Angiotensin II Type 1 Receptor Expression in Renal Transplant Biopsies and Anti-AT1R Antibodies in Serum Indicates the Risk of Transplant Loss

Effect of angiotensin II type 1 receptor antibodies on graft function and survival in paediatric kidney transplant recipients

HLA donor specific antibodies (DSA) are implicated in antibody-mediated rejection (AMR), graft dysfunction and failure in kidney transplant (KT) recipients. Non-HLA antibodies including angiotensin II type 1 receptor (AT1R) may also play a role in AMR, impact graft function and survival. Data is limited in paediatric KT cohorts. We aimed to assess the prevalence and effect of pre-transplant AT1R antibodies on rejection, graft function and survival in paediatric KT recipients. This was a retrospective cohort study conducted across two paediatric centres including KT recipients with a pre-transplant AT1R antibody level. Outcomes included rejection, de novo DSA formation, graft function, failure, proteinuria and hypertension. Of 71 individuals, 72% recorded a positive pre-transplant AT1R Ab level (≥17 U/mL). Over a median follow-up of 4.7 years, AT1R Ab positivity demonstrated a trend towards increased risk of rejection however was not statistically significant (HR 3.45, 95% CI 0.97-12.35, p-value 0.06). Sensitivity analysis with AT1R Ab levels of ≥25 U/mL (HR 2.05 95% CI 0.78-5.39, p-value 0.14) and ≥40 U/mL (HR 1.32, CI 95% 0.55-3.17, p-value 0.53) validated this. De novo DSA formation occurred more frequently with AT1R Ab positivity (41% vs. 20%, p-value 0.9). AT1R Ab was not associated with hypertension, proteinuria, graft failure or dysfunction. In conclusion, this cohort study demonstrated a high prevalence of pre-transplant AT1R Ab positivity (72%). AT1R Ab positivity demonstrated a trend towards increased risk of rejection and de novo DSA formation however did not meet statistical significance. There was no association between AT1R Ab and hypertension, proteinuria, graft failure or dysfunction.

Exploration of the shared gene signatures and biological mechanisms between ischemia-reperfusion injury and antibody-mediated rejection in renal transplantation

BACKGROUND

Antibody-mediated rejection (ABMR) plays a crucial role in graft loss during allogeneic renal transplantation. In renal transplantation, ischemia-reperfusion injury (IRI) is unavoidable, serves as a major contributor to acute rejection, and is linked to graft loss. However, the mechanisms underlying IRI and ABMR are unclear. Therefore, this study aimed to investigate the shared genetic characteristics and biological mechanisms between IRI and ABMR.

METHODS

Gene expressions for IRI (GSE43974) and ABMR (GSE129166 and GSE36059) were retrieved from the Gene Expression Omnibus database. The shared differentially expressed genes (DEGs) of IRI and ABMR were identified, and subsequent functional enrichment analysis was performed. Immune cell infiltration in ABMR and its relationship with the shared DEGs were investigated using the CIBERSORT method. Random forest analysis, a protein-protein interaction network, and Cytoscape were used to screen hub genes, which were subsequently subjected to gene set enrichment analysis, miRNA prediction, and transcription factors analysis. The survival analysis was performed through Kaplan-Meier curves. Finally, drug compound prediction was performed on the shared DEGs using the Drug Signature Database.

RESULTS

Overall, 27 shared DEGs were identified between the renal IRI and ABMR groups. Among these, 24 genes exhibited increased co-expression, whereas none showed decreased co-expression. The shared DEGs were primarily enriched in the inflammation signaling pathways. Notably, CD4 memory T cells were identified as potential critical mediators of IRI, leading to ABMR. Tumor necrosis factor alpha-induced protein 3 (TNFAIP3), interferon regulatory factor 1 (IRF1), and early growth response 2 (EGR2) were identified as key components in the potential mechanism that link IRI and ABMR. Patients undergoing renal transplantation with higher expression levels of TNFAIP3, IRF1, and EGR2 exhibited decreased survival rates compared to those with lower expression levels.

CONCLUSION

Inflammation is a key mechanism that links IRI and ABMR, with a potential role played by CD4 memory T cells. Furthermore, TNFAIP3, IRF1, and EGR2 are implicated in the underlying mechanism between IRI and ABMR.

Levels of angiotensin II type-1 receptor antibodies and endothelin-1 type-A receptor antibodies correlate with antibody-mediated rejection and poor graft function in kidney-transplantation patients

OBJECTIVE

Angiotensin II type-1 receptor antibodies (AT1R-Ab) and endothelin-1 type-A receptor antibodies (ETAR-Ab) are non-human leukocyte antigen (HLA) antibodies that can elicit adverse effects on kidney transplantation (KT) outcomes. We investigated the correlation between levels of AT1R-Ab and ETAR-Ab and postoperative outcomes in KT recipients.

METHODS

Pre-KT and post-KT serum from 79 patients was collected. Post-KT serum was collected within 1 year after KT or simultaneously as the biopsy. Levels of AT1R-Ab and ETAR-Ab were measured using enzyme-linked immunosorbent assay kits. AT1R-Ab >17.0 U/mL and ETAR-Ab >10.0 U/mL was considered to denote positivity according to manufacturer recommendations. We measured donor-specific antibodies against human leukocyte antigens (HLA-DSA) levels using LABScreen™ single-antigen kits.

RESULTS

Seventy-nine (54 men, 25 women) formed the study cohort. Seven (8.7%) patients were positive for AT1R-Ab, 25 (31.6%) patients were positive for both AT1R-Ab and ETAR-Ab, and 47 (59.5%) were negative for both antibodies at all time points. No patients died during the study period. Patients with both AT1R-Ab and ETAR-Ab were associated with a higher prevalence of antibody-mediated rejection (AMR) and lower estimated glomerular filtration rate, but not allograft loss or delayed graft function. AT1R-Ab were associated with T-cell-mediated rejection, but the association was not significant. HLA-DSA were associated significantly with a higher creatinine level in serum at 12 months and 24 months in patients with AT1R-Ab and/or ETAR-Ab.

CONCLUSIONS

AT1R-Ab, ETAR-Ab, and HLA-DSA were associated with a higher prevalence of AMR and decline in graft function. Measurement of levels of AT1R-Ab and ETAR-Ab in KT patients may be useful for stratification of immunological risk and identification of patients at a high risk of adverse graft outcome.

Relationship Between Angiotensin II Type 1 Receptor Antibody Positivity and Cytokine Gene Polymorphism in Renal Transplant Patients When Organ Rejection Occurs

OBJECTIVES

Kidney transplant remains the gold standard for the treatment of end-stage renal disease. Relationships between the presence of non-HLA antibodies, antibodies to AT1R, and cytokine gene polymorphisms with rejection have recently been shown. We sought to determine whether the presence of antibodies to AT1R and cytokine gene polymorphisms affected the development of rejection in pediatric and adult patients, whether a relationship is present between cytokine polymorphism and level of antibodies to AT1R, and whether their presence can be a biomarker pretransplant.

MATERIALS AND METHODS

Our study included 100 pediatric and adult kidney transplant patients plus 50 healthy controls. Levels of AT1R antibodies (by enzyme-linked immunosorbent assay) and gene polymorphisms of the cytokines transforming growth factor β, tumor necrosis factor α, interleukins 6 and 10, and interferon gamma cytokines (by sequence- specific primer-polymerase chain reaction) were studied retrospectively and evaluated with the SPSS statistical program.

RESULTS

We found no statistically significant relationship between levels of antibodies to AT1R and gene polymorphisms among the studied cytokines in patients with rejection compared with the healthy controls and patients with uneventful courses posttransplant. However, higher levels of antibodies to AT1R were observed in pediatric compared with adult transplant recipients (P < .001). A statistically significant relationship was also observed between transforming growth factor β1 C/C G/C low-release and interleukin 6 G/C high-release gene polymorphism and levels of antibodies to AT1R (P < .001).

CONCLUSIONS

Because we observed that some gene polymorphisms among the studied cytokines may affect AT1R antibody levels, future studies are needed to understand the mechanism of the relationship. In addition, studies with larger groups are required to sufficiently confirm that higher antibody levels are present in pediatric versus adult patients.

Biomarkers in Primary Focal Segmental Glomerulosclerosis in Optimal Diagnostic-Therapeutic Strategy

Focal segmental glomerulosclerosis (FSGS) involves podocyte injury. In patients with nephrotic syndrome, progression to end-stage renal disease often occurs over the course of 5 to 10 years. The diagnosis is based on a renal biopsy. It is presumed that primary FSGS is caused by an unknown plasma factor that might be responsible for the recurrence of FSGS after kidney transplantation. The nature of circulating permeability factors is not explained and particular biological molecules responsible for inducing FSGS are still unknown. Several substances have been proposed as potential circulating factors such as soluble urokinase-type plasminogen activator receptor (suPAR) and cardiolipin-like-cytokine 1 (CLC-1). Many studies have also attempted to establish which molecules are related to podocyte injury in the pathogenesis of FSGS such as plasminogen activator inhibitor type-1 (PAI-1), angiotensin II type 1 receptors (AT1R), dystroglycan(DG), microRNAs, metalloproteinases (MMPs), forkheadbox P3 (FOXP3), and poly-ADP-ribose polymerase-1 (PARP1). Some biomarkers have also been studied in the context of kidney tissue damage progression: transforming growth factor-beta (TGF-β), human neutrophil gelatinase-associated lipocalin (NGAL), malondialdehyde (MDA), and others. This paper describes molecules that could potentially be considered as circulating factors causing primary FSGS.

Angiotensin and Endothelin Receptor Structures With Implications for Signaling Regulation and Pharmacological Targeting

In conjunction with the endothelin (ET) type A (ETAR) and type B (ETBR) receptors, angiotensin (AT) type 1 (AT1R) and type 2 (AT2R) receptors, are peptide-binding class A G-protein-coupled receptors (GPCRs) acting in a physiologically overlapping context. Angiotensin receptors (ATRs) are involved in regulating cell proliferation, as well as cardiovascular, renal, neurological, and endothelial functions. They are important therapeutic targets for several diseases or pathological conditions, such as hypertrophy, vascular inflammation, atherosclerosis, angiogenesis, and cancer. Endothelin receptors (ETRs) are expressed primarily in blood vessels, but also in the central nervous system or epithelial cells. They regulate blood pressure and cardiovascular homeostasis. Pathogenic conditions associated with ETR dysfunctions include cancer and pulmonary hypertension. While both receptor groups are activated by their respective peptide agonists, pathogenic autoantibodies (auto-Abs) can also activate the AT1R and ETAR accompanied by respective clinical conditions. To date, the exact mechanisms and differences in binding and receptor-activation mediated by auto-Abs as opposed to endogenous ligands are not well understood. Further, several questions regarding signaling regulation in these receptors remain open. In the last decade, several receptor structures in the apo- and ligand-bound states were determined with protein X-ray crystallography using conventional synchrotrons or X-ray Free-Electron Lasers (XFEL). These inactive and active complexes provide detailed information on ligand binding, signal induction or inhibition, as well as signal transduction, which is fundamental for understanding properties of different activity states. They are also supportive in the development of pharmacological strategies against dysfunctions at the receptors or in the associated signaling axis. Here, we summarize current structural information for the AT1R, AT2R, and ETBR to provide an improved molecular understanding.

Molecular Effects of Auto-Antibodies on Angiotensin II Type 1 Receptor Signaling and Cell Proliferation

The angiotensin II (Ang II) type 1 receptor (AT1R) is involved in the regulation of blood pressure (through vasoconstriction) and water and ion homeostasis (mediated by interaction with the endogenous agonist). AT1R can also be activated by auto-antibodies (AT1R-Abs), which are associated with manifold diseases, such as obliterative vasculopathy, preeclampsia and systemic sclerosis. Knowledge of the molecular mechanisms related to AT1R-Abs binding and associated signaling cascade (dys-)regulation remains fragmentary. The goal of this study was, therefore, to investigate details of the effects of AT1R-Abs on G-protein signaling and subsequent cell proliferation, as well as the putative contribution of the three extracellular receptor loops (ELs) to Abs-AT1R signaling. AT1R-Abs induced nuclear factor of activated T-cells (NFAT) signaling, which reflects Gq/11 and Gi activation. The impact on cell proliferation was tested in different cell systems, as well as activation-triggered receptor internalization. Blockwise alanine substitutions were designed to potentially investigate the role of ELs in AT1R-Abs-mediated effects. First, we demonstrate that Ang II-mediated internalization of AT1R is impeded by binding of AT1R-Abs. Secondly, exclusive AT1R-Abs-induced Gq/11 activation is most significant for NFAT stimulation and mediates cell proliferation. Interestingly, our studies also reveal that ligand-independent, baseline AT1R activation of Gi signaling has, in turn, a negative effect on cell proliferation. Indeed, inhibition of Gi basal activity potentiates proliferation triggered by AT1R-Abs. Finally, although AT1R containing EL1 and EL3 blockwise alanine mutations were not expressed on the human embryonic kidney293T (HEK293T) cell surface, we at least confirmed that parts of EL2 are involved in interactions between AT1R and Abs. This current study thus provides extended insights into the molecular action of AT1R-Abs and associated mechanisms of interrelated pathogenesis.

Donor–Recipient Non-HLA Variants, Mismatches and Renal Allograft Outcomes: Evolving Paradigms

Despite significant improvement in the rates of acute allograft rejection, proportionate improvements in kidney allograft longevity have not been realized, and are a source of intense research efforts. Emerging translational data and natural history studies suggest a role for anti-donor immune mechanisms in a majority of cases of allograft loss without patient death, even when overt evidence of acute rejection is not identified. At the level of the donor and recipient genome, differences in highly polymorphic HLA genes are routinely evaluated between donor and recipient pairs as part of organ allocation process, and utilized for patient-tailored induction and maintenance immunosuppression. However, a growing body of data have characterized specific variants in donor and recipient genes, outside of HLA loci, that induce phenotypic changes in donor organs or the recipient immune system, impacting transplant outcomes. Newer mechanisms for “mismatches” in these non-HLA loci have also been proposed during donor–recipient genome interactions with transplantation. Here, we review important recent data evaluating the role of non-HLA genetic loci and genome-wide donor-recipient mismatches in kidney allograft outcomes.

Non-HLA Antibodies in Hand Transplant Recipients Are Connected to Multiple Acute Rejection Episodes and Endothelial Activation

The role of anti-HLA antibodies in transplant rejection is well-known but the injury associated with non-HLA antibodies is now widely discussed. The aim of our study was to investigate a role of non-HLA antibodies in hand allografts rejection. The study was performed on six patients after hand transplantation. The control group consisted of: 12 kidney transplant recipients and 12 healthy volunteers. The following non-HLA antibodies were tested: antibody against angiotensin II type 1 receptor (AT1R-Ab), antibody against endothelin-1 type-A-receptor (ETAR-Ab), antibody against protease-activated receptor 1 (PAR-1-Ab) and anti-VEGF-A antibody (VEGF-A-Ab). Chosen proinflammatory cytokines (Il-1, IL-6, IFNγ) were used to evaluate the post-transplant humoral response. Laboratory markers of endothelial activation (VEGF, sICAM, vWF) were used to assess potential vasculopathy. The patient with the highest number of acute rejections had both positive non-HLA antibodies: AT1R-Ab and ETAR-Ab. The same patient had the highest VEGF-A-Ab and very high PAR1-Ab. All patients after hand transplantation had high levels of laboratory markers of endothelial activation. The existence of non-HLA antibodies together with multiple acute rejections observed in patient after hand transplantation should stimulate to look for potential role of non-HLA antibodies in humoral injury in vascular composite allotransplantation.

The Summarized Assessment of Endothelin A Receptor Expression in Renal Transplant Compartments Associated with Antibody-Mediated Rejection

The occurrence of anti-endothelin A receptor antibodies may be useful in diagnosis of transplant damage. We noticed that the presence of the endothelin A receptor (ETA receptor) in biopsy compartments is yet to be defined. We decided therefore to analysed the presence and relevance of the ETA receptor in biopsy to define the cause. Our study aims to evaluate the expression of ETA receptors in renal recipients after a biopsy due to the worsening of transplant function.

METHODS

The expression of ETA receptors was analyzed in renal transplant biopsies using the immunohistochemical method. The evaluation of ETA receptors was performed on paraffin sections. ETA receptor expression was analyzed in four compartments of renal transplant biopsies: glomeruli; vessels; tubular epithelium; and interstitium. The assessment was presented using a three-step scale (0: lack of expression; 1: mild to moderate immunoreactivity; 2: high expression). The results of each compartment from a single biopsy were summarized and assessed in the context of antibody-mediated rejection (AMR).

RESULTS

We analyzed 156 patients who had a renal allograft biopsy after renal transplantation. For each patient, we created a summarized ETA receptor expression score. The summarized ETA receptor expression score analysis showed statistically significant differences in patients with and without AMR. In addition, we noticed that patients with AMR had a significantly higher mean summarized expression of ETA receptor score of 3.28 ± 1.56 compared to patients who had a biopsy for other reasons with a mean summarized ETA receptor expression score of 1.47 ± 1.35 (p < 0.000001). ROC analysis of the ETA receptor expression score for detecting AMR status showed that the most appropriate cut-off for the test of the chosen binary classifier is between 2 and 3 of the summarized ETA receptor expression score.

CONCLUSIONS

The expression of endothelin A receptors in renal transplant compartments may be associated with antibody-mediated rejection. The positive ETA receptor staining might be a vital feature in the diagnosis of damage in AMR. The summarized ETA receptor expression score seems to be an exciting diagnostic tool in transplant injury assessment.

Effect of anti-angiotensin II type-1 receptor antibodies on the outcomes of kidney transplantation: a systematic review and meta-analysis

BACKGROUND

Anti-angiotensin II type 1 receptor antibodies (AT1R-Abs) have been recognized as non-HLA antibodies associated with allograft rejection and poor allograft outcomes after kidney transplantation. The aim of this study was to assess the risk anti-AT1R-Abs pose for rejection and graft loss among kidney transplant populations.

METHODS

We systematically searched PubMed, EMBASE, and the Cochrane Library databases for relevant articles published from inception until June 2021 to identify all studies concerning the role AT1R-Abs play in the clinical outcome after kidney transplantation. Two reviewers independently identified studies, abstracted outcome data, and assessed the quality of the studies. The meta-analysis was summarized using the fixed-effects models or random-effects models, according to heterogeneity. The major outcomes included delayed graft function, acute rejection, graft loss, or patient death after transplantation.

RESULTS

Twenty-one eligible studies involving a total of 4,023 kidney transplantation recipients were included in the evaluation to identified. Meta-analysis results showed that the AT1R-Ab positive kidney transplant (KT) group had a greater incidence of antibody-mediated rejection (RR = 1.94, 95%CI: 1.61-2.33, P < 0.00001) and graft loss (RR = 2.37, 95%CI: 1.50-3.75, P = 0.0002) than did the AT1R-Abs negative KT group. There was no significant statistical difference in delayed graft function rate, T-cell mediated rejection, mixed rejection, acute cellular rejection, acute rejection, and patient death rate between AT1R-Ab positive KT and AT1R-Ab negative KT groups.

CONCLUSIONS

Our study shows that the presence of anti-AT1R-Abs was associated with a significantly higher risk of antibody-mediated rejection and graft loss in kidney transplantation. Future studies are still needed to evaluate the importance of routine anti-AT1R monitoring and therapeutic targeting. These results shows that assessment of anti-AT1R-Abs would be helpful in determining immunologic risk and susceptibility to immunologic events for recipients.

The role of non-HLA antibodies in solid organ transplantation: a complex deliberation

PURPOSE OF REVIEW

There is tremendous interest in understanding when, if, and how non-HLA antibodies contribute to allograft injury. Numerous non-HLA target antigens have been identified and sensitization to these targets have been associated with delayed allograft function, rejection, and allograft failure. This review focuses on the clinical utility of HLA antibody testing, highlighting the strengths and limitations of current clinical studies, and the need for defining characteristics to inform non-HLA antibody pathogenicity.

RECENT FINDINGS

Clinical studies continue to show associations between non-HLA antibodies and rejection and reduced allograft survival across multiple transplanted organ types. The worst clinical outcomes continue to be observed among recipients testing positive for both non-HLA and donor-specific HLA antibodies. Mechanistic insights from both animal and clinical studies support a model in which tissue injury accompanied by an inflammatory environment influence non-HLA antibody formation and pathogenicity.

SUMMARY

Immune triggers that lead to non-HLA antibody formation and pathogenicity are complex and poorly understood. The ability of non-HLA antibodies to mediate allograft injury may depend upon their affinity and strength (titer), target specificity, density of the target antigen, and synergy with donor-specific HLA antibodies.

Challenges of Diagnosing Antibody-Mediated Rejection: The Role of Invasive and Non-Invasive Biomarkers

Kidney transplantation is the best treatment modality for end-stage kidney disease, leading to improvement in a patient’s quality and quantity of life. With significant improvements in short-term outcomes, prolonging long-term allograft and patient survival remain ongoing challenges. The ability to monitor allograft function, immune tolerance and predict rejection accurately would enable personalization and better prognostication during post-transplant care. Though kidney biopsy remains the backbone of transplant diagnostics, emerging biomarkers can help detecting kidney allograft injury early enough to prevent permanent damage and detect injury before it is clinically apparent. In this review, we summarize the recent biomarkers that have shown promise in the prediction of acute rejection with a focus on antibody-mediated rejection in kidney transplantation.