Capillary Deposition of Complement C4d and C3d in Pediatric Renal Allograft Biopsies

Complement fragments are biomarkers of antibody-mediated endothelial injury

Antibody-mediated rejection (AbMR) adversely affects long-term graft survival in kidney transplantation. Currently, the diagnosis of AbMR requires a kidney biopsy, and detection of complement C4d deposition in the allograft is one of the diagnostic criteria. Complement activation also generates several soluble fragments which could potentially provide non-invasive biomarkers of the process. Furthermore, microvesicles released into the plasma from injured cells can serve as biomarkers of vascular injury. To explore whether soluble complement fragments or complement fragments bound to endothelial microvesicles can be used to non-invasively detect AbMR, we developed an in vitro model in which human endothelial cells were exposed to anti-HLA antibodies and complement sufficient serum. We found that complement fragments C4a and sC5b-9 were increased in the supernatants of cells exposed to complement-sufficient serum compared to cells treated complement-deficient serum. Furthermore, complement activation on the cell surface was associated with the release of microvesicles bearing C4 and C3 fragments. We next measured these analytes in plasma from kidney transplant recipients with biopsy-proven acute AbMR (n = 9) and compared the results with those from transplant recipients who also had impaired allograft function but who did not have AbMR (n = 30). Consistent with the in vitro results, complement fragments C4a and Ba were increased in plasma from patients with AbMR compared to control subjects (P < 0.001 and P < 0.01, respectively). Endothelial microvesicle counts were not increased in patients with AbMR, however, and the number of microvesicles with C4 and C3 bound to the surface was actually lower compared to control subjects (both P < 0.05). Our results suggest that plasma complement activation fragments may be useful as non-invasive biomarkers of antibody-mediated complement activation within the allograft. Complement-opsonized endothelial microvesicles are decreased in patients with AbMR, possibly due to enhanced clearance of microvesicles opsonized with C3 and C4 fragments.

Pathology of Lung Rejection: Cellular and Humoral Mediated

Acute rejection is an important risk factor for bronchiolitis obliterans syndrome, the clinical manifestation of chronic airway rejection in lung allograft recipients. Patients with acute rejection might be asymptomatic or present with symptoms that are not specific and can be also seen in other conditions. Clinical tests such as pulmonary function tests and imaging studies among others usually are abnormal; however, their results are also not specific for acute rejection. Histopathologic features of acute rejection in adequate samples of transbronchial lung biopsy of the lung allograft are currently the gold standard to assess for acute rejection in lung transplant recipients. Acute alloreactive injury can affect both the vasculature and the airways. Currently, the guidelines of the 2007 International Society of Heart and Lung Transplantation consensus conference are recommended for the histopathologic assessment of rejection. There are no specific morphologic features recognized to diagnose antibody-mediated rejection (AMR) in lung allografts. Therefore, the diagnosis of AMR currently requires a “triple test” including clinical features, serologic evidence of donor-specific antibodies, and pathologic findings supportive of AMR. Complement 4d deposition is used to support a diagnosis of AMR in many solid organ transplants; however, its significance for the diagnosis of AMR in lung allografts is not entirely clear. This chapter discusses the currently recommended guidelines for the assessment of cellular rejection of lung allografts and summarizes our knowledge about morphologic features and immunophenotypic tests that might help in the diagnosis of AMR.

Diagnosis of Acute Cellular Rejection and Antibody-Mediated Rejection on Lung Transplant Biopsies: A Perspective From Members of the Pulmonary Pathology Society

Context.—

The diagnosis and grading of acute cellular and antibody-mediated rejection (AMR) in lung allograft biopsies is important because rejection can lead to acute graft dysfunction and/or failure and may contribute to chronic graft failure. While acute cellular rejection is well defined histologically, no reproducible specific features of AMR are currently identified. Therefore, a combination of clinical features, serology, histopathology, and immunologic findings is suggested for the diagnosis of AMR.

Objective.—

To describe the perspective of members of the Pulmonary Pathology Society (PPS) on the workup of lung allograft transbronchial biopsy and the diagnosis of acute cellular rejection and AMR in lung transplant.

Data Sources.—

Reports by the International Society for Heart and Lung Transplantation (ISHLT), experience of members of PPS who routinely review lung allograft biopsies, and search of literature database (PubMed).

Conclusions.—

Acute cellular rejection should be assessed and graded according to the 2007 working formulation of the ISHLT. As currently no specific features are known for AMR in lung allografts, the triple test (clinical allograft dysfunction, donor-specific antibodies, pathologic findings) should be used for its diagnosis. C4d staining might be performed when morphologic, clinical, and/or serologic features suggestive of AMR are identified.

Practical Applications in Immunohistochemistry: Evaluation of Rejection and Infection in Organ Transplantation

CONTEXT

-Immunohistochemical analysis of tissue biopsy specimens is a crucial tool in diagnosis of both rejection and infection in patients with solid organ transplants. In the past 15 years, the concept of antibody-mediated rejection has been refined, and diagnostic criteria have been codified in renal, heart, pancreas, and lung allografts (with studies ongoing in liver, small intestine, and composite grafts), all of which include immunoanalysis for the complement split product C4d.

OBJECTIVES

-To review the general concepts of C4d biology and immunoanalysis, followed by organ-allograft-specific data, and interpretative nuances for kidney, pancreas, and heart, with discussion of early literature for lung and liver biopsies. Additionally, practical applications and limitations of immunostains for infectious organisms (Polyomavirus, Adenoviridae [adenovirus], and the herpes virus family, including Herpes simplex virus, Cytomegalovirus, Human herpes virus 8, and Epstein-Barr virus) are reviewed in the context of transplant recipients.

DATA SOURCES

-Our experience and published primary and review literature.

CONCLUSIONS

-Immunohistochemistry continues to have an important role in transplant pathology, most notably C4d staining in assessment of antibody-mediated rejection and assessment of viral pathogens in tissue. In all facets of transplant pathology, correlation of morphology with special studies and clinical data is critical, as is close communication with the transplant team.

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

In this study, we conducted a systematic review of the literature to re-evaluate the role of C4d in the diagnosis of acute antibody-mediated rejection of kidney allografts. Electronic databases were searched until September 2013. Eligible studies allowed derivation of diagnostic tables for the performance of C4d by immunofluorescence or immunohistochemistry with comparison to histopathological features of acute antibody-mediated rejection and/or donor-specific antibody (DSA) assays. Of 3492 unique abstracts, 29 studies encompassing 3485 indication and 868 surveillance biopsies were identified. Assessment of C4d by immunofluorescence and immunohistochemistry exhibited slight to moderate agreement with glomerulitis, peritubular capillaritis, solid-phase DSA assays, DSA with glomerulitis, and DSA with peritubular capillaritis. The sensitivity and specificity of C4d varied as a function of C4d and comparator test thresholds. Prognostically, the presence of C4d was associated with inferior allograft survival compared with DSA or histopathology alone. Thus, our findings support the presence of complement-dependent and -independent phenotypes of acute antibody-mediated rejection. Whether the presence of C4d in combination with histopathology or DSA should be considered for the diagnosis of acute antibody-mediated rejection warrants further study.

Sensitive solid-phase detection of donor-specific antibodies as an aid highly relevant to improving allograft outcomes

Transplant recipients who have had sensitizing events such as pregnancies, blood transfusions and previous transplants often develop antibodies directed against human leukocyte antigen (HLA)-molecules of the donor tissue. These pre-formed donor-specific antibodies (DSA) represent a high risk of organ failure as a consequence of antibody-mediated hyper-acute or acute allograft rejection. As a first assay to detect DSA, the complement-dependent lymphocytotoxicity assay (CDC) was established more than 40 years ago. However, this assay is characterized by several drawbacks such as a low sensitivity and a high susceptibility to various artificial factors generally not leading to valid and reliable outcomes under several circumstances that are reviewed in this article. Furthermore, only those antibodies that exert complement-fixing activity are detected. As a consequence, novel procedures that act independently of the complement system and that do not represent functional assays were generated in the format of solid phase assays (SPAs) (bead- or ELISA-based). In this article, we review the pros and cons of these sensitive SPA in comparison with the detection of DSA through the use of the traditional methods such as CDC and flow cytometric analyses. Potential drawbacks of the alternative methodological approaches comprising high background reactivity, susceptibility to environmental factors and the possible influence of subjective operators' errors concerning the interpretation of the results are summarized and critically discussed for each method. We provide a forecast on the future role of SPAs reliably excluding highly deleterious DSA, thus leading to an improved graft survival.

The continual presence of C3d but not IgG glomerular capillary deposition in stage I idiopathic membranous nephropathy in patients receiving corticosteroid treatment

Background

Pathologic diagnosis of stage I idiopathic membranous nephropathy (MN-I) requires electron microscopy or immunohistochemistry that shows a glomerular capillary staining pattern of IgG and C3. However, it is not uncommon that renal biopsy did not obtain sufficient material for electron microscopy and that IgG and C3 staining in glomeruli largely lost at biopsy due to corticosteroid treatment. Since C3d is one of the final degradation products of C3 that is more stable in vivo, we determine if C3d staining could be used as a novel immunohistochemical marker for MN-I.

Methods and results

74 MN-I patients with electron microscopy proven MN-I were examined by immunoperoxidase staining of C3d. Intensive C3d staining was present in glomerular capillary like the staining pattern of IgG and C3 in MN-I. Importantly, in 40 MN-I patients who underwent corticosteroid treatment at biopsy the intensity and glomerular capillary pattern of C3d staining remained largely intact while the staining for IgG had substantially reduced and the pattern of glomerular capillary staining became unrecognizable.

Conclusions

C3d glomerular capillary staining may be a novel marker for pathologic diagnosis of MN-I that is continuously present at biopsy in patient who has received corticosteroid treatment.

Virtual slides

The virtual slide(s) for this article can be found here: http://www.diagnosticpathology.diagnomx.eu/vs/2120780075734479

Surveillance biopsies in children post-kidney transplant

Surveillance biopsies are increasingly used in the post-transplant monitoring of pediatric renal allograft recipients. The main justification for this procedure is to diagnose early and presumably modifiable acute and chronic renal allograft injury. Pediatric recipients are theoretically at increased risk for subclinical renal allograft injury due to their relatively large adult-sized kidneys and their higher degree of immunological responsiveness. The safety profile of this procedure has been well investigated. Patient morbidity is low, with macroscopic hematuria being the most common adverse event. No patient deaths have been attributed to this procedure. Longitudinal surveillance biopsy studies have revealed a substantial burden of subclinical immunological and non-immunological injury, including acute cellular rejection, interstitial fibrosis and tubular atrophy, microvascular lesions and transplant glomerulopathy. The main impediment to the implementation of surveillance biopsies as the standard of care is the lack of demonstrable benefit of early histological detection on long-term outcome. The considerable debate surrounding this issue highlights the need for multicenter, prospective, and randomized studies.

Novel solid phase-based ELISA assays contribute to an improved detection of anti-HLA antibodies and to an increased reliability of pre- and post-transplant crossmatching

Antibodies directed against HLA antigens of a given organ donor represent the dominating reason for hyper-acute or acute allograft rejections. In order to select recipients without donor-specific antibodies, a standard crossmatch (CM) procedure, the complement-dependent cytotoxicity assay (CDC), was developed. This functional assay strongly depends on the availability of isolated vital lymphocytes of a given donor. However, the requirements of the donor’s material may often not be fulfilled, so that the detection of the antibodies directed against HLA molecules is either impaired or becomes completely impossible. To circumvent the disadvantages of the CDC procedure, enzyme-linked immunosorbent assay (ELISA)-based and other solid phase-based ELISA-related techniques have been designed to reliably detect anti-HLA antibodies in recipients. Due to the obvious advantages of these novel technologies, when compared with the classical CDC assay, there is an urgent need to implement them as complementary methods or even as a substitution for the conventional CDC crossmatch that is currently being applied by all tissue typing laboratories.

Expression of complement components differs between kidney allografts from living and deceased donors

A disparity remains between graft survival of renal allografts from deceased donors and from living donors. A better understanding of the molecular mechanisms that underlie this disparity may allow the development of targeted therapies to enhance graft survival. Here, we used microarrays to examine whole genome expression profiles using tissue from 53 human renal allograft protocol biopsies obtained both at implantation and after transplantation. The gene expression profiles of living-donor kidneys and pristine deceased-donor kidneys (normal histology, young age) were significantly different before reperfusion at implantation. Deceased-donor kidneys exhibited a significant increase in renal expression of complement genes; posttransplantation biopsies from well-functioning, nonrejecting kidneys, regardless of donor source, also demonstrated a significant increase in complement expression. Peritransplantation phenomena, such as donor death and possibly cold ischemia time, contributed to differences in complement pathway gene expression. In addition, complement gene expression at the time of implantation was associated with both early and late graft function. These data suggest that complement-modulating therapy may improve graft outcomes in renal transplantation.

The use of C3d and C4d immunohistochemistry on formalin-fixed tissue as a diagnostic adjunct in the assessment of inflammatory skin disease

BACKGROUND

Direct immunofluorescent (DIF) testing defines an important diagnostic adjunct in the classification of various inflammatory skin conditions; it requires fresh tissue, a laboratory equipped to perform the procedure, and a pathologist skilled in its interpretation. Although advances have been made in the development of antibodies that can be applied to paraffin-embedded tissue, there has been no reported success on the application of paraffin tissue-based immunohistochemistry as a potential substitute for DIF testing on skin biopsy material.

OBJECTIVE

We applied C3d and C4d immunohistochemistry on paraffin-embedded, formalin-fixed tissue to define a potential application of these two antibodies as a diagnostic adjunct in the evaluation of various inflammatory skin diseases.

DESIGN

A natural language search identified cases submitted for both light microscopic and DIF studies from July 2006 to August 2007. We prospectively included similar cases encountered from August 2007 to March 2008. We correlated the C3d and C4d staining pattern with the DIF and light microscopic findings.

RESULTS

All cases of scarring discoid lupus erythematosus (LE) (20/20) and systemic LE (5/5) showed prominent granular C3d along the dermoepidermal junction (DEJ) and a positive lupus band test result in the latter by DIF. All systemic LE cases demonstrated granular DEJ C4d with C3d or C4d in blood vessels (BV). There was a negative lupus band test result without DEJ C3d or C4d in all cases of subacute cutaneous lupus erythematosus (SCLE) (15/15). There were, however, deposits of C4d within epidermal keratinocytes (7/7), corresponding to IgG decoration of keratinocytes by DIF and the presence of anti-Ro antibodies. Dermatomyositis cases showed prominent mural C3d and C4d in BV corresponding to C5b-9 by DIF (12/12) and one case of hydroxyurea-induced dermatomyositis lacked this staining. Although by DIF all dermatomyositis cases had a negative lupus band test result, 25% of cases showed staining for C3d along the DEJ (3/9). Bullous pemphigoid cases demonstrated homogenous DEJ C3d (17/17) whereas C4d was characteristically negative; there was 100% concordance with linear IgG and C3d by DIF. Eighty two percent of pemphigus cases demonstrated prominent intercellular C3d and C4d, roughly mirroring the intercellular pattern for IgG and complement seen by DIF (9/11). Porphyria cases showed homogeneous and granular C3d (11/11) and C4d (7/11), mirroring the vascular immunoglobulin and C5b-9 by DIF. All cases of urticarial (5), leukocytoclastic (6), and lymphocytic (1) vasculitis exhibited prominent mural C3d and C4d in BV, whereas Henoch-Schönlein purpura (10/10) showed primarily mural BV C3d without C4d, with IgA by DIF. Three cases of relapsing polychondritis showed C3d and C4d within chondrocyte nuclei (3/3), in contrast to negative staining in chondrodermatitis nodularis helicis (0/2). Hypersensitivity reactions were negative for C3d and C4d.

LIMITATIONS

The small sample size in each category is a limitation. The lack of literature precedent with regard to immunohistochemical assessment of extracellular antigens on paraffin-embedded tissue in skin samples is another limitation of this study.

CONCLUSIONS

When correlated with the light microscopic and clinical findings, the C3d and C4d assay has significant application in the assessment of select inflammatory skin diseases including vasculopathic conditions, collagen vascular disease, and autoimmune vesiculobullous disorder. It may prompt further DIF testing or, in some instances, may even define a reasonable substitute for DIF and/or add to the morphologic assessment of a biopsy specimen submitted for routine light microscopic assessment primarily in the setting of autoimmune vesiculobullous disease and collagen vascular disease.

Chronic allograft nephropathy

Chronic allograft nephropathy (CAN) is the leading cause of renal allograft loss in paediatric renal transplant recipients. CAN is the result of immunological and nonimmunological injury, including acute rejection episodes, hypoperfusion, ischaemia reperfusion, calcineurin toxicity, infection and recurrent disease. The development of CAN is often insidious and may be preceded by subclinical rejection in a well-functioning allograft. Classification of CAN is histological using the Banff classification of renal allograft pathology with classic findings of interstitial fibrosis, tubular atrophy, glomerulosclerosis, fibrointimal hyperplasia and arteriolar hyalinosis. Although improvement in immunosuppression has led to greater 1-year graft survival rates, chronic graft loss remains relatively unchanged and opportunistic infectious complications remain a problem. Protocol biopsy monitoring is not current practice in paediatric transplantation for CAN monitoring but may have a place if new treatment options become available. Newer immunosuppression regimens, closer monitoring of the renal allograft and management of subclinical rejection may lead to reduced immune injury leading to CAN in the paediatric population but must be weighed against the risk of increased immunosuppression and calcineurin inhibitor nephrotoxicity.

H-Y antibody development associates with acute rejection in female patients with male kidney transplants

BACKGROUND

Human minor histocompatibility antigens (mHA) and clinically relevant immune responses to them have not been well defined in organ transplantation. We hypothesized that women with male kidney transplants would develop antibodies against H-Y, the mHA encoded on the Y-chromosome, in association with graft rejection.

METHODS

We tested sera from 118 consecutive transplant recipients with kidney biopsies. Antibodies that specifically recognized the recombinant H-Y antigens RPS4Y1 or DDX3Y were detected by IgG enzyme-linked immunosorbent assay and western blotting. Immunogenic epitopes were further identified using overlapping H-Y antigen peptides for both the H-Y proteins.

RESULTS

In the 26 female recipients of male kidneys, H-Y antibody development posttransplant (1) was more frequent (46%) than in other gender combinations (P<0.001), (2) showed strong correlation with acute rejection (P=0.00048), (3) correlated with plasma cell infiltrates in biopsied kidneys (P=0.04), and (4) did not correlate with C4d deposition or donor-specific anti-human leukocyte antigen (HLA) antibodies. Of the two H-Y antigens, RPS4Y1 was more frequently recognized (P=0.005).

CONCLUSION

This first demonstration of a strong association between H-Y antibody development and acute rejection in kidney transplant recipients shows that in solid organ allografts, humoral immune responses against well defined mHA have clear clinical correlates, can be easily monitored, and warrant study for possible effects on long-term graft function.

Four stages and lack of stable accommodation in chronic alloantibody-mediated renal allograft rejection in Cynomolgus monkeys

The etiology of immunologically mediated chronic renal allograft failure is unclear. One cause is thought to be alloantibodies. Previously in Cynomolgus monkeys, we observed a relationship among donor-specific alloantibodies (DSA), C4d staining, allograft glomerulopathy, allograft arteriopathy and progressive renal failure. To define the natural history of chronic antibody-mediated rejection and its effect on renal allograft survival, we now extend this report to include 417 specimens from 143 Cynomolgus monkeys with renal allografts. A subset of animals with long-term renal allografts made DSA (48%), were C4d positive (29%), developed transplant glomerulopathy (TG) (22%) and chronic allograft arteriopathy (CAA) (19%). These four features were highly correlated and associated with statistically significant shortened allograft survival. Acute cellular rejection, either Banff type 1 or 2, did not correlate with alloantibodies, C4d deposition or TG. However, endarteritis (Banff type 2) correlated with later CAA. Sequential analysis identified four progressive stages of chronic antibody-mediated rejection: (1) DSA, (2) deposition of C4d, (3) TG and (4) rising creatinine/renal failure. These new findings provide strong evidence that chronic antibody-mediated rejection develops without enduring stable accommodation, progresses through four defined clinical pathological stages and shortens renal allograft survival.

Acute antibody-mediated rejection of renal transplant: pathogenetic and diagnostic considerations

CONTEXT

Acute antibody-mediated rejection (AMR) has emerged recently as an important cause of graft failure.

OBJECTIVE

To review the pathogenetic, clinicopathologic, and diagnostic considerations of AMR.

DATA SOURCES

Review of literature and the authors' experience.

CONCLUSIONS

Acute antibody-mediated rejection is mediated by antibodies specific for donor antigens, which bind to target antigens and activate the complement system, culminating in tissue injury. The clinical manifestation of AMR is not specific, and transplant biopsy is needed for diagnosis. The glomeruli show thrombosis or neutrophils or mononuclear leukocytes in capillary lumens. The tubulointerstitial compartment shows edema, hemorrhage, necrosis, mild inflammation, and neutrophils or mononuclear leukocytes in the peritubular capillary lumens. The blood vessels show thrombosis, thrombotic microangiopathy, fibrinoid necrosis, or transmural vasculitis. Strong staining for C4d in the peritubular capillaries is characteristic. A definitive diagnosis of AMR requires (1) morphologic evidence of acute tissue injury, (2) immunopathologic evidence for antibody action, and (3) serologic evidence of circulating donor-specific antibodies. Acute antibody-mediated rejection should be suspected if some but not all 3 criteria are met. Since effective treatment is currently available, accurate and timely diagnosis of AMR is essential.

Sirolimus is not always responsible for new-onset proteinuria after conversion for chronic allograft nephropathy

An eight-yr-old combined liver and kidney transplant recipient for hyperoxaluria type I developed significant proteinuria and hypertension after conversion of a Tacrolimus, MMF, and corticosteroids-based immunosuppression to Sirolimus, low-dose Tacrolimus, and corticosteroids six and a half yr after the transplant for chronic allograft nephropathy. There was only one class I HLA match and the recipient had multiple blood exposures prior to transplantation. The patient was treated with combined hemodialysis and peritoneal dialysis while awaiting transplantation to reduce the oxalate load. A renal biopsy revealed a de novo transplant glomerulopathy that was associated with specific HLA antibodies unrelated to the donor (HLA DR 17 and 18). After reintroduction of MMF, these antibodies became undetectable and the proteinuria completely resolved. We hypothesize that HLA antibodies may cause transplant glomerulopathy even if they are not donor-specific. Their production appears more susceptible to MMF therapy. A thorough work-up of new-onset proteinuria after conversion to Sirolimus should be performed, including an immunological work-up and a renal biopsy.

Renal pathology in the pediatric transplant patient

Renal transplantation is a therapeutic goal for children with advanced chronic kidney disease. There are many causes of renal dysfunction in children with allografts--the transplanted kidney can develop a variety of morphologic alterations leading to dysfunction. Evaluation of the kidney biopsy is one of the best methods of determining the cause of graft dysfunction. Rejection is a major cause of renal allograft failure in children. The morphologic hallmarks of acute antibody-mediated and cell-mediated rejection and chronic allograft nephropathy have been codified in classification strategies that are useful in adults and children. Viral infection and Epstein-Barr virus-driven posttransplant lymphoproliferative disease also occur in the pediatric transplanted kidney. Drug toxicity from immunosuppressive agents also causes characteristic morphologic alterations in the renal allograft. As the survival of pediatric heart and liver transplant patients improves, the incidence of immunosuppression therapy-related disease in the native kidney in these patients will likely become more important clinically. In addition to renal lesions related to the allograft state, glomerular disease can recur or occur de novo in renal allografts. Here, we describe the pathology of the more common morphologic lesions in kidneys of children with a renal allograft.

Antibody-mediated renal allograft rejection: diagnosis and pathogenesis

Alloantibodies to HLA class I or II and other antigens expressed by endothelium cause a variety of effects on renal transplants, ranging from acute to chronic rejection, and even apparent graft acceptance (accommodation). Recognition of these conditions and appropriate therapy requires demonstration of C4d in biopsies, commonly confirmed by tests for circulating alloantibody. Substantial practical experience by pathologists in the interpretation and pitfalls of C4d stains are reviewed along with considerations of the clinical significance and pathologic mechanisms of the different effects of antibody on the endothelium of the renal allograft. Clinical trials will be needed to ascertain the optimal treatment for the newly appreciated conditions chronic humoral rejection and accommodation.

Banff '05 Meeting Report: differential diagnosis of chronic allograft injury and elimination of chronic allograft nephropathy ('CAN')

The 8th Banff Conference on Allograft Pathology was held in Edmonton, Canada, 15-21 July 2005. Major outcomes included the elimination of the non-specific term "chronic allograft nephropathy" (CAN) from the Banff classification for kidney allograft pathology, and the recognition of the entity of chronic antibody-mediated rejection. Participation of B cells in allograft rejection and genomics markers of rejection were also major subjects addressed by the conference.

Mechanisms and Role of HLA and non-HLA Alloantibodies

The role of alloantibodies against HLA and non-HLA targets is becoming increasingly recognized as critical in the pathogenesis of acute and chronic renal allograft outcomes. This review discusses the antigenic targets, the mechanisms of T and B cell activation that result in the production of antibody, the complement cascade, methods of antibody detection, and the evidence that alloantibody-mediated mechanisms are active in acute and chronic rejection.

Diagnostic value of C4d in renal biopsies

PURPOSE OF REVIEW

Capillary C4d is now an established marker of antibody-mediated rejection in graft biopsies. The technique is widely used to further define the clinical relevance of humoral alloreactivity in various patient subgroups. These include highly sensitized patients, recipients with late graft failure and also some with 'stable' graft function.

RECENT FINDINGS

The C4d technique compares favourably with other techniques that are explored, for example detection of C3d. Capillary C4d can be associated with any graft pathology, including transplant glomerulopathy. C4d is related to circulating alloantibodies but not autoantibodies, and is probably not derived from local sources. Presensitization and de-novo sensitization are important scenarios of humoral alloreactivity that require refined analysis and treatment.

SUMMARY

Detection of C4d in graft biopsies has emerged as an important tool that could substantiate the clinical significance of antibody-mediated rejections. The comprehensive analysis of humoral alloreactivity in the posttransplantation period is still ongoing and will hopefully result in improved patient care and better long-term graft survival.