Allergic and parasitic helminth immunity is characterized by infiltration of tissues with IL-4- and IL-13-expressing cells, including Th2 cells, eosinophils and basophils. Additionally, tissue macrophages assume a distinct phenotype, designated alternatively activated macrophages. Relatively little is known regarding the early factors that trigger these host responses, however we do know that these host responses depend on Stat6 signaling downstream of IL-4 and IL-13 receptors.
We identified two mammalian chitinase-like proteins, AMCase and Ym2 as being Stat6-dependent and upregulated during an immune response to the helminth, Nippostrongylus brasiliensis. AMCase is an enzymatically active chitinase that degrades chitin, while Ym2 is a chitinase-like protein that has lost enzymatic activity. In experiments using transgenic mice that overexpressed AMCase or Ym2, as well as antibody blockade experiments we concluded that neither AMCase nor Ym2 were proinflammatory. Thus, we took the approach of exploring the effects of chitin, a carbohydrate ligand of AMCase and Ym2 to ascertain their functions in immunity to chitin-containing pathogens and allergens.
Chitin is the second most abundant polysaccharide in nature, and it confers structural rigidity to fungi, crustaceans, helminths and insects. We found that chitin induced the tissue accumulation of IL-4-expressing innate immune cells, including eosinophils and basophils, when given to mice. Moreover, chitin mediated alternative macrophage activation in vivo and induced leukotriene B4 production, which was required for optimal immune cell recruitment. Importantly, we found that tissue infiltration of effector cells and alternative macrophage activation was abolished when chitin was treated with AMCase, but not Ym2. Therefore, we conclude that chitin is a recognition element for tissue infiltration by innate cells implicated in allergic and helminth immunity and this process is negatively regulated by a vertebrate chitinase.
This work implicates chitin, a common environmental biopolymer and component of aerosolized antigens as an pathogen associated molecular pattern that can trigger early inflammatory responses associated with allergic and parasitic immunity. In our model chitin induces alternative macrophage activation and recruitment of cells that can produce IL-4 and IL-13. IL-4/IL-13 expression in tissues leads to activation of Stat6-dependent genes, including AMCase and Ym2. While the function of Ym2 remains unclear at this time, AMCase appears to act in a negative feedback fashion to degrade chitin and dampen chitin-induced inflammation.