Spleen specific expression of an MMTV related transcript associated with the Mtv-6 locus in BALB/c mice

Expression pattern of mouse mammary tumor virus in transgenic mice carrying exogenous proviruses of different origins

To study the tissue specificity of mouse mammary tumor virus (MMTV) gene expression, we developed two series of transgenic mice, containing the MMTV proviral DNA of mammary (GR) and kidney (C3H-K) origin. The expression pattern in the MMTV(GR) transgenic mice is very similar to that observed in infected animals, e.g., a strong preference for viral expression in the lactating mammary glands and lower levels of expression in salivary glands, lymphoid tissues, and male reproductive organs. One line of transgenic mice carrying the C3H-K provirus has a similar expression pattern, indicating that MMTV(C3H-K), despite a striking alteration in the U3 region of its long terminal repeat, can be expressed in the same tissues as the wild-type MMTV.

Factors controlling the expression of mouse mammary tumour virus

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Retroviral super-antigens and T cells

For many years immunologists have been intrigued by a series of potent antigens encoded in the murine genome. These antigens, originally termed minor lymphocyte stimulating (Mls) antigens, are capable of inducing extremely strong T cell proliferative responses when presented in the context of MHC class II molecules. Recently, Mls antigens have been shown to stimulate T cells bearing particular T cell receptor V beta elements, leading to the designation of super-antigens. The endogenous expression of these super-antigens in mice results in the clonal elimination of large numbers of T cells in order to maintain self-tolerance. In this review we discuss the recent identification of endogenous super-antigens as retroviral gene products. In addition, we analyze the role of class II MHC molecules in the presentation of endogenous super-antigens to T cells. Finally, we discuss the dramatic effect of retroviral super-antigens on the T cell repertoire.

Endogenous superantigen expression is controlled by mouse mammary tumor proviral loci

Superantigens are defined by their ability to stimulate T cells based predominantly on their V beta expression and ability to delete T cells in the thymus when expressed endogenously. We show here that the expression of one endogenous superantigen, Etc-1, is controlled by the expression of the open reading frame region of the 3' long terminal repeat of the mouse mammary tumor proviral gene, Mtv-9. We show that Mtv-8 controls a superantigen with similar specificity, and that both Mtv-8 and Mtv-9 stimulate some V beta 17+ T cells. A third provirus, Mtv-6, controls a superantigen with specificity for V beta 3. Data presented raise the possibility that endogenous superantigens may compete for class II molecules in a single B cell.

Mouse mammary tumor virus (MMTV) infection in SWISS and RIII mice. Correlation between resistance to exogenous infection and anti-MMTV serum response

Host-virus relationships were examined in mice from the two mouse mammary tumor virus (MMTV)-infected strains SWISS MB+ and RIII, which harbour the same MMTV variant, and from the derived sublines Swiss MB- and RIIIf, which were freed of milk-borne MMTV by foster-nursing. These two strains are not phylogenetically related, the SWISS strain bearing the endogenous Mtv-3 locus in its DNA. In RIII and SWISS MB+ mice, the incidence of early mammary tumors, which was of 96% and 8%, respectively, was correlated to the level of MMTV expression in milk. In the SWISS MB-line, a non-coordinate expression of the provirus associated with the Mtv-3 locus was observed in the mammary glands, the salivary glands and the spleen. This expression was not tumorigenic and was characterized by the presence of the p28 gag antigen and the absence of the gp52 env antigen, except, however, in mammary glands of elder mice where traces of gp52 were found. In the mammary glands of SWISS MB+ mice, the expression of the Mtv-3 locus was masked by large amounts of antigens resulting from exogenous virus expression. RIIIf mice were MMTV-negative. Viral antigens coexisted with anti-MMTV antibodies in the serum of infected and tumor-bearing mice, but not in the form of immune complexes as verified by a method that allowed to detect specific antigen-containing-soluble immune complexes. An anti-MMTV serum reactivity was also detected in SWISS MB- and RIIIf mice. However, the serum response was higher in the two SWISS lines than in the two RIII lines. Except in tumor-bearing mice, the anti-MMTV response was not significantly modified by the presence of exogenous virus and thus resulted essentially from exposure to endogenous MMTV expression. In experimental infection studies, RIII mice were more susceptible to MMTV infection than SWISS mice. The correlation between resistance to MMTV infection and serum response to endogenous MMTV expression, suggests that the non-tumorigenic expression of an endogenous provirus can protect at least partially, against exogenous MMTV infection.

Transgenic mice carrying the mouse mammary tumor virus ras fusion gene: distinct effects in various tissues

Transgenic mice carrying the v-Ha-ras oncogene under the control of the mouse mammary tumor virus long terminal repeat were produced. These mice exhibit several phenotypes: mammary tumors, bilateral hyperplasia of the harderian lacrimal gland, primary bronchio-alveolar lung adenocarcinoma, and splenomegaly. High levels of the transgene RNA were detected in mammary, harderian, and lung tumors. Accumulation of cells of the myeloid lineages was found in enlarged spleens. This phenotype may represent an indirect effect of v-Ha-ras expression on myeloid progenitors. Our data illustrate the cell-specific effects of v-Ha-ras.

Construction and characterization of a packaging cell line for MMTV-based conditional retroviral vectors

A chimeric provirus in which the 5'LTR of a complete biologically active Mouse Mammary Tumour Virus (MMTV) proviral DNA has been replaced with the Rous Sarcoma Virus LTR has been constructed. Upon transfection into permissive cells, this provirus directs the synthesis of the MMTV gag and env structural proteins, but is impaired in packaging of the RNAs that encode these proteins. Supertransfection of these cells with MMTV based vector constructs results in the production of infectious recombinant virus at a higher efficiency than with previously described helper cell lines. Such a retroviral vector system based on MMTV will allow the study of the effects of conditional expression of inserted genes upon infected cells.

Transgenic mice carrying the mouse mammary tumor virus ras fusion gene: distinct effects in various tissues

Transgenic mice carrying the v-Ha-ras oncogene under the control of the mouse mammary tumor virus long terminal repeat were produced. These mice exhibit several phenotypes: mammary tumors, bilateral hyperplasia of the harderian lacrimal gland, primary bronchio-alveolar lung adenocarcinoma, and splenomegaly. High levels of the transgene RNA were detected in mammary, harderian, and lung tumors. Accumulation of cells of the myeloid lineages was found in enlarged spleens. This phenotype may represent an indirect effect of v-Ha-ras expression on myeloid progenitors. Our data illustrate the cell-specific effects of v-Ha-ras.

Spontaneous progression of hyperplastic outgrowths of the D1 lineage to mammary tumors: expression of mouse mammary tumor virus and cellular proto-oncogenes

Mammary cancer in mice is characterized by progression through defined stages of preneoplasia, with the most common preneoplastic stage being the hyperplastic alveolar nodule (HAN). We determined the relative levels of RNA expression of various cellular proto-oncogenes and endogenous mouse mammary tumor virus genes in outgrowths and tumors of three sublines of the transplantable D1 HAN preneoplastic outgrowth line. The three sublines differed in relative tumor-producing capabilities. Subline D1B produced a high incidence of tumors with short latency periods, whereas sublines D1C and D1D produced low incidences of tumors with long latency periods. No consistent alteration in proto-oncogene expression correlated with relative tumorigenicity, although tumors frequently contained higher levels of one or more proto-oncogene transcripts as compared with preneoplastic tissue. Slightly elevated (2- to 6-fold) levels of different oncogene transcripts were detected in 13 of 17 tumors as compared with outgrowth tissue, including abl (2 tumors), fps (5 tumors), Ha-ras (6 tumors), and Ki-ras (8 tumors). One tumor contained 45 times more Ki-ras-specific RNA than outgrowth tissue because of a comparable amplification of Ki-ras DNA sequences. Elevated levels of Ha-ras occurred more frequently in tumors of a high-incidence subline than in a less-aggressive subline (5/10 vs 1/7), but this difference was not statistically significant. However, consistent changes in MMTV expression accompanied progression from preneoplastic tissues to mammary tumors. All 17 tumors displayed reduced levels of the MMTV-specific long terminal repeat (LTR) transcript (1.6 kb) as compared with HAN tissue; tumors with moderate levels of LTR transcript expressed the 3.8-kb envelope message as well, one not detected in HANs. Expression of the LTR transcript is apparently influenced by factors in addition to the methylation status of endogenous mouse mammary tumor virus genes, which was similar in outgrowths and tumors. As the survey of representative proto-oncogenes failed to identify a uniform change between HAN and tumors, it is likely that other genes are involved in tumor progression in the mammary gland.

Endogenous mouse mammary tumor virus is expressed in several organs in addition to the lactating mammary gland

We studied the transcription of endogenous mouse mammary tumor virus (MMTV) in transgenic and normal strains of mice of different genetic backgrounds. Although the lactating mammary gland in all strains showed the highest level of MMTV expression, detectable levels of viral RNA were also found in the lungs, kidneys, salivary glands, seminal vesicles and/or prostate gland, testes, and lymphoid tissue in mice which contain different endogenous proviruses. Transcription in transgenic mice containing the MMTV long terminal repeat linked to the simian virus 40 early region also occurred in these same organs. These results indicate that the MMTV long terminal repeat functions in several organs in addition to the lactating mammary gland.

The mouse mammary tumor virus long terminal repeat directs expression in epithelial and lymphoid cells of different tissues in transgenic mice

A series of transgenic mice was developed that contained the simian virus 40 early region genes under the transcriptional control of the mouse mammary tumor virus long terminal repeat, including the promoter and glucocorticoid response elements. These mice all expressed the transgene in the epithelial cells of a number of different organs, such as lungs, kidneys, and prostate, salivary, and mammary glands, and in Leydig and lymphoid cells. Transcription of the chimeric gene was inducible by glucocorticoids, either after transfection into tissue culture cells or in cells cultured from animals carrying the transgene. Many, but not all, tissues which expressed the simian virus 40 sequences, as determined immunologically and by RNA analysis, developed into tumors, although they showed premalignant features. Since the mouse mammary tumor virus long terminal repeat is expressed in a number of different cell types when inherited through the germ line, the lactating mammary gland-specific transcription of endogenous proviruses must require other factors or sequences to achieve this specificity.

Mouse mammary tumor virus mediated transfer and expression of neomycin resistance to infected cultured cells

The mammary gland specific, glucocorticoid controlled expression of MMTV makes it an ideal candidate for the basis of a nonpromiscuous regulated retroviral vector system. We have previously constructed an MMTV proviral variant that gives rise to virus particles upon introduction into cultured cells. This was used to construct a defective MMTV provirus in which the MMTV env gene was replaced by the neomycin resistance gene under the control of the herpes simplex thymidine kinase promotor. The defective provirus was packaged after transfection into two distinct MMTV producing cell lines. Conditioned medium from these cells contains virus particles which are able to infect cells of fibroblast and epithelial origin and to confer neomycin resistance upon them. This indicates that the defective MMTV provirus contains the sequences required for packaging of the genomic viral RNA. Transfection of the same MMTV-neo recombinant provirus into the MoMLV packaging cell line, psi 2, did not result in any infectious virus particles. Thus the packaging signals for MMTV and MoMLV appear to be distinct. Analysis of the MMTV infected cells reveals the presence of the MMTV-neo recombinant provirus. Expression of both MMTV and neomycin is detectable and augmented when the infected cells are grown in the presence of glucocorticoid hormone.