Pathological changes in the spleens of gamma interferon receptor-deficient mice infected with murine gammaherpesvirus: a role for CD8 T cells

Indirect CD4+ T cell protection against mouse gamma-herpesvirus infection via interferon gamma

CD4+ T cells play a key role in γ-herpesvirus infection control. However, the mechanisms involved are unclear. Murine herpesvirus type 4 (MuHV-4) allows relevant immune pathways to be dissected experimentally in mice. In the lungs, it colonizes myeloid cells, which can express MHC class II (MHCII), and type 1 alveolar epithelial cells (AEC1), which lack it. Nevertheless, CD4+ T cells can control AEC1 infection, and this control depends on MHCII expression in myeloid cells. Interferon-gamma (IFNγ) is a major component of CD4+ T cell-dependent MuHV-4 control. Here, we show that the action of IFNγ is also indirect, as CD4+ T cell-mediated control of AEC1 infection depended on IFNγ receptor (IFNγR1) expression in CD11c+ cells. Indirect control also depended on natural killer (NK) cells. Together, the data suggest that the activation of MHCII+ CD11c+ antigen-presenting cells is key to the CD4+ T cell/NK cell protection axis. By contrast, CD8+ T cell control of AEC1 infection appeared to operate independently.

IMPORTANCE

CD4+ T cells are critical for the control of gamma-herpesvirus infection; they act indirectly, by recruiting natural killer (NK) cells to attack infected target cells. Here, we report that the CD4+ T cell/NK cell axis of gamma-herpesvirus control requires interferon-γ engagement of CD11c+ dendritic cells. This mechanism of CD4+ T cell control releases the need for the direct engagement of CD4+ T cells with virus-infected cells and may be a common strategy for host control of immune-evasive pathogens.

Latent gammaherpesvirus infection enhances type I IFN response and reduces virus spread in an influenza A virus co-infection model

Infections with persistent or latent viruses alter host immune homeostasis and have potential to affect the outcome of concomitant acute viral infections such as influenza A virus (IAV). Gammaherpesviruses establish life-long infections and require an on-going immune response to control reactivation. We have used a murine model of co-infection to investigate the response to IAV infection in mice latently infected with the gammaherpesvirus MHV-68. Over the course of infection, latently infected BALB/c mice showed less weight loss, clinical signs, pulmonary cellular infiltration and expression of inflammatory mediators than naïve mice infected with IAV and had significantly more activated CD8+ T cells in the lungs. Four days after IAV infection, virus spread in the lungs of latently infected animals was significantly lower than in naïve animals. By 7 days after IAV infection latently infected lungs express elevated levels of cytokines and chemokines indicating they are primed to respond to the secondary infection. Investigation at an early time point showed that 24 h after IAV infection co-infected animals had higher expression of IFNβ and Ddx58 (RIG-I) and a range of ISGs than mice infected with IAV alone suggesting that the type I IFN response plays a role in the protective effect. This effect was mouse strain dependent and did not occur in 129/Sv/Ev mice. These results offer insight into innate immune mechanisms that could be utilized to protect against IAV infection and highlight on-going and persistent viral infections as a significant factor impacting the severity of acute respiratory infections.

Age-associated B cells are long-lasting effectors that impede latent γHV68 reactivation

Age-associated B cells (ABCs; CD19⁺CD11c⁺T-bet⁺) are a unique population that are increased in an array of viral infections, though their role during latent infection is largely unexplored. Here, we use murine gammaherpesvirus 68 (γHV68) to demonstrate that ABCs remain elevated long-term during latent infection and express IFNγ and TNF. Using a recombinant γHV68 that is cleared following acute infection, we show that ABCs persist in the absence of latent virus, though their expression of IFNγ and TNF is decreased. With a fluorescent reporter gene-expressing γHV68 we demonstrate that ABCs are infected with γHV68 at similar rates to other previously activated B cells. We find that mice without ABCs display defects in anti-viral IgG2a/c antibodies and are more susceptible to reactivation of γHV68 following virus challenges that typically do not break latency. Together, these results indicate that ABCs are a persistent effector subset during latent viral infection that impedes γHV68 reactivation.

Age-associated B cells are long-lasting effectors that impede latent γHV68 reactivation

Age-associated B cells (ABCs; CD19⁺CD11c⁺T-bet⁺) are a unique population that are increased in an array of viral infections, though their role during latent infection is largely unexplored. Here, we use murine gammaherpesvirus 68 (γHV68) to demonstrate that ABCs remain elevated long-term during latent infection and express IFNγ and TNF. Using a recombinant γHV68 that is cleared following acute infection, we show that ABCs persist in the absence of latent virus, though their expression of IFNγ and TNF is decreased. With a fluorescent reporter gene-expressing γHV68 we demonstrate that ABCs are infected with γHV68 at similar rates to other previously activated B cells. We find that mice without ABCs display defects in anti-viral IgG2a/c antibodies and are more susceptible to reactivation of γHV68 following virus challenges that typically do not break latency. Together, these results indicate that ABCs are a persistent effector subset during latent viral infection that impedes γHV68 reactivation.

Emerging cellular and molecular determinants of idiopathic pulmonary fibrosis

Idiopathic pulmonary fibrosis (IPF), the most common form of idiopathic interstitial pneumonia, is a progressive, irreversible, and typically lethal disease characterized by an abnormal fibrotic response involving vast areas of the lungs. Given the poor knowledge of the mechanisms underpinning IPF onset and progression, a better understanding of the cellular processes and molecular pathways involved is essential for the development of effective therapies, currently lacking. Besides a number of established IPF-associated risk factors, such as cigarette smoking, environmental factors, comorbidities, and viral infections, several other processes have been linked with this devastating disease. Apoptosis, senescence, epithelial-mesenchymal transition, endothelial-mesenchymal transition, and epithelial cell migration have been shown to play a key role in IPF-associated tissue remodeling. Moreover, molecules, such as chemokines, cytokines, growth factors, adenosine, glycosaminoglycans, non-coding RNAs, and cellular processes including oxidative stress, mitochondrial dysfunction, endoplasmic reticulum stress, hypoxia, and alternative polyadenylation have been linked with IPF development. Importantly, strategies targeting these processes have been investigated to modulate abnormal cellular phenotypes and maintain tissue homeostasis in the lung. This review provides an update regarding the emerging cellular and molecular mechanisms involved in the onset and progression of IPF.

Control of B Cell Lymphoma by Gammaherpesvirus-Induced Memory CD8 T Cells

Persistent infection with gammaherpesviruses (γHV) can cause lymphomagenesis in immunocompromised patients. Murine γHV-68 (MHV-68) is an important tool for understanding immune factors contributing to γHV control; however, modeling control of γHV-associated lymphomagenesis has been challenging. Current model systems require very long incubation times or severe immune suppression, and tumor penetrance is low. In this report, we describe the generation of a B cell lymphoma on the C57BL/6 background, which is driven by the Myc oncogene and expresses an immunodominant CD8 T cell epitope from MHV-68. We determined MHV-68-specific CD8 T cells in latently infected mice use either IFN-γ or perforin/granzyme to control γHV-associated lymphoma, but perforin/granzyme is a more potent effector mechanism for lymphoma control than IFN-γ. Consistent with previous reports, CD4-depleted mice lost control of virus replication in persistently infected mice. However, control of lymphoma remained intact in the absence of CD4 T cells. Collectively, these data show the mechanisms of T cell control of B cell lymphoma in γHV-infected mice overlap with those necessary for control of virus replication, but there are also important differences. This study establishes a tool for further dissecting immune surveillance against, and optimizing adoptive T cell therapies for, γHV-associated lymphomas.

ORF6 and ORF61 Expressing MVA Vaccines Impair Early but Not Late Latency in Murine Gammaherpesvirus MHV-68 Infection

Gammaherpesviruses (γHV) are important pathogens causing persistent infections which lead to several malignancies in immunocompromised patients. Murine γHV 68 (MHV-68), a homolog to human EBV and KSHV, has been employed as a classical pathogen to investigate the molecular pathogenicity of γHV infections. γHV express distinct antigens during lytic or latent infection and antigen-specific T cells have a significant role in controlling the acute and latent viral infection, although the quality of anti-viral T cell responses required for protective immunity is not well-understood. We have generated recombinant modified vaccinia virus Ankara (recMVA) vaccines via MVA-BAC homologous recombination technology expressing MHV-68 ORF6 and ORF61 antigens encoding both MHC class I and II-restricted epitopes. After vaccination, we examined T cell responses before and after MHV-68 infection to determine their involvement in latent virus control. We show recognition of recMVA- and MHV-68-infected APC by ORF6 and ORF61 epitope-specific T cell lines in vitro. The recMVA vaccines efficiently induced MHV-68-specific CD8+ and CD4+ T cell responses after a single immunization and more pronounced after homologous prime/boost vaccination in mice. Moreover, we exhibit protective capacity of prophylactic recMVA vaccination during early latency at day 17 after intranasal challenge with MHV-68, but failed to protect from latency at day 45. Further T cell analysis indicated that T cell exhaustion was not responsible for the lack of protection by recMVA vaccination in long-term latency at day 45. The data support further efforts aiming at improved vaccine development against γHV infections with special focus on targeting protective CD4+ T cell responses.

Immune Control of γ-Herpesviruses

Vaccination against γ-herpesviruses has been hampered by our limited understanding of their normal control. Epstein–Barr virus (EBV)-transformed B cells are killed by viral latency antigen-specific CD8⁺ T cells in vitro, but attempts to block B cell infection with antibody or to prime anti-viral CD8⁺ T cells have protected poorly in vivo. The Doherty laboratory used Murid Herpesvirus-4 (MuHV-4) to analyze γ-herpesvirus control in mice and found CD4⁺ T cell dependence, with viral evasion limiting CD8⁺ T cell function. MuHV-4 colonizes germinal center (GC) B cells via lytic transfer from myeloid cells, and CD4⁺ T cells control myeloid infection. GC colonization and protective, lytic antigen-specific CD4⁺ T cells are now evident also for EBV. Subunit vaccines have protected only transiently against MuHV-4, but whole virus vaccines give long-term protection, via CD4⁺ T cells and antibody. They block infection transfer to B cells, and need include no known viral latency gene, nor any MuHV-4-specific gene. Thus, the Doherty approach of in vivo murine analysis has led to a plausible vaccine strategy for EBV and, perhaps, some insight into what CD8⁺ T cells really do.

LXR Alpha Restricts Gammaherpesvirus Reactivation from Latently Infected Peritoneal Cells

Gammaherpesviruses are ubiquitous viruses that establish lifelong infections. Importantly, these viruses are associated with numerous cancers and lymphoproliferative diseases. While risk factors for developing gammaherpesvirus-driven cancers are poorly understood, it is clear that elevated viral reactivation from latency often precedes oncogenesis. Here, we demonstrate that the liver X receptor alpha isoform (LXRα) restricts gammaherpesvirus reactivation in an anatomic-site-specific manner. We have previously demonstrated that deficiency of both LXR isoforms (α and β) leads to an increase in fatty acid and cholesterol synthesis in primary macrophage cultures, with a corresponding increase in gammaherpesvirus replication. Interestingly, expression of fatty acid synthesis genes was not derepressed in LXRα-deficient hosts, indicating that the antiviral effects of LXRα are independent of lipogenesis. Additionally, the critical host defenses against gammaherpesvirus reactivation, virus-specific CD8⁺ T cells and interferon (IFN) signaling, remained intact in the absence of LXRα. Remarkably, using a murine gammaherpesvirus 68 (MHV68) reporter virus, we discovered that LXRα expression dictates the cellular tropism of MHV68 in the peritoneal cavity. Specifically, LXRα^-/- mice exhibit reduced latency within the peritoneal B cell compartment and elevated latency within F4/80⁺ cells. Thus, LXRα restricts gammaherpesvirus reactivation through a novel mechanism that is independent of the known CD8⁺ T cell-based antiviral responses or changes in lipid synthesis and likely involves changes in the tropism of MHV68 in the peritoneal cavity.IMPORTANCE Liver X receptors (LXRs) are nuclear receptors that mediate cholesterol and fatty acid homeostasis. Importantly, as ligand-activated transcription factors, LXRs represent potential targets for the treatment of hypercholesterolemia and atherosclerosis. Here, we demonstrate that LXRα, one of the two LXR isoforms, restricts reactivation of latent gammaherpesvirus from peritoneal cells. As gammaherpesviruses are ubiquitous oncogenic agents, LXRs may represent a targetable host factor for the treatment of poorly controlled gammaherpesvirus infection and associated lymphomagenesis.

A Human Gain-of-Function STING Mutation Causes Immunodeficiency and Gammaherpesvirus-Induced Pulmonary Fibrosis in Mice

We previously generated STING N153S knock-in mice that have a human disease-associated gain-of-function mutation in STING. Patients with this mutation (STING N154S in humans) develop STING-associated vasculopathy with onset in infancy (SAVI), a severe pediatric autoinflammatory disease characterized by pulmonary fibrosis. Since this mutation promotes the upregulation of antiviral type I interferon-stimulated genes (ISGs), we hypothesized that STING N153S knock-in mice may develop more severe autoinflammatory disease in response to a virus challenge. To test this hypothesis, we infected heterozygous STING N153S mice with murine gammaherpesvirus 68 (γHV68). STING N153S mice were highly vulnerable to infection and developed pulmonary fibrosis after infection. In addition to impairing CD8⁺ T cell responses and humoral immunity, STING N153S also promoted the replication of γHV68 in cultured macrophages. In further support of a combined innate and adaptive immunodeficiency, γHV68 infection was more severe in Rag1^-/- STING N153S mice than in Rag1^-/- littermate mice, which completely lack adaptive immunity. Thus, a gain-of-function STING mutation creates a combined innate and adaptive immunodeficiency that leads to virus-induced pulmonary fibrosis.IMPORTANCE A variety of human rheumatologic disease-causing mutations have recently been identified. Some of these mutations are found in viral nucleic acid-sensing proteins, but whether viruses can influence the onset or progression of these human diseases is less well understood. One such autoinflammatory disease, called STING-associated vasculopathy with onset in infancy (SAVI), affects children and leads to severe lung disease. We generated mice with a SAVI-associated STING mutation and infected them with γHV68, a common DNA virus that is related to human Epstein-Barr virus. Mice with the human disease-causing STING mutation were more vulnerable to infection than wild-type littermate control animals. Furthermore, the STING mutant mice developed lung fibrosis similar to that of patients with SAVI. These findings reveal that a human STING mutation creates severe immunodeficiency, leading to virus-induced lung disease in mice.

Safety Profile of Good Manufacturing Practice Manufactured Interferon γ-Primed Mesenchymal Stem/Stromal Cells for Clinical Trials

Mesenchymal stem/stromal cells (MSCs) are widely studied by both academia and industry for a broad array of clinical indications. The collective body of data provides compelling evidence of the clinical safety of MSC therapy. However, generally accepted proof of therapeutic efficacy has not yet been reported. In an effort to generate a more effective therapeutic cell product, investigators are focused on modifying MSC processing protocols to enhance the intrinsic biologic activity. Here, we report a Good Manufacturing Practice‐compliant two‐step MSC manufacturing protocol to generate MSCs or interferon γ (IFNγ) primed MSCs which allows freshly expanded cells to be infused in patients on a predetermined schedule. This protocol eliminates the need to infuse cryopreserved, just thawed cells which may reduce the immune modulatory activity. Moreover, using (IFNγ) as a prototypic cytokine, we demonstrate the feasibility of priming the cells with any biologic agent. We then characterized MSCs and IFNγ primed MSCs prepared with our protocol, by karyotype, in vitro potential for malignant transformation, biodistribution, effect on engraftment of transplanted hematopoietic cells, and in vivo toxicity in immune deficient mice including a complete post‐mortem examination. We found no evidence of toxicity attributable to the MSC or IFNγ primed MSCs. Our data suggest that the clinical risk of infusing MSCs or IFNγ primed MSCs produced by our two‐step protocol is not greater than MSCs currently in practice. While actual proof of safety requires phase I clinical trials, our data support the use of either cell product in new clinical studies. Stem Cells Translational Medicine2017;6:1868–1879

Autophagy Genes Enhance Murine Gammaherpesvirus 68 Reactivation from Latency by Preventing Virus-Induced Systemic Inflammation

Host genes that regulate systemic inflammation upon chronic viral infection are incompletely understood. Murine gammaherpesvirus 68 (MHV68) infection is characterized by latency in macrophages, and reactivation is inhibited by interferon-γ (IFN-γ). Using a lysozyme-M-cre (LysMcre) expression system, we show that deletion of autophagy-related (Atg) genes Fip200, beclin 1, Atg14, Atg16l1, Atg7, Atg3, and Atg5, in the myeloid compartment, inhibited MHV68 reactivation in macrophages. Atg5 deficiency did not alter reactivation from B cells, and effects on reactivation from macrophages were not explained by alterations in productive viral replication or the establishment of latency. Rather, chronic MHV68 infection triggered increased systemic inflammation, increased T cell production of IFN-γ, and an IFN-γ-induced transcriptional signature in macrophages from Atg gene-deficient mice. The Atg5-related reactivation defect was partially reversed by neutralization of IFN-γ. Thus Atg genes in myeloid cells dampen virus-induced systemic inflammation, creating an environment that fosters efficient MHV68 reactivation from latency.

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Autophagy (Atg) genes in myeloid cells inhibit virus-triggered systemic inflammation

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Atg gene-regulated systemic inflammation inhibits herpesvirus reactivation

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Interferon-γ controls herpesvirus reactivation in the setting of Atg gene mutations

Viruses in Idiopathic Pulmonary Fibrosis. Etiology and Exacerbation

Viral infections are important contributors to exacerbation of asthma and chronic obstructive pulmonary disease; however, the role of viruses in the pathogenesis of idiopathic pulmonary fibrosis (IPF) is less clear. This likely reflects that fact that IPF acute exacerbations are defined clinically as "noninfectious," and little attention has been paid to the outcomes of patients with IPF with diagnosed infections. However, accumulating evidence suggests that infections (both bacterial and viral) may influence disease outcomes either as exacerbating agents or initiators of disease. Support for a viral role in disease initiation comes from studies demonstrating the presence of herpesviral DNA and epithelial cell stress in the lungs of asymptomatic relatives at risk for developing familial IPF. In addition, the number of studies that can associate viral (especially herpesviral) signatures in the lung with the development of IPF is steadily growing, and activated leukocyte signatures in patients with IPF provide further support for infectious processes driving IPF progression. Animal modeling has been used to better understand how a gamma herpesvirus infection can modulate the pathogenesis of lung fibrosis and has demonstrated that preceding infections appear to reprogram lung epithelial cells during latency to produce profibrotic factors, making the lung more susceptible to subsequent fibrotic insult, whereas exacerbations of existing fibrosis, or infections in susceptible hosts, involve active viral replication and are influenced by antiviral therapy. In addition, there is new evidence that bacterial burden in the lungs of patients with IPF may predict a poor prognosis.

The small noncoding RNAs (sncRNAs) of murine gammaherpesvirus 68 (MHV-68) are involved in regulating the latent-to-lytic switch in vivo

The human gammaherpesviruses Epstein-Barr virus (EBV) and Kaposi’s sarcoma-associated herpesvirus (KSHV), which are associated with a variety of diseases including tumors, produce various small noncoding RNAs (sncRNAs) such as microRNAs (miRNAs). Like all herpesviruses, they show two stages in their life cycle: lytic replication and latency. During latency, hardly any viral proteins are expressed to avoid recognition by the immune system. Thus, sncRNAs might be exploited since they are less likely to be recognized. Specifically, it has been proposed that sncRNAs might contribute to the maintenance of latency. This has already been shown in vitro, but the respective evidence in vivo is very limited. A natural model system to explore this question in vivo is infection of mice with murine gammaherpesvirus 68 (MHV-68). We used this model to analyze a MHV-68 mutant lacking the expression of all miRNAs. In the absence of the miRNAs, we observed a higher viral genomic load during late latency in the spleens of mice. We propose that this is due to a disturbed regulation of the latent-to-lytic switch, altering the balance between latent and lytic infection. Hence, we provide for the first time evidence that gammaherpesvirus sncRNAs contribute to the maintenance of latency in vivo.

Interplay of Murine Gammaherpesvirus 68 with NF-kappaB Signaling of the Host

Herpesviruses establish a chronic infection in the host characterized by intervals of lytic replication, quiescent latency, and reactivation from latency. Murine gammaherpesvirus 68 (MHV68) naturally infects small rodents and has genetic and biologic parallels with the human gammaherpesviruses (gHVs), Kaposi's sarcoma-associated herpesvirus and Epstein-Barr virus. The murine gammaherpesvirus model pathogen system provides a platform to apply cutting-edge approaches to dissect the interplay of gammaherpesvirus and host determinants that enable colonization of the host, and that shape the latent or lytic fate of an infected cell. This knowledge is critical for the development of novel therapeutic interventions against the oncogenic gHVs. The nuclear factor kappa B (NF-κB) signaling pathway is well-known for its role in the promotion of inflammation and many aspects of B cell biology. Here, we review key aspects of the virus lifecycle in the host, with an emphasis on the route that the virus takes to gain access to the B cell latency reservoir. We highlight how the murine gammaherpesvirus requires components of the NF-κB signaling pathway to promote replication, latency establishment, and maintenance of latency. These studies emphasize the complexity of gammaherpesvirus interactions with NF-κB signaling components that direct innate and adaptive immune responses of the host. Importantly, multiple facets of NF-κB signaling have been identified that might be targeted to reduce the burden of gammaherpesvirus-associated diseases.

Ablation of STAT3 in the B Cell Compartment Restricts Gammaherpesvirus Latency In Vivo

A challenging property of gammaherpesviruses is their ability to establish lifelong persistence. The establishment of latency in B cells is thought to involve active virus engagement of host signaling pathways. Pathogenic effects of these viruses during latency or following reactivation can be devastating to the host. Many cancers, including those associated with members of the gammaherpesvirus family, Kaposi’s sarcoma-associated herpesvirus and Epstein-Barr virus, express elevated levels of active host signal transducer and activator of transcription-3 (STAT3). STAT3 is activated by tyrosine phosphorylation in response to many cytokines and can orchestrate effector responses that include proliferation, inflammation, metastasis, and developmental programming. However, the contribution of STAT3 to gammaherpesvirus pathogenesis remains to be completely understood. This is the first study to have identified STAT3 as a critical host determinant of the ability of gammaherpesvirus to establish long-term latency in an animal model of disease. Following an acute infection, murine gammaherpesvirus 68 (MHV68) established latency in resident B cells, but establishment of latency was dramatically reduced in animals with a B cell-specific STAT3 deletion. The lack of STAT3 in B cells did not impair germinal center responses for immunoglobulin (Ig) class switching in the spleen and did not reduce either total or virus-specific IgG titers. Although ablation of STAT3 in B cells did not have a global effect on these assays of B cell function, it had long-term consequences for the viral load of the host, since virus latency was reduced at 6 to 8 weeks postinfection. Our findings establish host STAT3 as a mediator of gammaherpesvirus persistence.

The insidious ability of gammaherpesviruses to establish latent infections can have detrimental consequences for the host. Identification of host factors that promote viral latency is essential for understanding latency mechanisms and for therapeutic interventions. We provide the first evidence that STAT3 expression is needed for murine gammaherpesvirus 68 to establish latency in primary B cells during an active immune response to infection. STAT3 deletion in B cells does not impair adaptive immune control of the virus, but loss of STAT3 in B cells has a long-lasting impact on viral persistence. These results indicate a potential therapeutic benefit of STAT3 inhibitors for combating gammaherpesvirus latency and, thereby, associated pathologies.

CD8+ T Cell Response to Gammaherpesvirus Infection Mediates Inflammation and Fibrosis in Interferon Gamma Receptor-Deficient Mice

Idiopathic pulmonary fibrosis (IPF), one of the most severe interstitial lung diseases, is a progressive fibrotic disorder of unknown etiology. However, there is growing appreciation for the role of viral infection in disease induction and/or progression. A small animal model of multi-organ fibrosis, which involves murine gammaherpesvirus (MHV68) infection of interferon gamma receptor deficient (IFNγR-/-) mice, has been utilized to model the association of gammaherpesvirus infections and lung fibrosis. Notably, several MHV68 mutants which fail to induce fibrosis have been identified. Our current study aimed to better define the role of the unique MHV68 gene, M1, in development of pulmonary fibrosis. We have previously shown that the M1 gene encodes a secreted protein which possesses superantigen-like function to drive the expansion and activation of Vβ4+ CD8+ T cells. Here we show that M1-dependent fibrosis is correlated with heightened levels of inflammation in the lung. We observe an M1-dependent cellular infiltrate of innate immune cells with most striking differences at 28 days-post infection. Furthermore, in the absence of M1 protein expression we observed reduced CD8+ T cells and MHV68 epitope specific CD8+ T cells to the lungs-despite equivalent levels of viral replication between M1 null and wild type MHV68. Notably, backcrossing the IFNγR-/- onto the Balb/c background, which has previously been shown to exhibit weak MHV68-driven Vβ4+ CD8+ T cell expansion, eliminated MHV68-induced fibrosis-further implicating the activated Vβ4+ CD8+ T cell population in the induction of fibrosis. We further addressed the role that CD8+ T cells play in the induction of fibrosis by depleting CD8+ T cells, which protected the mice from fibrotic disease. Taken together these findings are consistent with the hypothesized role of Vβ4+ CD8+ T cells as mediators of fibrotic disease in IFNγR-/- mice.

The anti-fibrotic effect of inhibition of TGFβ-ALK5 signalling in experimental pulmonary fibrosis in mice is attenuated in the presence of concurrent γ-herpesvirus infection

TGFβ-ALK5 pro-fibrotic signalling and herpesvirus infections have been implicated in the pathogenesis and exacerbation of pulmonary fibrosis. In this study we addressed the role of TGFβ-ALK5 signalling during the progression of fibrosis in a two-hit mouse model of murine γ-herpesvirus 68 (MHV-68) infection on the background of pre-existing bleomycin-induced pulmonary fibrosis. Assessment of total lung collagen levels in combination with ex vivo micro-computed tomography (µCT) analysis of whole lungs demonstrated that MHV-68 infection did not enhance lung collagen deposition in this two-hit model but led to a persistent and exacerbated inflammatory response. Moreover, µCT reconstruction and analysis of the two-hit model revealed distinguishing features of diffuse ground-glass opacities and consolidation superimposed on pre-existing fibrosis that were reminiscent of those observed in acute exacerbation of idiopathic pulmonary fibrosis (AE-IPF). Virally-infected murine fibrotic lungs further displayed evidence of extensive inflammatory cell infiltration and increased levels of CCL2, TNFα, IL-1β and IL-10. Blockade of TGFβ-ALK5 signalling attenuated lung collagen accumulation in bleomycin-alone injured mice, but this anti-fibrotic effect was reduced in the presence of concomitant viral infection. In contrast, inhibition of TGFβ-ALK5 signalling in virally-infected fibrotic lungs was associated with reduced inflammatory cell aggregates and increased levels of the antiviral cytokine IFNγ. These data reveal newly identified intricacies for the TGFβ-ALK5 signalling axis in experimental lung fibrosis, with different outcomes in response to ALK5 inhibition depending on the presence of viral infection. These findings raise important considerations for the targeting of TGFβ signalling responses in the context of pulmonary fibrosis.

The role of myeloid cell activation and arginine metabolism in the pathogenesis of virus-induced diseases

When an antiviral immune response is generated, a balance must be reached between two opposing pathways: the production of proinflammatory and cytotoxic effectors that drive a robust antiviral immune response to control the infection and regulators that function to limit or blunt an excessive immune response to minimize immune-mediated pathology and repair tissue damage. Myeloid cells, including monocytes and macrophages, play an important role in this balance, particularly through the activities of the arginine-hydrolyzing enzymes nitric oxide synthase 2 (Nos2; iNOS) and arginase 1 (Arg1). Nitric oxide (NO) production by iNOS is an important proinflammatory mediator, whereas Arg1-expressing macrophages contribute to the resolution of inflammation and wound repair. In the context of viral infections, expression of these enzymes can result in a variety of outcomes for the host. NO has direct antiviral properties against some viruses, whereas during other virus infections NO can mediate immunopathology and/or inhibit the antiviral immune response to promote chronic infection. Arg1 activity not only has important wound healing functions but can also inhibit the antiviral immune response during some viral infections. Thus, depending on the specific virus and the tissue(s) involved, the activity of both of these arginine-hydrolyzing enzymes can either exacerbate or limit the severity of virus-induced disease. In this review, we will discuss a variety of viral infections, including HIV, SARS-CoV, LCMV, HCV, RSV, and others, where myeloid cells influence the control and clearance of the virus from the host, as well as the severity and resolution of tissue damage, via the activities of iNOS and/or Arg1. Clearly, monocyte/macrophage activation and arginine metabolism will continue to be important areas of investigation in the context of viral infections.

Establishment of murine gammaherpesvirus latency in B cells is not a stochastic event

Murid γ-herpesvirus-4 (MuHV-4) promotes polyclonal B cell activation and establishes latency in memory B cells via unclear mechanisms. We aimed at exploring whether B cell receptor specificity plays a role in B cell susceptibility to viral latency and how this is related to B cell activation. We first observed that MuHV-4-specific B cells represent a minority of the latent population, and to better understand the influence of the virus on non-MuHV-4 specific B cells we used the SW_HEL mouse model, which produce hen egg lysozyme (HEL)-specific B cells. By tracking HEL⁺ and HEL⁻ B cells, we showed that in vivo latency was restricted to HEL⁻ B cells while the two populations were equally sensitive to the virus in vitro. Moreover, MuHV-4 induced two waves of B cell activation. While the first wave was characterized by a general B cell activation, as shown by HEL⁺ and HEL⁻ B cells expansion and upregulation of CD69 expression, the second wave was restricted to the HEL⁻ population, which acquired germinal center (GC) and plasma cell phenotypes. Antigenic stimulation of HEL⁺ B cells led to the development of HEL⁺ GC B cells where latent infection remained undetectable, indicating that MuHV-4 does not benefit from acute B cell responses to establish latency in non-virus specific B cells but relies on other mechanisms of the humoral response. These data support a model in which the establishment of latency in B cells by γ-herpesviruses is not stochastic in terms of BCR specificity and is tightly linked to the formation of GCs.

Murid γ-herpesvirus-4 (MuHV-4) is a good model to study infectious mononucleosis in mice, in which the virus ultimately establishes life-long latency in B cells. Whereas several viral proteins have been shown to modulate B cell behavior, in the present study we aimed at clarifying the parameters that dictate the establishment of viral latency from the B cell perspective. Indeed, the B cell repertoire is highly diverse and it remains unknown whether latency takes place randomly in B cells. To study this question, we isolated latently infected B cells in which we observed a low frequency of virus-specific B cells, suggesting that viral latency is not restricted to this population. To better understand MuHV-4 influence on non-virus specific B cells, we then followed the fate of B cells specific for a foreign antigen, hen egg lysozyme (HEL). While in vitro experiments showed that HEL-specific B cells could be acutely infected by MuHV-4, these cells were resistant to MuHV-4 latent infection in vivo. These results suggest that while establishment of γ-herpesvirus latency is not restricted to virus-specific B cells, it does not take place randomly in B cells and relies on mechanisms that remain to be identified.

IFNγ and perforin cooperate to control infection and prevent fatal pathology during persistent gammaherpesvirus infection in mice

Infection with murine gammaherpesvirus 68 has become an accepted model for studying the virus/host interactions with regard to gammaherpesvirus infections. Previous studies using gene-deficient mice have revealed that neither IFNγ nor perforin is essential in controlling the outcome of infection or the virus load during chronic infection in C57BL/6 mice. However, pronounced multiorgan fibrosis and splenic atrophy are observed in mice lacking IFNγ or the IFNγ receptor. To study the interplay between perforin and IFNγ in controlling the virus-induced pathology and the viral load during chronic gammaherpesvirus infection, we infected IFNγ/perforin double-deficient C57BL/6 mice and followed the course of infection. While absence of perforin prevented the splenic atrophy in IFNγ-deficient mice, fibrosis did not disappear. Moreover, double-deficient mice developed extreme splenomegaly, were unable to control the viral load and displayed chronic immune activation. Thus, IFNγ and perforin act in concert to minimize pathology and control the viral load in mice chronically infected with MHV68. Furthermore, while certain aspect of the virus-induced pathology in IFNγ-deficient mice may be alleviated in double-deficient mice, other aspects are exaggerated, and the normal architecture of the spleen is completely destroyed. We believe that these findings add to the understanding of the virus/host interaction during chronic gammaherpes virus infection.

Enhanced response of T cells from murine gammaherpesvirus 68-infected mice lacking the suppressor of T cell receptor signaling molecules Sts-1 and Sts-2

The human gammaherpesviruses establish life-long infections that are associated with the development of lymphomas and neoplasms, especially in immunocompromised individuals. T cells play a crucial role in the control of gammaherpesvirus infection through multiple functions, including the direct killing of infected cells, production of cytokines such as interferon-γ (IFN-γ), and costimulation of B cells. Impaired T cell function in mice infected with murine gammaherpesvirus 68 (MHV68) leads to increased reactivation and pathologies, including a higher incidence of lymphoid hyperplasia. Here we report that the absence of Suppressor of TCR signaling −1 and −2 (Sts-1^-/-/2^-/-) during MHV68 infection leads to the generation of T cells with significantly heightened responses. Transient differences in the T and B cell response of infected Sts-1^-/-/2^-/- (Sts dKO) mice were also observed when compared to WT mice. However, these alterations in the immune response and the overall absence of Sts-1 and Sts-2 did not impact viral pathogenesis or lead to pathology. Acute lytic replication in the lungs, establishment of latency in the spleen and reactivation from latency in the spleen in the Sts dKO mice were comparable to WT mice. Our studies indicate that Sts-1 and Sts-2 are not required for the immune control of MHV68 in a normal course of gammaherpesvirus infection, but suggest that interference with negative regulators of T cell responses might be further explored as a safe and efficacious strategy to improve adoptive T cell therapy.

CD4 T cell responses in latent and chronic viral infections

The spectrum of tasks which is fulfilled by CD4 T cells in the setting of viral infections is large, ranging from support of CD8 T cells and humoral immunity to exertion of direct antiviral effector functions. While our knowledge about the differentiation pathways, plasticity, and memory of CD4 T cell responses upon acute infections or immunizations has significantly increased during the past years, much less is still known about CD4 T cell differentiation and their beneficial or pathological functions during persistent viral infections. In this review we summarize current knowledge about the differentiation, direct or indirect antiviral effector functions, and the regulation of virus-specific CD4 T cells in the setting of persistent latent or active chronic viral infections with a particular emphasis on herpes virus infections for the former and chronic lymphocytic choriomeningitis virus infection for the latter.

Ataxia telangiectasia mutated kinase controls chronic gammaherpesvirus infection

Gammaherpesviruses, such as Epstein-Barr virus (EBV), are ubiquitous cancer-associated pathogens that interact with DNA damage response, a tumor suppressor network. Chronic gammaherpesvirus infection and pathogenesis in a DNA damage response-insufficient host are poorly understood. Ataxia-telangiectasia (A-T) is associated with insufficiency of ataxia-telangiectasia mutated (ATM), a critical DNA damage response kinase. A-T patients display a pattern of anti-EBV antibodies suggestive of poorly controlled EBV replication; however, parameters of chronic EBV infection and pathogenesis in the A-T population remain unclear. Here we demonstrate that chronic gammaherpesvirus infection is poorly controlled in an animal model of A-T. Intriguingly, in spite of a global increase in T cell activation and numbers in wild-type (wt) and ATM-deficient mice in response to mouse gammaherpesvirus 68 (MHV68) infection, the generation of an MHV68-specific immune response was altered in the absence of ATM. Our finding that ATM expression is necessary for an optimal adaptive immune response against gammaherpesvirus unveils an important connection between DNA damage response and immune control of chronic gammaherpesvirus infection, a connection that is likely to impact viral pathogenesis in an ATM-insufficient host.

Murine gammaherpesvirus 68 infection protects lupus-prone mice from the development of autoimmunity

Gammaherpesvirus infections, such as those caused by EBV, have been suggested to promote the development of autoimmunity. To test this idea, we infected healthy WT and lupus-prone B6.Sle123 mice with an EBV-related and rodent-specific gammaherpesvirus, γHV68. Although acute γHV68 infection increased autoantibody levels for 4 to 6 wk, latent infection inhibited these responses for 1 y. The inhibition of autoantibody expression was only observed in B6.Sle123 females and not in males, which already displayed lower autoantibody titers. Contrary to the initial hypothesis, infection of young B6.Sle123 mice, both male and female, resulted in suppression of lymphoid activation and expansion and of glomerular inflammation and sclerosis, preserving kidney function. Moreover, γHV68 infection led to reduced autoantibody titers, lymphoid activation, and glomerular inflammation whether lupus-prone females were infected before or during disease manifestation. Finally, γHV68 infection also inhibited autoantibody production in the genetically distinct MRL/lpr lupus-prone mice. Our findings indicate that γHV68 infection strongly inhibits the development and progression of lupus-like disease in mice that spontaneously develop this condition mediating its beneficial effects at the humoral, cellular, and organ levels. The mechanisms by which the virus exerts this down-modulatory action are not yet clear, but appear to operate via reduced activation of dendritic cells, T cells, and B cells. Gammaherpesviruses coevolved with the vertebrate immune systems, establishing lifelong infections in humans and other mammals. Our findings that γHV68 infection prevents rather than exacerbates autoimmunity in mice suggest that infection with gammaherpesviruses may be protective rather than pathological in most individuals.

Strain-dependent requirement for IFN-γ for respiratory control and immunotherapy in murine gammaherpesvirus infection

Interferon-γ (IFN-γ) and perforin (pfp) are important effector mechanisms used by CD8 T cells to clear virus-infected cells. In this study, we used IFN-γ/pfp double knockout mice to address if these two effector molecules play redundant roles in the control of acute infection with murine gammaherpesvirus-68 (MHV-68) in BALB/C mice. Perforin knockout (KO) mice and wild-type mice cleared infectious virus from the lungs, even following high-dose infection. However, the IFN-γ KO and IFN-γ/pfp double knockout (DKO) groups had higher virus titers in the lungs at day 10 post-infection, and both groups had higher mortality rates. In IFN-γ/pfp DKO mice, the virus titer and mortality rate were significant higher than in IFN-γ KO mice, indicating a role for perforin in protection from disease. WT mice given IFN-γ blocking antibody also showed significantly higher viral titers. In contrast, IFN-γ KO mice on a C57BL/6 background controlled respiratory infection comparably to wild-type mice. These data show that perforin plays a redundant role in the control of virus replication, but IFN-γ plays an essential role in BALB/C mice infected with MHV-68. We conclude that there is a marked strain-dependent difference in the effector mechanisms needed to control acute MHV-68 infection between C57BL/6 and BALB/C mice. In addition we show that immune therapy that re-establishes viral control after spontaneous reactivation in CD4-deficient mice depends upon perforin in C57BL/6 mice but IFN-γ in BALB/C mice.

Malignant catarrhal fever induced by Alcelaphine herpesvirus 1 is characterized by an expansion of activated CD3+CD8+CD4- T cells expressing a cytotoxic phenotype in both lymphoid and non-lymphoid tissues

Alcelaphine herpesvirus 1 (AlHV-1) is carried by wildebeest asymptomatically. It causes a fatal lymphoproliferative disease named wildebeest-derived malignant catarrhal fever (WD-MCF) when cross-species transmitted to a variety of susceptible species of the Artiodactyla order. WD-MCF can be reproduced experimentally in rabbits. In a previous report, we demonstrated that WD-MCF induced by AlHV-1 is associated with a severe proliferation of CD8(+) T cells in the lymphoid tissues. Here, we further studied the mononuclear leukocytic populations in both the lymphoid (throughout the infection and at time of euthanasia) and non-lymphoid (at time of euthanasia) organs during WD-MCF induced experimentally in rabbits. To reach that goal, we performed multi-colour flow cytometry stainings. The results obtained demonstrate that the development of WD-MCF correlates in peripheral blood with a severe increase of CD8(+) cell percentages; and that CD3(+)CD8(+)CD4(-) T cells were the predominant cell type in both lymphoid and non-lymphoid organs at time of euthanasia. Further characterization of the mononuclear leukocytes isolated from both lymphoid and non-lymphoid tissues revealed that the CD8(+) T cells express high levels of the activation markers CD25 and CD44, produce high amount of gamma-interferon (IFN-γ) and perforin, and showed a reduction of interleukin-2 (IL-2) gene expression. These data demonstrate that the development of WD-MCF is associated with the expansion and infiltration of activated and cytotoxic CD3(+)CD8(+)CD4(-) T cells secreting high amount of IFN-γ but low IL-2.

Inhibition of NF-kappaB signaling reduces virus load and gammaherpesvirus-induced pulmonary fibrosis

Idiopathic pulmonary fibrosis (IPF) is a chronic progressive lung disorder of unknown etiology. Several studies have demonstrated an association between pulmonary infection with a herpesvirus and IPF. Based on those observations, we have developed a mouse model in which interferon (IFN)gammaR(-/-) mice infected intranasally with murine gammaherpesvirus 68 (MHV68) develop lung fibrosis. We hypothesize that viral load was a critical factor for the development of fibrosis. Because nuclear factor (NF)-kappaB signaling is required to efficiently establish gammaherpesvirus, latency we infected IFNgammaR(-/-) mice with a MHV68 virus that expresses a mutant dominant inhibitor of the NF-kappaB signaling pathway, called IkappaBalphaM. Striking differences were observed at the onset of the chronic infection, which correlated with a decreased virus load in mice infected with MHV68-IkappaBalphaM compared with mice infected with control MHV68 (MHV68-MR). IFNgammaR(-/-) mice infected with MHV68-IkappaBalphaM lacked vasculitis and fibrosis 15 to 120 days post infection. Inhibition of NF-kappaB in MHV68-infected cells of the lungs diminished the expression of the fibrocyte recruiting chemokines monocyte chemoattractant protein 1 (MCP-1) and CXCL12, ameliorated macrophage expression of markers of alternative activation, and failed to increase expression of the integrin alphavbeta6, which is implicated in the activation of the profibrotic factor TGF-beta. Thus, the inhibition of NF-kappaB signaling in the infected lung cells of IFNgammaR(-/-) mice reduces virus persistence and ameliorates profibrotic events. Host determinants of latency might therefore represent new therapeutic targets for gammaherpesvirus-associated pulmonary fibrosis.

Viral latency and its regulation: lessons from the gamma-herpesviruses

Latency is a state of cryptic viral infection associated with genomic persistence and highly restricted gene expression. Its hallmark is reversibility: under appropriate circumstances, expression of the entire viral genome can be induced, resulting in the production of infectious progeny. Among the small number of virus families capable of authentic latency, the herpesviruses stand out for their ability to produce such infections in every infected individual and for being completely dependent upon latency as a mode of persistence. Here, we review the molecular basis of latency, with special attention to the gamma-herpesviruses, in which the understanding of this process is most advanced.

The de novo methyltransferases DNMT3a and DNMT3b target the murine gammaherpesvirus immediate-early gene 50 promoter during establishment of latency

The role of epigenetic modifications in the regulation of gammaherpesvirus latency has been a subject of active study for more than 20 years. DNA methylation, associated with transcriptional silencing in mammalian genomes, has been shown to be an important mechanism in the transcriptional control of several key gammaherpesvirus genes. In particular, DNA methylation of the functionally conserved immediate-early replication and transcription activator (RTA) has been shown to regulate Epstein-Barr virus and Kaposi's sarcoma-associated herpesvirus Rta expression. Here we demonstrate that the murine gammaherpesvirus (MHV68) homolog, encoded by gene 50, is also subject to direct repression by DNA methylation, both in vitro and in vivo. We observed that the treatment of latently MHV68-infected B-cell lines with a methyltransferase inhibitor induced virus reactivation. In addition, we show that the methylation of the recently characterized distal gene 50 promoter represses activity in a murine macrophage cell line. To evaluate the role of de novo methyltransferases (DNMTs) in the establishment of these methylation marks, we infected mice in which conditional DNMT3a and DNMT3b alleles were selectively deleted in B lymphocytes. DNMT3a/DNMT3b-deficient B cells were phenotypically normal, displaying no obvious compromise in cell surface marker expression or antibody production either in naïve mice or in the context of nonviral and viral immunogens. However, mice lacking functional DNMT3a and DNMT3b in B cells exhibited hallmarks of deregulated MHV68 lytic replication, including increased splenomegaly and the presence of infectious virus in the spleen at day 18 following infection. In addition, total gene 50 transcript levels were elevated in the spleens of these mice at day 18, which correlated with the hypomethylation of the distal gene 50 promoter. However, by day 42 postinfection, aberrant virus replication was resolved, and we observed wild-type frequencies of viral genome-positive splenocytes in mice lacking functional DNMT3a and DNMT3b in B lymphocytes. The latter correlated with increased CpG methylation in the distal gene 50 promoter, which was restored to levels similar to those of littermate controls harboring functional DNMT3a and DNMT3b alleles in B lymphocytes, suggesting the existence of an alternative mechanism for the de novo methylation of the MHV68 genome. Importantly, this DNMT3a/DNMT3b-independent methylation appeared to be targeted specifically to the gene 50 promoter, as we observed that the promoters for MHV68 gene 72 (v-cyclin) and M11 (v-bcl2) remained hypomethylated at day 42 postinfection. Taken together, these data provide the first evidence of the importance of DNA methylation in regulating gammaherpesvirus RTA/gene 50 transcription during virus infection in vivo and provide insight into the hierarchy of host machinery required to establish this modification.

Pathogenesis of a model gammaherpesvirus in a natural host

Murine gammaherpesvirus 68 (MHV-68) infection of laboratory mice (Mus musculus) is an established model of gammaherpesvirus pathogenesis. The fact that M. musculus is not a host in the wild prompted us to reassess MHV-68 infection in wood mice (Apodemus sylvaticus), a natural host. Here, we report significant differences in MHV-68 infection in the two species: (i) following intranasal inoculation, MHV-68 replicated in the lungs of wood mice to levels approximately 3 log units lower than in BALB/c mice; (ii) in BALB/c mice, virus replication in alveolar epithelial cells was accompanied by a diffuse, T-cell-dominated interstitial pneumonitis, whereas in wood mice it was restricted to focal granulomatous infiltrations; (iii) within wood mice, latently infected lymphocytes were abundant in inducible bronchus-associated lymphoid tissue that was not apparent in BALB/c mice; (iv) splenic latency was established in both species, but well-delineated secondary follicles with germinal centers were present in wood mice, while only poorly delineated follicles were seen in BALB/c mice; and, perhaps as a consequence, (v) production of neutralizing antibody was significantly higher in wood mice. These differences highlight the value of this animal model in the study of MHV-68 pathogenesis.

Immune control of mammalian gamma-herpesviruses: lessons from murid herpesvirus-4

Many acute viral infections can be controlled by vaccination; however, vaccinating against persistent infections remains problematic. Herpesviruses are a classic example. Here, we discuss their immune control, particularly that of gamma-herpesviruses, relating the animal model provided by murid herpesvirus-4 (MuHV-4) to human infections. The following points emerge: (i) CD8(+) T-cell evasion by herpesviruses confers a prominent role in host defence on CD4(+) T cells. CD4(+) T cells inhibit MuHV-4 lytic gene expression via gamma-interferon (IFN-gamma). By reducing the lytic secretion of immune evasion proteins, they may also help CD8(+) T cells to control virus-driven lymphoproliferation in mixed lytic/latent lesions. Similarly, CD4(+) T cells specific for Epstein-Barr virus lytic antigens could improve the impact of adoptively transferred, latent antigen-specific CD8(+) T cells. (ii) In general, viral immune evasion necessitates multiple host effectors for optimal control. Thus, subunit vaccines, which tend to prime single effectors, have proved less successful than attenuated virus mutants, which prime multiple effectors. Latency-deficient mutants could make safe and effective gamma-herpesvirus vaccines. (iii) The antibody response to MuHV-4 infection helps to prevent disease but is suboptimal for neutralization. Vaccinating virus carriers with virion fusion complex components improves their neutralization titres. Reducing the infectivity of herpesvirus carriers in this way could be a useful adjunct to vaccinating naive individuals with attenuated mutants.

A novel Cre recombinase imaging system for tracking lymphotropic virus infection in vivo

Background

Detection, isolation, and identification of individual virus infected cells during long term infection are critical to advance our understanding of mechanisms of pathogenesis for latent/persistent viruses. However, current approaches to study these viruses in vivo have been hampered by low sensitivity and effects of cell-type on expression of viral encoded reporter genes. We have designed a novel Cre recombinase (Cre)-based murine system to overcome these problems, and thereby enable tracking and isolation of individual in vivo infected cells.

Methodology/Principal findings

Murine gammaherpesvirus 68 (MHV-68) was used as a prototypic persistent model virus. A Cre expressing recombinant virus was constructed and characterised. The virus is attenuated both in lytic virus replication, producing ten-fold lower lung virus titres than wild type virus, and in the establishment of latency. However, despite this limitation, when the sEGFP7 mouse line containing a Cre-activated enhanced green fluorescent protein (EGFP) was infected with the Cre expressing virus, sites of latent and persistent virus infection could be identified within B cells and macrophages of the lymphoid system on the basis of EGFP expression. Importantly, the use of the sEGFP7 mouse line which expresses high levels of EGFP allowed individual virus positive cells to be purified by FACSorting. Virus gene expression could be detected in these cells. Low numbers of EGFP positive cells could also be detected in the bone marrow.

Conclusions/Significance

The use of this novel Cre-based virus/mouse system allowed identification of individual latently infected cells in vivo and may be useful for the study and long-term monitoring of other latent/persistent virus infections.

gamma-Herpesvirus-induced protection against bacterial infection is transient

Herpesviruses are widely disseminated in the population and establish lifelong latency, which is associated with a variety of pathological consequences. A recent report showed that mice latently infected with either murine gamma-herpesvirus-68 (gammaHV68) or murine cytomegalovirus (mCMV), mouse pathogens genetically similar to the human herpesviruses, Epstein-Barr virus, Kaposi's sarcoma-associated herpesvirus, and cytomegalovirus, had enhanced resistance to subsequent bacterial infection, suggesting protective as well as deleterious effects of latency. Here we confirm that latent gammaHV68 infection confers protection against subsequent infection with Listeria monocytogenes. However, the effect is transient, lasting only a few months.

Murid herpesvirus-4 induces chronic inflammation of intrahepatic bile ducts in mice deficient in gamma-interferon signalling

AIM

Infection of gamma interferon receptor defective mice with murid herpesvirus-4 also known as murine gammaherpesvirus-68 results in multi-organ fibrosis. In this paper we characterise the pathological changes occurring in the liver in this model.

METHODS

Standard immunohistochemistry and in situ hybridisation techniques were used to identify the cellular changes and the presence of virus at different times post infection.

RESULTS

In liver sections from infected gamma interferon receptor defective mice sampled on day 16 to at least day 120, 79% showed proliferating intrahepatic bile ducts associated with a chronic mononuclear cell inflammation. Only 8% of wild type mice showed similar lesions. Coincident with the inflammatory response bile duct epithelial cells were positive for arginase 1. Around day 50 post infection onwards focal fibrotic lesions appeared in approximately 30% of gamma interferon receptor defective mice resulting in destruction of intrahepatic bile ducts. In contrast to the chronic persisting inflammatory response the presence of virus infected cells were only observed between day 12-20 post-infection.

CONCLUSION

Infection of gamma interferon receptor defective mice with a murine gammaherpesvirus initiates a chronic persisting inflammatory response with a pathological profile similar to the human fibrotic liver disorder Primary Sclerosing Cholangitis.

A gammaherpesvirus-secreted activator of Vbeta4+ CD8+ T cells regulates chronic infection and immunopathology

Little is known about herpesvirus modulation of T cell activation in latently infected individuals or the implications of such for chronic immune disorders. Murine gammaherpesvirus 68 (MHV68) elicits persistent activation of CD8(+) T cells bearing a Vbeta4(+) T cell receptor (TCR) by a completely unknown mechanism. We show that a novel MHV68 protein encoded by the M1 gene is responsible for Vbeta4(+) CD8(+) T cell stimulation in a manner reminiscent of a viral superantigen. During infection, M1 expression induces a Vbeta4(+) effector T cell response that resists functional exhaustion and appears to suppress virus reactivation from peritoneal cells by means of long-term interferon-gamma (IFNgamma) production. Mice lacking an IFNgamma receptor (IFNgammaR(-/-)) fail to control MHV68 replication, and Vbeta4(+) and CD8(+) T cell activation by M1 instead contributes to severe inflammation and multiorgan fibrotic disease. Thus, M1 manipulates the host CD8(+) T cell response in a manner that facilitates latent infection in an immunocompetent setting, but promotes disease during a dysregulated immune response. Identification of a viral pathogenecity determinant with superantigen-like activity for CD8(+) T cells broadens the known repertoire of viral immunomodulatory molecules, and its function illustrates the delicate balance achieved between persistent viruses and the host immune response.

Murine γ-Herpesvirus 68 Limits Naturally Occurring CD4+CD25+T Regulatory Cell Activity following Infection

During microbial infections, naturally occurring CD4+CD25+ T regulatory cells can suppress protective host responses or they can limit pathogen-induced inflammatory responses. The particular role played by these cells seems to depend upon the infectious agent being investigated. Gamma-herpesviruses are efficacious pathogens which are well-known for their ability to induce lymphoproliferative disease and to establish latency in the host. However, no studies have investigated the importance of naturally occurring CD4+CD25+ T regulatory cells during infection with these viruses. Using the murine model of gamma-herpesvirus infection, murine gamma-herpesvirus 68 (gammaHV-68), we were surprised to find that levels of the CD4+CD25+ T regulatory cell transcript, FoxP3, continued to decrease as viral latency increased and as the leukocytosis phase of the disease progressed. Consistent with these results, the decrease in FoxP3 protein expression followed similar kinetics. Along with the reduced expression of this regulatory T cell marker, we also observed diminished CD4+CD25+ T regulatory cell activity in these cells isolated from gammaHV-68-infected animals. Dendritic cells infected in vitro with gammaHV-68 did not alter the ability of normal CD4+CD25+ regulatory T cells to limit the proliferation of CD4+ Th cells following stimulation. Taken together, these studies demonstrate a decreased presence and activity of CD4+CD25+ T regulatory cells during the mononucleosis-like phase of this viral infection. These alterations in naturally occurring T regulatory cell function may help to explain the dysregulation of the host's immune response which allows the uncontrolled expansion of leukocytes as viral latency is established.

CD80 and CD86 control antiviral CD8+ T-cell function and immune surveillance of murine gammaherpesvirus 68

The interactions between CD80 and CD86 on antigen-presenting cells and CD28 on T cells serve as an important costimulatory signal in the activation of T cells. Although the simplistic two-signal hypothesis has been challenged in recent years by the identification of different costimulators, this classical pathway has been shown to significantly impact antiviral humoral and cellular immune responses. How the CD80/CD86-CD28 pathway affects the control of chronic or latent infections has been less well characterized. In this study, we investigated its role in antiviral immune responses against murine gammaherpesvirus 68 (MHV-68) and immune surveillance using CD80/CD86(-/-) mice. In the absence of CD80/CD86, primary antiviral CD8(+) T-cell responses and the induction of neutralizing antibodies were severely impaired. During long-term immune surveillance, the virus-specific CD8(+) T cells were impaired in IFN-gamma production and secondary expansion and exhibited an altered phenotype. Surprisingly, a low level of viral reactivation in the lung was observed, and this effect was independent of CD28 and CTLA-4. Thus, CD80 and CD86, signaling through CD28 and possibly another unidentified receptor, are required for optimal immune surveillance and antiviral immune responses to murine gammaherpesvirus.

Immune mechanisms in murine gammaherpesvirus-68 infection

The murine gamma-herpesvirus-68 (MHV-68) is a relative of the Kaposi's sarcoma-associated herpesvirus (KSHV) and Epstein-Barr virus (EBV) that infects mice. All these gamma-herpesviruses are subject to immune control, but limit the impact of this control through immune evasion. Molecular evasion mechanisms have been described in abundance. However, we can only speculate what EBV and KSHV immune evasion contributes to the viral lifecycle. With MHV-68, we can analyze in vivo the contribution of immunological and virological gene expression to pathogenesis. While the physiology of infection seems quite well conserved between these viruses, the pathologies associated with immune suppression are obviously very different. MHV-68 is therefore more suited to uncovering the basic biology of gamma-herpesvirus infection than to testing disease interventions. Nevertheless, it may make some useful predictions about effective strategies of vaccination and infection control. This review aims to outline our current state of knowledge and to highlight some limitations of the MHV-68 model as it stands, in the hope of stimulating constructive progress.

Murine gammaherpesvirus 68 open reading frame 45 plays an essential role during the immediate-early phase of viral replication

Murine gammaherpesvirus 68 (MHV-68) has been developed as a model for the human gammaherpesviruses Epstein-Barr virus and human herpesvirus 8/Kaposi's sarcoma-associated herpesvirus (HHV-8/KSHV), which are associated with several types of human diseases. Open reading frame 45 (ORF45) is conserved among the members of the Gammaherpesvirinae subfamily and has been suggested to be a virion tegument protein. The repression of ORF45 expression by small interfering RNAs inhibits MHV-68 viral replication. However, the gene product of MHV-68 ORF45 and its function have not yet been well characterized. In this report, we show that MHV-68 ORF45 is a phosphorylated nuclear protein. We constructed an ORF45-null MHV-68 mutant virus (45STOP) by the insertion of translation termination codons into the portion of the gene encoding the N terminus of ORF45. We demonstrated that the ORF45 protein is essential for viral gene expression immediately after the viral genome enters the nucleus. These defects in viral replication were rescued by providing ORF45 in trans or in an ORF45-null revertant (45STOP.R) virus. Using a transcomplementation assay, we showed that the function of ORF45 in viral replication is conserved with that of its KSHV homologue. Finally, we found that the C-terminal 23 amino acids that are highly conserved among the Gammaherpesvirinae subfamily are critical for the function of ORF45 in viral replication.

The role of IL-12, IL-23 and IFN-gamma in immunity to viruses

IL-12, IL-23 and IFN-γ form a loop and have been thought to play a crucial role against infectious viruses, which are the prototype of “intracellular” pathogens. In the last 10 years, the generation of knock-out (KO) mice for genes that control IL-12/IL-23-dependent IFN-γ-dependent mediated immunity (STAT1, IFN-γR1, IFNγR2, IL-12p40 and IL-12Rβ1) and the identification of patients with spontaneous germline mutations in these genes has led to a re-examination of the role of these cytokines in anti-viral immunity. We here review viral infections in mice and humans with genetic defects in the IL-12/IL-23-IFN-γ axis. A comparison of the phenotypes observed in KO mice and deficient patients suggests that the human IL-12/IL-23-IFN-γ axis plays a redundant role in immunity to most viruses, whereas its mouse counterparts play a more important role against several viruses.

Critical role of CD4 T cells in an antibody-independent mechanism of vaccination against gammaherpesvirus latency

We have previously demonstrated that it is possible to effectively vaccinate against long-term murine gammaherpesvirus 68 (gamma HV68) latency by using a reactivation-deficient virus as a vaccine (S. A. Tibbetts, J. S. McClellan, S. Gangappa, S. H. Speck, and H. W. Virgin IV, J. Virol. 77:2522-2529, 2003). Immune antibody was capable of recapitulating aspects of this vaccination. This led us to determine whether antibody is required for vaccination against latency. Using mice lacking antigen-specific antibody responses, we demonstrate here that antibody and B cells are not required for vaccination against latency. We also show that surveillance of latent infection in normal animals depends on CD4 and CD8 T cells, suggesting that T cells might be capable of preventing the establishment of latency. In the absence of an antibody response, CD4 T cells but not CD8 T cells are required for effective vaccination against latency in peritoneal cells, while either CD4 or CD8 T cells can prevent the establishment of splenic latency. Therefore, CD4 T cells play a critical role in immune surveillance of gammaherpesvirus latency and can mediate vaccination against latency in the absence of antibody responses.

Identification of a region of the virus genome involved in murine gammaherpesvirus 68-induced splenic pathology

Infection with the murine gammaherpesvirus MHV-68 has profound effects on splenic and mediastinal lymph node pathology in mice which lack the interferon-gamma receptor (IFN-gamma R(-/-)). In these mice MHV-68 infection causes fibrosis and loss of lymphocytes in the spleen and the mediastinal lymph node as well as interstitial pulmonary fibrosis and fibrotic changes in the liver. The changes are associated with transient elevated latent virus loads in the spleen. Four independent virus mutants with insertions and/or deletions in the left end of the genome fail to induce the pathological changes and establish latency at normal levels in the spleen. The data indicate that the pathology does not correlate with any of the known genes encoded within this region of the genome, genes M1-M4 and the eight vtRNAs. Northern analysis of mRNAs transcribed by wild-type and mutant viruses shows that at least two uncharacterized transcripts are encoded within this region. These transcripts are absent in the mutant viruses and are candidates for the virus genes responsible for the aberrant pathology in IFN-gamma R(-/-) mice.

Comparison of the Rta/Orf50 transactivator proteins of gamma-2-herpesviruses

The viral immediate-early transactivator Rta/Orf50 is necessary and sufficient to initiate Kaposi's sarcoma-associated herpesvirus/human herpesvirus 8 (KSHV/HHV-8) reactivation from latently infected cells. Since Rta/Orf50 is conserved among all known gamma-2-herpesviruses, we investigated whether the murine gamma-68-herpesvirus (MHV-68) and rhesus monkey rhadinovirus (RRV) homologs can functionally substitute for KSHV Rta/Orf50. (i) Our comparison of 12 KSHV promoters showed that most responded to all three Rta/Orf50proteins, but three promoters (vGPCR, K8, and gB) responded only to the KSHV Rta/Orf50 transactivator. Overall, the activation of KSHV promoters was higher with KSHV Rta than with the RRV and MHV-68 Rta. (ii) Only the primate Rta/Orf50 homologs were able to interfere with human p53-depedent transcriptional activation. (iii) Transcriptional profiling showed that the KSHV Rta/Orf50 was more efficient than it's homologs in inducing KSHV lytic transcription from the latent state. These results suggest that the core functionality of Rta/Orf50 is conserved and independent of its host, but the human protein has evolved additional, human-specific capabilities.

Cytokines and Costimulatory Molecules in the Immune Response to Murine Gammaherpesvirus-68

Murine gammaherpesvirus 68 (MHV-68) infection of mice provides a useful small animal model for studying gammaherpesvirus pathogenesis and immunity. Recent work has elucidated the cytokine and chemokine profiles during MHV-68 infection and has identified some of the costimulatory interactions that are important for an effective immune response to this virus. Several themes emerge from this work. There is a differential requirement for certain cytokines and costimulatory molecules in the acute and long-term control of MHV-68, and for control of the virus in different anatomical sites. CD4 T cell help is not required for short-term control of MHV-68 in the lung by cytotoxic CD8 T cells, but is essential for effective long-term control. Stimulation via CD40 is an important component of this CD4 T cell help, and interestingly, some of its effects appear to be independent of CD28. MHV-68 infection also increases the expression of several chemokines, which could potentially play important roles in leukocyte trafficking to sites of infection. However, to counter this response, MHV-68 has evolved strategies that enable it to evade or subvert the host chemokine system. Studying the role of cytokines and costimulatory molecules in immunity to MHV-68 may provide useful insights for the development of agents to control gammaherpesviruses that cause human disease.

Murine γ-Herpesvirus-68-Induced IL-12 Contributes to the Control of Latent Viral Burden, but Also Contributes to Viral-Mediated Leukocytosis

Early IFN-alpha/beta production, followed by the development of a viral-specific CTL response, are critical factors in limiting the level of murine gamma-herpesvirus-68 (gammaHV-68) infection. Development of a long-lived CTL response requires T cell help, and these CTLs most likely function to limit the extent of infection following reactivation. The importance of IL-12 in the development and/or activity of Th1 cells and CTLs is well documented, and we investigated the kinetics and magnitude of gammaHV-68-induced IL-12 production. Following intranasal infection, IL-12 and IL-23 mRNA expression was up-regulated in lung and spleen and lung, respectively, followed by increased levels of IL-12p40 in lung homogenates and sera. Exposure of cultured macrophages or dendritic cells to gammaHV-68 induced secretion of IL-12, suggesting that these cells might be responsible for IL-12 production in vivo. gammaHV-68 infection of mice made genetically deficient in IL-12p40 expression (IL-12p40(-/-)) resulted in a leukocytosis and splenomegaly that was significantly less than that observed in syngeneic C57BL/6 mice. IL-12p40(-/-) mice showed increased levels of infectious virus in the lung, but only at day 9 postinfection. Increased levels of latent virus in the spleen at day 15 postinfection were also observed in IL-12p40(-/-) mice when compared with syngeneic C57BL/6 mice. An overall reduction in gammaHV-68-induced IFN-gamma production was observed in IL-12p40(-/-) mice, suggesting that most of the viral-induced IFN-gamma in C57BL/6 mice was IL-12 dependent. Taken together, these results suggest that gammaHV-68-induced IL-12 contributes to the pathophysiology of viral infection while also functioning to limit viral burden.

Maintenance of gammaherpesvirus latency requires viral cyclin in the absence of B lymphocytes

Gammaherpesviruses establish a life-long chronic infection that is tightly controlled by the host immune response. We previously demonstrated that viruses lacking the gammaherpesvirus 68 (gammaHV68) viral cyclin (v-cyclin) exhibited a severe defect in reactivation from latency and persistent replication. In this analysis of chronic infection, we demonstrate that the v-cyclin is required for gammaHV68-associated mortality in B-cell-deficient mice. Furthermore, we identify the v-cyclin as the first gene product required for maintenance of gammaherpesvirus latency in vivo in the absence of B lymphocytes. While the v-cyclin was necessary for maintenance of latency in the absence of B cells, maintenance of v-cyclin-deficient viruses was equivalent to that of wild-type gammaHV68 in the presence of B cells. These results support a model in which maintenance of chronic gammaHV68 infection requires v-cyclin-dependent reactivation and reseeding of non-B-cell latency reservoirs in the absence of B cells and raise the possibility that B cells represent a long-lived latency reservoir maintained independently of reactivation. These results highlight distinct mechanisms for the maintenance of chronic infection in immunocompetent and B-cell-deficient mice and suggest that the different latency reservoirs have distinct gene requirements for the maintenance of latency.

A battle for survival: immune control and immune evasion in murine gamma-herpesvirus-68 infection

CD8(+) T cells are generally considered a key defence against herpesviruses. The murine gamma-herpesvirus-68 encodes two proteins that limit their efficacy. M3 neutralizes chemokines, while K3 downregulates MHC class I glycoproteins. The consequence of this evasion is that CD4(+) T cells are essential to the control of persistent infection.