Abstract

Kidneys are often targets of systemic vasculitis (SVs), being affected in many different forms and representing a possible sentinel of an underlying multi-organ condition. Renal biopsy still remains the gold standard for the identification, characterization and classification of these diseases, solving complex differential diagnosis thanks to the combined application of light microscopy (LM), immunofluorescence (IF) and electron microscopy (EM). Due to the progressively increasing complexity of renal vasculitis classification systems (e.g. pauci-immune vs immune complex related forms), a clinico-pathological approach is mandatory and adequate technical and interpretative expertise in nephropathology is required to ensure the best standard of care for our patients. In this complex background, the present review aims at summarising the current knowledge and challenges in the world of renal vasculitis, unveiling the potential role of the introduction of digital pathology in this setting, from the creation of hub-spoke networks to the future application of artificial intelligence (AI) tools to aid in the diagnostic and scoring/classification process.

Introduction

Systemic vasculitis (SVs) is a rare autoimmune disease affecting vessels (< 100/million people), and are characterized by an overall dismal prognosis if untreated (one-year mortality rate > 80%) ¹. Timely diagnosis is crucial, although their infrequent occurrence and heterogeneity are sources of diagnostic delay. This reflects the variability of underlying pathogenesis (primary vs secondary, mainly drug-related) ², clinical manifestations, histological aspects ³ (Tab. I), organs and size of vessels involved, with frequent overlapping clinico-pathological features among different forms. To partly overcome this complexity, efforts have been made to simplify the nosology of SVs through the definition of a vessel-size-based classification approach (2012 International Chapel Hill Consensus Conference, CHCC ⁴, with the 2018 update) ⁵, dividing these entities in large (LVV), medium (MVV) and small vessels vasculitis (SVV, Tab. II). In this complex landscape, kidneys often represent the target and sentinel of an underlying SV, due to its high vascularity composed of different size arteries and an intricate capillary tangle represented by the glomeruli. Although clinical and serological features may help in orienting toward suspect renal vasculitis, morphological characterization of kidney damage still represents the gold standard for diagnosis, thanks to the complementary employment of light microscopy (LM), immunofluorescence (IF) and electron microscopy (EM) on renal biopsy. This combined clinical-pathological approach allows the sub-classification of renal vasculitis (mainly glomerulonephritis in SVVs) into antineutrophil cytoplasmic antibody (ANCA) associated vasculitis (AAV) and immune complex vasculitis (ICV), with necrotizing medium artery vasculitis in MVV (e.g. polyarteritis nodosa) or secondary ischemic changes in renal artery vasculitis in LVV being much less frequent ⁶.

In this review, the histological features of the different renal vasculitis are discussed, detailing the methodological diagnostic approach with appropriate clinico-pathological correlations, and reporting the possible differential diagnosis of these challenging entities. Finally, the impact of digital pathology implementation with the creation of ultra-specialized hub and spoke networks and the application of artificial intelligence (AI) tools to assist in the classification of these forms are discussed as future perspectives.

Diagnostic approach and pathology of renal vasculitis

The protean manifestations of renal vasculitis are strictly related to the underlying pathogenesis. Based on the patterns of injury (Tab. III), a schematic classification of renal vasculitis can include pauci-immune (generally ANCA-associated), immunoglobulin-mediated/anti-GBM, immune complex-associated, and medium vessel arteritis forms (Fig. 1). Different pathology characteristics based on LM, IF and EM findings can significantly help in differentiating these forms, as further discussed herein (Fig. 2).

PAUCIMMUNE GLOMERULONEPHRITIS AND ANCA-ASSOCIATED VASCULITIS

ANCA targets specific proteins within the cytoplasmic granules of neutrophils and monocytes ⁷, with two different forms recognized based on the serum immunofluorescence pattern, defined as cytoplasmic (cANCA) and perinuclear (pANCA). The former generally consists of anti-proteinase-3 (PR3) antibodies, a 29-kDa neutral serine proteinase found in neutrophil azurophilic granules ⁸, the second underlying anti-myeloperoxidase (MPO) antibodies directed against elastase, cathepsin G, lactoferrin, lysozyme, and bactericidal/permeability-increasing protein ⁹, with clinical and histological differences described depending on the underlying autoantibody (Tab. IV). Although the reasons for the development of these autoantibodies remain largely unclear, tumor necrosis factor α (TNF-α) or interleukin 1 (IL-1) mediated priming allows the interaction of ANCA with their targets in neutrophils and monocytes, resulting in degranulation, production of reactive oxygen species, and release of proinflammatory cytokines such as IL-1 and IL-8 with potential tissue damage ¹⁰. Based on clinical, laboratory, and histological features, different clinical presentations of AAV are recognized: granulomatosis with polyangiitis (GPA)¹¹, microscopic polyangiitis (MPA) ¹², eosinophilic granulomatosis with polyangiitis (Churg-Strauss) ¹³ and renal limited vasculitis ¹⁴. Patients with PR3-ANCA are more frequently male and younger ¹⁵. They also tend to have a lower risk of renal disease, with more active but less chronic renal lesions, to have ear, nose, throat or ocular involvement more frequently than MPO-ANCA patients. Moreover, PR3-ANCA is associated with a higher relapse risk, a higher rate of complete remission at 6 months if treated with rituximab, and better renal and global survival ^16-17.

Kidney disease is prevalent in AAV and is the most significant predictor of mortality. The typical renal presentation is called rapidly progressive glomerulonephritis (RPGN), characterized by a sudden decline in kidney function, subnephrotic range proteinuria, microscopic hematuria, and hypertension over days to a few months. Its combination with pulmonary haemorrhage configures the pulmorenal syndrome, with high mortality if not promptly treated ¹⁸. Individuals with glomerular filtration rates (GFRs) below 50 mL/min have a 50% chance of death or kidney failure within 5 years ¹⁹. For these reasons, the Canadian Vasculitis Research Network (CanVasc) ²⁰ and the European League Against Rheumatism (EULAR/ERA-EDTA) ²¹ recommendations for the management of AAV recommend renal biopsy at diagnosis and at relapse, when possible, representing the gold standard for the diagnosis of renal AAV ²². Kidney biopsy typically reveals a pauci-immune focal necrotizing crescentic GN without significant immune complex deposition on immunofluorescence or electron microscopy. On light microscopy, active AAV typically presents extracapillary hypercellularity configuring cellular or fibrocellular crescents eventually associated with GBM and Bowman capsule rupture and fibrinoid necrosis (Fig. 3) ²³. Later in the disease, glomerular chronicity can manifest as segmental sclerosis with broad base adhesion to the Bowman capsule and the formation of fibrous crescents, finally leading to global sclerosis in the “fragmented’ pattern. Usually, AAV does not show significant endocapillary or mesangial hypercellularity, but frequent interstitial inflammation is noted, with mixed lymphocyte and plasma cell rich infiltrates with occasional polymorphonuclear cells (especially eosinophils in EGPA forms) ²⁴ and aggregates of epithelioid hystiocytes in the form of small non-necrotizing granulomas, often associated with the disruption of Bowman capsule. Finally, transmural necrotizing arteritis, eventually associated with peri-arterial granulomas, can be seen in a minority of cases ²⁵. Although the necrotizing crescentic GN is typically pauci-immune in up to 43-57% cases, traces of IgG or C3 may be detected by immunofluorescence within mesangial spaces and/or along the glomerular capillary walls ²⁶, sometimes corresponding to small and isolated electron dense deposits at electron microscopy in the same regions ²⁷. As reported by a recent Japanese study, ANCA-GN patients with glomerular C3 deposition on IF had worse renal and overall survival rates ²⁸, therefore the pathologist must draw attention to describe C3 positivity, even if of mild intensity, in renal biopsy, also in consideration of the new therapeutic approaches that selectively target different complement pathways. Light microscopy characteristics have been advocated as surrogate markers to stratify the prognosis of patients with AAV. In particular, Berden et al. proposed a four-tiered classification based on the prevalence of glomeruli with no lesions, sclerosis, crescents or mixed lesions (focal, sclerotic, crescentic, and mixed, respectively) ²². Although subsequent meta-analyses confirmed the prognostic role of this classification, further efforts have been made to improve these classification systems ²⁹. Among them, the recently proposed Renal Risk Score by Brix et al. merged clinical and histological information to obtain a reliable stratification of patients’ prognosis ³⁰. The presence of Bowman capsule rupture (BCR) to these systems further improved the risk prediction ^31-32. In cases of ANCA-associated vasculitis renal medulla becomes increasingly important because one can observe necrotizing medullary capillaritis and tubular red blood cells and RBC casts with acute tubular necrosis (Fig. 4). The presence of these medullary features, especially in cases of suboptimal renal biopsy with insufficient cortical tissue, should strongly suggest a diagnosis of ANCA associated vasculitis.

As previously reported by Raissian et al. 32% of pauci-immune GN showed an interstitial infiltrate with at least moderately increased IgG4 positive plasma cells ³³ (Fig. 5), and thus the pathologist should pay attention to report this peculiar plasma cell-rich infiltrate in the absence of storiform fibrosis, without considering it a IgG4-related tubulointerstitial nephritis. Renal biopsy findings can guide the management of patients, which should be based as well on the clinical picture, especially in cases with extrarenal involvement. Timely initiation of therapy is critical to prevent progression to end stage kidney disease (ESKD), evaluating treatment approaches on different clinical features (e.g. age, ANCAs phenotype, eGFR, histological features)³⁴. The value of ANCA in patients monitoring is controversial, since patients can have negative serology with active GPA and antibody titers do not necessarily correlate with disease activity, as demonstrated by persistently high titers after clinical remission ³⁵. PR3- ANCA increase during serial monitoring has been demonstrated to predict relapse, but diminished antibody levels are not a clear indicator of response to therapy ^36-37. Remission is better defined as stabilization or improvement in eGFR or serum creatinine level (Scr), and Scr level and recurrence of hematuria may help in the follow-up of AAV patients. The likelihood of possible active lesions in renal biopsies in patients without hematuria is very low and non-nephrotic proteinuria and microscopic hematuria may persist even in patients in stable remission and could be permanent due to structural damage from vasculitis. Therefore, disease progression or response to treatment cannot be determined by hematuria or proteinuria alone ³⁸.

IMMUNE COMPLEX-ASSOCIATED VASCULITIS

Immune complex-associated vasculitidies are characterized by the deposition of immunoglobulins and complement within vessel walls, preferentially of small caliber and capillaries, making kidneys among the most involved organs. According to the CHCC, disorders included in this group are IgA vasculitis (IgAV), cryoglobulinemic vasculitis (CV), and hypocomplementemic urticarial vasculitis ⁴.

IgA vasculitis (formerly known as Henoch-Schoenlein Purpura) is characterized by circulation and tissue deposition of abnormal IgA immunoglobulins. IgAV is the most common vasculitis in childhood with an annual incidence of 13-20 cases per 100,000 children and a peak incidence in autumn and winter, the onset being generally anticipated by a respiratory tract infection. In adults, IgAV occurs less frequently with an annual incidence of 0.8-1.8 per 10,000 and no seasonal correlation ³⁹. IgAV may present as a single-organ disease or as a systemic disease with multiorgan involvement, with renal-limited forms representing a challenging differential diagnosis with the most common IgA nephropathy (IgAN). The “classic triad” of organ involvement in IgAV includes skin involvement with palpable purpura usually in lower extremities, arthralgia, and gastrointestinal manifestations. Renal involvement is frequent, occurring in 45-85% of adult patients with IgAV, and is the most important independent prognostic factor, with up to 30% of adult patients progressing to ESKD and generally presenting with microscopic hematuria and subnephrotic proteinuria with or without renal function impairment ³⁹. Especially in adults, renal biopsy is mandatory to distinguish this relatively rare entity from the possible mimickers (e.g. IgAN or other vasculitis). On LM, IgAV can show the same protean manifestations of IgAN, ranging from mesangial hypercellularity and matrix expansion, endocapillary hypercellularity and crescents with fibrinoid necrosis in the active phases, glomerulosclerosis and tubulointerstitial fibrosis during the chronic progression ⁴⁰. Since LM features are quite unspecific, the diagnostic hallmark is represented by IF finding of dominant/co-dominant granular mesangial deposition of IgA, often associated with C3 but less frequent co-deposition of IgG. Electron microscopy may reveal electron dense deposits within mesangial and subendothelial spaces. In contrast to IgAN, the MEST-C score (Oxford classification) has not yet been proven to be prognostically relevant in the setting of IgAV, but significant efforts are directed towards the identification of prognostic scores in this rare renal vasculitis ⁴¹.

Cryoglobulinemic vasculitis (CV) is a small medium-vessel vasculitis due to cryoglobulins (immune complexes that precipitate in vitro at temperatures below 37°C and dissolve upon rewarming) ⁴². A classification system based on their composition has been proposed:

1. Type I (10-15%): monoclonal immunoglobulins, usually IgG or IgM, found in lymphoproliferative disorders (Waldenstrom’s macroglobulinemia, multiple myeloma, and monoclonal gammopathy of unknown significance, MGUS) ⁴³.
2. Type II (50-60%): usually correlated with hepatitis C virus (HCV) infection and are composed of monoclonal IgM with rheumatoid factor (RF) activity in association with polyclonal immunoglobulins (usually IgG); other causes include hepatitis B virus (HBV) or human immunodeficiency virus (HIV) infections, autoimmune diseases, and lymphoproliferative disorders.
3. Type III (25-30%): polyclonal IgM and IgG with RF activity and polyclonal, generally linked to autoimmune disorders and infections, mainly due to HCV.

Types II and III are associated with mixed cryoglobulinemia (MC) syndrome, accounting for about 75% of cases ⁴⁴. Laboratory findings also generally include hypocomplementemia, both for C3 and C4 fragments. Skin, joints, peripheral nerves, and kidneys are most frequently affected. Renal damage is present in 20-35% of patients at the time of diagnosis, and 10-35% present microscopic hematuria, mild proteinuria, and hypertension. Nephrotic or nephritic syndrome is present in 22% and 14% of cases, respectively. Eighty percent of patients also have extrarenal manifestations such as weakness, purpura, and arthralgia (Meltzer’s triad) ⁴⁵. Renal biopsy can help in the diagnosis of renal involvement, in the differentiation of the different types of CV and in distinguishing this disease from mimickers (e.g. lupus) ⁴⁶. On LM, a membranoproliferative glomerulonephritis (MPGN) pattern is characteristic, with lobular appearance of the tuft due to diffuse and global severe mesangial hypercellularity associated with endocapillary hypercellularity and duplication of the GBM at silver stain (double contours). Occasionally, subendothelial and intracapillary globular intensively PAS-positive deposits can be seen (so-called cryoplugs), whose staining characteristics may suggest a high content in IgM immunoglobulins. Extracapillary proliferation and crescents can be seen but are rare. In approximately 30% of the cases vasculitic lesions are described with vascular PAS-positive deposits, endoluminal accumulation of lymphocytes and fibrinoid necrotizing changes in more advanced stages of the disease ⁴⁷. Findings in IF may reflect the composition of the cryoglobulins depending on the clinical type, with IgG and/or IgM showing smudgy and coarsely granular positivity of different intensity, with co-deposition of C3 and polytypic or monotypic (most frequently kappa) light chains ⁴⁸. Electron microscopy reveals peculiar electron dense deposits located mainly in mesangial, subendothelial and intracapillary spaces, with characteristic substructure organisation in 12-20 nm randomly arranged and non-branching fibrils alternated with 20-35 nm microtubules ⁴⁹.

Hypocomplementemic urticarial vasculitis syndrome (HUVS) is a rare SVV associated with urticaria, hypocomplementemia and positivity of anti-C1q antibodies. In rare instances, HUVS can manifest as an immune-complex mediated glomerulonephritis with a membranoproliferative pattern of injury: a rapidly progressive form of glomerulonephritis with severe nephrotic syndrome due to a membranoproliferative pattern of glomerular injury with extensive crescent formation ⁵⁰.

ANTI-GBM GLOMERULONEPHRITIS AND GOODPASTURE SYNDROME

Anti-glomerular basement membrane (anti-GBM) disease is due to the formation of autoantibodies specifically directed against the NC1 domain of the α3 chain of type IV collagen, an antigen found in the GBM and alveolar basement membranes of the lung ⁵¹. As such, the disease can either affect kidneys or lung independently, or present as a pulmorenal syndrome also known as Goodpasture’s syndrome ⁵². Genetic landscape, mainly in the form of major histocompatibility complex (MHC) haplotypes, may predispose to (e.g. HLA-DR15 and HLA-DR4) or protect (HLA-DR1 and HLA-DR7) from the disease ⁵³, and external triggers (infection vs urinary tract obstruction/nephrolithiasis) are thought to elicit the development of autoantibodies ⁵⁴. RPGN is the most frequent renal presentation (80-90%), oligo-anuria can develop within a few days, while a slower deterioration of kidney function is observed in a smaller group of patients. Microscopic hematuria of glomerular origin and red blood cell casts are the most prominent features, with macrohematuria being rarer ⁵⁵. Both pulmonary and renal involvement occurs in 60-80% of the patients and pulmonary involvement may precede renal disease by weeks to months, whereas exclusive kidney disease is present in around 20-40% of cases. Other systemic symptoms are infrequent and suggest the coexistence of AVV ⁵⁶. Notably ANCA antibodies, mainly MPO, are found in 10-38% of patients with anti-GBM disease (“double positive”) ⁵⁷. Diagnosis is mainly based on the detection of circulating anti-GBM antibodies, although atypical cases can be characterized by more subtle clinical courses and the absence of circulating autoantibodies ⁵⁸. The formation of crescents in the absence of significant endocapillary/mesangial hypercellularity at LM is the histopathological hallmark of anti-GBM disease ⁵⁹. According to a large biopsy series, crescents are present in up to 95% of kidney biopsies with > 50% of glomeruli affected in the majority, and the degree of renal impairment at presentation correlating with the proportion of affected glomeruli⁶⁰. Crescents are usually uniform and temporally homogeneous, showing the same stage of activity and chronicity, conversely to what happens in AAV showing mixture of cellular, fibrocellular, and fibrous crescents of varying ages and activity levels ⁶¹. Fibrinoid necrosis is likely to occur in the underlying glomerular tuft of crescentic glomeruli. In severe disease due to rupture of Bowman’s capsule, peri-glomerular inflammation, progressing to granuloma formation, may be observed. Given the acuity of disease onset, interstitial fibrosis and tubular atrophy are uncommon in anti-GBM disease though interstitial inflammation may be observed. Immunofluorescence typically shows diffuse and global linear positivity for IgG (polytypic) and C3, the latter showing a granular pattern in most cases (75%). In case of scant/insufficient tissue for dedicated fresh/frozen IF, anti-GBM is one of the few glomerulonephritis that cannot be diagnosed on pronase IF due to lack in sensitivity ⁶². Electron microscopy can show GBM interruption, necrosis, and crescents, but no deposits are usually seen.

MEDIUM AND LARGE VESSEL VASCULITIS INVOLVING THE KIDNEY

Medium and large vessel vasculitis only rarely involve the renal parenchyma. Polyarteritis nodosa (PAN) is a necrotizing vasculitis generally involving medium-sized arteries, typically presenting with gastrointestinal, skin, articular and muscle manifestations, but occasionally affecting the kidney by definition without glomerulonephritis. Histology can show segmental fibrinoid necrosis with mural and perivascular infiltration of leukocytes, in the most severe form potentially causing cortical necrosis or parenchymal infarction ⁶. In time the acute arteritis evolves into a sclerotic process with fibrosis of the arterial wall, leading to renovascular hypertension. Even rarer is a renal presentation of Kawasaki disease ⁶³, whereas giant cell arteritis (GCA) and Takayasu arteritis (TA) usually affect the aorta and its major branches, with renal arteries being involved in 8-25% of cases ⁶⁴. Although granulomatosis of renal arteries has been documented without evident clinical manifestations, renal impairment can be present even if not adequately documented by histology ⁶⁵. The most frequent histological presentation is mainly ischemic changes to the renal parenchyma with partial collapse of the tuft to the vascular pole, increase of the Bowman space and microtubular modifications without appreciable fibrosis, mimicking a picture of renal artery stenosis..

Renal vasculitis pitfalls and differential diagnosis

The diagnosis of renal vasculitis can be challenging from clinical and histological points of view. Clinically, vasculitis is generally suspected in patients with systemic or constitutional symptoms in combination with single/multiorgan dysfunction such as palpable purpura, pulmonary infiltrates, microscopic hematuria, chronic inflammatory sinusitis, mononeuritis multiplex, unexplained ischemic events, and renal impairment or urinary abnormalities. Recognizing the complexities in diagnosing vasculitis is imperative due to the overlapping clinical presentations and the varying timeframes in which vessels of different calibers may be involved in the inflammatory disease. For these reasons, a correct correlation between histopathological and clinical aspects is fundamental in the diagnostic approach. Serology can often help in directing the clinical suspicion, although PR3-ANCA or MPO-ANCA can be found in a range of non-vasculitic conditions, especially in chronic infections such as endocarditis ⁶⁶, tuberculosis ⁶⁷, HIV ⁶⁸ and bartonellosis ³⁵. Since misdiagnosis of sub-acute infection as AAV and initiation of inappropriate immunosuppressive therapy can have devastating consequences, renal biopsy can be of help to distinguish among possible clinical mimickers. Autoimmune and other systemic diseases can have positive ANCA (e.g. rheumatoid arthritis ⁶⁹ or systemic lupus erythematosus) ⁷⁰ potentially involving the kidney as well, although the difference in LM, IF and EM findings can quite easily allow the differential diagnosis with paucimmune GN. Finally, 10% of patients with clinical features and pathology consistent with AAV are negative for ANCA. Although these patients may have a similar clinical course and response to treatment, they are more likely to have renal-limited disease or less severe systemic disease ⁷¹. Histological lesions of vasculitis may also be detected in other conditions, such as infections, neoplasms, drug reactions, or vasculopathies ⁷². Some of the ICV involving the kidney can represent challenging differential diagnosis with other common or rare IC-mediated glomerulonephritis unrelated to SV. This is the example of IgA-dominant infection related GN, a form often correlated with infections sustained by S. aureus especially from skin ulcers of elderly diabetic patients ⁷³. This entity typically shows segmental to global endocapillary hypercellularity with neutrophils and crescents, diffuse/global granular mesangial positivity for IgA (usually polytypic), with almost invariably present C3 deposition and mesangial/subendothelial deposits under electron microscopy, with peculiar dome-shaped, hump-like subepithelial electron dense deposits pointing at the infection-related nature of this disease (Fig. 6). The LM, IF and EM features allow the distinction of this entity from the more commonly encountered IgAN and IgAV, eventually aided by the clinical evidence of recent S. aureus infection and hypocomplementemia, avoiding useless and counterproductive immunosuppressive therapies. The distinction of cryoglobulin MPGN from other mimickers is based on the peculiar histological characteristics detailed in the previous section and on the clinical features (positive cryoprecipitate and hypocomplementemia for C3 and C4, with underlying lymphoproliferative/infectious or autoimmune disease). In detail, GN showing similar pattern of damage can be lupus nephritis in its diffuse/global proliferative form ⁷⁴, proliferative glomerulonephritis with monoclonal IgG deposits (PGMID) ⁷⁵, C3 glomerulonephritis ⁷⁶ and fibrillary GN ⁷⁷, this latter even showing structured deposits at EM with overlapping features with cryo plugs and occasionally having a DNAJB9 negative, monotypic nature, further complicating the differential diagnosis ⁷⁸. Unlike other organs/districts involved by AAV, in the kidney the detection of necrotizing arteritis is quite rare, accounting for about 10-15% of cases, and is not considered a required diagnostic criteria for the final diagnosis ⁷⁹. Although rare, recent evidence is pointing to a substantial underestimation of this finding, with possible significant negative prognostic role for necrotizing arteritis in the setting of ANCA-associated pauci-immune glomerulonephritis ⁸⁰, especially if associated with other established histological prognostic factors ⁸¹. The presence of transmural necrotizing arteritis affecting medium/large sized vessels, especially in the absence of crescentic vasculitis on renal biopsy, should strongly suggest a diagnosis of MVVs, mainly PAN, which can occasionally overlap with circulating ANCA (mainly MPO) positivity (Fig. 7).

Future perspectives and digital pathology

The rising complexity of this renal diagnostic field is progressively stressing the need for reliable expertise both from a technical and interpretative points of view for nephropathology centers. The creation of hub-spoke networks, facilitated by the introduction of digital pathology tools ⁸², is promising to improve the diagnostic output of our departments, increasing the quality of care for patients with renal vasculitis. The technological/instrumentation improvements ⁸³ and the increasing capabilities of artificial intelligence (AI) and computational softwares is further revolutionizing the nephropathology field ⁸⁴. In this setting, AI tools already proved to be able to reliably classify different glomerular lesions (e.g. mesangial/endocapillary hypercellularity, sclerosis and crescents), promising a significant help in the identification of elementary features of many GN. Although this is already a reality in some forms (e.g. MEST-C Oxford classification for IgAN) ⁸⁵, the development of robust assisting tools for renal vasculitis (e.g. ANCA-associated) ⁸⁶ is still lagging behind.

Conclusion

The diagnostic approach to renal vasculitis is based on a careful integration of clinical, laboratory and pathology data. Renal histology is heterogeneous and the complementary application of all the available nephropathology techniques, along with adequate expertise and the future assistance of digital pathology and AI tools, can help in extricating from complex differential diagnosis.

CONFLICTS OF INTEREST

The authors declare no conflicts of interest.

FUNDING

None.

AUTHORS’ CONTRIBUTIONS

VL and SC performed the review of the available literature and drafted the manuscript. AB provided iconography for the review. AB, BC and FP performed the critical revision of the first draft. All authors were involved in writing the paper and had final approval of the submitted and published versions.

ETHICAL CONSIDERATION

The research was conducted ethically, with all study procedures being performed in accordance with the requirements of the World Medical Association’s Declaration of Helsinki.

Figures and tables

Figure 1.Differences in the pathogenesis of renal vasculitis on the different groups of disease recognized and the relative structural and immune features that characterise the pathology of these forms. GBM, glomerular basement membrane.

Figure 2.Schematic diagnostic approach to the pathology of renal vasculitis.MPGN, membranoproliferative glomerulonephritis; GBM, glomerular basement membrane; MVV, medium vessel vasculitis; PAN, polyarteritis nodosa; AAV, ANCA-associated vasculitis; IgAV, IgA vasculitis; CV, cryoglobulinemic vasculitis; LN, lupus nephritis; TRI, tubuloreticular inclusions.

Figure 3.Spectrum of age of crescents in ANCA-associated vasculitis ranging from cellular crescents (A: PAS 40X; B: Trichrome stain 40X) with large area of fibrinoid necrosis (yellow circle, C: AFOG stain 40; D: PTAH stain 40X) tofibrocellular (E: PAS 40X; F: Trichrome stain 40X) and fibrous ones (G: PAS 20X; H: Trichrome stain 20X). Special stains like PTAH and AFOG highlight the presence of fibrinoid necrosis, respectively with an orange and blue color.PAS, periodic acid of Schiff; PTAH, Phosphotungstic acid haematoxylin; AFOG, Acid Fuchsin Orange G.

Figure 4.Additional morphologic features in systemic vasculitis: PAS staining (A-B) demonstrated examples of medullary capillaritis with a moderate neutrophilic interstitial infiltrate in a case of a patient with p-ANCA positive paucimmune glomerulonephritis. Trichrome staining (C-D) highlighted RBC casts with associated acute tubular necrosis in biopsies with ANCA-associated vasculitis.

Figure 5.Male patient, 66 years old, affected by lung squamous carcinoma. One year later the patient was admitted to the emrgency room for fatigue, malaise, nausea, and vomiting. Laboratory tests showed anaemia, subnephrotic proteinuria (1400 mg/24h), microhematuria and rapidly progressive decrease of renal function (serum creatinine from 1.3 to 8 mg/dl), normal complement levels and double positivity for pANCA and anti GBM. Renal biopsy demonstrated diffuse epithelial crescents with large areas of fibrinoid necrosis with a strong linear politypic positivity for IgG on immunofluorescence related to anti-glomerular basement disease. There was also a severe plasmacellular interstitial infiltrate until 70 elements/hpf (A-B: PAS staining) with a marked increase of IgG4 positive plasma cells (C: IgG immunohistochemistry; D: IgG4 staining) with a maximum of 48 IgG4 positive plasma cells/hpf without storiform fibrosis, correlated to associated p-ANCA positivity.

Figure 6.A challenging case in differential diagnosis with ICV. A male patient in his 60s with previous lung cancer under chemotherapy treatment recently suspended for skin rash, presented with acute kidney injury (AKI, serum creatinine from 1.1-1.4 to 2.3 mg/dl), microhematuria of glomerular origin and subnephrotic proteinuria (2.8 g/24h). A renal biopsy was performed, showing on LM (A) interruption of the Bowman space due to the accumulation of cells in the extracapillary space (H&E, 40X, left), causing the formation of cellular crescents associated with GBM break and fibrinoid necrosis (asterisk, JMS, 40X, center) and focal but global endocapillary hypercellularity (PAS, 40X, right). Immunofluorescence (B) showed global and diffuse granular positivity, mainly in the mesangial space and segmental within capillary loops, to IgA (3+), C3 (2+), with immunoglobulin light chains being lambda > kappa (1/2+ vs trace) and fibrinogen (3+) within fibrinoid necrosis areas. These features, along with the recent history of skin rashes, may be indicative of a form of IgA vasculitis (e.g. Schonlein-Henoch purpura). Electron microscopy (C) was performed in this case, confirming the presence of medium-sized electron dense deposits within the mesangial space (black arrowhead, left, 3400X), but segmental subepithelial deposits with peculiar dome shape were noted (white asterisk, right, x10,500), compatible with subepithelial humps, which are more common in infection-related glomerulonephritis. Additional clinical data available after EM was performed (S. aureus isolation from the skin rash/ulcers, C3 hypocomplementemia 71 mg/dl with normal C4 29 mg/dl) allowed the final correct diagnosis of IgA-dominant infection related glomerulonephritis.ICV, immune complex vasculitis; H&E, Hematoxylin and Eosin; GBM, glomerular basement membrane; JMS, Jones methenamine silver stain; PAS, periodic acid of Schiff.

Figure 7.Male patient in his 60s presenting to the emergency room with fatigue and malaise. Laboratory tests demonstrated anemia, subnephrotic proteinuria (700 mg/24h), microhematuria and rapidly progressive decrease in renal function (serum creatinine from 0.8 to 2 mg/dl), normal complement levels and positivity to pANCA (MPO 347 IU/ml). Renal biopsy was performed, showing no significant abnormalities at the glomerular level (A, from left to right: PAS, JMS, and Masson trichrome stains, 20X), with immunofluorescence being negative for all antisera. Medium/large arteries showed transmural inflammation with massive fucsinophilic and pale/eosinophilic material at Masson and JMS stains (black arrowheads, B, left and right, 20X, respectively), compatible with fibrinoid necrosis, for which a diagnosis of necrotizing arteritis of medium/large sized vessels was made. The subsequent development of skin rash, arthralgias, muscle deafness and abdominal pain, along with the prevalent arterial involvement at renal biopsy with substantial glomerular sparing, pointed to the final clinico-pathological diagnosis of polyarteritis nodosa.

References

1. Scott DGI, Watts RA. Epidemiology and clinical features of systemic vasculitis. Clin Exp Nephrol. 2013; 17(5):607-10. DOI
2. Savage CO, Harper L, Cockwell P. ABC of arterial and vascular disease: vasculitis. BMJ. 2000; 320(7245):1325-8. DOI
3. Lie JT. Illustrated histopathologic classification criteria for selected vasculitis syndromes. American College of Rheumatology Subcommittee on Classification of Vasculitis. Arthritis Rheum. 1990; 33(8):1074-87. DOI
4. Jennette JC, Falk RJ, Bacon PA. 2012 revised International Chapel Hill Consensus Conference Nomenclature of Vasculitides. Arthritis Rheum.. 2013; 65(1):1-11. DOI
5. Sunderkötter CH, Zelger B, Chen KR. Nomenclature of Cutaneous Vasculitis: Dermatologic Addendum to the 2012 Revised International Chapel Hill Consensus Conference Nomenclature of Vasculitides. Arthritis Rheumatol. 2018; 70(2):171-84. DOI
6. Jennette JC, Falk RJ. The pathology of vasculitis involving the kidney. Am J Kidney Dis. 1994; 24(1):130-41. DOI
7. Lai KN, Leung JC, Rifkin I. Effect of anti-neutrophil cytoplasm autoantibodies on the intracellular calcium concentration of human neutrophils. Lab Invest. 1994; 70(2):152-62.
8. Niles JL, Pan GL, Collins AB. Antigen-specific radioimmunoassays for anti-neutrophil cytoplasmic antibodies in the diagnosis of rapidly progressive glomerulonephritis. J Am Soc Nephrol. 1991; 2(1):27-36. DOI
9. Jennette JC, Wilkman AS, Falk RJ. Diagnostic predictive value of ANCA serology. Kidney Int. 1998; 53(3):796-8. DOI
10. Prendecki M, Pusey CD. Recent advances in understanding of the pathogenesis of ANCA-associated vasculitis. F1000Res [Internet]. 2018; 7DOI
11. Van der Woude FJ, Rasmussen N, Lobatto S. Autoantibodies against neutrophils and monocytes: tool for diagnosis and marker of disease activity in Wegener’s granulomatosis. Lancet. 1985; 1(8426):425-9. DOI
12. Falk RJ, Jennette JC. Anti-neutrophil cytoplasmic autoantibodies with specificity for myeloperoxidase in patients with systemic vasculitis and idiopathic necrotizing and crescentic glomerulonephritis. N Engl J Med. 1988; 318(25):1651-7. DOI
13. Sablé-Fourtassou R, Cohen P, Mahr A. Antineutrophil cytoplasmic antibodies and the Churg-Strauss syndrome. Ann Intern Med. 2005; 143(9):632-8. DOI
14. Yamaguchi M, Ito M, Sugiyama H. Association between renal-limited vasculitis and relapse of antineutrophil cytoplasmic antibody-associated vasculitis: A single-center retrospective cohort study in Japan. PLoS One.. 2022; 17(9):e0274483. DOI
15. Murosaki T, Sato T, Akiyama Y. Difference in relapse-rate and clinical phenotype by autoantibody-subtype in Japanese patients with anti-neutrophil cytoplasmic antibody-associated vasculitis. Mod Rheumatol.. 2017; 27(1):95-101. DOI
16. Mahr A, Katsahian S, Varet H. Revisiting the classification of clinical phenotypes of anti-neutrophil cytoplasmic antibody-associated vasculitis: a cluster analysis. Ann Rheum Dis. 2013; 72(6):1003-10. DOI
17. Windpessl M, Bettac EL, Gauckler Pha D. ANCA Status or Clinical Phenotype - What Counts More?. Curr Rheumatol Rep.. 2021; 23(6):37. DOI
18. Vanoli J, Riva M, Vergnano B. Granulomatosis with polyangiitis presenting with diffuse alveolar hemorrhage requiring extracorporeal membrane oxygenation with rapid multiorgan relapse: A case report. Medicine.. 2017; 96(13):e6024. DOI
19. Ni A, Chen L, Huang X. The risk factors for early mortality and end-stage renal disease in anti-neutrophil cytoplasmic antibody-associated glomerulonephritis: experiences from a single center. Clin Exp Med. 2021; 21(3):389-97. DOI
20. Mendel A, Ennis D, Go E. CanVasc Consensus Recommendations for the Management of Antineutrophil Cytoplasm Antibody-associated Vasculitis: 2020 Update. J Rheumatol. 2021; 48(4):555-66. DOI
21. Yates M, Watts RA, Bajema IM. EULAR/ERA-EDTA recommendations for the management of ANCA-associated vasculitis. Ann Rheum Dis. 2016; 75(9):1583-94. DOI
22. Berden AE, Ferrario F, Hagen EC. Histopathologic classification of ANCA-associated glomerulonephritis. J Am Soc Nephrol. 2010; 21(10):1628-36. DOI
23. Fogo AB, Lusco MA, Najafian B, Alpers CE. AJKD Atlas of Renal Pathology: Pauci-immune Necrotizing Crescentic Glomerulonephritis. Am J Kidney Dis. 2016; 68(5):e31-2. DOI
24. Reggiani F, L’Imperio V, Calatroni M. Renal involvement in eosinophilic granulomatosis with polyangiitis. Front Med. 2023; 10:1244651. DOI
25. Emmi G, Bettiol A, Gelain E. Evidence-Based Guideline for the diagnosis and management of eosinophilic granulomatosis with polyangiitis. Nat Rev Rheumatol. 2023; 19(6):378-93. DOI
26. Wang R, Wu Y, Zhang X. Clinicopathological Characteristics and Influencing Factors of Renal Vascular Lesions in Anti-neutrophil Cytoplasmic Autoantibody-Related Renal Vasculitis. Front Med. 2021; 8:710386. DOI
27. Haas M, Eustace JA. Immune complex deposits in ANCA-associated crescentic glomerulonephritis: a study of 126 cases. Kidney Int. 2004; 65(6):2145-52. DOI
28. Oba R, Kanzaki G, Sasaki T. Long-Term Renal Survival in Antineutrophil Cytoplasmic Antibody-Associated Glomerulonephritis with Complement C3 Deposition. Kidney Int Rep. 2021; 6:2661-2670. DOI
29. Sanchez-Alamo B, Schirmer JH, Hellmich B, Jayne D, Monti S, Tomasson G. Systematic literature review informing the 2022 update of the EULAR recommendations for the management of ANCA-associated vasculitis (AAV): Part 2 - Treatment of eosinophilic granulomatosis with polyangiitis and diagnosis and general management of AAV. RMD Open [Internet].. 2023; 9(2)DOI
30. Brix SR, Noriega M, Tennstedt P. Development and validation of a renal risk score in ANCA-associated glomerulonephritis. Kidney Int.. 2018; 94(6):1177-88. DOI
31. L’Imperio V, Vischini G, Pagni F, Ferraro PM. Bowman’s capsule rupture on renal biopsy improves the outcome prediction of ANCA-associated glomerulonephritis classifications. Ann Rheum Dis.. 2022; 81(6):e95. DOI
32. Solmaz D, Akar S. Response to: “Bowman’s capsule rupture on renal biopsy improves the outcome prediction of ANCA-associated glomerulonephritis classifications’ by L’Imperio. Ann Rheum Dis.. 2022; 81(6):e96. DOI
33. Raissian Y, Nasr SH, Larsen CP, Colvin RB, Smyrk TC. Diagnosis of IgG4-Related Tubulointerstitial Nephritis. J Am Soc Nephrol. 2011; 22:1343-1352. DOI
34. Chung SA, Langford CA, Maz M. 2021 American College of Rheumatology/Vasculitis Foundation Guideline for the Management of Antineutrophil Cytoplasmic Antibody-Associated Vasculitis. Arthritis Rheumatol. 2021; 73(8):1366-83. DOI
35. Bossuyt X, Cohen Tervaert JW, Arimura Y, lores-Suárez LF. Position paper: Revised 2017 international consensus on testing of ANCAs in granulomatosis with polyangiitis and microscopic polyangiitis. Nat Rev Rheumatol. 2017; 13(11):683-92. DOI
36. Karras A, Pagnoux C, Haubitz M. Randomised controlled trial of prolonged treatment in the remission phase of ANCA-associated vasculitis. Ann Rheum Dis. 2017; 76(10):1662-8. DOI
37. Walsh M, Flossmann O, Berden A. Risk factors for relapse of antineutrophil cytoplasmic antibody-associated vasculitis. Arthritis Rheum. 2012; 64(2):542-8. DOI
38. Vandenbussche C, Bitton L, Bataille P. Prognostic Value of Microscopic Hematuria after Induction of Remission in Antineutrophil Cytoplasmic Antibodies-Associated Vasculitis. Am J Nephrol. 2019; 49(6):479-86. DOI
39. Jauhola O, Ronkainen J, Koskimies O. Clinical course of extrarenal symptoms in Henoch-Schonlein purpura: a 6-month prospective study. Arch Dis Child. 2010; 95(11):871-6. DOI
40. Fogo AB, Lusco MA, Najafian B, Alpers CE. AJKD Atlas of Renal Pathology: IgA nephropathy. Am J Kidney Dis. 2015; 66(5):e33-4. DOI
41. Barbour SJ, Coppo R, Er L. Histologic and Clinical Factors Associated with Kidney Outcomes in IgA Vasculitis Nephritis. Clin J Am Soc Nephrol [Internet].. 2024. DOI
42. Brouet JC, Clauvel JP, Danon F, Klein M, Seligmann M. Biologic and clinical significance of cryoglobulins. A report of 86 cases. Am J Med. 1974; 57(5):775-88. DOI
43. L’Imperio V, Cazzaniga G, Vergani B. Monoclonal Gammopathy of Renal Significance: A Molecular Middle Earth between Oncology, Nephrology, and Pathology. Kidney Dis (Basel). 2022; 8(6):446-57. DOI
44. Saadoun D, Sellam J, Ghillani-Dalbin P. Increased risks of lymphoma and death among patients with non-hepatitis C virus-related mixed cryoglobulinemia. Arch Intern Med. 2006; 166(19):2101-8. DOI
45. Ferri C, Sebastiani M, Giuggioli D. Mixed cryoglobulinemia: demographic, clinical, and serologic features and survival in 231 patients. Semin Arthritis Rheum. 2004; 33(6):355-74. DOI
46. Fogo AB, Lusco MA, Najafian B, Alpers CE. AJKD Atlas of Renal Pathology: Cryoglobulinemic Glomerulonephritis. Am J Kidney Dis. 2016; 67(2):e5-7. DOI
47. Noris M, Daina E, Remuzzi G. Membranoproliferative glomerulonephritis: no longer the same disease and may need very different treatment. Nephrol Dial Transplant. 2023; 38(2):283-90. DOI
48. Santoriello D, Nasr SH. Novel approaches beyond standard immunofluorescence for kidney biopsies. Curr Opin Nephrol Hypertens. 2022; 31(3):221-7. DOI
49. Beddhu S, Bastacky S, Johnson JP. The clinical and morphologic spectrum of renal cryoglobulinemia. Medicine. 2002; 81(5):398-409. DOI
50. Ion O, Obrişcă B, Ismail G. Kidney Involvement in Hypocomplementemic Urticarial Vasculitis Syndrome-A Case-Based Review. J Clin Med Res [Internet].. 2020; 9(7)DOI
51. Sundaramoorthy M, Meiyappan M, Todd P, Hudson BG. Crystal structure of NC1 domains. Structural basis for type IV collagen assembly in basement membranes. J Biol Chem. 2002; 277(34):31142-53. DOI
52. Goodpasture EW. Landmark publication from The American Journal of the Medical Sciences: The significance of certain pulmonary lesions in relation to the etiology of influenza. Am J Med Sci. 2009; 338(2):148-51. DOI
53. Phelps RG, Rees AJ. The HLA complex in Goodpasture’s disease: a model for analyzing susceptibility to autoimmunity. Kidney Int. 1999; 56(5):1638-53. DOI
54. Pedchenko V, Bondar O, Fogo AB. Molecular architecture of the Goodpasture autoantigen in anti-GBM nephritis. N Engl J Med. 2010; 363(4):343-54. DOI
55. Hellmark T, Segelmark M. Diagnosis and classification of Goodpasture’s disease (anti-GBM). J Autoimmun.. 2014; 48-49:108-12. DOI
56. Reggiani F, L’Imperio V, Calatroni M. Goodpasture syndrome and anti-glomerular basement membrane disease. Clin Exp Rheumatol. 2023; 41(4):964-74. DOI
57. Levy JB, Hammad T, Coulthart A. Clinical features and outcome of patients with both ANCA and anti-GBM antibodies. Kidney Int. 2004; 66(4):1535-40. DOI
58. L’Imperio V, Ajello E, Pieruzzi F. Clinicopathological characteristics of typical and atypical anti-glomerular basement membrane nephritis. J Nephrol. 2017; 30(4):503-9. DOI
59. Fogo AB, Lusco MA, Najafian B, Alpers CE. AJKD Atlas of Renal Pathology: Anti-Glomerular Basement Membrane Antibody-Mediated Glomerulonephritis. Am J Kidney Dis. 2016; 68(5):e29-30. DOI
60. Sánchez-Agesta M, Rabasco C, Soler MJ. Anti-glomerular Basement Membrane Glomerulonephritis: A Study in Real Life. Front Med. 2022; 9:889185. DOI
61. Akhtar M, Taha NM, Asim M. Anti-glomerular Basement Membrane Disease: What Have We Learned?. Adv Anat Pathol. 2021; 28(1):59-65. DOI
62. Nasr SH, Fidler ME, Said SM. Paraffin Immunofluorescence: A Valuable Ancillary Technique in Renal Pathology. Kidney Int Rep. 2018; 3(6):1260-6. DOI
63. Kawasaki T. Kawasaki disease. Mucocutaneous lymph node syndrome or MCLS. Acta Pathol Jpn. 1982; 32:63-72.
64. Grayson PC, Maksimowicz-McKinnon K, Clark TM. Distribution of arterial lesions in Takayasu’s arteritis and giant cell arteritis. Ann Rheum Dis. 2012; 71(8):1329-34. DOI
65. Klingler AM. Giant cell arteritis; chronic renal disease. JAAPA.. 2010; 23(1):60-1. DOI
66. Langlois V, Lesourd A, Girszyn N. Antineutrophil Cytoplasmic Antibodies Associated with Infective Endocarditis. Medicine.. 2016; 95(3):e2564. DOI
67. Flores-Suárez LF, Cabiedes J, Villa AR. Prevalence of antineutrophil cytoplasmic autoantibodies in patients with tuberculosis. Rheumatology. 2003; 42(2):223-9. DOI
68. Cornely OA, Hauschild S, Weise C. Seroprevalence and disease association of antineutrophil cytoplasmic autoantibodies and antigens in HIV infection. Infection. 1999; 27(2):92-6. DOI
69. Locht H, Skogh T, Wiik A. Characterisation of autoantibodies to neutrophil granule constituents among patients with reactive arthritis, rheumatoid arthritis, and ulcerative colitis. Ann Rheum Dis. 2000; 59(11):898-903. DOI
70. Schnabel A, Csernok E, Isenberg DA. Antineutrophil cytoplasmic antibodies in systemic lupus erythematosus. Prevalence, specificities, and clinical significance. Arthritis Rheum. 1995; 38(5):633-7. DOI
71. Shah S, Havill J, Rahman MH, Geetha D. A historical study of American patients with anti-neutrophil cytoplasmic antibody negative pauci-immune glomerulonephritis. Clin Rheumatol. 2016; 35(4):953-60. DOI
72. Bajocchi G, Cavazza A. Histopathology of vasculitis. Reumatismo. 2018; 70(3):155-64. DOI
73. Paueksakon P, Najafian B, Alpers CE, Fogo AB. AJKD Atlas of Renal Pathology: IgA-Dominant Infection-Related Glomerulonephritis. Am J Kidney Dis.. 2024; 83(1):e1-2. DOI
74. Fogo AB, Lusco MA, Najafian B, Alpers CE. AJKD Atlas of Renal Pathology: Focal and Diffuse Lupus Nephritis (ISN/RPS Class III and IV). Am J Kidney Dis. 2017; 70(2):e9-11. DOI
75. Nasr SH, Satoskar A, Markowitz GS. Proliferative glomerulonephritis with monoclonal IgG deposits. J Am Soc Nephrol. 2009; 20(9):2055-64. DOI
76. Lusco MA, Fogo AB, Najafian B, Alpers CE. AJKD Atlas of Renal Pathology: Glomerulonephritis With Dominant C3. Am J Kidney Dis. 2015; 66(4):e25-6. DOI
77. Lusco MA, Fogo AB, Najafian B, Alpers CE. AJKD Atlas of Renal Pathology: Fibrillary Glomerulonephritis. Am J Kidney Dis. 2015; 66(4):e27-8. DOI
78. L’Imperio V, Barreca A, Vergani B. Destructuring glomerular diseases with structured deposits: challenges in the precision medicine era. J Nephrol. 2021; 34(6):2151-4. DOI
79. Endo A, Hoshino J, Suwabe T. Significance of small renal artery lesions in patients with antineutrophil cytoplasmic antibody-associated glomerulonephritis. J Rheumatol. 2014; 41(6):1140-6. DOI
80. Boudhabhay I, Delestre F, Coutance G. Reappraisal of Renal Arteritis in ANCA-associated Vasculitis: Clinical Characteristics, Pathology, and Outcome. J Am Soc Nephrol. 2021; 32(9):2362-74. DOI
81. L’Imperio V, Pagni F. Unveiling the Role of Additional Histological Parameters in ANCA-Associated Vasculitis. J Am Soc Nephrol. 2022; 33(6):1226-7. DOI
82. L’Imperio V, Brambilla V, Cazzaniga G. Digital pathology for the routine diagnosis of renal diseases: a standard model. J Nephrol. 2021; 34(3):681-8. DOI
83. L’Imperio V, Casati G, Cazzaniga G. Improvements in digital pathology equipment for renal biopsies: updating the standard model. J Nephrol [Internet].. 2023. DOI
84. Cazzaniga G, Rossi M, Eccher A. Time for a full digital approach in nephropathology: a systematic review of current artificial intelligence applications and future directions. J Nephrol [Internet].. 2023. DOI
85. Altini N, Rossini M, Turkevi-Nagy S. Performance and limitations of a supervised deep learning approach for the histopathological Oxford Classification of glomeruli with IgA nephropathy. Comput Methods Programs Biomed. 2023; 242:107814. DOI
86. Morris AD, Freitas DLD, Lima KMG. Automated Computational Detection of Disease Activity in ANCA-Associated Glomerulonephritis Using Raman Spectroscopy: A Pilot Study. Molecules [Internet].. 2022; 27(7)DOI
87. Lyons PA, Rayner TF, Trivedi S. Genetically distinct subsets within ANCA-associated vasculitis. N Engl J Med. 2012; 367(3):214-23. DOI