Module 023806: Advanced topics in Immunology



Module 806: Advanced topics in Immunology March 2004

Lecture #8 Dr Adrian Mountford.

Autoimmunity

Aims and objectives;

The aim of this lecture is to explain how the immune system can malfunction and begin to react against host tissues. It will provide a description of organ-specific and organ non-specific autoimmune diseases, examine the underlying immunological mechanism, the factors responsible for induction, describe possible therapies.

The lecture will build on your understanding of the basic mechanisms of the immune system as described in lectures 1-6. You will need to apply the information gained in these lectures to the specific examples of autoimmunity described in the current lecture and how they relate to real disease in the human population. Finally, you will be able to apply this information to understand how one might devise therapies to treat patients with autoimmune disease.

The lecture will cover the following topics;

• Definition

• Examples of organ-specific and systemic autoimmune diseases

• Experimental models

• The immunological basis of autoimmunity;

T cell intrinsic:

1. CD4+ lymphocytes

2. Costimulatory molecules

3. Fas and apoptosis

T cell extrinsic:

1. Immature / tolerogenic APCs

2. T regulatory cells; IL-10, TGFβ and CTLA-4.

• Factors linked to susceptibility

Sex

MHC

T cell receptor

• Mechanisms for the induction of autoimmunity

Release of sequestered antigens

1. Molecular mimicry

2. Inappropriate expression of MHC II

3. Polyclonal B cell activation

• Therapy

Immunosuppression by chemotherapy

Blockade of CD4/APC interactions

Cytokine therapy/blockade

Antigen-specific therapy.

General sources of Information;

Cellular and Molecular Immunology. Abbas and Lichtman. 5th Edn.

Immunobiology; The Immune system in health and disease. By Janeway, Travers, Walport and Capra. 5th Edition

Current Opinion in Immunology. This review journal has a selection of excellent articles on Autoimmunity in the last issue (Dec) of each year for 2000, 2001, 2002 and 2004

Nature Immunology. Special issue on Autoimmunity. Sept 2001 Vol 2 no. 9

Selected Papers;

Elson & Barker (2000) Helper T cells in antibody-mediated, organ-specific autoimmunity. Curr Opin Immunol. 12 664-669.

Gavin and Rudensky (2003) Control of immune homeostasis by naturally arising regulatory CD4+ T cells Curr Opin Immunol. 15:690-696.

Hill & Sarvetnick (2002). Cytokines:promoters and dampeners of autoimmunity. Curr Opin Immunol. 14, 783-790.

Nepom (2002). Therapy of autoimmune diseases:clinical trials and new biologics. Curr Opin Immunol 14, 812-815.

Pasare and Medzithov. (2003). Toll-like receptors:balancing host resistance with immune tolerance. Curr Opin Immunol. 15:677-682.

Walker and Abbas (2002) The enemy within: Keeping self-reactive T cells at bay in the periphery. Nature Reviews Immunology. 2: 11-19

Whitacre et al. (1999) A gender gap in Autoimmunity. Science, 283, 1277-78

Internet:

Lecture notes (# 1, 8 & 9).

Autoimmunity;

Definition

Erlich described the condition as ‘horror autotoxicus’

Stems from a failure of the immune response to tolerate ‘self’ components.

The host generates lymphocytes, which react with self-antigens. These cells can then destroy host tissues.

Originally, it was thought that all self-reactive lymphocytes (i.e. those with receptors which recognise epitopes expressed by host cells) were eliminated in the thymus early during embryonic development. Therefore, our immune response ordinarily is tolerant to self-antigens. However, autoimmunity stemmed from the failure to delete these self-reactive T and B lymphocytes.

Now it is realised that healthy individuals possess many self-reactive cells and ordinarily these autoimmune cells are not pathogenic. Indeed most people have significant quantities of natural autoantibodies.

It is thought that auto-reactive cells are normally regulated by anergy, suppression or apoptosis. So called T cell-intrinsic mechanisms.

T cell-extrinsic mechanisms include T regulatory cells (Th3) and tolerogenic dendritic cells.

Increasing evidence now suggests that a specialised population of regulatory T helper cells operate to limit unwanted/excessive immune reactions. Only if these regulatory mechanisms breakdown do self-reactive clones become activated.

In addition specialised tolerogenic dendritic cells (possibly imature) present antigen in the context of low costimulatory molecule expression and IL-10 production

Tissue destruction can be mediated by T cells or antibodies in combination with phagocytes or complement.

Examples of autoimmune diseases

Table 20.1 Kuby Immunology

Organ specific

Insulin-dependent diabetes mellitus Type I diabetes). (an example of cellular damage).

A relatively common autoimmune disease leading to the destruction of insulin-producing cells (beta cells) and consequently increased levels of blood sugar. First, cytotoxic T cells attack the beta cells leading to the production of proinflammatory cytokines (IFNγ, TNFα and IL-1β) and influx of macrophages. Cytokines and lytic products released by the activated macrophages lead to a classic DTH type response involving Th1 cells resulting in the destruction of the beta cells.

Graves’ Disease. (an example of stimulating auto-antibodies)

Auto-antibodies are produced against the receptor for thyroid-stimulating hormones thus mimicking the action of these hormones when they bind to the receptor. Since the auto-antibodies are not regulated they over stimulate the thyroid with overproduction of thyroid hormones.

Myasthenia Gravis. (an example of blocking auto-antibodies)

Auto-antibodies are produced against the acetylcholine receptor thus blocking the normal binding of acetylcholine. Complement-mediated degradation of these receptor results in progressive weakening of the skeletal muscles.

Systemic, or organ non-specific diseases.

Systemic Lupus Erythematosus (SLE)

Characterised by fever, weakness, arthritis, skin rashes, pleurisy and kidney disfunction. Individuals produce auto-antibodies against many host antigens including DNA, RBCs, platelets, leucocytes and clotting factors. Auto-antibodies bind with these antigens to form immune complexes, which activate the complement system leading to widespread tissue damage. Type III hypersentivity.

Multiple Sclerosis (MS)

Affects the CNS but the symptoms can range from numbness to paralysis or blindness. Usually diagnosed in people between 20-40 years old particularly those living in the Northern Hemisphere. If someone moves north they gain an increased risk of suffering from MS indicating an environmental component. Genetic influences are important since relatives of a sufferer have an increased risk of developing MS. Initial relapsing/remitting phase followed by chronic degenerative phase in a minority (30%) of patients

Trigger for inflammation is some form of damage leading to TNFa production which activates macrophages, T and B cells to breach the blood brain barrier. All express high levels of VLA-4 adhesion molecule leading to enhanced infiltration of cells into CNS region which normally has a poor immune context. Autoreactive T cells target the myelin basic protein on myelin sheath. T cells secrete cytokines which activate macrophages and astrocytes to release NO which further damages the sheath. Largely a Th 1 type phenomenon.

Rheumatoid Arthritis (RA)

A very common autoimmune disease affecting women from 40-60 years old. Causes chronic inflammation of the joints. Auto-antibodies are produced and bind to the Fc region of IgG. The auto-antibodies are usually of the IgM class and the resulting IgM-IgG complexes circulate until they are deposited in joints. In this site, complement is activated resulting in a type III hypersensitivity reaction.

Immunological basis of autoimmunity.

Experimental models;

Much of our understanding about autoimmunity has been derived from animal models of different autoimmune diseases.

In these models, the disease can occur spontaneously, or following induction experimentally.

1. In the first example, MRL/lpr/lpr mice develop a disease very much like SLE. These mice are deficient for the fas gene.

[fas is a cell surface molecule which when it binds to its ligand on a neighbouring cell transduces a signal which leads to the death of fas bearing cells by apoptosis. In this way, the host has a mechanism for the removal of hyperactivated T cells on which fas it highly upregulated.]

In these mice auto-reactive cells which might have been removed by apoptosis remain to cause SLE-like disease. These studies underline the possible role of fas and apoptosis in autoimmune diseases.

2. Nonobese diabetic (NOD) mice develop diabetes very closely related to human diabetes. There is a very strong evidence for development of disease and MHC alleles which vary between strains of mice.

3. The most widely used model is experimental autoimmune encephalomyelitis (EAE) which resembles chronic relapsing and remitting form of MS in humans. EAE is induced by immunisation of animals with myelin basic protein (MBP) in complete Freunds adjuvant. This causes infiltration of cells to the myelin sheath of the CNS leading to demyelination and paralysis.

Immunological basis of autoimmune disease

T cell intrinsic:

CD4+ cells.

1. Have shown that T cells from NOD mice can confer diabetes in x-irradiated (to remove host T cells) normal mice. Conversely, T cells from normal mice stop NOD mice from developing diabetes. These studies also show a requirement for CD4 cells of the Th1 type in disease development.

2. EAE can be transferred to normal animals by the transfer of T cells, or T cell clones, from animals immunised with MBP. Disease can also be prevented using antibodies against CD4.

3. Th1 cells cause exacerbation of EAE, whilst Th2 cells protect against induction of EAE and limit progression of established disease. Th1 cells secreting IL-2, IFNγ are present in the CNS at the height of disease.

4. Injection of IL-12 exacerbates EAE disease, whereas IL-4 causes EAE inhibition. Similarly, antibodies to IL-12 prevent the development of EAE.

Costimulatory molecules

The molecules are integral to the normal antigen processing and presentation pathway. There is now evidence that their expression is also related to autoimmune disease progression.

1. Expression of CD80 (B7.1) has been recorded early in MS progression linked to the development of Th1 like responses. [Links via CD28] Indeed CD80 is upregulated in many autoimmune diseases such as RA, and psoriasis. In contrast CD86 (B7.2) is not linked to the development of disease.

2. Costimulation via CTLA-4, which also binds via CD28 has a negatively regulatory role by downregulating autoreactive T cells.

Fas and the failure of apoptosis

In mice deficient for fas (CD95) or fas ligand, there is a failure to eliminate autoreactive B cells by apoptosis (programmed cell death) leading to the development of SLE like disease.

Although patients with SLE have normal levels of fas expression, defined mutations in the fas molecule have been found in certain groups of individuals with SLE indicating a possible disfunction in these people.

Apoptotic cells bearing fas are normally cleared by macrophages. However, if apoptosis is delayed to poor fas expression, the dying cells express nuclear (DNA) antigens on their surface. These cells can then be processed by DCs which then prime T cells with host nuclear antigens. In addition B cells with antibodies specific for nuclear material can also present material to T cells thus amplifying the immune response. This is now thought to be the major explanation for the induction of SLE.

T cell extrinsic:

Tolerogenic or Immature APCs.

Immature APCs express low levels of MHC II and other costimulatory molecules. As such they do not activate efficient T cell priming. This important in maintaining tolerance.

• Not activated via their TLRs, therefore they do not see self antigens as dangerous. However, in autoimmune conditions TLRs are activated by endogenous ligands e.g. DNA, or HSPs resulting from tissue damage. If host proteins are present this will activate priming of host T cells with specificities for host proteins.

• Tolerogenic APCs might have a unique phenotype and express certain co-stimulatory molecules which induce regulation. Such factors have not yet been identified.

• Thought that they might secrete IL-10

• Might be directly responsible for inducing T regulatory cells.

T regulatory cells (Th3? Tr1? TReg?)

Regulatory T ells, characterised by their secretion of TGFβ and IL-10 may be the hosts most efficient way of restricting unwanted immune responses.

• TGFβ when transgenically engineered in the beta cells of NOD mice completely prevented diabetes.

• Certain T cell clones capable of suppressing diabetes in NOD mice also secreted abundant TGFβ.

• IL-10 deficient mice develop spontaneous autoimmune colitis and RA. Similar effects occur if antibodies against IL-10 were administered to mice.

• IL-10 producing T cells can regulate IDDM and inhibit EAE.

Both IL-10 and TGFβ act to downregulated CD80 (required for efficient costimulation and upregulate CTLA-4 which reduces the activation of T cells.

• Regulatory cells are CD25+ (IL-2Receptor) and all express CTLA-4.

• Express GITR and FoxP3; both markers of regulatory T cells.

• The are hyporesponsive to antigen stimulation (i.e. they do not divide) but they secrete IL-10 and TGFβ.

• They can act in an antigen non-specific manner, so they can act on neighbouring cells with a different antigen specificity = Bystander suppression.

• Antigen-specific T regulatory cells also exist. Ag specific suppression.

Factors linked to disease susceptibility

1. Sex

Women are far more likely to suffer autoimmune diseases than men, although the ratio varies between diseases.

Female : Male

MS 2 : 1

RA 2-3 : 1

SLE 9 : 1

However, in the case of MS at least, the disease severity can be much greater in men than women.

Why?

• Women have higher levels of antibody in general.

• Women have greater numbers of CD4+ cells.

• Female mice tend to develop Th1 responses and where these are important are better able to resist infection (e.g. herpes simplex virus).

• Male mice tend to produce more Th2-like responses.

Sex hormones appear to be a likely factor.

• In humans, oestrogen is a factor in the development of SLE, although this not apparent in MS or RA.

This is confirmed in mice where oestrogen stimulates the production of autoantibodies in lpr mice. SLE in these mice can be modulated by the administration of anti-oestrogen drugs.

• Androgens such as testosterone are also implicated since female NOD are more susceptible to diabetes than males, and castration increases the risk of disease in male mice. Testosterone may be protective against MS, diabetes, and SLE.

• Sex hormones are similar to cytokines in the way that they operate (i.e. they are soluble factors which signal through a specific cell surface receptor to cause expression of individual gene products.) It is highly likely that hormones bind to receptors on the surface of immune cells and modulate their function. However, it is not known at present which immune cells, if any, express hormone receptors.

Factors linked to pregnancy:

• Prolactin which is expressed at very high levels in pregnant women is highly stimulatory of the immune system. Both T and B cells have prolactin receptors. There is some evidence that prolactin stimulates Th1 immune responses.

• During pregnancy the mother is in effect tolerating a foreign graft. Although women usually mount more Th1 like responses, during pregnancy the response becomes more skewed towards the Th2 pole. This is because hormones produced during pregnancy are generally anti-inflammatory (reduce Th1 responses) thereby suppressing the any form of rejection of the foetus. In this respect autoimmune diseases linked with Th1 responses (e.g. RA and MS) are often modulated.

• Cells from the foetus will enter the mothers blood circulation and may persist for decades. These long-lived cells are potent stimulators of autoimmune disease. Conversely, maternal cells in the child might be the cause of autoimmune disease in men.

2. MHC

Strong evidence exists to suggest that there is an association between an HLA allele and susceptibility to disease.

e.g. Individuals with HLA B27 have a 90 times greater risk of developing ankylosing spondylitis.

The reason may be the close linkage of TNFα and TNFβ genes with the HLA-B locus; thus expression of the 27 allele may cause greater levels of these two inflammatory cytokines to be produced so accelerating the tissue destruction in this disease.

But the MHC gene complex is complicated and expression of other alleles may confer an advantage.

Other linkages include;

IDDM DR4/DR3 20 x

MS DR2 5

SLE DR3 5

In the case of MS, the DR2 region of MHC was found to present the 85-99 peptide of MBP to T cells with the corresponding receptor

3. T cell-receptor

Usage of TCRs with certain Vα and Vβ domains has been linked to a number of autoimmune diseases.

Individuals with MS and myasthenia gravis tend to have restricted expression of TCR variable regions. This suggests that a single epitope is responsible for the expansion of the T cells causing autoimmune pathogenesis.

In the case of the 85-99 epitope of MBP, some single amino acid changes prevented activation of the T cells. However, other amino acid substitutions had little effect. Binding to the T cell receptor is therefore not completely specific to a given peptide allowing other similar peptides to bind and activate the T cell. This has great relevance for the concept of ‘molecular mimicry’ and ‘epitope spreading’, as discussed later.

Mechanisms for the induction of autoimmunity

1. Release of Sequestered Antigens.

Normally T cells reactive with self-antigens are deleted in the thymus during development. However, some antigens are expressed only in tissues which are not normally exposed to the immune system.

A good example is MBP (myelin basic protein) which is normally protected from the immune system by the blood-brain barrier.

In EAE, autoimmunity develops after deliberate injection of MBP, but in the normal state this can happen after physical injury or a viral/bacterial infection causes MBP to enter the circulation.

Other examples include, release of sperm antigens after vasectomy, lens protein after eye damage, or heart muscle antigens after a heart attack; all result in the formation of auto-antibodies.

2. Molecular mimicry.

A number of viruses and bacteria possess antigenic determinants which are identical or very similar to normal host components.

Human cytomegalovirus P D P L G R P D E D

HLA-DR molecule V T E L G R P D A E

Poliovirus VP2 S T T K E S R G T T

Acetylcholine receptor T V I K E S R G T K

Measles virus P3 E I S D N L G Q E

MBP E I S F K L G Q E

• More than 3% of virus-specific monoclonal antibodies cross-react with host tissues.

• Animals kept in clean, germ-free conditions do not develop autoimmune diseases.

• As explained earlier, a single TCR can bind several different related peptides provided that they share several key amino acids forming the critical residue which binds to the TCR. i.e. TCRs can be promiscuous!

Specific examples;

MBP and viral peptides

The MBP peptide 61-69 is highly homologous with P3 of measles virus.

Another peptide 66-75 shares homologies with a number of viruses (influenza, adenovirus, poliomyelitis, Epstein-Barr, hepatitis-B.

Experimentally, rabbits immunised with the hepatitis-B peptide developed auto-antibodies and T cells, and an EAE like condition was promoted in the CNS.

Heat shock proteins

This family of proteins is conserved to a high degree between the host and bacteria. Patients with RA have a large number of T cells and antibodies reactive with HSP.

In NOD mice, it is possible to transfer diabetes to a normal animal using a T cell clone which reacts with HSP. Moreover, patients with IDDM have anti-HSP antibodies which react with glutamic acid decarboxylase, an enzyme in the insulin-producing beta cells of the pancreas.

3. Inappropriate expression of MHC molecules.

Beta cells in the pancreas of patients with IDDM express high levels of MHC I and II whereas healthy cells express only low class I and no class II. [Class II would only be normally expressed on APCs].

Therefore, expression of class II may cause the beta cells to present self peptides thereby sensitising Th, or activating B cells.

Expression of class II may be directly upregulated by certain foreign components (echo of PAMPs here).

Expression of class II is also induced by IFNγ. This may be elicited following injury or infection causing the release of inflammatory cytokines such as TNFα.

Evidence comes from;

• patients with SLE who have high levels of IFNγ in their serum.

• Construction of a transgenic mouse with the IFNγ gene engineered downstream from an insulin-promoter region. This causes the beta cells to secrete IFNγ. The mice had upregulated MHC II and developed diabetes.

4. Polyclonal B cell activation.

Several Bacteria and viruses induce non-specific polyclonal B cell-activation leading to the production of IgM in the absence of Th cells. If the B cells activated in this way are self-reactive, the production of auto-antibodies ensues.

EBV (causative agent of glandular fever) is a major cause of polyclonal B cell activation and patients have auto-antibodies reactive with many host factors.

Treatment of autoimmune diseases.

Any treatment targeting the autoimmune response should leave the rest of the immune response intact.

1. Chemotherapy.

Most current therapies are non-specific and suppress the whole immune response. The drugs given (e.g. corticosteroids, cyclophosphamide), act by slowing the proliferation of lymphocytes. The severity of the symptoms is often greatly reduced, but this treatment increases the risk of infection and cancer.

Cyclosporin A is a better treatment because it blocks signal transduction by the TCR, thus it only inhibits activated T cells.

2. Blockade of APC/T cell interactions

• Blockade of MHC.

Synthetic peptides which differ by one amino acid from the pathogenic peptide can be used to bind the appropriate MHC and block binding.

• Anti-MHC antibody treatment.

The discovery that certain MHC alleles confer susceptibility to disease raises the possibility that antibodies against these specific MHC molecules might prevent antigen presentation by cells which express them.

• Anti-CD4 antibody treatment.

Monoclonal antibodies can be administered to the host to block or deplete the host of CD4 cells. Whilst this reduced autoimmune reactions, the host was more prone to infection etc. Successful at reducing MS lesions in the brain.

• Anti-IL-2R antibody treatment.

To improve the specificity of the above treatment, an antibody to IL-2 receptor α chain (which is expressed at high levels on autoimmune cells) has been used successfully to protect hosts against lethal EAE.

• Anti-TCR antibody treatment.

Antibodies specific for the Vβ 8.2 TCR prevented EAE induction by MBP.

• Manipulation of costimulatory molecules

a). blockade of CD80/86 / CD28 costimulation

b). blockade of CD154 to prevent binding of CD40.

The latter approach has mixed results in SLE patients with no response in more than 50% of patients. In MS patients there were side effects with higher numbers of thromboembolic events.

c). Use a synthetic fusion protein of CTLA-4 and Ig. which is a competitor for CD28 mediated events.

d). An antibody against CTLA-4 will upregulate CTLA-4 expression and lead to T cell regulation.

3. Cytokine therapy/blockade.

TNFα is one of the major inflammatory cytokines present at sites of autoimmune tissue destruction in RA. It removal leads to a decrease in the number of other proinflammatory cytokines such as IL-1, IL-6 and IL-8.

• Administration of antibody to TNFα (Inflixmab) lead to a marked decrease in clinical symptoms in RA patients. But at least one third were not affected indicating that another proinflammatory cytokine needs to be blocked (IL-1β).

• Administration of soluble TNF receptor (Etanercept. linked to a human IgG Fc molecule to ‘mop up’ excess TNFα have had some effects against RA.

• Administration of IL-1 receptor antagonist (IL-1ra). Blocks the IL-1 R and has good effects in animal and human trials of RA.

• Chemical inhibitors of TNFα have also been successful at reducing experimental EAE.

• Administration of regulatory cytokines (IL-10 and TGFβ) have lead to the decreases in the severity of disease in EAE and experimental thyroditis. But IL-10 may cause exacerbation of IDDM by enhancing inflammation.

• Administration of Th2-associated cytokines (e.g. IL-4 and IL-13) have also had significant effects in experimental models of RA.

• IFNβ has been used to treat MS patients. This causes an increase in the production of IL-10 as well as IL-1 receptor antagonist (IL-1ra) which is an inhibitor of IL-1 production.

•

4. Antigen-specific therapy

Aim to administer self antigens to induce tolerance to the same antigens.

• This would increase apoptosis of self-reactive T cells by over stimulating them (increased fas expression).

• Increase anergy. (i.e. T cells fail to respond and switch off).

• Cause the production of Th3 / Tr regulatory cells.

• Switch from pathogenic Th1 to non-pathogenic Th2 responses.

Key features of antigen-specific therapies are;

• Generally need to be at high dose.

• Need to be in soluble form (leading to inefficient costimulation).

• Use an intravenous route for systemic sensitization.

For example,

a). injection of mice with collagen II prevented the induction of RA.

b). injection of NOD mice with glutamic acid decarboxylase induces the production of Th2 and T regulatory cells which prevents IDDM.

Types of “antigen”

• Short peptide encoding epitope defined by T cells with a specific TCR.

• Can engineer peptide with MHC to enhance Ag processing.

• Can engineer as a chimera with as an chimera with an IG molecule to enhance Ag presentation.

• Synthetic Altered Peptide Ligand. APL.

• Delivery as naked DNA.

• Use glatiramer acetate (GA) which is a random copolymer and non-specifically engages the TCR to induce more cross-reactive Tr cells which down-regulate the immune response. Can be used to decrease EAE. Clinical trials have shown that GA decreases relapse rates in MS patients

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Self reactive T cell

Self reactive T cell

The same TCR is promiscuous for other viral epitopes which differ in their aa

T cell activated and leads to autoimmunity

if self ag exposed

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