Inflammasomes Induced by 7-Ketocholesterol and Other Stimuli in RPE and in Bone Marrow-Derived Cells Differ Markedly in Their Production of IL-1β and IL-18.

PURPOSE: The inflammatory process plays a major role in the pathogenesis of AMD, and recent data indicate the involvement of inflammasomes. Inflammasomes are intracellular structures that trigger inflammation by producing mature interleukin-(IL)-1β and IL-18. This study examined the capacity of 7-ketocholesterol (7KCh), an oxysterol that accumulates in the retinal pigmented epithelium (RPE) and choroid, to initiate inflammasome formation in RPE and bone marrow-derived cells.
METHODS: Tested cells included fetal human RPE (fhRPE), human ARPE-19 cells, primary human brain microglia cells, and human THP-1 monocyte cells. 7-Ketocholesterol and other compounds were added to the cell cultures, and their stimulatory effects were determined by quantitative PCR and release of cytokines, measured by ELISA and Western blotting.
RESULTS: 7-Ketocholesterol efficiently induced inflammasome formation by all primed cell populations, but secreted cytokine levels were higher in cultures of bone marrow-derived cells (microglia and THP-1 cells) than in RPE cultures. Interestingly, inflammasomes formed in cells of the two populations differed strikingly in their preferential production of the two cytokines. Thus, whereas bone marrow-derived cells produced levels of IL-1β that were higher than those of IL-18, the opposite was found with RPE cells, which secreted higher levels of IL-18. Importantly, Western blot analysis showed that IL-18, but not IL-1β, was expressed constitutively by RPE cells.
CONCLUSIONS: 7-Ketocholesterol efficiently stimulates inflammasome formation and is conceivably involved in the pathogenesis of AMD. In contrast to bone marrow-derived cells, RPE cells produced higher levels of IL-18 than IL-1β. Further, IL-18, a multifunctional cytokine, was expressed constitutively by RPE cells. These observations provide new information about stimuli and cells and their products assumed to be involved in the pathogenesis of AMD. Copyright 2015 The Association for Research in Vision and Ophthalmology, Inc.