Chitosan but Not Chitin Activates the Inflammasome by a Mechanism Dependent upon Phagocytosis

Abstract

Chitin is an abundant polysaccharide found in fungal cell walls, crustacean shells, and insect exoskeletons. The immunological properties of both chitin and its deacetylated derivative chitosan are of relevance because of frequent natural exposure and their use in medical applications. Depending on the preparation studied and the end point measured, these compounds have been reported to induce allergic responses, inflammatory responses, or no response at all. We prepared highly purified chitosan and chitin and examined the capacity of these glycans to stimulate murine macrophages to release the inflammasome-associated cytokine IL-1β. We found that although chitosan was a potent NLRP3 inflammasome activator, acetylation of the chitosan to chitin resulted in a near total loss of activity. The size of the chitosan particles played an important role, with small particles eliciting the greatest activity. An inverse relationship between size and stimulatory activity was demonstrated using chitosan passed through size exclusion filters as well as with chitosan-coated beads of defined size. Partial digestion of chitosan with pepsin resulted in a larger fraction of small phagocytosable particles and more potent inflammasome activity. Inhibition of phagocytosis with cytochalasin D abolished the IL-1β stimulatory activity of chitosan, offering an explanation for why the largest particles were nearly devoid of activity. Thus, the deacetylated polysaccharide chitosan potently activates the NLRP3 inflammasome in a phagocytosis-dependent manner. In contrast, chitin is relatively inert.

AMCase is a crucial regulator of type 2 immune responses to inhaled house dust mites

“…S4A). As reported previously (19), small chitin induced greater production of IL-1β rather than intermediate or large chitin, and we also confirmed that intermediate chitin fragmented by sonication could induce IL-1β to a greater extent than untreated intermediate chitin (Fig. 3C).…”

“…3C). The production of IL-1β was reduced in cytochalasin D pretreated macrophages, suggesting that IL-1β production by chitin is dependent on phagocytosis as described in a previous report (19). Interestingly, the production of IL-1β was reduced in macrophages from AMCase-ED mice upon the treatment of all sizes of chitin fragments whereas the production of TNFα was unaltered (Fig.…”

“…It is interesting that chitins of different sizes have different effects both in vivo and in vitro (19,21). It was previously reported that intranasal administration of chitin beads induced infiltration of IL-4-expressing cells in lung, but administration of chitin beads that were pretreated with recombinant AMCase failed to induce infiltration of IL-4-expressing cells in lung (11).…”

“…The role of chitin in the immune system is very controversial (19,21,37,38). One of the main reasons for this controversy is that it is difficult to extensively purify chitin (37,38).…”

“…Chitosan, a commercially available deacetylated chitin, but not chitin, has been reported to activate caspase-1 by the phagocytic pathway (19). It has also been shown that the size of chitosan was important for phagocytosis (19,20).…”