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

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.

TNF-alpha-dependent regulation of CXCR3 expression modulates neuronal survival during West Nile virus encephalitis

The chemokine CXCL10 exerts antiviral effects within the central nervous system (CNS) through the recruitment of virus-specific T cells. However, elevated levels of CXCL10 may induce neuronal apoptosis given its receptor, CXCR3, is expressed by neurons. Using a murine model of West Nile virus (WNV) encephalitis, we determined that WNV-infected neurons express TNF-alpha, which down-regulates neuronal CXCR3 expression via signaling through TNFR1. Down-regulation of neuronal CXCR3 decreased CXCL10-mediated calcium transients and delayed Caspase 3 activation. Loss of CXCR3 activation, via CXCR3-deficiency or pretreatment with TNF-alpha prevented neuronal apoptosis during in vitro WNV infection. These results suggest that neuronal TNF-alpha expression during WNV encephalitis may be an adaptive response to diminish CXCL10-induced death.

Regulatory T cells inhibit T cell proliferation and decrease demyelination in mice chronically infected with a coronavirus

Mice infected with the neurotropic JHM strain of mouse hepatitis virus (JHMV) develop acute and chronic demyelinating diseases with histopathological similarities to multiple sclerosis. The process of demyelination is largely immune-mediated, as immunodeficient mice (RAG1(-/-) mice) do not develop demyelination upon infection; however, demyelination develops if these mice are reconstituted with either JHMV-immune CD4 or CD8 T cells. Because myelin destruction is a consequence of the inflammatory response associated with virus clearance, we reasoned that decreasing the amount of inflammation would diminish clinical disease and demyelination. Given that regulatory T cells (Tregs) have potent anti-inflammatory effects, we adoptively transferred Tregs into infected C57BL/6 and RAG1(-/-) mice. In both instances, transfer of Tregs decreased weight loss, clinical scores, and demyelination. Transferred Tregs were not detected in the CNS of infected RAG1(-/-) mice, but rather appeared to mediate their effects in the draining cervical lymph nodes. We show that Tregs dampen the inflammatory response mediated by transferred JHMV-immune splenocytes in infected RAG1(-/-) mice by decreasing T cell proliferation, dendritic cell activation, and proinflammatory cytokine/chemokine production, without inducing apoptosis. By extension, decreasing inflammation, whether by Treg transfer or by otherwise enhancing the anti-inflammatory milieu, could contribute to improved clinical outcomes in patients with virus-induced demyelination.

Gamma interferon signaling in oligodendrocytes is critical for protection from neurotropic coronavirus infection

Neurotropic coronavirus induces acute encephalomyelitis and demyelination in mice. Infection of BALB/c (H-2(d)) mice expressing a dominant negative gamma interferon (IFN-gamma) receptor specifically in oligodendrocytes was examined to determine the influence of IFN-gamma signaling on pathogenesis. Inhibition of IFN-gamma signaling in oligodendrocytes increased viral load, infection of oligodendrocytes, oligodendrocyte loss, demyelination, and axonal damage resulting in increased mortality. IFN-gamma levels and the inflammatory response were not altered, although the level of tumor necrosis factor (TNF) mRNA was increased. These data indicate that IFN-gamma signaling by oligodendroglia reduces viral replication but affects both demyelination and tissue destruction in a host-specific manner.

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.

The Biology of Persistent Infection: Inflammation and Demyelination following Murine Coronavirus Infection of the Central Nervous System

Multiple Sclerosis (MS) is an immune-mediated demyelinating disease of humans. Although causes of MS are enigmatic, underlying elements contributing to disease development include both genetic and environmental factors. Recent epidemiological evidence has pointed to viral infection as a trigger to initiating white matter damage in humans. Mouse hepatitis virus (MHV) is a positive strand RNA virus that, following intracranial infection of susceptible mice, induces an acute encephalomyelitis that later resolves into a chronic fulminating demyelinating disease. Immune cell infiltration into the central nervous system is critical both to quell viral replication and instigate demyelination. Recent efforts by our laboratory and others have focused upon strategies capable of enhancing remyelination in response to viral-induced demyelination, both by dampening chronic inflammation and by surgical engraftment of remyelination - competent neural precursor cells.

Protective and pathologic roles of the immune response to mouse hepatitis virus type 1: implications for severe acute respiratory syndrome

Intranasal mouse hepatitis virus type 1 (MHV-1) infection of mice induces lung pathology similar to that observed in severe acute respiratory syndrome (SARS) patients. However, the severity of MHV-1-induced pulmonary disease varies among mouse strains, and it has been suggested that differences in the host immune response might account for this variation. It has also been suggested that immunopathology may represent an important clinical feature of SARS. Little is known about the host immune response to MHV-1 and how it might contribute to some of the pathological changes detected in infected mice. In this study we show that an intact type I interferon system and the adaptive immune responses are required for controlling MHV-1 replication and preventing morbidity and mortality in resistant C57BL/6J mice after infection. The NK cell response also helps minimize the severity of illness following MHV-1 infection of C57BL/6J mice. In A/J and C3H/HeJ mice, which are highly susceptible to MHV-1-induced disease, we demonstrate that both CD4 and CD8 T cells contribute to morbidity during primary infection, and memory responses can enhance morbidity and mortality during subsequent reexposure to MHV-1. However, morbidity in A/J and C3H/HeJ mice can be minimized by treating them with immune serum prior to MHV-1 infection. Overall, our findings highlight the role of the host immune response in contributing to the pathogenesis of coronavirus-induced respiratory disease.

Target-dependent B7-H1 regulation contributes to clearance of central nervous system infection and dampens morbidity

The neurotropic coronavirus JHM strain of mouse hepatitis virus persists in oligodendroglia despite the presence of virus-specific CD8 T cells. Expression of programmed death 1 (PD-1) and B7-H1 were studied during acute and persistent infection to examine whether this negative regulatory mechanism contributes to CNS viral persistence. The majority of CNS-infiltrating CD8 T cells expressed PD-1, with the highest levels on virus-specific CD8 T cells. Moreover, despite control of infectious virus, CD8 T cells within the CNS of persistently infected mice maintained high PD-1 expression. Analysis of virus-susceptible target cells in vivo revealed that B7-H1 expression was regulated in a cell type-dependent manner. Oligodendroglia and microglia up-regulated B7-H1 following infection; however, although B7-H1 expression on oligodendroglia was prominent and sustained, it was significantly reduced and transient on microglia. Infection of mice deficient in the IFN-gamma or IFN-alpha/beta receptor demonstrated that B7-H1 expression on oligodendroglia is predominantly regulated by IFN-gamma. Ab blockade of B7-H1 on oligodendroglia in vitro enhanced IFN-gamma secretion by virus-specific CD8 T cells. More efficient virus control within the CNS of B7-H1-deficient mice confirmed inhibition of CD8 T cell function in vivo. Nevertheless, the absence of B7-H1 significantly increased morbidity without altering demyelination. These data are the first to demonstrate glia cell type-dependent B7-H1 regulation in vivo, resulting in adverse effects on antiviral CD8 T cell function. However, the beneficial role of PD-1:B7-H1 interactions in limiting morbidity highlights the need to evaluate tissue-specific intervention strategies.

Interleukin-12 (IL-12), but not IL-23, deficiency ameliorates viral encephalitis without affecting viral control

The relative contributions of interleukin-12 (IL-12) and IL-23 to viral pathogenesis have not been extensively studied. IL-12p40 mRNA rapidly increases after neurotropic coronavirus infection. Infection of mice defective in both IL-12 and IL-23 (p40(-/-)), in IL-12 alone (p35(-/-)), and in IL-23 alone (p19(-/-)) revealed that the symptoms of coronavirus-induced encephalitis are regulated by IL-12. IL-17-producing cells never exceeded background levels, supporting a redundant role of IL-23 in pathogenesis. Viral control, tropism, and demyelination were all similar in p35(-/-), p19(-/-), and wild-type mice. Reduced morbidity in infected IL-12 deficient mice was also not associated with altered recruitment or composition of inflammatory cells. However, gamma interferon (IFN-gamma) levels and virus-specific IFN-gamma-secreting CD4 and CD8 T cells were all reduced in the central nervous systems (CNS) of infected p35(-/-) mice. Transcription of the proinflammatory cytokines IL-1beta and IL-6, but not tumor necrosis factor, were initially reduced in infected p35(-/-) mice but increased to wild-type levels during peak inflammation. Furthermore, although transforming growth factor beta mRNA was not affected, IL-10 was increased in the CNS in the absence of IL-12. These data suggest that IL-12 does not contribute to antiviral function within the CNS but enhances morbidity associated with viral encephalitis by increasing the ratio of IFN-gamma to protective IL-10.

Type I IFN-mediated protection of macrophages and dendritic cells secures control of murine coronavirus infection

The swift production of type I IFNs is one of the fundamental aspects of innate immune responses against viruses. Plasmacytoid dendritic cell-derived type I IFNs are of prime importance for the initial control of highly cytopathic viruses such as the mouse hepatitis virus (MHV). The aim of this study was to determine the major target cell populations of this first wave of type I IFNs. Generation of bone marrow-chimeric mice expressing the type I IFN receptor (IFNAR) on either hemopoietic or non-bone marrow-derived cells revealed that the early control of MHV depended mainly on IFNAR expression on hemopoietic cells. To establish which cell population responds most efficiently to type I IFNs, mice conditionally deficient for the IFNAR on different leukocyte subsets were infected with MHV. This genetic analysis revealed that IFNAR expression on LysM+ macrophages and CD11c+ dendritic cells was most important for the early containment of MHV within secondary lymphoid organs and to prevent lethal liver disease. This study identifies type I IFN-mediated cross-talk between plasmacytoid dendritic cells on one side and macrophages and conventional dendritic cells on the other, as an essential cellular pathway for the control of fatal cytopathic virus infection.

Production of protective gamma interferon by natural killer cells during early mouse hepatitis virus infection

Gamma interferon (IFN-gamma) plays a major role in the protection against lethal infection with mouse hepatitis virus A59. IFN-gamma production reaches a maximum level 2 days after viral inoculation, especially in liver immune cells. Among these cells, natural killer cells are the major producers of this cytokine. Transfer experiments indicated that the protective role of IFN-gamma is mediated through a direct effect on cells targeted by the virus rather than through indirect activation of T lymphocytes.

Memory CD4+ T-cell-mediated protection from lethal coronavirus encephalomyelitis

The antiviral role of CD4(+) T cells in virus-induced pathologies of the central nervous system (CNS) has not been explored extensively. Control of neurotropic mouse hepatitis virus (JHMV) requires the collaboration of CD4(+) and CD8(+) T cells, with CD8(+) T cells providing direct perforin and gamma interferon (IFN-gamma)-mediated antiviral activity. To distinguish bystander from direct antiviral contributions of CD4(+) T cells in virus clearance and pathology, memory CD4(+) T cells purified from wild type (wt), perforin-deficient (PKO), and IFN-gamma-deficient (GKO) immune donors were transferred to immunodeficient SCID mice prior to CNS challenge. All three donor CD4(+) T-cell populations controlled CNS virus replication at 8 days postinfection, indicating IFN-gamma- and perforin-independent antiviral function. Recipients of GKO CD4(+) T cells succumbed more rapidly to fatal disease than untreated control infected mice. In contrast, wt and PKO donor CD4(+) T cells cleared infectious virus to undetectable levels and protected from fatal disease. Recipients of all CD4(+) T-cell populations exhibited demyelination. However, it was more severe in wt CD4(+) T-cell recipients. These data support a role of CD4(+) T cells in virus clearance and demyelination. Despite substantial IFN-gamma-independent antiviral activity, IFN-gamma was crucial in providing protection from death. IFN-gamma reduced neutrophil accumulation and directed macrophages to white matter but did not ameliorate myelin loss.

Induction of class I antigen processing components in oligodendroglia and microglia during viral encephalomyelitis

Glia exhibit differential susceptibility to CD8 T cell mediated effector mechanisms during neurotropic coronavirus infection. In contrast to microglia, oligodendroglia are resistant to CD8 T cell perforin‐mediated viral control in the absence of IFNγ. Kinetic induction of MHC Class I expression by microglia and oligodendroglia in vivo was thus analyzed to assess responses to distinct inflammatory signals. Flow cytometry demonstrated delayed Class I surface expression by oligodendroglia compared with microglia. Distinct kinetics of Class I protein upregulation correlated with cell type specific transcription patterns of genes encoding Class I heavy chains and antigen processing components. Microglia isolated from naïve mice expressed high levels of these mRNAs, whereas they were near detection limits in oligodendroglia; nevertheless, Class I protein was undetectable on both cell types. Infection induced modest mRNA increases in microglia, but dramatic transcriptional upregulation in oligodendroglia coincident with IFNα or IFNγ mRNA increases in infected tissue. Ultimately mRNAs reached similar levels in both cell types at their respective time points of maximal Class I expression. Expression of Class I on microglia, but not oligodendroglia, in infected IFNγ deficient mice supported distinct IFN requirements for Class I presentation. These data suggest an innate immune preparedness of microglia to present antigen and engage CD8 T cells early following infection. The delayed, yet robust, IFNγ dependent capacity of oligodendroglia to express Class I suggests strict control of immune interactions to avoid CD8 T cell recognition and potential presentation of autoantigen to preserve myelin maintenance. © 2008 Wiley‐Liss, Inc.

Chemokines in and out of the central nervous system: much more than chemotaxis and inflammation

Actions of chemokines and the interaction with specific receptors go beyond their original, defined role of recruiting leukocytes to inflamed tissues. Chemokine receptor expression in peripheral elements and resident cells of the central nervous system (CNS) represents a relevant communication system during neuroinflammatory conditions. The following examples are described in this review: Chemokine receptors play important homeostatic properties by regulating levels of specific ligands in blood and tissues during healthy and pathological conditions; chemokines and their receptors are clearly involved in leukocyte extravasation and recruitment to the CNS, and current studies are directed toward understanding the interaction between chemokine receptors and matrix metalloproteinases in the process of blood brain barrier breakdown. We also propose novel functions of chemokine receptors during demyelination/remyelination, and developmental processes.

Mouse hepatitis virus infection of the CNS: a model for defense, disease, and repair

Viral infection of the central nervous system (CNS) results in varied outcomes ranging from encephalitis, paralytic poliomyelitis or other serious consequences. One of the principal factors that directs the outcome of infection is the localized innate immune response, which is proceeded by the adaptive immune response against the invading viral pathogen. The role of the immune system is to contain and control the spread of virus within the CNS, and paradoxically, this response may also be pathological. Studies with a neurotropic murine coronavirus, mouse hepatitis virus (MHV) have provided important insights into how the immune system combats neuroinvasive viruses, and have identified molecular and cellular mechanisms contributing to chronic disease in persistently infected mice.

CD4 T cells contribute to virus control and pathology following central nervous system infection with neurotropic mouse hepatitis virus

Replication of the neurotropic mouse hepatitis virus strain JHM (JHMV) is controlled primarily by CD8(+) T-cell effectors utilizing gamma interferon (IFN-gamma) and perforin-mediated cytotoxicity. CD4(+) T cells provide an auxiliary function(s) for CD8(+) T-cell survival; however, their direct contribution to control of virus replication and pathology is unclear. To examine a direct role of CD4(+) T cells in viral clearance and pathology, pathogenesis was compared in mice deficient in both perforin and IFN-gamma that were selectively reconstituted for these functions via transfer of virus-specific memory CD4(+) T cells. CD4(+) T cells from immunized wild-type, perforin-deficient, and IFN-gamma-deficient donors all initially reduced virus replication. However, prolonged viral control by IFN-gamma-competent donors suggested that IFN-gamma is important for sustained virus control. Local release of IFN-gamma was evident by up-regulation of class II molecules on microglia in recipients of IFN-gamma producing CD4(+) T cells. CD4(+) T-cell-mediated antiviral activity correlated with diminished clinical symptoms, pathology, and demyelination. Both wild-type donor CD90.1 and recipient CD90.2 CD4(+) T cells trafficked into the central nervous system (CNS) parenchyma and localized to infected white matter, correlating with decreased numbers of virus-infected oligodendrocytes in the CNS. These data support a direct, if limited, antiviral role for CD4(+) T cells early during acute JHMV encephalomyelitis. Although the antiviral effector mechanism is initially independent of IFN-gamma secretion, sustained control of CNS virus replication by CD4(+) T cells requires IFN-gamma.

T cell antiviral effector function is not dependent on CXCL10 following murine coronavirus infection

The chemokine CXCL10 is expressed within the CNS in response to intracerebral infection with mouse hepatitis virus (MHV). Blocking CXCL10 signaling results in increased mortality accompanied by reduced T cell infiltration and increased viral titers within the brain suggesting that CXCL10 functions in host defense by attracting T cells into the CNS. The present study was undertaken to extend our understanding of the functional role of CXCL10 in response to MHV infection given that CXCL10 signaling has been implicated in coordinating both effector T cell generation and trafficking. We show that MHV infection of CXCL10(+/+) or CXCL10(-/-) mice results in comparable levels of T cell activation and similar numbers of virus-specific CD4+ and CD8+ T cells. Subsequent analysis revealed no differences in T cell proliferation, IFN-gamma secretion by virus-specific T cells, or CD8+ T cell cytolytic activity. Analysis of chemokine receptor expression on CD4+ and CD8+ T cells obtained from MHV-immunized CXCL10(+/+) and CXCL10(-/-) mice revealed comparable levels of CXCR3 and CCR5, which are capable of responding to ligands CXCL10 and CCL5, respectively. Adoptive transfer of splenocytes acquired from MHV-immunized CXCL10(-/-) mice into MHV-infected RAG1(-/-) mice resulted in T cell infiltration into the CNS, reduced viral burden, and demyelination comparable to RAG1(-/-) recipients of immune CXCL10(+/+) splenocytes. Collectively, these data imply that CXCL10 functions primarily as a T cell chemoattractant and does not significantly influence T cell effector response following MHV infection.

Liver X receptor activation decreases the severity of experimental autoimmune encephalomyelitis

Agonists of liver X receptors (LXR), members of the nuclear hormone receptor superfamily, alter secretion of proinflammatory cytokines, suggesting potential antiinflammatory effects. A synthetic LXR agonist inhibited T-cell proliferation and cytokine release in a dose-dependent manner. Treatment of mice during induction of experimental autoimmune encephalomyelitis reduced clinical symptoms, central nervous system cellular inflammation, and major histocompatibility class II expression on microglia, as well as demyelination. In contrast to in vitro analysis, no reductions in peripheral neuroantigen specific T-cell responses were detected in comparing ligand and vehicle treated mice. These data suggest that LXR agonists play an important protective role in the regulation of T-cell-mediated inflammatory disease of the central nervous system.

CD8(+) T cell-mediated immune responses in West Nile virus (Sarafend strain) encephalitis are independent of gamma interferon

The flavivirus West Nile virus (WNV) can cause fatal encephalitis in humans and mice. It has recently been demonstrated, in an experimental model using WNV strain Sarafend and C57BL/6 mice, that both virus- and immune-mediated pathology is involved in WNV encephalitis, with CD8(+) T cells being the dominant subpopulation of lymphocyte infiltrates in the brain. Here, the role of activated WNV-immune CD8(+) T cells in mouse WNV encephalitis was investigated further. Passive transfer of WNV-immune CD8(+) T cells reduced mortality significantly and prolonged survival times of mice infected with WNV. Early infiltration of WNV-immune CD8(+) T cells into infected brains is shown, suggesting a beneficial contribution of these lymphocytes to recovery from encephalitis. This antiviral function was not markedly mediated by gamma interferon (IFN-gamma), as a deficiency in IFN-gamma did not affect mortality to two strains of WNV (Sarafend and Kunjin) or brain virus titres significantly. The cytolytic potential, as well as precursor frequency, of WNV-immune CD8(+) T cells were not altered by the absence of IFN-gamma. This was reflected in transfer experiments of WNV-immune CD8(+) T cells from IFN-gamma(-/-) mice into WNV-infected wild-type mice, which showed that IFN-gamma-deficient T cells were as effective as those from WNV-immune wild-type mice in ameliorating disease outcome. It is speculated here that one of the pleiotropic functions of IFN-gamma is mimicked by WNV-Sarafend-mediated upregulation of cell-surface expression of major histocompatibility complex antigens, which may explain the lack of phenotype of IFN-gamma(-/-) mice in response to WNV.

Virally activated CD8 T cells home to Mycobacterium bovis BCG-induced granulomas but enhance antimycobacterial protection only in immunodeficient mice

The effect of secondary infections on CD4 T-cell-regulated chronic granulomatous inflammation is not well understood. Here, we have investigated the effect of an acute viral infection on the cellular composition and bacterial protection in Mycobacterium bovis strain bacille Calmette-Guérin (BCG)-induced granulomas using an immunocompetent and a partially immunodeficient murine model. Acute lymphocytic choriomeningitis virus (LCMV) coinfection of C57BL/6 mice led to substantial accumulation of gamma interferon (IFN-gamma)-producing LCMV-specific T cells in liver granulomas and increased local IFN-gamma. Despite traffic of activated T cells that resulted in a CD8 T-cell-dominated granuloma, the BCG liver organ load was unaltered from control levels. In OT-1 T-cell-receptor (TCR) transgenic mice, ovalbumin (OVA) immunization or LCMV coinfection of BCG-infected mice induced CD8 T-cell-dominated granulomas containing large numbers of non-BCG-specific activated T cells. The higher baseline BCG organ load in this CD8 TCR transgenic animal allowed us to demonstrate that OVA immunization and LCMV coinfection increased anti-BCG protection. The bacterial load remained substantially higher than in mice with a more complete TCR repertoire. Overall, the present study suggests that peripherally activated CD8 T cells can be recruited to chronic inflammatory sites, but their contribution to protective immunity is limited to conditions of underlying immunodeficiency.

Control of coronavirus infection through plasmacytoid dendritic-cell-derived type I interferon

This study demonstrates a unique and crucial role of plasmacytoid dendritic cells (pDCs) and pDC-derived type I interferons (IFNs) in the pathogenesis of mouse coronavirus infection. pDCs controlled the fast replicating mouse hepatitis virus (MHV) through the immediate production of type I IFNs. Recognition of MHV by pDCs was mediated via TLR7 ensuring a swift IFN-α production following encounter with this cytopathic RNA virus. Furthermore, the particular type I IFN response pattern was not restricted to the murine coronavirus, but was also found in infection with the highly cytopathic human severe acute respiratory syndrome (SARS) coronavirus. Taken together, our results suggest that rapid production of type I IFNs by pDCs is essential for the control of potentially lethal coronavirus infections.

Gamma interferon plays a crucial early antiviral role in protection against West Nile virus infection

West Nile virus (WNV) causes a severe central nervous system (CNS) infection in humans, primarily in the elderly and immunocompromised. Prior studies have established an essential protective role of several innate immune response elements, including alpha/beta interferon (IFN-alpha/beta), immunoglobulin M, gammadelta T cells, and complement against WNV infection. In this study, we demonstrate that a lack of IFN-gamma production or signaling results in increased vulnerability to lethal WNV infection by a subcutaneous route in mice, with a rise in mortality from 30% (wild-type mice) to 90% (IFN-gamma(-/-) or IFN-gammaR(-/-) mice) and a decrease in the average survival time. This survival pattern in IFN-gamma(-/-) and IFN-gammaR(-/-) mice correlated with higher viremia and greater viral replication in lymphoid tissues. The increase in peripheral infection led to early CNS seeding since infectious WNV was detected several days earlier in the brains and spinal cords of IFN-gamma(-/-) or IFN-gammaR(-/-) mice. Bone marrow reconstitution experiments showed that gammadelta T cells require IFN-gamma to limit dissemination by WNV. Moreover, treatment of primary dendritic cells with IFN-gamma reduced WNV production by 130-fold. Collectively, our experiments suggest that the dominant protective role of IFN-gamma against WNV is antiviral in nature, occurs in peripheral lymphoid tissues, and prevents viral dissemination to the CNS.

Vaccine-induced memory CD8+ T cells cannot prevent central nervous system virus reactivation

Noncytopathic viruses use multiple strategies to evade immune detection, challenging a role for vaccine induced CTL in preventing microbial persistence. Recrudescence of neurotropic coronavirus due to loss of T cell-mediated immune control provided an experimental model to test T cell vaccination efficacy in the absence of Ab. Challenge virus was rapidly controlled in vaccinated Ab-deficient mice coincident with accelerated recruitment of memory CD8+ T cells and enhanced effector function compared with primary CD8+ T cell responses. In contrast to primary effectors, reactivated memory cells persisted in the CNS at higher frequencies and retained ex vivo cytolytic activity. Nevertheless, despite earlier and prolonged T cell-mediated control in the CNS of vaccinated mice, virus ultimately reactivated. Apparent loss of memory CD8+ effector function in vivo was supported by a prominent decline in MHC expression on CNS resident target cells, presumably reflecting diminished IFN-gamma. Severely reduced MHC expression on glial cells at the time of recrudescence suggested that memory T cells, although fully armed to exert antiviral activity upon Ag recognition in vitro, are not responsive in an environment presenting few if any target MHC molecules. Paradoxically, effective clearance of viral Ag thus affords persisting virus a window of opportunity to escape from immune surveillance. These studies demonstrate that vaccine-induced T cell memory alone is unable to control persisting virus in a tissue with strict IFN-dependent MHC regulation, as evident in immune privileged sites.

T-cells in human encephalitis

Encephalitis literally means inflammation of the brain. In general, this inflammation can result from a viral or bacterial infection in the brain itself or alternatively from a secondary autoimmune reaction against an infection or a tumor in the rest of the body. Besides this, encephalitis is present in (believed autoimmune) diseases with unknown etiology, such as multiple sclerosis or Rasmussen encephalitis (RE). This article summarizes the existing data on the role of T-cells in the pathogenesis of three types of human encephalitis: RE, paraneoplastic encephalomyelitis, and virus encephalitis. In all of them, T-cells play a major role in disease pathogenesis, mainly mediated by major histocompatiblity complex class I-restricted CD8⁺ T-lymphocytes.

Inhibition of interferon-gamma signaling in oligodendroglia delays coronavirus clearance without altering demyelination

Infection of the central nervous system (CNS) by the neurotropic JHM strain of mouse hepatitis virus (JHMV) induces an acute encephalomyelitis associated with demyelination. To examine the anti-viral and/or regulatory role of interferon-gamma (IFN-gamma) signaling in the cell that synthesizes and maintains the myelin sheath, we analyzed JHMV pathogenesis in transgenic mice expressing a dominant-negative IFN-gamma receptor on oligodendroglia. Defective IFN-gamma signaling was associated with enhanced oligodendroglial tropism and delayed virus clearance. However, the CNS inflammatory cell composition and CD8(+) T-cell effector functions were similar between transgenic and wild-type mice, supporting unimpaired peripheral and CNS immune responses in transgenic mice. Surprisingly, increased viral load in oligodendroglia did not affect the extent of myelin loss, the frequency of oligodendroglial apoptosis, or CNS recruitment of macrophages. These data demonstrate that IFN-gamma receptor signaling is critical for the control of JHMV replication in oligodendroglia. In addition, the absence of a correlation between increased oligodendroglial infection and the extent of demyelination suggests a complex pathobiology of myelin loss in which infection of oligodendroglia is required but not sufficient.

Mouse hepatitis virus pathogenesis in the central nervous system is independent of IL-15 and natural killer cells

Infection by the neurotropic JHM strain of mouse hepatitis virus (JHMV) results in an acute encephalomyelitis associated with demyelination. T cells are critical in controlling viral replication, but also contribute to central nervous system (CNS) pathogenesis. To reveal a role for innate effectors in anti-viral immunity and neurological disease, JHMV pathogenesis was studied in mice deficient in interleukin-15 (IL-15^−/−) and natural killer (NK) cells. Clinical disease, CNS inflammation and demyelination in infected IL-15^−/− mice were similar to wild-type mice. Despite the absence of NK cells and suboptimal CD8⁺ T cell responses, IL-15^−/− mice controlled JHMV replication as efficiently as wild-type mice. Similar kinetics of class I and class II upregulation on microglia further suggested no role of NK cells in regulating major histocompatibility complex (MHC) molecule expression on resident CNS cells. IL-15 and NK cells thus appear dispensable for anti-viral immunity and CNS pathogenesis during acute JHMV infection.

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

Several viruses infect the mammalian central nervous system (CNS), some with devastating consequences, others resulting in chronic or persistent infections associated with little or no overt pathology. Coronavirus infection of the murine CNS illustrates the contributions of both the innate immune response and specific host effector mechanisms that control virus replication in distinct CNS cell types. Despite T-cell-mediated control of acute virus infection, host regulatory mechanisms, probably designed to protect CNS integrity, contribute to the failure to eliminate virus. Distinct from cytolytic effector mechanisms expressed during acute infection, non-lytic humoral immunity prevails in suppressing infectious virus during persistence.

Immunopathogenesis of coronavirus infections: implications for SARS

The severe acute respiratory syndrome (SARS), which was first identified in 2003, is caused by a novel coronavirus: the SARS coronavirus (SARS-CoV).

Many features of the infection indicate that an excessive, but perhaps 'normal', immune response contributes to SARS.

Several coronaviruses cause diseases that result in considerable morbidity and mortality in animals. Some of these diseases are also immune mediated and provide insights into the pathogenesis of SARS.

Feline infectious peritonitis virus (FIPV) causes a fatal, immune-mediated disease of felines. Macrophage infection, lymphocyte depletion and antibody-dependent disease enhancement are hallmarks of this disease.

Infection with the murine coronavirus murine hepatitis virus (MHV) strain JHM results in immune-mediated demyelination. Similar to SARS, macrophage activation is a key component in the pathogenic process.

Another strain of MHV, MHV-3, causes a fatal, fulminant hepatitis. MHV-3 infection of macrophages, with subsequent activation and induction of expression of a novel procoagulant, fibrinogen-like protein 2 (FGL2), is required for severe disease.

Chickens that are infected with avian infectious bronchitis virus (IBV) develop respiratory and renal disease. An excessive innate immune response contributes to the pathogenic process in these animals.

To develop effective therapies for SARS will require understanding of the contributions of direct injury by virus and of the host immune response to pathogenesis. This requires further studies of the interactions of SARS-CoV with its target cells and necessitates the development of an animal model that reproduces the pulmonary infection that is observed in infected humans.

At the end of 2002, the first cases of severe acute respiratory syndrome (SARS) were reported, and in the following year, SARS resulted in considerable mortality and morbidity worldwide. SARS is caused by a novel species of coronavirus (SARS-CoV) and is the most severe coronavirus-mediated human disease that has been described so far. On the basis of similarities with other coronavirus infections, SARS might, in part, be immune mediated. As discussed in this Review, studies of animals that are infected with other coronaviruses indicate that excessive and sometimes dysregulated responses by macrophages and other pro-inflammatory cells might be particularly important in the pathogenesis of disease that is caused by infection with these viruses. It is hoped that lessons from such studies will help us to understand more about the pathogenesis of SARS in humans and to prevent or control outbreaks of SARS in the future.

Immune regulation of transgene expression in the brain: B cells regulate an early phase of elimination of transgene expression from adenoviral vectors

Cellular immune mechanisms that regulate viral gene expression within infected brain cells remain poorly understood. Previous work has shown that systemic immunization against adenovirus after vector delivery to the brain results in complete loss of brain cells infected by adenoviral vectors. Although T cells play an important role in this process, we demonstrate herein that B cells also significantly regulate transgene expression from the CNS. After the systemic immunization against adenovirus of animals injected via the brain with an adenoviral vector 30 days earlier, we uncovered substantial infiltration by CD19+ B cells of the area of the brain transduced by the virus. This suggests the involvement of B cells in the adaptive immune response-mediated loss of transduced cells from the brain. Confocal analysis of these brains demonstrated physical contacts between transduced brain cells and CD19+ cells. To test the hypothesis that B cells play a causal role in the loss of infected cells from the brain, we demonstrated that animals devoid of B cells were unable to eliminate transgene expression at early time points after immunization. This demonstrates that B cells play a necessary role in the loss of transgene expression at early, but not late, time points postimmunization. Thus, these data have important implications for our understanding of the role of B cells as immune effectors during the immune-mediated clearance of viral infections from the CNS, and also for understanding mechanisms operating in brain autoimmunity, as well as for the potential safety of clinical gene therapy for brain diseases.

CD8 T cells require gamma interferon to clear borna disease virus from the brain and prevent immune system-mediated neuronal damage

Borna disease virus (BDV) frequently causes meningoencephalitis and fatal neurological disease in young but not old mice of strain MRL. Disease does not result from the virus-induced destruction of infected neurons. Rather, it is mediated by H-2(k)-restricted antiviral CD8 T cells that recognize a peptide derived from the BDV nucleoprotein N. Persistent BDV infection in mice is not spontaneously cleared. We report here that N-specific vaccination can protect wild-type MRL mice but not mutant MRL mice lacking gamma interferon (IFN-gamma) from persistent infection with BDV. Furthermore, we observed a significant degree of resistance of old MRL mice to persistent BDV infection that depended on the presence of CD8 T cells. We found that virus initially infected hippocampal neurons around 2 weeks after intracerebral infection but was eventually cleared in most wild-type MRL mice. Unexpectedly, young as well as old IFN-gamma-deficient MRL mice were completely susceptible to infection with BDV. Moreover, neurons in the CA1 region of the hippocampus were severely damaged in most diseased IFN-gamma-deficient mice but not in wild-type mice. Furthermore, large numbers of eosinophils were present in the inflamed brains of IFN-gamma-deficient mice but not in those of wild-type mice, presumably because of increased intracerebral synthesis of interleukin-13 and the chemokines CCL1 and CCL11, which can attract eosinophils. These results demonstrate that IFN-gamma plays a central role in host resistance against infection of the central nervous system with BDV and in clearance of BDV from neurons. They further indicate that IFN-gamma may function as a neuroprotective factor that can limit the loss of neurons in the course of antiviral immune responses in the brain.

Astrocyte expression of a dominant-negative interferon-gamma receptor

Interferon-gamma (IFN-gamma) is a major proinflammatory cytokine, and binding to its nearly ubiquitous receptor induces a wide variety of biological functions. To explore the role(s) of IFN-gamma signaling in astrocytes, transgenic mice (GFAP/IFN-gammaR1DeltaIC) expressing a dominant-negative IFN-gamma receptor alpha chain under control of the astrocyte-specific glial fibrillary acid protein (GFAP) promoter were generated. Transgenic mice developed normally, had normal astrocyte numbers and distribution, and exhibited no clinically overt phenotype. Transgene mRNA expression was detected only in the CNS, and the transgene-encoded IFN-gamma receptor 1 colocalized with GFAP, which is consistent with astrocyte expression. Astrocytes from transgenic mice exhibited reduced IFN-gamma-induced signaling as measured by major histocompatibility class II induction. Neither CNS inflammation nor perforin-mediated clearance of a neurotropic mouse hepatitis virus from astrocytes was impaired following infection. Transgenic mice with impaired astrocyte responsiveness to IFN-gamma provide a model for studying the selective astrocyte-dependent effects of this critical cytokine in CNS immunopathology.

T cell-mediated restriction of intracerebral murine cytomegalovirus infection displays dependence upon perforin but not interferon-gamma

The authors have previously reported that adoptive transfer of splenocytes suppresses murine cytomegalovirus (MCMV) brain infection following intracerebroventricular injection of immunodeficient mice and that depletion of Thy 1.2⁺ T lymphocytes abolishes this suppressive effect. Here the authors report that splenocytes depleted of CD4⁺ T lymphocytes prior to adoptive transfer retained their ability to control viral expression in the brain. In sharp contrast, depletion of the CD8⁺ T-cell population prior to transfer abolished the suppressive effect, with sixfold greater expression in the brain than when undepleted splenocytes were used. The authors went on to examine the contributions of cytokine- and perforin-mediated mechanisms in controlling MCMV brain infection using splenocytes from major histocompatibility (MHC)-matched IFN-γ-knockout (GKO), and perforin-knockout (PKO) mice. When used in adoptive transfer studies, splenocytes from GKO mice controlled viral expression; however, cells from PKO mice could not control reporter gene expression or viral DNA replication in brain tissues. The authors have previously reported that the levels of the T-cell chemoattractant CXCL10 are highly elevated in the brains of MCMV-infected mice. Here the authors found that the receptor for this ligand, CXCR3, was not essential in mediating the suppressive effects of adoptive transfer. These data indicate that peripheral CD8⁺ T cells control MCMV brain infection through a perforin-mediated mechanism and that neither IFN-γ nor CXCR3 play a critical role in this neuroprotective response.

Important roles for gamma interferon and NKG2D in gammadelta T-cell-induced demyelination in T-cell receptor beta-deficient mice infected with a coronavirus

gammadelta T cells mediate demyelination in athymic (nude) mice infected with the neurotropic coronavirus mouse hepatitis virus strain JHM. Now, we show that these cells also mediate the same process in mice lacking alphabeta T cells (T-cell receptor beta-deficient [TCRbeta(-/-)] mice) and demyelination is gamma interferon (IFN-gamma) dependent. Most strikingly, our results also show a major role for NKG2D, expressed on gammadelta T cells, in the demyelinating process with in vivo blockade of NKG2D interactions resulting in a 60% reduction in demyelination. NKG2D may serve as a primary recognition receptor or as a costimulatory molecule. We show that NKG2D(+) gammadelta T cells in the JHM-infected central nervous system express the adaptor molecule DAP12 and an NKG2D isoform (NKG2D short), both required for NKG2D to serve as a primary receptor. These results are consistent with models in which gammadelta T cells mediate demyelination using the same effector cytokine, IFN-gamma, as CD8 T cells and do so without a requirement for signaling through the TCR.

Expression of a dominant negative IFN-gammareceptor on mouse oligodendrocytes

The interferon-gamma (IFN-gamma) receptor is expressed by all nucleated cells, and binding of its cognate ligand, IFN-gamma, induces a wide variety of biological functions. Transgenic mice expressing a dominant negative IFN-gamma receptor 1 (IFN-gammaR1DeltaIC) on oligodendrocytes under control of the myelin proteolipid protein promoter are described. The mRNA encoding the transgene was only detected in the nervous system and protein expression was confirmed by immunohistochemistry. Transgenic receptor expression does not alter myelination and the mice exhibited no clinically apparent phenotype. Consistent with the restricted nervous system expression of the transgene, no alterations in peripheral immune responses were detected. Flow cytometric analysis demonstrated constitutive expression of both the IFN-gammaR1DeltaIC transgene and the endogenous IFN-gamma receptor 2 at high levels on oligodendrocytes derived from the transgenic mice. These oligodendrocytes also exhibited decreased STAT1 phosphorylation in response to IFN-gamma, confirming dominant negative transgene function. Transgenic mice in which oligodendrocytes have a diminished ability to respond to IFN-gamma showed delayed virus clearance from oligodendroglia compared with wild-type mice. This model will allow evaluation of oligodendrocyte responses to this critical cytokine during CNS inflammation.

Virus-specific antibody, in the absence of T cells, mediates demyelination in mice infected with a neurotropic coronavirus

Mice infected with mouse hepatitis virus strain JHM develop an inflammatory demyelinating disease in the central nervous system with many similarities to human multiple sclerosis. The mouse disease is primarily immune-mediated because demyelination is not detected in JHM-infected mice lacking T or B cells but does occur after transfer of JHM-specific T cells. Although less is known about the ability of antibodies to mediate demyelination, the presence of oligoclonally expanded B cells and high concentrations of antibodies (against self or infectious agents) in the central nervous system of many multiple sclerosis patients suggests that antibodies may also contribute to myelin destruction. Here, we show that anti-JHM antibodies, in the absence of T or B cells, caused demyelination in JHM-infected mice. Anti-JHM antibody was detected adjacent to areas of demyelination, consistent with a direct interaction between antibody and infected cells. Demyelination was reduced by 85 to 90% in infected RAG1(-/-) mice lacking normal expression of activating Fc receptors (FcRgamma(-/-)) and by approximately 76% when complement was depleted by treatment with cobra venom factor. These data demonstrate that JHM-specific antibodies are sufficient to cause demyelination and that myelin destruction in the presence of anti-virus antibodies results from a combination of complement- and Fc receptor-dependent mechanisms.

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.

Expression of the mouse hepatitis virus receptor by central nervous system microglia

Detection of the mouse hepatitis virus receptor within the central nervous system (CNS) has been elusive. Receptor expression on microglia was reduced during acute infection and restored following immune-mediated virus control. Receptor down regulation was independent of neutrophils, NK cells, gamma interferon, or perforin. Infection of mice devoid of distinct inflammatory cells revealed CD4(+) T cells as the major cell type influencing receptor expression by microglia. In addition to demonstrating receptor expression on CNS resident cells, these data suggest that transient receptor down regulation on microglia aids in establishing persistence in the CNS by assisting virus infection of other glial cell types.