Purpose: Inducible nitric oxide (NO) synthase (iNOS) has been implicated in the pathogenesis of endotoxin-induced uveitis (EIU). This study was undertaken to localize the cells, in the eye, which express iNOS during EIU in the rat.
Methods: EIU was induced in Lewis rats by a single foot pad injection of 150 micrograms lipopolysaccharide (LPS) from Salmonella typhimurium. At different time intervals after LPS injection, the authors evaluated ocular inflammation (slit lamp observation), iNOS localization by in situ hybridization, and comparison of OX-42- and ED1-positive cell appearance and of glial response by specific immunohistochemistry.
Results: iNOS mRNA was not detected in the iris-ciliary body nor in the retina of control rats. It was detected strongly in the epithelial cells of the iris-ciliary body at 6 hours and also in stromal cells of the ciliary processes at 16 hours after LPS injection. In the neuroretina, iNOS mRNA was observed in the inner layers 16 hours after LPS injection. iNOS-positive cells were also present on the vitreous at this time. At 6 and approximately 16 hours after LPS injection, immunohistochemistry experiments revealed a large number of OX-42- and ED1-positive cells (microglia, macrophages, or polymorphonuclear leukocytes) colocalized in part with some iNOS-positive cells in the ciliary body and in the retina. Furthermore, expression of iNOS in Müller cells cannot be excluded.
Conclusions: These observations confirm that subcutaneous injection of endotoxin dramatically induces NOS mRNA expression in the eye, and they demonstrate that epithelial cells of the iris-ciliary body and cells infiltrating the anterior segment of the eye and the retina are the major source of NO. These results support the hypothesis that both inflammatory and resident ocular cells are involved in iNOS expression during EIU.