Identification of the hepatitis B virus factor that inhibits expression of the beta interferon gene

Infection of a leukemic cell line (K562) by hepatitis B virus induces cell growth inhibition

Hepatitis B virus inhibits the in vitro growth of the human leukemic cell line K562; however, the mechanism of this growth inhibition is not understood. One to 12 days after exposure, viral DNA and RNA were detected in K562 cells by Southern blot and reverse-transcriptase polymerase chain reaction analyses. Virus-containing serum that was heat-inactivated failed to inhibit cell growth and no viral DNA or RNA was detected in these cells. In addition, murine monoclonal antibodies directed to hepatitis B virus surface epitopes neutralized the virus-induced growth inhibition whereas antibodies to hepatitis B virus core epitopes failed to suppress the inhibition. These results indicate that in vitro infection of K562 cells by hepatitis B virus causes inhibition of hematopoietic cell line growth.

RNA- and DNA-binding activities in hepatitis B virus capsid protein: a model for their roles in viral replication

The hepatitis B virus capsid or core protein (p21.5) binds nucleic acid through a carboxy-terminal protamine region that contains nucleic acid-binding motifs organized into four repeats (I to IV). Using carboxy-terminally truncated proteins expressed in Escherichia coli, we detected both RNA- and DNA-binding activities within the repeats. RNA-binding and packaging activity, assessed by resolving purified E. coli capsids on agarose gels and disclosing their RNA content with ethidium bromide, required only the proximal repeat I (RRRDRGRS). Strikingly, a mutant in which four Arg residues replaced repeat I was competent to package RNA, demonstrating that Arg residues drive RNA binding. In contrast, probing immobilized core proteins with 32P-nucleic acid revealed an activity which (i) required more of the protamine region (repeats I and II), (ii) appeared to bind DNA better than RNA, and (iii) was apparently modulated by phosphorylation in p21.5 derived from Xenopus oocytes. Deletion analysis suggested that this activity may depend on an SPXX-type DNA-binding motif in repeat II. Similar motifs found in repeats III and IV may also function to bind DNA. On the basis of these observations, together with a reinterpretation of recent studies showing that capsid protein mutants cause defects in viral genome replication, we propose a model suggesting that hepadnavirus capsid proteins participate directly in the intracapsid reverse transcription of RNA into DNA. In this model, repeat I binds RNA whereas the distal repeats are progressively recruited to bind elongating DNA strands. The latter motifs may be required for replication to be energetically feasible.

Interferons in the persistence, pathogenesis, and treatment of HIV infection

Interferon (IFN) plays an important role in the treatment and pathogenesis of HIV disease. Recent studies show beneficial effects of IFN alpha in the treatment of HIV-associated Kaposi's Sarcoma and early HIV-infection. Moreover, cell culture studies support these beneficial effects. HIV infection of monocytes is blocked by IFN alpha administered at the time of viral challenge. The IFN alpha-treated cells show no evidence of HIV infection. Viral gene products produced in monocytes infected with HIV then treated with IFN alpha gradually decrease to baseline. Large quantities of proviral DNA are seen in the HIV-infected IFN alpha-treated cells with little evidence for viral transcription suggesting true microbiological latency. While most viral infections of cells result in IFN production, HIV is a notable exception. Indeed, HIV does not induce monocytes to produce IFN alpha and blocks its production following poly(I).poly(c) stimulation. This allows HIV yet another mechanism to evade an important host antiviral response. Paradoxically, the appearance of IFN activity in sera of HIV-infected patients is associated with disease progression, not resolution. Recent observations showing that the interaction between HIV-infected monocytes and PBMC results in the production of IFN alpha s with reduced anti-HIV activity may help explain this paradox. Thus, IFN alpha plays an important but complex role in HIV disease. The elucidation of cellular factors that regulate the antiretroviral effects of IFN alpha may lead to the development of novel therapeutic strategies for HIV infection.

Viral inhibition of the interferon system

In response to interferon (IFN), cells develop an antiviral state in which the replication of a wide spectrum of RNA and DNA viruses is inhibited. Viruses have evolved a variety of mechanisms to inhibit the production and action of the interferons. Interferon action may be blocked by inhibition of the post-receptor signalling pathway, which prevents the expression of a number of proteins with antiviral properties. Other viruses prevent the action of specific, interferon-induced antiviral systems. In particular, the action of the dsRNA-dependent protein kinase (DAI) is inhibited by a variety of different viruses, indicating the fundamental importance of this enzyme to the antiviral response.

Interferon alpha (IFN)-macrophage interactions in human immunodeficiency virus (HIV) infection: role of IFN in the tempo and progression of HIV disease

Components of the host immune response that constrain virus replication and affect long-lasting antiviral immunity following HIV infection are incompletely defined. IFNs are critical participants in host antiviral processes. While IFN induces significant anti-retroviral activities, they also serve as harbingers for poor clinical outcomes. Moreover, monocytes, a major cellular source of IFN and HIV in man, are poor producer cells for IFN following HIV infection. Indeed, HIV infection of monocytes results in a diminished production and induction of IFN. IFN is only produced during cell to cell contact between HIV-infected cells and uninfected PBMC. Analysis of the biologic activity of HIV-induced IFN(s) shows that it poorly restricts HIV replication. Thus, the role of IFN in HIV disease is complex and seemingly paradoxical. The diminished capacity of HIV-infected monocytes to produce IFN and the production of defective IFNs likely reflect specific viral adaptive mechanisms for persistent infection.