CD63+ extracellular vesicles from retinal pigment epithelial cells participate in crosstalk with macrophages in the innate inflammatory axis

Abstract

The aim of the study is to clarify the participation of extracellular vesicles (EV) secreted by murine primary retinal pigment epithelial (mpRPE) cells in the cell to cell communication with macrophages (Mps), firstly described by the authors in 2016. In ocular inflammation, Mps act as sources of tumor necrosis factor-α (TNF-α), an activator of RPE cells. TNF-α stimulates the production of monocyte chemotactic protein (MCP-1) by RPE cells, thereby causing greater recruitment of Mps to the sub-RPE space. Murine RAW 264.7 Mps cells were co-cultured with C57BL/6 mouse mpRPE cells, either together or separated in transwells, vertically or horizontally connectable, with 0.40 or 0.03 μm membrane filters. The association of EV with mpRPE or RAW 264.7 was quantified by fluorescence cell sorting (FACS) using Qdot655 streptavidin-conjugated biotinylated EV. Increased levels of CD63+ EV were detected in co-cultures by western blotting or FACS analysis, in accordance with the increased production of nanoparticles (50–150 nm) detected by Nanosight tracking analysis. The gene expressions of cytokines, MCP-1, IL-6, IL-8, and VEGF in mpRPE cells and the corresponding proteins were increased in co-cultures even in transwells, vertically connected with 0.40 μm membrane filters, while the repressed TNF-α protein production was not affected. Most of the CD63+ EVs produced by mpRPE cells in co-cultures were associated with Raw264.7, but not with mpRPE cells. Semi-purified CD63+ EV secreted from mpRPE cells, increased the secretion of MCP-1, IL-6, and VEGF in co-cultures with RAW 264.7. Culture chamber separation horizontally connected with 0.03 μm membrane filters reduced this increased secretion. Collectively, mpRPE derived CD63+ EV partly participate in the sub-retinal innate inflammation. To evaluate the functional damage of RPE cells upon chronic exposure to here defined EVs will be the critical issue to uncover their roles in chronic retinal degenerative diseases.