Novelties in diagnostics and therapy of antibody mediated rejection
Department of Nephrology and Intensive Care Medicine Campus Virchow-Klinikum, Center for Cardiovascular Research and Institute for Pathology Medical Faculty of the Charité Berlin
Correspondence to: Dr Duska Dragun, Department of Nephrology and Intensive Care Medicine, Charite Campus Virchow Clinic, Augustenburger Platz 1, 13353 Berlin, Germany. Email: duska.dragun{at}charite.de
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Abstract |
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 Top Abstract IntroductionDiagnostic criteria for AMRTherapy of AMR induced...Agonistic non-HLA antibodiesAT1R-AA - effectors of...References |
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Antibody mediated rejection (AMR) is becoming an increasing challenge in renal allotransplantation. AMR is usually associated with profound allograft dysfunction and inferior allograft survival. In the era of refined crossmatch-techniques and improved solid-phase assays, hyperacute rejections in presensitized patients are less common. Majority of AMRs occur due to de novo production of donor specific antibodies. AMR appears to stand for a spectrum of histologic changes paralled with immunohistochemical positivity for C4d along peritubular capillaries. Removal of antibodies with plasmapheresis or immunoadsorption in combination with neutralizing and immunomodulatory intravenous immunoglobulin are therapy standards in majority of transplant centers worldwide. The addition of anti-CD20 (rituximab) with the aim to reduce the number of B-cells may offer advantage in some cases. Apart from donor specific HLA-antibodies, non-HLA antibodies reacting to arterial antigens have been speculated to be responsible for rejections in some patients. More precise definition of antigen targets for non-HLA antibodies is emerging. The analysis of the subtle mechanistic and diagnostic differences among rejection subtypes will help to identify patients at particular risk and improve outcome of AMR.
Keywords: alloantibodies; apheresis; kidney transplantation; non-HLA antibodies; rejection
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Introduction |
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TopAbstractIntroductionDiagnostic criteria for AMRTherapy of AMR induced...Agonistic non-HLA antibodiesAT1R-AA - effectors of...References |
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Antibody mediated rejection (AMR) remains a diagnostic and therapeutic challenge. When AMR occurs in renal transplants, the process resists to conventional treatments and significant proportions of patients loose their allografts [1]. Depending on the circulating antibody titers, the specificity, and the magnitude of the host immune response, the alloantibody response can be clinically more or less significant [2]. Donor-specific anti-HLA alloantibodies initiate rejection through complement-mediated and antibody-dependent cell-mediated cytotoxicity [3]. The accumulation of the complement degradation product, C4d is generally regarded as a marker for an antibody-mediated allo-response and is associated with poor graft survival [4]. At present all C4d positive cases coinciding with detected donor specific antibodies by solid-phase assays or flow-crosmmatch techniques are classified as AMR [2]. Nevertheless, the diagnostic value and timing of various HLA and non-HLA antibody tests in the management of patients with or without allograft dysfunction is uncertain. In parallel, there has been increasing attention on development of alternative therapies that promptly and effectively control antibody mediated response, and maximize renal allograft survival. The choice of rational diagnostic modalities and/or optimal immunosuppressive regimen for AMR remains a subject of intensive research, clinical trials, and intense debate within a transplant community. There is also an increasing awareness that AMR forms a heterogeneous group of clinical, etiological, and histopathological entities. One way to improve outcome of AMR will also be to detail the subtle diagnostic and mechanistic differences in order to subcategorize patients who might respond to a particular therapy.
We aim to update on the recent advances in the field of AMR. A brief overview of our own findings on agonistic non-HLA antibodies will be also included.
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Diagnostic criteria for AMR |
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TopAbstractIntroductionDiagnostic criteria for AMRTherapy of AMR induced...Agonistic non-HLA antibodiesAT1R-AA - effectors of...References |
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Histologic diagnosis and grading according to Banff classification remains a gold standard to diagnose and classify rejection [5]. During the last decade, the Banff classification underwent and still undergoes dynamic modifications in attempt to unite morphology with causes and clinical course of rejection [5,6]. The classification of AMR seems to be particularly difficult. As more we learn about the AMR, it seems that the term AMR applies to a wide variety of more or less severe vascular and epithelial lesions in the allograft. The presence of donor-specific alloantibodies (DSA) directed toward class I and class II human leukocyte antigens (HLA) in serum indicates in accordance with the accumulation of C4d along peritubular capillaries a direct interaction of alloantibodies with the allograft [7].
H. Feucht from Munich published in 1993 a landmark paper that pioneered detection of C4d as a marker for antibody-mediated alloresponse. C4d is a fragment of C4b, an activation product of the classic complement pathway [8]. Since then, immunohistochemical detection of linear deposition of C4d on peritubular capillaries with polyclonal antibody on frozen or monoclonal antibody on paraffin embedded sections became an essential tool in diagnosis of AMR [9]. However, C4d positivity is despite positive HLA-DSA serology difficult to intereprete in the absence of morphologic changes and in absence of apparent allograft dysfunction [9]. Controversy also exists for C4d positivity and various AMR associated morphological findings in either indication or protocol biopsies. For example, 40-50% of rejections with poor prognosis are C4d negative, implicating involvement of non-complement fixing antibodies [10]. Another confusing issue is that circulating DSA and C4d positivity can often be seen in association with different degrees of cellular alloresponse ranging from no T-cell infiltrates to severe tubulo-interstitial rejection or even transplant endarteritis [11]. Sometimes, there are B-cell or plasma cell rich infiltrates in the biopsies [12]. In view of all these findings, we are beginning to cope with many faces of AMR. Once the AMR is diagnosed, it is of great therapeutic significance to determine whether or not the patient had developed so called "pure AMR" without T-cell infiltrates or any kind of mixed-rejection [11]. Serial measurement of anti-HLA DSA and control biopsies may represent important tools to ensure adequate follow-up.
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Therapy of AMR induced by anti-HLA DSA |
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TopAbstractIntroductionDiagnostic criteria for AMRTherapy of AMR induced...Agonistic non-HLA antibodiesAT1R-AA - effectors of...References |
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AMR requires different therapy than T-cell mediated rejection. Until very recently, the majority of transplant nephrologists relied on a "standard" approach based on tacrolimus, mycophenolate mofetil, steroids, and high- or low-dose intravenous immunoglobulin in combination with plasma exchange [13]. Akin to hematologists or rheumatologists, nephrologists are now focusing on the concept of B-cell depletion employing anti-CD20 antibody, rituximab for cases who do not respond to "standard treatment" [14]. However, the use of these modalities is empirical. One can however understand that it almost impossible to design the trial that leaves out any of these components. Immunoadsorption with protein A was tested in a randomized trial and reversed AMR in 5/5 cases, whereas 4/5 without early immunoadsroption lost their allografts [15].
As it has become obvious that all AMR episodes are not equal, the "standard treatment" may not be applicable for all cases. Design of future trials should consider refined serologic and histologic criteria.
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Agonistic non-HLA antibodies |
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TopAbstractIntroductionDiagnostic criteria for AMRTherapy of AMR induced...Agonistic non-HLA antibodiesAT1R-AA - effectors of...References |
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Recipients of HLA identical kidneys can develop features of refractory AMR with vascular pathology, implicating putative pathogenic antibodies that are not directed against the HLA system [16]. Unknown immune targets and consecutive lack of detection methods make non-HLA antibody mediated rejection particularly difficult to diagnose and treat. In renal transplant rejection, the presence of antibodies to non-HLA antigens has been associated with antibodies against endothelial cells, tubular epithelial cells, podocytes, mesangial cells and monocytes [17]. Many of these antibodies appear to be autoantibodies. Most attention has been focused on anti-endothelial cell antibodies (AECA). AECA represent an extremely heterogeneous family of antibodies reacting with different structures on endothelial cells, the functional effect of which remains to be determined [18]. Vascular bed specific antigen heterogeneity of endothelial cells precluded a development of standardized tests [18]. Moreover, initially detected target antigens of anti-endothelial cell antibodies were of unclear functional relevance. Determination of their antigen specificity is essential to clarify the pathogenic mechanisms by which they contribute to rejection or other forms of allograft injury.
We recently reported the presence of agonistic antibodies against the Angiotensin II type 1 receptor (AT1R-AA) in 16 recipients of renal allografts who had severe vascular rejection and malignant hypertension, but who did not have anti-HLA antibodies [19]. The clinical presentation of other 15 transplant recipients with AT1R-AA was at first similar to that of anti-HLA antibodies, albeit with more severe histology [19]. AT1R-AA have also been associated with preeclampsia [20] and malignant hypertension [21]. The clinical manifestation of rejection in our index-patient was so reminiscent of eclamptic crisis in pregnancy, a condition that she had developed two decades before transplantation, that we begun to prospectively study patients with similar clinical features. Transplants from all patients exhibited endarteritis, transmural arteritis and/or fibrinoid vascular necrosis (Banff IIb or Banff III rejection) as shown in a representative biopsy (Figure 1). Apart from arterial changes, we also noticed tubulitis and interstitial infiltrates, characteristic for acute cellular rejection. In contrast to anti-HLA antibody positive patients, AT1R-AA positivity was invariably associated with severe hypertension in all and seizures in some of patients. Most of patients (13/16) did not have hypertension before vascular rejection occurred, implying that the post-transplantation hypertension was secondary to rejection. AT1R-AA-related vascular rejections occurred during the first week after transplantation.
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Seven among total of 16 AT1R-AA patients were treated with combination therapy consisting of plasmapheresis, intravenous immune globulin infusions, and the AT1R-blocker, losartan. The treatment significantly prolonged allograft survival compared to AT1R-AA positive patients who received standard treatment option [19]. All 16 patients from our initial study had pre-formed AT1R-AA and some of them were treated either with ACE-inhibitors or AT1R-antagonists while on dialysis. We speculate that the absence or discontinuation of RAS blockade in combination with permissive factors surrounding renal transplantation may have predisposed them to develop AT1R-AA-related vascular rejection. Thus, continous pharmacologic blockade of AT1R in patients on waiting list for kidney transplantation detected as AT1R-AA positive may be a useful approach to prevent or attenuate vascular rejection. We have now established and validated a cell based ELISA in collaboration with biotech partnerts for detection of AT1R-AA in serum [22]. Pretransplantation screening for AT1R-AA detects a subset of ESRD patients who are similar but not identical with patients with anti-HLA-panel reacitivity.
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AT1R-AA – effectors of vascular damage |
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TopAbstractIntroductionDiagnostic criteria for AMRTherapy of AMR induced...Agonistic non-HLA antibodiesAT1R-AA - effectors of...References |
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In contrast to HLA-antibodies, AT1R-AA seem to operate through complement independent mechanisms as C4d was detected in biopsy specimens from only 5 of our 16 patients [19]. According to our working concept AT1R-AA bind to second extracellular loop of AT1R and act as allosteric receptor agonists via initiation of signal transduction cascades in endothelial and vascular smooth muscle cells. Consequent increasing DNA binding activity of transcription factors activated activator protein 1 (AP-1) and nuclear factor-
B (NF-B) is responsible for increased expression of their target genes involved in inflammatory responses and coagulation. Renal transplant biopsy specimens obtained during an AT1R-AA mediated rejection episode revealed intense diffuse Tissue Factor staining of epithelial, endothelial and mesangial cells in absence of complement activation. As human mononuclear cells express AT1R [23], an effect of AT1R-AA on T-cells or monocytes needs to be evaluated yet appears to be likely. Whether or not AT1R-AA-mediated process represents a "true-rejection" or an autoimmune phenomenon triggered in the permissive allogeneic and postiischemic inflammatory enviroment is subject of our current investigations. Our current working hypothesis is that factors surrounding the organ transplantation process may contribute to the overall reactivity of the target cells by enhancing AT1R-AA binding to AT1R. An allogeneic background, brain death associated "cytokine storm", reperfusion injury to the transplant and/or use of calcineurin inhibitors or steroids are probably permissive factors responsible for an increased AT1R density on target cells. Another possibility is that perioperative discontninuation of ACE-inhibitors or AT1R-blockers predisposes for AT1R-AA related pathologies. For example AT1R-AA positive patients who receive continuously AT1R-blockers or ACE-inhibitors together with intensified immunosuppression (depletional antibody induction, tacrolimus, MMF, and steroids) and are recipients of leaving donor kidneys seem not to be prone for development of fulminant AT1R-AA related rejection [24].
For the moment, we provide a new concept where autoimmune receptor activation induces severe vascular pathology in the situation of allogeneic transplantation. We are beginning to learn more about pathophysiologic mechanisms and optimization of diagnostic and therapeutic modalities for AT1R-AA positive patients.
Conflict of interest statement. None declared
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TopAbstractIntroductionDiagnostic criteria for AMRTherapy of AMR induced...Agonistic non-HLA antibodiesAT1R-AA - effectors of...References |
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