CX₃CL1 (fractalkine) and CX₃CR1 expression in myelin oligodendrocyte glycoprotein-induced experimental autoimmune encephalomyelitis: kinetics and cellular origin

Multiple sclerosis (MS) is a chronic inflammatory disease of the central nervous system (CNS). It is associated with local activation of microglia and astroglia, infiltration of activated macrophages and T cells, active degradation of myelin and damage to axons and neurons. The proposed role for CX₃CL1 (fractalkine) in the control of microglia activation and leukocyte infiltration places this chemokine and its receptor CX₃CR1 in a potentially strategic position to control key aspects in the pathological events that are associated with development of brain lesions in MS. In this study, we examine this hypothesis by analyzing the distribution, kinetics, regulation and cellular origin of CX₃CL1 and CX₃CR1 mRNA expression in the CNS of rats with an experimentally induced MS-like disease, myelin oligodendrocyte glycoprotein (MOG)-induced autoimmune encephalomyelitis (EAE).

The expression of CX₃CL1 and its receptor CX₃CR1 was studied with in situ hybridization histochemical detection of their mRNA with radio labeled cRNA probes in combination with immunohistochemical staining of phenotypic cell markers. Both healthy rat brains and brains from rats with MOG EAE were analyzed. In defined lesional stages of MOG EAE, the number of CX₃CR1 mRNA-expressing cells and the intensity of the in situ hybridization signal were determined by image analysis. Data were statistically evaluated by ANOVA, followed by Tukey\primes multiple comparison test.

Expression of CX₃CL1 mRNA was present within neuronal-like cells located throughout the neuraxis of the healthy rat. Expression of CX₃CL1 remained unaltered in the CNS of rats with MOG-induced EAE, with the exception of an induced expression in astrocytes within inflammatory lesions. Notably, the brain vasculature of healthy and encephalitic animals did not exhibit signs of CX₃CL1 mRNA expression. The receptor, CX₃CR1, was expressed by microglial cells in all regions of the healthy brain. Induction of MOG-induced EAE was associated with a distinct accumulation of CX₃CR1 mRNA expressing cells within the inflammatory brain lesions, the great majority of which stained positive for markers of the microglia-macrophage lineage. Analysis in time-staged brain lesions revealed elevated levels of CX₃CR1 mRNA in microglia in the periplaque zone, as well as a dramatically enhanced accumulation of CX₃CR1 expressing cells within the early-active, late-active and inactive, demyelinated lesions.

Our data demonstrate constitutive and regulated expression of the chemokine CX₃CL1 and its receptor CX₃CR1 by neurons/astrocytes and microglia, respectively, within the normal and inflamed rat brain. Our findings propose a mechanism by which neurons and reactive astrocytes may control migration and function of the surrounding microglia. In addition, the accumulation of CX₃CR1 expressing cells other than microglia within the inflammatory brain lesions indicate a possible role for CX₃CL1 in controlling invasion of peripheral leucocytes to the brain.