Specific infection of central nervous system white matter by a variant of gross murine leukemia virus

Exposure of neonatal Balb.B mice to a variant of Gross murine leukemia virus, termed WB91-GV, resulted in selective white matter infection within the central nervous system. Viral antigens were detected in brain sections of animals inoculated by either intracerebral or intraperitoneal routes, but were only seen in mice exposed within the first day after birth. This distinct tropism was confirmed by virus replication and gp70 expression in isolated glial cultures in vitro. Analysis of gp70 expression in highly enriched glial subpopulations indicated that oligodendrocytes and perhaps a subset of astrocytes were the targets of this infection.

Class II polytropic murine leukemia viruses (MuLVs) of AKR/J mice: possible role in the generation of class I oncogenic polytropic MuLVs

We examined the frequency of occurrence of polytropic murine leukemia viruses (MuLVs) in the spleens and thymuses of preleukemic AKR/J mice from 1 week to 6 months of age and analyzed the genomic RNAs of several polytropic isolates by RNase T1 oligonucleotide fingerprinting. Polytropic MuLVs were first detected in the spleens of 3-week-old mice and preceded the appearance of polytropic MuLVs in the thymus by over 1 month. At 4 months of age and older, nearly all mice expressed polytropic MuLVs in both organs. In contrast to previous studies which have identified class I polytropic MuLVs in AKR/J mice, fingerprint analysis of polytropic MuLVs from both young (3- to 4-week-old) and older (5- to 6-month-old) preleukemic mice indicated that a large proportion of viruses at both ages were class II polytropic MuLVs. All polytropic viruses (five isolates) analyzed from 3- to 4-week-old mice were recovered from spleen cells and were class II polytropic MuLVs. In older preleukemic mice, five of seven isolates were class II polytropic MuLVs and two were class I polytropic viruses. Class I and class II polytropic MuLVs were recovered from both the spleens and thymuses of older preleukemic mice. A detailed comparison of the class I and class II polytropic MuLVs from 5- to 6-month-old mice revealed that the nonecotropic gp70 sequences of most of the class I and class II MuLVs were identical, consistent with a common origin for these sequences. In contrast, the nonecotropic p15E sequences of class I MuLVs were clearly derived from different endogenous sequences than the nonecotropic p15E sequences of the class II MuLVs. The in vitro host ranges of class I and class II polytropic viruses were clearly distinguishable. Examination of the in vitro host range of several isolates suggested that the predominant polytropic viruses initially identified in the thymus (2 to 3 months of age) were class II polytropic viruses. The order of appearance of the class I and class II polytropic MuLVs and the identity of the gp70 oligonucleotides of these MuLVs suggested a model for the stepwise generation of class I polytropic MuLVs involving a class II polytropic MuLV intermediate.

Increase of cellular murine leukemia virus reverse transcriptase in interferon-treated cells

The addition of mouse interferon (IFN) to AKR murine leukemia virus (MuLV)-producing NIH3T3 cells inhibited the extracellular appearance of AKR MuLV when assayed for reverse transcriptase activity or infectious virus production. IFN treatment had no detectable effect on proviral DNA formation during infection nor on the level of viral RNA in virus-producing cells. However, addition of IFN did alter the level of cellular viral reverse transcriptase activity. Chromatography of extracts from virus-producing cells on poly(G)-Sepharose columns revealed two peaks of reverse transcriptase activity. Peaks I and II eluted at 0.45 M and 0.65 M NaCl, respectively, while the cellular DNA polymerase beta eluted earlier at 0.3 M NaCl. IFN treatment of these chronic virus producer cells resulted in a 5-fold increase in peak I whereas peak II and polymerase beta remained essentially unchanged. When reverse transcriptase from purified virions was similarly chromatographed on poly(G)-Sepharose, all of the enzymatic activity eluted as peak I. Thus, the reverse transcriptase in peak I from cell extracts appears to be the form which is present in mature virions. Contrary to the results with chronic virus-producing cells, IFN treatment prior to exogenous infection with MuLV did not alter levels of reverse transcriptase peaks I and II or polymerase beta. These results provide further evidence that the major effect of IFN occurs at the level of MuLV maturation and assembly.

AKR thymic lymphomas involving mink cell focus-inducing murine leukemia viruses have a common region of provirus integration

Newly acquired proviruses related to a mink cell focus-inducing murine leukemia virus were detected in low copy number in restriction endonuclease-digested DNAs from thymic lymphomas of AKR/J mice. These extra proviruses were not present in DNAs of either normal thymus or leukemic brain tissues. Extra tumor-specific DNA fragments generated by restriction endonucleases either were identical in size or fell into similar size classes, suggesting a common site(s) of provirus integration. Characterization of extra EcoRI DNA fragments for mink cell focus-inducing viral sequences revealed that all of them contained large terminal repeat sequences and that a significant number represented proviruses with deletions.

Molecular cloning of a highly leukemogenic, ecotropic retrovirus from an AKR mouse

SL3-3 is a leukemogenic, ecotropic retrovirus produced by a T-cell line derived from a spontaneous lymphoma of an AKR mouse. We have isolated a molecular clone of its DNA provirus from infected NIH 3T3 fibroblasts. Cloned proviral DNA produced infectious virus upon transfection onto NIH 3T3 cells. Virus derived by transfection induced lymphomas at high frequency in AKR/J, C3H(f)/Bi, CBA/J, and NFS/N mice. Heteroduplex and RNase T1 fingerprinting analyses showed that the genomes of SL3-3 and the non-leukemogenic virus, Akv, contain no major substitutions relative to one another and differ by only a few base changes. These results unambiguously show that SL3-3 is a highly leukemogenic virus and that major rearrangements of the genome relative to Akv are not required for virulence.

Isolation of a replication-defective murine leukaemia virus from cultured AKR leukaemia cells

Mice of the AKR strain are characterised by a high incidence of spontaneous thymic lymphomas. AKR chromosomes contain the genomes of ecotropic murine leukaemia virus (MuLV) at two loci, termed Akv-1 and Akv-2 (refs 2-6). Shortly after birth, the normal tissues of AKR mice begin to produce high levels of this XC-positive MuLV (ref. 7) (that is, one that forms XC plaques). A second class of MuLV, termed mink cell focus-inducing virus (MCF), is produced specifically by preleukaemic and leukaemic AKR thymocytes. Nowinski et al. have established a series of tissue culture lines from AKR leukaemias and reported that the resulting cell lines produce virus particles, but that these particles, surprisingly, do not give rise to XC plaques. We have analysed the virus particles produced by one of these cell lines, termed AKRSL2. We show here that, unlike most or all of the nonmalignant tissues in the AKR mouse, these cultured lymphoma cells produce very little non-defective ecotropic MuLV; however, they do produce replication-defective ecotropic MuLV.

Therapeutic activity of pretazettine, a narcissus alkaloid, on spontaneous AKR leukemia

A narcissus alkaloid, pretazettine hydrochloride (PTZ) has been shown to be active against spontaneous AKR leukemia. The long-term treatment with PTZ begining at 5--7 months of age of a group of AKR mice containing 10--20% of advanced leukemic mice significantly prolonged the life span of the group. The therapeutic effectiveness of PTZ has been compared with several standard antileukemic drugs. PTZ decreased the AKR virus titer in the circulating blood of mice and its antiviral activity in AKR virus infected NIH/3T3 cells has been confirmed by XC assay.

Retrovirus purification: method that conserves envelope glycoprotein and maximizes infectivity

A Sepharose 4B chromatographic method for purification of retroviruses is described which was less time consuming, increased purified virus yields, conserved viral glycoprotein, and increased recovery of biological infectivity in comparison with conventional sucrose gradient ultracentrifugation techniques.

A discrepancy in XC and oncogenicity assays for murine leukemia virus in AKR mice

Studies of murine leukemia virus expression in AKR mice are presented. Material from in vivo and in vitro sources of normal tissues and lymphomas was assayed for in vitro infectivity, using the XC plaque assay, and for oncogenicity, by assessing lymphoma-accelerating capacity after inoculation into newborn animals. Normal tissues from healthy young AKR mice up to 7 months of age were found to have XC but not oncogenic activity. XC activity persisted, and weak oncogenic activity appeared in older mice. Cocultivation of normal young cells with NIH Swiss mouse embryo cells did not result in the appearance of oncogenic activity, although XC virus increased in titer. A cell-free filtrate of a virus-accelerated lymphoma was studied for host range. Virus as measured by polymerase and gs antigen was found to be propagated on NIH Swiss mouse embryo and wild mouse embryo cells, but not on human rhabdomyosarcoma, normal rat kidney, rabbit corneal, and BALB/c embryo cells. Virus as measured by the XC assay grew better on NIH Swiss mouse than on BALB/c embryo cells. Both of these cell lines propagated virus as measured by the oncogenicity assay. Supernatants from an in vitro cell line from a virus-accelerated lymphoma did not produce XC plaques but were oncogenic. Those from two cell lines of spontaneous lymphomas were negative with both assays. Cultivation of supernatants from these cultured lymphoma cells with NIH Swiss mouse embryo cells resulted in material which produced small plaques on the XC assay. These findings are interpreted as showing the presence of two viruses in AKR mice. One is XC positive and present throughout life. The other is oncogenic, appears later in life, and could be a separate virus or a variant of the first one.

Qualitative and quantitative studies of AKR-type murine leukemia virus sequences in mouse DNA

Utilizing a single-stranded [3H]DNA probe highly representative of AKR viral 70S RNA, we have performed association kinetics experiments with cellular DNA in vast excess from 3 high-, 5 low- and 4 non-virus-yielding mouse strains. Our hybridization studies indicate that in the strains so far tested, the complete genome of the AKR-type MLV is present in the DNA of the embryos of both high- and low-virus-yielding mouse strains, while DNA of non-virus strains contains only a part of the genome. Furthermore, at least two populations of virus-specific DNA sequences can be identified (more abundant and less abundant species) according to their rate of association. Low-virus-yielding mouse strains contain a smaller number (1-2 copies) of the less abundant species, and thus a lower number of complete viral genome than do high-virus strains (3-4 copies). Non-virus-yielding strains are lacking these less abundant sequences in their genome. DNA from wild Mus musculus also contained viral sequences, the sample tested showing association kinetics identical to the non-virus-producing strains. Thus there is a good correlation between completeness of the AKR-type MLV genome in cellular DNA and the capacity of the cells to release AKR-type MLV. Mice of a non-virus-yielding strain made partially congenic for the AKR virus-inducing locus Akv-1 contained the complete virus genome, confirming that this locus consists of structural genes of the virus.

Semi-micro XC cell assay technique for murine leukemia virus

The XC cell assay employed in in vitro titration of murine leukemia viruses was modified for use as a semi-micro procedure.