Immune Complex-Mediated MPGN: Adult-Onset Nephrotic Syndrome with AKI-CKD and Hypertension

1. Understanding the Full Clinical Label

2. What is IC-MPGN?

• Immunoglobulin/Immune complex-mediated (IC-MPGN) - the type in this patient
• Complement-mediated (C3 glomerulopathy: C3GN and Dense Deposit Disease)
• Neither (thrombotic microangiopathy pattern)

3. Causes of IC-MPGN (Secondary Causes to Rule Out)

• Hepatitis C virus (most common, often with cryoglobulinemia)
• Hepatitis B virus
• Bacterial: infective endocarditis, infected ventriculo-atrial shunts, visceral abscesses, leprosy, meningococcal meningitis
• Protozoal: malaria, schistosomiasis, leishmaniasis, filariasis

• Systemic lupus erythematosus (lupus nephritis class III/IV can mimic MPGN)
• Sjogren syndrome
• Rheumatoid arthritis
• Mixed connective tissue disease

• Monoclonal gammopathy (plasma cell or B-cell dyscrasia)

• Goldman-Cecil Medicine, Table 107-8

4. Pathogenesis of the MPGN Pattern

1. Circulating immune complexes (antigen + IgG antibody) deposit in subendothelial locations and the mesangium
2. Complement activation (C1q, C4, C3 all deposited - hallmark of classical pathway activation)
3. Mesangial cells proliferate and extend outward along the capillary wall
4. New basement membrane is synthesized in response to subendothelial deposits
5. This creates the characteristic "tram-track" or double-contour appearance (GBM duplication/splitting) on PAS/silver stain
6. Leukocytes infiltrate capillary lumens, and the glomerulus takes on a lobular, hypercellular appearance
7. Crescents may form in severe cases, progressing to RPGN

• Robbins, Cotran & Kumar Pathologic Basis of Disease, p. 850

5. Histopathology (Renal Biopsy Findings)

6. Why Does This Present as Adult-Onset Nephrotic Syndrome?

• Massive subendothelial and mesangial immune deposits damage the glomerular filtration barrier (podocytes + GBM)
• Complement-mediated inflammation increases GBM permeability
• Result: heavy proteinuria (>3.5 g/day) → hypoalbuminemia → reduced oncotic pressure → edema + ascites
• Compensatory hepatic lipoprotein synthesis → hyperlipidemia and lipiduria (oval fat bodies, fatty casts)

7. Complement Levels as a Diagnostic Clue

• Both C3 and C4 are reduced (intermittently) in IC-MPGN - because classical pathway activation consumes both
• In C3 glomerulopathy (complement-mediated MPGN): only C3 is low (alternative pathway only)
• C3 nephritic factor (C3NeF) stabilizes C3 convertase and is found in complement-mediated subtypes

8. Complications Explained

A. AKI-on-CKD (Acute Kidney Injury on Chronic Kidney Disease)

• Ongoing immune complex deposition → persistent glomerular inflammation → glomerulosclerosis
• Tubulointerstitial damage from proteinuria (filtered proteins are toxic to tubular cells)
• ~50% of untreated patients develop end-stage kidney disease within 10-15 years
• Only 25-40% maintain normal renal function without treatment
• Spontaneous remission occurs in <10% of cases

• Severe nephrotic syndrome causing renal hypoperfusion (reduced effective circulating volume)
• Crescentic transformation (RPGN) - rapid GFR loss from >50% crescent formation
• Thrombotic events (renal vein thrombosis - nephrotic syndrome increases coagulation risk)
• Sepsis (infections that triggered MPGN can also precipitate AKI)
• Contrast nephropathy or nephrotoxic drugs in a susceptible CKD patient
• Goldman-Cecil Medicine, Prognosis section

B. Hypertension

1. Fluid retention: proteinuria → hypoalbuminemia → secondary hyperaldosteronism → Na+ and water retention → volume overload
2. RAAS activation: reduced renal perfusion (from volume redistribution in nephrotic syndrome) → renin release → angiotensin II → vasoconstriction + aldosterone
3. Reduced renal mass (CKD): loss of pressure-natriuresis mechanism
4. Endothelin and sympathetic activation: inflammatory cytokines from glomerular injury activate sympathetic outflow

9. Clinical Presentation Summary

• Edema (periorbital in the morning, pedal/ankle, ascites in severe cases)
• Frothy urine (proteinuria)
• Hematuria (often microscopic - RBCs and RBC casts from nephritic component)
• Hypertension (moderate to severe)
• Oliguria or reduced urine output if AKI is present
• Fatigue and anorexia from uremia (if CKD advanced)
• Features of the underlying cause (jaundice/right upper quadrant pain in HCV; joint pain/rash in SLE; pallor/bone pain in monoclonal gammopathy)

10. Investigations

11. Treatment

• HCV-associated IC-MPGN: Direct-acting antivirals (DAAs) - highly effective, protect against recurrence
• SLE: hydroxychloroquine + appropriate immunosuppression for LN class
• Monoclonal gammopathy: clone-directed therapy (rituximab, bortezomib)
• Bacterial infections: antibiotics + source control

• RAAS blockade: ACE inhibitor or ARB - first-line for blood pressure control AND antiproteinuric effect (target BP <130/80 mmHg)
• Salt restriction and loop diuretics for edema/fluid overload
• Statins for hyperlipidemia
• Anticoagulation: consider prophylactic anticoagulation if serum albumin <2.5 g/dL (high VTE risk)

• Prednisone 1 mg/kg/day (max 60-80 mg/day) for 12-16 weeks
• Mycophenolate mofetil 2-3 g/day may be added for 6-12 months if suboptimal response
• Refractory: Cyclophosphamide 2 mg/kg/day + prednisone 10 mg/day (3-6 months), OR Rituximab 1 g x2 doses 2 weeks apart + third dose at 6 months

• IV methylprednisolone pulse (1-3 g) followed by oral corticosteroids + oral cyclophosphamide (ANCA-vasculitis regimen)

12. Prognosis

• 50-60% of untreated patients progress to ESKD within 10-15 years
• With treatment: remission in 30-50% of observational studies
• Worse prognosis with: crescents, nephrotic-range proteinuria, elevated creatinine at presentation, persistent hypertension, and failure to achieve remission
• Post-transplant: recurrence rate 20-45%; graft loss in up to 50% of those with recurrence
• Goldman-Cecil Medicine, Prognosis section

Summary Diagram (Pathophysiologic Flow)

Antigenic trigger (HCV / bacteria / SLE / monoclonal protein)
        ↓
Circulating immune complexes (Ag-Ab-C3)
        ↓
Subendothelial + mesangial deposition in glomerulus
        ↓
Classical complement activation (C1q → C4 → C3 → C5-9 MAC)
        ↓
Mesangial proliferation + GBM duplication (tram-track)
        ↓
Glomerular filtration barrier disruption
        ↓
Heavy proteinuria (>3.5 g/day)
        ↓
┌─────────────────────┬────────────────────────┐
│ Nephrotic Syndrome  │   Nephritic Component  │
│ - Hypoalbuminemia   │   - Hematuria          │
│ - Edema             │   - RBC casts          │
│ - Hyperlipidemia    │   - Hypertension       │
│ - Lipiduria         │   - Oliguria/AKI       │
└─────────────────────┴────────────────────────┘
        ↓ (progressive glomerulosclerosis)
    CKD → ESKD (50% at 10-15 years)

• Robbins, Cotran & Kumar Pathologic Basis of Disease - MPGN Morphology and Pathogenesis (p. 850)
• Goldman-Cecil Medicine, International Edition - MPGN Classification, Treatment and Prognosis (Ch. 107)
• Comprehensive Clinical Nephrology, 7th Edition - IC-MPGN in Transplantation
• NEJM 2025 - Pegcetacoplan in C3G and IC-MPGN (PMID: 41337715)