Neutrophil Cathepsin G Regulates Dendritic Cell Production of IL-12 during Development of CD4 T Cell Responses to Antigens in the Skin

Contact hypersensitivity (CHS) is a CD8 T cell-mediated response to hapten skin sensitization and challenge. Sensitization of wild-type (WT) mice induces hapten-reactive effector CD8 T cells producing IFN-γ and IL-17- and IL-4-producing CD4 T cells that cannot mediate CHS. Although CXCR2-dependent Ly6G⁺ (neutrophil) cell recruitment into hapten-challenged skin is required to direct effector CD8 T cell infiltration into the challenge site to elicit CHS, 2,4-dinitrofluorobenezene (DNFB) sensitization of CXCR2^-/- mice and neutrophil-depleted WT mice induced both hapten-reactive CD4 and CD8 T cells producing IFN-γ and IL-17. CD4 T cell-mediated CHS responses were not generated during DNFB sensitization of neutrophil-depleted WT mice treated with anti-IL-12 mAb or neutrophil-depleted IL-12^-/- mice. Neutrophil depletion during DNFB sensitization of WT mice markedly increased IL-12-producing hapten-primed dendritic cell numbers in the skin-draining lymph nodes. Sensitization of mice lacking the neutrophil serine protease cathepsin G (CG)-induced hapten-reactive CD4 and CD8 T cells producing IFN-γ and IL-17 with elevated and elongated CHS responses to DNFB challenge. Induction of CHS effector CD4 T cells producing IFN-γ in neutrophil-depleted WT mice was eliminated by s.c. injection of active, but not inactivated, CG during sensitization. Thus, hapten skin sensitization induces neutrophil release of CG that systemically inhibits hapten-presenting dendritic cell production of IL-12 and the development of hapten-reactive CD4 T cells to IFN-γ-producing CHS effector cells.

An optimized method for enumerating CNS derived memory B cells during viral-induced inflammation

•

Memory B cell markers characterizing peripheral B cell phenotypes show more diverse expression patterns in the infected central nervous system (CNS).

•

TLR7/8 stimulation for 2 days prior to ELISPOT analysis achieves optimal conversion of CNS-derived memory B cells to ASC while minimizing cell loss.

•

In vitro stimulation allows simultaneous assessment of antibody secreting cell and memory B cell isotype, antigen specificity, and temporal alterations during CNS inflammation.

Background

CNS inflammation resulting from infection, injury, or neurodegeneration leads to accumulation of diverse B cell subsets. Although antibody secreting cells (ASC) within the inflamed CNS have been extensively examined, memory B cell (Bmem) characterization has been limited as they do not secrete antibody without stimulation. Moreover, unlike human Bmem, reliable surface markers for murine Bmem remain elusive.

New method

Using a viral encephalomyelitis model we developed a modified limiting dilution in vitro stimulation assay to convert CNS-derived virus specific Bmem into ASC.

Comparison with existing methods

Stimulation methods established for lymphoid tissue cells using prolonged stimulation with viral lysate resulted in substantial ASC loss and minimal Bmem to ASC conversion of CNS-derived cells. By varying stimulation duration, TLR activators, and culture supplements, we achieved optimal conversion by culturing cells with TLR7/8 agonist R848 in the presence of feeder cells for 2 days.

Results

Flow cytometry markers CD38 and CD73 characterizing murine Bmem from lymphoid tissue showed more diverse expression patterns on corresponding CNS-derived B cell subsets. Using the optimized TLR7/8 stimulation protocol, we compared virus-specific IgG Bmem versus pre-existing ASC within the brain and spinal cord. Increasing Bmem frequencies during chronic infection mirrored kinetics of ASC. However, despite initially similar Bmem and ASC accumulation, Bmem prevailed in the brain, but were lower than ASC in the spinal cord during persistence.

Conclusion

Simultaneous enumeration of antigen-specific Bmem and ASC using the Bmem assay optimized for CNS-derived cells enables characterization of temporal changes during microbial or auto-antigen induced neuroinflammation.

Differential Regulation of Self-reactive CD4+ T Cells in Cervical Lymph Nodes and Central Nervous System during Viral Encephalomyelitis

Viral infections have long been implicated as triggers of autoimmune diseases, including multiple sclerosis (MS), a central nervous system (CNS) inflammatory demyelinating disorder. Epitope spreading, molecular mimicry, cryptic antigen, and bystander activation have been implicated as mechanisms responsible for activating self-reactive (SR) immune cells, ultimately leading to organ-specific autoimmune disease. Taking advantage of coronavirus JHM strain of mouse hepatitis virus (JHMV)-induced demyelination, this study demonstrates that the host also mounts counteractive measures to specifically limit expansion of endogenous SR T cells. In this model, immune-mediated demyelination is associated with induction of SR T cells after viral control. However, their decline during persisting infection, despite ongoing demyelination, suggests an active control mechanism. Antigen-specific IL-10-secreting CD4⁺ T cells (Tr1) and Foxp3⁺ regulatory T cells (Tregs), both known to control autoimmunity and induced following JHMV infection, were assessed for their relative in vivo suppressive function of SR T cells. Ablation of Foxp3⁺ Tregs in chronically infected DEREG mice significantly increased SR CD4⁺ T cells within cervical lymph nodes (CLN), albeit without affecting their numbers or activation within the CNS compared to controls. In contrast, infected IL-27 receptor deficient (IL-27R^-/-) mice, characterized by a drastic reduction of Tr1 cells, revealed that SR CD4⁺ T cells in CLN remained unchanged but were specifically increased within the CNS. These results suggest that distinct Treg subsets limit SR T cells in the draining lymph nodes and CNS to maximize suppression of SR T-cell-mediated autoimmune pathology. The JHMV model is thus valuable to decipher tissue-specific mechanisms preventing autoimmunity.

CXCL13 promotes isotype-switched B cell accumulation to the central nervous system during viral encephalomyelitis

Elevated CXCL13 within the central nervous system (CNS) correlates with humoral responses in several neuroinflammatory diseases, yet its role is controversial. During coronavirus encephalomyelitis CXCL13 deficiency impaired CNS accumulation of memory B cells and antibody-secreting cells (ASC) but not naïve/early-activated B cells. However, despite diminished germinal center B cells and follicular helper T cells in draining lymph nodes, ASC in bone marrow and antiviral serum antibody were intact in the absence of CXCL13. The data demonstrate that CXCL13 is not essential in mounting effective peripheral humoral responses, but specifically promotes CNS accumulation of differentiated B cells.

Sustained TNF production by central nervous system infiltrating macrophages promotes progressive autoimmune encephalomyelitis

Background

Tumor necrosis factor (TNF) has pleiotropic functions during both the demyelinating autoimmune disease multiple sclerosis (MS) and its murine model experimental autoimmune encephalomyelitis (EAE). How TNF regulates disability during progressive disease remains unresolved. Using a progressive EAE model characterized by sustained TNF and increasing morbidity, this study evaluates the role of unregulated TNF in exacerbating central nervous system (CNS) pathology and inflammation.

Methods

Progressive MS was mimicked by myelin oligodendrocyte glycoprotein (MOG) peptide immunization of mice expressing a dominant negative IFN-γ receptor alpha chain under the human glial fibrillary acidic protein promoter (GFAPγR1∆). Diseased GFAPγR1∆ mice were treated with anti-TNF or control monoclonal antibody during acute disease to monitor therapeutic effects on sustained disability, demyelination, CNS inflammation, and blood brain barrier (BBB) permeability.

Results

TNF was specifically sustained in infiltrating macrophages. Anti-TNF treatment decreased established clinical disability and mortality rate within 7 days. Control of disease progression was associated with a decline in myelin loss and leukocyte infiltration, as well as macrophage activation. In addition to mitigating CNS inflammation, TNF neutralization restored BBB integrity and enhanced CNS anti-inflammatory responses.

Conclusions

Sustained TNF production by infiltrating macrophages associated with progressive EAE exacerbates disease severity by promoting inflammation and disruption of BBB integrity, thereby counteracting establishment of an anti-inflammatory environment required for disease remission.

An IL-27/Lag3 axis enhances Foxp3+ regulatory T cell-suppressive function and therapeutic efficacy

Foxp3-expressing regulatory T cells (Tregs) are central regulators of immune homeostasis and tolerance. As it has been suggested that proper Treg function is compromised under inflammatory conditions, seeking for a pathway that enhances or stabilizes Treg function is a subject of considerable interest. We report that interleukin (IL)-27, an IL-12 family cytokine known to have both pro- and anti-inflammatory roles in T cells, plays a pivotal role in enhancing Treg function to control T cell-induced colitis, a model for inflammatory bowel disease (IBD) in humans. Unlike wild-type (WT) Tregs capable of inhibiting colitogenic T-cell expansion and inflammatory cytokine expression, IL-27R-deficient Tregs were unable to downregulate inflammatory T-cell responses. Tregs stimulated with IL-27 expressed substantially improved suppressive function in vitro and in vivo. IL-27 stimulation of Tregs induced expression of Lag3, a surface molecule implicated in negatively regulating immune responses. Lag3 expression in Tregs was critical to mediate Treg function in suppressing colitogenic responses. Human Tregs also displayed enhanced suppressive function and Lag3 expression following IL-27 stimulation. Collectively, these results highlight a novel function for the IL-27/Lag3 axis in modulating Treg regulation of inflammatory responses in the intestine.

Self-reactive CD4(+) T cells activated during viral-induced demyelination do not prevent clinical recovery

Background

Microbial infections have been implicated in initiating and enhancing severity of autoimmune diseases including the demyelinating disease multiple sclerosis (MS). Nevertheless, the incidence of both acute and persisting viral infections without evidence of autoimmune sequelae suggests that this process is well controlled. The conditions promoting or stemming self-reactive (SR) T cells following viral-induced tissue damage thus need to be better defined. Using a non-fatal viral mouse model of encephalomyelitis associated with demyelination and disability, yet ultimate clinical improvement, this study set out to monitor uptake and presentation of endogenous myelin antigens, as well as induction and fate of SR T cells.

Methods

Activation and central nervous system (CNS) recruitment of myelin-specific CD4 T cells was analyzed by flow cytometry during encephalomyelitis induced by a glia tropic murine coronavirus. Potential antigen-presenting cells (APC) ingesting myelin were characterized by flow cytometry and their ability to activate SR T cells tested by co-culture with carboxyfluorescein succinimidyl ester (CFSE)-labeled myelin-specific CD4 T cells. Endogenous SR T cell kinetics was analyzed within both cervical lymph nodes and CNS by Enzyme-Linked ImmunoSpot (ELISPOT) following viral infection.

Results

The data demonstrate the presence of APC capable of activating SR T cells in both draining lymph nodes and the CNS temporally correlating with overt demyelination. While both the CNS-infiltrating myeloid population and microglia ingested myelin, only CNS-infiltrating APC were capable of presenting endogenous myelin antigen to SR T cells ex vivo. Finally, SR T cell activation from the endogenous T cell repertoire was most notable when infectious virus was controlled and paralleled myelin damage. Although SR T cell accumulation peaked in the persistently infected CNS during maximal demyelination, they were not preferentially retained. Their gradual decline, despite ongoing demyelination, suggested minimal re-stimulation and pathogenic function in vivo consistent with the lack of autoimmune symptoms.

Conclusions

The results demonstrate the potential for CNS tissue destruction to induce and recruit SR T cells to the injury site and support a host suppressive mechanism limiting development of autoimmunity.

Interleukin-10 is a critical regulator of white matter lesion containment following viral induced demyelination

Neurotropic coronavirus induces an acute encephalomyelitis accompanied by focal areas of demyelination distributed randomly along the spinal column. The initial areas of demyelination increase only slightly after the control of infection. These circumscribed focal lesions are characterized by axonal sparing, myelin ingestion by macrophage/microglia, and glial scars associated with hypertrophic astrocytes, which proliferate at the lesion border. Accelerated virus control in mice lacking the anti‐inflammatory cytokine IL‐10 was associated with limited initial demyelination, but low viral mRNA persistence similar to WT mice and declining antiviral cellular immunity. Nevertheless, lesions exhibited sustained expansion providing a model of dysregulated white matter injury temporally remote from the acute CNS insult. Expanding lesions in the absence of IL‐10 are characterized by sustained microglial activation and partial loss of macrophage/microglia exhibiting an acquired deactivation phenotype. Furthermore, IL‐10 deficiency impaired astrocyte organization into mesh like structures at the lesion borders, but did not prevent astrocyte hypertrophy. The formation of discrete foci of demyelination in IL‐10 sufficient mice correlated with IL‐10 receptor expression exclusively on astrocytes in areas of demyelination suggesting a critical role for IL‐10 signaling to astrocytes in limiting expansion of initial areas of white matter damage. GLIA 2015;63:2106–2120

Astrocyte response to IFN-γ limits IL-6-mediated microglia activation and progressive autoimmune encephalomyelitis

Background

Therapeutic modalities effective in patients with progressive forms of multiple sclerosis (MS) are limited. In a murine model of progressive MS, the sustained disability during the chronic phase of experimental autoimmune encephalomyelitis (EAE) correlated with elevated expression of interleukin (IL)-6, a cytokine with pleiotropic functions and therapeutic target for non-central nervous system (CNS) autoimmune disease. Sustained IL-6 expression in astrocytes restricted to areas of demyelination suggested that IL-6 plays a major role in disease progression during chronic EAE.

Methods

A progressive form of EAE was induced using transgenic mice expressing a dominant negative interferon-γ (IFN-γ) receptor alpha chain under control of human glial fibrillary acidic protein (GFAP) promoter (GFAPγR1Δ mice). The role of IL-6 in regulating progressive CNS autoimmunity was assessed by treating GFAPγR1Δ mice with anti-IL-6 neutralizing antibody during chronic EAE.

Results

IL-6 neutralization restricted disease progression and decreased disability, myelin loss, and axonal damage without affecting astrogliosis. IL-6 blockade reduced CNS inflammation by limiting inflammatory cell proliferation; however, the relative frequencies of CNS leukocyte infiltrates, including the Th1, Th17, and Treg CD4 T cell subsets, were not altered. IL-6 blockade rather limited the activation and proliferation of microglia, which correlated with higher expression of Galectin-1, a regulator of microglia activation expressed by astrocytes.

Conclusions

These data demonstrate that astrocyte-derived IL-6 is a key mediator of progressive disease and support IL-6 blockade as a viable intervention strategy to combat progressive MS.

Distinct CD4 T-cell effects on primary versus recall CD8 T-cell responses during viral encephalomyelitis

CD4 T-cell help is not a universal requirement for effective primary CD8 T cells but is essential to generate memory CD8 T cells capable of recall responses. This study examined how CD4 T cells affect primary and secondary anti-viral CD8 T-cell responses within the central nervous system (CNS) during encephalomyelitis induced by sublethal gliatropic coronavirus. CD4 T-cell depletion before infection did not impair peripheral expansion, interferon-γ production, CNS recruitment or initial CNS effector capacity of virus-specific CD8 T cells ex vivo. Nevertheless, impaired virus control in the absence of CD4 T cells was associated with gradually diminished CNS CD8 T-cell interferon-γ production. Furthermore, within the CD8 T-cell population short-lived effector cells were increased and memory precursor effector cells were significantly decreased, consistent with higher T-cell turnover. Transfer of memory CD8 T cells to reduce viral load in CD4-depleted mice reverted the recipient CNS CD8 T-cell phenotype to that in wild-type control mice. However, memory CD8 T cells primed without CD4 T cells and transferred into infected CD4-sufficient recipients expanded less efficiently and were not sustained in the CNS, contrasting with their helped counterparts. These data suggest that CD4 T cells are dispensable for initial expansion, CNS recruitment and differentiation of primary resident memory CD8 T cells as long as the duration of antigen exposure is limited. By contrast, CD4 T cells are essential to prolong primary CD8 T-cell function in the CNS and imprint memory CD8 T cells for recall responses.

IL-27 limits central nervous system viral clearance by promoting IL-10 and enhances demyelination

IL-27 is a pleiotropic member of the IL-6 and IL-12 cytokine family composed of the IL-27p28 and the EBV-induced gene 3. IL-27 and its receptor mRNA are both upregulated in the CNS during acute encephalomyelitis induced by the JHM strain of mouse hepatitis virus (JHMV) and sustained during viral persistence. Contributions of IL-27 to viral pathogenesis were evaluated by infection of IL-27Rα-chain-deficient (IL-27Rα(-/-)) mice. The absence of IL-27 signaling accelerated virus control within the CNS associated with increased IFN-γ secreting virus-specific CD4+ and CD8+ T cells. Abrogation of IL-27 signaling did not affect virus-specific CD8+ T cell-mediated IL-10 production or cytolytic activity or Foxp3+ regulatory T cell populations. However, IL-10 production by virus-specific CD4+ T cells was reduced significantly. Despite increased T cell-mediated antiviral function in IL-27Rα(-/-) mice, the virus persisted in the CNS at similar levels as in wild-type mice. Nevertheless, IL-27Rα(-/-) mice exhibited decreased clinical disease during persistence, coincident with less severe demyelination, the hallmark tissue damage associated with JHMV infection. Overall, these data demonstrate that in contrast to viral infections at other sites, IL-27 does not play a proinflammatory role during JHMV-induced encephalomyelitis. Rather, it limits CNS inflammation and impairs control of CNS virus replication via induction of IL-10 in virus-specific CD4+ T cells. Furthermore, in contrast to its protective role in limiting CNS autoimmunity and preventing immunopathology, these data define a detrimental role of IL-27 in promoting demyelination by delaying viral control.

PKR mediated regulation of inflammation and IL-10 during viral encephalomyelitis

Double-stranded RNA-dependent protein kinase (PKR) regulates antiviral activity, immune responses, apoptosis and neurotoxicity. Gliatropic coronavirus infection induced PKR activation in infected as well uninfected cells within the central nervous system (CNS). However, PKR deficiency only modestly increased viral replication and did not affect IFN-α/β or IL-1β expression. Despite reduced Il-6, Ccl5, and Cxcl10 mRNA, protein levels remained unaltered. Furthermore, PKR deficiency selectively reduced IL-10 production in CD4, but not CD8 T cells, without affecting CNS pathology. The results demonstrate the ability of PKR to balance neuroinflammation by selectively modulating key cytokines and chemokines in CNS resident and CD4 T cells.

Role of CD25(+) CD4(+) T cells in acute and persistent coronavirus infection of the central nervous system

The influence of CD25(+)CD4(+) regulatory T cells (Treg) on acute and chronic viral infection of the central nervous system (CNS) was examined using a glial tropic murine coronavirus. Treg in the CNS were highest during initial T cell mediated virus control, decreased and then remained relatively stable during persistence. Anti-CD25 treatment did not affect CNS recruitment of inflammatory cells. Viral control was initially delayed; however, neither the kinetics of viral control nor viral persistence were affected. By contrast, the absence of Treg during the acute phase resulted in increased demyelination during viral persistence. These data suggest that CNS inflammation, progression of viral control and viral persistence are relatively independent of CD25(+)CD4(+) Treg. However, their absence during acute infection alters the ability of the host to limit tissue damage.

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.

•

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

•

IL-21 promotes peripheral and CNS humoral immunity.

•

IL-21 promotes CNS viral control and prevents mortality.

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.

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.

IFN-γ signaling to astrocytes protects from autoimmune mediated neurological disability

Demyelination and axonal degeneration are determinants of progressive neurological disability in patients with multiple sclerosis (MS). Cells resident within the central nervous system (CNS) are active participants in development, progression and subsequent control of autoimmune disease; however, their individual contributions are not well understood. Astrocytes, the most abundant CNS cell type, are highly sensitive to environmental cues and are implicated in both detrimental and protective outcomes during autoimmune demyelination. Experimental autoimmune encephalomyelitis (EAE) was induced in transgenic mice expressing signaling defective dominant-negative interferon gamma (IFN-γ) receptors on astrocytes to determine the influence of inflammation on astrocyte activity. Inhibition of IFN-γ signaling to astrocytes did not influence disease incidence, onset, initial progression of symptoms, blood brain barrier (BBB) integrity or the composition of the acute CNS inflammatory response. Nevertheless, increased demyelination at peak acute disease in the absence of IFN-γ signaling to astrocytes correlated with sustained clinical symptoms. Following peak disease, diminished clinical remission, increased mortality and sustained astrocyte activation within the gray matter demonstrate a critical role of IFN-γ signaling to astrocytes in neuroprotection. Diminished disease remission was associated with escalating demyelination, axonal degeneration and sustained inflammation. The CNS infiltrating leukocyte composition was not altered; however, decreased IL-10 and IL-27 correlated with sustained disease. These data indicate that astrocytes play a critical role in limiting CNS autoimmune disease dependent upon a neuroprotective signaling pathway mediated by engagement of IFN-γ receptors.

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. © 2012 Wiley Periodicals, Inc.

IFN-γ protects from lethal IL-17 mediated viral encephalomyelitis independent of neutrophils

BACKGROUND

The interplay between IFN-γ, IL-17 and neutrophils during CNS inflammatory disease is complex due to cross-regulatory factors affecting both positive and negative feedback loops. These interactions have hindered the ability to distinguish the relative contributions of neutrophils, Th1 and Th17 cell-derived effector molecules from secondary mediators to tissue damage and morbidity.

METHODS

Encephalitis induced by a gliatropic murine coronavirus was used as a model to assess the direct contributions of neutrophils, IFN-γ and IL-17 to virus-induced mortality. CNS inflammatory conditions were selectively manipulated by adoptive transfer of virus-primed wild-type (WT) or IFN-γ deficient (GKO) memory CD4+ T cells into infected SCID mice, coupled with antibody-mediated neutrophil depletion and cytokine blockade.

RESULTS

Transfer of GKO memory CD4+ T cells into infected SCID mice induced rapid mortality compared to recipients of WT memory CD4+ T cells, despite similar virus control and demyelination. In contrast to recipients of WT CD4+ T cells, extensive neutrophil infiltration and IL-17 expression within the CNS in recipients of GKO CD4+ T cells provided a model to directly assess their contribution(s) to disease. Recipients of WT CD4+ T cells depleted of IFN-γ did not express IL-17 and were spared from mortality despite abundant CNS neutrophil infiltration, indicating that mortality was not mediated by excessive CNS neutrophil accumulation. By contrast, IL-17 depletion rescued recipients of GKO CD4+ T cells from rapid mortality without diminishing neutrophils or reducing GM-CSF, associated with pathogenic Th17 cells in CNS autoimmune models. Furthermore, co-transfer of WT and GKO CD4+ T cells prolonged survival in an IFN-γ dependent manner, although IL-17 transcription was not reduced.

CONCLUSIONS

These data demonstrate that IL-17 mediates detrimental clinical consequences in an IFN-γ-deprived environment, independent of extensive neutrophil accumulation or GM-CSF upregulation. The results also suggest that IFN-γ overrides the detrimental IL-17 effector responses via a mechanism downstream of transcriptional regulation.

Interferon-induced Ifit2/ISG54 protects mice from lethal VSV neuropathogenesis

Interferon protects mice from vesicular stomatitis virus (VSV) infection and pathogenesis; however, it is not known which of the numerous interferon-stimulated genes (ISG) mediate the antiviral effect. A prominent family of ISGs is the interferon-induced with tetratricopeptide repeats (Ifit) genes comprising three members in mice, Ifit1/ISG56, Ifit2/ISG54 and Ifit3/ISG49. Intranasal infection with a low dose of VSV is not lethal to wild-type mice and all three Ifit genes are induced in the central nervous system of the infected mice. We tested their potential contributions to the observed protection of wild-type mice from VSV pathogenesis, by taking advantage of the newly generated knockout mice lacking either Ifit2 or Ifit1. We observed that in Ifit2 knockout (Ifit2^−/−) mice, intranasal VSV infection was uniformly lethal and death was preceded by neurological signs, such as ataxia and hind limb paralysis. In contrast, wild-type and Ifit1^−/− mice were highly protected and survived without developing such disease. However, when VSV was injected intracranially, virus replication and survival were not significantly different between wild-type and Ifit2^−/− mice. When administered intranasally, VSV entered the central nervous system through the olfactory bulbs, where it replicated equivalently in wild-type and Ifit2^−/− mice and induced interferon-β. However, as the infection spread to other regions of the brain, VSV titers rose several hundred folds higher in Ifit2^−/− mice as compared to wild-type mice. This was not caused by a broadened cell tropism in the brains of Ifit2^−/− mice, where VSV still replicated selectively in neurons. Surprisingly, this advantage for VSV replication in the brains of Ifit2^−/− mice was not observed in other organs, such as lung and liver. Pathogenesis by another neurotropic RNA virus, encephalomyocarditis virus, was not enhanced in the brains of Ifit2^−/− mice. Our study provides a clear demonstration of tissue-, virus- and ISG-specific antiviral action of interferon.

In mammals, the first line of defense against virus infection is the interferon system. Viruses induce synthesis of interferon in the infected cells and its secretion to circulation. Interferon acts upon the as yet uninfected cells and protects them from oncoming infection by inducing the synthesis of hundreds of new proteins, many of which interfere with virus replication. Vesicular stomatitis virus (VSV), a virus similar to rabies virus, is very sensitive to interferon but it is not known which interferon-induced protein inhibits its replication. Here, we have identified a single interferon-induced protein as the protector of mice from death by VSV infection. Knocking out the gene encoding this protein, Ifit2, made mice very vulnerable to neuropathogenesis caused by VSV infection; a related protein, Ifit1, did not share this property. Moreover, Ifit2 failed to protect mice from another neurotropic virus, encephalomyocarditis virus, nor was it necessary for protecting organs other than brain from infection by VSV. Our observation that a single IFN-induced protein protects a specific organ from infection by a specific virus revealed an unexpected degree of specificity of the antiviral action of IFN.

Peritoneal macrophage from male and female SJL mice differ in IL-10 expression and macrophage maturation

Injection of proteins and particulate antigens into the peritoneal cavity of male SJL mice preferentially activates T cells secreting Th2 cytokines. Identical immunizations of females activate T cells secreting Th1 cytokines. CD11b(+)F4/80(hi) LPM and CD11b(+)F4/80(lo) SPM populations were compared between naive males and females to define their role in supporting differential Th1 versus Th2 T cell activation. No sex-dependent differences in the expression of MHC class II, costimulatory molecules, and MR were detected. Immunization induced influx of CD11b(lo)F4/80(lo) cells in both sexes. CD11b(lo)F4/80(lo) cells consist predominantly of Ly6C(hi) monocytes, which mature into a Ly6C(-) SPM subset. Following immunization, equivalent frequencies of LPM had taken up antigen. However, the CD11b(lo)F4/80(lo) population, which had taken up antigen, was decreased significantly in males compared with females. Similar to naïve macrophages, antigen-positive cells in immunized males and females exhibited no phenotypic differences. However, fewer Ly6C(-)F4/80(+) cells were present in males compared with females, consistent with the reduced number of antigen-positive cells. Furthermore, CD11b(lo)F4/80(lo) cells, which had taken up antigen in males, expressed increased IL-10 and limited IL-12 mRNA compared with the predominant IL-12 mRNA expression in female-derived, antigen-positive CD11b(lo)F4/80(lo) cells. IL-10 blockade increased the frequency of Ly6C(-)F4/80(+) cells in males to the frequency in females, suggesting that preferential activation of Th2 T cells in male SJL mice is associated with increased IL-10 expression and limited antigen presentation as a result of decreased macrophage maturation under the influence of IL-10.

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.

Increased T regulatory cells lead to development of Th2 immune response in male SJL mice

SJL mice represent a mouse model in which young adult females are susceptible to autoimmune disease, while age-matched males are relatively resistant. T cells primed in female SJL mice secrete cytokines associated with a Th1 phenotype. By contrast, T cells primed in males secrete cytokines associated with a Th2 phenotype. Activation of Th2-type T cells in males vs. Th1 cells in females correlates with increased CD4(+)CD25(+) T regulatory cells (Treg) in males. T cells primed in males depleted of CD4(+)CD25(+) T cells preferentially secrete IFN-γ and decreased IL-4 and IL-10 compared to CD4(+)CD25(+) T-cell-sufficient males, suggesting that Treg influence subsequent antigen-specific cytokine secretion. Treg from males and females exhibit equivalent in vitro T-cell suppression. Treg from males express increased CTLA-4 and CD62L and preferentially secrete IL-10. These data suggest that an increased frequency of IL-10 secreting Treg in male SJL mice may contribute resistance to autoimmune disease by favoring the development of Th2 immune responses.

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.

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.

CXCR3-dependent mechanisms are required for plasma cell recruitment into the CNS (94.18)

CXCR3 and its ligands, CXCL-9, -10, and -11 regulate T cell trafficking to sites of infection, but their role in recruiting antibody secreting cells (ASC) is unknown. Neurotropic coronavirus induces CXCR3 ligands early during infection, and CXCL10 remains elevated throughout CNS persistence. In addition, lymph nodes reveal increased CXCR3 expression on ASC prior to their accumulation in the CNS. CXCR3-dependent ASC recruitment was thus studied in CXCR3-/- mice, as well as following CXCL10 neutralization. Infected CXCR3-/- mice exhibited increased mortality and clinical disease. Uncontrolled CNS virus replication compared to wt mice coincided with a decrease in lymphocyte recruitment and increased T and B cell accumulation in lymph nodes. Both total and virus-specific ASC in the brain were reduced by ~80%. In contrast, CXCL10 inhibition in wt mice did not significantly alter CNS inflammation or morbidity. Furthermore, Ab treatment increased the relative frequency of B cells, CD138+ plasma blasts, and both total and virus-specific ASC in the CNS. Nevertheless, CD4 T cell percentages remained reduced. Thus, although the absence of CXCR3 reduced ASC recruitment to the CNS, CXCL10 blockade did not inhibit protective ASC accumulation. CXCL10-independent ASC recruitment to the CNS suggests CXCL9-mediated support of local CNS humoral immunity.

Supported by NIH AI 47249

PD-L1 expression in the CNS mitigates immune-mediated damage and morbidity at the cost of viral persistence (45.5)

Interactions of programmed death 1 (PD-1) with its ligand (PD-L1) contribute to negative regulation of T cell function and dampened anti-viral responses. Following neurotropic coronavirus infection, PD-L1 is prominently upregulated on oligodendroglia, but only modestly on microglia and astrocytes. Concomitant expression of PD-1 on CNS infiltrating virus-specific CD8 T cells implicated PD-1:PD-L1 interactions in contributing to viral persistence in oligodendrocytes. This notion was confirmed by infection of PD-L1 deficient mice (B7-H1-/-). Virus is controlled more rapidly with significantly reduced numbers of infected oligodendrocytes in B7-H1-/- mice compared to wild-type. Accelerated control of infectious virus correlated with enhanced IFN-gamma production, critical to mediate anti-viral function in oligodendroglia. However, the lack of PD-L1 significantly increased disease severity and mortality. High Granzyme B levels and expression of TNF-alpha and iNOS were associated with the enhanced morbidity. Moreover, exacerbated disease in B7-H1-/- mice was coupled with greater axonal damage, while myelin loss was similar. The data demonstrate a significant contribution of PD-L1 in hindering viral clearance in glia cells, while protecting CNS tissue from immune-mediated damage. Supported by NIH NS18146.

CD4 T cell derived IFN-gamma protects from lethal encephalomyelitis by inhibiting neutrophil infiltration independent of IL-17 (45.6)

Immunodeficient SCID recipients of virus-primed memory CD4 T cells survive CNS infection by neurotropic coronavirus (JHMV), whereas IFN-γ deficient (GKO) donor CD4 T cells substantially increase morbidity. Control of virus replication and demyelination are similar in both recipients compared to uncontrolled replication without demyelination in non-treated mice. However, distinct from wt CD4 T cell recipients, GKO recipients exhibit massive CNS neutrophil infiltration, prominent CD4 T cell localization to grey matter, and detection of IL-17 mRNA. To determine the relative role of IFN-γ, IL-17 and neutrophils in disease severity, equal populations of wt and GKO CD4 T cells were co-transferred to SCID mice. Despite the vast abundance of GKO versus wt CD4 T cells accumulating in the CNS at 8 days p.i. (80 vs 20 %), disease severity and survival was similar to protected recipients of wt CD4 T cells. Furthermore, the minor population of wt CD4 T cells in mixed recipients resulted in decreased CNS neutrophil infiltration, consistent with modestly elevated IFN-γ and reduced neutrophil-attracting chemokines. However, wt CD4 T cells did not reduce CNS IL-17 mRNA levels in recipients of mixed relative to GKO donor cells. The data suggest that IFN-γ dominates over IL-17 in controlling excessive neutrophil recruitment and thus protecting JHMV infected SCID recipients from premature death.

Gender-dependent differential antigen uptake by CD11bloF4/80lo macrophages from experimental autoimmune encephalomyelitis resistant male and susceptible female SJL mice (137.2)

Macrophages (MΦ) preferentially regulate induction of antigen-specific Th1 and Th2 immune responses to protein antigens in female and male SJL mice, respectively. Alexa Flour 488 conjugated ovalbumin (OVA) and self-quenched DQ-OVA that fluoresces following lysosomal degradation, were used to determine gender-dependent differences in antigen uptake and processing in peritoneal MΦ. Antigen uptake and processing are higher in CD11bhiF4/80hi compared to the CD11bloF4/80lo MΦ of both genders. CD11bhiF4/80hi MΦ in both genders exhibit similar levels of antigen uptake, processing and MHC class II expression. By contrast, CD11bloF4/80lo MΦ from males show increased antigen uptake and decreased MHC Class II expression compared to female CD11bloF4/80lo MΦ. Despite differences in antigen uptake and MHC class II expression, CD11bloF4/80lo MΦ from both genders show similar kinetics of antigen processing. CD11bloF4/80lo MΦ from both genders presented antigen to CD4+ T cells; however, increased IL-10 was induced during T cell activation by male CD11bloF4/80lo MΦ compared to female CD11bloF4/80lo MΦ. These data demonstrate increased antigen uptake, diminished Class II expression and preferential IL-10 secretion by CD11bloF4/80lo MΦ derived from Th2 responsive EAE resistant male SJL mice. This work was supported by National Institute of Health grant NS53824.

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.

Expression of CXC chemokine ligand 10 from the mouse hepatitis virus genome results in protection from viral-induced neurological and liver disease

Using the recombinant murine coronavirus mouse hepatitis virus (MHV) expressing the T cell-chemoattractant CXCL10 (MHV-CXCL10), we demonstrate a potent antiviral role for CXCL10 in host defense. Instillation of MHV-CXCL10 into the CNS of CXCL10-deficient (CXCL10(-/-)) mice resulted in viral infection and replication in both brain and liver. Expression of virally encoded CXCL10 within the brain protected mice from death and correlated with increased infiltration of T lymphocytes, enhanced IFN-gamma secretion, and accelerated viral clearance when compared with mice infected with an isogenic control virus, MHV. Similarly, viral clearance from the livers of MHV-CXCL10-infected mice was accelerated in comparison to MHV-infected mice, yet was independent of enhanced infiltration of T lymphocytes and NK cells. Moreover, CXCL10(-/-) mice infected with MHV-CXCL10 were protected from severe hepatitis as evidenced by reduced pathology and serum alanine aminotransferase levels compared with MHV-infected mice. CXCL10-mediated protection within the liver was not dependent on CXC-chemokine receptor 2 (CXCR2) signaling as anti-CXCR2 treatment of MHV-CXCL10-infected mice did not modulate viral clearance or liver pathology. In contrast, treatment of MHV-CXCL10-infected CXCL10(-/-) mice with anti-CXCL10 Ab resulted in increased clinical disease correlating with enhanced viral recovery from the brain and liver as well as increased serum alanine aminotransferase levels. These studies highlight that CXCL10 expression promotes protection from coronavirus-induced neurological and liver disease.

Distinct regulation of MHC molecule expression on astrocytes and microglia during viral encephalomyelitis

The potential interplay of glial cells with T cells during viral induced inflammation was assessed by comparing major histocompatibility complex molecule upregulation and retention on astrocytes and microglia. Transgenic mice expressing green fluorescent protein under control of the astrocyte‐specific glial fibrillary acidic protein promoter were infected with a neurotropic coronavirus to facilitate phenotypic characterization of astrocytes and microglia using flow cytometry. Astrocytes in the adult central nervous system up‐regulated class I surface expression, albeit delayed compared with microglia. Class II was barely detectable on astrocytes, in contrast to potent up‐regulation on microglia. Maximal MHC expression in both glial cell types correlated with IFN‐γ levels and lymphocyte accumulation. Despite a decline of IFN‐γ concomitant to virus clearance, MHC molecule expression on glia was sustained. These data demonstrate distinct regulation of both class I and class II expression by microglia and astrocytes in vivo following viral induced inflammation. Furthermore, prolonged MHC expression subsequent to viral clearance implies a potential for ongoing presentation. © 2007 Wiley‐Liss, Inc.

Mycobacteria-induced Gr-1+ subsets from distinct myeloid lineages have opposite effects on T cell expansion

Similar to the regulation of vasodilation, the balance between NO and superoxide (O2-) regulates expansion of activated T cells in mice. Reduction of suppressive NO levels by O2- is essential for T cell expansion and development of autoimmunity. In mice primed with heat-killed Mycobacterium, a splenocyte population positive for Gr-1 (Ly-6G/C) is the exclusive source of both immunoregulatory free radicals. Distinct Gr-1+ cell subpopulations were separated according to Ly-6G expression. In culture with activated T cells, predominantly monocytic Ly-6G- Gr-1+ cells produced T cell-inhibitory NO but no O2-. However, mostly granulocytic Ly-6G+ cells produced O2- simultaneously but had no measurable effect on proliferation. Recombination of the two purified Gr-1+ subpopulations restored controlled regulation of T cell proliferation through NO and O2- interaction. Coculture of p47phox-/- and inducible NO synthase-/- Gr-1+ cells confirmed this intercellular interaction. These data suggest that bacterial products induce development of distinct Gr-1+ myeloid lineages, which upon stimulation by activated T cells, interact via their respective free radical products to modulate T cell expansion.

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.

Altered neuroantigen-specific cytokine secretion in a Th2 environment reduces experimental autoimmune encephalomyelitis

Activation of Th2 cells suppresses clinical experimental autoimmune encephalitis (EAE), demyelination and expression of genes associated with Th1-mediated inflammation. Despite both reduced central nervous system inflammation and IFN-gamma induced MHC class II expression by microglia, the composition of CNS infiltrates in Th2-protected mice were similar to mice with EAE. Analysis of the CNS infiltrating cells by flow cytometry suggests that protection did not correlate with abrogation of CD4+ T cell recruitment, preferential recruitment of donor Th2 cells or an increased frequency of CD25+ CD4+ T cells. By contrast, protection correlated with an increased frequency of neuroantigen-specific Th2 cells infiltrating the CNS. These data suggest that a peripheral Th2 cytokine environment influences both potential antigen presenting cells as well as recruitment and/or retention of neuroAg-specific Th2 CD4+ T cells.

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.

Viral induced demyelination

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

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.

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.

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

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

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 and bystander CD8 T cells recruited during virus-induced encephalomyelitis

Neurotropic coronavirus-induced encephalitis was used to evaluate recruitment, functional activation, and retention of peripheral bystander memory CD8+ T cells. Mice were first infected with recombinant vaccinia virus expressing a non-cross-reactive human immunodeficiency virus (HIV) epitope, designated p18. Following establishment of an endogenous p18-specific memory CD8+ T-cell population, mice were challenged with coronavirus to directly compare recruitment, longevity, and activation characteristics of both primary coronavirus-specific and bystander memory populations trafficking into the central nervous system (CNS). HIV-specific memory CD8+ T cells were recruited early into the CNS as components of the innate immune response, preceding CD8+ T cells specific for the dominant coronavirus epitope, designated pN. Although pN-specific T-cell numbers gradually exceeded bystander p18-specific CD8+ T-cell numbers, both populations peaked concurrently within the CNS. Nevertheless, coronavirus-specific CD8+ T cells were preferentially retained. By contrast, bystander CD8+ T-cell numbers declined to background numbers following control of CNS virus replication. Furthermore, in contrast to highly activated pN-specific CD8+ T cells, bystander p18-specific CD8+ T cells recruited to the site of inflammation maintained a nonactivated memory phenotype and did not express ex vivo cytolytic activity. Therefore, analysis of host CD8+ T-cell responses to unrelated infections demonstrates that bystander memory CD8+ T cells can comprise a significant proportion of CNS inflammatory cells during virus-induced encephalitis. However, transient CNS retention and the absence of activation suggest that memory bystander CD8+ T cells may not overtly contribute to pathology in the absence of antigen recognition.

Expression of matrix metalloproteinases and their tissue inhibitor during viral encephalitis

Matrix metalloproteinases (MMPs) participate in remodeling the extracellular matrix and facilitate entry of inflammatory cells into tissues. Infection of the murine central nervous system (CNS) with a neurotropic coronavirus induces encephalitis associated with increased levels of mRNA encoding MMP-3 and MMP-12. Whereas virus-induced MMP-3 expression was restricted to CNS resident astrocytes, MMP-12 mRNA was expressed by both inflammatory cells and CNS resident cells. Immunosuppression increased both MMP-3 and MMP-12 mRNA levels in CNS resident cells, suggesting that the presence of virus rather than inflammation induced protease up-regulation. MMP activity is partially regulated by a small family of genes encoding tissue inhibitors of matrix metalloproteinases (TIMPs); among the TIMPs, only TIMP-1 mRNA expression increased in the CNS following coronavirus infection. During inflammation TIMP-1 mRNA was most prominently expressed by infiltrating cells. By contrast, in the immunosuppressed host TIMP-1 mRNA was expressed by CNS resident cells. Analysis of cytokine and chemokine mRNA induction within the infected CNS of healthy and immunocompromised mice suggested a possible correlation between increased viral replication and increased levels of beta interferon, MMP-3, MMP-12, and TIMP-1 mRNA. CD4+ T cells which localize to the perivascular and subarachnoid spaces were identified as the primary source of TIMP-1 protein. By contrast, protein expression was undetectable in astrocytes or CD8+ T cells, the primary antiviral effectors that localize to the CNS parenchyma in response to infection. These data suggest that in contrast to the results seen with MMPs, inhibition of protease activity via TIMP-1 expression correlates with the differential tissue distribution of T-cell subsets during acute coronavirus-induced encephalitis.

T cell expansion is regulated by activated Gr-1+ splenocytes

CD4+ T cell proliferation depends on the balance between NO and extra-cellular superoxide (O2-). By reducing NO bio-availability, O2- promotes splenic T cell proliferation and immune response intensity. Here, we show that spleen cells from naïve mice produced neither NO nor O2- during T cell activation, but Gr-1+ splenocytes from primed mice regulated Ag-specific T cell expansion via production of both molecules. Purified splenic Gr-1+ cells included mostly granulocytes at various stages of maturation, as well as monocytes. Activation or recruitment of regulatory Gr-1+ cells was dependent on immunization with CFA. Importantly, these regulatory cells were not detected in draining lymph nodes. These data suggest that innate Gr-1+ splenic cells regulate adaptive immunity.

IL-15-independent antiviral function of primary and memory CD8+ T cells

Memory CD8⁺ T cells are comprised of CD122^hi IL-15-dependent and CD122^lo IL-15-independent subsets. Induction and retention of IL-15-independent memory CD8⁺ T cells was assessed in IL-15^−/− and wild-type (wt) mice immunized with recombinant vaccinia virus (rVV) or Sindbis virus (rSIN) vectors expressing the identical foreign epitope. Both vectors induced epitope-specific CD8⁺ T cell expansion and function, independent of IL-15. Similar kinetics of rVV clearance confirmed effective CD8⁺ T cell function in IL-15^−/− mice. CD44^hi CD122^hi CD8⁺ T cells, mainly of the CD62L^−/lo phenotype, increased more dramatically and declined more rapidly in IL-15^−/− mice, independent of the vector. Rapid IL-15-independent memory CD8⁺ T cell expansion following challenge of immune mice compensated for the limited memory CD8⁺ populations in IL-15^−/− mice. However, despite expansion and expression of potent effector function, viral clearance was delayed in the absence of IL-15, coinciding with a rapid loss in cytolytic function.

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.