18.4 Systemic Lupus Erythematosus & Drug-Induced Lupus

Connective tissue diseases (CTDs) are multisystem autoimmune disorders characterized by chronic immune-mediated tissue injury, vascular abnormalities, and specific autoantibody signatures. For the MRCP(UK) Part 1 candidate, precise recall of clinical presentations, diagnostic serology, histopathology, and evidence-based pharmacotherapeutic guidelines is essential.

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1. Systemic Lupus Erythematosus (SLE): Pathophysiology & Clinical Spectrum

Systemic lupus erythematosus is a prototypical multisystem autoimmune disease characterized by polyclonal B-cell hyperreactivity, loss of tolerance to nuclear self-antigens, and defective clearance of apoptotic material.

Pathophysiology & Genetics

• Demographics: Striking female predilection (9:1 to 12:1 during childbearing years of 15–45), reflecting oestrogenic immune modulation. Significantly higher prevalence, earlier onset, and greater severity in patients of African-Caribbean, South Asian, and Hispanic ancestry.
• Immunopathogenesis: Ultraviolet (UV) radiation triggers keratinocyte apoptosis. Defective phagocytic clearance of apoptotic nuclear blebs exposes intracellular antigens (DNA, histones, ribonucleoproteins Ro/La/Sm) to plasmacytoid dendritic cells. Toll-like receptor (TLR-7, TLR-9) activation stimulates massive release of type I interferon (IFN-alpha). Autoreactive B cells produce antinuclear autoantibodies, forming circulating antigen-antibody complexes. These immune complexes deposit in capillary vascular beds of the skin, synovial membranes, pleura, pericardium, and renal glomeruli, fixing complement via the classical pathway (C1q -> C4 -> C2 -> C3) and causing intense inflammatory tissue destruction.
• Genetic Links: Strongly linked to HLA-DR2 and HLA-DR3, alongside rare monogenic homozygous early complement deficiencies (C1q, C1r/s, C4, C2), where impaired apoptotic clearance almost universally causes early-onset, severe lupus.

Clinical Manifestations

• Mucocutaneous:
  • Malar Rash (Butterfly Rash): Fixed, erythematous, macular or maculopapular eruption extending across the bridge of the nose and malar eminences. CRITICAL EXAM PEARL: The malar rash characteristically spares the nasolabial folds (distinguishing it from rosacea, seborrhoeic dermatitis, and perioral dermatitis, which involve the nasolabial folds).
  • Discoid Lupus Erythematosus (DLE): Well-demarcated erythematous scaly plaques with follicular plugging, central scarring, dermal atrophy, telangiectasia, and peripheral hyperpigmentation; causes irreversible cicatricial alopecia.
  • Photosensitivity: Elicitation of skin rash in sun-exposed areas within 24–48 hours of UV light exposure.
  • Mucosal Ulcers: Painless, shallow oral (palatal) or nasopharyngeal ulcers.
  • Alopecia: Non-scarring diffuse thinning or patchy hair loss.
• Musculoskeletal: Non-erosive polyarthritis or polyarthralgias involving small joints of the hands, wrists, and knees. In 10–15% of patients, chronic tenosynovial inflammation leads to Jaccoud's arthropathy—reversible joint subluxation, ulnar deviation, swan-neck deformities, and 'z-thumb' resulting from ligamentous laxity and capsule fibrosis without articular bone erosion on plain radiographs (unlike rheumatoid arthritis).
• Serositis: Pleurisy (pleuritic chest pain, pleural friction rub, exudative effusion with normal or mildly low glucose) and pericarditis (retrosternal pain worse supine, friction rub, pericardial effusion, rare cardiac tamponade). Libman-Sacks endocarditis presents with sterile verrucous vegetations on mitral and aortic valves.
• Lupus Nephritis: Major cause of morbidity and mortality, occurring in up to 50% of patients. Classified according to the ISN/RPS (International Society of Nephrology / Renal Pathology Society) classification:
  • Class I: Minimal mesangial lupus nephritis (normal LM, mesangial deposits on IF).
  • Class II: Mesangial proliferative lupus nephritis (mesangial hypercellularity).
  • Class III: Focal lupus nephritis (< 50% of glomeruli involved; active necrotizing lesions, subendothelial deposits).
  • Class IV: Diffuse proliferative lupus nephritis (>= 50% of glomeruli involved). The most common, aggressive, and severe histological subtype. Features extensive subendothelial immune deposits producing thick, rigid capillary loops ("wire-loop" lesions on light microscopy), cellular crescents, intracapillary hypercellularity, heavy proteinuria, microscopic haematuria with red cell casts, and rapid renal decline.
  • Class V: Membranous lupus nephritis. Characterized by uniform subepithelial immune deposits and thickened capillary walls; manifests with heavy, nephrotic-range proteinuria and peripheral oedema, often with preserved renal function.
  • Class VI: Advanced sclerosing lupus nephritis (> 90% globally sclerosed glomeruli).
  • Immunofluorescence Feature: Classic "full-house" pattern—simultaneous, intense granular capillary loop and mesangial staining for IgG, IgA, IgM, C3, and C1q.
• Neuropsychiatric Lupus (NPSLE): Ranging from seizures and acute psychosis to aseptic meningitis, transverse myelitis, peripheral neuropathy, and cognitive dysfunction.
• Haematological: Cytopenias driven by autoantibody-mediated peripheral destruction:
  • Autoimmune Haemolytic Anaemia (AIHA): Normocytic anaemia with reticulocytosis, positive direct antiglobulin (Coombs) test (IgG and C3d), elevated indirect bilirubin, high LDH, and undetectable haptoglobin.
  • Leukopenia (< 4.0 × 10⁹/L) and Lymphopenia (< 1.0 × 10⁹/L); lymphopenia tracks active disease.
  • Immune Thrombocytopenia (ITP): Anti-platelet autoantibodies causing isolated thrombocytopenia (< 100 × 10⁹/L).

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2. SLE Serology, Complement Dynamics & Disease Monitoring

Interpretation of the autoantibody profile is central to establishing diagnosis, predicting organ complications, and tracking disease flares in MRCP(UK) Part 1:

                          Diagnostic Autoantibody Tree in SLE
                                           |
         +---------------------------------+---------------------------------+
         |                                 |                                 |
        ANA                            Anti-dsDNA                         Anti-Sm
  > 95-98% Sensitive               > 95% Specific                    > 99% Specific
  Entry criterion / screening      Tracks disease flares             Highly specific for SLE;
  Low specificity                  and lupus nephritis               Titre remains constant
                                           |
         +---------------------------------+---------------------------------+
         |                                                                   |
   Anti-Ro (SSA) & Anti-La (SSB)                                    Complement Levels (C3, C4)
   - Subacute cutaneous lupus                                       - Markedly consumed / low
   - Secondary Sjögren's syndrome                                     during active disease flares
   - Congenital complete heart block in neonate                     - Tracks with rising anti-dsDNA

• Antinuclear Antibodies (ANA): Present in > 95–98% of SLE patients. Serves as the mandatory entry criterion for EULAR/ACR classification. Highly sensitive but poorly specific; a negative ANA essentially rules out SLE (except in rare anti-Ro-only positive subacute cutaneous lupus).
• Anti-double-stranded DNA (Anti-dsDNA): Present in 60–70% of patients. Highly specific (> 95%) for SLE. Crucially, anti-dsDNA titres fluctuate with disease activity: a sharp rise in anti-dsDNA titre directly heralds an impending systemic disease flare or active Class IV lupus nephritis.
• Anti-Smith (Anti-Sm): Directed against core proteins of small nuclear ribonucleoproteins (snRNPs). Highly specific (> 99%) for SLE, but only 20–30% sensitive. Unlike anti-dsDNA, anti-Sm titres do not correlate with disease activity or flares and remain positive indefinitely.
• Anti-Ro/SSA & Anti-La/SSB: Directed against ribonuclear protein complexes. Present in 30–50% of SLE patients. Closely associated with:
  1. Subacute Cutaneous Lupus Erythematosus (SCLE): Highly photosensitive, widespread non-scarring annular or papulosquamous polycyclic lesions.
  2. Secondary Sjögren's Syndrome.
  3. Neonatal Lupus Erythematosus & Congenital Heart Block: Maternal IgG anti-Ro (and anti-La) cross the placenta from gestational week 16 onwards, binding to fetal cardiac conduction tissue and triggering autoimmune myocarditis, permanent fibrosis, and congenital complete atrioventricular heart block in the fetus, alongside transient neonatal annular skin lesions and thrombocytopenia.
• Serum Complement Levels (C3 & C4): During active lupus flares, massive immune complex formation triggers activation and consumption of classical complement proteins. Depressed serum C3 and C4 levels, in concert with rising anti-dsDNA titres, are the pathognomonic biochemical markers of active lupus nephritis and systemic flares.
• Antiphospholipid Antibodies: Present in 30–40% of SLE patients: Lupus Anticoagulant (LA), anticardiolipin antibodies (aCL), and anti-beta-2-glycoprotein I (anti-β2GPI). Confer high risk of arterial/venous thrombosis and recurrent pregnancy loss (Antiphospholipid Syndrome [APS]). Paradoxical lab finding: Lupus anticoagulant prolongs the in vitro activated partial thromboplastin time (APTT) which fails to correct on 1:1 mixing study, yet causes in vivo hypercoagulability.

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3. Lupus Pharmacotherapy Protocols

Therapy is tailored according to organ severity, balancing disease remission against drug toxicity:

• Universal Baseline Therapy: Hydroxychloroquine (HCQ) is recommended for all patients with SLE regardless of disease severity, unless strictly contraindicated. HCQ reduces disease flares by > 50%, improves long-term survival, lowers cardiovascular event rates, prevents irreversible renal damage, and is safe throughout pregnancy. Dosed at <= 5 mg/kg actual body weight to prevent retinal toxicity.
• Mild / Cutaneous / Musculoskeletal Disease: Low-dose oral corticosteroids, topical steroids for rashes, NSAIDs (with caution regarding renal effects), or methotrexate/azathioprine for steroid-sparing control.
• Severe Organ-Threatening Disease (Active Lupus Nephritis Class III/IV):
  • Induction Remission Phase: High-dose pulsed intravenous methylprednisolone (500–750 mg IV daily for 3 consecutive days) followed by oral prednisolone, combined with either:
    1. Mycophenolate Mofetil (MMF) (2–3 g daily; preferred in non-Caucasian ethnicities, with superior efficacy and lower gonadal toxicity); OR
    2. Intravenous Cyclophosphamide (low-dose Euro-Lupus regimen: 500 mg IV every 2 weeks for 6 doses). Cyclophosphamide carries significant toxicities: myelosuppression, hemorrhagic cystitis (mitigated by hydration and mesna [sodium 2-mercaptoethanesulfonate]), and dose-dependent irreversible premature ovarian failure / infertility.
  • Maintenance Phase: Once remission is achieved (usually after 6 months), patients transition to maintenance with oral MMF or azathioprine, alongside low-dose prednisolone and background hydroxychloroquine, continued for at least 3–5 years.
• Targeted Biologics:
  • Belimumab: Fully human monoclonal antibody that binds and inhibits soluble B-lymphocyte stimulator (BLyS / BAFF), blocking B-cell survival. Approved as add-on therapy for active autoantibody-positive SLE and lupus nephritis.
  • Anifrolumab: Human monoclonal antibody targeting the type I interferon receptor subunit 1 (IFNAR1), indicated for moderate-to-severe SLE.

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4. Drug-Induced Lupus (DIL)

Drug-induced lupus is an idiosyncratic autoimmune reaction sharing clinical features with SLE, triggered by chronic exposure to specific pharmacological agents.

Causative Pharmacological Agents

• High-Risk Culprits: Procainamide (induces ANA in 50–70%, DIL in 20%), Hydralazine (induces DIL in 5–10%, particularly in slow hepatic acetylators), Isoniazid, Minocycline (commonly used for acne vulgaris in young patients), Quinidine, and D-penicillamine.
• Biologic Triggers: Anti-TNF agents (infliximab, adalimumab, etanercept; paradoxically induce anti-dsDNA antibodies and cutaneous lupus).

Clinical Features & Vital Diagnostic Contrasts

• Clinical Presentation: Gradual onset of fever, myalgias, arthralgias, symmetrical non-erosive polyarthritis, and serositis (pleurisy, pericarditis). Malar rash may occur.
• Organ Sparing: In stark contrast to idiopathic SLE, renal involvement (lupus nephritis) and central nervous system (CNS) disease are EXTREMELY RARE in drug-induced lupus.
• Serology:
  • Antinuclear Antibodies (ANA): Positive in > 95% (homogenous pattern).
  • Anti-Histone Antibodies: Positive in > 95% of cases; the hallmark serological marker of drug-induced lupus.
  • Anti-dsDNA & Anti-Sm: Consistently negative (except in anti-TNF-induced lupus, which can induce anti-dsDNA).
  • Serum Complement Levels (C3/C4): Characteristically NORMAL (not consumed, unlike idiopathic SLE flares).
• Prognosis & Management: Prompt cessation of the offending medication is curative. Clinical symptoms resolve within weeks, and autoantibody titres gradually normalize over several months. Short-term NSAIDs or low-dose oral corticosteroids can be used for acute symptomatic relief.

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