Dose-dependent differential regulation of cytokine secretion from macrophages by fractalkine

Although expression of the fractalkine (CX3CL1, FKN) is enhanced in inflamed tissues, it is detected at steady state in various organs such as the intestine, and its receptor CX3CR1 is highly expressed in resident-type dendritic cells and macrophages. We hypothesized that FKN might regulate the inflammatory responses of these cells. Therefore, murine macrophages were pretreated with FKN and then stimulated with LPS. We found that macrophages pretreated with 0.03 nM FKN but not with 3 nM FKN secreted 50% less TNF-alpha than did cells treated with LPS alone. Cells treated with 0.03 nM FKN and LPS also showed reduced phosphorylation of ERK1/2 and reduced NF-kappaB p50 subunit. Interestingly, the p65 subunit of NF-kappaB was translocated to the nuclei but redistributed to the cytoplasm in the early phase by forming a complex with peroxisome proliferator-activated receptor (PPAR) gamma. Exogenous 15-deoxy-Delta(12,14)-prostaglandin J2, a natural ligand for PPAR-gamma, also induced redistribution of p65 with decreased TNF-alpha secretion after LPS challenge. Pretreatment with 0.03 nM but not 3 nM FKN increased the cellular levels of 15-deoxy-Delta(12,14)-prostaglandin J2 as well as mRNA of PPAR-gamma. Requirement of PPAR-gamma for the effect of 0.03 nM FKN was confirmed by small interfering RNA of PPAR-gamma. In contrast, pretreatment with 3 nM FKN induced higher levels of IL-23 compared with cells pretreated with 0.03 nM FKN and produced TNF-alpha in a CX3CR1-dependent manner. These dose-dependent differential effects of FKN establish its novel role in immune homeostasis and inflammation.