Leukemogenicity of clonal isolates of murine leukemia viruses.

The leukemogenicity of three types of cloned, in vitro grown murine retroviruses was studied. Two Moloney virus clones caused leukemia, as did five clones of the B-tropic endogenous virus of BALB/c mice. Neither of two clones of N-tropic BALB/c virus caused leukemia in Fv-1n/n mice, and the viruses were not recoverable from the animals. The ability to induce leukemia therefore appeared to reside in the genome of at least certain nondefective murine retroviruses.     

Aberrant viruses in cells infected with murine sarcoma virus-feline leukemia virus.

This study describes an unusual type of virus seen when a feline leukemia virus (FeLV) pseudotype of murine sarcoma virus (MuSV) obtained by cocentrifugation procedures infected feline embryo cells (FEF) and two Crandell cat cell lines (CrFK1, CrFK2). When all three cell cultures were infected with MuSV-FeLV, only FEF and CrFK2 were transformed and only these showed normal and aberrant virus. The CrFK1 infected with MuSV-FeLV did not transform but did replicate normal type-C virus with a 50-A intermediate coat. The virus replicated in the two transformed lines showed three particles; a normal particle with a 50-A intermediate coat, a normal particle with a 100-A intermediate coat, and an aberrant particle with a 100-A intermediate coat.     

Lymphocytes and leukemia viruses: tropism and transtropism of murine leukemia virus.

The tropism of naturally occurring murine leukemia virus (MuLV) was investigated in short-term lymphocyte cultures. The tropism of MuLV was readily defined in fibroblast cultures, but not in lymphocyte cultures. Lymphocytes free of infectious MuLV could be infected across the tropism barrier by partially purified MuLv or by in vitro contact with MuLV-producing lymphocytes. Stimulation of lymphocytes was not required for this cross-infection and replication of MuLV. When cross-infected lymphocytes and was specifically associated with lymphocytes were stimulated in vitro by allogeneic cells, they facilitated MuLV infection of ordinarily non-permissive fibroblasts. This phenomenon (transtropism) required antigenically stimulated lymphocytes and was specifically associated with infection of the lymphocyte by MuLV across the tropism barrier. Thus in contrast with the resting lymphocyte, the transformed lymphocyte acquired the ability to disseminate infectious MuLV to nonpermissive cells. These findings suggest a novel relationship between lymphocytes and leukemia viruses. They indicate one mechanism whereby antigenic stimulation may enhance the development of virus-induced lymphoid neoplasms.     

Ultraviolet inactivation of murine leukemia and sarcoma viruses

The ultraviolet (UV) sensitivity of Friend leukemia virus and a murine sarcoma virus (MSV) was determined. The inactivation dose D₁₀ was 2,000 to 3,000 ergs/mm² for both. This value is close to the one obtained with Rous sarcoma virus, 3,000 ergs/mm² (Goldé et al., 1961). MSV produced by the cells incubated in 5-fluorouracil (5 × 10^?3 M) was significantly more UV sensitive than control MSV, suggesting the sensitization of the viral nucleic acid by the base analogue. The virus surviving UV irradiation showed a prolonged latent period, but it was due to the interference of the inactivated virions with the normal growth of surviving virions. When UV-irradiated MSV was titrated on the cells previously irradiated with UV, the slope of the inactivation curve was steeper than when the virus was titrated on the normal cells. This may suggest the complementation of the UV-damaged viral genome by the corresponding homologous host genome.     

DNA transfection of ecotropic murine leukemia viruses in mouse cell cultures.

DNA's were isolated from cells chronically infected with N-, B-, or NB-tropic murine leukemia viruses and tested for infectious activity in various mouse cell cultures. Early detection of the DNA transfection is facilitated by growing the DNA-recipient cells in medium containing 10(-6) M hydrocortisone. Appropriate shearing of the DNA preparations may increase the efficiency of the transfection. With these procedures virus production of the transfected cells can be detected by XC plaque assay as early as 4 days after DNA inoculation in NIH 3T3 cells. Susceptibility of the mouse cell cultures to DNA transfection does not parallel their susceptibility to virion infection. Progeny viruses derived from the transfection show the same N- or B-tropic host range property as do the parent viruses.     

Strain-dependent expression of endogenous mouse-tropic leukemia viruses in chemically induced murine leukemias.

The effects of two chemical carcinogens, nitrosobutylurea and 7,12-dimethylbenz-(a) anthracene, on the expression of endogenous N- and B-tropic viruses were studied. The mice used were from two inbred strains, C57BL/6 and DDD-Fvr, and from 13 partially inbred strains derived from the cross of C57BL/6 with DDD-Ffr. These mouse strains are classified into three groups by the pattern of expression of endogenous viruses in normal, aged mice: (1) negative for both viruses; (2) positive for N-tropic virus only; and (3) positive for both viruses. Mice were given the carcinogens, and were tested at various intervals by the UV-XC procedure for the viruses in the spleens and other enlarged organs. Irrespective of the presence or absence of leukemias, the carcinogen-treated mice showed the same pattern of expression of endogenous viruses as that of non-treated normal mice. It can be concluded that the activation of the endogenous viruses is dependent on the mouse strains and that the growth of the activated viruses is not a necessary step for the chemical induction of leukemias.     

H-2 antigens on murine leukemia cells and viruses

Electron microscopic analysis of the distribution of H-2 (b, d, k) antigens on the cell surface and viruses of Rauscher, Friend, Graffi, Mazurenko, Gross and SZ leukemia was carried out using the hybrid antibody test. H-2 antigens occupied different areas on the surface of the cells, but their average content per 100 randomly selected cells ranged from 13-24% in different leukemias. The amount of mature viral particles with H-2 antigens on their surface did not correspond to the average content of these antigens on leukemia cells and amounted to 7-15%. H-2 antigens never covered the entire surface of viral particles and the ferritin label, which made it possible to spot them, was not more than 5-7 molecules. The same was true of viral buds, the majority of which, even on membrane sites with H-2 antigens, were devoid of these antigens. Possible reasons for the absence of H-2 antigens on mature and budding viruses are discussed.     

Antiviral effect of N-phenylacetoaminomethylene-DL-p-nitrophenylalanine (A-101) on murine leukemia viruses.

N-Phenylacetoaminomethylene-DL-p-nitrophenylalanine (A-101), when administered ip to male DDY mice infected with Friend leukemia virus, significantly inhibited virus-induced splenomegaly, reduced viable virus titers in spleen and plasma, and significantly prolonged survival time. A-101 also inhibited multiplication of the Friend and Moloney viruses in tissue culture systems.     

Immunofluorescent study of the group-specific antigen of murine leukemia viruses

In the present work a method is described for obtaining monospecific antibodies to the group-specific antigen of the murine leukemia viruses (MuLV-gs). This antigen is demonstrated in normal and malignant tissues by the indirect immunofluorescence technique. The MuLV-gs was demonstrated in Rauscher leukemic spleen cells of BALB/c mice and in normal spleen cells of high-leukemic AKR and some low-leukemic mice. The MuLV-gs was shown in cells of methylcholanthrene-induced sarcomas and in cells of tumors induced with polyoma virus. The MuLV-gs was not revealed in III Af mouse sarcoma induced by Rous virus. The specificity of the reaction was confirmed by the specific inhibition of the fluorescence after neutralization of the antibodies by the partly-purified MuLV-gs. The results obtained with the immunofluorescence technique coincided with those of agar precipitation analysis.     

Group-specific antigen of murine leukemia viruses in mice of low-leukemic strains

The distribution of group-specific antigen of murine leukemia viruses (MuL V-gs) was studied in mice of low-leukemic strains: C57BL/6, C57BL/10Sn, BALB/c and others. A rabbit monospecific anti-MuLV-gs serum was used for the antigen detection. The analyses were carried out by an indirect immunoradioautography technique which permitted the detection of trace amounts of the antigen. MuLV-gs was found in the spleen of mice of all strains studied. It was present in mouse embryos as well as in adult animals.     

Antiviral effects of amino acid derivatives with the fluorene substituent on murine leukemia viruses

Two new amino acid derivatives with the fluorene substituent, when administered ip to female inbred ICR-CD1 mice inoculated with Friend murine leukemia virus, significantly inhibited virus-induced splenomegaly, reduced viable virus titers in spleen and plasma, and significantly prolonged survival time. These compounds also inhibited multiplication of the strains of the Friend and Moloney murine leukemia viruses in a cell culture system. The action of these compounds on murine leukemia virus was presumely different from that of tilorone.     

Leukemogenesis in vitro induced by thymus epithelial reticulum cells transmitting murine leukemia viruses.

The role of the thymus in induction of leukemia was studied in vitro. Curltivation of normal thymus cells on thymus epithelial reticulum cell monolayers that had been grown from radiation leukemia virus-induced leukemic thymuses rendered the thymocytes leukemic. C57BL/6 thymocytes were cultivated for 3 days on leukemic thymus reticulum monolayers, and 106 thymocytes were injected i.p. into young adult C57BL/6 mice. After 3 to 4 weeks all mice died of disseminated lymphatic leukemia. Mice given thymocytes that had been cultivated on thymus epithelial reticulum monolayers from normal mice did not develop lymphomas. The leukemic thymus epithelial reticulum cells were shown to produce thymotropic as well as ecotropic and xenotropic radiation leukemia virus. (Thymotropic virus has affinity for thymus lymphocytes but noes not infect fibroblasts.) The cells were brightly positive for murine leukemia virus group-specific antigen in immunofluorescence tests. Leukemic thymus epithelial reticulum cells produced ample infectious exotropic virus in the culture supernatant, although the cells were negative in the XC syncytia test. Upon infection of mouse fibroblasts with ecotropic virus produced by the leukemic reticulum cells, XC syncytia were readily obtained. Thymocytes that were cultivated on leukemic thymus reticulum cells became positive for murine leukemia virus group-specific antigen and produced syncytia in the XC test. Thus, in vitro lymphomagenesis of the thymocytes that were cultured on leukemic thymus reticulum cells was associated with their infection with thymotropic and ecotropic radiation leukemia virus.     

Immunotherapy of murine leukemia. I. protection against friend leukemia virus-induced disease by passive serum therapy

Passive serum therapy with chimpanzee anti-serum raised against Friend leukemia virus (FLV) or the major viral envelope glycoprotein of FLV, gp71, has been shown to be effective in protecting mice against the development of virus-induced disease. The heterologous serum was administered in four doses beginning 3 days after virus inoculation and the mice were followed for the development of splenomegaly, virus level, and the induction of a host anti-viral immune response. The latter was measured by an in vitro humoral cytotoxicity assay using FLV-releasing mouse cells as targets, as well as by direct radioimmunoassay for the presence of mouse antibodies capable of binding FLV gp71. The results indicate a close correlation between the development of anti-viral humoral immunity and protection against splenomegaly, with both of these parameters being inversely related to serum and spleen infectious virus titers. Recognition of and response to the administered heterologous immunoglobulin in by the recipient animal does not appear to represent an integral part of the mechanism leading to protection, as determined by the absence of mouse antibodies capable of binding chimpanzee γ-globulin, as well as by the equal efficacy of the passive serum transfer protocol in mice after induction of tolerance to chimpanzee γ-globulin. It is suggested that passive serum therapy protection may function via a mechanism involving the reduction of the virus burden below a level which is immunosuppressive, allowing the development of a host anti-viral immune response which is necessary for elimination of virus and/or infected cells leading to long-term resistance.