LCMV and the central nervous system: uncovering basic principles of CNS physiology and virus-induced disease

Congenitally acquired persistent lymphocytic choriomeningitis viral infection reduces neuronal progenitor pools in the adult hippocampus and subventricular zone

Lymphocytic choriomeningitis virus (LCMV) can be transmitted through congenital infection, leading to persistent infection of numerous organ systems including the central nervous system (CNS). Adult mice persistently infected with LCMV (LCMV-cgPi mice) exhibit learning deficits, such as poor performance in spatial discrimination tests. Given that deficits in spatial learning have been linked to defects in adult neurogenesis, we investigated the impact of congenital LCMV infection on generation of neuroblasts from neural progenitor cells within neurogenic zones of adult mice. In LCMV-cgPi mice, QPCR and immunohistochemistry detected presence of LCMV glycoprotein-coding RNA and nucleoprotein in the hippocampal dentate gyrus and subventricular zone (SVZ), sites of neurogenesis that harbor populations of neuroblasts. Numbers of neuroblasts were reduced in LCMV-cgPi mice, as determined by IHC quantification, and analysis of BrdU incorporation by flow cytometry revealed lower numbers of BrdU-labeled neuroblasts. Additionally, TUNEL assays performed in situ showed increased numbers of apoptotic cells in the two neurogenic regions. Next, neurosphere cultures were infected in vitro with LCMV and differentiated to create a population of cells that consisted of both transit amplifying cells and neuroblasts. Immunocytochemical and TUNEL assays revealed increased numbers of TUNEL-positive cells that express nestin, suggesting that the drop in numbers of neuroblasts was due to a combination of impaired proliferation and apoptosis of progenitor cells. LCMV-cgPi mice exhibited transcriptional up-regulation several cytokines and chemokines, including gamma-interferon inducible chemokines CXCL9 and CXCL10. Chronic up-regulation of these chemokines can facilitate a pro-inflammatory niche that may contribute to defects in neurogenesis.

Negative regulation of interferon-β gene expression during acute and persistent virus infections

The production of type I interferons (IFNs) in response to viral infections is critical for antiviral immunity. However, IFN production is transient, and continued expression can lead to inflammatory or autoimmune diseases. Thus, understanding the mechanisms underlying the negative regulation of IFN expression could lead to the development of novel therapeutic approaches to the treatment of these diseases. We report that the transcription factor IRF3 plays a central role in the negative regulation of interferon-β (IFNβ) expression during both acute and persistent (chronic) virus infections. We show that the degradation of IRF3 during acute infections, rather than the activation of transcriptional repressors, leads to the down regulation of IFNβ expression. We also show that the block to IFNβ expression in mouse embryonic fibroblasts that are persistently infected with Sendai virus (SeV) correlates with the absence of transcriptionally active IRF3. Remarkably, ongoing protein synthesis and viral replication are required to maintain repression of the IFNβ gene in persistently infected cells, as the gene can be activated by the protein synthesis inhibitor cycloheximide, or by the antiviral drug ribavirin. Finally, we show that the SeV V protein inhibits IRF3 activity in persistently infected cells. Thus, in conjunction with the known interference with STAT1 by the SeV C protein, both IFN activation and its signaling pathways are blocked in persistently infected cells. We conclude that the transcription factor IRF3 is targeted for turnover and inactivation through distinct mechanisms from both the host cells and virus, leading to the inhibition of IFNβ gene expression during acute and persistent viral infections. These observations show that IRF3 plays a critical role, not only in the activation of the IFNβ gene, but also in the controlling the duration of its expression. (284 words)

Altered central nervous system gene expression caused by congenitally acquired persistent infection with lymphocytic choriomeningitis virus

Neonatal infection of most mouse strains with lymphocytic choriomeningitis virus (LCMV) leads to a life-long persistent infection characterized by high virus loads in the central nervous system (CNS) in the absence of inflammation and tissue destruction. These mice, however, exhibit impaired learning and memory. The occurrence of cognitive defects in the absence of overt CNS pathology led us to the hypothesis that chronic virus infection may contribute to neuronal dysfunction by altering the host's gene expression profile. To test this hypothesis, we examined the impact of LCMV persistence on host gene expression in the CNS. To model the natural route of human congenital CNS infection observed with a variety of viruses, we established a persistently infected mouse colony where the virus was maintained via vertical transmission from infected mothers to offspring (LCMV-cgPi). LCMV-cgPi mice exhibited a lifelong persistent infection involving the CNS; the infection was associated with impaired spatial-temporal learning. Despite high viral loads in neurons of the brains of adult LCMV-cgPi mice, we detected changes in the host's CNS gene expression for only 75 genes, 56 and 19 being significantly induced and reduced, respectively. The majority of the genes induced in the brain of LCMV-cgPi mice were interferon (IFN)-stimulated genes (ISGs) and included the transcription factors STAT1 and IRF9, the ISG15 protease UBP43, and the glucocorticoid attenuated-response genes GARG16 and GARG49. Based on their crucial role in antiviral defense, these ISGs may play an important role in limiting viral spread and replication. However, since IFNs have also been implicated in adverse effects on neuronal function, the chronic induction of some ISGs may also contribute to the observed cognitive impairment.

Transgenic mice expressing the nucleoprotein of Borna disease virus in either neurons or astrocytes: decreased susceptibility to homotypic infection and disease

The nucleoprotein (N) of Borna disease virus (BDV) is the major target of the disease-inducing antiviral CD8 T-cell response in the central nervous system of mice. We established two transgenic mouse lines which express BDV-N in either neurons (Neuro-N) or astrocytes (Astro-N). Despite strong transgene expression, neurological disease or gross behavioral abnormalities were not observed in these animals. When Neuro-N mice were infected as adults, replication of BDV was severely impaired and was restricted to brain areas with a low density of transgene-expressing cells. Notably, the virus failed to replicate in the transgene-expressing granular and pyramidal neurons of the hippocampus (which are usually the preferred host cells of BDV). When Neuro-N mice were infected within the first 5 days of life, replication of BDV was not suppressed in most neurons, presumably because the onset of transgene expression in the brain occurred after these cells became infected with BDV. Astro-N mice remained susceptible to BDV infection, but they were resistant to BDV-induced neurological disorder. Unlike their nontransgenic littermates, Neuro-N mice with persistent BDV infection did not develop neurological disease after immunization with a vaccinia virus vector expressing BDV-N. In contrast to the situation in wild-type mice, this treatment also failed to induce N-specific CD8 T cells in the spleens of both transgenic mouse lines. Thus, while resistance to BDV infection in N-expressing neurons appeared to result from untimely expression of a viral nucleocapsid component, the resistance to BDV-induced neuropathology probably resulted from immunological tolerance.