Induction of an antiviral response by interferon requires thymidine kinase

"Antivirals" in the treatment of adult T cell leukaemia- lymphoma (ATLL)

Adult T cell leukaemia / lymphoma (ATLL) is a mature (post thymic) T cell lymphoma caused by the human T-lymphotropic virus type 1 (HTLV-1) infection. Overall survival in the aggressive subtypes (Acute Leukaemia and Lymphomatous) remains poor in part due to chemotherapy resistance. To improve treatment outcome for de novo disease, better induction therapies are required and since the pathogenic agent is known it would seem sensible to target the virus. In a recent meta-analysis the use of zidovudine and interferon alpha (ZDV/IFN) has been associated with improved response rates and prolonged overall survival in leukemic subtypes of ATLL (both acute and Chronic) confirmed in a multivariate analysis. In a more recent UK study the overall response rate for patients with aggressive ATLL treated with chemotherapy alone was 49 % compared to 81 % with combined first line therapy (chemotherapy with concurrent or sequential ZDV/IFN). Combined first line therapy prolonged median OS in acute (p = 0.0081) and lymphomatous ATLL (p = 0.001).These data support the use of low dose ZDV/IFN with chemotherapy as first line treatment for patients with newly diagnosed aggressive ATLL. Although the mechanisms of action are incompletely understood, some possible explanations for their efficacy will be discussed.

Effect of interferon-alpha2b administration on rat liver regeneration after partial hepatectomy

The purpose of the present study was to delineate the effect of interferon-alpha2b (IFN-alpha2b) administration on the liver regenerative capacity after partial hepatectomy in rats. The administration of IFN-alpha2b simultaneously with partial hepatectomy did not affect hepatic proliferation in a statistically significant manner. When IFN-alpha2b was administered either 2 or 12 hr postoperatively, an inhibition of hepatocyte proliferation was observed 24 hr postoperatively, while at further time intervals up to 48 hr, DNA synthesis remained similar to that observed in the simply partially hepatectomized rats. The enzyme thymidine kinase (TK), has been implicated in the suppression of proliferation in interferon-treated cell cultures. In all IFN-alpha2b-treated groups of rats, alterations of TK activity were observed without being correlated to the liver regenerative status. Additionally, the administration of the polyamine putrescine in partially hepatectomized rats treated at the time of surgery with IFN strongly enhanced TK activity, but did not affect DNA biosynthesis. In the above-mentioned in vivo model of controlled cellular proliferation, the administration of IFN-alpha2b affected the rate of hepatocyte proliferation depending on the time of its administration; this effect was not correlated to the enzymatic activity of TK, as inhibited TK activity is responsible for the suppressed DNA synthesis in in vitro systems.

Overexpression of a heterologous thymidine kinase delays apoptosis induced by factor deprivation and inhibitors of deoxynucleotide metabolism

Perturbing deoxyribonucleoside triphosphate (dNTP) metabolism with inhibitors of the de novo synthesis of dNTP causes apoptosis in the interleukin-3 (IL-3)-dependent pre-B cell line BAF3. Under these conditions apoptosis is prevented when deoxyribonucleosides for dNTP synthesis are supplied in the culture medium. On the other hand, removal of IL-3 from cultures of BAF3 cells resulted in down-regulation of thymidine kinase activity, rapid imbalance in dNTP levels, and apoptosis. In this study we show that overexpression of a heterologous thymidine kinase, herpes simplex virus thymidine kinase (TK), in BAF3 cells protects these cells from apoptosis induced by either inhibitors of dNTP synthesis or IL-3 deprivation. This protection against apoptosis is abrogated by 9-(4-hydroxybutyl)-N2-phenylguanine, a specific inhibitor of herpes simplex virus-1 TK. These results suggest that deoxyribonucleoside kinases, particularly TK, may be important in the regulation of apoptosis in hemopoietic cells.

Beta and gamma interferons act synergistically to produce an antiviral state in cells resistant to both interferons individually

We showed previously that the mouse fibroblastoid cell line Ltk-aprt- is resistant to the antiviral effects of beta interferon. This lack of response reflects a partial sensitivity to the interferon that is accompanied by a failure to activate expression of several interferon-regulated genes, although certain other genes respond in a normal manner. We show here that Ltk-aprt- cells were also unable to establish an antiviral state and to activate expression of 2,5-oligo(A) synthetase when treated with gamma interferon. Strikingly, however, treatment with a combination of beta interferon and gamma interferon provided complete protection against viral replication. Although the cells were completely insensitive to up to 250 U of the interferons per ml added singly, essentially complete protection from viral cytopathic effects was achieved when as little as 10 U of each of the interferons per ml were combined. Expression of 2,5-oligo(A) synthetase was also sensitive to this synergistic effect. Activation of an antiviral state could also be achieved by sequential treatment, first with gamma interferon and then with beta interferon. Partial protection against viral replication could be achieved by pretreatment with gamma interferon for as little as 1 h before incubation with beta interferon and could be blocked by the addition of specific antibodies or by cycloheximide, indicating that gamma interferon induces the synthesis of a protein which can act synergistically with a signal produced by the beta-interferon receptor. We suggest that Ltk-aprt- cells suffer from defects in one or more components of the gene activation pathways for both type I and type II interferons. Nonetheless, gamma interferon is able to activate the expression of a gene encoding a protein required for signal transduction. This protein acts synergistically with a transient signal produced in response to beta interferon, thereby activating the expression of a further group of genes.

Activation of metallothionein expression is potentiated by DNA sequences present in the herpes simplex virus thymidine kinase gene

A mouse cell line (Ltk-aprt-) which is resistant to the anti-viral effects of interferon also has a reduced ability to synthesize metallothionein on exposure to cadmium. Like the ability to respond to interferon, cadmium-induced metallothionein synthesis is restored to wild-type levels in clones obtained by introducing a thymidine kinase gene into Ltk-aprt-cells. Transfection of other genes does not have such an effect. Since metallothionein expression is also activated by interferon the results suggest that the regulation of several genes which are responsive to interferon can be modulated by specific sequences present in the Herpes virus thymidine kinase gene.

Genetically determined resistance to lethal murine cytomegalovirus infection is mediated by interferon-dependent and -independent restriction of virus replication

Susceptibility of 4-week-old mice of different strains to lethal murine cytomegalovirus (MCMV) infection was studied. Strains homozygous for H-2k and C57BL strains were resistant to greater than or equal to 10(5.5) PFU. B10.BR mice congenic for C57BL background genes and H-2k were about 10-fold more resistant than either C3H/HeN or C57BL strains. BALB/c mice (H-2d) were susceptible (50% lethal dose, 10(5.05) PFU). This susceptibility was dominant over resistance associated with H-2k but not that associated with C57BL background genes. The dominant susceptibility trait segregated in backcross mice as if carried by a single gene. Virus replication in spleen cells in vivo correlated with susceptibility to lethal infection. A similar trend was found in tests of salivary glands. Replication of MCMV in vitro in cultures of adherent spleen cells and primary mouse embryo cells correlated with replication in vivo. Neutralization of interferon (IFN) in cultures of adherent spleen cells reversed H-2k-linked restriction of viral replication but had minor effects on cells of other strains. Natural killer cell responses to infection were often higher in more resistant strains, but B10.BR mice developed minimal natural killer cell responses. Specific antibody and cytotoxic T cell responses in B10.BR mice were similar or lower than in other strains. Thus, resistance to lethal MCMV infection was not immunologically mediated, was dependent on and reflected by the capacity of cells from a given mouse strain to support replication in vivo and in vitro, and was IFN dependent and recessive if linked to H-2k but IFN independent when associated with C57BL background genes.

Intracellular human gamma-interferon triggers an antiviral state in transformed murine L cells

Interaction of human gamma-interferon (IFN-gamma) with a cell-surface receptor is known to be essential for the cell to become resistant to viral infection. Here we demonstrate that IFN-gamma, when present inside the cell, is also capable of inducing a permanent antiviral state. Mouse cells transformed with a truncated human cDNA encoding a mature IFN-gamma protein lacking the signal peptide accumulate high levels of intracellular human IFN-gamma. Not only do these cells acquire a permanent resistance to viral infection, they also exhibit all the biochemical characteristics normally observed after exposure to exogenous IFN. The observed loss of species specificity normally associated with IFN-gamma suggests that this restriction is strictly dependent on the interaction of the molecule with the cell-surface receptor.

Growth of wild-type and interferon-sensitive mengovirus in tk- L cells

Cells lacking thymidine kinase (tk) have been reported to fail to respond to the action of interferon (IFN). While some laboratories have confirmed this observation, others have failed to do so. We studied the effect of IFN on four freshly isolated tk- lines of mouse L cells infected with mengovirus. In all cases, normal antiviral activity was induced. The antiproliferative activity of IFN was studied using the parental L cell line, a tk- derivative, and a tk- (tk+) subline into which the tk gene of herpes simplex virus was introduced. All three lines had a doubling time of about 20 h. In all cases, 2,000 U/ml of IFN increased this time to about 50 h. In contrast to the above results, an IFN-sensitive mutant of mengovirus (is-1) grew much better in protected tk- cells than in protected normal cells. This phenomenon appears to be dependent on the fact that tk- cells were routinely maintained in medium containing 5-bromodeoxyuridine (BUDR). In the absence of this drug, the virus behaved normally. The implications of this observation are discussed.

Changes in structure and methylation pattern in a cluster of thymidine kinase genes

Cell line 101 is a thymidine kinase (TK)-positive derivative of Ltk- which contains ca. 20 copies of the herpes simplex virus TK gene organized in a tandem array. DNA methylation at three sites within the gene and flanking sequences was inversely correlated with expression: the sites were unmethylated in line 101, methylated in each of 4 TK-negative derivatives of 101, and unmethylated in each of 21 TK-positive derivatives derived from them. The same three sites were affected in most of the 20 copies of the TK gene, whereas other sites between them were not affected. Although the entire gene cluster was never lost, indicating that integration into the genome was stable, internal rearrangements occurred at a high frequency. The rearrangements had no obvious correlation with the state of methylation or with the expression of the genes.

Regulation of protein synthesis in lymphoblastoid cells during inhibition of cell proliferation by human interferons

Treatment of human lymphoblastoid (Daudi) cells with interferons inhibits cell proliferation in culture within 24 h. The failure of cell growth has been shown to be associated with impaired processing and decreased stability of newly replicated DNA. Because there is a close relationship between DNA replication and protein synthesis we have measured protein synthesis in intact Daudi cells. Protein synthesis declined steadily between 24 and 96 h after interferon treatment to a value which is only 20-30% of the rate in control cells. The enzyme 2',5'-oligo(A) synthetase is induced but our data do not support a role for the 2',5'-oligo(A)-activated ribonuclease in the control of translation in this system.

Changes in structure and methylation pattern in a cluster of thymidine kinase genes

Cell line 101 is a thymidine kinase (TK)-positive derivative of Ltk- which contains ca. 20 copies of the herpes simplex virus TK gene organized in a tandem array. DNA methylation at three sites within the gene and flanking sequences was inversely correlated with expression: the sites were unmethylated in line 101, methylated in each of 4 TK-negative derivatives of 101, and unmethylated in each of 21 TK-positive derivatives derived from them. The same three sites were affected in most of the 20 copies of the TK gene, whereas other sites between them were not affected. Although the entire gene cluster was never lost, indicating that integration into the genome was stable, internal rearrangements occurred at a high frequency. The rearrangements had no obvious correlation with the state of methylation or with the expression of the genes.

Purified recombinant human leukocyte interferons IFLrA and IFLrD induce human lupus inclusions in Raji and Daudi cells

Raji and Daudi are human B lymphoblastoid cell lines that readily form lupus inclusions (LIs; TRS) when grown in medium supplemented with leukocyte-, or fibroblast-derived interferon (IFN-alpha, -beta, respectively). WISH, MDBK, and GM2504 are three cell lines commonly used to measure antiviral activities. None of them form LIs in their antiviral response to alpha or immune (gamma)IFN. This distinguishes between the abilities of a cell to develop an antiviral state and to form LIs in response to IFN. Human (Hu) lymphoblastoid IFN and the two pure and homogeneous recombinant human IFN-alpha proteins IFLrA and IFLrD induce LIs in Raji cells and Daudi cells. In Daudi, a simultaneous inhibition of cell growth occurs. When compared by antiviral activities, IFLrA inhibits the growth of Daudi cells more, while IFLrD induces the greater frequency of LIs. According to molecular concentration, IFLrA and IFLrD at 133 X 10(-13) M induce LIs in Daudi cells to their maximum frequency. Growth inhibition for these same cell samples is also at maximum for IFLrA, but only 25% of maximum for IFLrD. Our results with Raji and Daudi cells provide evidence against a cause-and-effect relationship between these two biologic responses to IFN by Daudi cells. They also provide evidence for distinct, but interacting, intracellular pathways. This phenomenon is a new explanation for some of the biologic diversity shown for the HuIFNs-alpha.

Interferon--present and future prospects

The interferons are a group of proteins that have inspired a new era of investigation into biological modification. The interferons are now divided into subgroups characterized by chemical means and correspond to different biological responses which can be observed in terms of the inducer used, and the timing of the response. Identified originally as antiviral agents when homologous cell systems were treated prior to infection, new studies have extended these observations to place the interferons in a central role as a strong force in the regulation of immunologic responses. A marriage of interferonology and cell immunology is enlarging both our understanding of the action of these proteins and our ability to follow the course of an illness and eventually to control its outcome . Genetic engineering has provided a way to process quantities of interferon and provided the molecular sequence of all three classes of IFN including a model of the active site for IFN-alpha. The offshot of the technology developed to study the intracellular processes after interferon treatment have already led to increased sensitivity to detect virally treated diseases. Both the variety of the interferon inducers and the scope of parasites in which it can exert its influence provide a frontier of biological investigation which has at the root of its nature the very secret of life. In addition to cellular phenomena, the positive effects on tumor-bearing organisms and the ill effects on infant animals highlight the potential power of the interferons.

Thymidine kinase genes and the induction of anti-viral responses by interferon

A mouse fibroblast cell-line deficient in thymidine kinase (Ltk(-) aprt(-)) fails to show an anti-viral response when treated with interferon. After introduction of a viral tk gene into these cells the resultant clones showed normal responses to interferon. However, one such tk-containing clone (C6) spontaneously lost its ability to respond to interferon by inducing an antiviral state although it retained its ability to induce the enzyme oligo(2'-5' A)-synthetase. This sub-clone (6A) still expressed thymidine kinase activity but restriction endonuclease analysis indicated an alteration in the sequences flanking the exogenous viral tk gene. Our results suggest that a modification in the exogenous viral DNA sequences led to a loss of interferon sensitivity.