Neuroinflammation is defined as an inflammatory state within the central nervous system (CNS). Microglia conprise the resident tissue macrophages of the neuronal tissue. Upon viral infection of the CNS, microglia become activated and start to produce inflammatory mediators important for clearance of the virus, but an excessive neuroinflammation can harm nearby neuronal cells. Herpesviruses express several molecular mechanisms, which can modulate apoptosis of infected neurons, astrocytes and microglia but also divert immune response initiated by the infected cells. In this review we also describe the link between virus-related neuroinflammation, and development of neurodegenerative diseases.

(1) Background: Epigallocatechin gallate (EGCG) has been recognized as a flavonoid showing antiviral activity against various types of DNA and RNA viruses. In this work, we tested if EGCG-modified silver nanoparticles (EGCG-AgNPs) can become novel microbicides with additional adjuvant properties to treat herpes infections. (2) Methods: The anti-HSV and cytotoxic activities of EGCG-AgNPs were tested in human HaCaT and VK-2-E6/E7 keratinocytes. HSV-1/2 titers and immune responses after treatment with EGCG-AgNPs were tested in murine models of intranasal HSV-1 infection and genital HSV-2 infection. (3) Results: EGCG-AgNPs inhibited attachment and entry of HSV-1 and HSV-2 in human HaCaT and VK-2-E6/E7 keratinocytes much better than EGCG at the same concentration. Infected mice treated intranasally (HSV-1) or intravaginally (HSV-2) with EGCG-AgNPs showed lower virus titers in comparison to treatment with EGCG alone. After EGCG-AgNPs treatment, mucosal tissues showed a significant infiltration in dendritic cells and monocytes in comparison to NaCl-treated group, followed by significantly better infiltration of CD8+ T cells, NK cells and increased expression of IFN-α, IFN-γ, CXCL9 and CXCL10. (4) Conclusions: Our findings show that EGCG-AgNPs can become an effective novel antiviral microbicide with adjuvant properties to be applied upon the mucosal tissues.

Herpes simplex virus type 2 (HSV-2) is a sexually transmitted pathogen that causes a persistent infection in sensory ganglia. The infection manifests itself as genital herpes but in rare cases it can cause meningitis. In this study, we used a murine model of HSV-2 meningitis to show that Fas and FasL are induced within the CNS upon HSV-2 infection, both on resident microglia and astrocytes and on infiltrating monocytes and lymphocytes. Mice lacking Fas or FasL had a more severe disease development with significantly higher morbidity, mortality, and an overall higher CNS viral load. In parallel, these Fas/FasL-deficient mice showed a severely impaired infection-induced CNS inflammatory response with lower levels of infiltrating CD4+ T-cells, lower levels of Th1 cytokines and chemokines, and a shift in the balance between M1 and M2 microglia/monocytes. In vitro, we confirmed that Fas and FasL is required for the induction of leucocyte apoptosis, but also show that the Fas/FasL pathway is required for adequate cytokine and chemokine production by glial cells. In summary, our data show that the Fas/FasL cell death receptor pathway is an important defense mechanism in the spinal cord as it down-regulates HSV-2-induced inflammation while at the same time promoting adequate anti-viral immune responses against infection.

Herpes simplex virus-1 (HSV-1) is a neurotropic virus that can infect the brain, and an uncontrolled infection can lead to severe encephalitis. NO can exert both antiviral as well as cytotoxic effects in the central nervous system (CNS) depending on its concentration and site of infection. In this study, we report that treatment of an intranasal murine HSV-1 infection with aminoguanidine (AMG) decreases both neuroinflammation and neurodegeneration markers, but its positive effect depends on the time of treatment. Specifically, early treatment with AMG impaired the activation of microglia/monocytes, leading to decreased virus-specific antiviral response and higher viral titers in the brain. However, AMG treatment during the peak of brain infection significantly improved antiviral response, reduced inflammation and improved general clinical score. We also found that treatment with AMG decreased beta amyloid levels during both primary and latent infections and protected from the accumulation of phosphorylated Tau protein during early infection. Our findings position inducible nitric oxide synthetase (iNOS) as a potential therapeutic target for mitigating virus-induced neuroinflammation and neurodegeneration.

Many studies have shown an association between herpes simplex virus type 1 (HSV-1) infection and the development of neurodegeneration processes later in life, such as Alzheimer’s disease. The Fas/FasL death pathway plays an important role in the complex regulation of the local inflammatory response and mounting of the specific antiviral response in HSV-1 infection. Here, we applied a mouse model of latent HSV-1 neuroinfection to Fas- and FasL-deficient mice (lpr and gld) to explore whether the lack of functional Fas/FasL pathway protects from inflammation-related neurodegeneration. The latently infected Fas- and FasL-deficient mice (lpr and gld) were not protected from virus replication despite the accumulation of virus-specific cytotoxic T cells. However, the lack of Fas/FasL pathway decreased neuroinflammation- and neurodegeneration-related markers, including cognitive impairment, amyloid-β protein, and tau hyperphosphorylation. The use of a glucocorticoid, dexamethasone, to decrease neuroinflammation in wild-type mice did not protect from cognitive impairment, despite the improved antiviral response. Our data indicate that excessive neuroinflammation via the Fas/FasL pathway during HSV-1 infection is associated with neurodegeneration. Furthermore, the administration of immunomodulatory agents to ameliorate the outcome of HSV-1 latent infection should be restricted to the peak of neuroinflammation.