HSV Replication: Triggering and Repressing STING Functionality

MINT3 promotes STING activation and facilitates antiviral immune responses

Stimulator-of-interferon genes (STING) translocation is the rate-limiting step in the cGAS-STING signaling which detects cytosolic DNA and produces type I interferons. However, the mechanism by which this process is modulated remains to be further clarified. In the present study, we identified munc18-1-interacting protein 3 (MINT3) as a positive regulator of STING signaling. MINT3 promotes type I interferons production induced by herpes simplex virus-1 (HSV-1) infection and ISD or cGAMP stimulation in mouse peritoneal macrophages. Deficiency of Mint3 greatly inhibited STING and IRF3 activation in macrophages. Mint3 knockdown also attenuated STING and IRF3 activation in macrophages, human THP-1 cells, and RAW264.7 cells. Mechanistically, MINT3 interacted with STING, selectively enhanced its K63-linked polyubiquitination and facilitated STING translocation to the Golgi, resulting in the enhancement of the STING and TBK1 interaction. Furthermore, MINT3 also facilitated HSV-1-induced innate antiviral immune responses and impaired HSV-1 replication in vitro and in vivo. Interestingly, we showed that the expression of MINT3 was dramatically elevated during HSV-1 infection, and ISD stimulation in macrophages. Thus, we have revealed a feedback mechanism for the regulation of the cGAS-STING pathway, providing a promising therapeutic target for the treatment of disorders triggered by aberrant STING activity.

Nuclear scaffold attachment factor A functions as a potential viral recognition receptor involved in the antiviral immunity of grass carp (Ctenopharyngodon idella)

As one of the most primitive vertebrates, fish have evolved a distinct repertoire of viral recognition receptors. However, the existence of nuclear viral recognition receptors in fish remains uncertain. In this study, we identified a scaffold attachment factor A (SAFA), CiSAFA, as a potential nuclear recognition receptor for grass carp reovirus (GCRV) in grass carp (Ctenopharyngodon idella). CiSAFA shares high amino acid sequence similarity (70.3-99.6 %) and identity (56.1-98.6 %) with its counterparts from other vertebrates and contains three conserved domains, namely, SAP, SPRY, and AAA_33. On the basis of the subcellular location analysis, CiSAFA was found to localize to the nucleus. In vitro, CiSAFA can bind to poly(I:C) and induce interferon (IFN) expression. The expression data revealed that CiSAFA exhibited ubiquitous mRNA expression across all the tissues of grass carp. After GCRV infection, CiSAFA showed significantly upregulated mRNA expression levels and exhibited an expression pattern similar to that of IFN1 in the spleen and head kidney. The results of RNA immunoprecipitation indicated that CiSAFA might interact with dsRNA segment 6 of GCRV. The overexpression of CiSAFA significantly increased the expression levels of several representative antiviral genes, including interferon regulatory factor 3 (IRF3), IRF7, IFN1, and virus-induced gene 1 (GIG1), and inhibited GCRV replication. To our knowledge, this study represents the first discovery of a potential nuclear recognition receptor, CiSAFA, for GCRV in grass carp and reveals its antiviral immune mechanism against GCRV infection, which may provide new insight into host immune recognition system-virus interactions in fish.

Herpes simplex virus-1 targets the 2'-3'cGAMP importer SLC19A1 as an antiviral countermeasure

The extracellular addition of the STING agonist, 2-3cGAMP, induces an antiviral state that inhibits HSV-1 replication in a cell type dependent manner via the transportation of the cyclic-dinucleotide through the folate antiporter SLC19A1. To establish a successful infection, herpes simplex virus-1 (HSV-1), a ubiquitous virus with high seropositivity in the human population, must undermine a multitude of host innate and intrinsic immune defense mechanisms, including key players of the STimulator of INterferon Genes (STING) pathway. Herein, we report that HSV-1 infection results in the reduction of SLC19A1 transcription, translation, and importantly, the rapid removal of SLC19A1 from the cell surface of infected cells. Our data indicate SLC19A1 functions as a newly identified antiviral mediator for extracellular 2'-3'cGAMP which is undermined by HSV-1 protein ICP27. This work presents novel and important findings about how HSV-1 manipulates the host's immune environment for viral replication and discovers details about an important antiviral mechanism.

Co-option of mitochondrial nucleic acid-sensing pathways by HSV-1 UL12.5 for reactivation from latent infection

Although viruses subvert innate immune pathways for their replication, there is evidence they can also co-opt antiviral responses for their benefit. The ubiquitous human pathogen, Herpes simplex virus-1 (HSV-1), encodes a protein (UL12.5) that induces the release of mitochondrial nucleic acid into the cytosol, which activates immune-sensing pathways and reduces productive replication in nonneuronal cells. HSV-1 establishes latency in neurons and can reactivate to cause disease. We found that UL12.5 is required for HSV-1 reactivation in neurons and acts to directly promote viral lytic gene expression during initial exit from latency. Further, the direct activation of innate immune-sensing pathways triggered HSV-1 reactivation and compensated for a lack of UL12.5. Finally, we found that the induction of HSV-1 lytic genes during reactivation required intact RNA- and DNA-sensing pathways, demonstrating that HSV-1 can respond to and active antiviral nucleic acid-sensing pathways to reactivate from a latent infection.

Toward the Eradication of Herpes Simplex Virus: Vaccination and Beyond

Herpes simplex virus (HSV) has coevolved with Homo sapiens for over 100,000 years, maintaining a tenacious presence by establishing lifelong, incurable infections in over half the human population. As of 2024, an effective prophylactic or therapeutic vaccine for HSV remains elusive. In this review, we independently screened PubMed, EMBASE, Medline, and Google Scholar for clinically relevant articles on HSV vaccines. We identified 12 vaccines from our literature review and found promising candidates across various classes, including subunit vaccines, live vaccines, DNA vaccines, and mRNA vaccines. Notably, several vaccines-SL-V20, HF10, VC2, and mRNA-1608-have shown promising preclinical results, suggesting that an effective HSV vaccine may be within reach. Additionally, several other vaccines such as GEN-003 (a subunit vaccine from Genocea), HerpV (a subunit vaccine from Agenus), 0ΔNLS/RVx201 (a live-attenuated replication-competent vaccine from Rational Vaccines), HSV 529 (a replication-defective vaccine from Sanofi Pasteur), and COR-1 (a DNA-based vaccine from Anteris Technologies) have demonstrated potential in clinical trials. However, GEN-003 and HerpV have not advanced further despite promising results. Continued progress with these candidates brings us closer to a significant breakthrough in preventing and treating HSV infections.

Safety of non-replicative and oncolytic replication-selective HSV vectors

Herpes simplex virus type 1 (HSV-1) is a DNA virus and human pathogen used to construct promising therapeutic vectors. HSV-1 vectors fall into two classes: replication-selective oncolytic vectors for cancer therapy and defective non-replicative vectors for gene therapy. Vectors from each class can accommodate ≥30 kb of inserts, have been approved clinically, and demonstrate a relatively benign safety profile. Despite oncolytic HSV (oHSV) replication in tumors and elicited immune responses, the virus is well tolerated in cancer patients. Current non-replicative vectors elicit only limited immune responses. Seropositivity and immune responses against HSV-1 do not eliminate either the vector or infected cells, and the vectors can therefore be re-administered. In this review we highlight vectors that have been translated to the clinic and host-virus immune interactions that impact on the safety and efficacy of HSVs.

Co-option of mitochondrial nucleic acid sensing pathways by HSV-1 UL12.5 for reactivation from latent Infection

Although viruses subvert innate immune pathways for their replication, there is evidence they can also co-opt anti-viral responses for their benefit. The ubiquitous human pathogen, Herpes Simplex Virus-1 (HSV-1), encodes a protein (UL12.5) that induces the release of mitochondrial nucleic acid into the cytosol, which activates immune sensing pathways and reduces productive replication in non-neuronal cells. HSV-1 establishes latency in neurons and can reactivate to cause disease. We found that UL12.5 is required for HSV-1 reactivation in neurons and acts to directly promote viral lytic gene expression during initial exit from latency. Further, the direct activation of innate immune sensing pathways triggered HSV reactivation and compensated for a lack of UL12.5. Finally, we found that the induction of HSV-1 lytic genes during reactivation required intact RNA and DNA sensing pathways, demonstrating that HSV-1 can both respond to and active antiviral nucleic acid sensing pathways to reactivate from a latent infection.

Herpes Simplex Virus-1 targets the 2'-3'cGAMP importer SLC19A1 as an antiviral countermeasure

To establish a successful infection, herpes simplex virus-1 (HSV-1), a virus with high seropositivity in the human population, must undermine host innate and intrinsic immune defense mechanisms, including the stimulator of interferon genes (STING) pathway. Recently it was discovered that not only de novo produced intracellular 2'-3'cGAMP, but also extracellular 2'-3'cGAMP activates the STING pathway by being transported across the cell membrane via the folate transporter, SLC19A1, the first identified extracellular antiporter of this signaling molecule. We hypothesized that the import of exogenous 2'-3'cGAMP functions to establish an antiviral state like that seen with the paracrine antiviral activities of interferon. Further, to establish a successful infection, HSV-1 must undermine this induction of the STING pathway by inhibiting the biological functions of SLC19A1. Herein, we report that treatment of the monocytic cell line, THP-1 cells, epithelial cells (ARPE-19) and SH-SY5Y neuronal cell line with exogenous 2'-3'cGAMP induces interferon production and establishes an antiviral state. Using either pharmaceutical inhibition or genetic knockout of SLC19A1 blocks the 2'-3'cGAMP-induced antiviral state. Additionally, HSV-1 infection results in the reduction of SLC19A1 transcription, translation, and importantly, the rapid removal of SLC19A1 from the cell surface of infected cells. Our data indicate SLC19A1 functions as a newly identified antiviral mediator for extracellular 2'-3'cGAMP which is undermined by HSV-1. This work presents novel and important findings about how HSV-1 manipulates the host's immune environment for viral replication and discovers details about an antiviral mechanism which information could aid in the development of better antiviral drugs in the future.

Importance

HSV-1 has evolved multiple mechanisms to neutralize of the host's innate and intrinsic defense pathways, such as the STING pathway. Here, we identified an antiviral response in which extracellular 2'-3'cGAMP triggers IFN production via its transporter SLC19A1. Moreover, we report that HSV-1 blocks the functions of this transporter thereby impeding the antiviral response, suggesting exogenous 2'-3'cGAMP can act as an immunomodulatory molecule in uninfected cells to activate the STING pathway, and priming an antiviral state, similar to that seen in interferon responses. The details of this mechanism highlight important details about HSV-1 infections. This work presents novel findings about how HSV-1 manipulates the host's immune environment for viral replication and reveals details about a novel antiviral mechanism. These findings expand our understanding of how viral infections undermine host responses and may help in the development of better broad based antiviral drugs in the future.

Flavonoids with Anti-Herpes Simplex Virus Properties: Deciphering Their Mechanisms in Disrupting the Viral Life Cycle

The herpes simplex virus (HSV) is a double-stranded DNA human virus that causes persistent infections with recurrent outbreaks. HSV exists in two forms: HSV-1, responsible for oral herpes, and HSV-2, primarily causing genital herpes. Both types can lead to significant complications, including neurological issues. Conventional treatment, involving acyclovir and its derivatives, faces challenges due to drug resistance. This underscores the imperative for continual research and development of new drugs, with a particular emphasis on exploring the potential of natural antivirals. Flavonoids have demonstrated promise in combating various viruses, including those within the herpesvirus family. This review, delving into recent studies, reveals the intricate mechanisms by which flavonoids decode their antiviral capabilities against HSV. By disrupting key stages of the viral life cycle, such as attachment to host cells, entry, DNA replication, latency, and reactivation, flavonoids emerge as formidable contenders in the ongoing battle against HSV infections.

Therapeutic Potential of Biochanin A in Herpes Simplex Keratitis

Herpes simplex keratitis (HSK) is a blinding eye disease that is initiated by the herpes simplex virus type 1 (HSV-1). Resistance to acyclovir (ACV) and the side effects of corticosteroid drugs have become concerning issues, so it is crucial to develop new antivirals for treating HSK. In this study, we report that biochanin A (BCA), a naturally occurring flavonoid compound, provides multifaceted protective effects with anti-viral, anti-inflammatory, anti-oxidative stress and anti-apoptotic activities to alleviate HSK. The results show that BCA significantly inhibited HSV-1 replication in vitro and further proved that BCA principally influenced the early stage of virus infection. We reveal that BCA downregulated the expression of pro-inflammatory factors triggered by HSV-1, including TNF-α, RANTES, IL-1β and IL-6. Furthermore, BCA treatment alleviated oxidative stress and apoptotic arising from HSV-1 infection. Lastly, we induced HSK in male C57BL/6 mice and treated them with either BCA or phosphate buffer solution (PBS) eye drops. We observed the ocular surface lesions; determined the virus load in the tear fluid, corneas as well as trigeminal ganglions (TGs); and detected the levels of inflammation and apoptosis in the corneas simultaneously. These results show that BCA inhibits HSV-1 and alleviates the corneal lesion degree. Our study illustrates that BCA is a promising therapeutic approach for application in treating HSK.