Immunological Tolerance by Okkenhaug PDF

MVST & NST Part IB Pathology-Biology of Disease MT 2024 Prof Klaus Okkenhaug Lecture 9 Immunological Tolerance: The lack of adverse immune reactivity against self and innocuous antigen Once set in motion, immunological r...

MVST & NST Part IB Pathology-Biology of Disease MT 2024 Prof Klaus Okkenhaug Lecture 9 Immunological Tolerance: The lack of adverse immune reactivity against self and innocuous antigen Once set in motion, immunological responses can be costly in terms of the host's energy and biological resources and in the case of autoimmunity destructive, as explained in the next lecture. How are responses to self tissues and innocuous (harmless) antigens avoided? The response to an antigen should not damage the host. The immune system avoids reacting against your own tissues. It is said to be tolerant of them. Immunological tolerance is also necessary in order to avoid responding to innocuous substances. Tolerance is acquired, not hard-wired. In early immunology text-books it is generally stated that the immune system distinguishes self from non-self. This is not strictly true. The immune system is really trained to recognise infectious non-self. It took a long while to appreciate that injection of a protein can result in either a response, or tolerance, depending on the context. Strong adaptive immune responses almost always require the antigen to be injected in a mixture known as an adjuvant. This concept was referred by Janeway to as the Immunologists’ dirty little secret. How do adjuvants work? They cue the immune system that an infection is taking place. They convert soluble protein into particulate material, which is ingested by antigen presenting cells such as macrophages. Some adjuvants contain bacterial products, which stimulate macrophages or dendritic cells through Pattern Recognition Receptors. Complete Freund's Adjuvant (CFA) for example, contains ground-up mycobacteria. Tolerance is effected by a variety of mechanisms. Tolerance is generally thought of as Central, which occurs during lymphocyte development, or Peripheral, which occurs after the lymphocytes leave the primary organs. Central tolerance - T cells The mechanism for generating a useful repertoire of T cells is based on clonal selection. It is achieved by first generating T cell receptors irrespective of specificity. Second, by selecting the small number of receptors that work with self MHC molecules to see foreign antigens. This second goal is achieved by positively selecting those clones with some affinity for self MHC and negatively selecting clones that bind too strongly to self MHC plus peptide. In the thymus, negative selection leads to the deletion of thymocytes whose T cell receptors have ‘high affinity’ for self. This was covered in lecture 8. There are a number of factors that affect tolerance, such as timing, dose of antigen, amount of costimulation and location. A celebrated experiment illustrates two of these principles. Mice of strain A were injected at birth with bone marrow from strain B. Six weeks later they were grafted with skin from mouse B. The skin graft from mouse B was accepted whereas a 'third-party' graft from mouse C was rejected. The A mouse had acquired specific tolerance to mouse B. If on the other hand the B bone marrow cells were injected a week or so after birth tolerance was not achieved and grafts of skin from mouse 1 B were rejected. The interpretation at the time was that tolerance only operated early in life. Now we understand about central tolerance in the thymus we can reinterpret this experiment. Probably what happens is that bone marrow stem cells from mouse B establish chimerism in the host. Some of these cells differentiate into antigen presenting cells and migrate to the thymus where they tolerise developing thymocytes by deletion (central tolerance). Lifelong chimerism is needed to maintain tolerance but even a low level of chimerism is sufficient. If the transfer is done later the number and maturity of the peripheral T cell pool of the host is sufficient to destroy the donor stem cells before they can engraft. How can all relevant self-antigens be expressed in the thymus? One problem with central tolerance is that many antigens are not generally expressed in thymus or bone marrow. Some antigens are not expressed until after the immune system has matured (eg some genes are only expressed after puberty). Others are only expressed in specialised tissues, such as insulin in the pancreas. This limitation is circumvented to some degree by expression of a promiscuous transcription factor called AIRE, which turns on many 'peripheral' genes in the thymus, so that the developing T cells may be exposed to their products. Individuals lacking AIRE suffer from a wide range of autoimmune conditions. The condition is called APECED, autoimmune polyendocrinopathy-candidiasis-ectodermal dystrophy, sometimes also referred to as Autoimmune Polyglandular Syndrome Type 1 (APS-1). APECED/AIRE can live full lives, but can suffer from a wide variety of autoimmune syndromes as well as candidiasis. 2 Central tolerance - B cells In a perfect system B cells need T cell help and should not need to be tolerized. But, the immune system is not perfect. B cells that react to abundant antigens on self cells are eliminated as they develop. Self-reactive cells have an opportunity to avoid apoptosis by replacing the light chain in a process called receptor editing. High doses of soluble proteins result in anergy rather than apoptosis. Five mechanisms of T cell tolerance: Despite the elaborate system for purging autoreactive T cells in the thymus and B cells in the bone marrow, a substantial number of autoreactive T cells and B cells leave these organs and populate the secondary lymphoid tissues. These need to be tolerised against to prevent immune responses against self. Moreover, while negative selection in the thymus can eliminate T cells with high affinity for self-peptides, but cannot eliminate T cells that react against commensal bacteria. 1. Ignorance is the term used when potentially self-reactive T cells are not activated. This could be because antigens are hidden from the immune system in locations that are 3  not freely exposed to surveillance. These immunologically privileged sites include the brain, eye and testis. This organs have limited or no lymphatic drainage*. Split tolerance can be broken. An example is that damage to one eye can expose the immune system to eye-specific antigens (often in presence of PAMPs and/or DAMPs and lead to destruction of the second eye. This illustrates that ignorance is not complete and can be breached. *it has recently become appreciated that there is lymphatic drainage from the brain. Pregnancy As discussed above, there are several immunologically privileged sites. One special situation where tolerance operates is the placenta. The foetus and placenta share the child’s genotype with a ~50% contribution from the father. There may be several tolerance mechanisms involved, such as: 1. Physical barrier to the mother’s T cells. 2. Lack of MHC class I expression. Trophoblast cells that form the outer layer of the placenta in contact with maternal tissues do not express classical class I molecules and so are not targets for cytotoxic T cells. 3. Production of immunosuppressive factors such as α-fetoprotein and IDO (indoleamine 2,3-dioxygenase, a tryptophan catabolising enzyme). 4 2. Split Tolerance. This reflects the notion that, as many pathways in the immune system are interdependent, they do not all need to be tolerised. The most frequent situation is where T cell tolerance has been established but autoreactive B cells are still present. Without T cell help the B cells are 'helpless'. The explanation of this is that it takes 100- 1000- times more antigen to tolerise B cells than it does T cells. As a result, this type of split tolerance situation is reasonably common for self serum proteins. cytokines IL-4 etc IL-2 cross link CD40L CD40 mIg T cell CD28 B7 B cell TCR MHC CD4 CD40L CD28 TCR CD4 BCR antigen MHC CD40 immune B7 complex Dendritic cell Follicular dendritic cell 3. Anergy is a state of non-responsiveness. It can be induced in T cells if the receptor is engaged by the MHC molecule but the second signal is absent. The anergised cell does not die but biochemical changes take place so it no longer responds. Immature B cells may be anergised in a similar way by exposure to soluble antigen. Large amounts of soluble antigen lead to anergy if it is not cross-linked at the cell surface. 5 Dendritic cells (DCs) become activated to express high levels of the CD28-costimulatory ligands CD80 and CD86 upon exposure to Pathogen-associated molecular patterns (PAMPs). PAMPs also induce DCs to migrate to the draining lymph nodes where they present antigen to T cells. DCs and other antigen presenting cells that have not been activated by PAMPs induce tolerance instead of activation due to their lack of costimulatory ligands. 4. Treg suppression. Some autoreactive T cells appear to be prevented from reacting by the presence of other T cells, a phenomenon which has been termed Suppression or Regulation. Data call for a dominant effect of a special class of regulatory CD4 cells (Treg) that express high levels of the IL2-receptor CD25 and the transcription factor FOXP3 transcription factor, which may be used as a marker to distinguish them from other T cells. Natural Tregs are develop in the thymus. Tregs have an intermediate affinity for self antigens in MHC molecules and are not deleted by negative selection. The mechanism of action of Tregs is under intensive investigation. On contacting self- antigen presented by MHC class II molecules Tregs suppress the proliferation of naive T cells responding to autoantigens presented on the same antigen presenting cell. Their 6 suppressive effects are specific, and may require cell contact. Secretion of anti- inflammatory cytokines such as IL-10 and TGF-β may contribute to the down-modulation. Other mechanisms include induced activation of an enzyme (IDO) by DCs which inhibits T cell growth by depleting tryptophan, inhibition of proinflammatory cytokines, and signalling to the APC to decrease B7 co-receptor expression which is taken up by CTLA-4 which is highly expressed by Treg (and activated conventional T cells). Most Treg develop in the thymus (tTreg), but some also develop in the periphery, especially at mucosal surfaces in the lung and gut, These Treg are considered peripheral (pTreg) or induced (iTreg). Mechanisms proposed to favour their development include: the cytokine profile upon antigenic stimulation (TGF-β); chronic low-dose antigen exposure; and presentation of antigen by immature DCs. The latter may occur in the gut associated lymphoid tissue (GALT), a microenvironment rich in TGF-β where cells may be exposed to the microbiome and food antigens. Boys with a congenital lack of Treg due to inherited mutations in FOXP3 die by two years of age unless treated due to lymphoproliferation and autoimmunity. This X-linked syndrome is called IPEX (Immunodysregulation polyendocrinopathy enteropathy X-linked). Scurfy mice lack Foxp3 and only survive for 4-6 weeks after birth. In both cases, effector T cells undergo massive proliferation and cause lethal autoimmunity. Treg are therefore an essential mechanism of self-tolerance (rather than simply a back-up mechanism for negative selection). 5. T cell exhaustion. T cells exhaustion is sometimes considered a pathological phenomenon where T cells fail to clear a chronic infection or tumour. However, T cell exhaustion probably evolved to avoid prolonged and potentially harmful and/or futile T cell responses. Exhausted T cells express the co-receptor PD1. PD1 is related to CD28 but binds different ligands called PD-L1 and PD-L2. PD1 suppressed T cell signalling and 7 hence renders exhausted T cells less responsive to stimulation with antigen. There is evidence that T cell exhaustion also plays a role in reducing the severity of autoimmune diseases. Experimental tolerance A number of tolerisation regimes have been successfully applied under experimental conditions and with particular antigens. A clinically usable protocol is still elusive. Two examples of tolerising conditions that are showing promise: Costimulatory receptor blockade Because CD28 costimulation is essential for T cell activation, blocking this signal can lead to immunological tolerance. A drug called Abetacept work like this. Abetacept is and engineered soluble form of CTLA4 (and Fc domain replaces the transmembrane domain). Because CTLA4 binds B7.1 and B7.2 with greater affinity than does CD28, injection of Abetacept prevents T cells from receiving their costimulatory signal. Abetacept is use to treat rheumatoid arthritis and to prevent graft rejection (see also lecture 10). Cancer: For a long time it was believed that immunosuppression did not result in more tumours. In fact, the immune response is important in dealing with cancer, particularly with tumours associated with viruses (e.g. Kaposi's in immunosuppressed AIDS patients). Many tumours make altered protein antigens that could be presented by MHC class I. At the same time, many tumours that progress appear to have lost MHC class I expression by mutation or loss of fragments of chromosome. Current cancer therapies showing great promise are aimed at exploiting and enhancing immune responses to tumours. Two new treatments have recently been approved. One is an antibody against CTLA4. Normally, CTLA4 competes with CD28 and prevents costimulatory signals from being transmitted to the naïve T cells. Ipilimumab is an antibody against CTLA4 which prevents CTLA4 from binding to CD80 or CD86. This breaks an important mechanism of immune 8 tolerance and enhances signalling via CD28. Ipilimumab is approved for the treatment of metastatic melanoma. It works by activating a strong immune response against neoantigens expressed by the cancerous melanocytes. Typically, UV light (from too much exposure to the sun) causes mutations that alter self peptides. These proteins now look “foreign” to the immune system and are hence referred to a neoantigens. An even more effect therapy is anti-PD1 or anti PD-L1. PD1 is expressed by activated T cells, and like CTLA-4, delivers a negative signal to the T cells. PD-L1 can be expressed by the tumour cells and by macrophages in the tumour microenvironment. These therapies, which you will learn more about next term, have revolutionised cancer treatment and in some cases led to complete remission or cure. In general, cancers with more mutations have more altered proteins for the T cells to recognise and hence are more susceptible to immunotherapy with anti-CTLA4 or anti-PD1. These so-called checkpoint inhibitors have now been approved for more than a dozen different cancer types. Because these drugs break tolerance, they can also cause autoimmunity which is the subject of the next lecture. 9 MVST & NST Part IB Pathology-Biology of Disease MT 2024 Prof Klaus Okkenhaug Lecture 10 Autoimmunity Autoimmunity is an adverse immune response against self-antigens. Although the immune system has an elaborate system of checks and balances to ensure self tolerance, occasionally this system breaks down. When the immune system attacks host components causing pathological change, this is called autoimmunity. Many people experience an autoimmune reaction during their lifetime. Mostly these are short-lived, self-resolving sequelae of infection. However; in some 2-3% of individuals the reaction is chronic, debilitating and even life- threatening. It is these latter conditions, where serious immunopathology occurs, which are usually considered autoimmune disease. Affected tissues are generally inflamed and can be abnormally infiltrated with lymphocytes without any obvious signs of infection. Autoimmunity is generally considered to be due to failure of self- tolerance. The immune system seems to attack tissues as if they were infected. Antibodies (autoantibodies) or T cells (autoimmune T cells) are directed to antigens on target tissues, known as autoantigens. One potential explanation for the increase in autoimmune conditions in developed countries is that the immune system is no longer conditioned by early exposure to infection. The hygiene hypothesis was proposed to encapsulate this idea, supported by the realisation that autoimmunity has increased as infections such as measles, mumps and TB have declined in incidence. A more recent proposal is that it is not necessarily infection with pathogens that condition the immune system early in life but exposure to diverse commensals (‘friendly’ bacteria) that comprise your microbiome. It is still possible that some autoimmune conditions are caused by infectious organisms that have not been identified: the Cryptic Infection hypothesis. In this lecture we will consider: 1. The characteristics of autoimmune diseases. 2. Mechanisms of autoimmune pathology. 3. Predisposing factors 4. Initiation of disease. 10  The characteristics of autoimmune diseases There is a wide spectrum of diseases which may be classified as organ-specific or non-specific. Grave’s disease Organ-specific Hashimoto’s thyroiditis Pernicious anaemia Addison’s disease Insulin dependent diabetes mellitus Goodpasture’s syndrome Myasthenia gravis Multiple sclerosis Autoimmune haemolytic anaemia Idiopathic thrombocytopenic purpura Rheumatoid arthritis Scleroderma Systemic lupus erythematosis systemic In autoimmunity the immune system seems to be attacking cells as if they were infected. There may be antibodies (autoantibodies) and/or T cells (autoimmune T cells) to self-antigens (autoantigens) on target tissues. Common organs affected in organ-specific autoimmunity include thyroid (Hashimoto's thyroiditis, Grave's disease), adrenals, stomach (pernicious anaemia) and pancreas (type 1 diabetes). This may be because they are well vascularised and make organ- specific proteins. Non-organ specific targets include the skin (scleroderma), kidney (SLE) and joints (RA). Mechanisms of autoimmune pathology There are three general types of mechanism. These parallel Hypersensitivity types II (antibody), III (immune complex) and IV (cell-mediated) that will be covered in lecture 11. 11 1. Direct antibody mediated effects Almost all patients presenting with autoimmune conditions have some autoantibodies present in their serum. Some have an important, direct role in the disease. Others may be a secondary result of the tissue damage. Graves disease: This is due to antibody to TSH receptor. Unlike the natural ligand, TSH, the antibody is not subject to feedback inhibition, resulting in overproduction of thyroid hormones and hyperthyroidism. Graves may be a ‘Th2 type’ response, in which there is little inflammation or lymphocyte infiltration. In contrast, Hashimoto’s thyroiditis is mostly a Th1 response as CD4 and CD8 T cells infiltrate the organ. Antibodies are also generated against thyroid peroxidases, thyroidglobulin and THS receptors (in contrast to Grave’s disease, antibodies against the TSH receptor are blocking in Hashimoto’s disease). Hashimoto’s thyroiditis is much more common in women than men. Both Grave’s disease and Hashimoto’s thyroiditis may result in a characteristic goitre (swelling of the neck). In Myasthenia Gravis autoantibodies to the acetylcholine receptor diminish neuromuscular transmission from cholinergic neurons by blocking the binding of acetylcholine and by causing downregulation (degradation) of its receptor. Rheumatic fever is another example of direct tissue pathology following antibody binding. 12 2. Immune complex mediated effects. Immune complexes may be cleared by binding of complement components which then bind to complement receptor 1 on erythrocytes. These cells transport the bound complexes to the liver and spleen where the red cells are removed by phagocytes. Immune-complexes that are not removed efficiently are frequently implicated in autoimmune pathology. Systemic diseases such as SLE and vasculitis result from autoantibody-antigen complexes and their consequences. Certain organs are especially sensitive to immune complex deposition, particularly the kidney and vascular endothelium. In SLE (systemic lupus erythematosus), patients have a wide variety of anti- cytoplasmic and anti-nuclear auto-antibodies. The condition results in a characteristic 'butterfly' or ‘wolf’ rash on the face. There can be significant depletion of complement in these patients. In addition, complement deficiencies that impair immune clearance, such as C1, C2, C4 are predisposing factors. SLE is much more common in women than in men. 13 3. T cell-mediated (cellular autoimmune) diseases Autoimmune T cell-mediated damage leads to tissue destruction without requiring the production of autoantibody. Mechanisms may include: cytotoxicity by CD8 T cells; direct destruction by TNF; recruitment of macrophages and subsequent bystander killing; and induction of apoptosis by Fas ligand. Up to 3% of people in some populations suffer from RA, although numbers are decreasing as people give up smoking (see later). Multiple Sclerosis (MS) MS is caused by T cells that cross the blood-brain barrier and damage the myelin sheath that surrounds the nerves. This leads to impaired nerve transduction and hence impaired mobility and other neurological effects. Experimental autoimmune encephalitis (EAE) is a mouse model of multiple sclerosis. The mouse is immunised with myelin basic protein (MBP) in complete Freund's adjuvant. CD4+ T cells specific to MBP are isolated and can subsequently cause disease if injected into another animal, demonstrating the importance of cell- mediated immunity. Both Th1 and Th17 cells are thought to contribute. Type 1 Diabetes (T1D) Type 1 diabetes (T1D) is caused by T cells that attack the insulin-producing β cells in the pancreas. Insulin stimulates glucose uptake by muscles, liver and fat. As a consequence of β cell destruction, blood glucose levels can become dangerously high, causing neurological and vascular damage. Those affected by type 1 diabetes 14 therefore need to monitor glucose levels in inject insulin as needed. There is no cure, but research is focussed on transplantation of βislets as well as various strategies to induce immunological tolerance towards the β cells The non-obese diabetic (NOD) mouse is a spontaneous T1D model, which mimics human disease well. The islets of these mice are infiltrated with T cells and macrophages, with associated cytokine release and production of autoantibodies that could kill cells by ADCC. Like in humans the immune mechanisms lead to abnormalities in glucose metabolism and ketoacidosis, a breakdown product of fat. Switching bone marrow cells between NOD and normal mice confirmed that these cells were responsible. Some protection against disease was afforded by shifting animals from a germ-free environment. Rheumatoid arthritis (RA) RA is a complex disease caused both by CD4 T cells that react against antigens in the joints as well as by antibodies that can form immune complexes. 15  Disease autoantigen consequence Antibody to cell-surface or matrix antigens Autoimmune haemolytic Rh blood group antigens Destruction of RBC anaemia Goodpasture’s syndrome Collagen type IV Glomerulonephritis Grave’s disease TSH receptor hyperthyroidism Myasthenia gravis Acetylcholine receptor Progressive weakness Immune complex disease SLE DNA, histones, Glomerulonephritis ribosomes, snRNP, Vasculitis, arthritis scRNP Mixed essential Rheumatoid factor Systemic vasculitis cryoglobulinemia (antibody against IgG) (IgG insoluble at low temp) T cell mediated Type I diabetes (T1D or Pancreatic cell antigen β-cell destruction IDDM) Rheumatoid arthritis (RA) Synovial joint antigens Joint inflammation and destruction Multiple sclerosis (MS) Myelin basic protein Brain degeneration proteolipoprotein paralysis Genetic and environmental factors which predispose to autoimmune disease Most autoimmune diseases are associated with one or more HLA allotype. These are not encoded by ‘mutant’ alleles but are polymorphic variants in the normal population. A common way of expressing the way allotypes influence disease is the concept of Relative Risk. For example, in comparison with HLA-DQ6-ve people, HLA-DQ6+ves are 12 times more likely to develop multiple sclerosis. But HLA-DQ6 is common in normal healthy individuals. Conversely, not all patients who develop the disease have this allotype and only a fraction of DQ6+ves will succumb. 16 Note that alleles of polymorphic HLA genes are combined on haplotypes. For example, a common haplotype in Caucasian populations is A1-B8-DR3-DQ2. This haplotype is associated with a number of autoimmune diseases including T1D, SLE, Graves’ and Myasthenia Gravis. Determination of the causative allele requires careful analysis of the disease in individuals, in different populations, e.g. where B8, DR3 and DQ2 are individually distributed on different haplotypes. After this analysis it may turn out that multiple alleles contribute to susceptibility: e.g. DR3 and DQ2 may both individually confer different levels of risk. Most autoimmune conditions have an MHC association, but different HLA types are associated with different diseases. This situation most likely reflects binding 17 of different peptides to the grooves of HLA molecules. For example, a residue at position 57 of the β chain of HLA-DQ is protective if charged (aspartate) but not if hydrophobic (val, ser, ala), reflecting binding of different diabetogenic peptides. Other genetic factors Genetic susceptibility plays an important role in almost all autoimmune conditions. Genome wide association (GWAS) studies show that multiple genetic loci are involved in susceptibility to common autoimmune conditions such as diabetes (T1D) and rheumatoid arthritis. They are said to be multigenic disorders. Endocrine factors (sex differences) Most autoimmune diseases do not occur with equal frequency in males and females. For example Grave’s and Hashimoto’s are 4-5 times, and SLE 10 times, more common in females. Ankylosing Spondylitis is 3-4 more frequent in males. The increased susceptibility may be due in part to hormonal differences and in part due to genes found on the X chromosome which are imperfectly silenced (hence females express higher levels. One such gene is TLR7 which may promote susceptibility to Lupus. Primary biliary cirrhosis Systemic lupus erythematosus Mixed connective tissue disease Chronic active hepatitis 18 Environment Environmental factors also play a major role in autoimmune disease. The twin concordance rate is about 20-40% for common autoimmune diseases such as diabetes and rheumatoid arthritis. As mentioned above, a likely factor is childhood infection or exposure to microbes, but are infections protective or predisposing? In some cases there appears to be a direct effect: rheumatic fever can follow Streptococcal infection; reactive arthritis after Yersinia, Shigella or Chlamydia. Non- specific infection is known to cause a flare-up of MS. It is estimated that environmental factors contribute about 50% of the susceptibility to T1D. The MHC contributes about 25% and the other 25% comprises a variety of other genes. Initiation and perpetuation of autoimmune reactions Autoimmunity could be triggered by infection in a number of ways: Release of sequestered antigen e.g. In the case of autoimmune sympathetic ophthalmia, damage to one eye leads to subsequent autoimmune attack of the contralateral eye (see lecture 9). T cell tolerance may be bypassed in a number of ways, for example: Modification – generation of a neoantigen recognised by T cells. e.g. by modification of proteins, such as citrullination, by environmental factors, such as smoking. This results in breaking of tolerance to a self-antigen. Molecular mimicry Molecular mimicry is thought to occur if an antibody against an infectious agent can also cause an autoimmune response. In other words, the antibody is cross- reactive. The potent immune response elicited by the infectious agent may hence break immunological tolerance. A similar scenario can occur with T cells. Rheumatic fever is caused by cross-reactive antibodies to Streptococcus. 19 Potential triggers and causes of autoimmunity The mechanisms underlying autoimmunity remain incompletely understood but in the last decade some important clues have emerged from specific examples. In some cases, immune tolerance seems to be broken by modification of proteins. Rheumatoid arthritis (RA) may involve protein modification – in this case protein citrullination by peptidylarginine deiminase. Initially this may occur in the lung and there is a strong link between RA and smoking. Antibodies to the modified proteins: ACPA (anti-citrullinated protein antibodies) are present in most patients with RA. Coeliac disease is not strictly autoimmune as it is dependent on eating wheat. It involves presentation by specific HLA-DQ molecules of deamidated gliadin peptides. Removal of the antigen from the diet is generally curative. Another clue comes from periodontitis. Infection of teeth and gums with P. gingivalis can result in citrullination of epitopes which again result in ACPA. A mechanism affecting joint inflammation is the formation of immune complexes, although multiple elements of immune pathways may be involved. Pathogenesis of both periodontitis and RA involve bone erosion. They have some common risk factors, including smoking and aging. Periodontitis often precedes RA. Microbiota The bacteria in our gut help shape our immune repertoires and the differentiation states of our T cells. This is best demonstrated in mice. Germ-free (GF) mice have been bred in the complete absence of commensal bacteria. These do not develop several kinds of autoimmune diseases, presumably because the 20 pathogenic T cells have not been primed initially. Specific-pathogen free (SPF) mice have harbour microbiota, but do not contain pathogenic bacteria and viruses. Treatments for autoimmunity Organ-specific treatments may be available, such as thyroxine for hypothyroidism and insulin for type-1 diabetes. Immunosuppressive drugs such as steroids have a general effect on dampening immune responses. They may be effective but their long-term use invokes unwanted side-effects. Other immunosuppressive agents include cyclosporin and rapamycin. These are potent inhibitors of T cell activation. Antibodies to TNF-α or its receptor have proved to be effective in RA. TNF-α is generally produced by innate immune cells and therefore blocking its effect is considered anti-inflammatory. CTLA4-Ig is a fusion protein which binds CD80 and CD86 with high affinity, hence preventing costimulation of T cells via CD28. CTLA-4Ig is approved for the treatment of RA. 21 22 MVST & NST Part IB Pathology-Biology of Disease MT 2024 Prof Klaus Okkenhaug Lecture 11 Hypersensitivity Hypersensitivity - this term refers to immune responses that are damaging rather than helpful to the host. In other words, these are over-reactions of the immune system. In the previous lecture we considered the special cases of autoimmune diseases. In this lecture we consider hypersensitivity reactions against otherwise innocuous foreign antigens. Gell and Coombs proposed a classification scheme that defined four types of hypersensitivity reactions. The first three are mediated by antibodies, the fourth is mediated by T cells. Type I Hypersensitivity The rapid allergic reaction hayfever (allergic rhinitis), as well as eczema and asthma all result from type I hypersensitivity. The cause is contact with antigen to which the host has pre-existing IgE antibody. The IgE has been generated to the allergen, the agent triggering the response, through a TH2 type response. 23 MVST & NST Part IB Pathology-Biology of Disease MT 2024 Prof Klaus Okkenhaug Lecture 11 Mast cells are activated by cross-linking of FcεRI receptors via antigen binding to the bound IgE molecules. There is a rapid response as the mast cells degranulate to release histamine and serotonin. Some six hours later, secondary inflammatory mediators are released in the late response. Normally, IgE protects against infection with large multicellular pathogens, such as worms: IgE responses against allergens are maladaptive immune responses Common allergens include: Pollens: birch tree/ragweed/oil seed rape Foods: nuts/eggs/seafood Drugs: penicillin/aspirin Insect products: bee venom/house dust mite Animal hair: cat hair and dander. None of these would normally cause us harm – in these cases, the immune system is not protecting us against potential harm, but responding inappropriately to innocuous antigens. What property distinguishes allergens, which stimulate strong IgE responses, from other antigens? There is no single common factor for all, but they are often proteases. They are generally low MW and highly soluble, so they diffuse readily into mucus. They are also generally stable and can survive as a desiccated particle. They contain peptides that can bind MHC class II to prime T cells. The low dose encountered may favour IL-4-producing, Th2 responses. 24 MVST & NST Part IB Pathology-Biology of Disease MT 2024 Prof Klaus Okkenhaug Lecture 11 Sensitivity to allergens can be determined by the skin-prick test. A ‘wheal and flare’ reaction appears at the site of infection within a few minutes. The wheal is swelling (edema) and the subsequent redness (flare or erythema) is from increased blood flow. After 6 hours there may be a late-phase reaction where the swelling spreads to involve the surrounding tissue. Incidence and genetic susceptibility. Up to 30% of individuals in some populations exhibit type I hypersensitivity. Having two susceptible parents doubles the risk of an affected child, indicating a genetic component. Some people have multiple allergies, typically both hayfever and eczema. These atopic (atopy is a predisposed state) individuals have serum IgE raised 10-100 times the normal level. The selective advantage of such responses may relate to protection against parasites, common in tropical countries, where muscular contractions are needed to expel them from the gastrointestinal tract. Extreme cases can result if the antigen is directly injected into the blood stream. In systemic anaphylaxis the increased permeability of blood vessels results in extreme drop in blood pressure and anaphylactic shock, which can be fatal. Treatments include identification and avoidance of the antigen; as well as antihistamine and corticosteroids, which suppress leukocyte function. In some cases, desensitisation may be achieved by gradual exposure to increased dose of antigen, to convert Th2 to Th1 and/or iTreg responses. Asthma 25 MVST & NST Part IB Pathology-Biology of Disease MT 2024 Prof Klaus Okkenhaug Lecture 11 Asthma is a condition resulting in chronic inflammation of the airways characterised by increased TH2 lymphocytes, eosinophils, neutrophils and basophils, which amplify inflammation and airway remodeling. The disease has a complex aetiology. Some candidate genetic susceptibility loci include HLA class II, TCR and genes which affect the TH1/TH2 balance. Other genes affect the IgE receptor and cytokines including IL-4. In addition, are effects of non-immune genes such as those influencing smooth muscle cell behavior, bronchial physiology and tissue repair. Mice lacking the T-Bet transcription factor, which drives T cells to differentiate into TH1 cells, and suppresses the TH2 pathway, have increased levels of IL-4, IL-5 and IL-13 cytokines and have a disease similar to human asthma. 26 MVST & NST Part IB Pathology-Biology of Disease MT 2024 Prof Klaus Okkenhaug Lecture 11 A skin test is not suitable to test whether a person will respond to an allergen with asthma. Instead, breathing exhalation rate is tested after exposure to an inhaled allergen. After introducing the antigen, a response in minutes is due to degranulation of mast cells in the respiratory tract, armed with allergen-specific IgE. The released mediators constrict the bronchial smooth muscle to attempt expulsion of the offending material by coughing. This acute response ends after an hour or so. As in the skin test, a late response follows after about 6 hours, due to leukotrienes and other inflammatory mediators. This phase is most damaging and leads to recruitment of eosinophils and TH2 lymphocytes. If the antigen persists the condition may become chronic, whereby allergen-specific TH2 cells promote further IgE production and recruitment of eosinophils and neutrophils. The condition may deteriorate and the airways become occluded by mucus plugs. Re-exposure to antigen can trigger further attacks. The disease may be exacerbated by bacterial and viral infections, dominated by TH2 cells and a type IV hypersensitivity response (see later). 27 MVST & NST Part IB Pathology-Biology of Disease MT 2024 Prof Klaus Okkenhaug Lecture 11 Type II Hypersensitivity Type II hypersensitivity results from IgM or IgG antibody binding to cells or tissue antigens. Destruction of red blood cells (hemolytic anemia) or platelets (thrombocytopenia) can be uncommon side effects of drugs such as penicillin. The drug binds to the cell surface and is a target for antibodies in a minority of individuals. The cell-bound antibody triggers clearance of the cell by tissue macrophages in the spleen, which bear Fcγ receptors, or by complement lysis. 28 MVST & NST Part IB Pathology-Biology of Disease MT 2024 Prof Klaus Okkenhaug Lecture 11 Blood transfusion is the oldest form of transplantation and is an example of type II hypersensitivity. Blood groups are genetically variable (polymorphic) structures present on red blood cells. Some of these structures are only present on red blood cells and others are present on many or all tissue cells. The ABO blood group is special in that it is the only histocompatibility alloantigen for which pre-existing antibody is present in naive, previously untransplanted (untransfused) recipients. ABO blood grouping Molecules consist of a core H antigen The O (null) allele is unmodified H antigen Sugars may be attached to this core: The A allele adds a terminal N-acetylgalactosamine The B allele adds a terminal galactose AB indicates both modifications Phenotype Frequencies in European Caucasoid population A - 40 % B - 11 % AB - 4 % O - 45% 29 MVST & NST Part IB Pathology-Biology of Disease MT 2024 Prof Klaus Okkenhaug Lecture 11 Rhesus reaction Type II - the Rhesus reaction (haemolytic disease of the newborn - HDNB) 1st birth postpartum Subsequent pregnancy RhD+foetus RhD- RhD+foetus mother Anti-RhD RhD+ve RBC + + B + Anti-RhD lysis + Production of anti-RhD IgG postpartum IgG crosses placenta into Foetal circulation (IgM can RBC from RhD + foetus leak into not cross!). 2nd RhD+ baby affected the maternal circulation during birth The rhesus reaction is a different case of blood incompatibility. If the mother is Rhesus negative and the child is Rhesus positive the mother can produce antibodies to the Rhesus antigen. This happens because some Rh+ cells leak into the maternal circulation at birth. The IgG can cross the placenta and compromise the subsequent Rh+ baby. This may be circumvented by giving anti-Rh antibody (RhoGam) to the mother before she reacts to her child's red blood cells. This is presumed to mask or eliminate the Rh antigen 30 MVST & NST Part IB Pathology-Biology of Disease MT 2024 Prof Klaus Okkenhaug Lecture 11 before the mother has the chance to react to it. It may also suppress B cell reactivity by cross-linking the inhibitor FcγRIIB receptor. 31 MVST & NST Part IB Pathology-Biology of Disease MT 2024 Prof Klaus Okkenhaug Lecture 11 Type III Hypersensitivity Type III IgG hypersensitivity reactions occur when the antigen is soluble and in high quantities (low levels tend to produce IgE responses). Immune complexes form and are deposited in tissues. These trigger mast cells via the low affinity FcγRIII receptor. Complement is activated and polymorphs are attracted to the site of deposition, causing local tissue damage and inflammation. Common examples include post-infection complications such as arthritis and glomerulonephritis. The Arthus reaction is a local type III hypersensitivity reaction. This can be triggered in the skin of sensitized individuals who have IgG against the sensitising antigen. Other examples include pigeon fancier's lung or farmer's lung, where the antigen is inhaled. Serum sickness used to result when high doses of horse serum were used to treat pneumonia. The course of the response to this now out-dated treatment is shown above. This classic form of type III hypersensitivity is rarely seen now although injection of serum is still used as anti-snake venom. 32 MVST & NST Part IB Pathology-Biology of Disease MT 2024 Prof Klaus Okkenhaug Lecture 11 Type IV Hypersensitivity Delayed type hypersensitivity (DTH) is mediated by specific T cells, that release cytokines which in turn recruit mononuclear cells. The effect is usually maximal in 48-72 hours. A classical example is tuberculosis (the Mantoux test). The amount of antigen required for type IV reactions is generally 10-100 times more than for antibody-mediated hypersensitivity. Similar reactions take place in contact hypersensitivity. These are cutaneous responses to haptens, which form stable complexes with host proteins. For example, poison ivy, metal salts and small reactive chemicals. In many cases damage is through Th1 cells activating macrophages. Figure (above) Type IV hypersensitivity reactions are orchestrated by cytokines released by Th1 CD4 cells in response to antigen. Macrophages recruited to the site of inflammation by chemokines present antigen to T cells and amplify the response. IFN-γ and TNF-α activate macrophages, increasing the release of inflammatory mediators. Delayed type hypersensitivity: Certain haptens modify proteins and make them immunogenic. A T cell with low affinity for the unmodified peptide may react strongly to the chemically modified peptide. These reactions can be elicited by poison ivy, metals salts, small reactive chemical, black henna. 33 MVST & NST Part IB Pathology-Biology of Disease MT 2024 Prof Klaus Okkenhaug Lecture 11 The Mantoux test for Mycobacterium tuberculosis exposure One way to diagnose someone who might have had a past infection with mycobacterium tuberculosis is to inject a small amount of tuberculin under the skin. T cells will recognise the inoculum and mediate a type-I immune response leading to oedema. One limitation of this test is that individuals who have been vaccinated against Tb (eg with BCG), also react positively to this test. Some small drugs bind to MHC molecules and displace peptides For example, Abacavir sensitivity syndrome is a T cell mediated drug hypersensitivity that occurs in individuals possessing the HLA class I allele HLA-B*57. Some other drugs are specific to other HLA alleles. 34 MVST & NST Part IB Pathology-Biology of Disease MT 2024 Prof Klaus Okkenhaug Lecture 11 Summary of hypersensitivity reactions Type Descriptive Initiation mechanism Examples name/timing of symptom onset I IgE-mediated Ag induces cross-linking Systemic (2-30 mins) of IgE bound to mast cells anaphylaxis with release of vasoactive Local anaphylaxis mediators Hay fever/Asthma II Antibody-mediated Ab directed against cell- Blood transfusion reactions (5-8hrs) surface antigens mediates Autoimmune Cytotoxic cell destruction via ADCC Haemolytic anaemia hypersensitivity or complement III Immune-complex Ag-Ab complexes Arthus reaction mediated deposited at various sites (localised) hypersensitivity induces mast cell Serum sickness (2-8 hrs) degranulation via FcγRIII, PMN degranulation damages tissue IV Cell-mediated Memory TH1 cells release Contact dermatitis hypersensitivity cytokines that recruit and Tubercular lesions (24-72hrs) activate macrophages 35 MVST & NST Part IB Pathology-Biology of Disease MT 2024 Prof Klaus Okkenhaug Lecture 12 Transplantation In humans and animals, the situations when allogeneic cells (cells form a different host) come into contact with the immune system are either: Iatrogenic (effects of medical treatment), such as blood transfusion, or Natural, such as pregnancy in placental mammals. Transplantation is the introduction of biological material - organs, tissue, cells or fluids - into an organism. The main problem with transplanting tissue is that most cells express polymorphic surface antigens encoded by the MHC. Variation between the donor and recipient at the MHC results in rejection. Even if there is a perfect match other ‘minor’ antigens can be recognised by the immune system. Unlike ABO blood typing there are no universal donors. If tissue is mis-matched it is generally rejected. We can distinguish four possible relationships between transplanted donor material and the recipient, as illustrated below: Allogeneic, the most common type of transplants, display immunological memory. The immune system is primed by the allograft, upon the first encounter with antigen. When a recipient that has previously rejected a skin graft is regrafted with skin from the same donor the graft is rejected more rapidly, in a second-set reaction. Skin from a third-party donor grafted onto the same recipient would be rejected as for the first-set. The rapid second set rejection course can be transferred to normal or irradiated recipients by T cells from the original recipient. This shows that second-set rejection is caused by a memory immune response from clonally expanded and primed T cells specific for the donor skin. Memory T cells are produced alongside effector T cells in a primary immune response. Upon a second exposure to the same antigens memory T cells promote a more rapid, more effective response. This is because memory T cells to a specific antigen are more numerous than naïve T cells and they are more readily activated. 36 Rejection mechanisms A central role for recognition of transplanted tissue is played by T cells but other cells may be involved in rejection, including NK. However, in some cases antibodies are important, although their production may be initiated by T cells. T cell recognition may be through direct recognition of the donor MHC or indirect recognition, of an antigen presented by self MHC molecules. Generally, there types of rejection are recognised in allotransplants in terms of their kinetics: 1. Hyperacute rejection 2. Acute rejection 3. Chronic rejection 1. Hyperacute rejection Hyperacute rejection occurs very rapidly, within minutes or a few hours. It results from pre-existing antibody, such as in ABO incompatible transplants. Other antibodies, mostly anti-HLA can also come into play: 1. From previous organ transplants (e.g. children who have multiple transplants) 2. From pregnancy – at childbirth foetal cells enter maternal circulation and stimulate adaptive response to paternal HLA 3. From blood transfusion (matched for ABO but not HLA) The tissue is rejected rapidly as ABO and HLA antigens are expressed on the endothelial cells lining blood vessels. The tissue is damaged by complement activation, coagulation and leakage of fluids, as well as by aggregation of platelets that block the microvasculature. 37 Hyperacute rejection also takes place in Xenotransplants. This is a problem for companies hoping to use pigs as a source of organs for human recipients. There are several problems to overcome. First, we make natural IgM and IgG antibody to modified sugars on pig tissue. These transplants are said to be discordant. Second, complement does not function well across species. Normally complement is disabled on self tissues by the action of regulatory proteins such as decay accelerating factor (DAF). This does not work on pig tissue and the graft is attacked by the human complement. 2. Acute Graft Rejection This is the main immunological barrier to allotransplantation. It is caused by T cell recognition of the transplanted tissue. Note it is not an issue in blood transfusion as red blood cells do not carry MHC antigens. Acute rejection can be thought of as a type of Type IV hypersensitivity reaction as it involves the response of CD8 T cells to HLA class I differences and CD4 T cells to HLA class II differences. 38 There are two quite distinct modes of recognition: A. Direct recognition of allo MHC. Experimental transplantation showed that one genetic region dominated histocompatibility in within species grafts. This was the Major Histocompatibility Complex (MHC). This region exists in all vertebrates and it is highly polymorphic. In spite of attempts at matching, most organ transplants are performed across some HLA class I and/or class II differences. The recipient’s naïve T cell population contains clones of so-called alloreactive T cells that recognise HLA allotypes that are not shared with the recipient. Some of these clones are of the memory type and were initially stimulated and expanded in response to pathogens but cross-react with allogeneic HLA. Why do the products of the MHC cause such reproducible and rapid graft rejection? This is because naive individuals inadvertently have a high frequency of T cell receptors reactive with allo-MHC products, irrespective of the loaded peptide. Remember that negative selection in the thymus reduces the number of T cells that see self antigens but does not limit the number that respond to alloantigens. These receptors may not need to bind with a particularly high affinity to result in activation of the T cell. Unlike normal antigen presentation, which can be triggered by a small number of self-MHC molecules presenting a peptide, T cells in transplants could engage every MHC ligand on the cell. The recipient is sensitised by migration of allogeneic dendritic cells from the inflamed donor organ via the lymph to the host's secondary lymphoid tissue. Here they settle in the T cells zones and present donor MHC molecules, with donor peptides, to activate those circulating host T cells bearing the corresponding T cell receptors. The alloreactive T cells are carried back to the graft, which they attack through the direct pathway of allorecognition. The effector T cells migrate to the transplanted tissue and Th1 T cells activate the resident macrophages, which increase inflammation. A main mechanism of killing is through CD8 T cell cytotoxicity. B. Indirect recognition This is thought to be due to the uptake of allogeneic proteins by the recipient’s own antigen presenting cells (APC’s) and their presentation to T cells by self MHC molecules. Some of the donor dendritic cells that migrate to the secondary lymphoid tissue die by apoptosis or necrosis. Among the graft-derived peptides presented by the recipient’s APC’s may be some HLA molecules that differ from those of the host, as well 39 as some so-called non-MHC ‘minor’ or H antigens. Minor transplantation antigens are various proteins which vary in sequence and where one (at least) of the allomorphs contains a peptide which binds to the MHC of the recipient. Both HLA and non-HLA proteins different from those of the host are taken up by dendritic cells of the host (recipient) and processed so that peptides are presented by the recipient’s own HLA. As they are taken up by endocytosis they are mostly presented on HLA class II. Some of the peptides that differ in amino acid sequence from those of the host stimulate a CD4 T cell alloreaction. The responding T cells will be specific for a complex of a peptide derived from the donor bound to a recipient HLA class II allotype. Thus, in principle T cells activated in this way will be specific for self-HLA molecules plus donor peptides. Nevertheless they result in destruction of the graft by a variety of mechanisms. Some T cells will cross react with donor HLA plus peptide. Counter-intuitively, MHC sharing between donor and recipient in indirect recognition may increase reactivity as donor, as well as recipient, dendritic cells can prime recipient T cells for minor peptides. Other mechanisms of tissue destruction may include activation of macrophages, which produce lytic enzymes, production of antibody through B cell activation and subsequent immune complex formation and ADCC. Properties of minor antigens Rejection is slower than for direct recognition although many MHC minor differences combine to give rapid rejection. In experimental animals even grafts between animals of the same isogenic strain can be rejected, for example where the donor is male and the recipient female and the H-Y antigen is recognised. In human transplantation recognition of male minor antigens can play an important part in haemopoietic stem cell transplants. 40 3. Chronic rejection Rejection of matched organs can occur years after transplant. There has been no improvement in treatment of chronic rejection in the past 30 years. The mechanism is obscure but may relate to immune response against blood vessels. The blood supply to the organ is compromised, resulting in ischemia and loss of function. Chronic rejection may be due to a type III hypersensitivity reaction due to IgG antibodies against allogeneic HLA class I molecules on the graft, forming immune complexes that deposit in the blood vessels of the transplanted organ. It is also thought that indirect allo- recognition is important for chronic rejection as B cells binding shed donor HLA proteins are ‘helped’ by T cells. Kidney damage can also affect loss by non-immune mechanisms. 41 Particular transplant situations 1. Privileged sites Transplants at certain sites may be occur with little or no immune rejection. The most important of these is the cornea. The absence of lymphatic drainage is probably the critical common factor but some sites also lack vascularisation. 2. Vascularised solid organs These include: kidney, lung, liver, heart, pancreas 3. Haemopoietic stem cell transplants Also called bone marrow transplants. There are three sources of stem cells: 1. Peripheral blood - enriched by cytokine administration – CD34+ pluripotent stem cells are mobilised by GM-CSF. 2. Bone marrow 3. Cord blood. Influence of HLA-matching on on allograft survival. This varies significantly according to tissue transplanted Very important significant no effect Haemopoietic stem cell kidney, heart liver Initially, with immunosuppression it was thought that HLA matching for kidney transplants was not always necessary. However, data collected over decades shows that there is a precise relationship between the number of mis-matches and transplant survival. Note that this type of data is gathered from results of large numbers of transplants and mis-matches of some HLA alleles may be more permissible than others. A precise match is uncommon as there are at least six polymorphic genes (class I A,B,C and class II DR,DP,DQ) on each haplotype (shown below are just A, B and DR), each of which can exhibit one of different alleles. As mentioned above, for haemopoietic transplants, where the recipient’s immune system has been ablated, a sibling or parent sharing one haplotype can be advantageous to elicit a GvL response. There are many HLA alleles making matching difficult 42  There are many alleles at HLA loci making precise matching difficult A B DR 2 44 4 1 8 3 33 33 33 5 5 29 29 44 44 7 7 etc etc etc How is HLA matching determined? Traditionally, microcytoxicity was used. This test relies on sera with known anti-HLA antibodies that specifically recognize allomorphs of particular HLA loci. These sera were taken from multiparous women who had produced antibodies to their babies, although these were eventually superseded by monoclonal antibodies as they became available. The donor's blood cells were HLA typed by mixing them with a panel of such sera containing the anti-HLA antibodies. If the antibodies recognized their epitope on the MHC, in the presence of added complement the cells would be osmotically lysed and would take up a dye. Most tissue typing laboratories have now adopted DNA typing instead, although the microcytotoxicity test has had a major impact on histocompatibility testing. Before a transplant a mixed lymphocyte reaction (MLR) was performed by mixing leukocytes from recipient and donor in culture. After several days cells proliferated in response to MHC incompatibility. In vitro about 1% of cells respond to an allogeneic mismatch. Cross-matching – an exact match may be difficult and DNA typing may miss alleles where the gene is present but the protein is not expressed. It is important to determine if there are any preformed antibodies to potential donor HLA alloantigens in the recipient. 43 A positive cross-match indicates that the recipient has antibodies against HLA proteins carried by the donor. The most common method is to screen the recipient serum against panels of microbeads, each coupled with a specific HLA protein Cross –matching assay HLA molecules from single Recipient serum allele anti-HLA Luminex bead antibody fluorescent secondary antibody (anti-IgG) Immunosuppression is essential for clinical transplantation of organs such as kidneys (excluding privileged sites or autologous transplants). Some drugs may be given before the transplant, to condition the immune response in the patient. These include: Steroids such as prednisone- given for systemic immunosuppressive effects. Steroids have multiple mechanisms of action which mimic those of corticosteroids. They can have severe side-effects when used in high doses. Humanised antibody to CD52 on the surface of leukocytes results in long-lasting lymphopenia due to activation of complement. Other drugs may be given after the transplant. These include: Cytotoxic drugs such as azathioprine lead to death of rapidly dividing cells such as T cells Immunosuppressive drugs that target cell signalling pathways in lymphocytes e.g. cyclosporine, FK506, rapamycin. Many of these drugs were isolated from soil organisms. They block signal transduction for activation of T cells. The immunosuppressive drug needs to be maintained indefinitely. Transplantation is now accepted, successful and common in most societies. UK statistics (2021) # of Organ transplants 10 year graft survival >70% from deceased donor, >80% from kidney 2199 living donor Heart 169 >65% (>90% for infants) Lung 88 30-40% Liver 739 >60% 44 Cornea 2412 ~70% (5 years) Haematopoietic transplants These transplants are used to treat genetic immunodeficiencies, leukaemias and lymphomas. Unlike solid organ transplants there is generally no shortage of donors. No surgery is required as these are ‘liquid’ transplants. The host’s immune system is ablated (myeloablative therapy) by chemotherapy and radiation and the patient’s immune system is reconstituted with donor cells. In Allogeneic transplants the donor is healthy and HLA-matched, as far as possible. The recipient’s immune system is destroyed by irradiation and drugs. Problems arise as donor T cells respond to the recipient’s HLA molecules in a graft versus host (GvH) reaction. This is essentially a systemic type IV hypersensitivity reaction. T cells from the donor access recipient lymphoid tissue, interacting with host dendritic cells divide and proliferate. These T cells attack epithelial tissues inflamed by irradiation and chemotherapy. External signs include rash and raised or discoloured areas of the skin and eyes but internal organs may also be damaged. Autologous transplants help to overcome histo-incompatilibity. In this situation, self cells, which are of course perfectly histocompatible, are removed from the patient and any residual immune cells and leukaemia are ablated. The immune cells are then reinfused into the patient after removal of the tumour cells. A problem with autologous transplants is potential return of the cancer (relapse). More than 2000 people are in need of a bone marrow transplant annually. These can be used to correct inborn errors of immunity (also known as primary immunodeficiencies). More commonly, bone marrow transplants are used to reconstitute the immune system after chemotherapy and or whole-body irradiation, especially to treat leukaemia and lymphoma. Bone marrow transplants can cause graft versus host disease (GvHD) if the infused bone marrow cells contain NK cells and/or T cells. However, such cells can also help kill remaining leukaemic cells through a process called graft versus leukaemia (GvL). 45 Adoptive T cell therapy involves the isolation of tumour-reactive T cells which are expanded in cell culture before transferred back into the patient. This remains an experimental form of therapy. T cells can also be isolated from the patient and transduced with a Chimaeric antigen receptor (CAR). CARs have an antigen binding domain derived from an antibody on the extracellular side and TCR and costimulatory signalling domains on the intracellularly. CAR-T cells are now approved for the treatment of B cell lymphomas. CAR- T cells for other types of cancers are also under development. These represent a novel class of “living drugs”. 46 Therapeutic approaches Cyclosporin and Rapamycin are commonly used to prevent graft rejections (for mechanism of action, see Lecture 8: T cells). CTLA4-Ig: prevents CD28 costimulation and may induce tolerance toward the graft. Belatacept is approved for the prevention of kidney transplant rejection. Regulatory T cells Treg can be expanded and infused back into the patients with the aim to provide tolerance against a graft. This form of therapy has not yet been shown to be effective in the clinic. Autologous stem cells – regenerative medicine Technically challenging – may in the future be a source of artificially grown autologous organs and tissues. 47 48