Assay of C-type virus infectivity by measurement of RNA-dependent DNA polymerase activity

Molecular characterization of a neuropathogenic and nonerythroleukemogenic variant of Friend murine leukemia virus PVC-211

PVC-211 murine leukemia virus (MuLV) is a replication-competent, ecotropic type C retrovirus that was isolated after passage of the Friend virus complex through F344 rats. Unlike viruses in the Friend virus complex, it does not cause erythroleukemia but causes a rapidly progressive hind limb paralysis when injected into newborn rats and mice. We have isolated an infectious DNA clone (clone 3d) of this virus which causes neurological disease in animals as efficiently as parental PVC-211 MuLV. The restriction map of clone 3d is very similar to that of the nonneuropathogenic, erythroleukemogenic Friend murine leukemia virus (F-MuLV), suggesting that PVC-211 MuLV is a variant of F-MuLV and that no major structural alteration was involved in its derivation. Studies with chimeric viruses between PVC-211 MuLV clone 3d and wild-type F-MuLV clone 57 indicate that at least one determinant for neuropathogenicity resides in the 2.1-kb XbaI-ClaI fragment containing the gp70 coding region of PVC-211 MuLV. Compared with nonneuropathogenic ecotropic MuLVs, the env gene of PVC-211 MuLV encodes four unique amino acids in the gp70 protein. Nucleotide sequence analysis also revealed a deletion in the U3 region of the long terminal repeat (LTR) of PVC-211 MuLV clone 3d compared with F-MuLV clone 57. In contrast to the env gene of PVC-211 MuLV, particular sequences within the U3 region of the viral LTR do not appear to be required for neuropathogenicity. However, the changes in the LTR of PVC-211 MuLV may be responsible for the failure of this virus to cause erythroleukemia, because chimeric viruses containing the U3 region of F-MuLV clone 57 were erythroleukemogenic whereas those with the U3 of PVC-211 MuLV clone 3d were not.

Natural killer cell activity in a population of leukemia-prone wild mice (Mus musculus)

Natural cell-mediated cytotoxicity against YAC-I targets was measured in splenocytes from leukemia-prone wild mice trapped near Lake Casitas (LC) in southern California. Cytotoxicity was mediated by cells that were non-adherent to nylon wool, non-phagocytic and resistant to thy-1.2 antiserum plus complement. Natural MuLV viremia in LC mice did not impair splenic cytotoxicity against TAC-I target cells, Cells infected with amphotropic and ecotropic MuLV of wild mouse origin were not appreciably lysed by LC splenic effectors. Although variable levels of cytotoxicity were detected against TAC-1 by normal spleen cells, consistently low levels of cytotoxicity against allogenic LC lymphoma, sarcoma and carcinoma targets were found using the same splenocytes. These results indicate that LC mice possess splenocytes with the characteristics of natural killer (NK) cells as defined in inbred mice. The resistance of LC-derived targets to lysis by LC NK cells suggests that NK cells may not be involved in natural tumor immunosurveillance or that the development of spontaneous tumors may involve escape from NK-mediated effector mechanisms.

Naturally occurring humoral immunity to endogenous xenotropic and amphotropic type-C virus in the mouse

Natural humoral cytotoxic antibodies from 13- and 18-month-old BALB/c mice showed a virus-specific complement-dependent activity against target cells productively infected with xenotropic, amphotropic or ecotropic type-C viruses. The cytotoxic activity was lowest against ecotropic virus-shedding cells. Serum obtained from mice less than 12 months old had no such reactivity. The cytotoxic reactivity was found to reside solely in the immunoglobulin M fraction which yielded reactivity comparable to the unfractionated sera concerning both titer and relative reactivities to the target cells infected with different type-C viruses. In hyperimmune mouse serum cytotoxic reactivity resided in IgG and IgM fractions. Examination of serum from individual, normal 18-month-old BALB/c mice revealed that 80-90% of them were cytotoxic against virus non-producer mink target cells expressing gp70 or gaggene product. Absorption of sera from 18-month-old normal BALB/c mice with cells shedding Class II or Class III xenotropic virus, amphotropic virus, Rauscher-MuLV, or Class 1 murine leukemia virus indicate a closer amphotropic-FMR viral subtype specificity of the natural cytotoxic immune response as compared to the amphotropic-xenotropic or amphotropic-ecotropic specificity. The incidence and the level of measured humoral cytotoxic activity was sustained in the tumor-bearing animal up to 28 months of age as compared to the background established in the 18-month-old animal. However, in the non-tumor-bearing animal, the incidence and level of cytotoxic reactivity declined rather rapidly with aging. Te sustained cytotoxic reactivity of the serum from old tumored mice might be involved in the progression of the tumor.

Macromolecular requirements for abrogation of Fv-1 restriction by murine leukemia viruses

The molecular basis of abrogation of Fv-1 restriction in mouse cells by murine leukemia virus was investigated. Two different lines of experimentation indicated that high-molecular-weight viral RNA is required for abrogation. First, the decay of abrogating ability of virus stocks heated at 43 degrees C was quantitatively correlated with a loss of intact virion 35S RNA. Second, Act D virions, which lack such RNA although they contain normal structural proteins, failed to abrogate. These findings imply that abrogation does not result from the mere entry of virion structural proteins into a cell. Additional data indicate that the role of viral RNA in abrogation is not that of a template for DNA synthesis. Virus particles lacking reverse transcriptase activity as a result of either mutation or heat inactivation exhibit abrogating activity even though they do not synthesize detectable viral DNA. In addition, abrogation was shown to take place in the presence of cytosine arabinoside, an inhibitor of DNA synthesis. Thus, abrogation does not depend on viral or cellular DNA synthesis, and the role of viral RNA in this process must involve some other function. The nature of this viral function and its occurrence in Fv-1 permissive cells are discussed.

C-type RNA virus from hamster cells transformed in vitro by 1-B-D-arabinofuranosylcytosine

Evidence of type-C RNA viral activity in fetal hamster cells transformed in vitro by 1-B-D-arabinofuranosylcytosine (ara-C) after at least one in vivo passage is described. The virus possesses properties typical of other type-C RNA viruses, such as: a) morphology as determined with the electron microscope, b) presence of 70S RNA, c) enhanced expression following treatment with halogenated pyrimidines, d) group specific antigens of hamster type, and e) a buoyant density of 1.15 g per cm3. However, the virus particles are deficient in RNA-dependent DNA polymerase activity under conditions that easily detect Rauscher Leukemia virus and will infect neither hamster, rat, mouse, human nor rabbit cells. The possible role of this virus in chemical carcinogenesis of cultured hamster fetal cells is discussed.

Establishment of a cell line with associated Epstein-Barr-like virus from a leukemic orangutan

An Epstein-Barr virus like herpesvirus has been isolated from a lymphoid cell line derived from an orangutan with spontaneous myelomonocytic leukemia. Herpesvirus has not previously been isolated from this species of higher ape.

Revertants of human cells transformed by murine sarcoma virus

Revertants of nonproducer human osteosarcoma (NP/KHOS) cells induced by Kirsten murine sarcoma virus were isolated after incubating at high temperature (40.5 degrees C) overnight and subcloning at 36 degrees C. The morphologic variants, from which murine sarcoma virus could no longer be rescued, had growth properties similar to those of the nontransformed, parent human osteosarcoma cells and did not release RNA-dependent DNA polymerase activity. These revertants were nontumorigenic in nude mice. The revertants supported leukemia virus growth and showed an enhanced sensitivity to murine sarcoma virus superinfection. Thus, the revertants were from human cells transformed by an oncogenic RNA virus.

The co-carcinogenic activity of 4-nitropyridine-1-oxide (4-NPO) and prevention of transformation by type-specific anti-viral antibodies

Fischer rat embryo cells chronically infected with Rauscher murine leukemia virus, and known to be sensitive to transformation by potent chemical carcinogens, were transformed by the weak carcinogen 4-nitropyridine-1-oxide. Transformed cells grew in semi-solid agar and produced tumors in newborn Fischer rats. Transformation was inhibited by antisera specific for the ecotropic Rauscher murine leukemia virus, but not by antisera of equal toxicity specific for xenotropic Swiss mouse AT-124 virus.

Prevention of viral-chemical co-carcinogenesis in vitro by type-specific anti-viral antibody

Low passage Fischer rat embryo cultures, which are normally very resistant to transformation by 3-methylcholanthrene but are highly susceptible when chronically infected with the Rauscher murine leukemia virus, were completely protected from transformation by methylcholanthrene when treated with neutralizing antibody specific for the leukemia virus prior to and during treatment with methylcholanthrene. Sister cultures were not protected by neutralizing antibody specific for the B-tropic radiation leukemia virus. This demonstrates clearly a definite type specific role for Rauscher murine leukemia virus in the 2-methylcholanthrene transformation system in rat cells.

Physical and chemical properties of an oncornavirus associated with a murine adrenal carcinoma cell line

A type C oncornavirus has been isolated from a continuous cell line of murine adrenal carcinoma in culture. The particles have a buoyant density of 1.165 g/cm(3), exhibit typical type C morphology by electron microscopy, possess an RNA-dependent DNA polymerase, and have a high molecular weight RNA (6.1 x 10(6)) which can be denatured to a homogeneous lower molecular weight species (3.2 x 10(6)) when extracted from rapidly harvested "immature" virions. The virus is related antigenically to other mammalian oncornaviruses and exhibits a similar, although much more complex, sodium dodecyl sulfate gel electrophoretic profile of virion proteins when compared to the profiles of other type C RNA tumor viruses.

Deficiency of 60 to 70S RNA in murine leukemia virus particles assembled in cells treated with actinomycin D

Production of particles with the ultrastructural appearance of C-type virions persisted for at least 6 h in actinomycin D-treated cells infected with murine leukemia virus. This phenomenon occurred despite severe inhibition of viral RNA synthesis. Virus particles present in a 6-h harvest sedimented in sucrose gradients with the buoyant density characteristic of RNA tumor viruses (1.16 g/cm(3)) and exhibited high levels of reverse transcriptase activity in response to the exogenous template polyriboadenylic acid.oligo deoxythymidylic acid in the range of untreated controls. However, RNase-sensitive endogenous activity was only (1/5) the level found in controls. This observation correlated with a marked reduction in infectivity. Kinetic studies on the appearance of labeled RNA in banded virions revealed that within the first hour after addition of actinomycin D, particles contained 60 to 70S RNA and two low-molecular-weight RNA species corresponding to 8 and 4S RNA. After approximately 1 h of incubation with actinomycin D, 60 to 70S RNA could not be detected and 4S RNA was the predominant species. These findings suggest that murine leukemia virus particles assembled in the presence of actinomycin D are deficient in 60 to 70S viral RNA.

Isolation of a ribonucleoprotein structure from oncornaviruses

Detergent treatment releases from both avian and mammalian oncornaviruses a structure containing the viral RNA and a major arginine-rich protein, as well as reverse transcriptase activity.

Mutants of nonproducer cell lines transformed by murine sarcoma virus. II. Relationship of tumorigenicity to presence of viral markers and rescuable sarcoma genome

Tumorigenic and nontumorigenic mutants induced by a single 5'-bromodeoxyuridine (BrdU) treatment of a nonproducer (NP) tumorigenic cell line were isolated and characterized. Among the cloned derivatives were examples of virus-free and sarcoma virus-producing cell lines. Oncogenicity did not correlate with production of virus or ease of rescue of the sarcoma genome. All lines, including nononcogenic derivatives, retained the sarcoma genome. Phenotypic reversion of some cell mutants was observed after in vivo inoculation or long term in vitro cultivation. The M-50T cell line, obtained from a tumor induced by M-50 cells, had a sarcoma genome rescuable by direct superinfection; this was only achieved with parental M-50 cells by a cell fusion rescue technique. The M-43-2T cell, obtained from a single small static tumor induced by otherwise nononcogenic M-43-2 cells, shed sarcoma virus and became tumorigenic. M-58-4-48 became tumorigenic after passage 48 of the M-58-4 line, which was originally nontumorigenic. These observations of phenotypic reversion demonstrate that the presence of the sarcoma gene in cells is an essential but not sufficient condition of tumorigenesis.

5-Bromo-2'-deoxyuridine potentiation of transformation of rat-embryo cells induced in vitro by 3-methylcholanthrene: induction of rat leukemia virus gs antigen in transformed cells

Low-passage rat-embryo cells were not transformed by 3-methylcholanthrene or by 5-bromo-2' deoxyuridine. However, prior treatment with bromodeoxyuridine, followed by treatment with methylcholanthrene, resulted in cell transformation about three subpassages after removal of the carcinogen. RNA-directed DNA polymerase activity could not be detected in either normal or transformed cells. However, gs-1 antigen specific for rat C-type RNA virus was detected in cultures derived from bromodeoxyuridine-treated cells. No gs-1 antigen for the C-type RNA virus was detected in cultures that had not been treated with bromodeoxyuridine during the 25 subpassages of these experiments.High-passage rat-embryo cells, derived from a different cell pool, were transformed by either methylcholanthrene or dimethylbenzanthracene without prior infection with an exogenous virus, and without prior treatment with bromodeoxyuridine, gs-1 antigen for C-type RNA virus was also detected in 4 of 4 randomly selected transformed cell lines; the gs-1 antigen was not detected in any of 4 nontransformed control cultures. Considering these and previously published findings, we conclude that the lowpassage cells cannot be transformed by methylcholanthrene because of powerful cellular controls over the endogenous virus. Bromodeoxyuridine triggers some expressions of the endogenous virus; thus, the bromodeoxyuridine-treated cells are more susceptible to the transforming effects of methylcholanthrene. High-passage rat cells do not maintain perfect control over expression of their endogenous virus; the cell cultures are susceptible to the transforming effects of chemical carcinogens.

Karyotypic, virologic, and immunologic analyses of two continuous lymphocyte lines established from New Zealand black mice: possible relationship of chromosomal mosaicism to autoimmunity

Two continuous-suspension lymphocyte lines were isolated from the spleen and fibrosarcoma of a New Zealand Black female mouse. A C-type virus with a density of 1.16 g x cm(-3), 70S RNA, and RNA-directed DNA polymerase activity was isolated. The virus was infectious for NRK, NZB, (NZB x NZW)F(1), and (NZW x NZW)F(1) embryos, and for BALB/c 3T3 cells, but not for NIH Swiss cells. All cells from established lymphocyte cultures, as well as some embryo cells from New Zealand Black mice, showed karyotypic abnormalities. The possibility of chromosomal mosaicism is suggested.