Resistance of lymphocytic choriomeningitis virus to alpha/beta interferon and to gamma interferon

High-Affinity-Mediated Viral Entry Triggers Innate Affinity Escape Resulting in Type I IFN Resistance and Impaired T Cell Immunity

Increased receptor binding affinity may allow viruses to escape from Ab-mediated inhibition. However, how high-affinity receptor binding affects innate immune escape and T cell function is poorly understood. In this study, we used the lymphocytic choriomeningitis virus (LCMV) murine infection model system to create a mutated LCMV exhibiting higher affinity for the entry receptor α-dystroglycan (LCMV-GPH155Y). We show that high-affinity receptor binding results in increased viral entry, which is associated with type I IFN (IFN-I) resistance, whereas initial innate immune activation was not impaired during high-affinity virus infection in mice. Consequently, IFN-I resistance led to defective antiviral T cell immunity, reduced type II IFN, and prolonged viral replication in this murine model system. Taken together, we show that high-affinity receptor binding of viruses can trigger innate affinity escape including resistance to IFN-I resulting in prolonged viral replication.

Themis suppresses the effector function of CD8+ T cells in acute viral infection

CD8+ T cells play a central role in antiviral immune responses. Upon infection, naive CD8+ T cells differentiate into effector cells to eliminate virus-infected cells, and some of these effector cells further differentiate into memory cells to provide long-term protection after infection is resolved. Although extensively investigated, the underlying mechanisms of CD8+ T-cell differentiation remain incompletely understood. Themis is a T-cell-specific protein that plays critical roles in T-cell development. Recent studies using Themis T-cell conditional knockout mice also demonstrated that Themis is required to promote mature CD8+ T-cell homeostasis, cytokine responsiveness, and antibacterial responses. In this study, we used LCMV Armstrong infection as a probe to explore the role of Themis in viral infection. We found that preexisting CD8+ T-cell homeostasis defects and cytokine hyporesponsiveness do not impair viral clearance in Themis T-cell conditional knockout mice. Further analyses showed that in the primary immune response, Themis deficiency promoted the differentiation of CD8+ effector cells and increased their TNF and IFNγ production. Moreover, Themis deficiency impaired memory precursor cell (MPEC) differentiation but promoted short-lived effector cell (SLEC) differentiation. Themis deficiency also enhanced effector cytokine production in memory CD8+ T cells while impairing central memory CD8+ T-cell formation. Mechanistically, we found that Themis mediates PD-1 expression and its signaling in effector CD8+ T cells, which explains the elevated cytokine production in these cells when Themis is disrupted.

Recombinant lymphocytic choriomeningitis virus-based vaccine vector protects type I interferon receptor deficient mice from viral challenge

Reverse genetically engineered recombinant lymphocytic choriomeningitis virus (rLCMV) is a novel vaccine vector platform. Here, we investigate the safety and efficacy of rLCMV in mice lacking a functional type I interferon system with high susceptibility to viral infections. Propagation-deficient rLCMV vector expressing ovalbumin as a model antigen is cleared from type I interferon receptor-deficient mice (Ifnar^-/-) within seven days post vaccination. In Ifnar^-/-, induction of vaccine antigen specific T cells is delayed compared to wild type animals. However, immunization of Ifnar^-/- results in potent memory formation and generates multifunctional cytotoxic CD8⁺ T cells. Most importantly, Ifnar^-/- vaccinated with rLCMV are protected from a challenge with the aggressive LCMV Clone 13. Our data provide evidence for an excellent safety profile with maintained efficacy in immunocompromised animals.

Efficient Delivery and Replication of Oncolytic Virus for Successful Treatment of Head and Neck Cancer

Head and neck cancer has been treated by a combination of surgery, radiation, and chemotherapy. In recent years, the development of immune checkpoint inhibitors (ICIs) has made immunotherapy a new treatment method. Oncolytic virus (OV) therapy selectively infects tumor cells with a low-pathogenic virus, lyses tumor cells by the cytopathic effects of the virus, and induces anti-tumor immunity to destroy tumors by the action of immune cells. In OV therapy for head and neck squamous cell carcinoma (HNSCC), viruses, such as herpes simplex virus type 1 (HSV-1), vaccinia virus, adenovirus, reovirus, measles virus, and vesicular stomatitis virus (VSV), are mainly used. As the combined use of mutant HSV-1 and ICI was successful for the treatment of melanoma, studies are underway to combine OV therapy with radiation, chemotherapy, and other types of immunotherapy. In such therapy, it is important for the virus to selectively replicate in tumor cells, and to express the viral gene and the introduced foreign gene in the tumor cells. In OV therapy for HNSCC, it may be useful to combine systemic and local treatments that improve the delivery and replication of the inoculated oncolytic virus in the tumor cells.

Non-apoptotic TRAIL function modulates NK cell activity during viral infection

The role of death receptor signaling for pathogen control and infection-associated pathogenesis is multifaceted and controversial. Here, we show that during viral infection, tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) modulates NK cell activity independently of its pro-apoptotic function. In mice infected with lymphocytic choriomeningitis virus (LCMV), Trail deficiency led to improved specific CD8+ T-cell responses, resulting in faster pathogen clearance and reduced liver pathology. Depletion experiments indicated that this effect was mediated by NK cells. Mechanistically, TRAIL expressed by immune cells positively and dose-dependently modulates IL-15 signaling-induced granzyme B production in NK cells, leading to enhanced NK cell-mediated T cell killing. TRAIL also regulates the signaling downstream of IL-15 receptor in human NK cells. In addition, TRAIL restricts NK1.1-triggered IFNγ production by NK cells. Our study reveals a hitherto unappreciated immunoregulatory role of TRAIL signaling on NK cells for the granzyme B-dependent elimination of antiviral T cells.

Novel Functions of IFI44L as a Feedback Regulator of Host Antiviral Responses

We describe a novel function for the interferon (IFN)-induced protein 44-like (IFI44L) gene in negatively modulating innate immune responses induced after virus infections. Furthermore, we show that decreasing IFI44L expression impairs virus production and that IFI44L expression negatively modulates the antiviral state induced by an analog of double-stranded RNA (dsRNA) or by IFN treatment. The mechanism likely involves the interaction of IFI44L with cellular FK506-binding protein 5 (FKBP5), which in turn interacts with kinases essential for type I and III IFN responses, such as inhibitor of nuclear factor kappa B (IκB) kinase alpha (IKKα), IKKβ, and IKKε. Consequently, binding of IFI44L to FKBP5 decreased interferon regulatory factor 3 (IRF-3)-mediated and nuclear factor kappa-B (NF-κB) inhibitor (IκBα)-mediated phosphorylation by IKKε and IKKβ, respectively. According to these results, IFI44L is a good target for treatment of diseases associated with excessive IFN levels and/or proinflammatory responses and for reduction of viral replication.IMPORTANCE Excessive innate immune responses can be deleterious for the host, and therefore, negative feedback is needed. Here, we describe a completely novel function for IFI44L in negatively modulating innate immune responses induced after virus infections. In addition, we show that decreasing IFI44L expression impairs virus production and that IFI44L expression negatively modulates the antiviral state induced by an analog of dsRNA or by IFN treatment. IFI44L binds to the cellular protein FKBP5, which in turn interacts with kinases essential for type I and III IFN induction and signaling, such as the kinases IKKα, IKKβ, and IKKε. IFI44L binding to FKBP5 decreased the phosphorylation of IRF-3 and IκBα mediated by IKKε and IKKβ, respectively, providing an explanation for the function of IFI44L in negatively modulating IFN responses. Therefore, IFI44L is a candidate target for reducing virus replication.

Interferon-Induced Protein 44 Interacts with Cellular FK506-Binding Protein 5, Negatively Regulates Host Antiviral Responses, and Supports Virus Replication

Using multiple viral systems, and performing silencing approaches, overexpression approaches, and experiments in knockout cells, we report, for the first time, that interferon (IFN)-induced protein 44 (IFI44) positively affects virus production and negatively modulates innate immune responses induced after viral infections. Moreover, IFI44 is able to rescue poly(I·C)- and IFN-mediated inhibition of virus growth. Furthermore, we report a novel interaction of IFI44 with the cellular factor FK506-binding protein 5 (FKBP5), which binds to cellular kinases such as the inhibitor of nuclear factor kappa B (IκB) kinases (IKKα, IKKβ, and IKKε). Importantly, in the presence of FKBP5, IFI44 decreases the ability of IKKβ to phosphorylate IκBα and the ability of IKKε to phosphorylate interferon regulatory factor 3 (IRF-3), providing a novel mechanism for the function of IFI44 in negatively modulating IFN responses. Remarkably, these new IFI44 functions may have implications for diseases associated with excessive immune signaling and for controlling virus infections mediated by IFN responses.IMPORTANCE Innate immune responses mediated by IFN and inflammatory cytokines are critical for controlling virus replication. Nevertheless, exacerbated innate immune responses could be detrimental for the host and feedback mechanisms are needed to maintain the cellular homeostasis. In this work, we describe a completely novel function for IFI44 in negatively modulating the innate immune responses induced after viral infections. We show that decreasing IFI44 expression by using small interfering RNAs (siRNAs) or by generating knockout (KO) cells impairs virus production and increases the levels of IFN responses. Moreover, we report a novel interaction of IFI44 with the cellular protein FKBP5, which in turn interacts with kinases essential for type I and III IFN induction and signaling, such as the inhibitor of nuclear factor kappa B (IκB) kinases IKKα, IKKβ, and IKKε. Our data indicate that binding of IFI44 to FKBP5 decreased the phosphorylation of IRF-3 and IκBα mediated by IKKε and IKKβ, respectively, providing a likely explanation for the function of IFI44 in negatively modulating IFN responses. These results provide new insights into the induction of innate immune responses and suggest that IFI44 is a new potential antiviral target for reducing virus replication.

Monocyte-Derived Dendritic Cells Dictate the Memory Differentiation of CD8+ T Cells During Acute Infection

Monocyte-derived dendritic cells (moDCs) have been shown to robustly expand during infection; however, their roles in anti-infectious immunity remain unclear. Here, we found that moDCs were dramatically increased in the secondary lymphoid organs during acute LCMV infection in an interferon-γ (IFN-γ)-dependent manner. We also found that priming by moDCs enhanced the differentiation of memory CD8⁺ T cells compared to differentiation primed by conventional dendritic cells (cDCs) through upregulation of Eomesodermin (Eomes) and T cell factor-1 (TCF-1) expression in CD8⁺ T cells. Consequently, impaired memory formation of CD8⁺ T cells in mice that had reduced numbers of moDCs led to defective clearance of pathogens upon rechallenge. Mechanistically, attenuated interleukin-2 (IL-2) signaling in CD8⁺ T cells primed by moDCs was responsible for the enhanced memory programming of CD8⁺ T cells. Therefore, our findings unveil a specialization of the antigen-presenting cell subsets in the fate determination of CD8⁺ T cells during infection and pave the way for the development of a novel therapeutic intervention on infection.

Complexities of Type I Interferon Biology: Lessons from LCMV

Over the past decades, infection of mice with lymphocytic choriomeningitis virus (LCMV) has provided an invaluable insight into our understanding of immune responses to viruses. In particular, this model has clarified the central roles that type I interferons play in initiating and regulating host responses. The use of different strains of LCMV and routes of infection has allowed us to understand how type I interferons are critical in controlling virus replication and fostering effective antiviral immunity, but also how they promote virus persistence and functional exhaustion of the immune response. Accordingly, these discoveries have formed the foundation for the development of novel treatments for acute and chronic viral infections and even extend into the management of malignant tumors. Here we review the fundamental insights into type I interferon biology gained using LCMV as a model and how the diversity of LCMV strains, dose, and route of administration have been used to dissect the molecular mechanisms underpinning acute versus persistent infection. We also identify gaps in the knowledge regarding LCMV regulation of antiviral immunity. Due to its unique properties, LCMV will continue to remain a vital part of the immunologists' toolbox.

Exhaustion and Inflation at Antipodes of T Cell Responses to Chronic Virus Infection

Viruses that have coevolved with their host establish chronic infections that are well tolerated by the host. Other viruses, that are partly adapted to their host, may induce chronic infections where persistent replication and viral antigen expression occur. The former induce highly functional and resilient CD8T cell responses called memory inflation. The latter induce dysfunctional and exhausted responses. The reasons compelling T cell responses towards inflationary or exhausted responses are only partly understood. In this review we compare the two conditions and describe mechanistic similarities and differences. We also provide a list of potential reasons why exhaustion or inflation occur in different virus infections. We propose that T cell-mediated transcriptional repression of viral gene expression provides a critical feature of inflation that allows peaceful virus and host coexistence. The virus is controlled, but its genome is not eradicated. If this mechanism is not available, as in the case of RNA viruses, the virus and the host are compelled to an arms race. If virus proliferation and spread proceed uncontrolled for too long, T cells are forced to strike a balance between viral control and tissue destruction, losing antiviral potency and facilitating virus persistence.

Spatiotemporally restricted arenavirus replication induces immune surveillance and type I interferon-dependent tumour regression

Immune-mediated effector molecules can limit cancer growth, but lack of sustained immune activation in the tumour microenvironment restricts antitumour immunity. New therapeutic approaches that induce a strong and prolonged immune activation would represent a major immunotherapeutic advance. Here we show that the arenaviruses lymphocytic choriomeningitis virus (LCMV) and the clinically used Junin virus vaccine (Candid#1) preferentially replicate in tumour cells in a variety of murine and human cancer models. Viral replication leads to prolonged local immune activation, rapid regression of localized and metastatic cancers, and long-term disease control. Mechanistically, LCMV induces antitumour immunity, which depends on the recruitment of interferon-producing Ly6C+ monocytes and additionally enhances tumour-specific CD8+ T cells. In comparison with other clinically evaluated oncolytic viruses and to PD-1 blockade, LCMV treatment shows promising antitumoural benefits. In conclusion, therapeutically administered arenavirus replicates in cancer cells and induces tumour regression by enhancing local immune responses.

TCR independent suppression of CD8(+) T cell cytokine production mediated by IFNγ in vivo

CD8(+) memory T cells produce IFNγ within hours of secondary infection, but this is quickly terminated in vivo despite the presence of stimulatory viral antigen, suggesting that active suppression occurs. Herein, we investigated the in vivo effector function of CD8(+) memory T cells during successive encounters with viral antigen. CD8(+) T cells in immune mice receiving prior viral or peptide challenge failed to reproduce IFNγ during LCMV rechallenge. Surprisingly, this refractory state was induced even in memory cells that had not encountered their cognate antigen, indicating that the silencing of CD8(+) T cell responses is TCR-independent. Direct injection of IFNγ also suppressed the ability of virus-specific memory cells to respond to subsequent viral challenge. We propose the existence of a negative feedback loop whereby IFNγ, produced by memory CD8(+) T cells to combat viral challenge, acts - directly or indirectly - to limit its further production.

Interplay between regulatory T cells and PD-1 in modulating T cell exhaustion and viral control during chronic LCMV infection

T reg cells effectively maintain CD8 T cell exhaustion during chronic LCMV infection, but blockade of PD-1 is critical for elimination of infected cells.

Regulatory T (T reg) cells are critical for preventing autoimmunity mediated by self-reactive T cells, but their role in modulating immune responses during chronic viral infection is not well defined. To address this question and to investigate a role for T reg cells in exhaustion of virus-specific CD8 T cells, we depleted T reg cells in mice chronically infected with lymphocytic choriomeningitis virus (LCMV). T reg cell ablation resulted in 10–100-fold expansion of functional LCMV-specific CD8 T cells. Rescue of exhausted CD8 T cells was dependent on cognate antigen, B7 costimulation, and conventional CD4 T cells. Despite the striking recovery of LCMV-specific CD8 T cell responses, T reg cell depletion failed to diminish viral load. Interestingly, T reg cell ablation triggered up-regulation of the molecule programmed cell death ligand-1 (PD-L1), which upon binding PD-1 on T cells delivers inhibitory signals. Increased PD-L1 expression was observed especially on LCMV-infected cells, and combining T reg cell depletion with PD-L1 blockade resulted in a significant reduction in viral titers, which was more pronounced than that upon PD-L1 blockade alone. These results suggest that T reg cells effectively maintain CD8 T cell exhaustion, but blockade of the PD-1 inhibitory pathway is critical for elimination of infected cells.

Potent inhibition of Junín virus infection by interferon in murine cells

The new world arenavirus Junín virus (JUNV) is the causative agent of Argentine hemorrhagic fever, a lethal human infectious disease. Adult laboratory mice are generally resistant to peripheral infection by JUNV. The mechanism underlying the mouse resistance to JUNV infection is largely unknown. We have reported that interferon receptor knockout mice succumb to JUNV infection, indicating the critical role of interferon in restricting JUNV infection in mice. Here we report that the pathogenic and vaccine strains of JUNV were highly sensitive to interferon in murine primary cells. Treatment with low concentrations of interferon abrogated viral NP protein expression in murine cells. The replication of both JUNVs was enhanced in IRF3/IRF7 deficient cells. In addition, the vaccine strain of JUNV displayed impaired growth in primary murine cells. Our data suggested a direct and potent role of host interferon response in restricting JUNV replication in mice. The defect in viral growth for vaccine JUNV might also partially explain its attenuation in mice.

The new world arenavirus Junín virus (JUNV) is the causative agent of a lethal human infectious disease, Argentine hemorrhagic fever. Laboratory mice are used as models to study many viral diseases. However, adult laboratory mice are generally resistant to JUNV infection. Interferons are early immune regulatory molecules that induce potent anti-viral status in host cells and activate host immune cells to counteract virus infection. The activity of interferons relies on their cell surface receptors. We have previously reported that mutant mice with defect in interferon receptors succumbed to challenge with JUNV, highlighting the critical role of interferon in restricting JUNV infection in mice. Here we further study the basis of mouse resistance to JUNV infection and report that the replication of both pathogenic JUNV and its vaccine strains are highly sensitive to type I IFN treatment in mouse cells. However, both strains replicate efficiently in Africa green monkey-derived Vero cells and human cells when treated with high doses of interferon. Additionally, the vaccine strain replicates less efficiently in mouse cells compared with the pathogenic strain, which might partially explain its attenuation in mice. Our new findings help better understand the JUNV-host interaction.

Nucleoprotein-specific nonneutralizing antibodies speed up LCMV elimination independently of complement and FcγR

CD8(+) T cells have an essential role in controlling lymphocytic choriomeningitis virus (LCMV) infection in mice. Here, we examined the contribution of humoral immunity, including nonneutralizing antibodies (Abs), in this infection induced by low virus inoculation doses. Mice with impaired humoral immunity readily terminated infection with the slowly replicating LCMV strain Armstrong but showed delayed virus elimination after inoculation with the faster replicating LCMV strain WE and failed to clear the rapidly replicating LCMV strain Docile, which is in contrast to the results obtained with wild-type mice. Thus, the requirement for adaptive humoral immunity to control the infection was dependent on the replication speed of the LCMV strains used. Ab transfers further showed that LCMV-specific IgG Abs isolated from LCMV immune serum accelerated virus elimination. These Abs were mainly directed against the viral nucleoprotein (NP) and completely lacked virus neutralizing activity. Moreover, mAbs specific for the LCMV NP were also able to decrease viral titers after transfer into infected hosts. Intriguingly, neither C3 nor Fcγ receptors were required for the antiviral activity of the transferred Abs. In conclusion, our study suggests that rapidly generated nonneutralizing Abs specific for the viral NP speed up virus elimination and thereby may counteract T-cell exhaustion.

Timing and magnitude of type I interferon responses by distinct sensors impact CD8 T cell exhaustion and chronic viral infection

Type I interferon (IFN-I) promotes antiviral CD8(+)T cell responses, but the contribution of different IFN-I sources and signaling pathways are ill defined. While plasmacytoid dendritic cells (pDCs) produce IFN-I upon TLR stimulation, IFN-I is induced in most cells by helicases like MDA5. Using acute and chronic lymphocytic choriomeningitis virus (LCMV) infection models, we determined that pDCs transiently produce IFN-I that minimally impacts CD8(+)T cell responses and viral persistence. Rather, MDA5 is the key sensor that induces IFN-I required for CD8(+)T cell responses. In the absence of MDA5, CD8(+)T cell responses to acute infection rely on CD4(+)T cell help, and loss of both CD4(+)T cells and MDA5 results in CD8(+)T cell exhaustion and persistent infection. Chronic LCMV infection rapidly attenuates IFN-I responses, but early administration of exogenous IFN-I rescues CD8(+)T cells, promoting viral clearance. Thus, effective antiviral CD8(+)T cell responses depend on the timing and magnitude of IFN-I production.

Type I interferons: diversity of sources, production pathways and effects on immune responses

Type I interferons (IFN-I) were first described over 50 years ago as factors produced by cells that interfere with virus replication and promote an antiviral state. Innate and adaptive immune responses to viruses are also greatly influenced by IFN-I. In this article we discuss the diversity of cellular sources of IFN-I and the pathways leading to IFN-I production during viral infections. Finally, we discuss the effects of IFN-I on cells of the immune system with emphasis on dendritic cells.

Evaluation of the immunomodulatory and antiviral effects of the cytokine combination IFN-α and IL-7 in the lymphocytic choriomeningitis virus and Friend retrovirus mouse infection models

Existing therapies for chronic viral infections are still suboptimal or have considerable side effects, so new therapeutic strategies need to be developed. One option is to boost the host's immune response with cytokines. We have recently shown in an acute ex vivo HIV infection model that co-administration of interferon (IFN)-α and interleukin (IL)-7 allows us to combine the potent anti-HIV activity of IFN-α with the beneficial effects of IL-7 on T-cell survival and function. Here we evaluated the effect of combining IFN-α and IL-7 on viral replication in vivo in the chronic lymphocytic choriomeningitis virus (LCMV) and acute Friend retrovirus (FV) infection models. In the chronic LCMV model, cytokine treatment was started during the early replication phase (i.e., on day 7 post-infection [pi]). Under the experimental conditions used, exogenous IFN-α inhibited FV replication, but had no effect on viral replication in the LCMV model. There was no therapeutic benefit of IL-7 either alone or in combination with IFN-α in either of the two infection models. In the LCMV model, dose-dependent effects of the cytokine combination on T-cell phenotype/function were observed. It is possible that these effects would translate into antiviral activity in re-challenged mice. It is also possible that another type of IFN-α/β or induction of endogenous IFN-α/β alone or in combination with IL-7 would have antiviral activity in the LCMV model. Furthermore, we cannot exclude that some effect on viral titers would have been seen at later time points not investigated here (i.e., beyond day 34 pi). Finally, IFN-α/IL-7 may inhibit the replication of other viruses. Thus it might be worth testing these cytokines in other in vivo models of chronic viral infections.

Viral replicative capacity is the primary determinant of lymphocytic choriomeningitis virus persistence and immunosuppression

The Clone 13 (Cl13) strain of lymphocytic choriomeningitis virus is widely studied as a model of chronic systemic viral infection. Here, we used reverse genetic techniques to identify the molecular basis of Cl13 persistence and immunosuppression, the characteristics differentiating it from the closely related Armstrong strain. We found that a single-point mutation in the Cl13 polymerase was necessary and partially sufficient for viral persistence and immunosuppression. A glycoprotein mutation known to enhance dendritic cell targeting accentuated both characteristics but when introduced alone, failed to alter the phenotype of the Armstrong strain. The decisive polymerase mutation increased intracellular viral RNA load in plasmacytoid dendritic cells, which we identified as a main initial target cell type in vivo, and increased viremia in the early phase of infection. These findings establish the enhanced replicative capacity as the primary determinant of the Cl13 phenotype. Viral persistence and immunosuppression can, thus, represent a direct consequence of excessive viral replication overwhelming the host's antiviral defense.

Inhibition of the type I interferon antiviral response during arenavirus infection

Arenaviruses merit interest both as tractable experimental model systems to study acute and persistent viral infections, and as clinically-important human pathogens. Several arenaviruses cause hemorrhagic fever (HF) disease in humans. In addition, evidence indicates that the globally-distributed prototypic arenavirus lymphocytic choriomeningitis virus (LCMV) is a human pathogen of clinical significance in congenital infections, and also poses a great danger to immunosuppressed individuals. Arenavirus persistence and pathogenesis are facilitated by their ability to overcome the host innate immune response. Mammalian hosts have developed both membrane toll-like receptors (TLR) and cytoplasmic pattern recognition receptors (PRRs) that recognize specific pathogen-associated molecular patterns (PAMPs), resulting in activation of the transcription factors IRF3 or IRF7, or both, which together with NF-κB and ATF-2/c-JUN induce production of type I interferon (IFN-I). IFN-I plays a key role in host anti-microbial defense by mediating direct antiviral effects via up-regulation of IFN-I stimulated genes (ISGs), activating dendritic cells (DCs) and natural killer (NK) cells, and promoting the induction of adaptive responses. Accordingly, viruses have developed a plethora of strategies to disrupt the IFN-I mediated antiviral defenses of the host, and the viral gene products responsible for these disruptions are often major virulence determinants. IRF3- and IRF7-dependent induction of host innate immune responses is frequently targeted by viruses. Thus, the arenavirus nucleoprotein (NP) was shown to inhibit the IFN-I response by interfering with the activation of IRF3. This NP anti-IFN activity, together with alterations in the number and function of DCs observed in mice chronically infected with LCMV, likely play an important role in LCMV persistence in its murine host. In this review we will discuss current knowledge about the cellular and molecular mechanisms by which arenaviruses can subvert the host innate immune response and their implications for understanding HF arenaviral disease as well as arenavirus persistence in their natural hosts.

Neutralization of IFNgamma defeats haemophagocytosis in LCMV-infected perforin- and Rab27a-deficient mice

Hereditary haemophagocytic lymphohistiocytosis (HLH) is a fatal inflammatory disease and treatments currently may lead to serious side effects. There is a pressing need for effective, less toxic treatments for this disease. Previous reports have suggested that interferon γ (IFNγ) has a role in the pathogenesis of HLH. Here, we report that blocking IFNγ had a therapeutic effect in two different murine models of human hereditary HLH (perforin-deficient and Rab27a-deficient mice, both infected with lymphocytic choriomeningitis virus). Therapeutic administration of an anti-IFNγ antibody induced recovery from haemophagocytosis in both genetic models, as evidenced by increased survival in perforin-deficient mice and correction of blood cytopenia, moderation of body temperature changes, decreased cytokinaemia, restoration of splenic architecture and reduced haemophagocytosis in the liver of both murine models. Involvement of the central nervous system in Rab27a-deficient mice was prevented by anti-IFNγ therapy. Hepatic T-cell infiltrates and virus persisted, with no detectable harm during the time course of these studies. These data strongly suggest that neutralization of IFNγ could be used in humans to safely alleviate the clinical manifestations of haemophagocytosis.

Model with two types of CTL regulation and experiments on CTL dynamics

Recently, we developed a mathematical model of interaction between the HIV and the immune system to match various dynamic experiments carried out in HIV-infected humans and SIV-infected macaques. The model includes helper cell-dependent and helper cell-independent cytotoxic lymphocytes (CTLs) and predicts two stable steady states, a state with a high virus load and few helper cells, and another state with a low virus load and many helper cells. Here we upgrade the model to take into account recent reports on the link between the activation status of infected cells and their ability to produce virus, the effect of helper cells at the time of priming on CTL differentiation, and virus dynamics in unvaccinated macaques with a broad genetic background acutely infected with SIVmac251. We also discuss in detail the experimental justification of the CTL block and the robustness of model predictions with respect to the hypothesis of two CTL subtypes.

Multiple mechanisms contribute to impairment of type 1 interferon production during chronic lymphocytic choriomeningitis virus infection of mice

Type 1 IFNs, innate cytokines with important effector and immunomodulatory properties, are rapidly induced in the acute phase of many virus infections; however, this is generally a transient response that is not sustained during virus persistence. To gain insight into mechanisms that can contribute to down-regulation of type 1 IFN production during virus persistence, we analyzed type 1 IFN production during acute and chronic lymphocytic choriomeningitis virus (LCMV) infection. High-level type 1 IFN production was transiently up-regulated in cells including plasmacytoid and conventional dendritic cells (DCs) following LCMV infection of mice, but LCMV persistence was associated with only low-level type 1 IFN production. Nonetheless, chronically infected mice were able to up-regulate type 1 IFN production in response to TLR3, 7, and 9 ligands, albeit less efficiently than uninfected mice. Splenic DC numbers in mice chronically infected with LCMV were decreased, and the remaining cells exhibited a reduced response to TLR stimulation. LCMV-infected cell lines efficiently up-regulated type 1 IFN production following TLR ligation and infection with a DNA virus, but exhibited a defect in type 1 IFN induction following infection with Sendai, an RNA virus. This block in type 1 IFN production by infected cells, together with abnormalities in DC numbers and functions, likely contribute to the low-level type 1 IFN production in mice chronically infected with LCMV. Impairment of type 1 IFN production may both promote virus persistence and impact on host immunocompetence. Understanding the mechanisms involved may assist in development of strategies for control of virus persistence and superinfection.

Genomic and biological characterization of aggressive and docile strains of lymphocytic choriomeningitis virus rescued from a plasmid-based reverse-genetics system

Arenaviruses include several causative agents of haemorrhagic fever disease in humans. In addition, the prototypic arenavirus lymphocytic choriomeningitis virus (LCMV) is a superb model for the study of virus-host interactions, including the basis of viral persistence and associated diseases. There is little understanding about the molecular mechanisms concerning the regulation and specific role of viral proteins in modulating arenavirus-host cell interactions either associated with an acute or persistent infection, and associated disease. Here, we report the genomic and biological characterization of LCMV strains 'Docile' (persistent) and 'Aggressive' (not persistent) recovered from cloned cDNA via reverse genetics. Our results confirmed that the cloned viruses accurately recreated the in vivo phenotypes associated with the corresponding natural Docile and Aggressive viral isolates. In addition, we provide evidence that the ability of the Docile strain to persist is determined by the nature of both S and L RNA segments. Thus, our findings provide the foundation for studies aimed at gaining a detailed understanding of viral determinants of LCMV persistence in its natural host, which may aid in the development of vaccines to prevent or treat the diseases caused by arenaviruses in humans.

Lymphocytic choriomeningitis infection of the central nervous system

Viral infection of the central nervous system (CNS) can result in a multitude of responses including pathology, persistence or immune clearance. Lymphocytic choriomeningitis virus (LCMV) is a powerful model system to explore these potential outcomes of CNS infection due to the diversity of responses that can be achieved after viral inoculation. Several factors including tropism, timing, dose and variant of LCMV in combination with the development or suppression of the corresponding immune response dictates whether lethal meningitis, chronic infection or clearance of LCMV in the CNS will occur. Importantly, the functionality and positioning of the LCMV-specific CD8+ T cell response are critical in directing the subsequent outcome of CNS LCMV infection. Although a basic understanding of LCMV and immune interactions in the brain exists, the molecular machinery that shapes the balance between pathogenesis and clearance in the LCMV-infected CNS remains to be elucidated. This review covers the various outcomes of LCMV infection in the CNS and what is currently known about the impact of the virus itself versus the immune response in the development of disease or clearance.

A ML29 reassortant virus protects guinea pigs against a distantly related Nigerian strain of Lassa virus and can provide sterilizing immunity

Lassa virus (LASV) is responsible for the deaths of thousands of people in West Africa annually. Genetic diversity among LASV strains is the highest among the Arenaviridae and represents a great challenge for vaccine development. Guinea pigs vaccinated with a ML29 reassortant vaccine experienced sterilizing immunity and complete protection when challenged on day 30 either with homologous virus or with the distantly related Nigerian isolate. Simultaneous vaccination-challenge or challenge on day 2 after vaccination also protected 60-100% of the animals against both strains, but without sterilizing immunity. These results indicate that simultaneous replication of ML29 and LASV attenuates the virulence of LASV infection.

Altered central nervous system gene expression caused by congenitally acquired persistent infection with lymphocytic choriomeningitis virus

Neonatal infection of most mouse strains with lymphocytic choriomeningitis virus (LCMV) leads to a life-long persistent infection characterized by high virus loads in the central nervous system (CNS) in the absence of inflammation and tissue destruction. These mice, however, exhibit impaired learning and memory. The occurrence of cognitive defects in the absence of overt CNS pathology led us to the hypothesis that chronic virus infection may contribute to neuronal dysfunction by altering the host's gene expression profile. To test this hypothesis, we examined the impact of LCMV persistence on host gene expression in the CNS. To model the natural route of human congenital CNS infection observed with a variety of viruses, we established a persistently infected mouse colony where the virus was maintained via vertical transmission from infected mothers to offspring (LCMV-cgPi). LCMV-cgPi mice exhibited a lifelong persistent infection involving the CNS; the infection was associated with impaired spatial-temporal learning. Despite high viral loads in neurons of the brains of adult LCMV-cgPi mice, we detected changes in the host's CNS gene expression for only 75 genes, 56 and 19 being significantly induced and reduced, respectively. The majority of the genes induced in the brain of LCMV-cgPi mice were interferon (IFN)-stimulated genes (ISGs) and included the transcription factors STAT1 and IRF9, the ISG15 protease UBP43, and the glucocorticoid attenuated-response genes GARG16 and GARG49. Based on their crucial role in antiviral defense, these ISGs may play an important role in limiting viral spread and replication. However, since IFNs have also been implicated in adverse effects on neuronal function, the chronic induction of some ISGs may also contribute to the observed cognitive impairment.

Perforin-deficient CD8+ T cells mediate fatal lymphocytic choriomeningitis despite impaired cytokine production

Intracerebral (i.c.) infection with lymphocytic choriomeningitis virus (LCMV) is one of the most studied models for virus-induced immunopathology, and based on results from perforin-deficient mice, it is currently assumed that fatal disease directly reflects perforin-mediated cell lysis. However, recent studies have revealed additional functional defects within the effector T cells of LCMV-infected perforin-deficient mice, raising the possibility that perforin may not be directly involved in mediating lethal disease. For this reason, we decided to reevaluate the role of perforin in determining the outcome of i.c. infection with LCMV. We confirmed that the expansion of virus-specific CD8(+) T cells is unimpaired in perforin-deficient mice. However, despite the fact that the virus-specific CD8(+) effector T cells in perforin-deficient mice are broadly impaired in their effector function, these mice invariably succumb to i.c. infection with LCMV strain Armstrong, although a few days later than matched wild-type mice. Upon further investigation, we found that this delay correlates with the delayed recruitment of inflammatory cells to the central nervous system (CNS). However, CD8(+) effector T cells were not kept from the CNS by sequestering in infected extraneural organ sites such as liver or lungs. Thus, the observed dysfunctionality regarding the production of proinflammatory mediators probably results in the delayed recruitment of effector cells to the CNS, and this appears to be the main explanation for the delayed onset of fatal disease in perforin-deficient mice. However, once accumulated in the CNS, virus-specific CD8(+) T cells can induce fatal CNS pathology despite the absence of perforin-mediated lysis and reduced capacity to produce several key cytokines.

Reciprocal immunomodulation in a schistosome and hepatotropic virus coinfection model

Human coinfection with the helminth parasite Schistosoma mansoni and hepatitis B and hepatitis C viruses is associated with increased hepatic viral burdens and severe liver pathology. In this study we developed a murine S. mansoni/lymphocytic choriomeningitis virus (LCMV) coinfection model that reproduces the enhanced viral replication and liver pathology observed in human coinfections, and used this model to explore the mechanisms involved. Viral coinfection during the Th2-dominated granulomatous phase of the schistosome infection resulted in induction of a strong LCMV-specific T cell response, with infiltration of high numbers of LCMV-specific IFN-gamma-producing CD8+ cells into the liver. This was associated with suppression of production of the Th2 cytokines dominant during S. mansoni infection and a rapid increase in morbidity, linked to hepatotoxicity. Interestingly, the liver of coinfected mice was extremely susceptible to viral replication. This correlated with a reduced intrahepatic type I IFN response following virus infection. Schistosome egg Ags were found to suppress the type I IFN response induced in murine bone marrow-derived dendritic cells by polyinosinic-polycytidylic acid. These results suggest that suppression of the antiviral type I IFN response by schistosome egg Ags in vivo predisposes the liver to enhanced viral replication with ensuing immunopathological consequences, findings that may be paralleled in human schistosome/hepatotropic virus coinfections.

Interferon alfacon-1 protects hamsters from lethal pichinde virus infection

Hemorrhagic fever of arenaviral origin is a frequently fatal infectious disease of considerable priority to the biodefense mission. Historically, the treatment of arenaviral infections with alpha interferons has not yielded favorable results. Here we present evidence that interferon alfacon-1, a nonnaturally occurring bioengineered alpha interferon approved for the treatment of chronic hepatitis C, is active against Pichinde and Tacaribe arenaviruses in cell culture. In the hamster model of Pichinde virus (PCV) infection, interferon alfacon-1 treatment significantly protected animals from death, prolonged the survival of those that eventually died, reduced virus titers, and limited liver damage characteristic of PCV-induced disease. Moreover, interferon alfacon-1 also demonstrated therapeutic activity, to a lesser degree, when the initiation of treatment was delayed up to 2 days post-virus challenge. Despite the observed advantages of interferon alfacon-1 therapy, efforts to stimulate the immune system with the known interferon inducer poly(I:C12U) (Ampligen) offered only limited protection against lethal PCV challenge. Taken together, these data suggest that the increased potency of the bio-optimized interferon alfacon-1 molecule may be critical to the observed antiviral effects. These data are the first report demonstrating efficacious treatment of acute arenaviral disease with alpha interferon therapy, and further study is warranted.

Dichotomy between survival and lytic gene expression in RNase L- and PKR-deficient mice transduced with an adenoviral vector expressing murine IFN-beta following ocular HSV-1 infection

The present study investigated the role of interferon-inducible pathways in herpes simplex virus type 1-infected mice transduced with an adenoviral vector expressing murine interferon-beta (Ad:IFN-beta). Wild type mice or RNase L(-/-) mice deficient in responses to 2'-5' oligoadenylate synthetase activation, or lacking RNA-dependent protein kinase and transduced with Ad:IFN-beta showed enhanced survival following HSV-1 infection. The protective effect was associated with a reduction in viral gene expression in the cornea and trigeminal ganglion in wild type mice as well as the trigeminal ganglion of RNase L(-/-) mice. However, the efficacy of Ad:IFN-beta was lost in the corneas of RNase L(-/-) mice and significantly diminished in both the cornea and trigeminal ganglion as measured by viral gene expression in RNA-dependent protein kinase deficient mice. Collectively, the data suggest survival rates of viral-infected mice do not reflect the replication capacity as measured by herpes simplex virus type one lytic gene expression.

Porcine reproductive and respiratory syndrome virus field isolates differ in in vitro interferon phenotypes

Type I interferons (IFN-α and -β) play an important role in the innate host defense against viral infection by inducing antiviral responses. In addition to direct antiviral activities, type I IFN serves as an important link between the innate and adaptive immune response through multiple mechanisms. Therefore, the outcome of a viral infection can be affected by IFN induction and the IFN sensitivity of a virus. North American porcine reproductive and respiratory syndrome virus (PRRSV) field isolates were studied with regard to IFN-α sensitivity and induction in order to understand the role of type I IFN in PRRSV pathogenesis. PRRSV isolates were differentially sensitive to porcine recombinant IFN-α (rIFN-α) and varied in their ability to induce IFN-α in porcine alveolar macrophages (PAM) cultures as measured by a porcine IFN-α specific ELISA on cell culture supernatants. Fifty-two plaques were purified from three PRRSV isolates (numbers 3, 7, and 12) and tested for IFN sensitivity and IFN induction. Plaque-derived populations were composed of heterogeneous populations in terms of IFN-inducing capacity and sensitivity to rIFN-α. When macrophages infected with isolates 3, 7, or 12 were treated with polycytidylic acid (polyI:C), IFN-α production was enhanced. Cells infected with isolate 3 and treated with polyI:C showed the most consistent and strongest enhancement of IFN-α production. It was demonstrated that the relatively low concentrations of IFN-α produced by isolate 3 contributed to the enhanced IFN-α synthesis in response to polyI:C. Isolates 7 and 12 significantly suppressed the enhanced IFN-α production by isolate 3 in polyI:C treated cells. To determine if suppression was at the level of IFN-α transcription, quantitative RT-PCR was performed for IFN-α mRNA and compared to GAPDH and cyclophilin mRNA quantification. However, the relative number of IFN-α transcript copies did not correlate with IFN-α protein levels, suggesting a post-transcriptional mechanism of suppression. In summary, these results demonstrate that PRRSV field isolates differ both in IFN-α sensitivity and induction. Furthermore, a PRRSV field isolate strongly enhance polyI:C-induced IFN-α production in PAM cultures and this priming effect was suppressed by other PRRSV isolates.

Human macrophages, but not dendritic cells, are activated and produce alpha/beta interferons in response to Mopeia virus infection

Lassa virus (LV) and Mopeia virus (MV) are closely related members of the Arenavirus genus, sharing 75% amino acid sequence identity. However, LV causes hemorrhagic fever in humans and nonhuman primates, whereas MV cannot induce disease. We have previously shown that antigen-presenting cells (APC)-macrophages (MP) and dendritic cells (DC)-sustain high replication rates of LV but are not activated, suggesting that they play a role in the immunosuppression observed in severe cases of Lassa fever. Here, we infected human APC with MV and analyzed the cellular responses induced. MV infection was productive in MP and even more so in DC. Apoptosis was not induced in either cell type. Moreover, unlike DC, MP were early and strongly activated in response to MV, as shown by the increased surface expression of CD86, CD80, CD54, CD40, and HLA-abc and by the production of mRNA encoding alpha interferon (IFN-alpha), IFN-beta, tumor necrosis factor alpha and interleukin-6. In addition, MV-infected MP produced less of the virus than DC, which was related to the fact that these cells secreted IFN-alpha. Thus, the strong activation of MP is probably a major event in the control of MV infection and may be involved in the induction of an adaptive immune response in infected hosts. These results may explain the difference in pathogenicity between LV and MV.

Deliberate removal of T cell help improves virus-neutralizing antibody production

The B cell response to lymphocytic choriomeningitis virus is characterized by a CD4(+) T cell-dependent polyclonal hypergammaglobulinemia and delayed formation of virus-specific neutralizing antibodies. Here we provide evidence that, paradoxically, because of polyclonal B cell activation, virus-specific T cell help impairs the induction of neutralizing antibody responses. Experimental reduction in CD4(+) T cell help in vivo resulted in potent neutralizing antibody responses without impairment of CD8(+) T cell activity. These unexpected consequences of polyclonal B cell activation may affect vaccine strategies and the treatment of clinically relevant chronic bacterial, parasitic and viral infections in which hypergammaglobulinemia is regularly found.

Underwhelming the immune response: effect of slow virus growth on CD8+-T-lymphocyte responses

The speed of virus replication has typically been seen as an advantage for a virus in overcoming the ability of the immune system to control its population growth. Under some circumstances, the converse may also be true: more slowly replicating viruses may evoke weaker cellular immune responses and therefore enhance their likelihood of persistence. Using the model of lymphocytic choriomeningitis virus (LCMV) infection in mice, we provide evidence that slowly replicating strains induce weaker cytotoxic-T-lymphocyte (CTL) responses than a more rapidly replicating strain. Conceptually, we show a "bell-shaped" relationship between the LCMV growth rate and the peak CTL response. Quantitative analysis of human hepatitis C virus infections suggests that a reduction in virus growth rate between patients during the incubation period is associated with a spectrum of disease outcomes, from fulminant hepatitis at the highest rate of viral replication through acute resolving to chronic persistence at the lowest rate. A mathematical model for virus-CTL population dynamics (analogous to predator [CTL]-prey [virus] interactions) is applied in the clinical data-driven analysis of acute hepatitis B virus infection. The speed of viral replication, through its stimulus of host CTL responses, represents an important factor influencing the pathogenesis and duration of virus persistence within the human host. Viruses with lower growth rates may persist in the host because they "sneak through" immune surveillance.

Inhibition of different Lassa virus strains by alpha and gamma interferons and comparison with a less pathogenic arenavirus

The high pathogenicity of Lassa virus is assumed to involve resistance to the effects of interferon (IFN). We have analyzed the effects of alpha IFN (IFN-alpha), IFN-gamma, and tumor necrosis factor alpha (TNF-alpha) on replication of Lassa virus compared to the related, but less pathogenic, lymphocytic choriomeningitis virus (LCMV). Three low-passage Lassa virus strains (AV, NL, and CSF), isolated from humans with mild to fulminant Lassa fever, were tested. Lassa virus replication was inhibited by IFN-alpha and IFN-gamma, but not TNF-alpha, in Huh7 and Vero cells. The degree of IFN sensitivity of a Lassa virus isolate did not correlate with disease severity in human patients. Furthermore, cytokine effects observed for Lassa virus and LCMV (strains CH-5692, Armstrong, and WE) were similar. To address the mechanisms involved in the IFN effect, we used cell lines in which overexpression of IFN-stimulated proteins promyelocytic leukemia protein (PML) and Sp100 could be induced. Both proteins reside in PML bodies, a cellular target of the LCMV and Lassa virus Z proteins. Overexpression of PML or Sp100 did not affect replication of either virus. This, together with the previous finding that PML knockout facilitates LCMV replication in vitro and in vivo (M. Djavani, J. Rodas, I. S. Lukashevich, D. Horejsh, P. P. Pandolfi, K. L. Borden, and M. S. Salvato, J. Virol. 75:6204-6208, 2001; W. V. Bonilla, D. D. Pinschewer, P. Klenerman, V. Rousson, M. Gaboli, P. P. Pandolfi, R. M. Zinkernagel, M. S. Salvato, and H. Hengartner, J. Virol. 76:3810-3818, 2002), describes PML as a mediator within the antiviral pathway rather than as a direct effector protein. In conclusion, the high pathogenicity of Lassa virus compared to LCMV is probably not due to increased resistance to the effects of IFN-alpha or IFN-gamma. Both cytokines inhibit replication which is relevant for the design of antiviral strategies against Lassa fever with the aim of enhancing the IFN response.

Do thymically and strictly extrathymically developing T cells generate similar immune responses?

If present in sufficient numbers, could extrathymic T cells substitute for thymus-derived T cells? To address this issue, we studied extrathymic T cells that develop in athymic mice under the influence of oncostatin M (OM). In this model, extensive T-cell development is probably due to amplification of a minor pathway of T-cell differentiation taking place only in the lymph nodes. Extrathymic CD4 T cells expanded poorly and were deficient in providing B-cell help after infection with vesicular stomatitis virus (VSV) and lymphocytic choriomeningitis virus (LCMV). Compared with classic T cells, stimulated extrathymic CD8 T cells produced copious amounts of interferon gamma (IFN-gamma), and their expansion was precocious but of limited amplitude because of a high apoptosis rate. Consequently, although extrathymic cytotoxic T lymphocytes (CTLs) responded to LCMV infection, as evidenced by the expansion of GP33-41 tetramer-positive CD8 T cells, they were unable to eradicate the virus. Our data indicate that the site of development impinges on T-cell quality and function and that extrathymic T cells functionally cannot substitute for classical thymic T cells.

A unique mechanism for innate cytokine promotion of T cell responses to viral infections

The kinetics of CD8 T cell IFN-gamma responses as they occur in situ are defined here during lymphocytic choriomeningitis virus (LCMV) infections, and a unique mechanism for the innate cytokines IFN-alphabeta and IL-18 in promoting these responses is defined. Infections of mice with Armstrong or WE strains of LCMV induced an unexpectedly early day 4 IFN-gamma response detectable in serum samples and spleen and liver homogenates. Production of IFN-gamma was MHC class I/CD8 dependent, but did not require IL-12, NK cells, TCR-gammadelta T cells, MHC class II, or CD4 T cells. Peak response required specific Ag recognition, as administration of antagonist peptide partially impaired day 4 IFN-gamma induction, and viral peptide stimulation enhanced CD8 T cell IFN-gamma expression in culture. The IFN-gamma response was associated with IL-18 and IFN-alphabeta expression. Furthermore, both factors augmented peptide-driven IFN-gamma production in culture, and mice lacking IL-18 or IFN-alphabeta functions had reduced day 4 IFN-gamma. Collectively, these results demonstrate that during viral infections, there is a dramatic in vivo CD8 T cell response preceding maximal expansion of these cells, and that the mechanism supporting this response is dependent on endogenous innate cytokines. Because stimulation by microbial products is linked to innate cytokine expression, the studies also suggest a pathway for precisely limiting T cell functions to times of need.

Innate immune responses to LCMV infections: natural killer cells and cytokines

Although much remains to be learned, the study of early responses to LCMV infections of mice has contributed to the basic understanding of the regulation of a variety of important innate immune responses. Major discoveries have included the appreciation of the levels of type 1 IFNs induced during endogenous responses to viral infections, the importance of IFN-alpha/beta for induction of NK cell cytotoxicity, and the roles for IFN-alpha/beta in regulating the expression of other innate cytokines, i.e., IL-12 and IFN-gamma produced by NK cells (Fig. 11). Taken together with the characterization of adaptive responses to LCMV, a paradigm is emerging for a possible initial to innate to adaptive response cascade during infections with viruses eliciting endogenous expression of high levels of IFN-alpha/beta. The results not only advance the understanding of endogenous responses to viral infections and how they are balanced to achieve the best possible outcome for the host, but also give insights into possible consequences of therapeutic intervention with type 1 IFNs.

Effects of promyelocytic leukemia protein on virus-host balance

The cellular promyelocytic leukemia protein (PML) associates with the proteins of several viruses and in some cases reduces viral propagation in cell culture. To examine the role of PML in vivo, we compared immune responses and virus loads of PML-deficient and control mice infected with lymphocytic choriomeningitis virus (LCMV) and vesicular stomatitis virus (VSV). PML(-/-) mice exhibited accelerated primary footpad swelling reactions to very-low-dose LCMV, higher swelling peaks upon high-dose inoculation, and higher viral loads in the early phase of systemic LCMV infection. T-cell-mediated hepatitis and consequent mortality upon infection with a hepatotropic LCMV strain required 10- to 100-times-lower inocula despite normal cytotoxic T-lymphocyte reactivity in PML(-/-) mice. Furthermore, PML deficiency rendered mice 10 times more susceptible to lethal immunopathology upon intracerebral LCMV inoculation. Accordingly, 10-times-lower VSV inocula elicited specific neutralizing-antibody responses, a replication-based effect not observed with inactivated virus or after immunization with recombinant VSV glycoprotein. These in vivo observations corroborated our results showing more virus production in PML(-/-) fibroblasts. Thus, PML is a contributor to innate immunity, defining host susceptibility to viral infections and to immunopathology.

Critical role for perforin-, Fas/FasL-, and TNFR1-mediated cytotoxic pathways in down-regulation of antigen-specific T cells during persistent viral infection

Viral persistence following infection with invasive strains of lymphocytic choriomeningitis virus (LCMV) can be achieved by selective down-regulation of virus-specific T lymphocytes. High viral burden in the onset of infection drives responding cells into functional unresponsiveness (anergy) that can be followed by their physical elimination. In this report, we studied down-regulation of the virus-specific CD8(+)-T-cell response during persistent infection of adult mice with LCMV, with emphasis on the role of perforin-, Fas/FasL-, or tumor necrosis factor receptor 1 (TNFR1)-mediated cytolysis in regulating T-cell homeostasis. The results reveal that the absence of perforin, Fas-ligand, or TNFR1 has no significant effect on the kinetics of proliferation and functional inactivation of virus-specific CD8(+) T cells in the onset of chronic LCMV infection. However, these molecules play a critical role in the homeostatic regulation of T cells, influencing the longevity of the virus-specific CD8(+)-T-cell population once it has become anergic. Thus, CD8(+) T cells specific to the dominant LCMV NP(396-404) epitope persist in an anergic state for at least 70 days in perforin-, FasL-, or TNFR1-deficient mice, but they were eliminated by day 30 in C57BL/6 controls. These effects were additive as shown by a deficit of apoptotic death of NP(396-404) peptide-specific CD8(+) T cells in mice lacking both perforin and TNFR1. This suggests a role for perforin-, FasL-, and TNFR1-mediated pathways in down-regulation of the antiviral T cell response during persistent viral infection by determining the fate of antigen-specific T cells. Moreover, virus-specific anergic CD8(+) T cells in persistently infected C57BL/6 mice contain higher levels of Bcl-2 and Bcl-XL than functionally intact T cells generated during acute LCMV infection. In the case of proapoptotic factors, Bax expression did not differ between T-cell populations and Bad was below the limit of detection in all samples. As expression of the Bcl-2 family members controls susceptibility to apoptosis, this finding may provide a molecular basis for the survival of anergic cells under conditions of prolonged antigen stimulation.

Concerted action of the FasL/Fas and perforin/granzyme A and B pathways is mandatory for the development of early viral hepatitis but not for recovery from viral infection

Cytotoxic T lymphocytes (CTL) play a major role in the recovery from primary viral infections and the accompanying tissue injuries. However, it is unclear to what extent the two main cytolytic pathways, perforin-granzyme A and B exocytosis and Fas ligand (FasL)-Fas interaction, contribute to these processes. Here we have employed mouse strains with either spontaneous mutations or targeted gene defects in one or more components of either of the two cytolytic pathways to analyze the molecular basis of viral clearance and induction of hepatitis during lymphocytic choriomeningitis virus infection. Our results reveal that viral clearance is solely dependent on perforin but that virus-induced liver damage only occurs when both the FasL/Fas and the perforin pathways, including granzymes A and B, are simultaneously activated. The finding that development of hepatitis but not viral clearance is dependent on the concomitant activation of FasL-Fas and perforin-granzymes may be helpful in designing novel strategies to prevent hepatic failures during viral infections.

Critical role for alpha/beta and gamma interferons in persistence of lymphocytic choriomeningitis virus by clonal exhaustion of cytotoxic T cells

Under conditions of high antigenic load during infection with invasive lymphocytic choriomeningitis virus (LCMV) strains, virus can persist by selective clonal exhaustion of antigen-specific CD8(+) T cells. In this work we studied the down-regulation of the virus-specific CD8(+)-T-cell response during a persistent infection of adult mice, with particular emphasis on the contribution of the interferon response in promoting host defense. Studies were conducted by infecting mice deficient in receptors for type I (alpha/beta interferon [IFN-alpha/beta]), type II (IFN-gamma), and both type I and II IFNs with LCMV isolates that vary in their capacity to induce T-cell exhaustion. The main conclusions of this study are as follows. (i) IFNs play a critical role in LCMV infection by reducing viral loads in the initial stages of infection and thus modifying both the extent of CD8(+)-T-cell exhaustion and the course of infection. The importance of IFNs in this context varies with the biological properties of the LCMV strain. (ii) An inverse correlation exists between antigen persistence and responsiveness of virus-specific CD8(+) T cells. This results in distinct programs of activation or tolerance (functional unresponsiveness and/or physical elimination of antigen-specific cells) during acute and chronic virus infections, respectively. (iii) A successful immune response associated with definitive viral clearance requires an appropriate balance between cellular and humoral components of the immune system. We discuss the role of IFNs in influencing virus-specific T cells that determine the outcome of persistent infections.

Low level viral persistence after infection with LCMV: a quantitative insight through numerical bifurcation analysis

Many important viruses persist at very low levels in the body in the face of host immunity, and may influence the maintenance of this state of 'infection immunity'. To analyse low level viral persistence in quantitative terms, we use a mathematical model of antiviral cytotoxic T lymphocyte (CTL) response to lymphocytic choriomeningitis virus (LCMV). This model, described by a non-linear system of delay differential equations (DDEs), is studied using numerical bifurcation analysis techniques for DDEs. Domains where low level LCMV coexistence with CTL memory is possible, either as an equilibrium state or an oscillatory pattern, are identified in spaces of the model parameters characterising the interaction between virus and CTL populations. Our analysis suggests that the coexistence of replication competent virus below the conventional detection limit (of about 100 pfu per spleen) in the immune host as an equilibrium state requires the per day relative growth rate of the virus population to decrease at least 5-fold compared to the acute phase of infection. Oscillatory patterns in the dynamics of persisting LCMV and CTL memory, with virus population varying between 1 and 100 pfu per spleen, are possible within quite narrow intervals of the rates of virus growth and precursor CTL population death. Whereas the virus replication rate appears to determine the stability of the low level virus persistence, it does not affect the steady-state level of the viral population, except for very low values.

Noncytolytic control of viral infections by the innate and adaptive immune response

This review describes the contribution of noncytolytic mechanisms to the control of viral infections with a particular emphasis on the role of cytokines in these processes. It has long been known that most cell types in the body respond to an incoming viral infection by rapidly secreting antiviral cytokines such as interferon alpha/beta (IFN-alpha/beta). After binding to specific receptors on the surface of infected cells, IFN-alpha/beta has the potential to trigger the activation of multiple noncytolytic intracellular antiviral pathways that can target many steps in the viral life cycle, thereby limiting the amplification and spread of the virus and attenuating the infection. Clearance of established viral infections, however, requires additional functions of the immune response. The accepted dogma is that complete clearance of intracellular viruses by the immune response depends on the destruction of infected cells by the effector cells of the innate and adaptive immune system [natural killer (NK) cells and cytotoxic T cells (CTLs)]. This notion, however, has been recently challenged by experimental evidence showing that much of the antiviral potential of these cells reflects their ability to produce antiviral cytokines such as IFN-gamma and tumor necrosis factor (TNF)-alpha at the site of the infection. Indeed, these cytokines can purge viruses from infected cells noncytopathically as long as the cell is able to activate antiviral mechanisms and the virus is sensitive to them. Importantly, the same cytokines also control viral infections indirectly, by modulating the induction, amplification, recruitment, and effector functions of the immune response and by upregulating antigen processing and display of viral epitopes at the surface of infected cells. In keeping with these concepts, it is not surprising that a number of viruses encode proteins that have the potential to inhibit the antiviral activity of cytokines.

Role of the promyelocytic leukemia protein PML in the interferon sensitivity of lymphocytic choriomeningitis virus

Lymphocytic choriomeningitis virus (LCMV) induces type I interferon (alpha and beta interferon [IFN-alpha and IFN-beta]) upon infection and yet is sensitive to the addition of type II interferon (gamma interferon [IFN-gamma]) to the culture media. This sensitivity is biologically important because it correlates inversely with the ability of certain LCMV strains to persist in mice (D. Moskophidis, M. Battegay, M. A. Bruendler, E. Laine, I. Gresser, and R. M. Zinkernagel, J. Virol. 68:1951-1955, 1994). The cellular oncoprotein PML is induced by both IFN-alpha/beta and IFN-gamma, and PML binds the LCMV Z protein and becomes redistributed within cells from nucleus to cytoplasm upon LCMV infection. In the present study, increased PML expression results in diminished LCMV replication, implicating PML in the IFN sensitivity of LCMV. Virus production in PML -/- murine embryonic fibroblasts (MEF) exceeds virus production in PML +/+ MEF, and this difference is exacerbated by IFN treatment that upregulates PML expression. IFN-gamma also diminishes LCMV production in PML -/- cells; therefore, viral IFN sensitivity is not entirely due to PML. Both viral mRNA production and viral protein production decrease as PML expression increases. Here we propose that PML reduces LCMV transcription through its interaction with the Z protein.

Impairment of CD4(+) T cell responses during chronic virus infection prevents neutralizing antibody responses against virus escape mutants

We have shown previously that neutralizing antibodies (nAbs) are important contributors to the long-term immune control of lymphocytic choriomeningitis virus infection, particularly if cytotoxic T cell responses are low or absent. Nevertheless, virus escape from the nAb response due to mutations within the surface glycoprotein gene may subsequently allow the virus to persist. Here we show that most of the antibody-escape viral mutants retain their immunogenicity. We present evidence that the failure of the infected host to mount effective humoral responses against emerging neutralization-escape mutants correlates with the rapid loss of CD4(+) T cell responsiveness during the establishment of viral persistence. Similar mechanisms may contribute to the persistence of some human pathogens such as hepatitis B and C viruses, and human immunodeficiency virus.

Host factors influencing viral persistence

With the aim of characterizing the antiviral immune response to a non-cytocidal virus, we studied the outcome of lymphocytic choriomeningitis virus infection in a number of gene knockout mouse strains. Two virus strains differing markedly in their capacity to spread and replicate inside the murine host were used. Our results reveal that very different outcomes may be observed depending on virus strain and immunocompetence of the host. Thus while CD4+ cells are not critical during the initial phase of virus control, infectious virus reappear in mice lacking CD4+ cells, B cells or CD40 ligand. Reappearance of virus is associated with impaired long-term CD8+ T-cell mediated immune surveillance, and the time to virus resurgence is inversely correlated to the replication rate of the virus. Our studies also reveal that interferon-gamma is a central cytokine, and depending on the rate of virus replication, mice lacking the ability to produce interferon-gamma may develop either a severe, mostly fatal, T-cell mediated wasting syndrome or a chronic infection characterized by long-term coexistence of antiviral cytotoxic T lymphocytes and infectious virus. Mathematical modelling indicates that these different outcomes may be explained in relatively simple mathematical terms. This suggests that modelling may be used as a means to predict critical host and virus parameters. Therefore, combining mathematical modelling with precise, quantitative, in vivo analyses looks to be a promising approach in addressing central quantitative issues in immunobiology.