Coronavirus infection of the central nervous system: host-virus stand-off

Asymmetry of Cell Division in CFSE-Based Lymphocyte Proliferation Analysis

Flow cytometry-based analysis of lymphocyte division using carboxyfluorescein succinimidyl ester (CFSE) dye dilution permits acquisition of data describing cellular proliferation and differentiation. For example, CFSE histogram data enable quantitative insight into cellular turnover rates by applying mathematical models and parameter estimation techniques. Several mathematical models have been developed using different types of deterministic or stochastic approaches. However, analysis of CFSE proliferation assays is based on the premise that the label is halved in the two daughter cells. Importantly, asymmetry of protein distribution in lymphocyte division is a basic biological feature of cell division with the degree of the asymmetry depending on various factors. Here, we review the recent literature on asymmetric lymphocyte division and CFSE-based lymphocyte proliferation analysis. We suggest that division- and label-structured mathematical models describing CFSE-based cell proliferation should take into account asymmetry and time-lag in cell proliferation. Utilization of improved modeling algorithms will permit straightforward quantification of essential parameters describing the performance of activated lymphocytes.

Development of transgenic mice expressing a coronavirus-specific public CD4 T cell receptor

Mice that are transgenic (Tg) for T cell receptor (TCR) expression are used extensively to analyze longitudinal T cell responses during effector and memory phases of the T cell response. Generation of TCR Tg mice generally requires T cell stimulation and cloning in vitro prior to amplification, processes which introduce biases into selection of the TCR that is ultimately chosen for TCR Tg mouse generation. Here we describe an alternative approach that involves no T cell stimulation or propagation in vitro. We generated mice that were transgenic for a TCR responding to a CD4 T cell epitope (epitope M133) that is immunodominant in mice infected with a neurotropic coronavirus, the JHM strain of mouse hepatitis virus. The CD4 T cell response to epitope M133 is of particular interest because it may be pathogenic, protective or regulatory, depending upon the physiological setting. We applied an iterative process in which we identified a TCR-β chain expressed by all mice that were examined (‘public sequence’). This TCR-β chain was introduced into bone marrow cells with a lentivirus vector, generating TCR-β retrogenic mice. A TCR-α chain that paired with this TCR-β was then identified and used to generate a second set of TCR (α/β) retrogenic mice. After demonstrating that these cells were functional and responded to epitope M133, these TCR chains were used to generate an epitope M133-specific TCR Tg mouse. This method should be generally useful for engineering TCR Tg mice without introduction of bias caused by in vitro manipulation and propagation.

IL-21 optimizes T cell and humoral responses in the central nervous system during viral encephalitis

Acute coronavirus encephalomyelitis is controlled by T cells while humoral responses suppress virus persistence. This study defines the contribution of interleukin (IL)-21, a regulator of T and B cell function, to central nervous system (CNS) immunity. IL-21 receptor deficiency did not affect peripheral T cell activation or trafficking, but dampened granzyme B, gamma interferon and IL-10 expression by CNS T cells and reduced serum and intrathecal humoral responses. Viral control was already lost prior to humoral CNS responses, but demyelination remained comparable. These data demonstrate a critical role of IL-21 in regulating CNS immunity, sustaining viral persistence and preventing mortality.

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IL-21 optimizes CNS CD4 and CD8 T cell responses during viral encephalomyelitis.

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IL-21 promotes peripheral and CNS humoral immunity.

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IL-21 promotes CNS viral control and prevents mortality.

Protective efficacy of VP1-specific neutralizing antibody associated with a reduction of viral load and pro-inflammatory cytokines in human SCARB2-transgenic mice

Hand-foot-mouth diseases (HFMD) caused by enterovirus 71 (EV71) and coxsackievirus 16 (CVA16) in children have now become a severe public health issue in the Asian-Pacific region. Recently we have successfully developed transgenic mice expressing human scavenger receptor class B member 2 (hSCARB2, a receptor of EV71 and CVA16) as an animal model for evaluating the pathogenesis of enterovirus infections. In this study, hSCARB2-transgenic mice were used to investigate the efficacy conferred by a previously described EV71 neutralizing antibody, N3. A single injection of N3 effectively inhibited the HFMD-like skin scurfs in mice pre-infected with clinical isolate of EV71 E59 (B4 genotype) or prevented severe limb paralysis and death in mice pre-inoculated with 5746 (C2 genotype). This protection was correlated with remarkable reduction of viral loads in the brain, spinal cord and limb muscles. Accumulated viral loads and the associated pro-inflammatory cytokines were all reduced. The protective efficacy of N3 was not observed in animals challenged with CVA16. This could be due to dissimilarity sequences of the neutralizing epitope found in CVA16. These results indicate N3 could be useful in treating severe EV71 infections and the hSCARB2-transgenic mouse could be used to evaluate the protective efficacy of potential anti-enterovirus agent candidates.

Maturation of lymph node fibroblastic reticular cells from myofibroblastic precursors is critical for antiviral immunity

The stromal scaffold of the lymph node (LN) paracortex is built by fibroblastic reticular cells (FRCs). Conditional ablation of lymphotoxin-β receptor (LTβR) expression in LN FRCs and their mesenchymal progenitors in developing LNs revealed that LTβR-signaling in these cells was not essential for the formation of LNs. Although T cell zone reticular cells had lost podoplanin expression, they still formed a functional conduit system and showed enhanced expression of myofibroblastic markers. However, essential immune functions of FRCs, including homeostatic chemokine and interleukin-7 expression, were impaired. These changes in T cell zone reticular cell function were associated with increased susceptibility to viral infection. Thus, myofibroblasic FRC precursors are able to generate the basic T cell zone infrastructure, whereas LTβR-dependent maturation of FRCs guarantees full immunocompetence and hence optimal LN function during infection.

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Novel transgenic mouse model that targets FRCs in adult lymph nodes

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FRC-specific ablation of the LTβR did not abrogate LN development

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Myofibroblastic FRC precursors generate the basic infrastructure of the adult LN

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LTβR-mediated FRC maturation is critical for the maintenance of immunocompentence

Intrathecal humoral immunity to encephalitic RNA viruses

The nervous system is the target for acute encephalitic viral infections, as well as a reservoir for persisting viruses. Intrathecal antibody (Ab) synthesis is well documented in humans afflicted by infections associated with neurological complications, as well as the demyelinating disease, multiple sclerosis. This review focuses on the origin, recruitment, maintenance, and biological relevance of Ab-secreting cells (ASC) found in the central nervous system (CNS) following experimental neurotropic RNA virus infections. We will summarize evidence for a highly dynamic, evolving humoral response characterized by temporal alterations in B cell subsets, proliferation, and differentiation. Overall local Ab plays a beneficial role via complement-independent control of virus replication, although cross or self-reactive Ab to CNS antigens may contribute to immune-mediated pathogenesis during some infections. Importantly, protective Ab exert anti-viral activity not only by direct neutralization, but also by binding to cell surface-expressed viral glycoproteins. Ab engagement of viral glycoproteins blocks budding and mediates intracellular signaling leading to restored homeostatic and innate functions. The sustained Ab production by local ASC, as well as chemokines and cytokines associated with ASC recruitment and retention, are highlighted as critical components of immune control.

The chemokine receptor CXCR2 and coronavirus-induced neurologic disease

Inoculation with the neurotropic JHM strain of mouse hepatitis virus (MHV) into the central nervous system (CNS) of susceptible strains of mice results in an acute encephalomyelitis in which virus preferentially replicates within glial cells while excluding neurons. Control of viral replication during acute disease is mediated by infiltrating virus-specific T cells via cytokine secretion and cytolytic activity, however sterile immunity is not achieved and virus persists resulting in chronic neuroinflammation associated with demyelination. CXCR2 is a chemokine receptor that upon binding to specific ligands promotes host defense through recruitment of myeloid cells to the CNS as well as protecting oligodendroglia from cytokine-mediated death in response to MHV infection. These findings highlight growing evidence of the diverse and important role of CXCR2 in regulating neuroinflammatory diseases.

Viral models of multiple sclerosis: neurodegeneration and demyelination in mice infected with Theiler's virus

Multiple sclerosis (MS) is a complex inflammatory disease of unknown etiology that affects the central nervous system (CNS) white matter, and for which no effective cure exists. Indeed, whether the primary event in MS pathology affects myelin or axons of the CNS remains unclear. Animal models are necessary to identify the immunopathological mechanisms involved in MS and to develop novel therapeutic and reparative approaches. Specifically, viral models of chronic demyelination and axonal damage have been used to study the contribution of viruses in human MS, and they have led to important breakthroughs in our understanding of MS pathology. The Theiler's murine encephalomyelitis virus (TMEV) model is one of the most commonly used MS models, although other viral models are also used, including neurotropic strains of mouse hepatitis virus (MHV) that induce chronic inflammatory demyelination with similar histological features to those observed in MS. This review will discuss the immunopathological mechanisms involved in TMEV-induced demyelinating disease (TMEV-IDD). The TMEV model reproduces a chronic progressive disease due to the persistence of the virus for the entire lifespan in susceptible mice. The evolution and significance of the axonal damage and neuroinflammation, the importance of epitope spread from viral to myelin epitopes, the presence of abortive remyelination and the existence of a brain pathology in addition to the classical spinal cord demyelination, are some of the findings that will be discussed in the context of this TMEV-IDD model. Despite their limitations, viral models remain an important tool to study the etiology of MS, and to understand the clinical and pathological variability associated with this disease.

Epstein-Barr virus-induced gene 3 negatively regulates neuroinflammation and T cell activation following coronavirus-induced encephalomyelitis

Epstein-Barr virus-induced gene 3 (EBI3) associates with p28 and p35 to form the immunomodulatory cytokines IL-27 and IL-35, respectively. Infection of EBI3-/- mice with the neuroadapted JHM strain of mouse hepatitis virus (JHMV) resulted in increased mortality that was not associated with impaired ability to control viral replication but enhanced T cell and macrophage infiltration into the CNS. IFN-γ secretion from virus-specific CD4+ and CD8+ T cells isolated from infected EBI3-/- mice was augmented while IL-10 expression muted in comparison to infected WT mice. These data demonstrate a regulatory role for EBI3-associated cytokines in controlling host responses following CNS viral infection.

Astrocyte-derived CXCL10 drives accumulation of antibody-secreting cells in the central nervous system during viral encephalomyelitis

Microbial infections of the central nervous system (CNS) are often associated with local accumulation of antibody (Ab)-secreting cells (ASC). By providing a source of Ab at the site of infection, CNS-localized ASC play a critical role in acute viral control and in preventing viral recrudescence. Following coronavirus-induced encephalomyelitis, the CNS accumulation of ASC is chemokine (C-X-C motif) receptor 3 (CXCR3) dependent. This study demonstrates that CNS-expressed CXCR3 ligand CXCL10 is the critical chemokine regulating ASC accumulation. Impaired ASC recruitment in CXCL10(-/-) but not CXCL9(-/-) mice was consistent with reduced CNS IgG and κ-light chain mRNA and virus-specific Ab. Moreover, the few ASC recruited to the CNS in CXCL10(-/-) mice were confined to the vasculature, distinct from the parenchymal localization in wild-type and CXCL9(-/-) mice. However, neither CXCL9 nor CXCL10 deficiency diminished neutralizing serum Ab, supporting a direct role for CXCL10 in ASC migration. T cell accumulation, localization, and effector functions were also not affected in either CXCL9(-/-) or CXCL10(-/-) mice, consistent with similar control of infectious virus. There was also no evidence for dysregulation of chemokines or cytokines involved in ASC regulation. The distinct roles of CXCL9 and CXCL10 in ASC accumulation rather coincided with their differential localization. While CXCL10 was predominantly expressed by astrocytes, CXCL9 expression was confined to the vasculature/perivascular spaces. These results suggest that CXCL10 is critical for two phases: recruitment of ASC to the CNS vasculature and ASC entry into the CNS parenchyma.

Transgenic CCL2 expression in the central nervous system results in a dysregulated immune response and enhanced lethality after coronavirus infection

Chemokine (C-C motif) ligand 2 (CCL2), a chemoattractant for macrophages, T cells, and cells expressing CCR2, is upregulated during acute and chronic inflammation. CCL2 has been implicated in both proinflammatory and anti-inflammatory responses and has been suggested as a target for therapy in some inflammatory disorders. To examine the role of CCL2 during virus infection, we infected mice transgenically expressing CCL2 in the central nervous system (CCL2 Tg) with an attenuated neurotropic coronavirus (rJ2.2 strain of mouse hepatitis virus). Infection of wild-type mice with rJ2.2 results in mild acute encephalitis, followed by a nonlethal, chronic demyelinating disease. Proinflammatory innate and adaptive immune responses mediate virus clearance. In marked contrast, CCL2 Tg mice infected with rJ2.2 ineffectively cleared virus and rapidly succumbed to the infection. CCL2 Tg mice mounted a dysregulated immune response, characterized by augmented accumulation of regulatory Foxp3(+)CD4(+) T cells and of nitric-oxide- and YM-1-expressing macrophages and microglia, suggestive of mixed M1/M2 macrophage activation. Further, macrophages from infected CCL2 Tg brains relative to non-Tg controls were less activated/mature, expressing lower levels of major histocompatibility complex class II (MHC-II), CD86, and CD40. Collectively, these results show that persistent CCL2 overexpression establishes and sustains an immunological milieu that is both inflammatory and immunosuppressive and predisposes mice to a defective immune response to a minimally lethal virus.

Enhanced CD8 T-cell anti-viral function and clinical disease in B7-H1-deficient mice requires CD4 T cells during encephalomyelitis

Background

Anti-viral CD8 T-cell activity is enhanced and prolonged by CD4 T-cell-mediated help, but negatively regulated by inhibitory B7-H1 interactions. During viral encephalomyelitis, the absence of CD4 T cells decreases CD8 T cell activity and impedes viral control in the central nervous system (CNS). By contrast, the absence of B7-H1 enhances CD8 T-cell function and accelerates viral control, but increases morbidity. However, the relative contribution of CD4 T cells to CD8 function in the CNS, in the absence of B7-H1, remains unclear.

Methods

Wild-type (WT) and B7-H1^−/− mice were infected with a gliatropic coronavirus and CD4 T cells depleted to specifically block T helper function in the CNS. Flow cytometry and gene expression analysis of purified T-cell populations from lymph nodes and the CNS was used to directly monitor ex vivo T-cell effector function. The biological affects of altered T-cell responses were evaluated by analysis of viral control and spinal-cord pathology.

Results

Increased anti-viral activity by CD8 T cells in the CNS of B7-H1^−/− mice was lost upon depletion of CD4 T cells; however, despite concomitant loss of viral control, the clinical disease was less severe. CD4 depletion in B7-H1^−/− mice also decreased inducible nitric oxide synthase expression by microglia and macrophages, consistent with decreased microglia/macrophage activation and reduced interferon (IFN)-γ. Enhanced production of IFN-γ, interleukin (IL)-10 and IL-21 mRNA was seen in CD4 T cells from infected B7-H1^−/− compared with WT mice, suggesting that over-activated CD4 T cells primarily contribute to the increased pathology.

Conclusions

The local requirement of CD4 T-cell help for CD8 T-cell function is not overcome if B7-H1 inhibitory signals are lost. Moreover, the increased effector activity by CD8 T cells in the CNS of B7-H1^−/− mice is attributable not only to the absence of B7-H1 upregulation on major histocompatibility complex class I-presenting resident target cells, but also to enhanced local CD4 T-cell function. B7-H1-mediated restraint of CD4 T-cell activity is thus crucial to dampen both CD8 T-cell function and microglia/macrophage activation, thereby providing protection from T-cell-mediated bystander damage.

Oligodendroglia are limited in type I interferon induction and responsiveness in vivo

Type I interferons (IFNα/β) provide a primary defense against infection. Nevertheless, the dynamics of IFNα/β induction and responsiveness by central nervous system (CNS) resident cells in vivo in response to viral infections are poorly understood. Mice were infected with a neurotropic coronavirus with tropism for oligodendroglia and microglia to probe innate antiviral responses during acute encephalomyelitis. Expression of genes associated with the IFNα/β pathways was monitored in microglia and oligodendroglia purified from naïve and infected mice by fluorescent activated cell sorting. Compared with microglia, oligodendroglia were characterized by low basal expression of mRNA encoding viral RNA sensing pattern recognition receptors (PRRs), IFNα/β receptor chains, interferon sensitive genes (ISG), as well as kinases and transcription factors critical in IFNα/β signaling. Although PRRs and ISGs were upregulated by infection in both cell types, the repertoire and absolute mRNA levels were more limited in oligodendroglia. Furthermore, although oligodendroglia harbored higher levels of viral RNA compared with microglia, Ifnα/β was only induced in microglia. Stimulation with the double stranded RNA analogue poly I:C also failed to induce Ifnα/β in oligodendroglia, and resulted in reduced and delayed induction of ISGs compared with microglia. The limited antiviral response by oligodendroglia was associated with a high threshold for upregulation of Ikkε and Irf7 transcripts, both central to amplifying IFNα/β responses. Overall, these data reveal that oligodendroglia from the adult CNS are poor sensors of viral infection and suggest they require exogenous IFNα/β to establish an antiviral state.

Differential effects of IL-12 on Tregs and non-Treg T cells: roles of IFN-γ, IL-2 and IL-2R

Complex interactions between effector T cells and Foxp3⁺ regulatory T cells (Treg) contribute to clinical outcomes in cancer, and autoimmune and infectious diseases. Previous work showed that IL-12 reversed Treg-mediated suppression of CD4⁺Foxp3⁻ T cell (Tconv) proliferation. We and others have also shown that Tregs express T-bet and IFN-γ at sites of Th1 inflammation and that IL-12 induces IFN-γ production by Tregs in vitro. To investigate whether loss of immunosuppression occurs when IFN-γ is expressed by Tregs we treated mouse lymphocyte cultures with IL-12. IFN-γ expression did not decrease the ability of Tregs to suppress Tconv proliferation. Rather, IL-12 treatment decreased Treg frequency and Foxp3 levels in Tregs. We further showed that IL-12 increased IL-2R expression on Tconv and CD8 T cells, diminished its expression on Tregs and decreased IL-2 production by Tconv and CD8 T cells. Together, these IL-12 mediated changes favored the outgrowth of non-Tregs. Additionally, we showed that treatment with a second cytokine, IL-27, decreased IL-2 expression without augmenting Tconv and CD8 T cell proliferation. Notably, IL-27 only slightly modified levels of IL-2R on non-Treg T cells. Together, these results show that IL-12 has multiple effects that modify the balance between Tregs and non-Tregs and support an important role for relative levels of IL-2R but not for IFN-γ expression in IL-12-mediated reversal of Treg immunosuppression.

Clearance of virus infection from the CNS

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

Animal models of multiple sclerosis: the good, the bad and the bottom line

Multiple sclerosis is a debilitating inflammatory demyelinating disease of the CNS. In this perspective, the author examines the strengths and weaknesses of the numerous animal models that are currently being used to analyze the pathogenesis of this disease with an eye toward the precise pathological aspect that each model recapitulates.

Multiple sclerosis (MS) is a spontaneous, acquired, inflammatory demyelinating disease of the human CNS. Because it involves a complex interaction between two of the most intricate biological systems, immune system and CNS, animal modeling has been critical for addressing MS pathogenesis. MS models were originally developed serendipitously more than 75 years ago. Immune-mediated, toxic, viral and genetic models of demyelination are now used to understand the manifold aspects of MS. MS treatments evolved in part from animal model research, and further progress is envisaged in large part because these systems have been continually refined and their use focused on questions whose relevance was established by studying the human disease.

The spleen is the major source of antidonor antibody-secreting cells in murine heart allograft recipients

Antibody-mediated allograft rejection is an increasingly recognized problem in clinical transplantation. However, the primary location of donor-specific alloantibody (DSA)-producing cells after transplantation have not been identified. The purpose of this study was to test the contribution of allospecific antibody-secreting cells (ASCs) from different anatomical compartments in a mouse transplantation model. Fully MHC-mismatched heart allografts were transplanted into three groups of recipients: nonsensitized wild type, alloantigen-sensitized wild-type and CCR5(-/-) mice that have exaggerated alloantibody responses. We found that previous sensitization to donor alloantigens resulted in the development of antidonor alloantibody (alloAb) with accelerated kinetics. Nevertheless, the numbers of alloantibody-secreting cells and the serum titers of antidonor IgG alloantibody were equivalent in sensitized and nonsensitized recipients 6 weeks after transplantation. Regardless of recipient sensitization status, the spleen contained higher numbers of donor-reactive ASCs than bone marrow at days 7-21 after transplantation. Furthermore, individual spleen ASCs produced more antidonor IgG alloantibody than bone marrow ASCs. Taken together, our results indicate that the spleen rather than bone marrow is the major source of donor-reactive alloAb early after transplantation in both sensitized and nonsensitized recipients.

Regulatory T cells in arterivirus and coronavirus infections: do they protect against disease or enhance it?

Regulatory T cells (T_regs) are a subset of T cells that are responsible for maintaining peripheral immune tolerance and homeostasis. The hallmark of T_regs is the expression of the forkhead box P3 (FoxP3) transcription factor. Natural regulatory T cells (nT_regs) are a distinct population of T cells that express CD4 and FoxP3. nTregs develop in the thymus and function in maintaining peripheral immune tolerance. Other CD4⁺, CD4^-CD8^-, and CD8⁺CD28^- T cells can be induced to acquire regulatory function by antigenic stimulation, depending on the cytokine milieu. Inducible (or adaptive) T_regs frequently express high levels of the interleukin 2 receptor (CD25). Atypical T_regs express FoxP3 and CD4 but have no surface expression of CD25. Type 1 regulatory T cells (Tr1 cells) produce IL-10, while T helper 3 cells (Th3) produce TGF-β. The function of inducible T_regs is presumably to maintain immune homeostasis, especially in the context of chronic inflammation or infection. Induction of T_regs in coronaviral infections protects against the more severe forms of the disease attributable to the host response. However, arteriviruses have exploited these T cell subsets as a means to dampen the immune response allowing for viral persistence. T_reg induction or activation in the pathogenesis of disease has been described in both porcine reproductive and respiratory syndrome virus, lactate dehydrogenase elevating virus, and mouse hepatitis virus. This review discusses the development and biology of regulatory T cells in the context of arteriviral and coronaviral infection.

Protective role of Toll-like Receptor 3-induced type I interferon in murine coronavirus infection of macrophages

Toll-like Receptors (TLRs) sense viral infections and induce production of type I interferons (IFNs), other cytokines, and chemokines. Viral recognition by TLRs and other pattern recognition receptors (PRRs) has been proven to be cell-type specific. Triggering of TLRs with selected ligands can be beneficial against some viral infections. Macrophages are antigen-presenting cells that express TLRs and have a key role in the innate and adaptive immunity against viruses. Coronaviruses (CoVs) are single-stranded, positive-sense RNA viruses that cause acute and chronic infections and can productively infect macrophages. Investigation of the interplay between CoVs and PRRs is in its infancy. We assessed the effect of triggering TLR2, TLR3, TLR4, and TLR7 with selected ligands on the susceptibility of the J774A.1 macrophage cell line to infection with murine coronavirus (mouse hepatitis virus, [MHV]). Stimulation of TLR2, TLR4, or TLR7 did not affect MHV production. In contrast, pre-stimulation of TLR3 with polyinosinic-polycytidylic acid (poly I:C) hindered MHV infection through induction of IFN-β in macrophages. We demonstrate that activation of TLR3 with the synthetic ligand poly I:C mediates antiviral immunity that diminishes (MHV-A59) or suppresses (MHV-JHM, MHV-3) virus production in macrophages.

CXCR2 signaling and host defense following coronavirus-induced encephalomyelitis

Inoculation of the neurotropic JHM strain of mouse hepatitis virus (JHMV) into the central nervous system (CNS) of susceptible strains of mice results in wide-spread replication within glial cells accompanied by infiltration of virus-specific T lymphocytes that control virus through cytokine secretion and cytolytic activity. Virus persists within white matter tracts of surviving mice resulting in demyelination that is amplified by inflammatory T cells and macrophages. In response to infection, numerous cytokines/chemokines are secreted by resident cells of the CNS and inflammatory leukocytes that participate in both host defense and disease. Among these are the ELR-positive chemokines that are able to signal through CXC chemokine receptors including CXCR2. Early following JHMV infection, ELR-positive chemokines contribute to host defense by attracting CXCR2-expressing cells including polymorphonuclear cells to the CNS that aid in host defense through increasing the permeability the blood-brain-barrier (BBB). During chronic disease, CXCR2 signaling on oligodendroglia protects these cells from apoptosis and restricts the severity of demyelination. This review covers aspects related to host defense and disease in response to JHMV infection and highlights the different roles of CXCR2 signaling in these processes.

Olig1 function is required for remyelination potential of transplanted neural progenitor cells in a model of viral-induced demyelination

Multiple sclerosis (MS) is a chronic inflammatory disease of the central nervous system (CNS) resulting in cumulative neurologic deficits associated with progressive myelin loss. We have previously shown that transplantation of neural progenitor cells (NPCs) into mice persistently infected with the JHM strain of mouse hepatitis virus (JHMV) results in enhanced differentiation into oligodendrocyte progenitor cells (OPCs) that is associated with remyelination and axonal sparing. The current study examines the contributions of the transcription factor Olig1 on NPC differentiation and remyelination. Under defined conditions, NPCs preferentially differentiate into oligodendroglia whereas NPCs isolated from Olig1-deficient (Olig1-/-) mice exhibit enhanced differentiation into astrocytes. Transplantation of Olig1-/- and Olig1+/+ NPCs into JHMV-infected mice resulted in similar cell survival, proliferation, and selective migration to areas of demyelination. However, only recipients of wild type NPCs exhibited extensive remyelination compared to mice receiving Olig1-/- NPCs. In vivo characterization of NPCs revealed that Olig1+/+ NPCs preferentially differentiated into NG2-positive OPCs and formed processes expressing myelin basic protein that encircled axons. In contrast, the majority of transplanted Olig1-/- NPCs differentiated into GFAP-positive cells consistent with the astrocyte lineage. These results indicate that exogenous NPCs contribute to improved clinical and histological outcome and this is associated with remyelination by this donor population. Further, these findings reveal that Olig1function is required for the remyelination potential of NPCs after transplant, through specification and/or maintenance of oligodendroglial identity.

Regulatory T cells selectively preserve immune privilege of self-antigens during viral central nervous system infection

Regulatory T cells (Tregs) are important for the attenuation of immune reactions. During viral CNS infections, however, an indiscriminate maintenance of CNS immune privilege through Treg-mediated negative regulation could prevent autoimmune sequelae but impair the control of viral replication. We analyzed in this study the impact of Tregs on the development of acute viral encephalomyelitis, T cell-mediated antiviral protection, and prevention of CNS autoimmunity following intranasal infection with the gliatropic mouse hepatitis virus strain A59. To assess the contribution of Tregs in vivo, we specifically depleted CD4(+)Foxp3(+) T cells in a diphtheria toxin-dependent manner. We found that depletion of Tregs had no impact on viral distribution and clearance and did not significantly alter virus-specific CD4(+) and CD8(+) T cell responses. However, Treg depletion led to a more severe CNS inflammation associated with neuronal damage. Dissection of the underlying immunopathological mechanisms revealed the elaborate Treg-dependent regulation of self-reactive CD4(+) T cell proliferation within the CNS-draining lymph node and downtuning of CXCR3 expression on T cells. Taken together, these results suggest that Tregs preserve CNS immune privilege through selective control of CNS-specific Th cells while keeping protective antiviral immunity fully operative.

Inflammatory monocytes damage the hippocampus during acute picornavirus infection of the brain

BACKGROUND

Neuropathology caused by acute viral infection of the brain is associated with the development of persistent neurological deficits. Identification of the immune effectors responsible for injuring the brain during acute infection is necessary for the development of therapeutic strategies that reduce neuropathology but maintain immune control of the virus.

METHODS

The identity of brain-infiltrating leukocytes was determined using microscopy and flow cytometry at several acute time points following intracranial infection of mice with the Theiler's murine encephalomyelitis virus. Behavioral consequences of immune cell depletion were assessed by Morris water maze.

RESULTS

Inflammatory monocytes, defined as CD45hiCD11b++F4/80+Gr1+1A8-, and neutrophils, defined as CD45hiCD11b+++F4/80-Gr1+1A8+, were found in the brain at 12 h after infection. Flow cytometry of brain-infiltrating leukocytes collected from LysM: GFP reporter mice confirmed the identification of neutrophils and inflammatory monocytes in the brain. Microscopy of sections from infected LysM:GFP mice showed that infiltrating cells were concentrated in the hippocampal formation. Immunostaining confirmed that neutrophils and inflammatory monocytes were localized to the hippocampal formation at 12 h after infection. Immunodepletion of inflammatory monocytes and neutrophils but not of neutrophils only resulted in preservation of hippocampal neurons. Immunodepletion of inflammatory monocytes also preserved cognitive function as assessed by the Morris water maze.

CONCLUSIONS

Neutrophils and inflammatory monocytes rapidly and robustly responded to Theiler's virus infection by infiltrating the brain. Inflammatory monocytes preceded neutrophils, but both cell types were present in the hippocampal formation at a timepoint that is consistent with a role in triggering hippocampal pathology. Depletion of inflammatory monocytes and neutrophils with the Gr1 antibody resulted in hippocampal neuroprotection and preservation of cognitive function. Specific depletion of neutrophils with the 1A8 antibody failed to preserve neurons, suggesting that inflammatory monocytes are the key effectors of brain injury during acute picornavirus infection of the brain. These effector cells may be important therapeutic targets for immunomodulatory or immunosuppressive therapies aimed at reducing or preventing central nervous system pathology associated with acute viral infection.

STAT1-independent control of a neurotropic measles virus challenge in primary neurons and infected mice

Neurons are chiefly nonrenewable; thus, cytolytic immune strategies to clear or control neurotropic viral infections could have lasting neurologic consequences. IFN-γ is a potent antiviral cytokine that is critical for noncytolytic clearance of multiple neurotropic viral infections, including measles virus (MV); however, the downstream pathways through which IFN-γ functions in neurons have not been defined. Unlike most cell types studied to date in which IFN-γ affects gene expression via rapid and robust activation of STAT1, basal STAT1 levels in primary hippocampal neurons are constitutively low, resulting in attenuated STAT1 activation and consequently slower kinetics of IFN-γ-driven STAT1-dependent gene expression. Given this altered expression and activation of STAT1 in neurons, we sought to determine whether STAT1 was required for IFN-γ-mediated protection from infection in neurons. To do so, we evaluated the consequences of MV challenge of STAT1-deficient mice and primary hippocampal neurons explanted from these mice. Surprisingly, the absence of STAT1 did not restrict the ability of IFN-γ to control viral infection either in vivo or ex vivo. Moreover, the canonical IFN-γ-triggered STAT1 gene expression profile was not induced in STAT1-deficient neurons, suggesting that IFN-γ regulates neuronal STAT1-independent pathways to control viral replication.

T helper cell- and CD40-dependent germline IgM prevents chronic virus-induced demyelinating disease

Generation of antiviral IgM is usually considered as a marker of a short-lived initial antibody response that is replaced by hypermutated and more-efficient IgG. However, once viruses have established a particular niche for their persistence (e.g., within the CNS), the immune system has to specifically mobilize a broad range of antimicrobial effectors to contain the pathogen in the long term. Infection of the CNS with the mouse hepatitis virus (MHV) provides a unique model situation in which the extent of inflammatory CNS disease is determined by the balance between antiviral immune control, viral replication, and immune-mediated damage. We show here that whereas antibody- or B cell-deficient mice failed to contain MHV CNS infection and developed progressive demyelinating disease, germline IgM produced in activation-induced cytidine deaminase-deficient mice (aicda(-/-)) provided long-term protection against the chronic multiple sclerosis-like disease. Furthermore, we found that appropriate B-cell activation within the CNS-draining lymph node and subsequent CXCR3-mediated migration of antiviral IgM-secreting cells to the infected CNS was dependent on CD40-mediated interaction of B cells with T helper cells. These data indicate that the CD40-mediated collaboration of T and B cells is critical to secure neuroprotective IgM responses during viral CNS infection.

Diseases Involving Myelin

This chapter focuses on multiple sclerosis and the acquired and inherited peripheral neuropathies. The integrity of myelin sheaths is dependent upon the normal functioning of the myelin-forming oligodendrocytes in the central nervous system (CNS) and Schwann cells in the peripheral nervous system (PNS) as well as on the viability of the axons that they ensheath. Neuronal death inevitably leads to degeneration of axons and secondary degeneration of the myelin surrounding them. Failure of synthesis of normal myelin proteins or lipids is referred to as hypomyelination or dysmyelination. Primary demyelination involves the destruction of myelin with relative sparing of axons, whereas secondary demy­elination includes those disorders in which myelin is involved only after damage to neurons and axons occurs. The integrity of myelin sheaths is dependent upon the normal functioning of myelin-forming oligodendrocytes in the central nervous system (CNS) and Schwann cells in the peripheral nervous system (PNS) as well as on the viability of the axons that they ensheath.

Coronavirus pathogenesis

Coronaviruses infect many species of animals including humans, causing acute and chronic diseases. This review focuses primarily on the pathogenesis of murine coronavirus mouse hepatitis virus (MHV) and severe acute respiratory coronavirus (SARS-CoV). MHV is a collection of strains, which provide models systems for the study of viral tropism and pathogenesis in several organs systems, including the central nervous system, the liver, and the lung, and has been cited as providing one of the few animal models for the study of chronic demyelinating diseases such as multiple sclerosis. SARS-CoV emerged in the human population in China in 2002, causing a worldwide epidemic with severe morbidity and high mortality rates, particularly in older individuals. We review the pathogenesis of both viruses and the several reverse genetics systems that made much of these studies possible. We also review the functions of coronavirus proteins, structural, enzymatic, and accessory, with an emphasis on roles in pathogenesis. Structural proteins in addition to their roles in virion structure and morphogenesis also contribute significantly to viral spread in vivo and in antagonizing host cell responses. Nonstructural proteins include the small accessory proteins that are not at all conserved between MHV and SARS-CoV and the 16 conserved proteins encoded in the replicase locus, many of which have enzymatic activities in RNA metabolism or protein processing in addition to functions in antagonizing host response.

Central nervous system recruitment of effector memory CD8+ T lymphocytes during neuroinflammation is dependent on α4 integrin

Clonally expanded CD8⁺ T lymphocytes are present in multiple sclerosis lesions, as well as in the cerebrospinal fluid of patients with multiple sclerosis. In experimental autoimmune encephalomyelitis, CD8⁺ T lymphocytes are found in spinal cord and brainstem lesions. However, the exact phenotype of central nervous system-infiltrating CD8⁺ T lymphocytes and the mechanism by which these cells cross the blood–brain barrier remain largely unknown. Using cerebrospinal fluid from patients with multiple sclerosis, spinal cord from experimental autoimmune encephalomyelitis and coronavirus-induced encephalitis, we demonstrate that central nervous system-infiltrating CD8⁺ T lymphocytes are mostly of the effector memory phenotype (CD62L⁻ CCR7⁻ granzymeB^hi). We further show that purified human effector memory CD8⁺ T lymphocytes transmigrate more readily across blood-brain barrier-endothelial cells than non-effector memory CD8⁺ T lymphocytes, and that blood-brain barrier endothelium promotes the selective recruitment of effector memory CD8⁺ T lymphocytes. Furthermore, we provide evidence for the recruitment of interferon-γ- and interleukin-17-secreting CD8⁺ T lymphocytes by human and mouse blood-brain barrier endothelium. Finally, we show that in vitro migration of CD8⁺ T lymphocytes across blood-brain barrier-endothelial cells is dependent on α4 integrin, but independent of intercellular adhesion molecule-1/leucocyte function-associated antigen-1, activated leucocyte cell adhesion molecule/CD6 and the chemokine monocyte chemotactic protein-1/CCL2. We also demonstrate that in vivo neutralization of very late antigen-4 restricts central nervous system infiltration of CD8⁺ T lymphocytes in active immunization and adoptive transfer experimental autoimmune encephalomyelitis, and in coronavirus-induced encephalitis. Our study thus demonstrates an active role of the blood-brain barrier in the recruitment of effector memory CD8⁺ T lymphocytes to the CNS compartment and defines α4 integrin as a major contributor of CD8⁺ T lymphocyte entry into the brain.

The critical role of antigen-presentation-induced cytokine crosstalk in the central nervous system in multiple sclerosis and experimental autoimmune encephalomyelitis

Multiple sclerosis (MS) is a debilitating disease of the central nervous system (CNS) that has been extensively studied using the animal model experimental autoimmune encephalomyelitis (EAE). It is believed that CD4(+) T lymphocytes play an important role in the pathogenesis of this disease by mediating the demyelination of neuronal axons via secretion of proinflammatory cytokines resulting in the clinical manifestations. Although a great deal of information has been gained in the last several decades about the cells involved in the inflammatory and disease mediating process, important questions have remained unanswered. It has long been held that initial neuroantigen presentation and T cell activation events occur in the immune periphery and then translocate to the CNS. However, an increasing body of evidence suggests that antigen (Ag) presentation might initiate within the CNS itself. Importantly, it has remained unresolved which antigen presenting cells (APCs) in the CNS are the first to acquire and present neuroantigens during EAE/MS to T cells, and what the conditions are under which this takes place, ie, whether this occurs in the healthy CNS or only during inflammatory conditions and what the related cytokine microenvironment is comprised of. In particular, the central role of interferon-γ as a primary mediator of CNS pathology during EAE has been challenged by the emergence of Th17 cells producing interleukin-17. This review describes our current understanding of potential APCs in the CNS and the contribution of these and other CNS-resident cells to disease pathology. Additionally, we discuss the question of where Ag presentation is initiated and under what conditions neuroantigens are made available to APCs with special emphasis on which cytokines may be important in this process.

CXCR4 signaling regulates remyelination by endogenous oligodendrocyte progenitor cells in a viral model of demyelination

Following intracranial infection with the neurotropic JHM strain of mouse hepatitis virus (JHMV), susceptible mice will develop widespread myelin destruction that results in pathological and clinical outcomes similar to those seen in humans with the demyelinating disease Multiple Sclerosis (MS). Partial remyelination and clinical recovery occurs during the chronic phase following control of viral replication yet the signaling mechanisms regulating these events remain enigmatic. Here we report the kinetics of proliferation and maturation of oligodendrocyte progenitor cells (OPCs) within the spinal cord following JHMV-induced demyelination and that CXCR4 signaling contributes to the maturation state of OPCs. Following treatment with AMD3100, a specific inhibitor of CXCR4, mice recovering from widespread demyelination exhibit a significant (P < 0.01) increase in the number of OPCs and fewer (P < 0.05) mature oligodendrocytes compared with HBSS-treated animals. These results suggest that CXCR4 signaling is required for OPCs to mature and contribute to remyelination in response to JHMV-induced demyelination. To assess if this effect is reversible and has potential therapeutic benefit, we pulsed mice with AMD3100 and then allowed them to recover. This treatment strategy resulted in increased numbers of mature oligodendrocytes, enhanced remyelination, and improved clinical outcome. These findings highlight the possibility to manipulate OPCs in order to increase the pool of remyelination-competent cells that can participate in recovery.

MMP9 deficiency does not decrease blood-brain barrier disruption, but increases astrocyte MMP3 expression during viral encephalomyelitis

Expression of matrix metalloproteinases (MMPs), especially MMP9 correlates with blood–brain barrier (BBB) disruption during many neuroinflammatory diseases. During neurotropic coronavirus virus (JHMV) induced encephalomyelitis, MMP9 activity is restricted to neutrophils. Furthermore, myeloid cell depletion implicated MMP9 in facilitating leukocyte central nervous system (CNS) infiltration via loss of BBB integrity. The requirement of MMP9 in BBB disruption was thus assessed in JHMV infected MMP9 deficient (MMP9^−/−) mice. Depletion of neutrophils reduced CNS accumulation of monocytes and T cells, albeit without affecting overall pathogenesis. By contrast, infected MMP9^−/− mice revealed no differences in CNS leukocyte infiltration, composition or localization, consistent with BBB disruption similar to wild‐type (WT) mice. Unimpaired T cell mediated virus control supported an unexpectedly redundant role of MMP9 in promoting leukocyte access to the brain parenchyma. Although MMP9 deficiency did not expand the overall limited pattern of MMP expression during JHMV infection, it coincided with MMP3 upregulation. MMP3 expression remained largely confined to astrocytes, similar to WT mice. These data demonstrate that neutrophil‐derived MMP9 is not the sole mediator facilitating parenchymal leukocyte entry via BBB disruption during viral encephalomyelitis. Moreover, significantly enhanced MMP3 expression by astrocytes in infected MMP9^−/− mice suggests an active role of resident cells in participating and potentially collaborating with infiltrating cells in regulating BBB permeability. Overall, these results highlight the complexity of targeting individual MMPs as a strategy to regulate inflammation. © 2011 Wiley‐Liss, Inc.

IFN-γ- and IL-10-expressing virus epitope-specific Foxp3(+) T reg cells in the central nervous system during encephalomyelitis

Pathogen-specific Foxp3⁺ T reg cells can be identified on the basis of cytokine production, are detected in naive T cell populations, and exhibit suppressive ability toward effector T cells with the same antigen specificity.

Foxp3⁺ CD4 regulatory T cells (T reg cells) are important in limiting immunopathology in infections. However, identifying pathogen-specific epitopes targeted by these cells has been elusive. Using MHC class II/peptide tetramers and intracellular cytokine staining, we identify T reg cells recognizing two virus-specific CD4 T cell epitopes in the coronavirus-infected central nervous system as well as naive T cell precursor pools. These T reg cells are detected at the same time as effector T cells (T eff cells) exhibiting the same specificity and can suppress T eff cell proliferation after stimulation with cognate peptide. These virus-specific T reg cells may be especially effective in inhibiting the immune response during the peak of infection, when virus antigen is maximal. Furthermore, these T reg cells express both IL-10 and IFN-γ after peptide stimulation. IFN-γ expression is maintained during both acute and chronic phases of infection. Identification of T reg cell target epitopes by cytokine production is also applicable in autoimmune disease because myelin oligodendrocyte glycoprotein–specific Foxp3⁺ T reg cells express IL-10 and IL-17 at the peak of disease in mice with experimental autoimmune encephalomyelitis. These results show that pathogen epitope-specific Foxp3⁺ T reg cells can be identified on the basis of cytokine production.

Surgical transplantation of mouse neural stem cells into the spinal cords of mice infected with neurotropic mouse hepatitis virus

Mice infected with the neurotropic JHM strain of mouse hepatitis virus (MHV) develop pathological and clinical outcomes similar to patients with the demyelinating disease Multiple Sclerosis (MS). We have shown that transplantation of NSCs into the spinal cords of sick mice results in a significant improvement in both remyelination and in clinical outcome. Cell replacement therapies for the treatment of chronic neurologic diseases are now a reality and in vivo models are vital in understanding the interactions between the engrafted cells and host tissue microenvironment. This presentation provides an adapted method for transplanting cells into the spinal cord of JHMV-infected mice. In brief, we provide a procedure for i) preparation of NSCs prior to transplant, ii) pre-operative care of mice, iii) exposure of the spinal cord via laminectomy, iv) stereotactic injection of NSCs, and iv) post-operative care.

Virally expressed interleukin-10 ameliorates acute encephalomyelitis and chronic demyelination in coronavirus-infected mice

The absence of interleukin-10 (IL-10), a potent anti-inflammatory cytokine results in increased immune-mediated demyelination in mice infected with a neurotropic coronavirus (recombinant J2.2-V-1 [rJ2.2]). Here, we examined the therapeutic effects of increased levels of IL-10 at early times after infection by engineering a recombinant J2.2 virus to produce IL-10. We demonstrate that viral expression of IL-10, which occurs during the peak of virus replication and at the site of disease, enhanced survival and diminished morbidity in rJ2.2-infected wild-type B6 and IL-10(-/-) mice. The protective effects of increased IL-10 levels were associated with reductions in microglial activation, inflammatory cell infiltration into the brain, and proinflammatory cytokine and chemokine production. Additionally, IL-10 increased both the frequency and number of Foxp3(+) regulatory CD4 T cells in the infected central nervous system. Most strikingly, the ameliorating effects of IL-10 produced during the first 5 days after infection were long acting, resulting in decreased demyelination during the resolution phase of the infection. Collectively, these results suggest that the pathogenic processes that result in demyelination are initiated early during infection and that they can be diminished by exogenous IL-10 delivered soon after disease onset. IL-10 functions by dampening the innate or very early T cell immune response. Further, they suggest that early treatment with IL-10 may be useful adjunct therapy in some types of viral encephalitis.

Shifting hierarchies of interleukin-10-producing T cell populations in the central nervous system during acute and persistent viral encephalomyelitis

Interleukin-10 (IL-10) mRNA is rapidly upregulated in the central nervous system (CNS) following infection with neurotropic coronavirus and remains elevated during persistent infection. Infection of transgenic IL-10/green fluorescent protein (GFP) reporter mice revealed that CNS-infiltrating T cells were the major source of IL-10, with minimal IL-10 production by macrophages and resident microglia. The proportions of IL-10-producing cells were initially similar in CD8(+) and CD4(+) T cells but diminished rapidly in CD8(+) T cells as the virus was controlled. Overall, the majority of IL-10-producing CD8(+) T cells were specific for the immunodominant major histocompatibility complex (MHC) class I epitope. Unlike CD8(+) T cells, a large proportion of CD4(+) T cells within the CNS retained IL-10 production throughout persistence. Furthermore, elevated frequencies of IL-10-producing CD4(+) T cells in the spinal cord supported preferential maintenance of IL-10 production at the site of viral persistence and tissue damage. IL-10 was produced primarily by the CD25(+) CD4(+) T cell subset during acute infection but prevailed in CD25(-) CD4(+) T cells during the transition to persistent infection and thereafter. Overall, these data demonstrate significant fluidity in the T-cell-mediated IL-10 response during viral encephalitis and persistence. While IL-10 production by CD8(+) T cells was limited primarily to the time of acute effector function, CD4(+) T cells continued to produce IL-10 throughout infection. Moreover, a shift from predominant IL-10 production by CD25(+) CD4(+) T cells to CD25(-) CD4(+) T cells suggests that a transition to nonclassical regulatory T cells precedes and is retained during CNS viral persistence.

CXCR3-dependent plasma blast migration to the central nervous system during viral encephalomyelitis

Immunoglobulin in cerebral spinal fluid and antibody secreting cells (ASC) within the central nervous system (CNS) parenchyma are common hallmarks of microbial infections and autoimmune disorders. However, the signals directing ASC migration into the inflamed CNS are poorly characterized. This study demonstrates that CXCR3 mediates CNS accumulation of ASC during neurotropic coronavirus-induced encephalomyelitis. Expansion of CXCR3-expressing ASC in draining lymph nodes prior to accumulation within the CNS was consistent with their recruitment by sustained expression of CXCR3 ligands during viral persistence. Both total and virus-specific ASC were reduced greater than 80% in the CNS of infected CXCR3(-/-) mice. Similar T cell CNS recruitment and local T cell-dependent antiviral activity further indicated that the ASC migration defect was T cell independent. Furthermore, in contrast to the reduction of ASC in the CNS, neither virus-specific ASC trafficking to bone marrow nor antiviral serum antibody was reduced relative to levels in control mice. Impaired ASC recruitment into the CNS of infected CXCR3(-/-) mice coincided with elevated levels of persisting viral RNA, sustained infectious virus, increased clinical disease, and mortality. These results demonstrate that CXCR3 ligands are indispensable for recruitment of activated ASC into the inflamed CNS and highlight their local protective role during persistent infection.

Multistrain influenza protection induced by a nanoparticulate mucosal immunotherapeutic

All commercial influenza vaccines elicit antibody responses that protect against seasonal infection, but this approach is limited by the need for annual vaccine reformulation that precludes efficient responses against epidemic and pandemic disease. In this study we describe a novel vaccination approach in which a nanoparticulate, liposome-based agent containing short, highly conserved influenza-derived peptides is delivered to the respiratory tract to elicit potent innate and selective T cell-based adaptive immune responses. Prepared without virus-specific peptides, mucosal immunostimulatory therapeutic (MIT) provided robust, but short-lived, protection against multiple, highly lethal strains of influenza in mice of diverse genetic backgrounds. MIT prepared with three highly conserved epitopes that elicited virus-specific memory T-cell responses but not neutralizing antibodies, termed MITpep, provided equivalent, but more durable, protection relative to MIT. Alveolar macrophages were more important than dendritic cells in determining the protective efficacy of MIT, which induced both canonical and non-canonical antiviral immune pathways. Through activation of airway mucosal innate and highly specific T-cell responses, MIT and MITpep represent novel approaches to antiviral protection that offer the possibility of universal protection against epidemic and pandemic influenza.

Highly activated cytotoxic CD8 T cells express protective IL-10 at the peak of coronavirus-induced encephalitis

Acute viral encephalitis requires rapid pathogen elimination without significant bystander tissue damage. In this article, we show that IL-10, a potent anti-inflammatory cytokine, is produced transiently at the peak of infection by CD8 T cells in the brains of coronavirus-infected mice. IL-10(+)CD8 and IL-10(-)CD8 T cells interconvert during acute disease, possibly based on recent Ag exposure. Strikingly, IL-10(+)CD8 T cells were more highly activated and cytolytic than IL-10(-)CD8 T cells, expressing greater levels of proinflammatory cytokines and chemokines, as well as cytotoxic proteins. Even though these cells are highly proinflammatory, IL-10 expressed by these cells was functional. Furthermore, IL-10 produced by CD8 T cells diminished disease severity in mice with coronavirus-induced acute encephalitis, suggesting a self-regulatory mechanism that minimizes immunopathological changes.

Coronavirus pathogenesis

Coronaviruses infect many species of animals including humans, causing acute and chronic diseases. This review focuses primarily on the pathogenesis of murine coronavirus mouse hepatitis virus (MHV) and severe acute respiratory coronavirus (SARS-CoV). MHV is a collection of strains, which provide models systems for the study of viral tropism and pathogenesis in several organs systems, including the central nervous system, the liver, and the lung, and has been cited as providing one of the few animal models for the study of chronic demyelinating diseases such as multiple sclerosis. SARS-CoV emerged in the human population in China in 2002, causing a worldwide epidemic with severe morbidity and high mortality rates, particularly in older individuals. We review the pathogenesis of both viruses and the several reverse genetics systems that made much of these studies possible. We also review the functions of coronavirus proteins, structural, enzymatic, and accessory, with an emphasis on roles in pathogenesis. Structural proteins in addition to their roles in virion structure and morphogenesis also contribute significantly to viral spread in vivo and in antagonizing host cell responses. Nonstructural proteins include the small accessory proteins that are not at all conserved between MHV and SARS-CoV and the 16 conserved proteins encoded in the replicase locus, many of which have enzymatic activities in RNA metabolism or protein processing in addition to functions in antagonizing host response.

Factors supporting intrathecal humoral responses following viral encephalomyelitis

Central nervous system (CNS) infections and autoimmune inflammatory disorders are often associated with retention of antibody-secreting cells (ASC). Although beneficial or detrimental contributions of ASC to CNS diseases remain to be defined, virus-specific ASC are crucial in controlling persistent CNS infection following coronavirus-induced encephalomyelitis. This report characterizes expression kinetics of factors associated with ASC homing, differentiation, and survival in the spinal cord, the prominent site of coronavirus persistence. Infection induced a vast, gamma interferon (IFN-γ)-dependent, prolonged increase in chemokine (C-X-C motif) ligand 9 (CXCL9), CXCL10, and CXCL11 mRNA, supporting a role for chemokine (C-X-C motif) receptor 3 (CXCR3)-mediated ASC recruitment. Similarly, CD4 T cell-secreted interleukin-21, a critical regulator of both peripheral activated B cells and CD8 T cells, was sustained during viral persistence. The ASC survival factors B cell-activating factor of the tumor necrosis factor (TNF) family (BAFF) and a proliferating-inducing ligand (APRIL) were also significantly elevated in the infected CNS, albeit delayed relative to the chemokines. Unlike IFN-γ-dependent BAFF upregulation, APRIL induction was IFN-γ independent. Moreover, both APRIL and BAFF were predominantly localized to astrocytes. Last, the expression kinetics of the APRIL and BAFF receptors coincided with CNS accumulation of ASC. Therefore, the factors associated with ASC migration, differentiation, and survival are all induced during acute viral encephalomyelitis, prior to ASC accumulation in the CNS. Importantly, the CNS expression kinetics implicate rapid establishment, and subsequent maintenance, of an environment capable of supporting differentiation and survival of protective antiviral ASC, recruited as plasmablasts from lymphoid organs.

Pro-inflammatory cytokines derived from West Nile virus (WNV)-infected SK-N-SH cells mediate neuroinflammatory markers and neuronal death

Background

WNV-associated encephalitis (WNVE) is characterized by increased production of pro-inflammatory mediators, glial cells activation and eventual loss of neurons. WNV infection of neurons is rapidly progressive and destructive whereas infection of non-neuronal brain cells is limited. However, the role of neurons and pathological consequences of pro-inflammatory cytokines released as a result of WNV infection is unclear. Therefore, the objective of this study was to examine the role of key cytokines secreted by WNV-infected neurons in mediating neuroinflammatory markers and neuronal death.

Methods

A transformed human neuroblastoma cell line, SK-N-SH, was infected with WNV at multiplicity of infection (MOI)-1 and -5, and WNV replication kinetics and expression profile of key pro-inflammatory cytokines were analyzed by plaque assay, qRT-PCR, and ELISA. Cell death was measured in SK-N-SH cell line in the presence and absence of neutralizing antibodies against key pro-inflammatory cytokines using cell viability assay, TUNEL and flow cytometry. Further, naïve primary astrocytes were treated with UV-inactivated supernatant from mock- and WNV-infected SK-N-SH cell line and the activation of astrocytes was measured using flow cytometry and ELISA.

Results

WNV-infected SK-N-SH cells induced the expression of IL-1β, -6, -8, and TNF-α in a dose- and time-dependent manner, which coincided with increase in virus-induced cell death. Treatment of cells with anti-IL-1β or -TNF-α resulted in significant reduction of the neurotoxic effects of WNV. Furthermore treatment of naïve astrocytes with UV-inactivated supernatant from WNV-infected SK-N-SH cell line increased expression of glial fibrillary acidic protein and key inflammatory cytokines.

Conclusion

Our results for the first time suggest that neurons are one of the potential sources of pro-inflammatory cytokines in WNV-infected brain and these neuron-derived cytokines contribute to WNV-induced neurotoxicity. Moreover, cytokines released from neurons also mediate the activation of astrocytes. Our data define specific role(s) of WNV-induced pro-inflammatory cytokines and provide a framework for the development of anti-inflammatory drugs as much-needed therapeutic interventions to limit symptoms associated with WNVE.

Deficient incorporation of spike protein into virions contributes to the lack of infectivity following establishment of a persistent, non-productive infection in oligodendroglial cell culture by murine coronavirus

Infection of mouse oligodendrocytes with a recombinant mouse hepatitis virus (MHV) expressing a green fluorescence protein facilitated specific selection of virus-infected cells and subsequent establishment of persistence. Interestingly, while viral genomic RNAs persisted in infected cells over 14 subsequent passages with concomitant synthesis of viral subgenomic mRNAs and structural proteins, no infectious virus was isolated beyond passage 2. Further biochemical and electron microscopic analyses revealed that virions, while assembled, contained little spike in the envelope, indicating that lack of infectivity during persistence was likely due to deficiency in spike incorporation. This type of non-lytic, non-productive persistence in oligodendrocytes is unique among animal viruses and resembles MHV persistence previously observed in the mouse central nervous system. Thus, establishment of such a culture system that can recapitulate the in vivo phenomenon will provide a powerful approach for elucidating the mechanisms of coronavirus persistence in glial cells at the cellular and molecular levels.

The pathogenesis of murine coronavirus infection of the central nervous system

Mouse hepatitis virus (MHV) is a positive-strand RNA virus that causes an acute encephalomyelitis that later resolves into a chronic fulminating demyelinating disease. Cytokine production, chemokine secretion, and immune cell infiltration into the central nervous system are critical to control viral replication during acute infection. Despite potent antiviral T-lymphocyte activity, sterile immunity is not achieved, and MHV chronically persists within oligodendrocytes. Continued infiltration and activation of the immune system, a result of the lingering viral antigen and RNA within oligodendrocytes, lead directly to the development of an immune-mediated demyelination that bears remarkable similarities, both clinically and histologically, to the human demyelinating disease multiple sclerosis. MHV offers a unique model system for studying host defense during acute viral infection and immune-mediated demyelination during chronic infection.

Autocrine interferon priming in macrophages but not dendritic cells results in enhanced cytokine and chemokine production after coronavirus infection

Coronaviruses efficiently inhibit interferon (IFN) induction in nonhematopoietic cells and conventional dendritic cells (cDC). However, IFN is produced in infected macrophages, microglia, and plasmacytoid dendritic cells (pDC). To begin to understand why IFN is produced in infected macrophages, we infected bone marrow-derived macrophages (BMM) and as a control, bone marrow-derived DC (BMDC) with the coronavirus mouse hepatitis virus (MHV). As expected, BMM but not BMDC expressed type I IFN. IFN production in infected BMM was nearly completely dependent on signaling through the alpha/beta interferon (IFN-α/β) receptor (IFNAR). Several IFN-dependent cytokines and chemokines showed the same expression pattern, with enhanced production in BMM compared to BMDC and dependence upon signaling through the IFNAR. Exogenous IFN enhanced IFN-dependent gene expression in BMM at early times after infection and in BMDC at all times after infection but did not stimulate expression of molecules that signal through myeloid differentiation factor 88 (MyD88), such as tumor necrosis factor (TNF). Collectively, our results show that IFN is produced at early times postinfection (p.i.) in MHV-infected BMM, but not in BMDC, and primes expression of IFN and IFN-responsive genes. Further, our results also show that BMM are generally more responsive to MHV infection, since MyD88-dependent pathways are also activated to a greater extent in these cells than in BMDC.

Coronaviruses cause diseases with various degrees of severity in humans, including severe acute respiratory syndrome (SARS). In domestic and companion animals, coronaviruses induce interferon (IFN) in only a few cell types. In particular, macrophages, which are known to have both protective and pathogenic roles in coronavirus infections, express IFN while dendritic cells do not. Little is known about the basis of these cell-specific differences in IFN induction. Here, we show that an animal coronavirus, mouse hepatitis virus, induces IFN and other IFN-responsive molecules in macrophages, but not in dendritic cells, via a feedback loop that is dependent upon low-level IFN expression at early times after infection. This pathway of cellular activation may be a useful target for modulating macrophage function in order to selectively enhance the antivirus immune response and diminish the pathogenic role of these cells in SARS and other coronavirus infections.

Enhanced antiviral T cell function in the absence of B7-H1 is insufficient to prevent persistence but exacerbates axonal bystander damage during viral encephalomyelitis

The T cell inhibitory ligand B7-H1 hinders T cell-mediated virus control, but also ameliorates clinical disease during autoimmune and virus-induced CNS disease. In mice infected with gliatropic demyelinating coronavirus, B7-H1 expression on oligodendroglia delays virus control, but also dampens clinical disease. To define the mechanisms by which B7-H1 alters pathogenic outcome, virus-infected B7-H1-deficient (B7-H1(-/-)) mice were analyzed for altered peripheral and CNS immune responses. B7-H1 deficiency did not affect peripheral T or B cell activation or alter the magnitude or composition of CNS-infiltrating cells. However, higher levels of IFN-γ mRNA in CNS-infiltrating virus-specific CD8 T cells as well as CD4 T cells contributed to elevated IFN-γ protein in the B7-H1(-/-) CNS. Increased effector function at the single-cell level was also evident by elevated granzyme B expression specifically in virus-specific CNS CD8 T cells. Although enhanced T cell activity accelerated virus control, 50% of mice succumbed to infection. Despite enhanced clinical recovery, surviving B7-H1(-/-) mice still harbored persisting viral mRNA, albeit at reduced levels compared with wild-type mice. B7-H1(-/-) mice exhibited extensive loss of axonal integrity, although demyelination, a hallmark of virus-induced tissue damage, was not increased. The results suggest that B7-H1 hinders viral control in B7-H1 expressing glia cells, but does not mediate resistance to CD8 T cell-mediated cytolysis. These data are the first, to our knowledge, to demonstrate that B7-H1-mediated protection from viral-induced immune pathology associated with encephalomyelitis resides in limiting T cell-mediated axonal bystander damage rather than direct elimination of infected myelinating cells.

A systems immunology approach to plasmacytoid dendritic cell function in cytopathic virus infections

Plasmacytoid dendritic cell (pDC)-mediated protection against cytopathic virus infection involves various molecular, cellular, tissue-scale, and organism-scale events. In order to better understand such multiscale interactions, we have implemented a systems immunology approach focusing on the analysis of the structure, dynamics and operating principles of virus-host interactions which constrain the initial spread of the pathogen. Using high-resolution experimental data sets coming from the well-described mouse hepatitis virus (MHV) model, we first calibrated basic modules including MHV infection of its primary target cells, i.e. pDCs and macrophages (Mphis). These basic building blocks were used to generate and validate an integrative mathematical model for in vivo infection dynamics. Parameter estimation for the system indicated that on a per capita basis, one infected pDC secretes sufficient type I IFN to protect 10(3) to 10(4) Mphis from cytopathic viral infection. This extremely high protective capacity of pDCs secures the spleen's capability to function as a 'sink' for the virus produced in peripheral organs such as the liver. Furthermore, our results suggest that the pDC population in spleen ensures a robust protection against virus variants which substantially down-modulate IFN secretion. However, the ability of pDCs to protect against severe disease caused by virus variants exhibiting an enhanced liver tropism and higher replication rates appears to be rather limited. Taken together, this systems immunology analysis suggests that antiviral therapy against cytopathic viruses should primarily limit viral replication within peripheral target organs.

Cell replacement therapies to promote remyelination in a viral model of demyelination

Persistent infection of the central nervous system (CNS) of mice with the neuroadapted JHM strain of mouse hepatitis (MHV) is characterized by ongoing demyelination mediated by inflammatory T cells and macrophages that is similar both clinically and histologically with the human demyelinating disease multiple sclerosis (MS). Although extensive demyelination occurs in mice persistently infected with MHV there is only limited remyelination. Therefore, the MHV model of demyelination is a relevant model for studying disease and evaluating therapeutic approaches to protect cells of the oligodendrocyte lineage and promote remyelination. This concept is further highlighted as the etiology of MS remains enigmatic, but viruses have long been considered as potential triggering agents in initiating and/or maintaining MS symptoms. As such, understanding mechanisms associated with promoting repair within the CNS in the context of a persistent viral infection is critical given the possible viral etiology of MS. This review focuses on recent studies using either mouse neural stem cells (NSCs) or human oligodendrocyte progenitor cells (OPCs) derived from human embryonic stem cell (hESC) to promote remyelination in mice persistently infected with MHV. In addition, the potential role for chemokines in positional migration of transplanted cells is addressed.