Patients with steroid refractory acute vascular rejection develop agonistic antibodies targeting angiotensin II type 1 receptor

Antibody-mediated rejection after kidney transplantation in children; therapy challenges and future potential treatments

Antibody‐mediated rejection (AMR) remains one of the most critical problems in renal transplantation, with a significant impact on patient and graft survival. In the United States, no treatment has received FDA approval jet. Studies about treatments of AMR remain controversial, limited by the absence of a gold standard and the difficulty in creating large, multi‐center studies. These limitations emerge even more in pediatric transplantation because of the limited number of pediatric studies and the occasional use of some therapies with unknown and poorly documented side effects. The lack of recommendations and the unsharp definition of different forms of AMR contribute to the challenging management of the therapy by pediatric nephrologists. In an attempt to help clinicians involved in the care of renal transplanted children affected by an AMR, we rely on the latest recommendations of the Transplantation Society (TTS) for the classification and treatment of AMR to describe treatments available today and potential new treatments with a particular focus on the pediatric population.

Antibodies against Angiotensin II Type 1 and Endothelin 1 Type A Receptors in Cardiovascular Pathologies

Angiotensin II receptor type 1 (AT1R) and endothelin-1 receptor type A (ETAR) are G-protein-coupled receptors (GPCRs) expressed on the surface of a great variety of cells: immune cells, vascular smooth cells, endothelial cells, and fibroblasts express ETAR and AT1R, which are activated by endothelin 1 (ET1) and angiotensin II (AngII), respectively. Certain autoantibodies are specific for these receptors and can regulate their function, thus being known as functional autoantibodies. The function of these antibodies is similar to that of natural ligands, and it involves not only vasoconstriction, but also the secretion of proinflammatory cytokines (such as interleukin-6 (IL6), IL8 and TNF-α), collagen production by fibroblasts, and reactive oxygen species (ROS) release by fibroblasts and neutrophils. The role of autoantibodies against AT1R and ETAR (AT1R-AAs and ETAR-AAs, respectively) is well described in the pathogenesis of many medical conditions (e.g., systemic sclerosis (SSc) and SSc-associated pulmonary hypertension, cystic fibrosis, and allograft dysfunction), but their implications in cardiovascular diseases are still unclear. This review summarizes the current evidence regarding the effects of AT1R-AAs and ETAR-AAs in cardiovascular pathologies, highlighting their roles in heart transplantation and mechanical circulatory support, preeclampsia, and acute coronary syndromes.

Intimal Arteritis and Microvascular Inflammation Are Associated With Inferior Kidney Graft Outcome, Regardless of Donor-Specific Antibodies

Background: The prognostic role of intimal arteritis of kidney allografts in donor-specific antibody negative (DSA–) antibody-mediated rejection (ABMR) remains unclear.

Methods: Seventy-two out of 881 patients who had undergone kidney transplantation from 2014 to 2017 exhibited intimal arteritis in biopsies performed during the first 12 months. In 26 DSA negative cases, the intimal arteritis was accompanied by tubulointerstitial inflammation as part of T cell-mediated vascular rejection (TCMRV, N = 26); intimal arteritis along with microvascular inflammation occurred in 29 DSA negative (ABMRV/DSA–) and 19 DSA positive cases (ABMRV, DSA+, N = 17). In 60 (83%) patients with intimal arteritis, the surveillance biopsies after antirejection therapy were performed. Hundred and two patients with non-vascular ABMR with DSA (ABMR/DSA+, N = 55) and without DSA (ABMR/DSA–, N = 47) served as controls. Time to transplant glomerulopathy (TG) and graft failure were the study endpoints.

Results: Transplant glomerulopathy -free survival at 36 months was 100% in TCMRV, 85% in ABMR/DSA–, 65% in ABMRV/DSA-, 54% in ABMR/DSA+ and 31% in ABMRV/DSA+ (log rank p < 0.001). Death-censored graft survival at 36 months was 98% in ABMR/DSA-, 96% in TCMRV, 86% in ABMRV/DSA–, 79% in ABMR/DSA+, and 64% in ABMRV/DSA+ group (log rank p = 0.001). In surveillance biopsies, the resolution of rejection was found in 19 (90%) TCMRV, 14 (58%) ABMRV/DSA–, and only 4 (27%) ABMRV/DSA+ patients (p = 0.006). In the multivariable model, intimal arteritis as part of ABMR represented a significant risk for TG development (HR 2.1, 95% CI 1.2–3.8; p = 0.012) regardless of DSA status but not for graft failure at 36 months.

Conclusions: Intimal arteritis as part of ABMR represented a risk for early development of TG regardless of the presence or absence of DSA. Intimal arteritis in DSA positive ABMR represented the high-risk phenotype.

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.

C4d-negative antibody-mediated rejection with high anti-angiotensin II type I receptor antibodies in absence of donor-specific antibodies

AIMS

Acute antibody-mediated rejection can occur in absence of circulating donor-specific antibodies. Agonistic antibodies targeting the anti-angiotensin II type 1 receptor (anti-AT1 R) are emerging as important non-human leucocyte antigen (HLA) antibodies. Elevated levels of anti-angiotensin II receptor antibodies were first observed in kidney transplant recipients with malignant hypertension and allograft rejection. They have now been studied in three separate kidney transplant populations and associate to frequency of rejection, severity of rejection and graft failure.

METHODS

We report 11 cases of biopsy-proven, Complement 4 fragment d (C4d)-negative, acute rejection occurring without circulating donor-specific anti-HLA antibodies. In eight cases, anti-angiotensin receptor antibodies were retrospectively examined. The remaining three subjects were identified from our centre's newly instituted routine anti-angiotensin receptor antibody screening.

RESULTS

All subjects fulfilled Banff 2013 criteria for antibody-mediated rejection and all responded to anti-rejection therapy, which included plasma exchange and angiotensin receptor blocker therapy.

CONCLUSIONS

These cases support the routine assessment of anti-AT1 R antibodies in kidney transplant recipients to identify subjects at risk. Further studies will need to determine optimal assessment protocol and the effectiveness of pre-emptive treatment with angiotensin receptor blockers.

Long-term follow-up of non-HLA and anti-HLA antibodies: incidence and importance in renal transplantation

BACKGROUND

Detection of antibody-mediated injury is becoming increasingly important in post-transplant patient care. The role of donor-specific anti-human leukocyte antigen (HLA) antibodies in kidney transplant damage is known, whereas the significance of non-HLA antibodies remains an unresolved concern. The aim of the study was to determine the presence and influence on renal function of non-HLA and anti-HLA antibodies in stable patients at 5 years after kidney transplantation.

METHODS

We evaluated the antibodies in 35 consecutive patients with stable renal function at 5 years after transplantation.

RESULTS

Pretransplant screening for donor-specific antibodies by CDC cross-matches was negative in all patients. Anti-endothelial cell antibodies (AECA), anti-angiotensin II type 1 receptor antibodies (anti-AT1R), and anti-endothelin receptor antibodies (anti-ETAR) were assayed as non-HLA antibodies. Non-HLA antibodies were observed in 12 (34%) patients, including AECA (n = 5; 14%), anti- AT1R (n = 6; 17%), anti-ETAR (n = 4; 11%), and both anti-AT1R and anti-ETAR (n = 3). Among 13 (37%) patients with anti-HLA antibodies, 7 also had both non-HLA antibodies: AECA (n = 1), anti-AT1R (n = 3), and anti-ETAR (n = 3). The antibody-negative group (n = 13) showed significantly better renal function than the antibody-positive group (non-HLA and/or anti-HLA; n = 22). Biopsy-proven acute rejection had occurred in 2 of 13 (15%) antibody-negative versus 8 of 22 (36%) antibody-positive patients. These preliminary data revealed an high prevalence of autoantibody and alloantibody production among stable patients at 5 years after kidney transplantation.

CONCLUSION

Simultaneous production of these antibodies and their association with reduced renal function suggests that active humoral immune responses are poorly controlled by immunosuppression.

Receptor-activating autoantibodies and disease: preeclampsia and beyond

The research reviewed in this article provides examples of autoantibody-mediated receptor activation that likely contributes to disease. The classic example is Graves' hyperthyroidism, in which autoantibodies activate the thyroid-stimulating hormone receptor resulting in overproduction of thyroid hormones. Other compelling examples come from the cardiovascular literature and include agonistic autoantibodies targeting the cardiac β(1)-adrenergic receptor, which are associated with dilated cardiomyopathy. Autoantibodies capable of activating α(1)-adrenergic receptors are associated with refractory hypertension and cardiomyopathy. A prominent example is preeclampsia, a hypertensive disease of pregnancy, characterized by the presence of autoantibodies that activate the major angiotensin receptor, AT(1). AT(1) receptor-activating autoantibodies are also observed in kidney transplant recipients suffering from severe vascular rejection and malignant hypertension. AT(1) receptor-activating autoantibodies and antibodies that activate the endothelin-1 receptor, ET(A), are prevalent in individuals diagnosed with systemic sclerosis. Thus, the presence of agonistic autoantibodies directed to G protein-coupled receptors has been observed in numerous cardiovascular disease states. Rapidly emerging evidence indicates that receptor-activating autoantibodies contribute to disease, and that efforts to detect and remove these pathogenic autoantibodies or block their actions will provide promising therapeutic possibilities.

Inhibition of the renin-angiotensin system and target organ protection

The renin-angiotensin system (RAS) is involved in the pathological mechanisms of target organ damage, as well as in the induction of hypertension. RAS inhibition by angiotensin converting enzyme (ACE) inhibitors and angiotensin (Ang) II receptor blockers can prevent tissue damage by inhibition of Ang II type 1 receptor signaling. A beneficial effect of RAS inhibition on the heart, vasculature and kidney in cardiovascular disease has been reported. However, RAS inhibition can also prevent fibroproliferative diseases and damage of other tissues, such as brain, adipose tissue and muscle, because local RAS has an important role in tissue damage compared with circulating RAS. Moreover, other players, such as Ang II type 2 receptor signaling, aldosterone and ACE2 have been highlighted. Furthermore, there has also been a focus on the emerging concept of regulation of RAS, such as receptor-interacting proteins and receptor modifications, in the new discovery of therapeutic agents for tissue protection. The RAS has a pivotal role in various target organ damage, with complicated mechanisms; therefore, blockade of RAS may be therapeutically effective in preventing organ damage, as well as in having an antihypertensive effect.

Gene polymorphisms of the renin-angiotensin-aldosterone system and angiotensin II type 1-receptor activating antibodies in renal rejection

Steroid refractory acute rejection (SRAR) is a major vital factor in renal transplantation recipients. The pathogenesis of SRAR may involve both immune and non-immune mechanisms. A decreased renal allograft function has also been associated with increased activity of renin-angiotensin-aldosterone system (RAS), which may be genetically determined. A total 206 renal transplant recipients, 116 males and 90 females, were included. The effects of gene polymorphisms of the four components of RAS including angiotensinogen (AGT), angiotensin-converting enzyme (ACE), angiotensin type 1 receptor (AT1R), and aldosterone synthase (CYP11B2) were investigated in 19 cases of renal transplant patients with SRAR. The association between SRAR and the activating antibodies targeting the AT1R were also investigated. Genotyping was performed for the M235T-AGT, the I/D-ACE, the A1166C-AT1R, and the -344T/C-CYP11B2 gene polymorphisms using polymerase chain reaction. Our results showed that renal allograft recipients with SRAR had significantly higher occurrences of the DD genotype of ACE and CC genotype of AT1R than recipients without SRAR. The other genetic polymorphisms of the RAS were not associated with SRAR. Activating antibodies targeting the AT1R were detected in the sera from 14 SRAR victims with malignant hypertension and without anti-HLA antibodies. This study provides evidence that determination before transplantation of the polymorphism of the gene encoding components of RAS may help identify patients who are at risk for SRAR. The detection of the antibodies of AT1R may contribute to the prevention of SRAR.

Emerging Concepts of Regulation of Angiotensin II Receptors

Angiotensin (Ang) II exerts its important physiological functions through 2 distinct receptor subtypes, type 1 (AT 1 ) and type 2 (AT 2 ) receptors. Recently, new evidence has accumulated showing the existence of several novel receptor interacting proteins and various angiotensin II receptor activation mechanisms beyond the classical actions of receptors for Ang II. These associated proteins could contribute not only to Ang II receptors’ functions, but also to influencing pathophysiological states. Receptor dimerization of Ang II receptors such as homodimer, heterodimer, and complex formation with other G protein-coupled receptors has also been focused on as a new mechanism of their activation or inactivation. Moreover, ligand-independent receptor activation systems such as mechanical stretch for the AT 1 receptor have also been revealed. These emerging concepts of regulation of Ang II receptors and a new insight into future drug discovery are discussed in this review.

Agonist autoantibodies against the angiotensin AT1 receptor in renal and hypertensive disorders

Previous studies demonstrated the significance of an agonistic angiotensin II receptor AT1 autoantibody (AT1-AA) in preeclampsia. Because of its ability to release calcium in vascular smooth muscle cells, stimulate reactive oxygen species, and initiate proinflammatory processes, this antibody was thought to be important in the etiology and pathogenesis of preeclampsia. Recent investigations, however, have broadened and refined the pathobiological relevance of this antibody and refuted its role as the primary cause for preeclampsia. Because AT1-AA has been linked to an impaired uteroplacental perfusion and has been detected in patients with renal allograft rejection, its occurrence and function seem to be wider and more complex. This review summarizes current knowledge about the generation, function, and clinical importance of AT1-AA.

Angiotensin II type 1-receptor activating antibodies in renal-allograft rejection

BACKGROUND

Antibodies against HLA antigens cause refractory allograft rejection with vasculopathy in some, but not all, patients.

METHODS

We studied 33 kidney-transplant recipients who had refractory vascular rejection. Thirteen had donor-specific anti-HLA antibodies, whereas 20 did not. Malignant hypertension was present in 16 of the patients without anti-HLA antibodies, 4 of whom had seizures. The remaining 17 patients had no malignant hypertension. We hypothesized that activating antibodies targeting the angiotensin II type 1 (AT1) receptor might be involved.

RESULTS

Activating IgG antibodies targeting the AT1 receptor were detected in serum from all 16 patients with malignant hypertension and without anti-HLA antibodies, but in no other patients. These receptor-activating antibodies are subclass IgG1 and IgG3 antibodies that bind to two different epitopes on the second extracellular loop of the AT1 receptor. Tissue factor expression was increased in renal-biopsy specimens from patients with these antibodies. In vitro stimulation of vascular cells with an AT1-receptor-activating antibody induced phosphorylation of ERK 1/2 kinase and increased the DNA binding activity of the transcription factors activator protein 1 (AP-1) and nuclear factor-kappaB. The AT1 antagonist losartan blocked agonistic AT1-receptor antibody-mediated effects, and passive antibody transfer induced vasculopathy and hypertension in a rat kidney-transplantation model.

CONCLUSIONS

A non-HLA, AT1-receptor-mediated pathway may contribute to refractory vascular rejection, and affected patients might benefit from removal of AT1-receptor antibodies or from pharmacologic blockade of AT1 receptors.