Autoimmune diseases Flashcards
Negative selection could adopt two approaches. What are these and what are their consequences?
- Rigorous approach: low risk of AI, poor repertoire and high susceptibility to infection
- Permissive approach: broad repertoire, low risk of infection, high risk of AI
- some potentially AI T cells are produced but corrected via peripheral tolerance
Define the following peripheral tolerance mechanisms
- Immunological hierarchy
- Antigen segregation
- Peripheral anergy
- Regulatory T cells
- Cytokine deviation
- Clonal exhaustion
- CD4 T cell will not be activated unless antigen is presented in an inflammatory context with TLR ligation
- Physical barriers to sequestered antigen ‘immunological privilege’
- Weak signalling between APC/CD4 T cell without co-stimulation causes T cells to become APC/CD4 T cell
- CD25+FoxP3+ T cells and other types of Treg calls actively suppress immune responses by cytokine and juxtacrine signalling
- Change in T cell phenotype e.g. TH1 and TH2 may reduce inflammation
- Apoptosis post-activation by activation-induced cell death
AID’s are classified as ‘organ specific’ and ‘non-organ specific’
Give two examples of each
ORGAN SPECIFIC
- T1DM
- Pemphigus, pemphigoid
- Graves
- Hashimotos
- AI cytopenias (haemolytic anaemia, thrombocytopenia)
NON-ORGAN SPECIFIC
- SLE
- RA
Outline the pathogenesis of haemolytic anaemia
- FCR+ cells in fixed mononuclear phagocyte system –> phagocytosis + RBC destruction
- Complement activation and intravascular haemolysis –> lysis + RBC destruction
How does thrombocytopenia present?
- Rash
- Bruising
Autoantibodies provide a pathogenic mechanism for AID
What kind of hypersensitivity is this?
What is the criteria?
- Type 2 hypersensitivity: a disease where an antibody is clearly pathogenic
- Disease transference by infusion of serum, or during gestation causing fetal problems
- Removal of antibody by plasmapheresis beneficial
- Pathogenic antibody can be identified and characterised
Graves is an autoimmune disease that arise because of pathogenic antibodies
Describe this disease (include symptoms, type of disease and pathophysiology)
Graves
- symptoms of hyperthyroidism: tachycardia, palpitations, tremor, anxiety, heat tolerance
- goitre
- graves ophthalmopathy (retro-orbital inflammation)
- characteristic of antibody-mediated disease: neonatal hyperthyroidism, serum transfers disease, antibody detected and characterised
AI B cells make ABs against TSH receptor stimulating thyroid hormone production –> less production of TSH by pituitary (feedback), no effect of Auto-AB production –> excessive TH production
Myasthenia gravis is an autoimmune disease that arise because of pathogenic antibodies
Describe its presentation and pathophysiology
- Ptosis at rest, worse when opening/closing eyes quickly
- Muscle weakness and fatigability
- Eyelids, facial weakness, chewing, talking, swallowing [most affected]
- Normal communication between nerve and muscle interrupted at NMJ. Antibodys block/alter/destroy receptors for ACh at NMJ preventing muscle from contracting (no Na+ influx_
- ACh receptors internalised and degraded
Spontaneous urticaria is an autoimmune disease that arise because of pathogenic antibodies
Describe its presentation and pathophysiology
(spontaneous rash)
- Manifests with hives and swelling
- IgG Fc3R1 antibody crosslinks mast cell receptor causing degranulation
T cells provide a pathogenic mechanism for AID
What kind of hypersensitivity is this?
What is the criteria?
Type 4 hypersensitivity:
- tissue damage directly mediated by T cell dependent mechanisms. T cells activate macrophages and other elements of innate immunity
- CD8 T cells damage tissue directly
- Most difficult to demonstrate. Models rely on sensitised genetically susceptible animals
What kind of AI disease is Hashimotos?
Describe its presentation and pathophysiology
Give two more examples of this kind of AI disease
Type 4 hypersensitivity (T cells)
- commonest cause of hypothyroidism in industrialised countries
- women, >30
- AI destruction of thyroid; organ infiltrated by CD4 and CD8 T cells
Coeliac, T1DM
Consider the aetiological factors for AID
What is the evidence for the involvement of genetics?
Evidence:
- rare monogenic disorders
- mouse models rely on susceptible strains e.g. NOD mice
- familial association attributable to HLA
What kind of AID is APACED (AI polyglandular syndrome candidiasis and ectodermal dystrophy)
- Aetiology?
- Key presentation and pathophysiology
- Monogenic AI disorder
- AIRE gene regulates ectopic expression of tissue specific antigens in thymus becomes mutated which results in a failure in negative selection
- Candidiasis
Results from antibodies to IL17 (important in host defence against fungi at mucosal surfaces)
What kind of AID is DiGeorge syndrome?
- Aeitology
- Prevalence?
- Pathophysiology
- Phenotype?
Monogenic AI disorder
- microdeletions on chromosome 22
- 1/200
- Failed migration of 3rd and 4th branchial arches
- Full phenotype: absent parathyroids (low Ca, tetany), cleft palate, congenital heart defects, thymic aplasia (low T cells, immunodeficiency)
- Variable presentation: huge spectrum of immunodeficiency from mild SCID-like, AI common
What kind of AID is IPEX (immunodysregulation polyendocrinopathy enteropathy X-linked)?
- Aeitology
- Pathophysiology
- Phenotype?
Monogenic AI disorder
- rare X linked mutation affecting FoxP3 gene
- prevents production of CD4, CD25, FOXP3, Treg
- Inflammatory bowel disease, dermatitis, organ specific AI
What is the pathophysiology of classical complement deficiency?
Presentation?
- immune complexes are cleared by phagocytes; process enhanced by phagocyte Fc receptors and C3B receptors
- deficiency of C1Q/C2/C4 predispose to lupus and recurrent bacteria infections because immune complexes are not cleared
Describe the HLA system
- APCs present processed peptide to T cells in combination with highly polymorphic MHC (HLA) molecules
- encoded by HLA system on chromosome 6
Class 1: A, B, C
Class 2: DR, DP, DQ - Diverse group of genes producing diverse group of receptors
- Strong association between expression of HLA molecules and AID
Consider coeliac disease
- What is it?
- Prevalence?
- Manifestations
- Histopathology
- Common inflammatory disease of small bowel with GI and extra-GI features
- <1% of population; women; majority undiagnosed
- characteristic of AID but triggered by exogenous antigen (gluten)
- Main manifestation: malabsorption (loose stool, weight loss, vitamin deficiency, anaemia, poor growth in children)
- Histopathology: total villous atrophy, crypt hyperplasia and lymphocyte infiltration in advanced disease
Consider coeliac disease
- Which HLA molecules are expressed?
- Pathophysiology
- express HLA-DQ2, HLA-DQ8 (or both)
- dietary gliadin (wheat, rye, barley) is degraded by guy tissue transglutaminase 2 enzyme during digestion producing gliadin peptides. HLA/DQ2/8 molecules present these peptides to T cells if the appropriate T cell receptors present
- the damage mediated by T cells not antibodys (although produced)
- inflammation resolves with strict gluten avoidance
Infection is a non-genetic factor in AI
Molecular mimicry provides an explanation for this. Describe it
Molecular mimicry: epitopes relevant to the pathogen are shared with host antigens
- Viral infection: presentation of viral peptides to CD4 t cell via MHC2 causing T cell activation
- the viral peptides happens to be similar to a host-derived peptide; the T cell would normally recognise these peptides but not react
- The activated T cell now reacts strongly to self-peptide and initiates inflammation
- Process depends on having correct MHC molecules to present this critical epitope that is common to both virus + host (inherited)
- dependent on having the correct T cell to recognise it
Describe two examples of molecular mimicry
AI haemolysis after mycoplasma pneumonaiae
- mycoplasma antigen has homolgy to ‘I’ antigen on RBCs
- IgM antibody to mycoplasma may cause transient haemolysis
Rheumatic fever: inflammatory disease occuring after streptococcal infection affecting heart, joints, skin, brain,
- antistreptococcal antibodies cross-react with connective tissue
Describe the general presentation of T1DM
Lack of insulin impairs cellular uptake of glucose –> polyuria, polydipsia, polyphagia, weight loss
- typical childhood onset, prevalence = 0.8%
- differentiate from monogenic and T2DM
What is the evidence that supports T1DM as an autoimmune disease?
- islet cell antibodies detectable for months-years before onset (not pathological)
- HLA associations
- mouse models
- early pancreatic biopsy shows CD4/8 T cell infiltration
- by the time patient has established T1DM, no active inflammation in pancreatic biopsy
Describe the progression of T1DM
- B cell mass rises slightly and platos during ‘genetic susceptibility’
- Environmental trigger
- Autoantibody insulinitis casues a decline in B cell mass
- Clinical onset when b cell mass lowest
- Final stage: loss of c peptide
Describe the genetics of T1DM
100% concordance in monozygotic twins
HLA class 2 alleles genetic risk factor
- DR3/DR4 risk = 6
- DR3 + DR4 risk= 15
- believed to be required to present relevant islet cell antigens to CD4 T cells
Describe the possible molecular mimicry cause of T1DM
Some evidence for Cox Sackie virus (strong immune response, infection precipitates AI diabetes in mouse models) and protein 2c from Cox Sackie virus has homology with islet cell antigen GAD
It is important to detect antibody’s in blood in diagnosis of autoimmune
Detail how indirect immunofluorescence is used
INCUBATE
- Patient serum containing (or not) relevant antibodies
- Glass slide with tissue of interest harvested from animal source
DETECT
- add detection antibody labelled with fluorescent marker
READ
- look for fluorescence under microscope
Why is it important to diagnose T1DM?
- At increased risk of ketoacidosis
- They need insulin
- Monogenic and T2DM require different management
It is important to detect antibody’s in blood in diagnosis of autoimmune disease
Detail how immunoassay (ELISA) is used
- Antigen is bound to plastic in well
WASH - Block - using powdered milk/BSA. Blocking blocks gaps between antigens so that nothing else binds
WASH - Add patient serum: specific antibody will bind antigen
WASH - Use labelled antibody (enzyme linked) which will bind to Fc portion of antibody 1 antigen
- Substate for antibody 2 is added and converted by enzyme into coloured product
- Rate of colour formation is proportionate to amount of specific antibody 1
It is important to detect antibody’s in blood in diagnosis of autoimmune disease
How are ELISA results interpreted?
- First have to generate a standard curve using a negative (add no serum or a serum with known no AB) and a positive (known serums with AB) control
- Optical density is read and curve control value plotted. The optical density of the patient is then used to derive specific antibody concentrations
- Only use linear aspect of curve as that is the relationship between [Antibody 1] and colour
It is important to detect antibody’s in blood in diagnosis of autoimmune disease
What is the antigen bound to the well in the ELISA of Rheumatoid arthritis
If positive, what are the next steps?
CCP
- Urgent referral to rheumatology for immunomodulation BEFORE joint damage
It is important to detect antibody’s in blood in diagnosis of autoimmune disease
Comment on the technical validity of the ELISA
- Failure of any control leads to invalidation of assay
DIRECT IMMUNOFLUORESCENCE is used to detect antibodies bound to tissue
Detail the steps
- PREPARE TISSUE BIOPSY: Take a biopsy of affected tissue e.g. skin. If damage mediated by antibody, antibody will already be suck to its antigen in tissue
- DETECT: Add detection antibody labelled with fluorescent marker
- READ: look for fluorescence under microscope
There are two types of bullous skin disease
What are they and how do they compare?
PEMPHIGOID
- thick walled bullae, rarely on mucus membranes
- target: antigen at dermo-epidermal junction
- linear deposition of antibody, which activated complement producing skin dehiscence + tense blisters
PEMPHIGUS
- thin walled bullae on skin and mucus membranes (RUPTURE)
- target: intercellular cement protein desmoglein 3 in superficial skin layers
- Both fulfil criteria for AB-mediated disease
How is coeliac disease diagnosed?
Antibodies binding to endomysium of smooth muscle fibres
- indirect immunofluorescence with monkey oesophagus shows characteristic endomysial stairing pattern
- Modern immunoassays use antigens from tissue transglutaminase expressed in recombinant systems
- HLA typing (absence of HLA DQ2/8 makes coeliac disease unlikely)
Describe what happens in pernicious anaemia
How can this be demonstrated?
- Normally vitamin B12 is absorbed in terminal ileum with co factor (intrinsic factor) which is secreted by gastric parietal cells
- AI destruction of the gastric parietal cells causes lack of absorption of B12
- For 2 years this is undiscovered as liver stores 2 years supply of B12, once depleted –> anaemia, neuro symptoms, subfertility
Auto-antibodies demonstrated by indirect immunofluorescence
Consider treatment options for autoimmune disease
What are the principles of management at the moment?
Manage the consequences in preference to treating underlying immunology as immunosuppressive drugs are toxic and by the time the clinical disease becomes apparent the damage is already done so immunosuppression is unhelpful
Consider treatment options for autoimmune disease
When would immunomodulation be appropriate?
What is used in it?
Some non-organ specific diseases/ Muti-system AID
- systemic corticosteriods
- small molecule immunosuppressive drugs
- high dose intravenous immunoglobulin
Consider treatment options for autoimmune disease
What does plasmapharesis achieve?
Removes antibodies from bloodstream therefore useful in antibody mediated diseases
