Recent Developments in the Pathogenesis and the Pathological Classification of IgA Nephropathy

IgA nephropathy (IgAN), also known as Berger’s disease, is the most common primary glomerular disease world wide and a significant cause of end-stage renal disease (ESRD) in many parts of the world (Floege & Feehally, 2000; Geddes et al., 2003; Julian et al., 1988). It was first described in 1968 in Paris, France (Berger & Hingalis, 1968), but now the disease is known to have a cosmopolitan distribution (Ballardie et al., 1987; Chandrika, 2009; D’Amico, 1985; Koyama et al., 1997; Lai et al., 1999; Mubarak, 2009; Power et al., 185; Rivera et al., 2004; Schena, 1997; Seedat et al., 1988; Sinniah, 1980; Strata P, et al., 1995; Yahya et al., 1997). Significant advancements have been made during last four decades in understanding the etiology, pathogenesis, classification, and treatment of this enigmatic disease. In this chapter, we discuss briefly about the historical background, epidemiology, etiology, pathogenesis, pathology, treatment, and recent developments in the diagnosis and prognosis of IgAN with particular emphasis on a recent clinicopathological classification developed through international collaboration by the members of International IgAN network in conjunction with renal pathology society (RPS). This classification is simple, easy to apply, fairly reproducible, and clinically pre-validated, and provides a role model for classifying other renal diseases in similar manner.


Introduction
IgA nephropathy (IgAN), also known as Berger's disease, is the most common primary glomerular disease world wide and a significant cause of end-stage renal disease (ESRD) in many parts of the world (Floege & Feehally, 2000;Geddes et al., 2003;Julian et al., 1988).It was first described in 1968 in Paris, France (Berger & Hingalis, 1968), but now the disease is known to have a cosmopolitan distribution (Ballardie et al., 1987;Chandrika, 2009;D'Amico, 1985;Koyama et al., 1997;Lai et al., 1999;Mubarak, 2009;Power et al., 185;Rivera et al., 2004;Schena, 1997;Seedat et al., 1988;Sinniah, 1980;Strata P, et al., 1995;Yahya et al., 1997).Significant advancements have been made during last four decades in understanding the etiology, pathogenesis, classification, and treatment of this enigmatic disease.In this chapter, we discuss briefly about the historical background, epidemiology, etiology, pathogenesis, pathology, treatment, and recent developments in the diagnosis and prognosis of IgAN with particular emphasis on a recent clinicopathological classification developed through international collaboration by the members of International IgAN network in conjunction with renal pathology society (RPS).This classification is simple, easy to apply, fairly reproducible, and clinically pre-validated, and provides a role model for classifying other renal diseases in similar manner.

Historical background
IgAN was first described by Berger and Hingalis in France in 1968 as a renal glomerular disease characterized by the predominant or co-dominant IgA containing immune complex deposits in the mesangium of the glomeruli (Berger & Hingalis, 1968).Thus, its diagnosis required, and still so, the histopathologic and immunoflourescence (IF) evaluation of the invasive procedure of renal biopsy.No blood or urinary test has been developed yet to replace the renal biopsy for its accurate diagnosis.Traditionally, the disease was considered to be a benign disease, due mainly to its very slow rate of progression, as is apparent from its earlier synonym of "benign familial hematuria" and to be confined to France.However, soon it was realized, as evidence accumulated from subsequent long term follow-up studies from different parts of the world, that both the above assumptions about IgAN were wrong, as will be discussed below in the epidemiology and the prognosis of the disease.

Etiology
The vast majority of cases of IgAN are primary or idiopathic, and as their name suggests, their etiology is still not known (Barratt et al., 2004;Donadio & Grande, 2002;Floege & Feehally, 2000;Galla, 1995).The disease is, in most cases, a sporadic disorder.However, cases of familial and secondary IgAN are well documented and these have proved very useful for the insight they have provided for a better understanding of the pathogenetic mechanisms underlying the disease (Barratt et al., 2004;Donadio & Grande, 2002;Galla, 1995).A list of these secondary causes is shown in table 1.

The pathogenetic mechanisms
The mechanism of development of IgAN is still largely unresolved.However, many advances have been made, especially during the last decade in unraveling some of the steps involved in the pathogenetic pathway of the disease.This has been made possible by the numerous experimental studies conducted on animal models and clinical studies in humans on the sporadic as well as rare genetic forms of the disease (Barratt et al., 2004;Donadio & Grande, 2002;Floege & Feehally, 2000;Galla, 1995).The postulated mechanisms of pathogenesis are shown in Fig. 1.

Increased formation of IgA1 in bone marrow
Binding of IgA1 to mesangial cells

Increased mesangial matrix formation and cell proliferation
Fig. 1.Schematic diagram showing the main steps involved in the pathogenesis of IgA nephropathy.The final common step consists of deposition of polymers of IgA1 molecules or their complexes in the mesangium.These, then induce injury and stimulate mesangial cells and recruited inflammatory cells to release mediators which result in the morphological expression of glomerular abnormaliteis.
As is apparent from this figure, the final common pathway in the process consists of an accumulation of an abnormally galactosylated polymers of IgA1 (pIgA1) molecules and/or complexes in the mesangial regions of the glomeruli.It is plausible that numerous pathogenetic pathways are involved and converge on this central and unifying stage in the pathway (Barratt et al., 2004).The likely events of pathogenesis implicated in different studies run the whole span of IgA immune system abnormalities from increased production of aberrantly galactosylated pIgA1 molecules by a putative abnormal subclone of B lymphocytes, to decreased clearance by the liver, to host immune response to abnormal pIgA1 molecules, to physicochemical mechanisms leading to abnormal accumulation of pIgA1 in the mesangium (Coppo & Amore, 2004).These bind to and interact with mesangial cells leading to their activation and proliferation.Activated mesangial cells as well as infiltrating inflammatory cells, not only proliferate but also produce proinflammatory cytokines, chemokines, and growth factors, which initiate tissue injury.A number of cytokines have been studied in detail including platelet derived growth factor (PDGF) βchain and transforming growth factor-β (TGF-β) for their roles in the ultimate expression of the disease (Barratt et al., 2004;Donadio & Grande, 2002;Floege & Feehally, 2000;Galla, 1995).There is also evidence of local complement activation induced by alternative pathway triggered by pIgA1 molecules in the mesangium.There are species differences in the molecular structure of IgA and unique features of human IgA1 have prevented development of satisfactory animal models for the early stages of IgAN.A fully humanized mouse model of disease is still awaited to better extrapolate the steps of the pathogenesis to clinical disease in humans.It is probable that events downstream of pIgA1 deposition, which result in glomerular inflammation and scarring, are not specific to IgAN but generic to many immune complex mediated forms of glomerulonephritis (Barrat et al., 2004).There is also preliminary evidence in favor of genetic factors in the predisposition, development, and progression of the disease (Hsu et al., 2000).However, there is no single "IgA gene" involved, and it is likely that multiple interacting genes will eventually be discovered that predispose to the disease.More recently, IgAN has also been categorized as a form of auto-immune disease, in which autoantibodies of the IgG class are formed against aberrantly galactosylated IgA1 molecules.A complete understanding of the pathogenesis along with etiology is necessary if we are to develop a specific and targeted therapy for the disease.Future molecular biologic studies will certainly help in this discovery of missing steps in the pathogenesis (Barratt et al., 2004;Donadio & Grande, 2002;Floege & Feehally, 2000;Galla, 1995;Novak et al., 2001).

Pathology
The diagnosis of IgAN on renal biopsy requires a detailed pathological evaluation of the biopsy by light microscopy (LM), IF, and electron microscopy (EM).In particular, IgAN is a diagnosis of IF microscopy.The features of IgAN on each of these investigations are described below:

Light microscopy
IgAN is primarily an immune-complex mediated GN, and like lupus nephritis manifests a wide range of morphological appearances on LM.Thus, on LM, IgAN is quite heterogenous.The range of morphologic alterations seen on renal biopsy under the LM spans the whole spectrum from minor changes in the glomeruli and the surrounding parenchyma to full blown crescentic GN (CresGN), except for pure membranous pattern.The list of morphological patterns seen in IgAN is shown in table 2 and in figures 2 to 6.All the above patterns of glomerular injury in IgAN are unified by the presence of predominant or codominant IgA deposits in the glomerular mesangium on IF microscopy (Galla, 1995).The reason for this varying phenotypic expression of the disease in different patients is not completely known, but may be related to both the abnormalities of IgA molecule and the host factors.This phenotypic variability o f I g A N o n L M i s e x p r e s s e d a s c l i n i c a l heterogeneity of the disease, but the clinical histologic correlation is not perfect.This variability in morphology of the disease has led to efforts to classify the disease into prognostic groups.(Hass, 1997;Manno et al., 2007).More recently, a new pathological classification of IgAN, termed "The Oxford classification of IgAN" has been formulated and published (Cattran et al., 2009;Roberts et al., 2009).It is unique and unprecedented type of classification that was validated before it was formulated and published, in contrast to the traditional approach to disease classifications.It is very simple and based on only four pathological features on renal biopsy and is easy to apply in routine practice.This classification is pre-validated, evidence based, and of good interobserver reproducibility and most important of all, proves independent prognostic value of pathological features on renal biopsy.Although, pre-validated, the scheme does require more validation studies in routine practice in prospective cohorts and in different study populations.The reasons are; although the sample size in the original study cohort was large, it was limited in scope, the retrospective design of the study, the use of only LM assessment for the classification, and most important of all, the unaddressed issue of treatment strategies.LM features are of independent prognostic value in IgAN and have been investigated by many researchers for classification purpose.

Electron microscopy
Ultra-structurally, the main finding apart from the secondary changes of mesangial proliferation and other alterations, is the deposition of large, frequently triangular shaped, electron-dense deposits, most often, in the mesangium and para-mesangial area of the glomeruli (Fig. 8, 9).The deposits may on rare occasions be found in other locations within the glomeruli.The amount of deposits varies from case to case, and in some cases, massive deposits are found throughout the mesangium, extending into the sub-endothelial location.Peripheral capillary wall deposits in subepithelial, subendothelial, and intramembranous locations are found in approximately one fourth to one third of cases of IgAN.The deposits in these loci are more common in histologically severe forms of the disease, such as cresGN and rare in the mild forms of the disease.Electron-dense deposits may not occasionally be found in typical cases of IgAN diagnosed on IF microscopy.This does not rule out the diagnosis of IgAN, as the disease is a diagnosis of IF microscopy.Moreover, the deposits may be patchy and not included in the scanty material usually examined under the EM.
In addition to the electron-dense deposits, the glomeruli show expansion of the mesangium and mesangial cell proliferation.These alterations parallel those seen on LM.There is also focal to diffuse loss of foot processes of podocytes, especially in those cases presenting with nephrotic range proteinuria.Focal areas of irregular thinning of GBM are seen in many cases of IgAN; these are not synonymous with concurrent IgAN and thin basement membrane disease (TBMD), and may contribute to the hematuria.Occasionally, IgAN co-exists with other glomerulopathies, such as TBMD, minimal change disease (MCD), membranous GN (MN), etc.In those cases, features of both diseases will be found on ultrastructural study.No prognostic importance is attached with the ultrastructural alterations in IgAN.
Fig. 6.High-power view showing a glomerulus with marked extracapillary cellular proliferation forming a crescent.The glomerular tufts are collapsed.This morphological pattern is rare in IgA nephropathy, but is well described, and leads rapidly to renal failure.

Clinical presentation
Similar to its pathology and clinical course, IgAN is also heterogeneous in clinical presentation.It presents in a variety of ways.It can occur at any age, but majority of cases are seen in adolescents and young adults.The disease is more common in males than females, the male to female varying from less than 2:1 in Japan and other oriental studies to more than 6:1 in the studies from northern Europe and United States.The classic presentation is with painless usually episodic gross or persistent microscopic hematuria, usually developing concurrently with upper respiratory tract infection (hence the use of synonym of synpharyngitic GN), gastroenteritis, or pneumonia.The presenting illness of episodic, grossly visible hematuria is more common in younger patients, whereas that of microscopic hematuria and proteinuria is more frequent in older individuals.Proteinuria is usually mild and usually associated with persistent microscopic hematuria.Occasional cases present with nephrotic range proteinuria.IgAN may rarely present with acute or chronic renal failure.Hypertension is rarely diagnosed at the time of presentation, but its frequency increases as the disease duration is increased or when patients develop ESRD (Barratt et al., 2004;Donadio & Grande, 2002;Floege & Feehally, 2000;Galla, 1995).

Risk of progression and classification systems
Just like its morphology, the disease is also heterogeneous in its clinical evolution.Although the disease was traditionally considered as benign process, it is now known to lead to a slowly progressive decline in renal function with ESRD developing in upto 30-40% of patients 20 years after initial presentation (Barratt et al., 2004;Donadio & Grande, 2002;Galla, 1995;Geddes et al., 2003).Long term follow up studies show variable rates of disease progression in different parts of the world (Geddes et al., 2003).Many attempts have been made in recent past in identifying clinical, familial, laboratory, immunological, genetic, and morphologic features on renal biopsies which can predict the outcome in individual patients.However, many of these parameters have been investigated by simple univariate analysis, and the independent prognostic value of these factors has not been established.Among these, the predictive value of serial estimations of clinical and laboratory parameters is proved beyond any doubt.But whether, pathological features on renal biopsy are of any significant and independent predictive value has remained controversial till very recently.This is because of the fact that almost all pathologic studies carried out by different investigators in different parts of the world have produced conflicting results.Some investigators tried to incorporate the various morphological features observed under LM into a pathologic classification of IgAN, but none succeeded in achieving international approval (Hass, 1997;Lee et al., 2005;Manno et al., 2007).More recently, an international working group of nephrologists and nephropathologists with keen interest and expertise in IgAN have promulgated a novel classification of IgAN, termed "The Oxford classification of IgAN" (Cattran et al., 2009;Roberts et al., 2009).This scheme was developed by using a novel and unique approach to the pathological classification of IgAN, inasmuch as no arbitrary classes or grades are constructed, as in previous classifications.Instead, specific pathological variables of prognostic importance independent of clinical data at the time of biopsy and follow up have been identified and scored by a rigorous iterative methodology of first defining the lesions and then testing for reproducibility and ease of scoring and finally testing them for their predictive power.This type of rigorous effort has never been employed previously in the classification of renal diseases (Cattran et al., 2009;Roberts et al., 2009).The study cohort for the final study validation project consisted of 265 adults and children with biopsy-proven IgAN from eight different countries from four continents, with a median follow-up period of 5 years.The study cohort can be considered as fairly but not completely representative of the entire clinicopathological spectrum of IgAN.It included both males and females, children and adults, nearly all racial groups, and nearly all grades of severity of the disease.However, there was a marked underrepresentation of some grades of disease, for example, very mild cases and very severe cases were very few.This classification is based on detailed analysis of retrospective clinical data obtained on these patients in concert with intense and detailed pathological review of their renal biopsy material for the identified prognostic pathological features.Thus, the classification was validated before its formulation.A set of six distinct pathology variables were identified, from around 24 original pathological features tested, on the basis of sufficient reproducibility among the nephropathologists, least propensity for sampling error, and the ease of scoring in routine practice while avoiding strong co-linearity shown in the final analysis to have independent predictive value for the final renal outcome even after taking into account all clinical and laboratory parameters available at the time of biopsy as well as during follow up.These pathological variables along with their proposed scoring (so called MEST score), which are recommended by the Oxford group, to be included in the pathology report of renal biopsy specimens from patients with IgAN, are shown in Table 3.These variables showed the independent prognostic value and a significant correlation with the clinical outcome in both univariate and multivariate analysis.The classification could not address the prognostic value of crescents due to their low prevalence in the enrolled cohort which did not include rapidly progressive cases.This classification is clinically pre-validated, evidence based with acceptable interobserver reproducibility, and most important of all, relatively simple to apply in routine practice (Cattran et al., 2009;Mubarak, 2009;Roberts et al., 2009).The case mix was fairly varied; the cases were collected from eight different centers from four main continents, to ensure international participation and consensus development.However, there are certain limitations of the classification scheme; the classification was developed by dedicated, research-oriented nephropathologists with special interest in IgAN and on a limited repertoire of cases.As discussed previously, both mild (non-progressive) and severe (rapidly-progressive) ends of IgA spectrum were not enrolled.It was retrospective by design and the impact of treatment was not explored in detail.
There is definitely a scope for further refinement and validation of the scheme in other prospective cohorts of patients.It needs to be validated in routine practice among practicing pathologists throughout the world on a wide range of cases (Herzenberg et al., 2011).Moreover, the original classification is based purely on light microscopic assessment of pathological features on renal biopsies (Mubarak, 2009).No correlation with IF or EM variables was carried out during the development of this classification.More recently, the question of clinical value of immunohistologic findings in the context of the original study patients' cohort has been addressed by the core group of the researchers involved in the development of the original classification (Bellur et al., 2011).They conclude that the glomerular location of IgA and the presence of IgG correlate with mesangial and endocapillary cellularity.It is likely that the immunohistologic findings may be added to the classification in near future as more data accumulate.

Pathological features Score
1. Mesangial hypercellularity Present in ≤50% of the glomeruli M0 Present in >50% of the glomeruli M1 2. Endocapillary hypercellularity A b s e n t E 0 P r e s e n t E 1 3. Segmental glomerulosclerosis A b s e n t S 0 P r e s e n t S 1 4. Tubular atrophy/interstitial fibrosis ≤ 2 5 % o f t h e c o r t e x T 0 26-50% of the cortex T1 > 5 0 % o f t h e c o r t e x T 2 Table 3.The four key pathological features that are recommended by the Oxford group to be included in the standardized pathology reports for patients with IgA nephropathy.
There is also need to investigate the effect of additional features which had enrollment bias in the original study ( rapidly progressive and non-progressive cases) and to investigate the combined prognostic value of adding clinical data to biological scores (e.g.proteinuria at the time of renal biopsy or during follow-up), and the last, but not the least, to detect for each lesion, the most effective treatment and the point of no return when no treatment is effective.

Future prospects
During recent past, a series of important advancements in the areas of molecular pathogenesis and experimental therapy have taken place, reflected in a molecular paradigm shift in the techniques and approaches applied to the study of IgAN (Novak et al., 2001).
Ongoing and future investigations in this area will lead to the development of new noninvasive molecular tests for the diagnosis and prognosis, and the application of these discoveries to the rational design of tailored therapeutic agents.
www.intechopen.comRecent Developments in the Pathogenesis and the Pathological Classification of IgA Nephropathy 125

Fig. 2 .
Fig. 2. Medium-power view showing a glomerulus with minor changes on light microscopy.Although rare, this pattern can be seen in IgA nephropathy.The surrounding tubules and interstitium show no significant pathology.(Jones' methenamine silver, ×200).

Fig. 3 .
Fig. 3. Medium-power view showing two glomeruli showing mild prominence of the mesangium, but there is no significant increase in the mesangial cellularity in another case of IgA nephropathy.Surrounding parenchyma is unremarkable.(Jones' methenamine silver, ×200).

Fig. 4 .
Fig. 4. High-power view showing a glomerulus with segmental and moderate degree mesangial hypercellularity.The patchy involvement of the glomeruli is characteristic of IgA nephropathy.Peripheral capillary walls are thin and capillary lumena patent.The later features distinguishes this pattern from mesangiocapillary pattern.This is the typical and the most common morphological pattern seen in IgA nephropathy.(PAS, ×400).

Fig. 5 .
Fig. 5. High-power view showing a segmental scar with adhesion formation with Bowman's capsule.The unscarred portion of the glomerulus shows mild mesangial hypercellularity.This morphologic pattern is common in IgA nephropathy and is distinguished from idiopathic focal segmental glomerulosclerosis by the presence of IgA deposits on immunoflourescence and electron microscopy.(PAS, ×400).
Fig. 7. High-power view showing a glomerulus with diffuse, granular, strong (3+ on a scale of 0 to 3+) deposits of IgA in the mesangial regions.Focally, the deposits are extending into the peripheral capillary walls.(FITC-conjugated IgA, ×400).

Fig. 8 .
Fig. 8. Low-power electron photomicrograph showing mild mesangial proliferation associated with many large, triangular shaped, electron dense deposits in the mesangium.(Lead citrate & Uranyl acetate, ×5000).The clinical course in other patients is non-progressive, while in a minority of individuals it leads to a rapidly progressive course to ESRD.Most of the research is focused on identifying the predictive factors for the subset of patients with slowly progressive forms of the disease.Long term follow up studies show variable rates of disease progression in different parts of the world(Geddes et al., 2003).Many attempts have been made in recent past in identifying clinical, familial, laboratory, immunological, genetic, and morphologic features on renal biopsies which can predict the outcome in individual patients.However, many of these parameters have been investigated by simple univariate analysis, and the independent prognostic value of these factors has not been established.Among these, the predictive value of serial estimations of clinical and laboratory parameters is proved beyond any doubt.But whether, pathological features on renal biopsy are of any significant and independent predictive value has remained controversial till very recently.This is because of the fact that almost all pathologic studies carried out by different investigators in different parts of the world have produced conflicting results.Some investigators tried to incorporate the various morphological features observed under LM into a pathologic classification of IgAN, but none succeeded in achieving international approval(Hass, 1997;Lee et al., 2005;Manno et al., 2007).More recently, an international working group of nephrologists and nephropathologists with keen interest and expertise in IgAN have promulgated a novel classification of IgAN, termed "The Oxford classification of IgAN"(Cattran et al., 2009;Roberts et al., 2009).This scheme was developed by using a novel and unique approach to the pathological classification of IgAN, inasmuch as no arbitrary classes or grades are constructed, as in

Diseases of the gastrointestinal tract: celiac
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Table 1 .
List of diseases/conditions associated with IgA deposits in the kidney.