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URL: https://pubmed.ncbi.nlm.nih.gov/28147283/

⇱ Immune Escape via a Transient Gene Expression Program Enables Productive Replication of a Latent Pathogen - PubMed


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Abstract

How type I and II interferons prevent periodic reemergence of latent pathogens in tissues of diverse cell types remains unknown. Using homogeneous neuron cultures latently infected with herpes simplex virus 1, we show that extrinsic type I or II interferon acts directly on neurons to induce unique gene expression signatures and inhibit the reactivation-specific burst of viral genome-wide transcription called phase I. Surprisingly, interferons suppressed reactivation only during a limited period early in phase I preceding productive virus growth. Sensitivity to type II interferon was selectively lost if viral ICP0, which normally accumulates later in phase I, was expressed before reactivation. Thus, interferons suppress reactivation by preventing initial expression of latent genomes but are ineffective once phase I viral proteins accumulate, limiting interferon action. This demonstrates that inducible reactivation from latency is only transiently sensitive to interferon. Moreover, it illustrates how latent pathogens escape host immune control to periodically replicate by rapidly deploying an interferon-resistant state.

Keywords: gene expression; herpesvirus; interferon; latency; neurons; reactivation.

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Figures

👁 Figure 1
Figure 1. Suppression of HSV-1 reactivation by exogenous IFNβ or IFNγ in a neuron cell-intrinsic manner
A) Protocol for establishing latency and reactivating HSV-1 using homogenous neuronal cultures isolated from rat SCGs. SCGs were isolated, dissociated and cultured for 6 d with mitotic inhibitors to yield a homogenous cultured neuron population (Camarena et al., 2010). Cultures latently-infected with HSV-1 EGFP were established using acyclovir (ACV) to suppress productive virus growth. ACV was removed after 6 d and reactivation induced by treating with the PI3-K inhibitor LY294002 (LY) +/− IFN. After 20 h, LY was removed and incubation continued +/− IFN. Reactivation was quantified by determining the percentage of EGFP-positive wells detected by fluorescence microscopy at 4 d post-treatment. (B) Neurons were treated with 20μM LY +/− 100 U/ml IFNβ for 20 h. At 20h, LY was removed, neurons were cultured +/− IFN, and reactivation measured as described in (A). (C) As in B except that 100 U/ml IFNγ was used. (D) As in B and C except that the amount of infectious virus produced was quantified by plaque assay using Vero cells. (E) As in B except neurons were treated +/− 10 μM JAK Inhibitor for 20 h. At 20h, media containing LY was removed and replaced with media containing IFN and / or JAK Inhibitor. (F) As in E except that 100 U/ml IFNγ was used.
👁 Figure 2
Figure 2. Inhibition of reactivation by IFN despite enforced interruption of PI3K-Akt-mTORC1 signaling
(A) Latently-infected neurons were treated for 20 h with 20 μM LY alone or with 100 U/ml IFNβ or IFNγ. Total protein was isolated, fractionated by SDS-PAGE in a 17.5% gel and analyzed by immunoblotting. Slower-migrating, low abundance hyper-phosphorylated 4E-BP1 isoforms indicate active PI3K-Akt-mTORC1 signaling. Accumulation of faster-migrating, hypophosphorylated 4E-BP1 indicates that PI3-K-Akt-mTORC1 signaling is inhibited. Tubulin is a loading control. (B) Total protein isolated from latently-infected neurons treated as in A was separated by SDS-PAGE and analyzed by immunoblotting using STAT1 and DAXX antibodies. (C) Neurons transduced with a lentiviral vector expressing a doxycycline (dox)-inducible, constitutively-active 4E-BP1 (AA) mutant unresponsive to mTORC1 were treated with dox +/− 100 U/ml IFNβ or IFNγ. HSV-1 UL30 mRNA accumulation measured by qRT-PCR was plotted relative to untreated controls.
👁 Figure 3
Figure 3. Suppression of reactivation by IFN is limited to a temporal window that closes before Phase II
(A, B) Latently-infected neurons were treated with 20 μM LY alone or in the presence of 100 U/ml IFNβ or IFNγ. After 20 h, accumulation of HSV-1 lytic Phase I transcripts UL30 (A) or ICP27 (B) was analyzed by qRT-PCR. (C–E) Latently-infected neurons induced to reactivate with LY for 20 h (shaded arrow) were treated with 100 U/ml (D) IFNβ or (E) IFNγ for different periods: during and after the LY pulse (IFN for Phase I & II), only during the LY pulse (IFN for Phase I only), or only after the LY pulse when Phase I has already occurred (IFN for Phase II only). Reactivation was scored 4 d after LY application.
👁 Figure 4
Figure 4. Treatment with IFNβ and IFNγ elicit unique profiles of neuronal gene expression in reactivating neurons
Volcano plots of genes (blue and orange points) differentially expressed following IFNβ (A) or IFNγ (B) treatment. Dotted line indicates y=0.01, vertical solid lines indicate the × = −0.5 and × = 0.5 thresholds. (C) Clustered heat maps of all differentially expressed genes showing log2 fold-change values in either condition. (D) Venn diagram depicting significant overlaps between genes whose expression changes in response to IFNβ or IFNγ. The significance of overlap was calculated using a hypergeomeric test. (E–F) The top 6 pathways enriched among genes induced uniquely by (E) IFNβ or (F) IFNγ. (G) Interferon induced genes were compared to a reference transcriptomic dataset of CNS cell types (Zhang et al, 2014). Heat map depicts the top 10 neuron-enriched genes and respective enrichment scores for each cell type.
👁 Figure 5
Figure 5. IFNβ but not IFNγ counters HSV-1 reactivation even in the presence of the viral protein ICP0
ICP0 was ectopically expressed from a dox-inducible promoter using a recombinant adenovirus vector in neurons latently-infected with HSV-1. (A) The percentage of wells in which HSV-1 had reactivated −/+ 100 U/ml IFNβ or IFNγ was scored 4 d after inducing ICP0 expression. After 4 d of dox treatment, total protein was isolated from (B) neurons not latently-infected with HSV-1 or (C) latently-infected with HSV-1 and ICP0 accumulation was measured by immunoblotting. (D) Model depicting how a viral genome-wide expression burst or virion protein delivery represent different strategies to counter anti-viral defenses and promote productive viral growth during acute infection or reactivation. During acute infection, the tegument delivers a subset of viral proteins from different kinetic classes into newly infected cells that antagonize host cell-intrinsic immunity and foster reproductive replication. Unlike acute infection, which begins with virion entry and tegument deposition, reactivation begins with de novo expression of latent HSV-1 genomes organized in compacted chromatin within neuronal nuclei. Instead of delivering a preformed tegument loaded with viral proteins into the host, reactivation Phase I in neurons results in a genome-wide burst of viral gene expression allowing the simultaneous accumulation of viral proteins of all kinetic classes without viral DNA replication or infectious virus production. Both strategies achieve a similar function by providing a toolkit of viral proteins capable of countering cell-intrinsic host defenses, including responses to IFN and ISGs, and stimulating productive virus growth.

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