Monday, November 6, 2006

Asthma – the Chitin hypothesis

drj writes "Chitin is a polysaccharide that forms fungal cell walls and the exoskeleton of insects and crustacea. Some 'lower' organisms use enzymes that degrade chitin, chitinases, for defense. Chitinases are also found in mammals but their function is unknown. Here, Zhu et al. show that acidic mammalian chitinase (AMCase) accumulates in the lung epithelium of human asthmatics and in mice treated to induce asthma. In mice, the accumulation depended on "type 2" (allergic) helper T lymphocytes and their characteristic cytokines, IL-4 and IL-13. Most promising was the reduction in inflammation caused by blocking AMCase with specific antibodies, suggesting it is a good target for therapy. The authors discuss the potential links between chitin-containing insects, hygiene, and asthma."
PubMed Acidic Mammalian Chitinase in Asthmatic Th2 Inflammation and IL-13 Pathway Activation Zhou Zhu, Tao Zheng, Robert J. Homer, Yoon-Keun Kim, Ning Yuan Chen, Lauren Cohn, Qutayba Hamid, Jack A. EliasScience304: 1678-82 Zhu et al., Science 304:1678-82, April 2004


Anonymous said...

Hygiene logic?
The observation that chitin stimulates Th1 responses [article refs 7 and 15] would seem to suggest that exposure to chitin-containing insects would increase Th1 responses, reducing asthma, which is a Th2-mediated disease. Instead, early exposure to cockroaches is associated with increased asthma in children (Children's Health Study []). As these authors suggest, chitinases might reduce the Th1 bias and favor the Th2 response, but this is a secondary effect. If chitin stimulates Th1 immune responses then shouldn’t cockroach exposure decrease the incidence of asthma?

Anonymous said...

Re:Hygiene logic?
Cockroach exposure in the young, as opposed to an adult, might favor a Th2 response. Immunization of neonatal mice in the absence of strong Th1-inducing agents such as DNA vaccines or CPG motifs generally results in a Th2-skewed response PubMed Abstract []