Abstract and Introduction
Abstract
Advances in multimodal immunotherapy have significantly reduced acute rejection rates and substantially improved 1-year graft survival following renal transplantation. However, long-term (10-year) survival rates have stagnated over the past decade. Recent studies indicate that antibody-mediated rejection (ABMR) is among the most important barriers to improving long-term outcomes. Improved understanding of the roles of acute and chronic ABMR has evolved in recent years following major progress in the technical ability to detect and quantify recipient anti-HLA antibody production. Additionally, new knowledge of the immunobiology of B cells and plasma cells that pertains to allograft rejection and tolerance has emerged. Still, questions regarding the classification of ABMR, the precision of diagnostic approaches, and the efficacy of various strategies for managing affected patients abound. This review article provides an overview of current thinking and research surrounding the pathophysiology and diagnosis of ABMR, ABMR-related outcomes, ABMR prevention and treatment, as well as possible future directions in treatment.
Introduction
The widespread use of potent and specific immunosuppressive agents has significantly reduced acute cellular rejection rates and substantially improved 1-year graft survival following renal transplantation. Substantial improvement of long-term (10-year) outcomes, however, has not been realized. A recent analysis of more than 250,000 North American renal transplant recipients showed that despite modest improvements in long-term graft survival between 1989 and 2005, and improvements in graft half-life in the past decade for both living and deceased donor transplants, high attrition rates persist that stubbornly limit recent progress.
The ongoing therapeutic challenge is to achieve effective and safe immunosuppression and avoid unwanted toxicities to produce enduring renal allograft function. The incidence of hyperacute rejection caused by preexisting anti-HLA donor-specific antibodies (DSA) has been nearly eliminated by crossmatch and compatibility matching strategies. Similarly, the incidence of acute T cell–mediated injury has been significantly reduced with the effective multimodal application of immunosuppressive agents. However, acute and chronic antibody-mediated rejection (ABMR) are playing an increasingly critical role in kidney allograft loss and are considered among the most important barriers that limit long-term outcomes.
Although the cellular and molecular pathways that regulate ABMR are still under investigation, new knowledge of humoral immunobiology indicate that B cell and plasma cell activation results in the generation of DSA, which bind to HLA or non-HLA molecules on the endothelium. Antibody binding to endothelium and subsequent cellular activation involving complement-dependent and -independent pathways leads to the recruitment of natural killer (NK) cells, polymorphonuclear neutrophils and macrophages, which contribute to capillaritis and eventual tissue injury (Figure 1). The morphologic nature of endothelial cell injury in acute ABMR demonstrates platelet aggregation, thrombotic microangiopathy (TMA) and neutrophilic accumulation, resulting in an early pattern of cellular necrosis and a relatively rapid decline in allograft function. Chronic ABMR results from a repetitive pattern of chronic thrombotic events and inflammatory changes, which result in cellular injury and repair. It manifests as late transplant glomerulopathy (TG) and results in a decline in renal function. In addition to pathology mediated directly by antibodies, recent evidence suggests that B cells and plasma cells may themselves influence rejection or tolerance. The clinical picture of ABMR has become increasingly complex, with questions abounding regarding its classification, the precision of diagnostic approaches, and the efficacy of various therapeutic strategies for safely and effectively managing affected patients. This article provides an overview of current progress in clinical and translational research surrounding ABMR pathophysiology and ABMR-related outcomes, prevention, treatment and future directions.
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Figure 1.
Mechanisms of donor-specific antibody-mediated endothelial injury in renal allografts. Anti-MHC antibodies may either result in direct injury to the capillary endothelium or in indirect injury via complement fixation or recruitment of inflammatory cells with Fc receptors. In cases with donor-specific antibodies that lack C4d deposition, endothelial injury and cellular recruitment could be important mediators. Poly, polymorphonuclear cell. Reproduced with permission from Farkash and Colvin (15).
Defining and Diagnosing ABMR
The first description of acute ABMR identified two distinct features: neutrophils in peritubular capillaries (PTCs) and de novo antidonor HLA class I antibodies. Around the same time, C4d, a degradation product of the complement pathway that binds covalently to the endothelium, was identified as a stable marker of antidonor humoral activity. Subsequently, the correlation between DSA, histologic findings of microcapillary injury and diffuse (>50%) C4d deposits in the PTCs were described in acute ABMR. C4d and DSA were also linked to the histopathologic features of chronic ABMR. Since 2003, the Banff Working Group classification system for renal allograft biopsies has differentiated T cell–mediated rejection (TCMR) from ABMR. The most recent Banff 2013 diagnosis of ABMR, published in this issue of the journal, requires histologic evidence of acute or chronic tissue injury, evidence of current/recent antibody interaction with vascular endothelium and serologic evidence of the presence of circulating DSA. Importantly, C4d staining is no longer a requirement for the diagnosis of ABMR ( Table 1 ).
C4d and the Diagnosis of ABMR
C4d is a split product of C4 activation and has no known biological action. It may be activated by the classical and lectin complement pathways and serves as a footprint of antibody–antigen interactions on the surface of endothelial cells. Although useful, C4d has significant limitations for the diagnosis of ABMR, not least because of methodological issues (immunoperoxidase vs. immunofluorescence, frozen vs. paraffin), poor understanding of the meaning of minimal and focal staining, and its waxing and waning deposition. Staining depends on the density of the capillary network, with poor sensitivity in chronic settings, and C4d positivity has been reported in the absence of other evidence of graft injury. Furthermore, C4d staining may not be associated with measurable DSA in the case of non-HLA antibodies or antibodies absorbed by the allograft.
Overall, the sensitivity of C4d is low, and its expression depends on the density of PTCs. In this regard, a number of studies have established the concept of C4d-negative acute and chronic ABMR. Loupy et al reported that C4d staining waxed and waned and was not a sensitive indicator of parenchymal disease in the first year after transplant. In this study, 55% of C4d-negative biopsies with ABMR had evidence of concomitant capillary inflammation. Sis et al described that 60% of kidneys with high endothelial activation and injury transcripts (ENDATs) and chronic ABMR or graft loss were C4d negative. Findings were confirmed by another study in which 63% of late kidney failures after biopsy were attributable to ABMR, but many were C4d negative. A recent microarray study from Sellarés et al concluded that changes in ABMR-associated gene expression (mostly in endothelial or NK cells) correlated with the presence of capillary lesions or DSA and may predict graft failure independent of C4d staining. Together, these observations point to the low sensitivity of C4d for the diagnosis of humoral rejection and support the addition of novel biomarkers of capillary inflammation and endothelial injury, including NK cells and macrophages to the diagnosis algorithm of ABMR. This recommendation was officially acknowledged at the 11th Banff Conference on Allograft Pathology (Figure 2) and was incorporated into the new Banff 2013 diagnostic criteria for ABMR (Table 1).
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Figure 2.
Acute and chronic definitions of ABMR based on C4d positivity. ABMR, antibody-mediated rejection; ATN, acute tubular necrosis; DSA, donor-specific antibodies; IF, immunofluorescence; IFTA, interstitial fibrosis and tubular atrophy; IHC, immunohistochemistry; PTC, peritubular capillary. Reproduced with permission from Mengel et al (21).
DSA and the Diagnosis of ABMR
Terasaki et al identified HLA antibodies in the serum of patients after transplantation nearly 45 years ago. However, the importance of a low-strength antibody that is undetectable by cell-based methodology was not recognized until studies from the same group, three decades later, discovered a strong association between HLA antibodies detected by solid-phase assays and graft failure.
DSA may be directed against HLA or other endothelial cell antigens, and its presence is required for the diagnosis of acute and chronic active ABMR. In addition, there is growing evidence supporting the roles of preformed and de novo DSA as independent risk factors for acute and chronic ABMR and graft loss. A recent systematic review and meta-analysis demonstrated that the presence of DSA before transplantation was associated with a twofold greater risk of acute rejection and a 75% greater risk of graft loss. Despite these findings, our understanding of the biological relevance of DSA remains limited. In vitro studies suggest that anti-HLA class I alloantibodies result in endothelial cell injury and activation through both complement-dependent and complement-independent pathways. However, little is known about signal transduction in response to class II antibodies or the pathogenesis of DSA-induced renal allograft injury in actual patients. It is important to note that not all DSA fix complement or cause ABMR and, conversely, not all episodes of acute graft injury with capillary inflammation and C4d deposition are associated with DSA being detectable with standard assays. In fact, the majority of patients with DSA maintain normal kidney function for years and have long-term outcomes similar to nonsensitized patients.
Another important limitation is that currently available HLA antibody tests are qualitative and have not been cleared by the US Food and Drug Administration (FDA) for quantitative measurements. More studies are needed to identify risk stratification strategies on the basis of semiquantitative measures of DSA and calculated panel reactive antibody (PRA), subclasses of immunoglobulin G anti-HLA antibodies, and C1q complement-fixing DSA. Pending the results of collaborative standardization studies, consensus guidelines on the testing and clinical management issues associated with HLA and non-HLA antibodies in transplantation were recently published. These recommendations are intended to provide guidance on the use and clinical application of contemporary methods for HLA antibody detection.
ABMR Classification and Phenotypes
The 2011 Banff meeting report and a 2010 workshop held by the FDA both noted the confusion generated by reports on acute and chronic ABMR, and emphasized the importance of correctly defining ABMR phenotypes. In the Banff report, two principal phenotypes of acute ABMR were defined: (1) ABMR phenotype 1 in the presensitized patient, occurring early posttransplant; and (2) ABMR phenotype 2, which develops from the emergence of de novo DSA in the late posttransplant period and is thought to be mostly related to nonadherence or inadequate immunosuppression. However, additional characteristics—including the nature of the antibody; the significance of C4d; the severity of microcapillary injury, gene transcripts, molecular and cellular signatures; and the pathology and function of the allograft—are relevant, were included in the Banff 2013 criteria, and may subsequently affect the design of clinical trials of patients with ABMR (Table 1).