Late-Onset Chronic Inflammatory Encephalopathy in Immune-Competent and Severe Combined Immune-Deficient (SCID) Mice with Astrocyte-Targeted Expression of Tumor Necrosis Factor

Abstract

To examine the role of tumor necrosis factor (TNF)-alpha in the pathogenesis of degenerative disorders of the central nervous system (CNS), transgenic mice were developed in which expression of murine TNF-alpha was targeted to astrocytes using a glial fibrillary acidic protein (GFAP)-TNF-alpha fusion gene. In two independent GFAP-TNFalpha transgenic lines (termed GT-8 or GT-2) adult (>4 months of age) animals developed a progressive ataxia (GT-8) or total paralysis affecting the lower body (GT-2). Symptomatic mice had prominent meningoencephalitis (GT-8) or encephalomyelitis (GT-2) in which large numbers of B cells and CD4+ and CD8+ T cells accumulated at predominantly perivascular sites. The majority of these lymphocytes displayed a memory cell phenotype (CD44high, CD62Llow, CD25-) and expressed an early activation marker (CD69). Parenchymal lesions contained mostly CD45+ high, MHC class II+, and Mac-1+ cells of the macrophage microglial lineage with lower numbers of neutrophils and few CD4+ and CD8+ T cells. Cerebral expression of the cellular adhesion molecules ICAM-1, VCAM-1, and MAdCAM as well as a number of alpha- and beta-chemokines was induced or upregulated and preceded the development of inflammation, suggesting an important signaling role for these molecules in the CNS leukocyte migration. Degenerative changes in the CNS of the GFAP-TNFalpha mice paralleled the development of the inflammatory lesions and included primary and secondary demyelination and neurodegeneration. Disease exacerbation with more extensive inflammatory lesions that contained activated cells of the macrophage/microglial lineage occurred in GFAP-TNFalpha mice with severe combined immune deficiency. Thus, persistent astrocyte expression of murine TNF-alpha in the CNS induces a late-onset chronic inflammatory encephalopathy in which macrophage/microglial cells but not lymphocytes play a central role in mediating injury.

Metallothioneins Are Upregulated in Symptomatic Mice with Astrocyte-Targeted Expression of Tumor Necrosis Factor-α

“…However, such upregulation mainly occurred in clinically affected animals. This could be somewhat surprising since the presymptomatic mice also have TNF-␣ production, which is lower than that of symptomatic mice but substantially higher than that of control mice (59). It is feasible to think of a threshold effect of the TNF-␣ concentration on MT-I ϩ II induction, but considering the magnitude of the difference in the TNF-␣ expression of the presymptomatic and the symptomatic mice and the above-discussed results in liver cells, it seems likely that TNF-␣ would act in concert with other factors rather than exert a direct effect on MT synthesis.…”

“…This induction was dramatic in the cerebellum, thus paralleling the expression of the GFAP-TNF␣ gene, which is the highest in the cerebellum (59). Moreover, the endogenous TNF-␣ gene is also coordinately induced in the GFAP-TNF␣ mice (59).…”

“…The GT-8 transgenic mice used in this study show a characteristic physical presentation that includes symptoms of ataxia from 6 months of age or older, the severity of which increases progressively; after the onset of motor impairment, the mice display muscle spasms and become progressively cachectic (59). We have analyzed MT expression in both presymptomatic and symptomatic mice defined by these clinical criteria.…”

“…* P at least Ͻ0.05 vs the control mice of the same age. significant activation of microglia, recruitment of macrophages, and the appearance of GFAP-positive reactive astrocytes (see (59) for a detailed analysis). In agreement with the MT-I expression, MT-I ϩ II immunoreactivity was increased in the GFAP-TNF␣ mice especially in the cerebellum (Fig.…”

“…It is feasible to think of a threshold effect of the TNF-␣ concentration on MT-I ϩ II induction, but considering the magnitude of the difference in the TNF-␣ expression of the presymptomatic and the symptomatic mice and the above-discussed results in liver cells, it seems likely that TNF-␣ would act in concert with other factors rather than exert a direct effect on MT synthesis. The CNS of symptomatic mice suffers a dramatic infiltration of lymphocytes and macrophages (59), which could produce a number of factors driving MT synthesis. Moreover, during the inflammatory response in the CNS an increased oxidative stress could be expected (27), which, in turn, could be coupled to MT-I ϩ II upregulation (17,18).…”