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

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.

Effect of Viral Strain and Host Age on Clinical Disease and Viral Replication in Immunocompetent Mouse Models of Chikungunya Encephalomyelitis

The alphavirus chikungunya virus (CHIKV) represents a reemerging public health threat as mosquito vectors spread and viruses acquire advantageous mutations. Although primarily arthritogenic in nature, CHIKV can produce neurological disease with long-lasting sequelae that are difficult to study in humans. We therefore evaluated immunocompetent mouse strains/stocks for their susceptibility to intracranial infection with three different CHIKV strains, the East/Central/South African (ECSA) lineage strain SL15649 and Asian lineage strains AF15561 and SM2013. In CD-1 mice, neurovirulence was age- and CHIKV strain-specific, with SM2013 inducing less severe disease than SL15649 and AF15561. In 4-6-week-old C57BL/6J mice, SL15649 induced more severe disease and increased viral brain and spinal cord titers compared to Asian lineage strains, further indicating that neurological disease severity is CHIKV-strain-dependent. Proinflammatory cytokine gene expression and CD4+ T cell infiltration in the brain were also increased with SL15649 infection, suggesting that like other encephalitic alphaviruses and with CHIKV-induced arthritis, the immune response contributes to CHIKV-induced neurological disease. Finally, this study helps overcome a current barrier in the alphavirus field by identifying both 4-6-week-old CD-1 and C57BL/6J mice as immunocompetent, neurodevelopmentally appropriate mouse models that can be used to examine CHIKV neuropathogenesis and immunopathogenesis following direct brain infection.

EGR1 Upregulation during Encephalitic Viral Infections Contributes to Inflammation and Cell Death

Early growth response 1 (EGR1) is an immediate early gene and transcription factor previously found to be significantly upregulated in human astrocytoma cells infected with Venezuelan equine encephalitis virus (VEEV). The loss of EGR1 resulted in decreased cell death but had no significant impact on viral replication. Here, we extend these studies to determine the impacts of EGR1 on gene expression following viral infection. Inflammatory genes CXCL3, CXCL8, CXCL10, TNF, and PTGS2 were upregulated in VEEV-infected cells, which was partially dependent on EGR1. Additionally, transcription factors, including EGR1 itself, as well as ATF3, FOS, JUN, KLF4, EGR2, and EGR4 were found to be partially transcriptionally dependent on EGR1. We also examined the role of EGR1 and the changes in gene expression in response to infection with other alphaviruses, including eastern equine encephalitis virus (EEEV), Sindbis virus (SINV), and chikungunya virus (CHIKV), as well as Zika virus (ZIKV) and Rift Valley fever virus (RVFV), members of the Flaviviridae and Phenuiviridae families, respectively. EGR1 was significantly upregulated to varying degrees in EEEV-, CHIKV-, RVFV-, SINV-, and ZIKV-infected astrocytoma cells. Genes that were identified as being partially transcriptionally dependent on EGR1 in infected cells included ATF3 (EEEV, CHIKV, ZIKV), JUN (EEEV), KLF4 (SINV, ZIKV, RVFV), CXCL3 (EEEV, CHIKV, ZIKV), CXCL8 (EEEV, CHIKV, ZIKV, RVFV), CXCL10 (EEEV, RVFV), TNF-α (EEEV, ZIKV, RVFV), and PTGS2 (EEEV, CHIKV, ZIKV). Additionally, inhibition of the inflammatory gene PTGS2 with Celecoxib, a small molecule inhibitor, rescued astrocytoma cells from VEEV-induced cell death but had no impact on viral titers. Collectively, these results suggest that EGR1 induction following viral infection stimulates multiple inflammatory mediators. Managing inflammation and cell death in response to viral infection is of utmost importance, especially during VEEV infection where survivors are at-risk for neurological sequalae.

Between immunomodulation and immunotolerance: The role of IFNγ in SARS-CoV-2 disease

Interferons have prominent roles in various pathophysiological conditions, mostly related to inflammation. Interferon-gamma (IFNγ) was, initially discovered as a potent antiviral agent, over 50 years ago, and has recently garnered renewed interest as a promising factor involved in both innate and adaptive immunity. When new disease epidemics appear such as SARS-CoV (severe acute respiratory syndrome coronavirus), MERS-CoV (Middle East respiratory syndrome coronavirus), IAV (Influenza A virus), and in particular the current SARS-CoV-2 pandemic, it is especially timely to review the complexity of immune system responses to viral infections. Here we consider the controversial roles of effectors like IFNγ, discussing its actions in immunomodulation and immunotolerance. We explore the possibility that modulation of IFNγ could be used to influence the course of such infections. Importantly, not only could endogenous expression of IFNγ influence the outcome, there are existing IFNγ therapeutics that can readily be applied in the clinic. However, our understanding of the molecular mechanisms controlled by IFNγ suggests that the exact timing for application of IFNγ-based therapeutics could be crucial: it should be earlier to significantly reduce the viral load and thus decrease the overall severity of the disease.

Human Antibodies Protect against Aerosolized Eastern Equine Encephalitis Virus Infection

Eastern equine encephalitis virus (EEEV) is one of the most virulent viruses endemic to North America. No licensed vaccines or antiviral therapeutics are available to combat this infection, which has recently shown an increase in human cases. Here, we characterize human monoclonal antibodies (mAbs) isolated from a survivor of natural EEEV infection with potent (<20 pM) inhibitory activity of EEEV. Cryo-electron microscopy reconstructions of two highly neutralizing mAbs, EEEV-33 and EEEV-143, were solved in complex with chimeric Sindbis/EEEV virions to 7.2 Å and 8.3 Å, respectively. The mAbs recognize two distinct antigenic sites that are critical for inhibiting viral entry into cells. EEEV-33 and EEEV-143 protect against disease following stringent lethal aerosol challenge of mice with highly pathogenic EEEV. These studies provide insight into the molecular basis for the neutralizing human antibody response against EEEV and can facilitate development of vaccines and candidate antibody therapeutics.

Production of Noncapped Genomic RNAs Is Critical to Sindbis Virus Disease and Pathogenicity

Mosquito-transmitted alphaviruses have been the cause of widespread outbreaks of disease that can range from mild illness to lethal encephalitis or severe polyarthritis. There are currently no safe and effective vaccines or therapeutics with which to prevent or treat alphaviral disease, highlighting the need to better understand alphaviral pathogenesis to develop novel antiviral strategies. This report reveals production of noncapped genomic RNAs (ncgRNAs) to be a novel determinant of alphaviral virulence and offers insight into the importance of inflammation to pathogenesis. Taken together, the findings reported here suggest that the ncgRNAs contribute to alphaviral pathogenesis through the sensing of the ncgRNAs during alphaviral infection and are necessary for the development of severe disease.

Alphaviruses are positive-sense RNA viruses that utilize a 5′ cap structure to facilitate translation of viral proteins and to protect the viral RNA genome. Nonetheless, significant quantities of viral genomic RNAs that lack a canonical 5′ cap structure are produced during alphaviral replication and packaged into viral particles. However, the role/impact of the noncapped genomic RNA (ncgRNA) during alphaviral infection in vivo has yet to be characterized. To determine the importance of the ncgRNA in vivo, the previously described D355A and N376A nsP1 mutations, which increase or decrease nsP1 capping activity, respectively, were incorporated into the neurovirulent AR86 strain of Sindbis virus to enable characterization of the impact of altered capping efficiency in a murine model of infection. Mice infected with the N376A nsP1 mutant exhibited slightly decreased rates of mortality and delayed weight loss and neurological symptoms, although levels of inflammation in the brain were similar to those of wild-type infection. Although the D355A mutation resulted in decreased antiviral gene expression and increased resistance to interferon in vitro, mice infected with the D355A mutant showed significantly reduced mortality and morbidity compared to mice infected with wild-type virus. Interestingly, expression of proinflammatory cytokines was found to be significantly decreased in mice infected with the D355A mutant, suggesting that capping efficiency and the production of ncgRNA are vital to eliciting pathogenic levels of inflammation. Collectively, these data indicate that the ncgRNA have important roles during alphaviral infection and suggest a novel mechanism by which noncapped viral RNAs aid in viral pathogenesis.

Evaluation in Swine of a Recombinant African Swine Fever Virus Lacking the MGF-360-1L Gene

The African swine fever (ASF) pandemic is currently affecting pigs throughout Eurasia, resulting in significant swine production losses. The causative agent, ASF virus (ASFV), is a large, structurally complex virus with a genome encoding more than 160 genes. The function of most of those genes remains unknown. Here, we presented the previously uncharacterized ASFV gene MGF360-1L, the first gene in the genome. The kinetic studies of virus RNA transcription demonstrated that the MGF360-1L gene was transcribed as a late virus protein. The essentiality of MGF360-1L to virus replication was evaluated by developing a recombinant ASFV lacking the gene (ASFV-G-ΔMGF360-1L). In primary swine macrophage cell cultures, ASFV-G-ΔMGF360-1L showed similar replication kinetics as the parental highly virulent field isolate Georgia2007 (ASFV-G). Domestic pigs experimentally infected with ASFV-G-ΔMGF360-1L presented with a clinical disease indistinguishable from that caused by ASFV-G, demonstrating that MGF360-1L was not involved in virulence in swine, the natural host of ASFV.

Disruption of Zika Virus xrRNA1-Dependent sfRNA1 Production Results in Tissue-Specific Attenuated Viral Replication

The Zika virus (ZIKV), like other flaviviruses, produces several species of sub-genomic RNAs (sfRNAs) during infection, corresponding to noncoding RNA fragments of different lengths that result from the exonuclease degradation of the viral 3′ untranslated region (UTR). Over the course of infection, these sfRNAs accumulate in the cell as a result of an incomplete viral genome degradation of the 3′ UTR by the host 5′ to 3′ exoribonuclease, Xrn1. The halting of Xrn1 in the 3′ UTR is due to two RNA pseudoknot structures in the 3′ UTR, termed exoribonuclease-resistant RNA1 and 2 (xrRNA1&2). Studies with related flaviviruses have shown that sfRNAs are important for pathogenicity and inhibiting both mosquito and mammalian host defense mechanisms. However, these investigations have not included ZIKV and there is very limited data addressing how sfRNAs impact infection in a whole animal model or specific tissues. In this study, we generate a sfRNA1-deficient ZIKV (X1) by targeted mutation in the xrRNA1 3′ UTR structure. We find that the X1 virus lacks the production of the largest ZIKV sfRNA species, sfRNA1. Using the X1 virus to infect adult Ifnar1^−/− mice, we find that while the lack of sfRNA1 does not alter ZIKV replication in the spleen, there is a significant reduction of ZIKV genome replication in the brain and placenta compared to wild-type ZIKV infection. Despite the attenuated phenotype of the X1 ZIKV, mice develop a robust neutralizing antibody response. We conclude that the targeted disruption of xrRNA1 results in tissue-specific attenuation while still supporting robust neutralizing antibody responses. Future studies will need to investigate the tissue-specific mechanisms by which ZIKV sfRNAs influence infection and may utilize targeted xrRNA mutations to develop novel attenuated flavivirus vaccine approaches.

A Rewarding Career Unraveling the Pathogenesis of Viral Infections

My great-grandparents were immigrants from Sweden and settled as farmers in Iowa and Illinois. My father, the oldest of six children, was the first in his family to go to college and had careers as a petroleum geologist and an academic. My mother, the youngest of four children, had older siblings in education, and she focused on early childhood education. My childhood in Oklahoma with two younger sisters was happy and comfortable, and public school prepared me well. My career trajectory into virology did not involve much if any advance planning but was characterized by recognizing the fascinating puzzles of virus diseases, being in good places at the right time, taking advantage of opportunities as they presented themselves, and being surrounded by great mentors, colleagues, trainees, and family. Most of my career was spent studying two diseases caused by RNA viruses, alphavirus encephalomyelitis and measles, and was enriched with several leadership opportunities.

Astrocyte Reactivity: Subtypes, States, and Functions in CNS Innate Immunity

Astrocytes are neural parenchymal cells that ubiquitously tile the central nervous system (CNS). In addition to playing essential roles in healthy tissue, astrocytes exhibit an evolutionarily ancient response to all CNS insults, referred to as astrocyte reactivity. Long regarded as passive and homogeneous, astrocyte reactivity is being revealed as a heterogeneous and functionally powerful component of mammalian CNS innate immunity. Nevertheless, concepts about what astrocyte reactivity comprises and what it does are incomplete and sometimes controversial. This review discusses the goal of differentiating reactive astrocyte subtypes and states based on composite pictures of molecular expression, cell morphology, cellular interactions, proliferative state, normal functions, and disease-induced dysfunctions. A working model and conceptual framework is presented for characterizing the diversity of astrocyte reactivity.

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.

Type II interferon signaling in the brain during a viral infection with age-dependent pathogenesis

Viral infections of the central nervous system (CNS) often cause disease in an age-dependent manner, with greater neuropathology during the fetal and neonatal periods. Transgenic CD46+ mice model these age-dependent outcomes through a measles virus infection of CNS neurons. Adult CD46+ mice control viral spread and survive the infection in an interferon gamma (IFNγ)-dependent manner, whereas neonatal CD46+ mice succumb despite similar IFNγ expression in the brain. Thus, we hypothesized that IFNγ signaling in the adult brain may be more robust, potentially due to greater basal expression of IFNγ signaling proteins. To test this hypothesis, we evaluated the expression of canonical IFNγ signaling proteins in the neonatal and adult brain, including the IFNγ receptor, Janus kinase (JAK) 1/2, and signal transducer and activator of transcription-1 (STAT1) in the absence of infection. We also analyzed the expression and activation of STAT1 and IFNγ-stimulated genes during MV infection. We found that neonatal brains have equivalent or greater JAK/STAT1 expression in the hippocampus and the cerebellum than adults. IFNγ receptor expression varied by cell type in the brain but was widely expressed on neuronal and glial cells. During MV infection, increased STAT1 expression and activation correlated with viral load in the hippocampus regardless of age, but not in the cerebellum where viral load was consistently undetectable in adults. These results suggest the neonatal brain is capable of initiating IFNγ signaling during a viral infection, but that downstream STAT1 activation is insufficient to limit viral spread.

Visualization of cell-type dependent effects of anti-E2 antibody and interferon-gamma treatments on localization and expression of Broccoli aptamer-tagged alphavirus RNAs

Sindbis virus (SINV) is an alphavirus that causes age-dependent encephalomyelitis in mice. Within 7-8 days after infection infectious virus is cleared from neurons through the antiviral effects of antibody and interferon-gamma (IFNγ), but RNA persists. To better understand changes in viral RNA associated with immune-mediated clearance we developed recombinant strains of SINV that have genomic and subgenomic viral RNAs tagged with the Broccoli RNA aptamer that binds and activates a conditional fluorophore for live cell imaging of RNA. Treatment of SINV-Broccoli-infected cells with antibody to the SINV E2 glycoprotein had cell type-specific effects. In BHK cells, antibody increased levels of intracellular viral RNA and changed the primary location of genomic RNA from the perinuclear region to the plasma membrane without improving cell viability. In undifferentiated and differentiated AP7 (dAP7) neuronal cells, antibody treatment decreased levels of viral RNA. Occasional dAP7 cells escaped antibody-mediated clearance by not expressing cell surface E2 or binding antibody to the plasma membrane. IFNγ decreased viral RNA levels only in dAP7 cells and synergized with antibody for RNA clearance and improved cell survival. Therefore, analysis of aptamer-tagged SINV RNAs identified cell type- and neuronal maturation-dependent responses to immune mediators of virus clearance.

CpG Oligonucleotides Protect Mice From Alphavirus Encephalitis: Role of NK Cells, Interferons, and TNF

Arboviruses including alphavirus are responsible for most emerging infectious diseases worldwide. Recent outbreaks of chikungunya virus serve as a stark reminder to their pathogenic potential. There are no vaccines or therapeutics currently available to contain alphavirus outbreaks. In this study we evaluated the effect of immunomodulatory CpG ODN on the clinical progression of neurotropic Sindbis virus infection. Neonatal C57Bl-6 mice challenged with Sindbis virus AR339 (25 PFU Subcutaneous) infect neurons in the CNS leading to the development of ataxia, seizures, paralysis, and death. We show that systemic administration of CpG ODN modulates the cytokine and chemokine gene expression levels in the CNS and ultimately protects neonatal mice from lethal neurotropic infection. The protection conferred by CpG ODN is controlled by innate immune response and T and B cells were dispensable. Further, protection required Type I, Type II interferons, and TNF as well as functional NK cells, but did not involve iNOS. This study confirms that administration of innate immune modulators can be used as a strategy to boost host innate immune responses and protect against neurotropic viruses reducing their pathogenic footprint.

Interferon-Gamma Modulation of the Local T Cell Response to Alphavirus Encephalomyelitis

Infection of mice with Sindbis virus (SINV) provides a model for examining the role of the immune response to alphavirus infection of the central nervous system (CNS). Interferon-gamma (IFN-γ) is an important component of this response, and we show that SINV-infected differentiated neurons respond to IFN-γ in vitro by induction of antiviral genes and suppression of virus replication. To determine the in vivo effects of IFN-γ on SINV clearance and T cell responses, C57BL/6 mice lacking IFN-γ or IFN-γ receptor-1 were compared to wild-type (WT) mice after intracranial SINV infection. In WT mice, IFN-γ was first produced in the CNS by natural killer cells and then by CD4+ and CD8+ T cells. Mice with impaired IFN-γ signaling initiated clearance of viral RNA earlier than WT mice associated with CNS entry of more granzyme B-producing CD8+ T cells. However, these mice established fewer CD8+ tissue-resident memory T (TRM) cells and were more likely to experience reactivation of viral RNA synthesis late after infection. Therefore, IFN-γ suppresses the local development of granzyme B-expressing CD8+ T cells and slows viral RNA clearance but promotes CD8+ TRM cell establishment.

Effect of exogenous expression of IFN-γ on the new world alphavirus replication and infection

Aim: IFN-γ plays an important role in control of the old world alphavirus infection. However, the role of IFN-γ in the infection by the new world alphaviruses is not well characterized. Materials & methods: Ad5-mIFN-γ, a recombinant, replication-deficient human adenovirus, was constructed to express mouse IFN-γ (mIFN-γ) and a mouse, lethal challenge model of the new world alphavirus western equine encephalitis virus (WEEV) was used. Results: A single-dose injection of Ad5-mIFN-γ produced a high level of mIFN-γ in mice. Cells inoculated with Ad5-mIFN-γ restricted the replication of WEEV. A single-dose injection of Ad5-mIFN-γ delayed the WEEV infection and extended the survival time in mice. Conclusion: IFN-γ restricts the WEEV infection.

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.

Protease-activated receptor 4 protects mice from Coxsackievirus B3 and H1N1 influenza A virus infection

PAR4 is expressed by a variety of cells, including platelets, cardiac, lung and immune cells. We investigated the contribution of PAR4 to viral infections of the heart and lung. Toll-like receptor (TLR) 3-dependent immune responses were analyzed after co-stimulation of PAR4 in murine bone-marrow derived macrophages, embryonic fibroblasts and embryonic cardiomyocytes. In addition, we analyzed Coxsackievirus B3 (CVB3) or H1N1 influenza A virus (H1N1 IAV) infection of PAR4^-/- (ΔPAR4) and wild-type (WT) mice. Lastly, we investigated the effect of platelet inhibition on H1N1 IAV infection. In vitro experiments revealed that PAR4 stimulation enhances the expression of TLR3-dependent CXCL10 expression and decreases TLR3-dependent NFκB-mediated proinflammatory gene expression. Furthermore, CVB3-infected ΔPAR4 mice exhibited a decreased anti-viral response and increased viral genomes in the heart leading to more pronounced CVB3 myocarditis compared to WT mice. Similarly, H1N1 IAV-infected ΔPAR4 mice had increased immune cell numbers and inflammatory mediators in the lung, and increased mortality compared with infected WT controls. The study showed that PAR4 protects mice from viral infections of the heart and lung.

Neuroinflammation During RNA Viral Infections

Neurotropic RNA viruses continue to emerge and are increasingly linked to diseases of the central nervous system (CNS) despite viral clearance. Indeed, the overall mortality of viral encephalitis in immunocompetent individuals is low, suggesting efficient mechanisms of virologic control within the CNS. Both immune and neural cells participate in this process, which requires extensive innate immune signaling between resident and infiltrating cells, including microglia and monocytes, that regulate the effector functions of antiviral T and B cells as they gain access to CNS compartments. While these interactions promote viral clearance via mainly neuroprotective mechanisms, they may also promote neuropathology and, in some cases, induce persistent alterations in CNS physiology and function that manifest as neurologic and psychiatric diseases. This review discusses mechanisms of RNA virus clearance and neurotoxicity during viral encephalitis with a focus on the cytokines essential for immune and neural cell inflammatory responses and interactions. Understanding neuroimmune communications in the setting of viral infections is essential for the development of treatments that augment neuroprotective processes while limiting ongoing immunopathological processes that cause ongoing CNS disease.

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.

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.

Death and gastrointestinal bleeding complicate encephalomyelitis in mice with delayed appearance of CNS IgM after intranasal alphavirus infection

Central nervous system (CNS) infection of C57BL/6 mice with the TE strain of Sindbis virus (SINV) provides a valuable animal model for studying the pathogenesis of alphavirus encephalomyelitis. While SINV TE inoculated intracranially causes little mortality, 20-30 % of mice inoculated intranasally (IN) died 8 to 11 days after infection, the period during which immune cells typically infiltrate the brain and clear infectious virus. To examine the mechanism behind the mortality, mice infected IN with SINV TE were monitored for evidence of neurological disease, and those with signs of severe disease (moribund) were sacrificed and tissues collected. Mice showing the usual mild signs of encephalomyelitis were concurrently sacrificed to serve as time-matched controls (sick). Sixty-eight per cent of the moribund mice, but none of the sick mice, showed upper gastrointestinal bleeding due to gastric ulceration. Clinical disease and gastrointestinal pathology could not be attributed to direct viral infection of tissues outside of the CNS, and brain pathology and inflammation were comparable in sick and moribund mice. However, more SINV antigen was present in the brains of moribund mice, and clearance of infectious virus from the CNS was delayed compared to sick mice. Lower levels of SINV-specific IgM and fewer B220⁺ B cells were present in the brains of moribund mice compared to sick mice, despite similar levels of antiviral IgM and IgG in serum. These findings highlight the importance of the local antibody response in determining the outcome of viral encephalomyelitis and offer a model system for understanding individual variation in this response.

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.