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For the Good of the Gut: Can Parasitic Worms Treat Autoimmune Diseases?

Helminths could suppress immune disorders by promoting healthy mucus production in the intestine
human-whipworm-eggs



Kimberley Evason, UCSF

In 2007, parasite immunologist P'ng Loke sat down for lunch at a University of California, San Francisco, cafeteria with an inquisitive man who had called him earlier that week. Their chosen topic of conversation would deprive many people of an appetite, but the scientist and his guest shared an intellectual hunger for a stomach-churning subject: gut worms—specifically, tiny worm-like parasitic organisms called helminths that live nestled in the gastrointestinal tracts of their hosts.

Loke was fully prepared to answer the man's questions about the parasites he knew so well, but what he did not realize was that his companion had more than just questions—he had worms burrowed in his intestinal walls, worms he had deliberately swallowed. Together, Loke and the worm-wrangler embarked on a research project, the results of which appear today in the December 2010 issue of Science Translational Medicine.

The 35-year-old man who had lunch with Loke was quite healthy in 2007. But only a few years earlier he was in the throes of an inflammatory bowel disease known as ulcerative colitis. An autoimmune disease, ulcerative colitis inflames the colon and leaves it rife with open sores; patients experience intense abdominal pain, vomiting, diarrhea, rectal bleeding and weight loss. While searching for treatments, the man discovered the work of Joel Weinstock, a gastroenterologist, parasitologist and immunologist at Tufts University who has pioneered research on helminthic therapy—treating autoimmune diseases by deliberately infesting patients with parasitic worms, such as whipworm and hookworm.

The results of Loke's new case study—the most recent of only five studies that investigate helminthic therapy in people instead of animals—suggest that helminths may ease the symptoms of autoimmune diseases by increasing mucus production.

"It's a unique study—there's nothing like it before," says Weinstock, who was not involved in the new research. "In this case they had a very unique patient—one who was self-infecting with helminths." Clinical trials on helminthic therapy are particularly difficult to arrange because helminths are live pathogens and have not been officially approved as therapeutic agents by any governmental agency, although the U.S. Food and Drug Administration has granted pig whipworm (Trichuris suis) the status of Investigational New Drug. In contrast to human whipworm (Trichuris trichiura), the porcine variety cannot survive inside the human gut for very long.

"The researchers noticed a specific pattern of behavior, cycling between remission and active disease depending on when the patient infected himself with helminths," Weinstock adds. "This is not a double-blind study, but the pattern is highly suggestive that the worms helped this patient. The major point of this paper is the potential mechanism—mucus production—which has not been looked at properly before."

The Might of Mucus

In the new study, Loke—who is now with New York University—analyzed the man's medical records prior to 2007 and personally tracked the man's health from 2007 onwards. In 2004 the man swallowed a vial of salty liquid brimming with 500 human whipworm eggs, which he obtained from a parasitologist in Thailand. Three months later, he slurped down another 1,000 eggs. The larvae hatched and matured within his gastrointestinal tract, burying their heads in the intestinal wall. By mid-2005, he was virtually symptom free and required no medical treatment for his colitis, except occasional anti-inflammatory drugs to suppress flare-ups. The nearly complete dismissal of colitis symptoms is especially striking because human whipworm infection can itself cause digestive problems, including diarrhea, abdominal pain, nausea, vomiting and, in extreme cases, rectal prolapse. Severe infections can also cause anemia and stunt the growth of children.

In 2008, the number of whipworm eggs in the man's stool began to dwindle, dropping from more than 15,000 per gram to fewer than 7,000 per gram. As the eggs disappeared, the symptoms of colitis returned. So the man infected himself with another 2,000 whipworm eggs and, a few months later, his symptoms practically vanished once again. Repeated colonoscopies revealed that wherever worms colonized his colon, the symptoms of colitis were significantly reduced or nonexistent.

During the 2008 relapse, the researchers found that immune cells in tissues with active colitis produced large quantities of an inflammatory signaling molecule named interluekin-17 (IL-17), but very little IL-22, the latter of which has been linked to wound healing and mucus production. When worms recolonized the colon, however, immune cells began manufacturing much more IL-22. Blood profiling and genetic analysis further revealed that tissues in which helminths thrived increased carbohydrate metabolism—a prerequisite for mucus production.

"Ulcerative colitis is often associated with decreased mucus production and the worms seem to somehow restore mucus production, possibly by inducing a population of immune cells that make IL-22," Loke says. "It's possible the mucus serves as a defensive barrier between bacteria and the gut that prevents bacteria from causing inflammation and crossing over into other tissues." Autoimmune diseases generally occur when the immune system overreacts to benign—and even beneficial—organisms living within the body. In the case of colitis, researchers suspect the reaction is directed toward the bacteria in the gut. Loke thinks that the human body may boost mucus production when it detects helminths as a defense against the parasites; for a patient with ulcerative colitis, the extra mucus may also help calm an excessively aggressive immune system.

"We saw an association with remission and immune cells that make IL-22, but we don't know for sure if these immune cells are actually induced by worms," Loke says. "You can't tell with a sample size of one," which is especially susceptible to the placebo effect. Still, Loke adds, "the results seems quite compelling, especially when you consider the background—all the animal studies and clinical trials that show worms can suppress colitis and other autoimmune disorders."

Mounting Evidence

In fact, in numerous animal studies, helminth infestation has protected rodents against colitis, asthma, rheumatoid arthritis, food allergies and type 1 diabetes.

Researchers have conducted few human studies, but most have shown promise. In a clinical trial published in 2005 in the journal Gut, Weinstock asked 29 participants with Crohn's disease (another autoimmune inflammatory bowel condition) to ingest 2,500 pig whipworm eggs every three weeks for six months. Twenty-three patients (79.3 percent) improved significantly, and 21 (72.4 percent) experienced remission. Both the researchers and participants, however, knew exactly what treatment they were receiving, which makes excluding a placebo effect impossible.

In a controlled clinical trial published in 2005 in Gastroenterology, Weinstock and his colleagues gave 52 participants with colitis 2,500 pig whipworm eggs or a placebo every two weeks for three months. Thirteen of the 29 patients (44.8 percent) who received whipworm eggs improved, compared with only four of the 23 participants (17.4 percent) who received the placebo.

Weinstock and his collaborators point to these trials as experimental evidence that fits a global pattern: immune disorders are much rarer in less developed countries where helminthic infestation is widespread than in industrialized countries where much smaller populations host helminths. The "old friends hypothesis" proposes that the human immune system cannot learn to regulate itself without exposure to common pathogens like helminths that have coevolved with people and that modern hygienic practices deprive people of this necessary exposure, possibly explaining the relatively higher and more recent prevalence of immune diseases in industrialized countries like the U.S.

Loke plans to continue researching helminthic therapy in people and in monkeys. "We are talking about doing a small trial of, say, 10 people and basically doing colonoscopies on them before and after giving them pig whipworm," he says. Loke also mentions that colitis plagues many juvenile monkeys in primate research centers and that he has received a pilot grant to treat diseased monkeys with human whipworm, an as-yet-unpublished experiment that is already returning promising results.

"When I first sat down to lunch with the guy who called me and he started telling me his story, I was really quite skeptical," Loke recalls. "But now I am completely changing my mind about helminthic therapy."

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