Immunology of Multiple Sclerosis Discussion
Why would the immune system “turn” on patients? I figured I would write a couple of thoughts regarding the cells and mechanisms involved in how the immune system functions. For many of you this may be obvious, but I thought I would review some of the concepts here at a very high level, with the hope of stimulating ideas/discussions.
The immune system shares with the nervous system the features of: memory, self-nonself recognition, specificity, and “knowing” when to respond or not respond based on quantity of stimulus.
For example, the immune system is does not “blindly” attack everything that is foreign. As you know, sometimes the immune system does not even attack just what is foreign but alson turns against the body, such as in autoimmune conditions like multiple sclerosis, or type 1 diabetes, or rheumatoid arthritis, where the immune system attacks myelin basic protein, GAD, and collagen I, respectively.
Usually the immune system selects what it should attack or not attack based on recognition of what is “self” and what is “non-self”. Classically, this process is thought to occur before birth. The main cells of the “adaptive immune system”, the part of the immune system that learns based on exposure, are the T cells and the B cells. The T cells can either be killer T cells (CD8), which recognize other cells that are virally infected or cancerous, and CD4 T cells, which provide chemical signals (cytokines) for recruitment of other immune cells (T helper cells). Of CD4 cells broadly speaking, there are also T helper 1 cells, that contribute to immunity to cancer and viruses, Th2 cells that fight parasitic infections, and Th17 cells that cause inflammation. There are also T suppressor/T regulatory cells, which inhibit ongoing T cell responses. The B cells, on the other hand, are responsible for making antibodies, proteins that bind to bacteria or other cells and inhibit their activity or kill them.
Lets first discuss how the T cells tell “self” from “non-self”. The process is called thymic selection. Each T cell has the capability of making approximately 10(10)-10(15) different types of receptors, which can each recognize different “antigens” or in other words, molecules found on other things. This means that in the thymus T cells are made that can recognize pretty much anything in the universe ! So how is it then then the T cells that are made in the thymus do not attack the body?
What happens is that in the thymus, many of the proteins or chemical compositions that are present in the body, or will be present in the body in the future as the person ages, are actually expressed in the thymus before birth. So T cells that are generated before birth, when they encounter proteins in the thymus, they are instructed to die. This causes the T cells that escape the thymus (because they didn’t recognize “self”) to be able to recognize everything else that is not the body.
So if T cells are deleted that recognize the body, then why do some people get autoimmunity?
There are a couple reasons. Firstly the protein in the thymus that causes expression of every single body protein, called AIRE (stands for “autoimmune regulator”), is sometimes mutated. Patients with a mutation in AIRE do not express all of the body proteins in the thymus and as a result autoreactive T cells enter the body and eventually cause autoimmunity. Mind you, this is a pretty rare cause of autoimmunity. AIRE mutations are found primarily in patients with the disease Autoimmune polyendocrinopathy-candidiasis-ectodermal dystrophy (APECED) which has autoimmunity against tissues that make hormones.
Another cause of autoimmunity is that the T cells that escape from the thymus, although at time of escape do not recognize “self”, upon encounter with a protein that is “non-self” they start to mutate so as to recognize “self”. The ability of the T cell to mutate is critical since many pathogens mutate…so if the T cells could not recognize the mutated pathogens, then they would eventually kill the host.
So why is it then that not everyone gets autoimmunity? One reason is that recognition of proteins by T cells is not enough to stimulate the T cell. The T cell also requires what are called “second signals”. These are signals associated with “Danger”. For example, the immune system intrinsically “knows” that certain things are potentially harmful for the host. Bacteria have unique components that are not found in mammalian cells, such as lipopolysaccharide, which stimulate cells of the body such as dendritic cells, to provide a “second signal” to the T cell. So if a bacteria has a “non-self” protein on it, but is not stimulating the “danger signal”, then the immune response will not be activated. Only if there is: a) a unique nonself protein; and b) a “danger signal”, will be immune system be activated.
One way in which autoimmunity occurs is when there is inflammation at a site in the body that expresses proteins that may be recognized by the “self-reactive” T cells.
Let me provide an animal model example that proves this point. Pamela Ohashi, now at the Ontario Cancer Institute, was the first author of a paper (Ohashi et al. Ablation of “tolerance” and induction of diabetes by virus infection in viral antigen transgenic mice. 1991 Apr 19;65(2):305-17) in which the cellular basis for self-nonself recognition was elucidated.
Ohashi developed two types of mice. The first type expressed a foreign protein called LCMV glycoprotein (GP) selectively in the pancreas. She did this by genetic engineering of the mouse so that the “foreign” protein is present in the mouse from fetal stages all the way to adulthood. How as we discussed before, T cells capable of reacting against GP may theoretically be deleted by AIRE in the thymus. Even if these cells are not deleted, they theoretically would not be able to cause autoimmunity (in this case very easily measurable by insulin levels because the autoantigen is in the pancreas) because there is no “danger signal”. When she examined these mice, they grew old and as expected did not develop autoimmunity.
She then developed a second strain of mice that have T cells that recognize the GP protein. These mice, called T cell receptor (TCR) transgenic mice have a large quantity of their T cells capable of recognizing the GP protein. When T cells from the TCR-trangenic mice are added to pancreatic cells that express GP, the pancreatic cells are destroyed in vitro.
So when she bred the mice, so that the offspring had: a) GP expressed in the pancreas; and b) Large numbers of T cell recognizing GP, the animals did not get diabetes !!
When she examined these mice, she found that because there was no “danger signal”, despite the large number of circulating autoreactive T cells, the mice did not get diabetes.
When she infected the mice with the LCMV virus, which causes a potent “danger signal” then self-tolerance was lost and the animals rapidly developed diabetes.
Perhaps more representative of human autoimmunity are model systems in which administration of an autoantigen is given in the presence of a “danger signal”. For example, if one wants to induce a disease that mimics multiple sclerosis, one takes myelin basic protein, or peptides of it, mixes it with adjuvant (serves as the “danger signal”) and administers it into mice or rats of certain genetic backgrounds. The disease that follows resembles multiple sclerosis in terms of cellular pathology (infiltration of T cells and macrophages in the central nervous system), as well as physically in that it culminates in paralysis. Some animal models of multiple sclerosis actually have a relapse-remitting component, thus further resembling human disease.
The interesting thing is that this sort of “induced” autoimmunity is not specific to multiple sclerosis but can be seen in other conditions. For example, administration of type I collagen to mice on the SJL background together with adjuvant leads to induction of a disease similar to human rheumatoid arthritis, called “collagen induced arthritis”. Even autoimmune cardiomyopathy can be induced (autoimmunity against heart muscle) by injection of myosin peptides together with adjuvant.
So the question is, how could multiple sclerosis actually start in people? and how could we even have a chance of curing it?
Of course the answer is not known. There are some indications that various pathogens may be associated with initiation of disease. Unfortunately, conclusive studies do not seem to support this. One of the problems may be that a lot of pathogens are still not known because they can not be cultured. Various cell wall deficient bacteria may be involved which are difficult to detect, or which cause a “hit and run” in the sense that they induce the autoimmune disease but are subsequently cleared by the immune system.
Some interesting work in this area was conducted at Vanderbilt University in the early 1990s. There was speculation that Chlamydia pneumoniae was involved in multiple sclerosis because it was found in the cerebral spinal fluid of some patients, and other patients had high levels of antibody to it (Sriram et al. Chlamydia pneumoniae infection of the central nervous system in multiple sclerosis. Ann Neurol 1999 Jul;46(1):6-14). So the group of Dr. Sriram actually starting infecting mice with Chlamydia pneumoniae and inducing EAE by immunization with self-antigen. As seen in the figure below, administration of the live bacteria result in acceleration of disease.
So at present it is still not known what causes multiple sclerosis in patients. We do know that there are inflammatory components that appear to be associated with its onset, and also that there are certain genetic factors that contribute.
The next question is, even if we do not know how it starts, what type of treatments could work against it?
The obvious concept is that since MS is an immunologically mediated disease, it would make sense that suppression of the immune system would be therapeutically useful. Indeed, this is how steroids work. Numerous other agents that nonspecifically suppress the immune system are used in the treatment of MS, for example, cyclosporine, azathioprine and methotrexate. These agents have various side effects in part because they knock-down the majority of T cell functions of the immune system.
Another class of agents used in the treatment of MS are biological response modifiers, the first generation being inhibitors of the protein Tumor Necrosis Factor-alpha (TNF-a). Originally TNF was identified independently by Beutler and Cerami. Beutler found that mice injected with the immune stimulant BCG made a protein that could cause necrosis of tumors, whereas Cerami found that mice bearing tumors had a protein that was responsible for weight-loss (cachexia). Eventually it was found that both of them were working with the same protein. Interestingly, while the protein ended up not have profound anti-tumor activity in humans, it was discovered that the protein played a significant role in the various inflammatory conditions that affect humans such as rheumatoid arthritis. Interestingly antibodies to TNF-alpha instead of inhibiting multiple sclerosis actually appeared to accelerate the disease (van Oosten et al. Increased MRI activity and immune activation in two multiple sclerosis patients treated with the monoclonal anti-tumor necrosis factor antibody cA2. Neurology 1996 Dec;47(6):1531-4). This led to the search for other modulators of the immune system.
Currently used biologics or biological response modifiers are interferon beta and glatiramer acetate. Although their mechanism of action is not entirely known even today, they appear to reduce severity of multiple sclerosis. Other agents include antibodies such as natalizumab (Tysabri). These agents do not suppress all aspects of the immune system, so have a better toxicity profile as compared to chemotherapeutics, however, are still relatively nonspecific and have adverse effects. Newer treatments such as anti-CD25 antibodies are also in clinical trials, but still are not antigen-specific.
Antigen-specific approaches towards treatment of multiple sclerosis would specifically involve the neutralization of the T cells that are causing damage to myelin but not other T cells. The level of immune specificity in multiple sclerosis is very high, as explained in this video, that demonstrates only certain T cell clones act as “driver clones” in that they maintain the autoreactive phenotype. One way of killing these autoreactive cells is to use the immune system to kill them by vaccination with expanded autoreactive cells.
Another way is to “trick” the autoreactive cells into believing that they are no longer necessary. For example, the immune system intrinsically knows that if there is too high concentrations of a protein that it is attacking, more often than not, it should not be attacking it. So what happens when the body is overloaded with an autoreactive antigen, the T cells “decide” that they should stop their reactions against it. This is the concept of “high dose” tolerance. An interesting study from the NIH demonstrated that repeated administration of myelin basic protein into mice after induction of experimental allergic encephalomyelitis actually inhibited disease progression (McFarland et al. Amelioration of autoimmune reactions by antigen-induced apoptosis of T cells. Adv Exp Med Biol. 1995;383:157-66).
The concept of adding to the body something that could exacerbate an autoimmune response sounds risky, however, this was successfully used by the Canadian company BioMS in that they have performed clinical trials using peptides from myelin basic protein and administering them intravenously. The company is currently performing pivotal Phase III trials in patients with secondary progressive multiple sclerosis.
Treatment of multiple sclerosis would ideally involve the combination of immune modulation and regeneration. Both of these are important since current approaches do not address the fact that after damage to the central nervous system has occurred, the natural regenerative capacity, while present, is still relatively limited.
We have previously published results of 3 multiple sclerosis patients being administered their own fat derived stem cells, and subsequently having positive responses. This procedure, which was described in a collaborative publication with the University of California San Diego, appears to be working via concurrent immune modulation and induction of regeneration.