Pathogenesis of Systemic Lupus Erythematosus (SLE)

1. Disease Phases

1. Genetic/epigenetic susceptibility phase - inherited risk
2. Subclinical immune dysregulation - early immune abnormalities
3. Asymptomatic autoimmunity - autoantibodies appear (on average 3.4 years before diagnosis; ANA, anti-Ro, anti-La, antiphospholipid antibodies first; anti-Sm and anti-RNP later; anti-dsDNA ~2.7 years before diagnosis)
4. Clinical SLE - triggered by additional environmental hits
5. Relapsing-remitting course - with accrual of organ damage over years

• Firestein & Kelley's Textbook of Rheumatology, p. 237; Rheumatology 2-Volume Set (Elsevier 2022)

2. Genetic Susceptibility

• HLA associations: HLA-DR2 and HLA-DR3 carry an odds ratio of 2-3; both together ~5x risk
• Complement deficiencies: C1q, C2, or C4 deficiency in ~5% of SLE patients - these impair clearance of immune complexes and apoptotic cells, and impair B cell tolerance
• FcγRIIB polymorphism: reduced inhibitory signaling on B cells, allowing unchecked activation
• GWAS genes: IRF3, IRF5, IRF7, STAT4, PTPN22, BLK, TNFAIP3, TREX1, DNase I
• Monogenic lupus: rare mutations in complement components, DNase I, TREX1 (encoding a 3'-5' DNA exonuclease), SAMHD1 - all cause inability to degrade self-DNA/RNA
• Sex: 10:1 female-to-male ratio; sex hormones (estrogen) influence immune responses; X-linked genes relevant (TLR7 is X-linked - its duplication causes lupus in mouse models)
• Concordance: ~25-50% in monozygotic twins vs ~5% in dizygotic - strong genetic but not purely genetic basis
• Cellular and Molecular Immunology (Abbas), p. 1273-1274; Rheumatology 2022

3. Environmental Triggers

• Rheumatology 2022 (Elsevier), Table 139.1

4. Central Initiating Mechanism: Impaired Clearance of Apoptotic Cells

• UV light and other stressors cause apoptosis of cells
• Normally, apoptotic cells are rapidly phagocytosed by macrophages
• In SLE, clearance is defective: reduced complement (C1q opsonizes apoptotic debris), reduced DNase I, reduced CRP, macrophage dysfunction
• Nuclear material (DNA, histones, ribonucleoproteins) from uncleared apoptotic cells becomes accessible as self-antigens
• These nucleic acid-containing complexes activate TLR7 (RNA) and TLR9 (DNA) on B cells and plasmacytoid dendritic cells (pDCs)
• Result: break in B cell tolerance and massive production of antinuclear autoantibodies
• Cellular and Molecular Immunology, p. 1274; Rheumatology 2022

5. The Type I Interferon Axis

• Plasmacytoid DCs (pDCs) are the major source of IFN-α
• TLR7/TLR9 activation by nucleic acid-containing immune complexes triggers pDC IFN-α production
• The IFN-α gene signature (upregulation of IFN-stimulated genes in peripheral blood) is found in 50-80% of SLE patients and correlates with disease activity
• IFN-α drives:
  • Dendritic cell maturation and enhanced antigen presentation
  • B cell class switching and autoantibody production
  • T cell activation and effector differentiation
  • Upregulation of BLyS/BAFF (B cell survival factor)
• In the presymptomatic phase: IFN-α activity rises ~2 years before clinical diagnosis
• Key feedback loop: autoantibody-antigen immune complexes are taken up by pDCs via FcγRIIa, reactivating TLR9/7 and driving more IFN-α - a self-amplifying cycle
• Cellular and Molecular Immunology, p. 1274-1275; Rheumatology 2022, p. 477

6. B Cell Dysregulation and Autoantibody Production

• SLE B cells have a lower activation threshold, defective tolerance checkpoints (both central and peripheral), and increased survival signals
• Key autoantibodies and their pathogenic roles:

• Immune complex deposition in glomeruli, joints, and skin triggers complement activation → inflammation and tissue damage
• BLyS/BAFF is elevated in SLE and promotes B cell survival and class switching; belimumab (anti-BLyS) is FDA-approved for SLE
• Cellular and Molecular Immunology; Rheumatology 2022, p. 2900-2901

7. T Cell Abnormalities

• Reduced IL-2 production: CD4+ T cells produce less IL-2, which normally maintains regulatory T cells (Tregs); Treg dysfunction allows unchecked autoreactive responses
• Lower activation threshold: abnormal TCR signaling via substitution of FcεRIγ for CD3ζ chain, enhanced calcium mobilization
• Tfh (T follicular helper) cell expansion: drives germinal center reactions and high-affinity autoantibody maturation
• Th17 expansion: IL-17-producing cells are increased, especially in patients with renal involvement; IL-17 promotes neutrophil recruitment and tissue inflammation
• CD8+ T cell exhaustion: impaired cytotoxic T cell function
• Abnormal DNA methylation in T cells: hypomethylation of immune genes (ITGAL/CD11a, perforin, CD70) causes overexpression and autoreactivity
• Rheumatology 2022, Key Points; Firestein & Kelley's

8. Innate Immune Dysregulation

Neutrophils and Low-Density Granulocytes (LDGs)

• SLE patients have a distinct population of LDGs (low-density granulocytes) in peripheral blood
• LDGs have a striking type I IFN gene signature and produce far more IFN-α than normal neutrophils
• LDGs and activated neutrophils form NETs (Neutrophil Extracellular Traps):
  • NETs consist of DNA decorated with antimicrobial proteins (LL-37, histones, elastase, MPO)
  • LL-37-DNA complexes are potent activators of pDCs → massive IFN-α release
  • NETs expose modified self-antigens, stimulating autoantibody production
  • NETs cause direct vascular endothelial damage → vasculopathy in SLE
• Impaired NET clearance (due to anti-NET antibodies, DNase I deficiency) perpetuates the cycle

Dendritic Cells

• Plasmacytoid DCs are the main IFN-α producers
• TLR7/9 activation by RNA- and DNA-containing immune complexes triggers IFN-α
• Conventional DCs (myeloid DCs) are reduced in circulation (trafficked to lymph nodes) but are potent antigen presenters

Macrophages

• Impaired efferocytosis (phagocytosis of apoptotic cells) - "tingible body" macrophages in germinal centers are reduced
• Macrophage activation contributes to cytokine storm in severe SLE

Basophils

• Basophils infiltrate lymph nodes in SLE patients with an activated phenotype (upregulated CD63, CD203c, CXCR4)
• They produce IL-4 which promotes autoantibody production by B cells and Th17 differentiation
• Basophil depletion reduces disease in murine models
• Firestein & Kelley's Rheumatology, pp. 237-238 (NETs/LDG section)

9. Complement System in Tissue Damage

• Immune complex deposition in glomeruli → classical pathway complement activation
• C3b and C4b opsonize complexes; C5a recruits neutrophils
• Paradox: complement deficiency (C1q, C2, C4) predisposes to SLE, yet complement activation mediates tissue damage in established disease
  • C1q normally opsonizes apoptotic cells for clearance; its deficiency leads to accumulation of autoantigen and autoimmunity
  • Once immune complexes form, complement-mediated inflammation drives nephritis and vasculitis

10. Epigenetic Dysregulation

• DNA hypomethylation in SLE CD4+ T cells - affects genes including ITGAL (CD11a/CD18), perforin, CD70, and a widespread IFN-hypomethylation signature in CD4+, CD19+, and CD14+ cells
• Histone modifications: altered acetylation and methylation patterns
• MicroRNAs: miR-21 and miR-148a downregulate DNMT1, promoting hypomethylation
• Drugs like procainamide and hydralazine inhibit DNMT1 → drug-induced lupus via demethylation
• Firestein & Kelley's Rheumatology, p. 4227

11. Cytokine Network

Summary: Integrated Pathogenic Loop

Genetic susceptibility + Environmental triggers
         ↓
Impaired apoptotic cell clearance → Exposure of nuclear antigens
         ↓
TLR7/9 activation (by RNA/DNA complexes) on pDCs and B cells
         ↓
IFN-α surge (type I IFN signature) + NETosis by LDGs
         ↓
B cell activation → Antinuclear autoantibody production
T cell activation → Tfh expansion → Germinal center hyperactivity
         ↓
Immune complex formation (anti-dsDNA + DNA; anti-Sm + snRNP, etc.)
         ↓
Deposition in kidneys, skin, joints, vessels
         ↓
Complement activation → Inflammation → Organ damage
         ↓
Tissue damage releases more nuclear antigens → Amplification loop

• Cellular and Molecular Immunology (Abbas), pp. 1273-1275
• Rheumatology, 2-Volume Set (Elsevier 2022), Chapter 139, pp. 2761-2924
• Firestein & Kelley's Textbook of Rheumatology, pp. 237-238, 4227-4246