Transplant accommodation in highly sensitized patients: a potential role for Bcl-xL and alloantibody

Current Status of Genetically Engineered Pig to Monkey Kidney Xenotransplantation in Korea

BACKGROUND

In South Korea, pig-to-nonhuman primate trials of solid organs have only been performed recently, and the results are not sufficiently satisfactory to initiate clinical trials. Since November 2011, we have performed 30 kidney pig-to-nonhuman primate xenotransplantations at Konkuk University Hospital.

METHODS

Donor αGal-knockout-based transgenic pigs were obtained from 3 institutes. The knock-in genes were CD39, CD46, CD55, CD73, and thrombomodulin, and 2-4 transgenic modifications with GTKO were done. The recipient animal was the cynomolgus monkey. We used the immunosuppressants anti-CD154, rituximab, anti-thymocyte globulin, tacrolimus, mycophenolate mofetil, and steroids.

RESULTS

The mean survival duration of the recipients was 39 days. Except for a few cases for which survival durations were <2 days because of technical failure, 24 grafts survived for >7 days, with an average survival duration of 50 days. Long-term survival was observed 115 days after the removal of the contralateral kidney, which is currently the longest-recorded graft survival in Korea. We confirmed functioning grafts for the surviving transplanted kidneys after the second-look operation, and no signs of hyperacute rejection were observed.

CONCLUSIONS

Although our survival results are relatively poor, they are the best-recorded results in South Korea, and the ongoing results are improving. With the support of government funds and the volunteering activities of clinical experts, we aim to further improve our experiments and contribute to the commencement of clinical trials of kidney xenotransplantation in Korea.

A Fresh Glimpse into Cartilage Immune Privilege

The increasing prevalence of degenerative cartilage disorders in young patients is a growing public concern worldwide. Cartilage's poor innate regenerative capacity has inspired the exploration and development of cartilage replacement treatments such as tissue-engineered cartilages and osteochondral implants as potential solutions to cartilage loss. The clinical application of tissue-engineered implants is hindered by the lack of long-term follow-up demonstrating efficacy, biocompatibility, and bio-integration. The historically reported immunological privilege of cartilage tissue was based on histomorphological observations pointing out the lack of vascularity and the presence of a tight extracellular matrix. However, clinical studies in humans and animals do not unequivocally support the immune-privilege theory. More in-depth studies on cartilage immunology are needed to make clinical advances such as tissue engineering more applicable. This review analyzes the literature that supports and opposes the concept that cartilage is an immune-privileged tissue and provides insight into mechanisms conferring various degrees of immune privilege to other, more in-depth studied tissues such as testis, eyes, brain, and cancer.

Protection of transplants against antibody-mediated injuries: from xenotransplantation to allogeneic transplantation, mechanisms and therapeutic insights

Long-term allograft survival in allotransplantation, especially in kidney and heart transplantation, is mainly limited by the occurrence of antibody-mediated rejection due to anti-Human Leukocyte Antigen antibodies. These types of rejection are difficult to handle and chronic endothelial damages are often irreversible. In the settings of ABO-incompatible transplantation and xenotransplantation, the presence of antibodies targeting graft antigens is not always associated with rejection. This resistance to antibodies toxicity seems to associate changes in endothelial cells phenotype and modification of the immune response. We describe here these mechanisms with a special focus on endothelial cells resistance to antibodies. Endothelial protection against anti-HLA antibodies has been described in vitro and in animal models, but do not seem to be a common feature in immunized allograft recipients. Complement regulation and anti-apoptotic molecules expression appear to be common features in all these settings. Lastly, pharmacological interventions that may promote endothelial cell protection against donor specific antibodies will be described.

Prospective multi-centric study to analyze pre-transplant compatibility algorithm for live-related donor kidney transplant in Indian setting: the "Delhi approach"!

BACKGROUND

Since no single test is always accurate and sensitive, two or more tests are used to increase the precision of evaluation. Different algorithms have been proposed by centers in Leiden, Basel, Vienna and Minnesota, etc. With an intention to develop an optimal algorithm for India, we evaluated pre-transplant compatibility tests for live-donor kidney transplants. Three tests complement dependent cyto-toxicity cross-match (CDCXM), flow-cytometry cross-match (FCXM) and anti-HLA antibody screening (HAS) were performed and confirmed by the anti-HLA antibody identification (HAI) assay in a multi-centric trial (three transplant centers) in India.

MATERIALS AND METHODS

All prospective recipients (and their potential donors) underwent low-resolution HLA typing as well as CDCXM, FCXM and HAS assays. In addition, HAI {single antigen bead assay; (SAB)} was done for all recipients to identify possible anti-HLA antibodies. In a virtual cross-match (VXM), antibody specificity was mapped to donor HLA type to determine donor-specific antibodies (DSA). Only patients without DSA were cleared for the transplant. Alternatively, patients with DSA were offered an exchange in the kidney paired donation (KPD) program. The screening results (CDCXM, FCXM, and HAS) were analyzed, individually as well as in combination of screening assays (CDCXM+HAS, CDCXM+FCXM, and FCXM+HAS) and the results were compared with those from the HAI test.

RESULTS

Out of 100 patients, 69 were males and 31 were females; 85 recipients (85%) underwent a kidney transplant. The sensitivity of CDCXM was only 12.1% and the specificity of CDCXM was 100%; whereas the sensitivity of FCXM was 84.8% and the specificity of FCXM was 89.6%. The sensitivity and specificity of class I HAS was 88.2% and 84.3%, respectively. The sensitivity and specificity class II HAS was 88.0% and 80.0%, respectively. However, when both class I/II HAS were tested together the sensitivity increased to 97.0% and the specificity to 82.1%. Similarly, the sensitivity of combined FCXM+HAS had the sensitivity of 100% and the specificity of 76.1%; CDCXM+FCXM had the sensitivity of 84.8% and the specificity of 89.6% and CDCXM+HAS assays reached 97% with the specificity of 82.1%.

CONCLUSIONS

Our results showed that the algorithm of FCXM with HAS produced the best sensitivity of 100%. The specificity of 76.1% indicate that the combined FCXM+HAS assays may detect up to 24.9% false positive results. We suggest that these false-positives may be easily resolved by performing the virtual crossmatch based on HAI (SAB) results. In our reflex testing algorithmic approach only 49% patients needed HAI (SAB). Finally, our results suggested that the CDCXM assay may be discontinued in pre-transplant workup owing to its very low sensitivity (12.1%).

CD5-Positive B Lymphocytes after Kidney Transplantation

Kidney transplantation is the treatment of choice for end-stage kidney diseases. Unfortunately, kidney allograft recipients rarely develop tolerance or accommodation and require life-long immunosuppression. Among many other regulatory mechanisms, CD5+ B lymphocytes (mainly B-1a) seem to be involved in the process of allograft acceptance. These cells are the major source of natural, low-affinity antibodies, which are polyreactive. Thus, we hypothesized that CD5+ B cells could be referred to as a biomarker in those patients who developed accommodation towards kidney allotransplant. In this study, 52 low-immunized kidney transplant recipients were evaluated for transplant outcome up to 8 y post-transplant. The follow up included anti-HLA antibodies, B cells phenotype and cytokines. We have identified a cohort of recipients who produced alloantibodies (Abs+), which was associated with increased levels of CD5+ B cells, mainly during the first year after transplantation but also later on. Importantly, creatinine levels were comparable between Abs+ and Abs− allorecipients at 2 years after the transplantation and graft survival rate was comparable between these groups even eight years post-transplant. So, it seems that despite the presence of alloantibodies the graft function was sustained when the level of CD5+ B cells was increased. Targeting CD5+ B cells may be a valuable therapeutic option to increase transplant success. The phenotype can be also tried as a biomarker to increase the effectiveness of individualized post-transplant treatments.

B cells in transplant tolerance and rejection: friends or foes?

Our understanding of the role of B cells in organ transplantation remains incomplete and continues to grow. The majority of research has focused on the detrimental role of antibodies that drive the development of pathogenesis of the transplanted organ. However, it has been shown that not all donor-specific antibodies are harmful and in some circumstances can even promote tolerance through the mechanism of accommodation. Furthermore, B cells can have effects on transplanted organs through their interaction with T cells, namely antigen presentation, cytokine production, and costimulation. More recently, the role and importance of Bregs was introduced to the field of transplantation. Due to this functional and ontogenetic heterogeneity, targeting B cells in transplantation may bring undesired immunologic side effects including increased rejection. Therefore, the selective control of B cells that contribute to the humoral response against donor antigens will continue to be an important and challenging area of research and potentially lead to improved long-term transplant outcomes.

PCR Array Technology in Biopsy Samples Identifies Up-Regulated mTOR Pathway Genes as Potential Rejection Biomarkers After Kidney Transplantation

Background: Antibody-mediated rejection (AMR) is the major cause of kidney transplant rejection. The donor-specific human leukocyte antigen (HLA) antibody (DSA) response to a renal allograft is not fully understood yet. mTOR complex has been described in the accommodation or rejection of transplants and integrates responses from a wide variety of signals. The aim of this study was to analyze the expression of the mTOR pathway genes in a large cohort of kidney transplant patients to determine its possible influence on the transplant outcome. Methods: A total of 269 kidney transplant patients monitored for DSA were studied. The patients were divided into two groups, one with recipients that had transplant rejection (+DSA/+AMR) and a second group of recipients without rejection (+DSA/-AMR and -DSA/-AMR, controls). Total RNA was extracted from kidney biopsies and reverse transcribed to cDNA. Human mTOR-PCR array technology was used to determine the expression of 84 mTOR pathway genes. STRING and REVIGO software were used to simulate gene to gene interaction and to assign a molecular function. Results: The studied groups showed a different expression of the mTOR pathway related genes. Recipients that had transplant rejection showed an over-expressed transcript (≥5-fold) of AKT1S1, DDIT4, EIF4E, HRAS, IGF1, INS, IRS1, PIK3CD, PIK3CG, PRKAG3, PRKCB (>12-fold), PRKCG, RPS6KA2, TELO2, ULK1, and VEGFC, compared with patients that did not have rejection. AKT1S1 transcripts were more expressed in +DSA/-AMR biopsies compared with +DSA/+AMR. The main molecular functions of up-regulated gene products were phosphotransferase activity, insulin-like grown factor receptor and ribonucleoside phosphate binding. The group of patients with transplant rejection also showed an under-expressed transcript (≥5-fold) of VEGFA (>15-fold), RPS6, and RHOA compared with the group without rejection. The molecular function of down-regulated gene products such as protein kinase activity and carbohydrate derivative binding proteins was also analyzed. Conclusions: We have found a higher number of over-expressed mTOR pathway genes than under-expressed ones in biopsies from rejected kidney transplants (+DSA/+AMR) with respect to controls. In addition to this, the molecular function of both types of transcripts (over/under expressed) is different. Therefore, further studies are needed to determine if variations in gene expression profiles can act as predictors of graft loss, and a better understanding of the mechanisms of action of the involved proteins would be necessary.

Current protocols and outcomes of ABO-incompatible kidney transplantation

One of the principal obstacles in transplantation from living donors is that approximately 30% are immunologically incompatible because of the presence in the recipient of antibodies directed against the human leukocyte antigen system of the donor or because of the incompatibility of the ABO system. The aim of this review is to describe the more recent data from the literature on the different protocols used and the clinical outcomes of ABO-incompatible kidney transplantation. Two different strategies are used to overcome these barriers: desensitization of the recipient to remove the antibodies and to prevent their rebound after transplantation and the exchange of organs between two or more pairs. The largest part of this review is dedicated to describing the techniques of desensitization. Even if the first reports of successful renal transplantation between ABO-incompatible pairs have been published by 1980, the number of ABO-incompatible transplants increased substantially in this century because of our improved knowledge of the immune system and the availability of new drugs. Rituximab has substantially replaced splenectomy. The technique of apheresis has improved and more recently a tailored desensitization proved to be the more efficient strategy avoiding an excess of immunosuppression with the related side effects. Recent reports document outcomes for such transplantation similar to the outcomes of standard transplantation.

Mechanisms of organ transplant injury mediated by B cells and antibodies: Implications for antibody-mediated rejection

Recent adjustments to the histological diagnosis and the introduction of molecular classification are providing renewed support for the paradigm that antibody-mediated rejection (ABMR) is an important clinical problem for which there is an urgent need for better therapies. Acute ABMR is observed when the graft is exposed to rapid increases in high-titer donor-specific antibodies (DSA) that are most often generated as anamnestic responses in sensitized recipients or de novo responses in nonsensitized patients who are nonadherent. Chronic ABMR is associated with slower increases in DSA, which may be high or low titer and transient or persistent. These DSA elicit cycles of injury and repair that manifest as multilamination of the peritubular capillary basement membrane or arteriopathy manifesting as intimal fibrosis. Mitigating the problem of AMBR requires the anamnestic and de novo DSA responses to be prevented and established DSA responses to be reversed. To this end, a better understanding of the immunobiology of DSA production is necessary and also the development of assays capable of detecting early humoral immune responses.Recent advances in understanding the immunobiology of B cells and areas requiring further investigation that might lead to new therapies or better diagnosis are discussed in this review.

Vascular Signaling in Allogenic Solid Organ Transplantation - The Role of Endothelial Cells

Graft rejection remains the major obstacle after vascularized solid organ transplantation. Endothelial cells, which form the interface between the transplanted graft and the host’s immunity, are the first target for host immune cells. During acute cellular rejection endothelial cells are directly attacked by HLA I and II-recognizing NK cells, macrophages, and T cells, and activation of the complement system leads to endothelial cell lysis. The established forms of immunosuppressive therapy provide effective treatment options, but the treatment of chronic rejection of solid organs remains challenging. Chronic rejection is mainly based on production of donor-specific antibodies that induce endothelial cell activation—a condition which phenotypically resembles chronic inflammation. Activated endothelial cells produce chemokines, and expression of adhesion molecules increases. Due to this pro-inflammatory microenvironment, leukocytes are recruited and transmigrate from the bloodstream across the endothelial monolayer into the vessel wall. This mononuclear infiltrate is a hallmark of transplant vasculopathy. Furthermore, expression profiles of different cytokines serve as clinical markers for the patient’s outcome. Besides their effects on immune cells, activated endothelial cells support the migration and proliferation of vascular smooth muscle cells. In turn, muscle cell recruitment leads to neointima formation followed by reduction in organ perfusion and eventually results in tissue injury. Activation of endothelial cells involves antibody ligation to the surface of endothelial cells. Subsequently, intracellular signaling pathways are initiated. These signaling cascades may serve as targets to prevent or treat adverse effects in antibody-activated endothelial cells. Preventive or therapeutic strategies for chronic rejection can be investigated in sophisticated mouse models of transplant vasculopathy, mimicking interactions between immune cells and endothelium.

The five dimensions of B cell tolerance

B cell tolerance has been generally understood to be an acquired property of the immune system that governs antibody specificity in ways that avoid auto-toxicity. As useful as this understanding has proved, it fails to fully explain the existence of auto-reactive specificities in healthy individuals and contribution these may have to health. Mechanisms underlying B cell tolerance are considered to select a clonal repertoire that generates a collection of antibodies that do not bind self, ie tolerance operates more or less in three dimensions that largely spare autologous cells and antigens. Yet, most B lymphocytes in humans and probably in other vertebrates are auto-reactive and absence of these auto-reactive B cells is associated with disease. We suggest that auto-reactivity can be embodied by extending the concept of tolerance by two further dimensions, one of time and circumstance and one that allows healthy cells to actively resist injury. In this novel concept, macromolecular recognition by the B cell receptor leading to deletion, anergy, receptor editing or B cell activation is extended by taking account of the time of development of normal immune responses (4th dimension) and the accommodation (or tolerance) of normal cells to bound antibody, activation of complement, and interaction with inflammatory cells (fifth dimension). We discuss how these dimensions contribute to understanding B cell biology in health or disease.

Outstanding questions in transplantation: B cells, alloantibodies, and humoral rejection

Over the past three decades, improved immunosuppression has significantly reduced T cell-mediated acute rejection rates, but long-term graft survival rates have seen only marginal improvement. The cause of late graft loss has been under intense investigation, and chronic antibody-mediated rejection (AMR) has been identified as one of the leading causes, thus providing a strong rationale for basic science investigation into donor-specific B cells and antibodies in transplantation and ways to mitigate their pathogenicity. In 2018, the American Society of Transplantation launched a community-wide online discussion of Outstanding Questions in Transplantation, and the topic of B cell biology and donor-specific antibody prevention emerged as a major area of interest to the community, leading to a highly engaged dialogue, with comments from basic and translational scientists as well as physicians (http://community.myast.org/communities/community-home/digestviewer). We have summarized this discussion from a bedside to bench perspective and have organized this review into outstanding questions within the paradigm that AMR is a leading cause of graft loss in the clinic, and points of view that challenge aspects of this paradigm. We also highlight opportunities for basic and translational scientists to contribute to the resolution of these questions, mapping important future directions for the transplant research field.

Non-canonical B cell functions in transplantation

B cells are differentiated to recognize antigen and respond by producing antibodies. These activities, governed by recognition of ancillary signals, defend the individual against microorganisms and the products of microorganisms and constitute the canonical function of B cells. Despite the unique differentiation (e.g. recombination and mutation of immunoglobulin gene segments) toward this canonical function, B cells can provide other, "non-canonical" functions, such as facilitating of lymphoid organogenesis and remodeling and fashioning T cell repertoires and modifying T cell responses. Some non-canonical functions are exerted by antibodies, but most are mediated by other products and/or direct actions of B cells. The diverse set of non-canonical functions makes the B cell as much as any cell a central organizer of innate and adaptive immunity. However, the diverse products and actions also confound efforts to weigh the importance of individual non-canonical B cell functions. Here we shall describe the non-canonical functions of B cells and offer our perspective on how those functions converge in the development and governance of immunity, particularly immunity to transplants, and hurdles to advancing understanding of B cell functions in transplantation.

Constitutive expression of the anti-apoptotic Bcl-2 family member A1 in murine endothelial cells leads to transplant tolerance

Anti-apoptotic genes, including those of the Bcl-2 family, have been shown to have dual functionality inasmuch as they inhibit cell death but also regulate inflammation. Several anti-apoptotic molecules have been associated with endothelial cell (EC) survival following transplantation; however, their exact role has yet to be elucidated in respect to controlling inflammation. In this study we created mice expressing murine A1 (Bfl-1), a Bcl-2 family member, under the control of the human intercellular adhesion molecule 2 (ICAM-2) promoter. Constitutive expression of A1 in murine vascular ECs conferred protection from cell death induced by the proinflammatory cytokine tumour necrosis factor (TNF)-α. Importantly, in a mouse model of heart allograft transplantation, expression of A1 in vascular endothelium increased survival in the absence of CD8⁺ T cells. Better graft outcome in mice receiving an A1 transgenic heart correlated with a reduced immune infiltration, which may be related to increased EC survival and reduced expression of adhesion molecules on ECs. In conclusion, constitutive expression of the anti-apoptotic molecule Bfl1 (A1) in murine vascular ECs leads to prolonged allograft survival due to modifying inflammation.

Endothelial Cells in Antibody-Mediated Rejection of Kidney Transplantation: Pathogenesis Mechanisms and Therapeutic Implications

Antibody-mediated rejection (AMR) has been identified as a main obstacle for stable immune tolerance and long survival of kidney allografts. In spite of new insights into the underlying mechanisms of AMR, accurate diagnosis and efficient treatment are still challenges in clinical practice. Endothelium is the first barrier between recipients' immune systems and grafts in vascularized organ transplants. Considering that endothelial cells express a number of antigens that can be attacked by various allo- and autoantibodies, endothelial cells act as main targets for the recipients' humoral immune responses. Importantly, emerging evidence has shown that endothelial cells in transplants could also initiate protective mechanisms in response to immune injuries. A better understanding of the role of endothelial cells during the pathogenesis of AMR might provide novel therapeutic targets. In the present review, we summarize the antigens expressed by endothelial cells and also discuss the activation and accommodation of endothelial cells as well as their clinical implications. Collectively, the progress discussed in this review indicates endothelial cells as promising targets to improve current diagnosis and therapeutic regimens for AMR.

Deceased donor kidney transplantation across donor-specific antibody barriers: predictors of antibody-mediated rejection

BACKGROUND

Apheresis-based desensitization allows for successful transplantation across major immunological barriers. For donor-specific antibody (DSA)- and/or crossmatch-positive transplantation, however, it has been shown that even intense immunomodulation may not completely prevent antibody-mediated rejection (ABMR).

METHODS

In this study, we evaluated transplant outcomes in 101 DSA+ deceased donor kidney transplant recipients (transplantation between 2009 and 2013; median follow-up: 24 months) who were subjected to immunoadsorption (IA)-based desensitization. Treatment included a single pre-transplant IA session, followed by anti-lymphocyte antibody and serial post-transplant IA. In 27 cases, a positive complement-dependent cytotoxicity crossmatch (CDCXM) was rendered negative immediately before transplantation. Seventy-four of the DSA+ recipients had a negative CDCXM already before IA.

RESULTS

Three-year death-censored graft survival in DSA+ patients was significantly worse than in 513 DSA- recipients transplanted during the same period (79 versus 88%, P = 0.008). Thirty-three DSA+ recipients (33%) had ABMR. While a positive baseline CDCXM showed only a trend towards higher ABMR rates (41 versus 30% in CDCXM- recipients, P = 0.2), DSA mean fluorescence intensity (MFI) in single bead assays significantly associated with rejection, showing 20 versus 71% ABMR rates at <5000 versus >15 000 peak DSA MFI. The predictive value of MFI was moderate, with the highest accuracy at a median of 13 300 MFI (after cross-validation: 0.72). Other baseline variables, including CDC assay results, human leukocyte antigen mismatch, prior transplantation or type of induction treatment, did not add independent predictive information.

CONCLUSIONS

IA-based desensitization failed to prevent ABMR in a considerable number of DSA+ recipients. Assessing DSA MFI may help stratify risk of rejection, supporting its use as a guide to organ allocation and individualized treatment.

ABO incompatible renal transplants and decreased likelihood for developing immune responses to HLA and kidney self-antigens

Immune responses to HLA and tissue-restricted self-antigens (SAgs) have been proposed to play a role in the pathogenesis of renal allograft (KTx) rejection. However, ABO incompatible (ABOi) KTx recipients (KTxR) following depletion of antibodies (Abs) to blood group antigens had fewer rejections. To determine the mechanisms, pre- and post-transplant sera from ABOi (n=18) and ABO-compatible (ABOc) (n=45) KTxR were analyzed for Abs against HLA class I and II by LABScreen single antigen assay. The development of Abs to SAgs was measured by ELISA. Immunity to Collagen IV (Col-IV) and cytokines induced were measured by ELISPOT. While 8/45 (18%) ABOc KTxR developed new donor specific antibodies to HLA (DSA) following transplantation, 0/18 ABOi KTxR developed DSA. ABOi KTxR failed to develop Abs to kidney SAgs (Col-IV and fibronectin (FN)). In contrast, 7 ABOc KTxR developed Abs to both Col-IV and FN. Col-IV stimulation of lymphocytes from ABOc KTxR demonstrated increased IFNγ, IL-17 and decreased IL-10. In contrast ABOi recipients following stimulation with antigens resulted in more IL10 and reduced IFN-γ and IL17 production. At one year, the GFR in ABOi KTxR were significantly better (p<0.04) than ABOc KTxR. De novo DSA and immune responses to SAgs are reduced or absent in ABOi KTxR which we propose leads to less acute rejection and better long term function following ABOi KTx.

Difference in outcomes after antibody-mediated rejection between abo-incompatible and positive cross-match transplantations

Graft survival seems to be worse in positive cross-match (HLAi) than in ABO-incompatible (ABOi) transplantation. However, it is not entirely clear why these differences exist. Sixty-nine ABOi, 27 HLAi and 10 combined ABOi+HLAi patients were included in this retrospective study, to determine whether the frequency, severity and the outcome of active antibody-mediated rejection (AMR) were different. Five-year death-censored graft survival was better in ABOi than in HLAi and ABOi+HLAi patients (99%, 69% and 64%, respectively, P = 0.0002). Features of AMR were found in 38%, 95% and 100% of ABOi, HLAi and ABOi+HLAi patients that had a biopsy, respectively (P = 0.0001 and P = 0.001). After active AMR, a declining eGFR and graft loss were observed more frequently in HLAi and HLAi+ABOi than in ABOi patients. The poorer prognosis after AMR in HLAi and ABOi+HLAi transplantations was not explained by a higher severity of histological lesions or by a less aggressive treatment. In conclusion, ABOi transplantation offers better results than HLAi transplantation, partly because AMR occurs less frequently but also because outcome after AMR is distinctly better. HLAi and combined ABOi+HLAi transplantations appear to have the same outcome, suggesting there is no synergistic effect between anti-A/B and anti-HLA antibodies.

Optimising long-term graft survival: establishing the benefit of targeting B lymphocytes

Kidney transplants do not last for the natural lifespan of most recipients. Of the reasons why transplants fail, damage by the immune system is the commonest cause. Understanding how the immune system recognises transplanted organs has increased significantly in recent years, but there is little insight into how organs are damaged, and no still no way of suppressing immune-mediated damage without exposing patients to the detrimental effects of long-term immunosuppression. In this article, we review the role of antibodies and B cells in immune-mediated damage of kidney transplants, and discuss the potential for manipulation of B cells to improve clinical outcomes.

Vascular endothelium as a target of immune response in renal transplant rejection

This review of clinical and experimental studies aims at analyzing the interplay between graft endothelium and host immune system in renal transplantation, and how it affects the survival of the graft. Graft endothelium is indeed the first barrier between self and non-self that is encountered by host lymphocytes upon reperfusion of vascularized solid transplants. Endothelial cells (EC) express all the major sets of antigens (Ag) that elicit host immune response, and therefore represent a preferential target in organ rejection. Some of the Ag expressed by EC are target of the antibody-mediated response, such as the AB0 blood group system, the human leukocyte antigens (HLA), and MHC class I related chain A antigens (MICA) systems, and the endothelial cell-restricted Ag; for each of these systems, the mechanisms of interaction and damage of both preformed and de novo donor-specific antibodies are reviewed along with their impact on renal graft survival. Moreover, the rejection process can force injured EC to expose cryptic self-Ag, toward which an autoimmune response mounts, overlapping to the allo-immune response in the damaging of the graft. Not only are EC a passive target of the host immune response but also an active player in lymphocyte activation; therefore, their interaction with allogenic T-cells is analyzed on the basis of experimental in vitro and in vivo studies, according to the patterns of expression of the HLA class I and II and the co-stimulatory molecules specific for cytotoxic and helper T-cells. Finally, as the response that follows transplantation has proven to be not necessarily destructive, the factors that foster graft endothelium functioning in spite of rejection, and how they could be therapeutically harnessed to promote long-term graft acceptance, are described: accommodation that is resistance of EC to donor-specific antibodies, and endothelial cell ability to induce Foxp3+ regulatory T-cells, that are crucial mediators of tolerance.

Using individual DSA titers to assess for accommodation after late humoral rejection

Management of late humoral rejection remains challenging, and DSA may persist. A case report illustrates how individual DSA titers using solid-phase-based assays may help to assess for accommodation. A male cystinosis patient received a cadaveric renal transplant at the age of 12 yr with a daclizumab, tacrolimus, MMF, and steroids-based immunosuppression. After three acute rejection episodes over the first eight months, interstitial fibrosis/tubular atrophy (IF/TA) was diagnosed on biopsy, while the immunosuppression was left unchanged with a high target exposure for both tacrolimus and MPA. One yr later, AMR type III (C4d and DSA positive) was treated with daily plasmapheresis, IVIG 100 mg/kg and pulse steroids 5 mg/kg. DSA (DR 53, DQ4, and DQ 2) were not responding until the plasma volume was increased to 2.5 plasma volumes. A second rise of creatinine confirmed worse humoral rejection; daily plasma exchange was resumed, and two doses of rituximab (375 mg/m(2)) were given. Subsequently, all DSA dropped, but only DR53 DSA remained unchanged, whereas the DQ antibodies rebounded to very strong titers. With a follow-up of over 120 days after recovery of the CD19 count, off all additional treatment and on identical immunosuppression with tacrolimus and MMF and prednisone, the patient's creatinine remained stable between 45 and 50 um while DQ DSA remain strong to very strong. We conclude that the patient is in a state of accommodation. DSA titers should be monitored when managing late humoral rejection.

Graft microvascular disease in solid organ transplantation

Alloimmune inflammation damages the microvasculature of solid organ transplants during acute rejection. Although immunosuppressive drugs diminish the inflammatory response, they do not directly promote vascular repair. Repetitive microvascular injury with insufficient regeneration results in prolonged tissue hypoxia and fibrotic remodeling. While clinical studies show that a loss of the microvascular circulation precedes and may act as an initiating factor for the development of chronic rejection, preclinical studies demonstrate that improved microvascular perfusion during acute rejection delays and attenuates tissue fibrosis. Therefore, preservation of a functional microvasculature may represent an effective therapeutic strategy for preventing chronic rejection. Here, we review recent advances in our understanding of the role of the microvasculature in the long-term survival of transplanted solid organs. We also highlight microvessel-centered therapeutic strategies for prolonging the survival of solid organ transplants.

Increased intragraft rejection-associated gene transcripts in patients with donor-specific antibodies and normal biopsies

We investigated why some donor-specific antibody-positive patients do not develop antibody-mediated rejection. Of 71 donor-specific antibody-positive patients, 46 had diagnosis of antibody-mediated rejection and 25 had normal biopsies. Fifty donor-specific antibody-negative patients with normal biopsies were used as a control group. A subgroup of 61 patients with available biopsy and 64 with blood samples were analyzed by microarrays. Both donor-specific antibody-positive/antibody-mediated rejection-positive and negative biopsies showed increased expression of gene transcripts associated with cytotoxic T cells, natural killer cells, macrophages, interferon-gamma, and rejection compared to donor-specific antibody-negative biopsies. Regulatory T-cell transcripts were upregulated in donor-specific antibody-positive/antibody-mediated rejection-positive and B-cell transcripts in donor-specific antibody-positive/antibody-mediated rejection-negative biopsies. Whole-blood gene expression analysis showed increased immune activity in only donor-specific antibody-positive/antibody-mediated rejection-positive but not negative patients. During a median follow-up of 36 months, 4 donor-specific antibody-positive/antibody-mediated rejection-negative patients developed antibody-mediated rejection, 12 continued to have donor-specific antibody, but 9 lost their donor-specific antibody. Gene expression profiles did not predict the development of antibody-mediated rejection or the persistence of donor-specific antibody. Thus, donor-specific antibody-positive/antibody-mediated rejection-negative patients had increased rejection-associated gene transcripts in their allografts despite no histologic findings of rejection but not in their blood. This was found in both biopsy and blood samples of donor-specific antibody-positive/antibody-mediated rejection-positive patients.

Complement modulation in solid-organ transplantation

The complement system is a major constituent of the innate immune system. It has a critical role in defense against pathogens but dysregulation of complement activation may lead to tissue injury and modulate the adaptive immune response. In organ transplantation, local complement activation is involved in hyper-acute rejection and antibody-mediated rejection. This last decade, interest in complement activation has increased due to new insights into the pathophysiology of antibody-mediated rejection, but also since the availability of news drugs that target terminal complement activation. In this review, we discuss our current understanding of how local complement activation induces acute and chronic graft injury, and review recent advances in clinical trials that block complement activation using the anti-C5 monoclonal antibody, eculizumab. Finally, we discuss how complement-targeted therapy may be integrated into our current immunosuppressive regimen and what type of patient will benefit most from this therapy.

The impact of de novo donor-specific anti-human leukocyte antigen antibodies on 5-year renal transplant outcome

Numerous studies have shown that circulating donor-specific antibodies targeting human leukocyte antigen (HLA) are associated with accelerated renal transplant failure, but many patients with these antibodies have good graft function. The aim of our study was to investigate the long-term graft function and survival in patients with de novo post-transplant donor-specific anti-HLA antibodies (DSA). Our prospective study included 78 consecutive recipients with a negative crossmatch before transplantation. Recipient serum samples were assayed for DSA in week 2 and 1, 3, 6, 9, 12 months after transplantation using a complement-dependent lymphocytotoxic technique with donor lymphocytes. Additionally, patients with DSA and stable renal function in the first year were tested with a more sensitive flow-panel-reactive antibody. DSA were present in 34 (44%) of our patients during the first 12 months after transplantation. Biopsy-proved acute rejection occurred in 11 DSA-positive and 10 DSA-negative patients. Seven DSA-positive patients had antibody-mediated rejection and no DSA-negative ones developed humoral rejection. The serum creatinine level in DSA-positive patients was significantly higher (2.48 vs 1.43 mg/dL) in year 5. The 13 (38%) DSA-positive patients with good graft function in month 12 were stable during a 5-year follow-up: their serum creatinine was 1.46 ± 0.4 in year 1 and 1.56 ± 0.4 mg/dL in year 5 and nobody lost their allograft. One- and 5- year graft survivals were appropriately 85% and 59% in DSA-positive patients compared to 93% and 93% in DSA-negative patients. To sum up, post-transplant DSA had a significant influence on kidney function and graft survival but in 38% of patients the presence of DSA did not decrease a 5-year renal function. A good renal allograft function in the presence of DSA in the first year after transplantation and cessation of their production in the subsequent years may be a good prognostic marker for a long-term allograft function and survival.

Molecular deciphering of the ABO system as a basis for novel diagnostics and therapeutics in ABO incompatible transplantation

In recent years ABO incompatible kidney transplantation (KTx) has become a more or less clinical routine procedure with graft and patient survival similar to those of ABO compatible transplants. Antigen-specific immunoadsorption (IA) for anti-A and anti-B antibody removal constitutes in many centers an important part of the treatment protocol. ABO antibody titration by hemagglutination is guiding the treatment; both if the recipient can be transplanted as well as in cases of suspected rejections if antibody removal should be performed. Despite the overall success of ABO incompatible KTx, there is still room for improvements and an extension of the technology to include other solid organs. Based on an increased understanding of the structural complexity and tissue distribution of ABH antigens and the fine epitope specificity of the ABO antibody repertoire, improved IA matrices and ABO antibody diagnostics should be developed. Furthermore, understanding the molecular mechanisms behind accommodation of ABO incompatible renal allografts could make it possible to induce long-term allograft acceptance also in human leukocyte antigen (HLA) sensitized recipients and, perhaps, also make clinical xenotransplantation possible.

Over-expression of heme oxygenase-1 does not protect porcine endothelial cells from human xenoantibodies and complement-mediated lysis

Accommodated organs can survive in the presence of anti-organ antibodies and complement. Heme oxygenase-1 (HO-1) is essential to ensure accommodation in concordant xenotransplant models. However, whether induction of HO-1 over-expression could protect porcine endothelial cells (PECs) against human xenoantibodies and complement-mediated lysis and induce an in vitro accommodation is still unknown. The SV40-immortalized porcine aorta-derived endothelial cell line (iPEC) was pre-incubated with 20, 50, or 80 μmol/L of cobalt-protoporphyrins IX (CoPPIX) for 24 h, and the HO-1 expression in iPECs was analyzed by using Western blotting. CoPPIX-treated or untreated iPECs were incubated with normal human AB sera, and complement-dependent cytotoxicity (CDC) was measured by both flow cytometry and lactate dehydrogenase (LDH) release assay. In vitro treatment with CoPPIX significantly increased the expression of HO-1 in iPECs in a dose-dependent manner. Over-expression of HO-1 was successfully achieved by incubation of iPECs with either 50 or 80 μmol/L of CoPPIX. However, HO-1 over-expression did not show any protective effects on iPECs against normal human sera-mediated cell lysis. In conclusion, induction of HO-1 over-expression alone is not enough to protect PECs from human xenoantibodies and complement-mediated humoral injury. Additionally, use of other protective strategies is needed to achieve accommodation in pig-to-primate xenotransplantation.

Solid phase detection of C4d-fixing HLA antibodies to predict rejection in high immunological risk kidney transplant recipients

Protocols for recipient desensitization may allow for successful kidney transplantation across major immunological barriers. Desensitized recipients, however, still face a considerable risk of antibody-mediated rejection (AMR), which underscores the need for risk stratification tools to individually tailor treatment. Here, we investigated whether solid phase detection of complement-fixing donor-specific antibodies (DSA) has the potential to improve AMR prediction in high-risk transplants. The study included 68 sensitized recipients of deceased donor kidney allografts who underwent peritransplant immunoadsorption for alloantibody depletion (median cytotoxic panel reactivity: 73%; crossmatch conversion: n = 21). Pre and post-transplant sera were subjected to detection of DSA-triggered C4d deposition ([C4d]DSA) applying single-antigen bead (SAB) technology. While standard crossmatch and [IgG]SAB testing failed to predict outcomes in our desensitized patients, detection of preformed [C4d]DSA (n = 44) was tightly associated with C4d-positive AMR [36% vs. 8%, P = 0.01; binary logistic regression: odds ratio: 10.1 (95% confidence interval: 1.6-64.2), P = 0.01]. Moreover, long-term death-censored graft survival tended to be worse among [C4d]DSA-positive recipients (P = 0.07). There were no associations with C4d-negative AMR or cellular rejection. [C4d]DSA detected 6 months post-transplantation were not related to clinical outcomes. Our data suggest that pretransplant SAB-based detection of complement-fixing DSA may be a valuable tool for risk stratification.

Cardiac antibody-mediated rejection

Many factors limit short- and long-term survival after pediatric heart transplantation. Historically, attention had been directed toward T-cell responses and acute cellular rejection. Presence of pretransplant antibodies against HLA is associated with increased donor wait times and poor post-transplant outcomes. Therapies aimed to mitigate circulating antibodies include plasmapheresis, protein A immunoadsorption columns, intravenous immune globulin, rituximab, and bortezomib. The negative effects of B cells, HLA antibodies, and AMR and potential interventions are the focus of this review article.

HLA class I: an unexpected role in integrin β4 signaling in endothelial cells

The production of anti-donor antibodies to HLA class I and class II antigens following transplantation is associated with development of transplant vasculopathy and graft loss. Antibodies against HLA class I (HLA-I) molecules are thought to contribute to transplant vasculopathy by triggering signals that elicit the activation and proliferation of endothelial cells. The proximal molecular events that regulate HLA-I dependent signal transduction are not well understood. We demonstrated a mutual dependency between HLA-I and integrin β4 to stimulate signal transduction and cell proliferation. Similarly, we found that integrin β4-mediated cell migration was dependent upon its interactions with HLA-I molecules. Since integrin β4 has been implicated in angiogenesis and tumor formation, associations between integrin β4 and HLA-I may play an important role in cancer. Further characterization of interactions between HLA-I and integrin β4 may lead to the development of therapeutic strategies for the treatment and prevention of chronic allograft rejection and cancer.

Intragraft gene expression in positive crossmatch kidney allografts: ongoing inflammation mediates chronic antibody-mediated injury

We studied intragraft gene expression profiles of positive crossmatch (+XM) kidney transplant recipients who develop transplant glomerulopathy (TG) and those who do not. Whole genome microarray analysis and quantitative rt-PCR were performed on RNA from protocol renal allograft biopsies in three groups: (1) +XM/TG+ biopsies before and after TG; (2) +XM/NoTG; and (3) negative crossmatch kidney transplants (control). Microarray comparisons showed few differentially expressed genes between paired biopsies from +XM/TG+ recipients before and after the diagnosis of TG. Comparing +XM/TG+ and control groups, significantly altered expression was seen for 2447 genes (18%) and 3200 genes (24%) at early and late time points, respectively. Canonical pathway analyses of differentially expressed genes showed inflammatory genes associated with innate and adaptive immune responses. Comparing +XM/TG+ and +XM/NoTG groups, 3718 probe sets were differentially expressed but these were over-represented in only four pathways. A classic accommodation phenotype was not identified. Using rt-PCR, the expression of inflammatory genes was significantly increased in +XM/TG+ recipients compared to the +XM/NoTG and control groups. In conclusion, pretransplant donor-specific anti-HLA antibodies results in a gene expression profile characterized by inflammation and cellular infiltration and the majority of +XM grafts are exposed to chronic injury.

The role of complement in the early immune response to transplantation

The complement system is a key element of the innate immune system, and the production of complement components can be divided into central (hepatic) and peripheral compartments. Essential complement components such as C3 are produced in both of these compartments, but until recently the functional relevance of the peripheral synthesis of complement was unclear. Here, we review recent findings showing that local peripheral synthesis of complement in a transplanted organ is required for the immediate response of the donor organ to tissue stress and for priming alloreactive T cells that can mediate transplant rejection. We also discuss recent insights into the role of complement in antibody-mediated rejection, and we examine how new treatment strategies that take into account the separation of central and peripheral production of complement are expected to make a difference to transplant outcome.

Mechanism of accommodation in a sensitized human leukocyte antigen transgenic murine cardiac transplant model

BACKGROUND

Presence of donor-specific antibodies (Abs) is detrimental to posttransplant allograft function. Some sensitized recipients have successfully undergone transplantation after pretransplant conditioning regimen using plasmapheresis and/or intravenous immunoglobulin therapy, but underlying mechanisms that confer such allograft protection are undefined.

METHODS

We developed a single human leukocyte antigen (HLA)-mismatched heterotopic murine heart transplant model (HLA-A2 into HLA-A2-sensitized-C57BL/6) to determine whether pretreatment of donors with low concentration of HLA class I (W6/32) or control Ab (C1.18.4) will confer protection. Expression levels of survival genes, Bcl-2 and heme oxygenase-1, were analyzed by gene array analysis and quantitative real-time polymerase chain reaction. Expression levels of cytokine panel were analyzed by Luminex. Role of Bcl-2 in the induction of allograft protection was analyzed by silencing the Bcl-2 expression in the donor hearts using a small hairpin (shRNA) specific for Bcl-2.

RESULTS

Control Ab-pretreated hearts were rejected in less than 5 days demonstrating hemorrhage, Ab, and C4 deposition. In contrast, W6/32-pretreated hearts were rejected at 15 days (P<0.05) that was prolonged to 25 days with antilymphocyte serum treatment. W6/32-pretreated hearts on day 5 exhibited increased expression of Bcl-2 (5.5-folds), Bcl-xl (5.5-folds), and heme oxygenase-1 (4.4-folds); decreased expression of ICAM-1, VCAM-1 (3.2-fold), along with reduced levels of cytokines interleukin (IL)-1β (4.4-folds), tumor necrosis factor α (3.7-folds), IL-6 (7.5-folds), IL-12 (2.3-folds) and chemokines monocyte chemotactic protein 1 (4.5-folds), MIG (4.4-folds), MIP-1α (3.4-folds), and IL-8 (3.1-folds). Silencing of Bcl-2 in accommodated hearts before transplant resulted in loss of protection with rejection (9±3 vs. 15±2days, P<0.05).

CONCLUSION

Pretreatment of hearts with low levels of anti-HLA Abs increases expression of antiapoptotic genes that inhibits caspases, leading to decreased inflammatory cytokines and chemokines, which promote allograft survival.

Transplant accommodation--are the lessons learned from xenotransplantation pertinent for clinical allotransplantation?

"Accommodation" refers to a vascularized transplant that has acquired resistance to antibody-mediated rejection (AMR). The term was coined in 1990, but the phenomenon was first described after clinical ABO-incompatible (ABOi) renal transplantation in the 1980s and is recognized as a common outcome in this context today. Because of the absence, until recently of reliable animal models of allograft accommodation, it has been studied extensively by investigators in the xenotransplantation field. With recent advances in the ability to recognize and diagnose AMR in human organs, the growth of desensitization programmes for transplantation into sensitized recipients and the availability of therapies that have the potential to promote accommodation, it is timely to review the literature in this area, identifying lessons that may inform preclinical and clinical studies in the future.

Clinical relevance of pretransplant HLA class II donor-specific antibodies in renal transplantation patients with negative T-cell cytotoxicity crossmatches

Background

We evaluated the clinical relevance of pretransplant donor-specific HLA antibodies (DSA) in renal transplantation patients who had negative T-cell cytotoxicity crossmatches.

Methods

From 328 consecutive renal transplant recipients, we selected 28 patients who had positive pretransplant (historical or at the time of transplantation) flow cytometry crossmatches, but negative T-cell cytotoxicity crossmatches at the time of transplantation. The presence of DSA and its level at the time of transplantation were retrospectively tested using Luminex single antigen assays.

Results

DSA was present in 16 (57.1%) of 28 patients. Biopsy-proven acute rejection (9 patients) occurred more frequently in patients with DSA than in those without DSA (56.3% vs. 0.0%; P=0.003). The positivity rate of class II DSA was significantly higher in patients with antibody-mediated rejection (AMR) than in those without AMR (100% vs. 21.7%; P=0.003). However, the positivity rate of class I DSA was not different between the two groups (40% vs. 40.9%). Among patients with class II DSA, those with AMR tended to have higher antibody levels (median fluorescence intensity, MFI) than those without AMR (16,359 vs. 5,910; P=0.056). A cut-off MFI value of 4,487 for class II DSA predicted the occurrence of AMR with good sensitivity and specificity (100% and 87.0%).

Conclusions

In patients with negative T-cell cytotoxicity crossmatches, the presence of class II DSA and its level at the time of transplantation were associated with the occurrence of AMR. Pretransplant DSA measurement with Luminex single antigen assay would be useful in renal transplantation.

Plasmapheresis in immunologic renal disease

Plasmapheresis has been used in the management of immunologic renal disease for the last 40 years. The rationale behind this approach is to remove pathogenic immune mediators, such as autoantibodies and immune complexes, from the circulation. There may also be benefit in depleting proinflammatory molecules, such as complement components and coagulation factors. Initial experience was gained in Goodpasture's disease, in which antiglomerular basement membrane antibodies were known to be pathogenic. More recently, a role for autoantibodies has become clear in small-vessel systemic vasculitis and some cases of hemolytic uremic syndrome/thrombotic thrombocytopenic purpura. Removal of immune complexes is thought to be important in cryoglobulinemia and systemic lupus erythematosus. Plasmapheresis is used in renal transplantation for the treatment of acute antibody-mediated rejection, and for desensitization of patients with preformed anti-HLA antibodies or those receiving an ABO-incompatible transplant. Although many of the early studies were uncontrolled, there has been an increasing number of randomized controlled trials in recent years. The aim of this article is to summarize current indications for the use of plasmapheresis in immunologic renal disease.

ABO Incompatible Kidney Transplantation-Current Status and Uncertainties

In the past, ABO blood group incompatibility was considered an absolute contraindication for kidney transplantation. Progress in defined desensitization practice and immunologic understanding has allowed increasingly successful ABO incompatible transplantation during recent years. This paper focused on the history, disserted outcomes, desensitization modalities and protocols, posttransplant immunologic surveillance, and antibody-mediated rejection in transplantation with an ABO incompatible kidney allograft. The mechanism underlying accommodation and antibody-mediated injury was also described.

Chronic alloantibody mediated rejection

Alloantibodies clearly cause acute antibody mediated rejection, and all available evidence supports their pathogenic etiology in the development of chronic alloantibody mediated rejection (CAMR). But the slow evolution of this disease, the on-going immunosuppression, the variations in titer of alloantibodies, and variation in antigenic targets all complicate identifying which dynamic factors are most important clinically and pathologically. This review highlights the pathological factors related to the diagnosis of CAMR, the time course and natural history of this disease. What is known about CAMR pathogenesis is discussed including alloantibodies, the role of complement, gene activation, and Fc effector cell function. Therapy, which is problematic for this disease, is also discussed, including on-going and potential therapies and their limitations.

Complement inhibition enables renal allograft accommodation and long-term engraftment in presensitized nonhuman primates

Protection against humoral injury mediated by donor-specific antibodies (DSA), also known as accommodation, may allow for long-term allograft survival in presensitized recipients. In the present study, we determined the role of complement in renal allograft accommodation in donor skin-presensitized nonhuman primates under conventional immunosuppression. Donor skin allografts were transplanted to presensitized recipients 14 days prior to renal transplantation. Renal allografts not receiving any immunosuppressive treatment developed accelerated rejection with predominantly humoral injury, which was not prevented using conventional cyclosporine (CsA) triple therapy. Inhibition of complement activation with the Yunnan-cobra venom factor (Y-CVF) successfully prevented accelerated antibody-mediated rejection and resulted in successful accommodation and long-term renal allograft survival in most presensitized recipients. Accommodation in this model was associated with the prevention of the early antibody responses induced against donor antigens by complement inhibition. Some antiapoptotic proteins and complement regulatory proteins, including Bcl-2, CD59, CD46 and clusterin, were upregulated in the surviving renal allografts. These results suggest that the complement inhibition-based strategy may be valuable alternative in future clinical cross-match positive or ABO-incompatible transplantation.

The link between major histocompatibility complex antibodies and cell proliferation

Experimental evidence indicates that donor-specific antibodies targeting major histocompatibility complex classes I and II molecules can elicit the key features of transplant vasculopathy by acting on the graft vasculature in 3 ways: directly activating proliferative, prosurvival, and migratory signaling in the target endothelial and smooth muscle cells; increasing expression of mitogenic factors in vascular endothelial cells, creating a potential proliferative autocrine loop; and promoting recruitment of inflammatory cells that produce mitogenic factors and elicit chronic inflammation, proliferation, and fibrosis. Here, we review the experimental literature showing the complement and Fc-independent effects of major histocompatibility complex classes I and II antibodies on graft vascular cells that may directly contribute to the proliferative aspect of transplant vasculopathy.

The utility of comprehensive assessment of donor specific anti-HLA antibodies in the clinical management of pediatric kidney transplant recipients

Advances in anti-HLA antibody characterization have had a major impact on kidney transplantation. Comprehensive characterization of DSA has improved protocols for allosensitized transplant candidates, increasing access to acceptable donors. Sensitive and specific solid phase antibody detection assays have given important insight into the clinical characteristics of antibody-mediated allograft injury, resulting in the development of a classification system for acute AMR. In addition, important insights into the nature of chronic antibody-mediated allograft rejection have been achieved as a result of improvements in DSA characterization. Finally, assays initially developed as tools to detect DSA in the clinical setting have been employed in innovative research protocols, allowing the investigation of new therapeutic approaches.

Pretransplant IgG subclasses of donor-specific human leukocyte antigen antibodies and development of antibody-mediated rejection

BACKGROUND

The subclass of IgG antibodies contributes to their capability to activate complement. It is currently unknown whether the pretransplant IgG subclass composition allows distinguishing harmful from presumably irrelevant donor-specific human leukocyte antigen (HLA) antibodies (HLA-DSA) detected by single-antigen flow beads (SAFB).

METHODS

Seventy-four patients transplanted in the presence of HLA-DSA were investigated. HLA-DSA characteristics were not different between patients experiencing antibody-mediated rejection (AMR) (n=40) and patients who did not (n=34) experience AMR. Sera were reanalyzed using SAFB with IgG subclass-specific reporter antibodies.

RESULTS

The 74 patients had in total 141 HLA-DSA. IgG1 was the predominant subclass (78%), followed by IgG2 (49%), IgG3 (36%), and IgG4 (20%). When grouped according to the complement-activating capability, only 4 of 74 patients (5%) had exclusively weak/no complement-activating HLA-DSA (i.e., IgG2 and IgG4), 21 of 74 patients (28%) had isolated strong complement-activating HLA-DSA (i.e., IgG1 and IgG3), and 46 of 74 patients (62%) had a mixture of both. There was no difference between the strong complement-activating and the mixture group regarding incidence of AMR (57% vs. 54%; P=0.81), phenotypes of AMR (P=0.70), and death-censored allograft survival at 5 years (78% vs. 78%; P=0.74). Interestingly, patients with exclusively weak/no complement-activating HLA-DSA (n=4) had a numerically lower incidence of AMR (25%) and no allograft loss has occurred yet.

CONCLUSION

In 90% of patients, pretransplant HLA-DSA are composed of isolated strong or a mixture of strong and weak/no complement-activating IgG subclasses. Because outcomes in these two groups were similar, pretransplant IgG subclass analysis is likely not providing substantial value beyond the standard IgG SAFB assay for pretransplant risk stratification.

Immunohistochemical markers of tissue injury in biopsies with transplant glomerulitis

Transplant glomerulitis is associated with suboptimal graft function. To understand its pathogenesis and to assess the parameters of potential prognostic value, we immunostained 25 paraffin-embedded allograft biopsies showing glomerulitis for markers of complement activation (C4d), cytotoxicity (Granzyme-B), apoptosis (Bcl-XL, Bcl-2, and Fas-L), and endothelial injury (von Willebrand factor). Staining was semiquantitatively assessed in different anatomical compartments, and comparison was made with 40 control allograft biopsies without glomerulitis. Biopsies with glomerulitis had more frequent incidence of "mixed" T-cell and antibody-mediated rejection compared with controls [8/25 (32%) versus 4/40 (10%), P = .046]. Furthermore, they had higher glomerular capillary-C4d scores (1.9 ± 1.1 versus 1.2 ± 1.2, P = .015), which tended to persist when biopsies showing transplant glomerulopathy were excluded. Higher glomerular capillary-C4d scores were observed in samples with versus without donor-specific antibody (2.5 ± 0.9 versus 1.2 ± 1.2, P = .01). Compared with controls, biopsies with glomerulitis had more intraglomerular (4.8 ± 4.5 versus 0.9± 0.8 cells/glomerulus, P < .001) and interstitial mainly peritubular capillary (6.1 ± 4.1 versus 3.2 ± 3.4 cells/hpf, P = .002) Granzyme-B(+) leukocytes. Higher mesangial-von Willebrand factor scores were noted in the glomerulitis group (1.8 ± 1.0 versus 0.8 ± 0.8, P = .003) and correlated with the percentage of inflamed glomeruli (r = 0.54, P < .001). Interstitial-von Willebrand factor was associated with a higher peritubular capillaritis score (interstitial-von Willebrand factor: 1.6 ± 1.2 versus no interstitial-von Willebrand factor: 0.6 ± 0.9, P = .02). Glomerular capillary-Bcl-XL was not associated with accommodation. Finally, no difference in Bcl-2 or Fas-L was observed upon comparing glomerulitis to controls. In conclusion, glomerular injury in transplant glomerulitis appears to be mediated by complement activation and cellular cytotoxicity. Mesangial- or interstitial-von Willebrand factor identified cases with more severe microcirculation injury.

Antibody-mediated allograft rejection: the emerging role of endothelial cell signalling and transcription factors

The presence of antibodies against class I human leukocyte antigens (HLA) can cause the development of chronic allograft rejection. Although endothelial cell activation has been identified as a main effector, the mechanisms mediating this process are not fully understood. Exposure of endothelium to antibodies against HLA antigens induces cell activation which promotes rejection. This activation process can be divided into two phases: an early response in which intracellular signalling proteins and transcription factors are activated, and a later change in protein expression and cell function. In this review, antibody-mediated endothelial signalling and the role of transcription factors in organ transplantation will be described with a particular focus on their potential to mediate some of the graft-damaging effects of anti-HLA class I antibodies.

Novel C1q assay reveals a clinically relevant subset of human leukocyte antigen antibodies independent of immunoglobulin G strength on single antigen beads

It has been known for 40 years that cytotoxic human leukocyte antigen (HLA) antibodies are associated with graft rejection. However, the complement-dependent cytotoxicity assay (CDC) used to define these clinically deleterious antibodies suffers from a lack of sensitivity and specificity. Recently, methods exploiting immunoglobulin G (IgG) antibody binding to HLA single antigen beads (SAB) have overcome sensitivity and specificity drawbacks but introduced a new dilemma: which of the much broader set of antibodies defined by these methods are clinically relevant. To address this, we developed a complement-fixing C1q assay on the HLA SAB that combines sensitivity, specificity, and functional potential into one assay. We compared the CDC, IgG, and C1q assays on 96 sera having 2,118 defined antibodies and determined that CDC detects only 19% of complement-fixing antibodies detected by C1q, whereas C1q detects only 47% of antibodies detected by IgG. In the same patient, there is no predictability by IgG mean fluorescence intensity (MFI) as to which of the antibodies will bind C1q because fixation is independent of MFI values. In 3 clinical studies, C1q(+) antibodies appear to be more highly correlated than those detected by IgG alone for antibody-mediated rejection in hearts as well as for kidney transplant glomerulopathy and graft failure.