Evidence that deficient IFN-gamma production is a biological basis of herpes simplex virus type-2 neurovirulence

RIPK3 and caspase 8 collaborate to limit herpes simplex encephalitis

Invasion of the brain by herpes simplex virus 1 (HSV1) can lead to the development of herpes simplex encephalitis (HSE) that is often associated with significant morbidity and mortality regardless of therapeutic intervention. Both virus and host immune factors dictate HSE onset and progression. Because programmed cell death pathways including necroptosis are important antiviral defense mechanisms in HSV1-associated peripheral diseases, they might also play critical roles in HSV1 neuropathogenesis. HSV1-encoded ICP6 prevents receptor-interacting protein kinase 3 (RIPK3)-mediated necroptosis during infection of human cells, but it also acts as a species-dependent inducer of necroptosis in murine cells and thereby restricts virus replication. We therefore used an established mouse model of HSE to investigate RIPK3-mediated necroptosis impact on HSV1 neuropathogenesis. Following corneal HSV1 inoculation, RIPK3 knockout mice showed increased susceptibility to HSE when compared with wildtype mice indicating RIPK3 helps to limit HSE progression. RIPK3-mediated defense against HSE was found to be independent of the kinase domain necessary to drive necroptosis implicating that a death independent function of RIPK3 protects against HSE. Conversely the pro-necroptotic kinase function RIPK3 served to limit viral replication in corneal tissue implicating a tissue-specific RIPK3 function in limiting HSV1. Further evaluation of the kinase-independent mechanism to restrict HSE revealed that the RIPK3 signaling partner, caspase 8, contributes to limiting HSE neuropathogenesis. Increased HSE susceptibility from loss of caspase 8 and RIPK3 correlated with decreased levels of chemokines, cytokines, and antiviral lymphocytes recruitment to the brain. We conclude that RIPK3 contributes toward host control of HSV1 replication in a tissue-specific fashion. Whereas RIPK3-mediated necroptosis restricts virus replication within the cornea, kinase-independent induction of inflammation by RIPK3 in collaboration with caspase 8 restricts virus replication within the brain during HSE neuropathogenesis.

Viral genes and cellular markers associated with neurological complications during herpesvirus infections

Despite the importance of neurological disorders associated with herpesviruses, the mechanism by which these viruses influence the central nervous system (CNS) has not been definitively established. Owing to the limitations of studying neuropathogenicity of human herpesviruses in their natural host, many aspects of their pathogenicity and immune response are studied in animal models. Here, we present an important model system that enables studying neuropathogenicity of herpesviruses in the natural host. Equine herpesvirus type 1 (EHV-1) is an alphaherpesvirus that causes a devastating neurological disease (EHV-1 myeloencephalopathy; EHM) in horses. Like other alphaherpesviruses, our understanding of virus neuropathogenicity in the natural host beyond the essential role of viraemia is limited. In particular, information on the role of different viral proteins for virus transfer to the spinal cord endothelium in vivo is lacking. In this study, the contribution of two viral proteins, DNA polymerase (ORF30) and glycoprotein D (gD), to the pathogenicity of EHM was addressed. Furthermore, different cellular immune markers, including alpha-interferon (IFN-α), gamma-interferon (IFN-γ), interleukin-10 (IL-10) and interleukin-1 beta (IL-1β), were identified to play a role during the course of the disease.

Herpes simplex type 2 encephalitis and methotrexate medication: a fortuitous or causative association in a patient with spondyloarthritis?

It is unclear whether immunosuppression is a risk factor for herpes encephalitis. Herein, we describe a rare case of herpes simplex virus type 2 encephalitis in a patient treated with low-dose methotrexate for HLA-B27-associated spondyloarthritis. The patient was successfully treated with acyclovir but presented sequelae of encephalitis. Here we discuss the possible role of low-dose methotrexate therapy as a risk factor of neurological herpes reactivation and severe disease. The host-related and viral risk factors are also addressed.

Development of a glycoprotein D-expressing dominant-negative and replication-defective herpes simplex virus 2 (HSV-2) recombinant viral vaccine against HSV-2 infection in mice

Using the T-REx (Invitrogen, California) gene switch technology and a dominant-negative mutant polypeptide of herpes simplex virus 1 (HSV-1)-origin binding protein UL9, we previously constructed a glycoprotein D-expressing replication-defective and dominant-negative HSV-1 recombinant viral vaccine, CJ9-gD, for protection against HSV infection and disease. It was demonstrated that CJ9-gD is avirulent following intracerebral inoculation in mice, cannot establish detectable latent infection following different routes of infection, and offers highly effective protective immunity against primary HSV-1 and HSV-2 infection and disease in mouse and guinea pig models of HSV infections. Given these favorable safety and immunological profiles of CJ9-gD, aiming to maximize levels of HSV-2 glycoprotein D (gD2) expression, we have constructed an ICP0 null mutant-based dominant-negative and replication-defective HSV-2 recombinant, CJ2-gD2, that contains 2 copies of the gD2 gene driven by the tetracycline operator (tetO)-bearing HSV-1 major immediate-early ICP4 promoter. CJ2-gD2 expresses gD2 as efficiently as wild-type HSV-2 infection and can lead to a 150-fold reduction in wild-type HSV-2 viral replication in cells coinfected with CJ2-gD2 and wild-type HSV-2 at the same multiplicity of infection. CJ2-gD2 is avirulent following intracerebral injection and cannot establish a detectable latent infection following subcutaneous (s.c.) immunization. CJ2-gD2 is a more effective vaccine than HSV-1 CJ9-gD and a non-gD2-expressing dominant-negative and replication-defective HSV-2 recombinant in protection against wild-type HSV-2 genital disease. Using recall response, we showed that immunization with CJ2-gD2 elicited strong HSV-2-specific memory CD4(+) and CD8(+) T-cell responses. Collectively, given the demonstrated preclinical immunogenicity and its unique safety profiles, CJ2-gD2 represents a new class of HSV-2 replication-defective recombinant viral vaccines in protection against HSV-2 genital infection and disease.

Effector CD4+ T-cell involvement in clearance of infectious herpes simplex virus type 1 from sensory ganglia and spinal cords

In primary infection, CD8(+) T cells are important for clearance of infectious herpes simplex virus (HSV) from sensory ganglia. In this study, evidence of CD4(+) T-cell-mediated clearance of infectious HSV type 1 (HSV-1) from neural tissues was also detected. In immunocompetent mice, HSV-specific CD4(+) T cells were present in sensory ganglia and spinal cords coincident with HSV-1 clearance from these sites and remained detectable at least 8 months postinfection. Neural CD4(+) T cells isolated at the peak of neural infection secreted gamma interferon, tumor necrosis factor alpha, interleukin-2 (IL-2), or IL-4 after stimulation with HSV antigen. HSV-1 titers in neural tissues were greatly reduced over time in CD8(+) T-cell-deficient and CD8(+) T-cell-depleted mice, suggesting that CD4(+) T cells could mediate clearance of HSV-1 from neural tissue. To examine possible mechanisms by which CD4(+) T cells resolved neural infection, CD8(+) T cells were depleted from perforin-deficient or FasL-defective mice. Clearance of infectious virus from neural tissues was not significantly different in perforin-deficient or FasL-defective mice compared to wild-type mice. Further, in spinal cords and brains after vaginal HSV-1 challenge of chimeric mice expressing both perforin and Fas or neither perforin nor Fas, virus titers were significantly lower than in control mice. Thus, perforin and Fas were not required for clearance of infectious virus from neural tissues. These results suggest that HSV-specific CD4(+) T cells are one component of a long-term immune cell presence in neural tissues following genital HSV-1 infection and play a role in clearance of infectious HSV-1 at neural sites, possibly via a nonlytic mechanism.

The lack of RNA-dependent protein kinase enhances susceptibility of mice to genital herpes simplex virus type 2 infection

Mice deficient in RNA-dependent protein kinase (PKR-/-) or deficient in PKR and a functional 2',5'-oligoadenylate synthetase (OAS) pathway (PKR/RL-/-) are more susceptible to genital herpes simplex virus type 2 (HSV-2) infection than wild-type mice or mice that are deficient only in a functional OAS pathway (RL-/-) as measured by survival over 30 days. The increase in susceptibility correlated with an increase in virus titre recovered from vaginal tissue or brainstem of infected mice during acute infection. There was also an increase in CD45+ cells and CD8+ T cells residing in the central nervous system of HSV-2-infected PKR/RL-/- mice in comparison with RL-/- or wild-type control animals. In contrast, there was a reduction in the HSV-specific CD8+ T cells within the draining lymph node of the PKR/RL-/- mice. Collectively, activation of PKR, but not of OAS, contributes significantly to the local control and spread of HSV-2 following genital infection.

Estradiol is required for a proper immune response to bacterial and viral pathogens in the female brain

Although the neuroprotective effects of estrogens are well recognized, the exact mechanisms involved in the ability of these sex steroids to protect the cerebral tissue still remain unclear. We tested in our study the hypothesis that estradiol (E(2)) modulates the innate immune response and expression of genes encoding proteins that a provide survival signal to neurons during infection. Mice received a single systemic or cerebral injection of LPS to trigger a robust but transient inflammatory reaction in the brain. The endotoxin increased transcriptional activation of genes encoding TLR2, TNF-alpha, and IL-12 in microglial cells. Expression of these transcripts was largely inhibited in the brain of ovariectomized mice at time 24 h postchallenge. E(2) replacement therapy totally rescued the ability of the endotoxin to trigger microglial cells and these permissive effects of E(2) are mediated via the estrogen receptor (ER)alpha. Indeed, ERalpha-deficient mice exhibited an inappropriate reaction to LPS when compared with ERbeta-deficient and wild-type mice. This defective innate immune response was also associated with a widespread viral replication and neurodegeneration in ovariectomized mice inoculated intranasally with HSV-2. These data provide evidence that interaction of E(2) with their nuclear ERalpha plays a critical role in the control of cytokines involved in the transfer from the innate to adaptive immunity. This transfer is deviant in mice lacking E(2), which allows pathogens to hide from immune surveillance and exacerbates neuronal damages during viral encephalitis.

Valacyclovir treatment ameliorates the persistently increased pentylenetetrazol-induced seizure susceptibility in mice with herpes simplex virus type 1 infection

Herpes simplex virus type 1 (HSV-1) is an important pathogen related to epilepsy. We have shown previously that corneal inoculation of mice with HSV-1 causes acute spontaneous behavioral and electrophysiological seizures and increases hippocampal excitability and kainite-induced seizure susceptibility. In this study, we aimed to determine whether early-life HSV-1 infection in mice might cause short- and long-term enhanced susceptibility to pentylenetetrazol (PTZ)-induced seizures and to evaluate whether early antiviral drug therapy was effectively ameliorating this deficit. Seizure threshold was calculated by the latency of onset of the myoclonic jerk, generalized clonus, and maximal tonic-clonic convulsion. We demonstrate that the localization of viral antigens was predominantly within the bilateral temporal areas (amygdala, piriform, and entorhinal cortex) of HSV-1-infected mice. We also present evidence that mice of all HSV-1-infected groups had a shorter latency and higher severity to PTZ-induced seizures than in age-matched, mock-infected controls. Treatment of HSV-1-infected mice with valacyclovir, a potent inhibitor of HSV-1 replication, produced a dose-dependent decrease in the signs of neurological deficits, pathological damages, and PTZ-induced seizure severity. Our results are consistent with the hypothesis that early-life HSV-1 infection leads to persistent enhancement of neuronal excitability in limbic circuits, which could result in an overall increased propensity to induce seizures later in life. Additionally, prompt optimal antiviral therapy effectively decreases seizure susceptibility in HSV-1-infected mice by limiting the level of viral replication and inflammatory response induced by virus. The present study provides not only experimental evidence, but also a new therapeutic strategy in HSV-1-associated human epilepsy.

Age- and host-dependent control of Borna disease virus spread in the developing brains of gerbils and rats

Borna disease virus (BDV) is a non-cytolytic, neurotropic RNA virus that has a broad host range in warm-blooded animals, probably including humans. Recently, we have demonstrated that the neonatal gerbil is a unique model for analyzing BDV-induced acute neurological disease. In this report, to understand the effects of the brain development of gerbils in BDV-induced neuropathogenesis, as well as to investigate the host-dependent differences in BDV propagation and pathogenesis in the brains, we performed experimental infection of BDV using two different infant rodent models, gerbils and rats. We demonstrated here that most of the gerbils infected with BDV on postnatal days (PD) 14, but not on PD1 and PD7, could survive neurological disorders during the observation period of PD85. Interestingly, the levels of BDV RNA and antigen in surviving PD14 inoculated gerbil brains were extremely low, whereas diseased gerbils and both PD7 and PD14 inoculated rats contained significant amounts of BDV antigen in the central nervous system, suggesting that PD14 gerbils successfully controlled BDV spread in the brain. Furthermore, the viral distribution, as well as the expression levels of cytokine and CD8 mRNAs, in the brains was markedly different between the rodent models and between diseased and non-diseased statuses of the gerbils. These results demonstrated that developmentally regulated and host-specific factors could contribute to the prevention of BDV spread in developing animal brains. Studies using different animal systems would provide novel insights into the mechanisms of host defense responses to neurotropic virus infections.

Quantitation of viral load in neonatal herpes simplex virus infection and comparison between type 1 and type 2

Neonatal herpes simplex virus (HSV) infection is a severe disease with high mortality and morbidity in spite of the development of effective anti-viral therapies. The viral load in neonatal herpes simplex virus (HSV) infection was measured retrospectively in 37 patients. HSV DNA copy numbers in serum and cerebrospinal fluid (CSF) were quantified using a real-time PCR assay. Patients with disseminated infection had a higher viral load in their sera. whereas patients with central nervous system (CNS) infection exhibited a higher viral load in the CSF. The viral load was significantly higher in the serum of patients who died later. Interestingly, patients with HSV type-2 infection exhibited more CNS involvement and neurological impairment, together with a high viral load in the CSF, than did HSV type-1 patients. These results suggest that quantitation of HSV viral load may be useful for assessing the prognosis, and may provide additional information for the management of neonatal HSV infection.

Intranasal herpes simplex virus type 2 inoculation causes a profound thymidine kinase dependent cerebral inflammatory response in the mouse hindbrain

The herpes simplex virus (HSV) has the ability to replicate in the central nervous system (CNS), which may cause fatal encephalitis. The present study investigated the activity of the nuclear factor kappa B (NF-kappa B) and the pattern of cytokine/chemokine gene expression across the brain of HSV-infected mice and the role of the viral thymidine kinase (TK) in mediating these effects. Mice were killed 1-8 days after intranasal inoculation with either HSV-2 TK-competent or TK-deficient clinical isolates. Animals infected with the TK-competent virus exhibited first signs of infection at day 5 postinoculation, whereas severe signs of sickness were observed between day 6 and 8. A robust hybridization signal was found in the brain of these animals for the gene encoding the inhibitory factor kappa B alpha (I kappa B alpha, index of NF-kappa B activity), toll-like receptor 2 (TLR2), tumour necrosis factor alpha (TNF-alpha) and monocyte chemoattractant protein-1 (MCP-1) in numerous regions of the pons and medulla. The levels of expression of these genes increased 4 days after the inoculation and peaked at day 6 within the endothelium of the brain capillaries and cells of myeloid origin. A robust signal for the TK gene and its encoding protein was detected selectively within the regions that exhibited expression of the immune molecules. In contrast, animals that received the TK-deficient virus did not show any signs of sickness or cerebral inflammation or HSV replication within the cerebral tissue. The present data provide clear evidence that HSV-2 has the ability to trigger a profound inflammatory response in a pattern that follows the viral TK-dependent HSV replication in neurons. Such neurovirulence occurring in the hindbrain is proposed here to be directly responsible for neurodegeneration and to lead to the cerebral innate immune response, which in turn could play a key role in fatal HSV-2-induced encephalitis.

Cytokines in experimental herpes simplex virus infection

Herpes simplex virus (HSV) causes productive and latent forms of infection in humans and experimental animals. The primary infection and reactivation of the latent infection evoke an immune response in the host organism, involving activities of macrophages, CD4+ and CD8+ lymphocytes, and B lymphocytes. Strong cytokine responses are associated with the acute and recurrent phases of HSV infection. Also, during the latent phase of HSV infection in the sensory ganglia, expression of certain cytokines can be detected. The cytokine response to HSV infection is dominated by proinflammatory and Th1 type cytokines; however, Th2 type cytokines such as interleukin-4 also are expressed in the infected tissue. The use of novel HSV-derived, cytokine-expressing gene therapy vectors necessitates studies on the possible modulation of the host responses by the virus-encoded cytokine transgenes. This review focuses on the roles of certain Th1 and Th2 type cytokines in different phases of the experimental HSV infections.

Herpes simplex virus type 1 infection induces upregulation of interleukin-23 (p19) mRNA expression in trigeminal ganglia of BALB/c mice

We investigated the expression kinetics of several cytokines in trigeminal ganglia (TG) and in brains of BALB/c mice during the course of ocular herpes simplex virus type 1 (HSV-1) infection. All mice recovered from the infection within 2 weeks. The quantitative rapid real-time RT-PCR method was used to analyze interleukin-4 (IL-4), interferon-gamma (IFN-gamma), IL-12p35, IL-12p40, and the recently described IL-23 (p19) mRNA in TG, brain, and splenocyte samples. In TG, we found elevated expression of mRNA for IL-23 (p19) from early acute infection (day 3) to the beginning of the latent phase (day 14). The increase was not detected in brain or in the spleen. IL-4 expression occurred in both TG and brain from the beginning of the experiment to the latent phase. During the latent phase (days 14 and 31), IL-4 expression was significantly elevated in the brain when compared with the uninfected controls (p < 0.05). Considerable expression of IFN-gamma mRNA was detected in TG of mice during acute HSV-1 infection. The expression of IL-23 was detected also in the brains of the mice, even though no significant changes were found during the acute HSV-1 infection. This is, to our knowledge, the first report to show elevated expression of IL-23 (p19) mRNA (p < 0.05) during viral infection in TG of mice.

Contribution of T cells to mortality in neurovirulent Sindbis virus encephalomyelitis

Intranasal inoculation of C57BL/6 mice with a neurovirulent strain of Sindbis virus (SV) results in fatal encephalomyelitis. Mice with selective immune deficiencies were studied to determine the role of the immune response in fatal outcome. Mortality was decreased in mice deficient in alphabeta, but not gammadelta, T cells demonstrating a contribution of alphabeta T cells. Mice lacking either CD4+ or CD8+ T cells also had reduced mortality and mice lacking interferon (IFN)-gamma were completely protected. Clearance of infectious virus was identical in mice without T cells or IFN-gamma, but clearance of viral RNA was delayed compared to normal mice. Mice unable to produce antibody, perforin, Fas, TNF-alpha receptor1, IL-6 or IL-12 were not protected. These data suggest that T cells contribute to fatal acute viral encephalomyelitis through the production of IFN-gamma.

Stress-associated immunomodulation and herpes simplex virus infections

Stress has been shown to modulate an individual's immune system through the release of certain signal molecules such as catecholamines, cytokines and glucocorticoids. These signal molecules can significantly alter the host immune system and leave it susceptible to a primary or recurrent viral infection. Focusing on herpes simplex virus types-1 and -2 as examples, the authors explain how stress-associated immunomodulation can influence the recurrence of herpes simplex viral infections. Specific signal molecules such as epinephrine, interleukin-6, cyclic adenosine monophosphate, glucocorticoids and prostaglandins are upregulated during episodes of acute and chronic stress and have been implicated as effectors of herpes simplex viral reactivation and recurrent disease. The authors suggest that the release of immunomodulating signal molecules due to stress can compromise the host's cellular immune response and trigger herpes simplex viral reactivation.

A pathogenic role of Th2 responses on the severity of encephalomyelitis induced in mice by herpes simplex virus type 2 infection

A pathogenic role of Th2 cells and their cytokine products (IL-4 and IL-10, Th2 cytokines) on the development of herpes simplex myelitis (HSM) was studied in mice exposed to footpad injection of herpes simplex virus type 2 (HSV-2). Morbidity and mortality of mice with HSM (HSM mice) increased when they were treated with a mixture of Th2 cytokines. Additionally, survival rates of HSM mice increased when they were treated with a mixture of mAbs for Th2 cytokines. As compared with HSM mice treated with saline, the growth of HSV-2 in spinal cords of HSM mice treated with the mixture of Th2 cytokines increased. Th2 cells (myelitis-associated Th2 cells, MTh2 cells) were demonstrated among cerebrospinal fluid cells from HSM mice. After the stimulation with HSV-2 antigen (Ag), MTh2 cells from HSM mice previously treated with the mixture of Th2 cytokines produced enhanced amounts of Th2 cytokines into their culture fluids, as compared with the amount of Th2 cytokines produced by MTh2 cells. Th2 cells were also demonstrated in mononuclear cells from spleens of HSM mice. When compared with HSM mice inoculated with splenic CD4(+) T cells from normal mice, morbidity and mortality of HSM mice inoculated with MTh2 cells markedly increased. These results indicated that the severity of HSM induced in mice by footpad injection of HSV-2 was influenced by MTh2 cells or Th2 cytokines released from these MTh2 cells. Th2 responses manifested in mice by HSV-2 infection may act as a pathogenic enhancer of HSM severities.

Evidence for deficiencies in intracerebral cytokine production, adhesion molecule induction, and T cell recruitment in herpes simplex virus type-2 infected mice

We examined the intracerebral T cell response in mice infected with neurovirulent HSV-2 strains and an avirulent HSV-1. In HSV-2-infected brains, (i) IL-1beta, TNF-alpha and IFN-gamma mRNA expression was low, (ii) ICAM-1 and VCAM-1 were not induced, (iii) few CD4+ or CD8+ cells were detected. By contrast, in HSV-1-infected brains, (i) cytokine mRNA expression was high, (ii) adhesion molecules were strongly expressed, (iii) many T cells were detected. We suggest that deficient T cell extravasation into HSV-2-infected brain regions is caused by negligible ICAM-1 and VCAM-1 expression, which is due to low expression of critical cytokines.