Asthma Nexus

Ever since human chitinase was discovered in the 1990s, its presence has been somewhat of an enigma. Chitinase breaks down chitin--the stuff of insect exoskeletons and parasite cell walls. Yet mammals themselves make no chitin.

Now, researchers at Yale University and McGill University, in Quebec, have discovered that one human chitinase (acidic mammalian chitinase) is a key player in the acute inflammation pathway of asthma [Science, 304, 1678 (2004)].

The group, led by Jack A. Elias at Yale, first recognized a strong similarity between the classic antiparasitic response of prokaryotes and simple eukaryotes and the exaggerated inflammation reaction in humans (called the Th2 response) that leads to asthma.

Because chitinase breaks down chitin in parasite cell walls, it is highly involved in the antiparasitic reaction, "so we thought chitinase might also be upregulated in human asthma," says Qutayba Hamid, a specialist in asthma at McGill.

The group induced a Th2 response in mice and, indeed, saw levels of acidic mammalian chitinase rise. They also compared lung tissues of human asthma sufferers with normal patients. Only asthma patients had high levels of chitinase in their epithelial and macrophage cells.

Further, one of the symptoms of acute asthma is airway acidification, and acidic mammalian chitinase functions best at pH 2.3. Elias' group suspects that current steroid therapy works partly because it raises the pH of the airways. More generally, they say, any drug that hampers the activity of acidic mammalian chitinase will likely lead to milder inflammation in asthma sufferers.

The fact that some asthma attacks are triggered by exposure to house dust mites (and thus their chitin) may also be related to chitinase activity, Hamid says. But experiments haven't looked at that yet.