Viral clearance during hepatitis B virus (HBV) infection has been thought to reflect the destruction of infected hepatocytes by CD8(+) T lymphocytes. However, in this study, HBV DNA was shown to largely disappear from the liver and the blood of acutely infected chimpanzees long before the peak of T cell infiltration and most of the liver disease. These results demonstrate that noncytopathic antiviral mechanisms contribute to viral clearance during acute viral hepatitis by purging HBV replicative intermediates from the cytoplasm and covalently closed circular viral DNA from the nucleus of infected cells.

Summary Viral nucleic acids often trigger an innate immune response in infected cells. Many viruses, including hepatitis C virus (HCV), have evolved mechanisms to evade intracellular recognition. Nevertheless, HCV-permissive cells can trigger a viral RNA-, TLR7- and cell contact-dependent compensatory interferon response in nonpermissive plasmacytoid dendritic cells (pDCs). Here we report that these events are mediated by transfer of HCV RNA-containing exosomes from infected cells to pDCs. The exosomal viral RNA transfer is dependent on the endosomal sorting complex (ESCRT) machinery and on Annexin A2, an RNA-binding protein involved in membrane vesicle trafficking, and it is suppressed by exosome release inhibitors. Further, purified concentrated HCV RNA-containing exosomes are sufficient to activate pDCs. Thus, vesicular sequestration and exosomal export of viral RNA may serve both as a viral strategy to evade pathogen-sensing within infected cells and as a host strategy to induce an unopposed innate response in replication-nonpermissive by-stander cells.

Hepatitis B virus (HBV) is a prototype for liver-specific pathogens in which the failure of the immune system to mount an effective response leads to chronic infection. Our understanding of the immune response to HBV is incomplete, largely due to the narrow host restriction of this pathogen and the limitations of existing experimental models. We have developed a murine model for studying human HBV replication, immunogenicity, and control. After transfection of hepatocytes in vivo with a replicationcompetent, over-length, linear HBV genome, viral antigens and replicative intermediates were synthesized and virus was secreted into the blood. Viral antigens disappeared from the blood as early as 7 days after transfection, coincident with the appearance of antiviral antibodies. HBV transcripts and replicative intermediates disappeared from the liver by day 15, after the appearance of antiviral CD8 ؉ T cells. In contrast, the virus persisted for at least 81 days after transfection of NOD͞Scid mice, which lack functional T cells, B cells, and natural killer (NK) cells. Thus, the outcome of hydrodynamic transfection of HBV depends on the host immune response, as it is during a natural infection. The methods we describe will allow the examination of viral dynamics in a tightly controlled in vivo system, the application of mutagenesis methods to the study of the HBV life cycle in vivo, and the dissection of the immune response to HBV using genetically modified mice whose immunoregulatory and immune effector functions have been deleted or overexpressed. In addition, this methodology represents a prototype for the study of other known and to-be-discovered liver-specific pathogens.H epatitis B virus (HBV) is a human hepadnavirus that causes acute and chronic hepatitis and hepatocellular carcinoma (1). Although an effective vaccine has been available for two decades, an estimated 350 million people worldwide are chronically infected. A significant proportion of chronic infections terminate in hepatocellular carcinoma, leading to more than one million deaths annually (2).A reproducible tissue culture model of HBV infection does not exist, nor is HBV infectious for immunologically well-defined laboratory animals. Much of our current understanding of the viral life cycle after HBV infection is derived from studies of duck HBV (DHBV) (3) and woodchuck HBV (WHV) (4) infection in their natural hosts, HBV-infected chimpanzees (5-7), and HBV transgenic mice (8-10). For various reasons, however, none of these models is ideal. DHBV (11) and WHV (12, 13) are genetically divergent from HBV, and immunological studies in genetically outbred and immunologically uncharacterized ducks and woodchucks are difficult. Chimpanzee experiments are limited by cost, availability, and ethical considerations, whereas transgenic mice are immunologically tolerant to the virus, thereby compromising the greatest potential strength of a mouse model of HBV infection. We present here work describing a recently developed mouse model that alleviates many of thes...

The virological and cellular consequences of persistent hepatitis C virus (HCV) infection have been elusive due to the absence of the requisite experimental systems. Here, we report the establishment and the characteristics of persistent in vitro infection of human hepatoma-derived cells by a recently described HCV genotype 2a infectious molecular clone. Persistent in vitro infection was characterized by the selection of viral variants that displayed accelerated expansion kinetics, higher peak titers, and increased buoyant densities. Sequencing analysis revealed the selection of a single adaptive mutation in the HCV E2 envelope protein that was largely responsible for the variant phenotype. In parallel, as the virus became more aggressive, cells that were resistant to infection emerged, displaying escape mechanisms operative at the level of viral entry, HCV RNA replication, or both. Collectively, these results reveal the existence of coevolutionary events during persistent HCV infection that favor survival of both virus and host.The hepatitis C virus (HCV) is a hepatotropic, positivestranded RNA virus that causes acute and chronic hepatitis. Because most infections become persistent, HCV chronically infects more than 170 million people worldwide, many of whom will develop liver cirrhosis and hepatocellular carcinoma (15). HCV is thought to be noncytopathic in vivo, and the pathogenesis of the associated hepatitis is assumed to reflect destruction of HCV infected cells by cytotoxic CD8 ϩ T cells (9). HCV is the sole member of the genus Hepacivirus in the Flaviviridae family. Its 9.6-kb RNA genome encodes a long open reading frame that is co-and posttranslationally cleaved by cellular and viral proteases into structural (core, E1, E2, and p7) and nonstructural (NS2, NS3, NS4A, NS4B, NS5A, and NS5B) proteins (2). The viral life cycle and the host-virus interactions that determine the outcome of HCV infection have been difficult to study due to the absence of a tissue culture model of HCV infection. Recently, several groups (16,20,25,29,31) have developed cell culture models of HCV infection that release HCV particles that are infectious for human hepatoma-derived cell lines. The most robust of these in vitro infections are based on the extraordinary replicative capacity of the genotype 2a JFH-1 strain of HCV, which replicates efficiently in vitro without requiring adaptive mutations (14). Importantly, cell culture-derived JFH-1 and a chimeric virus expressing the structural region of the related J6 strain of HCV and the nonstructural region of JFH-1 are infectious for chimpanzees and uPA-SCID mice reconstituted with human hepatocytes (17,25).At present, the cell culture system has been used primarily to study the early steps of HCV infection. For example, we and others (16, 31) have reported that primary HCV infection can be inhibited by blocking the interaction between the HCV E2 glycoprotein and the cellular protein CD81, an important coreceptor for HCV entry (3,13,18,30). In the current study, we used the cell c...

Background and Rationale Approximately 50% of patients with chronic hepatitis C (CHC) have ongoing expression of interferon stimulated genes (ISGs) in the liver. It is unclear why this endogenous antiviral response is inefficient in eradicating the infection. Several viral escape strategies have been identified in vitro, including inhibition of interferon (IFN) induction and ISG mRNA translation. The in vivo relevance of these mechanisms is unknown, because reliable methods to identify hepatitis C virus (HCV)-infected cells in human liver are lacking. We developed a highly sensitive in situ hybridization (ISH) system capable of HCV RNA and ISG mRNA detection in human liver biopsies and applied it to study the interaction of HCV with endogenous IFN system. Main Results We simultaneously monitored HCV RNA and ISG mRNA using HCV isolate- and ISG mRNA-specific probes in liver biopsy sections from 18 CHC patients. The signals were quantified at the single cell resolution in a series of random high-power fields. The proportion of infected hepatocytes ranged from 1 to 54% and correlated with viral load, but not with HCV genotype or ISG expression. Infected cells occurred in clusters, pointing to cell-to-cell spread as the predominant mode of HCV transmission. ISG mRNAs were readily detected in HCV-infected cells, challenging previously proposed mechanisms of viral interference with the immune system. Conversely, infected cells and neighboring cells showed increased ISG mRNA levels, demonstrating that the stimulus driving ISG expression originates from HCV infected hepatocytes. Conclusion HCV infection in human hepatocytes during CHC does not efficiently interfere with IFN induction, IFN signaling or transcription of ISG mRNA.

To better define the mechanism(s) likely responsible for viral clearance during hepatitis B virus (HBV) infection, viral clearance was studied in a panel of immunodeficient mouse strains that were hydrodynamically transfected with a plasmid containing a replication-competent copy of the HBV genome. Neither B cells nor perforin were required to clear the viral DNA transcriptional template from the liver. In contrast, the template persisted for at least 60 days at high levels in NOD/Scid mice and at lower levels in the absence of CD4 + and CD8 + T cells, NK cells, Fas, IFN-gamma (IFN-γ), IFN-alpha/beta receptor (IFN-α/βR1), and TNF receptor 1 (TNFR1), indicating that each of these effectors was required to eliminate the transcriptional template from the liver. Interestingly, viral replication was ultimately terminated in all lineages except the NOD/Scid mice, suggesting the existence of redundant pathways that inhibit HBV replication. Finally, induction of a CD8 + T cell response in these animals depended on the presence of CD4 + T cells. These results are consistent with a model in which CD4 + T cells serve as master regulators of the adaptive immune response to HBV; CD8 + T cells are the key cellular effectors mediating HBV clearance from the liver, apparently by a Fas-dependent, perforinindependent process in which NK cells, IFN-γ, TNFR1, and IFN-α/βR play supporting roles. These results provide insight into the complexity of the systems involved in HBV clearance, and they suggest unique directions for analysis of the mechanism(s) responsible for HBV persistence.H epatitis B virus (HBV) is a noncytopathic human hepadnavirus that causes acute and chronic hepatitis and hepatocellular carcinoma (1). Approximately 350 million people worldwide are chronically infected by HBV, which greatly increases the risk of hepatocellular carcinoma (HCC) and causes more than 1 million deaths annually (2). Because HBV is not infectious in small animal or tissue culture models, systematic examination of the host-virus interactions during HBV infection has been difficult. The full spectrum of immunological requirements for HBV clearance is not completely defined.Our current understanding is based to a large extent on comparison of the immune responses mounted against HBV in patients who clear and who fail to clear HBV (3), experiments in which potential effectors of clearance (e.g., HBsAg-specific CD8 + T cells, recombinant IFN-γ and TNF-α) have been adoptively transferred to or induced in HBV transgenic mice (4-8), and a limited number of experiments conducted in HBVinfected chimpanzees (9, 10, 18). Collectively, these studies have led to the current model for clearance of acute HBV infection, namely (i) that viral clearance during HBV infection is associated with entry of CD8 + T cells into the liver, the production of IFN-γ, and the induction of inflammatory liver disease (reviewed in ref.3); (ii) that IFN-γ production, viral clearance, and liver disease are all impaired in the absence of CD8 + T cells (11); and (iii) that noncy...

The genome sequence of the genetically tractable, mesophilic, hydrogenotrophic methanogen Methanococcus maripaludis contains 1,722 protein-coding genes in a single circular chromosome of 1,661,137 bp. Of the protein-coding genes (open reading frames [ORFs]), 44% were assigned a function, 48% were conserved but had unknown or uncertain functions, and 7.5% (129 ORFs) were unique to M. maripaludis. Of the unique ORFs, 27 were confirmed to encode proteins by the mass spectrometric identification of unique peptides. Genes for most known functions and pathways were identified. For example, a full complement of hydrogenases and methanogenesis enzymes was identified, including eight selenocysteine-containing proteins, with each being paralogous to a cysteine-containing counterpart. At least 59 proteins were predicted to contain iron-sulfur centers, including ferredoxins, polyferredoxins, and subunits of enzymes with various redox functions. The methanogenic Archaea (methanogens) occupy a unique metabolic niche, as they produce methane, which is a useful energy source and a powerful greenhouse gas. These organisms are found in diverse anaerobic habitats, ranging from aquatic and marine sediments to sewage digesters and the rumens and large intestines of herbivores and other mammals (127). In these habitats, the degradation of organic matter results in the production of H 2 and other intermediates by fermentative organisms. By maintaining an extremely low partial pressure of H 2 , the methanogens keep fermentative pathways energetically favorable. In addition, some methanogens may occupy niches where hydrogen is produced predominately by geothermal reactions.Metabolically, methanogens are divided into those that specialize in CO 2 reduction and those that also use acetate and/or methyl compounds. The former group, the hydrogenotrophs, use H 2 as an electron donor to reduce CO 2 to methane. Many hydrogenotrophic species can substitute formate or certain low-molecular-weight alcohols and ketones for H 2 . Complete genome sequences have been published for three hydrogenotrophic methanogens, Methanocaldococcus jannaschii (13), Methanothermobacter thermautotrophicus (105), and Methanopyrus kandleri (104), all of which are thermophiles or hyperthermophiles. Of the methanogens that utilize acetate and methyl compounds, complete genome sequences have been published for two species, Methanosarcina acetivorans (26) and Methanosarcina mazei (19), both of which are mesophiles. In addition, partial sequences have been published for two psychrophiles, the hydrogenotroph Methanogenium frigidum and the methylotroph Methanolobus burtonii (97).Genome sequences of methanogens have answered many questions, but they have inspired many others. More than half of the genes in Methanocaldococcus jannaschii lack a predicted function (13), and this proportion has not declined significantly as other methanogen sequences have been determined. The proportions of genes of unknown functions, which are either homologous to other genes of unknown function...

The intrahepatic immune environment is normally biased towards tolerance. Nonetheless, effective antiviral immune responses can be induced against hepatotropic pathogens. To examine the immunological basis of this paradox we studied the ability of hepatocellularly expressed hepatitis B virus (HBV) to activate immunologically naïve HBV-specific CD8+ T cell receptor (TCR) transgenic T cells after adoptive transfer to HBV transgenic mice. Intrahepatic priming triggered vigorous in situ T cell proliferation but failed to induce interferon gamma production or cytolytic effector function. In contrast, the same T cells differentiated into cytolytic effector T cells in HBV transgenic mice if Programmed Death 1 (PD-1) expression was genetically ablated, suggesting that intrahepatic antigen presentation per se triggers negative regulatory signals that prevent the functional differentiation of naïve CD8+ T cells. Surprisingly, coadministration of an agonistic anti-CD40 antibody (αCD40) inhibited PD-1 induction and restored T cell effector function, thereby inhibiting viral gene expression and causing a necroinflammatory liver disease. Importantly, the depletion of myeloid dendritic cells (mDCs) strongly diminished the αCD40 mediated functional differentiation of HBV-specific CD8+ T cells, suggesting that activation of mDCs was responsible for the functional differentiation of HBV-specific CD8+ T cells in αCD40 treated animals. These results demonstrate that antigen-specific, PD-1-mediated CD8+ T cell exhaustion can be rescued by CD40-mediated mDC-activation.