Expression and Localization of the Inducible Isoform of Nitric Oxide Synthase in Nasal Polyps

Nitric oxide (NO) and peroxynitrite play an important role in pathophysiology of several airway diseases. An inducible isoform of nitric oxide synthase (iNOS) is known to be expressed in the nasal mucosa in allergic and chronic rhinitis. Few reports exist, however, on the expression of iNOS in nasal polyps. We detected and localized iNOS expression in nasal polyp tissue. Nasal polyps were obtained from 10 patients following polypectomy, and divided into allergic and infectious groups based on clinical presentation and laboratory testing. One nasal mucosa of the inferior turbinate was also obtained from a cadaver without nasal disease. iNOS expression was studied by immunohistochemistry under light and electron microscopy.Immunoreactivity for iNOS was localized to the mucosal epithelium, inflammatory cells, vascular endothelium and smooth muscle, and nasal gland. Strong immunoreactivity was shown in the mucosal epithelium of both groups, and weak to moderate reactivity in the mucosal epithelium of the inferior turbinate. Vascular endothelium and smooth muscle of both groups sometimes showed weak to moderate immunoreactivity. Nasal glands of both groups sometimes showed weak immunoreactivity.A significant difference between allergic and infectious groups was observed in predominant types of inflammatory cells. Neutrophils were predominant in the infectious group (p<0.01), and eosinophils in the allergic group (p<0.0001). About 50%-53% in allergic and 42% in infectious groups-of inflammatory cells showed positive immunoreactivity for iNOS. Immunoreactive cells were neutrophils, eosinophils, and macrophages. Lymphocytes, plasma cells, and mast cells invariably reacted negatively. A significant difference between allergic and infectious groups was observed in predominant iNOS-immunoreactive cells. Ratios of immunoreactive neutrophils to all neutrophils (p<0.05) and to all inflammatory cells (p<0.05) were significantly higher in the infectious group. The ratio of immunoreactive eosinophils to all inflammatory cells was significantly higher in the allergic group (p<0.0001), while the ratio of immunoreactive eosinophils to all eosinophils did not differ between infectious and allergic groups. The ratios of immunoreactive macrophages to all macrophages and to all inflammatory cells did not differ significantly between groups. Electron microscopy showed that degenerated cells with pyknotic nuclei were located next to immunoreactive eosinophils, suggesting the cytotoxicity of NO, peroxynitrite, or superoxide.