How can lupus nephritis class III, class IV, and class V be distinguished on gross (macroscopic) and microscopic (light, immunofluorescence, and electron) examination of a kidney biopsy?

Distinguishing Lupus Nephritis Classes III, IV, and V on Kidney Biopsy

The key to distinguishing these classes lies in identifying the pattern and location of immune deposits and the presence or absence of leukocyte infiltration: Class III/IV show subendothelial deposits with leukocyte influx (proliferative), while Class V shows subepithelial deposits without leukocyte influx (membranous). 1

Macroscopic (Gross) Examination

Gross examination of kidney biopsies in lupus nephritis typically shows no distinguishing features between classes III, IV, and V. The kidney may appear normal or show non-specific changes. The diagnosis requires microscopic evaluation with light microscopy (LM), immunofluorescence (IF), and electron microscopy (EM).

Microscopic Examination

Light Microscopy Features

Class III (Focal Lupus Nephritis):

• <50% of glomeruli affected with active or inactive focal, segmental, or global endocapillary or extracapillary glomerulonephritis
• Segmental endocapillary hypercellularity is the hallmark feature 2
• May show fibrinoid necrosis (fibrin with GBM disruption and/or mesangial matrix lysis) 1
• Active lesions include endocapillary hypercellularity, karyorrhexis, fibrinoid necrosis, wire loops, and cellular crescents

Class IV (Diffuse Lupus Nephritis):

• ≥50% of glomeruli affected with active or inactive diffuse, segmental, or global endocapillary or extracapillary glomerulonephritis
• Global endocapillary hypercellularity is characteristic of Class IVG 2
• Wire loops (thickened capillary walls from subendothelial deposits visible by LM) are hallmarks 2
• Hyaline thrombi and double contours (basement membrane duplication) are prominent features 2
• Class IVS shows intermediate characteristics between Class III and IVG 2
• Note: The 2018 revision eliminated the S/G subdivisions for classification purposes, though these distinctions should still be described 1

Class V (Membranous Lupus Nephritis):

• Diffuse thickening of glomerular capillary walls
• No significant endocapillary hypercellularity or leukocyte infiltration 1
• Basement membrane "spikes" may be visible on silver stains
• Abundant mesangial deposits distinguish it from primary membranous nephropathy 3

Immunofluorescence Microscopy

Critical distinguishing feature: Location of immune deposits

Class III:

• Mesangial and peripheral (capillary wall) pattern most common 4
• IgG, C3, and C1q are typically present
• "Full house" pattern (IgG, IgA, IgM, C3, C1q) is characteristic of lupus

Class IV:

• Peripheral (capillary wall) pattern is by far the most common 4
• Intense granular staining along capillary walls
• IgG, C3, and C1q predominate
• Full house pattern frequently present

Class V:

• Membranous pattern with granular deposits along capillary walls
• Predominantly subepithelial location
• May have mesangial deposits as well 3
• Full house pattern can be present

Electron Microscopy

This is the definitive method for distinguishing classes based on deposit location:

Class III/IV:

• Subendothelial deposits (between endothelium and basement membrane) are the defining feature 1
• These deposits are associated with leukocyte influx (neutrophils and monocytes/macrophages) 1
• Mesangial deposits are also present
• The subendothelial location is what drives the proliferative response

Class V:

• Subepithelial deposits (between basement membrane and podocytes) are the defining feature 1
• No leukocyte influx into capillaries 1
• Mesangial deposits are typically abundant (unlike primary membranous nephropathy) 3
• May have intramembranous deposits

Mixed Class III/IV + V:

• Shows both subendothelial AND subepithelial deposits 1
• Leukocyte influx is present (from the proliferative component)
• This combination requires treatment as proliferative disease 3

Practical Algorithm for Classification

1. First, assess glomerular involvement by LM:

   • <50% glomeruli affected → Consider Class III
   • ≥50% glomeruli affected → Consider Class IV
   • Diffuse capillary wall thickening without proliferation → Consider Class V

2. Second, perform IF to identify deposit pattern:

   • Peripheral/capillary wall dominant → Class III/IV or V
   • Membranous pattern alone → Class V

3. Third, use EM to determine deposit location (DEFINITIVE):

   • Subendothelial deposits + leukocyte influx → Class III or IV (based on extent)
   • Subepithelial deposits without leukocyte influx → Class V
   • Both subendothelial and subepithelial → Mixed Class III/IV + V

Common Pitfalls

• Don't rely on LM alone: Wire loops and hyaline thrombi suggest Class IV, but EM is required for definitive classification
• Mesangial deposits are present in all classes: Their presence doesn't distinguish between classes
• Class V can coexist with Class III/IV: Always look for both subendothelial and subepithelial deposits on EM 3
• Prior steroid treatment reduces deposits: Interpretation must consider treatment history, as steroids reduce subendothelial and mesangial deposits 5
• Segmental vs. global distinction has poor reproducibility: The 2018 revision eliminated this as a classification criterion for Class IV, though it should still be described 1