Gene expression profiling of myelin-phagocytosing macrophages

Multiple sclerosis is a chronic, inflammatory, demyelinating disease of the central nervous system in which macrophages and microglia play a central role. During active multiple sclerosis foamy macrophages and microglia, containing degenerated myelin, are abundantly found in demyelinated areas. Recent studies have described an altered macrophage phenotype after myelin internalization. However, by which mechanisms myelin affects the phenotype of macrophages and how this phenotype can influence lesion progression is unclear. Here we demonstrate, by using genome wide gene expression analysis, that myelin-phagocytosing macrophages have an enhanced expression of genes in pathways involved in migration, phagocytosis and inflammation. More interestingly, we show that myelin internalization induces the expression of genes involved in liver X receptor signaling and cholesterol efflux. In vitro validation shows that myelin-phagocytosing macrophages indeed have an increased capacity to dispose intracellular cholesterol. Additionally, myelin suppresses the production of pro-inflammatory mediators, like nitric oxide and IL-6, by macrophages in a similar manner as a liver X receptor agonist. Our data show that myelin modulates the phenotype of macrophages by nuclear receptor activation, which may subsequently affect lesion progression in multiple sclerosis. Rat peritoneal macrophages were left untreated (n=5) or treated with isolated myelin (n=5) for 3 days. Both untreated and myelin-treated macrophages were subsequently stimulated with IFNү and IL1-β for 9 hours.