Serum amyloid P deposition is a sensitive and specific feature of membranous-like glomerulopathy with masked IgG kappa deposits

Diagnostic Challenges and Emerging Pathogeneses of Selected Glomerulopathies

Recent progress in glomerular immune complex and complement-mediated diseases have refined diagnostic categories and informed mechanistic understanding of disease development in pediatric patients. Herein, we discuss selected advances in 3 categories. First, membranous nephropathy antigens are increasingly utilized to characterize disease in pediatric patients and include phospholipase A2 receptor (PLA2R), Semaphorin 3B (Sema3B), neural epidermal growth factor-like 1 (NELL1), and protocadherin FAT1, as well as the lupus membranous-associated antigens exostosin 1/2 (EXT1/2), neural cell adhesion molecule 1 (NCAM1), and transforming growth factor beta receptor 3 (TGFBR3). Second, we examine advances in techniques for paraffin and light chain immunofluorescence (IF), including the former's function as a salvage technique and their necessity for diagnosis in adolescent cases of membranous-like glomerulopathy with masked IgG kappa deposits (MGMID) and proliferative glomerulonephritis with monotypic Ig deposits (PGNMID), respectively. Finally, progress in understanding the roles of complement in pediatric glomerular disease is reviewed, with specific attention to overlapping clinical, histologic, and genetic or functional alternative complement pathway (AP) abnormalities among C3 glomerulopathy (C3G), infection-related and post-infectious GN, "atypical" post-infectious GN, immune complex mediated membranoproliferative glomerulonephritis (IC-MPGN), and atypical hemolytic uremic syndrome (aHUS).

Diabetic with Nephrotic Syndrome: A Case of "Masked" Membranous Nephropathy - A Case Report

Distinguishing nondiabetic renal disease (NDKD) from diabetic nephropathy (DN) is of paramount importance in choosing treatment modalities and determining renal prognosis. Nearly 40% of the patients with diabetes are likely to have NDKD. We report a case of a patient with diabetes with a massive nephrotic range of proteinuria that was labeled as DN based on LM and IF, but paraffin IF confirmed the presence of masked MN.

Chasing the Zebra: a case of membranous-like Glomerulopathy with SSA/RO52 deposits and no overt connective tissue disease

BACKGROUND

The nature of the deposits in immune-mediated glomerulonephritis with a membranous pattern and masked IgG-Kappa deposits (MGMID) remains still to be elucidated.

CASE PRESENTATION

We present a case of 33-year-old woman developing a continuous asymptomatic proteinuria (0.8-1 g/24 h) with no overt connective tissue diseases. She tested positive at high titers for SSA antibodies (Ro52 838 UI/mL, Ro60 2716 UI/mL) and at the kidney biopsy histological findings were compatible with an immune-mediated glomerulonephritis with a membranous pattern and masked IgG-Kappa deposits. Also, we demonstrated a positive immunohistochemistry staining for anti-Ro52-SSA antibodies, with a granular positivity in mesangium and along rare glomerular capillaries. To date, only one case of a patient with overt diagnosis of Sjögren's syndrome with MGMID has been described but a pathogenic role for SSA and SSB antibodies has never been proven.

CONCLUSIONS

In this case, we described for the first time by immunohistochemistry a Ro52+ granular positivity in the mesangium and glomerular capillaries, potentially paving the way for a better understanding of MGMID.

Monoclonal Gammopathy of Renal Significance: A Molecular Middle Earth between Oncology, Nephrology, and Pathology

Background

The renal biopsy represents a cornerstone in the definition of monoclonal gammopathy of renal significance (MGRS), helping in identifying patients with sub-detectable neoplastic clones (MGUS) that would deserve aggressive chemotherapies. However, the rising complexity of this onco-nephrology field is significantly challenging the daily work of nephrologists and nephropathologists, leading to the formation of ultra-specialized international centers with dedicated personnel/instrumentation and stressing the need for a better understanding of the underlying molecular landscape of these entities.

Summary

In this setting, the application of proteomic techniques, some with in situ capabilities (e.g., MALDI-MS imaging), for the investigation of the most challenging MGRS is progressively shedding light on the pathobiology of these diseases, providing new insights in the diagnosis and prognosis of these cases. This transformation is further enhanced by the application of next-generation digital pathology platforms, leading to a significant improvement of the cultural background for physicians thanks to second opinions, database and atlas creation, enhancement of diagnostic reports, with obvious repercussions for patients both in terms of turnaround time and appropriateness.

Key Messages

The present review is aimed at bridging the gap between clinical questions (i.e., a better characterization of MGRS) and the molecular landscape of onco-nephrology entities.

Novel approaches beyond standard immunofluorescence for kidney biopsies

PURPOSE OF REVIEW

Immunofluorescence on frozen tissue (IF-F) utilizing antibodies against immunoglobulin (Ig) heavy and light chains (IgA, IgG and IgM, kappa and lambda) and components of classical and alternative complement pathways (C1q, C3c and C4) is the standard of renal pathology. However, conventional IF-F has limitations, particularly in nephropathies associated with organized and/or monoclonal Ig deposits. This review will discuss new applications of established methods beyond conventional IF-F and recent novel immunohistochemical methods.

RECENT FINDINGS

The combined application of paraffin immunofluorescence (IF-P) and IgG subtype staining excluded monotypic deposits in 62-66% of DNA J homolog subfamily B member 9-associated fibrillary glomerulonephritis (FGN) with apparent monotypic deposits by IF-F, whereas IF-P unmasks IgG deposits in a subset of cases of immunotactoid glomerulopathy. A novel IF technique targeting epitopes at the junction of the Ig heavy and light chains was introduced and unmasked polytypic deposits in a subset of glomerulonephritis with apparent monotypic deposits on IF-F. A recent study described the successful application of co-detection by indexing (CODEX) multiplexed IF to visualize more than a dozen target antigens within a single kidney tissue section. Finally, immunohistochemical protocols for detection of the novel antigens in membranous nephropathy have already entered the clinical practice of renal pathology.

SUMMARY

Novel ancillary techniques in renal pathology have the potential to significantly enhance our ability to evaluate renal biopsies.

Monoclonal Gammopathy-Related Kidney Diseases

Monoclonal gammopathies occur secondary to a broad range of clonal B lymphocyte or plasma cell disorders, producing either whole or truncated monoclonal immunoglobulins. The kidneys are often affected by these monoclonal proteins, and, although not mutually exclusive, can involve the glomeruli, tubules, interstitium, and vasculature. The nephrotoxic potential of these monoclonal proteins is dependent on a variety of physicochemical characteristics that are responsible for the diverse clinicopathologic manifestations, including glomerular diseases with organized deposits, glomerular diseases with granular deposits, and other lesions, such as C3 glomerulopathy and thrombotic microangiopathy with unique pathophysiologic features. The diseases that involve primarily the tubulointerstitial and vascular compartments are light chain cast nephropathy, light chain proximal tubulopathy, crystal-storing histiocytosis, and crystalglobulin-induced nephropathy with distinct acute and chronic clinicopathologic features. The diagnosis of a monoclonal gammopathy-related kidney disease is established by identification of an underlying active or more commonly, low-grade hematologic malignancy, serologic evidence of a monoclonal gammopathy when detectable, and most importantly, monoclonal protein-induced pathologic lesions seen in a kidney biopsy, confirming the association with the monoclonal protein. Establishing a diagnosis may be challenging at times, particularly in the absence of an overt hematologic malignancy, with or without monoclonal gammopathy, such as proliferative glomerulonephritis with monoclonal immunoglobulin deposits. Overall, the treatment is directed against the underlying hematologic disorder and the potential source of the monoclonal protein.

An update of proliferative glomerulonephritis with monoclonal immunoglobulin deposits

As aging increases, monoclonal gammopathy is becoming more common, and monoclonal gammopathy of renal significance (MGRS) is gaining attention due to frequent renal involvement. Among MGRS, proliferative glomerulonephritis with monoclonal immunoglobulin deposits (PGNMID) is a special category. The disease was first described in 2004, and the research history on it is relatively short. Compared with other MGRS, the detection rate of circulating clones is lower in patients with PGNMID, which is easy to be missed and misdiagnosed in clinical work. In this review, the etiology and clinical features of PGNMID are discussed. It is noted that PGNMID is not only associated with MGRS, but also with malignancy, infection, and other factors. PGNMID is not a disease exclusive to the elderly, young people can also develop this disease. Due to the low detection rate of circulating clones in most patients, confirmation of the disease needs to be combined with renal pathology, which emphasizes the importance of completing light and heavy chain subtype staining. Treatment options for patients with PGNMID differ by etiology. For MGRS-associated PGNMID, the current treatment is primarily empirical and more research evidence is needed to fill the treatment gap.

How Times Have Changed! A Cornucopia of Antigens for Membranous Nephropathy

The identification of the major target antigen phospholipase A2 receptor (PLA2R) in the majority of primary (idiopathic) cases of membranous nephropathy (MN) has been followed by the rapid identification of numerous minor antigens that appear to define phenotypically distinct forms of disease. This article serves to review all the known antigens that have been shown to localize to subepithelial deposits in MN, as well as the distinctive characteristics associated with each subtype of MN. We will also shed light on the novel proteomic approaches that have allowed identification of the most recent antigens. The paradigm of an antigen normally expressed on the podocyte cell surface leading to in-situ immune complex formation, complement activation, and subsequent podocyte injury will be discussed and challenged in light of the current repertoire of multiple MN antigens. Since disease phenotypes associated with each individual target antigens can often blur the distinction between primary and secondary disease, we encourage the use of antigen-based classification of membranous nephropathy.

Neural cell adhesion molecule 1 is a novel autoantigen in membranous lupus nephritis

Membranous lupus nephritis is a frequent cause of nephrotic syndrome in patients with systemic lupus erythematosus. It has been shown in phospholipase A2 receptor positive membranous nephropathy that known antibodies can be detected within sera, determination of the target autoantigen can have diagnostic significance, inform prognosis, and enable non-invasive monitoring of disease activity. Here we utilized mass spectrometry for antigen discovery in laser captured microdissected glomeruli from formalin-fixed paraffin embedded tissue and tissue protein G immunoprecipitation studies to interrogate immune complexes from frozen kidney biopsy tissue. We identified neural cell adhesion molecule 1 (NCAM1) to be a target antigen in some cases of membranous lupus nephritis and within rare cases of primary membranous nephropathy. The prevalence of NCAM1 association was 6.6% of cases of membranous lupus nephritis and in 2.0% of primary membranous nephropathy cases. NCAM1 was found to colocalize with IgG within glomerular immune deposits by confocal microscopy. Additionally, serum from patients with NCAM1-associated membranous nephropathy showed reactivity to NCAM1 recombinant protein on Western blotting and by indirect immunofluorescence assay, demonstrating the presence of circulating antibodies. Thus, we propose that NCAM1 is a target autoantigen in a subset of patients with membranous lupus nephritis. Future studies are needed to determine whether anti-NCAM1 antibody levels correlate with disease activity or response to therapy.

Membranous Nephropathy With Monoclonal IgM Lambda Deposits in a Patient With IgM Monoclonal Gammopathy: A Case Report

We report a case of membranous nephropathy with monoclonal immunoglobulin (Ig)M lambda deposits in a patient with IgM monoclonal gammopathy, in whom histological changes were observed on repeat renal biopsy. A 72-year-old Japanese woman was referred to our hospital because of massive proteinuria. A prominent increase in monoclonal IgM lambda level was identified, and she was diagnosed as having IgM monoclonal gammopathy of undetermined significance. Renal biopsy showed glomerular subepithelial electron-dense deposits that were found to be granular deposits of IgM lambda but not kappa or IgG by immunofluorescence staining, resulting in a diagnosis of membranous nephropathy with monoclonal IgM deposits. The second biopsy, which was performed 2 years later because of exacerbation of her nephrotic syndrome, demonstrated less immunofluorescence staining of IgM, and dominant IgG2 deposition without light chain restriction. Interestingly, immunostaining for thrombospondin-type-1-domain-containing-7A was positive in both renal biopsy tissues, although the second biopsy showed clearly stronger immunoreactivity. The effect of steroid therapy was limited; however, rituximab treatment improved both the hematological and renal abnormalities. Solitary deposition of IgM in membranous nephropathy is a quite rare condition. To our knowledge, this is the first case of monoclonal gammopathy of renal significance presenting as membranous nephropathy with monoclonal IgM deposits, in which chronological immunohistochemical changes were observed and rituximab therapy was effective.

Membranous nephropathy with masked polyclonal IgG deposits associated with primary Sjögren's syndrome

Tubulointerstitial nephritis and renal tubular acidosis are well-known renal involvements with primary Sjögren's syndrome. However, several types of glomerulonephritis such as membranoproliferative glomerulonephritis and membranous nephropathy are also known to develop in patients with this syndrome. We here report a case of membranous nephropathy that developed 8 years after a diagnosis of primary Sjögren's syndrome in a female patient. Interestingly, the deposition was not identified by routine immunofluorescence using snap frozen tissue, but was revealed by immunofluorescence on formalin-fixed paraffin-embedded sections treated with proteinase K. We further performed immunofluorescence analysis on the treated paraffin-embedded sections with the identified antigen but found that the deposited IgG was not monoclonal and that serum amyloid P, a sensitive marker for membranous-like glomerulopathy with masked IgG κ deposits, was not evident in the glomeruli. To the best of our knowledge, this report depicted the first case of masked polyclonal IgG deposits and further analysis is needed to clarify the underlying mechanisms of IgG masking and possible association with autoantibodies.