Chitin induces accumulation in tissue of innate immune cells associated with allergy

Allergic and parasitic worm immunity is characterized by infiltration of tissues with interleukin (IL)-4- and IL-13-expressing cells, including T-helper-2 cells, eosinophils and basophils(1). Tissue macrophages assume a distinct phenotype, designated alternatively activated macrophages(2). Relatively little is known about the factors that trigger these host responses. Chitin, a widespread environmental biopolymer of N-acetyl-beta-D-glucosamine, provides structural rigidity to fungi, crustaceans, helminths and insects(3). Here, we show that chitin induces the accumulation in tissue of IL-4- expressing innate immune cells, including eosinophils and basophils, when given to mice. Tissue infiltration was unaffected by the absence of Toll-like-receptor-mediated lipopolysaccharide recognition but did not occur if the injected chitin was pre-treated with the IL-4- and IL-13-inducible mammalian chitinase, AMCase(4), or if the chitin was injected into mice that overexpressed AMCase. Chitin mediated alternative macrophage activation in vivo and the production of leukotriene B-4, which was required for optimal immune cell recruitment. Chitin is a recognition element for tissue infiltration by innate cells implicated in allergic and helminth immunity and this process can be negatively regulated by a vertebrate chitinase.