MACROPHAGES AND MHC CLASS-II POSITIVE CELLS IN THE CHOROID DURING ENDOTOXIN-INDUCED UVEITIS

Aims/background - Endotoxin induced uveitis has been regarded as a mod el for acute anterior uveitis and until now Little was known about cho roidal involvement. The aim of this study was to investigate changes i n macrophages and MHC class II positive cells in the choroid of Lewis rats during endotoxin induced uveitis. Methods - Choroid-sclera wholem ounts were isolated from normal Lewis rats and at different time point s - 4, 8, 16, 24, 48, 72, and 96 hours, and 7, 10, and 14 days after a footpad injection of 200 mu g of lipopolysaccharide (LPS). Immunohist ochemistry was performed using the monoclonal antibodies ED1 (monocyte s, macrophages, dendritic cells), and OX6 (MHC class II antigen). Resu lts - In normal rats, two layers of macrophages were identified in the choroid; a layer located immediately beneath the retinal pigment epit helium (RPE) and a layer bordering the sclera. The density of ED1 posi tive cells in the layer bordering the RPE cells was 902 (SD 132) cells /mm(2) whereas the scleral layer had a cell density of 389 (73) cells/ mm(2). Based on morphology, positive cells could be divided into two m ain categories; pleomorphic/round cells and dendritiform cells with va rying appearances, with the latter being predominant in normal eyes. A network of MHC class II positive dendritic cells was found in the cho roid, beneath the RPE, with a density of 659 (96) cells/mm(2). No MHC class II positive cells were found in the macrophage layer bordering t he sclera. LPS injection caused a massive influx of ED1 positive macro phages in the area below the RPE cells but did not result in an influx of macrophages at the scleral side of the choroid. The infiltrate rea ched a maximum at 16 hours following LPS injection and decreased at 96 hours. The morphology of the infiltrating cells was pleomorphic/round at early stages of inflammation and changed into a dendritiform cell population later. The number of MHC class II positive cells on the ant erior side of the choroid increased 8 hours after injection and reache d a peak at 72-96 hours. MHC class II positive cells were not observed in the vicinity of the sclera at any time after LPS injection. Both r esident and MHC class II positive dendritic cell numbers returned to n ormal values at day 14 following LPS injection. Conclusions - These re sults indicate that the choroid is severely inflamed after systemic LP S administration to Lewis rats and suggests that endotoxin induced uve itis may serve as a model for generalised uveitis in humans.