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Background: Chitin is an abundant polysaccharide found in fungi, insects, and parasitic nematodes. Innate immune host defense against chitin-containing pathogens include production of chitinases. In human lower airways, acidic mammalian chitinase (AMCase) is produced in epithelial cells via a Th2-specific, IL-13–dependent pathway, and may act as an inflammatory mediator in asthma. The role of AMCase in chronic rhinosinusitis (CRS) has not been studied previously. Methods: Eleven controls and 22 subjects with medically recalcitrant CRS were prospectively enrolled before undergoing endoscopic sinus surgery. RNA was extracted from surgically obtained ethmoid mucosa, and real-time PCR was used to determine expression of AMCase, eotaxin, and IL-13. Subjects were followed for at least 6 months postoperatively to assess for polyp recurrence. Based on the presence or absence of polyps, the subjects were classified as either recalcitrant or responsive to therapy. Results: AMCase mRNA was detected in the sinus mucosa of 72% of control subjects and in 72% of patients with eosinophilic CRS with nasal polyps (CRSwNP). The expression of AMCase was significantly greater in recalcitrant CRSwNP than it was in treatment-responsive CRSwNP. There was no significant difference in IL-13 expression between these two groups. Conclusion: AMCase may be an important mediator in the pathogenesis of Th2 inflammatory diseases of the respiratory tract. Failure of medical and surgical therapy in CRSwNP is associated with significantly increased expression of AMCase, but not the Th2 cytokines IL-13 and eotaxin. Additional studies are needed to determine the potential of AMCase as a therapeutic target in CRSwNP.