Immunopathology II: Autoimmune Diseases pg. 7
Haleigh Stidham
1. There are several different ways that we think about diseases, especially the broad category of how immune mechanisms can cause or be associated with a particular disease. One way to categorize that is the classical hypersensitivity mechanisms in which you focus on the immune process and what kind of immune interaction that is going on that is associated with the disease.
The fundamental way to think about it is what antigen is driving that response. If you know what antigen is driving that response, it gives you a much better insight into what I can do about it. If it’s an infection, you know the bug and can give an antibiotic to kill that infection. If it’s an environmental agent, like someone with asthma, you can figure out what in the environment is causing this and get rid of that. It’s a lot more effective than just treating the end-stage symptoms of the problem. You have to know what is driving it, rather than what is the mechanism.
In a situation where there is an occult infection, you know there is an infection but you don’t know what the bug is. It could be that it is an immune response from the host that is not tolerant of its own tissues. Depending on which tissue that is, you can get rid of it. Until you know what the antigen is, you can’t decide that it’s an auto-antigen. Several years ago, they discovered Helicobacter pylori was the cause of stomach ulcers. What we do know is that we can tell the difference if it is localized to a particular organ or not, versus non-organ specific.
2. This is an early classification scheme. Generally if it is organ specific, then the immune response is antigen specific. You have an immune response to that organ specific antigen. There can be overlaps in terms of clinical syndromes. An example is blood vessels which are in all organs. Blood vessels are all over, so you will have a systemic response. Non-organ specific diseases operate by immune complex disposition. Immune complexes deposit in specific places. Those diseases are small vessel vasculitis, glomerulonephritis, and in the synovium of joints. Diseases like Lupus seem to be a basic defect in the B cells that produce antibodies.
3. This is a diagram of the way T cells work and the concept of tolerance. Basically, you tolerate your own tissues. Whatever is present when the lymphocytes grow up, then that is a working definition of tolerance. It’s not necessarily something that is genetically a part of that animal because T and B cells can adapt to it. It is a failure of tolerance that is the cause of an autoimmune disease.
4. There are experimental insights as to how tolerance can be broken and result in an immune response to the system you are looking at. One is that if you covalently link or conjugate a self-epitope to another molecule that is immunogenic, then when the person responds to the immunogenic epitope, the immune system gets mixed up and starts reacting to its self-epitope. An example is that if you immunize a rabbit with a cow’s thyroid gland, some of the molecules between the rabbit and the cow are the same and some are different. The rabbit responds to the epitopes of the cow and activates T cells which haven’t been touched experimentally. The normal mechanisms that would prevent this self attack, get messed up and you start reacting to a self-epitope. This is a theme in how you produce an autoimmune response.
Ankylosing Spondylitis (AS) is associated with a particular Shigella bacterial infection. It produces a toxin that has a similar molecular structure to the joint synovium in the vertebral column. Shigella is an intestinal infection, and you get diarrhea. The immune response responds to the bacteria, and it gets immunized to a response in your own body. The result is arthritis and inflammation in the joint. Over a period of time it can destroy the joint. The idea is that the infection ‘ticks off’ the system, and then it induces the autoimmune response. You don’t have to directly couple the epitope. The Shigella toxin and AS are not directly linked to the synovial component. If you mix an antigen called acetylcholine receptor into an adjuvant made up of oil and mycobacterium to make an emulsion, and then you inject it all together. You can get an immune response to the adjuvant and get an immune response to the acetylcholine. Even though acetylcholine is not a foreign protein, it was mixed up in this strong immune response and can go on to cause an autoimmune disease.
Another example of how we think tolerance might be broken is by the exposure of “hidden” antigens. Meaning, things that the immune system doesn’t normally see. An example is injury/trauma to the testes. The body normally doesn’t see the testes because they are sequestered from the blood, and they aren’t tolerant. After injury, both testes will actually get an immune response and an inflammatory lesion because the T cells normally don’t see the testes. This also occurs in the eye called Sympathetic Ophthalmitis in which you get an injury to one eye and the other eye can get uveitis and other inflammatory situations. This happens because you release an antigen and the immune response crosses over and finds that antigen in a different spot.
Finally, there is a number of different genetic patterns of responsiveness. Historically, the big site of genetic lesions has been the Major Histocompatability Complex (HLA antigens in humans). Certain people that have a certain genetic makeup, for instance, HLA-B28 is highly associated with AS. If you have HLA-B28, you are 90 times more likely to get AS if you are infected with that particular Shigella. You can even put that protein in a mouse, and infect the mouse with the Shigella bacteria. Then even the mouse can get AS. There is something about the molecular shape of that HLA-B28 that makes the immune system easily confused in terms of specificity.
5. Infection and tissue damage activates the innate immune response and that activates other inflammatory mediators. That inflammatory milieu can lead to an overreaction. Another example of genetic diseases are SNPs (pronounced “snips”), or small genetic changes that occur all over the genome. The TNF-alpha gene has these SNPs. If you have that particular genetic version of TNF-alpha, then you are more likely to get Rheumatoid Arthritis (RA). It is thought that TNF is an inflammatory mediator, and you can tick it off and create an autoimmune disease. If you don’t have that genetic makeup of TNF, then you are much less likely to set off an autoimmune disease.
6. To induce an organ specific autoimmune disease, you need some kind of local inciting response. It could be an infection, a hypersensitivity to a drug, or trauma. This results in an immune response. The second issue is the malfunction of the normal mechanism that regulates inflammatory lesions. After the malfunction of immune regulation, then the immune response continues and you make the disease. If you are looking at a very rugged tissue type, it may take a long time before you actually get a symptomatic disease. If it is a tissue type that is very sensitive, there are few cells and don’t regrow, then it is fairly soon that the disease shows up. The disease results in tissue destruction and loss of function of the organ that was attacked.
7. Don’t memorize all these names…I wanted to give you an idea that there are a lot of different organ specific autoimmune diseases. If you pick an organ, there are the same pathophysiological events occurring that result in damage and loss of function of that tissue. Some are reasonably common like juvenile diabetes. An uncommon one is idiopathic Addison’s Disease. John F. Kennedy had this disease, which is an immune attack against the cortical cells in the adrenal glands, and this prevents the correct amount of gluccocorticoids to be produced and end up with hormonal problems.
8. Multiple Sclerosis (MS). This is an immune destruction of the myelin in the CNS. The basic function of myelin is like insulation of the nerves in your brain. If you destroy this insulation, then it is easy to get cross over and causes all kinds of problems. The histopathology depends on what stage the lesion is in. A viral infection usually precedes MS. Children who go to daycare and are exposed to a lot of childhood infections seem to be more resistant to MS. Children who stay at home and never get infected, as adults seem to be more susceptible to MS because their immune system isn’t a good.
A very well studied animal model called EAE that stands for Experimental Allergic Encephalomyelitis shows was Vircal (spelling?) the pathologist saw….
9. Named Multiple Sclerosis because sclerosis means “scar.” So there are multiple scars within the central nervous system. So if someone has MS for a long time and you look at their spinal cord at autopsy, it will look like this. It’s stained with myelin, and that’s the dark color. A normal person’s spinal cord will look like this (dark colored the whole way down). Lesions from MS have destroyed the myelin, and it’s a lighter color on the stain. MS skips around the spinal column.
10. This is a higher power staining of the myelin fibers. Myelin deficient areas (light colored areas) are where there was an immune response and the myelin was destroyed. The immune response burned out after the myelin was all gone in that one spot and you are left with a scar. But then those T cells go to another area and destroy new myelin and that causes a new scar.
11. This is the brain of a person who had MS. This little spot (dark spot on the right side of brain near the midline in the white matter) would have no myelin in it if it was stained. But the other white matter around it is myelin and will stain. So the little spot is not grey matter, it’s a place where there is an inflammatory lesion and that’s called a plaque. It will turn into a scar over time where there is no myelin. Whatever nerve transmission is trying to get through the scar, it will be faulty.
12. If you do an MRI, this is the pattern you get. The white spots are plaques. Like popcorn kernels going off in the brain. You can do several MRIs over time and in one place it will be there, and the next time it will be gone and there will be a new one some place else in the brain. So over time, these lesions get initiated; they grow up; they burn out the myelin in that one spot, and the whole thing caves in on itself and you are left with a scar. Some people have rapidly progressing MS, or it can go two to three years without any symptoms at all. No one knows why it appears in one spot or another. Symptomatically, it migrates in terms of neurological space. One time your left arm will hurt, the next time you lose bladder control, next time you have cold sensations on one side of the body, lose bowel control, or you can’t walk. Therefore, the course of MS is highly variable.
13. If you take Myelin Basic Protein (MBP) and add an adjuvant with Mycobacteria and a thick viscous oil, it makes an emulsion. Then inject this emulsion into a mouse. The mice get the disease called EAE. It usually affects high limb paralysis, the primary symptoms. There is a big inflammatory lesion in the spinal column. It doesn’t have the small focal lesions in the cerebrum like in MS.
If you take T cells from mice that have that lesion from the spleen and isolate them in vitro, you can inject it into another mouse that wasn’t immunized with MBP. Then the new mouse will get the same disease. Adaptive transfer, where you take the immune component and put it in a different animal and reproduce the disease, is a very convincing experiment that caused the disease. Mice immunized like this get antibodies to MBP. If you isolate these antibodies and inject it into another mouse, nothing happens. The mouse has the antibodies but you don’t get lesions and you don’t get paralysis. Those antibodies might be helpful in the diagnosis, but in that situation, it doesn’t cause the disease. This is a hard experiment to do in people, so we presume from these animal models that this is what is going on.
14. Louis Pasteur did an experiment when he was trying to make the rabies vaccine. He injected the foam from a dog or rat that had rabies into the spinal cord of a rabbit. He knew rabies was a neurological disease, and if he put it into a rabbit, the rabbit would grow the virus. He took the infected rabbit and was going make a vaccine from it. He killed the antigen with heat pasteurization, and used that antigen to immunize people to prevent them from getting rabies. Turn out, he gave people EAE. People actually got an immune response to the MBP, just like in the mouse experiment. The same thing happened in both humans and mice. This is a picture of a mouse that is dragging himself around because his hind limbs are paralyzed with EAE.
15. A second example is Insulin Dependent Diabetes Mellitus (DM), which is an immune destruction to all the beta cells in the Islets of Langerhan. There aren’t very many Beta cells in your body, and all they do is make insulin. They don’t regrow, so if you damage these cells, they won’t grow back. So if you kill these cells off, you are deficient in insulin. You can take insulin, but you have to time it just right. It is a real serious disease to be insulin-deficient. It took a long time to figure that the destruction of the beta cells was actually an immune response. At autopsies, there are no islets. We didn’t find out it was an autoimmune disease until a person who was just diagnosed with DM was in a car wreck. The autopsy showed that there was lymphocytic infiltrates in all of the islets. They finally figured out that DM is caused by immune destruction in these beta cells.
16. NOD stands for “Non-obese diabetic.” This is a normal islet, (left pic) and it has all these pink, puffy beta cells ready to make insulin. In this lesion (right pic) called “insulitis,” there is a lymphocytic activated macrophage infiltrate in the islet that is destroying the beta cells. In NOD, this happens spontaneously. Because the target is very fragile, you have a deficiency disease right away.