Covert infection of a mouse mammary tumor cell line with Mycoplasma hyorhinis: cosedimentation with mouse mammary tumor virus in sucrose density gradients

Supernatant fluids from cultures of a mouse mammary tumor (MT) cell line were found to produce a specific cell detachment effect when inoculated into HeLa cells. The cell detachment factor (CDG) responsible for this effect was examined. Repeated attempts to cultivate this CDF in bacteriologic, fungal, and mycoplasma media were unsuccessful. However, using the DNA fluorochrome staining technique and specific immunofluorescent staining procedures, the CDF was identified positively as a noncultivable strain of Mycoplasma hyorhinis. It was also noted that this CDF could be labeled with [3H]uridine in MT cell cultures, concentrated, and banded at a density of 1.18 g/cm3 when centrifuged to equilibrium in a 20 to 60% sucrose gradient. Using a multiple antibiotic treatment regimen, the MT cells were "cured" of the M. hyorhinis contaminant. Re-infection of these cells with an exogenous strain of M. hyorhinis resulted in the same cell detachment effect, and this strain when labeled with [3H]uridine also sedimented at a density of 1.18 g/cm3. The salient feature of these studies is that M. hyorhinis sediments at the same density of mouse mammary tumor virus (MMTV) in sucrose density gradients. This was demonstrated by sucrose density gradient analyses of a purified sample of MMTV, assaying for reverse transcriptase activity, and a [3H]uridine labeled sample of the M. hyorhinis present in the MT cell cultures.

Use of the immune adherence hemagglutination test for titration of breast cancer patients' sea cross-reacting with purified mouse mammary tumor virus

Ninety-two sera from patients with breast cancer, 42 sera from patients with neoplastic diseases other than breast cancer and 59 sera from apparently healthy women were examined by means of the immune adherence hemagglutination (IAHA) test using purified mouse mammary tumor virus (MMTV) fron RII mouse milk. It was found that 36.4% (34/96) of the sera from breast cancer patients, 7.1% (3/42) of the sera from patients with other neoplastic diseases and 5.1% (3/59) of the sera from apparently healthy women showed a positive reaction. Among the IAHA positive sera from breast cancer patients, 82.9% (29/35) showed a titer of more than 1:16. On the other hand, none of the positive sera from patients with cancers other than breast cancer showed a titer of more than 1:16. The sera from 4 breast cancer patients, which showed a positive reaction with RII MMTV in the IAHA test, were tested to examine the specificity of the reaction by using milk samples from sources other than RII mice, including C57BL mice, dogs, cattle and humans. None of the 4 sera showed a positive reaction with milk samples from sources other than the RIII mouse.

[Detection of MMTV gp52 in C3H and C3Hf strain mice]

Immunofluorescence in paraffine sections demonstrated the difference in expression of the membrane glycoprotein of mouse mammary tumor virus (gp52) in two mouse sublines: C3H and C3Hf. In the C3Hf the glycoprotein becomes detectable in the mammary gland in the second part of pregnancy while in C3H line mature females before pregnancy. No viral glycoprotein could be found in the spleen, liver, kidneys and other organs in males and females of the two sublines or in C3H embryos.

Isolation of a series of novel variants of murine mammary tumor viruses with broadened host ranges

We have previously isolated mouse mammary tumor virus (MMTV) host range variants by serial virus passage in feline cells. These variants productively infect cells of a broad range of species but replicate most efficiently in feline cells. We report here the isolation of a series of novel MMTV host range variants that have the ability to replicate with high efficiency in murine, rat, canine and human cells, respectively; these variants were isolated by serial virus passage in cells of each respective species. These new variants, furthermore, all retained their ability to efficiently replicate in feline cells, and each exhibited unique host range properties. The novel MMTV variants obtained from murine, rat, feline, canine, and human cells showed no overt evidence of recombination with endogenous type-C viruses in that they retained their antigenic reactivities in group-specific radioimmunoassays for MMTV polypeptides, and were unreactive for type-C virus proteins when tested by radioimmunoassays and DNA polymerase assays. These novel MMTV host range variants now broaden the spectrum of studies that can be undertaken involving MMTV replication and the initiation and promotion of virus-mediated mammary cell transformation.

Sequence homology of nucleic acids from human breast cancer cells and complementary DNA's from murine mammary tumor virus and Mason-Pfizer monkey virus

Simultaneous presence of murine mammary tumor virus- and Mason-Pfizer monkey virus-specific sequences has been detected in nucleic acids isolated from some human breast tumors and from MCF-7 cells, a well-characterized human breast cancer cell line. Carefully characterized long complementary DNA transcripts were used in the molecular hybridization experiments. From the data that are presently available, it would appear that when homology is detected with one of the mammary tumor probes the other also generally shows shows homology. Among all the complementary DNA-RNA hybrids only three, all murine mammary tumor virus hybrids, show Tm values close to 80 degrees. The rest of the hybrids are low melting with shallow slopes for their Crt curves, indicating partial and imperfect hybrids in the majority of cases. Low levels of weak hybrid formation are also detectable with the tumor DNA's. The present experiments cannot ascertain whether the hybridizing sequences from Mason-Pfizer monkey virus and murine mammary tumor virus code for specific viral functions in their natural hosts. Annealing experiments using gene specific cDNA's would be required for fully characterizing these sequences.

The cause of species differences in mammary tumourigenesis: significance of mammary gland DNA synthesis

The causes of species differences in mammary tumourigenesis are briefly discussed from the genetical and environmental points of view, although no firm conclusions can be drawn. Distinct differences in mammary gland DNA synthesis were found between female mice and rats, which behave very differently in mammary tumourigenesis. Based on the importance of mammary gland DNA synthesis in mammary tumourigenesis, the differences in mammary gland DNA synthesis between species is hypothesized to be a major cause of the species differences in mammary tumourigenesis.

Simultaneous production of casein and mammary tumor virus in mouse mammary epithelial cells grown on floating collagen gels

Simultaneous production of casein and mammary tumor virus (MTV) was analyzed in monolayer cultures of mammary epithelial cells from pregnant BALB/cfC3H mice. A comparison of the 2 cell culture substrata, plastic culture dish and floating collagen gel, showed that the latter supported a much higher degree of simultaneous casein and MTV production in the presence of insulin, cortisol and prolactin in serum-free culture medium. Importance of floating collagen gel was further shown by delaying the flotation of gels. When the release of gels was delayed, there were concomitant delays in the increase of casein and MTV production. These results indicate that hormones, nature of substratum and flotation regulate the degree of differentiation of mammary epithelial cells in vitro.

Large-scale production of mouse mammary tumor virus in the absence of endogenous murine leukemia virus

A system for the large-scale production and purification of mouse mammary tumor virus in the absence of detectable endogenous murine leukemia virus is described. By utilizing the Mm5mt/c1 cell line established from an adenocarcinoma of a C3H mouse, the continuous production of over 25,000 liters of mouse mammary tumor virus-containing tissue culture fluids has been achieved. By the strict adherence to well-defined tissue culture conditions, mammary tumor virus production was accomplished without the expression of murine leukemia virus. Various biochemical and immunological systems were established for the rapid and precise detection of the endogenous leukemia virus, the expression of which could be enhanced under conditions of culture stress.

Isolation of host-range variants of mouse mammary tumor viruses that efficiently infect cells in vitro

Host-range variants of mouse mammary tumor virus (MMTV) have been isolated that have the ability to productively infect cells in vitro with high efficiency (at multiplicities of infection </=1) and with extremely short latent periods to the production of de novo virus (as short as 4 days after infection). These variants of the highly oncogenic MMTV of RIII, C3H, and GR mice were obtained by serial virus passage in feline cells. The resultant variant stocks react in group-specific radioimmunoassays for the MMTV major external glycoprotein (gp52) and major internal protein (p28), possess a protein profile similar to that of wild-type MMTV, and contain a virion-associated DNA polymerase with a magnesium cation preference. Addition of dexamethasone and insulin to culture media enhances the titer of de novo MMTV to levels of approximately 10(10) particles per 75-cm(2) flask (containing 5 x 10(6) cells) per 24 hr. Variant stocks exhibit no evidence of contamination with either murine or feline type C retroviruses, as assayed by various techniques. The variants of MMTV derived from C3H and RIII mice exhibit differential host ranges that include the ability to productively infect feline, canine, bat, mink, murine, and human cells. Use of these MMTV host-range variants now facilitates the study of the complete replicative cycle of MMTV as well as an elucidation of the interaction of MMTV with various hormones, physical or chemical carcinogens, and tumor promoters in the initiation and promotion of mammary neoplasia.

Suppressed murine mammary tumor virus (MuMTV) expression in RIII female mice treated neonatally with goat antiserum to MuMTV

Passive immunization of newborn inbred RIII (R3) mice with the globulin fraction of goat antiserum to murine mammary tumor virus (MuMTV) successfully suppressed MuMTV expression in the milk of some of the treated mice throughout nine successive lactations. No mammary tumors developed in the MuMTV-suppressed mice during the first 9 months, whereas all untreated R3 female breeders expressed MuMTV in the milk of the third lactation, and all developed tumors before 9 months of age (mode and median: 189 days).

Transmission of mammary tumor virus in mouse strain DD: further support for the uniqueness of strain GR

Outcrosses between high tumor mouse strain DD and low-incidence strains BALB/c and C57BL to produce reciprocal F1 susceptible offspring of both types resulted in high incidences of mammary tumors in (DD female X BALB/c male)F1 and (DD female X C57BL male)F1 females but not in the reciprocal hybrids with DD as the male parent. The finding that transmission of murine mammary tumor virus (MuMTV) in DD was through the milk, as most often observed in mouse strains, further supported the uniqueness of strain GR in which MuMTV is transmitted by either parent.

Mammary tumors and mammary tumor virus expression in hybrid mice of strains C57BL and GR

Mammary tumorigenesis in genetic crosses between the high mammary tumor incidence GR and the low incidence C57BL mouse strains is highly correlated with murine mammary tumor virus expression in milk. Although the F1 and first backcross females had a mammary tumor incidence which was consistent with a single dominant gene segregation, the tumor incidence in the critical second backcross segregants disproved the single gene hypothesis. Genetic factors were clearly involved in regulation of virus expression which in turn correlated with both tumor incidence and tumor latency; these complex phenotypes are however best explained as threshold or quasicontinuous characters. As predicted from this model, the age specific incidence of mammary tumors showed a broad peak at 14-19 mo of age with no evidence of an early or late phase. Hematopoietic tumors showed no correlation with virus expression or mammary tumorigenesis suggesting different etiologies for these tumors.

Is there a role for actin in virus budding?

Electrophoretic data from both sodium dodecyl sulfate-polyacrylamide gels (SDS-PAGE) and acid-urea gels reveal a protein in purified murine mammary tumor virus (MuMTV) which co-migrates with purified chick skeletal muscle actin. 125I-labeling of intact and disrupted virus preparations shows that the actin-like protein is not artifactually adsorbed to the outside of virions during isolation. Quantitative SDS-PAGE and examination of negatively stained preparations show that the actin cannot be accounted for by a contaminating population of virus-free vesicles. The ultrastructure of mammary epithelial cells and of Rous sarcoma virus-transformed chick embryo fibroblasts shows that virus extrusion is associated with filament-containing cellular processes. In particular, MuMTV is released from the ends of long microvilli which contain a bundle of 6-8-nm microfilaments and share other structural features with intestinal microvilli. We suggest that virus nucleoids require an interaction with host cell contractile proteins for their extrusion from the cell.

Factors interfering with cellular immunological responses to the murine mammary tumor virus in tumor-bearing mice

T-cell-enriched spleen cell fractions from BALB/c mice bearing virally induced mammary tumors become unresponsive with progressive tumor growth in the lymphocyte blastogenesis test, using purified mammary tumor virus (MTV) as an antigen. reactivity can be restored by mild trypsinization or extensive washing of the cells. When reactive cells from mice immunized with MTV were incubated with the wash fluid of the initially unresponsive cells. MTV-specific blastogenesis was inhibited. Washings of normal cells had no such effect. Leukocytes from tumor-bearing mice also become unresponsive to MTV in the leukocyte adherence inhibition (LAI) assay. Trypsinization or extensive washing does not restore reactivity. However, such treatment of reactive cells from immunized animals abolishes a positive reaction in the LAI assay, using MTV as an antigen. The washings of spleen cells from tumor-bearing mice inhibit the reactivity of leukocytes from immunized animals. Sera from these mice could block the reaction in both cellular immunological tests. The inhibiting factors appeared early after the onset of tumor growth. By means of the Sepharose bead immunofluorescence assay it was established that wash fluid and sera with blocking activity contain MTV antigens as well as anti-MTV antibodies.

Characterization of mouse mammary tumor viruses propagated in heterologous cells

Mouse mammary tumor viruses (MMTV) from three different strains of mice have been used to establish productive infections in feline and mink cell lines. The virions that are released by these cells compete completely in a radioimmunoassay for the major virion surface glycoprotein of MMTV (gp52), thus demonstrating that antigenic determinants of gp52 are viral coded. Competitive molecular hybridization studies have shown that the 60 to 70 S RNA's of MMTV's propagated in feline cells contain all the nucleic acid sequences found in 60 to 70 S RNA from MMTV synthesized by murine cells. The virion buoyant densities in sucrose and cesium chloride, virion sedimentation coefficient, divalent cation requirement of the virion DNA polymerase, and morphology of MMTV's synthesized in heterologous cells are similar to those of MMTV's grown in murine cells. Cultures of MMTV-infected feline cells have continuously released between 0.1 and 1.0 microgram of virus per 10(7) cells (75-sq cm flask) per day during the 60-week observation period. No detectable feline or murine type C viruses were produced by these cultures.

Levels of mammary tumor virus in hormone-dependent and -independent mouse mammary tumor cells

Levels of mammary tumor virus particles (types A and B) and levels of the virus antigen were assayed in hormone-dependent and -independent mammary tumors of GR mice. Various transplant generations of seven separate tumor lines were investigated. The results indicated that the tumors consisted of different cell clones, each of which exhibited a separate progressive expression and subsequent loss of the mammary tumor virus. When the tumors were transplanted, levels of B particles first declined in the hormone-dependent cells, but in later transplant generations, the B particle content of the autonomous cells also dropped. In some tumor lines, this was accompanied by a decrease in viral antigens and/or A particles, but in other lines these concentrations remained high. One tumor line (line V) that remained hormone-dependent throughout nine transplantations was practically devoid of B particles but contained high levels of A particles and mammary tumor antigen.

The major polypeptides of the murine-mammary-tumor virus isolated by plant-lectin affinity chromatography

Solubilized polypeptides of the murine mammary tumor virus (MuMT virus) were chromatographed on a column of immobilised concanavalin A. The unbound viral material was rechromatographed on phosphocellulose, resulting in the isolation of the major proteins with a molecular weight of 28000 (p28) and 12000 (p12) respectively. The adsorbed glycopolypeptides after elution with methyl alpha-D-mannopyranoside were subjected to gel filtration. The major glycoprotein with a molecular weight of 52000 (gp52) was obtained in an almost pure form. However, a considerable part of gp52 elutes together with a glycoprotein with a molecular weight of 36000 (gp36), suggesting that in addition to the free form of gp52 a complex exists of gp52 plus gp36.