• Evidence for a genetic basis:
  • 6x increased risk of SLE among first-degree relatives of an index case
  • 11-12x increased risk of related autoimmunity (including thyroid disease) among first-degree relatives of an index case
  • The risk of SLE among monozygotic twins is 25-50% [risk among dizygotic twins is only 1-2%]
  • Monogenic forms of SLE have been identified (genes: DNASE1, TREX1)
  • The female predominance of SLE
• Genome-wide association studies (GWAS) have identified several groups of genes associated with systemic lupus
  • Genes involving the classical complement pathway (C2, C4, C1q) – highest risk!
  • Genes involving interferon regulation and Toll-like receptor (TLR) repression
  • Genes involving B cell and dendritic cell function

• Ultraviolet light
• Infections: Epstein-Barr virus, parvovirus
• Drugs: hydralazine, isoniazid, procainamide
• Toxins

Immunological anomalies in lupus:

• High serum levels of Type 1 interferon, overexpression of genes relating to interferon 1
• Increased anergic B cell response
• Increased plasmablast response
• Increased plasmacytoid dendritic cells (PDCs), which are professional antigen-presenting cells
  • Widespread antigen presentation overwhelms natural immunological tolerance
• Impaired T-reg function
• Increased low-density neutrophils, which produce neutrophil extracellular traps (NETs)

What is type 1 interferon, and what is the role of TLRs?¹

• Toll-like receptors (TLRs) are an evolutionarily ancient system inherited from invertebrates
• They recognize ‘patterns’ from invading viral nucleic acids (often identified by their hypomethylation)
• The recognition of viral nucleic acids sets off a cascade of cytokine signaling, eventually resulting in the production of type 1 interferon (T1 IFN), which is an important anti-viral cytokine
• It aids the transformation of peripheral mononuclear cells into PDCs
• These PDCs produce large amounts of type 1 interferon
• T1 IFN, in turn, can transform other granulocytes into PDCs, thus creating a self-sustaining process

What is an anergic B cell?²

• In the process of V(D)J recombination, an enormous number of B cells with different specificities are generated; some of these cells, however, are autoreactive
• One of the ways in which the body’s natural tolerance mechanisms deal with such autoreactive B cells is to render them irresponsive (‘anergic’) to antigenic stimulation
• Such anergic B cells remain in circulation but have a much shorter lifespan and are eventually culled
• However, in the presence of very high levels of B-cell activating factor (BAFF), as occurs in lupus, such anergic B cells can survive and generate auto-antibodies

• DNA/RNA release occurs in response to normal cellular injury, viral infections, environmental factors such as UV light, or increased production of NETs
• The mechanisms by which such DNA/RNA lead to autoimmunity include:
  1. Abnormalities in the metabolic pathways that clear DNA/RNA, resulting in accumulation of nucleic acids and likely stimulation of TLR-mediated interferon production
     • Associated with Mendelian forms of lupus
  2. Abnormalities in complement-mediated opsonization of apoptotic debris (including DNA/RNA), preventing clearance
     • Genetic variants increase the risk of lupus 5-10-fold
  3. Recognition of human DNA by TLRs due to abnormalities in genes that normally repress their activity
     • Genetic variants increase the risk of lupus 1.3-1.4-fold
  4. Increased production of type 1 interferon via TLR signaling due to irregularities in genes that regulate type 1 interferon
     • Genetic variants increase the risk of lupus 1.2-1.3-fold
  5. Increased production of autoantibodies due to defects in T cell and B cell function
     • Genetic variants increase the risk of lupus 1.1-1.2-fold

• Thus, there appears to be a step-wise hierarchy of risk influencing the eventual development of lupus

• The critical step in lupus nephritis is the occurrence of antigen-antibody complexes in the kidney
  • Previously, it was thought that autoantibodies bind to circulating antigens, and these then deposit in the kidney
  • Recently, it has been demonstrated that antibodies may form at tertiary lymphoid tissues within the kidney itself³
  • Antibodies can deposit onto nuclear antigens that have become trapped in the kidney
• These antigen-antibody complexes then set off an inflammatory cascade, which recruits neutrophils
  • These neutrophils produce inflammatory cytokines, apoptosis, and make NETs, resulting in further antigen exposure within the kidney

• CRP is a complement protein and therefore does not correlate well with lupus activity
• ESR reflects an inflammatory state and correlates better with lupus activity
• ANAs
  • Initially thought to occur in 98% of cases at presentation
  • True prevalence closer to 85%, with detection sometimes requiring serial ANAs to be done over some time
  • Thus, though ANA-negative lupus is unusual, it should not exclude the diagnosis
  • Titres of 1:80 or less are seen in 5% of the general population
  • Therefore, only high titers, in the range of 1:240, should be considered to have clinical significance
• dsDNA
  • Highly specific and associated with lupus nephritis
  • dsDNA has a high negative predictive value and a low positive predictive value
  • dsDNA may be detected by Crithidia luciliae indirect fluorescence assay or by ELISA
    • Crithidia assay is more specific
    • ELISA sometimes detects low-affinity dsDNA that is not pathogenic
  • C1q antibodies
    • Associated with proliferative LN and may be pathogenic
  • C3, C4
    • At a population level, the reliability of C3 and C4 for tracking disease activity is poor
    • The membrane attack complex (C5b-9) tracks best with lupus nephritis but is currently limited to the research environment
  • Anti-Smith antibody
    • When present, it is strongly predictive of lupus
    • However, it cannot be used to monitor disease activity

• Urinary biomarkers in lupus (APRIL, BAFF) are currently of limited utility

References
1. Crow MK. Type I Interferon in the Pathogenesis of Lupus. J. Immunol. 192, 5459–5468 (2014). [https://www.jimmunol.org/content/192/12/5459] 2. Isnardi I, Ng YS, Menard L, Meyers G, Saadoun D, et al. Complement receptor 2/CD21− naive human B cells contain mostly autoreactive unresponsive clones. Blood 115, 5026–5036 (2010). [https://ashpublications.org/blood/article/115/24/5026/27217/Complement-receptor-2-CD21-human-naive-B-cells] 3. Robson KJ, Kitching AR. Tertiary lymphoid tissue in kidneys: understanding local immunity and inflammation. Kidney Int. 98, 280–283 (2020). [https://www.kidney-international.org/article/S0085-2538(20)30504-4/fulltext]