Overview & Classification Evolution
MPGN is a pattern of glomerular injury characterized by endocapillary proliferation and thickening of the GBM. The 2016 classification fundamentally reorganized MPGN based on immunofluorescence patterns and pathogenic mechanisms rather than EM appearance alone.
Traditional Classification (Pre-2016)
- Type I: Subendothelial deposits
- Type II (DDD): Intramembranous "dense" deposits
- Type III: Subendothelial + subepithelial deposits
Modern Classification (2016+)
I. Immune-Complex MPGN: Dominant Ig deposits on IF. Associated with lupus, post-infectious GN, cryoglobulinemia.
II. Complement-Mediated (C3-Dominant): C3-dominant/exclusive on IF. Subdivided into DDD and C3-GN.
📚 Key Point: Modern MPGN classification prioritizes IF pattern and genetic/molecular mechanism over EM appearance. This shift has enabled targeted therapy for complement-mediated disease.
Pathophysiology: Complement Dysregulation
Alternative Pathway Activation (C3-Mediated MPGN)
Results from uncontrolled alternative complement pathway activation:
- Factor H mutations (most common; impairs C3 regulation)
- Factor B mutations (alternative pathway amplification)
- C3 mutations (rare, highly penetrant)
- CFHR1-CFHR5 mutations (factor H-related proteins)
- Acquired: Anti-FH antibodies, C3 nephritic factors
Dense Deposit Disease (DDD) Distinctive Features
- Intramembranous "ribbon-like" electron-dense deposits
- Often associated with C3-nephritic factors or complement gene mutations
- More aggressive course: 70% progression to ESRD at 10 years without treatment
Clinical Presentation
- Hematuria: Microscopic or gross; persistent or recurrent
- Proteinuria: Mild to nephrotic (0.5–10 g/day)
- Hypertension: Common
- Edema: Variable depending on proteinuria severity
Diagnosis
Critical Complement Distinction
| Finding | Immune-Complex MPGN | C3-MPGN |
|---|
| C3 | Usually low | Low/normal (dominant pattern) |
| C4 | Low (classical pathway) | Normal |
| IF Pattern | Ig-dominant (IgG, IgM, IgA) ± C3 | C3-dominant or C3-exclusive |
| Serologies | Often positive (ANA, HCV, cryoglobulins) | Negative |
📚 Clinical Pearl: Low C3 + hematuria + proteinuria + seronegative workup is classic for C3-MPGN. Immune-complex MPGN typically has positive serology and may be reversible if the underlying disease is treated.
Biopsy: LM Hallmarks
- Endocapillary proliferation with membranoproliferative pattern
- GBM duplication ("tram-track" appearance)
- Mesangial expansion
- Leukocyte infiltration (especially immune-complex MPGN)
Treatment Approaches
Immune-Complex MPGN: Address the Underlying Cause
- Post-infectious GN: Supportive care; often recovers
- Lupus GN: MMF + corticosteroids per SLE guidelines
- Cryoglobulinemia: Direct-acting antivirals for HCV; rituximab for vasculitis
C3-Mediated MPGN: Complement-Targeted Therapy
📚 Key Point: Complement inhibition has revolutionized C3-MPGN/DDD treatment. Pegcetacoplan and emerging Factor D inhibitors show promise for inducing remission and halting progression.
- Pegcetacoplan (C3 inhibitor): FDA-approved for C3-MPGN; can induce remission/slow progression
- Iptacopan (Factor D inhibitor): Emerging evidence in C3-MPGN trials
- Eculizumab (C5 inhibitor): Limited efficacy in C3-GN alone; may help in some DDD cases
Supportive Care (All Patients)
- ACE-I/ARB to reduce proteinuria
- BP control (<120/80 mmHg target)
- Pneumococcal and meningococcal vaccines (especially before complement inhibitors)
Prognosis
| Subtype | 10-Year Renal Survival | Key Factors |
|---|
| Post-infectious | 85–95% | Age <50, preserved GFR |
| Lupus-associated | 70–85% | Lupus activity; nephritis class |
| Cryoglobulinemia-related | 60–75% | HCV viral load; treatment response |
| DDD (untreated) | 30% | 70% progress to ESRD by 10 years |
| C3-GN with mutations | 40–50% | Without complement-targeted therapy |
⚠️ Post-Transplant Recurrence: C3-MPGN/DDD has a 50–80% recurrence rate post-transplant. Consider complement-targeted therapy prophylactically; monitor with protocol biopsies.
Special Considerations
Monoclonal Immunoglobulin Deposition (MGRS)
Some MPGN cases present with monoclonal IgG/IgM deposits (kappa or lambda restricted). May represent Monoclonal Gammopathy of Renal Significance (MGRS). Requires hematology evaluation and plasma cell-directed therapy.
Clinical Pearls & Practice Points
- Always interpret complement levels with immunofluorescence together — C3 dominance + low C3 is pathognomonic for C3-MPGN
- Post-infectious MPGN may resolve spontaneously — avoid aggressive immunosuppression
- Genetic testing for complement mutations increasingly recommended in C3-MPGN
- Complement inhibition represents a paradigm shift for C3-MPGN/DDD
- DDD morphology portends aggressive disease — early complement-targeted agents recommended
References
- Sethi S, Fervenza FC. MPGN — a new look at an old entity. NEJM. 2012;366(12):1119-1131. PubMed
- Pickering MC, et al. C3 glomerulopathy: consensus report. Kidney Int. 2013;84(6):1079-1089. PubMed
- Bomback AS, et al. Eculizumab for dense deposit disease and C3 glomerulopathy. CJASN. 2012;7(5):748-756. PubMed
- Jokiranta TS. HUS and atypical HUS. Blood. 2017;129(21):2847-2856. PubMed
- Riedel M, et al. MPGN type I and III. Pediatr Nephrol. 2012;27(9):1503-1514. PubMed Search
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