6.6 Systemic Lupus Erythematosus

Systemic lupus erythematosus is the prototypical multisystem autoimmune disease, and it is a reliable source of board items because almost every organ system can be the presenting complaint. Mastery rests on four things: applying the classification criteria without treating them as diagnostic checkboxes, reading the autoantibody and complement profile correctly, knowing which histologic class of lupus nephritis demands immunosuppression, and separating a genuine disease flare from infection, thrombosis, and drug-induced disease. Systemic sclerosis, Sjogren syndrome, the inflammatory myopathies, and the vasculitides are developed in the sections that follow.

1. Systemic Lupus Erythematosus (SLE)

Pathophysiology

SLE is a systemic autoimmune disease characterized by loss of immune tolerance to self-nuclear antigens, defective clearance of apoptotic nuclear debris, persistent activation of plasmacytoid dendritic cells with upregulation of Type I Interferon (IFN-α) signaling, and production of pathogenic autoantibodies. Deposition of insoluble antigen-antibody immune complexes in microvascular beds (Type III hypersensitivity) activates the classical complement pathway (consuming C3 and C4) and recruits inflammatory leukocytes, causing tissue damage in the kidneys, skin, serosa, and central nervous system.

2019 EULAR / ACR Classification Criteria for SLE

Diagnosis requires the obligatory entry criterion: Antinuclear Antibodies (ANA) titer >= 1:80 on HEp-2 cells (sensitivity >98%; a negative ANA makes SLE extraordinarily unlikely). If positive, calculate weighted scores across 7 clinical and 3 immunologic domains (must achieve >= 10 points with at least 1 clinical domain):

Pharmacotherapy for SLE

• Hydroxychloroquine (HCQ): MANDATORY baseline therapy for ALL SLE patients indefinitely, regardless of disease severity. HCQ reduces flare frequency, prevents organ damage accrual, decreases thrombotic events, improves lipid profiles, and significantly improves overall survival. Dose: <= 5.0 mg/kg actual body weight/day (typically 200–400 mg daily). Ophthalmology screening: baseline exam within year 1, then annual screening after 5 years.
• Systemic Glucocorticoids: Used as bridging therapy at the lowest effective dose for acute flares, then minimized and withdrawn where possible. The 2023 EULAR update lowered the acceptable chronic maintenance dose to <= 5 mg/day prednisone equivalent, down from 7.5 mg/day in the 2019 version, because cumulative glucocorticoid exposure is a dominant driver of irreversible organ damage in lupus.
• Lupus Nephritis Management (Class III / IV Proliferative Nephritis):
  • Induction Therapy: High-dose IV Methylprednisolone pulses + Mycophenolate Mofetil (MMF) 2–3 g/day OR low-dose IV Cyclophosphamide (Euro-Lupus protocol: 500 mg q2 weeks x 6 doses).
  • Triple Therapy Add-ons: Addition of Belimumab (anti-BAFF / BLyS monoclonal antibody) or Voclosporin (novel calcineurin inhibitor) significantly accelerates renal remission.
  • Maintenance Therapy: MMF 1–2 g/day or Azathioprine 2 mg/kg/day for >=3–5 years.
• Targeted Biologics:
  • Belimumab (Benlysta): Fully human monoclonal antibody that binds and inhibits soluble B-lymphocyte stimulator (BLyS / BAFF), inducing B-cell apoptosis. Approved for active systemic SLE and lupus nephritis.
  • Anifrolumab (Saphnelo): Human monoclonal antibody directed against the Type I Interferon Receptor subunit 1 (IFNAR1). Approved for moderate-to-severe systemic SLE (markedly improves refractory cutaneous and articular manifestations).

Drug-Induced Lupus (DIL)

• Offending Medications: Hydralazine, Procainamide, Isoniazid, Minocycline, Anti-TNF agents (Infliximab/Etanercept), Quinidine, Methyldopa.
• Clinical Presentation: Fever, arthralgias, myalgias, serositis (pleuritis, pericarditis), and cutaneous rashes developing after months of drug therapy. Spares the central nervous system and kidneys.
• Serologic Profile: Anti-Histone Antibodies positive in >95%; ANA positive; normal serum complement levels; negative anti-dsDNA and negative anti-Sm.
• Management: Discontinuation of the offending drug leads to complete resolution of clinical symptoms within weeks and serologic normalization within months. Short-course NSAIDs or low-dose steroids can be used for symptomatic relief.

2. Lupus Nephritis, Disease Monitoring, Antiphospholipid Antibodies & Pregnancy

Histologic Classification of Lupus Nephritis

Lupus nephritis develops in roughly 40% of patients and is frequently asymptomatic at onset, which is why a urinalysis with microscopy and a spot urine protein-to-creatinine ratio belong at every visit. Any patient with proteinuria above 0.5 g/day, an active urinary sediment, or an unexplained rise in creatinine needs a kidney biopsy — serology cannot predict the histologic class, and the class determines whether immunosuppression helps at all.

Two refinements matter. Classes III and IV may occur combined with Class V ("mixed" disease), which is treated as proliferative nephritis. And the 2018 ISN/RPS revision retained Classes I through VI but replaced the old A, A/C, and C subdivisions with modified NIH activity (0-24) and chronicity (0-12) indices: the activity index predicts response to immunosuppression, while the chronicity index predicts irreversible loss of kidney function and argues against escalating toxic therapy.

Monitoring Disease Activity

• Anti-double-stranded DNA titer and C3/C4 track activity in many patients, especially those with nephritis. A rising anti-dsDNA with falling complement accompanies or precedes a flare.
• Anti-Smith, anti-Ro/SSA, anti-La/SSB, and anti-RNP do not track disease activity and should not be repeated once established. The antinuclear antibody should never be repeated to follow disease — the titer does not vary meaningfully with activity, and re-testing generates confusion rather than information.
• Treat-to-target: the 2023 EULAR recommendations set remission or low disease activity as the goal, keep hydroxychloroquine in essentially every patient, and push for early use of immunosuppressive and biologic agents and combination therapy in nephritis specifically to spare glucocorticoids.
• Flare versus infection is the recurring bedside decision, and it is complicated by the fact that immunosuppressed patients can have both at once. A pattern of low C3 and C4, rising anti-dsDNA, an active urinary sediment, and leukopenia favors a lupus flare. A pattern of markedly elevated C-reactive protein, neutrophilia, and normal complement favors infection — C-reactive protein is characteristically only mildly elevated in a pure lupus flare, with serositis and arthritis as the notable exceptions. When the picture is ambiguous, treat empirically for infection while investigating.

Antiphospholipid Antibodies in Lupus

Thirty to forty percent of lupus patients carry antiphospholipid antibodies, and roughly a third of those develop clinical antiphospholipid syndrome.

• Positivity must be confirmed on two occasions at least 12 weeks apart to exclude transient infection-related antibodies. Triple positivity — lupus anticoagulant plus anticardiolipin plus anti-beta-2-glycoprotein I — carries the highest thrombotic risk.
• The lupus anticoagulant paradox: it prolongs the activated partial thromboplastin time in vitro and fails to correct on a 1:1 mixing study, yet causes thrombosis, not bleeding, in vivo.
• Warfarin, not a direct oral anticoagulant, is the agent of choice after arterial thrombosis and in triple-positive antiphospholipid syndrome; randomized trials found excess arterial thrombotic events with rivaroxaban in high-risk patients.
• Libman-Sacks endocarditis produces sterile verrucous vegetations on both surfaces of the mitral or aortic valve, is associated with antiphospholipid antibodies, and may embolize or cause valvular regurgitation.

Lupus and Pregnancy

• Conceive during at least 6 months of quiescent disease; active nephritis at conception is the strongest predictor of adverse maternal and fetal outcome.
• Continue hydroxychloroquine throughout pregnancy. Stopping it provokes flares and increases the risk of fetal congenital heart block in anti-Ro-positive women.
• Compatible in pregnancy: hydroxychloroquine, azathioprine, tacrolimus and cyclosporine, low-dose prednisone, and certolizumab if a biologic is required. Contraindicated: mycophenolate mofetil (teratogenic — switch to azathioprine at least 3 months before conception), cyclophosphamide, methotrexate, and ACE inhibitors and angiotensin receptor blockers.
• Add low-dose aspirin from 12 weeks for preeclampsia prophylaxis in every lupus pregnancy.
• Anti-Ro/SSA and anti-La/SSB cross the placenta and cause neonatal lupus: a photosensitive annular rash and cytopenias that resolve as maternal antibody clears, and congenital complete heart block, which is permanent and often requires pacing. Serial fetal echocardiography is used in anti-Ro-positive pregnancies.
• Nephritis flare versus preeclampsia is a classic discrimination. A flare favors low complement, rising anti-dsDNA, an active sediment with red cell casts, and extrarenal lupus features. Preeclampsia favors normal complement, an elevated uric acid, elevated transaminases, and thrombocytopenia after 20 weeks, with resolution after delivery.

Cutaneous Subsets and Long-Term Risk

• Acute cutaneous lupus: the photosensitive malar rash that spares the nasolabial folds; non-scarring.
• Subacute cutaneous lupus: annular-polycyclic or papulosquamous plaques on the sun-exposed upper trunk and arms, strongly anti-Ro/SSA-associated and frequently drug-induced by hydrochlorothiazide, terbinafine, proton pump inhibitors, calcium channel blockers, and tumor necrosis factor inhibitors. Non-scarring.
• Discoid lupus: indurated scaly plaques with follicular plugging, central atrophy, scarring alopecia, and lasting dyspigmentation. Only about 5-10% of isolated discoid disease progresses to systemic lupus.
• Rigorous photoprotection and smoking cessation belong in every plan — smoking worsens cutaneous disease and reduces hydroxychloroquine efficacy.
• Mortality in lupus is bimodal: early deaths come from active disease and infection, while cardiovascular disease is the leading cause of late mortality, with accelerated atherosclerosis driven by chronic inflammation and glucocorticoid exposure. Manage lipids and blood pressure aggressively, screen for osteoporosis in steroid-treated patients, and complete vaccinations — including pneumococcal, influenza, COVID-19, recombinant zoster, and HPV — before starting immunosuppression wherever possible.