Expression of toll-like receptor 4 in uvea-resident tissue macrophages during endotoxin-induced uveitis

To investigate the dynamics and distribution of toll-like receptor 4 (TLR4)-positive cells and resident tissue macrophages in the uvea during endotoxin-induced uveitis (EIU) in Wistar rats.

Wistar rats (n=40) received a footpad injection of 200 microg of Vibrio cholera lipopolysaccharide (LPS), and the intensity of anterior segment inflammation was evaluated after the LPS injection. Ten rats each were killed 6, 12, 24 and 48 h after injection. Ten normal Wistar rats were killed as controls (0 h). The iris-ciliary body complex and choroids from each eye were removed and subdivided into segments. Immunohistochemical localization of TLR4 and a resident tissue macrophage marker, cluster of differentiation 163 (CD163), was performed on whole mount isolated iris-ciliary body complexes and choroids. TLR4+ and CD163+ cells in the iris were manually counted, and the cell density (cells/mm(2)) was calculated. The distribution patterns and phenotypes of cells expressing these two proteins were further characterized by double-labeled immunofluorescence studies.

The iris-ciliary body complex did not express TLR4 in normal rats. TLR4+ cells were detectable in the iris stroma 6 h after injection, and the number significantly increased (p<0.001 by one-way ANOVA) 12, 24, and 48 h after injection. The morphology of TLR4+ cells hardly changed 12-48 h after injection. CD163 was expressed in the uvea in all rats. During the inflammatory response phase (0-48 h after injection), the proportion of CD163+ tissue macrophages having a round morphology increased (p<0.001 by one-way ANOVA) concurrently with a decrease in the proportion of dendritiform CD163+ cells. These changes occurred mainly in the macrophages located in the stroma bordering the iris endothelial layer. Double-labeling immunofluorescence demonstrated the co-expression of TLR4 and CD163 in round stroma cells with TLR4 located at the cell membrane and CD163 in the cytoplasm. TLR4+ cells could not be detected in choroids in any of the rats.

The results of the present study indicate that TLR4 expression increased in the iris and iris tissue macrophages expressed TLR4 during EIU. This has significant implications for the understanding of ocular inflammation and for interpreting the potential role of Gram-negative bacteria in the pathogenesis of acute anterior uveitis.