A systematic review of the role of C4d in the diagnosis of acute antibody-mediated rejection

Enzymatic conversion of human blood group A kidneys to universal blood group O

ABO blood group compatibility restrictions present the first barrier to donor-recipient matching in kidney transplantation. Here, we present the use of two enzymes, FpGalNAc deacetylase and FpGalactosaminidase, from the bacterium Flavonifractor plautii to enzymatically convert blood group A antigens from the renal vasculature of human kidneys to 'universal' O-type. Using normothermic machine perfusion (NMP) and hypothermic machine perfusion (HMP) strategies, we demonstrate blood group A antigen loss of approximately 80% in as little as 2 h NMP and HMP. Furthermore, we show that treated kidneys do not bind circulating anti-A antibodies in an ex vivo model of ABO-incompatible transplantation and do not activate the classical complement pathway. This strategy presents a solution to the donor organ shortage crisis with the potential for direct clinical translation to reduce waiting times for patients with end stage renal disease.

Targeting the Complement Pathway in Kidney Transplantation

The complement system is paramount in the clearance of pathogens and cell debris, yet is increasingly recognized as a key component in several pathways leading to allograft injury. There is thus a growing interest in new biomarkers to assess complement activation and guide tailored therapies after kidney transplantation (KTx). C5 blockade has revolutionized post-transplant management of atypical hemolytic uremic syndrome, a paradigm of complement-driven disease. Similarly, new drugs targeting the complement amplification loop hold much promise in the treatment and prevention of recurrence of C3 glomerulopathy. Although unduly activation of the complement pathway has been described after brain death and ischemia reperfusion, any clinical attempts to mitigate the ensuing renal insults have so far provided mixed results. However, the intervention timing, strategy, and type of complement blocker need to be optimized in these settings. Furthermore, the fast-moving field of ex vivo organ perfusion technology opens new avenues to deliver complement-targeted drugs to kidney allografts with limited iatrogenic risks. Complement plays also a key role in the pathogenesis of donor-specific ABO- and HLA-targeted alloantibodies. However, C5 blockade failed overall to improve outcomes in highly sensitized patients and prevent the progression to chronic antibody-mediated rejection (ABMR). Similarly, well-conducted studies with C1 inhibitors in sensitized recipients yielded disappointing results so far, in part, because of subtherapeutic dosage used in clinical studies. The emergence of new complement blockers raises hope to significantly reduce the negative effect of ischemia reperfusion, ABMR, and nephropathy recurrence on outcomes after KTx.

The Diagnostic Significance of C3d Antigen in Kidney and Skin Histopathology - The Current State-Of-The-Art and Practical Examples

The aim of this narrative review is to summarize recent knowledge about the diagnostic significance of immunobiological detection of C3d with a focus on renal and skin tissue biopsies. We completed the present narrative review with our own experiences with preparation and practical use of monoclonal C3d antibodies at a small national level.

The role of the mesangium in glomerular function

When discussing glomerular function, one cell type is often left out, the mesangial cell (MC), probably since it is not a part of the filtration barrier per se. The MCs are instead found between the glomerular capillaries, embedded in their mesangial matrix. They are in direct contact with the endothelial cells and in close contact with the podocytes and together they form the glomerulus. The MCs can produce and react to a multitude of growth factors, cytokines, and other signaling molecules and are in the perfect position to be a central hub for crosstalk communication between the cells in the glomerulus. In certain glomerular diseases, for example, in diabetic kidney disease or IgA nephropathy, the MCs become activated resulting in mesangial expansion. The expansion is normally due to matrix expansion in combination with either proliferation or hypertrophy. With time, this expansion can lead to fibrosis and decreased glomerular function. In addition, signs of complement activation are often seen in biopsies from patients with glomerular disease affecting the mesangium. This review aims to give a better understanding of the MCs in health and disease and their role in glomerular crosstalk and inflammation.

C4d Staining Is Present in Normal Placentas From Pregnancies Prior to Pregnancy Loss Associated With Chronic Histiocytic Intervillositis and Is Reduced by Immunomodulatory Therapy in Subsequent Pregnancies

INTRODUCTION

Chronic histiocytic intervillositis (CHI) is associated with adverse pregnancy outcomes and high recurrence risk. Recent studies suggest that CHI may represent a host-vs-graft rejection, and that C4d immunostain can be used as a marker for complement activation and antibody-mediated rejection in the CHI.

MATERIALS AND METHODS

This retrospective cohort study focused on 5 fetal autopsy cases associated with CHI (5 index cases) from 5 women. We analyzed placentas from the index cases (fetal autopsy cases associated with CHI) and placentas from the women's previous and subsequent pregnancies. We assessed the presence and extent of CHI and C4d immunostaining in these placentas. We evaluated each available placenta and graded the severity of CHI as either <50% or ≥50%. Additionally, we conducted C4d immunostaining on one representative section from each placenta and graded the staining levels as follows: 0+ for staining <5%; 1+ for staining between 5% and <25%; 2+ for staining between 25% and <75%; and 3+ for staining ≥75%.

RESULTS

Three of the 5 women had pregnancies prior to their index cases (fetal autopsy cases associated with CHI). Despite the absence of CHI in their initial pregnancies, the placentas displayed positive C4d staining with grades of 1+, 3+, and 3+, respectively. These results suggest the presence of complement activation and antibody-mediated rejection in placentas from their prior pregnancies without CHI. Three of the 5 women received immunomodulatory therapy after experiencing pregnancy losses associated with CHI. After treatment, 2 of these women had live births at 35 and 37 gestational weeks, respectively, while the third had a stillbirth at 25 gestational weeks. The severity of CHI and the degree of C4d staining in the placentas decreased in all 3 cases following immunomodulatory therapies. Specifically, the level of C4d staining decreased from 3+ to 2+, 2+ to 0+, and 3+ to 1+ in these 3 cases, respectively.

DISCUSSION

In women with a history of recurrent pregnancy loss associated with CHI, C4d immunostaining was present in the placentas from their previous non-CHI pregnancies, suggesting activation of the classical complement pathway and antibody-mediated reaction in their prior non-CHI pregnancies before the development of CHI in subsequent pregnancies. Immunomodulatory therapy may improve pregnancy outcomes by reducing complement activation, as shown by the reduction of C4d immunopositivity in the placentas after immunomodulatory treatment. Although we believe that the study provides valuable insights, we acknowledge that there are limitations to the findings. Therefore, to further elucidate the pathogenesis of CHI, additional research efforts with a collaborative and multidisciplinary approach are necessary.

Proposed Definitions of Antibody-Mediated Rejection for Use as a Clinical Trial Endpoint in Kidney Transplantation

Antibody-mediated rejection (AMR) is caused by antibodies that recognize donor human leukocyte antigen (HLA) or other targets. As knowledge of AMR pathophysiology has increased, a combination of factors is necessary to confirm the diagnosis and phenotype. However, frequent modifications to the AMR definition have made it difficult to compare data and evaluate associations between AMR and graft outcome. The present paper was developed following a Broad Scientific Advice request from the European Society for Organ Transplantation (ESOT) to the European Medicines Agency (EMA), which explored whether updating guidelines on clinical trial endpoints would encourage innovations in kidney transplantation research. ESOT considers that an AMR diagnosis must be based on a combination of histopathological factors and presence of donor-specific HLA antibodies in the recipient. Evidence for associations between individual features of AMR and impaired graft outcome is noted for microvascular inflammation scores ≥2 and glomerular basement membrane splitting of >10% of the entire tuft in the most severely affected glomerulus. Together, these should form the basis for AMR-related endpoints in clinical trials of kidney transplantation, although modifications and restrictions to the Banff diagnostic definition of AMR are proposed for this purpose. The EMA provided recommendations based on this Broad Scientific Advice request in December 2020; further discussion, and consensus on the restricted definition of the AMR endpoint, is required.

Allorecognition and the spectrum of kidney transplant rejection

Detection of mismatched human leukocyte antigens by adaptive immune cells is considered as the main cause of transplant rejection, leading to either T-cell mediated rejection or antibody-mediated rejection. This canonical view guided the successful development of immunosuppressive therapies and shaped the diagnostic Banff classification for kidney transplant rejection that is used in clinics worldwide. However, several observations have recently emerged that question this dichotomization between T-cell mediated rejection and antibody-mediated rejection, related to heterogeneity in the serology, histology, and prognosis of the rejection phenotypes. In parallel, novel insights were obtained concerning the dynamics of donor-specific anti-human leukocyte antigen antibodies, the immunogenicity of donor-recipient non-human leukocyte antigen mismatches, and the autoreactivity against self-antigens. Moreover, the potential of innate allorecognition was uncovered, as exemplified by natural killer cell-mediated microvascular inflammation through missing self, and by the emerging evidence on monocyte-driven allorecognition. In this review, we highlight the gaps in the current classification of rejection, provide an overview of the expanding insights into the mechanisms of allorecognition, and critically appraise how these could improve our understanding and clinical approach to kidney transplant rejection. We argue that consideration of the complex interplay of various allorecognition mechanisms can foster a more integrated view of kidney transplant rejection and can lead to improved risk stratification, targeted therapies, and better outcome after kidney transplantation.

Evaluation of Early Renal Allograft Dysfunction from Living Donors among Egyptian Patients (Histopathological and Immunohistochemical Study)

BACKGROUND: Early renal graft dysfunction is a major problem in the early post-transplantation period and is considered a major cause of graft loss. Clinical diagnosis based on the clinical criteria alone is unreliable; therefore, biopsy remains the gold standard to differentiate between rejection and non-rejection causes. AIM: This study was designed to identify and differentiate between causes of early graft dysfunction during the first post-transplantation month and to correlate between histological lesions and immunohistochemistry (IHC) for accurate diagnosis and a better outcome. MATERIALS AND METHODS: A total of 163 renal allograft biopsies, performed in the first post-transplantation month over 6 years, were included in the study. New sections were prepared from the paraffin blocks and stained with conventional stains. Additional sections were prepared and treated by complement fragment 4d (C4d) and cluster differentiation 3 (CD3) antibodies for IHC evaluation. RESULTS: All the studied cases were from living donors. The mean patient age was 39 years with predominant males. The clinical indication for most biopsies (94.5%) was impaired graft function. Acute rejection (AR) was the main diagnostic category observed in (98/163, 60.1%); out of which, T cell-mediated rejection (TCMR) was observed in (62/98, 63.2%). Drug toxicity was suspected in (53/163, 32.5%), acute tubular injury (ATI) not otherwise specified (nos) in (21/163, 12.9%), and other lesions including thrombotic microangiopathy were observed in the remaining biopsies. The most common cause of graft dysfunction in the 1st and 2nd weeks was AR representing. A significant correlation was seen between mild glomerulitis (g1) and mild peritubular capillaritis (PTC) 1, on the one side, and negative C4d staining, on the other side. No significant correlation was seen between moderate glomerulitis (g2) and moderate ptc2 at one side and positive C4d staining at the other side reflecting the poor association between the microvascular inflammation (“g” and “ptc” scores) and C4d positivity (r = 0.2). Missed mild tubulitis (t1) was found in a single case and missed moderate tubulitis (t2) was found in a single case detected by CD3 IHC. CONCLUSION: AR and drug toxicity account for the majority of early graft dysfunction, however, other pathological lesions, per se or coincide with them may be the cause. The significance of g2 per se as a marker for diagnosis of antibody-mediated rejection requires further study. Considering C4d score 1 (by IHC) positive; also requires further study with follow-up.

Targeting the Complement Pathway in Malignant Glioma Microenvironments

Malignant glioma is a highly fatal type of brain tumor, and its reoccurrence is largely due to the ordered interactions among the components present in the complex microenvironment. Besides its role in immune surveillance and clearance under physiological conditions, the complement system is expressed in a variety of tumor types and mediates the interactions within the tumor microenvironments. Recent studies have uncovered the broad expression spectrum of complement signaling molecules in the tumor microenvironment and various tumor cells, in particular, malignant glioma cells. Involvement of the complement system in tumor growth, immunosuppression and phenotype transition have also been elucidated. In this review, we enumerate the expression and function of complement molecules in multiple tumor types reported. Moreover, we elaborate the complement pathways in glioma cells and various components of malignant glioma microenvironments. Finally, we summarize the possibility of the complement molecules as prognostic factors and therapeutic targets in the treatment of malignant glioma. Specific targeting of the complement system maybe of great significance and value in the future treatment of multi-type tumors including malignant glioma.

Glomerular C4d in Post-Transplant IgA Nephropathy is associated with decreased allograft survival

Background

Glomerulonephritis (GN), including post-transplant IgAN (post-Tx IgAN) is an important contributor to decreased long-term allograft survival. The immunopathological detection of the complement degradation product C4d in glomeruli (C4dG) has been recently described as a risk factor in native kidney IgAN, however little is known about C4dG deposition in post-Tx IgAN. We hypothesized that glomerular C4d may indicate a more aggressive disease course and worse allograft survival in patients with post-Tx IgAN.

Methods

In this retrospective study we assessed the presence and clinical relevance of C4dG in patients with post-transplant IgAN. We analyzed 885 renal allograft recipients, including 84 patients with post-transplant GN. All patients were transplanted between January 1999 and April 2006 and underwent at least one biopsy for differnt causes. The primary endpoint was death-censored graft survival, with a median follow-up of 9.6 (IQR 3.8–13.2) years.

Results

The prevalence of post-Tx GN was 9.5%. Twenty-seven patients with post-Tx IgAN were included. C4dG positive patients (N = 18, 66.7%) had significantly worse allograft survival compared to C4dG negative post-Tx IgAN patients and patients without post-Tx IgAN [C4dG positive: 27.8% vs. 55.6% and 66.0%; log-rank: p = 0.01]. C4dG remained a significant risk factor (HR 2.22, 95% CI 1.27–3.87) for allograft loss even after adjustment for T cell mediated rejection (TCMR) and antibody mediated rejection.

Conclusion

Glomerular C4d deposition is an independent risk factor for worse graft-survival in patients with post-Tx IgAN, even after adjusting for other risk factors such as antibody mediated rejection.  Assessment of glomerular C4d deposition may provide a valuable prognostic risk assessment tool to identify high risk patients in post-Tx IgAN.

Graphic abstract

Electronic supplementary material

The online version of this article (10.1007/s40620-020-00914-x) contains supplementary material, which is available to authorized users.

The immunohistochemical expression of von Willebrand factor, T-cadherin, and Caveolin-1 is increased in kidney allograft biopsies with antibody-mediated injury

BACKGROUND

There are only a few reports evaluating the applicability of endothelial-damage markers analysis by immunohistochemistry (IHC) in kidney allograft samples. This study analyzed the expression of Caveolin-1 (Cav), von Willebrand factor (Vwf), and T-cadherin (Cad) in kidney biopsies and their association with antibody-mediated injury.

METHODS

In this retrospective study, 114 cases with antibody-mediated changes (Banff, 2020) and 72 with interstitial fibrosis/tubular atrophy were selected. IHC for Cav, Vwf and Cad was performed and evaluated according to their qualitative expression in peritubular capillaries. The cases were grouped according to the presence of microvascular inflammation (MVI), donor-specific antibodies (DSA), C4d positivity and antibody-mediated rejection (AMR). A level of significance < 0.05 was adopted.

RESULTS

Vwf expression was associated with MVI (p < 0.001), DSA (p = 0.016), C4d (p < 0.001) and AMR (p < 0.001), and was higher in DSA+/C4d+ cases despite MVI (p < 0.001). The expression of Cad correlated with MVI (p = 0.015), C4d (p = 0.005) and AMR (p = < 0.001). Cad was more expressed in chronic AMR compared with acute/active cases (p = 0.001). Cav expression was associated with MVI (p = 0.029) and AMR (p = 0.016) and was also higher in chronic AMR (p = 0.049). A combined score of Vwf and Cad was higher in AMR when compared with C4d without rejection and IF/TA cases (p < 0.001).

CONCLUSION

Vwf, Cad and Cav expression shows association with antibody-mediated injury and may be helpful to support AMR diagnosis.

Immunostaining Patterns of Posttransplant Liver Biopsies Using 2 Anti-C4d Antibodies

Histopathologic diagnosis of antibody-mediated rejection in posttransplant liver biopsies is challenging. The recently proposed diagnostic criteria by the Banff Working Group on Liver Allograft Pathology require positive C4d immunohistochemical staining to establish the diagnosis. However, the reported C4d staining patterns vary widely in different studies. One potential explanation may be due to different antibody preparations used by different investigators. In this study, posttransplant liver biopsies from 69 patients histopathologically diagnosed with acute cellular rejection, chronic rejection, or recurrent hepatitis C were immunohistochemically stained using 2 polyclonal anti-C4d antibodies. On the basis of the distribution of C4d immunoreactivity, 5 different staining patterns were observed: portal vein and capillary, hepatic artery, portal stroma, central vein, and sinusoids. The frequency, extent, and intensity of positive C4d staining with the 2 antibody preparations differed significantly for portal veins/capillaries and central veins, but not for hepatic arteries and portal stroma. Positive sinusoidal staining was seen in only 1 case. There were no significant differences in the frequency, extent, and intensity of positive C4d staining among the acute cellular rejection, chronic rejection, and recurrent hepatitis C groups with the 2 anti-C4d antibodies. These data show that different anti-C4d antibodies can show different staining patterns, which may lead to different interpretation. Caution is thus needed when selecting C4d antibodies for clinical use to aid in the diagnosis of antibody-mediated rejection.

De novo HLA Class II antibodies are associated with the development of chronic but not acute antibody-mediated rejection after liver transplantation - a retrospective study

Donor-specific antibodies (DSA) cause antibody-mediated rejection (AMR); however, their pathogenic role has not yet been adequately investigated after liver transplantation. The aim of our study was to analyse the clinical significance of DSA and complement-binding DSA for the prediction of AMR after liver transplantation. Our cohort included 120 liver recipients with assessed protocol biopsies one year post-transplant. All patients had defined HLA-specific and complement-binding (C1q + and C3d+) antibodies before and in regular intervals after transplantation. The incidence of DSA was evaluated in relation with clinical and histopathological data in the liver allografts. A higher occurrence of acute AMR was observed in recipients with preformed complement-binding DSA to HLA Class I antigens. Patients who developed chronic AMR had more frequently de novo-produced antibodies against HLA Class II antigens (P = 0.0002). A correlation was also found between de novo-formed C1q + and C3d+-binding antibodies to HLA Class II antigens and the development of chronic AMR (P = 0.043). Our study implies that preformed complement-binding DSA to HLA Class I antigens are related to increased risk of acute antibody-mediated rejection, while chronic AMR is more frequent in patients with de novo-produced antibodies to HLA Class II antigens after liver transplantation.

The Role of Complement in Organ Transplantation

The current immunosuppressive protocols used in transplant recipients have improved short-term outcomes, but long-term allograft failure remains an important clinical problem. Greater understanding of the immunologic mechanisms that cause allograft failure are needed, as well as new treatment strategies for protecting transplanted organs. The complement cascade is an important part of the innate immune system. Studies have shown that complement activation contributes to allograft injury in several clinical settings, including ischemia/reperfusion injury and antibody mediated rejection. Furthermore, the complement system plays critical roles in modulating the responses of T cells and B cells to antigens. Therapeutic complement inhibitors, therefore, may be effective for protecting transplanted organs from several causes of inflammatory injury. Although several anti-complement drugs have shown promise in selected patients, the role of these drugs in transplantation medicine requires further study.

The role of complement in antibody mediated transplant rejection

Antibody mediated transplant rejection (AMR) is a major cause of long-term allograft failure, and currently available treatments are of limited efficacy for treating the disease. AMR is caused by donor specific antibodies (DSA) that bind to antigens within the transplanted organ. DSA usually activate the classical pathway of complement within the allograft, and complement activation is believed to be an important cause of tissue injury in AMR. Several new clinical assays may improve our ability to identify patients at risk of AMR. Complement inhibitory drugs have also been tested in selected patients and in small series. Better understanding of the role of complement activation in the pathogenesis of AMR will likely improve our ability to diagnose the disease and to develop novel treatments.

Complement Markers in Blood and Urine: No Diagnostic Value in Late Silent Antibody-Mediated Rejection

Background

Antibody-mediated rejection (AMR) is a major cause of kidney allograft failure. Its molecular mechanisms are multifaceted and may include a role of complement activation via the classical pathway. Here, we investigated whether noninvasive complement monitoring adds predictive power to the diagnosis of AMR in the setting of donor-specific antibody (DSA) positivity.

Methods

In this cross-sectional study, 741 kidney transplant recipients with stable graft function ≥180 days posttransplantation were screened for the presence of human leukocyte antigen (HLA) alloantibodies. Eighty-three of 111 DSA-positive recipients underwent protocol biopsies and were tested for blood and urinary levels of complement proteins (C1q, C4, C3) and activation products (C4d, C3a, C5a, C5b-9).

Results

Forty-seven recipients were diagnosed with AMR, and 21 were C4d-positive. While biopsy-confirmed AMR (and C4d) associated with DSA-binding strength (IgG mean fluorescence intensity of the immunodominant DSA versus AMR; area under the receiver operating characteristic curve: 0.76), tested complement markers did not have any predictive value for rejection (area under the receiver operating characteristic curve: 0.49-0.56). There were, however, tight correlations between complement activation products in urine and protein/creatinine ratio (ρ = 0.44-0.64; P < 0.001). Analysis of death-censored graft survival over a median of 60 months revealed no independent associations with levels of complement markers in blood or urine.

Conclusions

Complement patterns in blood and urine failed to identify AMR in late biopsies and may have no relevant diagnostic value in this particular context.

The therapeutic challenge of late antibody-mediated kidney allograft rejection

Late antibody‐mediated rejection (ABMR) is a cardinal cause of kidney allograft failure, manifesting as a continuous and, in contrast with early rejection, often clinically silent alloimmune process. While significant progress has been made towards an improved understanding of its molecular mechanisms and the definition of diagnostic criteria, there is still no approved effective treatment. In recent small randomized controlled trials, therapeutic strategies with promising results in observational studies, such as proteasome inhibitor bortezomib, anti‐C5 antibody eculizumab, or high dose intravenous immunoglobulin plus rituximab, had no significant impact in late and/or chronic ABMR. Such disappointing results reinforce a need of new innovative treatment strategies. Potential candidates may be the interference with interleukin‐6 to modulate B cell alloimmunity, or innovative compounds that specifically target antibody‐producing plasma cells, such as antibodies against CD38. Given the phenotypic heterogeneity of ABMR, the design of adequate systematic trials to assess the safety and efficiency of such therapies, however, is challenging. Several trials are currently being conducted, and new developments will hopefully provide us with effective ways to counteract the deleterious impact of antibody‐mediated graft injury. Meanwhile, the weight of evidence would suggest that, when approaching using existing treatments for established antibody‐mediated rejection, “less may be more”.

Detection of Complement-binding Donor-specific Antibodies, Not IgG-antibody Strength Nor C4d Status, at Antibody-mediated Rejection Diagnosis Is an Independent Predictor of Kidney Graft Failure

BACKGROUND

Antibody-mediated rejection (AMR) remains associated with reduced kidney graft survival and no clear prognostic marker is available.

METHODS

We investigated whether donor-specific antibodies (DSA) ability to bind C1q in comparison with AMR C4d status, both indirect signs of complement activation, improve risk stratification at time of AMR. Hence, among 467 patients in whom 1 or more graft biopsies were performed between 2008 and 2015, we included 56 with AMR according to Banff '15 criteria. Using concurrent sera, we prospectively identified DSA by single-antigen beads (IgG and C1q) assays.

RESULTS

Antibody-mediated rejection C4d (+) (n = 28) was associated with preformed DSA (P = 0.007), whereas DSA C1q (+) (n = 25) cases had stronger IgG-DSA (P < 0.001). At AMR, graft function was similar between DSA C1q groups, but in the first year after, it improved in DSA C1q (-), whereas a steady decline was observed in DSA C1q (+) cases, remaining significantly lower from 1 year until 4 years after AMR. DSA C1q (+) was significantly associated with reduced graft survival (P = 0.021), whereas AMR C4d (+) was not (P = 0.550). Importantly, a similar negative impact of DSA C1q (+) on graft survival was observed within AMR C4d (+) (P = 0.040) and (-) (P = 0.036), cases. In multivariable analysis, DSA C1q (+) (hazard ratio, 3.939, P = 0.005) and de novo DSA (hazard ratio, 4.409, P = 0.033) were independent predictors of graft failure, but stronger IgG-DSA was not. Similar results were obtained considering C1q-DSA and IgG-DSA strength as continuous variables.

CONCLUSIONS

C1q-DSA assessment at AMR can be a valuable tool in detecting patients with higher risk of graft failure.

C5b9 Deposition in Glomerular Capillaries Is Associated With Poor Kidney Allograft Survival in Antibody-Mediated Rejection

C4d deposition in peritubular capillaries (PTC) reflects complement activation in antibody-mediated rejection (ABMR) of kidney allograft. However, its association with allograft survival is controversial. We hypothesized that capillary deposition of C5b9-indicative of complement-mediated injury-is a severity marker of ABMR. This pilot study aimed to determine the frequency, location and prognostic impact of these deposits in ABMR. We retrospectively selected patients diagnosed with ABMR in two French transplantation centers from January 2005 to December 2014 and performed C4d and C5b9 staining by immunohistochemistry. Fifty-four patients were included. Median follow-up was 52.5 (34.25-73.5) months. Thirteen patients (24%) had C5b9 deposits along glomerular capillaries (GC). Among these, seven (54%) had a global and diffuse staining pattern. Twelve of the C5b9+ patients also had deposition of C4d in GC and PTC. C4d deposits along GC and PTC were not associated with death-censored allograft survival (p = 0.42 and 0.69, respectively). However, death-censored allograft survival was significantly lower in patients with global and diffuse deposition of C5b9 in GC than those with a segmental pattern or no deposition (median survival after ABMR diagnosis, 6 months, 40.5 months and 44 months, respectively; p = 0.015). Double contour of glomerular basement membrane was diagnosed earlier after transplantation in C5b9+ ABMR than in C5b9- ABMR (median time after transplantation, 28 vs. 85 months; p = 0.058). In conclusion, we identified a new pattern of C5b9+ ABMR, associated with early onset of glomerular basement membrane duplication and poor allograft survival. Complement inhibitors might be a therapeutic option for this subgroup of patients.

Molecular Assessment of C4d-Positive Renal Transplant Biopsies Without Evidence of Rejection

Introduction

Immunohistochemical staining for C4d in peritubular capillaries has been part of antibody-mediated rejection (AbMR) definition in the Banff Classification for Allograft Pathology since 2003. However, it has limited sensitivity and specificity, therefore the clinical significance of C4d-positive biopsies without evidence of rejection (C4d+ WER) is unknown. We investigated the transcript levels of genes associated with AbMR in C4d+ WER biopsies from both ABO-compatible and incompatible renal transplant patients.

Methods

RNA was extracted from formalin-fixed paraffin-embedded renal transplant biopsies (n = 125) and gene expression analysis of 35 AbMR-associated transcripts carried out using the NanoString nCounter system.

Results

AbMR-associated transcripts were significantly increased in samples with AbMR or suspicious AbMR. A subgroup of 17 of 35 transcripts that best distinguished AbMR from C4d-negative biopsies without evidence of rejection was used to study C4d+ WER samples. There was no differential expression between C4d-negative and C4d+ WER from both ABO-incompatible and -compatible transplants. The geometric mean of 17 differentially expressed genes was used to assign the C4d+ WER biopsies a high- or low-AbMR transcript score. Follow-up biopsies showed AbMR within 1 year of initial biopsy in 5 of 7 high-AbMR transcript patients but only 2 of 46 low-AbMR transcript patients. In multivariate logistic regression analysis, elevated transcript levels in a C4d+ WER biopsy were associated with increased odds for biopsy-proven AbMR on follow-up (P = 0.032, odds ratio 16.318), whereas factors including donor-specific antibody (DSA) status and time since transplantation were not.

Conclusion

Gene expression analysis in C4d+ WER samples has the potential to identify patients at higher risk of developing AbMR.

Anti-C1s monoclonal antibody BIVV009 in late antibody-mediated kidney allograft rejection-results from a first-in-patient phase 1 trial

The classical pathway (CP) of complement may contribute to the pathogenesis of antibody-mediated rejection (ABMR). Selective CP blockade may be a promising strategy to counteract rejection. The objective of this first-in-patient phase 1b trial was to evaluate the safety/tolerability and CP-blocking potential of 4 weekly doses (60 mg/kg) of the anti-C1s antibody BIVV009 in complement-mediated disorders. Here we describe the results in a cohort of 10 stable kidney transplant recipients (median of 4.3 years posttransplantation) with late active ABMR and features of CP activation, such as capillary C4d or complement-fixing donor-specific antibodies (DSA). During 7 weeks follow-up, no severe adverse events were reported, and BIVV009 profoundly inhibited overall and DSA-triggered CP activation in serum. Five of 8 C4d-positive recipients turned C4d-negative in 5-week follow-up biopsies, while another 2 recipients showed a substantial decrease in C4d scores. There was, however, no change in microcirculation inflammation, gene expression patterns, DSA levels, or kidney function. In conclusion, we demonstrate that BIVV009 effectively blocks alloantibody-triggered CP activation, even though short-course treatment had no effect on indices of activity in late ABMR. This initial trial provides a valuable basis for future studies designed to clarify the therapeutic value of CP blockade in transplantation. ClinicalTrials.gov NCT#02502903.

The role of C4d deposition in the diagnosis of antibody-mediated rejection after lung transplantation

Antibody-mediated rejection (AMR) is an increasingly recognized form of lung rejection. C4d deposition has been an inconsistent finding in previous reports and its role in the diagnosis has been controversial. We conducted a retrospective single-center study to characterize cases of C4d-negative probable AMR and to compare these to cases of definite (C4d-positive) AMR. We identified 73 cases of AMR: 28 (38%) were C4d-positive and 45 (62%) were C4d-negative. The two groups had a similar clinical presentation, and although more patients in the C4d-positive group had neutrophilic capillaritis (54% vs. 29%, P = .035), there was no significant difference in the presence of other histologic findings. Despite aggressive antibody-depleting therapy, 19 of 73 (26%) patients in the overall cohort died within 30 days, but there was no significant difference in freedom from chronic lung allograft dysfunction (CLAD) or survival between the two groups. We conclude that AMR may cause allograft failure, but that the diagnosis requires a multidisciplinary approach and a high index of suspicion. C4d deposition does not appear to be a necessary criterion for the diagnosis, and although some cases may respond initially to therapy, there is a high incidence of CLAD and poor survival after AMR.

Transplant glomerulopathy

In the renal allograft, transplant glomerulopathy represents a morphologic lesion and not a specific diagnosis. The hallmark pathologic feature is glomerular basement membrane reduplication by light microscopy or electron microscopy in the absence of immune complex deposits. Transplant glomerulopathy results from chronic, recurring endothelial cell injury that can be mediated by HLA alloantibodies (donor-specific antibodies), various autoantibodies, cell-mediated immune injury, thrombotic microangiopathy, or chronic hepatitis C. Clinically, transplant glomerulopathy may be silent, detectable on protocol biopsy, or present with overt manifestations, including up to nephrotic range proteinuria, hypertension, and declining glomerular filtration rate. In either case, transplant glomerulopathy is associated with reduced graft survival. This review details the morphologic features of transplant glomerulopathy found on light microscopy, immunofluorescence microscopy, and electron microscopy. The pathophysiology of the causes and risk factors are discussed. Clinical manifestations are emphasized and potential therapeutic modalities are examined.

Antibody-mediated rejection: New approaches in prevention and management

Despite the success of desensitization protocols, antibody-mediated rejection (AMR) remains a significant contributor to renal allograft failure in patients with donor-specific antibodies. Plasmapheresis and high-dose intravenous immunoglobulin have proved to be effective treatments to prevent and treat AMR, but irreversible injury in the form of transplant glomerulopathy can commonly manifest months to years later. There is an unmet need to improve the outcomes for patients at risk for AMR. Updated Banff criteria now take into account the increasing understanding of the complex and heterogeneous nature of AMR phenotypes, including the timing of rejection, subclinical and chronic AMR, C4d-negative AMR, and antibody-mediated vascular rejection. Treatment for AMR is not standardized, and there is little in the way of evidence-based treatment guidelines. Refining more precisely the mechanisms of injury responsible for different AMR phenotypes and establishing relevant surrogate endpoints to facilitate more informative studies will likely allow for more accurate determination of prognosis and efficacious intervention using new therapeutic approaches. In addition to plasma exchange and intravenous immunoglobulin, a number of other add-on therapies have been tried in small studies without consistent benefit, including anti-CD20, proteasome inhibitors, complement inhibitors, anti-interleukin-6 receptor blockers, and immunoglobulin G-degrading enzyme of Streptococcus pyogenes (called IdeS).

Persistent C4d and antibody-mediated rejection in pediatric renal transplant patients

Pediatric renal transplant recipient survival continues to improve, but ABMR remains a significant contributor to graft loss. ABMR prognostic factors to guide treatment are lacking. C4d staining on biopsies, diagnostic of ABMR, is associated with graft failure. Persistent C4d+ on follow-up biopsies has unknown significance, but could be associated with worse outcomes. We evaluated a retrospective cohort of 17 pediatric renal transplant patients diagnosed with ABMR. Primary outcome at 12 months was a composite of ≥50% reduction in eGFR, transplant glomerulopathy, or graft failure. Secondary outcome was the UPCR at 12 months. We used logistic and linear regression modeling to determine whether persistent C4d+ on follow-up biopsy was associated with the outcomes. Forty-one percent reached the primary outcome at 12 months. Persistent C4d+ on follow-up biopsy occurred in 41% and was not significantly associated with the primary outcome, but was significantly associated with the secondary outcome (estimate 0.22, 95% CI 0.19-0.25, P < .001), after controlling for confounding factors. Persistent C4d+ on follow-up biopsies was associated with a higher UPCR at 12 months. Patients who remain C4d+ on follow-up biopsy may benefit from more aggressive or prolonged ABMR treatment.

Blockade of HLA Antibody-Triggered Classical Complement Activation in Sera From Subjects Dosed With the Anti-C1s Monoclonal Antibody TNT009-Results from a Randomized First-in-Human Phase 1 Trial

Background

Complement may play a key role in antibody-mediated rejection. A promising therapeutic approach may be classical pathway (CP) inhibition at the level of early component C1.

Methods

In this first-in-human, double-blind, randomized placebo-controlled phase 1 trial, we evaluated the safety and complement inhibitory effect of TNT009, a humanized monoclonal anti-C1s antibody. Sixty-four adult healthy volunteers received either single (n = 48; 7 consecutive cohorts, 0.3-100 mg/kg) or 4 weekly infusions (n = 16; 2 consecutive cohorts, 30 and 60 mg/kg per infusion) of TNT009 or placebo. To assess the effect of treatment on complement activity, sera from dosed subjects were analyzed in a CP activation assay evaluating C3d deposition on HLA-coated microbeads spiked with alloantibodies.

Results

Single doses of TNT009 at 3 to 100 mg/kg uniformly and profoundly inhibited HLA antibody-mediated C3d deposition (≥86% after 60 minutes), whereby the duration of CP inhibition (2-14 days) was dose-dependent. Four weekly doses persistently blocked complement for 5 to 6 weeks. Ex vivo serum CP activity was profoundly inhibited when TNT009 concentrations exceeded 20 μg/mL. Infusions were well tolerated without serious or severe adverse events.

Conclusions

Treatment with TNT009 was safe and potently inhibited CP activity. Future studies in patients are required to assess the potential of TNT009 for preventing or treating antibody-mediated rejection.

In a double-blind, randomized, placebo-controlled phase 1 trial in adult healthy volunteers, the authors report that TNT009, an anti-C1s monoclonal antibody, significantly inhibits complement classical pathway activation with an excellent tolerance.

Plasma C4d+ Endothelial Microvesicles Increase in Acute Antibody-Mediated Rejection

BACKGROUND

Antibody-mediated rejection (AMR) is a major cause of kidney allograft loss. Currently, AMR diagnosis relies on biopsy which is an invasive procedure. A noninvasive biomarker of acute AMR could lead to early diagnosis and treatment of this condition and improve allograft outcome. Microvesicles are membrane-bound vesicles released from the cell surface after injury. We hypothesized that because AMR is associated with allograft endothelial injury and C4d deposition, plasma microvesicles positive for endothelial (CD144) marker and C4d are increased in this condition.

METHODS

We studied microvesicle concentration in the plasma of 95 kidney transplant patients with allograft dysfunction and compared with 23 healthy volunteers. Biopsy diagnosis and scoring was performed using Banff classification.

RESULTS

In the 28 subjects with AMR, the density of C4d+/CD144+ microvesicles was on average 11-fold (P = 0.002) higher than transplant recipients with no AMR and 24-fold (P = 0.008) than healthy volunteers. Densities of C4d+ and C4d+/annexin V+ (C4d+/AVB+) microvesicles were also increased in AMR patients compared with no AMR and healthy subjects. C4d+/AVB+ microvesicles correlated with AMR biopsy severity. Nine patients with acute AMR that received treatment showed a mean 72% decrease (P = 0.01) in C4d+/CD144+ microvesicle concentration compared with pretreatment values.

CONCLUSIONS

Quantification of plasma C4d+ microvesicles provides information about presence of AMR, its severity and response to treatment in transplant patients.

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.

Antibody-Mediated Rejection After Liver Transplant

Antibody-mediated rejection (AMR) in liver transplants is a field in its infancy compared with its allograft cohorts of the kidney and lung. Acute AMR is diagnosed based on specific clinical and histopathologic criteria: serum donor specific antibodies, C4d staining, histopathologic findings on liver biopsy, and exclusion of other entities. In contrast, the histologic features of chronic AMR are not as specific and it is a more challenging diagnosis to make. Treatments of acute and chronic AMR include some combination of steroids, immune-modulating agents, intravenous immunoglobulin, plasmapheresis, and proteasome inhibitors.

Torque Teno Virus Load-Inverse Association With Antibody-Mediated Rejection After Kidney Transplantation

BACKGROUND

Antibody-mediated rejection (AMR) represents one of the cardinal causes of late allograft loss after kidney transplantation, and there is great need for noninvasive tools improving early diagnosis of this rejection type. One promising strategy might be the quantification of peripheral blood DNA levels of the highly prevalent and apathogenic Torque Teno virus (TTV), which might mirror the overall level of immunosuppression and thus help determine the risk of alloimmune response.

METHODS

To assess the association between TTV load in the peripheral blood and AMR, 715 kidney transplant recipients (median, 6.3 years posttransplantation) were subjected to a systematical cross-sectional AMR screening and, in parallel, TTV quantification.

RESULTS

Eighty-six of these recipients had donor-specific antibodies and underwent protocol biopsy, AMR-positive patients (n = 46) showed only 25% of the TTV levels measured in patients without AMR (P = 0.003). In a generalized linear model, higher TTV levels were associated with a decreased risk for AMR after adjustment for potential confounders (risk ratio 0.94 per TTV log level; 95% confidence interval 0.90-0.99; P = 0.02).

CONCLUSIONS

Future studies will have to clarify whether longitudinal assessment of TTV load might predict AMR risk and help guide the type and intensity of immunosuppression to prevent antibody-mediated graft injury.

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.

2016 Comprehensive Update of the Banff Working Group on Liver Allograft Pathology: Introduction of Antibody-Mediated Rejection

The Banff Working Group on Liver Allograft Pathology reviewed and discussed literature evidence regarding antibody-mediated liver allograft rejection at the 11th (Paris, France, June 5-10, 2011), 12th (Comandatuba, Brazil, August 19-23, 2013), and 13th (Vancouver, British Columbia, Canada, October 5-10, 2015) meetings of the Banff Conference on Allograft Pathology. Discussion continued online. The primary goal was to introduce guidelines and consensus criteria for the diagnosis of liver allograft antibody-mediated rejection and provide a comprehensive update of all Banff Schema recommendations. Included are new recommendations for complement component 4d tissue staining and interpretation, staging liver allograft fibrosis, and findings related to immunosuppression minimization. In an effort to create a single reference document, previous unchanged criteria are also included.

Peritubular Capillary Basement Membrane Multilayering in Renal Allograft Biopsies of Patients With De Novo Donor-Specific Antibodies

BACKGROUND

Severe peritubular capillary basement membrane multilayering (PTCBML) is part of the Banff definition of chronic antibody-mediated rejection. We retrospectively investigated whether assessment of the mean number of layers of basement membrane (BM) around peritubular capillaries (PTC) can be used in a cohort of patients with de novo donor-specific antibodies (dnDSA) as an early marker to predict long-term antibody-mediated injury.

METHODS

This is a retrospective cohort study with 151 electron microscopy samples from 54 patients with dnDSA, assessed at around 1 year after transplantation, for a mean number of BM layers around PTC and in serial biopsies. Graft survival and time to transplant glomerulopathy (TG) development were estimated in survival analyses.

RESULTS

We found that a mean PTCBML count greater than 2.5 layers assessed in a sample of 25 PTCs around 1 year after transplantation is indicative of the development of TG in patients with dnDSA (P = 0.001). In addition, in patients with serial biopsies available for electron microscopy analysis, we could distinguish 2 groups: patients with a mean PTCBML count of 2.5 or less on all biopsies, and patients who developed greater than 2.5 layers at any time after transplantation. The latter group reflected dnDSA patients at risk for TG development (P < 0.001). In patients with dnDSA, PTCBML score added significantly to the sensitivity and specificity of prediction of TG compared with microcirculation injury score alone.

CONCLUSIONS

Our results highlight the potential value of assessing the mean number of BM in PTC for early prediction of progression to chronic antibody-mediated injury.

2013 Banff Criteria for Chronic Active Antibody-Mediated Rejection: Assessment in a Real-Life Setting

Significant changes in the criteria for chronic active antibody-mediated rejection (CAABMR) were made in the Banff 2013 classification. These modifications expanded the number of patients diagnosed with CAABMR, with undetermined clinical significance. We compared the 2007 and 2013 criteria for the composite end point of death-censored graft failure or doubling of serum creatinine in 123 patients meeting the criterion related to the morphologic evidence of chronic tissue injury. In all, 18% and 36% of the patients met the 2007 and 2013 criteria, respectively. For the criterion related to antibody interaction with endothelium, only 25% were positive based on the 2007 definition compared with 82% using the 2013 definition. Cox modeling revealed that a 2013 but not a 2007 diagnosis was associated with the composite end point (adjusted hazard ratio 2.5 [95% confidence interval (CI) 1.2-5.2] vs. 1.6 [95% CI 0.7-3.8], respectively). The 2013 criterion based on both the C4d score and the glomerulitis plus peritubular capillaritis score (g+ptc) was more strongly associated with the end point than the 2007 criterion based only on C4d; however, when dissected by component, only the C4d component was significant. The association with clinical outcomes improved with the 2013 criteria. This is related to the new C4d threshold but not to the g+ptc ≥2 component.

Evaluation of Renal Allograft Biopsies for Graft Dysfunction and Relevance of C4d Staining in Antibody Mediated Rejection

INTRODUCTION

Biopsy remains gold standard for diagnosis of Graft Dysfunction (GD). It guides clinical management, provides valuable insights into pathogenesis of early and late allograft injury and is indispensable for distinguishing rejection from non- rejection causes of GD.

AIM

The primary aim of the study was to evaluate the diverse histomorphological lesions in renal allograft biopsy (RAB). Further, we determined the frequency of peritubular capillary (PTC) C4d positivity and its correlation with microvascular inflammation in Antibody Mediated Rejection (AMR).

MATERIALS AND METHODS

This was a prospective study on RAB over a period of 2 months. Histopathological evaluation was undertaken as per revised Banff'13 schema. Immunohistochemistry was performed to detect PTC C4d deposition.

RESULTS

Sixty five diagnostic biopsies were evaluated. Mean patient age was 34 years and males were predominant. The time interval between graft biopsy and transplantation ranged from 5 days to 8 years, with 52.3% biopsies belonging to period of ≤ 6 months post-transplant. Immune injuries were observed in 40 biopsies out of which AMR was observed in 35 biopsies. Calcineurin inhibitor toxicity (CNI Toxicity) was the second commonest cause observed in 12 biopsies and other lesions including de novo glomerulopathies were observed in the remaining biopsies. The sensitivity of C4d in detecting acute AMR was 55% and chronic AMR was 23.5.

CONCLUSION

AMR and CNI Toxicity account for majority of graft dysfunction. C4d is not as sensitive a marker of AMR, as was initially thought. Higher proportion of moderate microvascular inflammation is found in diffuse C4d positive cases compared to focal C4d positive cases.

The emerging role of complement inhibitors in transplantation

The role of complement in the biology of kidney transplantation is becoming more and more significant, especially but not only because we now have access to drugs inhibiting complement. After describing the main characteristics of complement biology, both activation of the complement cascade and the many regulatory factors, we will review the precise role of complement in kidney transplant biology. Complement activation has been involved in ischemia-reperfusion injury, in the recurrence of several diseases such as atypical hemolytic uremic syndrome, C3 glomerulopathies, and antiphospholipid syndrome, as well as the process of antibody-mediated rejection, either acute or chronic. There are many potentially interesting drugs interfering with complement inhibition that have been or may be studied in kidney transplantation. Currently, the bulk of data concerns eculizumab, a monoclonal antibody blocking the complement cascade at the C5. Its efficacy has been demonstrated in the treatment and prevention of recurrence of atypical hemolytic uremic syndrome with an overall good safety profile. Although it has been reported to be efficacious to prevent antibody-mediated rejection, properly designed trials are currently being performed to state this efficacy. In addition, randomized trials are, in the process, regarding the prevention of ischemia-reperfusion injury after kidney transplantation.

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.

Innate immunity in solid organ transplantation: an update and therapeutic opportunities

Innate immunity is increasingly recognized as a major player in transplantation. In addition to its role in inflammation in the early post-transplant period, innate immunity shapes the differentiation of cells of adaptive immunity, with a capacity to promote either rejection or tolerance. Emerging data indicate that innate allorecognition, a characteristic previously limited to lymphocytes, is involved in allograft rejection. This review briefly summarizes the physiology of each component of the innate immune system in the context of transplantation and presents the current or promising therapeutic applications, such as cellular, anticomplement and anticytokine therapies.

Complement Interception Across Humoral Incompatibility in Solid Organ Transplantation: A Clinical Perspective

The humoral barrier in transplant biology is the result of preformed donor-specific antibodies (DSAs), directed either against human leukocyte antigens (HLA) or non-HLA antigens such as blood group (ABO) molecules. The term "sensitization" applies to patients carrying these antibodies. Transplantation is widely accepted as a life-saving opportunity for patients with terminal end-organ disease. However, in sensitized patients, transplant outcome is hampered by antibody-mediated rejection (AMR) as a consequence of DSA exposure. Furthermore, sensitized patients have limited access to "matched" organs from the both living and deceased donor pool.Considering the crucial role of the complement system in the pathophysiology of AMR and the availability of complement intervention therapeutics, there is a growing interest in complement-targeting strategies. This review highlights the emerging importance of monitoring and modulation of the complement system in the context of enabling transplantation across humoral incompatibility in sensitized recipients with preformed anti-HLA or natural anti-ABO antibodies. It also discusses the significance of the complement system in the induction of accommodation and further emphasizes current and future perspectives of novel complement therapeutics.

Detection of C3d-binding donor-specific anti-HLA antibodies at diagnosis of humoral rejection predicts renal graft loss

Antibody-mediated rejection (AMR) is a major cause of kidney graft loss, yet assessment of individual risk at diagnosis is impeded by the lack of a reliable prognosis assay. Here, we tested whether the capacity of anti-HLA antibodies to bind complement components allows accurate risk stratification at the time of AMR diagnosis. Among 938 kidney transplant recipients for whom a graft biopsy was performed between 2004 and 2012 at the Lyon University Hospitals, 69 fulfilled the diagnosis criteria for AMR and were enrolled. Sera banked at the time of the biopsy were screened for the presence of donor-specific anti-HLA antibodies (DSAs) and their ability to bind C1q and C3d using flow bead assays. In contrast with C4d graft deposition, the presence of C3d-binding DSA was associated with a higher risk of graft loss (P<0.001). Despite similar trend, the difference did not reach significance with a C1q-binding assay (P=0.06). The prognostic value of a C3d-binding assay was further confirmed in an independent cohort of 39 patients with AMR (P=0.04). Patients with C3d-binding antibodies had worse eGFR and higher DSA mean fluorescence intensity. In a multivariate analysis, only eGFR <30 ml/min per 1.73 m(2) (hazard ratio [HR], 3.56; 95% confidence interval [CI], 1.46 to 8.70; P=0.005) and the presence of circulating C3d-binding DSA (HR, 2.80; 95% CI, 1.12 to 6.95; P=0.03) were independent predictors for allograft loss at AMR diagnosis. We conclude that assessment of the C3d-binding capacity of DSA at the time of AMR diagnosis allows for identification of patients at risk for allograft loss.