Inefficient Type I Interferon-Mediated Antiviral Protection of Primary Mouse Neurons Is Associated with the Lack of Apolipoprotein L9 Expression

Abstract

We examined the antiviral response promoted by type I interferons (IFN) in primary mouse neurons. IFN treatment of neuron cultures strongly upregulated the transcription of IFN-stimulated genes but conferred a surprisingly low resistance to infection by neurotropic viruses such as Theiler's murine encephalomyelitis virus (TMEV) or vesicular stomatitis virus (VSV). Response of primary mouse neurons to IFN treatment was heterogeneous, as many neurons failed to express the typical IFN response marker Mx1 after IFN treatment. This heterogeneous response of primary neurons correlated with a low level of basal expression of IFN-stimulated genes, such as Stat1, that are involved in signal transduction of the IFN response. In addition, transcriptomic analysis identified 15 IFN-responsive genes whose expression was low in IFN-treated primary neurons compared to that of primary fibroblasts derived from the same mice (Dhx58, Gvin1, Sp100, Ifi203 isoforms 1 and 2, Irgm2, Lgals3bp, Ifi205, Apol9b, Ifi204, Ifi202b, Tor3a, Slfn2, Ifi35, Lgals9). Among these genes, the gene coding for apolipoprotein L9b (Apol9b) displayed antiviral activity against Theiler's virus when overexpressed in L929 cells or in primary neurons. Accordingly, knocking down Apol9b expression in L929 cells increased viral replication. Therefore, we identified a new antiviral protein induced by interferon, ApoL9b, whose lack of expression in primary neurons likely contributes to the high sensitivity of these cells to viral infection. IMPORTANCE The type I interferon (IFN) response is an innate immune defense mechanism that is critical to contain viral infection in the hostuntil an adaptive immune response can be mounted. Neurons are a paradigm for postmitotic, highly differentiated cells. Our data show that primary mouse neurons that are exposed to type I interferon remain surprisingly susceptible to viral infection. On one hand, the low level of basal expression of some factors in neurons might prevent a rapid response of these cells. On the other hand, some genes that are typically activated by type I interferon in other cell types are expressed at much lower levels in neurons. Among these genes is the gene encoding apolipoprotein L9, a protein that proved to have antiviral activity against the neurotropic Theiler's murine encephalomyelitis virus. Our data suggest important functional differences in the IFN response mounted by specific cell populations.

Type I Interferons Link Viral Infection to Enhanced Epithelial Turnover and Repair

“…Interestingly, increased expression of ApoLs has been observed in cervical and breast cancer (Ahn et al, 2004; Jung et al, 2005). Apol9b has been implicated in anti-viral protection in L929 cells and primary neurons, but the extent and mechanism of such protection was unclear (Kreit et al, 2014). In this report, we show that Apol9a can be secreted and promotes epithelial proliferation.…”

piRNAs derived from ancient viral processed pseudogenes as transgenerational sequence-specific immune memory in mammals

“…Notably neurons, like germ cells, are permissive to retrotransposition (Upton et al 2015) and relatively nonresponsive to type I interferons (Lin et al 2013;Kreit et al 2014). Transcripts of the murine PIWI-clade partner of most pachytene piRNAs (MIWI) can be detected in a distribution overlapping highly BDV-susceptible cells in the cerebellum, dentate gyrus, and olfactory bulb (Lein et al 2007;Ackermann et al 2010).…”

The brain parenchyma has a type I interferon response that can limit virus spread

“…However, there have been no studies that resolved whether an innate immune response mounted within the brain parenchyma can limit viral spread, especially along neuronal circuitry. Studies using in vitro-cultured neurons have attempted to address whether an innate immune response can restrict virus spread among neurons but produced conflicting results, including for VSV (57,58). These previous studies raised the issue of whether virus spread among neurons in the immunologically distinct parenchyma can be limited by the innate immune response.…”

“…Previous studies have demonstrated that viral infection can stimulate expression of ISGs and proinflammatory cytokines in cultured CNS cells and in the brain (8,10,55,56) and that both neurons and glia are responsive to IFN treatment (57)(58)(59). Additionally, studies tracking virus spread into the brain from peripheral sites of inoculation have highlighted the importance of the innate immune response in preventing high virus load within the brain (14,50,60,61).…”