Flow cytometry based detection of HLA alloantibody mediated classical complement activation

On a Long and Winding Road: Alloantibodies in Organ Transplantation

Today we know that both the humoral and the cellular arm of the immune system are engaged in severe immunological challenges. A close interaction between B and T cells can be observed in most "natural" challenges, including infections, malignancies, and autoimmune diseases. The importance and power of humoral immunity are impressively demonstrated by the current coronavirus disease 2019 pandemic. Organ transplant rejection is a normal immune response to a completely "artificial" challenge. It took a long time before the multifaceted action of different immunological forces was recognized and a unified, generally accepted opinion could be formed. Here, we address prominent paradigms and paradigm shifts in the field of transplantation immunology. We identify several instances in which the transplant community missed a timely paradigm shift because essential, available knowledge was ignored. Moreover, we discuss key findings that critically contributed to our understanding of transplant immunology but sometimes developed with delay and in a roundabout way, as was the case with antibody-mediated rejection-a main focus of this article. These include the discovery of the molecular principles of histocompatibility, the recognition of the microcirculation as a key interface of immune damage, the refinement of alloantibody detection, the description of C4d as a footmark of endothelium-bound antibody, and last but not least, the developments in biopsy-based diagnostics beyond conventional morphology, which only now give us a glimpse of the enormous complexity and pathogenetic diversity of rejection.

Role of complement in patients with autoimmune hemolytic anemia and platelet transfusion refractoriness

The complement is a key player of the innate immune response. It provides defense mechanisms that are not specific, but very efficient at neutralizing any invader, accounting for 4% of the proteins in the peripheral blood. Nevertheless, there is a dark side to the complement system, as it may activate its machinery against healthy cells such as peripheral blood red blood cells and platelets resulting in undesired hemolysis and thrombocytopenia, respectively. Understanding and identifying the role of complement in these settings allow physicians to adjust their diagnostic and therapeutic modalities accordingly. The role of complement in the pathophysiology and management of autoimmune hemolytic anemia and of alloimmune-mediated thrombocytopenia is under investigation and discussed.

Chimeric Proteins Containing MAP-1 and Functional Domains of C4b-Binding Protein Reveal Strong Complement Inhibitory Capacities

The complement system is a tightly regulated network of proteins involved in defense against pathogens, inflammatory processes, and coordination of the innate and adaptive immune responses. Dysregulation of the complement cascade is associated with many inflammatory disorders. Thus, inhibition of the complement system has emerged as an option for treatment of a range of different inflammatory diseases. MAP-1 is a pattern recognition molecule (PRM)-associated inhibitor of the lectin pathway of the complement system, whereas C4b-binding protein (C4BP) regulates both the classical and lectin pathways. In this study we generated chimeric proteins consisting of MAP-1 and the first five domains of human C4BP (C4BP¹⁻⁵) in order to develop a targeted inhibitor acting at different levels of the complement cascade. Two different constructs were designed and expressed in CHO cells where MAP-1 was fused with C4BP¹⁻⁵ in either the C- or N-terminus. The functionality of the chimeric proteins was assessed using different in vitro complement activation assays. Both chimeric proteins displayed the characteristic Ca²⁺-dependent dimerization and binding to PRMs of native MAP-1, as well as the co-factor activity of native C4BP. In ELISA-based complement activation assays they could effectively inhibit the lectin and classical pathways. Notably, MAP-1:C4BP¹⁻⁵ was five times more effective than rMAP-1 and rC4BP¹⁻⁵ applied at the same time, emphasizing the advantage of a single inhibitor containing both functional domains. The MAP-1/C4BP chimeras exert unique complement inhibitory properties and represent a novel therapeutic approach targeting both upstream and central complement activation.

Effect of the Anti-C1s Humanized Antibody TNT009 and Its Parental Mouse Variant TNT003 on HLA Antibody-Induced Complement Activation-A Preclinical In Vitro Study

The classic pathway (CP) of complement is believed to significantly contribute to alloantibody-mediated transplant injury, and targeted complement inhibition is currently considered to be a promising approach for preventing rejection. Here, we investigated the mode of action and efficacy of the humanized anti-C1s monoclonal antibody TNT009 and its parental mouse variant, TNT003, in preclinical in vitro models of HLA antibody-triggered CP activation. In flow cytometric assays, we measured the attachment of C1 subcomponents and C4/C3 split products (C4b/d, C3b/d) to HLA antigen-coated flow beads or HLA-mismatched aortic endothelial cells and splenic lymphocytes. Anti-C1s antibodies profoundly inhibited C3 activation at concentrations >20 μg/mL, in both solid phase and cellular assays. While C4 activation was also prevented, this was not the case for C1 subcomponent attachment. Analysis of serum samples obtained from 68 sensitized transplant candidates revealed that the potency of inhibition was related to the extent of baseline CP activation. This study demonstrates that anti-C1s antibodies TNT009 and TNT003 are highly effective in blocking HLA antibody-triggered complement activation downstream of C1. Our results provide the foundation for clinical studies designed to investigate the potential of TNT009 in the treatment or prevention of complement-mediated tissue injury in sensitized transplant recipients.

Diagnostic Contribution of Donor-Specific Antibody Characteristics to Uncover Late Silent Antibody-Mediated Rejection-Results of a Cross-Sectional Screening Study

BACKGROUND

Circulating donor-specific antibodies (DSA) detected on bead arrays may not inevitably indicate ongoing antibody-mediated rejection (AMR). Here, we investigated whether detection of complement-fixation, in parallel to IgG mean fluorescence intensity (MFI), allows for improved prediction of AMR.

METHODS

Our study included 86 DSA+ kidney transplant recipients subjected to protocol biopsy, who were identified upon cross-sectional antibody screening of 741 recipients with stable graft function at 6 months or longer after transplantation. IgG MFI was analyzed after elimination of prozone effect, and complement-fixation was determined using C1q, C4d, or C3d assays.

RESULTS

Among DSA+ study patients, 44 recipients (51%) had AMR, 24 of them showing C4d-positive rejection. Although DSA number or HLA class specificity were not different, patients with AMR or C4d + AMR showed significantly higher IgG, C1q, and C3d DSA MFI than nonrejecting or C4d-negative patients, respectively. Overall, the predictive value of DSA characteristics was moderate, whereby the highest accuracy was computed for peak IgG MFI (AMR, 0.73; C4d + AMR, 0.71). Combined analysis of antibody characteristics in multivariate models did not improve AMR prediction.

CONCLUSIONS

We estimate a 50% prevalence of silent AMR in DSA+ long-term recipients and conclude that assessment of IgG MFI may add predictive accuracy, without an independent diagnostic advantage of detecting complement-fixation.

From Humoral Theory to Performant Risk Stratification in Kidney Transplantation

The purpose of the present review is to describe how we improve the model for risk stratification of transplant outcomes in kidney transplantation by incorporating the novel insights of donor-specific anti-HLA antibody (DSA) characteristics. The detection of anti-HLA DSA is widely used for the assessment of pre- and posttransplant risks of rejection and allograft loss; however, not all anti-HLA DSA carry the same risk for transplant outcomes. These antibodies have been shown to cause a wide spectrum of effects on allografts, ranging from the absence of injury to indolent or full-blown acute antibody-mediated rejection. Consequently, the presence of circulating anti-HLA DSA does not provide a sufficient level of accuracy for the risk stratification of allograft outcomes. Enhancing the predictive performance of anti-HLA DSA is currently one of the most pressing unmet needs for facilitating individualized treatment choices that may improve outcomes. Recent advancements in the assessment of anti-HLA DSA properties, including their strength, complement-binding capacity, and IgG subclass composition, significantly improved the risk stratification model to predict allograft injury and failure. Although risk stratification based on anti-HLA DSA properties appears promising, further specific studies that address immunological risk stratification in large and unselected populations are required to define the benefits and cost-effectiveness of such comprehensive assessment prior to clinical implementation.

Detection of HLA Antibodies in Organ Transplant Recipients - Triumphs and Challenges of the Solid Phase Bead Assay

This review outlines the development of human leukocyte antigen (HLA) antibody detection assays and their use in organ transplantation in both antibody screening and crossmatching. The development of sensitive solid phase assays such as the enzyme-linked immunosorbent assay technique, and in particular the bead-based technology has revolutionized this field over the last 10-15 years. This revolution however has created a new paradigm in clinical decision making with respect to the detection of low level pretransplant HLA sensitization and its clinical relevance. The relative sensitivities of the assays used are discussed and the relevance of conflicting inter-assay results. Each assay has its advantages and disadvantages and these are discussed. Over the last decade, the bead-based assay utilizing the Luminex® fluorocytometer instrument has become established as the "gold standard" for HLA antibody testing. However, there are still unresolved issues surrounding this technique, such as the presence of denatured HLA molecules on the beads which reveal cryptic epitopes and the issue of appropriate fluorescence cut off values for positivity. The assay has been modified to detect complement binding (CB) in addition to non-complement binding (NCB) HLA antibodies although the clinical relevance of the CB and NCB IgG isotypes is not fully resolved. The increase sensitivity of the Luminex® bead assay over the complement-dependent cytotoxicity crossmatch has permitted the concept of the "virtual crossmatch" whereby the crossmatch is predicted to a high degree of accuracy based on the HLA antibody specificities detected by the solid phase assay. Dialog between clinicians and laboratory staff on an individual patient basis is essential for correct clinical decision making based on HLA antibody results obtained by the various techniques.

Antibody Subclass Repertoire and Graft Outcome Following Solid Organ Transplantation

Long-term outcomes in solid organ transplantation are constrained by the development of donor-specific alloantibodies (DSA) against human leukocyte antigen (HLA) and other targets, which elicit antibody-mediated rejection (ABMR). However, antibody-mediated graft injury represents a broad continuum, from extensive complement activation and tissue damage compromising the function of the transplanted organ, to histological manifestations of endothelial cell injury and mononuclear cell infiltration but without concurrent allograft dysfunction. In addition, while transplant recipients with DSA as a whole fare worse than those without, a substantial minority of patients with DSA do not experience poorer graft outcome. Taken together, these observations suggest that not all DSA are equally pathogenic. Antibody effector functions are controlled by a number of factors, including antibody concentration, antigen availability, and antibody isotype/subclass. Antibody isotype is specified by many integrated signals, including the antigen itself as well as from antigen-presenting cells or helper T cells. To date, a number of studies have described the repertoire of IgG subclasses directed against HLA in pretransplant patients and evaluated the clinical impact of different DSA IgG subclasses on allograft outcome. This review will summarize what is known about the repertoire of antibodies to HLA and non-HLA targets in transplantation, focusing on the distribution of IgG subclasses, as well as the general biology, etiology, and mechanisms of injury of different humoral factors.

C1Q Assay Results in Complement-Dependent Cytotoxicity Crossmatch Negative Renal Transplant Candidates with Donor-Specific Antibodies: High Specificity but Low Sensitivity When Predicting Flow Crossmatch

The aim of the present study was to describe the association of positive flow cross match (FXM) and C1q-SAB. Methods. In this observational, cross-sectional, and comparative study, patients included had negative AHG-CDC-XM and donor specific antibodies (DSA) and were tested with FXM. All pretransplant sera were tested with C1q-SAB assay. Results. A total of 50 donor/recipient evaluations were conducted; half of them had at least one C1q+ Ab (n = 26, 52%). Ten patients (20.0%) had DSA C1q+ Ab. Twenty-five (50%) FXMs were positive. Factors associated with a positive FXM were the presence of C1q+ Ab (DSA C1q+ Ab: OR 27, 2.80–259.56, P = 0.004, and no DSA C1q+ Ab: OR 5, 1.27–19.68, P = 0.021) and the DSA LABScreen-SAB MFI (OR 1.26, 95% CI 1.06–1.49, P = 0.007). The cutoff point of immunodominant LABScreen SAB DSA-MFI with the greatest sensitivity and specificity to predict FXM was 2,300 (sensitivity: 72% and specificity: 75%). For FXM prediction, DSA C1q+ Ab was the most specific (95.8%, 85–100) and the combination of DSA-MFI > 2,300 and C1q+ Ab was the most sensitive (92.0%, 79.3–100). Conclusions. C1q+ Ab and LABScreen SAB DSA-MFI were significantly associated with FXM. DSA C1q+ Ab was highly specific but with low sensitivity.

Transplant immuno-diagnostics: crossmatch and antigen detection

Identifying and monitoring donor-directed anti-human leukocyte antigen antibodies are a rapidly evolving area of solid organ transplantation. Donor-specific antibodies dictate pre-transplant donor choice and donor-recipient matching and underlie much acute and chronic allograft rejection and loss. The evolution of available technology has driven this progress. Early, labor-intensive, whole-cell assays based on complement-dependent cytotoxicity suffered from poor sensitivity and specificity, technical challenges and lack of precision. Sequential improvement in assay performance included anti-human immunoglobulin-enhanced, complement-dependent cytotoxicity techniques followed by cell-based flow cytometry. However, variable specificity and sensitivity inherent in cell-based testing continued to limit flow cytometry. The introduction of solid-phase assays led to a second revolution in histocompatibility testing with the use of purified antigens bound to artificial surfaces rather than whole cells. These techniques augmented sensitivity and specificity to detect even low-titer antibodies to previously undetected antigens. Identification of complement-activating antibodies is being introduced, but current technology is in the developmental stage. While the detection of alloantibodies has improved dramatically, our comprehension of their importance remains imperfect. Variability in methodology and a lack of standardization limits the clinical application of these tests. In spite of the hurdles that remain, antibody-mediated rejection has become a key target to improve graft survival.

Improved flow cytometry based cytotoxicity and binding assay for clinical antibody HLA crossmatching

The presence of preformed donor-specific HLA antibodies leads to early antibody mediated kidney allograft rejection. Therefore, detection and avoidance of donor reactive HLA antibodies prior to transplantation is of outmost importance in order to minimize the risk of rejection. Detection of pre-formed HLA antibodies is currently performed using complement-dependent cytotoxicity (CDC) assay alone or together with a flow cytometry based crossmatch (FCXM). This study was initiated to further evaluate our recently developed flow cytometry based procedure for determination of both cytotoxicity of and IgG binding to donor-derived lymphocytes by HLA antibodies. Highly enriched immuno-magnetic bead purified T and B lymphocytes were used as target cells for patient sera using 96-well plates. Importantly, the assay shows high sensitivity and specificity as determined by HLA typed donor cells and serum with defined HLA antibody IgG and C1q. Based on this and additional data generated in this paper, such as evaluation of appropriate serum and complements incubation times and assay reproducibility and stability, will enable us to more rapidly implement this assay in our clinical laboratory routines. In addition, we demonstrate that FCtox crossmatching of deceased donor cells has superior specificity compared to conventional CDC assay especially regarding high frequencies of false-positive reactions.

Characteristics of Circulating Donor Human Leukocyte Antigen-specific Immunoglobulin G Antibodies Predictive of Acute Antibody-mediated Rejection and Kidney Allograft Failure

BACKGROUND

Characteristics of pretransplant antibodies directed at donor human leukocyte antigen (HLA) donor-specific antibodies (DSA) associated with adverse outcomes in kidney transplant recipients are being elucidated but uncertainties exist.

METHODS

We prospectively screened pretransplant sera from 543 kidney recipients using single antigen bead assays and identified 154 patients with and 389 without DSA. We investigated the association of DSA features to acute rejection and graft failure.

RESULTS

One-year acute rejection incidence was higher in DSA-positive group (P < 0.001), primarily due to antibody-mediated rejection (AMR, 13% vs. 1.8%, P < 0.001) and not T cell-mediated rejection (ACR, 5% vs.6%, P = 0.65). The sum of mean fluorescence intensity of DSA (DSA MFI-Sum) of 6,000 or higher (OR, 18; 95% CI, 7.0-47; P < 0.001) and the presence of DSA against both HLA class I and II (OR, 39; 95% CI, 14-106; P < 0.0001) predicted 1-year AMR, independent of other covariates. Calculated panel reactive antibody and a positive flow cytometry cross-match result were associated with AMR by bivariate analysis but neither was an independent predictor in a multivariable regression analysis that included DSA-MFI-Sum or HLA DSA class. In multivariable Cox proportional hazards models, the covariate-adjusted hazard ratio for graft failure was 2.03 (95%CI, 1.05-3.92; P = 0.04) for DSA MFI-Sum of 6,000 or higher and 2.23 (95% CI, 1.04-4.80; P = 0.04) for class I and II DSA. Prediction of graft failure was not independent of AMR.

CONCLUSION

Our study suggests that DSA MFI-Sum and HLA class of DSA are characteristics predictive of AMR and graft failure. The elevated risk of graft failure in those with the identified features of DSA is attributable to increased risk of AMR.

Detection of alloantibody-mediated complement activation: A diagnostic advance in monitoring kidney transplant rejection?

OBJECTIVE

Antibody-mediated rejection (ABMR) is an important cause of kidney allograft injury. In the last two decades, detection of complement split product C4d along transplant capillaries, a footprint of antibody-mediated classical complement activation, has evolved as a useful diagnostic marker of ABMR. While it was recognized that ABMR may occur also in the absence of C4d, numerous studies have shown that C4d deposition may indicate a more severe rejection phenotype associated with poor graft survival. Such studies suggest a possible diagnostic benefit of ex vivo monitoring the complement-activating capability of circulating alloantibodies.

DESIGN AND METHODS

We reviewed the literature between 1993 and 2015, focusing on in vivo (biopsy work-up) and in vitro detection (modified bead array technology) of HLA antibody-triggered classical complement activation in kidney transplantation.

RESULTS

Precise HLA antibody detection methods, in particular Luminex-based single antigen bead (SAB) assays, have provided a valuable basis for the design of techniques for in vitro detection of HLA antibody-triggered complement activation reflected by C1q, C4 or C3 split product deposition to the bead surface. Establishing such assays it was recognized that deposition of complement products to SAB, which critically depends on antibody binding strength, may be a cardinal trigger of the prozone effect, a troublesome in vitro artifact caused by a steric interference with IgG detection reagents. False-low IgG results, especially on SAB with extensive antibody binding, have to be considered when interpreting studies analyzing the diagnostic value of complement in relation to standard IgG detection. Levels of complement-fixing donor-specific antibodies (DSA) were shown to correlate with the results of standard crossmatch tests, suggesting potential application for crossmatch prediction. Moreover, while the utility of pre-transplant complement detection, at least in crossmatch-negative transplant recipients, is controversially discussed, a series of studies have shown that the appearance of post-transplant complement-fixing DSA may be associated with C4d deposition in transplant capillaries and a particular risk of graft failure.

CONCLUSIONS

The independent value of modified single antigen bead assays, as compared to a careful analysis of standard IgG detection, which may be affected considerably by complement dependent artifacts, needs to be clarified. Whether they have the potential to improve the predictive accuracy of our current diagnostic repertoire warrants further study.

[Ten years' experience in the research of abdominal compartment syndrome (2004-2014)]

Intra-abdominal hypertension and abdominal compartment syndrome are frequent findings among severe surgical ill patients. In spite of the fast diagnostic methods and effective therapeutic procedures the mortality is high. The causing factors lead to increased intra-abdominal pressure and abdominal compartment syndrome. It can be defined as adverse physiologic consequences that occur as a result of an acute increase in the intra-abdominal pressure. The most common causes are retroperitoneal haemorrhage, pancreatitis, bowel obstruction, tense ascites, peritonitis and serious visceral edema due to massive fluid resuscitation. The affected systems are cardiovascular, respiratory, renal, central nervous systems, splanchnic organs, and finally the whole body. The diagnostic method is the intra-abdominal pressure monitoring. The bases of the treatment are adequate fluid resuscitation, non-surgical management and decompression. The authors review the topic including the international and Hungarian references based on their ten years experience.

Bead arrays for antibody and complement profiling reveal joint contribution of antibody isotypes to C3 deposition

The development of antigen arrays has provided researchers with great tools to identify reactivities against self or foreign antigens from body fluids. Yet, these approaches mostly do not address antibody isotypes and their effector functions even though these are key points for a more detailed understanding of disease processes. Here, we present a bead array-based assay for a multiplexed determination of antigen-specific antibody levels in parallel with their properties for complement activation. We measured the deposition of C3 fragments from serum samples to reflect the degree of complement activation via all three complement activation pathways. We utilized the assay on a bead array containing native and citrullinated peptide antigens to investigate the levels of IgG, IgM and IgA autoantibodies along with their complement activating properties in serum samples of 41 rheumatoid arthritis patients and 40 controls. Our analysis revealed significantly higher IgG reactivity against the citrullinated fibrinogen β and filaggrin peptides as well as an IgA reactivity that was exclusive for citrullinated fibrinogen β peptide and C3 deposition in rheumatoid arthritis patients. In addition, we characterized the humoral immune response against the viral EBNA-1 antigen to demonstrate the applicability of this assay beyond autoimmune conditions. We observed that particular buffer compositions were demanded for separate measurement of antibody reactivity and complement activation, as detection of antigen-antibody complexes appeared to be masked due to C3 deposition. We also found that rheumatoid factors of IgM isotype altered C3 deposition and introduced false-positive reactivities against EBNA-1 antigen. In conclusion, the presented bead-based assay setup can be utilized to profile antibody reactivities and immune-complex induced complement activation in a high-throughput manner and could facilitate the understanding and diagnosis of several diseases where complement activation plays role in the pathomechanism.

For anti-HLA-specific donor antibodies detection by flow cytometry cytotoxic crossmatches comparison of methods

Anti-HLA-specific donor antibodies induce rapid, irreversible destruction of the transplant (hyperacute rejection) that today happens rarely due to immunologic studies-prospective crossmatch-of patients awaiting the kidney graft. The usual approach for pretransplant donor/recipient evaluation is based on 2 methods: (1) the cytotoxic complement crossmatch (CDC) and (2) the flow cytometric crossmatch (FCX). The CDC crossmatch is positive when complement-fixing antibodies are present, an absolute contraindication to kidney transplantation. The more sensitive FCX-positive crossmatch detects low concentrations of unable to fix performed antibodies complement. It is an "index" of possible damage due to accelerated rejection. The target of our study was to develop a cytotoxic flow cytometry crossmatch (cFCX) that detected cytotoxic antibodies move sensitively than the traditional CDC method and also was less subjective and more standardized for interpretation studying sera from 23 patients; the cFCX showed the requested efficiency characteristics even in an emergency. In addition, the new method permited one to calculate a cutoff for positivity (average value of the negative control + 2 standard deviations), assuring an "objective" interpretation of the results that agreed with the CDC but was more sensitive and accurate allowing solution of ambiguous results for cases of "doubt"-positive CDC crossmatch. Furthermore, our aim was to correlate the effect of the strength of the anti-HLA antibodies determined by mean fluorescence intensity value of LabScreen Single Antigen beads with results of CDC, cFCX, and FCX methods.

Consensus guidelines on the testing and clinical management issues associated with HLA and non-HLA antibodies in transplantation

BACKGROUND

The introduction of solid-phase immunoassay (SPI) technology for the detection and characterization of human leukocyte antigen (HLA) antibodies in transplantation while providing greater sensitivity than was obtainable by complement-dependent lymphocytotoxicity (CDC) assays has resulted in a new paradigm with respect to the interpretation of donor-specific antibodies (DSA). Although the SPI assay performed on the Luminex instrument (hereafter referred to as the Luminex assay), in particular, has permitted the detection of antibodies not detectable by CDC, the clinical significance of these antibodies is incompletely understood. Nevertheless, the detection of these antibodies has led to changes in the clinical management of sensitized patients. In addition, SPI testing raises technical issues that require resolution and careful consideration when interpreting antibody results.

METHODS

With this background, The Transplantation Society convened a group of laboratory and clinical experts in the field of transplantation to prepare a consensus report and make recommendations on the use of this new technology based on both published evidence and expert opinion. Three working groups were formed to address (a) the technical issues with respect to the use of this technology, (b) the interpretation of pretransplantation antibody testing in the context of various clinical settings and organ transplant types (kidney, heart, lung, liver, pancreas, intestinal, and islet cells), and (c) the application of antibody testing in the posttransplantation setting. The three groups were established in November 2011 and convened for a "Consensus Conference on Antibodies in Transplantation" in Rome, Italy, in May 2012. The deliberations of the three groups meeting independently and then together are the bases for this report.

RESULTS

A comprehensive list of recommendations was prepared by each group. A summary of the key recommendations follows. Technical Group: (a) SPI must be used for the detection of pretransplantation HLA antibodies in solid organ transplant recipients and, in particular, the use of the single-antigen bead assay to detect antibodies to HLA loci, such as Cw, DQA, DPA, and DPB, which are not readily detected by other methods. (b) The use of SPI for antibody detection should be supplemented with cell-based assays to examine the correlations between the two types of assays and to establish the likelihood of a positive crossmatch (XM). (c) There must be an awareness of the technical factors that can influence the results and their clinical interpretation when using the Luminex bead technology, such as variation in antigen density and the presence of denatured antigen on the beads. Pretransplantation Group: (a) Risk categories should be established based on the antibody and the XM results obtained. (b) DSA detected by CDC and a positive XM should be avoided due to their strong association with antibody-mediated rejection and graft loss. (c) A renal transplantation can be performed in the absence of a prospective XM if single-antigen bead screening for antibodies to all class I and II HLA loci is negative. This decision, however, needs to be taken in agreement with local clinical programs and the relevant regulatory bodies. (d) The presence of DSA HLA antibodies should be avoided in heart and lung transplantation and considered a risk factor for liver, intestinal, and islet cell transplantation. Posttransplantation Group: (a) High-risk patients (i.e., desensitized or DSA positive/XM negative) should be monitored by measurement of DSA and protocol biopsies in the first 3 months after transplantation. (b) Intermediate-risk patients (history of DSA but currently negative) should be monitored for DSA within the first month. If DSA is present, a biopsy should be performed. (c) Low-risk patients (nonsensitized first transplantation) should be screened for DSA at least once 3 to 12 months after transplantation. If DSA is detected, a biopsy should be performed. In all three categories, the recommendations for subsequent treatment are based on the biopsy results.

CONCLUSIONS

A comprehensive list of recommendations is provided covering the technical and pretransplantation and posttransplantation monitoring of HLA antibodies in solid organ transplantation. The recommendations are intended to provide state-of-the-art guidance in the use and clinical application of recently developed methods for HLA antibody detection when used in conjunction with traditional methods.

Preformed complement-activating low-level donor-specific antibody predicts early antibody-mediated rejection in renal allografts

BACKGROUND.: Donor-specific anti-HLA antibodies (DSA) are a major cause of alloimmune injury. Transplant recipients with negative complement-dependent cytotoxic crossmatch (CDC-XM) and donor cell-based flow cytometric crossmatch (flow-XM) but low level DSA (i.e., by Luminex) have worse outcomes compared with nonsensitized patients. The aim of this study was to establish whether complement-activating ability in this low-level DSA, present before transplantation, as determined by this technique is important in dictating pathogenicity. METHODS.: We retrospectively studied 52 patients with preformed DSA detected by single-antigen flow cytometric fluorescent beads (SAFBs). Patients were transplanted using a steroid-sparing regimen consisting of alemtuzumab induction, 1 week of corticosteroids and tacrolimus monotherapy.Fifteen (29%) of 52 patients experienced antibody-mediated rejection (AMR), whereas 37 (71%) patients did not. There were no demographic differences between patients with AMR and those without. Pretransplant sera were retested using a modified (SAFB) assay, which detects the presence of the complement fragment C4d as a result of DSA-induced complement activation. RESULTS.: C4d+DSA were detected in 10 (19%) of 52 patients. Biopsy-proven AMR occurred in 7 (70%) of the 10 patients with C4d+DSA and in 8 (19%) of 42 patients with C4d-DSA. AMR-free survival was worse in patients with C4d+DSA (P<0.001). CONCLUSIONS.: The ability of preformed, low-level, DSA to trigger C4d fixation in vitro in patients with negative conventional crossmatch tests is predictive for AMR. C4d SAFB is potentially a powerful tool for risk stratification prior to transplantation and may allow identification of unacceptable donor antigens, or patients who may require enhanced immunosuppression.

C1q assay for the detection of complement fixing antibody to HLA antigens

Solid phase Luminex(®) and flow cytometric single antigen bead assays offer exquisite sensitivity and specificity for HLA antibody detection. Unlike the historical complement-dependent cytotoxicity (CDC) method, these assays do not distinguish complement fixing from non-complement fixing antibody, the former of which are considered the most clinically relevant in the peri-transplant period. This chapter describes a novel solid phase C1q binding assay to distinguish HLA antibodies that can bind the first component of complement (C1q). These antibodies have the capacity to initiate the complement cascade irrespective of whether that actually occurs. The C1q assay detects many more complement fixing antibodies than are observed by the less sensitive and less specific CDC assay.

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.

Complement activation is not required for obliterative airway disease induced by antibodies to major histocompatibility complex class I: Implications for chronic lung rejection

BACKGROUND

The role of non-complement activating antibodies (ncAbs) to mismatched donor human leukocyte antigen (HLA) in the pathogenesis of chronic lung rejection is not known. We used a murine model of obliterative airway disease (OAD) induced by Abs to major histocompatibility major histocompatibility complex (MHC) class I and serum from donor-specific Abs developed in human lung transplant (LTx) recipients to test the role of ncAbs in the development of OAD and bronchiolitis obliterans syndrome (BOS).

METHODS

Anti-MHC ncAbs were administered intrabronchially in B.10 mice or in C3 knockout (C3KO) mice. Lungs were analyzed by histopathology. Lymphocytes secreting interleukin (IL)-17, interferon-γ, or IL-10 to collagen V and K-α1 tubulin (Kα1T) were enumerated by enzyme-linked immunospot assay. Serum antibodies to collagen V and Kα1T were determined by enzyme-linked immunosorbent assay. Cytokine and growth factor expression in lungs was determined by real-time polymerase chain reaction. Donor-specific Abs from patients with BOS and control BOS-negative LTx recipients were analyzed by C1q assay.

RESULTS

Administration of ncAbs in B.10 mice or C3KO resulted in OAD lesions. There were significant increases in IL-17- and interferon-γ-secreting cells to collagen V and Kα1T, along with serum Abs to these antigens. There was also augmented expression of monocyte chemotactic protein-1, IL-6, IL-1β, vascular endothelial growth factor, transforming growth factor-β, and fibroblastic growth factor in mice administered ncAbs by Day 3. Among 5 LTx recipients with BOS, only 1 had C1q binding donor-specific Abs.

CONCLUSION

Complement activation by Abs to MHC class I is not required for development of OAD and human BOS. Therefore, anti-MHC binding to epithelial and endothelial cells can directly activate pro-fibrotic and pro-inflammatory cascades leading to immune response to self-antigens and chronic rejection.

The complement-mediated prozone effect in the Luminex single-antigen bead assay and its impact on HLA antibody determination in patient sera

The Luminex xMAP system has become an important tool for HLA antibody screening and identification in sera of transplant patients. Recently, the Luminex single antigen bead assay was shown to be prone to an artefact, the so called prozone phenomenon: Sera with high titer HLA antibodies gave negative results when tested neat, but reacted strongly positive after 1:10 dilution. We also observed such a phenomenon and found that it was most likely caused by the complement component 1 (C1) by competitively displacing the detection antibodies. In this article we review the complement-mediated prozone effect and other mechanisms of interference with solid phase assays, and we discuss possible consequences for HLA antibody testing with the Luminex SAB assay.

Antibodies, isotypes and complement in allograft rejection

PURPOSE OF REVIEW

Classical complement activation is a key step in the process of antibody-mediated rejection. Emphasizing novel diagnostic strategies, this study will discuss recent studies highlighting the particular relevance of alloantibodies with complement-fixing ability.

RECENT FINDINGS

Reinforcing the pivotal role of complement, numerous studies have shown tight associations of capillary C4d deposition, a 'footprint' of alloantibody-triggered complement activation, with the occurrence of allograft injury. Distribution patterns of immunoglobulin isotypes or subclasses, which strongly differ in their ability to activate complement, may not adequately reflect the actual pathogenetic relevance of detected allosensitization. This fact may be explained by the finding that other variables, such as antibody-binding density or a synergism of antibodies against different epitopes of the same antigen, may contribute to complement activation. An attractive approach to distinguish between complement-fixing and presumably less harmful noncomplement-fixing alloreactivities could be the detection of C4d deposition in vitro. Applying such techniques, recent studies have shown that human leukocyte antigen reactivity with C4d-fixing ability, in contrast to noncomplement-fixing sensitization, may strongly predict antibody-mediated rejection and inferior graft survival.

SUMMARY

Considering the pivotal role of complement, technologies that uncover the complement-fixing ability of alloantibodies may be of particular interest for the selective detection of deleterious sensitization.

Hidden perils in a highly sensitized kidney transplant recipient

Highly sensitised patients are at increased risk for antibody mediated rejection (AMR) and reduced graft survival. Highly sensitive assays for detecting recipient preformed anti-HLA antibodies have been developed and identify high immunological risk donors. A 62yo male with end stage renal failure secondary to glomerulonephritis received a T-cell crossmatch negative, deceased donor, renal transplant mismatched at 3 of 6 HLA loci. A donor specific antibody (DSAb) to DR17 (MFI 2073) was present. Given his advancing age, multiple medical comorbidities and broad HLA sensitisation the transplant was accepted, however, shortly before transplantation two atypical results were made available. Firstly a B-cell crossmatch was performed and found to be negative in current serum but strongly positive in peak serum, secondly a further potential DSAb was predicted based on linkage disequilibrium with known donor HLA typing. The donor HLA typing would not be clarified until after the transplant. Despite the increased risk of AMR the transplant proceeded with pre-emptive plasma exchange. The patient developed severe AMR requiring extensive therapy. Incomplete prospective donor HLA typing can generate uncertainty in the interpretation of the virtual crossmatch performed for deceased donor transplants. This may result in clinically relevant sequelae. Advances in antibody detection techniques need to be matched by timely donor HLA typing for its full benefit to be realised.

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.

C1q-fixing human leukocyte antigen antibodies are specific for predicting transplant glomerulopathy and late graft failure after kidney transplantation

BACKGROUND

Human leukocyte antigen (HLA) antibodies, especially those that fix complement, are associated with antibody-mediated rejection and graft failure. The C1q assay on single antigen beads detects a subset of HLA antibodies that can fix complement and precede C4d deposition. The aim of this study was to determine whether C1q-fixing antibodies distinguish de novo donor-specific antibodies (DSA) that are clinically relevant and harmful.

METHODS

We retrospectively studied 31 of 274 kidney transplant recipients who had pretransplant and concurrent biopsy and serum specimens, 13 with C4d-positive and 18 with C4d-negative staining. We measured IgG and C1q DSA pretransplant and at the time of biopsy using single antigen bead assays. We identified 13 recipients who developed de novo DSA by IgG or C1q and examined associations with C4d deposition, transplant glomerulopathy, and graft failure.

RESULTS

Testing for DSA by IgG is more sensitive for C4d deposition (IgG: 100%, 95% confidence interval [CI] 0.60-1; C1q: 75%, 95% CI 0.36-0.96). Testing for DSA by C1q is more specific for transplant glomerulopathy (C1q: 81%, 95% CI 0.57-0.94; IgG: 67%, 95% CI 0.43-0.85) and graft loss (C1q: 79%, 95% CI 0.54-0.93; IgG: 63%, 95% CI 0.39-0.83). Absence of de novo DSA by IgG and C1q has a high negative predictive value for the absence of C4d deposition (IgG: 100%, 95% CI 0.73-1; C1q: 88%, 95% CI 0.62-0.98), transplant glomerulopathy (IgG: 100%, 95% CI 0.73-1; C1q: 100%, 95% CI 0.77-1), and graft failure (IgG: 86%, 95% CI 0.56-0.97; C1q: 88%, 95% CI 0.62-0.98).

CONCLUSION

Monitoring patients with the C1q assay, which detects antibodies that fix complement, offers a minimally invasive means of identifying patients at risk for transplant glomerulopathy and graft loss.

C4d-fixing capability of low-level donor-specific HLA antibodies is not predictive for early antibody-mediated rejection

BACKGROUND

Recent studies indicate that not all low-level donor-specific human leukocyte antigen (HLA) antibodies (HLA-DSA) (i.e., positive by solid-phase assays, negative by complement-dependent cytotoxic-crossmatch) have a detrimental clinical impact. The aim of this study was to investigate whether the pretransplant C4d-fixing capability allows distinguishing harmful from presumably clinically irrelevant HLA-DSA.

METHODS

We retrospectively investigated 64 patients with low-level HLA-DSA detected by single-antigen flow beads (SAFB). Thirty-four of 64 patients (53%) experienced early antibody-mediated rejection (AMR), whereas 30 patients (47%) did not. HLA-DSA characteristics (i.e., number, class, and strength), frequency of retransplants, and immunosuppressive regimens were not different in these two groups. Pretransplant sera were reanalyzed using a modified SAFB assay measuring C4d fixation induced by HLA antibodies.

RESULTS

C4d-fixing HLA-DSA were observed in 11 of 64 patients (17%). AMR occurred in 6 of 11 patients (55%) with C4d-fixing HLA-DSA and in 28 of 53 patients (53%) without C4d-fixing HLA-DSA (P=1.0). Positive C4d staining in peritubular capillaries was detected in 6 of 11 patients (55%) with C4d-fixing HLA-DSA and in 21 of 53 patients (40%) without C4d-fixing HLA-DSA (P=0.50).

CONCLUSION

The pretransplant capability of low-level HLA-DSA to trigger C4d fixation in vitro is not predictive for early AMR or C4d deposition in peritubular capillaries in vivo. This argues against pretransplant C4d SAFB testing to define the clinical relevance of low-level HLA-DSA.

Clinical relevance of preformed C4d-fixing and non-C4d-fixing HLA single antigen reactivity in renal allograft recipients

Donor-specific alloantibodies (DSA), especially those fixing complement, may pose a particular immunologic risk to transplant recipients. To assess the clinical impact of C4d- or non-C4d-fixing (IgG) HLA sensitization, pretransplant sera obtained from 338 kidney allograft recipients prescreened by FlowPRA were retrospectively evaluated by Luminex single antigen (SA) testing using a novel fluorescent-labeled anti-C4d reagent for detection of antibody-triggered C4d deposition in addition to IgG binding. Recipients with [IgG]DSA (n = 39) showed a substantially higher rate of C4d positive rejection (33%) than 16 patients with [IgG] non-DSA (0%) or 283 antibody-negative patients (4%, multivariate analysis excluding retransplantation because of high co-linearity: P < 0.0001), and adversely affected 5-year death-censored graft survival (74% vs. 81% and 90%, respectively, multivariate model: P < 0.05). [C4d] DSA (n = 21) and [C4d] non-DSA (n = 25) increased rates of C4d positive rejections to a similar extent (24% and 28% vs. 4% in recipients without C4d-fixing reactivity; multivariate analysis: P <or= 0.002) with a trend towards adverse 5-year graft survival (76% and 76% vs. 90%; P <or= 0.2). In conclusion, Luminex-based characterization of HLA sensitization may be a useful strategy for risk stratification. Possibly as a result of intensified immunosuppression in presensitized recipients, identification of C4d-fixing DSA was not associated with a further increase of rejection and graft loss rates.

Mechanisms of complement activation, C4d deposition, and their contribution to the pathogenesis of antibody-mediated rejection

Complement split products have emerged as useful markers of antibody-mediated rejection in solid organ transplants. One split product, C4d, is now widely accepted as a marker for antibody-mediated rejection in renal and cardiac allografts. This review summarizes the rationale for the use of C4d as a marker of antibody-mediated rejection, along with the clinical evidence supporting its use in the clinical diagnosis of antibody-mediated rejection. Antibody-independent mechanisms by which C4d can be activated by the classical and lectin pathways of complement activation are also identified. Finally, mechanisms by which complement activation stimulates effector cells (neutrophils, monocytes, macrophages, platelets, and B and T lymphocytes) as well as target cells (endothelial cells) are discussed in relation to antibody-mediated allograft rejection.

Review article: Luminex technology for HLA antibody detection in organ transplantation

Since its inception in the early 1960s, the serologically based complement-dependent cytotoxicity (CDC) assay has been the cornerstone technique for the detection of human leucocyte antigen (HLA) antibodies, not only in pre-transplant renal patients, but also in other forms of organ transplantation. Recently, solid phase assays have been developed and introduced for this purpose, and in particular the Flow-based bead assays such as the Luminex system. This latter assay has proved to be far more sensitive than the CDC assay and has revealed pre-sensitization in potential transplant recipients not detected by other methods of HLA antibody detection. However, the clinical implications of this increased sensitivity have not been convincingly demonstrated until recently. This technology for HLA antibody detection permits the evaluation of the clinical importance of antibodies directed at, for example, HLA-DPB1 and HLA-DQA1, which has not been possible to date. There are Luminex issues, however, requiring resolution such as the ability to distinguish between complement fixing and non-complement fixing antibodies and determination of their relative clinical significance. Luminex technology will permit a re-evaluation of the role of HLA antibodies in both early and late antibody-mediated rejection.

Posttransplant HLA alloreactivity in stable kidney transplant recipients-incidences and impact on long-term allograft outcomes

Humoral alloreactivity is well established to predict adverse allograft outcomes. However, in some recipients, alloantibodies may also occur in the absence of graft dysfunction. We evaluated if and how often complement- and noncomplement-fixing alloantibodies are detectable in stable recipients and whether, in this context, they affect long-term outcomes. Sera obtained from 164 kidney transplant recipients at 2, 6 and 12 months were evaluated by FlowPRA screening and single-antigen testing for detection of IgG- or C4d-fixing HLA panel reactivity and donor-specific antibodies (DSA). Applying stringent criteria, we selected 34 patients with an uneventful 1-year course (no graft dysfunction or rejection) and excellent graft function at 12 months [estimated glomerular filtration rate (eGFR) >or=60 mL/min and proteinuria <or=0.5 g/24 h]. Nine (27%) and 5 (15%) of these recipients tested positive by [IgG] and [C4d]FlowPRA screening, respectively. In five cases, DSA were identified. Frequencies of positive test results and DSA binding intensities were not significantly lower than those documented for patients who did not fulfill the above criteria. In recipients with an excellent 1-year course, FlowPRA reactivity was not associated with lower eGFR or increased protein excretion during 68-month median follow-up. Our results suggest cautious interpretation of antibody monitoring in patients with normal-functioning grafts.

Anti-major histocompatibility complex class I-related chain A antibodies in organ transplantation

Candidate trigger antigens for alloreactive responses have been appearing continuously in the organ transplant scenario. Major histocompatibility complex class I-related chain A (MICA) is a polymorphic gene family, located near the HLA-B locus on chromosome 6, that encode a 62-kd cell surface glycoprotein. Endothelial cells, in addition to many cell lines, express MICA, whereas resting lymphocytes do not, making this polymorphic molecule a target for both cellular and humoral immune responses. Major histocompatibility complex class I-related chain A antigens are able to elicit the synthesis of alloantibodies in transplant recipients. These antibodies have been found in association with irreversible allograft rejection, an increased frequency of acute rejection episodes, and a significantly lower deceased donor graft survival, as well as in the eluates from rejected grafts. This review summarizes currently available information on MICA in the transplant setting. Undoubtedly, the questions that have surfaced surpass in excess the currently available answers.

In vitro detection of C4d-fixing HLA alloantibodies: associations with capillary C4d deposition in kidney allografts

Capillary C4d deposition is a valuable marker of antibody-mediated rejection (AMR). In this analysis, flow cytometric detection of alloantibody-triggered C4d deposition to HLA antigen-coated microparticles ([C4d]FlowPRA) was evaluated for its value as a marker for C4d deposition in renal allografts. For comparative analysis, 105 first renal biopsies performed for graft dysfunction and an equal number of concurrent sera were subjected to immunohistochemistry and [C4d] plus standard [IgG]FlowPRA, respectively. C4d deposition/fixation was detected in 17 biopsies and, applying [C4d]FlowPRA HLA class I and II screening, also in a small number of corresponding sera (N = 20). IgG reactivity detected by standard [IgG]FlowPRA was more frequent (49% of sera). Comparing [C4d]FlowPRA screening with capillary C4d staining, we found a high level of specificity (0.92 [95% confidence interval: 0.86-0.98]), which far exceeded that calculated for [IgG]FlowPRA (0.60 [0.50-0.70]). [IgG]FlowPRA screening, however, turned out to be superior in terms of sensitivity (0.94 [0.83-1.05] vs. 0.76 [0.56-0.97] calculated for C4d-fixing panel reactivity). Remarkably, posttransplant single antigen testing for identification of complement-fixing donor-specific alloreactivities failed to improve the predictive value of FlowPRA-based serology. In conclusion, our results suggest that detection of complement-fixing HLA panel reactivity could provide a specific tool for monitoring of C4d-positive AMR.

[Experimental Application of Whole Blood Flow Cytometry to HLA Crossmatch for Renal Transplantation.]

BACKGROUND

The lymphocytes separated from whole blood are used in HLA flow cytometry crossmatch (FCXM) for renal transplantation. In this study, the methodology of whole blood flow cytometry was applied to FCXM, omitting lymphocyte separation step.

METHODS

In the 20 cases (including positive 5 cases) of T cell FCXM for renal transplantation, the standard assay using the separated mononuclear cells (MNC) was compared with the two variant assays using whole blood. In the latter assay, the donor whole blood was incubated with the excessive recipient serum. The red cells were lysed (lysed whole blood, LWB). Otherwise, instead of red cell lysis, the signals of T cells among whole blood (WB) were acquired using fluorescence triggering. The sample/negative control mean fluorescence intensity (MFI) ratio was calculated for the interpretation.

RESULTS

The MFI ratio of the 20 cases by MNC, LWB and WB assay were 4.9+/-8.1, 5.4+/-9.7 and 4.8+/-7.8, respectively. Both LWB and WB assay were not significantly different from MNC assay (P= 0.313, 0.831, respectively, paired t-test). The qualitative determinations were concordant in all cases, except for one case which was weakly positive with MFI ratio 2.2 by LWB assay.

CONCLUSIONS

The assays using whole blood were comparable to the standard assay in FCXM for renal transplantation. This study indirectly supports that the variant methods can be used reliably in the case of the MNC preparation erroneously mixed with other blood cells.

C4d fixing, luminex binding antibodies - a new tool for prediction of graft failure after heart transplantation

The standard method to detect pretransplant antibodies has been the complement dependent cytotoxicity (CDC) test of donor leukocytes. Solid phase assays to detect HLA antibodies in pretransplant serum reveal a greater number of sensitized patients, but their clinical impact is less certain. Here we have developed a method of detecting C4d fixing HLA antibodies on Luminex beads. Pretransplant serum from 565 cardiac transplant patients was retrospectively tested for the presence of HLA antibodies using CDC, HLA coated Luminex beads and C4d deposition on Luminex beads, and the results correlated with graft survival. Whereas 5/565 patients had CDC positive donor specific antibodies (DSA) before their transplant, this number was increased by 19 using Luminex beads. The 1-year survival of CDC -ve/Luminex +ve patients with DSA (n = 19) was 42% compared with 77% for CDC -ve/Luminex +ve without DSA (n = 39, p = 0.0039). Fixation of C4d (22/67 Luminex positive sera) had a negative effect on graft outcome; 1-year graft survival was, C4d +ve/DSA +ve (n = 11) 20%, C4d +ve/DSA -ve (n = 11) 91%, C4d -ve DSA +ve (n = 13) 54%, C4d -ve DSA -ve (n = 32) 75%, compared with 75% for antibody-negative patients (p = 0.0002). In conclusion, detection of Luminex +ve DSA in pretransplant serum provides a powerful negative predictor of graft survival, especially if they bind C4d.