Lupus Nephritis: The Significant Contribution of Electron Microscopy

Abstract

Background: Systemic lupus erythematosus (SLE) represents a principal prototype of a multisystemic autoimmune disease with the participation of both cell- and antibody-mediated mechanisms causing significant renal impairment. A renal biopsy diagnosis is the gold standard for clinical renal disease in SLE, which includes a broad range of indications.

Summary: Renal disease in SLE can involve glomerular, tubulointerstitial, and/or vascular compartments, none of which are mutually exclusive. In most instances, the basic pathogenetic mechanism involves tissue deposition of immune complexes and/or cell-mediated mechanisms, identified by light microscopy, immunohistochemical methods, and electron microscopy (EM), evoking intraglomerular proliferative, inflammatory, and other tissue responses. These produce a spectrum of histologic lesions, depending on the participation of a wide range of clinical triggers, namely, genetic, serological, and immunological factors, correlating with their underlying pathogenetic potential. In addition to light and immunofluorescence microscopy, EM in this setting facilitates an accurate diagnosis, assesses disease activity, delineates subclasses, differentiates from primary forms of non-lupus renal lesions, identifies organized deposits, and rarely, identifies other forms of nonimmune complex lesions such as podocytopathies, amyloidosis, and thrombotic microangiopathy.

Key messages: EM findings that are distinctive for most of the renal lesions in SLE include immune complex and nonimmune complex diseases as well as overlapping entities. Routine ultrastructural examination not only provides significant diagnostic and prognostic information from both initial and repeat renal biopsies from lupus patients but also contributes toward the understanding of the underlying pathophysiology of the disease process.

Keywords: Electron microscopy; Lupus nephritis; Lupus podocytopathy; Lupus vasculopathy; Organized deposits; Systemic lupus erythematosus; Thrombotic microangiopathy.

Fig. 1

Minimal mesangial LN (ISN/RPS class 1). a Normal glomerular appearance without mesangial hypercellularity (periodic acid-Schiff, ×400). b Variable/scant, granular IgG deposits in the mesangial areas (IF, ×400). c–f Finely granular, scant to small mesangial electron dense deposits (arrows), with relatively preserved foot processes, no significant changes of the mesangial cells, matrix, or glomerular basement membranes (c–f ×6,000). LN, lupus nephritis; IF, immunofluorescence; ISN/RPS, International Society of Nephrology/Renal Pathology Society.

Fig. 2

Mesangial proliferative LN (ISN/RPS class 2). a Global moderate mesangial expansion due to hypercellularity and increased matrix (periodic acid-Schiff, ×400). b Global granular mesangial IgG deposits (IF, ×400). c Conspicuous granular mesangial electron dense deposits, mesangial hypercellularity, normal glomerular basement membranes, with intact foot processes (×6,000). d Rare subepithelial electron dense deposits (arrows) within the capillary basement membranes with localized foot process effacement, but preserved in areas with mesangial hypercellularity and deposits (×6,000). e Small areas of subendothelial expansion by electron dense deposits adjacent to the mesangium (arrows) may be seen, but still categorized as class 2 (×6,000). LN, lupus nephritis; IF, immunofluorescence; ISN/RPS, International Society of Nephrology/Renal Pathology Society.

Fig. 3

Severe mesangial proliferative LN (ISN/RPS class 2). a Glomerulus showing global severe mesangial cell proliferation with mild narrowing of capillary lumina and no endocapillary hypercellularity (in all of the glomeruli examined) (periodic acid-Schiff, ×400). b–e In addition to granular mesangial electron dense deposits, there are small, well-defined subendothelial electron dense deposits in the capillary basement membranes (b, e, arrows) and inflammatory cells in glomerular capillary lumina (d, e), suggesting the consideration of LN of higher activity (class 3 or class 4), which may be unsampled or in progression (often clinical presentation and serology may also be helpful to determine the activity). Scattered rare small subepithelial and intramembranous deposits (c, d, arrows) may also be noted in this setting without indicating a change in the class of LN (×6,000 for b, ×10,000 for c, ×8,000 for d, e). LN, lupus nephritis; ISN/RPS, International Society of Nephrology/Renal Pathology Society.

Fig. 4

Focal proliferative LN (ISN/RPS class 3). a Focal segmental glomerular endocapillary hypercellularity, overlaid by a small fibrocellular crescent with adhesion to the Bowman capsule and the rest of the glomerulus, shows focal mesangial prominence and patent capillary lumina (periodic acid-Schiff, ×200). b Segmental lesional IgG localization within the capillary walls and mainly mesangial deposits in the rest of the glomerulus (×200). c Focal, small subendothelial and mesangial deposits are noted in some capillary basement membranes (arrows), in 1 capillary loop, in a case of class 3 LN. Fairly widespread subendothelial deposits should lead to a suspicion of an active class of lupus (class 3 or class 4) (×8,000). d Endocapillary hypercellularity is characterized mainly by inflammatory cells occluding the glomerular capillary lumen, which may also be associated with subendothelial deposits (arrows) and overlaid by FC (×6,000). LN, lupus nephritis; FC, fibrocellular crescent; ISN/RPS, International Society of Nephrology/Renal Pathology Society.

Fig. 5

Pauci-immune focal proliferative nephritis in systemic lupus. a, b Segmental fibrinoid, necrotizing lesion without significant mesangial or endocapillary hypercellularity that usually shows scant mesangial deposits by IF staining for immunoglobulins and complement components (not shown here) (PAS, PASM ×400). A small proportion of these patients may have positive ANCA serology. c Glomeruli often contain some baseline minimal to mild glomerular mesangial electron dense deposits (arrows), confirming the underlying lupus disease (×2,500). d, e Extracapillary fibrin aggregate with a characteristic bundled, fibrillary substructure and tactoids in low and high magnification, within this lesion (×2,500, ×8,000). IF, immunofluorescence.

Fig. 6

Diffuse proliferative LN (ISN/RPS class 4). a Two glomeruli showing relatively global glomerular abundant eosinophilic capillary wall deposits, “wireloops” or “hyaline thrombi” in the lumina, with only focal endocapillary hypercellularity (periodic acid-Schiff, ×200). b Strong granular IgG deposition along the capillary walls, mesangial areas, and focal luminal globular aggregates, corresponding to the “hyaline thrombi,” having a “full house pattern” of composition (×400). c A single glomerular capillary loop distended with endocapillary hypercellularity containing lymphocytes and monocytes, trapping scattered and aggregates of electron dense deposits and focal subendothelial deposits (arrows). d A portion of a glomerulus demonstrating 2 intraluminal granular aggregates termed “hyaline thrombi” composed of immune deposits (arrows) (×6,000). e Same glomerulus as in (c) with abundant mesangial deposits (arrows) admixed with matrix, cells with adjacent glomerular endocapillary hypercellularity (×6,000). f Two glomerular capillary loops with circumferential subendothelial electron dense deposits (arrows) and mesangial deposits that appear in continuity with the subendothelial deposits, representing the ultrastructural correlate of “wireloop” lesions, with preserved lamina densa. The foot processes are generally preserved and no significant subepithelial deposits are identified (×7,000). g A “wireloop” lesion with massive finely granular subendothelial electron dense deposits on 1 side of the glomerular capillary basement membrane has infiltrating monocytes within the lumen (×6,000). h A higher magnification image of a mesangial area containing nodular aggregates of electron dense deposits, displacing cells and admixed with matrix at the periphery (×10,000). LN, lupus nephritis; ISN/RPS, International Society of Nephrology/Renal Pathology Society.

Fig. 7

Diffuse LN (ISN/RPS class 4). a This is another variant of diffuse lupus GN where the glomerular capillary walls are globally thickened by massive eosinophilic/PAS+, subendothelial deposits, with focal obstructive intracapillary “hyalin thrombi” having a smooth outer contour, suggesting the absence of subepithelial deposits. A small proportion of cases show this unique feature involving all of the glomeruli in a given biopsy without significant hypercellularity (PAS, ×400). b Strong IgG deposition is localized in the capillary walls in the same distribution as seen by LM (×400). c, d EM findings show corresponding large, confluent subendothelial markedly electron dense deposits involving most capillary subendothelial zones displacing the endothelial cells and partly filling capillary lumina, as well as expansive mesangial deposits replacing the matrix and cells (×2,000, ×3,000). GN, glomerulonephritis; LN, lupus nephritis; EM, electron microscopy; LM, light microscopy; ISN/RPS, International Society of Nephrology/Renal Pathology Society.

Fig. 8

Diffuse proliferative LN (ISN/RPS class 4). a Two glomeruli display lobular architecture with global endocapillary hypercellularity including an influx of inflammatory cells almost occluding the capillary lumina, irregular thickening of the peripheral capillary walls displaying focal double contours indicative of a membranoproliferative (MPGN) pattern of glomerular injury in lupus, one of which is overlaid by a small FC. This pattern is not seen very often (PAS ×200). b Glomerulus showing mainly stretches of subendothelial IgG deposits which vary from 1 capillary loop to another having a smooth outer contour, suggesting no obvious subepithelial deposits and focal mesangial deposits (×400). c A portion of a glomerulus, where at least 2 capillary loops are markedly thickened by double contours, and extensive cellular interposition (arrows), which is in continuity with the adjacent mesangial area, while a separate loop is uninvolved (×1,800). d Higher magnification of the previous image showing subendothelial cellular interposition, where the original basement membrane is above (arrows) infiltrated by coarsely granular electron dense deposits and trapping some cellular debris with totally effaced foot processes. Another layer of basement membrane material is noted below the interposed cells, which is lined in the inner aspect by endothelial cells, giving rise to the double contours by LM (×600). e EM of a glomerulus from a different lupus patient with diffuse LN and focal MPGN, demonstrating a circumferential cellular interposition of a capillary loop (arrows) with small deposits and new basement membrane formation in the inner aspect, in continuity with the mesangial area containing significant deposits. Note the adjacent 2 capillary loops do not show the cellular interposition or thickening (×3,000). LN, lupus nephritis; EM, electron microscopy; FC, fibrocellular crescent; LM, light microscopy; ISN/RPS, International Society of Nephrology/Renal Pathology Society.

Fig. 9

Membranous LN (ISN/RPS class 5). a–c Glomerular capillary wall irregular thickening, mild to severe by PAS+ deposits showing suggestion of spikes in some capillary loops, with varying amount of mesangial hypercellularity, but without endocapillary hypercellularity, in all the glomeruli within a given biopsy (per­iodic acid-Schiff, ×400). d Global granular IgG in the glomerular capillary walls with a “full house” pattern is observed (×400). e, f Numerous small or confluent, fine to coarsely granular subepithelial or intramembranous electron dense deposits are seen frequently interspersed by basement membrane spikes. Some of these deposits may penetrate the thickness of the glomerular basement membrane with attenuation of lamina densa. The presence of such “penetrating” deposits, as well as mesangial deposits, favors lupus membranous GN over primary membranous GN. Rare subendothelial deposits may occur (arrows), the extent of which will determine whether there is a more active component and a combined class 4 and class 5 should be considered. The foot processes are totally effaced but may also show some degenerative changes and denudation, due to the extensive basement membrane remodeling (×6,000). g An earlier stage of glomerular subepithelial deposits, which appear finely granular, discrete, and frequently interspersed with basement membrane spikes, showing an intact or even thickening of the lamina densa (×6,000). h, i Higher magnification of the glomerular capillary basement membranes showing numerous subepithelial and intramembranous deposits penetrating deep into the matrix with marked attenuation of the lamina densa. Small subendothelial deposits (arrows) may be found, without affecting the class designation (×15,000). j This image shows mainly intramembranous and focal subepithelial electron dense deposits in varying stages of resorption (lucencies or rarefaction, partially dense, clear, granular) (arrows) with persistent foot process effacement. These findings reflect resolving deposits and chronic evolution of the disease process, as seen in the various stages of primary membranous GN (×8,000). GN, glomerulonephritis; LN, lupus nephritis; ISN/RPS, International Society of Nephrology/Renal Pathology Society.

Fig. 10

Sclerosing LN (ISN/RPS class 6). a Most of the glomeruli are sclerotic or almost completely sclerosed in this advanced class of LN, but mild focal residual cellularity may be observed (per­iodic acid-Schiff, ×400). b Immunoglobulins (IgG in this illustration) or complement components are either lost (arrows) or segmentally preserved (arrowheads). c EM of a portion of a sclerotic glomerulus showing matrix expansion, wrinkled/collapsed capillary basement membranes, and residual scattered, ill-defined, granular electron dense deposits (arrows) (×8,000). LN, lupus nephritis; EM, electron microscopy; ISN/RPS, International Society of Nephrology/Renal Pathology Society.

Fig. 11

Proliferative and membranous LN (combined ISN/RPS classes 4 and 5). a Two glomeruli with segmental endocapillary hypercellularity and marked irregularly thickened capillary walls, infiltrating inflammatory cells and mild to moderate mesangial expansion (periodic acid-Schiff, ×400). b Strong granular IgG deposits along the glomerular capillary walls and mesangial areas (×400). This pattern of staining can also be observed in isolated class 4 or class 5 LN. EM is the best way to identify this combined lesion when associated with global diffuse LN. c–e Glomerular capillaries show abundant granular dense deposits by EM in both subepithelial and subendothelial (arrows) locations of the glomerular basement membranes, some being superficial, others with basement membrane spikes, and a few fused over making them intramembranous and focal in the mesangial areas. The lamina densa may be largely intact or show focal “penetrating” deposits (×4,000, ×7,000 for c, d, ×8,000 for e). d This image also demonstrates the massive subendothelial and intraluminal accumulation of immune deposits. LN, lupus nephritis; EM, electron microscopy; ISN/RPS, International Society of Nephrology/Renal Pathology Society.

Fig. 12

Lupus podocytopathy. Minimal change disease. A young female SLE patient of 2 years duration, with no specific treatment, presented with a sudden onset of nephrotic syndrome. a Glo­merulus with preserved capillary architecture and patent lumina (PAS ×200). b Occasionally, there may be mild to moderate glomerular mesangial hypercellularity and mild increased matrix in this setting (×400). FSGS. This may develop in a few SLE patients presenting with active lupus serology and nephrotic syndrome. c Glomerulus with minimal mesangial thickening and an area of segmental capillary tuft collapse with sclerosis and capsular adhesion at the periphery, resembling the NOS form of primary FSGS with minimal to mild mesangial immune deposits by IF (PAS ×200). d, e The EM findings are similar in both minimal change disease and FSGS. Portions of 2 glomeruli showing normal thickness of basement membranes, total effacement of foot processes (arrows) with epithelial swelling, and microvillous transformation in the urinary space. The endothelial fenestrations are largely preserved. However, the mesangial areas appear prominent with finely granular small electron dense deposits (arrows) (×1,800). EM, electron microscopy; FSGS, focal segmental glomerulosclerosis; IF, immunofluorescence.

Fig. 13

Lupus podocytopathy. Collapsing glomerulopathy. This is a 35-year-old female patient who presented with severe nephrotic syndrome (10 g/24 h) and renal insufficiency (creatinine 2.5 mg/dL) with a recent diagnosis of SLE and high titers of ANA and anti-dsDNA. a, b Nearly 50% of the glomeruli showed segmental (a) or global (b) marked wrinkling and collapse with loss of patency of the lumina, but without an increase in intraglomerular cellularity or matrix, overlaid by hyperplastic and variably vacuolated epithelial cells in the areas of collapse (×400). There were mild tubulointerstitial changes initially, which progressed in a few months to significant tubular epithelial injury, vacuolization, and focal microcystic tubular changes. c Only focal fine IgG staining in the mesangial areas is found (×400). d, e On EM, the collapsed portions of the glomeruli show marked thickening and wrinkling of the capillary basement membranes, leading to mostly occlusion of the lumina with total foot process effacement and rare subepithelial, intramembranous, and mesangial densities, suggesting evidence of occasional immune deposits in the setting of lupus (d ×6,000, e ×12,000). f Glomerular findings adjacent to the areas of collapse, in addition to diffuse foot process effacement, demonstrate the severe visceral epithelial/podocyte injury changes, such as small and large confluent intracytoplasmic vacuolization with varying degenerative cytoplasmic changes (arrows) and condensation of the nuclear chromatin, and focal separation from the basement membranes. There is also total loss of endothelial fenestrations in the capillaries (×3,000). EM, electron microscopy; SLE, systemic lupus erythematosus.

Fig. 14

Glomerular organized deposits in LN. a–c These are from 1 case where the glomerular capillary basement membranes show extensive subendothelial and mesangial deposits (arrows) that are totally transformed as tubulofibrillar appearance, occurring in bundles that are irregularly arranged, ranging from 20 to 60 nm in diameter, without any evidence of a background of granular deposits. The overlying basement membrane is mild to moderately thickened with denudation or effacement of foot processes. Such deposits have also been seen exclusively in the subepithelial areas in other rare LN cases in our files (a ×10,000, b ×20,000, c ×60,000). d Another case with similar randomly arranged tubulofibrillar deposits, now admixed with a background of finely granular deposits (arrows) found in the capillary basement membranes and the mesangial area (×10,000). e High magnification EM image of glomerular deposits in different cases showing randomly arranged tubulofibrillar deposits along with a 2nd form of organized partial fingerprint deposit (arrows) in the case field (×60,000). EM, electron microscopy.

Fig. 15

Spectrum of organized fingerprint glomerular deposits in LN. a, b, e A case of diffuse LN with some granular subendothelial and mesangial deposits, where significant portions of them are transformed into mostly fragmented or partial “fingerprint-like” patterns (arrows), composed of concentric curved dark and light lines with a diameter of 10–15 nm (a ×25,000, b ×40,000, c, e ×60,000). c Deposits on both sides of the glomerular basement membrane with a fingerprint appearance (×40,000). d High magnification of an aggregate of fully formed “fingerprint-like” deposits in a glomerulus with LN (×40,000). LN, lupus nephritis.

Fig. 16

Cryoglobulin and other organized glomerular deposits in LN. a Extensive subendothelial randomly arranged fibrillar deposits showing short rigid fibrils with a solid cross section measuring 10–18 nm (×40,000). b Subendothelial deposits composed of haphazard aggregates of large hollow tubular deposits (arrows) in a patient where circulating cryoglobulins were identified (×40,000). c, d A case of diffuse LN with deposits having an unusual loose somewhat mottled appearance (arrows), throughout the glomerulus. Occasionally, such an appearance may be seen in patients who were treated with immunosuppressive therapy for a short duration (×10,000). LN, lupus nephritis.

Fig. 17

Tubuloreticular inclusion. a–c A TRI body characterized by reticular aggregates of cytomembranous structures of uniform diameter (arrows) found within the glomerular and sometimes peritubular capillary endothelial cell smooth endoplasmic reticulum, readily seen in active LN cases. The glomerular capillary basement membrane in (a) shows both subepithelial and subendothelial immune complex deposits (a ×20,000, b ×15,000, ×15,000). d Hematoxylin body: This is rarely seen as an “in vitro” phenomenon, where the bare neutrophil or endothelial nuclei are exposed to circulating ANA, leading to swelling and homogeneous condensation of the chromatin, assuming an unusual morphology of enlargement and hyperchromatism with extremely dark osmiophilic staining. By EM, it represents an acellular mass of amorphous material, which is identified in the glomerular capillary lumen (×8,000). e Concentric laminated myelin-like structures (“zebra bodies”) within podocytes, overlying a capillary loop with subepithelial electron dense deposits. This patient having systemic lupus was treated with chloroquine, presenting with proteinuria. The kidney biopsy shows class 5 membranous LN. These abnormal podocyte lipid inclusions are reminiscent of Fabry disease, due to the interference of chloroquine with the cellular lysosomal processing of phospholipids, a known complication of chloroquine therapy in some lupus patients (×3,000). LN, lupus nephritis; EM, electron microscopy; TRI, tubuloreticular inclusion.

Fig. 18

Active and chronic TIN, associated with lupus GN. a Active and chronic TIN, associated with class 4 lupus GN noted within the glomeruli included, composed of mainly large aggregates of mononuclear inflammatory cell infiltrate lymphocytes, a small proportion of plasma cells and macrophages, and focal tubulitis (PAS, ×200). b Positive Ig in glomerulus (left), as well as granular staining along the tubular basement membranes and interstitial space. c EM image showing nonspecific chronic tubulointerstitial changes, including tubular epithelial injury, numerous granular electron dense deposits of pale density within the tubular basement membranes with irregular thickening, and infiltrating inflammatory cells (×4,000). d Mild chronic TIN, associated with lupus GN. d Active and chronic TIN, associated with class 4 lupus GN (left) with focal tubulitis (PAS, ×200). e Positive deposits of Ig in the glomeruli but not in the tubulointerstitial compartment (×200). f Ultrastructurally, the tubules show nonspecific chronic tubulointerstitial changes, including tubular cell injury, thickened and focal laminated tubular basement membrane without immune deposits, interstitial fibrosis, and interstitial inflammation (×4,000). GN, glomerulonephritis; EM, electron microscopy; TIN, tubulointerstitial nephritis.

Fig. 19

Lupus vascular lesion: lupus vasculopathy. a A preglomerular arteriole discloses intimal PAS+ immune complex deposits with a flocculent appearance (arrows), without associated inflammation, accompanied by proliferative LN seen in the adjacent glomerulus (PAS ×400). b Intimal IgG deposits in a small arteriole (arrows) corresponding to the light microscopic finding, often showing a “full house” pattern by IF. c, d Segmental granular electron dense deposits having varied density (arrows) seen mainly restricted to the intimal layer and focally involving the medial layer (c ×6,000, d ×8,000). e An arteriole displaying exclusively intimal immune deposits (arrows), surrounded by 1 or 2 layers of smooth muscle medial layer (×1,800). f High magnification of a small arterial wall with small deposits (arrows) infiltrating in between the smooth muscle cells in the media. These may represent the usual uncomplicated vascular immune deposits detected by IF in many active LN cases (×6,000). g A peritubular capillary with abundant granular deposits in the basal lamina (×12,000). IF, immunofluorescence; LN, lupus nephritis.

Fig. 20

TMA in lupus: a, b This is a pediatric case of diffuse LN presenting with acute renal failure and severe clinical TMA features. The glomeruli show global endocapillary hypercellularity and focal capillary microthrombi (arrows) (a) and an occluded small artery and arteriolar thrombus trapping fragmented RBCs (arrows), with moderate ischemic collapse in an adjacent glomerulus (b) (a PAS ×200, b H&E ×200). c Same case as above, a glomerular capillary loop showing variable subendothelial widening with a lucent space and focal subendothelial deposits (arrows). The rest of the basement membrane is generally intact with relatively preserved foot processes (×12,000). d This is a lupus patient with detectable antiphospholipid antibodies presenting with a mild rise in creatinine and subnephrotic proteinuria without any evidence of immune complex-mediated glomerular lesions. Healing glomerular TMA lesion with organized subendothelial space containing increased matrix material trapping focal cellular debris (×6,000). e Another lupus patient with antiphospholipid antibodies, proteinuria, and mesangial LN, showing incidental intracapillary fibrin tactoids in 1 loop (arrows), while the adjacent capillary shows a foamy macrophage trapped in the lumen (×6,000). f, g Intrarenal arterioles from 2 different lupus patients with kidney biopsies, showing the loose edematous intimal expansion (arrows) with trapped dark fragments of RBCs and occluded by a thrombus with disintegrating cellular elements in (f). The other is completely occluded by the intimal mucoid swelling limited by the intact internal elastic lamina, with total separation of the endothelial cells, containing remnants of myointimal cellular elements in g (arrows) (f ×2,000, g ×1,800). LN, lupus nephritis; RBCs, red blood cells; TMA, thrombotic microangiopathy.