TREM2 is down-regulated by HSV1 in microglia and involved in antiviral defense in the brain

Immunological control of viral infections in the brain exerts immediate protection and also long-term maintenance of brain integrity. Microglia are important for antiviral defense in the brain. Here, we report that herpes simplex virus type 1 (HSV1) infection of human induced pluripotent stem cell (hiPSC)-derived microglia down-regulates expression of genes in the TREM2 pathway. TREM2 was found to be important for virus-induced IFNB induction through the DNA-sensing cGAS-STING pathway in microglia and for phagocytosis of HSV1-infected neurons. Consequently, TREM2 depletion increased susceptibility to HSV1 infection in human microglia-neuron cocultures and in the mouse brain. TREM2 augmented STING signaling and activation of downstream targets TBK1 and IRF3. Thus, TREM2 is important for the antiviral immune response in microglia. Since TREM2 loss-of-function mutations and HSV1 serological status are both linked to Alzheimer's disease, this work poses the question whether genetic or virus-induced alterations of TREM2 activity predispose to post-infection neurological pathologies

ER stress induces caspase-2-tBID-GSDME-dependent cell death in neurons lytically infected with herpes simplex virus type 2

Neurotropic viruses, including herpes simplex virus (HSV) types 1 and 2, have the capacity to infect neurons and can cause severe diseases. This is associated with neuronal cell death, which may contribute to morbidity or even mortality if the infection is not controlled. However, the mechanistic details of HSV-induced neuronal cell death remain enigmatic. Here, we report that lytic HSV-2 infection of human neuron-like SH-SY5Y cells and primary human and murine brain cells leads to cell death mediated by gasdermin E (GSDME). HSV-2-induced GSDME-mediated cell death occurs downstream of replication-induced endoplasmic reticulum stress driven by inositol-requiring kinase 1α (IRE1α), leading to activation of caspase-2, cleavage of the pro-apoptotic protein BH3-interacting domain death agonist (BID), and mitochondria-dependent activation of caspase-3. Finally, necrotic neurons released alarmins, which activated inflammatory responses in human iPSC-derived microglia. In conclusion, lytic HSV infection in neurons activates an ER stress-driven pathway to execute GSDME-mediated cell death and promote inflammation.</p

Sensing of HSV-1 by the cGAS-STING pathway in microglia orchestrates antiviral defence in the CNS

Herpes simplex encephalitis (HSE) is the most common form of acute viral encephalitis in industrialized countries. Type I interferon (IFN) is important for control of herpes simplex virus (HSV-1) in the central nervous system (CNS). Here we show that microglia are the main source of HSV-induced type I IFN expression in CNS cells and these cytokines are induced in a cGAS-STING-dependent manner. Consistently, mice defective in cGAS or STING are highly susceptible to acute HSE. Although STING is redundant for cell-autonomous antiviral resistance in astrocytes and neurons, viral replication is strongly increased in neurons in STING-deficient mice. Interestingly, HSV-infected microglia confer STING-dependent antiviral activities in neurons and prime type I IFN production in astrocytes through the TLR3 pathway. Thus, sensing of HSV-1 infection in the CNS by microglia through the cGAS-STING pathway orchestrates an antiviral program that includes type I IFNs and immune-priming of other cell types

STING agonists enable antiviral cross-talk between human cells and confer protection against genital herpes in mice

<div><p>In recent years, there has been an increasing interest in immunomodulatory therapy as a means to treat various conditions, including infectious diseases. For instance, Toll-like receptor (TLR) agonists have been evaluated for treatment of genital herpes. However, although the TLR7 agonist imiquimod was shown to have antiviral activity in individual patients, no significant effects were observed in clinical trials, and the compound also exhibited significant side effects, including local inflammation. Cytosolic DNA is detected by the enzyme cyclic GMP-AMP (2’3’-cGAMP) synthase (cGAS) to stimulate antiviral pathways, mainly through induction of type I interferon (IFN)s. cGAS is activated upon DNA binding to produce the cyclic dinucleotide (CDN) 2’3’-cGAMP, which in turn binds and activates the adaptor protein Stimulator of interferon genes (STING), thus triggering type I IFN expression. In contrast to TLRs, STING is expressed broadly, including in epithelial cells. Here we report that natural and non-natural STING agonists strongly induce type I IFNs in human cells and in mice <i>in vivo</i>, without stimulating significant inflammatory gene expression. Systemic treatment with 2’3’-cGAMP reduced genital herpes simplex virus (HSV) 2 replication and improved the clinical outcome of infection. More importantly, local application of CDNs at the genital epithelial surface gave rise to local IFN activity, but only limited systemic responses, and this treatment conferred total protection against disease in both immunocompetent and immunocompromised mice. In direct comparison between CDNs and TLR agonists, only CDNs acted directly on epithelial cells, hence allowing a more rapid and IFN-focused immune response in the vaginal epithelium. Thus, specific activation of the STING pathway in the vagina evokes induction of the IFN system but limited inflammatory responses to allow control of HSV2 infections <i>in vivo</i>.</p></div

Systemic treatment with STING agonists confers protection against genital HSV2 infection.

<p>Wildtype and <i>cGas</i>^-/- mice were treated with 2’3’-cGAM(PS)₂ (Rp/Sp) (125 μg/mouse) and infected intravaginally with HSV2 (6.7×10⁴ p.f.u.). (<b>A</b>) Illustration of the timeline for the treatment regimens. (<b>B, C</b>) Overall survival for HSV2-infected and treated wildtype (<b>B</b>) or <i>cGas</i>^<i>-</i>/- (<b>C</b>) mice. n = 6–10. * = p<0,05 compared to mock. <b>(D</b>) HSV2 titer (TCID50) in vaginal washes collected 48 hours post infection. n = 6–10. * = p<0,05 compared to mock in the same genotype. Statistics, (<b>B</b>, <b>C</b>) Log-rank test with Holm-Bonferroni correction. (<b>D</b>) One-way ANOVA of log₁₀-transformed data with Dunnett’s multiple comparisons test.</p

STING agonists induces type I IFN expressing <i>in vivo</i>.

<p>Equimolar (1.687x10^-7 mol) doses of STING agonists 2’3’-cGAMP (121 μg/mouse), 3’3’c-diAMP (119 μg/mouse), 2’3’-cGAM(PS)₂ (125 μg/mouse), 3’3’-cAIMP (111 μg/mouse) or DMXAA (95 μg/mouse, STING activation by DMXAA requires two molecules (3.374x10^-7 mol)) were administrated to mice i.p. Samples were collected 6 hours later for further analysis. (<b>A</b>) Levels of phosphorylated STAT1 (pSTAT1), STING, viperin and ISG15 were determined by Western blotting of spleen, vagina and brain tissues. Vinculin was used as loading control. n = 5, two representative samples are shown. (<b>B</b>) IFNα and IFNβ in the serum determined by ELISA. n = 3–5. * = p<0.05 compared to mock. (<b>C</b>) Expression for <i>Ifnb</i> and <i>Mx1</i> mRNA in tissues samples from vagina, spleen and brain, normalized to GAPDH. n = 3–5. * = p<0,05 compared to mock. <b>(D</b>) Mice were perfused prior to isolation of brain samples and gene expressions of <i>Ifnb</i> and <i>Mx1</i> mRNA were measured. n = 3–5. * = p<0,05 compared to mock. Statistics, (<b>B-D</b>) Kruskal-Wallis test with Dunn’s multiple comparisons test.</p

HSV2 and STING agonists induce antiviral genes in vaginal epithelium.

<p>Mice were injected i.p. with 2’3’-cGAM(PS)₂ (125 μg/mouse) or infected with HSV2 (6.7×10⁴ p.f.u.). Tissues were isolated from mice 6 h and 24 h after CDN stimulation and HSV2 infection, respectively. Paraffin sections of the vaginal tissues were stained for viperin (red) and HSV2 (green). DAPI (blue) marks the nuclei and the dotted white lines mark basal membrane between the epithelium and stroma. White arrows highlight examples of viperin positive cells, and arrowheads mark examples of HSV2-infected cells. L = lumen, E = epithelium, S = stroma. n = 3. One representative picture is shown for each staining and treatment group.</p