Fibrillary and Immunotactoid Glomerulonephritis

Rupali S. Avasare

Samar M. Said

Samih H. Nasr

FIBRILLARY GLOMERULONEPHRITIS

Introduction/General Considerations

Fibrillary glomerulonephritis (FGN) is a rare glomerular disease first described by Rosenmann and Eliakim in 1977¹ and recognized as a distinct entity by Duffy et al in 1983.² Alpers et al coined the term “fibrillary glomerulonephritis” in 1987 when describing seven patients with noncongophilic fibrillar glomerular deposits.³ Traditionally, FGN has been defined by glomerular deposition of Congo red negative, randomly oriented fibrils that lack a hollow center at magnification less than 30,000 and stain with immunoglobulins (Igs) by immunofluorescence (IF).⁴,⁵,⁶,⁷,⁸,⁹ Some early studies lumped FGN with the rarer entity of immunotactoid glomerulonephritis (ITG) characterized by glomerular deposition of larger microtubules with parallel alignment.¹⁰,¹¹ However, many studies published over the past two decades unequivocally showed that these two entities are different histologically, histochemically, and clinically, and thus are considered distinct conditions.⁴,⁵,⁸,¹² FGN was initially thought to be idiopathic, but recent series highlighted the association with autoimmune disease, malignancy, or hepatitis C in some patients. Kidney prognosis is poor. There is no curative therapy for FGN, but rituximab has been recently shown to stabilize kidney function and reduce proteinuria in some patients. The pathogenesis of FGN remains unknown. Recently, a highly sensitive and specific tissue marker for FGN, DnaJ homolog subfamily B member 9 (DNAJB9), has been identified, which has greatly facilitated the accurate diagnosis of this disease and distinction from other forms of glomerulonephritis, although a pathogenetic role for this protein in FGN has not been established.¹²,¹³,¹⁴

Pathogenesis

The etiology of FGN remains enigmatic. Glomerular deposits in most cases stain for IgG, kappa (κ) light chain (LC), and lambda (λ) LC, and exhibit IgG subclass restriction (usually IgG4). Ultrastructural immunogold labeling studies revealed localization of IgG, κ, and λ to the FGN fibrils. Based on these observations, it has been assumed that this disease is an immune complex-type glomerulonephritis in which the IgG deposits are polymerized into fibrils due to their homogenous nature.⁵,¹⁵,¹⁶

Recently, through the use of laser microdissection-assisted liquid chromatography-tandem mass spectrometry (LMD/MS-MS), two research
groups from the Mayo Clinic and the University of Washington identified DNAJB9 in the glomeruli of almost all tested cases of FGN but not in cases of renal amyloidosis, a large variety of non-FGN glomerular diseases, or in normal controls.¹²,¹³,¹⁴ DNAJB9 was the fourth most abundant protein identified in FGN glomeruli, and FGN glomeruli exhibited a 6-fold overabundance of DNAJB9 compared to amyloidosis glomeruli.¹³ Both groups also showed colocalization of DNAJB9 and IgG in the mesangium and glomerular basement membrane (GBM) by dual IF.¹³,¹⁴ Ultrastructural immunogold labeling revealed localization of DNAJB9 to individual FGN fibrils, but not to ITG microtubules or amyloid fibrils.¹² DNAJB9 immunostaining exhibited 98% sensitivity and greater than 99% specificity for FGN¹² (Visual Abstract 9.1). These studies established DNAJB9 as the first tissue biomarker for FGN.

DNAJB9 is a member of the DNAJ family of proteins, which are co-chaperones to heat shock protein 70s (Hsp70). DNAJB9 and other co-chaperones recognize specific sequences in their unfolded protein clients, which either facilitates their proper folding or their degradation as part of the unfolded protein response (UPR).¹⁷,¹⁸ Other known functions of DNAJB9 include inhibiting the pro-apoptotic function of p53 and therefore protecting against cell death,¹⁹,²⁰ and governing the integrity of hematopoietic stem cells during endoplasmic reticulum (ER) stress.²¹ DNAJB9 is ubiquitously expressed at low levels in all human tissues, with the highest expression in liver, placenta, and kidney.¹²,²² Because of the high sensitivity and specificity of DNAJB9 for FGN and its overabundance in FGN glomeruli, DNAJB9 has been proposed to be an autoantigen in FGN.¹²,¹⁴ One proposed theory is that a misfolded DNAJB9 molecule is formed (possibly facilitated by protein posttranslational modification) and deposited in glomeruli through entrapment or interaction with glomerular constituents, which then triggers an autoimmune response.²³,²⁴ However, no circulating anti-DNAJB9 autoantibody has been found thus far in patients with FGN, and therefore this theory remains to be proven. Given its role in binding misfolded proteins during ER stress, the alternative possibility is that DNAJB9 secondarily binds to misfolded IgG molecules by recognizing aggregation-prone motifs.²³,²⁴ This binding could be critical for the IgG aggregates to undergo fibrillogenesis. The source of DNAJB9 production in FGN patients is yet to be determined, but potentially plasma cells could be the source of secreted DNAJB9 bound to misfolded IgG.²³ LMD/MS-MS analysis of FGN glomeruli did not detect other components of UPR,¹⁴,²³ and DNAJB9 is not transcriptionally upregulated in glomeruli of FGN patients,²⁵ and thus localized ER stress and activation of UPR at the level of glomeruli is unlikely to be driving the pathogenesis of FGN. Serum DNAJB9 levels are 4-fold higher in FGN patients than in controls,²⁶ but the levels inversely correlate with eGFR and do not correlate with disease severity or response to therapy,²⁷ favoring that the increased levels are likely in part due to decreased elimination by the kidney. Further studies are needed to understand whether FGN is a primary autoimmune disease, a result of UPR abnormalities, or a combination of these or other factors.

Pathology

Light Microscopy

Most cases of FGN show mesangial proliferative and sclerosing glomerulonephritis (MesGN) pattern, characterized by mesangial expansion by deposits, mesangial hypercellularity, and an increase in mesangial matrix (Figure 9.1A-C).⁷,⁸,¹²,²⁸ A smaller percentage of biopsies exhibit a membranoproliferative glomerulonephritis (MPGN) pattern with GBM double contours and cellular
interposition,⁵,⁷,⁸,¹²,²⁸ which is almost always associated with mesangial sclerosis, deposits, and hypercellularity.⁵,⁶,⁷,¹²,²⁸ This pattern is likely an advanced stage of disease, as a transition from MesGN to MPGN over time has been documented.²⁸ Rarely encountered patterns of injury in FGN include a membranous GN pattern, owing to global thickening of the GBM by subepithelial and intramembranous infiltration of fibrils with spike formation,⁷,⁸,²⁸,²⁹ and endocapillary proliferative GN characterized by leukocyte infiltration and endocapillary hypercellularity leading to occlusion of the peripheral capillaries (Figure 9.1D).⁷,²⁸ Focal crescents are encountered in 17% to 50% of cases,⁵,⁶,⁸,¹² whereas a diffuse crescentic GN pattern (ie, crescents >50% of glomeruli involved) is rarer, occurring in only 5% of cases (Figure 9.1D). The glomerular mesangial deposits appear glassy, variably periodic acid-Schiff-positive, and silver negative (Figure 9.1B, C). By traditional definitions of FGN, the deposits should be Congo red negative; however, recent studies showed that up to 4% of cases of FGN are Congo red positive (Figure 9.2).²⁹,³⁰ A concurrent glomerulopathy is encountered in 17% of FGN cases, most commonly diabetic nephropathy followed by IgA nephropathy.³¹

FIGURE 9.1: Light microscopy findings in fibrillary glomerulonephritis. A, On hematoxylin and eosin stain, the glomerulus shows mesangial hypercellularity and expansion (original magnification, ×400). B, The glomerulus shown exhibits global mesangial expansion by moderately periodic acid-Schiff (PAS)-positive immune deposits and mesangial sclerosis. There is also segmental mesangial hypercellularity (upper right). (Original magnification ×400). C, The deposits are silver negative and segmentally extend into the glomerular basement membranes. The glomerular basement membranes do not show duplication (Jones methenamine silver stain, ×400). D, A glomerulus exhibits a circumferential cellular crescent. The underlying glomerular tuft shows endocapillary hypercellularity with intracapillary infiltrating mononuclear cells (Jones methenamine silver stain, ×400).

FIGURE 9.2: Congophilic fibrillary glomerulonephritis. A, The two glomeruli exhibit Congo red positivity in mesangium and globally along the glomerular basement membranes (×200). B, When examining the Congo red slide under polarized light, the glomerular deposits show red/yellow/apple green birefringence (×200).

Immunofluorescence and Immunohistochemistry

By IF, the vast majority of FGN cases show mesangial and glomerular capillary wall positivity for IgG and C3, while staining for IgA, IgM, and C1q is less common and less intense when present (Figure 9.3). Extraglomerular deposits by IF are common in FGN (30%-49% of cases), and in most cases involve rare tubular basement membranes, peritubular capillaries, or arterioles (Figure 9.4).¹²,²⁸ The texture of glomerular deposits is typically smudgy, but a small percentage of cases exhibit linear staining of the GBM and can be associated with crescents
mimicking anti-GBM nephritis.³²,³³ IgG subclass analysis in most cases of FGN exhibits exclusive or dominant staining for IgG4 (Figure 9.5), whereas a minority show dominant or codominant staining for IgG1. As stated above, the vast majority of FGN cases (98% in one study) exhibit strong smudgy mesangial and glomerular capillary staining for DNAJB9 (Figure 9.6A).¹² Of note, rare cases of
immunoglobulin-negative FGN have been described, and these cases can be confirmed by DNAJB9 immunohistochemical staining.³⁴

FIGURE 9.3: Immunofluorescence findings in fibrillary glomerulonephritis. There is bright, smudgy, global mesangial and glomerular capillary wall staining for IgG, C3, κ light chain, and λ light chain (×200 for all).

FIGURE 9.4: Extraglomerular involvement in fibrillary glomerulonephritis. There is focal, bright, smudgy to semilinear tubular basement membrane staining for IgG (immunofluorescence, ×400).

FIGURE 9.5: IgG subclass immunofluorescence staining in fibrillary glomerulonephritis. Most cases of fibrillary glomerulonephritis exhibit IgG4 subclass dominant or restricted deposits. The glomeruli show global, smudgy glomerular basement membrane and mesangial positivity for IgG4 (×200). Glomeruli were negative for IgG1, IgG2, and IgG3 (not shown).

FIGURE 9.6: Immunohistochemical staining for DNAJB9 in fibrillary glomerulonephritis and immunotactoid glomerulopathy. A, A case of fibrillary glomerulonephritis showing strong smudgy staining of the mesangium and glomerular basement membrane for DNAJB9. Focal staining of tubular basement membranes (short arrows) and peritubular capillaries (long arrow) are also evident (×200). B, A case of immunotactoid glomerulopathy showing negative glomerular staining for DNAJB9 (×100).

By standard IF on frozen tissue (IF-F), roughly 90% of FGN cases exhibit polyclonal IgG deposits (ie, positivity for both κ and λ LCs) (Figure 9.3). Most of the remaining cases (˜8%-9%) show LC restriction (ie, staining for κ LC or λ LC). These cases were traditionally considered monoclonal lesions and were included within the spectrum of pathologic lesions associated with monoclonal gammopathy of renal significance (MGRS).³⁵ Most of these cases are DNAJB9 positive, similar to the polytypic variant. However, two recent studies have questioned the inclusion of DNAJB9-associated monotypic FGN as an MGRS lesion as they showed (a) cases with λ LC restriction frequently exhibit staining for both λ LC and κ LC when IF is performed on pronase-digested, paraffin tissue (IF-P); (b) these cases not uncommonly exhibit staining for more than one IgG subclass, excluding monoclonal deposits; and (c) the incidence of clinical evidence of monoclonal gammopathy in these cases (even in cases confirmed by IgG subclass staining and IF-P) is very small (comparable to polyclonal FGN).³⁶,³⁷ Finally, rare cases (˜1%) of FGN are DNAJB9-negative and associated with glomerular deposition of truncated Ig γ heavy chain, and these cases are associated with MGRS.¹²,³⁸

Electron Microscopy

On electron microscopy, FGN is characterized by glomerular deposition of randomly oriented fibrils that measure 12 to 30 nm in thickness (Figure 9.7).⁴,⁵,⁶,³⁹ FGN fibrils virtually always permeate the mesangial matrix, and in most cases infiltrate the lamina densa of the GBMs, with a tendency to concentrate in the outer aspect of the GBM, occasionally associated with spike-like projections into the urinary space (Figure 9.7). Extraglomerular FGN fibrils (involving rare tubular basement membranes, peritubular capillaries, arterioles, or interstitium) are identified in up to 19% of cases if extensive ultrastructural search for them is performed. FGN fibrils (supported by positive staining for IgG and DNAJB9) have been documented in the spleen¹² but not in other organs.

Only gold members can continue reading. Log In or Register to continue