Chapter 13: Allergy and hypersensitivity
Chapter 13: Allergy and hypersensitivity
When people think about allergy they usually think about the sensitivity to cats, pollen, etc… and those fall under type I hypersensitivities and those are the ones that are mediated by IgE bound mast cells, but there are many other kinds of hypersensitivities or allergies, and those include those that are mediated by IgG, or what if someone made an antibody that recognizes a cell surface molecules, perhaps on the red blood cell, and if that individual was able to fix complement or the red blood cell got phagocytized, that individual would be depleted of red blood cells, they would be anemic. If you make an antibody against platelets, you can’t clot. Some of these are considered autoimmune diseases but they fall under the umbrella of type II hypersensitivities. Type III hypersensitivities for the most part, tend to be an antibody response against a foreign protein, and for the most part this appeared when people were treated with foreign proteins. In the 50’s and earlier it was hoped that if you immunized a horse with some pathogen, you could produce antibodies in that horse that would be beneficial to a person should they be infected with that pathogen, so serum sickness developed, because people responded to the differences in the antigenic components on the antibody molecules. The sequence of antibody molecules tends to be the same, but different species tend to have different sequences and people made antibodies against these foreign proteins. People would develop antibodies that bound other antibodies and they would precipitate and cause kidney damage, and damage to the tissue where those antibodies were found. Today it is more a problem with getting monoclonal antibodies to treat various disorders, so we humanize antibodies. The business end of the antibody is the variable domains of the heavy and light chains, genetically in the lab, you can splice human heavy and light chains to the variable regions that have the binding capacity that you want. Let’s say you want an antibody against CD3, a protein present on all T-cells, and anti-CD3 was one of the original treatments for the prevention of graft rejection. So the Fab and splice it onto the constant domain of the heavy and light chain for humans. So you get mouse monoclonal Fab and splice it onto human constant domains. People will not react against these antibodies, and these antibodies can be very beneficial. Herceptin is one example, it works by binding to EGFR, and it is used for treating women with breast cancer. There are many other monoclonal antibodies that are in clinical use all the time. If you have tumor that has a tumor marker that is distinct from all the other cells, it may be due to a state of differentiation, a viral gene product, or a lineage marker; if you have a monoclonal antibody against that marker you can use it to treat that tumor. But in order to use these antibodies you must humanize them to avoid the development of serum sickness against mouse monoclonals. Type IV hypersensitivities are all the T-cell mediated reactions; this is what most people think about when they think about T-cells and allergy. The TH1 response to poison ivy results in the characteristic rash which gets worse every time, and we will talk aout the different ways you can intervene to prevent the symptoms indicative of these various types of allergies.
Among the many things that are inside mast cell is histamine, and why do we think about taking antihistamines. There is no histamine in the T-cell that is associated with type IV hypersensitivities, there is no histamine in any of the downstream effector cells, because the T-cell sees it’s antigen on class II molecules, it makes its cytokines, the APC makes its cytokines and chemokines and you get an inflammatory response. Another one of the ways that this cell is important is in the tuberculin test. This is a skin test, and if you have ever been exposed to tuberculosis or the vaccine BCG, and you haven’t gotten the disease, you’ve simply been exposed, you have generated many T-lymphocytes that are memory cells for peptides associated with TB. If someone were to inject into your skin some of these peptides, the APCs in your skin would process the peptides and the T-cells doing immune surveillance will respond and the site of injection will be red and raised. If you have not been exposed to BCG or TB then you will not get a positive test. If you have been exposed and have a positive reaction you are generally sent for an x-ray to see if you have TB in the lungs. If you do, you will be given an antibiotic course for 18 months to eliminate that infection. If the lung x-ray is clear, it is marked on your chart that you are skin test positive and x-ray negative and you will be skin test positive for the rest of your life. 90% of individuals who are exposed to TB never convert on x-ray, because they have an intact T-cell compartment and they are able to handle the infection. Those who are susceptible to TB are individuals who are TH2 biased immune response and those people who are immunodeficient whether it is HIV associated immune deficiency or immune deficiency due to something else.
TH2 hypersensitivities are associated with other aspects of asthma, some skin diseases, like atopic dermatitis.
What turns on type I hypersensitivities? There are food allergies, pet allergies, allergies to pollen, allergies to the excrement of insects whether it be cockroaches or dust mites. You aren’t allergic to dust, you are allergic to the mites that live on dust. In all cases it’s originally a protein because you have to turn on a TH2 cells which instructs the B-cell to make IgE antibody. Any site in the body can be a site where you have type I hypersensitivities. If you are allergic to one of the coloring agents that are put in detergent and you wash your clothes in it, you won’t have a problem the first time, but 2 to 3 weeks later you will have developed IgE antibodies in the detergent that is still on the clothes when you wear them and it can cause hives. If you inhale something you are allergic to, and a lot of people inhale pollen, there are the upper respiratory symptoms and the asthma. If you are allergic to something and you eat it, there is the GI component of allergy. Sometimes the food allergy is manifest when food has been metabolized and taken out of the gut and goes into circulation and you see a skin allergy. By keeping diaries you can correlate the skin allergy with eating something. It is rare that people are sensitive to chicken or meat but they may be sensitive to egg components, and ingesting a food allergen can result in systemic anaphylaxis. Insect bites, a lot of people become allergic to insect bites, people develop a local reaction that lasts longer than the initial inflammation, so that 48 hours after they are bitten they develop a local response. The venom from insects can result in systemic anaphylaxis, and when people are very sensitive to bee stings they have to walk around with an epipen, and people with severe food allergies need to walk around with adrenaline in injectable form.
Most allergies are just nuisances. Those people that develop asthma and don’t treat the asthma however, can die of asthmatic attacks. With many allergens, the more you are exposed to them, the worse the reaction gets because you induce more clones of T-cells that are specific for it, you expand the number of plasma cells making IgE antibodies so that all the mast cells have IgE antibodies, and because you have local TH2 responses, the cytokines the TH2 cells release recruit more mast cells to the site.
The mediators of allergy cause changes in vascular permeability, if its local, you get swelling, if it is in other sites it can cause constriction of organs. If your larynx becomes swollen the ability to exchange air can be compromised. Circulatory collapse can occur with intravenous coagulation. There are other forms of urticaria, or the skin reaction, some people are allergic to cold, and if they immerse their hands in ice water, they will develop the allergic flare. The cold triggers the mast cells without IgE. The mast cells degranulate and you get the response.
Rhinoconjunctivitis, this is an allergic reaction in the upper respiratory tract and your eyes. If you get pollen in your eyes, they get itchy and red, washing your face can help and there are local drops that you can put in to help stem the reaction. Asthma causes bronchiole constriction and the airways get inflamed and over time fibrin gets deposited and it is irreversible.
To elicit an allergy you need a protein and the protein activates TH2 cell, and it is often proteases that elicit these responses. TH2 cells are induced by small molecular weight produts. So for instance, pollen grains release this small protease, which diffuses through the mucous, they are highly soluble and they are stable, and they reach mast cells. These reactions require T-cell priming, you don’t get this reaction the first time you are exposed to the allergen. After you are exposed the first time, the T-cells expand and induce antibody production, and then if you are re-exposed you have an allergic reaction.
So if you are exposed to pollen, the protease in pollen, Der p 1 will diffuse through the mucous in the airway, eat through the tight junctions between the columnar epithelial cells and it cross links the receptor on the mast cells, and the mast cell degranulates and you get two waves of reactions. The vesicles contain many things, not just histamine. When the mast cell gets cross linked it makes more IL-4, and this helps to make more antibody.
Over the years people have attempted to find why some people are more allergic than others. They have attempted to map susceptibility to MS, arthritis, and other autoimmune diseases. Pg. 5 slide 1 shows the various sites that have been mapped as increasing susceptibility to allergy. Atopic dermatitis, psoriasis, which is T-cell mediated, and autoimmune diseases, can be the result of certain alleles in many different genes. There is not one gene associated with susceptibility, and depending on your lineage you can inherit some of these susceptibilities. These susceptibilities include things like regulation of the production of IL-4. If the promoter for IL-4 has higher avidity, and you make more IL-4 under low conditions then you will have more TH2 cells. If your IgE promoter is more efficient you will make more IgE. A variant in the structure of the IL-4 receptor, the IgE receptor can be even more avid, certain structural variants of MHC can render individuals more susceptible. Each person has 6 class II alleles, and some of these are better able to present the peptides from one allergen or another. If you inherit one which is better at presenting a specific kind of pollen then it is more likely that you will develop an allergy, however the microenvironment in which the T-cells are educated, determines whether it will be a TH1 response or a TH2 response, so the nature of that APC, whether it is more likely to produce IL-12 or IL-4 will be tremendously important in polarizing the response. IL-12 producing APC will lead to a TH1 response and will not result in a hypersensitivity. However if an individuals immune system is polarized TH2 like, then they are more likely to develop TB and hypersensitivities, but they won’t develop MS or rheumatoid arthritis because of their TH2 bias. β adrenergic receptor variants can lead to higher susceptibility to allergies. 5 LO is the enzyme that takes lipids from the membrane and turns them into leukotrienes. Leukotrienes are very important in the pathology associated with asthma. Cyclooxygenase or COX takes that same lipid molecule and make prostaglandins from it instead of leukotrienes, and there are other enzymes that make other products but they are less important in the acute allergies. So if you make PGE2, prostaglandin E2, one of the things that you respond to is local pain, another is fever. So when COX is the dominant enzyme people have fever, when 5 lipoxygenase, 5 LO is the dominant enzyme, people make more leukotrienes. There are two basic types among leukotrienes and there are two receptors, one for each type, and one set induce the production of mucous and the deposition of fibrin. The other set recruits neutrophils to a site, but the other leukotrienes are associated with the production of mucous and inflammation. Because of that, there have been several drugs developed that block 5 LO or block the ability of leukotrienes to interact with their receptor. Singulair is one drug that blocks 5 LO; as a result they don’t produce mucous and don’t get local inflammation in the lung.
If someone inherits a susceptibility to allergy, the susceptibility alone doesn’t do anything. If you are living in a place that has none of the pollen that you could make IgE to, you will not have asthma symptoms. If however you have a high susceptibility and you are in an area in which the allergen is present you will have a reaction.
One hypothesis about allergies is the hygiene hypothesis, and that hypothesis is that in the past humans lived in a reasonably dirty environment, there was no bottled water, refrigeration and children lived in close contact with each other. When people were exposed to all these pathogens, the challenges to their immune system, they developed strong immune systems that were TH1 biased, because the weak ones died early. Those that survived had the TH1 bias and fewer allergies. As you probably know, the frequency of allergy and asthma has gone up in recent years, and is worse in some urban environments than others, and it is worse in some urban environments for many reasons. There are more cockroaches, pollution, and there is a synergy between low levels of inflammation induced by allergens and the low levels of inflammation induced by pollution. Another thing in the polluted air is ozone, and this contributes to some of the immune responses, and the hydrocarbons released by cars can cause immune responses. These pollutants can irritate the lungs and result in a higher frequency of asthma. So all these contribute to susceptibilities, so if you have the gene, but not the antigen, then you’re fine. The best thing to do is to avoid your allergen if you are allergic to something.
One of the components of the hygiene hypothesis has been the susceptibility of individuals to RSV. RSV is a common, acute respiratory viral infection but it elicits in most people a TH2 response. Years ago, there was a terrible experience, people thought they were developing a great vaccine against RSV, they killed the virus, and injected people to make them immune. It elicited antibody, but these were non-neutralizing antibody, and it elicited a TH2 response and when people were infected with RSV they died. Normally with RSV it can result in hospitalization of about 10% of the babies who get it under the age of 2, and a fair number will die if they aren’t treated appropriately, and a monoclonal antibody is used against a G protein of the RSV to treat it. In many kids, RSV causes an inapparent infection and some children develop an IFNγ response, but young animals don’t develop an IFNγ response, they develop a TH2 response and if they get reinfected, and because they don’t make neutralizing antibody, and they don’t make IFN-γ they make IL-4 and they have eosinophils and neutrophils in their lungs, they can have red cells in their lungs which shows that the permeability into the vasculature has been increased and there can be vascular leaks into the lungs and mice and people can die of these infections. Those individuals who were older or biased to a TH1 response do just fine, they don’t lose weight and they don’t have the eosinophils and neutrophils in their lungs. Those children that were immunized with the vaccine had the IL-4 TH2 response.
What happens when mast cells degranulate? It depends where the mast cell is located. In airways, because of the leukotrienes there is mucous secretion and a change in the diameter of the airways. There is congestion, blockage, wheezing, coughing, and people can struggle to breathe because of the bronchocongestion, so people take bronchodilators. In the GI tract mast cell degranulation, in the lower GI tract results in increased peristalsis and if it’s in the upper GI tract you vomit. Mast cell degranulation in blood vessels causes dilation and increased local blood flow, fluid can leak into tissues and cause swelling, and sometimes you can have intravascular coagulation.
What’s in mast cells? There are many things besides histamine, but histamine is one of the toxic mediators. Heprin is an anti-coagulant but it can also cause smooth muscle cell contraction and it is present in mast cells. There are many enzymes and some of these can not only remodel the allergen but can also change local tissue structure. There are many cytokines that amplify the TH2 response, some of them tell the bone marrow to make more cells, others can cause local inflammation, TNFα does lots of things depending on how much is released. There are chemokines which attract cells, prostaglandins, leukotrienes, which cause vascular permeability, mucous deposition and smooth muscle cell contraction. Platelet activating factor is another lipid mediator and it attracts leukocytes. All of these things are happening at once, so antihistamine alone won’t work, it may help you breathe better locally, and depending on whether your taking a sedating antihistamine you may have less cognitive abilities. Claritin and allegra are non-sedating antihistamines but there are many sedating antihistamines. This is because there are two histamine receptors, one is present in the periphery and the other is present in the brain and those of you that take over the counter sleeping pills are taking a sedating antihistamine.
Why do some over the counter antihistamines get bundled with pseudefrin? Allegra D and Claritin D are some examples. The Sudafed or pseudefrin will help with the local swelling in the nose and you will not produce as much mucous, you will be able to breathe more easily even though the antihistamine doesn’t help that.
For other allergies you want to block the leukotrienes and the 5 LO antagonists or the receptor antagonists are used to do that, but what you really want to do is avoid the allergen. If you can’t avoid the allergen, prevent mast cell degranulation. Sodium Chromalin will prevent mast cell degranulation, it’s drug name is nasochrome, and it is available over the counter. Sodium chromalin blocks the signal transduction from the epsilon receptor. For the pulmonary consequences you can take intal, and this requires a prescription, but intal is not a bronchodilator. If you need a bronchodilator then you have had mast cell degranulation, and the cytokines from the mast cells have caused broncoconstriction. You can’t use a bronchodilator too often, if you need a bronchodilator then you are also depositing fibrin and ultimately the lungs will not be elastic, and acute allergies or asthma attacks after fibrin deposition could kill.
Eosinophils are also recruited, they don’t have IgE like the mast cells however they can also be activated in these reactions. As you can see, they produce a lot of common inflammatory mediators (pg. 7 slide 2 and pg. 8 slide 1) there is the IL-3 and proteases, although not the same proteases. Eosinophils secrete matrix metalloproteinase and this will remodel the basement membrane. Collagenase will do the same thing. Eosinophils produce neurotoxins which cause local pain. Eosinophils are also important in response to parasites, all these things are important for killing parasites. Major basic protein which is secreted by eosinophils is toxic to parasites but it also causes mast cell degranulation. Eosinophils also secrete chemokines which recruit more cells to the area, they release leukotrienes as well.
Why are there two phases of reaction? If someone has an acute allergy and you expose them to their allergen in their lung, and you measure the ability to exchange air, and that’s what the assay on pg. 8 slide 2 measures, you will find that the ability to exchange air drops dramatically and immediately when they are exposed to allergen. However, after their exposure they begin to recover, and this first reaction is the histamine associated reaction. The late phase reaction is the leukotriene assocated reaction, it used to be called slow reactive factor, and the late phase is just as severe. Often when you are exposed to an allergen it isn’t just a puff, you are constantly exposed. Those people who have allergies and don’t know to what, will go to doctors and get skin tests. The doctor will inject small bits of allergen into the skin in order to determine what an individual is sensitive to.
Allergy shots can help allergy sufferers by helping those affected to make IgG antibodies. You can get immunized in a way that will produce TH1 responses to allergen, then you will make more IgG than IgE and if you have enough IgG around the IgG will bind the cat dander and prevent it from binding the IgE on the mast cell. Allergy shots help skew the reaction toward the TH1, IgG response. For people who have severe life threatening allergies, there is a monoclonal antibody that neutralizes the ε receptor, but that is something that individuals will have to take for the rest of their life, and it is very expensive. This is useful for individuals who are allergic to peanuts and other foods that can cause severe allergies to even residual amounts of the food.
Pg. 9 slide 1 shows some of what happens in an allergic reaction. If you eat an allergen and it sensitizes your mast cells, the mast cells degranulate, you get smooth muscle contractions. If it is respiratory you can see swelling of blood vessels and contraction of smooth muscle, but there is also production of mucous which isn’t shown on the slide. In skin you get local inflammation, and the skin does swell and become red.
Acute responses are the result of the mast cell degranulation and this recruits more cells and cause the contraction of the smooth muscle, and the more cells start releasing more inflammatory molecules and depositing fibrin.
In pg. 10 slide 2 you can see the comparison of a normal lung biopsy from a mouse, to a biopsy from a mouse that lacks TH1 cells. If you remember we talked about all the transcription factors that were important to delineate different cell types. So IL-12 binds to its receptor and STAT 4 gets activated and STAT 4 induces the production of T bet and Tbet is associated with the TH1 phenotype. A knockout for T bet produces only TH2 cells and so they make huge allergic responses resulting in tremendous inflammation of the airways, and lots of collagen gets deposited in the airways and the airways become inflexible and if you have a lot of basement membrane deposited, the lung loses flexibility. In this model (pg. 11 slide 1) there is hyperdeposition, eosinophils are also here, lots of cells are infiltrating and these are skin biopsies.
This (Pg. 11 slide 2) looks at some of the new models we have in mice of genes associated with different kinds of allergies. If you overexpress caspase 1, and caspase 1 is not only involved in the apoptosis cascade, but in maturing IL-1 to active IL-1, so it works on zymogens, and it results in lots of allergies. In animals that are STAT 6 deficient, and STAT 6 is an important signal transduction molecule which IL-4 requires for signaling. In the absence of STAT 6 you don’t get IgE because IL-4 is important for the production of IgE. IL-18 is another cytokine matured by caspase 1 and in IL-18 animals, although they overexpress caspase 1, there is no dermatitis, and the amount of IgE is normal. So these are just some models that people use to look at the relation of certain genes and susceptibility.
So what are the risk factors in developing food allergies? An immature mucosal immune system, that is one that is TH2 biased. Early introduction of solid foods can lead to food allergies as will IgA deficiency, hereditary increase in mucosal permeability, inadequate challenge of intestinal immune system with commensal flora, which can occur if you live in a very clean environment. An impaired enteric nervous system is another risk factor. You need the enteric nervous system to have normal gut function, and there are many aspects to the enteric nervous system. Gastrointestinal infection with parasites, which will lead to a TH2 response also is a risk factor.
Ciliac disease is an allergy to some components of flour. On pg. 12 slide 2 you can see normal jejunum compared with the jejunum of an individual with ciliac disease. Like any other allergy if you avoid your allergen you can do very well. In ciliac disease the protein comes in and it turns on the response and the activated cells result in tissue damage. For some people there is also an antibody component.
What can you do for your allergies? Antihistamines, antileukotrienes, you can blockade the IgE receptor with a monoclonal antibody. There are ways to inhibit CD40 or the cytokines with monoclonal antibodies or pseudo proteins. Basically you want to polarize the response to a TH1 response instead of TH2 and you polarize by activating the APCs with adjuvants such as TLR 10 ligand (CPGs).
Serum sickness is manifested when a protein, whether it is a foreign antibody or some other protein that you have IgG against, and you get complexes formed of various sizes and these can precipitate out and start the complement cascade and C3a, C4a and C5a have that profound vascular affect, and C3a and C5a can also activate mast cells, but they mostly recruit more cells to the area, as you can see on pg. 15 slide 1 there is space between the epithelial cells and the surrounding tissue is red and swollen. Remember that serum sickness is IgG not IgE and if it happens intravascularly then among other things you can get intravascular clots, if these are small complexes that are circulated in the blood they get filtered out at the kidney and you can get kidney damage, where you get fixation of complement in the kidney. If you inject the foreign antibody it is cleared and the antibody complexes happen as it is cleared, you get vasculitis, nephritis, fever, and you get more antibody against the protein so that the next time the response is more rapid and worse.
Type IV hypersensitivities are often thought of in terms of poison ivy or poison oak. There are a number of sensitizing chemicals that people in the laboratory or the chemical industry get exposed to, there are small metal ions that people can be exposed to, and some people can even have type IV copper sensitivities. If the reaction is really bad you can get blistering vesicles. Gliadin is one of the antigens involved in celiac disease, insect venom can also cause type IV hypersensitivities.
Pg. 16 slide 2 shows what happens in a tuberculin reaction. Tuberculin protein is presented to the T-cell and all you need is one T-cell, it makes all the cytokines and chemokines and this results in the response. The response takes 24-48 hours to occur after the injection, whereas some of the other responses occur within minutes or hours after exposure. Immediate hypersensitivities, and delayed hypersensitivities are the T-cell responses.
T-cells make chemokines and cytokines. The chemokines recruit cells. IFNγ causes maturation of many cells and they start expressing adhesion molecules, it activate macrophages and endothelial cells and APCs and then these activated cells release mediators. TNF causes local tissue destruction, expression of adhesion molecules, recruitment of cells. IL-3 and GMCSF signal the bone marrow to make more cells. The T-cells are also to kill the APC and shut down the reaction.
Contact sensitizers like dinitrophenol and trinitrophenol, copper, chromium and nickel, which bind onto proteins and activate langerhaans cells in the area which in turn activate T-cells and cause the huge reaction. This is basically the same as the reaction to tuberculin except that this is triggered by a metal or a sensitizing molecules binding a self protein.
For poison ivy, calamine lotion doesn’t do much at all. Steroid help because they prevent future cells from infiltrating, you still have the cells that already got there but no new cells will arrive. Ice helps by cooling it down, it also diminishes the size of the blood vessels. For the pain Tylenol helps. Local steroids are the best things for type IV hypersensitivities. If it’s a really bad case you can get systemic steroids. Let’s say you have a susceptibility to poison ivy, and you are hiking, what do you do? You can put creams on your skin that prevent the poison ivy oil from sensitizing your skin, you can also cover up with clothes and then wash your clothes when you get home.
Autoinflammatory diseases are mostly rare, one that isn’t is Crohn’s disease. This can be caused by a number of mutations including NOD-2. NOD molecules are intracellular sensors for pathogens, if you don’t have NOD-2 or you have an alternative form with a mutation, then the host doesn’t respond properly to normal bacterial colonization of the bowel. Individuals can get granulomas and disease in their bowel and it can perforate which results in the leakage of the commensal bacteria into the peritoneum. Persons who suffer from Crohn’s have tremendous pain and they don’t digest food properly. To diminish the inflammation you can use remicaid to neutralize TNF, because this is a T-cell response. Most of the other autoinflammatory diseases are really rare. In Crohn’s you can see tremendous cellular inflammation, and many of the cells are damaged by T-cell attacks.