A 28-year-old Indian man with systemic lupus erythematosus presents with nephrotic syndrome and hematuria. Kidney biopsy shows diffuse proliferative glomerulonephritis with subendothelial immune deposits and wire-loop lesions. Which feature best distinguishes lupus nephritis (Class IV) from membranoproliferative glomerulonephritis (MPGN)?

Explanation

## Distinguishing Lupus Nephritis (Class IV) from MPGN ### Key Discriminating Features **Key Point:** The 'full-house' immunofluorescence pattern (IgG, IgA, IgM, C1q, and C3 all positive) combined with wire-loop lesions and subendothelial deposits is pathognomonic for lupus nephritis and distinguishes it from MPGN. ### Wire-Loop Lesions Wire-loop lesions are: - Thickened capillary walls with subendothelial immune complex deposits - Visible on light microscopy as wire-like thickening of capillary loops - Pathognomonic for lupus nephritis - Absent in MPGN (which shows intramembranous or subendothelial deposits without this appearance) ### Comparison Table: Lupus Nephritis (Class IV) vs. MPGN | Feature | Lupus Nephritis (Class IV) | MPGN | | --- | --- | --- | | **Light Microscopy** | Diffuse proliferative, wire-loop lesions, hyaline thrombi | Proliferative, double contour (tram-track) | | **Immunofluorescence** | Full-house (IgG, IgA, IgM, C1q, C3) | C3-dominant, IgM variable | | **Electron Microscopy** | Subendothelial deposits (humps) | Intramembranous or subendothelial | | **Serum Complement** | Low C3, C4 (consumed) | Low C3 (MPGN Type II) or normal (Type I) | | **Systemic Features** | Malar rash, photosensitivity, ANA+, anti-dsDNA+ | None (primary MPGN) | | **Hematuria Pattern** | Dysmorphic RBCs, RBC casts (nephritic) | Dysmorphic RBCs, RBC casts (nephritic) | | **Clinical Presentation** | Nephrotic + nephritic (mixed) | Nephrotic or nephritic | ### Full-House Immunofluorescence **High-Yield:** The 'full-house' pattern is virtually diagnostic of lupus nephritis: - **IgG** — positive (most prominent) - **IgA** — positive (distinguishes from other diseases) - **IgM** — positive - **C1q** — positive (complement activation via classical pathway) - **C3** — positive (often with C1q, unlike post-infectious GN) MPGN typically shows: - **C3-dominant** pattern (C3 > immunoglobulin) - IgM may be present but not the full spectrum - C1q usually absent ### Pathophysiology **Lupus Nephritis:** 1. Circulating immune complexes (anti-dsDNA, anti-nucleosome) deposit in glomeruli 2. Subendothelial location → wire-loop lesions 3. Classical complement pathway activation (C1q, C4 consumption) 4. Proliferative inflammation **MPGN:** 1. Immune complex deposition (post-infectious, cryoglobulins, membranoproliferative pattern) 2. Alternative or classical pathway activation 3. Intramembranous or subendothelial deposits without wire-loops **Mnemonic:** **FULL-HOUSE** = Fluorescence pattern with all Immunoglobulins and Complement components positive, Lupus-specific, Unique to SLE, Subendothelial deposits, Helps distinguish from MPGN, Outcome varies by class, Urgent treatment needed, Serology confirms (anti-dsDNA) **Clinical Pearl:** Wire-loop lesions are so characteristic of lupus that their presence should immediately raise suspicion for SLE even if serology is initially negative. They represent acute immune complex-mediated injury and indicate active lupus nephritis. [cite:Robbins 10e Ch 20]