Fas ligand mRNA expression in the mouse central nervous system

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Abstract

Fas ligand (FasL) expressing cells delete Fas bearing T cells, thereby enabling privileged immune status in the brain. Although the presence of FasL immunoreactivity has been shown in various cell types in the central nervous system, the precise in vivo distribution of FasL mRNA in mammals is not known. Accordingly, we localized intense FasL mRNA signals in neuroglial cells mainly within the white matter regions. Using a combined labeling technique of immunocytochemistry and in situ hybridization, we confirmed that FasL signals were due to neuroglial cells rather than neurons. This study shows that FasL mRNA is constitutively expressed in the normal mouse brain, and suggests that the Fas/FasL system protects the CNS from immunological damage.

Introduction

The previous concept of immune privilege in the brain is based on the presence of the blood–brain barrier (BBB), which is known to inhibit immune cell entrance, and the absence of lymphatic vessels connecting the brain with adjacent lymphatic organs (Medawar, 1948). According to this concept, immune competent cells cannot enter the brain nor antigens leave the brain to induce immune response. However, recent data suggest that activated T cells continuously cross the BBB, and solicit immunological responses Hickey et al., 1991, Wekerle, 1993. This means that the BBB might not be the only mechanical barrier to blood cells and that another unknown mechanism counteracts the free access of activated T cells into brain tissue. The Fas/Fas ligand (FasL) system is believed to be such an alternative pathway and to be involved in the deletion of autoimmune cells in central nervous system (CNS) (Choi et al., 1999). Fas, a member of the tumor necrosis factor receptor (TNFr) family Yonehara et al., 1989, Itoh et al., 1991, Cleveland and Ihle, 1995, can transfer an apoptotic signal in specific cells when ligated by either an agonistic antibody or its ligand, FasL Trauth et al., 1989, Itoh et al., 1991, Itoh and Nagata, 1993, Tartaglia et al., 1993, Hofmann and Tschopp, 1995, Zipp et al., 1999. Even though resting T cells express low levels of Fas, the expression of Fas increases within hours of activation of its antigen receptor Miyawaki et al., 1992, Owen-Schaub et al., 1992, which suggests the possibility that Fas positive T cells are deleted by contacting naturally occurring FasL expressing cells in CNS. In this context, the cellular identities of FasL expressing cells are important in the study of the immune mechanism(s) occurring in CNS. A previous in vitro study (Choi et al., 1999) and an immunocytochemical study (Bechmann et al., 1999) showed the presence of FasL positive (FasL+) signals in various brain cells, such as astrocytes and neurons under different circumstances. However, in the normal mouse brain, the in vivo localization of FasL expressing cells remained to be proven. Thus in the present study, we performed in situ hybridization to demonstrate the expression of FasL mRNA in the mouse brain. These findings could provide the basis for the further study of the function of FasL mRNA, which is of considerable importance to the understanding of the immune mechanism of the mammalian CNS.

Section snippets

Animals and tissue processing

The adult ICR mice were treated in accordance with the “Principles of laboratory animal care” (NIH publication No. 86-24, revised 1985). The nomenclature and the maps of brain areas involved in this study were based on the atlases of Paxinos and Watson (1986) and Swanson (1992). After the mice had been anesthetized with diethyl ether, the brains of the decapitated mice were frozen rapidly in 2-methyl butane pre-cooled to its freezing point with liquid nitrogen, and specimens were sliced into 12

Results

Several adult mouse tissues including liver, muscle, skin and brain were examined by Northern blot hybridization using FasL cDNA as a probe. As seen in Fig. 1A, in the various mouse tissues, the forebrain and hindbrain showed a remarkable FasL mRNA band at ca. 2 kb, though weaker signals were found in the muscle and skin, we were unable to localized any positive signals in the liver tissues. These results were similar to those of a previous study, which found no FasL mRNA signals in the liver,

Discussion

The Sertoli cells of the testis show intense FasL expression, suggesting that passenger lymphocytes cannot explain the presence of FasL signals in all adult nonlymphoid immune privileged organs (French et al., 1996). This means that some resident cells express FasL signals constitutively, delete Fas+ T cells by apoptotic cell death, and thereby, confer an immune-privileged status upon some immune privileged organs. Furthermore, the coexpression of Fas/FasL in the same cells suggests that the

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    These authours contributed equally to this study.

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