Immunofluorescence microscopy in experimentally induced, type B hepatitis in the chimpanzee

Hepatitis B Virus DNA Integration and Clonal Expansion of Hepatocytes in the Chronically Infected Liver

Human hepatitis B virus (HBV) can cause chronic, lifelong infection of the liver that may lead to persistent or episodic immune-mediated inflammation against virus-infected hepatocytes. This immune response results in elevated rates of killing of virus-infected hepatocytes, which may extend over many years or decades, lead to fibrosis and cirrhosis, and play a role in the high incidence of hepatocellular carcinoma (HCC) in HBV carriers. Immune-mediated inflammation appears to cause oxidative DNA damage to hepatocytes, which may also play a major role in hepatocarcinogenesis. An additional DNA damaging feature of chronic infections is random integration of HBV DNA into the chromosomal DNA of hepatocytes. While HBV DNA integration does not have a role in virus replication it may alter gene expression of the host cell. Indeed, most HCCs that arise in HBV carriers contain integrated HBV DNA and, in many, the integrant appears to have played a role in hepatocarcinogenesis. Clonal expansion of hepatocytes, which is a natural feature of liver biology, occurs because the hepatocyte population is self-renewing and therefore loses complexity due to random hepatocyte death and replacement by proliferation of surviving hepatocytes. This process may also represent a risk factor for the development of HCC. Interestingly, during chronic HBV infection, hepatocyte clones detected using integrated HBV DNA as lineage-specific markers, emerge that are larger than those expected to occur by random death and proliferation of hepatocytes. The emergence of these larger hepatocyte clones may reflect a survival advantage that could be explained by an ability to avoid the host immune response. While most of these larger hepatocyte clones are probably not preneoplastic, some may have already acquired preneoplastic changes. Thus, chronic inflammation in the HBV-infected liver may be responsible, at least in part, for both initiation of HCC via oxidative DNA damage and promotion of HCC via stimulation of hepatocyte proliferation through immune-mediated killing and compensatory division.

Molecular biology of hepatitis B virus infection

Human hepatitis B virus (HBV) is the prototype of a family of small DNA viruses that productively infect hepatocytes, the major cell of the liver, and replicate by reverse transcription of a terminally redundant viral RNA, the pregenome. Upon infection, the circular, partially double-stranded virion DNA is converted in the nucleus to a covalently closed circular DNA (cccDNA) that assembles into a minichromosome, the template for viral mRNA synthesis. Infection of hepatocytes is non-cytopathic. Infection of the liver may be either transient (<6 months) or chronic and lifelong, depending on the ability of the host immune response to clear the infection. Chronic infections can cause immune-mediated liver damage progressing to cirrhosis and hepatocellular carcinoma (HCC). The mechanisms of carcinogenesis are unclear. Antiviral therapies with nucleoside analog inhibitors of viral DNA synthesis delay sequelae, but cannot cure HBV infections due to the persistence of cccDNA in hepatocytes.

Kinetics of hepadnavirus loss from the liver during inhibition of viral DNA synthesis

Hepadnaviruses replicate by reverse transcription, which takes place in the cytoplasm of the infected hepatocyte. Viral RNAs, including the pregenome, are transcribed from a covalently closed circular (ccc) viral DNA that is found in the nucleus. Inhibitors of the viral reverse transcriptase can block new DNA synthesis but have no direct effect on the up to 50 or more copies of cccDNA that maintain the infected state. Thus, during antiviral therapy, the rates of loss of cccDNA, infected hepatocytes (1 or more molecules of cccDNA), and replicating DNAs may be quite different. In the present study, we asked how these losses compared when woodchucks chronically infected with woodchuck hepatitis virus were treated with L-FMAU [1-(2-fluoro-5-methyl-beta-L-arabinofuranosyl) uracil], an inhibitor of viral DNA synthesis. Viremia was suppressed for at least 8 months, after which drug-resistant virus began replicating to high titers. In addition, replicating viral DNAs were virtually absent from the liver after 6 weeks of treatment. In contrast, cccDNA declined more slowly, consistent with a half-life of approximately 33 to 50 days. The loss of cccDNA was comparable to that expected from the estimated death rate of hepatocytes in these woodchucks, suggesting that death of infected cells was one of the major routes for elimination of cccDNA. However, the decline in the actual number of infected hepatocytes lagged behind the decline in cccDNA, so that the average cccDNA copy number in infected cells dropped during the early phase of therapy. This observation was consistent with the possibility that some fraction of cccDNA was distributed to daughter cells in those infected hepatocytes that passed through mitosis.

Combination therapy with lamivudine and adenovirus causes transient suppression of chronic woodchuck hepatitis virus infections

Treatment of hepatitis B virus carriers with the nucleoside analog lamivudine suppresses virus replication. However, rather than completely eliminating the virus, long-term treatment often ends in the outgrowth of drug-resistant variants. Using woodchucks chronically infected with woodchuck hepatitis virus (WHV), we investigated the consequences of combining lamivudine treatment with immunotherapy mediated by an adenovirus superinfection. Eight infected woodchucks were treated with lamivudine and four were infected with approximately 10(13) particles of an adenovirus type 5 vector expressing beta-galactosidase. Serum samples and liver biopsies collected following the combination therapy revealed a 10- to 20-fold reduction in DNA replication intermediates in three of four woodchucks at 2 weeks after adenovirus infection. At the same time, covalently closed circular DNA (cccDNA) and viral mRNA levels both declined about two- to threefold in those woodchucks, while mRNA levels for gamma interferon and tumor necrosis factor alpha as well as for the T-cell markers CD4 and CD8 were elevated about twofold. Recovery from adenovirus infection was marked by elevation of sorbitol dehydrogenase, a marker for hepatocyte necrosis, as well as an 8- to 10-fold increase in expression of proliferating cell nuclear antigen, a marker for DNA synthesis, indicating significant hepatocyte turnover. The fact that replicative DNA levels declined more than cccDNA and mRNA levels following adenovirus infection suggests that the former decline either was cytokine induced or reflects instability of replicative DNA in regenerating hepatocytes. Virus titers in all four woodchucks were only transiently suppressed, suggesting that the effect of combination therapy is transient and, at least under the conditions used, does not cure chronic WHV infections.

Hepatitis B virus biology

Hepadnaviruses (hepatitis B viruses) cause transient and chronic infections of the liver. Transient infections run a course of several months, and chronic infections are often lifelong. Chronic infections can lead to liver failure with cirrhosis and hepatocellular carcinoma. The replication strategy of these viruses has been described in great detail, but virus-host interactions leading to acute and chronic disease are still poorly understood. Studies on how the virus evades the immune response to cause prolonged transient infections with high-titer viremia and lifelong infections with an ongoing inflammation of the liver are still at an early stage, and the role of the virus in liver cancer is still elusive. The state of knowledge in this very active field is therefore reviewed with an emphasis on past accomplishments as well as goals for the future.

Inhibition of duck hepatitis B virus replication by hypericin

Hypericin was found to be active against a member of the hepatitis B virus family, duck hepatitis B virus (DHBV). After a single 1 h incubation with hypericin, cells stably-transfected with a clone of DHBV stopped producing infectious virus for several days, though virus-like particles continued to be released into the culture medium. Characterization of these virions revealed a buoyant density characteristic of infectious virus preparations and lower than that of virus cores, suggesting that the particles were enveloped. Western blot analysis suggested, however, that the viral preS protein in surface antigen particles and, by inference, in virions, was present in covalently cross-linked aggregates. Evidence of a similar level of aggregation of the core subunit of virion nucleocapsids was not found, nor was there evidence of a similar high level of aggregation of cell-associated core and preS proteins. Hypericin was only slightly virucidal against DHBV and culture medium from treated cultures did not block initiation of infection when added to DHBV susceptible cultures prior to a challenge with infectious DHBV. Thus, the primary antiviral activity of hypericin against DHBV replication appears to be exerted at a late step in viral morphogenesis.

The problem of antiviral therapy for chronic hepadnavirus infections

Attempts at antiviral therapy of patients with active liver disease as a consequence of chronic hepatitis B virus infection have been moderately successful. The molecular and cellular basis for a successful outcome in these patients is not understood and the same therapies do not appear to benefit carriers that still have fairly normal livers and only a moderate hepatitis as a result of the immune response to the infection. Most carriers fall into this latter classification, at least during the early years of infection, and a therapy that could be successfully applied before extensive liver damage had occurred would presumably reduce the risk of subsequent liver damage and the progression to primary hepatocellular carcinoma. Traditionally, it has been assumed that the primary reason that individuals become chronically infected is that the cytotoxic T-cell response and/or antibody-dependent killing of infected hepatocytes is insufficient to clear the infection. Less attention has been focused on the role of the antibody response in the generation of virus-neutralizing antibodies as the possible major deficiency predisposing some individuals to become carriers. However, carriers normally are antigenemic for HBsAg and virus, and carriers with only antibodies to these structures in their circulation are virtually unknown. In addition, it is usually assumed that the hepatocyte, the major target of infection, does not spontaneously turn over and that, in the absence of an immune response to the infected cell, hepatocellular viability is unaffected.(ABSTRACT TRUNCATED AT 250 WORDS)

HBsAg-like structures in immunosuppressed mice inoculated with human hepatitis B virus

Thymectomised and irradiated DBA/2 mice were injected intraperitoneally with human serum containing high titer of HBsAg, and were positive for HBsAg. Through the entire experiment neither degenerative and inflammatory lesions nor hepatitis B virus antigens could be detected in the liver of these animals by histomorphology and immunofluorescence, respectively. The sera of all these mice were negative for HBsAg by radioimmunoassay. By electron microscopy, however, increasing amounts of filaments and round particles measuring 20-22 nm in diameter could be observed in the endoplasmic reticulum of the mouse hepatocytes from the 8th day following injection. From the 90th day after inoculation the number of the filaments increased in an extreme degree. After fixation with KMnO4 and EDTA preferential staining, the filaments proved to be highly electrondense. According to the authors the filaments observed in mouse livers are lipoproteins produced by the hepatocytes in response to HBV inoculation. The appearance of the filaments is HBsAg-like, though their immunological characteristics become modified.

IgM antibody to the hepatitis B core antigen in acute hepatitis determined by SPRIA--diagnostic value

A solid phase radio-immunoassay (SPRIA) was developed for the detection of anti-HBc IgM. The assay proved sensitive and easy to perform and rheumatoid factor did not affect the test results. Anti-HBc IgM titres were followed in consecutive samples from 15 patients after uncomplicated acute hepatitis B. In the acute phase anti-HBc IgM titres ranged from 10(-5) to 10(-7) (mean 10(-6.4)). One year after onset of disease ten of the 15 had titres below 10(-4) and between two and three years after onset most patients had titres 10(-3). Anti-HBc IgM titres were determined in six episodes of acute hepatitis B, all HBsAg negative but anti-HBc positive in the first samples obtained (within 8 days) and developing anti-HBs during convalescence. Acute phase anti-HBc IgM titres in these patients ranged between 10(-5.5) and 10(-7) (mean 10(-6.5)) and were thus identical with HBsAg positive cases. When acute phase sera from 168 episodes of acute hepatitis primarily classified as non-A, non-B, were tested for anti-HBc IgM titres above 10(-5), sera from 13 episodes were positive and in seven of these hepatitis B diagnosis could be confirmed by rising anti-HBs titres in convalescence. Sera from four of the 13 patients contained HBeAg, which was thus demonstrated in the absence of HBsAg. The results show that testing for anti-HBc IgM is important for a true non-A, non-B diagnosis.