Retinal Pigment Epithelial Phagocytosis and Metabolism Differ from Those of Macrophages

Abstract

The purpose of this study is to compare primary human retinal pigment epithelium (RPE) cells with respect to particle uptake and further processing steps with immunological phagocytes for a better understanding of the possible role of RPE cells in triggering autoimmune diseases in the eye. We investigated the similarities of human RPE and monocytes/macrophages studying the uptake of fluorescein- and europium-labeled synthetic microparticles and microbial pathogens by human and bovine RPE cultures and a permanent RPE cell line (CRL). The uptake was monitored by laser scanning microscopy, flow cytometry and time-resolved fluorescence analysis; for comparison, macrophages and a macrophage-like cell line (MonoMac6) were used. A size-dependent uptake was seen in primary RPE cultures as well as in CRL, showing a preferential uptake of smaller beads followed by Staphylococcus aureus and Escherichia coli. Opsonization with serum caused a modest increase in bacteria uptake, but in contrast to macrophages, the classical complement receptors were not found on RPE cells. Living bacteria were also ingested in a time-dependent manner, but, as no intracellular overgrowth was observed, we further investigated the oxidative ability of RPE as a possible mechanism for microbial suppression. Unlike macrophages/granulocytes, no respiratory burst was detected in RPE cells, but, comparable to MonoMac6, IFN-γ induced neopterin in the human RPE. Interestingly a diurnal rhythm of phagocytosis was observed which was influenced by light exposure suggesting that RPE cells maintain their circadian rhythm also in cell culture to a certain extent. This study further demonstrates that in addition to similar phagocytic properties the RPE still shows substantial metabolic differences in comparison to blood-derived phagocytes.

Mitochondrial function and redox control in the aging eye: Role of MsrA and other repair systems in cataract and macular degenerations

“…RPE are the most actively phagocytozing cells in the body. This process may lead to generation of ROS, although some work suggests that the process is different to that of macrophages (Irschick, et al, 2004). As the RPE ages, oxidation of lipids and other cellular components result in an accumulation of indigestible material lipofuscin in the lysosomes, leading to their enlargement and formation of lipofuscin granules.…”

Clearance of dying ARPE-19 cells by professional and nonprofessional phagocytesin vitro- implications for age-related macular degeneration (AMD)

“…Wet AMD -the more advanced form of the disease -involves proliferation of blood vessels (neovascularization) through the blood-retina barrier and subretinal exudate formation (Green & Enger 1993). This stage is marked by professional phagocytes (macrophages and dendritic cells) involvement, the price of which is the accumulation of chemoattractant-rich drusen with the ability of initiating low-grade inflammation (Forrester 2003;Irschick et al 2004).…”

Phagocytosis of Human Retinal Pigment Epithelial Cells: Evidence of a Diurnal Rhythm, Involvement of the Cytoskeleton and Interference of Antiviral Drugs

“…We attribute this to the relatively small size of the used particles as compared to T lymphocytes and apoptotic cell material. In a previous study we found a size-dependent uptake in primary human RPE cultures as well as in ARPE-19 cells, showing a preferential uptake of smaller beads as compared to larger microbes [7] .…”

“…Additionally RPE cells were in contrast to macrophages not capable of inducing intracellular oxidative mechanisms upon ingestion of bacteria [7] although like in macrophages ␥ -interferon stimulation leads to induction of neopterin also in RPE cells [7] . The present study was undertaken to establish primarily whether a diurnal rhythm is involved in phagocytosis by human RPE cells.…”