Expression of CXCL4 in microglia in vitro and in vivo and its possible signaling through CXCR3.

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Signaling through chemokine receptor CXCR3 in the brain has been implicated in various brain diseases, as CXCR3 and its ligands are found under these conditions. Recently, a new chemokine ligand for CXCR3 was reported. In humans, an alternatively spliced variant of CXCR3 expressed on microvascular endothelial cells, named CXCR3b, was shown to bind CXCL4. In the periphery, the cellular expression and functions of CXCL4 are well described but in the brain its expression and function are unknown. Here, we show that brain microglia are a cellular source of CXCL4 in vitro and in vivo under neurodegenerating conditions. Microglial migration induced by CXCL4 is absent in CXCR3-deficient microglia, indicating a role of CXCR3. CXCL4 furthermore attenuates lipopolysaccharide-induced microglial phagocytosis and nitric oxide production in microglia and BV-2 cells. Based on these findings, it is proposed that locally released CXCL4 may control microglia responses.