L7 Infection and Immunity Autoimmunity

Overview

This lecture introduces autoimmunity as a breakdown of immune tolerance, then works through the epidemiology, genetics, immunopathology, clinical presentation and treatment of three example diseases: rheumatoid arthritis (RA), multiple sclerosis (MS) and systemic lupus erythematosus (SLE), before closing with a discussion of why autoimmune disease disproportionately affects women.

History and definition of autoimmunity

• 1901: Ehrlich coined “horror autotoxicus” for the idea that the organism would not endanger itself by making toxic autoantibodies against itself, rejecting the hypothesis that autoimmunity was possible.
• 1904: Donath and Landsteiner identified the first damaging autoantibody, an autohemolysin.
• 1915-1945: an “eclipse period” for the concept of autoimmunity.
• 1945-1965: rediscovery of autoimmunity.
• 1964: the concept of autoimmune disease became accepted.
• Autoimmunity is a dysfunction of the immune system arising from failure of tolerance:
  • Central tolerance failure: failure of negative selection or receptor editing of self-reactive developing B cells in the bone marrow, or negative selection of self-reactive developing T cells in the thymus.
  • Peripheral tolerance failure: failure of anergy induction, apoptosis or suppression of mature autoreactive B or T cells.
  • Normally, non-autoreactive B cells pass through the bone marrow and periphery unaffected, while autoreactive B cells are removed centrally by clonal deletion or receptor editing in the bone marrow, and any that escape are removed peripherally by clonal deletion or anergy.

Overview of autoimmune disease and risk factors

• 80 distinct autoimmune diseases exist, affecting up to 5% of the population; some are common (T1D, RA, vitiligo, MS) but many are rare.

• Risk factors:
  • Sex: female much greater than male (F >> M).
  • Genetics: often genes encoding immune system molecules.
  • Epigenetics: influenced by lifestyle/environment, e.g. smoking, bacterial or viral infection, medications, geographic location.
  • Having one autoimmune disease: ~25% of patients with an autoimmune disease go on to develop additional autoimmune diseases.
• Two major groups of autoimmune disease:
  • Organ-specific: target antigen expression restricted to a specific cell, organ or tissue, e.g. T1D (pancreatic beta cells), Graves’ disease (thyroid).
  • Systemic: autoantigen found in multiple cells, organs or tissues, e.g. RA (connective tissue), SLE (antinuclear antibodies, ANA).

Rheumatoid arthritis (RA)

Epidemiology, genetics, aetiology

• One of the most prevalent chronic inflammatory diseases; NZ 2020/21 prevalence in people 15+ was 2.3% (95,000 people).
• Adjusted prevalence ratios (statistically significant marked ): men vs women 0.56; Māori vs non-Māori 1.85*; Pacific vs non-Pacific 1.39; Asian vs non-Asian 0.44; most vs least deprived 1.81*; disabled vs non-disabled adults 2.71*.
• Primarily affects joints but has extra-articular manifestations (e.g. vasculitis, interstitial lung disease); decreases physical function and quality of life; carries socioeconomic burden (direct medical costs, reduced work capacity, decreased societal participation).

• 100 genetic loci associated with RA risk; positive family history increases risk 3-5x; epigenetics (DNA methylation, histone acetylation) contribute to pathogenesis.

• Environmental risk factors: smoking, low socioeconomic status, possible infectious triggers.

Immunology and pathogenesis

• Seropositivity (autoantibodies) is present in 60-80% of patients at diagnosis and is associated with more severe symptoms and joint damage.
• Anti-citrullinated peptide antibodies (ACPA, also called anti-CCP): bind citrullinated residues on self-proteins (e.g. fibronectin, fibrinogen, type II collagen), forming immune complexes; these bind rheumatoid factor (an anti-Fc IgG autoantibody), which leads to complement activation and macrophage activation. ACPAs can be detected up to 10 years before diagnosis and may directly activate macrophages/osteoclasts.
• Rheumatoid factor structure: a pentameric IgM-like antibody that binds the Fc portion of IgG.
• Pathway to RA (Smolen, Aletaha and McInnes):
  1. Pre-arthritis stage: susceptibility genes (MHC shared epitope, PADI, PTPN22) + environmental factors (recurrent exposure to viral/bacterial/commensal agents, smoking, silica, affecting oral, bronchial or gut microbiome) + epigenetic modifications + post-translational modifications (acetylation, carbamylation, citrullination, methylation, phosphorylation, sumoylation, ubiquitination) combine.
  2. Loss of tolerance: autoantibodies (ACPA, RF) form; dendritic cells, T cells and B cells are activated in lymph nodes/secondary lymphoid tissue.
  3. Asymptomatic synovitis: cellular infiltrate in the joint.
  4. Transition to manifest, symptomatic arthritis: swelling, pain, stiffness, joint damage.
• Joint inflammation: leucocyte infiltration into the synovial compartment, together with inflammatory fibroblasts and osteoclastogenesis, promotes articular destruction. Key cytokine mediators are TNF, IL-6 and GM-CSF, acting via JAK/STAT signalling. Compared to a normal joint, the RA joint shows osteoclasts, fibroblasts, macrophages, dendritic cells, T cells, B cells, plasma cells, extensive angiogenesis, mast cells, hyperplastic synovial lining and neutrophil infiltration, forming a destructive pannus.

Clinical presentation and diagnosis

• Diagnosis requires synovitis in at least one joint, absence of an alternative diagnosis, and some of: number/site of involved joints, serological abnormality (RF/ACPA), elevated acute-phase response (CRP, ESR), symptom duration.
• Extra-articular manifestations may include vasculitis (inflammation of blood vessels) and interstitial lung disease.

Treatment

• csDMARDs (conventional synthetic): methotrexate, prednisone, sulfasalazine, hydroxychloroquine, leflunomide, cyclosporine.
• bDMARDs (biologic, PHARMAC-funded): infliximab/adalimumab/etanercept (anti-TNF/TNF blocker), rituximab (anti-B-cell/CD20), tocilizumab (anti-IL6R).
• tsDMARDs (targeted synthetic): tofacitinib and upadacitinib (JAK inhibitors); upadacitinib is PHARMAC-funded.
• Infliximab is a full antibody (Fab/Fc regions, heavy and light chains, complement-binding region); etanercept is a fusion protein of the human p75 TNF receptor with an Fc region.
• Immunosuppressive drugs increase infection risk, so vaccinations should be kept up to date; live vaccines should be avoided in patients on bDMARDs (e.g. use Shingrix rather than Zostavax to prevent shingles).

Multiple sclerosis (MS)

Epidemiology, genetics, aetiology

• MS is a chronic inflammatory demyelinating CNS disease.
• ~4000 people in NZ have MS (100 per 100,000); 3x higher prevalence in the South Island than the North Island; females 2.5-3x more affected than males.
• Age of onset 20-40 years; 25 years after onset, 50% require permanent wheelchair use.
• Genetic links: variants of class II HLA-DRB1, TNFR1, IL-2R and IL-7R genes; concordance rate of 30-40% in monozygotic twins; possible epigenetic changes associated with MS.
• Environmental risk factors: low vitamin D, EBV exposure, smoking.
• It is unknown whether the immune dysfunction leading to MS arises in the periphery or in the CNS.

Immune dysregulation inside the CNS

• Early lesions: invading peripheral immune cells, leakage of the blood-brain barrier, macrophage-dominant infiltrate with fewer T cells, B cells and plasma cells, Th1 and Th17 CD4+ T cells, IL-17-producing CD8+ MAIT cells; lesions are focal, with otherwise normal-appearing white matter. Soluble mediators recruit immune cells from the subarachnoid space into the perivascular space and CNS parenchyma; TH1/IFNγ and TH17/IL-17 cells act on axons, causing degradation of myelin protein and demyelination, with antibody and complement also contributing.
• Late lesions: diffuse T and B cell infiltrates, microglia and astrocyte activation, pronounced grey and white matter atrophy, diffuse myelin reduction and axonal injury. Meningeal tertiary lymphoid-like structures (with clonally expanded B cells and follicular dendritic cells) promote glia limitans damage and astrocyte dysfunction; oligodendrocyte progenitor cells experience metabolic stress and energy deficiency, with glutamate accumulation, ROS/RNS and ionic imbalance leading to neuroaxonal and oligodendrocyte damage/death, driving further neurodegeneration at distal sites.

Clinical presentation and diagnosis

• Heterogeneous presentation: sensory and visual disturbances, motor impairment, fatigue, pain, cognitive deficits. Symptom regions include central (fatigue, cognitive impairment, depression, unstable mood), visual (nystagmus, optic neuritis, diplopia), speech (dysarthria), throat (dysphagia), musculoskeletal (weakness, spasms, ataxia), sensation (pain, hypoesthesias, paraesthesias), bowel (incontinence, diarrhoea or constipation), urinary (incontinence, frequency or retention).
• CNS inflammation and demyelination appear as white and grey matter lesions on MRI in brain and spinal cord; inflammation and neurodegeneration are closely associated at all lesion sites and disease stages. Demyelinated white matter areas can be partially repaired by remyelination, but ongoing disease leads to gradual neuroaxonal loss and disability.
• First episode of neurological symptoms = “clinically isolated syndrome”. A second episode, or subclinical MRI changes at least 30 days apart, can lead to a diagnosis of relapsing-remitting MS (“McDonald criteria”). Brain MRI is very sensitive for monitoring disease activity and treatment efficacy; spinal cord MRI is less sensitive.

Disease progression

• 80% of patients with relapsing-remitting MS (RRMS) develop secondary progressive disease.
• 10% of MS patients have primary progressive disease from onset.
• Disease course runs from pre-symptomatic disease through clinically isolated syndrome, relapsing-remitting disease (with inflammatory relapses), to secondary progressive or primary progressive disease, with disability increasing as brain volume, axonal integrity and neurological function decline.

Treatment

All target the immune system.

• First-line disease-modifying therapies: IFNbeta (Avonex, Betaferon), glatiramer acetate (Copaxone), dimethyl fumarate (Tecfidera), teriflunomide (Aubagio). Thought to dampen the Th1/Th17 response, skew towards a Th2 response, increase Treg-mediated suppression, or inhibit rapidly dividing cells.
• Newer disease-modifying therapies: natalizumab (Tysabri, for relapsing-remitting MS, blocks a molecule required for lymphocyte entry into the CNS), ocrelizumab (binds CD20 on B cells), fingolimod (Gilenya, prevents lymphocyte egress from secondary lymphoid organs).
• Acute relapses are managed with methylprednisolone.
• There are no treatments that promote remyelination.
• Natalizumab carries a risk of progressive multifocal leukoencephalopathy, caused by JC virus.

Systemic lupus erythematosus (SLE)

Epidemiology

• A complex, chronic, multi-system autoimmune disease with diverse clinical features, flares and periods of remission.
• 8-9x more common in women than men; typical age of onset 20-45 years; juvenile SLE is less common.
• Māori and Pacific New Zealanders have an age-adjusted SLE rate of 50.63 per 100,000 versus 14.6 per 100,000 for NZ Europeans (1983 data).
• Characteristic malar (“butterfly”) rash across the nose and cheeks.

Genetic, epigenetic and environmental risk factors

• Deficiencies in complement cascade components are well known to predispose to SLE.
• Dozens of other confirmed susceptibility loci, in genes for: antigen processing/presentation (HLA, TAP1/2); clearance of apoptotic debris (Dnase1); leukocyte cell surface receptors (FCGRI/II/III, ITGAM); cell signalling and gene transcription (LYN, BLK, PTPN22, STAT4, IRF5).
• Environmental risk factors: sunlight exposure, cigarette smoking, infection, vitamin D deficiency, exogenous oestrogen, and (rarely) TNFα inhibitors.
• Disease severity is associated with pregnancy, the menstrual cycle and oral contraceptive use, and decreases after menopause.
• Two epigenetic hallmarks of SLE: DNA hypomethylation and reactivation of the inactive X chromosome.

Pathogenesis and immunopathology

• Pathway to SLE: genes (C1q, C2, C4, HLA-D2/3/8, MBL, FcR2A/3A/2B, IL-10, MCP-1, PTPN22) combined with environment (UV exposure, sex, infection?) drive an abnormal immune response, in which antigen activates dendritic cells, T cells and B cells, and defective suppressive networks allow production of autoantibodies and immune complexes. These autoantibodies/immune complexes activate complement (C3, C3a), causing inflammation (rash, nephritis, arthritis, leukopenia, CNS disease, carditis, clotting abnormalities, etc.), which becomes chronic inflammation and chronic oxidation, ultimately causing damage: renal failure, atherosclerosis, pulmonary fibrosis, stroke, and damage from treatment.
• Immunopathology in more detail (cyclical): nucleic-acid-containing immune complexes drive loss of B cell tolerance and B cell hyperactivity (autoreactive B and T cell interactions producing switched autoantibodies of IgM, IgE and IgG isotypes via plasma cells and dendritic cells), which feeds inflammation (IL-3, M-CSF, GM-CSF acting on the spleen; IL-6, BAFF, IL-1, IFN, TNF acting on eosinophils/neutrophils/macrophages), driving haematopoiesis and acute-phase protein production by the liver, generating pathogenic autoantibodies that activate myeloid effectors, causing immune complex deposition and tissue injury/destruction (e.g. in the kidney). Genetics, environment, diet, stress and ageing modify this cycle.
• Key features: loss of B cell tolerance, B cell hyperactivity, production of anti-nuclear antibodies (ANAs, present in 90% of SLE patients on average 2-3 years before onset; includes anti-DNA and anti-RNA binding protein antibodies), T cell-mediated isotype switching and pro-inflammatory cytokine production, immune complex formation. 30-70% of patients develop life-limiting renal disease.

Clinical presentation

• Fatigue, weakness, loss of appetite, weight loss, skin rashes (most commonly the malar/butterfly rash), hair loss, mouth and nose ulcers, red sore dry eyes.

Diagnosis (SLICC classification, revised 2012)

• Clinical criteria: acute or subacute cutaneous lupus; chronic cutaneous lupus; oral ulcers; nonscarring alopecia; synovitis involving 2+ joints; serositis involving lungs or heart; renal involvement; neurological involvement; haemolytic anaemia; leukopenia or lymphopenia; thrombocytopenia.
• Immunologic criteria: raised ANA level; raised anti-dsDNA antibody level; presence of anti-Sm; positive antiphospholipid antibody; low complement levels; positive direct Coombs’ test.
• SLE is diagnosed if a patient has, over time, either: 4 criteria including ≥1 clinical and ≥1 immunological criterion, OR biopsy-proven lupus nephritis plus antinuclear antibodies or anti-dsDNA antibodies.

Treatment

• Immunosuppressives: azathioprine, mycophenolate, cyclophosphamide, and systemic corticosteroids (short-term/induction phase).
• Hydroxychloroquine (acting as an anti-inflammatory).
• Methotrexate.
• Targeted biologic: rituximab (anti-B-cell/CD20).

Why are females more susceptible to autoimmunity?

• Females mount more vigorous T and B cell responses than males; correspondingly, males are more susceptible to infection.
• This may be an evolutionary product of the reproductive role of women:
  • During pregnancy, immune responses are attenuated and become more Th2-like to prevent fetal rejection; this worsens antibody-mediated disease (lupus) but improves inflammatory disease (RA, MS).
  • At other times, more robust immunity protects offspring (e.g. antibody transfer through lactation).
• Sex hormones: oestrogen is associated with increased immunity.

Autoimmune disease results from loss of self-tolerance against erythrocyte surface antigens, cell surface receptors and hormone receptors alike (i.e. all of the above) — self-tolerance breakdown is not restricted to one antigen class.

Summary

• Autoimmunity is caused by breakdown of central and peripheral tolerance, so that immune cells and molecules react against self; disease may be organ-specific or systemic.
• Risk factors include genetic (mostly immune-related genes), environmental and host factors; there is no single known specific cause.
• Autoimmunity is treated through broad immunosuppression or targeted immune suppression (e.g. antibodies targeting immune cells or cytokines, inhibitors of immune cell signalling, inhibitors of immune cell trafficking).

Self-test

1. Distinguish central tolerance from peripheral tolerance, giving an example mechanism of each.
2. List four risk factors for developing autoimmune disease.
3. Distinguish organ-specific from systemic autoimmune disease, with one example of each.
4. Describe the role of ACPA and rheumatoid factor in the pathogenesis of rheumatoid arthritis.
5. Describe the stages of the pathway from pre-arthritis to symptomatic arthritis in RA.
6. List the three main cytokine mediators of joint inflammation in RA and the signalling pathway they act through.
7. Distinguish csDMARDs, bDMARDs and tsDMARDs in RA treatment, with one example drug of each.
8. Why should live vaccines be avoided in patients on bDMARDs, and what alternative is given for shingles prevention?
9. Describe the differences between early and late MS CNS lesions.
10. What are the McDonald criteria used for in MS diagnosis?
11. What proportion of RRMS patients develop secondary progressive disease, and what proportion of MS patients have primary progressive disease from onset?
12. Describe the mechanisms of action of the first-line MS disease-modifying therapies.
13. What serious risk is associated with natalizumab, and what causes it?
14. List the two epigenetic hallmarks of SLE.
15. Describe the immunopathological cycle linking B cell hyperactivity to tissue injury in SLE.
16. What are the SLICC criteria required to diagnose SLE?
17. A 28-year-old woman presents with a malar rash, joint pain and fatigue; blood tests show a raised ANA and anti-dsDNA antibody level. Explain, using the SLICC criteria, why this supports a diagnosis of SLE.
18. Explain why pregnancy tends to worsen lupus but improve rheumatoid arthritis and MS.
19. Comparing RA, MS and SLE, what do their pathogenesis pathways have in common in terms of the roles of genetics, environment and loss of tolerance?

Answers

Reveal answers

1. Central tolerance removes self-reactive lymphocytes during development: negative selection or receptor editing of self-reactive developing B cells in the bone marrow, and negative selection of self-reactive developing T cells in the thymus. Peripheral tolerance acts on mature autoreactive B or T cells that escape central tolerance, inducing anergy (unresponsiveness), apoptosis or suppression.
2. Any four of: sex (female much more than male), genetics (often immune-related genes), epigenetics (influenced by smoking, infection, medications, geography), and already having one autoimmune disease (~25% develop additional ones).
3. Organ-specific disease targets an antigen restricted to one cell/organ/tissue, e.g. T1D (pancreatic beta cells) or Graves’ disease (thyroid). Systemic disease targets an autoantigen present in multiple cells/organs/tissues, e.g. RA (connective tissue) or SLE (antinuclear antibodies).
4. ACPA bind citrullinated residues on self-proteins (e.g. fibronectin, fibrinogen, type II collagen), forming immune complexes; these bind rheumatoid factor (an anti-Fc IgG autoantibody), leading to complement activation and macrophage activation. ACPAs can be detected up to 10 years before diagnosis and may also directly activate macrophages/osteoclasts.
5. Pre-arthritis stage: susceptibility genes, environmental factors, epigenetic and post-translational modifications combine → loss of tolerance (ACPA/RF autoantibodies, DC/T/B cell activation in lymph nodes) → asymptomatic synovitis (cellular infiltrate) → transition to symptomatic arthritis (swelling, pain, stiffness, damage).
6. TNF, IL-6 and GM-CSF, acting via JAK/STAT signalling.
7. csDMARDs are conventional small-molecule drugs, e.g. methotrexate. bDMARDs are biologics targeting specific immune molecules/cells, e.g. infliximab (anti-TNF). tsDMARDs are targeted synthetic small molecules, e.g. tofacitinib (JAK inhibitor).
8. Immunosuppressive bDMARDs increase infection risk, and live vaccines could cause disease in an immunosuppressed patient; Shingrix (non-live) is used instead of Zostavax (live) to prevent shingles.
9. Early lesions are focal with macrophage-dominant infiltrate, fewer T/B cells and plasma cells, Th1/Th17 CD4+ T cells and IL-17-producing CD8+ MAIT cells, in otherwise normal-appearing white matter. Late lesions show diffuse T and B cell infiltrates, microglia/astrocyte activation, pronounced grey and white matter atrophy, and diffuse myelin reduction and axonal injury.
10. They are used to diagnose relapsing-remitting MS from a second clinical episode or subclinical MRI changes occurring at least 30 days apart from the first (clinically isolated syndrome).
11. 80% of RRMS patients develop secondary progressive disease; 10% of MS patients have primary progressive disease.
12. They are thought to dampen the Th1/Th17 immune response, skew it towards a Th2 response, increase suppression by Tregs, or inhibit rapidly dividing cells.
13. Progressive multifocal leukoencephalopathy, caused by JC virus.
14. DNA hypomethylation and reactivation of the inactive X chromosome.
15. Nucleic-acid-containing immune complexes drive B cell hyperactivity/loss of B cell tolerance, producing switched autoantibodies via plasma cells and DCs; this drives inflammation (via IL-3, M-CSF, GM-CSF, IL-6, BAFF, IL-1, IFN, TNF), which drives haematopoiesis and acute-phase protein production in the liver, generating pathogenic autoantibodies that activate myeloid effectors, causing immune complex deposition and tissue injury (e.g. kidney).
16. Either 4 criteria including at least 1 clinical and at least 1 immunological criterion, or biopsy-proven lupus nephritis plus antinuclear antibodies or anti-dsDNA antibodies.
17. The malar rash and joint pain (synovitis) satisfy clinical criteria, and the raised ANA and anti-dsDNA satisfy immunological criteria; this meets the SLICC requirement of ≥4 criteria including at least one clinical and one immunological criterion.
18. Pregnancy attenuates immune responses and skews them towards a Th2-like profile to prevent fetal rejection; this worsens antibody-mediated disease like lupus but improves inflammatory diseases like RA and MS.
19. In each, susceptibility genes (often immune-related) combine with environmental triggers to produce loss of tolerance and autoantibody/autoreactive lymphocyte activity, which then drives chronic inflammation and tissue-specific or systemic damage.