Impact and production of Non-HLA-specific antibodies in solid organ transplantation

Complement component C3 and C5b-9 deposition on hypoxia reperfused endothelial cells by non-HLA antibodies against RhoGDI2: A player involved in graft failure?

Antibodies against Rho GDP-dissociation inhibitor 2 (RhoGDI2) are associated with inferior graft survival in transplant patients receiving a kidney from deceased donors. Although this suggests that these antibodies contribute to graft injury because of ischemia, it remains unknown whether they are also pathogenically involved in the process of graft loss. To study this, we firstly analyzed the IgG subclass profile of anti-RhoGDI2 antibodies in kidney transplant recipients, and whether antibody titers change over time or because of acute rejection. Next, we investigated the expression of RhoGDI2 on primary kidney and lung endothelial cells (ECs) upon hypoxia reperfusion. In addition, the complement-fixing properties of anti-RhoGDI2 antibodies were studied using imaging flow cytometry. Anti-RhoGDI2 antibodies in patients are mainly IgG1, and titers remained stable and seemed not be changed because of rejection. Antibodies against RhoGDI2, which surface expression seemed to increase upon hypoxia reperfusion, co-localized with C3 on ECs. Binding of human IgG1 monoclonal anti-RhoGDI2 antibodies as well as patient derived antibodies, resulted in complement activation, suggesting that these antibodies are complement fixing. This study suggested a potential pathogenic role of anti-RhoGDI2 antibodies in kidney graft loss. During ischemia reperfusion, the ability of these antibodies to fix complement could be one of the mechanisms resulting in tissue injury.

Role of the Immune System in Renal Transplantation, Types of Response, Technical Approaches and Current Challenges

Advances over the last decades have made renal transplantation an important therapy for patients with end-stage renal disease, as the incidences of acute rejection and short-term transplant loss have been significantly reduced. However, long-term transplant survival remains a challenge in the renal transplantation community. The main causes of long-term graft loss are acute and chronic rejection, as well as the complications related to immunosuppression therapy. In spite of the breakthroughs achieved in recent years, histology is the gold standard technique to confirm the activation of the immune system against the graft with all the ensuing problems that taking biopsies brings to immunosuppressed patients. For this reason, several assays have been developed to try to monitor the immune function, but they show serious constraints owing to the fact that they require substantial laboratory work, they are not clinically available and they provide controversial results, so the combination of multiple assays is often needed to obtain a reliable diagnosis. Thus, the aim of this review is to perform a retrospective study of the immune system in renal transplantation, with special emphasis on the cutting-edge technological developments for monitoring, classification and early detection of rejection episodes in order to contribute to a better adjustment of immunosuppressive therapies and, hence, to a more personalized medicine that improves the quality of life of patients.

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.

Novel allosteric ligands of the angiotensin receptor AT1R as autoantibody blockers

While orthosteric ligands of the angiotensin II (AngII) type 1 receptor (AT1R) are available for clinical and research applications, allosteric ligands are not known for this important G protein-coupled receptor (GPCR). Allosteric ligands are useful tools to modulate receptor pharmacology and subtype selectivity. Here, we report AT1R allosteric ligands for a potential application to block autoimmune antibodies. The epitope of autoantibodies for AT1R is outside the orthosteric pocket in the extracellular loop 2. A molecular dynamics simulation study of AT1R structure reveals the presence of a druggable allosteric pocket encompassing the autoantibody epitope. Small molecule binders were then identified for this pocket using structure-based high-throughput virtual screening. The top 18 hits obtained inhibited the binding of antibody to AT1R and modulated agonist-induced calcium response of AT1R. Two compounds out of 18 studied in detail exerted a negative allosteric modulator effect on the functions of the natural agonist AngII. They blocked antibody-enhanced calcium response and reactive oxygen species production in vascular smooth muscle cells as well as AngII-induced constriction of blood vessels, demonstrating their efficacy in vivo. Our study thus demonstrates the feasibility of discovering inhibitors of the disease-causing autoantibodies for GPCRs. Specifically, for AT1R, we anticipate development of more potent allosteric drug candidates for intervention in autoimmune maladies such as preeclampsia, bilateral adrenal hyperplasia, and the rejection of organ transplants.

Association of non-HLA antibodies against endothelial targets and donor-specific HLA antibodies with antibody-mediated rejection and graft function in pediatric kidney transplant recipients

BACKGROUND

Non-HLA antibodies against endothelial targets have been implicated in the pathogenesis of antibody-mediated rejection (ABMR), but data in pediatric patients are scarce.

METHODS

We retrospectively analyzed a carefully phenotyped single-center (University Children's Hospital Heidelberg, Germany) cohort of 62 pediatric kidney transplant recipients (mean age at transplantation, 8.6 ± 5.0 years) at increased risk of graft function deterioration. Patients had received their transplant between January 1, 1999, and January 31, 2010. We examined at time of late index biopsies (more than 1-year post-transplant, occurring after January 2004) the association of antibodies against the angiotensin II type 1 receptor (AT₁R), the endothelin type A receptor (ET_AR), the MHC class I chain-like gene A (MICA), and vimentin in conjunction with overall and complement-binding donor-specific HLA antibodies (HLA-DSA) with graft histology and function.

RESULTS

We observed a high prevalence (62.9%) of non-HLA antibody positivity. Seventy-two percent of HLA-DSA positive patients showed additional positivity for at least one non-HLA antibody. Antibodies against AT₁R, ET_AR, and MICA were associated with the histological phenotype of ABMR. The cumulative load of HLA-DSA and non-HLA antibodies in circulation was related to the degree of microinflammation in peritubular capillaries. Non-HLA antibody positivity was an independent non-invasive risk factor for graft function deterioration (adjusted hazard ratio 6.38, 95% CI, 2.11-19.3).

CONCLUSIONS

Our data indicate that the combined detection of antibodies to HLA and non-HLA targets may allow a more comprehensive assessment of the patients' immune responses against the kidney allograft and facilitates immunological risk stratification.

Antibodies against HLA cross-reactivity groups: From single antigen bead assay to immunoinformatics interpretation of epitopes

Identification of anti-human leukocyte antigen (HLA) antibodies (Abs) is based on Luminex™ technology. We used bioinformatics to (i) study the correlations of mean fluorescence intensities (MFIs) for all the possible allele pairs, and (ii) determine the degree of epitope homology between HLA antigens. Using MFI data on anti-HLA Abs from 6000 Luminex™ assays, we provide an updated overview of class I and II HLA antigen cross-reactivity in which each node corresponded to an allele and each link corresponded to a strong correlation between two alleles (Spearman's ρ > 0.8). We compared these correlations with the serological groups and the results of an epitope analysis. The strongest correlations concerned allele-specific Abs directed against the same antigen. For the HLA-A locus, the highest values of Spearman's ρ reflected broad specificity. For the HLA-B locus, graphs defined the HLA-Bw4 public epitope, and correlations between HLA-A and -B alleles were only present for beads with the same Bw4 public epitope. For the HLA-C locus, we identified two groups that differed with regard to their KIR ligand subclassification. Lastly, the HLA-DRB1 subgroups were part of a network. In the epitope analysis, Spearman's ρ was related to the number of matched epitopes within pairs of alleles. The combination of Spearman's ρ with simple, undirected graphing constitutes an effective tool for understanding routinely encountered cross-reactivity profiles. Based on this model, we have implemented an online data visualization tool available at http://cusureau.pythonanywhere.com/.

Kidney re-transplantation in a child across the barrier of persisting angiotensin II type I receptor antibodies

BACKGROUND

Approximately 20% of antibody-mediated rejection (ABMR) episodes in the absence of donor-specific antibodies against human leucocyte antigens (HLA-DSA) in pediatric and adult kidney transplant recipients are associated with, and presumably caused by, antibodies against the angiotensin type 1 receptor (AT₁R-Ab). While the role of AT₁R-Ab for ABMR and graft failure is increasingly recognized, there is little information available on the management of these patients for re-transplantation over the barrier of persisting AT₁R-Ab.

CASE

We report on a male patient with kidney failure in infancy due to obstructive uropathy who had lost his first kidney transplant due to AT₁R-Ab-mediated chronic ABMR. Because this antibody persisted during 4 years of hemodialysis, for the 2nd kidney transplantation (living-related transplantation from his mother), he underwent a desensitization regimen consisting of 15 plasmapheresis sessions, infusions of intravenous immunoglobulin G and thymoglobulin, as well as pharmacological blockade of the Angiotensin II (AT II) pathway by candesartan. This intense desensitization regimen transiently decreased elevated AT₁R-Ab titers, resulting in stable short-term kidney allograft function. The subsequent clinical course, however, was complicated by acute cellular rejection and chronic ABMR due to persistent AT₁R-Ab and de novo HLA-DSA, which shortened allograft survival to a period of only 4 years.

CONCLUSION

This case highlights the difficulty of persistently decreasing elevated AT₁R-Ab titers by a desensitization regimen for re-transplantation and the detrimental effect of the interplay between AT₁R-Ab and HLA-DSA on kidney transplant survival.

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.