Production of human-inducible nitric oxide synthase in radicular cysts

To examine if nitric oxide (NO) is produced in radicular cysts, NO synthase (NOS) production was analyzed. Periapical tissues were removed from patients at the time of endodontic surgery. Frozen tissue sections were histologically evaluated with hematoxylin-eosin staining. Production of human-inducible NOS (iNOS) in apical cysts was then immunohistochemically examined. Immunoreactive human iNOS was widely distributed in epithelial cells, endothelial cells, fibroblasts, macrophages, or polymorphonuclear leukocytes. Remarkably, iNOS-positive cells were significantly present around blood vessels, and cells residing apart from the blood vessels showed weak or no iNOS production, suggesting that only cells around blood vessels could be stimulated for iNOS synthesis. These data demonstrated the possibility that several, but not all, cells could be stimulated to synthesize iNOS in inflamed tissues. In the presence of iNOS, NO can be produced spontaneously in periapical lesions and may play a crucial role in the regulation of chronic infection.