of a 69-year-old male patient whose kidney allograft was rejected as hyperacute, despite the absence of pretransplant HLA-specific antibodies. To place findings from previous studies in a kidney-related context, we performed crossmatch assays with primary renal endothelial cells. The patient's serum was reactive with primary renal ECs, demonstrated by antibody binding and complement-dependent cytotoxicity. Antibodies from this patient did not react with lymphocytes nor were HLA donor-specific antibodies (DSAs) found. Two years later, the patient successfully received a second kidney transplant after treatment with rituximab and plasmapheresis before and after transplantation. We demonstrated that the removal of antibodies against non-HLA EC-specific molecules can be monitored using a primary renal EC crossmatch test, possibly contributing to a successful transplantation outcome.

Antigen and Cell-Based Assays for the Detection of Non-HLA Antibodies

To date, human leukocyte antigens (HLA) have been the major focus in the approach to acute and chronic antibody-mediated rejection (AMBR) in solid-organ transplantation. However, evidence from the clinic and published studies has shown that non-HLA antibodies, particularly anti-endothelial cell antibodies (AECAs), are found either in the context of AMBR or synergistically in the presence of donor-specific anti-HLA antibodies (DSA). Numerous studies have explored the influence of AECAs on clinical outcomes, yet the determination of the exact clinical relevance of non-HLA antibodies in organ transplantation is not fully established. This is due to highly heterogeneous study designs including differences in testing methods and outcome measures. Efforts to develop reliable and sensitive diagnostic non-HLA antibody tests are continuously made. This is essential considering the technical difficulties of non-HLA antibody assays and the large variation in reported incidences of antibodies. In addition, it is important to take donor specificity into account in order to draw clinically relevant conclusions from non-HLA antibody assays. Here, we provide an overview of non-HLA solid-phase and cell-based crossmatch assays for use in solid-organ transplantation that are currently available, either in a research setting or commercially.

The Pre-Transplant Non-HLA Antibody Burden Associates With the Development of Histology of Antibody-Mediated Rejection After Kidney Transplantation

Background

Many kidney allografts fail due to the occurrence of antibody-mediated rejection (ABMR), related to donor-specific anti-HLA antibodies (HLA-DSA). However, the histology of ABMR can also be observed in patients without HLA-DSA. While some non-HLA antibodies have been related to the histology of ABMR, it is not well known to what extent they contribute to kidney allograft injury. Here we aimed to investigate the role of 82 different non-HLA antibodies in the occurrence of histology of ABMR after kidney transplantation.

Methods

We included all patients who underwent kidney transplantation between 2004-2013 in a single center and had biobanked serum. Pre- and post-transplant sera (n=2870) were retrospectively tested for the presence of 82 different non-HLA antibodies using a prototype bead assay on Luminex (Immucor, Inc). A ratio was calculated between the measured MFI value and the cut-off MFI defined by the vendor for each non-HLA target.

Results

874 patients had available pretransplant sera and were included in this analysis. Of them, 133 (15.2%) received a repeat kidney allograft, and 100 (11.4%) had pretransplant HLA-DSA. In total, 204 (23.3%) patients developed histology of ABMR after kidney transplantation. In 79 patients (38.7%) the histology of ABMR was explained by pretransplant or de novo HLA-DSA. The multivariable Cox analysis revealed that only the broadly non-HLA sensitized (number of positive non-HLA antibodies) patients and those with the highest total strength of the non-HLA antibodies (total ratios of the positive non-HLA antibodies) were independently associated with increased rates of histology of ABMR after transplantation. Additionally, independent associations were found for antibodies against TUBB (HR=2.40; 95% CI 1.37 – 4.21, p=0.002), Collagen III (HR=1.67; 95% CI 1.08 – 2.58, p=0.02), VCL (HR=2.04; 95% CI 1.12 – 3.71, p=0.02) and STAT6 (HR=1.47; 95% CI 1.01 – 2.15, p=0.04). The overall posttransplant non-HLA autoreactivity was not associated with increased rates of ABMRh.

Conclusions

This study shows that patients highly and broadly sensitized against non-HLA targets are associated with an increased risk of ABMR histology after kidney transplantations in the absence of HLA-DSA. Also, some pretransplant non‐HLA autoantibodies are individually associated with increased rates of ABMR histology. However, whether these associations are clinically relevant and represent causality, warrants further studies.

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.

A Multi-Marker Test for Analyzing Paired Genetic Data in Transplantation

Emerging evidence suggests that donor/recipient matching in non-HLA (human leukocyte antigen) regions of the genome may impact transplant outcomes and recognizing these matching effects may increase the power of transplant genetics studies. Most available matching scores account for either single-nucleotide polymorphism (SNP) matching only or sum these SNP matching scores across multiple gene-coding regions, which makes it challenging to interpret the association findings. We propose a multi-marker Joint Score Test (JST) to jointly test for association between recipient genotype SNP effects and a gene-based matching score with transplant outcomes. This method utilizes Eigen decomposition as a dimension reduction technique to potentially increase statistical power by decreasing the degrees of freedom for the test. In addition, JST allows for the matching effect and the recipient genotype effect to follow different biological mechanisms, which is not the case for other multi-marker methods. Extensive simulation studies show that JST is competitive when compared with existing methods, such as the sequence kernel association test (SKAT), especially under scenarios where associated SNPs are in low linkage disequilibrium with non-associated SNPs or in gene regions containing a large number of SNPs. Applying the method to paired donor/recipient genetic data from kidney transplant studies yields various gene regions that are potentially associated with incidence of acute rejection after transplant.

Plasma cell biology: Foundations for targeted therapeutic development in transplantation

Solid organ transplantation is a life-saving procedure for patients with end-stage organ disease. Over the past 70 years, tremendous progress has been made in solid organ transplantation, particularly in T-cell-targeted immunosuppression and organ allocation systems. However, humoral alloimmune responses remain a major challenge to progress. Patients with preexisting antibodies to human leukocyte antigen (HLA) are at significant disadvantages in regard to receiving a well-matched organ, moreover, those who develop anti-HLA antibodies after transplantation face a significant foreshortening of renal allograft survival. Historical therapies to desensitize patients prior to transplantation or to treat posttransplant AMR have had limited effectiveness, likely because they do not significantly reduce antibody levels, as plasma cells, the source of antibody production, remain largely unaffected. Herein, we will discuss the significance of plasma cells in transplantation, aspects of their biology as potential therapeutic targets, clinical challenges in developing strategies to target plasma cells in transplantation, and lastly, novel approaches that have potential to advance the field.

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.

Impact of SIRPα polymorphism on transplant outcomes in HLA-identical living donor kidney transplantation

Signal-regulatory protein α (SIRPα), a polymorphic inhibitory membrane-bound receptor, and its ligand CD47 have recently been implicated in the modulation of innate immune allorecognition in murine models. Here, we investigate the potential impact of SIRPα donor-recipient mismatches on graft outcomes in human kidney transplantation. To eliminate the specific role of HLA-matching in alloresponse, we genotyped the two most common variants of SIRPα in a cohort of 55 HLA-identical, biologically-related, donor-recipient pairs. 69% of pairs were SIRPα identical. No significant differences were found between donor-recipient SIRPα-mismatch status and T cell-mediated rejection/borderline changes (25.8% vs. 25%) or slow graft function (15.8% vs. 17.6%). A trend towards more graft failure (GF) (23.5% vs. 5.3%, P = .06), interstitial inflammation (50% vs. 23%, P = .06) and significant changes in peritubular capillaritis (ptc) (25% vs. 0%, P = .02) were observed in the SIRPα-mismatched group. Unexpectedly, graft-versus-host (GVH) SIRPα-mismatched pairs exhibited higher rates of GF and tubulitis (38% vs. 5%, P = .031 and .61 ± .88 vs. 0, P = .019; respectively). Whether the higher prevalence of ptc in SIRPα-mismatched recipients and the higher rates of GF in GVH SIRPα-mismatched pairs represent a potential role for SIRPα in linking innate immunity and alloimmune rejection requires further investigation in larger cohorts.

The emerging field of non-human leukocyte antigen antibodies in transplant medicine and beyond

The major medical advances in our knowledge of the human leukocyte antigen (HLA) system have allowed us to uncover several gaps in our understanding of alloimmunity. Although the non-HLA system has long sparked the interest of the transplant community, recognition of the role of immunity to non-HLA antigenic targets has only emerged recently. In this review, we will provide a comprehensive summary of the paradigm-changing concept of immunity to the non-HLA angiotensin II type 1 receptor (AT1R), discovered by Duška Dragun et al., that began from careful bedside clinical observations, to validated detection of anti-AT1R antibodies and lead to clinical intervention. This scientific approach has also allowed the recognition of broader pathogenicity of anti-AT1R antibodies across multiple organ transplants and in other human diseases, the integration of both non-HLA and HLA systems to understand their immunologic effects on organ allografts, and the identification of future directions for therapeutic intervention to modulate immunity to AT1R. Rationally designed successful interventions to target AT1R system provide an exemplar for other non-HLA antibodies to cross borders between medical specialties, will generate new avenues in translational research beyond transplantation, and will foster the development of new and reliable tools to improve our understanding of non-HLA immunity and ultimately allow us to improve patient care.

Humanized Mouse Model as a Novel Approach in the Assessment of Human Allogeneic Responses in Organ Transplantation

The outcome of organ transplantation is largely dictated by selection of a well-matched donor, which results in less chance of graft rejection. An allogeneic immune response is the main immunological barrier for successful organ transplantation. Donor and recipient human leukocyte antigen (HLA) mismatching diminishes outcomes after solid organ transplantation. The current evaluation of HLA incompatibility does not provide information on the immunogenicity of individual HLA mismatches and impact of non-HLA-related alloantigens, especially in vivo. Here we demonstrate a new method for analysis of alloimmune responsiveness between donor and recipient in vivo by introducing a humanized mouse model. Using molecular, cellular, and genomic analyses, we demonstrated that a recipient's personalized humanized mouse provided the most sensitive assessment of allogeneic responsiveness to potential donors. In our study, HLA typing provided a better recipient-donor match for one donor among two related donors. In contrast, assessment of an allogeneic response by mixed lymphocyte reaction (MLR) was indistinguishable between these donors. We determined that, in the recipient's humanized mouse model, the donor selected by HLA typing induced the strongest allogeneic response with markedly increased allograft rejection markers, including activated cytotoxic Granzyme B-expressing CD8+ T cells. Moreover, the same donor induced stronger upregulation of genes involved in the allograft rejection pathway as determined by transcriptome analysis of isolated human CD45+cells. Thus, the humanized mouse model determined the lowest degree of recipient-donor alloimmune response, allowing for better selection of donor and minimized immunological risk of allograft rejection in organ transplantation. In addition, this approach could be used to evaluate the level of alloresponse in allogeneic cell-based therapies that include cell products derived from pluripotent embryonic stem cells or adult stem cells, both undifferentiated and differentiated, all of which will produce allogeneic immune responses.

LIMS1 Risk Genotype and T-Cell Mediated Rejection in Kidney Transplant Recipients

BACKGROUND

This study aims to examine the association of LIM Zinc Finger Domain Containing 1 (LIMS1) genotype with allograft rejection in an independent kidney transplant cohort.

METHODS

We genotyped 841 kidney transplant recipients for LIMS1 rs893403 variant by Sanger sequencing followed by PCR confirmation of the deletion. Recipients who were homozygous for LIMS1 rs893403 genotype GG were compared to AA/AG genotypes. The primary outcome was T-cell mediated (TCMR) or antibody mediated rejection (ABMR) and secondary outcome was allograft loss.

RESULTS

After a median follow-up of 11.4 years, the rate of TCMR was higher in recipients with the GG (n = 200) compared to AA/AG (n = 641) genotypes [25 (12.5%) vs 35 (5.5%); p = 0.001] while ABMR did not differ by genotype [18 (9.0%) vs 62 (9.7%)]. Recipients with GG genotype had 2.4-times higher risk of TCMR than those who did not have this genotype (adjusted hazard ratio (aHR), 1.442.434.12, p = 0.001). A total of 189 (22.5%) recipients lost their allografts during follow up. Kaplan-Meier estimates of 5-year (94.3% vs. 94.4%, p = 0.99) and 10-year graft survival rates (86.9% vs. 83.4%, p = 0.31) did not differ significantly in those with GG compared to AA/AG groups.

CONCLUSIONS

Our study demonstrates that recipient LIMS1 risk genotype is associated with increased risk of TCMR after kidney transplantation, confirming the role of LIMS1 locus in allograft rejection. These findings may have clinical implications for the prediction and clinical management of kidney transplant rejection by pretransplant genetic testing of recipients and donors for LIMS1 risk genotype.

ARHGDIB and AT1R autoantibodies are differentially related to the development and presence of chronic antibody-mediated rejection and fibrosis in kidney allografts

The role of non-HLA autoantibodies in chronic-active antibody-mediated rejection (c-aABMR) of kidney transplants is largely unknown. In this study, the presence and clinical relevance of non-HLA autoantibodies using a recently developed multiplex Luminex-based assay were investigated. Patients with a kidney allograft biopsy at least 6 months after transplantation with a diagnosis of c-aABMR (n = 36) or no rejection (n = 21) were included. Pre-transplantation sera and sera at time of biopsy were tested for the presence of 14 relevant autoantibodies. A significantly higher signal for autoantibodies against Rho GDP-dissociation inhibitor 2 (ARHGDIB) was detected in recipients with c-aABMR as compared to recipients with no rejection. However, ARHGDIB autoantibodies did not associate with graft survival. Levels of autoantibodies against angiotensin II type 1-receptor (AT1R) and peroxisomal trans-2-enoyl-CoA reductase (PECR) were increased in recipients with interstitial fibrosis in their kidney biopsy. Only the signal for AT1R autoantibody showed a linear relationship with the degree of interstitial fibrosis and was associated with graft survival. In conclusion, anti-ARHGDIB autoantibodies are increased when c-aABMR is diagnosed but are not associated with graft survival, while higher levels of AT1R autoantibody are specifically associated with the presence of interstitial fibrosis and graft survival.

Maintenance Immunosuppression in Solid Organ Transplantation: Integrating Novel Pharmacodynamic Biomarkers to Inform Calcineurin Inhibitor Dose Selection

Calcineurin inhibitors, the primary immunosuppressive therapy used to prevent alloreactivity of transplanted organs, have a narrow therapeutic index. Currently, treatment is individualized based on clinical assessment of the risk of rejection or toxicity guided by trough concentration monitoring. Advances in immune monitoring have identified potential markers that may have value in understanding calcineurin inhibitor pharmacodynamics. Integration of these markers has the potential to complement therapeutic drug monitoring. Existing pharmacokinetic-pharmacodynamic (PK-PD) data is largely limited to correlation between the biomarker and trough concentrations at single time points. Immune related gene expression currently has the most evidence supporting PK-PD integration. Novel biomarker-based approaches to pharmacodynamic monitoring including development of enhanced PK-PD models are proposed to realize the full clinical benefit.

Recent Advances on Biomarkers of Early and Late Kidney Graft Dysfunction

New biomarkers of early and late graft dysfunction are needed in renal transplant to improve management of complications and prolong graft survival. A wide range of potential diagnostic and prognostic biomarkers, measured in different biological fluids (serum, plasma, urine) and in renal tissues, have been proposed for post-transplant delayed graft function (DGF), acute rejection (AR), and chronic allograft dysfunction (CAD). This review investigates old and new potential biomarkers for each of these clinical domains, seeking to underline their limits and strengths. OMICs technology has allowed identifying many candidate biomarkers, providing diagnostic and prognostic information at very early stages of pathological processes, such as AR. Donor-derived cell-free DNA (ddcfDNA) and extracellular vesicles (EVs) are further promising tools. Although most of these biomarkers still need to be validated in multiple independent cohorts and standardized, they are paving the way for substantial advances, such as the possibility of accurately predicting risk of DGF before graft is implanted, of making a “molecular” diagnosis of subclinical rejection even before histological lesions develop, or of dissecting etiology of CAD. Identification of “immunoquiescent” or even tolerant patients to guide minimization of immunosuppressive therapy is another area of active research. The parallel progress in imaging techniques, bioinformatics, and artificial intelligence (AI) is helping to fully exploit the wealth of information provided by biomarkers, leading to improved disease nosology of old entities such as transplant glomerulopathy. Prospective studies are needed to assess whether introduction of these new sets of biomarkers into clinical practice could actually reduce the need for renal biopsy, integrate traditional tools, and ultimately improve graft survival compared to current management.

What have we learned about how to prevent and treat antibody-mediated rejection in kidney transplantation?

Antibody-mediated rejection (ABMR) in kidney transplantation is a major cause of late graft loss, and despite all efforts to date the "standard of care" remains plasmapheresis, IVIg, and steroids, which itself is based on low quality evidence. This review focuses on the risk factors leading to memory and de novo donor-specific antibody (DSA)-associated ABMR, the optimal prevention strategies for ABMR, and advances in  adjunctive and emerging therapies for ABMR. Because new agents require regulatory approval via a Phase 3 randomized control trial (RCT), an overview of progress in innovative trial design for ABMR is provided. Finally, based on the insights gained in the biology of ABMR, current knowledge gaps are identified for future research that could significantly affect our understanding of how to optimally treat ABMR.

Genetic Engineering of the Kidney to Permanently Silence MHC Transcripts During ex vivo Organ Perfusion

Organ gene therapy represents a promising tool to correct diseases or improve graft survival after transplantation. Polymorphic variation of the major histocompatibility complex (MHC) antigens remains a major obstacle to long-term graft survival after transplantation. Previously, we demonstrated that MHC-silenced cells are protected against allogeneic immune responses. We also showed the feasibility to silence MHC in the lung. Here, we aimed at the genetic engineering of the kidney toward permanent silencing of MHC antigens in a rat model. We constructed a sub-normothermic ex vivo perfusion system to deliver lentiviral vectors encoding shRNAs targeting β2-microglobulin and the class II transactivator to the kidney. In addition, the vector contained the sequence for a secreted nanoluciferase. After kidney transplantation (ktx), we detected bioluminescence in the plasma and urine of recipients of an engineered kidney during the 6 weeks of post-transplant monitoring, indicating a stable transgene expression. Remarkably, transcript levels of β2-microglobulin and the class II transactivator were decreased by 70% in kidneys expressing specific shRNAs. Kidney genetic modification did not cause additional cell death compared to control kidneys after machine perfusion. Nevertheless, cytokine secretion signatures were altered during perfusion with lentiviral vectors as revealed by an increase in the secretion of IL-10, MIP-1α, MIP-2, IP-10, and EGF and a decrease in the levels of IL-12, IL-17, MCP-1, and IFN-γ. Biodistribution assays indicate that the localization of the vector was restricted to the graft. This study shows the potential to generate immunologically invisible kidneys showing great promise to support graft survival after transplantation and may contribute to reduce the burden of immunosuppression.