Interferon (IFN) responses are critical for controlling herpes simplex virus 1 (HSV-1). The importance of neuronal IFN signaling in controlling acute and latent HSV-1 infection remains unclear. ...Compartmentalized neuron cultures revealed that mature sensory neurons respond to IFNβ at both the axon and cell body through distinct mechanisms, resulting in control of HSV-1. Mice specifically lacking neural IFN signaling succumbed rapidly to HSV-1 corneal infection, demonstrating that IFN responses of the immune system and non-neuronal tissues are insufficient to confer survival following virus challenge. Furthermore, neurovirulence was restored to an HSV strain lacking the IFN-modulating gene, γ34.5, despite its expected attenuation in peripheral tissues. These studies define a crucial role for neuronal IFN signaling for protection against HSV-1 pathogenesis and replication, and they provide a novel framework to enhance our understanding of the interface between host innate immunity and neurotropic pathogens.
Herpes simplex virus 1 (HSV-1) establishes lifelong infection in the neurons of trigeminal ganglia (TG), cycling between productive infection and latency. Neuronal antiviral responses are driven by ...type I interferon (IFN) and are crucial to controlling HSV-1 virulence. Autophagy also plays a role in this neuronal antiviral response, but the mechanism remains obscure. In this study, HSV-1 infection of murine TG neurons triggered unusual clusters of autophagosomes, predominantly in neurons lacking detectable HSV-1 antigen. Treatment of neurons with IFN-β induced a similar response, and cluster formation by infection or IFN treatment was dependent upon an intact IFN-signaling pathway. The autophagic clusters were decorated with both ISG15, an essential effecter of the antiviral response, and p62, a selective autophagy receptor. The autophagic clusters were not induced by rapamycin or starvation, consistent with a process of selective autophagy. While clusters were triggered by other neurotropic herpesviruses, infection with unrelated viruses failed to induce this response. Following ocular infection in vivo, clusters formed exclusively in the infected ophthalmic branch of the TG. Taken together, our results show that infection with HSV and antiviral signaling in TG neurons produce an unorthodox autophagic response. This autophagic clustering is associated with antiviral signaling, the presence of viral genome, and the absence of HSV protein expression and may therefore represent an important neuronal response to HSV infection and the establishment of latency.
Herpes simplex virus type 1 (HSV-1) is a ubiquitous virus and a significant cause of morbidity and some mortality. It is the causative agent of benign cold sores, but it can also cause blindness and life-threatening encephalitis. The success of HSV-1 is largely due to its ability to establish lifelong latent infections in neurons and to occasionally reactivate. The exact mechanisms by which neurons defend against virus infection is poorly understood, but such defense is at least partially mediated by autophagy, an intracellular pathway by which pathogens and other unwanted cargoes are degraded. The study demonstrates and investigates a new autophagic structure that appears to be specific to the interaction between neurotropic herpesviruses and murine primary sensory neurons. This work may therefore have important implications for our understanding of latency and reactivation.
During viral infection, pattern recognition receptors (PRRs) and their associated adaptors recruit TANK-binding kinase 1 (TBK1) to activate interferon regulatory factor 3 (IRF3), resulting in ...production of type I interferons (IFNs). ICP0 and ICP34.5 are among the proteins encoded by herpes simplex virus 1 (HSV-1) that modulate type I IFN signaling. We constructed a recombinant virus (ΔXX) that lacks amino acids 87 to 106, a portion of the previously described TBK1-binding domain of the γ34.5 gene (D. Verpooten, Y. Ma, S. Hou, Z. Yan, and B. He, J Biol Chem 284:1097-1105, 2009, https://doi.org/10.1074/JBC.M805905200). These 20 residues are outside the γ34.5 beclin1-binding domain (BBD) that interacts with beclin1 and regulates autophagy. Unexpectedly, ΔXX showed no deficit in replication
in a variety of tissues and showed virulence comparable to that of wild-type and marker-rescued viruses following intracerebral infection. ΔXX was fully capable of mediating the dephosphorylation of eIF2α, and the virus was capable of controlling the phosphorylation of IRF3. In contrast, a null mutant in γ34.5 failed to control IRF3 phosphorylation due to an inability of the mutant to sustain expression of ICP0. Our data show that while γ34.5 regulates IRF3 phosphorylation, the TBK1-binding domain itself has no impact on IRF3 phosphorylation or on replication and pathogenesis in mice.
Interferons (IFNs) are potent activators of a variety of host responses that serve to control virus infections. The
have evolved countermeasures to IFN responses. Herpes simplex virus 1 (HSV-1) encodes the multifunctional neurovirulence protein ICP34.5. In this study, we investigated the biological relevance of the interaction between ICP34.5 and TANK-binding kinase 1 (TBK1), an activator of IFN responses. Here, we establish that although ICP34.5 binds TBK1 under certain conditions through a TBK1-binding domain (TBD), there was no direct impact of the TBD on viral replication or virulence in mice. Furthermore, we showed that activation of IRF3, a substrate of TBK1, was independent of the TBD. Instead, we provided evidence that the ability of ICP34.5 to control IRF3 activation is through its ability to reverse translational shutoff and sustain the expression of other IFN inhibitors encoded by the virus. This work provides new insights into the immunomodulatory functions of ICP34.5.
The fetal/neonatal period represents both a unique window of opportunity for interventions as well as vulnerability to a number of viral infections. While
such as herpes simplex virus (HSV) are ...highly prevalent and typically of little consequence among healthy adults, they are among the most consequential infections of early life. Despite treatment with antiviral drugs, neonatal HSV (nHSV) infections can still result in significant mortality and lifelong neurological morbidity. Fortunately, newborns in our pathogen-rich world inherit some of the protection provided by the maternal immune system in the form of transferred antibodies. Maternal seropositivity, resulting in placental transfer of antibodies capable of neutralizing virus and eliciting the diverse effector functions of the innate immune system are associated with dramatically decreased risk of nHSV. Given this clear epidemiological evidence of reduced risk of infection and its sequelae, we present what is known about the ability of monoclonal antibody therapies to treat or prevent HSV infection and explore how effective antibody-based interventions in conjunction with antiviral therapy might reduce early life mortality and long-term morbidity.
After infecting peripheral sites, herpes simplex virus (HSV) invades the nervous system and initiates latent infection in sensory neurons. Establishment and maintenance of HSV latency require host ...survival, and entail repression of productive cycle (“lytic”) viral gene expression. We find that a neuron-specific microRNA, miR-138, represses expression of ICP0, a viral transactivator of lytic gene expression. A mutant HSV-1 (M138) with disrupted miR-138 target sites in ICP0 mRNA exhibits enhanced expression of ICP0 and other lytic proteins in infected neuronal cells in culture. Following corneal inoculation, M138-infected mice have higher levels of ICP0 and lytic transcripts in trigeminal ganglia during establishment of latency, and exhibit increased mortality and encephalitis symptoms. After full establishment of latency, the fraction of trigeminal ganglia harboring detectable lytic transcripts is greater in M138-infected mice. Thus, miR-138 is a neuronal factor that represses HSV-1 lytic gene expression, promoting host survival and viral latency.
Display omitted
•miR-138 is a neuron-specific microRNA•miR-138 represses expression of the HSV-1 lytic gene transactivator, ICP0•Mutating miR-138 target sites in ICP0 causes higher lytic gene expression in neurons•Mutating these miR-138 sites results in increased host morbidity and mortality
Herpes simplex virus forms latent infections in neurons, but how neurons contribute to latency has remained largely undefined. Pan et al. identify a neuron-specific microRNA, miR-138, that reduces the expression of a viral gene activator and “lytic” viral gene expression, and promotes host survival and viral latency.
Our current understanding of HSV latency is based on a variety of clinical observations, and in vivo, ex vivo, and in vitro model systems, each with unique advantages and drawbacks. The criteria for ...authentically modeling HSV latency include the ability to easily manipulate host genetics and biological pathways, as well as mimicking the immune response and viral pathogenesis in human infections. Although realistically modeling HSV latency is necessary when choosing a model, the cost, time requirement, ethical constraints, and reagent availability are also equally important. Presently, there remains a pressing need for in vivo models that more closely recapitulate human HSV infection. While the current in vivo, ex vivo, and in vitro models used to study HSV latency have limitations, they provide further insights that add to our understanding of latency. In vivo models have shed light on natural infection routes and the interplay between the host immune response and the virus during latency, while in vitro models have been invaluable in elucidating molecular pathways involved in latency. Below, we review the relative advantages and disadvantages of current HSV models and highlight insights gained through each.
Natural interferon-producing cells (IPCs) specialize in the production of high levels of type 1 interferons (IFNs) in response to encapsulated DNA and RNA viruses. Here we demonstrate that the ...secretion of type 1 IFN in response to herpes simplex virus type 1 (HSV-1) in vitro is mediated by the toll-like receptor 9 (TLR9)/MyD88 pathway. Moreover, IPCs produce interleukin-12 (IL-12) in response to HSV-1 in vitro, which is also dependent on TLR9/ MyD88 signaling. Remarkably, though TLR9/MyD88-deficiency abrogates IPC responses to HSV-1 in vitro, mice lacking either MyD88 or TLR9 are capable of controlling HSV-1 replication in vivo after local infection, demonstrating that TLR9- and MyD88-independent pathways in cells other than IPCs can effectively compensate for defective IPC responses to HSV-1.
STING is a protein in the cytosolic DNA and cyclic dinucleotide sensor pathway that is critical for the initiation of innate responses to infection by various pathogens. Consistent with this, herpes ...simplex virus 1 (HSV-1) causes invariable and rapid lethality in STING-deficient (STING(-/-)) mice following intravenous (i.v.) infection. In this study, using real-time bioluminescence imaging and virological assays, as expected, we demonstrated that STING(-/-) mice support greater replication and spread in ocular tissues and the nervous system. In contrast, they did not succumb to challenge via the corneal route even with high titers of a virus that was routinely lethal to STING(-/-) mice by the i.v. route. Corneally infected STING(-/-) mice also showed increased periocular disease and increased corneal and trigeminal ganglia titers, although there was no difference in brain titers. They also showed elevated expression of tumor necrosis factor alpha (TNF-α) and CXCL9 relative to control mice but surprisingly modest changes in type I interferon expression. Finally, we also showed that HSV strains lacking the ability to counter autophagy and the PKR-driven antiviral state had near-wild-type virulence following intracerebral infection of STING(-/-) mice. Together, these data show that while STING is an important component of host resistance to HSV in the cornea, its previously shown immutable role in mediating host survival by the i.v. route was not recapitulated following a mucosal infection route. Furthermore, our data are consistent with the idea that HSV counters STING-mediated induction of the antiviral state and autophagy response, both of which are critical factors for survival following direct infection of the nervous system.
HSV infections represent an incurable source of morbidity and mortality in humans and are especially severe in neonatal and immunocompromised populations. A key step in the development of an immune response is the recognition of microbial components within infected cells. The host protein STING is important in this regard for the recognition of HSV DNA and the subsequent triggering of innate responses. STING was previously shown to be essential for protection against lethal challenge from intravenous HSV-1 infection. In this study, we show that the requirement for STING depends on the infection route. In addition, STING is important for appropriate regulation of the inflammatory response in the cornea, and our data are consistent with the idea that HSV modulates STING activity through inhibition of autophagy. Our results elucidate the importance of STING in host protection from HSV-1 and demonstrate the redundancy of host protective mechanisms, especially following mucosal infection.
ABSTRACT
Herpes simplex virus (HSV) latency in neurons remains poorly understood, and the heterogeneity of the sensory nervous system complicates mechanistic studies. In this study, we used primary ...culture of adult trigeminal ganglion (TG) mouse neurons in microfluidic devices and an
in vivo
model to examine the subtypes of sensory neurons involved in HSV latency. HSV-infected neurofilament heavy-positive (NefH
+
) neurons were more likely to express latency-associated transcripts (LATs) than infected neurofilament heavy-negative (NefH
−
) neurons. This differential expression of the LAT promoter correlated with differences in HSV-1 early infection that manifested as differences in the efficiency with which HSV particles reached the cell body following infection at the distal axon.
In vivo
, we further identified a specific subset of NefH
+
neurons which coexpressed calcitonin gene-related peptide α (NefH
+
CGRP
+
neurons) as the sensory neuron subpopulation with the highest LAT promoter activity following HSV-1 infection. Finally, an early-phase reactivation assay showed HSV-1 reactivating in NefH
+
CGRP
+
neurons, although other sensory neuron subpopulations were also involved. Together, these results show that sensory neurons expressing neurofilaments exhibit enhanced LAT promoter activity. We hypothesize that the reduced efficiency of HSV-1 invasion at an early phase of infection may promote efficient establishment of latency in NefH
+
neurons due to initiation of the antiviral state preceding arrival of the virus at the neuronal cell body. While the outcome of HSV-1 infection of neurons is determined by a broad variety of factors
in vivo
, neuronal subtypes are likely to play differential roles in modulating the establishment of latent infection.
IMPORTANCE
Two pivotal properties of HSV-1 make it a successful pathogen. First, it infects neurons, which are immune privileged. Second, it establishes latency in these neurons. Together, these properties allow HSV to persist for the lifetime of its host. Neurons are diverse and highly organized cells, with specific anatomical, physiological, and molecular characteristics. Previous work has shown that establishment of latency by HSV-1 does not occur equally in all types of neurons. Our results show that the kinetics of HSV infection and the levels of latency-related gene expression differ in certain types of neurons. The neuronal subtype infected by HSV is therefore a critical determinant of the outcome of infection and latency.
PDF
