Expression of the terminal protein region of hepatitis B virus inhibits cellular responses to interferons alpha and gamma and double-stranded RNA

Expression of the P-protein of the human hepatitis B virus in a vaccinia virus system and detection of the nucleocapsid-associated P-gene product by radiolabelling at newly introduced phosphorylation sites

Hepatitis B virus (HBV) contains a particle-associated DNA polymerase/reverse transcriptase activity encoded by the P (pol) open reading frame. Due to its low abundance, the corresponding protein has so far escaped direct detection and structural analysis. As a first step to overcome these difficulties, a series of recombinant vaccinia viruses was constructed and used for the synthesis in human hepatoma cells of both the authentic full length protein and of its functional domains. Pulse chase experiments demonstrated that the P-proteins had very short half lives in striking contrast to the viral core protein expressed in parallel with the same system. No evidence was obtained for a specific proteolytic processing of the P-protein as occurring with retroviral pol gene products. Overexpression of P-protein by recombinant vaccinia viruses was then employed to develop a highly sensitive detection method based on the in vitro phosphorylation of newly introduced target sites for protein kinase A. The usefulness of this method was demonstrated in the analysis of encapsidated P-gene products that were transiently expressed from an appropriately modified HBV genome. The results obtained indicate that the P-protein acts unprocessed, at least during the initial steps of nucleocapsid assembly and reverse transcription, and that a fraction of the P-protein molecules is linked as such to the viral DNA. Direct detection of the hepadnaviral P-protein by in vitro phosphorylation should greatly facilitate future analyses on P-protein structure and function.

High-frequency mutagenesis of human cells and characterization of a mutant unresponsive to both alpha and gamma interferons

2fTGH is a human cell line containing the selectable marker guanine phosphoribosyltransferase regulated by alpha interferon (IFN-alpha). Two IFN-alpha-unresponsive mutants were isolated previously at a low frequency (ca. 10(-8)) by selecting mutagenized 2fTGH cells in selective medium containing 6-thioguanine and IFN-alpha. By using five rounds of mutagenesis, mutants can be isolated at an appreciably higher frequency, greater than 3 x 10(-7). Five new mutants have been isolated, and all are recessive, as are the two mutants we described previously. The seven mutants are in four complementation groups (U1-U4). Since several different types of mutants unresponsive to IFN-alpha have been isolated with high frequency, related approaches may succeed with other cytokines or growth factors. Mutants in the two new complementation groups U3 and U4 are unresponsive to IFN-alpha and, surprisingly, also unresponsive to IFN-gamma. They are also partially defective in response to double-stranded RNA. These results indicate that the signaling pathways for the two types of IFN and double-stranded RNA share common components or that their function depends on common enzymes or transcription factors. IFN receptors are unaffected in mutants U3A and U4A. A major defect appears to be in the synthesis or activation of E, the transcription factor mediating the primary response to type I (alpha/beta) IFNs. Band-shift complementation assays show that U3A contains the E gamma subunit but does not contain an active E alpha subunit after treatment with IFN-alpha.

Modulation of MHC antigen expression by viruses and oncogenes

It is becoming increasingly clear that regulation of MHC antigen expression by viruses and oncogenes, leading to either immune evasion or autoimmunity, is widespread and important in disease. At a recent meeting*, which brought together workers interested in tumour immunology, viral infection and the MHC, a number of mechanisms for the regulation of MHC antigen expression were revealed and the importance of balanced expression of MHC gene products to effective immunity was underlined.

Inhibition of the cellular response to interferons by products of the adenovirus type 5 E1A oncogene

Expression of the E1A oncogene of adenovirus type 5 inhibits the response of interferon (IFN)-inducible constructs to Type I (alpha,beta) and II (gamma) IFNs in transient transfection assays. In human cell lines stably expressing E1A mRNA and protein acquisition of an antiviral state and the induction of a number of genes in response to alpha- and gamma-IFNs is inhibited. A short IFN-stimulable response element (ISRE) present in the 5' flanking region of a number of genes mediates induction by alpha- and gamma-IFNs. In cells expressing E1A there is a substantial reduction in the levels of the ISRE-binding factors E and M, inducible by alpha-IFN, and of factor G, inducible by gamma-IFN. In E1A-expressing cells the E alpha subunit of factor E is activated normally in response to alpha-IFN; the defect is in the production or activation of the E gamma subunit. The inhibitory activity of E1A is lost upon deletion of the CR1 domain. The induction of HLA class II genes by gamma-IFN, which involves a different DNA response element(s), and of beta-IFN mRNA in response to double-stranded RNA are also inhibited by E1A. An essential component(s) of a number of signalling pathways must, therefore, be subject, directly or indirectly, to inhibition by E1A.

The control of interferon-inducible gene expression

The alpha beta interferons (IFNs) transiently induce genes through an IFN-stimulable DNA response element (ISRE). IFN-cell surface receptor interaction triggers the cytoplasmic activation of the complex primary transcription factor E, which on translocation and interaction with the ISRE initiates transcription. Whether E is activated directly through the receptor(s) or through a more classical second message pathway(s) and the roles of additional factors in the alpha beta and gamma IFN responses remain to be established. Meanwhile analysis of mutants has revealed complexity and overlap in the alpha, beta and gamma IFN response pathways and the products of at least two viruses have been shown to inhibit IFN-inducible gene expression.