Contributions of the lymphocytic choriomeningitis virus glycoprotein and polymerase to strain-specific differences in murine liver pathogenicity

Durable lymphocyte subset elimination upon a single dose of AAV-delivered depletion antibody dissects immune control of chronic viral infection

To interrogate the role of specific immune cells in infection, cancer, and autoimmunity, immunologists commonly use monoclonal depletion antibodies (depletion-mAbs) or genetically engineered mouse models (GEMMs). To generate a tool that combines specific advantages and avoids select drawbacks of the two methods, we engineered adeno-associated viral vectors expressing depletion mAbs (depletion-AAVs). Single-dose depletion-AAV administration durably eliminated lymphocyte subsets in mice and avoided accessory deficiencies of GEMMs, such as marginal zone defects in B cell-deficient animals. Depletion-AAVs can be used in animals of different genetic backgrounds, and multiple depletion-AAVs can readily be combined. Exploiting depletion-AAV technology, we showed that B cells were required for unimpaired CD4+ and CD8+ T cell responses to chronic lymphocytic choriomeningitis virus (LCMV) infection. Upon B cell depletion, CD8+ T cells failed to suppress viremia, and they only helped resolve chronic infection when antibodies dampened viral loads. Our study positions depletion-AAVs as a versatile tool for immunological research.

Cellular and transcriptional impacts of Janus kinase and/or IFN-gamma inhibition in a mouse model of primary hemophagocytic lymphohistiocytosis

Background

Primary hemophagocytic lymphohistiocytosis (pHLH) is an inherited inflammatory syndrome driven by the exuberant activation of interferon-gamma (IFNg)-producing CD8 T cells. Towards this end, ruxolitinib treatment or IFNg neutralization (aIFNg) lessens immunopathology in a model of pHLH in which perforin-deficient mice (Prf1-/-) are infected with Lymphocytic Choriomeningitis virus (LCMV). However, neither agent completely eradicates inflammation. Two studies combining ruxolitinib with aIFNg report conflicting results with one demonstrating improvement and the other worsening of disease manifestations. As these studies used differing doses of drugs and varying LCMV strains, it remained unclear whether combination therapy is safe and effective.

Methods

We previously showed that a ruxolitinib dose of 90 mg/kg lessens inflammation in Prf1-/- mice infected with LCMV-Armstrong. To determine whether this dose controls inflammation induced by a different LCMV strain, we administered ruxolitinib at 90mg/kg to Prf1-/- mice infected with LCMV-WE. To elucidate the impacts of single agent versus combination therapy, Prf1-/- animals were infected with LCMV, treated or not with ruxolitinib, aIFNg or both agents, and analyzed for disease features and the transcriptional impacts of therapy within purified CD8 T cells.

Results

Ruxolitinib is well-tolerated and controls disease regardless of the viral strain used. aIFNg, administered alone or with ruxolitinib, is most effective at reversing anemia and reducing serum IFNg levels. In contrast, ruxolitinib appears better than aIFNg, and equally or more effective than combination therapy, at lessening immune cell expansion and cytokine production. Each treatment targets distinct gene expression pathways with aIFNg downregulating IFNg, IFNa, and IL-6-STAT3 pathways, and ruxolitinib downregulating IL-6-STAT3, glycolysis, and reactive oxygen species pathways. Unexpectedly, combination therapy is associated with upregulation of genes driving cell survival and proliferation.

Conclusions

Ruxolitinib is tolerated and curtails inflammation regardless of the inciting viral strain and whether it is given alone or in combination with aIFNg. When administered at the doses used in this study, the combination of ruxolitinb and aIFNg appears no better than treatment with either drug alone in lessening inflammation. Further studies are warranted to elucidate the optimal doses, schedules, and combinations of these agents for the treatment of patients with pHLH.

Antibody bivalency improves antiviral efficacy by inhibiting virion release independently of Fc gamma receptors

Across the animal kingdom, multivalency discriminates antibodies from all other immunoglobulin superfamily members. The evolutionary forces conserving multivalency above other structural hallmarks of antibodies remain, however, incompletely defined. Here, we engineer monovalent either Fc-competent or -deficient antibody formats to investigate mechanisms of protection of neutralizing antibodies (nAbs) and non-neutralizing antibodies (nnAbs) in virus-infected mice. Antibody bivalency enables the tethering of virions to the infected cell surface, inhibits the release of virions in cell culture, and suppresses viral loads in vivo independently of Fc gamma receptor (FcγR) interactions. In return, monovalent antibody formats either do not inhibit virion release and fail to protect in vivo or their protective efficacy is largely FcγR dependent. Protection in mice correlates with virus-release-inhibiting activity of nAb and nnAb rather than with their neutralizing capacity. These observations provide mechanistic insights into the evolutionary conservation of antibody bivalency and help refining correlates of nnAb protection for vaccine development.

Vectored antibody gene delivery restores host B and T cell control of persistent viral infection

Passive antibody therapy and vectored antibody gene delivery (VAGD) in particular offer an innovative approach to combat persistent viral diseases. Here, we exploit a small animal model to investigate synergies of VAGD with the host's endogenous immune defense for treating chronic viral infection. An adeno-associated virus (AAV) vector delivering the lymphocytic choriomeningitis virus (LCMV)-neutralizing antibody KL25 (AAV-KL25) establishes protective antibody titers for >200 days. When therapeutically administered to chronically infected immunocompetent wild-type mice, AAV-KL25 affords sustained viral load control. In contrast, viral mutational escape thwarts therapeutic AAV-KL25 effects when mice are unable to mount LCMV-specific antibody responses or lack CD8⁺ T cells. VAGD augments antiviral germinal center B cell and antibody-secreting cell responses and reduces inhibitory receptor expression on antiviral CD8⁺ T cells. These results indicate that VAGD fortifies host immune defense and synergizes with B cell and CD8 T cell responses to restore immune control of chronic viral infection.

Persistent RNA virus infection is short-lived at the single-cell level but leaves transcriptomic footprints

Several RNA viruses can establish life-long persistent infection in mammalian hosts, but the fate of individual virus-infected cells remains undefined. Here we used Cre recombinase-encoding lymphocytic choriomeningitis virus to establish persistent infection in fluorescent cell fate reporter mice. Virus-infected hepatocytes underwent spontaneous noncytolytic viral clearance independently of type I or type II interferon signaling or adaptive immunity. Viral clearance was accompanied by persistent transcriptomic footprints related to proliferation and extracellular matrix remodeling, immune responses, and metabolism. Substantial overlap with persistent epigenetic alterations in HCV-cured patients suggested a universal RNA virus-induced transcriptomic footprint. Cell-intrinsic clearance occurred in cell culture, too, with sequential infection, reinfection cycles separated by a period of relative refractoriness to infection. Our study reveals that systemic persistence of a prototypic noncytolytic RNA virus depends on continuous spread and reinfection. Yet undefined cell-intrinsic mechanisms prevent viral persistence at the single-cell level but give way to profound transcriptomic alterations in virus-cleared cells.

Opposing effects of T cell receptor signal strength on CD4 T cells responding to acute versus chronic viral infection

A hallmark of adaptive immunity is CD4 T cells’ ability to differentiate into specialized effectors. A long-standing question is whether T cell receptor (TCR) signal strength can dominantly instruct the development of Th1 and T follicular helper (Tfh) cells across distinct infectious contexts. We characterized the differentiation of murine CD4 TCR transgenic T cells responding to altered peptide ligand lymphocytic choriomeningitis viruses (LCMV) derived from acute and chronic parental strains. We found that TCR signal strength exerts opposite and hierarchical effects on the balance of Th1 and Tfh cells responding to acute versus persistent infection. TCR signal strength correlates positively with Th1 generation during acute but negatively during chronic infection. Weakly activated T cells express lower levels of markers associated with chronic T cell stimulation and may resist functional inactivation. We anticipate that the panel of recombinant viruses described herein will be valuable for investigating a wide range of CD4 T cell responses.

Chronic Viral Infection Promotes Efficient Germinal Center B Cell Responses

Persistent viral infections subvert key elements of adaptive immunity. To compare germinal center (GC) B cell responses in chronic and acute lymphocytic choriomeningitis virus infection, we exploit activation-induced deaminase (AID) fate-reporter mice and perform adoptive B cell transfer experiments. Chronic infection yields GC B cell responses of higher cellularity than acute infections do, higher memory B cell and antibody secreting cell output for longer periods of time, a better representation of the late B cell repertoire in serum immunoglobulin, and higher titers of protective neutralizing antibodies. GC B cells of chronically infected mice are similarly hypermutated as those emerging from acute infection. They efficiently adapt to viral escape variants and even in hypermutation-impaired AID mutant mice, chronic infection selects for GC B cells with hypermutated B cell receptors (BCRs) and neutralizing antibody formation. These findings demonstrate that, unlike for CD8⁺ T cells, chronic viral infection drives a functional, productive, and protective GC B cell response.

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Chronic viral infection elicits potent and sustained germinal center (GC) responses

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Chronic infection triggers prolonged plasma cell and memory B cell output from GCs

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GC B cells hypermutate efficiently and are potently selected in chronic infection

Hepatocyte-intrinsic type I interferon signaling reprograms metabolism and reveals a novel compensatory mechanism of the tryptophan-kynurenine pathway in viral hepatitis

The liver is a central regulator of metabolic homeostasis and serum metabolite levels. Hepatocytes are the functional units of the liver parenchyma and not only responsible for turnover of biomolecules but also act as central immune signaling platforms. Hepatotropic viruses infect liver tissue, resulting in inflammatory responses, tissue damage and hepatitis. Combining well-established in vitro and in vivo model systems with transcriptomic analyses, we show that type I interferon signaling initiates a robust antiviral immune response in hepatocytes. Strikingly, we also identify IFN-I as both, sufficient and necessary, to induce wide-spread metabolic reprogramming in hepatocytes. IFN-I specifically rewired tryptophan metabolism and induced hepatic tryptophan oxidation to kynurenine via Tdo2, correlating with altered concentrations of serum metabolites upon viral infection. Infected Tdo2-deficient animals displayed elevated serum levels of tryptophan and, unexpectedly, also vast increases in the downstream immune-suppressive metabolite kynurenine. Thus, Tdo2-deficiency did not result in altered serum homeostasis of the tryptophan to kynurenine ratio during infection, which seemed to be independent of hepatocyte-intrinsic compensation via the IDO-axis. These data highlight that inflammation-induced reprogramming of systemic tryptophan metabolism is tightly regulated in viral hepatitis.

Viral hepatitis is responsible for more than one million annual deaths worldwide and may progress to liver cirrhosis and hepatocellular carcinoma. The main metabolic cell type of the liver is the hepatocyte. In viral hepatitis, type I interferon (IFN-I) signaling rewires hepatocyte metabolism and serum metabolites to shape disease pathophysiology – an immune-regulatory circuit that might be therapeutically exploited. Here, we show that hepatocyte-intrinsic antiviral IFN-I signaling is both necessary and sufficient to induce wide-spread metabolic changes in hepatocytes. We identify a IFN-I-mediated induction of the hepatic kynurenine pathway via the rate-limiting and liver-specific enzyme TDO2, which controls serum homeostasis of tryptophan by converting it into kynurenine. Loss of TDO2 triggers a so far unknown compensatory mechanism, resulting in a vast increase of circulating kynurenine independent of hepatocyte intrinsic activity of the related IDO-enzymes. This study provides new insights into how inflammation reprograms metabolism of the liver and the kynurenine pathway during viral hepatitis.

Brain-resident memory T cells generated early in life predispose to autoimmune disease in mice

Epidemiological studies associate viral infections during childhood with the risk of developing autoimmune disease during adulthood. However, the mechanistic link between these events remains elusive. We report that transient viral infection of the brain in early life, but not at a later age, precipitates brain autoimmune disease elicited by adoptive transfer of myelin-specific CD4⁺ T cells at sites of previous infection in adult mice. Early-life infection of mouse brains imprinted a chronic inflammatory signature that consisted of brain-resident memory T cells expressing the chemokine (C-C motif) ligand 5 (CCL5). Blockade of CCL5 signaling via C-C chemokine receptor type 5 prevented the formation of brain lesions in a mouse model of autoimmune disease. In mouse and human brain, CCL5⁺ T_RM were located predominantly to sites of microglial activation. This study uncovers how transient brain viral infections in a critical window in life might leave persisting chemotactic cues and create a long-lived permissive environment for autoimmunity.

Population structure of Lassa Mammarenavirus in West Africa

Lassa mammarenavirus (LASV) is the etiologic agent of Lassa fever. In endemic regions in West Africa, LASV genetic diversity tends to cluster by geographic area. Seven LASV lineages are recognized, but the role of viral genetic determinants on disease presentation in humans is uncertain. We investigated the geographic structure and distribution of LASV in West Africa. We found strong spatial clustering of LASV populations, with two major east–west and north–south diversity gradients. Analysis of ancestry components indicated that known LASV lineages diverged from an ancestral population that most likely circulated in Nigeria, although alternative locations, such as Togo, cannot be excluded. Extant sequences carrying the largest contribution of this ancestral population include the prototype Pinneo strain, the Togo isolates, and a few viruses isolated in Nigeria. The LASV populations that experienced the strongest drift circulate in Mali and the Ivory Coast. By focusing on sequences form a single LASV sublineage (IIg), we identified an ancestry component possibly associated with protection from a fatal disease outcome. Although the same ancestry component tends to associate with lower viral loads in plasma, the small sample size requires that these results are treated with extreme caution.

The lipid-sensor TREM2 aggravates disease in a model of LCMV-induced hepatitis

Lipid metabolism is increasingly being appreciated to affect immunoregulation, inflammation and pathology. In this study we found that mice infected with lymphocytic choriomeningitis virus (LCMV) exhibit global perturbations of circulating serum lipids. Mice lacking the lipid-sensing surface receptor triggering receptor expressed on myeloid cells 2 (Trem2 ^-/-) were protected from LCMV-induced hepatitis and showed improved virus control despite comparable virus-specific T cell responses. Non-hematopoietic expression of TREM2 was found to be responsible for aggravated hepatitis, indicating a novel role for TREM2 in the non-myeloid compartment. These results suggest a link between virus-perturbed lipids and TREM2 that modulates liver pathogenesis upon viral infection. Targeted interventions of this immunoregulatory axis may ameliorate tissue pathology in hepatitis.

Evolutionary analysis of Old World arenaviruses reveals a major adaptive contribution of the viral polymerase

The Old World (OW) arenavirus complex includes several species of rodent-borne viruses, some of which (i.e., Lassa virus, LASV and Lymphocytic choriomeningitis virus, LCMV) cause human diseases. Most LCMV and LASV infections are caused by rodent-to-human transmissions. Thus, viral evolution is largely determined by events that occur in the wildlife reservoirs. We used a set of human- and rodent-derived viral sequences to investigate the evolutionary history underlying OW arenavirus speciation, as well as the more recent selective events that accompanied LASV spread in West Africa. We show that the viral RNA polymerase (L protein) was a major positive selection target in OW arenaviruses and during LASV out-of-Nigeria migration. No evidence of selection was observed for the glycoprotein, whereas positive selection acted on the nucleoprotein (NP) during LCMV speciation. Positively selected sites in L and NP are surrounded by highly conserved residues, and the bulk of the viral genome evolves under purifying selection. Several positively selected sites are likely to modulate viral replication/transcription. In both L and NP, structural features (solvent exposed surface area) are important determinants of site-wise evolutionary rate variation. By incorporating several rodent-derived sequences, we also performed an analysis of OW arenavirus codon adaptation to the human host. Results do not support a previously hypothesized role of codon adaptation in disease severity for non-Nigerian strains. In conclusion, L and NP represent the major selection targets and possible determinants of disease presentation; these results suggest that field surveys and experimental studies should primarily focus on these proteins.

Interferon-driven deletion of antiviral B cells at the onset of chronic infection

Inadequate antibody responses and perturbed B cell compartments represent hallmarks of persistent microbial infections, but the mechanisms whereby persisting pathogens suppress humoral immunity remain poorly defined. Using adoptive transfer experiments in the context of a chronic lymphocytic choriomeningitis virus (LCMV) infection of mice, we have documented rapid depletion of virus-specific B cells that coincided with the early type I interferon response to infection. We found that the loss of activated B cells was driven by type I interferon (IFN-I) signaling to several cell types including dendritic cells, T cells and myeloid cells. Intriguingly, this process was independent of B cell-intrinsic IFN-I sensing and resulted from biased differentiation of naïve B cells into short-lived antibody-secreting cells. The ability to generate robust B cell responses was restored upon IFN-I receptor blockade or, partially, when experimentally depleting myeloid cells or the IFN-I-induced cytokines interleukin 10 and tumor necrosis factor alpha. We have termed this IFN-I-driven depletion of B cells "B cell decimation". Strategies to counter "B cell decimation" should thus help us better leverage humoral immunity in the combat against persistent microbial diseases.

Brain-resident memory T cells represent an autonomous cytotoxic barrier to viral infection

During viral infections, brain tissue–resident memory T cells (bTRM) prevent fatal brain infection after acquiring perforin- and IFN-γ–dependent effector functions through a pathway that involves presentation of cognate antigen on MHC-I.

Tissue-resident memory T cells (T_RM) persist at sites of prior infection and have been shown to enhance pathogen clearance by recruiting circulating immune cells and providing bystander activation. Here, we characterize the functioning of brain-resident memory T cells (bT_RM) in an animal model of viral infection. bT_RM were subject to spontaneous homeostatic proliferation and were largely refractory to systemic immune cell depletion. After viral reinfection in mice, bT_RM rapidly acquired cytotoxic effector function and prevented fatal brain infection, even in the absence of circulating CD8⁺ memory T cells. Presentation of cognate antigen on MHC-I was essential for bT_RM-mediated protective immunity, which involved perforin- and IFN-γ–dependent effector mechanisms. These findings identify bT_RM as an organ-autonomous defense system serving as a paradigm for T_RM functioning as a self-sufficient first line of adaptive immunity.

Attenuated viral hepatitis in Trem1-/- mice is associated with reduced inflammatory activity of neutrophils

TREM1 (Triggering Receptor Expressed on Myeloid Cells 1) is a pro-inflammatory receptor expressed by phagocytes, which can also be released as a soluble molecule (sTREM1). The roles of TREM1 and sTREM1 in liver infection and inflammation are not clear. Here we show that patients with hepatitis B virus (HBV) or hepatitis C virus (HCV) infection manifest elevated serum levels of sTREM1. In mice, experimental viral hepatitis induced by infection with Lymphocytic Choriomeningitis Virus (LCMV)-WE was likewise associated with increased sTREM1 in serum and urine, and with increased TREM1 and its associated adapter molecule DAP12 in the liver. Trem1−/− mice showed accelerated clearance of LCMV-WE and manifested attenuated liver inflammation and injury. TREM1 expression in the liver of wild-type mice was mostly confined to infiltrating neutrophils, which responded to LCMV by secretion of CCL2 and TNF-α, and release of sTREM1. Accordingly, the production of CCL2 and TNF-α was decreased in the livers of LCMV-infected Trem1−/− mice, as compared to LCMV-infected wildtype mice. These findings indicate that TREM1 plays a role in viral hepatitis, in which it seems to aggravate the immunopathology associated with viral clearance, mainly by increasing the inflammatory activity of neutrophils.

Superoxide Dismutase 1 Protects Hepatocytes from Type I Interferon-Driven Oxidative Damage

Tissue damage caused by viral hepatitis is a major cause of morbidity and mortality worldwide. Using a mouse model of viral hepatitis, we identified virus-induced early transcriptional changes in the redox pathways in the liver, including downregulation of superoxide dismutase 1 (Sod1). Sod1(-/-) mice exhibited increased inflammation and aggravated liver damage upon viral infection, which was independent of T and NK cells and could be ameliorated by antioxidant treatment. Type I interferon (IFN-I) led to a downregulation of Sod1 and caused oxidative liver damage in Sod1(-/-) and wild-type mice. Genetic and pharmacological ablation of the IFN-I signaling pathway protected against virus-induced liver damage. These results delineate IFN-I mediated oxidative stress as a key mediator of virus-induced liver damage and describe a mechanism of innate-immunity-driven pathology, linking IFN-I signaling with antioxidant host defense and infection-associated tissue damage. VIDEO ABSTRACT.

Novel mechanism of arenavirus-induced liver pathology

Viral hemorrhagic fevers (VHFs) encompass a group of diseases with cardinal symptoms of fever, hemorrhage, and shock. The liver is a critical mediator of VHF disease pathogenesis and high levels of ALT/AST transaminases in plasma correlate with poor prognosis. In fact, Lassa Fever (LF), the most prevalent VHF in Africa, was initially clinically described as hepatitis. Previous studies in non-human primate (NHP) models also correlated LF pathogenesis with a robust proliferative response in the liver. The purpose of the current study was to gain insight into the mechanism of liver injury and to determine the potential role of proliferation in LF pathogenesis. C57Bl/6J mice were infected with either the pathogenic (for NHPs) strain of lymphocytic choriomeningitis virus (LCMV, the prototypic arenavirus), LCMV-WE, or with the non-pathogenic strain, LCMV-ARM. As expected, LCMV-WE, but not ARM, caused a hepatitis-like infection. LCMV-WE also induced a robust increase in the number of actively cycling hepatocytes. Despite this increase in proliferation, there was no significant difference in liver size between LCMV-WE and LCMV-ARM, suggesting that cell cycle was incomplete. Indeed, cells appeared arrested in the G₁ phase and LCMV-WE infection increased the number of hepatocytes that were simultaneously stained for proliferation and apoptosis. LCMV-WE infection also induced expression of a non-conventional virus receptor, AXL-1, from the TAM (TYRO3/AXL/MERTK) family of receptor tyrosine kinases and this expression correlated with proliferation. Taken together, these results shed new light on the mechanism of liver involvement in VHF pathogenesis. Specifically, it is hypothesized that the induction of hepatocyte proliferation contributes to expansion of the infection to parenchymal cells. Elevated levels of plasma transaminases are likely explained, at least in part, by abortive cell cycle arrest induced by the infection. These results may lead to the development of new therapies to prevent VHF progression.

The role of Kupffer cells in hepatitis B and hepatitis C virus infections

Globally, over 500 million people are chronically infected with the hepatitis B virus (HBV) or hepatitis C virus (HCV). These chronic infections cause liver inflammation, and may result in fibrosis/cirrhosis or hepatocellular carcinoma. Albeit that HBV and HCV differ in various aspects, clearance, persistence, and immunopathology of either infection depends on the interplay between the innate and adaptive responses in the liver. Kupffer cells, the liver-resident macrophages, are abundantly present in the sinusoids of the liver. These cells have been shown to be crucial players to maintain homeostasis, but also contribute to pathology. However, it is important to note that especially during pathology, Kupffer cells are difficult to distinguish from infiltrating monocytes/macrophages and other myeloid cells. In this review we discuss our current understanding of Kupffer cells, and assess their role in the regulation of anti-viral immunity and disease pathogenesis during HBV and HCV infection.

CD4+ T-cell help is required for effective CD8+ T cell-mediated resolution of acute viral hepatitis in mice

Cytotoxic CD8+ T cells are essential for the control of viral liver infections, such as those caused by HBV or HCV. It is not entirely clear whether CD4+ T-cell help is necessary for establishing anti-viral CD8+ T cell responses that successfully control liver infection. To address the role of CD4+ T cells in acute viral hepatitis, we infected mice with Lymphocytic Choriomeningitis Virus (LCMV) of the strain WE; LCMV-WE causes acute hepatitis in mice and is cleared from the liver by CD8+ T cells within about two weeks. The role of CD4+ T-cell help was studied in CD4+ T cell-lymphopenic mice, which were either induced by genetic deficiency of the major histocompatibility (MHC) class II transactivator (CIITA) in CIITA−/− mice, or by antibody-mediated CD4+ cell depletion. We found that CD4+ T cell-lymphopenic mice developed protracted viral liver infection, which seemed to be a consequence of reduced virus-specific CD8+ T-cell numbers in the liver. Moreover, the anti-viral effector functions of the liver-infiltrating CD8+ T cells in response to stimulation with LCMV peptide, notably the IFN-γ production and degranulation capacity were impaired in CIITA−/− mice. The impaired CD8+ T-cell function in CIITA−/− mice was not associated with increased expression of the exhaustion marker PD-1. Our findings indicate that CD4+ T-cell help is required to establish an effective antiviral CD8+ T-cell response in the liver during acute viral infection. Insufficient virus control and protracted viral hepatitis may be consequences of impaired initial CD4+ T-cell help.

Neuroprotective intervention by interferon-γ blockade prevents CD8+ T cell-mediated dendrite and synapse loss

Neurons are postmitotic and thus irreplaceable cells of the central nervous system (CNS). Accordingly, CNS inflammation with resulting neuronal damage can have devastating consequences. We investigated molecular mediators and structural consequences of CD8(+) T lymphocyte (CTL) attack on neurons in vivo. In a viral encephalitis model in mice, disease depended on CTL-derived interferon-γ (IFN-γ) and neuronal IFN-γ signaling. Downstream STAT1 phosphorylation and nuclear translocation in neurons were associated with dendrite and synapse loss (deafferentation). Analogous molecular and structural alterations were also found in human Rasmussen encephalitis, a CTL-mediated human autoimmune disorder of the CNS. Importantly, therapeutic intervention by IFN-γ blocking antibody prevented neuronal deafferentation and clinical disease without reducing CTL responses or CNS infiltration. These findings identify neuronal IFN-γ signaling as a novel target for neuroprotective interventions in CTL-mediated CNS disease.

Cell surface molecules involved in infection mediated by lymphocytic choriomeningitis virus glycoprotein

The glycoprotein (GP) of lymphocytic choriomeningitis virus (LCMV), the prototype arenavirus, is a promising envelope protein of lentiviral pseudotype vectors for gene therapy. The distribution of dystroglycan, a known receptor for LCMV, cannot explain the narrow tropism of LCMV-GP-pseudotypes. Here, we examined whether infection of LCMV-GP-pseudotypes was affected by the expression of four cell surface molecules-Axl and Tyro3 (from the TAM family) and DC-SIGN and LSECtin (from the C-type lectin family)-that are known receptors of Lassa virus, another arenavirus. All four molecules enhanced LCMV-GP-pseudotype infection of cells. These results help explain the tropism of LCMV-GP-pseudotypes and further our understanding of LCMV infection in animals.

Autoreactive T cells bypass negative selection and respond to self-antigen stimulation during infection

Autoimmunity occurs because central and peripheral tolerance mechanisms fail to tolerize T cells with weak self-reactivity to tissue-restricted antigen.

Central and peripheral tolerance prevent autoimmunity by deleting the most aggressive CD8⁺ T cells but they spare cells that react weakly to tissue-restricted antigen (TRA). To reveal the functional characteristics of these spared cells, we generated a transgenic mouse expressing the TCR of a TRA-specific T cell that had escaped negative selection. Interestingly, the isolated TCR matches the affinity/avidity threshold for negatively selecting T cells, and when developing transgenic cells are exposed to their TRA in the thymus, only a fraction of them are eliminated but significant numbers enter the periphery. In contrast to high avidity cells, low avidity T cells persist in the antigen-positive periphery with no signs of anergy, unresponsiveness, or prior activation. Upon activation during an infection they cause autoimmunity and form memory cells. Unexpectedly, peptide ligands that are weaker in stimulating the transgenic T cells than the thymic threshold ligand also induce profound activation in the periphery. Thus, the peripheral T cell activation threshold during an infection is below that of negative selection for TRA. These results demonstrate the existence of a level of self-reactivity to TRA to which the thymus confers no protection and illustrate that organ damage can occur without genetic predisposition to autoimmunity.

The proapoptotic Bcl-2 family member Bim plays a central role during the development of virus-induced hepatitis

The proapoptotic Bcl-2 homolog Bim was shown to control the apoptosis of both T cells and hepatocytes. This dual role of Bim might be particularly relevant for the development of viral hepatitis, in which both the sensitivity of hepatocytes to apoptosis stimuli and the persistence of cytotoxic T cells are essential factors for the outcome of the disease. The relevance of Bim in regulating survival of cytotoxic T cells or induction of hepatocyte death has only been investigated in separate systems, and their relative contributions to the pathogenesis of T cell-mediated hepatitis remain unclear. Using the highly dynamic model system of lymphocytic choriomeningitis virus-mediated hepatitis and bone marrow chimeras, we found that Bim has a dual role in the development of lymphocytic choriomeningitis virus-induced, T cell-mediated hepatitis. Although the absence of Bim in parenchymal cells led to markedly attenuated liver damage, loss of Bim in the lymphoid compartment moderately enhanced hepatitis. However, when both effects were combined in Bim(-/-) mice, the effect of Bim deficiency in the lymphoid compartment was overcompensated for by the reduced sensitivity of Bim(-/-) hepatocytes to T cell-induced apoptosis, resulting in the protection of Bim(-/-) mice from hepatitis.

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.

T cells can mediate viral clearance from ependyma but not from brain parenchyma in a major histocompatibility class I- and perforin-independent manner

Viral infection of the central nervous system can lead to disability and death. Yet the majority of viral infections with central nervous system involvement resolve with only mild clinical manifestations, if any. This is generally attributed to efficient elimination of the infection from the brain coverings, i.e. the meninges, ependyma and chorioplexus, which are the primary targets of haematogeneous viral spread. How the immune system is able to purge these structures from viral infection with only minimal detrimental effects is still poorly understood. In the present work we studied how an attenuated lymphocytic choriomeningitis virus can be cleared from the central nervous system in the absence of overt disease. We show that elimination of the virus from brain ependyma, but not from brain parenchyma, could be achieved by a T cell-dependent mechanism operating independently of major histocompatibility class I antigens and perforin. Considering that cytotoxic T lymphocyte-mediated cytotoxicity is a leading cause of viral immunopathology and tissue damage, our findings may explain why the most common viral intruders of the central nervous system rarely represent a serious threat to our health.

CD4+ T cell help improves CD8+ T cell memory by retained CD27 expression

CD4+ T cell help during the priming of CD8+ T lymphocytes imprints the capacity for optimal secondary expansion upon re-encounter with antigen. Helped memory CD8+ T cells rapidly expand in response to a secondary antigen exposure, even in the absence of T cell help and, are most efficient in protection against a re-infection. In contrast, helpless memory CTL can mediate effector function, but secondary expansion is reduced. How CD4+ T cells instruct CD8+ memory T cells during priming to undergo efficient secondary expansion has not been resolved in detail. Here, we show that memory CTL after infection with lymphocytic choriomeningitis virus are CD27(high) whereas memory CTL primed in the absence of CD4+ T cell have a reduced expression of CD27. Helpless memory CTL produced low amounts of IL-2 and did not efficiently expand after restimulation with peptide in vitro. Blocking experiments with monoclonal antibodies and the use of CD27(-/-) memory CTL revealed that CD27 ligation during restimulation increased autocrine IL-2 production and secondary expansion. Therefore, regulating CD27 expression on memory CTL is a novel mechanism how CD4+ T cells control CTL memory.

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.

Aggravation of viral hepatitis by platelet-derived serotonin

More than 500 million people worldwide are persistently infected with hepatitis B virus or hepatitis C virus. Although both viruses are poorly cytopathic, persistence of either virus carries a risk of chronic liver inflammation, potentially resulting in liver steatosis, liver cirrhosis, end-stage liver failure or hepatocellular carcinoma. Virus-specific T cells are a major determinant of the outcome of hepatitis, as they contribute to the early control of chronic hepatitis viruses, but they also mediate immunopathology during persistent virus infection. We have analyzed the role of platelet-derived vasoactive serotonin during virus-induced CD8(+) T cell-dependent immunopathological hepatitis in mice infected with the noncytopathic lymphocytic choriomeningitis virus. After virus infection, platelets were recruited to the liver, and their activation correlated with severely reduced sinusoidal microcirculation, delayed virus elimination and increased immunopathological liver cell damage. Lack of platelet-derived serotonin in serotonin-deficient mice normalized hepatic microcirculatory dysfunction, accelerated virus clearance in the liver and reduced CD8(+) T cell-dependent liver cell damage. In keeping with these observations, serotonin treatment of infected mice delayed entry of activated CD8(+) T cells into the liver, delayed virus control and aggravated immunopathological hepatitis. Thus, vasoactive serotonin supports virus persistence in the liver and aggravates virus-induced immunopathology.