Splenocyte plaque assay for the detection of murine leukemia virus

Expression of endogenous murine leukaemia viruses in AKR/J streaker mice

MICE of the AKR/J strain show high rates of spontaneous thymic lymphoma and are chronically infected with murine leukaemia viruses. Total thymectomy in young mice of this strain results in a marked decrease in the incidence of leukaemia. The finding of athymic mutant mice on the AKR/J background (referred to as streaker mice) is of importance in the study of genetic factors which influence virus expression and leukaemogenesis. Here we report that mice of both normal and mutant genotypes vary in the expression of a particular class of murine leukaemia virus. This lower virus expression in conjunction with the absence of a thymus leads to a lower tumour incidence in streaker mice.

Differential expression in human and mouse cells of human immunodeficiency virus pseudotyped by murine retroviruses

Expression of cell surface CD4 influences susceptibility of cells to human immunodeficiency virus (HIV) infection; however, some CD4-positive human and mouse cells are still resistant to HIV infection. To search for mechanisms of resistance to HIV independent of CD4 expression, HIV expression was studied in human and mouse cells normally resistant to HIV infection by introducing infectious virus by transfection of HIV DNA or infection with HIV pseudotyped with amphotropic or polytropic murine leukemia viruses. The results indicated that even when barriers to viral entry were bypassed, mouse NIH 3T3 cells and Dunni cells still showed a marked reduction in number of cells expressing HIV compared with the human cells studied, although the intensity of immunostaining of individual positive mouse cells was indistinguishable from that seen on permissive human cell lines. CD4 expression in mouse cells or human brain or skin cells did not influence the number of HIV foci observed after transfection with HIV DNA or infection with pseudotyped HIV. These results suggested that in addition to a block in the usual HIV fusion and entry process, CD4-positive mouse cells differed from human cells in exhibiting partial resistance to HIV infection which acted at a postpenetration step in the infection cycle. This resistance was partially overcome when mouse cells were infected by direct exposure to human lymphocytes producing HIV pseudotyped by amphotropic murine leukemia virus.

Development of a sensitive quantitative focal assay for human immunodeficiency virus infectivity

Accurate and sensitive quantitation of infectious human immunodeficiency virus (HIV) has been difficult to achieve. In this report, a quantitative focal immunoassay (FIA) for HIV was developed using human HeLa cells rendered susceptible to HIV infection by introduction of the CD4 gene via a retrovirus vector. Infected cells were identified by using human anti-HIV antibodies or mouse monoclonal antibodies specific for HIV together with secondary fluorescein- or peroxidase-conjugated antibody specific for mouse or human immunoglobulins. The assay identified cells infected with either wild-type or culture-adapted HIV isolates and was capable of detecting 1 positive cell in 10(6) cells. The FIA was also effective at detecting cell-free HIV, and in contrast to assays using A3.01, CEM, and other human leukemia cells, the FIA detected most wild-type HIV isolates. HIV neutralization could be determined by using the FIA, and two monoclonal antibodies reactive with HIV gp120 were found to neutralize only the LAV-IIIB strain of HIV. These monoclonal antibodies, as well as antibodies in serum samples from patients with acquired immune deficiency syndrome, were able to inhibit the spread of HIV infection in human lymphocyte suspension cultures but not in CD4-positive HeLa cells growing attached to plastic dishes.

Expression of murine leukemia viruses in B-cell lymphomas of CWD/Agl mice

The inbred mouse strain CWD/Agl has a high incidence of spontaneous B-cell lymphomas characterized by gross splenomegaly and lymph node enlargement. The endogenous ecotropic retrovirus of CWD/Agl mice is expressed in the spleen within the first 2 weeks of age and in the thymus by 1 month of age. Endogenous xenotropic virus is expressed in the spleen and bone marrow of the earliest age group examined (4 months). Restriction enzyme analysis of DNA extracted from tumorous tissues suggests that mink cell focus-forming viruses are not required for B-cell lymphomagenesis in CWD/Agl mice. CWD/Agl mice provide an important new experimental model for the study of B-cell lymphoma.

Established cell lines of mouse marrow adherent cells producing differentiation factor(s) for the granulocyte-macrophage lineage

Two continuous cell lines derived from long-term cultures of AKR mouse bone marrow adherent cells were isolated. These cell lines release colony stimulating activity (CSA), a factor that induces in vitro differentiation of granulocyte-macrophage progenitor cells. The colony forming cells and cluster forming cells in mouse marrow responsive to CSA from mouse lung conditioned medium MLCM). Colony forming cells were characterized by analysis of their density distribution after equilibrium centrifugation in density gradient. Cluster forming cells were characterized by analyzing the progeny of individual clusters after transfer to fresh semisolid culture medium containing MLCM. The results obtained indicate that the CSA from cell line conditioned medium closely compares with the CSA from MLCM in terms of the populations of colony and cluster forming cells stimulated.

Expression of murine leukemia viruses in the highly lymphomatous BXH-2 recombinant inbred mouse strain

Among 12 recombinant inbred strains of mice derived from crossing two strains, C57BL/6J and C3H/HeJ, which have a low incidence of neoplastic disease, one strain (BXH-2) has been found to have a high incidence of lymphoma, of non-T-cell origin, at an early age. The BXH-2 strain carries the Fv-1b allele and spontaneously expresses a B-tropic murine leukemia virus beginning at as early as 10 days of gestation and continuing throughout their life. No significant differences in ecotropic virus titers were observed at any age tested (16 to 17 days of gestation through 7 months), whereas xenotropic virus was first detected in lymphoid tissues of 2-month-old mice and virus titers increased with age. Dual tropic virus(es), which induced cytopathic changes on mink lung cells, was isolated from BXH-2 lymphomatous tissues. Unlike AKR mink lung focus-forming virus (N-tropic recombinant), BXH-2 dual tropic virus is B tropic and induces cytopathic changes in mouse fibroblast cultures as well. The BXH-2 mouse provides a model system for studying the role of replication-competent viruses in spontaneously occurring leukemias of non-T-cell lineage and neurological disease.

H-2-mediated resistance to an ecotropic type C retrovirus: localization of controlling genes and ontogenic studies of resistance

H-2 recombinant mice of the C57BL/10 series were used to study resistance to infection by an ecotropic (B-tropic) type C retrovirus. The gene(s) responsible for resistance was mapped to I-E/I-C subregions. Ontogenic studies indicated that this I-region mediated resistance is not present in the neonate but develops with age, and is firmly established within 2 weeks after birth. This temporary neonatal susceptibility in mice of resistant genotype is not the result of active suppression of the immune response.

Leukemogenesis, immune responsiveness, and murine leukemia virus expression in congenic AKR/J mice differing at H-2

In this study we examined the leukemia incidence, ectropic and xenotropic murine leukemia virus expression, and immune responsiveness of congenic AKR/J mice differing at the H locus. Congenic AKR.L-H-2b/1 mice, bearing the H-2b haplotype derived from C57L/J, were found to have a significant delay in time of death due to leukemia relative to that of AKR/J (H-2k) mice. The expression of ecotropic murine leukemia virus was found to be identical in both strains. The expression of xenotropic murine leukemia virus did vary, however, with the AKR.L-H-2b/1 mice showing a significantly reduced level of virus expression relative to AKR/J mice. In addition to these observations, we found that the AKR.L-H-2b/1 mice have an enhanced blastogenic responsiveness to phytohemagglutinin and to specific antigen to which they had previously been sensitized. Concomitant enhanced antibody response was not found. We suggest that the stronger cellular response, relative to AKR/J, may contribute to the delay in leukemia onset and to reduced xenotropic virus expression observed with the congenic mice.

Expression of leukemogenic recombinant viruses associated with a recessive gene in HRS/J mice

HRS/J inbred mice carry a mutant autosomal recessive gene (hr), which in homozygotes coincides with susceptibility to spontaneous thymic leukemia. Unlike their heterozygote (hr/+) littermates, hr/hr homozygotes express high levels of xenotropic virus during the preleukemic period, and viruses with a broadened host range (termed polytropic viruses) can be isolated from their preleukemic and leukemic tissues. Because hr/hr and hr/+ mice are otherwise genetically identical, the virological differences between them support the role of polytropic viruses in the generation of thymic leukemia. In the present report we show that the HRS/J polytropic viruses are env gene recombinants with unique oligonucleotide and peptide maps. These polytropic viruses appear to arise by recombination between ecotropic virus and an unidentified genome related, but not identical to, the endogenous xenotropic viruses. Moreover, polytropic viruses not only accelerate leukemogenesis in HRS/J mice, but also induce thymic leukemia in the low leukemia strain CBA/J. By contrast, cloned ecotropic and xenotropic viruses have no leukemogenic action.

Effects of thymosin on cytotoxicity and virus production in AKR mice

The cytotoxic potential and ability to produce endogenous ecotropic virus(es) of thymic-, splenic-, and lymph node-derived lymphocytes were significantly (and differentially) altered following thymosin treatment of young (nonleukemic) and aged (preleukemic) AKR mice. Thymosin administration markedly enhanced the cytotoxic potential of both splenic and lymph node lymphocytes from aged animals, but effected a moderate increase only in the spleens of young mice. Thymosin enhanced the production of endogenous virus(es) by thymocytes from aged mice, with the opposite effect noted in young animals. Splenectomy generally reduced the cytotoxic activity and virus titers of lymphocytes from both groups, and thymosin administration further reduced the cytotoxic response. These findings probably reflect overall effects on both regulatory lymphocyte subpopulations and different endogenous viruses in young and aged mice.

Absence of ecotropic or recombinant murine leukaemia virus in preleukaemic and leukaemic X-irradiated NZB mice

NZB mice X-irradiated with a single dose of 630 R when they were 1-month old developed a high incidence of histologically defined lymphocytic leukaemia 8--25 weeks later. We have screened for murine leukaemia viruses (MuLV) in the lymphoid tissues of 8 of these leukaemic mice, and in 8 "preleukaemic", apparently healthy NZBs killed 1 month post irradiation. Xenotropic, but not ecotropic or recombinant MuLV, was detected by in vitro co-cultivation of bone marrow, spleen and thymus with selectively permissive cell lines, followed by the immunofluorescence test for MuLV gs antigen, and the XC test. Our results are not consistent, therefore, with the concept that the factor causing the leukaemias was an oncogenic virus activated by X-irradiation.

Azathioprine-induced lymphocytic neoplasms of NZB mice lack ecotropic murine leukaemia virus

NZB mice injected intramuscularly throughout a 6-month period with the immunosuppressant azathioprine (Imuran) developed lymphocytic lymphomas 6--7 months after treatment was initiated. These malignancies were quite distinct from the reticulum-cell neoplasia which occurs spontaneously in the strain, and were readily transplantable to NZB or histocompatible BALB/c recipients. Xenotropic, but not ecotropic murine leukaemia virus (MuLV) was detected in leukaemic tissues of some donor and recipient NZBs when tested in vitro by co-cultivation with permissive cell lines, genome rescue, XC and viral polymerase assays. Virus filtrates prepared from donor leukaemic tissues were non-pathogenic when injected into newborn C3H mice. These results are evidence against a mandatory ecotropic MuLV genome in lymphocytic neoplasia.

Relationship between Moloney MSV tumor resistance and endogenous virogene expression in AKR mouse strain and its hybrids

We have examined the induction of Moloney-MSV tumors in AKR and other mouse strains in relation to endogenous virus expression. All virus-free strains so far tested were tumor-susceptible, while AKR was resistant. The selectivity of MSV tumor resistance, characteristic of AKR mice, was associated with AKR virogens segregation in first backcross to MuLV-negative/MSV-susceptible mice, and in a few second backcross families. Stronger evidence that AKR ecotropic virogene expression is the major, but not the sole, determinant of M-MSV tumor resistance in AKR mice was obtained in strains partially congeneic for akv-2 viral gene and in recombinant inbred lines derived from an original cross between AKR and C57L mice. These mice or lines, in which inheritance of endogenous viral genes had occurred, at the same time inherited the ability to show strong resistance to tumor induction by an exogenous oncogenic virus. This finding suggests that ecotropic endogenous viruses can exert a beneficial effect in their hosts providing some protective functions. Although the association between AKR-MuLV and M-MSV resistance is definite, the mechanisms by which MuLV+ mice are refractory to M-MSV tumors have still to be elucidated.