By identifying risk factors for autoimmune disease, researchers hope to predict and even prevent its onset
Wendy Brown feels good, but she's worried it won't last. Two years ago, Brown felt herself losing strength. She had trouble lifting things and getting up from a seated position, and when she pushed herself, her muscles groaned in pain. Those problems came to a head in July 2001, when she traveled from her home in Palm Coast, Fla., for a family vacation in Hungry Horse, Mont. a small mountain town.
One day, at a picturesque spot in the town, Brown's relatives piled into an old stagecoach for a family photo.
"My mother was 60 and she just climbed right in, but I didn't have the strength to pull myself up. That's when I knew something was definitely wrong," says Brown, who was 31 at the time. Within two months of returning from the trip, she needed a wheelchair to get around.
Doctors told her she had polymyositis (PM), an autoimmune disease that weakens the torso, upper arms and thighs. Autoimmune diseases occur when the immune system attacks the body's own tissues, and in PM, that attack is directed against muscle.
|Wendy Brown (lower center) was the only family member who couldn't climb aboard this stagecoach in 2001. She has polymyositis.|
PM, like many other autoimmune diseases, usually requires lifelong treatment with drugs that suppress the immune system. But remarkably, after just 15 months on prednisone (a powerful immunosuppressant), Brown's PM went into remission. She's been without symptoms and without need for treatment since January, and she hopes to stay that way.
But no one knows whether Brown is in the clear, because no one knows what causes PM or most other autoimmune diseases.
The need for answers is urgent, for Brown and millions of others. Although PM is rare, as a group, autoimmune diseases affect between 5 and 8 percent of the U.S. population. More common diseases in that group include rheumatoid arthritis and type 1 diabetes. Less common, lesser-known ones include PM and other neuromuscular diseases covered by MDA: dermatomyositis (DM), inclusion-body myositis (IBM), myasthenia gravis (MG), Lambert-Eaton myasthenic syndrome (LEMS) and, often, the thyroid-related muscle disorders. (See chart.)
Although each of these diseases is different on the surface, researchers are learning that they share common risk factors. By identifying those risk factors, they hope one day to predict who will get an autoimmune disease and who stands the best chance of permanent remission (which, so far, has proved rare).
Nature vs. nurture
One clue to the shared origins of different autoimmune diseases has come from studies of families.
Autoimmune diseases aren't inherited along predictable lines as are, for example, the muscular dystrophies. However, the genes that determine how the immune system is likely to behave are thought to affect the chance of developing an autoimmune disease.
"It's common to see certain families where there are multiple members with different autoimmune diseases," says Frederick Miller, a rheumatologist (specialist in inflammatory diseases of the muscles and joints) and an expert on autoimmune diseases at the National Institute of Environmental Health Sciences (NIEHS), a branch of the National Institutes of Health in Bethesda, Md.
"We've done a study in patients with PM or DM, and found that first-degree family members [parents, children and siblings] had an increased risk of having an autoimmune disease but it was unlikely to be myositis," he says. The relatives were more likely to have more common autoimmune diseases, such as arthritis or diabetes.
1. A bacterium invades the body, displaying proteins on its surface. The immune system sees these as antigens, requiring defensive action.
2. An antigen-presenting cell (APC) of the immune system engulfs the bacterium.
3. The bacterium begins to break up inside the APC, and its antigens, nestled inside a major histocompatibility complex (MHC), are displayed on the cells surface like a flag.
4. T-cells "see" the displayed antigen and begin secreting cytokines, which help antigen-presenting cells destroy the bacteria they've engulfed. They also send signals to B-cells, instructing them to start making antibodies to the displayed antigens.
The exact risk for people who have relatives with autoimmune diseases isn't clear, but (unsurprisingly) the closer the relative, the higher the risk. Miller estimates that for identical twins who share 100 percent of their genes when one has an autoimmune disease, the other has a 20 percent to 40 percent chance of getting the same disease.
Clearly, genes play a role in autoimmune disease, and the same genes probably are involved in different autoimmune diseases. But other factors are at work, too.
"The concept is that autoimmune diseases share a number of common genetic risk factors," Miller says. "Those factors might get you into the theater of autoimmune disease, and then other genes and environmental factors may usher you to your particular seat in that theater."
For example, subtle variations in genes that control the functions of the immune system might cause susceptibility to an autoimmune disease, and a "hit" from the environment such as an infection might provide the final trigger.
To understand how the immune system goes astray in autoimmune disease, it's important to know a few things about its normal activity.
When infectious diseases attack our bodies, our immune systems defend us by mustering a cellular army, in which the essential soldiers are T-cells and B-cells. Collectively known as lymphocytes, both cell types are born in the bone marrow, but T-cells mature in the thymus, a gland located just below the throat.
Both cell types travel through the bloodstream, watching for viruses, bacteria and other disease-causing "germs," but they attack by different means. T-cells act as the mobile infantry and communications network for the army, and B-cells provide the long-range artillery.
T-cells use a protein on their surface, called the T-cell receptor, to latch onto foreign antigens, which are (usually) proteins or sugars that belong to the invading germ. Attachment of the receptor to the antigen stimulates the T-cell to attack by releasing cytokines distress signals that call B-cells and other immune cells to the fray. Once they get the signal from T-cells, B-cells begin releasing antibodies, proteins that stick to the antigen and either mark its bearer for destruction or attract additional troops.
In autoimmune diseases, "The main problem is either antibodies or T-cells that are directed against a self-antigen [a protein that belongs to cells in the body]," says Premkumar Christadoss, an immunologist and MDA grantee who studies MG at the University of Texas in Galveston. "During this process, cytokines are released, and they further contribute to the problem," he says.
This isnt a matter of good immune cells turning bad. Our immune systems don't come equipped with T-cells or B-cells designed to recognize specific antigens. Instead, they make a greater diversity of immune cells than we really need: Random gene rearrangements inside B-cells and T-cells create billions of distinct antibodies and T-cell receptors with the potential to match an equal number of antigens.
Cells that encounter and recognize foreign antigens multiply rapidly, mount an attack, and retain a "memory" of the encounter. Cells that react to self-antigens normally kill themselves or become inactive but sometimes they survive and cause trouble.
The self-antigen or antigens are known in some, but not all, autoimmune diseases. (See chart.)
For example, MG occurs when the immune system targets proteins essential to the connection between nerve cells and muscle cells. Normally, the nerve cell releases a chemical called acetylcholine (ACh), which stimulates muscle contraction by attaching to a docking site on the muscle cell, called the ACh receptor. Some 85 percent of people with MG have antibodies to the ACh receptor detectable in their blood, and a small fraction have antibodies to MuSK a protein that helps organize ACh receptors on the muscle cell surface. (For more about MG, see Managing Myasthenia, May-June 2003.)
Many genes carry the instructions for making T-cell receptors, antibodies and other components of the immune system, and all of them have the potential to influence who gets autoimmune diseases. But the genes behind a set of proteins called the major histocompatibility complex (MHC) appear to be especially important.
|Molecular mimicry is the theory that some foreign antigens might have a similar appearance to self-antigens. T-cells or B-cells that attack the foreign antigen might be provoked by the self-antigen, too.|
T-cells actually can't "see" antigens without help from these proteins, which exist in two flavors. MHC type 1 is found on the surface of most cells in the body, and MHC type 2 is found only on the surface of antigen-presenting cells.
When a cell gets infected with a virus, it sheds the infectious bugs antigens and directs them to T-cells using MHC type 1. The MHC holds out the antigen like a hand waving a red flag, waiting for a T-cell to respond. This is a signal that says, "Kill me before I spread the infection!"
Antigen-presenting cells actually ingest foreign antigens, such as those from bacteria, and spit them out into MHC type 2 a signal that puts T-cells on the alert and causes them to release cytokines.
MHC proteins themselves are important antigens sometimes called human leukocyte antigens (HLA) because the genes responsible for making them vary in their exact "code" from one person to the next. MHC differences mark our cells as our own (creating the possibility of rejection during an organ transplant), ensure that we're not all wiped out by a single nasty germ, and apparently leave some of us vulnerable to autoimmune disease.
Some MHCs, Christadoss explains, probably have a peculiar way of displaying antigens that makes them more likely to trigger reactions to self-antigens. One MHC variant, called HLA-DR3, "imparts susceptibility to a lot of autoimmune diseases," including PM, DM and MG, he says.
Several other genes also appear to influence susceptibility to autoimmune disease. For example, certain variants of the gene encoding cytotoxic-T-lymphocyte-associated protein 4 (CTLA-4) have been associated with type 1 diabetes and thyroid disease. CTLA-4 sits on the surface of T-cells, and normally decreases their responses to antigens.
Infections, injuries and interlopers
When someone has a genetic makeup compatible with autoimmune disease, what's the environmental trigger that pushes him or her over the edge? Unfortunately, researchers dont have a specific answer in most cases, but they have a few theories that apply to all autoimmune diseases.
One theory is molecular mimicry. This is the idea that, to T-cells and B-cells, certain viral or bacterial antigens might appear similar to self-antigens. Once the cells react to the foreign antigen, they might indiscriminately attack the self-antigen, too. There's especially good evidence for this phenomenon in Guillain-Barré syndrome, a nerve disease caused by antibodies that cross-react to certain sugars found on nerve cells and on the bacterium Campylobacter jejuni.
Another idea concerns hidden antigens antigens found only in tissues that are under limited surveillance by the immune system, such as the eyes and brain or the inside of a muscle cell.
"An injury or infection [of those tissues] could release a hidden antigen, and the immune system having never seen it before might react to it, thinking its a foreign antigen," Christadoss explains.
Although cancer isn't a part of most autoimmune diseases, it might trigger some, such as LEMS, by presenting previously hidden antigens to the immune system. LEMS is caused by antibodies to a protein that's normally found only in nerve cells. Some 60 percent of LEMS cases are associated with small-cell lung cancer, and in those cases, the cancerous lung cells produce the nerve cell protein. Successful treatment of the cancer usually leads to full recovery from LEMS.
A relatively new theory suggests that mixing of maternal and fetal cells during pregnancy might trigger some autoimmune diseases.
Ann Reed, a pediatric immunologist and MDA grantee at the Mayo Clinic in Rochester, Minn., has found evidence for this phenomenon called microchimerism in juvenile DM. (In Greek mythology, a chimera is a monster made of different animal body parts; in medicine, chimerism refers to the mixture of donor and recipient cells that persists after a bone marrow transplant.)
In work supported by MDA, Reed says, "We started looking at children with DM and found that theyre more likely to have maternal cells present in their blood than are controls [siblings or unrelated children who dont have DM]." She believes the maternal cells might be immature lymphocytes that have lain dormant for some time, but eventually react to the child's tissue.
The reaction can occur in the opposite direction as well. Fetal cells can persist in a woman's blood for up to 30 years after a pregnancy, and researchers have found that the cells are more likely to persist in women with the autoimmune disease scleroderma than in controls. Reed is planning a new study to look for persistence of fetal cells in women with adult-onset DM.
Miller, who heads the newly established Environmental Autoimmunity Group at NIEHS, has begun one of the largest clinical studies ever to examine the combined roles of genetic and environmental risk factors in autoimmune disease.
He's seeking 400 pairs of twins, or siblings who are close in age, one of whom has rheumatoid arthritis, lupus erythematosus, systemic sclerosis, or myositis (PM, DM or IBM) while the other doesn't have an autoimmune disease.
He'll analyze blood samples for variations in the MHC and other genes, evidence of microchimerism, and exposure to various infections and chemicals. He'll also collect information using a questionnaire that asks about medical history and stressful life events. (For more information, see www.clinicaltrials.gov. Travel to NIEHS isn't necessary to participate in the study.)
Miller's research began in April and is expected to last five years. He says the study is designed to ask, "Why did one sibling develop the disease while the other one didn't?"
The answer, he believes, "will not only help us predict the course of autoimmune diseases, but also help us prevent them by screening individuals at risk. That's the ultimate goal, which I suspect is a ways off."
He adds, "A better understanding of mechanisms of the disease could also lead to better treatments." Indeed, recent insights into the causes of autoimmune disease have already led to lab experiments and clinical trials aimed at more effective treatments with fewer side effects. (See "Better Treatments Through Better Targets." )
Wendy Brown and her sister Fran (who doesn't have an autoimmune disease) have already given blood samples and filled out questionnaires for Miller's study.
"I'm participating to help find out why people get [these diseases] and to help find a cure, whether it's in my lifetime or somebody else's," Brown says. "I wouldn't want anybody else to go through what I did."
Using immunosuppressant drugs to treat autoimmune diseases is like using a sledgehammer when a carefully aimed rubber mallet might do the trick.
The drugs, now commonly used in diseases such as DM, PM and MG, offer a lifesaving treatment, but their broadly dampening effects on immune cells leave the body vulnerable to infection. They also can hinder the function of other cell types, causing undesirable and occasionally serious side effects.
For example, prednisone can cause weight gain, cataracts and osteoporosis (bone loss), while methotrexate, a common treatment for PM and DM, can damage the lungs, kidneys or liver if its not used carefully.
As scientists learn more about the mechanisms behind autoimmune diseases, they're beginning to investigate new drugs that specifically target troublemaking immune cells or chemicals, while sparing the rest of the body.
Many of these drugs are actually monoclonal antibodies (mabs) that are custom-made to stick to the cell or chemical and block its effects. (Monoclonal means that the antibodies were all derived from the same type, or "clone," of B-cells.)
One target of these new-generation drugs is tumor necrosis factor (TNF), a cytokine (protein released from immune cells) named for its ability to kill tumor cells that display abnormal antigens. TNF also helps mobilize immune cells and can thus stimulate autoimmune reactions. The protein appears to skyrocket in several autoimmune diseases, including rheumatoid arthritis and myositis.
Pharmaceutical companies began developing drugs to inhibit TNF in the early 1990s, and two TNF blockers are now on the market for rheumatoid arthritis: infliximab (Remicade) is an antibody against TNF, and etanercept (Enbrel) is a free-floating version of the TNF receptor, TNFs docking site on immune cells. It acts as a decoy, keeping TNF away from the cells.
Both drugs are under investigation for myositis Enbrel against DM at the MDA Clinic at St. Josephs Hospital in Phoenix, and Remicade against DM and PM at the National Institute of Arthritis, Musculoskeletal and Skin Diseases (NIAMS) in Bethesda, Md.
Meanwhile, MDA grantee Premkumar Christadoss is collaborating with Matthew Meriggioli, director of the section on neuromuscular disease at Rush University in Chicago, to test the mechanisms of etanercepts action in a small number of MG patients.
"If this pilot study looks promising, we hope to proceed with a larger trial," Meriggioli says. "I believe I have seen evidence for improvement in most patients, but I have not formally analyzed the data yet."
Another promising target is complement, a system of more than 20 proteins in the blood. The complement proteins spring into action when they "see" a cell marked by antibodies, assembling into a killing machine that punches a hole in the cells surface. This reaction appears to spin out of control in some autoimmune diseases, especially DM.
In a small clinical trial completed in 2001, a complement inhibitor called eculizumab appeared to improve the skin rash, but not the muscle weakness, associated with DM. The maker of eculizumab, Alexion Pharmaceuticals, hasn't announced any follow-up trials, but has acquired an "orphan drug" designation, which gives it government incentives to pursue development of the drug for DM.
Meanwhile, Christadoss believes that "eliminating part of the complement pathway might be a way to treat MG." In laboratory experiments, hes found that mice with genetic deficiencies in the complement system are resistant to developing MG.
*Scientists disagree on whether this is an autoimmune disease.