Characterization of intracellular viral RNA in interferon-treated cells chronically infected with murine leukemia virus

Human primary T lymphocytes have a low capacity to amplify MLV-based amphotropic RCR and the virions produced are largely noninfectious

The presence of replication-competent retrovirus (RCR) in retroviral-based gene therapy products poses a potential safety risk for patients. Therefore, RCR testing of clinical gene therapy products and monitoring of patients enrolled in gene therapy trials is required to assure viral safety. The requirement to test ex vivo-transduced cells originates from the presumed amplification of adventitious RCR during the transduction procedure. However, data on the capacity of different cell types to do so are lacking. In this study, we sought to analyze the amplification potential of primary human T lymphocytes after infection with amphotropic MLV-based RCR. The total number of viral particles produced after 1 or 2 weeks was measured by a quantitative 4070A env-specific RT-PCR assay. The fraction of infectious replication-competent viral particles was analyzed in the PG-4 S+L- assay. From this study, we conclude that the total number of viral particles RCR produced by T lymphocytes is 2-4 logs lower than the number produced by NIH-3T3 cells. Surprisingly, less than 1% of the viral particles produced by primary T lymphocytes appeared to be infectious, while nearly all virions produced by NIH-3T3 were. We conclude that primary human T lymphocytes are low producers of MLV-based amphotropic RCR.

Multiple effects of interferon on the replication of human immunodeficiency virus type 1

In this review, I shall summarize the major findings about the effect of IFN on the replication of HIV-1 virus in model systems in vitro and will describe the known molecular mechanisms involved in the IFN-mediated inhibition of HIV-1 replication. Finally, I shall relate these findings to the unique features of the HIV-1 replication cycle.

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.

Inhibition of the synthesis of Moloney leukemia virus structural proteins by N-methylisatin-beta-4',4'-diethylthiosemicarbazone

The mechanism of inhibition of Moloney leukemia virus by N-methylisatin-beta-4',4'-diethylthiosemicarbazone was studied. Experiments that used [3H]leucine for short-pulse labeling in the presence of the drug resulted in a 71% inhibition in the synthesis of Pr-80, the polypeptide precursor of the gag viral proteins. The radioactive pulse products of the polypeptide precursors after a further 2-h chase period showed a normal cleavage of the precursors, with the formation of a reduced amount of final mature viral structural proteins. The experimental evidence indicated that at the inhibitory concentration of 17 microM N-methylisatin-beta-4',4'-diethylthiosemicarbazone, the amount of intracellular viral RNA was not affected, whereas the activities of reverse transcriptase and the other viral protein syntheses were suppressed.

Effect of interferon on the penetration of murine leukemia virus and the binding of its genome RNA to polyribosomes at the early stage of infection

The effect of interferon (IFN) on the adsorption, penetration and subsequent binding of the incoming genome RNA of Moloney murine leukemia virus (MLV) to polyribosomes, was studied in NIH/3T3 mouse fibroblasts. Virus adsorption was assayed by determining reverse transcriptase activity in the inoculating virus stock and in the cell membrane fraction before and after 45 minutes of infection. Both measurements suggested that IFN had no effect on virus adsorption. Virus penetration was determined by measuring the amount of viral RNA in the cell cytoplasm at 45 minutes after infection. This amount was remarkably lower in IFN-treated than in untreated cells. This reduction was not due to inhibition of a possible induction of endogenous viral genetic information by the penetrating virions, but was proved to be a direct effect of IFN on virus penetration, which was related to the IFN-induced antiviral state. The effect of IFN on binding of parental genome RNA to polyribosomes was then investigated by analysing Crt hybridization kinetics of polyribosomal viral RNA at different time intervals after infection. While in untreated control cells maximal binding occurred at 3 hours postinfection, this maximal binding was observed in IFN-treated cells at 5 hours postinfection. The distribution of viral RNA molecules between sub-cytoplasmic fractions at 3 hours after infection was, in IFN-treated cells, significantly different from that observed in the untreated cells.

Accumulation and breakdown of RNA-deficient intracellular virus particles in interferon-treated NIH 3T3 cells chronically producing Moloney murine leukemia virus

Interferon treatment of NIH 3T3 cells chronically infected with Moloney murine leukemia virus inhibited about 95% of virus release. This inhibition was accompanied by a three- to twofold accumulation of intracellular virions. However, this accumulation could be demonstrated only be exogenous reverse transcriptase reaction assay or radioactive labeling of the assembled viral proteins. It could not be shown by the endogenous reverse transcriptase reaction assay, which depended on endogenous viral RNA, or by labeling the encapsidated viral RNA. It was therefore evident that most of the intracellular virions accumulated in interferon-treated cells were RNA deficient. Hybridization analysis revealed that these virions were deficient of genomic viral RNA, whereas size analysis by gel electrophoresis suggested that the deficiency of 4S RNA normally packaged in Moloney murine leukemia virus was even stronger. Our data also suggested that this RNA deficiency was not due to degradation of the encapsidated RNA, but more likely to a defect in virus assembly. RNA-lacking intracellular virions were unstable; they were found to collapse before being released.

Effect of interferon on the formation and release of intracellular virions in NIH/3T3 cells chronically infected with Moloney murine leukemia virus

Using transmission electron microscopy, we demonstrate in this study the existence of intracellular Moloney murine leukemia virus particles in cytoplasmic vacuoles of chronically infected NIH/3T3 cells. Interferon (IFN) treatment for 48 h resulted in a 3-fold higher accumulation of these vacuolar virions. Intracellular virions and their accumulation in IFN-treated cells could also be shown by radioactive labeling of the cells for 48 h and isolation of labeled virus particles from the cytoplasmic fraction of the cells by banding in sucrose gradient. These virions contained typical viral RNA, proteins and glycoproteins, as determined by gel-electrophoresis. Despite their higher accumulation, the formation rate of the intracellular virions, as measured by their pulse labeling for 1 h with S35-methionine, was found to be remarkably lower in IFN-treated cells. In order to elucidate this discrepancy, we followed the kinetics of the intracellular and extracellular virus appearance. We found that the inhibitory effect of IFN on the release of virus particles to the culture medium was much stronger than its effect on the formation of the intracellular virions. This finding can explain the accumulation of intracellular virus particles in IFN-treated cells despite their reduced formation rate.

Effect of interferon on assembly of intracellular Moloney murine leukemia virus particles in chronically infected 3T3/NIH cells

The effect of interferon (IFN) on virus release and on assembly of intracellular virions in 3T3/NIH cells chronically infected with Moloney murine leukemia virus was studied by short labeling with 3H-leucine (viral proteins), 3H-glucosamine (viral glycoproteins) and 3H-uridine (vital RNA). With all of these labels, IFN pretreatment was found to strongly inhibit extracellular virus release. No difference was found between the extent of labeling of viral proteins and glycoproteins of intracellular virions. Incorporation of 3H-uridine into intracellular virions was strongly reduced by the IFN pretreatment. Since it is rather unlikely that encapsidated RNA is significantly degraded during the relatively short time of label incorporation, this finding suggests that IFN interferes with packaging of viral RNA. The effect of IFN on virus release and on 3H-uridine labeling of intracellular virion were found to develop with the same kinetics, though the maximal inhibition of virus release was stronger.

Interferon inhibits establishment of fibroblast infection with avian retroviruses

Pretreatment of chick embryo fibroblasts (CEF) with low doses of homologous interferon (16 u/ml) drastically inhibits cell transformation by, and replication of Rous sarcoma virus (RSV). Treatment of chick cells with 16 u/ml of interferon before de novo infection with a transformation defective (td) mutant-RSV, also resulted in a reduction of extracellular virus particles. This was determined by infectivity titrations, virus associated reverse transcriptase (RT) activity and measurement of metabolically radioactively labelled virus particles. The viral proteins pr 180, pr 76, p 27, p 19 and p 12 were still synthesized in interferon-treated cells in an unaltered form, although at slightly reduced levels. No difference in the pattern of structural proteins could be detected between virus particles harvested from cells treated with interferon and from control cells. In contrast to de novo infected cells, concentrations of interferon as high as 200 u/ml had no influence on the reversible transformation of cloned fibroblasts infected with a temperature sensitive mutant of RSV. In addition, fibroblasts infected with td-SR-RSV-D before addition of interferon showed only a marginal effect on formation of infectious virus even after treatment with 200-500 u/ml of interferon. This was not caused by interferon-resistance of the td-SR-RSV-D infected cells since viral protein synthesis by superinfecting Vesicular stomatitis virus (VSV) was as sensitive to interferon as in cells not preinfected with retrovirus. Our results support the notion that exogenous infection of fibroblasts with avian retrovirus is inhibited by interferon during an early phase of the replication cycle.

Intracellular production of virus particles and viral components in NIH/3T3 cells chronically infected with Moloney murine leukemia virus: effect of interferon

The effect of interferon on the biochemical properties and the maturation process of intracellular viral particles isolated from the cytoplasmic fraction of NIH/3T3 cells chronically infected with Moloney murine leukemia virus was investigated. By labeling these virions with either [35S]methionine or [3H]glucosamine, we demonstrated that they contain the same viral proteins and glycoproteins found in extracellular virions. Interferon treatment was found to reduce the rate of intracellular virus assembly. This effect was not a consequence of an interferon inhibition of viral RNA synthesis or its translation or a consequence of an interference with the posttranslational cleavage processing of viral precursor proteins, since all of these steps were not affected by interferon. However, the reduced rate of virus assembly could be attributed to the inhibition of viral protein glycosylation observed in interferon-treated cells. Nevertheless, despite this reduced rate, virus particles accumulated in interferon-treated cells. This accumulation was probably due to the strong inhibition of their final release from such cells.