Diagnostic significance of peritubular capillary basement membrane multilaminations in kidney allografts: old concepts revisited

Transplant Glomerulopathy: Importance of Ultrastructural Examination

Background

Transplant glomerulopathy (TG) is a morphologic alteration in glomeruli of renal allografts, characterized by glomerular basement membrane reduplications.

Summary

TG is associated with progressive chronic allograft dysfunction and proteinuria and is a diagnostic feature of chronic antibody-mediated rejection (ABMR) in patients positive for donor-specific antibodies, according to the Banff schema for renal allograft pathology. It is a definitive endpoint in clinical trials and interventional studies for ABMR, but the lesion can also occur in the absence of definitive alloimmune injury, as a consequence of chronic thrombotic microangiopathy, and in some cases in association with hepatitis C infection. This review discusses the pathophysiology and clinical presentation of TG, the diagnostic features by light microscopy, and focuses on the sequential ultrastructural stages of the lesion. The differential diagnosis of TG, and Banff grading of the lesion, are reviewed. Clinicopathological indications for performing routine ultrastructural examination of renal allograft biopsies are discussed.

Key Messages

TG can be diagnosed at an early stage by electron microscopy, before histological features are apparent, emphasizing the importance of ultrastructural examination of renal allograft biopsies for an early diagnosis, when therapeutic intervention may be beneficial.

Clinical-pathological correlations in post-transplant thrombotic microangiopathy

AIMS

Post-transplant thrombotic microangiopathy (TMA) is a rare and clinically challenging finding in renal transplant biopsies. In addition to recurrent atypical haemolytic uraemic syndrome, TMA in renal transplants is associated with various conditions, such as calcineurin inhibitor (CNI) treatment, antibody-mediated rejection (ABMR), viral infections, sepsis, pregnancy, malignancies, and surgery. The therapeutic implications of this diagnosis are considerable. In order to better understand post-transplant TMA and to identify histological or clinical differences between associated causes, we conducted a multicentre retrospective study.

METHODS AND RESULTS

Clinical parameters and transplant renal biopsy findings from 81 patients with TMA were analysed. Biopsies from 38 patients were also analysed with electron microscopy. On the basis of clinical-pathological correlation, TMA was attributed to a main aetiology, whenever possible. TMA occurred at a median of 30 days post-transplantation. Systemic features of TMA were present in only 18% of cases. Twenty-two per cent of cases were attributed to CNI and 11% to ABMR. Although other potentially contributing factors were found in 56% of patients, in most cases (63%) no clearly attributable cause of TMA was identified. Histological differences between groups were minimal. The detection of ultrastructural features that are usually associated with ABMR may help to establish ABMR as the cause of TMA.

CONCLUSIONS

Although CNI and ABMR appear to be the main contributors to post-transplant TMA, the aetiology of most cases is probably multifactorial, and TMA cannot be unequivocally attributed to a single underlying aetiology. Morphological features of TMA are not discriminating, but electron microscopy may help to identify ABMR-associated TMA.

Banff survey on antibody-mediated rejection clinical practices in kidney transplantation: Diagnostic misinterpretation has potential therapeutic implications

The aim of this study was to determine how the Banff antibody-mediated rejection (ABMR) classification for kidney transplantation is interpreted in practice and affects therapy. The Banff Antibody-Mediated Injury Workgroup electronically surveyed clinicians and pathologists worldwide regarding diagnosis and treatment for 6 case-based scenarios. The participants' (95 clinicians and 72 renal pathologists) assigned diagnoses were compared to the Banff intended diagnoses (reference standard). The assigned diagnoses and reference standard differed by 26.1% (SD 28.1%) for pathologists and 34.5% (SD 23.3%) for clinicians. The greatest discordance between the reference standard and clinicians' diagnosis was when histologic features of ABMR were present but donor-specific antibody was undetected (49.4% [43/87]). For pathologists, the greatest discordance was in the case of acute/active ABMR C4d staining negative in a positive crossmatch transplant recipient (33.8% [23/68]). Treatment approaches were heterogeneous but linked to the assigned diagnosis. When acute/active ABMR was diagnosed by the clinician, treatment was recommended 95.3% (SD 18.4%) of the time vs only 77.7% (SD 39.2%) of the time when chronic active ABMR was diagnosed (P < .0001). In conclusion, the Banff ABMR classification is vulnerable to misinterpretation, which potentially has patient management implications. Continued efforts are needed to improve the understanding and standardized application of ABMR classification in the transplant community.

The relationship between pathologic lesions of active and chronic antibody-mediated rejection in renal allografts

The Banff classification of renal allograft pathology defines specific morphologic lesions that are used in the diagnosis of active (glomerulitis, peritubular capillaritis, endarteritis) and chronic (transplant glomerulopathy, peritubular capillary basement membrane multilayering, transplant arteriopathy) antibody-mediated rejection (ABMR). However, none of these individual lesions are specific for ABMR, and for this reason Banff requires 1 or more additional findings, including C4d deposition in peritubular capillaries, presence of circulating donor-specific antibodies (DSAs), and/or expression in the tissue of transcripts strongly associated with ABMR, for a definitive diagnosis of ABMR to be made. In addition, while animal studies examining serial biopsies have established the progression of morphologic lesions of active to chronic ABMR as well as intermediate forms (chronic active ABMR) exhibiting features of both, clear documentation that lesions of chronic ABMR require the earlier presence of corresponding active and intermediate lesions is less well established in human renal allografts. This review examines temporal relationships between key morphologic lesions of active and chronic ABMR in biopsies of human grafts, likely intermediate forms, and findings for and possibly against direct and potentially interruptible progression from active to chronic lesions.

A method to reduce variability in scoring antibody-mediated rejection in renal allografts: implications for clinical trials - a retrospective study

Poor reproducibility in scoring antibody-mediated rejection (ABMR) using the Banff criteria might limit the use of histology in clinical trials. We evaluated the reproducibility of Banff scoring of 67 biopsies by six renal pathologists at three institutions. Agreement by any two pathologists was poor: 44.8-65.7% for glomerulitis, 44.8-67.2% for peritubular capillaritis, and 53.7-80.6% for chronic glomerulopathy (cg). All pathologists agreed on cg0 (n = 20) and cg3 (n = 9) cases, however, many disagreed on scores of cg1 or cg2. The range for the incidence of composite diagnoses by individual pathologists was: 16.4-22.4% for no ABMR; 17.9-47.8% for active ABMR; and 35.8-59.7% for chronic, active antibody-mediated rejection (cABMR). A "majority rules" approach was then tested in which the scores of three pathologists were used to reach an agreement. This increased consensus both for individual scores (ex. 67.2-77.6% for cg) and for composite diagnoses (ex. 74.6-86.6% cABMR). Modeling using these results showed that differences in individual scoring could affect the outcome assessment in a mock study of cABMR. We conclude that the Banff schema has high variability and a majority rules approach could be used to adjudicate differences between pathologists and reduce variability in scoring in clinical trials.

An unusual pattern of peritubular capillary injury involving apoptosis in a renal transplant patient

Microvascular injury is an important factor in renal allograft survival. Repeated episodes of endothelial injury from chronic antibody-mediated rejection typically manifest at the ultrastructural level as circumferential multilayering of remodeled glomerular basement membrane material and peritubular capillary basal lamina. In contrast to this typical pattern of microvascular injury, a renal transplantation case is presented in which focally dilated and multilayered segments of peritubular capillary basal lamina bearing lipid droplets were interspersed with ultrastructurally normal unilayered segments of basal lamina devoid of lipid droplets. Glomerular basement membranes were not affected by this process. The peak incidence of lipid droplets within the peritubular capillary walls coincided with a peak in apoptotic activity within the allograft. Lesser amounts of the same lipidic material were identified in the mesangial matrix and an arteriolar wall. Mesangial electron-dense deposits were detected at two weeks posttransplantation and their appearance coincided with elevated immunological activity in the glomeruli, as determined by immunofluorescence microscopy. The unusual ultrastructure and immunological activity observed in this case may reflect a process of impaired apoptotic clearance within the allograft. The six biopsies from a single patient are discussed in the setting of a highly sensitized renal transplant recipient who received prophylactic terminal complement blockade by eculizumab. The findings may be relevant to the study of apoptosis, efferocytosis, microvascular injury, eculizumab, rejection, lupus, and drug-related disease.

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.

Local CD34-positive capillaries decrease in mouse models of kidney disease associating with the severity of glomerular and tubulointerstitial lesions

BACKGROUND

The renal vasculature plays important roles in both homeostasis and pathology. In this study, we examined pathological changes in the renal microvascular in mouse models of kidney diseases.

METHODS

Glomerular lesions (GLs) in autoimmune disease-prone male BXSB/MpJ-Yaa (Yaa) mice and tubulointerstitial lesions (TILs) in male C57BL/6 mice subjected to unilateral ureteral obstruction (UUO) for 7 days were studied. Collected kidneys were examined using histopathological techniques. A nonparametric Mann-Whitney U test (P < 0.05) was performed to compare healthy controls and the experimental mice. The Kruskal-Wallis test was used to compare three or more groups, and multiple comparisons were performed using Scheffe's method when significant differences were observed (P < 0.05).

RESULTS

Yaa mice developed severe autoimmune glomerulonephritis, and the number of CD34+ glomerular capillaries decreased significantly in GLs compared to that in control mice. However, UUO-treated mice showed severe TILs only, and CD34+ tubulointerstitial capillaries were decreased significantly in TILs with the progression of tubulointerstitial fibrosis compared to those in untreated control kidneys. Infiltrations of B-cells, T-cells, and macrophages increased significantly in the respective lesions of both disease models (P < 0.05). In observations of vascular corrosion casts by scanning electron microscopy and of microfil rubber-perfused thick kidney sections by fluorescence microscopy, segmental absences of capillaries were observed in the GLs and TILs of Yaa and UUO-treated mice, respectively. Further, transmission electron microscopy revealed capillary endothelial injury in the respective lesions of both models. The numbers of CD34+ glomerular and tubulointerstitial capillaries were negatively correlated with all examined parameters in GLs (P < 0.05) and TILs (P < 0.01), respectively.

CONCLUSIONS

From the analysis of mouse models, we identified inverse pathological correlations between the number of local capillaries in GLs and TILs and the severity of kidney diseases.

Rethinking peritubular capillary basement membrane multilayering in renal transplant pathology: a case report

BACKGROUND

Severe multilayering (ML) of the peritubular capillary basement membranes in kidney allografts is considered to be an ultrastructural hallmark of chronic antibody-mediated rejection (CAMR). We describe here the unexpected findings in a young male adolescent with underlying focal segmental glomerulosclerosis who underwent a living-related donor transplant procedure, a case which brought into question the specificity of ML.

METHODS

The patient received a kidney from his mother, whose donor screening was unremarkable. He developed nephrotic-range proteinuria shortly after the procedure. Biopsies performed within the first 6 months after transplantation demonstrated ML (5-6 layers).

RESULTS

Since there were no other criteria for CAMR, electron microscopic analysis of the baseline biopsy was performed, which in retrospect also demonstrated ML. The donor is still asymptomatic after 7 years of follow-up, with normal renal function and no proteinuria.

CONCLUSIONS

We discuss the phenomenon of ML in renal disease and together with the findings in our case would like to draw attention to the fact that ML in the setting of renal transplantation is not specific to CAMR, as it can exist in several kidney diseases and even in asymptomatic donors.

Glomerular C4d deposits can mark structural capillary wall remodelling in thrombotic microangiopathy and transplant glomerulopathy: C4d beyond active antibody-mediated injury: a retrospective study

Peritubular capillary C4d (ptc-C4d) usually marks active antibody-mediated rejection, while pseudolinear glomerular capillary C4d (GBM-C4d) is of undetermined diagnostic significance, especially when seen in isolation without concurrent ptc-C4d. We correlated GBM-C4d with structural GBM abnormalities and active antibody-mediated rejection in 319 renal transplant and 35 control native kidney biopsies. In kidney transplants, ptc-C4d was associated with GBM-C4d in 97% by immunofluorescence microscopy (IF) and 61% by immunohistochemistry (IHC; P < 0.001). Transplant glomerulopathy correlated with GBM-C4d (P < 0.001) and presented with isolated GBM-C4d lacking ptc-C4d in 69% by IF and 40% by IHC. Strong isolated GBM-C4d was found post year-1 in repeat biopsies with transplant glomerulopathy. GBM-C4d staining intensity correlated with Banff cg scores (rs = 0.45, P < 0.001). Stepwise exclusion and multivariate logistic regression corrected for active antibody-mediated rejection showed significant correlations between GBM duplication and GBM-C4d (P = 0.001). Native control biopsies with thrombotic microangiopathies demonstrated GBM-C4d in 92% (IF, P < 0.001) and 35% (IHC). In conclusion, pseudolinear GBM-C4d staining can reflect two phenomena: (i) structural GBM changes with duplication in native and transplant kidneys or (ii) active antibody-mediated rejection typically accompanied by ptc-C4d. While ptc-C4d is a dynamic 'etiologic' marker for active antibody-mediated rejection, isolated strong GBM-C4d can highlight architectural glomerular remodelling.

Refinement of the criteria for ultrastructural peritubular capillary basement membrane multilayering in the diagnosis of chronic active/acute antibody-mediated rejection

Chronic active/acute antibody-mediated rejection (cABMR) is the main cause of late renal allograft loss. Severe peritubular capillary basement membrane multilayering (PTCML) assessed on electron microscopy is one diagnostic feature of cABMR according to the Banff 2013 classification. We aimed to refine the PTCML criteria for an earlier diagnosis of cABMR. We retrospectively investigated ultrastructural features of 159 consecutive renal allografts and 44 nonallografts. The presence of serum donor-specific antibodies at the time of biopsy of allografts was also examined. Forty-three patients (27.0%) fulfilled the criteria of cABMR, regardless of PTCML, and comprised the cABMR group. Forty-one patients (25.8%) did not exhibit cABMR features and comprised the non-cABMR allograft control group. In addition, 15 zero-day wedge resections and 29 native kidney biopsies comprised the nonallograft control group. When the diagnostic accuracies of various PTCML features were assessed using the cABMR and non-cABMR allograft control groups, ≥4 PTCML, either circumferential or partial, in ≥2 peritubular capillaries of the three most affected capillaries exhibited the highest AUC value (0.885), greater than the Banff 2013 classification (0.640). None of the nonallograft control groups exhibited PTCML features. We suggest that ≥4 PTCML in ≥2 peritubular capillaries of the three most affected cortical capillaries represents the proper cutoff for cABMR.

Current pathological perspectives on chronic rejection in renal allografts

Chronic rejection in renal transplantation clinically manifests as slow deterioration in allograft function and is a major contributor of late renal graft loss. Most cases of chronic rejection involve chronic antibody-mediated rejection (ABMR) triggered by the interaction of donor-specific alloantibodies with endothelial cells of the microcirculation. The evolution of the Banff classification involved a major revision of the ABMR criteria during the 2000s and led to the inclusion of detailed pathological characteristics of chronic ABMR in the 2013 Banff scheme, including microcirculation damage observed as newly formed basement membranes and arterial fibrous intimal proliferation. Inflammation of microvasculature including glomeruli and/or peritubular capillaries is also seen in substantial cases of chronic ABMR, defined as chronic active ABMR. Chronic active T cell-mediated rejection (TCMR) results from chronic T cell-mediated injury involving renal arteries but is less characterized under the current Banff classification, mainly due to the expanding histological criteria of chronic active ABMR. Characteristics shared by these two chronic rejection types can potentially cause diagnostic confusion. Hence, the diagnostic criteria or categories of chronic renal rejection require amendment of the current Banff classification. Assessment of rejection cases with molecular phenotyping advanced the mechanistic understanding of various dysfunctions in renal allograft, including ABMR and TCMR. Identification of disease-specific changes in gene expression by immunohistological studies, especially in chronic ABMR, has already been validated by several studies, warranting potential application to the pathological diagnostic process. This review provides an overview of current pathological perspectives on chronic rejection of renal allografts and future directions.

Regardless of etiology, progressive renal disease causes ultrastructural and functional alterations of peritubular capillaries

Progressive renal diseases are associated with rarefaction of peritubular capillaries, but the ultrastructural and functional alterations of the microvasculature are not well described. To study this, we analyzed different time points during progressive kidney damage and fibrosis in 3 murine models of different disease etiologies. These models were unilateral ureteral obstruction, unilateral ischemia-reperfusion injury, and Col4a3-deficient mice, we analyzed ultrastructural alterations in patient biopsy specimens. Compared with kidneys of healthy mice, we found a significant and progressive reduction of peritubular capillaries in all models analyzed. Ultrastructurally, compared with the kidneys of control mice, focal widening of the subendothelial space and higher numbers of endothelial vacuoles and caveolae were found in fibrotic kidneys. Quantitative analysis showed that peritubular capillary endothelial cells in fibrotic kidneys had significantly and progressively reduced numbers of fenestrations and increased thickness of the cell soma and lamina densa of the capillary basement membrane. Similar ultrastructural changes were also observed in patient's kidney biopsy specimens. Compared with healthy murine kidneys, fibrotic kidneys had significantly increased extravasation of Evans blue dye in all 3 models. The extravasation could be visualized using 2-photon microscopy in real time in living animals and was mainly localized to capillary branching points. Finally, fibrotic kidneys in all models exhibited a significantly greater degree of interstitial deposition of fibrinogen. Thus, peritubular capillaries undergo significant ultrastructural and functional alterations during experimental progressive renal diseases, independent of the underlying injury. Analyses of these alterations could provide read-outs for the evaluation of therapeutic approaches targeting the renal microvasculature.

Peritubular capillary basement membrane multilayering in early and advanced transplant glomerulopathy: quantitative parameters and diagnostic aspects

The ultrastructural quantitative aspects of peritubular capillary basement membrane multilayering (PTCBML) were examined in 57 kidney transplant biopsies with transplant glomerulopathy (TG). The measurements included three cutoffs [permissive: 1 PTC with 5 basement membrane (BM) layers, intermediate: 3 PTCs with 5 layers or 1 PTC with 7 layers, strict: 1 PTC with 7 layers and 2 PTCs with 5 layers] and the mean number of BM layers (PTC_circ). Two groups were assigned, namely patients with mild TG (Banff cg1a and cg1b) and those with moderate-to-severe TG (cg2 and cg3). Their respective clinical, serological, and morphological characteristics were then compared. The clinical data revealed that mild TG corresponded to early chronic antibody-mediated rejection (cABMR), while moderate-to-severe TG corresponded to the advanced stage of the disease. The permissive threshold displayed the lowest specificity (73 %) and the highest sensitivity (83 %) for moderate-to-severe TG, and its corresponding PTC_circ value was 3 layers. In contrast, the strict threshold-adopted by the Banff 2013 classification-displayed a specificity and sensitivity of 93 and 52 %, respectively, and the corresponding PTC_circ was 4 layers. In mild TG, 26 % of the cases met the permissive cutoff and 6 % the strict cutoff. Mild TG was associated with a lower PTC_circ (2.6 layers vs 4.5 layers in moderate-to-severe TG; p < 0.0001). Amongst the various criteria, the permissive criterion was associated most frequently with mild TG, and had prognostic relevance. Because of this, we propose its usage as a marker of early cABMR-induced PTCBML if non-alloimmune causes of PTCBML can be ruled out.

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.

The Banff Conferences on renal allograft pathology – the latest 2013 report

The Banff Conference diagnostic categories and their criteria for renal biopsy interpretation were created in 1991 by a group comprising nephrologists, pathologists, transplant surgeons and immunologists. These guidelines are widely used in many countries. Every two years, participants at these meetings present and discuss research findings that have added to our knowledge of allograft pathology. From the start, features of T-cell mediated rejection were established. This was followed by discovery of C4d staining in biopsy tissue and better characterisation of antibody mediated rejection. The formation of working groups to look into problematic areas has allowed better refinements to be made to the classification scheme. The latest Banff 2013 report is significant for the inclusion of a C4d-negative category under humoral rejection. Together with the realisation that endarteritis may be antibody mediated, this latest report will greatly impact how pathologists interpret the allograft biopsy.

Medullary Microvascular Thrombosis and Injury in Sickle Hemoglobin C Disease

Sickle cell nephropathy is a common complication in patients with sickle cell hemoglobinopathies. In these disorders, polymerization of mutated hemoglobin S results in deformation of red blood cells, which can cause endothelial cell injury in the kidney that may lead to thrombus formation when severe or manifest by multilayering of the basement membranes (glomerular and/or peritubular capillaries) in milder forms of injury. As the injury progresses, the subsequent ischemia, tubular dysfunction, and glomerular scarring can result in CKD or ESRD. Sickle cell nephropathy can occur in patients with homozygous hemoglobin SS or heterozygous hemoglobin S (hemoglobin SC, hemoglobin S/β(0)-thalassemia, and hemoglobin S/β(+)-thalassemia). Clinical manifestations resulting from hemoglobin S polymerization are often milder in patients with heterozygous hemoglobin S. These patients may not present with clinically apparent acute sickle cell crises, but these milder forms can provide a unique view of the kidney injury in sickle cell disease. Here, we report a patient with hemoglobin SC disease who showed peritubular capillary and vasa recta thrombi and capillary basement membrane alterations primarily involving the renal medulla. This patient highlights the vascular occlusion and endothelial cell injury in the medulla that contribute to sickle cell nephropathy.

The significance of histological diagnosis in renal allograft biopsies in 2014

In 2014, the renal allograft biopsy still represents the best available diagnostic 'gold' standard to assess reasons for allograft dysfunction. However, it is well recognized that histological lesion observed in the biopsy is of limited diagnostic specificity and that the Banff classification as the international diagnostic standard represents mere expert consensus. Here, we review the role of the renal allograft biopsy in different clinical and diagnostic settings. To increase diagnostic accuracy and to compensate for lack of specificity, the interpretation of biopsy pathology needs to be within the clinical context, primarily defined by time post-transplantation and patient-specific risk profile. With this in mind, similar histopathological patterns will lead to different conclusions with regard to diagnosis, disease grading and staging and thus to patient-specific clinical decision-making. Consensus generation for such integrated diagnostic approach, preferably including new molecular tools, represents the next challenge to the transplant community on its way to precision medicine in transplantation.

Emerging Concepts and Controversies in Renal Pathology: C4d-Negative and Arterial Lesions as Manifestations of Antibody-Mediated Transplant Rejection

The consensus classification of antibody-mediated rejection (AMR) of renal allografts developed at the Sixth Banff Conference on Allograft Pathology, in 2001, identified three findings necessary for the diagnosis of active AMR: histologic evidence, antibodies against the graft, and capillary C4d deposition. Morphologic and molecular studies have noted evidence of microvascular injury, which, in the presence of donor-specific antibodies (DSAs) but the absence of C4d deposition, is associated with development of transplant glomerulopathy and graft loss. Recent studies suggest that intimal arteritis may in some cases be a manifestation of DSA-induced graft injury. These newly recognized lesions of AMR have now been incorporated into a revised Banff diagnostic schema.

An updated Banff schema for diagnosis of antibody-mediated rejection in renal allografts

PURPOSE OF REVIEW

To introduce the updated Banff schema for antibody-mediated renal allograft rejection and related revisions to definitions within this schema agreed upon during and immediately subsequent to the 2013 Banff Conference on Allograft Pathology.

RECENT FINDINGS

The original Banff schema for diagnosis of acute and chronic, active antibody-mediated rejection (ABMR) in renal allografts, formulated at the 2001 and 2007 Banff Conferences, has been of great assistance to pathologists and clinicians faced with an increasing awareness of the role of donor-specific alloantibodies (DSAs) in producing graft injury. This schema requires histologic (primarily microvascular inflammation and transplant glomerulopathy), immunohistologic (C4d in peritubular capillaries), and serologic (circulating DSA) evidence for a definitive diagnosis of ABMR. Still, like other Banff classifications, the 2001/2007 schema for renal ABMR is a working classification subject to revision based on new data. Increasing evidence for C4d-negative ABMR and antibody-mediated arterial lesions led to the development of a consensus at the 2013 Banff Conference for updating the schema to include these lesions. Definitions and thresholds for glomerulitis and chronic glomerulopathy were also revised to improve interobserver agreement and correlation with clinical, molecular, and serologic data.

SUMMARY

From a consensus reached at the 2013 Banff Conference, an updated schema for diagnosis of acute/active and chronic, active ABMR has been developed that accounts for recent data supporting the existence of C4d-negative ABMR and antibody-mediated intimal arteritis.

Banff 2013 meeting report: inclusion of c4d-negative antibody-mediated rejection and antibody-associated arterial lesions

The 12th Banff Conference on Allograft Pathology was held in Comandatuba, Brazil, from August 19-23, 2013, and was preceded by a 2-day Latin American Symposium on Transplant Immunobiology and Immunopathology. The meeting was highlighted by the presentation of the findings of several working groups formed at the 2009 and 2011 Banff meetings to: (1) establish consensus criteria for diagnosing antibody-mediated rejection (ABMR) in the presence and absence of detectable C4d deposition; (2) develop consensus definitions and thresholds for glomerulitis (g score) and chronic glomerulopathy (cg score), associated with improved inter-observer agreement and correlation with clinical, molecular and serological data; (3) determine whether isolated lesions of intimal arteritis ("isolated v") represent acute rejection similar to intimal arteritis in the presence of tubulointerstitial inflammation; (4) compare different methodologies for evaluating interstitial fibrosis and for performing/evaluating implantation biopsies of renal allografts with regard to reproducibility and prediction of subsequent graft function; and (5) define clinically and prognostically significant morphologic criteria for subclassifying polyoma virus nephropathy. The key outcome of the 2013 conference is defining criteria for diagnosis of C4d-negative ABMR and respective modification of the Banff classification. In addition, three new Banff Working Groups were initiated.

The specificity of acute and chronic microvascular alterations in renal allografts

The diagnosis of an antibody-mediated rejection (AMR) is made when there is evident histologic injury in the presence of detectable donor-specific alloantibodies (DSA) and diffuse peritubular capillary C4d staining (C4d-pos). In the presence of only detectable DSA or C4d-pos, the tissue injury is currently considered "presumptive" for antibody causation. In acute antibody-mediated rejection (AAMR), diagnostic morphologic features include microvascular inflammation (MVI), specifically glomerulitis and peritubular capillaritis. In the case of chronic active AMR (CAAMR), these inflammatory lesions have progressed to chronic microvascular injury, transplant glomerulopathy (TG) and peritubular capillary basement membrane multilayering (PTCBMML). Either TG or PTCBMML is sufficient morphological evidence for a diagnosis of CAAMR. Unfortunately, these lesions are not specific. MVI, TG, and PTCBMML are found in the setting of cell-mediated immunity, as well as in association with non-alloimmune mechanisms. The available treatments for AMR and CMR are different, and it is important to ascertain the dominant mechanism when approaching an individual patient. At present, no gold standard exists to establish the specific pathogenesis in the more ambiguous cases. We detail here the differential diagnosis of MVI, TG, and PTCBMML.

Membranous glomerulopathy in renal allograft: an ultrastructural study of 17 cases

BACKGROUND

Membranous glomerulopathy is a common complication of renal allograft. However, its incidence and prognosis are not well defined, because an undetermined number of them pass undiagnosed under the generic epigraph of chronic allograft nephropathy.

MATERIALS AND METHODS

To assess the diagnostic refinement supplied by electron microscopy to conventional light and immunofluorescence procedures the authors reviewed 17 cases of electron microscopy-confirmed membranous glomerulonephritis in kidney allograft. In addition, they searched for other features of graft injury, particularly lesions associated with alloimmune reaction, in order to evaluate the contribution of each lesion to the long-term outcome of the allograft.

RESULTS

In 4 of the 17 cases of their series the diagnosis of membranous glomerulopathy was made by electron microscopy. In addition, in 5 samples, lesions of chronic alloimmune rejection were present (in 4 cases the diagnosis was based on electron microscopy findings). At the end point of the study, 3 of the 5 patients with chronic alloimmune injury were in dialysis, 1 had died with functioning allograft, and the fifth suffered severe renal failure but was not in dialysis. On the other hand, 3 of the 12 patients without evidence of alloimmune injury had returned to the dialysis program.

CONCLUSIONS

Electron microscopy is a useful tool in the assessment of renal allograft pathology and can provide additional morphological features of prognostic relevance.