Light Microscopy Immunofluorescence and Electron Microscopy

Histologically, the glomerular lesion in all four clinicopathologic categories is identical and is characterized by fibrinoid necrosis and crescent formation (Figs. 9.4 to 9.6). In less than 10% of specimens, the glomerulonephritis may be accompanied by necrotizing arteritis (Fig. 9.7) (usually in the interlobular arteries) or medullary angiitis affecting the vasa rectae (Fig. 9.8).

By light microscopy and electron microscopy, pauci-immune crescentic glomerulonephritis cannot be distinguished from anti-GBM crescentic glomerulonephritis; however, immunofluorescence microscopy readily distinguishes the two. Pauci-immune crescentic glomerulonephritis, by definition, has no or low-intensity immunostaining for immunoglobulin; however, often there is some staining for immunoglobulin (12). A reasonable

Table 9.2. Names and definitions of vasculitis adopted by the Chapel Hill Consensus Conference on the Nomenclature of Systemic Vasculitis

Large-vessel vasculitis

Giant cell arteritis Granulomatous arteritis of the aorta and its major branches, with a predilection for the extracranial branches of the carotid artery. Often involves the temporal artery. Usually occurs in patients older than 50 and often is associated with polymyalgia rheumatica. Takayasu arteritis Granulomatous inflammation of the aorta and its major branches. Usually occurs in patients younger than 50.

Medium-sized vessel vasculitis

Polyarteritis nodosa

Kawasaki disease

Necrotizing inflammation of medium-sized or small arteries8 without glomerulonephritis or vasculitis in arterioles, capillaries or venules.

Arteritis involving large, medium-sized, and small arteries, and associated with mucocutaneous lymph node syndrome. Coronary arteries are often involved. Aorta and veins may be involved. Usually occurs in children.

Small-vessel vasculitis Wegener's granulomatosis

Microscopic polyangiitis

Granulomatous inflammation involving the respiratory tract, and necrotizing vasculitis affecting small to medium-sized vessels, e.g., capillaries, venules, arterioles, and arteries. Necrotizing glomerulonephritis is common. Churg-Strauss syndrome Eosinophil-rich and granulomatous inflammation involving the respiratory tract and necrotizing vasculitis affecting small to medium-sized vessels, and associated with asthma and blood eosinophilia. Necrotizing vasculitis with few or no immune deposits affecting small vessels, i.e., capillaries, venules, or arterioles. Necrotizing arteritis involving small and medium-sized arteries may be present. Necrotizing glomerulonephritis is very common. Pulmonary capillaritis often occurs.

Vasculitis with IgA-dominant immune deposits affecting small vessels, i.e., capillaries, venules, or arterioles. Typically involves skin, gut and glomeruli, and is associated with arthralgias or arthritis. Vasculitis with cryoglobulin immune deposits affecting small vessels, i.e., capillaries, venules, or arterioles, and associated with cryoglobulins in serum. Skin and glomeruli are often involved.

Henoch-Schonlein purpura

Cryoglobulinemic vasculitis a Large artery refers to the aorta and the largest branches directed toward major body regions (e.g., to the extremities and the head and neck); medium-sized artery refers to the main visceral arteries (e.g., renal, hepatic, coronary, and mesenteric arteries), and small artery refers to the distal arterial radicals that connect with arterioles (e.g., renal interlobular arteries). Note that some small and large vessel vasculitides may involve medium-sized arteries, but large and medium-sized vessel vasculitides do not involve vessels smaller than arteries.

Modified from Jennette et al. (11), with permission.

Figure 9.4. Glomerulus from a patient with Wegener's granulomatosis demonstrating segmental fibrinoid necrosis and early cellular crescent formation (H&E).

Figure 9.5. Glomerulus from a patient with microscopic polyangiitis demonstrating a cellular crescent at the top of the image and a small irregular fuch-sinophilic (red) focus of fibrinoid necrosis near the bottom of the image (Masson trichrome stain).

Figure 9.6. Glomerulus from a patient with ANCA-positive renal-limited disease showing a large cellular crescent with extensive destruction of the glomerular tuft (Jones silver stain).

Figure 9.5. Glomerulus from a patient with microscopic polyangiitis demonstrating a cellular crescent at the top of the image and a small irregular fuch-sinophilic (red) focus of fibrinoid necrosis near the bottom of the image (Masson trichrome stain).

Microscopic PolyangiitisLeukocytoclasia
Figure 9.7. Interlobular artery in a renal biopsy from a patient with microscopic polyangiitis showing circumferential fibrinoid necrosis with associated leukocyte infiltration and leukocytoclasia (H&E).

approach is to draw the line at 2 + or less immunoglobulin staining on a scale of 0 to 4 + for pauci-immune disease. Pauci-immune crescentic glomerulonephritis often has irregular segmental or global staining for fibrin at sites of fibrinoid necrosis and crescent formation (Fig. 9.9). Electron microscopy may show no electron-dense deposits, or there may be a few small electron-dense deposits, especially if immunofluorescence micros-

Leukocytoclasia
Figure 9.8. Medullary vasa recta in a renal biopsy from a patient with Wegener's granulomatosis showing angiitis with leukocytoclasia (H&E).
Lupus Nephrotis Fluroescent Stain Igg
Figure 9.9. Glomerulus from a patient with ANCA crescentic glomerulonephritis irregular staining of a large crescent by direct immunofluorescence microscopy using an antibody specific for fibrin.

copy revealed staining for immunoglobulin. Glomerular basement membrane breaks often can be identified.

Microscopic polyangiitis is necrotizing vasculitis with few or no immune deposits affecting small vessels, that is, capillaries, venules, or arterioles (11,13). Necrotizing arteritis occurs in some but not all patients. Approximately 90% of patients with microscopic polyangiitis have glomerulonephritis. Hemorrhagic pulmonary alveolar capillaritis is common in patients with microscopic polyangiitis. Histologically, the acute vascular lesions, for example, affecting dermal venules or small visceral arteries, are characterized by segmental fibrinoid necrosis, and mural and perivascular neutrophilic infiltration with leukocytoclasia (Figs. 9.7 and 9.8). Within a few days, the predominant inflammatory cells in the vasculitic lesions evolve from neutrophils to mononuclear leukocytes, and the fibrinoid necrosis transforms into fibrosis.

Wegener's granulomatosis is characterized by granulomatous inflammation that frequently is accompanied by necrotizing vasculitis affecting capillaries, venules, arterioles, and small to medium-sized arteries (11). Necrotizing granulomatous inflammation is observed most often in the upper and lower respiratory tract, but occasionally in other tissues, such as the orbit, skin, and kidneys. The granulomatous lesions typically have extensive necrosis with infiltrating mononuclear and polymorphonuclear leukocytes with scattered multinucleated giant cells. Necrotizing glomeru-lonephritis is common (Fig. 9.4). The vasculitis in the lungs and elsewhere can involve arteries, arterioles, veins, venules, and capillaries, and can be granulomatous or nongranulomatous. The latter is histologically identical to the necrotizing vasculitis of microscopic polyangiitis and Churg-Strauss syndrome.

Churg-Strauss syndrome is characterized by eosinophil-rich granuloma-tous inflammation involving the respiratory tract and necrotizing vasculitis affecting small to medium-sized vessels that is associated with asthma and blood eosinophilia (11). The vasculitis of Churg-Strauss syndrome cannot be definitively differentiated by histology from the vasculitis of Wegener's granulomatosis or microscopic polyangiitis; however, there is a tendency for more eosinophils among the infiltrating leukocytes. Likewise, the nec-rotizing granulomatous inflammation of Churg-Strauss syndrome resembles that of Wegener's granulomatosis but tends to have more eosinophils. The vasculitis of Churg-Strauss syndrome most often affects the lungs, heart, peripheral nervous system, skin, gut, and kidneys. The pauci-immune focal necrotizing glomerulonephritis of Churg-Strauss syndrome usually is less severe than the glomerulonephritis in Wegener's granulomatosis or microscopic polyangiitis, but is histologically indistinguishable.

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