Immunopathology of the Outer Retina: Crosstalk With the Immune System.

Abstract

The retinal pigment epithelium (RPE) forms the outer blood-retina barrier. This barrier separates the retina from the choriocapillaris, as well as the immune system. The RPE contributes to the immune privilege of the eye by communicating with the local and systemic immune systems via various receptors and soluble factors. We recently described the expression and upregulation of the transcription factor FoxP3 in stressed RPE cells in vivo and in vitro. FoxP3 was initially described as being specific to regulatory T cells. Based on the hypothesis that different FoxP3 variants are expressed in RPE cells, we investigated the subcellular localization of FoxP3 using a panel of nine antibodies raised against different FoxP3 epitopes in ARPE-19 cells. We investigated cytokine secretion and FoxP3 expression in stressed and unstressed ARPE-19 cells. Culture supernatants from ARPE-19 cells treated with LPS or with mechanical disruption of the cell layer were investigated for cytokine secretion using a multiplex assay. The cells were then stained with immunofluorescent antibodies that target different FoxP3 domains and phosphorylation sites to detect the intracellular localization of FoxP3. Detection of FoxP3 at the subcellular level by identifying different epitopes on the protein. Stressed ARPE-19 cells upregulate inflammatory cytokines (IL-6 and MCP-1), which correspond with FoxP3 localization patterns in the cytoplasm and nucleus (phosphorylated FoxP3), depending on the epitope detected by the antibodies. Stressed RPE cells switch from a tolerogenic to an effector phenotype, yet they immediately re-establish immune privilege by upregulating the transcription factor FoxP3, similar to T cells in the immune system. These different FoxP3 detection patterns suggest an RPE-specific role in maintaining the immune barrier, as well as in other non-T cell tissues that express FoxP3.