Autoimmunity (Rheumatoid Arthritis) Flashcards
What is the issue with the adaptive immune system?
• Adaptive immune system has high diversity due to somatic recombination of VDJ segments in lymphoid organs, and somatic hypermutation (of BCR) in peripheral organs.
− Adpative provides capacity to recognise any chemical structure → BUT! Potential for autoimmunity!
Why is an effective immune system a compromise?
- Need removal of dangerous self-reactivity but without impairing the capacity for broad recognition and effective defense
- If removal were too rigorous – holes in the repertoire
- Some self-reactivity is normal – limited by mechanisms of tolerance
What are the 3 mechanisms of immunological tolerance?
- Inactivation of potential self-reactive clones → death, receptor editing, anergy
- Immune regulation → suppression or functional deviation
- Antigenic epitopes not available for recognition → ignorance, sequestration, immune privilege
Describe central tolerance in B cells
• No self reaction:
− Immature B cells that don’t have strong reactivity to self antigen allowed to mature. Leave the bone marrow and carried to the spleen.
• Clonal deletion/receptor editing:
− Strong, multi-valent cross-linking by self antigen
− Interval before cell death where the B cell may be rescued by further gene rearrangement reaplcing the autoreactive receptor with one that isn’t.
− Upon strong cross-linking, self-reactive light chain is deleted and RAG expression continues, with light chain-rearrangement continuing until a non-autoreactive receptor produced
− Cells that remain autoreactive when all V and J segments are exhausted undergo clonal deletion by apoptosis.
• Anergy:
− Weakly cross-linking self antigen with low valence
− B cells enter a state of permanent unresponsiveness
− Anergic B cells cannot be activated by their antigen, even with T cell help
− Migration of anergic B cells within lymphoid organs is impaired (they are excluded from follicles), and they cannot compete with immunocompetent B cells
− Without T cell survival signals, they eventually die
• Clonal ignorance:
− Have affinity for self antigen, but do not sense and respond to it
− Antigen may not be accessible, may be in low concentration, or may bind so weakly that it does not generate an activating signal
− Some ignorant cells are activated under certain conditions such as inflammation or when antigen becomes available and reaches an unusually high concentration – so they should not be considered inert.
Describe the 3 checkpoint stages of central tolerance in T cells
Checkpoint 1: Quality check → Cortex
• Has the TCR-beta gene rearrangement resulted in a functional pre-TCR expression?
• Occurs at the DN3 stage
• If it hasn’t, the cell fails to receive a survival signal and is deleted by apoptosis
Checkpoint 2: Positive selection → Moving towards the medulla
• Can the rearranged abTCR recognise self MHC?
• DP thymocytes encounter MHC-I or MHC-II on the surface of cTECs
• Thymocytes who can bind this antigen receive a survival signal
• Cells that cannot bind can be recovered by receptor editing, or die by apoptosis
• Only 10-30% make it to the next stage
Checkpoint 3: Negative selection → Medulla
• Does the abTCR recognise self-MHC and peptide too strongly?
• Antigen presented by mTECs
• T cell clones deleted if they bind too strongly
What presents antigen in the thymus for central T cell tolerance?
cTECs:
• Constiutive MHCI and II expression
• MHC 1 peptides → processed by thymoproteasome
• MHC II peptides → processed by
− Cathepsin L in the endosome
− Thymus specific serine protease (TSSP) in the endosome or lysosome
− Constitutive macroautophage
mTEC:
• Constitutive MHC I and II expression
• MHC I peptides → processed by the housekeeping proteasome and immunoproteasome
• MHC II peptides → processed by capthepsin S and macroautophagy
What are the sources of antigen for central T cell tolerances?
mTECS:
• They are inefficient at uptake and presentation of exogenous antigen
• They therefore express AIRE → allows for promiscuous expression of tissue specific genes not normally expressed in the thymus
• Genetic deficiency of AIRE results in autoimmune polyendocrinopathy
• mTECS also perform macroautophagy, so their cellular contents can be degraded and expressed on MHCs
DCs:
• Uptake tissue restricted antigens shed form mTECs
• Uptake of blood-bourne antigen
• Cross-presentation of ingested antigen
Summarise the affinity hypothesis of T cell selection
- T cells that fail to make a functional TCR → die at checkpoint 1
- TCR that cannot recognise self-MHC → die at checkpoint 2 – positive selection
- TCR that recognizes self-MHC and self-antigen too strongly → die at checkpoint 3 – negative selection
- TCR that recognizes self-MHC and doesn’t bind self antigen too strongly → good
- T cells that recognise self-MHC and bind self-antigen moderately (not too strong that they are deleted) → become Foxp3+ TRegs
Describe peripheral tolerance in B cells
• Mainly due to lack of T cell help
- DC sees an antigen in the periphery, presents it and migrates to the lymph node
- Activates a T cell in the lymph node
- Activated T cells move to the B cell region of the lymph node
- Some of the antigen will have drained to the lymph node, and been taken up and presented by B cells
- If the T cell sees that the B cell is presenting the same antigen as the DC → gives it costimulation to become a plasma cell
- If however, a self antigen had arrived at the lymph node, the B cell would have presented it but there would be no T cell help, because a DC would not have recognized the antigen as dangerous, so wouldn’t have activated the T cells.
Another thing to consider is activation in germinal centres:
- Competition for limited antigen and Tfh cells drives selection for highest binders
- Failure to bind and present antigen = failure to receive survival signals
- Binding of soluble self antigen = apoptosis
- T follicular regulators = antigen specific suppression of B cells
Describe peripheral tolerance in T cells
• Antigen must be presented on a DC in an activation context to initiate naïve T cell activation
• Presentation in the absence of danger (without signal 2 or 3) is tolerogenic → leads to energy or regulation
• Clonal anergy:
− Presence of signal 1 without 2 or 3 renders a T cell anergic
− It is unresponsive to antigen – doesn’t turn into an effector cell
• Regulation → peripheral induced TReg
− Presence of signal 1 with immunosuppressive signal 3 skews the T cell to become a Treg, rather than an effector
− Number of different TRegs depending on the cytokine that is signal 3
− TGFB → Foxp3+ iTreg
− IL-10 → Tr1 (Foxp3-)
How regulatory T cells regulate:
• Production of anti-inflammatory cytokines → TGFB, IL-10
• Expression of inhibitor receptors → CTLA-4
• Modulation of DCs
• Outcompete effector T cells for resources → CD25 mops up IL-2 (acting as a cytokine sink)
• Direct killing of T cells using granzyme and perforin.
What is autoimmunity?
- Loss of tolerance to self antigen, resulting in specific adaptive immune responses to self-antigen.
- Loss of central tolerance likely occurs all the time, but is normally kept in check by peripheral tolerance
- It may or may not cause disease → you can have an autoimmune response resulting in autoantibodies that circulate for years without causing disease
What is autoimmune disease?
- Tissue response and damage resulting from autoimmunity
- Self antigen cant be cleared, resulting in chronic pathology
- Results from the failure of peripheral tolerance and dysregulated activity of normal immune effector functions → there is nothing new or overactive, it is just dysregulated
What may contribute to B cell-mediated loss of tolerance and autoimmune disease?
− Non-specific, T cell independent activation of B cells by microbial mitogens, eg) LPS, DNA and other TLR ligands
− molecular mimicry → T cell dependent. Sequence similarities between foreign and self-peptides are sufficient to result in the activation of autoreactive T or B cells by pathogen derived peptides.
− eg) Rheumatic fever
➢ Immune response to Streptococcal cell surface M-antigen generates antibodies that cross-react with self-glycoproteins in the heart, skin and connective tissue
➢ In susceptible individuals (doesn’t happen in everyone who gets infected), this gives rise to autoimmune disease where tissue damage is mediated by complement, neutrophils and macrophages
− genetic variation associated with altered signaling pathways
− eg) altered activation thresholds and ineffective negative selection
− or to enhanced germinal centre activity (e.g. somatic hypermutation leading to auto-reactivity) and preferential selection by autoantigens during the generation of memory B cells and plasma cells.
− Cell-type specific epigenetic changes resulting in altered gene expression in cells of the immune system.
What may contribute to T cell-mediated loss of tolerance and autoimmune disease?
− impaired generation of TRegs
− post-translational modification of self-proteins or altered availability providing neo-antigens
− Altered availability: may have tissue damage resulting in antigen release from immune privileged sites. T cells wont have seen these during central tolerance
− Altered self:
➢ Post-translational modification, eg) citrullination of Arg residues implicated in RA
➢ Chemical modification, eg) oxidation of type II collagen in RA joints
− Molecular mimicry: foreign antigen and self protein have similar epitopes, and be taken up and presented by APC. The T cell generated against the foreign antigen may be able to react to self antigen.
− genetic variation associated with altered signaling pathways
− Cell-type specific epigenetic changes resulting in altered gene expression in cells of the immune system.
What causes the transition from autoimmunity to chronic autoimmune disease?
Early:
Activation phase:
• May have some initial tissue damage or cell death
• Autoreactive B and T cells respond to autoantigen that is released, and produce autoantibodies
• This may occur at relatively low levels
Later:
Chronic phase:
• Further tissue damage occurs
• This breaks down sequestration
• Autoantibodies bind self-antigens and non-specific effectors (macrophages, neutrophils) are attracted
• Release of inflammatory mediators
• Self-destructive process perpetuated
• Epitope spreading
− refers to the development of an immune response to epitopes distinct from, and noncross-reactive with, the initial activating epitope
− Thee ability of the immune system to attack multiple targets on a pathogen has obvious advantages, but it can result in escalating autoimmunity.
− Epitope spreading in diabetes:
➢ Initial responses are towards central epitopes of glutamic acid decarboxylase (GAD)
➢ As the damage progresses, the response spreads to other GAD epitopes → intramolecular epitope spreading
➢ The response may also spread to other antigens, eg) insulin, HSP, carboxypeptidase H → intermolecular epitope spreading
− Epitope spreading in SLE:
➢ Nucleosomes may be recognized by a B cell, taken up, processed and antigen preented to a T cell
➢ This could result in antibodies against the histone or the DNA component
➢ Tend to see histone first, then DNA later
Describe the pathology of autoimmune diseases
- Primary pathology → a direct consequence of the autoimmune response
- Secondary pathology → arises as a consequence of the primary pathology
eg)
T1DM:
• Primary → autoimmune destruction of pancreatic beta cells
• Secondary → metabolic effects due to lack of insulin
Hashimotos thyroiditis:
• Primary → destruction of the thyroid tissue
• Secondary → hypothyroidism – causes fatigue, intolerance of cold, dry skin, mental impairment
• There is organ specific and organ non-specific disease
− Organ specific → eg, Hashimotos thyroiditis
− Organ non-specific → eg, SLE, RA
What are the immunological features that cause autoimmune disease? (T Cells/Autoantibody)
• Autoantibodies − In the serum − Deposited in tissues • Cellular infiltrate − T cells, B cells, Monocytes, NK cells, DCs, Neutrophils
• Most autoimmune diseases involves the generation of autoreactive CD4+ T cells
− These drive other immune effector functions (B cells and CTLs) and recruit and amplify innate effector cells
• However, in some conditions, autoantibodies are the primary mediators due to:
− Complement mediated lysis
− eg) Autoimmune haemolytic anaemia
− Anti-RBC autoantibodies bind to RBCs. Complement is fixed and leads to generation of the MAC. Results in lysis of the RBC
− Opsonisation
− Eg) Autoimmune thrombocytopenia
− Anti-platelet autoantibodies binds to the platelet, acting as an opsonin, leading to phagocytosis of the platelets
− Antibody mediated stimulation of function
− Eg) Graves disease (hyperthyroidism)
− In a normal situation, the pituitary secretes TSH, which acts on the thyroid to secrete thyroid hormone. These then act back on the pituitary as negative eedback, suppressing further TH synthesis.
− In Graves disease, autoantibodies against the TSH receptor stimulate thyroid hormone production
− Antibody mediated inhibition of function
− Eg) Myasthenia gravis → muscle weakness caused by inhibition of signals to the muscle
− Antagonist → autoantibodies blocks binding to the nicotinic acetylcholine receptor
− Removal of receptor → autoantibody triggers receptor internalization
− Antibody mediated deposition of immune complexes
− Eg) SLE → symptoms include butterfly rash, arthritis, vasculitis, glomerulonephritis
− Immune complexes deposit in small blood vessel walls (kidney, joints, skin) and initiate inflammatory reactions
In autoimmune disease, autoantibodies against nuclear antigen are prevalent - describe some features of this.
Antibodies against nuclear antigen: • Particular prevalent in SLE, where almost all patients have autoantibodies against nuclear antigens, but seen in other diseases and also healthy controls (highlighting the fact you can have autoantibodies without disease) − SLE → 99% of patients − Scleroderma → 80% − RA → 50% − Relatives of SLE patients → 20% − Healthy controls → 5% • Antigens include: − dsDNA – renal disease – 40-60% prevalence − Ribonuclear protein – connective tissue disease – 30-40% − Phospholipids – thrombocytopenia – 30% − Histones – SLE
• These antigens may be exposed due to impaired clearance following apoptosis and NETosis (apoptosis that occurs in neutrophils. Can contribute to phagocyte clearance but releases the contents of the neutrophil into the periphery)
− Usually, apoptotic cells are silently cleared by complement, but in SLE this is defective
As stated before, high affinity autoantibody generation may also arise because of intrinsic B cell defects due to genetic variants.
Overexpression of IFNa and BAFF anti-nuclear antibody production in SLE:
• Genetic variant of the FcyRII – can influence the ability to recognise immune complexes
• When plasmacytoid DCs take up the nuclear DNA/antibody immune complex, it triggers the production of type I IFN
• This can activate monocytes and DCs to produce BAFF – present at very high amounts in SLE patients due to altered gene expression control
• Can result in loss of negative feedback loop that would normal result in death of autoreactive B cells and can enhance their survival
What is the concordance rate in autoimmune disease?
In autoimmune disease:
• Concordance in identical twins > non-identical, but concordance in identical twins is 50% → genetic and environmental factors
eg) In RA:
• Unrelated individuals → 0.5-1%
• First degree relatives → 3-5%
• Identical twins → 15%
Other than concordance, what else indicates an environmental influence in autoimmune disease, and what are potential triggers?
- Rapid increase in incidence over the last 40-50 years too quick to be explained by genetics
- Geographical movements – populations moving from areas of low to high incidence reflect the incidence of the new location.
Possible environmental triggers:
• Infections → microbial pathogens may resemble self-antigen
• Drugs → may bind to self-epitope, yielding a neoantigen
• Toxins (UV, pesticides, smoking) → cause damage to tissues, expose sites for autoantibody mediated damage, give rise to protein modifications and neoantigens
• Diet → Through alterations of epigenome and effects on gut microbiome
• Obesity → adipokines are proinflammatory
• Hygeine → lack of challenge to immune system by parasites
• Vaccines → due to adjuvants
Name some single gene deficiencies giving rise to autoimmune syndromes.
AIRE
Autoimmune polyendocrine syndrome. T-cells escape negative selection due to decreased expression of self-antigen in thymus
Foxp3
IPEX . Decreased generation of TRegs
FAS
Autoimmune lymphoproliferative syndrome. Failure of apoptosis of self-reactive T and B cells
C1q
SLE. Failure to activate classical pathway of complement, leading to poor immune complex clearance
Other than single gene deficiences, what other general genetic associations have been found for autoimmune disease
Sex-related genes are also a risk factor:
• 75% autoimmune diseases occur in females
− SLE – 9:1 female to male
− Graves – 7:1
− Arthritis – 2:1
− Ankylosing spondylitis – 2:1 male to female
• Symptoms usually arise during childbearing years – symptoms can be altered during pregnancy
Most autoimmune diseases are polygenic:
• Susceptibility influenced by genetic variation
− Eg) naturally occurring SNPs , not mutations
− May directly affect protein function, or influence gene expression and hence cell phenotype
− Some conditions associated with allelic variants in large numbers of genes
− >30 genes contribute to SLE
− >40 contribute to RA
• Individual variants account for only a small proportion of genetic risk
− Each make a subtle contribution to dysregulation of immune function
− Requirement of allelic combinations (epistasis) to confer substantial risk
− epigenetics , which can be altered by the environment, can contribute
− non-coding transcripts, eg, miRNA can also influence gene expression
• Some gene variants affect susceptibility to multiple diseases, suggesting common pathways underlying the aberrant immune response
− eg) disease associated variants of PTPN22 and CTLA4 modulate the signaling pathways of T cells
Influence of MHC polymorphism:
• Strongest genetic associations are with MHC alleles (usually class II) → often 50% of genetic risk
− HLA-DR3 predisposes for Graves, HLA-DR7 is protective
How have GWAS been used to identify genetic factors in autoimmune disease?
• Scan the whole genome for SNPs associated with autoimmune disease
− 100s loci identified, many containing non-coding transcripts and regulatory elements
− 2/3 of these influence multiple conditions, suggesting effects on common pathways such as cytokine signaling and antigen presentation
• New algorithm PICS used to fine-map SNPs associated with 21 autoimmune diseases, in conjunction with epigenetic mapping to determine the active elements:
− 90% of causal SNPs are non-coding
− 60% map to immune cell enhancers, many of these CD4 enhancers that undergo acetylatio
