Lupus Nephritis

• Autoantibody Generation and Immune Complex Formation: SLE is characterized by defective central and peripheral tolerance leading to production of anti-double-stranded DNA (anti-dsDNA) and anti-nucleosome antibodies, the primary pathogenic antibodies in lupus nephritis. These circulating immune complexes (CICs) deposit in the glomerular basement membrane (GBM) and mesangium through several mechanisms: (1) in situ formation where antibodies directly bind to planted antigens (nucleosomal antigens, C1q, histones) already present in the glomerulus; (2) circulating complex deposition where preformed CICs deposit from the circulation in areas of capillary hemodynamic stress; and (3) lattice formation involving optimal antigen-antibody ratios. The quantity and avidity of anti-dsDNA antibodies correlate with disease activity and glomerular complement deposition, making these antibodies both pathogenic mediators and clinical biomarkers.

• Complement Activation and Glomerular Inflammation: Immune complex deposition activates the classical complement pathway through C1q binding, leading to sequential activation of C3 and C5 convertases and membrane attack complex (MAC) formation. This generates powerful chemoattractants (C3a, C5a) that recruit neutrophils and monocytes into the glomerulus, perpetuating inflammation. Notably, lupus nephritis demonstrates "full house" immunofluorescence with deposits of IgG, IgA, IgM, C3, C1q, and fibrinogen—a distinctive pattern reflecting global complement activation. Genetic variations in complement components (particularly C1q deficiency or mutations) and regulatory proteins (Factor H, Factor I polymorphisms) increase susceptibility to both SLE and severe lupus nephritis. Local complement activation also generates C5a and MAC, which directly injure glomerular endothelial cells, podocytes, and parietal epithelial cells through complement-mediated cytotoxicity and generation of reactive oxygen species (ROS).

• Glomerular Resident Cell Activation and Perpetuation: Mesangial cells and glomerular endothelial cells become activated through immune complex engagement of Fc receptors (FcγRIIa, FcγRIIb) and toll-like receptors (TLR7, TLR9), leading to production of inflammatory cytokines (TNF-α, IL-1β, IL-6, GM-CSF) and chemokines (MCP-1, RANTES). This amplifies leukocyte recruitment and activation. Additionally, immune complexes and complement fragments activate resident dendritic cells and macrophages to produce interferon-α (IFN-α), perpetuating adaptive immunity within the kidney. Podocyte injury occurs through both immune complex deposition on the subepithelial surface (in membranous lupus nephritis) and direct complement-mediated toxicity, resulting in cytoskeletal disruption, foot process effacement, and proteinuria. The net result is progressive glomerular injury manifesting histologically as proliferative inflammation, crescents, necrosis, and basement membrane thickening depending on disease phenotype.

• Genetic and Environmental Susceptibility Interplay: Multiple genetic polymorphisms increase lupus nephritis risk, including variations in PTPN22 (protein tyrosine phosphatase), ITGAM (integrin αM), and complement genes. Environmental triggers (particularly infections and UV exposure) can activate innate immunity and breach tolerance mechanisms in genetically susceptible individuals. Hormonal factors, particularly estrogen, enhance B cell activation and autoantibody production, explaining the higher incidence in women of reproductive age.

• SLE (Primary Cause): Lupus nephritis occurs exclusively as a manifestation of SLE by definition, though not all SLE patients develop renal involvement. The four ACR/EULAR SLE classification criteria most relevant to lupus nephritis are: (1) persistent proteinuria ≥0.5 g/day or cellular casts; (2) acute glomerulonephritis with proteinuria OR hematuria plus cellular casts; (3) positive anti-dsDNA antibodies; and (4) low serum complement C3 or C4. Patients meeting SLE criteria with glomerulonephritis on biopsy are defined as having lupus nephritis.

• High-Risk SLE Phenotypes: Certain demographic and clinical features increase lupus nephritis risk. African American, Hispanic, Asian, and Native American ancestry carry 2-5 fold increased risk compared to Caucasians, likely reflecting both genetic and socioeconomic/healthcare access factors. Males with SLE have higher risk of severe lupus nephritis despite lower overall SLE prevalence. Early-onset SLE (age <16 years) and SLE with high anti-dsDNA titers and hypocomplementemia at diagnosis portend higher risk of renal involvement.

• Disease Activity and Serologic Markers: Active SLE (evidenced by elevated anti-dsDNA antibodies and depressed C3/C4 complement levels) is the primary driver of lupus nephritis development and flares. Persistent proteinuria, hematuria, or elevated serum creatinine in an SLE patient indicates either active lupus nephritis or alternative renal pathology requiring investigation.

• Secondary Risk Modifiers: Infections (particularly viral reactivation), UV exposure, medications (NSAIDs, hydralazine, procainamide), pregnancy, and non-adherence to immunosuppression increase lupus nephritis activity or renal disease progression.

• Asymptomatic Urinary Abnormalities (Most Common Presentation): Approximately 50-60% of lupus nephritis patients present with hematuria and/or proteinuria detected on routine screening in the setting of known SLE. Microscopic hematuria may precede SLE diagnosis. This asymptomatic presentation reflects early glomerular immune complex deposition and complement activation triggering leak of red blood cells and proteins across damaged filtration barrier before sufficient inflammation accumulates to cause symptoms.

• Nephrotic Syndrome: Occurs in 20-30% of lupus nephritis patients, most commonly with membranous (WHO Class V) or proliferative lupus nephritis with prominent subepithelial immune complex deposition. Patients present with nephrotic-range proteinuria (>3.5 g/day), hypoalbuminemia, edema (periorbital, peripheral, sacral), and hyperlipidemia. Pathophysiologically, podocyte injury from immune complex deposition and complement-mediated toxicity causes foot process effacement and loss of charge-based filtration barrier selectivity, allowing albumin leakage. Nephrotic lupus nephritis carries elevated thrombotic risk from loss of anticoagulant proteins (protein C, protein S) and increased thrombin generation.

• Acute Nephritic Syndrome: Presents with rapid onset hematuria (cola-colored urine), hypertension, edema, and rapidly rising serum creatinine. This reflects acute glomerular inflammation with crescent formation and necrosis, particularly in proliferative lupus nephritis (WHO Classes III-IV). Patients may have flank pain or other constitutional symptoms of SLE flare.

• Rapidly Progressive Glomerulonephritis (RPGN): Occurs in approximately 5-10% of lupus nephritis cases, presenting with rapid decline in renal function over days to weeks, often accompanied by severe hypertension, hematuria with RBC casts, and oliguria. This severe phenotype reflects crescentic lupus nephritis with segmental necrosis and extracapillary proliferation. Histologically may show cellular or fibrocellular crescents. RPGN lupus nephritis represents a medical emergency requiring aggressive immunosuppression.

• Lupus with Secondary ANCA-Associated Vasculitis: Rare but increasingly recognized phenotype where SLE patients develop concurrent anti-neutrophil cytoplasmic antibody (ANCA) positivity, typically MPO-ANCA, leading to necrotizing crescentic glomerulonephritis ("superimposed" vasculitis). This presents with rapid GFR decline, hematuria, and RBC casts, histologically showing both immune complex deposits (lupus) and fibrinoid necrosis with absent immune deposits (vasculitis pattern).

• Systemic Manifestations Accompanying Renal Disease: Most SLE patients with lupus nephritis have concurrent or recent systemic manifestations including photosensitive malar rash, arthritis/arthralgia, serositis (pleuritis, pericarditis), hematologic cytopenias, or constitutional symptoms (fever, fatigue, weight loss). These provide clinical context for diagnosis and suggest active SLE driving renal involvement.

• Physical Examination Findings: Hypertension (present in 50% at diagnosis, increases with disease progression), peripheral edema (nephrotic cases), signs of systemic SLE (malar rash, photosensitive dermatitis, oral ulcers, lymphadenopathy), and signs of extrarenal manifestations guide diagnosis.

• Clinical Suspicion and Serologic Screening: Lupus nephritis should be suspected in any SLE patient with new or worsening proteinuria, hematuria, rising serum creatinine, or systemic SLE activity. The diagnostic workup begins with: (1) urinalysis with microscopy showing hematuria (may be dysmorphic RBCs indicating glomerular origin), proteinuria, and cellular casts (WBC casts, RBC casts, or granular casts suggest active glomerulonephritis); (2) 24-hour urine protein or urine protein-to-creatinine ratio (UPCR) quantifying proteinuria (nephrotic range >3.5 g/day, non-nephrotic 0.5-3.5 g/day); (3) serum creatinine and calculated GFR assessing renal function; (4) serum albumin and lipid panel when nephrotic syndrome present; (5) Anti-dsDNA antibodies with high titers correlating with lupus nephritis activity (sensitivity 70-90% in active lupus nephritis, though 30-40% specificity for renal involvement specifically).

• Complement Measurements: Serum C3 and C4 levels <normal indicate active complement consumption from immune complex activation. Persistently low complement (particularly C3) at lupus nephritis diagnosis predicts worse prognosis and higher treatment intensity needed. C3 normalization during therapy indicates response and immunologic remission. Very low C1q (measured at specialized labs) additionally correlates with severe proliferative lupus nephritis and predicts worse outcomes. Up to 20% of lupus nephritis patients maintain normal complement despite active disease ("normalization phenotype"), limiting specificity but not sensitivity of complement depression.

• Renal Biopsy - Gold Standard Diagnosis: Percutaneous kidney biopsy under ultrasound guidance remains the definitive diagnostic test, revealing histopathologic classification and enabling prognosis stratification. Biopsy is indicated in: (1) SLE patients with unexplained renal dysfunction; (2) persistent proteinuria >0.5 g/day; (3) hematuria with casts; (4) to differentiate lupus nephritis from other causes (drug-induced glomerulonephritis, thrombotic microangiopathy, other GN); (5) before initiating major immunosuppression to confirm diagnosis and determine therapy intensity. The WHO classification (modified by ISN/RPS in 2003) categorizes lupus nephritis into six classes based on light microscopy, immunofluorescence, and electron microscopy findings:

• Class I (Minimal Mesangial): Normal by light microscopy, mesangial immune deposits on IF. Excellent prognosis, ~5% progress to higher classes, minimal treatment needed.

• Class II (Mesangial Proliferative): Mesangial hypercellularity/expansion on LM, mesangial dominant IF deposits. 10-15% progress, generally good prognosis with conservative management.

• Class III (Focal Proliferative): <50% of glomeruli with endocapillary or extracapillary proliferation, often with necrosis and crescent formation. Associated with worse outcomes; requires intensive immunosuppression. IF shows full house pattern. Two sub-categories: IIIa (with subendothelial deposits) and IIIb (with subepithelial "humps").

• Class IV (Diffuse Proliferative) - Most Common and Severe: ≥50% of glomeruli with proliferation, often with necrosis, crescents, and wire-loop lesions. Represents most aggressive lupus nephritis, highest risk of ESRD (40-60% progression without treatment), mandates intensive immunosuppression. Two sub-categories based on segmentation: IVa (segmental, <75% capillary loops) and IVb (global, ≥75% capillary loops).

• Class V (Membranous): Subepithelial "dome-shaped" immune complex deposits on EM, GBM thickening mimicking membranous nephropathy. Presents with nephrotic syndrome (60-70% of Class V patients). Can coexist with proliferative disease (Class III+V or IV+V, termed "mixed"). Class V alone has better prognosis than Class IV, though nephrotic syndrome carries thrombotic risk.

• Class VI (Advanced Sclerosing): >90% glomerulosclerosis indicating advanced chronic disease with irreversible injury. Represents end-stage lupus nephritis, generally unresponsive to further immunosuppression; supportive care and renal replacement therapy required.

• Immunofluorescence (IF) Findings: The "full house" pattern (deposits of IgG, IgA, IgM, C3, C1q, and fibrinogen) is highly specific for lupus nephritis (present in >90% of lupus nephritis, uncommon in other GN). Individual IF patterns include: dominant IgG deposition, granular IF distribution (vs linear in anti-GBM disease), and C3 dominant pattern (suggests C3 nephropathy if no other lupus features present).

• Electron Microscopy (EM) Findings: Reveals subendothelial, mesangial, and subepithelial immune complex deposits. "Subepithelial humps" (dense deposits on podocyte side of GBM) are characteristic but not pathognomonic. Dense intramembranous deposits and "ribbon-like" GBM deposits also described. Wire-loop lesions (thickened capillary wall from subendothelial deposits) on LM correspond to subendothelial EM deposits.

• Activity and Chronicity Indices: Beyond WHO classification, most pathologists score lupus nephritis biopsies using Activity Index (reflects reversible inflammation: endocapillary proliferation, extracapillary proliferation/crescents, necrosis, hyaline thrombi, leukocyte exudation, interstitial inflammation) and Chronicity Index (reflects irreversible damage: glomerulosclerosis, fibrous crescents, tubular atrophy, interstitial fibrosis). Higher activity scores predict response to therapy; higher chronicity scores predict worse long-term outcomes despite treatment. These indices independently predict progression and guide treatment intensity.

• Differential Diagnosis: Alternative diagnoses to exclude with biopsy:
• Other causes of GN in SLE (secondary membranous from medications, IgA nephropathy, thrombotic microangiopathy, ANCA-associated vasculitis)
• Drug-induced lupus (antihistone antibodies, absence of anti-dsDNA)
• **Acute tubular necrosis

Stabilize first

• Rapidly progressive/crescentic disease (emergency): pulse IV glucocorticoids (methylprednisolone) started before biopsy results return if RPGN is clinically evident; treat hyperkalemia, volume overload, and uremia, with dialysis if refractory. Plasma exchange is not routine for lupus nephritis and is reserved for concurrent thrombotic microangiopathy/TTP, anti-GBM antibodies, or catastrophic antiphospholipid syndrome.

Background therapy for everyone (ACR and EULAR SLE recommendations)

• Antimalarial: hydroxychloroquine for all SLE patients unless contraindicated — reduces flares, thrombosis, and renal progression; requires baseline and annual ophthalmologic screening.
• Antiproteinuric/renoprotective: RAAS blockade (ACE inhibitor or ARB) for proteinuria and blood pressure control, plus statin therapy for lipid risk; KDIGO endorses adding an SGLT2 inhibitor (e.g., dapagliflozin) for persistent proteinuric CKD once immunologic disease is stable. Avoid NSAIDs.

Class-directed immunosuppression (KDIGO 2024 lupus nephritis guideline)

• Class I–II: no renal-indication immunosuppression; treat extrarenal lupus and control proteinuria.
• Class III/IV (± V) — initial therapy: glucocorticoids (tapered aggressively) plus either mycophenolate mofetil or low-dose IV cyclophosphamide (Euro-Lupus regimen). KDIGO favors triple therapy — adding belimumab (anti-BAFF) or a calcineurin inhibitor, typically voclosporin or tacrolimus, to MMF — to raise complete renal response rates and limit steroid exposure.
• Maintenance: mycophenolate or azathioprine continued for at least 3 years with low-dose or discontinued steroids; abrupt withdrawal provokes flare.
• Pure Class V: immunosuppression (steroid + MMF ± CNI) if nephrotic-range proteinuria; antiproteinuric therapy alone if subnephrotic.
• Class VI: no immunosuppression for renal indication — supportive care, dialysis, or transplant (transplant after disease quiescence; recurrence is uncommon).
• Refractory disease: switch MMF↔cyclophosphamide, or use rituximab (anti-CD20).

Contraindicated/avoid

• Mycophenolate and cyclophosphamide are teratogenic — switch to azathioprine or tacrolimus before conception; ACE inhibitors/ARBs are contraindicated in pregnancy. Consider GnRH agonist (leuprolide) for ovarian protection with cyclophosphamide, and avoid live vaccines during immunosuppression.

Disease-related

• End-stage renal disease: driven by unresolved proliferative inflammation and, more importantly, by chronicity (glomerulosclerosis, tubular atrophy, interstitial fibrosis). Signalled by rising creatinine with a high chronicity index and failure of proteinuria to fall below ~0.5–0.8 g/day by 12 months — the strongest predictor of long-term renal survival.
• Rapidly progressive glomerulonephritis (EMERGENCY): crescent formation with fibrinoid necrosis; signalled by creatinine doubling over days to weeks with RBC casts and oliguria. Requires pulse steroids without waiting.
• Venous thromboembolism/renal vein thrombosis: urinary loss of antithrombin and protein S with increased thrombin generation, amplified in membranous (Class V) disease and by coexisting antiphospholipid antibodies. Signalled by new flank pain, gross hematuria, or sudden proteinuria increase.
• Thrombotic microangiopathy (EMERGENCY): from antiphospholipid antibodies or complement dysregulation; signalled by schistocytes, thrombocytopenia, and LDH elevation out of proportion to nephritis.
• Accelerated atherosclerosis and hypertension: chronic inflammation plus steroid and nephrotic dyslipidemia; cardiovascular disease is a leading late cause of death.

Treatment-related

• Infection (EMERGENCY when septic): the leading early cause of death; combined steroid, mycophenolate/cyclophosphamide, and B-cell depletion. Add Pneumocystis jirovecii prophylaxis (TMP-SMX) with intensive regimens; screen for hepatitis B before rituximab (reactivation risk) and be alert for PML.
• Cyclophosphamide toxicity: acrolein metabolite causes hemorrhagic cystitis (prevent with hydration and mesna) and later urothelial carcinoma; also premature ovarian failure, azoospermia, myelosuppression, and leukemia.
• Glucocorticoid toxicity: avascular necrosis of the femoral head (new groin pain with normal radiographs — get MRI), osteoporosis, hyperglycemia, cataracts, adrenal suppression.
• Hydroxychloroquine retinopathy: bull's-eye maculopathy, dose- and duration-dependent, detected by screening before symptoms.
• Calcineurin inhibitor nephrotoxicity: vasoconstrictive GFR decline and hypertension mimicking a renal flare — a biopsy or drug-level check distinguishes them.
• Mycophenolate embryopathy: orofacial and ear malformations; pregnancy exposure is a preventable disaster.

• "Wire-loop" capillary loops = subendothelial deposits = proliferative disease: the buzzword points to Class IV (diffuse proliferative), the most common and most severe class, and the one requiring aggressive immunosuppression.
• "Full house" immunofluorescence (IgG, IgA, IgM, C3, C1q) is the single most specific histologic clue that a glomerulonephritis is lupus rather than idiopathic. Deposits are granular — a linear IF stem is anti-GBM disease, the classic distractor.
• Best next step in an SLE patient with proteinuria ≥0.5 g/day, an active sediment, or unexplained creatinine rise: renal biopsy — before committing to immunosuppression. Serologies cannot assign a class, and class determines therapy.
• The association examiners test: anti-dsDNA titers rise and C3/C4 fall with active nephritis, so they are the serologic activity markers. Anti-Smith is the most specific antibody for SLE but does not track disease activity — a frequent trap.
• Class V (membranous) = nephrotic syndrome + hypercoagulability: think renal vein thrombosis when flank pain, gross hematuria, or an abrupt proteinuria jump appears; loss of antithrombin and protein S is the mechanism.
• Hydroxychloroquine belongs in essentially every answer about SLE with nephritis as background therapy; the toxicity to know is bull's-eye maculopathy.
• Cyclophosphamide + hematuria and suprapubic pain = hemorrhagic cystitis from acrolein → mesna and hydration, not "lupus flare."
• Pregnancy swap: mycophenolate and cyclophosphamide are teratogenic and ACE inhibitors/ARBs are contraindicated — the correct answer is azathioprine (with hydroxychloroquine continued), not "stop all therapy."
• Drug-induced lupus (hydralazine, procainamide, isoniazid) gives anti-histone antibodies and characteristically spares the kidney — if the stem has nephritis, it is not drug-induced lupus.