Control of Sindbis virus infection by antibody in interferon-deficient mice

Interferon regulatory factor 7 modulates virus clearance and immune responses to alphavirus encephalomyelitis

IMPORTANCE

Viral encephalomyelitis outcome is dependent on host responses to neuronal infection. Interferon (IFN) is an important component of the innate response, and IFN regulatory factor (IRF) 7 is an inducible transcription factor for the synthesis of IFN-α. IRF7-deficient mice develop fatal paralysis after CNS infection with Sindbis virus, while wild-type mice recover. Irf7 -/- mice produce low levels of IFN-α but high levels of IFN-β with induction of IFN-stimulated genes, so the reason for this difference is not understood. The current study shows that Irf7 -/- mice developed inflammation earlier but failed to clear virus from motor neuron-rich regions of the brainstem and spinal cord. Levels of IFN-γ and virus-specific antibody were comparable, indicating that IRF7 deficiency does not impair expression of these known viral clearance factors. Therefore, IRF7 is either necessary for the neuronal response to currently identified mediators of clearance or enables the production of additional antiviral factor(s) needed for clearance.

Immune responses to Tilapia lake virus infection: what we know and what we don't know

Tilapia lake virus (TiLV) is a novel contagious pathogen associated with a lethal disease affecting and decimating tilapia populations on several continents across the globe. Fish viral diseases, such as Tilapia lake virus disease (TiLVD), represent a serious threat to tilapia aquaculture. Therefore, a better understanding of the innate immune responses involved in establishing an antiviral state can help shed light on TiLV disease pathogenesis. Moreover, understanding the adaptive immune mechanisms involved in mounting protection against TiLV could greatly assist in the development of vaccination strategies aimed at controlling TiLVD. This review summarizes the current state of knowledge on the immune responses following TiLV infection. After describing the main pathological findings associated with TiLVD, both the innate and adaptive immune responses and mechanisms to TiLV infection are discussed, in both disease infection models and in vitro studies. In addition, our work, highlights research questions, knowledge gaps and research areas in the immunology of TiLV infection where further studies are needed to better understand how disease protection against TiLV is established.

Immunopathogenesis of alphaviruses

Alphaviruses, members of the enveloped, positive-sense, single-stranded RNA Togaviridae family, represent a reemerging public health threat as mosquito vectors expand into new geographic territories. The Old World alphaviruses, which include chikungunya virus, Ross River virus, and Sindbis virus, tend to cause a clinical syndrome characterized by fever, rash, and arthritis, whereas the New World alphaviruses, which consist of Venezuelan equine encephalitis virus, eastern equine encephalitis virus, and western equine encephalitis virus, induce encephalomyelitis. Following recovery from the acute phase of infection, many patients are left with debilitating persistent joint and neurological complications that can last for years. Clues from human cases and studies using animal models strongly suggest that much of the disease and pathology induced by alphavirus infection, particularly atypical and chronic manifestations, is mediated by the immune system rather than directly by the virus. This review discusses the current understanding of the immunopathogenesis of the arthritogenic and neurotropic alphaviruses accumulated through both natural infection of humans and experimental infection of animals, particularly mice. As treatment following alphavirus infection is currently limited to supportive care, understanding the contribution of the immune system to the disease process is critical to developing safe and effective therapies.

Both ADP-Ribosyl-Binding and Hydrolase Activities of the Alphavirus nsP3 Macrodomain Affect Neurovirulence in Mice

Macrodomain (MD), a highly conserved protein fold present in a subset of plus-strand RNA viruses, binds to and hydrolyzes ADP-ribose (ADPr) from ADP-ribosylated proteins. ADPr-binding by the alphavirus nonstructural protein 3 (nsP3) MD is necessary for the initiation of virus replication in neural cells, whereas hydrolase activity facilitates replication complex amplification. To determine the importance of these activities for pathogenesis of alphavirus encephalomyelitis, mutations were introduced into the nsP3 MD of Sindbis virus (SINV), and the effects on ADPr binding and hydrolase activities, virus replication, immune responses, and disease were assessed. Elimination of ADPr-binding and hydrolase activities (G32E) severely impaired in vitro replication of SINV in neural cells and in vivo replication in the central nervous systems of 2-week-old mice with reversion to wild type (WT) (G) or selection of a less compromising change (S) during replication. SINVs with decreased binding and hydrolase activities (G32S and G32A) or with hydrolase deficiency combined with better ADPr-binding (Y114A) were less virulent than WT virus. Compared to the WT, the G32S virus replicated less well in both the brain and spinal cord, induced similar innate responses, and caused less severe disease with full recovery of survivors, whereas the Y114A virus replicated well, induced higher expression of interferon-stimulated and NF-κB-induced genes, and was cleared more slowly from the spinal cord with persistent paralysis in survivors. Therefore, MD function was important for neural cell replication both in vitro and in vivo and determined the outcome from alphavirus encephalomyelitis in mice.IMPORTANCE Viral encephalomyelitis is an important cause of long-term disability, as well as acute fatal disease. Identifying viral determinants of outcome helps in assessing disease severity and developing new treatments. Mosquito-borne alphaviruses infect neurons and cause fatal disease in mice. The highly conserved macrodomain of nonstructural protein 3 binds and can remove ADP-ribose (ADPr) from ADP-ribosylated proteins. To determine the importance of these functions for virulence, recombinant mutant viruses were produced. If macrodomain mutations eliminated ADPr-binding or hydrolase activity, viruses did not grow. If the binding and hydrolase activities were impaired, the viruses grew less well than the wild-type virus, induced similar innate responses, and caused less severe disease, and most of the infected mice recovered. If binding was improved, but hydrolase activity was decreased, the virus replicated well and induced greater innate responses than did the WT, but clearance from the nervous system was impaired, and mice remained paralyzed. Therefore, macrodomain function determined the outcome of alphavirus encephalomyelitis.

To Go or Stay: The Development, Benefit, and Detriment of Tissue-Resident Memory CD8 T Cells during Central Nervous System Viral Infections

CD8 T cells coordinate immune defenses against viral infections of the central nervous system (CNS). Virus-specific CD8 T cells infiltrate the CNS and differentiate into brain-resident memory CD8 T cells (CD8 bT_RM). CD8 bT_RM are characterized by a lack of recirculation and expression of phenotypes and transcriptomes distinct from other CD8 T cell memory subsets. CD8 bT_RM have been shown to provide durable, autonomous protection against viral reinfection and the resurgence of latent viral infections. CD8 T cells have also been implicated in the development of neural damage following viral infection, which demonstrates that the infiltration of CD8 T cells into the brain can also be pathogenic. In this review, we will explore the residency and maintenance requirements for CD8 bT_RM and discuss their roles in controlling viral infections of the brain.

Interferon Control of Neurotropic Viral Infections

Interferons (IFNs) comprise a pleiotropic family of signaling molecules that are often the first line of defense against viral infection. Inflammatory responses induced by IFN are generally well tolerated during peripheral infections; yet, the same degree of inflammation during infection of the central nervous system (CNS) could be catastrophic. Thus, IFN responses must be modified within the CNS to ensure host survival. In this review, we discuss emerging principles highlighting unique aspects of antiviral effects of IFN protection following neurotropic viral infection, chiefly using new techniques in rodent models. Evaluation of these unique responses provides insights into how the immune system eradicates or controls pathogens, while avoiding host damage. Defining these principles may have direct impact on the development of novel clinical approaches.

Interferon regulatory factors 3 and 7 have distinct roles in the pathogenesis of alphavirus encephalomyelitis

Interferon (IFN) regulatory factors (IRFs) are important determinants of the innate response to infection. We evaluated the role(s) of combined and individual IRF deficiencies in the outcome of infection of C57BL/6 mice with Sindbis virus, an alphavirus that infects neurons and causes encephalomyelitis. The brain and spinal cord levels of Irf7, but not Irf3 mRNAs, were increased after infection. IRF3/5/7-/- and IRF3/7-/- mice died within 3-4 days with uncontrolled virus replication, similar to IFNα receptor-deficient mice, while all wild-type (WT) mice recovered. IRF3-/- and IRF7-/- mice had brain levels of IFNα that were lower, but brain and spinal cord levels of IFNβ and IFN-stimulated gene mRNAs that were similar to or higher than WT mice without detectable serum IFN or increases in Ifna or Ifnb mRNAs in the lymph nodes, indicating that the differences in outcome were not due to deficiencies in the central nervous system (CNS) type I IFN response. IRF3-/- mice developed persistent neurological deficits and had more spinal cord inflammation and higher CNS levels of Il1b and Ifnγ mRNAs than WT mice, but all mice survived. IRF7-/- mice died 5-8 days after infection with rapidly progressive paralysis and differed from both WT and IRF3-/- mice in the induction of higher CNS levels of IFNβ, tumour necrosis factor (TNF) α and Cxcl13 mRNA, delayed virus clearance and more extensive cell death. Therefore, fatal disease in IRF7-/- mice is likely due to immune-mediated neurotoxicity associated with failure to regulate the production of inflammatory cytokines such as TNFα in the CNS.

Type I Interferons in NeuroHIV

Infection with Human Immunodeficiency Virus (HIV)-1 continues to cause HIV-associated neurocognitive disorders despite combined antiretroviral therapy. Interferons (IFNs) are important for any antiviral immune response, but the lasting production of IFNα causes exhaustive activation leading eventually to progression to AIDS. Expression of IFNα in the HIV-exposed central nervous system has been linked to cognitive impairment and inflammatory neuropathology. In contrast, IFNβ exerts anti-inflammatory effects, appears to control, at least temporarily, lentiviral infection in the brain and provides neuroprotection. The dichotomy of type I IFN effects on HIV-1 infection and the associated brain injury will be discussed in this review, because the underlying mechanisms require further investigation to allow harnessing these innate immune factors for therapeutic purposes.

Genetic control of alphavirus pathogenesis

Alphaviruses, members of the positive-sense, single-stranded RNA virus family Togaviridae, represent a re-emerging public health concern worldwide as mosquito vectors expand into new geographic ranges. Members of the alphavirus genus tend to induce clinical disease characterized by rash, arthralgia, and arthritis (chikungunya virus, Ross River virus, and Semliki Forest virus) or encephalomyelitis (eastern equine encephalitis virus, western equine encephalitis virus, and Venezuelan equine encephalitis virus), though some patients who recover from the initial acute illness may develop long-term sequelae, regardless of the specific infecting virus. Studies examining the natural disease course in humans and experimental infection in cell culture and animal models reveal that host genetics play a major role in influencing susceptibility to infection and severity of clinical disease. Genome-wide genetic screens, including loss of function screens, microarrays, RNA-sequencing, and candidate gene studies, have further elucidated the role host genetics play in the response to virus infection, with the immune response being found in particular to majorly influence the outcome. This review describes the current knowledge of the mechanisms by which host genetic factors influence alphavirus pathogenesis and discusses emerging technologies that are poised to increase our understanding of the complex interplay between viral and host genetics on disease susceptibility and clinical outcome.

Following Acute Encephalitis, Semliki Forest Virus is Undetectable in the Brain by Infectivity Assays but Functional Virus RNA Capable of Generating Infectious Virus Persists for Life

Alphaviruses are mosquito-transmitted RNA viruses which generally cause acute disease including mild febrile illness, rash, arthralgia, myalgia and more severely, encephalitis. In the mouse, peripheral infection with Semliki Forest virus (SFV) results in encephalitis. With non-virulent strains, infectious virus is detectable in the brain, by standard infectivity assays, for around ten days. As we have shown previously, in severe combined immunodeficient (SCID) mice, infectious virus is detectable for months in the brain. Here we show that in MHC-II^-/- mice, with no functional CD4 T-cells, infectious virus is also detectable in the brain for long periods. In contrast, in the brains of CD8^-/- mice, virus RNA persists but infectious virus is not detectable. In SCID mice infected with SFV, repeated intraperitoneal administration of anti-SFV immune serum rapidly reduced the titer of infectious virus in the brain to undetectable, however virus RNA persisted. Repeated intraperitoneal passive transfer of immune serum resulted in maintenance of brain virus RNA, with no detectable infectious virus, for several weeks. When passive antibody transfer was stopped, antibody levels declined and infectious virus was again detectable in the brain. In aged immunocompetent mice, previously infected with SFV, immunosuppression of antibody responses many months after initial infection also resulted in renewed ability to detect infectious virus in the brain. In summary, antiviral antibodies control and determine whether infectious virus is detectable in the brain but immune responses cannot clear this infection from the brain. Functional virus RNA capable of generating infectious virus persists and if antibody levels decline, infectious virus is again detectable.

Germ Line IgM Is Sufficient, but Not Required, for Antibody-Mediated Alphavirus Clearance from the Central Nervous System

Sindbis virus (SINV) infection of neurons in the brain and spinal cord in mice provides a model system for investigating recovery from encephalomyelitis and antibody-mediated clearance of virus from the central nervous system (CNS). To determine the roles of IgM and IgG in recovery, we compared the responses of immunoglobulin-deficient activation-induced adenosine deaminase-deficient (AID^-/-), secretory IgM-deficient (sIgM^-/-), and AID^-/- sIgM^-/- double-knockout (DKO) mice with those of wild-type (WT) C57BL/6 mice for disease, clearance of infectious virus and viral RNA from brain and spinal cord, antibody responses, and B cell infiltration into the CNS. Because AID is essential for immunoglobulin class switch recombination and somatic hypermutation, AID^-/- mice produce only germ line IgM, while sIgM^-/- mice secrete IgG but no IgM and DKO mice produce no secreted immunoglobulin. After intracerebral infection with the TE strain of SINV, most mice recovered. Development of neurologic disease occurred slightly later in sIgM^-/- mice, but disease severity, weight loss, and survival were similar between the groups. AID^-/- mice produced high levels of SINV-specific IgM, while sIgM^-/- mice produced no IgM and high levels of IgG2a compared to WT mice. All mice cleared infectious virus from the spinal cord, but DKO mice failed to clear infectious virus from brain and had higher levels of viral RNA in the CNS late after infection. The numbers of infected cells and the amount of cell death in brain were comparable. We conclude that antibody is required and that either germ line IgM or IgG is sufficient for clearance of virus from the CNS.IMPORTANCE Mosquito-borne alphaviruses that infect neurons can cause fatal encephalomyelitis. Recovery requires a mechanism for the immune system to clear virus from infected neurons without harming the infected cells. Antiviral antibody has previously been shown to be a noncytolytic means for alphavirus clearance. Antibody-secreting cells enter the nervous system after infection and produce antiviral IgM before IgG. Clinical studies of human viral encephalomyelitis suggest that prompt production of IgM is associated with recovery, but it was not known whether IgM is effective for clearance. Our studies used mice deficient in production of IgM, IgG, or both to characterize the antibody necessary for alphavirus clearance. All mice developed similar signs of neurologic disease and recovered from infection. Antibody was necessary for virus clearance from the brain, and either early germ line IgM or IgG was sufficient. These studies support the clinical observation that prompt production of antiviral antibody is a determinant of outcome.

Age has a role in driving host immunopathological response to alphavirus infection

Alphaviruses are a group of arthropod-borne pathogens capable of causing a wide spectrum of clinical symptoms, ranging from milder symptoms like rashes, fever and polyarthralgia, to life-threatening encephalitis. This genus of viruses is prevalent globally, and can infect patients across a wide age range. Interestingly, disease severity of virus-infected patients is wide-ranging. Definitions of the pathogenesis of alphaviruses, as well as the host factors influencing disease severity, remain limited. The innate and adaptive immune systems are important host defences against alphavirus infections. Several reports have highlighted the roles of specific immune subsets in contributing to the immune pathogenesis of these viruses. However, immunosenescence, a gradual deterioration of the immune system brought about by the natural advancement of age, affects the functional roles of these immune subsets. This phenomenon compromises the host's ability to defend against alphavirus infection and pathogenesis. In addition, the lack of maturity in the immune system in newborns and infants also results in more severe disease outcomes. In this review, we will summarize the subtle yet diverse physiological changes in the immune system during aging, and how these changes underlie the differences in disease severity for common alphaviruses.

Glutamine antagonist-mediated immune suppression decreases pathology but delays virus clearance in mice during nonfatal alphavirus encephalomyelitis

Infection of weanling C57BL/6 mice with the TE strain of Sindbis virus (SINV) causes nonfatal encephalomyelitis associated with hippocampal-based memory impairment that is partially prevented by treatment with 6-diazo-5-oxo-l-norleucine (DON), a glutamine antagonist (Potter et al., J Neurovirol 21:159, 2015). To determine the mechanism(s) of protection, lymph node and central nervous system (CNS) tissues from SINV-infected mice treated daily for 1 week with low (0.3mg/kg) or high (0.6mg/kg) dose DON were examined. DON treatment suppressed lymphocyte proliferation in cervical lymph nodes resulting in reduced CNS immune cell infiltration, inflammation, and cell death compared to untreated SINV-infected mice. Production of SINV-specific antibody and interferon-gamma were also impaired by DON treatment with a delay in virus clearance. Cessation of treatment allowed activation of the antiviral immune response and viral clearance, but revived CNS pathology, demonstrating the ability of the immune response to mediate both CNS damage and virus clearance.

Interferon gamma modulation of disease manifestation and the local antibody response to alphavirus encephalomyelitis

Infection of mice with Sindbis virus (SINV) produces encephalomyelitis and provides a model for examination of the central nervous system (CNS) immune response to alphavirus infection. Clearance of infectious virus is accomplished through a cooperative effort between SINV-specific antibody and IFN-γ, but the regulatory interactions are poorly understood. To determine the effects of IFN-γ on clinical disease and the antiviral immune response, C57BL/6 mice lacking IFN-γ (Ifng-/-) or IFN-γ receptor (Ifngr1-/-) were studied in comparison to WT mice. Maximum production of Ifng mRNA and IFN-γ protein in the CNS of WT and Ifngr1-/- mice occurred 5-7 days after infection, with higher levels of IFN-γ in Ifngr1-/- mice. Onset of clinical disease was earlier in mice with impaired IFN-γ signalling, although Ifngr1-/- mice recovered more rapidly. Ifng-/- and Ifngr1-/- mice maintained body weight better than WT mice, associated with better food intake and lower brain levels of inflammatory cytokines. Clearance of infectious virus from the spinal cords was slower, and CNS, but not serum, levels of SINV-specific IgM, IgG2a and IgG2b were lower in Ifngr1-/- and Ifng-/- mice compared to WT mice. Decreased CNS antiviral antibody was associated with lower expression of mRNAs for B-cell attracting chemokines CXCL9, CXCL10 and CXCL13 and fewer B cells in the CNS. Therefore, IFN-γ signalling increases levels of CNS pro-inflammatory cytokines, leading to clinical disease, but synergistically clears virus with SINV-specific antibody at least in part by increasing chemokine production important for infiltration of antibody-secreting B cells into the CNS.

Host translation shutoff mediated by non-structural protein 2 is a critical factor in the antiviral state resistance of Venezuelan equine encephalitis virus

Most previous studies of interferon-alpha/beta (IFN-α/β) response antagonism by alphaviruses have focused upon interruption of IFN-α/β induction and/or receptor signaling cascades. Infection of mice with Venezuelan equine encephalitis alphavirus (VEEV) or Sindbis virus (SINV) induces serum IFN-α/β, that elicits a systemic antiviral state in uninfected cells successfully controlling SINV but not VEEV replication. Furthermore, VEEV replication is more resistant than that of SINV to a pre-existing antiviral state in vitro. While host macromolecular shutoff is proposed as a major antagonist of IFN-α/β induction, the underlying mechanisms of alphavirus resistance to a pre-existing antiviral state are not fully defined, nor is the mechanism for the greater resistance of VEEV. Here, we have separated viral transcription and translation shutoff with multiple alphaviruses, identified the viral proteins that induce each activity, and demonstrated that VEEV nonstructural protein 2-induced translation shutoff is likely a critical factor in enhanced antiviral state resistance of this alphavirus.

Immune Responses to Viruses in the CNS

For recovery from infection, the immune response in the central nervous system (CNS) must eliminate or control virus replication without destroying nonrenewable, essential cells. Thus, upon intracellular virus detection, the infected cell must initiate clearance pathways without triggering neuronal cell death. As a result, the inflammatory response must be tightly regulated and unique mechanisms contribute to the immune response in the CNS. Early restriction of virus replication is accomplished by the innate immune response upon activation of pattern recognition receptors in resident cells. Infiltrating immune cells enter from the periphery to clear virus. Antibodies and interferon-γ are primary contributors to noncytolytic clearance of virus in the CNS. Lymphocytes are retained in the CNS after the acute phase of infection presumably to block reactivation of virus replication.

Manipulation of host factors optimizes the pathogenesis of western equine encephalitis virus infections in mice for antiviral drug development

While alphaviruses spread naturally via mosquito vectors, some can also be transmitted as aerosols making them potential bioterrorism agents. One such pathogen, western equine encephalitis virus (WEEV), causes fatal human encephalitis via multiple routes of infection and thus presumably via multiple mechanisms. Although WEEV also produces acute encephalitis in non-human primates, a small animal model that recapitulates features of human disease would be useful for both pathogenesis studies and to evaluate candidate antiviral therapies. We have optimized conditions to infect mice with a low passage isolate of WEEV, thereby allowing detailed investigation of virus tropism, replication, neuroinvasion, and neurovirulence. We find that host factors strongly influence disease outcome, and in particular, that age, gender, and genetic background all have significant effects on disease susceptibility independent of virus tropism or replication within the central nervous system. Our data show that experimental variables can be adjusted in mice to recapitulate disease features known to occur in both non-human primates and humans, thus aiding further study of WEEV pathogenesis and providing a realistic therapeutic window for antiviral drug delivery.

Differentiation of neurons restricts Arbovirus replication and increases expression of the alpha isoform of IRF-7

Susceptibility to alphavirus infection is age dependent, and host maturation is associated with decreased virus replication and less severe encephalitis. To identify factors associated with maturation-dependent restriction of virus replication, we studied AP-7 rat olfactory bulb neuronal cells, which can differentiate in vitro. Differentiation was associated with a 150- to 1,000-fold decrease in replication of the alphaviruses Sindbis virus and Venezuelan equine encephalitis virus, as well as La Crosse bunyavirus. Differentiation delayed synthesis of SINV RNA and protein but did not alter the susceptibility of neurons to infection or virion maturation. Additionally, differentiation slowed virus-induced translation arrest and death of infected cells. Differentiation of uninfected AP-7 neurons was associated with changes in expression of antiviral genes. Expression of key transcription factors was increased, including interferon regulatory factor 3 and 7 (IRF-3 and IRF-7) and STAT-1, suggesting that neuronal maturation may enhance the capacity for antiviral signaling upon infection. IRF-7 produced by undifferentiated AP-7 neurons was exclusively the short dominant negative γ-isoform, while that produced by differentiated neurons was the full-length α-isoform. A similar switch in IRF-7 isoforms also occurred in the brains of maturing C57BL/6J mice. Silencing of IRF expression did not improve virus multiplication in differentiated neurons. Therefore, neuronal differentiation is associated with upregulation of transcription factors that activate antiviral signaling, but this alone does not account for maturation-dependent restriction of virus replication.

IMPORTANCE

Viral encephalomyelitis is an important cause of age-dependent morbidity and mortality. Because mature neurons are not readily regenerated, recovery from encephalitis suggests that mature neurons utilize unique antiviral mechanisms to block infection and/or clear virus. To identify maturational changes in neurons that may improve outcome, we compared immature and mature cultured neurons for susceptibility to three encephalitic arboviruses and found that replication of Old World and New World alphaviruses and a bunyavirus was reduced in mature compared to immature neurons. Neuronal maturation was associated with increased baseline expression of interferon regulatory factor 3 and 7 mRNAs and production of distinct isoforms of interferon regulatory factor 7 protein. Overall, our studies identified maturational changes in neurons that likely contribute to assembly of immunoregulatory factors prior to infection, a more rapid antiviral response, increased resistance to virus infection, and improved survival.

Deliberate attenuation of chikungunya virus by adaptation to heparan sulfate-dependent infectivity: a model for rational arboviral vaccine design

Mosquito-borne chikungunya virus (CHIKV) is a positive-sense, single-stranded RNA virus from the genus Alphavirus, family Togaviridae, which causes fever, rash and severe persistent polyarthralgia in humans. Since there are currently no FDA licensed vaccines or antiviral therapies for CHIKV, the development of vaccine candidates is of critical importance. Historically, live-attenuated vaccines (LAVs) for protection against arthropod-borne viruses have been created by blind cell culture passage leading to attenuation of disease, while maintaining immunogenicity. Attenuation may occur via multiple mechanisms. However, all examined arbovirus LAVs have in common the acquisition of positively charged amino acid substitutions in cell-surface attachment proteins that render virus infection partially dependent upon heparan sulfate (HS), a ubiquitously expressed sulfated polysaccharide, and appear to attenuate by retarding dissemination of virus particles in vivo. We previously reported that, like other wild-type Old World alphaviruses, CHIKV strain, La Réunion, (CHIKV-LR), does not depend upon HS for infectivity. To deliberately identify CHIKV attachment protein mutations that could be combined with other attenuating processes in a LAV candidate, we passaged CHIKV-LR on evolutionarily divergent cell-types. A panel of single amino acid substitutions was identified in the E2 glycoprotein of passaged virus populations that were predicted to increase electrostatic potential. Each of these substitutions was made in the CHIKV-LR cDNA clone and comparisons of the mutant viruses revealed surface exposure of the mutated residue on the spike and sensitivity to competition with the HS analog, heparin, to be primary correlates of attenuation in vivo. Furthermore, we have identified a mutation at E2 position 79 as a promising candidate for inclusion in a CHIKV LAV.

With the adaptation of chikungunya virus (CHIKV) to transmission by the Aedes albopictus mosquito, a pandemic has occurred resulting in four to six million human infections, and the virus continues to become endemic in new regions, most recently in the Caribbean. CHIKV can cause debilitating polyarthralgia, lasting for weeks to years, and there are currently no licensed vaccines or antiviral therapies available. While an investigational live-attenuated vaccine (LAV) exists, problems with reactogenicity have precluded its licensure. The purpose of the current study was to: i) devise an in vitro passage procedure that reliably generates a panel of CHIKV envelope glycoprotein mutations for screening as vaccine candidates; ii) determine the position of the mutations in the three-dimensional structure of the alphavirus spike complex and their effect on electrostatic potential; iii) determine the attenuation characteristics of each mutation in a murine model of CHIKV musculoskeletal disease; and iv) to identify in vitro assays examining the dependency of infection upon HS that correlate with attenuation and localization in the glycoprotein spike. This approach provides a paradigm for the rational design of future LAVs for CHIKV and other mosquito-borne viruses, by deliberately selecting and combining attenuating processes.

Mice deficient in interferon-gamma or interferon-gamma receptor 1 have distinct inflammatory responses to acute viral encephalomyelitis

Interferon (IFN)-gamma is an important component of the immune response to viral infections that can have a role both in controlling virus replication and inducing inflammatory damage. To determine the role of IFN-gamma in fatal alphavirus encephalitis, we have compared the responses of wild type C57BL/6 (WTB6) mice with mice deficient in either IFN-gamma (GKO) or the alpha-chain of the IFN-gamma receptor (GRKO) after intranasal infection with a neuroadapted strain of sindbis virus. Mortalities of GKO and GRKO mice were similar to WTB6 mice. Both GKO and GRKO mice had delayed virus clearance from the brain and spinal cord, more infiltrating perforin(+) cells and lower levels of tumor necrosis factor (TNF)-alpha and interleukin (IL)-6 mRNAs than WTB6 mice. However, inflammation was more intense in GRKO mice than WTB6 or GKO mice with more infiltrating CD3(+) T cells, greater expression of major histocompatibility complex-II and higher levels of interleukin-17A mRNA. Fibroblasts from GRKO embryos did not develop an antiviral response after treatment with IFN-gamma, but showed increases in TNF-alpha, IL-6, CXCL9 and CXCL10 mRNAs although these increases developed more slowly and were less intense than those of WTB6 fibroblasts. These data indicate that both GKO and GRKO mice fail to develop an IFN-gamma-mediated antiviral response, but differ in regulation of the inflammatory response to infection. Therefore, GKO and GRKO cannot be considered equivalent when assessing the role of IFN-gamma in CNS viral infections.

An attenuating mutation in a neurovirulent Sindbis virus strain interacts with the IPS-1 signaling pathway in vivo

The AR86 strain of Sindbis virus causes lethal neurologic disease in adult mice. Previous studies have identified a virulence determinant at nonstructural protein (nsP) 1 position 538 that regulates neurovirulence, modulates clearance from the CNS, and interferes with the type I interferon pathway. The studies herein demonstrate that in the absence of type I interferon signaling, the attenuated mutant exhibited equivalent virulence to S300 virus. Furthermore, both S300 and nsP1 T538I viruses displayed similar neurovirulence and replication kinetics in IPS-1-/- mice. TRIF dependent signaling played a modest role in protecting against disease by both S300 and nsP1 T538I, but did not contribute to control of nsP1 T538I replication within the CNS, while MyD88 played no role in the disease process. These results indicate that the control of the nsP1 T538I mutant virus is largely mediated by IPS-1-dependent RLR signaling, with TRIF-dependent TLR signaling also contributing to protection from virus-induced neurologic disease.

Clearance of virus infection from the CNS

Viruses that cause encephalomyelitis infect neurons and recovery from infection requires noncytolytic clearance of virus from the nervous system to avoid damaging these irreplaceable cells. Several murine model systems of virus infection have been used to identify clearance mechanisms. Quantitative analysis of Sindbis virus clearance over 6 months shows three phases: day 5-7, clearance of infectious virus, but continued presence of viral RNA; day 8-60, decreasing levels of viral RNA; day 60-180, maintenance of viral RNA at low levels. Antiviral antibody and interferon-γ have major roles in clearance with a likely role for IgM as well as IgG antibody. Long-term residence of virus-specific immune cells in the nervous system is necessary to prevent virus reactivation.

Cross-protective immunity against o'nyong-nyong virus afforded by a novel recombinant chikungunya vaccine

Emerging mosquito-borne alphavirus infections caused by chikungunya virus (CHIKV) or o'nyong-nyong virus (ONNV) are responsible for sporadic and sometimes explosive urban outbreaks. Currently, there is no licensed vaccine against either virus. We have developed a highly attenuated recombinant CHIKV candidate vaccine (CHIKV/IRES) that in preclinical studies was demonstrated to be safe, immunogenic and efficacious. In this study we investigated the potential of this vaccine to induce cross-protective immunity against the antigenically related ONNV. Our studies demonstrated that a single dose of CHIKV/IRES elicited a strong cross-neutralizing antibody response and conferred protection against ONNV challenge in the A129 mouse model. Moreover, CHIKV/IRES immune A129 dams transferred antibodies to their offspring that were protective, and passively transferred anti-CHIKV/IRES immune serum protected AG129 mice, independently of a functional IFN response. These findings highlight the potential of the CHIKV/IRES vaccine to protect humans against not only CHIKV but also against ONNV-induced disease.

Complexity of the microglial activation pathways that drive innate host responses during lethal alphavirus encephalitis in mice

Microglia express multiple TLRs (Toll-like receptors) and provide important host defence against viruses that invade the CNS (central nervous system). Although prior studies show these cells become activated during experimental alphavirus encephalitis in mice to generate cytokines and chemokines that influence virus replication, tissue inflammation and neuronal survival, the specific PRRs (pattern recognition receptors) and signalling intermediates controlling microglial activation in this setting remain unknown. To investigate these questions directly in vivo, mice ablated of specific TLR signalling molecules were challenged with NSV (neuroadapted Sindbis virus) and CNS viral titres, inflammatory responses and clinical outcomes followed over time. To approach this problem specifically in microglia, the effects of NSV on primary cells derived from the brains of wild-type and mutant animals were characterized in vitro. From the standpoint of the virus, microglial activation required viral uncoating and an intact viral genome; inactivated virus particles did not elicit measurable microglial responses. At the level of the target cell, NSV triggered multiple PRRs in microglia to produce a broad range of inflammatory mediators via non-overlapping signalling pathways. In vivo, disease survival was surprisingly independent of TLR-driven responses, but still required production of type-I IFN (interferon) to control CNS virus replication. Interestingly, the ER (endoplasmic reticulum) protein UNC93b1 facilitated host survival independent of its known effects on endosomal TLR signalling. Taken together, these data show that alphaviruses activate microglia via multiple PRRs, highlighting the complexity of the signalling networks by which CNS host responses are elicited by these infections.

Probing the attenuation and protective efficacy of a candidate chikungunya virus vaccine in mice with compromised interferon (IFN) signaling

Chikungunya virus (CHIKV) is a mosquito-borne alphavirus that causes explosive outbreaks of febrile illness associated with rash, and painful arthralgia. The CHIK vaccine strain 181/clone25 (181/25) developed by the United States Army Medical Research Institute of Infectious Diseases (USAMRIID) was shown to be well-tolerated and highly immunogenic in phase I and II clinical trials although it induced transient arthralgia in some healthy adult volunteers. In an attempt to better understand the host factors that are involved in the attenuating phenotype of CHIK 181/25 vaccine virus we conducted studies in interferon (IFN)-compromised mice and also evaluated its immunogenic potential and protective capacity. Infection of AG129 mice (defective in IFN-α/β and IFN-γ receptor signaling) with CHIK 181/25 resulted in rapid mortality within 3-4 days. In contrast, all infected A129 mice (defective in IFN-α/β receptor signaling) survived with temporary morbidity characterized by ruffled appearance and body weight loss. A129 heterozygote mice that retain partial IFN-α/β receptor signaling activity remained healthy. Infection of A129 mice with CHIK 181/25 induced significant levels of IFN-γ and IL-12 while the inflammatory cytokines, TNFα and IL-6 remained low. A single administration of the CHIK 181/25 vaccine provided both short-term and long-term protection (38 days and 247 days post-prime, respectively) against challenge with wt CHIKV-La Reunion (CHIKV-LR). This protection was at least partially mediated by antibodies since passively transferred immune serum protected both A129 and AG129 mice from wt CHIKV-LR and 181/25 virus challenge. Overall, these data highlight the importance of IFNs in controlling CHIK 181/25 vaccine and demonstrate the ability of this vaccine to elicit neutralizing antibody responses that confer short-and long-term protection against wt CHIKV-LR challenge.

Recovery from viral encephalomyelitis: immune-mediated noncytolytic virus clearance from neurons

Viral encephalomyelitis is caused by virus infections of neurons in the brain and spinal cord. Recovery is dependent on immune-mediated control and clearance of virus from these terminally differentiated essential cells. Preservation of neuronal function is essential for prevention of neurologic sequelae such as paralysis, seizures and cognitive deficits. Using the model system of Sindbis virus-induced encephalomyelitis in mice, we have shown that immune-mediated clearance of infectious virus from neurons is a noncytolytic process. The major effectors are antibody to the E2 surface glycoprotein produced by B cells, and interferon-gamma produced by T cells. These effectors work in synergy, but neuronal populations differ in their responses to each. Virus is least likely to be cleared from brain neurons and most likely to be cleared from motor neurons in the cervical and thoracic regions of the spinal cord. Because the infected neurons are not eliminated, viral RNA persists and long-term control is needed to prevent virus reactivation. Virus-specific antibody-secreting cells residing in the nervous system after recovery from infection are likely to be important for long-term control.

A determinant of Sindbis virus neurovirulence enables efficient disruption of Jak/STAT signaling

Previous studies with Venezuelan equine encephalitis virus and Sindbis virus (SINV) indicate that alphaviruses are capable of suppressing the cellular response to type I and type II interferons (IFNs) by disrupting Jak/STAT signaling; however, the relevance of this signaling inhibition toward pathogenesis has not been investigated. The relative abilities of neurovirulent and nonneurovirulent SINV strains to downregulate Jak/STAT signaling were compared to determine whether the ability to inhibit IFN signaling correlates with virulence potential. The adult mouse neurovirulent strain AR86 was found to rapidly and robustly inhibit tyrosine phosphorylation of STAT1 and STAT2 in response to IFN-γ and/or IFN-β. In contrast, the closely related SINV strains Girdwood and TR339, which do not cause detectable disease in adult mice, were relatively inefficient inhibitors of STAT1/2 activation. Decreased STAT activation in AR86-infected cells was associated with decreased activation of the IFN receptor-associated tyrosine kinases Tyk2, Jak1, and Jak2. To identify the viral factor(s) involved, we infected cells with several panels of AR86/Girdwood chimeric viruses. Surprisingly, we found that a single amino acid determinant, threonine at nsP1 position 538, which is required for AR86 virulence, was also required for efficient disruption of STAT1 activation, and this determinant fully restored STAT1 inhibition when it was introduced into the avirulent Girdwood background. These data indicate that a key virulence determinant plays a critical role in downregulating the response to type I and type II IFNs, which suggests that the ability of alphaviruses to inhibit Jak/STAT signaling relates to their in vivo virulence potential.

Encephalitic alphaviruses

This review will cover zoonotic, encephalitic alphaviruses in the family Togaviridae. Encephalitic alphaviruses, i.e. Western- (WEEV), Eastern- (EEEV), Venezuelan equine encephalitis virus (VEEV) and, more rarely, Ross River virus, Chikungunya virus and Highlands J virus (HJV), are neuroinvasive and may cause neurological symptoms ranging from mild (e.g., febrile illness) to severe (e.g., encephalitis) in humans and equines. Among the naturally occurring alphaviruses, WEEV, EEEV and VEEV have widespread distributions in North, Central and South America. WEEV has found spanning the U.S. from the mid-West (Michigan and Illinois) to the West coast and extending to Canada with human cases reported in 21 states. EEEV is found along the Gulf (Texas to Florida) and Atlantic Coast (Georgia to New Hampshire), as well as in the mid-West (Wisconsin, Illinois and Michigan) and in Canada, with human cases reported in 19 states. In contrast, transmission of VEEV occurs predominantly in Central and South America. As with their geographical distribution, equine encephalitis viruses differ in their main mosquito vector species and their zoonotic potential.

CD4+ T cells provide protection against acute lethal encephalitis caused by Venezuelan equine encephalitis virus

Studying the mechanisms of host survival resulting from viral encephalitis is critical to the development of vaccines. Here we have shown in several independent studies that high dose treatment with neutralizing antibody prior to intranasal infection with Venezuelan equine encephalitis virus had an antiviral effect in the visceral organs and prolonged survival time of infected mice, even in the absence of alphabeta T cells. Nevertheless, antibody treatment did not prevent the development of lethal encephalitis. On the contrary, the adoptive transfer of primed CD4(+) T cells was necessary to prevent lethal encephalitis in mice lacking alphabeta T cell receptor.

Replicon particles of Venezuelan equine encephalitis virus as a reductionist murine model for encephalitis

Venezuelan equine encephalitis virus (VEE) replicon particles (VRP) were used to model the initial phase of VEE-induced encephalitis in the mouse brain. VRP can target and infect cells as VEE, but VRP do not propagate beyond the first infected cell due to the absence of the structural genes. Direct intracranial inoculation of VRP into mice induced acute encephalitis with signs similar to the neuronal phase of wild-type VEE infection and other models of virus-induced encephalitis. Using the previously established VRP-mRNP tagging system, a new method to distinguish the host responses in infected cells from those in uninfected bystander cell populations, we detected a robust and rapid innate immune response in the central nervous system (CNS) by infected neurons and uninfected bystander cells. Moreover, this innate immune response in the CNS compromised blood-brain barrier integrity, created an inflammatory response, and directed an adaptive immune response characterized by proliferation and activation of microglia cells and infiltration of inflammatory monocytes, in addition to CD4(+) and CD8(+) T lymphocytes. Taken together, these data suggest that a naïve CNS has an intrinsic potential to induce an innate immune response that could be crucial to the outcome of the infection by determining the composition and dynamics of the adaptive immune response. Furthermore, these results establish a model for neurotropic virus infection to identify host and viral factors that contribute to invasion of the brain, the mechanism(s) whereby the adaptive immune response can clear the infection, and the role of the host innate response in these processes.

Noncytolytic clearance of sindbis virus infection from neurons by gamma interferon is dependent on Jak/STAT signaling

The alphavirus Sindbis virus (SINV) causes encephalomyelitis in mice by infecting neurons of the brain and spinal cord. The outcome is age dependent. Young animals develop fatal disease, while older animals recover from infection. Recovery requires noncytolytic clearance of SINV from neurons, and gamma interferon (IFN-gamma) is an important contributor to clearance in vivo. IFN-gamma-dependent clearance has been studied using immortalized CSM14.1 rat neuronal cells that can be differentiated in vitro. Previous studies have shown that differentiated, but not undifferentiated, cells develop prolonged SINV replication and respond to IFN-gamma treatment with noncytolytic clearance of virus preceded by suppression of genomic viral RNA synthesis and reactivation of cellular protein synthesis. To determine the signaling mechanisms responsible for clearance, the responses of SINV-infected differentiated neurons to IFN-gamma were examined. IFN-gamma treatment of SINV-infected differentiated CSM14.1 cells, AP-7 olfactory neuronal cells, and primary dorsal root ganglia neurons triggered prolonged Stat-1 Tyr(701) phosphorylation, Stat-1 Ser(727) phosphorylation, and transient Stat-5 phosphorylation. Inhibition of Jak kinase activity with Jak inhibitor I completely reversed the neuroprotective and antiviral activities of IFN-gamma in differentiated cells. We conclude that activation of the Jak/Stat pathway is the primary mechanism for IFN-gamma-mediated clearance of SINV infection from mature neurons.

IFN-gamma-mediated suppression of coronavirus replication in glial-committed progenitor cells

The neurotropic JHM strain of mouse hepatitis virus (JHMV) replicates primarily within glial cells following intracranial inoculation of susceptible mice, with relative sparing of neurons. This study demonstrates that glial cells derived from neural progenitor cells are susceptible to JHMV infection and that treatment of infected cells with IFN-γ inhibits viral replication in a dose-dependent manner. Although type I IFN production is muted in JHMV-infected glial cultures, IFN-β is produced following IFN-γ-treatment of JHMV-infected cells. Also, direct treatment of infected glial cultures with recombinant mouse IFN-α or IFN-β inhibits viral replication. IFN-γ-mediated control of JHMV replication is dampened in glial cultures derived from the neural progenitor cells of type I receptor knock-out mice. These data indicate that JHMV is capable of infecting glial cells generated from neural progenitor cells and that IFN-γ-mediated control of viral replication is dependent, in part, on type I IFN secretion.

Intra-peritoneal injection of polyclonal anti-interferon alpha antibodies cross the blood brain barrier and neutralize interferon alpha

The central nervous system (CNS) is known to be an immunologically privileged organ in the body largely because the blood brain barrier (BBB) prevents the flow of large molecules, proteins, and cells from crossing into the CNS from the periphery. These restrictive properties of the BBB have made it difficult to treat CNS diseases. In this study, mice were infected intracranially (i.c.) with Sindbis virus (SV) and then treated either i.c. or intraperitoneally (i.p.) with neutralizing antibodies against interferon alpha (IFNalpha). SV infected control mice received i.p. saline. Antibodies against mouse IFNalpha were detected in the brain tissue of mice that received i.p. and i.c. injections of the antibody. ELISA analysis showed that both i.c. and i.p. antibody treated mice had significantly decreased levels of IFNalpha in the brain tissue. Also, mice that received IFNalpha neutralizing antibodies showed decreased presence of protein kinase R (PKR) measured by immunohistochemical densitometry, indicating the antibody successfully inhibited IFNalpha. The data shows that antibodies are capable of crossing the BBB and inhibiting IFNalpha, indicating that it is possible to target molecules of interest in the CNS with peripheral antibody treatment.

Type I interferons are essential in controlling neurotropic coronavirus infection irrespective of functional CD8 T cells

Neurotropic coronavirus infection induces expression of both beta interferon (IFN-beta) RNA and protein in the infected rodent central nervous system (CNS). However, the relative contributions of type I IFN (IFN-I) to direct, cell-type-specific virus control or CD8 T-cell-mediated effectors in the CNS are unclear. IFN-I receptor-deficient (IFNAR(-/-)) mice infected with a sublethal and demyelinating neurotropic virus variant and those infected with a nonpathogenic neurotropic virus variant both succumbed to infection within 9 days. Compared to wild-type (wt) mice, replication was prominently increased in all glial cell types and spread to neurons, demonstrating expanded cell tropism. Furthermore, increased pathogenesis was associated with significantly enhanced accumulation of neutrophils, tumor necrosis factor alpha, interleukin-6, chemokine (C-C motif) ligand 2, and IFN-gamma within the CNS. The absence of IFN-I signaling did not impair induction or recruitment of virus-specific CD8 T cells, the primary adaptive mediators of virus clearance in wt mice. Despite similar IFN-gamma-mediated major histocompatibility complex class II upregulation on microglia in infected IFNAR(-/-) mice, class I expression was reduced compared to that on microglia in wt mice, suggesting a synergistic role of IFN-I and IFN-gamma in optimizing class I antigen presentation. These data demonstrate a critical direct antiviral role of IFN-I in controlling virus dissemination within the CNS, even in the presence of potent cellular immune responses. By limiting early viral replication and tropism, IFN-I controls the balance of viral replication and immune control in favor of CD8 T-cell-mediated protective functions.

Non-pathogenic Sindbis virus causes hemorrhagic fever in the absence of alpha/beta and gamma interferons

The role of interferon-gamma (IFNgamma) in antiviral innate immune responses during acute alphavirus infection is not well defined. We examined the contribution of IFNgamma to the protection of adult mice from Sindbis virus (SB)-induced disease by comparing subcutaneous infection of mice lacking receptors for either IFNalpha/beta (A129), IFNgamma (G129) or both (AG129) to normal mice (WT129). While neither G129 nor WT129 mice exhibited clinical signs of disease, infection of A129 or AG129 mice was fatal with AG129 mice succumbing more rapidly. Furthermore, AG129 mice developed signs of viral hemorrhagic fever (VHF), including extensive hepatocellular damage, inflammatory infiltrates in multiple organs and vascular leakage, which were significantly delayed and/or partially ameliorated during fatal A129 infections. We conclude that: (i) IFNalpha/beta is the primary mediator of innate immunity to SB infection, however; (ii) IFNgamma is directly antiviral in vivo, acting before the adaptive immune response appears and; (iii) development of VHF may involve viral suppression of both IFNalpha/beta and IFNgamma responses.

Neuroprotective interventions targeting detrimental host immune responses protect mice from fatal alphavirus encephalitis

Systemic treatment with the tetracycline derivative, minocycline, attenuates neurologic deficits in animal models of amyotrophic lateral sclerosis, hypoxic-ischemic brain injury, and multiple sclerosis. Inhibition of microglial activation within the CNS is 1 mechanism proposed to underlie the beneficial effects of the drug in these systems. Given the widening scope of acute viral encephalitis caused by mosquito-borne pathogens, we investigated the therapeutic effects of minocycline in a murine model of fatal alphavirus encephalomyelitis in which widespread microglial activation is known to occur. We found that minocycline conferred significant protection against both paralysis and death, even when started after viral challenge and despite having no effect on CNS virus replication or spread. Further studies demonstrated that minocycline inhibited early virus-induced microglial activation and that diminished CNS production of the inflammatory mediator, interleukin (IL)-1beta, contributed to its protective effect. Therapeutic blockade of IL-1 receptors also conferred significant protection in our model, validating the importance of the IL-1 pathway in disease pathogenesis. We propose that interventions targeting detrimental host immune responses arising from activated microglia may be of benefit in humans with acute viral encephalitis caused by related mosquito-borne pathogens. Such treatments could conceivably act through neuroprotective rather than antiviral mechanisms to generate these clinical effects.

Efficient replication, and evolution of Sindbis virus genomes with non-canonical 3'A/U-rich elements (NC3ARE) in neonatal mice

Sindbis virus (SIN) is a mosquito-transmitted animal RNA virus. We previously reported that SIN genomes lacking a canonical 19 nt 3'CSE undergo novel repair processes in BHK cells to generate a library of stable atypical SIN genomes with non-canonical 3'A/U-rich elements (NC3AREs) adjacent to the 3' poly(A) tail [1]. To determine the stability and evolutionary pressures on the SIN genomes with NC3AREs to regain a 3'CSE, five representative SIN isolates and a wild type SIN were tested in newborn mice. The key findings of this study are: (a) all six SIN isolates, including those that have extensive NC3AREs in the 3'NTRs, replicate well and produce high titer viremia in newborn mice; (b) 7-9 successive passages of these isolates in newborn mice produced comparable levels of viremia; (c) while all isolates produced only small-sized plaques during primary infection in animals, both small- and large-sized plaques were generated in all other passages; (d) polymerase stuttering occurs on select 3' oligo(U) motifs to add more U residues within the NC3AREs; (e) the S3-8 isolate with an internal UAUUU motif in the 3'poly(A) tail maintains this element even after 9 passages in animals; (f) despite differences in 3'NTRs and variable tissue distribution, all SIN isolates appear to produce similar tissue pathology in infected animals. Competition experiments with wt SIN and atypical SIN isolates in BHK cells show dominance of wt SIN. As shown for BHK cells in culture, the 3'CSE of the SIN genome is not required for virus replication and genome stability in live animals. Since the NC3AREs of atypical SIN genomes are not specific to SIN replicases, alternate RNA motifs of alphavirus genome must confer specificity in template selection. These studies fulfill the need to confirm the long-term viability of atypical SIN genomes in newborn mice and offer a basis for exploring the use of atypical SIN genomes in biotechnology.

Synergistic roles of antibody and interferon in noncytolytic clearance of Sindbis virus from different regions of the central nervous system

Sindbis virus (SINV) is an alphavirus that causes infection of neurons and encephalomyelitis in adult immunocompetent mice. Recovery can occur without apparent neurological damage. To better define the factors facilitating noncytolytic clearance of SINV in different regions of the central nervous system (CNS) and the roles of innate and adaptive immune responses at different times during infection, we have characterized SINV infection and clearance in the brain, brain stem, and spinal cords of severe combined immunodeficiency (SCID) and C57BL/6 (wild-type [WT]) mice and mice deficient in beta interferon (IFN-beta) (BKO), antibody (muMT), IFN-gamma (GKO), IFN-gamma receptor (GRKO), and both antibody and IFN-gamma (muMT/GKO). WT mice cleared infectious virus by day 8, while SCID mice had persistent virus replication at all sites. For 3 days after infection, BKO mice had higher titers at all sites than WT mice, despite similar IFN-alpha production, but cleared virus similarly. GKO and GRKO mice cleared infectious virus from all sites by days 8 to 10 and, like WT mice, displayed transient reactivation at 12 to 22 days. muMT mice did not clear virus from the brain, and clearance from the brain stem and lumbar spinal cord was delayed, followed by reactivation. Eighty-one days after infection, muMT/GKO mice had not cleared virus from any site, but titers were lower than for SCID mice. These studies show that IFN-beta is independently important for early control of CNS virus replication, that antiviral antibody is critical for clearance from the brain, and that both antibody and IFN-gamma contribute to prevention of reactivation after initial clearance.

Critical roles for both STAT1-dependent and STAT1-independent pathways in the control of primary dengue virus infection in mice

Dengue virus (DEN), a flavivirus, causes dengue fever and dengue hemorrhagic fever/dengue shock syndrome, the most common mosquito-borne viral illnesses in humans worldwide. In this study, using STAT1(-/-) mice bearing two different mutant stat1 alleles in the 129/Sv/Ev background, we demonstrate that IFNR-dependent control of primary DEN infection involves both STAT1-dependent and STAT1-independent mechanisms. The STAT1 pathway is necessary for clearing the initial viral load, whereas the STAT1-independent pathway controls later viral burden and prevents DEN disease in mice. The STAT1-independent responses in mice with primary DEN infection included the early activation of B and NK cells as well as the up-regulation of MHC class I molecules on macrophages and dendritic cells. Infection of bone marrow-derived dendritic cell cultures with either DEN or Sindbis virus, another positive-strand RNA virus, confirmed the early vs late natures of the STAT1-dependent and STAT1-independent pathways. Collectively, these data begin to define the nature of the STAT1-dependent vs the STAT1-independent pathway in vivo.

Gamma interferon-dependent, noncytolytic clearance of sindbis virus infection from neurons in vitro

Due to the nonrenewable nature of neurons, recovery from viral infection of the central nervous system requires noncytopathic mechanisms for control of virus replication. Recovery from alphavirus encephalitis can occur without apparent neurological damage through the effects of antibody and gamma interferon (IFN-gamma). To establish an in vitro cell culture system that will allow the study of mechanisms of IFN-gamma-mediated control of Sindbis virus (SINV) replication in neurons, we have characterized the susceptibility to SINV infection and IFN-gamma responsiveness of two neuronal cell lines that can be differentiated in vitro: CSM14.1, a rat nigral cell line, and NSC34, a mouse motor neuron cell line. Undifferentiated CSM14.1 and NSC34 cells were permissive for SINV and susceptible to virus-induced cell death. With differentiation, CSM14.1 cells reduced virus replication and became progressively resistant to virus-induced cell death, resulting in prolonged virus replication. NSC34 cells did not differentiate completely and became only partially resistant to SINV infection. Both CSM14.1 and NSC34 cells responded to pretreatment with IFN-gamma by decreasing SINV replication. Differentiated CSM14.1 cells treated 24 h after infection with IFN-gamma responded with increased cell viability and clearance of infectious virus. IFN-gamma treatment sequentially altered the ratio of genomic to subgenomic viral RNA synthesis, promoted recovery of cellular protein synthesis, reduced viral protein synthesis, and inhibited viral RNA transcription within 24 h after treatment. We conclude that CSM14.1 cells provide an excellent model for the study of IFN-gamma-mediated noncytolytic clearance of SINV from mature neurons.

The role of IL-12, IL-23 and IFN-gamma in immunity to viruses

IL-12, IL-23 and IFN-γ form a loop and have been thought to play a crucial role against infectious viruses, which are the prototype of “intracellular” pathogens. In the last 10 years, the generation of knock-out (KO) mice for genes that control IL-12/IL-23-dependent IFN-γ-dependent mediated immunity (STAT1, IFN-γR1, IFNγR2, IL-12p40 and IL-12Rβ1) and the identification of patients with spontaneous germline mutations in these genes has led to a re-examination of the role of these cytokines in anti-viral immunity. We here review viral infections in mice and humans with genetic defects in the IL-12/IL-23-IFN-γ axis. A comparison of the phenotypes observed in KO mice and deficient patients suggests that the human IL-12/IL-23-IFN-γ axis plays a redundant role in immunity to most viruses, whereas its mouse counterparts play a more important role against several viruses.

Sindbis virus translation is inhibited by a PKR/RNase L-independent effector induced by alpha/beta interferon priming of dendritic cells

The tropism of Sindbis virus (SB) for cells of the dendritic cell (DC) lineage and the virulence of SB in vivo are largely determined by the efficacy of alpha/beta interferon (IFN-alpha/beta)-mediated antiviral responses. These responses are essentially intact in the absence of PKR and/or RNase L (K. D. Ryman, L. J. White, R. E. Johnston, and W. B. Klimstra, Viral Immunol. 15:53-76, 2002). In the present studies, we investigated the nature of antiviral effects and identity of antiviral effectors primed by IFN-alpha/beta treatment of bone marrow-derived DCs (BMDCs) generated from mice deficient in PKR and RNase L (TD). IFN-alpha/beta priming exerted significant antiviral activity at very early stages of SB replication and most likely inhibited the initial translation of infecting genomes. The early effect targeted cap-dependent translation as protein synthesis from an SB-like and a simple RNA were inhibited by interferon treatment, but an encephalomyocarditis virus internal ribosome entry site-driven element exhibited no inhibition. Phosphorylation of the alpha subunit of eukaryotic translation initiation factor 2 was defective after virus infection of TD cells, suggesting other mechanisms of translation inhibition. To identify components of these alternative antiviral pathway(s), we have compared global gene regulation in BMDCs derived from normal 129 Sv/Ev, IFNAR1-/-, and TD mice following infection with SB or treatment with IFN-alpha/beta. Candidate effectors of alternative antiviral pathways were those genes induced by virus infection or IFN-alpha/beta treatment in 129 Sv/Ev and TD-derived BMDC but not in virus-infected or IFN-alpha/beta-treated IFNAR1-/- cells. Statistical analyses of gene array data identified 44 genes that met these criteria which are discussed.

Persistent infection with ebola virus under conditions of partial immunity

Ebola hemorrhagic fever in humans is associated with high mortality; however, some infected hosts clear the virus and recover. The mechanisms by which this occurs and the correlates of protective immunity are not well defined. Using a mouse model, we determined the role of the immune system in clearance of and protection against Ebola virus. All CD8 T-cell-deficient mice succumbed to subcutaneous infection and had high viral antigen titers in tissues, whereas mice deficient in B cells or CD4 T cells cleared infection and survived, suggesting that CD8 T cells, independent of CD4 T cells and antibodies, are critical to protection against subcutaneous Ebola virus infection. B-cell-deficient mice that survived the primary subcutaneous infection (vaccinated mice) transiently depleted or not depleted of CD4 T cells also survived lethal intraperitoneal rechallenge for >/==" BORDER="0">25 days. However, all vaccinated B-cell-deficient mice depleted of CD8 T cells had high viral antigen titers in tissues following intraperitoneal rechallenge and died within 6 days, suggesting that memory CD8 T cells by themselves can protect mice from early death. Surprisingly, vaccinated B-cell-deficient mice, after initially clearing the infection, were found to have viral antigens in tissues later (day 120 to 150 post-intraperitoneal infection). Furthermore, following intraperitoneal rechallenge, vaccinated B-cell-deficient mice that were transiently depleted of CD4 T cells had high levels of viral antigen in tissues earlier (days 50 to 70) than vaccinated undepleted mice. This demonstrates that under certain immunodeficiency conditions, Ebola virus can persist and that loss of primed CD4 T cells accelerates the course of persistent infections. These data show that CD8 T cells play an important role in protection against acute disease, while both CD4 T cells and antibodies are required for long-term protection, and they provide evidence of persistent infection by Ebola virus suggesting that under certain conditions of immunodeficiency a host can harbor virus for prolonged periods, potentially acting as a reservoir.

DNA vaccination against mutant huntingtin ameliorates the HDR6/2 diabetic phenotype

Immunization against extracellular neurotoxic proteins has shown promise in the treatment of several neurodegenerative disorders. We sought to determine whether immunization against mutant huntingtin, the intracellular protein that causes Huntington's disease (HD), could slow disease progression in the HD mouse model HDR6/2. DNA vaccination was used to present the mutant intracellular antigen to the immune system in a physiological context. Assay of a peripheral biomarker, pancreatic insufficiency, was used as an initial test of efficacy. DNA vaccination with a 5' fragment of the HD cDNA prevented development of the HDR6/2 diabetic phenotype. Insulin staining demonstrated that HDR6/2 diabetes may be caused by a severe pancreatic insulin deficiency. Immunoresponsive HDR6/2 mice showed increased insulin staining more closely resembling wild-type levels. These observations suggest that DNA vaccination against toxic intracellular proteins may be therapeutic.

Gamma interferon is critical for neuronal viral clearance and protection in a susceptible mouse strain following early intracranial Theiler's murine encephalomyelitis virus infection

We evaluated the role of gamma interferon (IFN-gamma) in protecting neurons from virus-induced injury following central nervous system infection. IFN-gamma(-/-) and IFN-gamma(+/+) mice of the resistant major histocompatibility complex (MHC) H-2(b) haplotype and intracerebrally infected with Theiler's murine encephalomyelitis virus (TMEV) cleared virus infection from anterior horn cell neurons. IFN-gamma(+/+) H-2(b) mice also cleared virus from the spinal cord white matter, whereas IFN-gamma(-/-) H-2(b) mice developed viral persistence in glial cells of the white matter and exhibited associated spinal cord demyelination. In contrast, infection of IFN-gamma(-/-) mice of the susceptible H-2(q) haplotype resulted in frequent deaths and severe neurologic deficits within 16 days of infection compared to the results obtained for controls. Morphologic analysis demonstrated severe injury to spinal cord neurons in IFN-gamma(-/-) H-2(q) mice during early infection. More virus RNA was detected in the brain and spinal cord of IFN-gamma(-/-) H-2(q) mice than in those of control mice at 14 and 21 days after TMEV infection. Virus antigen was localized predominantly to anterior horn cells in infected IFN-gamma(-/-) H-2(q) mice. IFN-gamma deletion did not affect the humoral response directed against the virus. However, the level of expression of CD4, CD8, class I MHC, or class II MHC in the central nervous system of IFN-gamma(-/-) H-2(q) mice was lower than those in IFN-gamma(+/+) H-2(q) mice. Finally, in vitro analysis of virus-induced death in NSC34 cells and spinal motor neurons showed that IFN-gamma exerted a neuroprotective effect in the absence of other aspects of the immune response. These data support the hypothesis that IFN-gamma plays a critical role in protecting spinal cord neurons from persistent infection and death.

Immune responses to RNA-virus infections of the CNS

A successful outcome for the host of virus infection of the central nervous system (CNS) requires the elimination of the virus without damage to essential non-renewable cells, such as neurons. As a result, inflammatory responses must be tightly controlled, and many unique mechanisms seem to contribute to this control. In addition to being important causes of human disease, RNA viruses that infect the CNS provide useful models in which to study immune responses in the CNS. Recent work has shown the importance of innate immune responses in the CNS in controlling virus infection. And advances have been made in assessing the relative roles of cytotoxic T cells, antibodies and cytokines in the clearance of viruses from neurons, glial cells and meningeal cells.

Effects of PKR/RNase L-dependent and alternative antiviral pathways on alphavirus replication and pathogenesis

Type I interferons (IFN-alpha/beta) rapidly confer resistance to alphavirus infection in macrophages and dendritic cells (DC) as evidenced by the dramatically increased susceptibility of these cells in mice with the IFNAR1 subunit of the IFN-alpha/beta receptor ablated (IFNAR1-/-). Normal adult mice develop only a subclinical Sindbis virus infection, whereas infected IFNAR1-/- mice rapidly succumb to a fatal disease. Here, we investigated the individual and combined contributions of the two best characterized INF-alpha/beta-mediated antiviral pathways to the control of Sindbis virus replication: (1) the coupled 2-5A synthetase/RNase L pathway and (2) the double-stranded RNA-dependent protein kinase (PKR) pathway. Surprisingly, mice deficient in PKR, RNase L, and Mx-1 (triply-deficient [TD]) developed only subclinical infection. Although the permissivity of cells in lymph nodes draining the inoculation site was increased in the absence of PKR/RNase L, systemic dissemination of the virus infection was restricted by an alternative IFN-alpha/beta receptor-dependent mechanism. In vitro, suppression of early virus protein synthesis and virion production in primary bone marrow-derived dendritic cells (BMDC) was largely dependent on the PKR pathway. However, later in infection virion production was reduced even in the absence of PKR/RNase L by an IFN-alpha/beta receptor-dependent mechanism. Priming of BMDC with IFN-alpha/beta or IFN-gamma resulted in dose-dependent restriction of virus replication, largely independent of PKR and/or RNase L expression.

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.

Noncytolytic control of viral infections by the innate and adaptive immune response

This review describes the contribution of noncytolytic mechanisms to the control of viral infections with a particular emphasis on the role of cytokines in these processes. It has long been known that most cell types in the body respond to an incoming viral infection by rapidly secreting antiviral cytokines such as interferon alpha/beta (IFN-alpha/beta). After binding to specific receptors on the surface of infected cells, IFN-alpha/beta has the potential to trigger the activation of multiple noncytolytic intracellular antiviral pathways that can target many steps in the viral life cycle, thereby limiting the amplification and spread of the virus and attenuating the infection. Clearance of established viral infections, however, requires additional functions of the immune response. The accepted dogma is that complete clearance of intracellular viruses by the immune response depends on the destruction of infected cells by the effector cells of the innate and adaptive immune system [natural killer (NK) cells and cytotoxic T cells (CTLs)]. This notion, however, has been recently challenged by experimental evidence showing that much of the antiviral potential of these cells reflects their ability to produce antiviral cytokines such as IFN-gamma and tumor necrosis factor (TNF)-alpha at the site of the infection. Indeed, these cytokines can purge viruses from infected cells noncytopathically as long as the cell is able to activate antiviral mechanisms and the virus is sensitive to them. Importantly, the same cytokines also control viral infections indirectly, by modulating the induction, amplification, recruitment, and effector functions of the immune response and by upregulating antigen processing and display of viral epitopes at the surface of infected cells. In keeping with these concepts, it is not surprising that a number of viruses encode proteins that have the potential to inhibit the antiviral activity of cytokines.