BioEd Online

Being infected with parasitic worms might not sound all that desirable. But it seems that worms may protect against asthma and other allergies by cranking up the production of cells that calm the immune system.

Parasitic worms infect some 2 billion people across the world. For many, the worms cause no obvious symptoms, but others suffer from dysentery, stunted growth and malnutrition. Now it seems there might be a good side.

Regions of the world with high rates of parasite infection also have low rates of asthma, points out Rick Maizels of the University of Edinburgh, UK, who led the study. Previous research in Gabon, for example, has shown that children infected with worms have a reduced allergic response (see ' The worm has turned').

Researchers had suspected that this effect was brought about by increased production of calming regulatory T cells. But no one had proven the link. "This is the paper we've been waiting for," says Thomas Wynn, a parasitologist at the US National Institute of Allergy and Infectious Disease near Washington DC.

Diet of worms

Maizels and his team pinned down the issue by studying mice infected with Heligmosomoides polygyrus, a 5-millimetre worm that wraps itself around tiny projections in the gut called villi, through which the intestine absorbs nutrients.

The infected mice increased their production of regulatory T cells. And when the researchers transferred these regulatory T cells to uninfected mice, the cells migrated to the lungs and protected the animals from an asthma-like response to dust-mites¹.

The work, which appears in The Journal of Experimental Medicine, could lead to compounds that mimic the protective effect of parasitic worms without making people ill. "The parasite is obviously producing some molecule or molecules that could be therapeutically very valuable," says Maizels.

Happy together

The findings also reveal the exquisitely delicate control that parasites exert on their hosts. Maizels says parasites have evolved to damp the immune system to make a safe home for themselves. Without such protection, the host would kill the worms.

Not only have parasites adapted to humans, we might also have adapted to them. "The immune system has probably evolved to take into account downmodulation by parasites," says Maizels.

Without parasites, some people might be overly vulnerable to immune problems, so good hygiene may be partly responsible for the spike in asthma in developed countries.