Questions: Proliferative Glomerulonephritis: Immune Deposition and Glomerular Cell Proliferation

Question 1

Two patients both have immune complex deposits in their glomeruli. Patient A has subendothelial deposits; Patient B has subepithelial deposits. Which patient is more likely to present with severe acute inflammatory nephritis, and why?

• A. Patient B, because subepithelial deposits are closer to the urinary filtrate and cause more podocyte damage
• B. Patient A, because subendothelial deposits sit inside the capillary wall, accessible to circulating complement and leukocytes that drive aggressive inflammation
• C. Both equally, because immune complex size determines inflammation severity, not location
• D. Patient A, because the endothelium directly contacts deposits and releases inflammatory mediators independent of complement

Answer: (B)

Location determines inflammatory behavior. Subendothelial deposits lie between the endothelium and the GBM — inside the capillary wall and directly accessible to circulating complement proteins, neutrophils, and monocytes. Classical complement activation generates C3a and C5a, powerful chemoattractants that recruit leukocytes, causing the active cellular inflammation characteristic of diffuse proliferative nephritis. Subepithelial deposits (between GBM and podocytes) are on the urinary side of the GBM, shielded from circulating immune effectors, producing slower proteinuria without overt inflammatory infiltration.

Question 2

Why is rapidly progressive glomerulonephritis (RPGN) treated as a medical emergency even before biopsy results are complete?

• A. Because RPGN always progresses to nephrotic syndrome if untreated, which is more difficult to treat
• B. Because glucocorticoids are only effective before crescent formation begins, and biopsies take weeks to process
• C. Because crescent formation can obliterate glomerular function within days to weeks, and each day of delay corresponds to permanent nephron loss
• D. Because RPGN triggers a systemic autoimmune cascade that affects other organs if not treated within 48 hours

Answer: (C)

Crescents — structures formed by proliferating parietal epithelial cells and macrophages filling Bowman's space — functionally obliterate individual glomeruli. When >50% of glomeruli are involved, GFR can fall catastrophically within days to weeks. Unlike most nephropathies that progress over months to years, RPGN can result in dialysis dependence within weeks. Treatment delay is directly measured in permanent glomerular destruction, which is why aggressive immunosuppression begins empirically before the full biopsy workup is complete.

Question 3

In proliferative glomerulonephritis, the location of immune complex deposition within the glomerulus predicts the clinical severity of the disease.

• T. True
• F. False

Answer: True

This is the core architectural insight of proliferative GN. Subendothelial deposits (inside the capillary wall, as in diffuse proliferative lupus nephritis) access complement and leukocytes → aggressive active nephritis. Subepithelial deposits (outside the GBM, shielded from circulating effectors, as in membranous nephropathy) → slower proteinuria without cellular infiltration. Mesangial deposits → mesangial cell expansion. Location predicts phenotype.

Question 4

Crescent formation in the glomerulus is produced exclusively by proliferating glomerular endothelial and mesangial cells.

• T. True
• F. False

Answer: False

Crescents are built from proliferating parietal epithelial cells (lining Bowman's capsule) and infiltrating macrophages, scaffolded by fibrin that leaks into Bowman's space when the GBM ruptures. Endothelial and mesangial cell proliferation is characteristic of proliferative GN more broadly, but crescent formation specifically involves the parietal epithelium and fibrin scaffold — this is what makes crescentic GN a distinct and severe pathological entity.

Question 5

Explain why subendothelial immune complex deposits cause more aggressive inflammation than subepithelial deposits, even though both contain similar antibody-antigen complexes.

Model answer: The GBM acts as a physical barrier. Subendothelial deposits lie inside the capillary wall, where circulating complement proteins and leukocytes can directly contact them. Complement activation generates C3a and C5a, recruiting neutrophils and monocytes that release proteases and reactive oxygen species, destroying the filtration barrier. Subepithelial deposits sit between the GBM and podocytes on the urinary side — effectively outside the capillary and shielded from circulating immune effectors. They disrupt podocyte foot processes (causing proteinuria) but without leukocyte-driven destruction, the disease is slower and less inflammatory.

This spatial logic — access to circulating immune effectors — is why deposit location is the key variable in proliferative GN. It also explains why subepithelial diseases (like membranous nephropathy) are characterized by proteinuria and nephrotic syndrome rather than hematuria and the inflammatory nephritic syndrome seen with subendothelial disease.