Isolation and characterization of virus-resistant mouse embryo fibroblasts

Somatic cell mutants resistant to retrovirus replication: intracellular blocks during the early stages of infection

To identify cellular functions involved in the early phase of the retroviral life cycle, somatic cell mutants were isolated after selection for resistance to infection. Rat2 fibroblasts were treated with chemical mutagens, and individual virus-resistant clones were recovered after selection for resistance to infection. Two clones were characterized in detail. Both mutant lines were resistant to infection by both ecotropic and amphotropic murine viruses, as well as by human immunodeficiency virus type 1 pseudotypes. One clone showed a strong block to reverse transcription of the retroviral RNA, including formation of the earliest DNA products. The second clone showed normal levels of viral DNA synthesis but did not allow formation of the circular DNAs normally found in the nucleus. Cell fractionation showed that the viral preintegration complex was present in a form that could not be extracted under conditions that readily extracted the complex from wild-type cells. The results suggest that the DNA was trapped in a nonproductive state and excluded from the nucleus of the infected cell. The properties of these two mutant lines suggest that host gene products play important roles both before and after reverse transcription.

Isolation and characterization of a novel mutant mouse cell line resistant to Newcastle disease virus: constitutive interferon production and enhanced interferon sensitivity

In our attempt to isolate mutant cell lines resistant to Newcastle disease virus (NDV) we developed an improved procedure for enrichment of NDV-resistant cells from mouse FM3A cells and isolated a novel NDV-resistant mutant cell line, Had-2, with characteristics different from Had-1, a previously reported NDV-receptor-deficient mutant strain. Had-2 cells adsorbed NDV normally but the accumulation of viral mRNAs and proteins was inhibited. Had-2 cells had to be grown at higher cell densities in order to be NDV-resistant, and it was revealed that they did not exhibit NDV-resistance when grown at lower cell densities. A conditioned medium prepared from a culture of Had-2 cells grown at high cell density was able to make a low-density culture NDV-resistant. The activity of the conditioned medium to induce NDV-resistance was completely neutralized by addition of both anti interferon (IFN)-alpha and anti IFN-beta antibodies, indicating that Had-2 cells were constitutively releasing IFNs, though their levels were rather low. Had-2 cells were also characterized by an increased sensitivity to IFNs as compared with the parental FM3A cells, since the conditioned medium containing IFNs did not render FM3A cells resistant to NDV.

Interferon-mediated inhibition of virus penetration

Pretreatment of mouse L cells with mouse interferon (IFN) inhibits the penetration of vesicular stomatitis virus without affecting viral adsorption. The inhibition of virus uptake by IFN is dose dependent and, at the highest dose tested (1,000 units/ml), reaches 65%; 24 hr of treatment with IFN are required for maximal effect. A similar inhibition of uptake of virus occurs in human diploid fibroblasts and primary chicken embryo fibroblasts treated with homologous IFN. No significant inhibition occurs when cells are treated with heterologous IFN. These results document a previously unrecognized antiviral effect of IFN--namely, inhibition at the level of viral uptake.

Spontaneous production of human interferon

Several established lines of human lymphoblastoid cells were evaluated for abilities to produce interferons. Some cell lines were able to produce interferon when induced with either Newcastle disease virus or Sendai virus, whereas others failed to produce detectable interferon when so induced. However, several cell lines were able to spontaneously produce interferon without induction. Spontaneously produced interferon was liberated by cells only during logarithmic growth phase, reaching levels ranging from about 10 reference units/ml of growth medium for some cell lines to 1000 reference units/ml for others. The interferons produced by induced lymphoblastoid cells and the spontaneously produced interferons were all characterized as type I human leukocyte interferon by high levels of cross-species antiviral activities on bovine cells and by neutralizations by antiserum to human leukocyte interferon but not by antiserum to human fibroblast interferon. However, analysis by electrophoresis in sodium dodecyl sulfate/polyacrylamide gels revealed that spontaneously produced interferon was less size heterogeneous than human leukocyte interferon, migrating as a single band of activity with a peak at 20,000 daltons, whereas human leukocyte interferon contained peaks of major activity at 23,000 and 18,000 daltons and virus-induced Namalva lymphoblastoid cell interferon migrated predominantly as the 18,000-dalton form. Also, although neither virus-induced primary leukocyte interferon nor any of the virus-induced lymphoblastoid cell interferons were neutralized by antiserum to mouse interferon, all of the spontaneously produced interferons were neutralized by antiserum to mouse interferons. These data suggest significant structural similarities between the active cores of certain interferons from phylogenetically diverse animal species.

A murine cell possessing a dominant mutation affecting the regulation of interferon production: characterization by intraspecific hybrids

Using intraspecific hybrids, we have demonstrated the dominant nature of two phenotypic markers present in a mutant mouse 3T6 cell line, designated 3T6-VrB2. These are, resistance to virus infection (Vr) and semiconstitutive synthesis of interferon (IFsc). Hybrids were formed by polyethylene glycol-mediated fusion between 3T6-VrB2, or its parent 3T6, and 2TG0-13, a triply marked derivative of mouse 3T3 cells. When tested for the Vr marker, 3T6-VrB2 X 2TG0-13 hybrid clones displayed a level of resistance to virus infection which was equal to or greater than that of 3T6-VrB2. Similarly, when tested for the IFsc marker, these hybrid clones were found to possess the capacity to confer an interferon-induced antiviral state in mouse L929 cells upon cocultivation. By comparison, clones derived from 3T6 X 2TG0-13 fusions produced high levels of virus and failed to confer an interferon-induced antiviral state in L929 cells.

Interferon-associated, dsRNA-dependent enzyme activities in a mutant 3T6 cell engaged in the semiconstitutive synthesis of interferon

Cytoplasmic extracts of untreated cultures of a virus-resistant mutant of mouse 3T6 cells, designated 3T6-VrB2, contain two double-stranded, RNA-activated enzyme activities associated with interferon action. These are the synthesis of a low molecular weight oligonucleotide inhibitor of cell-free protein synthesis from ATP, and the phosphorylation of a 67,000 dalton polypeptide by transfer of the gamma phosphate of ATP. Basal levels of both enzyme activities are detectable in extracts of untreated parental 3T6 cells, and are greatly enhanced upon interferon pretreatment. A procedure was developed, using a nonionic detergent to effect cell lysis, which allowed the analysis of the protein kinase activity from as few as 2 x 10(7) cells. Using this procedure, direct proportionalities were demonstrated between the concentration of interferon to which 3T6 cells were exposed, and both the level of protein kinase activity and the magnitude of the antiviral state were established in these cells. Furthermore, untreated cultures of 3T6-VrB2 exhibited both an antiviral state and an intracellular protein kinase activity equal to that of cultures of the parental 3T6 cells pretreated with a single concentration of mouse interferon.

Murine interferon system regulation: isolation and characterization of a mutant 3T6 cell engaged in the semiconstitutive synthesis of interferon

We describe the isolation and characterization of a virus-resistant mutant of murine 3T6 cells. The mutant, designated 3T6-VrB2, displays a high degree of resistance to infection by members of the toga-, rhabdo- and picornavirus classes. The level of this resistance to infection is similar to the parent 3T6 pretreated with approximately 100 lU/ml of interferon. Upon co-cultivation of 3T6-VrB2 cells with interferon-sensitive mouse cells, an antiviral state is induced in the latter cells as measured by a reduction of virus yield following infection. The nature of the induction is defined by a series of experiments using anti-mouse interferon antiserum. In the presence of this antiserum, the ability of the mutant to induce an antiviral state in interferon-sensitive mouse cells upon co-cultivation is eliminated. Additionally, growth of the mutant cells in the presence of this antiserum causes a reversal of the virus-resistant phenotype. Our results indicate that 3T6-VrB2 contains a mutation affecting the regulation of the murine interferon system such that the cell is engaged in the semiconstitutive synthesis of interferon.

The selection of virus-resistant Chinese hamster ovary cells

We have selected Chinese hamster ovary (CHO) cells resistant to infection by encephalomycarditis (EMC) virus. Thus far, we have obtained five lines resistant to EMC, all of which manifest different phenotypes. Three of the five are not persistently infected with virus, while two lines produce infectious virus and grow in its presence. The nonpersistently infected lines exhibit different resistance profiles to the other viruses we have tested, and they are stable in nonselective growth conditions. Their resistance appears to be due to a genetic alteration in the cell.