Mouse hepatitis virus-specific cytotoxic T lymphocytes protect from lethal infection without eliminating virus from the central nervous system

Viral mouse models used to study multiple sclerosis: past and present

Multiple sclerosis (MS) is a common inflammatory demyelinating disease of the central nervous system. Although the etiology of MS is unknown, genetics and environmental factors, such as infections, play a role. Viral infections of mice have been used as model systems to study this demyelinating disease of humans. Three viruses that have long been studied in this capacity are Theiler’s murine encephalomyelitis virus, mouse hepatitis virus, and Semliki Forest virus. This review describes the viruses themselves, the infection process, the disease caused by infection and its accompanying pathology, and the model systems and their usefulness in studying MS.

Hypothesis for potential pathogenesis of SARS-CoV-2 infection-a review of immune changes in patients with viral pneumonia

Coronavirus disease 2019 (COVID-19) is an infectious disease caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) with droplets and contact as the main means of transmission. Since the first case appeared in Wuhan, China, in December 2019, the outbreak has gradually spread nationwide. Up to now, according to official data released by the Chinese health commission, the number of newly diagnosed patients has been declining, and the epidemic is gradually being controlled. Although most patients have mild symptoms and good prognosis after infection, some patients developed severe and die from multiple organ complications. The pathogenesis of SARS-CoV-2 infection in humans remains unclear. Immune function is a strong defense against invasive pathogens and there is currently no specific antiviral drug against the virus. This article reviews the immunological changes of coronaviruses like SARS, MERS and other viral pneumonia similar to SARS-CoV-2. Combined with the published literature, the potential pathogenesis of COVID-19 is inferred, and the treatment recommendations for giving high-doses intravenous immunoglobulin and low-molecular-weight heparin anticoagulant therapy to severe type patients are proposed.

Vaccination strategies to combat novel corona virus SARS-CoV-2

The 2019-novel coronavirus disease (COVID-19) is caused by SARS-CoV-2 is transmitted from human to human has recently reported in China. Now COVID-19 has been spread all over the world and declared epidemics by WHO. It has caused a Public Health Emergency of International Concern. The elderly and people with underlying diseases are susceptible to infection and prone to serious outcomes, which may be associated with acute respiratory distress syndrome (ARDS) and cytokine storm. Due to the rapid increase of SARS-CoV-2 infections and unavailability of antiviral therapeutic agents, developing an effective SAR-CoV-2 vaccine is urgently required. SARS-CoV-2 which is genetically similar to SARS-CoV and Middle East respiratory syndrome coronavirus (MERS-CoV) is an enveloped, single and positive-stranded RNA virus with a genome comprising 29,891 nucleotides, which encode the 12 putative open reading frames responsible for the synthesis of viral structural and nonstructural proteins which are very similar to SARS-CoV and MERS-CoV proteins. In this review we have summarized various vaccine candidates i.e., nucleotide, subunit and vector based as well as attenuated and inactivated forms, which have already been demonstrated their prophylactic efficacy against MERS-CoV and SARS-CoV, so these candidates could be used as a potential tool for the development of a safe and effective vaccine against SARS-CoV-2.

Activated GL7+ B cells are maintained within the inflamed CNS in the absence of follicle formation during viral encephalomyelitis

Central nervous system (CNS) inflammation associated with viral infection and autoimmune disease results in the accumulation of B cells in various differentiation stages. However, the contribution between peripheral and CNS activation remains unclear. During gliatropic coronavirus induced encephalomyelitis, accumulation of protective antibody secreting cells is preceded by infiltration of B cells with a naïve and early differentiation phenotype (Phares et al., 2014). Investigation of the temporal dynamics of B cell activation in draining cervical lymph nodes (CLN) and the CNS revealed that peak CNS infiltration of early activated, unswitched IgD⁺ and IgM⁺ B cells coincided with polyclonal activation in CLN. By contrast, isotype-switched IgG⁺ B cells did not accumulate until peripheral germinal center formation. In the CNS, unswitched B cells were confined to the perivascular space and meninges, with only rare B cell clusters, while isotype-switched B cells localized to parenchymal areas. Although ectopic follicle formation was not observed, more differentiated B cell subsets within the CNS expressed the germinal center marker GL7, albeit at lower levels than CLN counterparts. During chronic infection, CNS IgD^int and IgD^- B cell subsets further displayed sustained markers of proliferation and CD4 T cell help, which were only transiently expressed in the CLN. A contribution of local CD4 T cell help to sustain B cell activation was supported by occasional B cells adjacent to T cells. The results suggest that accumulation of differentiated B cell subsets within the CNS is largely dictated by peripheral activation, but that local events contribute to their sustained activation independent of ectopic follicle formation.

T-cell immunity of SARS-CoV: Implications for vaccine development against MERS-CoV

Over 12 years have elapsed since severe acute respiratory syndrome (SARS) triggered the first global alert for coronavirus infections. Virus transmission in humans was quickly halted by public health measures and human infections of SARS coronavirus (SARS-CoV) have not been observed since. However, other coronaviruses still pose a continuous threat to human health, as exemplified by the recent emergence of Middle East respiratory syndrome (MERS) in humans. The work on SARS-CoV widens our knowledge on the epidemiology, pathophysiology and immunology of coronaviruses and may shed light on MERS coronavirus (MERS-CoV). It has been confirmed that T-cell immunity plays an important role in recovery from SARS-CoV infection. Herein, we summarize T-cell immunological studies of SARS-CoV and discuss the potential cross-reactivity of the SARS-CoV-specific immunity against MERS-CoV, which may provide useful recommendations for the development of broad-spectrum vaccines against coronavirus infections.

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T-cell epitopes identified throughout the SARS-CoV proteome may act as candidates for vaccine development.

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Both SARS-CoV and MERS-CoV-recovered donors have had long-lasting memory T-cell immunity.

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The structures of HLA/SARS-CoV-epitopes illuminate the molecular bases of cellular immunogenicity.

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Potential cross-T-cell immune reactivities of SARS-CoV and MERS-CoV benefit vaccine development.

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.

A mechanism of virus-induced demyelination

Myelin forms an insulating sheath surrounding axons in the central and peripheral nervous systems and is essential for rapid propagation of neuronal action potentials. Demyelination is an acquired disorder in which normally formed myelin degenerates, exposing axons to the extracellular environment. The result is dysfunction of normal neuron-to-neuron communication and in many cases, varying degrees of axonal degeneration. Numerous central nervous system demyelinating disorders exist, including multiple sclerosis. Although demyelination is the major manifestation of most of the demyelinating diseases, recent studies have clearly documented concomitant axonal loss to varying degrees resulting in long-term disability. Axonal injury may occur secondary to myelin damage (outside-in model) or myelin damage may occur secondary to axonal injury (inside-out model). Viral induced demyelination models, has provided unique imminent into the cellular mechanisms of myelin destruction. They illustrate mechanisms of viral persistence, including latent infections, virus reactivation and viral-induced tissue damage. These studies have also provided excellent paradigms to study the interactions between the immune system and the central nervous system (CNS). In this review we will discuss potential cellular and molecular mechanism of central nervous system axonal loss and demyelination in a viral induced mouse model of multiple sclerosis.

Pathogenesis of murine coronavirus in the central nervous system

Murine coronavirus (mouse hepatitis virus, MHV) is a collection of strains that induce disease in several organ systems of mice. Infection with neurotropic strains JHM and A59 causes acute encephalitis, and in survivors, chronic demyelination, the latter of which serves as an animal model for multiple sclerosis. The MHV receptor is a carcinoembryonic antigen-related cell adhesion molecule, CEACAM1a; paradoxically, CEACAM1a is poorly expressed in the central nervous system (CNS), leading to speculation of an additional receptor. Comparison of highly neurovirulent JHM isolates with less virulent variants and the weakly neurovirulent A59 strain, combined with the use of reverse genetics, has allowed mapping of pathogenic properties to individual viral genes. The spike protein, responsible for viral entry, is a major determinant of tropism and virulence. Other viral proteins, both structural and nonstructural, also contribute to pathogenesis in the CNS. Studies of host responses to MHV indicate that both innate and adaptive responses are crucial to antiviral defense. Type I interferon is essential to prevent very early mortality after infection. CD8 T cells, with the help of CD4 T cells, are crucial for viral clearance during acute disease and persist in the CNS during chronic disease. B cells are necessary to prevent reactivation of virus in the CNS following clearance of acute infection. Despite advances in understanding of coronavirus pathogenesis, questions remain regarding the mechanisms of viral entry and spread in cell types expressing low levels of receptor, as well as the unique interplay between virus and the host immune system during acute and chronic disease.

Cytopathy of an infiltrating monocyte lineage during the early phase of infection with murinecoronavirus in the brain

Viral spread during the early stages after infection was compared between a highly neurovirulent mouse hepatitis virus (MHV), JHMV cl-2 strain (cl-2), and its low-virulent mutant, soluble-receptor-resistant (srr)7. The infection of cells with srr7 (soluble-receptor-resistant mutant 7) is dependent on a known MHV receptor (MHVR), carcinoembryonic cell adhesion molecule 1a, whereas cl-2 shows MHVR-independent infection. Initial viral antigens were detected between 12 and 24 h post-inoculation (p.i) in the infiltrating cells that appeared in the subarachnoidal space of mouse brains infected with viruses. There were no significant differences in the intensity or spread of viral antigens in the inflammatory cells between the two viruses. However, 48 h after infection with cl-2, viral antigen-positive cells in the grey matter with the shape of neurons, which do not express MHVR, were detected, while srr7 infection was observed primarily in the white matter. Some of the viral antigen-positive inflammatory cells found in the subarachnoidal space during the early phase of infection reacted with anti-F4/80 or anti-CD11b monoclonal antibodies. Syncytial giant cells (SGCs) expressing viral and CD11b antigens were also detected among these inflammatory cells. These antigen-positive cells appeared in the subarachnoidal space prior to viral antigen spread into the brain parenchyma, indicating that viral encephalitis starts with the infection of infiltrating monocytes which express MHVR. Furthermore, the observation indicates that viral infection has cytopathic effects on the monocyte lineage, which plays a critical role in innate immunity, leading to the rapid spread of viruses during the early stage of infection.

Regulation of proinflammatory cytokine expression in primary mouse astrocytes by coronavirus infection

Previous studies have shown that proinflammatory cytokines, such as tumor necrosis factor alpha (TNF-alpha) and interleukin 6 (IL-6), are differentially induced in primary mouse astrocytes by mouse hepatitis virus strain A59 (MHV-A59) and MHV-2. However, the signaling events that trigger TNF-alpha and IL-6 induction in these cells upon MHV infection remain unknown. In this study, we explored the potential signaling events. We found that induction of TNF-alpha and IL-6 occurred as early as 2 h postinfection and was completely dependent on viral replication. Using inhibitors specific for three mitogen-activated protein kinases, we showed that induction of TNF-alpha and IL-6 by MHV-A59 infection was mediated through activation of the Janus N-terminal kinase signaling pathway, but not through the extracellular signal-regulated kinase or p38 signaling pathway. This finding was further confirmed with knockdown experiments using small interfering RNAs specific for Janus N-terminal kinase. Interestingly, while nuclear factor kappaB (NF-kappaB), a key transcription factor required for the expression of proinflammatory cytokines in most cell types, was activated in astrocytes during MHV-A59 infection, disruption of the NF-kappaB pathway by peptide inhibitors did not significantly inhibit TNF-alpha and IL-6 expression. Furthermore, experiments using chimeric viruses demonstrated that the viral spike and nucleocapsid proteins, which play important roles in MHV pathogenicity in mice, are not responsible for the differential induction of the cytokines. These results illustrate the complexity of the regulatory mechanism by which MHV induces proinflammatory cytokines in primary astrocytes.

SARS vaccines: where are we?

In this review, the current state of vaccine development against human severe acute respiratory syndrome (SARS) coronavirus, focusing on recently published data is assessed. We discuss which strategies have been assessed immunologically and which have been evaluated in SARS coronavirus challenge models. We discuss inactivated vaccines, virally and bacterially vectored vaccines, recombinant protein and DNA vaccines, as well as the use of attenuated vaccines. Data regarding the correlates of protection, animal models and the available evidence regarding potential vaccine enhancement of SARS disease are discussed. While there is much evidence that various vaccine strategies against SARS are safe and immunogenic, vaccinated animals still display significant disease upon challenge. Current data suggest that intranasal vaccination may be crucial and that new or combination strategies may be required for good protective efficacy against SARS in humans.

Neuroprotective effect of apolipoprotein D against human coronavirus OC43-induced encephalitis in mice

Apolipoprotein D (apoD) is a lipocalin upregulated in the nervous system after injury or pathologies such as Alzheimer's disease, Parkinson's disease, and multiple sclerosis. We previously demonstrated that apoD protects against neuropathology by controlling the level of peroxidated lipids. Here, we further investigated the biological function of apoD in a mouse model of acute encephalitis. Our results show that apoD transcript and protein are upregulated during acute encephalitis induced by the human coronavirus OC43 (HCoV-OC43) infection. The apoD upregulation coincides with glial activation, and its expression returns to normal levels when the virus is cleared, concomitantly to a resolved glial reactivity. In addition, the overexpression of human apoD in the neurons of Thy-1/ApoD transgenic mice results in a threefold increase of the number of mice surviving to HCoV-OC43 infection. This increased survival rate is correlated with an upregulated glial activation associated with a limited innate immune response (cytokines, chemokines) and T-cell infiltration into infected brains. Moreover, the protection seems to be associated with a restricted phospholipase A2 activity. These data reveal a role for apoD in the regulation of inflammation and suggest that it protects from HCoV-OC43-induced encephalitis, most likely through the phospholipase A2 signaling pathways.

Severe acute respiratory syndrome vaccine efficacy in ferrets: whole killed virus and adenovirus-vectored vaccines

Although the 2003 severe acute respiratory syndrome (SARS) outbreak was controlled, repeated transmission of SARS coronavirus (CoV) over several years makes the development of a SARS vaccine desirable. We performed a comparative evaluation of two SARS vaccines for their ability to protect against live SARS-CoV intranasal challenge in ferrets. Both the whole killed SARS-CoV vaccine (with and without alum) and adenovirus-based vectors encoding the nucleocapsid (N) and spike (S) protein induced neutralizing antibody responses and reduced viral replication and shedding in the upper respiratory tract and progression of virus to the lower respiratory tract. The vaccines also diminished haemorrhage in the thymus and reduced the severity and extent of pneumonia and damage to lung epithelium. However, despite high neutralizing antibody titres, protection was incomplete for all vaccine preparations and administration routes. Our data suggest that a combination of vaccine strategies may be required for effective protection from this pathogen. The ferret may be a good model for SARS-CoV infection because it is the only model that replicates the fever seen in human patients, as well as replicating other SARS disease features including infection by the respiratory route, clinical signs, viral replication in upper and lower respiratory tract and lung damage.

Priming of CD8+ T cells during central nervous system infection with a murine coronavirus is strain dependent

Virus-specific CD8(+) T cells are critical for protection against neurotropic coronaviruses; however, central nervous system (CNS) infection with the recombinant JHM (RJHM) strain of mouse hepatitis virus (MHV) elicits a weak CD8(+) T-cell response in the brain and causes lethal encephalomyelitis. An adoptive transfer model was used to elucidate the kinetics of CD8(+) T-cell priming during CNS infection with RJHM as well as with two MHV strains that induce a robust CD8(+) T-cell response (RA59 and SJHM/RA59, a recombinant A59 virus expressing the JHM spike). While RA59 and SJHM/RA59 infections resulted in CD8(+) T-cell priming within the first 2 days postinfection, RJHM infection did not lead to proliferation of naïve CD8(+) T cells. While all three viruses replicated efficiently in the brain, only RA59 and SJHM/RA59 replicated to appreciable levels in the cervical lymph nodes (CLN), the site of T-cell priming during acute CNS infection. RJHM was unable to suppress the CD8(+) T-cell response elicited by RA59 in mice simultaneously infected with both strains, suggesting that RJHM does not cause generalized immunosuppression. RJHM was also unable to elicit a secondary CD8(+) T-cell response in the brain following peripheral immunization against a viral epitope. Notably, the weak CD8(+) T-cell response elicited by RJHM was unique to CNS infection, since peripheral inoculation induced a robust CD8(+) T-cell response in the spleen. These findings suggest that the failure of RJHM to prime a robust CD8(+) T-cell response during CNS infection is likely due to its failure to replicate in the CLN.

Antiviral CD8 T cells recognize borna disease virus antigen transgenically expressed in either neurons or astrocytes

Borna disease virus (BDV) can persistently infect the central nervous system (CNS) of mice. The infection remains nonsymptomatic as long as antiviral CD8 T cells do not infiltrate the infected brain. BDV mainly infects neurons which reportedly carry few, if any, major histocompatibility complex class I molecules on the surface. Therefore, it remains unclear whether T cells can recognize replicating virus in these cells or whether cross-presentation of viral antigen by other cell types is important for immune recognition of BDV. To distinguish between these possibilities, we used two lines of transgenic mice that strongly express the N protein of BDV in either neurons (Neuro-N) or astrocytes (Astro-N). Since these animals are tolerant to the neo-self-antigen, we adoptively transferred T cells with specificity for BDV N. In nontransgenic mice persistently infected with BDV, the transferred cells accumulated in the brain parenchyma along with immune cells of host origin and efficiently induced neurological disease. Neurological disease was also observed if antiviral T cells were injected into the brains of Astro-N or Neuro-N but not nontransgenic control mice. Our results demonstrate that CD8 T cells can recognize foreign antigen on neurons and astrocytes even in the absence of infection or inflammation, indicating that these CNS cell types are playing an active role in immune recognition of viruses.

Maturation and localization of macrophages and microglia during infection with a neurotropic murine coronavirus

Macrophages and microglia are critical in the acute inflammatory response and act as final effector cells of demyelination during chronic infection with the neutrotropic MHV‐JHM strain of mouse hepatitis virus (MHV‐JHM). Herein, we show that “immature” F4/80⁺Ly‐6C^hi monocytes are the first cells, along with neutrophils, to enter the MHV‐JHM‐infected central nervous system (CNS). As the infection progresses, macrophages in the CNS down‐regulate expression of Ly‐6C and CD62L, consistent with maturation, and a higher frequency express CD11c, a marker for dendritic cells (DCs). Microglia also express CD11c during this phase of the infection. CD11c⁺ macrophages in the infected CNS exhibit variable properties of immature antigen‐presenting cells (APCs), with modestly increased CD40 and MHC expression, and equivalent potent antigen uptake when compared with CD11c^‐ macrophages. Furthermore, CDllc⁺ and F4/80⁺ macrophages and microglia are localized to areas of demyelination, in some instances directly associated with damaged axons. These results suggest that chronic CNS infection results in the appearance of CD11c‐expressing macrophages from the blood that exhibit properties of immature APCs, are closely associated with areas of demyelination, and may act as final effectors of myelin destruction.

Fas ligand interactions contribute to CD8+ T-cell-mediated control of West Nile virus infection in the central nervous system

West Nile virus (WNV) is a neurotropic flavivirus that causes encephalitis, most frequently in elderly and immunocompromised humans. Previous studies demonstrated that CD8+ T cells utilize perforin-dependent cytolytic mechanisms to limit WNV infection. Nonetheless, the phenotype of perforin-deficient CD8+ T cells was not as severe as that of an absence of CD8+ T cells, suggesting additional effector control mechanisms. In this study, we evaluated the contribution of Fas-Fas ligand (FasL) interactions to CD8+ T-cell-mediated control of WNV infection. Notably, the cell death receptor Fas was strongly upregulated on neurons in culture and in vivo after WNV infection. gld mice that were functionally deficient in FasL expression showed increased susceptibility to lethal WNV infection. Although antigen-specific priming of CD8+ T cells in peripheral lymphoid tissues was normal in gld mice, increased central nervous system (CNS) viral burdens and delayed clearance were observed. Moreover, the adoptive transfer of WNV-primed wild-type but not gld CD8+ T cells to recipient CD8(-/-) or gld mice efficiently limited infection in the CNS and enhanced survival rates. Overall, our data suggest that CD8+ T cells also utilize FasL effector mechanisms to contain WNV infection in Fas-expressing neurons in the CNS.

Modulation of microglia and CD8(+) T cell activation during the development of stress-induced herpes simplex virus type-1 encephalitis

The central nervous system (CNS) has been shown to be vulnerable to a variety of insults in animals exposed to glucocorticoids. For example, psychological stress, a known inducer of glucocorticoid production, enhances the susceptibility of mice to herpes simplex virus type-1 (HSV-1) infection and results in the development of HSV-1 encephalitis (HSE). To determine the immune mechanisms by which stress promotes the development of HSE, we examined the role of the glucocorticoid receptor (GR) and the N-methyl-d-aspartate (NMDA) receptor in the development of HSE. Our findings demonstrate that blockade of either the GR or the NMDA receptor enhances survival following HSV-1 infection in stressed mice to levels similar to non-stressed mice. Subsequent studies determined the effect of GR and NMDA receptor blockade on immune function by specifically examining both microglia and CD8(+) T cell activation. Stress inhibited the expression of MHC class I by microglia and other brain-derived antigen presenting cells (CD45(hi)) independent of either the glucocorticoid receptor or the NMDA receptor, suggesting that stress-induced suppression of MHC class I expression in the brain does not affect survival during HSE. Blockade of the NMDA receptor, however, diminished HSV-1-induced increases in class I expression by CD45(hi) cells, suggesting that blockade of the NMDA receptor may limit CNS inflammation. Also, while CD8(+) T cell activation and function in the brain were not affected by stress, the number of CD8(+) T cells in the superficial cervical lymph nodes (SCLN) was decreased in stressed mice via GR-mediated mechanisms. These findings indicate that stress-induced hypocellularity is mediated by the GR while NMDA receptor activation is responsible for enhancing CNS inflammation. The combined effects of GR-mediated hypocellularity of the SCLN and NMDA receptor-mediated CNS inflammation during stress promote the development of HSE.

Screening and identification of severe acute respiratory syndrome-associated coronavirus-specific CTL epitopes

Severe acute respiratory syndrome (SARS) is a highly contagious and life-threatening disease that emerged in China in November 2002. A novel SARS-associated coronavirus was identified as its principal etiologic agent; however, the immunopathogenesis of SARS and the role of special CTLs in virus clearance are still largely uncharacterized. In this study, potential HLA-A*0201-restricted spike (S) and nucleocapsid protein-derived peptides were selected from an online database and screened for potential CTL epitopes by in vitro refolding and T2 cell-stabilization assays. The antigenicity of nine peptides which could refold with HLA-A*0201 molecules was assessed with an IFN-gamma ELISPOT assay to determine the capacity to stimulate CTLs from PBMCs of HLA-A2(+) SARS-recovered donors. A novel HLA-A*0201-restricted decameric epitope P15 (S411-420, KLPDDFMGCV) derived from the S protein was identified and found to localize within the angiotensin-converting enzyme 2 receptor-binding region of the S1 domain. P15 could significantly enhance the expression of HLA-A*0201 molecules on the T2 cell surface, stimulate IFN-gamma-producing CTLs from the PBMCs of former SARS patients, and induce specific CTLs from P15-immunized HLA-A2.1 transgenic mice in vivo. Furthermore, significant P15-specific CTLs were induced from HLA-A2.1-transgenic mice immunized by a DNA vaccine encoding the S protein; suggesting that P15 was a naturally processed epitope. Thus, P15 may be a novel SARS-associated coronavirus-specific CTL epitope and a potential target for characterization of virus control mechanisms and evaluation of candidate SARS vaccines.

Both spike and background genes contribute to murine coronavirus neurovirulence

Various strains of mouse hepatitis virus (MHV) exhibit different pathogenic phenotypes. Infection with the A59 strain of MHV induces both encephalitis and hepatitis, while the highly neurovirulent JHM strain induces a fatal encephalitis with little, if any, hepatitis. The pathogenic phenotype for each strain is determined by the genetic composition of the viral genome, as well as the host immune response. Using isogenic recombinant viruses with A59 background genes differing only in the spike gene, we have previously shown that high neurovirulence is associated with the JHM spike protein, the protein responsible for attachment to the host cell receptor (J. J. Phillips, M. M. Chua, G. F. Rall, and S. R. Weiss, Virology 301:109-120, 2002). Using another set of isogenic recombinant viruses with JHM background genes expressing either the JHM or A59 spike, we have further investigated the roles of viral genes in pathogenesis. Here, we demonstrate that the high neurovirulence of JHM is associated with accelerated spread through the brain and a heightened innate immune response that is characterized by high numbers of infiltrating neutrophils and macrophages, suggesting an immunopathogenic component to neurovirulence. While expression of the JHM spike is sufficient to confer a neurovirulent phenotype, as well as increased macrophage infiltration, background genes contribute to virulence as well, at least in part, by dictating the extent of the T-cell immune response.

Long-lived memory T lymphocyte responses against SARS coronavirus nucleocapsid protein in SARS-recovered patients

The nucleocapsid (N) protein is a structural component of severe acute respiratory syndrome (SARS) coronavirus (SARS-CoV) and can induce antibody responses in SARS patients during infection. However, it is not known whether SARS-CoV N protein can induce a long persistence of memory T-cell response in human. In this study, we found that peripheral blood mononuclear cells (PBMCs) from fully recovered SARS individuals rapidly produced IFN-gamma and IL-2 following stimulation with a pool of overlapping peptides that cover the entire N protein sequence. The N-specific IFN-gamma(+)CD4(+) T cells were mainly composed of CD45RA(-)CCR7(+)CD62L(-) cells, whereas IFN-gamma(+)CD8(+) memory T cells were mostly contained within CD45RA(+)CCR7(-)CD62L(-) cell population. Epitope mapping study indicated that a cluster of overlapping peptides located in the C-terminal region (amino acids [aa] 331 to 362) of N protein contained at least two different T-cell epitopes. The results indicated that human memory T-cell responses specific for SARS-CoV N protein could persist for 2 years in the absence of antigen, which would be a valuable for the design of effective vaccines against SARS-CoV and for basic studies of human T-cell memory.

Microglial cells initiate vigorous yet non-protective immune responses during HSV-1 brain infection

Central nervous system (CNS) infection with herpes simplex virus (HSV)-1 triggers neuroinflammatory responses leading to peripheral immune cell infiltration into the brain. Previous in vitro studies from our laboratory, using primary human brain cells, implicated microglia as the cellular source of infection-induced chemokines, such as CXC ligand 10 (CXCL10) and CC ligand 2 (CCL2). Here, we evaluated the role of microglial cells in HSV-induced neuroimmune responses using an in vivo murine model of herpes encephalitis. Data obtained during this study demonstrated robust levels of CXCL10, CCL2 and CXCL9 detectable in the brains of infected BALB/c mice between 5 and 8 days post-infection (p.i.). Microglial cells were identified as a source of this HSV-induced chemokine production. Additional experiments established that induction of these immune mediators preceded the presence of CD3, CD4, CD8, and CD45 mRNA in the brain, and immunohistochemical analysis confirmed the presence of infiltrating CD3(+) cells. Further analysis suggested that microglia-derived chemokines drive peripheral immune cell chemotaxis, as antibodies to CXCL10 and CCL2 blocked the migration of murine splenocytes toward HSV-infected microglia by approximately 59.3+/-4.1% and 17.5+/-1.4%, respectively. Taken together, these results demonstrate that a vigorous microglia-driven cascade of pro-inflammatory immune responses is not sufficient to protect susceptible mice from HSV-1 brain infection.

Viral induced demyelination

Viral induced demyelination, in both humans and rodent models, has provided unique insights into the cell biology of oligodendroglia, their complex cell-cell interactions and mechanisms of myelin destruction. They illustrate mechanisms of viral persistence, including latent infections in which no infectious virus is readily evident, virus reactivation and viral-induced tissue damage. These studies have also provided excellent paradigms to study the interactions between the immune system and the central nervous system (CNS). Although of interest in their own right, an understanding of the diverse mechanisms used by viruses to induce demyelination may shed light into the etiology and pathogenesis of the common demyelinating disorder multiple sclerosis (MS). This notion is supported by the persistent view that a viral infection acquired during adolescence might initiate MS after a long period of quiescence. Demyelination in both humans and rodents can be initiated by infection with a diverse group of enveloped and non-enveloped RNA and DNA viruses (Table 1). The mechanisms that ultimately result in the loss of CNS myelin appear to be equally diverse as the etiological agents capable of causing diseases which result in demyelination. Although demyelination can be a secondary result of axonal loss, in many examples of viral induced demyelination, myelin loss is primary and associated with axonal sparing. This suggests that demyelination induced by viral infections can result from: 1) a direct viral infection of oligodendroglia resulting in cell death with degeneration of myelin and its subsequent removal; 2) a persistent viral infection, in the presence or absence of infectious virus, resulting in the loss of normal cellular homeostasis and subsequent oligodendroglial death; 3) a vigorous virus-specific inflammatory response wherein the virus replicates in a cell type other than oligodendroglia, but cytokines and other immune mediators directly damage the oligodendroglia or the myelin sheath; or 4) infection initiates activation of an immune response specific for either oligodendroglia or myelin components. Virus-induced inflammation may be associated with the processing of myelin or oligodendroglial components and their presentation to the host's own T cell compartment. Alternatively, antigenic epitopes derived from the viral proteins may exhibit sufficient homology to host components that the immune response to the virus activates autoreactive T cells, i.e. molecular mimicry. Although it is not clear that each of these potential mechanisms participates in the pathogenesis of human demyelinating disease, analysis of the diverse demyelinating viral infections of both humans and rodents provides examples of many of these potential mechanisms.

Comparative evaluation of two severe acute respiratory syndrome (SARS) vaccine candidates in mice challenged with SARS coronavirus

Two different severe acute respiratory syndrome (SARS) vaccine strategies were evaluated for their ability to protect against live SARS coronavirus (CoV) challenge in a murine model of infection. A whole killed (inactivated by beta-propiolactone) SARS-CoV vaccine and a combination of two adenovirus-based vectors, one expressing the nucleocapsid (N) and the other expressing the spike (S) protein (collectively designated Ad S/N), were evaluated for the induction of serum neutralizing antibodies and cellular immune responses and their ability to protect against pulmonary SARS-CoV replication. The whole killed virus (WKV) vaccine given subcutaneously to 129S6/SvEv mice was more effective than the Ad S/N vaccine administered either intranasally or intramuscularly in inhibiting SARS-CoV replication in the murine respiratory tract. This protective ability of the WKV vaccine correlated with the induction of high serum neutralizing-antibody titres, but not with cellular immune responses as measured by gamma interferon secretion by mouse splenocytes. Titres of serum neutralizing antibodies induced by the Ad S/N vaccine administered intranasally or intramuscularly were significantly lower than those induced by the WKV vaccine. However, Ad S/N administered intranasally, but not intramuscularly, significantly limited SARS-CoV replication in the lungs. Among the vaccine groups, SARS-CoV-specific IgA was found only in the sera of mice immunized intranasally with Ad S/N, suggesting that mucosal immunity may play a role in protection for the intranasal Ad S/N delivery system. Finally, the sera of vaccinated mice contained antibodies to S, further suggesting a role for this protein in conferring protective immunity against SARS-CoV infection.

Preferential infection of mature dendritic cells by mouse hepatitis virus strain JHM

Mouse hepatitis virus strain JHM (MHV-JHM) causes acute encephalitis and acute and chronic demyelinating diseases in mice. Dendritic cells (DCs) are key cells in the initiation of innate and adaptive immune responses, and infection of these cells could potentially contribute to a dysregulated immune response; consistent with this, recent results suggest that DCs are readily infected by another strain of mouse hepatitis virus, the A59 strain (MHV-A59). Herein, we show that the JHM strain also productively infected DCs. Moreover, mature DCs were at least 10 times more susceptible than immature DCs to infection with MHV-JHM. DC function was impaired after MHV-JHM infection, resulting in decreased stimulation of CD8 T cells in vitro. Preferential infection of mature DCs was not due to differential expression of the MHV-JHM receptor CEACAM-1a on mature or immature cells or to differences in apoptosis. Although we could not detect infected DCs in vivo, both CD8(+) and CD11b(+) splenic DCs were susceptible to infection with MHV-JHM directly ex vivo. This preferential infection of mature DCs may inhibit the development of an efficient immune response to the virus.

Maintenance of CD8+ T cells during acute viral infection of the central nervous system requires CD4+ T cells but not interleukin-2

The JHM strain of mouse hepatitis virus (JHMV) is rapidly cleared from the central nervous system (CNS) by CD8(+) T cells. In the absence of CD4(+) T cells, fewer CD8(+) T cells are found within the CNS in association with a coordinate increase in apoptotic lymphocytes. Previous data suggested that CD4(+) T cells may support CD8(+) T cells through secretion of interleukin-2 (IL-2). To determine the in vivo role of IL-2 during CNS infection, IL-2 signaling was inhibited via administration of a neutralizing IL-2-specific monoclonal antibody (mAb). In contrast to depletion of CD4(+) T cells, inhibition of IL-2 signaling did not influence CD8(+) T cell infiltration, effector cell function or survival within the CNS. These data suggest that the cellular immune response to acute neurotropic JHMV infection requires a distinct CD4(+) T cell component, but is independent of a requirement for IL-2 for induction, activation, recruitment, and/or maintenance of CD8(+) T cells within the CNS during acute infection.

Receptor-independent spread of a highly neurotropic murine coronavirus JHMV strain from initially infected microglial cells in mixed neural cultures

Although neurovirulent mouse hepatitis virus (MHV) strain JHMV multiplies in a variety of brain cells, expression of its receptor carcinoembryonic antigen cell adhesion molecule 1 (CEACAM 1) (MHVR) is restricted only in microglia. The present study was undertaken to clarify the mechanism of an extensive JHMV infection in the brain by using neural cells isolated from mouse brain. In contrast to wild-type (wt) JHMV, a soluble-receptor-resistant mutant (srr7) infects and spreads solely in an MHVR-dependent fashion (F. Taguchi and S. Matsuyama, J. Virol. 76:950-958, 2002). In mixed neural cell cultures, srr7 infected a limited number of cells and infection did not spread, although wt JHMV induced syncytia in most of the cells. srr7-infected cells were positive for GS-lectin, a microglia marker. Fluorescence-activated cell sorter analysis showed that about 80% of the brain cells stained with anti-MHVR antibody (CC1) were also positive for GS-lectin. Pretreatment of those cells with CC1 prevented virus attachment to the cell surface and also blocked virus infection. These results show that microglia express functional MHVR that mediates JHMV infection. As expected, in microglial cell-enriched cultures, both srr7and wt JHMV produced syncytia in a majority of cells. Treatment with CC1 of mixed neural cell cultures and microglia cultures previously infected with wt virus failed to block the spread of infection, indicating that wt infection spreads in an MHVR-independent fashion. Thus, the present study indicates that microglial cells are the major population of the initial target for MHV infection and that the wt spreads from initially infected microglia to a variety of cells in an MHVR-independent fashion.

Increased epitope-specific CD8+ T cells prevent murine coronavirus spread to the spinal cord and subsequent demyelination

CD8+ T cells are important for clearance of neurotropic mouse hepatitis virus (MHV) strain A59, although their possible role in A59-induced demyelination is not well understood. We developed an adoptive-transfer model to more clearly elucidate the role of virus-specific CD8+ T cells during the acute and chronic phases of infection with A59 that is described as follows. C57BL/6 mice were infected with a recombinant A59 virus expressing the gp33 epitope, an H-2Db-restricted CD8+ T-cell epitope encoded in the glycoprotein of lymphocytic choriomeningitis virus, as a fusion with the enhanced green fluorescent protein (RA59-gfp/gp33). P14 splenocytes (transgenic for a T-cell receptor specific for the gp33 epitope) were transferred at different times pre- and postinfection (p.i.). Adoptive transfer of P14 splenocytes 1 day prior to infection with RA59-gfp/gp33, but not control virus lacking the gp33 epitope, RA59-gfp, reduced weight loss and viral replication and spread in the brain and to the spinal cord. Furthermore, demyelination was significantly reduced compared to that in nonrecipients. However, when P14 cells were transferred on day 3 or 5 p.i., no difference in acute or chronic disease was observed compared to that in nonrecipients. Protection in mice receiving P14 splenocytes prior to infection correlated with a robust gp33-specific immune response that was not observed in mice receiving the later transfers. Thus, an early robust CD8+ T-cell response was necessary to reduce virus replication and spread, specifically to the spinal cord, which protected against demyelination in the chronic phase of the disease.

The Role of T Cells in Corona-Virus-Induced Demyelination

Mice infected with neurotropic strains of coronavirus develop acute encephalomyelitis and eliminate infectious virus. However, control of acute infection is incomplete resulting in persistence of viral RNA in the central nervous system (CNS) associated with ongoing primary demyelination. A high prevalence of virus specific CD8 and CD4 T cells within the CNS correlates with ex vivo cytolytic activity and IFN-γ secretion, which are both required for virus reduction during the acute infection. Although most infected cell types are susceptible to perforin mediated clearance, IFN-γ is required for controlling infection of oligodendrocytes. Furthermore, by enhancing class I expression and inducing class II expression within resident CNS cells IFN-γ optimizes T cell receptor dependent functions. In addition to its direct anti viral activity, these multifactorial effects make IFN-γ more essential than perforin for viral control. CD4 T cells enhance CD8 T cell expansion, survival and effectiveness. Although both CD8 and CD4 T cells are retained within the CNS during persistence, they cannot control viral recrudescence in the absence of humoral immunity. Demyelination can be mediated by either CD8 or CD4 T cells; however, although a variety of effector molecules have been excluded, a dominant common denominator remains elusive. Thus concerted efforts to control infection coincide with a variety of potential mechanisms causing chronic demyelinating disease.

Differential regulation of primary and secondary CD8+ T cells in the central nervous system

T cell accumulation and effector function following CNS infection is limited by a paucity of Ag presentation and inhibitory factors characteristic of the CNS environment. Differential susceptibilities of primary and recall CD8+ T cell responses to the inhibitory CNS environment were monitored in naive and CD8+ T cell-immune mice challenged with a neurotropic coronavirus. Accelerated virus clearance and limited spread in immunized mice was associated with a rapid and increased CNS influx of virus-specific secondary CD8+ T cells. CNS-derived secondary CD8+ T cells exhibited increased cytolytic activity and IFN-gamma expression per cell compared with primary CD8+ T cells. However, both Ag-specific primary and secondary CD8+ T cells demonstrated similar contraction rates. Thus, CNS persistence of increased numbers of secondary CD8+ T cells reflected differences in the initial pool size during peak inflammation rather than enhanced survival. Unlike primary CD8+ T cells, persisting secondary CD8+ T cells retained ex vivo cytolytic activity and expressed high levels of IFN-gamma following Ag stimulation. However, both primary and secondary CD8+ T cells exhibited reduced capacity to produce TNF-alpha, differentiating them from effector memory T cells. Activation of primary and secondary CD8+ T cells in the same host using adoptive transfers confirmed similar survival, but enhanced and prolonged effector function of secondary CD8+ T cells in the CNS. These data suggest that an instructional program intrinsic to T cell differentiation, rather than Ag load or factors in the inflamed CNS, prominently regulate CD8+ T cell function.

Characterization of heteroclite subgenomic RNAs associated with PRRSV infection

In this study, porcine reproductive and respiratory syndrome virus (PRRSV) heteroclite (uncommon forms) RNAs were characterized. Nucleotide sequencing of 11 additional defective RNA species verified that heteroclites are formed between the 5' and 3' termini of PRRSV at short stretches of identity, with variability seen between the junction sites utilized. Northern blot and RT-PCR analyses indicated that heteroclite RNA species were likely to be packaged into purified virions. To study whether heteroclite RNAs and viral genomic RNAs could be packaged into the same virions, PRRSV strain VR-2332 was purified by sucrose density gradient centrifugation. RT-PCR amplification of the viral RNAs isolated from three distinct gradient bands, using genomic- and heteroclite-specific primer pairs, demonstrated that heteroclite RNAs could not be readily dissociated from genomic RNA. Partial segregation of full-length and larger heteroclite genomes to the upper two gradient bands was seen, but smaller species could be found in all three fractions. These results strongly suggest that heteroclite RNAs retain the PRRSV RNA packaging signal. In vitro transcription and translation of one heteroclite cDNA clone verified that the RNA could express a predicted 32.6 kDa protein, indicating that these RNA species have the potential to produce abnormal proteins in infected cells.

S protein of severe acute respiratory syndrome-associated coronavirus mediates entry into hepatoma cell lines and is targeted by neutralizing antibodies in infected patients

The severe acute respiratory syndrome-associated coronavirus (SARS-CoV) causes severe pneumonia with a fatal outcome in approximately 10% of patients. SARS-CoV is not closely related to other coronaviruses but shares a similar genome organization. Entry of coronaviruses into target cells is mediated by the viral S protein. We functionally analyzed SARS-CoV S using pseudotyped lentiviral particles (pseudotypes). The SARS-CoV S protein was found to be expressed at the cell surface upon transient transfection. Coexpression of SARS-CoV S with human immunodeficiency virus-based reporter constructs yielded viruses that were infectious for a range of cell lines. Most notably, viral pseudotypes harboring SARS-CoV S infected hepatoma cell lines but not T- and B-cell lines. Infection of the hepatoma cell line Huh-7 was also observed with replication-competent SARS-CoV, indicating that hepatocytes might be targeted by SARS-CoV in vivo. Inhibition of vacuolar acidification impaired infection by SARS-CoV S-bearing pseudotypes, indicating that S-mediated entry requires low pH. Finally, infection by SARS-CoV S pseudotypes but not by vesicular stomatitis virus G pseudotypes was efficiently inhibited by a rabbit serum raised against SARS-CoV particles and by sera from SARS patients, demonstrating that SARS-CoV S is a target for neutralizing antibodies and that such antibodies are generated in SARS-CoV-infected patients. Our results show that viral pseudotyping can be employed for the analysis of SARS-CoV S function. Moreover, we provide evidence that SARS-CoV infection might not be limited to lung tissue and can be inhibited by the humoral immune response in infected patients.

Perforin and gamma interferon-mediated control of coronavirus central nervous system infection by CD8 T cells in the absence of CD4 T cells

Infection of the central nervous system (CNS) with the neurotropic JHM strain of mouse hepatitis virus produces acute and chronic demyelination. The contributions of perforin-mediated cytolysis and gamma interferon (IFN-gamma) secretion by CD8(+) T cells to the control of infection and the induction of demyelination were examined by adoptive transfer into infected SCID recipients. Untreated SCID mice exhibited uncontrolled virus replication in all CNS cell types but had little or no demyelination. Memory CD8(+) T cells from syngeneic wild-type (wt), perforin-deficient, or IFN-gamma-deficient (GKO) donors all trafficked into the infected CNS in the absence of CD4(+) T cells and localized to similar areas. Although CD8(+) T cells from all three donors suppressed virus replication in the CNS, GKO CD8(+) T cells expressed the least antiviral activity. A distinct viral antigen distribution in specific CNS cell types revealed different mechanisms of viral control. While wt CD8(+) T cells inhibited virus replication in all CNS cell types, cytolytic activity in the absence of IFN-gamma suppressed the infection of astrocytes, but not oligodendroglia. In contrast, cells that secreted IFN-gamma but lacked cytolytic activity inhibited replication in oligodendroglia, but not astrocytes. Demyelination was most severe following viral control by wt CD8(+) T cells but was independent of macrophage infiltration. These data demonstrate the effective control of virus replication by CD8(+) T cells in the absence of CD4(+) T cells and support the necessity for the expression of distinct effector mechanisms in the control of viral replication in distinct CNS glial cell types.

Effects of an epitope-specific CD8+ T-cell response on murine coronavirus central nervous system disease: protection from virus replication and antigen spread and selection of epitope escape mutants

Both CD4(+) and CD8(+) T cells are required for clearance of the murine coronavirus mouse hepatitis virus (MHV) during acute infection. We investigated the effects of an epitope-specific CD8(+) T-cell response on acute infection of MHV, strain A59, in the murine CNS. Mice with CD8(+) T cells specific for gp33-41 (an H-2D(b)-restricted CD8(+) T-cell epitope derived from lymphocytic choriomeningitis glycoprotein) were infected with a recombinant MHV-A59, also expressing gp33-41, as a fusion protein with enhanced green fluorescent protein (EGFP). By 5 days postinfection, these mice showed significantly (approximately 20-fold) lower titers of infectious virus in the brain compared to control mice. Furthermore mice with gp33-41-specific CD8(+) cells exhibited much reduced levels of viral antigen in the brain as measured by immunohistochemistry using an antibody directed against viral nucleocapsid. More than 90% of the viruses recovered from brain lysates of such protected mice, at 5 days postinfection, had lost the ability to express EGFP and had deletions in their genomes encompassing EGFP and gp33-41. In addition, genomes of viruses from about half the plaques that retained the EGFP gene had mutations within the gp33-41 epitope. On the other hand, gp33-41-specific cells failed to protect perforin-deficient mice from infection by the recombinant MHV expressing gp33, indicating that perforin-mediated mechanisms were needed. Virus recovered from perforin-deficient mice did not exhibit loss of EGFP expression and the gp33-41 epitope. These observations suggest that the cytotoxic T-cell response to gp33-41 exerts a strong immune pressure that quickly selects epitope escape mutants to gp33-41.

Experimental Models of Virus-Induced Demyelination

This chapter reviews two of the most widely studied animal models of virus-induced demyelinating disease. These are Theiler's murine encephalomyelitis virus and murine hepatitis virus. Both viruses produce acute inflammatory encephalitis that is followed by chronic central-nervous-system (CNS) demyelinating disease. The clinical and pathologic correlates of virus-induced demyelination are largely immune mediated. Furthermore, several pathologic mechanisms have been proposed to explain the development of myelin damage and neurologic deficits, and each of the proposed mechanisms may play a role in disease progression depending on the genetic constitution of the infected animal. The induction of demyelinating disease by virus may be directly relevant to human MS. Several viruses are known to cause demyelination in humans and viral infection is an epidemiologic factor that is consistently associated with clinical exacerbation of MS. It is suggested that viral infection may be a cause of MS, although no specific virus has been identified as a causative agent.

The dual role of CD8+ T lymphocytes in the development of stress-induced herpes simplex encephalitis

Despite the generally restrictive nature of the blood-brain barrier (BBB), circulating lymphocytes can infiltrate into the central nervous system (CNS) during a variety of disease states. Although the contributions of these lymphocytes to CNS-associated disease have been identified in some viral models, the factors which govern this infiltration following herpes simplex virus (HSV) infection remain to be elucidated. We have developed a murine model of HSV encephalitis (HSE) to define the relationship among psychological stress, the recruitment of HSV-specific T cells into the CNS, and the development of HSE. Naive mice, as well as mice that had been vaccinated with a recombinant vaccinia virus (rVVESgB498-505) that elicits the generation of HSV-1 gB498-505-specific CD8(+) T cells, were infected intranasally (i.n.) with HSV-1 McIntyre. Beginning one day prior to HSV-1 infection and continuing for a total of 9 days, naive and vaccinated mice were exposed to a well-established stressor, restraint stress. Naive, stressed mice exhibited increased symptoms of HSE and HSE-associated mortality as compared to non-stressed controls. A concomitant increase in CD4(+) and CD8(+) T cells in the brain was observed throughout the infection, with CD8(+) T cells outnumbering CD4(+) T cells. The development of HSE in these naive, stressed mice was accompanied by a delayed infiltration of gB498-505-specific CD8(+) T cells after HSV spread into the brain. In contrast, both stressed and non-stressed rVVESgB498-505-vaccinated mice possessed gB498-505-specific CD8(+) T cells prior to HSV challenge and were protected against HSE despite having detectable HSV-1 DNA in the brain. Together, these findings suggest that a delayed infiltration of CD8(+) T cells into the brain may promote HSE in naive mice, while the presence of HSV-specific CD8(+) T cells in the brain prior to HSV challenge is protective, possibly by limiting HSV replication and spread within the CNS.

Control of central nervous system viral persistence by neutralizing antibody

Replication of the neurotropic JHM strain of mouse hepatitis virus within the central nervous system is controlled by cellular immunity. However, following initial clearance, virus reactivates in the absence of humoral immunity. Viral recrudescence is prevented by the transfer of antiviral antibody (Ab). To characterize the specificity and biological functions of Ab critical for maintaining viral persistence, monoclonal Abs specific for the viral spike, matrix, and nucleocapsid proteins were transferred into infected B-cell-deficient mice following initial virus clearance. Neutralizing immunoglobulin G (IgG) but not IgA anti-spike Ab suppressed virus recrudescence, reduced viral antigen in most cell types except oligodendroglia, and was associated with reduced demyelination. Nonneutralizing monoclonal Abs specific for the spike, matrix, and nucleocapsid proteins did not prevent recrudescence, demonstrating that neutralization is critical for maintaining JHM mouse hepatitis virus persistence within the central nervous system. Ab-mediated protection was not associated with alterations in virus-specific T-cell function or inflammation. Furthermore, neutralizing Ab delayed but did not prevent virus recrudescence. These data indicate that following acute viral clearance cellular immunity is ineffective in controlling virus recrudescence and suggest that the continued presence of neutralizing Ab is the essential effector in maintaining viral persistence within the central nervous system.

Perforin-mediated effector function within the central nervous system requires IFN-gamma-mediated MHC up-regulation

CD8(+) T cells infiltrating the CNS control infection by the neurotropic JHM strain of mouse hepatitis virus. Differential susceptibility of infected cell types to clearance by perforin or IFN-gamma uncovered distinct, nonredundant roles for these antiviral mechanisms. To separately evaluate each effector function specifically in the context of CD8(+) T cells, pathogenesis was analyzed in mice deficient in both perforin and IFN-gamma (PKO/GKO) or selectively reconstituted for each function by transfer of CD8(+) T cells. Untreated PKO/GKO mice were unable to control the infection and died of lethal encephalomyelitis within 16 days, despite substantially higher CD8(+) T cell accumulation in the CNS compared with controls. Uncontrolled infection was associated with limited MHC class I up-regulation and an absence of class II expression on microglia, coinciding with decreased CD4(+) T cells in CNS infiltrates. CD8(+) T cells from perforin-deficient and wild-type donors reduced virus replication in PKO/GKO recipients. By contrast, IFN-gamma-deficient donor CD8(+) T cells did not affect virus replication. The inability of perforin-mediated mechanisms to control virus in the absence of IFN-gamma coincided with reduced class I expression. These data not only confirm direct antiviral activity of IFN-gamma within the CNS but also demonstrate IFN-gamma-dependent MHC surface expression to guarantee local T cell effector function in tissues inherently low in MHC expression. The data further imply that IFN-gamma plays a crucial role in pathogenesis by regulating the balance between virus replication in oligodendrocytes, CD8(+) T cell effector function, and demyelination.

Kinetics of virus-specific CD8+ -T-cell expansion and trafficking following central nervous system infection

CD8+ T cells control acute infection of the central nervous system (CNS) by neurotropic mouse hepatitis virus but do not suffice to achieve sterile immunity. To determine the lag between T-cell priming and optimal activity within the CNS, the accumulation of virus-specific CD8+ T cells in the CNS relative to that in peripheral lymphoid organs was assessed by using gamma interferon-specific ELISPOT assays and class I tetramer staining. Virus-specific CD8+ T cells were first detected in the cervical lymph nodes. Expansion in the spleen was delayed and less pronounced but also preceded accumulation in the CNS. The data further suggest peripheral acquisition of cytolytic function, thus enhancing CD8+ -T-cell effector function upon cognate antigen recognition in the CNS.

Virus demyelination

A number of viruses can initiate central nervous system (CNS) diseases that include demyelination as a major feature of neuropathology. In humans, the most prominent demyelinating diseases are progressive multifocal leukoencephalopathy, caused by JC papovirus destruction of oligodendrocytes, and subacute sclerosing panencephalitis, an invariably fatal childhood disease caused by persistent measles virus. The most common neurological disease of young adults in the developed world, multiple sclerosis, is also characterized by lesions of inflammatory demyelination; however, the etiology of this disease remains an enigma. A viral etiology is possible, because most demyelinating diseases of known etiology in both man and animals are viral. Understanding of the pathogenesis of virus-induced demyelination derives for the most part from the study of animal models. Studies with neurotropic strains of mouse hepatitis virus, Theiler's virus, and Semliki Forest virus have been at the forefront of this research. These models demonstrate how viruses enter the brain, spread, persist, and interact with immune responses. Common features are an ability to infect and persist in glial cells, generation of predominantly CD8(+) responses, which control and clear the early phase of virus replication but which fail to eradicate the infection, and lesions of inflammatory demyelination. In most cases demyelination is to a limited extent the result of direct virus destruction of oligodendrocytes, but for the most part is the consequence of immune and inflammatory responses. These models illustrate the roles of age and genetic susceptibility and establish the concept that persistent CNS infection can lead to the generation of CNS autoimmune responses.

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.

Murine coronavirus spike glycoprotein mediates degree of viral spread, inflammation, and virus-induced immunopathology in the central nervous system

The mouse hepatitis virus (MHV) spike glycoprotein is a major determinant of neurovirulence. We investigated how alterations in spike affect neurovirulence using two isogenic recombinant viruses differing exclusively in spike. S(4)R, containing the MHV-4 spike gene, is dramatically more neurovirulent than S(A59)R, containing the MHV-A59 spike gene (J. J. Phillips, M. M. Chua, E. Lavi, and S. R. Weiss, 1999, J. Virol. 73, 7752-7760). We examined the contribution of differences in cellular tropism, viral spread, and the immune response to infection to the differential neurovirulence of S(4)R and S(A59)R. MHV-4 spike-mediated neurovirulence was associated with extensive viral spread in the brain in both neurons and astrocytes. Infection of primary hippocampal neuron cultures demonstrated that S(4)R spread more rapidly than S(A59)R and suggested that spread may occur between cells in close physical contact. In addition, S(4)R infection induced a massive influx of lymphocytes into the brain, a higher percentage of CD8(+) T cells, and a higher frequency of MHV-specific CD8(+) T cells relative S(A59)R infection. Despite this robust and viral-specific immune response to S(4)R infection, infection of RAG1-/- mice suggested that immune-mediated pathology also contributes to the high neurovirulence of S(4)R.

Matrix metalloproteinase expression correlates with virulence following neurotropic mouse hepatitis virus infection

The relationship(s) between viral virulence and matrix metalloproteinase (MMP) expression in the central nervous system (CNS) of mice undergoing lethal and sublethal infections with neurotropic mouse hepatitis virus was investigated. Lethal infection induced increased levels of MMP-3 and MMP-12 mRNAs as well as that of tissue inhibitor of matrix metalloproteinases 1 (TIMP-1) compared to sublethal infection. Increased induction of MMP, TIMP, and chemokine expression correlated with increased virus replication but not with inflammatory cell infiltration. Infection of immunosuppressed mice suggested that expression of most MMP, TIMP, and chemokine mRNA was induced primarily in CNS-resident cells. By contrast, MMP-9 protein activity was associated with the infiltration of neutrophils into the CNS. These data indicate an association between the magnitude of inflammatory gene expression within the CNS and viral virulence.

A review of coronavirus infection in the central nervous system of cats and mice

Feline infectious peritonitis (FIP) is a common cause of death in cats. Management of this disease has been hampered by difficulties identifying the infection and determining the immunological status of affected cats and by high variability in the clinical, pathological, and immunological characteristics of affected cats. Neurological FIP, which is much more homogeneous than systemic effusive or noneffusive FIP, appears to be a good model for establishing the basic features of FIP immunopathogenesis. Very little information is available about the immunopathogenesis of neurologic FIP, and it is reasonable to use research from the well-characterized mouse hepatitis virus (MHV) immune-mediated encephalitis system, as a template for FIP investigation, and to contrast findings from the MHV model with those of FIP. It is expected that the immunopathogenic mechanisms will have important similarities. Such comparative research may lead to better understanding of FIP immunopathogenesis and rational prospects for management of this frustrating disease.

Regulation of T cell responses during central nervous system viral infection

This chapter focuses on the contribution of T cells to the pathogenesis of neurologic disease and discusses specific examples of how individual T cell effector functions can be regulated during central nervous system's (CNS) viral infections. T cells can serve a variety of functions as part of the host immune response during CNS viral infection. They can participate directly in viral clearance from the brain, or they can promote the survival of the host without exerting any direct effect on virus replication. Only a small number of T cells infiltrate the brain under normal circumstances. This paucity of immune surveillance of baseline is one of several reasons why the CNS has often been characterized as an “immunologically privileged” site. T cell-mediated lysis of infected cells has been demonstrated to be an important mechanism of viral clearance from tissues other than the CNS. In several well-characterized animal models of CNS viral infection, part of the elicited T cell response actually contributes to the pathology and adverse outcome of disease. Neurotropic lymphocytic choriomeningitis virus infection of adult mice is the premier example of this phenomenon.

Mechanisms of central nervous system viral persistence: the critical role of antibody and B cells

Contributions of humoral and cellular immunity in controlling neurotropic mouse hepatitis virus persistence within the CNS were determined in B cell-deficient J(H)D and syngeneic H-2(d) B cell+ Ab-deficient mice. Virus clearance followed similar kinetics in all mice, confirming initial control of virus replication by cellular immunity. Nevertheless, virus reemerged within the CNS of all Ab-deficient mice. In contrast to diminished T cell responses in H-2(b) B cell-deficient muMT mice, the absence of B cells or Ab in the H-2(d) mice did not compromise expansion, recruitment into the CNS, or function of virus-specific CD4+ and CD8+ T cells. The lack of B cells and lymphoid architecture thus appears to manifest itself on T cell responses in a genetically biased manner. Increasing viral load did not enhance frequencies or effector function of virus-specific T cells within the CNS, indicating down-regulation of T cell responses. Although an Ab-independent antiviral function of B cells was not evident during acute infection, the presence of B cells altered CNS cellular tropism during viral recrudescence. Reemerging virus localized almost exclusively to oligodendroglia in B cell+ Ab-deficient mice, whereas it also replicated in astrocytes in B cell-deficient mice. Altered tropism coincided with distinct regulation of CNS virus-specific CD4+ T cells. These data conclusively demonstrate that the Ab component of humoral immunity is critical in preventing virus reactivation within CNS glial cells. B cells themselves may also play a subtle role in modulating pathogenesis by influencing tropism.