- Email: [email protected]
Histologic Classification of IgA Nephropathy: Past, Present, and Future
Histologic Classification of IgA Nephropathy: Past, Present, and Future D1X XMaria F.S. Soares, MD, PhD,D2X X and Ian D3X X S.D. Roberts, MBChB, FRCPathD4X X Summary: IgA nephropathy (IgAN) is the most common glomerulonephritis worldwide. Since its description in 1968, a number of histologic descriptions and classification systems have emerged, the most recent of which is the Oxford Classification of IgAN. We present a historical panorama of histologic classifications of IgAN and discuss the most recent developments, updates, and future challenges of the Oxford Classification of IgAN. Semin Nephrol 38:477 484 Ó 2018 Published by Elsevier Inc. Keywords: IGA nephropathy, classification, diagnosis, pathology
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gA nephropathy (IgAN) is the most common glomerulonephritis worldwide. Clinical presentation and outcome are highly variable and reflect the diverse light microscopic features. There have been a number of attempts to classify the histological changes to provide clinically relevant information, the most recent of them an international consensus classification—the Oxford Classification of IgAN—which is a truly evidence-based classification of glomerular disease.1 This article provides an overview of the histologic classifications of IgAN, the current Oxford Classification of IgAN, and discusses future challenges and developments.
TAGEDH1A HISTORY OF IgA NEPHROPATHY The mid-1960s set the background for the increase of commercially available immunofluorescence antibodies and the incorporation of immunofluorescence microscopy to diagnostic medicine.2 In 1967, Antoine et al3 presented a panorama of immunofluorescence patterns in renal lesions, among which he identified a number of patients with chronic glomerulonephritis or with purpuric lesions whose biopsy specimens showed glomerular deposits of IgA. In 1968, Berger and Hinglais4 described 25 patients with recurrent hematuria and mesangial IgA deposits that surmounted IgG deposits. This finding was groundbreaking at the time because IgG was thought to be the main immunoglobulin of pathogenic importance in glomerulonephritis, and was a consequence of the availability of purified anti-IgA antibody at Hopital Necker.5 Thirty cases were added to this series in the following year, amounting to 55 cases of a glomerulonephritis with a myriad of focal segmental lesions, but a
common denominator of strong and dominant IgA fluorescence.6 Of note, 22 of the patients had a synpharingitic presentation, and 1 patient progressed to end-stage renal disease (ESRD). At the time, the newly described syndrome drew little attention from the Nephrology community and was thought for a while to be limited to France. In the early 1970s, reports from North America, Europe, Asia, and Australia reignited the interest in patients with hematuria, focal nephritis, and mesangial IgA deposits. The eponym of Berger’s disease was introduced in 1973, and by 1975 the defining features of IgA nephropathy were consolidated.4 From these early descriptions of the condition, it was clear that IgAN showed hematuria and red blood cell casts, a slow but relentless evolution, with ESRD developing in some patients, an estimated 50% risk of recurrence in patients who had undergone a transplant, and a variable histopathologic picture, with proliferative glomerulonephritis, predominantly mesangial, but also focal and segmental, which overlapped with renal findings in HenochSch€onlein purpura (HSP), as outlined by Meadow et al7 in 1972.
TAGEDH1HISTOLOGIC MARKERS OF OUTCOME IN IgAN: EARLY STUDIES
Department of Cellular Pathology, Oxford University Hospitals NHS Foundation Trust, John Radcliffe Hospital, Oxford, United Kingdom. Financial disclosure and conflict of interest statements: none. Address reprint requests to Ian S. D. Roberts, MBChB, FRCPath, Department of Cellular Pathology, John Radcliffe Hospital, Headley Way, Headington, Oxford OX3 9DU, UK. E-mail: [email protected] 0270-9295/ - see front matter © 2018 Published by Elsevier Inc. https://doi.org/10.1016/j.semnephrol.2018.05.017
Increasing interest in the histopathology of IgAN followed in the 1980s. It was clear at that time that IgAN was primarily a glomerular disease, but many questions remained: Are glomerular lesions relevant to decreased renal function? Is there a marker of severity or irreversibility of immunologic damage? Do patterns of glomerular disease change during the clinical course? Is it possible to create an objective measurement of histologic damage and an algorithm to predict renal survival?8 In 1982, Lee et al9 used morphologic markers to predict the progression of renal disease in IgAN. Their fivetier classification system was based on the Meadow et al7 classification for Henoch-Sch€onlein nephritis and focused on the overall histologic activity in a given
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biopsy. It combined the severity of mesangial hypercellularity, glomerular sclerosis, crescents, and tubulointerstitial changes in a single score. This schema was applied in 20 patients with IgAN, 13 of whom were followed up for a mean of 2.8 years. All patients with diffuse proliferative lesions (grade IV) or chronic advanced lesions (grade V) on the initial biopsy developed ESRD. Patients with normal histology or mild to moderate lesions (grades II and III) had a benign course without deterioration of renal function. This publication suggested the usefulness of pathologic markers of disease activity and chronicity to predict the clinical course in IgAN. A number of reports by Droz et al10 in 1984, Levy et al11 in 1985, D’Amico et al12 in 1986, Magil and Ballon13 in 1987, and Lee et al14 in 1987 confirmed Lee’s observations9 in more than 900 pediatric and adult patients. These studies showed a worse clinical outcome in patients with extensive glomerular sclerosis, tubular atrophy, and interstitial fibrosis. Of note, the cohort of 292 patients followed up by D’Amico et al12 showed that, in addition to the histopathologic features described earlier, proteinuria more than 1 g/d, crescents, and the degree of intracapillary hypercellularity were indicators of an adverse outcome. In 1990, Bogenschutz et al15 analyzed 239 patients with nonproteinuric IgAN. In this series, glomerular findings were classified as minimal lesions, and as mild or moderate to severe mesangial proliferation. This report highlighted the fact that interstitial fibrosis and tubular atrophy might occur independently from glomerular lesions and are individual predictive markers of renal functional loss. A later report of 18 IgAN patients by Haas16 in 1996 focused on patients with focal segmental glomerulosclerosis (FSGS)-like lesions without crescents or interstitial chronic damage, a category that did not fit into any of Lee’s classes8,9. Serum creatinine and proteinuria values among these patients did not differ from typical FSGS patients, and renal survival did not differ when compared with typical FSGS and other IgAN patients despite previous observations of nephrotic proteinuria being a marker of bad outcome.13 The accumulated evidence encouraged further refinement of Lee’s classification.
TAGEDH1HAAS CLASSIFICATION OF IgAN In 1997, Haas17 published a new histologic grading system based on his observations of 244 patients with IgAN. The Haas17 study aimed to establish relevant clinicopathologic correlations and to identify potential histopathologic markers of outcome. The schema borrowed features of both the Lee system8,9 and the World Health Organization classification of lupus nephritis, and recognized FSGS-like lesions as part of the spectrum of IgAN. Five classes were described. A comparison between the Lee8,9 and Haas17 classes is presented in Table 1. Nearly 25% of the cases showed no signs of an active proliferative glomerulonephritis (classes I and II); class III (focal proliferative glomerulonephritis) included the majority of cases in Haas’s series17. A smaller percentage of patients (14%) showed diffuse proliferative glomerulonephritis (class IV) and chronic glomerulonephritis (class V); these patients had the highest serum creatinine levels and a higher prevalence of hypertension at the time of the biopsy. Outcome was measured by time to irreversible ESRD, defined by a requirement of renal replacement therapy. Notwithstanding the existence of five classes, only three different groups were recognized regarding clinical outcome: classes I and II with excellent prognosis, class III with intermediate prognosis, and classes IV and V with poor prognosis. The presence of crescents impacted negatively on renal survival in subclasses III and IV only when cases were not controlled by serum creatinine levels at the time of biopsy. Crescents were not classified according to their nature (cellular, fibrocellular, or fibrous) in this series. Serum creatinine level at the time of biopsy was predictive of renal survival in patients with classes IV and V, but was a less effective marker of renal survival in patients with class III. This finding might reflect the fact that higher serum creatinine level at presentation is usually a result of extensive baseline chronic damage and thus a herald of a less favorable prognosis. The impact of interstitial fibrosis and tubular atrophy on renal survival varied in different classes: although in class III the presence of interstitial fibrosis and tubular atrophy in 10% of the cortex was predictive of lower renal survival, in class IV this percentage was 20%.
Table 1. Lee and Haas Classification Systems Grade
Lee
Haas
I II
Mostly normal glomeruli <50% of glomeruli with mesangial hypercellularity and sclerosis Rare small crescents Diffuse mesangial proliferation Occasional adhesions and small crescents Diffuse marked mesangial proliferation 45% of glomeruli with crescents Frequent segmental and global sclerosis Severe mesangial proliferation >45% of glomeruli with crescents Frequent segmental and global sclerosis
Minimal histologic lesion Focal segmental glomerulosclerosis
III IV V
Proliferative glomerulonephritis in 50% of glomeruli Crescents might be present Proliferative glomerulonephritis in >50% of glomeruli Crescents might be present 40% glomerular sclerosis and/or tubular atrophy
Pathology of IgAN
Despite their ease of use, these single-grade classification systems were found to be inferior to clinical parameters—mean arterial pressure (MAP) and 24-hour proteinuria—in predicting renal functional decline over 2 to 3 years of follow-up evaluation.18 Patients categorized as class II in the Haas classification17 were found to have nephrotic proteinuria in 83% of cases; however, these patients did not fare worse. It should be emphasized that most clinicopathological studies performed at that time lacked consistent inclusion criteria and end points; most studies, including Haas’s series,17 used renal survival as a measure of outcome. Although the Haas system17 incorporated clinicopathological correlations and assimilated FSGS-like lesions and chronic glomerulonephritis into the spectrum of subclasses in IgAN, it suffered from the same shortcomings as previous schema,7-9 as it did not provide precise histopathological definitions; in particular, proliferative glomerulonephritis was a vague term that encompassed mesangial, endocapillary, and extracapillary hypercellularity. These observations prompted an international group of pathologists and nephrologists to develop a reproducible and clinically relevant IgAN classification system.
TAGEDH1THE OXFORD CLASSIFICATION OF IGANTAGEDN Although other classifications of glomerular pathology have relied heavily on expert opinion to define and group lesions into classes, the Oxford Classification of IgAN pioneered the use of an evidence-based approach to histologic classification.1,19,20 The evidence was derived from a clinicopathologic study of 206 adult and 59 pediatric patients from eight countries around the world. Their renal biopsy specimens were scored by a minimum of three pathologists by means of an unbiased and thorough list of glomerular, tubulointerstitial, and vascular lesions. All patients were followed up for at least 3 years; defined end points were either progression to ESRD or a 50% decrease of estimated glomerular filtration rate (eGFR). The rate of renal functional decline was an additional outcome measure. Despite ethnic and age diversity, the Oxford Classification cohort was rather homogeneous in clinical terms. Patients with extreme presentations of the disease were excluded: those with clinically benign disease (proteinuria, <0.5 g/d) and those with advanced disease (eGFR, <30 mL/min and patients who had been followed up for <1 year, including patients reaching end stage during this period). The exclusion of these subgroups aimed to achieve a more accurate estimate of the rate of loss of renal function and to enrich the cohort for patients reaching an end point. A 4-year effort of developing consensus definitions and scoring histologic slides aimed to identify reproducible and clinically meaningful lesions. Four histologic variables were found to be both reproducible and independently associated with clinical outcome: mesangial
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hypercellularity (M0, 50% glomeruli; M1, >50% of glomeruli showing mesangial hypercellularity); endocapillary hypercellularity (E0, absent; E1, present); segmental glomerulosclerosis (S0, absent; S1, present); and tubular atrophy/interstitial fibrosis (T0, absent or involving 25% of the cortex; T1, 26%-50% of the cortex; T2, >50% of the cortex). M, S, and T scores were found to be independent predictors of either rate of loss of renal function or renal survival in the entire cohort, using a multivariate analysis that included initial eGFR and proteinuria, and follow-up MAP and proteinuria. The E score was found to predict outcome independently in patients who did not receive steroid/immunosuppressive therapy. These scores comprise the MEST score. The prognostic value of the MEST criteria has been validated in adults and children by more than 20 studies, including more than 7,000 Asian, European, and North American patients.21-31 T score is consistently the strongest predictor of clinical outcome in these validation studies; T1/2 reflects advanced chronic damage and latestage disease at the time of diagnosis and therefore a shorter time to end stage. The Validation of the Oxford Classification of IgA Nephropathy (VALIGA) study was a large validation study that included 1,147 IgAN patients from 13 European countries.32 Its broad inclusion criteria allowed extrapolation of the Oxford Classification criteria to patients who were not addressed in the original Oxford Classification study, that is, patients with minimal proteinuria (<0.5 g/d) or poor renal function at presentation (eGFR < 30 mL/min). Patients with low initial proteinuria showed lower MAP, less frequent MEST lesions, and had 90% 5-year renal survival. In this subgroup, the presence of M1 and/or E1 predicted progression to higher levels of proteinuria (>1 and 2 g/ d). Patients with a low eGFR were significantly older, presented with greater initial proteinuria (2.4 g/24 h) and MAP, showed more frequent M1, S1, and T1-2 lesions, and had an average renal survival of 50% at 5 years. It must be emphasized that, as is the case for most clinicopathologic studies of IgAN, the Oxford Classification study was retrospective and uncontrolled for treatment. Rather than reflecting the natural history of IgAN, most series depict the current reality of nonrandom steroid/immunosuppressive therapy that is influenced by a number of clinical and histologic variables. Several studies have shown that patients with endocapillary hypercellularity and crescents are more likely to receive immunosuppression, an explanation for why these lesions were not found to be independent predictors of renal survival in most validation studies. The prognostic value of histologic variables, not surprisingly, is blunted by immunosuppression; after treatment of active inflammatory and proliferative lesions, these no longer predict renal outcome. This effect has been noted particularly in patients with endocapillary hypercellularity (E1 lesions): the presence of E1 lesions did not influence the rate of
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renal function decline in patients who received steroids. Several validation studies have shown an interaction between endocapillary hypercellularity, crescents, and therapy; patients whose biopsy specimens showed these lesions have a better outcome if treated with steroids. Only two studies have been able to determine the natural history of E1 lesions in patients who did not receive steroid/immunosuppressive therapy: both concluded that endocapillary hypercellularity is an independent predictor of loss of renal function in untreated patients.21,33 The VALIGA study confirmed the prognostic values of mesangial hypercellularity (M1 lesions), including in patients with low initial proteinuria and in pediatric IgAN; among the 174 children included in the VALIGA study cohort, M1 was associated with lower renal survival and increasing proteinuria, and to be the strongest predictor of eGFR slope. Similarly to endocapillary lesions and crescents,31 the predictive value of mesangial hypercellularity was attenuated by immunosuppression, indicating that M1 lesions are possibly steroid-responsive, especially in children. Many studies have linked crescents to worse renal outcome in univariate analysis; however, the prognostic impact of crescents was less pronounced when multivariate analysis was performed.34 Studies that have included patients with poor renal function at presentation or rapid progression to end-stage disease have concluded that the presence of crescents is predictive of a poor outcome.29,35 In a Japanese cohort of 702 patients, subanalyzed according to the inclusion criteria used in the Oxford Classification study,23 showed the importance of cellular and fibrocellular crescents and T score as predictors of less favorable outcome in patients with low eGFR at presentation.
TAGEDH1THE PRESENT: 2016 UPDATE OF THE OXFORD CLASSIFICATIONTAGEDN The first Oxford Conference on IgAN was held in 2014 and was attended by 74 nephrologists and pathologists from 16 countries on 5 continents. At this meeting, the current understanding of histologic and other prognostic
markers in IgAN was reviewed. The goal of the meeting was in part to optimize prognostication and also to identify how histology might be used to guide therapy. After the meeting, areas requiring further study were identified and six working groups were set up: mesangial and endocapillary hypercellularity scoring, focusing on reproducibility; crescents; segmental sclerosis; probabilistic modeling, to include clinical, histologic, and other laboratory data; pediatric IgAN; and biomarkers in IgAN. Their activity is now a joint endeavor of the International IgAN Network (IIgANN) and the Renal Pathology Society (RPS). New published data from two of these working groups led to a revision of the Oxford Classification in 2016. Provisional data from the other working groups has significant implications for the way in which biopsy specimens should be evaluated and scored, and how the histologic data are used in prognostic modeling. The crescents working group performed a re-analysis of combined data from 4 cohorts: the original Oxford Classification, VALIGA, and large validation studies from Japan and China. The multivariate analysis included initial eGFR, time-averaged MAP and proteinuria, and MEST scores. A total of 3,096 IgAN patients were assembled36; 36% of the biopsy specimens showed cellular and/or fibrocellular crescents, which in most cases were seen in only a small number of glomeruli (91% of the cases showed crescents in <25% of the glomeruli). In agreement with previous observations,29,30,32 the presence of crescents portended a higher likelihood of immunosuppression. This study concluded that the presence of any crescents was associated with a worse renal outcome only in those patients not receiving immunosuppression. Patients whose biopsy specimens showed crescents in more than 25% of glomeruli had a worse outcome irrespective of treatment. This evidence led to the incorporation of C (crescents) scores in the revised Oxford Classification: C0 (absence of crescents), C1 (crescents in 1%-24% of glomeruli), and C2 (crescents in 25% of glomeruli). The updated scoring system is presented in Table 2. The Oxford Classification of IgAN now includes 5 variables, Mesangial
Table 2. 2016 Oxford Classification of IgAN Variable
Definition
Score
M
Mesangial hypercellularity
E
Endocapillary hypercellularity
S
Segmental sclerosis/adhesions/synechiae
T
Tubular atrophy and interstitial fibrosis
C
Crescents
M0 M1 E0 E1 S0 S1 T0 T1 T2 C0 C1 C2
Absent to 50% of glomeruli >50% of glomeruli Absent Present Absent Present Absent to 25% of the cortex 25%-50% of the cortex >50% of the cortex Absent 1%-24% of the glomeruli 25% of glomeruli
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Figure 1. Lesions in which pathologists showed a high level of agreement in the RPS reproducibility study (periodic acid Schiff stain). (A) Mesangial hypercellularity with more than 4 nuclei surrounded by periodic acid Schiff positive matrix in a peripheral mesangial area. (B) Endocapillary hypercellularity with occlusion of capillary lumina by cells. (C) A fibrocellular crescent. (D) Segmental sclerosis with tuft adhesions.
hypercellularity, Endocapillary hypercellularity, Segmental Sclerosis, Tubular atrophy / interstitial fibrosis, and Crescents (MEST)-C.37 Representative glomerular lesions are shown in Figure 1. The Segmental Sclerosis Working Group addressed the issue of whether subclassification of segmental sclerosing lesions was of clinical value.38 Biopsy specimens from the original Oxford Classification cohort were reviewed and all segmental sclerosing lesions were assessed for various features. There was good to excellent reproducibility for the identification of glomerular tip lesions, podocyte hypertrophy, hyalinosis, and perihilar lesions. Those features (tip lesions and podocyte hypertrophy), which typically are seen in primary podocytopathies, were found to be associated with a higher level of initial proteinuria and worse renal survival in multivariate analysis. Patients whose biopsy specimens showed these changes had a better outcome if treated with steroids. It therefore was recommended that the report for biopsy specimens scored as S1 should include this additional information: segmental glomerulosclerosis with or without podocytopathic features.
TAGEDH1THE FUTURETAGEDN Prognostic Modeling The Prognostic Modeling Working Group’s aim is to optimize outcome prediction for individual patients by developing a risk prediction model that incorporates
histologic, clinical, and other data. An initial study,39 including more than 900 patients from three different cohorts (Oxford Classification, North American, and VALIGA), showed that a model based on a combination of MEST criteria and initial clinical data performed as well as 2-year clinical data in predicting the risk of reaching a combined end point of ESRD or 50% loss of eGFR. This earlier risk prediction was independent of treatment with renin-angiotensin system blockade or immunosuppression. The group now is refining the prediction model using accumulated data from several large IgAN patient cohorts in Asia and Europe. This was a large and detailed IgAN data set and will enable risk stratification according to ethnicity in both adults and children. The ultimate goal is to develop web-based and mobile application calculators that may be used in the management of individual patients.
Use of Renal Biopsy to Guide Therapy One of the central aims of the Oxford Classification study and the IIgANN/RPS working groups is to identify histologic lesions that are responsive to immunosuppression and thus use the biopsy in guiding therapeutic decisions. There is evidence from several of the large retrospective studies described earlier that endocapillary hypercellularity and crescents (E1 and C1/2) are associated with more rapid loss of renal function in the absence of immunosuppression, and patients with these lesions have a better outcome if treated with steroids. What is
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needed is confirmatory evidence from prospective randomized clinical trials (RCTs). Recent trials of standard steroid therapy40-42 and newer therapeutic approaches such as targeted-release budesonide43 have lacked either histologic data or analysis of the impact of histology on response to therapy. However, re-analysis of one RCT of steroid therapy to include the biopsy findings, scored using the Oxford Classification, showed that E1 was associated with a greater reduction in proteinuria in those patients treated with steroids.44 Repeat biopsy studies, with a second biopsy after immunosuppressive therapy, have shown that endocapillary hypercellularity, necrosis, and crescents are treatment-responsive lesions.45 There are a number of current and planned RCTs of newer therapeutic agents that include histologic analysis, and one with a second post-treatment biopsy. Data from these RCTs will clarify the role of histology in guiding therapy. IgA Vasculitis/HSP Nephritis The Oxford Classification of IgAN has not been validated in IgA vasculitis/HSP nephritis. Although HSP nephritis and IgAN share similar immunohistologic findings, HSP nephritis shows more proliferative, crescentic, and necrotizing lesions.46,47 One study has addressed the prognostic value of the Oxford Classification in HSP nephritis.48 In this study of 61 patients, endocapillary hypercellularity, tubular atrophy/interstitial fibrosis, and crescents in 50% or more of glomeruli were found to be predictive of an adverse renal outcome. An IIgANN/RPS Working Group currently is performing a retrospective study of more than 300 patients with HSP nephritis from North America, Asia, and Europe. This will determine whether an IgAN prediction model including MEST-C criteria predicts long-term renal outcome, and whether any histologic lesions are associated with therapeutic response.
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An online study of reproducibility of scoring glomerular lesions in IgAN among members of the RPS was performed in 2017 (Soares et al, unpublished data). Pathologists were provided with links to digital slides and a web-based response system through the RPS. This study included a training element; once pathologists had submitted their scores, they were provided with a summary of the responses from other pathologists around the world, and to the annotated slides, highlighting difficult lesions and providing advice on how to score them. The study identified certain areas of difficulty that included the scoring of endocapillary hypercellularity in the presence of segmental sclerosis, and in the distinction of cellular crescents from pseudocrescents associated with podocytopathic segmental sclerosing lesions (Fig. 2). The exercise will be repeated in 2018 to determine whether the training element has achieved convergence in pathologists’ scores. Interobserver variation in the interpretation of glomerular and other lesions in the renal biopsy specimen may be improved by providing clear lesion definitions and illustrated guidance. An RPS Working Group currently is developing consensus definitions that can be applied to any glomerular disease, not just IgAN.
Reproducibility of the Oxford Classification The scoring of histologic lesions that comprise the MEST-C criteria were found to be highly reproducible in the original Oxford Classification study among a small group of pathologists who worked closely together to resolve differences in the evaluation of problematic lesions. This does not mean, however, that the interobserver agreement will be similarly good between pathologists working in different units around the world who have not had the opportunity to discuss sources of difficulty. Data from the VALIGA study, in which biopsy specimens initially were scored by local pathologists from the 55 centers around Europe, before central scoring in Oxford, showed differences in the scoring of M and E lesions that had a significant impact on the clinical value of these scores.49
Figure 2. Lesions in which pathologists showed poor agreement in the RPS reproducibility study (periodic acid Schiff stain). (A) A segmental lesion in which capillaries are obliterated largely by increased matrix (S1). However, there is increased cellularity within this lesion, which many pathologists scored as endocapillary hypercellularity (E1). There was poor agreement for the E score in this biopsy specimen. (B) An extracapillary lesion that was interpreted variously as segmental sclerosis with overlying epithelial hypercellularity (pseudocrescent), and as a true inflammatory cellular crescent.
Pathology of IgAN
New Approaches to Biopsy Evaluation The poor reproducibility of the E score has led to a search for alternative methods for assessing inflammatory glomerular lesions in IgAN. Quantification of glomerular macrophages using immunohistochemistry for CD68 in an Oxford patient cohort has shown a close correlation between the maximum glomerular macrophage count and E score, with macrophage quantification being highly reproducible.50 CD68 immunohistochemistry to identify macrophages in extracapillary lesions is also a potential tool for differentiating true inflammatory crescents from pseudocrescents. There is growing evidence that the complement system mediates glomerular injury in IgAN and that immunohistochemistry for complement components might be used as a marker of disease activity. IgA activates complement through the alternative and lectin pathways. Genome-wide association studies in IgAN have identified three susceptibility loci at 1q32 containing the CFH and CFHR1-5 genes,51 and the ITGAM-ITGAX locus at 16p11 containing the complement receptor 4 gene, involved in leukocyte cell adhesion, migration, and complement phagocytosis.52 Genetic variants in CFH, CFHR3, and CFHR1 are associated with circulating C3 levels and mesangial C3 deposition.53 Mesangial deposition of mannose binding lectin and C4d, markers of the activation of the lectin pathway, is associated with more severe histologic lesions.54 Higher serum and glomerular levels of factor H related protein 5, shown by immunohistochemistry, are associated with progressive disease in IgAN.55 The potential clinical role of immunohistochemistry for complement components requires further study; in particular it remains to be determined whether it provides added value to the MEST-C criteria.
TAGEDH1CONCLUSIONSTAGEDN The value and acceptance of a histopathologic classification system is measured by its ease of use, reproducibility, and ability to provide clinically relevant information.56 Rather than being static, classification systems should be constantly reviewed and updated, with incorporation of new developments and knowledge as they emerge. This has always been the spirit of the Oxford Classification of IgA Nephropathy and will be led in the future by the IIgANN/RPS Working Groups. Studies over the past decade have identified those histologic lesions that are associated with progressive IgAN in the absence of immunosuppression, and shown which lesions are treatment-responsive. In the near future, methods for assessing renal biopsy specimens will be refined, accurate prediction models that incorporate clinical and histologic data will be developed, and the role
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of the renal biopsy in guiding standard therapy and the use of new therapeutic agents will be determined.
TAGEDH1REFERENCESTAGEDN 1. Cook T, Roberts IS, Feehally J, Barratt J. Evidence-based classification of glomerular pathology. Kidney Int. 2014;86:1059. 2. Feehally J, Levy M, Monteiro RC. Jean Berger (1930-2011). Kidney Int. 2011;80:437–8. 3. Antoine B, Neveu I, Berger J. Applications de l’immunofluorescence en nephrologie: Quelques problemes de methodologie. In: Actualites Nephrologiques de l’H^ opital Necker II. Paris: Flammarion. 1967;p 135. 4. Berger J, Hinglais N. Les depots intercapillaires d’IgA-IgG. J Urol Nephrol. 1968;74:694–5. 5. Feehally J, Cameron JS. IgA nephropathy: progress before and since Berger. Am J Kidney Dis. 2011;58:310–9. 6. Berger J. IgA glomerular deposits in renal disease. Transpl Proc. 1969;1:939–44. 7. Meadow SR, Glasgow EF, White RHR, Moncrieff MW, et al. Schoenlein-Henoch nephritis. Q J Med. 1972;41:241–58. 8. Lee SMK. Prognostic indicators of progressive renal disease in IgA nephropathy: emergence of a new histologic grading system. Am J Kidney Dis. 1997;29:953–8. 9. Lee SM, Rao VM, Franklin WA, Schiffer MS, et al. IgA nephropathy: morphologic predictors of progressive renal disease. Hum Pathol. 1982;13:314–22. 10. Droz D, Kramar A, Nawar T, Noel LH. Primary IgA nephropathy: prognostic factors. Contrib Nephrol. 1984;40:202–7. 11. Levy M, Gonzalez-Burchard G, Broyer M, et al. Berger’s disease in children. Natural history and outcome. Medicine (Baltimore). 1985;64:157–80. 12. D’Amico G, Minetti L, Ponticelli C, et al. Prognostic indicators in idiopathic IgA mesangial nephropathy. Q J Med. 1986;59:363– 78. 13. Magil AB, Ballon HS. IgA nephropathy. Evaluation of prognostic factors in patients with moderate disease. Nephron. 1987;47:246– 52. 14. Lee HS, Koh HI, Lee HB, Park HC. IgA nephropathy in Korea: a morphological and clinical study. Clin Nephrol. 1987;27:131–40. 15. Bogenschutz O, Bohle A, Batz C, Wehrmann M, et al. IgA nephritis: on the importance of morphological and clinical parameters in the long-term prognosis of 239 patients. Am J Nephrol. 1990;10:137–47. 16. Haas M. IgA nephropathy histologically resembling focal segmental glomerulosclerosis: a clinicopathologic study of 18 cases. Am J Kidney Dis. 1996;28:365–71. 17. Haas M. Histologic subclassification of IgA nephropathy: a clinicopathologic study of 244 cases. Am J Kidney Dis. 1997;29:829– 42. 18. Bartosik LP, Lajoie G, Sugar L, et al. Predicting progression in IgA nephropathy. Am J Kidney Dis. 2001;38:728–35. 19. Working Group of the International IgA Nephropathy Network and the Renal Pathology SocietyRoberts IS, Cook HT, Troyanov S, et al. The Oxford classification of IgA nephropathy: pathology definitions, correlations, and reproducibility. Kidney Int. 2009;76:546–56. 20. Working Group of the International IgA Nephropathy Network and the Renal Pathology SocietyCattran DC, Coppo R, Cook HT, et al. The Oxford classification of IgA nephropathy: rationale, clinicopathological correlations, and classification. Kidney Int. 2009;76:534–45. 21. Shi SF, Wang SX, Jiang L, et al. Pathologic predictors of renal outcome and therapeutic efficacy in IgA nephropathy: validation
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of the Oxford classification. Clin J Am Soc Nephrol. 2011;9:2175–84. Katafuchi R, Ninomiya T, Nagata M, et al. Validation study of Oxford classification of IgA nephropathy: the significance of extracapillary proliferation. Clin J Am Soc Nephrol. 2011; 6:2806–13. Kang SH, Choi SR, Park HS, et al. The Oxford classification as a predictor of prognosis in patients with IgA nephropathy. Nephrol Dial Transplant. 2012;27:252–8. Shima Y, Nakanishi K, Hama T, et al. Validity of the Oxford classification of IgA nephropathy in children. Ped Nephrol. 2012; 27:783–92. Zeng CH, Le W, Zhang M, et al. A multicenter application and evaluation of the Oxford classification of IgA nephropathy in adult Chinese patients. Am J Kidney Dis. 2012;60:812–20. Lee H, Yi SH, Seo MS, et al. Validation of the Oxford Classification of IgA nephropathy: a single center study in Korean adults. Korean J Intern Med. 2012;27:293–300. Lv J, Shi S, Xu D, et al. Evaluation of the Oxford classification of IgA nephropathy: a systematic review and meta-analysis. Am J Kidney Dis. 2013;62:891–9. Alamartine E, Sauron C, Laurent B, et al. The use of Oxford classification of IgA nephropathy to predict renal survival. Clin J Am Soc Nephrol. 2011;6:2384–8. Edstrom Halling S, Soderberg MP, Berg UB. Predictors of outcome in paediatric IgA nephropathy with regard to clinical and histopathological variables (Oxford Classification). Nephrol Dial Transplant. 2012;27:715–22. Herzenberg AM, Fogo AB, Reich HN, et al. Validation of the Oxford classification of IgA nephropathy. Kidney Int. 2011; 80:310–7. Working Group of the International IgA Nephropathy Network and the Renal Pathology Society. The Oxford IgA nephropathy clinicopathological classification is valid for children as well as adults. Kidney Int. 2010;77:921–7. Coppo R, Troyanov S, Bellur S, et al. Validation of the Oxford classification of IgA nephropathy in cohorts with different presentations and treatments. Kidney Int. 2014;86:828–36. Chakera A, MacEwen C, Bellur SS, et al. Prognostic value of endocapillary hypercellularity in IgA nephropathy patients with no immunosuppression. J Nephrol. 2016;29:367–75. Roberts ISD. Oxford classification of immunoglobulin A nephropathy: an update. Curr Opin Nephrol Hypertens. 2013;22:281–6. Walsh M, Sar A, Lee D, et al. Histopathologic features aid in predicting risk for progression in IgA nephropathy. Clin J Am Soc Nephrol. 2010;5:425–30. Haas M, Verhave JC, Liu ZH, et al. A multicenter study of the predictive value of crescents in IgA nephropathy. J Am Soc Nephrol. 2017;28:691–701. Trimarchi H, Barratt J, Cattran DC, et al. Oxford classification of IgA nephropathy 2016: an update from the IgA nephropathy classification working group. Kidney Int. 2017;91:1014–21. Bellur SS, Lepeytre F, Vorobyeva O, et al. Evidence from Oxford classification cohort supports the clinical value of subclassification of focal segmental glomerulosclerosis in IgA nephropathy. Kidney Int. 2017;91:235–43. Barbour SJ, Espino-Hernandez G, Reich H, et al. Oxford derivation, North-American validation and VALIGA consortia. The MEST score provides earlier risk prediction in IgA nephropathy. Kidney Int. 2016;89:167–75. Eitner F, Ackermann D, Hilgers RD, Floege J. Supportive Versus Immunosuppressive Therapy of Progressive IgA nephropathy (STOP) IgAN trial: rationale and study protocol. J Nephrol. 2008;21:284–9.
M.F.S. Soares and I.S.D. Roberts 41. Pozzi C. Pro: STOP immunosuppression in IgA nephropathy? Nephrol Dial Transplant. 2016;31:1766–70. 42. Lv J, Zhang H, Wong MG, Jardine MJ, et al. TESTING Study Group: effect of oral methylprednisolone on clinical outcomes in patients with IgA nephropathy: the TESTING randomized clinical trial. JAMA. 2017;318:432–42. 43. Fellstr€ om BC, Barratt J, Cook H, Coppo R, et al. NEFIGAN trial investigators: targeted release of budenosine versus placebo in patients with IgA nephropathy (NEFIGAN): a double-blind, randomised, placebo-controlled phase 2b trial. Lancet. 2017;389: 2117–27. 44. Lv J, Zhang H, Chen Y, Li G, et al. Combination therapy of prednisone and ACE inhibitor versus ACE-inhibitor therapy alone in patients with IgA nephropathy: a randomised controlled trial. Am J Kidney Dis. 2009;53:26–32. 45. Shen XH, Liang SS, Chen HM, Le WB, et al. Reversal of active glomerular lesions after immunosuppressive therapy in patients with IgA nephropathy: a repeat-biopsy based observation. J Nephrol. 2015;28:441–9. 46. Komatsu H, Fujimoto S, Yoshikawa N, Kitamura H, et al. Clinical manifestations of Henoch-Schonlein purpura nephritis and IgA nephropathy: comparative analysis of data from the Japan Renal Biopsy Registry (J-RBR). Clin Exp Nephrol. 2016;20:552–60. 47. Davin JC, Ten Berge IJ, Weening JJ. What is the difference between IgA nephropathy and Henoch-Schonlein purpura? Kidney Int. 2001;59:823–34. 48. Kim CH, Lim BJ, Bae YS, Kwon YE, et al. Using the Oxford classification of IgA nephropathy to predict long-term outcomes of Henoch-Schonlein purpura nephritis in adults. Mod Pathol. 2014;27:972–82. 49. Roberts I, Bellur S, Troyanov T, Cook, Coppo R. Reproducibility of the Oxford classification of IgA nephropathy and impact of scoring on prognostic value of the schema: evidence from the VALIGA study cohort. Virchows Arch. 2014;465(Suppl 1):S1– S379. 50. Soares MF, MacEwen C, Chakera A, Bellur S, Roberts I. Relationship between glomerular macrophage numbers and the Oxford classification of IgA nephropathy. Paper presented at: 14th International Symposium on IgA Nephropathy; 2016 Sept 15-18; Tours, France. 51. Gharavi AG, Kiryluk K, Choi M, Li Y, et al. Genome-wide association study identifies susceptibility loci for IgA nephropathy. Nat Genet. 2011;43:321–7. 52. Kiryluk K, Li Y, Scolari F, Sanna-Cherchi S, et al. Discovery of new risk loci for IgA nephropathy implicates genes involved in immunity against intestinal pathogens. Nat Genet. 2014;46:1187– 96. 53. Zhu L, Zhai YL, Wang FM, Hou P, et al. Variants in complement factor H and complement factor H-related protein genes, CFHR3 and CFHR1, affect complement activation in IgA nephropathy. J Am Soc Nephrol. 2015;26:1195–204. 54. Roos A, Rastaldi MP, Calvaresi N, et al. Glomerular activation of the lectin pathway of complement in IgA nephropathy is associated with more severe renal disease. J Am Soc Nephrol. 2006;17:1724–34. 55. Medjeral-Thomas NR, Constantinou N, Lomax-Browne HJ, Cook TH, Pickering MC. Glomerular FHR5 associates with severity in IgA nephropathy. Paper presented at: ASN Kidney Week; 2017 Oct 31 Nov 5; New Orleans, LA. 56. Haas M, Rastaldi MP, Fervenza FC. Histologic classification of glomerular diseases: clinicopathologic correlations, limitations exposed by validation studies, and suggestions for modification. Kidney Int. 2014;85:779–93.
I
gA nephropathy (IgAN) is the most common glomerulonephritis worldwide. Clinical presentation and outcome are highly variable and reflect the diverse light microscopic features. There have been a number of attempts to classify the histological changes to provide clinically relevant information, the most recent of them an international consensus classification—the Oxford Classification of IgAN—which is a truly evidence-based classification of glomerular disease.1 This article provides an overview of the histologic classifications of IgAN, the current Oxford Classification of IgAN, and discusses future challenges and developments.
TAGEDH1A HISTORY OF IgA NEPHROPATHY The mid-1960s set the background for the increase of commercially available immunofluorescence antibodies and the incorporation of immunofluorescence microscopy to diagnostic medicine.2 In 1967, Antoine et al3 presented a panorama of immunofluorescence patterns in renal lesions, among which he identified a number of patients with chronic glomerulonephritis or with purpuric lesions whose biopsy specimens showed glomerular deposits of IgA. In 1968, Berger and Hinglais4 described 25 patients with recurrent hematuria and mesangial IgA deposits that surmounted IgG deposits. This finding was groundbreaking at the time because IgG was thought to be the main immunoglobulin of pathogenic importance in glomerulonephritis, and was a consequence of the availability of purified anti-IgA antibody at Hopital Necker.5 Thirty cases were added to this series in the following year, amounting to 55 cases of a glomerulonephritis with a myriad of focal segmental lesions, but a
common denominator of strong and dominant IgA fluorescence.6 Of note, 22 of the patients had a synpharingitic presentation, and 1 patient progressed to end-stage renal disease (ESRD). At the time, the newly described syndrome drew little attention from the Nephrology community and was thought for a while to be limited to France. In the early 1970s, reports from North America, Europe, Asia, and Australia reignited the interest in patients with hematuria, focal nephritis, and mesangial IgA deposits. The eponym of Berger’s disease was introduced in 1973, and by 1975 the defining features of IgA nephropathy were consolidated.4 From these early descriptions of the condition, it was clear that IgAN showed hematuria and red blood cell casts, a slow but relentless evolution, with ESRD developing in some patients, an estimated 50% risk of recurrence in patients who had undergone a transplant, and a variable histopathologic picture, with proliferative glomerulonephritis, predominantly mesangial, but also focal and segmental, which overlapped with renal findings in HenochSch€onlein purpura (HSP), as outlined by Meadow et al7 in 1972.
TAGEDH1HISTOLOGIC MARKERS OF OUTCOME IN IgAN: EARLY STUDIES
Department of Cellular Pathology, Oxford University Hospitals NHS Foundation Trust, John Radcliffe Hospital, Oxford, United Kingdom. Financial disclosure and conflict of interest statements: none. Address reprint requests to Ian S. D. Roberts, MBChB, FRCPath, Department of Cellular Pathology, John Radcliffe Hospital, Headley Way, Headington, Oxford OX3 9DU, UK. E-mail: [email protected] 0270-9295/ - see front matter © 2018 Published by Elsevier Inc. https://doi.org/10.1016/j.semnephrol.2018.05.017
Increasing interest in the histopathology of IgAN followed in the 1980s. It was clear at that time that IgAN was primarily a glomerular disease, but many questions remained: Are glomerular lesions relevant to decreased renal function? Is there a marker of severity or irreversibility of immunologic damage? Do patterns of glomerular disease change during the clinical course? Is it possible to create an objective measurement of histologic damage and an algorithm to predict renal survival?8 In 1982, Lee et al9 used morphologic markers to predict the progression of renal disease in IgAN. Their fivetier classification system was based on the Meadow et al7 classification for Henoch-Sch€onlein nephritis and focused on the overall histologic activity in a given
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biopsy. It combined the severity of mesangial hypercellularity, glomerular sclerosis, crescents, and tubulointerstitial changes in a single score. This schema was applied in 20 patients with IgAN, 13 of whom were followed up for a mean of 2.8 years. All patients with diffuse proliferative lesions (grade IV) or chronic advanced lesions (grade V) on the initial biopsy developed ESRD. Patients with normal histology or mild to moderate lesions (grades II and III) had a benign course without deterioration of renal function. This publication suggested the usefulness of pathologic markers of disease activity and chronicity to predict the clinical course in IgAN. A number of reports by Droz et al10 in 1984, Levy et al11 in 1985, D’Amico et al12 in 1986, Magil and Ballon13 in 1987, and Lee et al14 in 1987 confirmed Lee’s observations9 in more than 900 pediatric and adult patients. These studies showed a worse clinical outcome in patients with extensive glomerular sclerosis, tubular atrophy, and interstitial fibrosis. Of note, the cohort of 292 patients followed up by D’Amico et al12 showed that, in addition to the histopathologic features described earlier, proteinuria more than 1 g/d, crescents, and the degree of intracapillary hypercellularity were indicators of an adverse outcome. In 1990, Bogenschutz et al15 analyzed 239 patients with nonproteinuric IgAN. In this series, glomerular findings were classified as minimal lesions, and as mild or moderate to severe mesangial proliferation. This report highlighted the fact that interstitial fibrosis and tubular atrophy might occur independently from glomerular lesions and are individual predictive markers of renal functional loss. A later report of 18 IgAN patients by Haas16 in 1996 focused on patients with focal segmental glomerulosclerosis (FSGS)-like lesions without crescents or interstitial chronic damage, a category that did not fit into any of Lee’s classes8,9. Serum creatinine and proteinuria values among these patients did not differ from typical FSGS patients, and renal survival did not differ when compared with typical FSGS and other IgAN patients despite previous observations of nephrotic proteinuria being a marker of bad outcome.13 The accumulated evidence encouraged further refinement of Lee’s classification.
TAGEDH1HAAS CLASSIFICATION OF IgAN In 1997, Haas17 published a new histologic grading system based on his observations of 244 patients with IgAN. The Haas17 study aimed to establish relevant clinicopathologic correlations and to identify potential histopathologic markers of outcome. The schema borrowed features of both the Lee system8,9 and the World Health Organization classification of lupus nephritis, and recognized FSGS-like lesions as part of the spectrum of IgAN. Five classes were described. A comparison between the Lee8,9 and Haas17 classes is presented in Table 1. Nearly 25% of the cases showed no signs of an active proliferative glomerulonephritis (classes I and II); class III (focal proliferative glomerulonephritis) included the majority of cases in Haas’s series17. A smaller percentage of patients (14%) showed diffuse proliferative glomerulonephritis (class IV) and chronic glomerulonephritis (class V); these patients had the highest serum creatinine levels and a higher prevalence of hypertension at the time of the biopsy. Outcome was measured by time to irreversible ESRD, defined by a requirement of renal replacement therapy. Notwithstanding the existence of five classes, only three different groups were recognized regarding clinical outcome: classes I and II with excellent prognosis, class III with intermediate prognosis, and classes IV and V with poor prognosis. The presence of crescents impacted negatively on renal survival in subclasses III and IV only when cases were not controlled by serum creatinine levels at the time of biopsy. Crescents were not classified according to their nature (cellular, fibrocellular, or fibrous) in this series. Serum creatinine level at the time of biopsy was predictive of renal survival in patients with classes IV and V, but was a less effective marker of renal survival in patients with class III. This finding might reflect the fact that higher serum creatinine level at presentation is usually a result of extensive baseline chronic damage and thus a herald of a less favorable prognosis. The impact of interstitial fibrosis and tubular atrophy on renal survival varied in different classes: although in class III the presence of interstitial fibrosis and tubular atrophy in 10% of the cortex was predictive of lower renal survival, in class IV this percentage was 20%.
Table 1. Lee and Haas Classification Systems Grade
Lee
Haas
I II
Mostly normal glomeruli <50% of glomeruli with mesangial hypercellularity and sclerosis Rare small crescents Diffuse mesangial proliferation Occasional adhesions and small crescents Diffuse marked mesangial proliferation 45% of glomeruli with crescents Frequent segmental and global sclerosis Severe mesangial proliferation >45% of glomeruli with crescents Frequent segmental and global sclerosis
Minimal histologic lesion Focal segmental glomerulosclerosis
III IV V
Proliferative glomerulonephritis in 50% of glomeruli Crescents might be present Proliferative glomerulonephritis in >50% of glomeruli Crescents might be present 40% glomerular sclerosis and/or tubular atrophy
Pathology of IgAN
Despite their ease of use, these single-grade classification systems were found to be inferior to clinical parameters—mean arterial pressure (MAP) and 24-hour proteinuria—in predicting renal functional decline over 2 to 3 years of follow-up evaluation.18 Patients categorized as class II in the Haas classification17 were found to have nephrotic proteinuria in 83% of cases; however, these patients did not fare worse. It should be emphasized that most clinicopathological studies performed at that time lacked consistent inclusion criteria and end points; most studies, including Haas’s series,17 used renal survival as a measure of outcome. Although the Haas system17 incorporated clinicopathological correlations and assimilated FSGS-like lesions and chronic glomerulonephritis into the spectrum of subclasses in IgAN, it suffered from the same shortcomings as previous schema,7-9 as it did not provide precise histopathological definitions; in particular, proliferative glomerulonephritis was a vague term that encompassed mesangial, endocapillary, and extracapillary hypercellularity. These observations prompted an international group of pathologists and nephrologists to develop a reproducible and clinically relevant IgAN classification system.
TAGEDH1THE OXFORD CLASSIFICATION OF IGANTAGEDN Although other classifications of glomerular pathology have relied heavily on expert opinion to define and group lesions into classes, the Oxford Classification of IgAN pioneered the use of an evidence-based approach to histologic classification.1,19,20 The evidence was derived from a clinicopathologic study of 206 adult and 59 pediatric patients from eight countries around the world. Their renal biopsy specimens were scored by a minimum of three pathologists by means of an unbiased and thorough list of glomerular, tubulointerstitial, and vascular lesions. All patients were followed up for at least 3 years; defined end points were either progression to ESRD or a 50% decrease of estimated glomerular filtration rate (eGFR). The rate of renal functional decline was an additional outcome measure. Despite ethnic and age diversity, the Oxford Classification cohort was rather homogeneous in clinical terms. Patients with extreme presentations of the disease were excluded: those with clinically benign disease (proteinuria, <0.5 g/d) and those with advanced disease (eGFR, <30 mL/min and patients who had been followed up for <1 year, including patients reaching end stage during this period). The exclusion of these subgroups aimed to achieve a more accurate estimate of the rate of loss of renal function and to enrich the cohort for patients reaching an end point. A 4-year effort of developing consensus definitions and scoring histologic slides aimed to identify reproducible and clinically meaningful lesions. Four histologic variables were found to be both reproducible and independently associated with clinical outcome: mesangial
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hypercellularity (M0, 50% glomeruli; M1, >50% of glomeruli showing mesangial hypercellularity); endocapillary hypercellularity (E0, absent; E1, present); segmental glomerulosclerosis (S0, absent; S1, present); and tubular atrophy/interstitial fibrosis (T0, absent or involving 25% of the cortex; T1, 26%-50% of the cortex; T2, >50% of the cortex). M, S, and T scores were found to be independent predictors of either rate of loss of renal function or renal survival in the entire cohort, using a multivariate analysis that included initial eGFR and proteinuria, and follow-up MAP and proteinuria. The E score was found to predict outcome independently in patients who did not receive steroid/immunosuppressive therapy. These scores comprise the MEST score. The prognostic value of the MEST criteria has been validated in adults and children by more than 20 studies, including more than 7,000 Asian, European, and North American patients.21-31 T score is consistently the strongest predictor of clinical outcome in these validation studies; T1/2 reflects advanced chronic damage and latestage disease at the time of diagnosis and therefore a shorter time to end stage. The Validation of the Oxford Classification of IgA Nephropathy (VALIGA) study was a large validation study that included 1,147 IgAN patients from 13 European countries.32 Its broad inclusion criteria allowed extrapolation of the Oxford Classification criteria to patients who were not addressed in the original Oxford Classification study, that is, patients with minimal proteinuria (<0.5 g/d) or poor renal function at presentation (eGFR < 30 mL/min). Patients with low initial proteinuria showed lower MAP, less frequent MEST lesions, and had 90% 5-year renal survival. In this subgroup, the presence of M1 and/or E1 predicted progression to higher levels of proteinuria (>1 and 2 g/ d). Patients with a low eGFR were significantly older, presented with greater initial proteinuria (2.4 g/24 h) and MAP, showed more frequent M1, S1, and T1-2 lesions, and had an average renal survival of 50% at 5 years. It must be emphasized that, as is the case for most clinicopathologic studies of IgAN, the Oxford Classification study was retrospective and uncontrolled for treatment. Rather than reflecting the natural history of IgAN, most series depict the current reality of nonrandom steroid/immunosuppressive therapy that is influenced by a number of clinical and histologic variables. Several studies have shown that patients with endocapillary hypercellularity and crescents are more likely to receive immunosuppression, an explanation for why these lesions were not found to be independent predictors of renal survival in most validation studies. The prognostic value of histologic variables, not surprisingly, is blunted by immunosuppression; after treatment of active inflammatory and proliferative lesions, these no longer predict renal outcome. This effect has been noted particularly in patients with endocapillary hypercellularity (E1 lesions): the presence of E1 lesions did not influence the rate of
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renal function decline in patients who received steroids. Several validation studies have shown an interaction between endocapillary hypercellularity, crescents, and therapy; patients whose biopsy specimens showed these lesions have a better outcome if treated with steroids. Only two studies have been able to determine the natural history of E1 lesions in patients who did not receive steroid/immunosuppressive therapy: both concluded that endocapillary hypercellularity is an independent predictor of loss of renal function in untreated patients.21,33 The VALIGA study confirmed the prognostic values of mesangial hypercellularity (M1 lesions), including in patients with low initial proteinuria and in pediatric IgAN; among the 174 children included in the VALIGA study cohort, M1 was associated with lower renal survival and increasing proteinuria, and to be the strongest predictor of eGFR slope. Similarly to endocapillary lesions and crescents,31 the predictive value of mesangial hypercellularity was attenuated by immunosuppression, indicating that M1 lesions are possibly steroid-responsive, especially in children. Many studies have linked crescents to worse renal outcome in univariate analysis; however, the prognostic impact of crescents was less pronounced when multivariate analysis was performed.34 Studies that have included patients with poor renal function at presentation or rapid progression to end-stage disease have concluded that the presence of crescents is predictive of a poor outcome.29,35 In a Japanese cohort of 702 patients, subanalyzed according to the inclusion criteria used in the Oxford Classification study,23 showed the importance of cellular and fibrocellular crescents and T score as predictors of less favorable outcome in patients with low eGFR at presentation.
TAGEDH1THE PRESENT: 2016 UPDATE OF THE OXFORD CLASSIFICATIONTAGEDN The first Oxford Conference on IgAN was held in 2014 and was attended by 74 nephrologists and pathologists from 16 countries on 5 continents. At this meeting, the current understanding of histologic and other prognostic
markers in IgAN was reviewed. The goal of the meeting was in part to optimize prognostication and also to identify how histology might be used to guide therapy. After the meeting, areas requiring further study were identified and six working groups were set up: mesangial and endocapillary hypercellularity scoring, focusing on reproducibility; crescents; segmental sclerosis; probabilistic modeling, to include clinical, histologic, and other laboratory data; pediatric IgAN; and biomarkers in IgAN. Their activity is now a joint endeavor of the International IgAN Network (IIgANN) and the Renal Pathology Society (RPS). New published data from two of these working groups led to a revision of the Oxford Classification in 2016. Provisional data from the other working groups has significant implications for the way in which biopsy specimens should be evaluated and scored, and how the histologic data are used in prognostic modeling. The crescents working group performed a re-analysis of combined data from 4 cohorts: the original Oxford Classification, VALIGA, and large validation studies from Japan and China. The multivariate analysis included initial eGFR, time-averaged MAP and proteinuria, and MEST scores. A total of 3,096 IgAN patients were assembled36; 36% of the biopsy specimens showed cellular and/or fibrocellular crescents, which in most cases were seen in only a small number of glomeruli (91% of the cases showed crescents in <25% of the glomeruli). In agreement with previous observations,29,30,32 the presence of crescents portended a higher likelihood of immunosuppression. This study concluded that the presence of any crescents was associated with a worse renal outcome only in those patients not receiving immunosuppression. Patients whose biopsy specimens showed crescents in more than 25% of glomeruli had a worse outcome irrespective of treatment. This evidence led to the incorporation of C (crescents) scores in the revised Oxford Classification: C0 (absence of crescents), C1 (crescents in 1%-24% of glomeruli), and C2 (crescents in 25% of glomeruli). The updated scoring system is presented in Table 2. The Oxford Classification of IgAN now includes 5 variables, Mesangial
Table 2. 2016 Oxford Classification of IgAN Variable
Definition
Score
M
Mesangial hypercellularity
E
Endocapillary hypercellularity
S
Segmental sclerosis/adhesions/synechiae
T
Tubular atrophy and interstitial fibrosis
C
Crescents
M0 M1 E0 E1 S0 S1 T0 T1 T2 C0 C1 C2
Absent to 50% of glomeruli >50% of glomeruli Absent Present Absent Present Absent to 25% of the cortex 25%-50% of the cortex >50% of the cortex Absent 1%-24% of the glomeruli 25% of glomeruli
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Figure 1. Lesions in which pathologists showed a high level of agreement in the RPS reproducibility study (periodic acid Schiff stain). (A) Mesangial hypercellularity with more than 4 nuclei surrounded by periodic acid Schiff positive matrix in a peripheral mesangial area. (B) Endocapillary hypercellularity with occlusion of capillary lumina by cells. (C) A fibrocellular crescent. (D) Segmental sclerosis with tuft adhesions.
hypercellularity, Endocapillary hypercellularity, Segmental Sclerosis, Tubular atrophy / interstitial fibrosis, and Crescents (MEST)-C.37 Representative glomerular lesions are shown in Figure 1. The Segmental Sclerosis Working Group addressed the issue of whether subclassification of segmental sclerosing lesions was of clinical value.38 Biopsy specimens from the original Oxford Classification cohort were reviewed and all segmental sclerosing lesions were assessed for various features. There was good to excellent reproducibility for the identification of glomerular tip lesions, podocyte hypertrophy, hyalinosis, and perihilar lesions. Those features (tip lesions and podocyte hypertrophy), which typically are seen in primary podocytopathies, were found to be associated with a higher level of initial proteinuria and worse renal survival in multivariate analysis. Patients whose biopsy specimens showed these changes had a better outcome if treated with steroids. It therefore was recommended that the report for biopsy specimens scored as S1 should include this additional information: segmental glomerulosclerosis with or without podocytopathic features.
TAGEDH1THE FUTURETAGEDN Prognostic Modeling The Prognostic Modeling Working Group’s aim is to optimize outcome prediction for individual patients by developing a risk prediction model that incorporates
histologic, clinical, and other data. An initial study,39 including more than 900 patients from three different cohorts (Oxford Classification, North American, and VALIGA), showed that a model based on a combination of MEST criteria and initial clinical data performed as well as 2-year clinical data in predicting the risk of reaching a combined end point of ESRD or 50% loss of eGFR. This earlier risk prediction was independent of treatment with renin-angiotensin system blockade or immunosuppression. The group now is refining the prediction model using accumulated data from several large IgAN patient cohorts in Asia and Europe. This was a large and detailed IgAN data set and will enable risk stratification according to ethnicity in both adults and children. The ultimate goal is to develop web-based and mobile application calculators that may be used in the management of individual patients.
Use of Renal Biopsy to Guide Therapy One of the central aims of the Oxford Classification study and the IIgANN/RPS working groups is to identify histologic lesions that are responsive to immunosuppression and thus use the biopsy in guiding therapeutic decisions. There is evidence from several of the large retrospective studies described earlier that endocapillary hypercellularity and crescents (E1 and C1/2) are associated with more rapid loss of renal function in the absence of immunosuppression, and patients with these lesions have a better outcome if treated with steroids. What is
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needed is confirmatory evidence from prospective randomized clinical trials (RCTs). Recent trials of standard steroid therapy40-42 and newer therapeutic approaches such as targeted-release budesonide43 have lacked either histologic data or analysis of the impact of histology on response to therapy. However, re-analysis of one RCT of steroid therapy to include the biopsy findings, scored using the Oxford Classification, showed that E1 was associated with a greater reduction in proteinuria in those patients treated with steroids.44 Repeat biopsy studies, with a second biopsy after immunosuppressive therapy, have shown that endocapillary hypercellularity, necrosis, and crescents are treatment-responsive lesions.45 There are a number of current and planned RCTs of newer therapeutic agents that include histologic analysis, and one with a second post-treatment biopsy. Data from these RCTs will clarify the role of histology in guiding therapy. IgA Vasculitis/HSP Nephritis The Oxford Classification of IgAN has not been validated in IgA vasculitis/HSP nephritis. Although HSP nephritis and IgAN share similar immunohistologic findings, HSP nephritis shows more proliferative, crescentic, and necrotizing lesions.46,47 One study has addressed the prognostic value of the Oxford Classification in HSP nephritis.48 In this study of 61 patients, endocapillary hypercellularity, tubular atrophy/interstitial fibrosis, and crescents in 50% or more of glomeruli were found to be predictive of an adverse renal outcome. An IIgANN/RPS Working Group currently is performing a retrospective study of more than 300 patients with HSP nephritis from North America, Asia, and Europe. This will determine whether an IgAN prediction model including MEST-C criteria predicts long-term renal outcome, and whether any histologic lesions are associated with therapeutic response.
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An online study of reproducibility of scoring glomerular lesions in IgAN among members of the RPS was performed in 2017 (Soares et al, unpublished data). Pathologists were provided with links to digital slides and a web-based response system through the RPS. This study included a training element; once pathologists had submitted their scores, they were provided with a summary of the responses from other pathologists around the world, and to the annotated slides, highlighting difficult lesions and providing advice on how to score them. The study identified certain areas of difficulty that included the scoring of endocapillary hypercellularity in the presence of segmental sclerosis, and in the distinction of cellular crescents from pseudocrescents associated with podocytopathic segmental sclerosing lesions (Fig. 2). The exercise will be repeated in 2018 to determine whether the training element has achieved convergence in pathologists’ scores. Interobserver variation in the interpretation of glomerular and other lesions in the renal biopsy specimen may be improved by providing clear lesion definitions and illustrated guidance. An RPS Working Group currently is developing consensus definitions that can be applied to any glomerular disease, not just IgAN.
Reproducibility of the Oxford Classification The scoring of histologic lesions that comprise the MEST-C criteria were found to be highly reproducible in the original Oxford Classification study among a small group of pathologists who worked closely together to resolve differences in the evaluation of problematic lesions. This does not mean, however, that the interobserver agreement will be similarly good between pathologists working in different units around the world who have not had the opportunity to discuss sources of difficulty. Data from the VALIGA study, in which biopsy specimens initially were scored by local pathologists from the 55 centers around Europe, before central scoring in Oxford, showed differences in the scoring of M and E lesions that had a significant impact on the clinical value of these scores.49
Figure 2. Lesions in which pathologists showed poor agreement in the RPS reproducibility study (periodic acid Schiff stain). (A) A segmental lesion in which capillaries are obliterated largely by increased matrix (S1). However, there is increased cellularity within this lesion, which many pathologists scored as endocapillary hypercellularity (E1). There was poor agreement for the E score in this biopsy specimen. (B) An extracapillary lesion that was interpreted variously as segmental sclerosis with overlying epithelial hypercellularity (pseudocrescent), and as a true inflammatory cellular crescent.
Pathology of IgAN
New Approaches to Biopsy Evaluation The poor reproducibility of the E score has led to a search for alternative methods for assessing inflammatory glomerular lesions in IgAN. Quantification of glomerular macrophages using immunohistochemistry for CD68 in an Oxford patient cohort has shown a close correlation between the maximum glomerular macrophage count and E score, with macrophage quantification being highly reproducible.50 CD68 immunohistochemistry to identify macrophages in extracapillary lesions is also a potential tool for differentiating true inflammatory crescents from pseudocrescents. There is growing evidence that the complement system mediates glomerular injury in IgAN and that immunohistochemistry for complement components might be used as a marker of disease activity. IgA activates complement through the alternative and lectin pathways. Genome-wide association studies in IgAN have identified three susceptibility loci at 1q32 containing the CFH and CFHR1-5 genes,51 and the ITGAM-ITGAX locus at 16p11 containing the complement receptor 4 gene, involved in leukocyte cell adhesion, migration, and complement phagocytosis.52 Genetic variants in CFH, CFHR3, and CFHR1 are associated with circulating C3 levels and mesangial C3 deposition.53 Mesangial deposition of mannose binding lectin and C4d, markers of the activation of the lectin pathway, is associated with more severe histologic lesions.54 Higher serum and glomerular levels of factor H related protein 5, shown by immunohistochemistry, are associated with progressive disease in IgAN.55 The potential clinical role of immunohistochemistry for complement components requires further study; in particular it remains to be determined whether it provides added value to the MEST-C criteria.
TAGEDH1CONCLUSIONSTAGEDN The value and acceptance of a histopathologic classification system is measured by its ease of use, reproducibility, and ability to provide clinically relevant information.56 Rather than being static, classification systems should be constantly reviewed and updated, with incorporation of new developments and knowledge as they emerge. This has always been the spirit of the Oxford Classification of IgA Nephropathy and will be led in the future by the IIgANN/RPS Working Groups. Studies over the past decade have identified those histologic lesions that are associated with progressive IgAN in the absence of immunosuppression, and shown which lesions are treatment-responsive. In the near future, methods for assessing renal biopsy specimens will be refined, accurate prediction models that incorporate clinical and histologic data will be developed, and the role
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of the renal biopsy in guiding standard therapy and the use of new therapeutic agents will be determined.
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