Systemic Lupus Erythematosus (SLE)

Definition and Overview

Epidemiology

• Prevalence: ~20-150 per 100,000 population; incidence varies widely by region and ethnicity.
• Sex and age: Strongly female-predominant, with a 9:1 female-to-male ratio in reproductive years; this ratio narrows in late-onset (>50 years) disease. Peak onset is between 15-45 years, though it can occur at any age.
• Ethnicity: More prevalent and severe in African American, Hispanic, and Asian populations compared to Caucasians.
• Pediatric SLE has an average age of onset of ~12 years; non-Caucasian children are disproportionately affected.

Pathogenesis

Key Mechanisms

1. Defective clearance of apoptotic debris Nuclear material from apoptotic cells is exposed and not cleared efficiently. This serves as the source of self-antigens (dsDNA, histones, ribonucleoproteins).
2. Autoantibody formation and immune complex deposition
   • B cells, aided by autoreactive T cells, produce IgG autoantibodies against nuclear antigens.
   • These form circulating immune complexes that deposit in glomeruli, skin, joints, and blood vessels.
   • Immune complex deposition activates the complement cascade, causing C3 and C4 consumption and local tissue damage.
   • In the kidney (lupus nephritis), cationic nuclear antigens (e.g., histones) may also bind the glomerular basement membrane directly, triggering in situ immune complex formation.
3. Type I interferon axis A dysregulated type I interferon (IFN-α/β) system is central to SLE pathogenesis. Plasmacytoid dendritic cells are chronically activated by nucleic acid-containing immune complexes via TLRs (TLR7, TLR9), producing large amounts of IFN-α, which drives further B-cell and T-cell activation.
4. T-cell dysfunction and DNA methylation defects
   • CD4+ T cells in SLE have impaired DNA methylation (reduced DNMT1 activity), which causes overexpression of LFA-1 (CD11a/CD18), converting normal T-helper cells into autoreactive, cytotoxic, pro-inflammatory T cells.
   • The Th17 subset is expanded; Treg function is reduced.
5. Complement deficiency
   • Paradoxically, inherited deficiencies of early complement components (C1q, C2, C4) predispose to SLE by impairing apoptotic debris clearance.
   • Acquired complement deficiencies (low C3, C4) occur due to ongoing consumption and contribute to disease activity.
6. Hormonal factors
   • Estrogens amplify B-cell and T-cell hyperresponsiveness, explaining the female predominance during reproductive years.
   • Exogenous hormones (OCP, HRT) and even some drugs can trigger lupus-like disease (Drug-Induced Lupus - DIL) via epigenetic mechanisms similar to idiopathic SLE.

Clinical Features

Constitutional

• Fatigue, fever (unexplained recurrent), weight loss, malaise, anorexia, myalgia, arthralgia.

Mucocutaneous (skin involved in majority of patients)

• Malar ("butterfly") rash - erythematous rash across cheeks and nose bridge, sparing the nasolabial folds; characteristic of Acute Cutaneous Lupus Erythematosus (ACLE). Highly associated with systemic disease.
• Discoid lupus - scarring, atrophic plaques with follicular plugging; chronic cutaneous LE (CCLE); can occur without systemic disease.
• Subacute cutaneous LE (SCLE) - annular or papulosquamous lesions; associated with anti-Ro/SSA antibodies; often photosensitive and drug-induced.
• Photosensitivity - skin eruption triggered by UV light.
• Oral ulcers - usually painless.
• Alopecia - diffuse non-scarring or scarring.
• LE panniculitis (lupus profundus), chilblain LE, and other subtypes also exist.

Musculoskeletal

• Non-erosive symmetric arthritis/arthralgia (the most common manifestation overall).
• Jaccoud arthropathy (reducible deformities without erosions).

Renal - Lupus Nephritis

• Occurs in 20-75% of patients; up to 50% of children may progress to end-stage kidney disease.
• Presents with hematuria, proteinuria, cellular casts, hypertension.
• WHO/ISN classification (Classes I-VI) guides treatment; kidney biopsy is diagnostic and prognostic.

Neuropsychiatric (NPSLE)

• Seizures, psychosis, cognitive dysfunction, headache, cerebrovascular events, peripheral neuropathy, myelopathy.

Cardiovascular

• Pericarditis (most common cardiac manifestation), myocarditis, Libman-Sacks endocarditis (non-bacterial verrucous endocarditis), accelerated atherosclerosis.
• Immune complex and complement deposition in cardiac tissue drives inflammatory change; autoantibody-mediated endothelial dysfunction and dyslipidemia accelerate atherogenesis.

Pulmonary

• Pleuritis/pleural effusion (most common pulmonary manifestation), pneumonitis, pulmonary hypertension, shrinking lung syndrome.

Hematologic

• Hemolytic anemia (Coombs-positive), leukopenia, lymphopenia, thrombocytopenia.

Antiphospholipid Syndrome (APS)

• Present in ~30-40% of SLE patients; associated with thrombosis (arterial and venous) and recurrent pregnancy loss.
• Diagnosed by lupus anticoagulant, anticardiolipin antibodies, and/or anti-beta2-glycoprotein I antibodies.

Autoantibodies in SLE

Diagnosis and Classification Criteria

1. ACR Criteria (1997) - requires ≥4 of 11:

2. SLICC Criteria - requires ≥4 items (at least 1 clinical + 1 immunologic) OR biopsy-proven lupus nephritis with positive ANA or anti-dsDNA:

• Clinical: acute cutaneous, chronic cutaneous, oral ulcers, alopecia, synovitis, serositis, renal, neurologic, hemolytic anemia, leukopenia, thrombocytopenia.
• Immunologic: ANA, anti-dsDNA, anti-Smith, antiphospholipid antibodies, low C3/C4/CH50, direct Coombs test (without hemolytic anemia).

Treatment

All Patients

• Hydroxychloroquine (HCQ) 5 mg/kg/day - recommended for ALL SLE patients regardless of severity. Effective for rash, photosensitivity, arthritis, alopecia, and malaise. Reduces disease flares, long-term damage, and thrombotic risk in APS. Has been shown to delay transition to full SLE classification.

Mild Disease (mucocutaneous, musculoskeletal, mild cytopenias, serositis)

• NSAIDs - for arthritis/arthralgia, fever, mild serositis; caution with hepatic/renal toxicity.
• Topical corticosteroids - for localized skin disease.
• Methotrexate - for musculoskeletal and skin manifestations.

Moderate Disease (not responding to standard therapy, low C3/C4, high anti-dsDNA)

• Oral corticosteroids (prednisone, usual dose 0.5-1 mg/kg/day) with gradual taper.
• Steroid-sparing agents: Azathioprine, Mycophenolate mofetil (MMF).

Severe/Life-Threatening Disease (nephritis, CNS, vasculitis, severe cytopenias, pneumonitis)

• High-dose corticosteroids - prednisone 1-2 mg/kg/day PO, or IV pulse methylprednisolone 500-1000 mg/day for 3-5 days in life-threatening situations.
• Cyclophosphamide - for life-threatening manifestations; significant side effects (infertility, hemorrhagic cystitis, malignancy).
• MMF - may be as effective as cyclophosphamide for certain classes of lupus nephritis with fewer side effects; preferred in younger patients wishing to preserve fertility.

Biologics

• Belimumab (anti-BLyS/BAFF) - FDA-approved (2012) for autoantibody-positive SLE on standard therapy; additionally approved (2020) for lupus nephritis.
• Rituximab (anti-CD20) - used in refractory SLE; effective in observational studies though placebo-controlled trials have been disappointing.
• Voclosporin (calcineurin inhibitor, no drug-level monitoring required) - FDA-approved (2021) for lupus nephritis in combination with standard therapy; markedly reduces proteinuria.

Monitoring and Prognosis

• Disease activity is tracked using validated scores (SLEDAI, BILAG).
• Anti-dsDNA levels and complement (C3, C4) are followed as biomarkers of activity - rising anti-dsDNA + falling complement signals a flare.
• Causes of death: historically infection and active disease; increasingly, cardiovascular disease (accelerated atherosclerosis) and drug-related toxicity.
• Hydroxychloroquine reduces long-term damage accumulation and may even delay SLE onset in at-risk individuals.

Drug-Induced Lupus (DIL)

• Classic DIL drugs: Procainamide, hydralazine (mechanism: inhibition of T-cell DNA methylation, converting helper T cells into autoreactive cytotoxic cells).
• Others: Isoniazid, minocycline, methyldopa, chlorpromazine.
• Biologics: TNF inhibitors (adalimumab, infliximab, etanercept) and IFN-α can induce ANA/anti-dsDNA and a lupus-like syndrome.
• DIL typically resolves after stopping the offending drug; differs from idiopathic SLE in having prominent anti-histone antibodies and less renal/CNS disease.

Summary Table