Renal Digest
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    Immunotactoid glomerulopathy
     Key features:
 
Clinical: Commonly associated with B-cell lymphoproliferative disorders; often presents with nephrotic range proteinuria
Light microscopy: Congo red-negative extracellular paraprotein deposition, usually with MPGN-like pattern of glomerular injury; mesangioproliferative or membranous patterns are less common
Immunofluorescence: Monoclonal immunoglobulin reactivity (e.g., IgG/kappa)
Electron microscopy: Parallel arrays of microtubules or fibrils, usually greater than 30 nm (10-90 nm) in diameter


Definition:  
Immunotactoid glomerulopathy is a glomerular disease characterized by deposition of abnormal proteins (paraproteins) that are Congo red negative by light microscopy and show microtubular or fibrillary substructure by electron microscopy. The microtubules are usually greater than 30 nm in diameter, sometimes with central core, and are arranged parallel to the glomerular basement membranes. There is usually monoclonal immunoglobulin reactivity by immunofluorescence. Clinically, immunotactoid glomerulopathy is strongly associated with B-cell lymphoproliferative disorders.

Synonym:
Paraprotein deposition disease

Differential diagnosis:
Fibrillary glomerulonephritis
Immunoglobulin A (IgA) nephropathy, mesangioproliferative type
Membranoproliferative glomerulonephritis (MPGN), idiopathic, type I
Cryoglobulin-associated glomerulonephritis
Lupus nephritis, focal (class III)

Etiology:  
Deposition of abnormal monoclonal protein in patients with paraproteinemia and lymphoproliferative disorder

Clinical:  
Usually occurs in patients over 60 years of age
Usually presents with nephrotic range proteinuria; other presenting symptoms vary, from nephrotic syndrome, hematuria, acute renal failure, rapidly progressive nephritis, chronic renal insufficiency

Histopathology:  
Commonly, there is mesangial expansion with increased mononuclear inflammatory cells and matrix and peripheral capillary loop thickening (MPGN-like pattern of injury); rarely, the predominant pattern can be mesangioproliferative (if the deposition is not involving capillary loops) or, even less commonly, predominantly membranous
Proliferative changes, such as increased endocapillary proliferation or crescent formation, can be seen on rare occasions
Congo red stain is negative
Silver stain may reveal “moth eaten” appearance (non-reactive deposits admixed with reactive matrix)

Immunofluorescence:   

      Monoclonal (kappa or lambda) reactivity of immunoglobulins (usually IgG)
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Electron microscopy:  
Visceral epithelial cells: Focal, sometimes marked effacement of visceral epithelial cell foot processes
Glomerular basement membranes: Microtubular deposits, frequently in parallel arrangements, can be seen in subepithelial and intramembranous locations, extending to the paramesangial and mesangial compartment; the basement membrane can be affected with deposits in various degrees. The fibrils or microtubules are non-branching, usually greater than 30 nm (10-90 nm) in diameter
Glomerular endothelial cells: Show loss of fenestrations and other non-specific changes; they do not contain tubuloreticular structures
Mesangium: Usually expanded by matrix and organized microtubular deposits

Clinical differential diagnosis:  
Diseases that present with nephrotic syndrome (membranous nephropathy, amyloidosis, and idiopathic (primary) focal and segmental glomerulosclerosis)
Diseases with acute renal failure and rapidly progressive nephritis (crescentic glomerulonephritides)

Pathologic differential diagnosis:  
Immune complex-mediated glomerulonephritides with MPGN pattern of injury; these include idiopathic MPGN, autoimmune diseases (such as lupus class IV, MCTD, RA, SS) and chronic infections (hepatitis B and C, hepatitis C-related cryoglobulinemia, endocarditis, shunt infections, parasitic infections)
Fibrillary glomerulopathy
Chronic thrombotic angiopathies
Diabetic glomerulosclerosis
IgA nephropathy
Membranous glomerulopathy
Lupus nephritis
Postinfectious glomerulonephritis
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Immunotactoid glomerulopathy: subepithelial microtubular deposits, with parallel arrangement; EM, 38,000x
Subepithelial, parallely arranged microtubular deposits; EM, 14,000x
Subepithelial deposits of various sizes, without basement membrane `spikes`, in immunotactoid glomerulopathy; EM, 8,900x
Substructural organization of deposits in immunotactoid glomerulopathy; EM, 28,000x
Mesangial microtubular deposits in immunotactoid glomerulopathy; EM, 18,000x
Very thickened capillary loop due to a presence of organized intramembranous deposits, in a case of immunotactoid glomerulopathy; EM, 8,000x
Peculiar subepithelial deposit with curvilinear and stright shape of long fibrils; EM, 30,000x
Subepithelial and intramembranous organized deposits in immunotactoid glomerulopathy. Note complete effacement of visceral epithelial cell foot processes; EM, 15,000x
Organized mesangial deposits in a patient with circulating IgG-kappa paraprotein and lymphoproliferative disorder; EM, 25,000x
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