Quantitative Determination of Mammary Tumor Virus in Individual Samples of Mouse Milk

The possibility to determine quantitatively the intact mammary tumor virus (MTV) in milk of mice carrying milk-transmitted MTV has been assayed by a method that allows direct comparison between individual milk samples. The method is based on (a) the measure of light scattering of partially purified MTV preparations, (b) the use of milk from genetically identical MTV free mice as blank and (c) the quantitative reference to the total protein content of whole milk. The sensitivity, specificity and reproducibility of the procedure, as well as the requirement of appropriate quantitative references, are illustrated and discussed. BALB/c (MTV free), BALB/cfC3H, and BALB/cfRIII mice have been used.

Quantitative Comparison of Milk-released C3H and RIII Mammary Tumor Viruses in Infected BALB/c Hosts

The C3H and RIII mammary tumor viruses (MTV) carried by BALB/cfC3H and BALB/cfRIII breeding females have been quantified and compared in milk samples, after partial purification with a sucrose density gradient. The samples were collected at identical times during the first 3 lactation periods from individual mice (6 per strain), standardized for age at delivery and size of litter. Milk samples from 6 MTV negative BALB/c controls have also been analyzed. Data for comparison are expressed in optical density units (ODU) and refer to the protein content of the whole milk using MTV-negative BALB/c milk as blank. The results have shown 1) an increase of MTV released through milk in each MTV-carrying female from the first (average ODU, 0.542) to the second (1.351) and third (2.105) lactation, 2) individual variations, and 3) a significant difference in release between C3H and RIIl MTV, the latter being more than double (average ODU, 1.801) in respect to the former (0.847). The apparent discrepancy between these results and the bioactivity in BALB/c mice of C3H and RIII MTV, significantly lower for the latter, is discussed.

Stability of Characters of Mammary Tumors in Balb/cfRIII Mice

The biological and morphological characteristics of mammary tumors in BALB/c mice infected with RIII mammary tumor virus (MTV) by foster nursing have substantially kept stable and unchanged after 20 years, 53 inbred generations and transfer of the strain from Perugia to Pisa. This suggests that the causal MTV keeps unaltered in time its biological properties.

Of mice and men: a comparative study of cancer-associated fibroblasts in mammary carcinoma

INTRODUCTION

The initial clinical experience from targeted therapy for breast cancer has been mixed. While important progress has been made in the care of a subset of patients characterized by amplification of HER2 through the use of trastuzumab, other targeted therapies have failed to improve the outcome for large, unselected groups of patients. Thus, efforts to find prognostic or predictive biomarkers to enable tailored therapy are highly warranted. Genetically engineered mouse models of human cancer provide a convenient setting in which to perform explorative studies. However, there is a paucity of comparative studies between mouse and human tumours in order to validate the use of mouse models as discovery tools.

MATERIALS AND METHODS

Here, we have compared the localization of markers for cancer-associated fibroblasts in the MMTV-PyMT mouse model of mammary carcinoma with that of human breast cancer. The expression of α-smooth muscle actin, platelet-derived growth factor receptor-α, and fibroblast-specific protein-1 was assessed by immunostaining of sections from tumours of MMTV-PyMT mice. Information about the distribution of the same markers in human breast cancer was derived from the publicly available database the Human Protein Atlas.

RESULTS

Both mouse and human mammary carcinomas were infused by a rich fibrotic stroma. While no marker was capable of identifying all stromal fibroblasts, the expression pattern of each marker was remarkably similar in mouse and human.

DISCUSSION

We conclude that the MMTV-PyMT mouse model of breast cancer will have utility as a discovery tool for biomarkers of cancer-associated fibroblasts during malignant conversion.

Mouse Mammary Tumor Biology: A Short History

For over a century, mouse mammary tumor biology and the associated Mouse mammary tumor virus (MMTV) have served as the foundation for experimental cancer research, in general, and, in particular, experimental breast cancer research. Spontaneous mouse mammary tumors were the basis for studies of the natural history of neoplasia, oncogenic viruses, host responses, endocrinology, and neoplastic progression. However, lacking formal proof of a human mammary tumor virus, the preeminence of the mouse model faded in the 1980s. Since the late 1980s, genetically engineered mice (GEM) have proven extremely useful for studying breast cancer and have become the animal model for human breast cancer. Hundreds of mouse models of human breast cancer have been developed since the first demonstration, in 1984, that the mouse mammary gland could be molecularly targeted and used to test the oncogenicity of candidate human genes. Now, very few scientists can avoid using a mouse model to test the biology of their favorite gene. The GEM have attracted a new generation of molecular and cellular biologists eager to apply their skills to these surrogates of the human disease. Newcomers often enter the field without an appreciation of the origins of mouse mammary tumor biology and the basis for many of the prevailing concepts. Our purpose in writing this short history of mouse mammary tumor biology is to provide a historical perspective for the benefit of the newcomers. If Einstein was correct in that "we stand on the shoulders of giants," the neophytes should meet their giants.

Viruses and human breast cancer

There are well-established risk factors for breast cancer, most of which relate to estrogens and growth hormones in females. These include early-age menarche, late-age menopause, postmenopausal obesity and use of hormone therapy. However, these factors do not account for the sixfold difference in breast cancer incidence and mortality between countries and the fact that these differences dramatically lessen after migration; nor do they account for male breast cancer. Accordingly, hormone-responsive viruses have become major suspects as etiological agents for human breast cancer. Human papillomaviruses, mouse mammary tumor virus and Epstein-Barr virus are the prime candidate viruses as causes of human breast cancer. Human papillomaviruses and the mouse mammary tumor virus have hormone responsive elements that appear to be associated with enhanced replication of these viruses in the presence of corticosteroid and other hormones. This biological phenomenon is particularly relevant because of the hormone dependence of breast cancer. Viral genetic material for each of these candidate viruses has been identified by polymerase chain reaction in breast tumors but rarely in normal breast tissue controls. Pooled data from controlled studies show substantial odds ratios for the presence of viral genetic material in breast tumors compared with normal controls. These and additional data provide substantial, but not conclusive, evidence that human papillomavirus, the mouse mammary tumor virus and Epstein-Barr virus may have a role in the etiology of human breast cancer. If conclusive evidence for a role of these viruses in breast carcinogenesis can be developed, there is a practical possibility of primary prevention.

Presence of mouse mammary tumour-like virus gene sequences may be associated with morphology of specific human breast cancer

BACKGROUND

Mouse mammary tumour virus (MMTV) has a proven role in breast carcinogenesis in wild mice and genetically susceptible in-bred mice. MMTV-like env gene sequences, which indicate the presence of a replication-competent MMTV-like virus, have been identified in some human breast cancers, but rarely in normal breast tissues. However, no evidence for a causal role of an MMTV-like virus in human breast cancer has emerged, although there are precedents for associations between specific histological characteristics of human cancers and the presence of oncogenic viruses.

AIM

To investigate the possibility of an association between breast cancer and MMTV-like viruses.

METHODS

Histological characteristics of invasive ductal human breast cancer specimens were compared with archival MMTV-associated mammary tumours from C3H experimental mice. The presence of MMTV-like env DNA sequences in the human breast cancer specimens was determined by polymerase chain reaction and confirmed by Southern hybridisation.

RESULTS

MMTV-like env gene sequences were identified in 22 of 59 (37.3%) human breast cancer specimens. Seventeen of 43 (39.5%) invasive ductal carcinoma breast cancer specimens and 4 of 16 (25%) ductal carcinoma in situ specimens had some histological characteristics, which were similar to MMTV-associated mouse mammary tumours. However, these similarities were not associated with the presence or absence of MMTV-like gene sequences in the human breast tumour specimens. A significant (p = 0.05) correlation was found between the grade of the human breast cancer and similarity to the mouse mammary tumours. The lower the grade, the greater the similarity.

CONCLUSION

Some human breast cancer specimens, in which MMTV-like env DNA sequences have been identified, were shown to have histological characteristics (morphology) similar to MMTV-associated mouse mammary tumours. These observations are compatible with, but not conclusive of, an association between the presence of MMTV-like env DNA sequences and some human breast cancers.

Clonal isolation of different strains of mouse mammary tumor virus-like DNA sequences from both the breast tumors and non-Hodgkin's lymphomas of individual patients diagnosed with both malignancies

PURPOSE

In a previous study, we had detected the presence of mouse mammary tumor virus (MMTV)-like envelope (ENV) gene sequences in both the breast tumors and non-Hodgkin's lymphoma tissue of two of our breast tumor patients who had been diagnosed simultaneously with both malignancies. The aim of this study was to determine if MMTV-like DNA sequences are present in the breast tumors and non-Hodgkin's lymphomas of additional patients suffering from both malignancies and if so to characterize these sequences in detail.

EXPERIMENTAL DESIGN

DNA was extracted from formalin-fixed, paraffin-embedded tissue sample blocks of breast tumors and non-Hodgkin's lymphomas from patients suffering from both malignancies. A 250-bp region of the MMTV ENV gene and a 630-bp region of the MMTV long terminal repeat (LTR) open reading frame (ORF) that encodes the MMTV superantigen (sag) gene were amplified by PCR from the isolated DNA. Amplified products were analyzed by Southern blotting, cloned, and sequenced.

RESULTS

MMTV-like ENV and LTR sequences were detected in both the breast tumors and non-Hodgkin's lymphomas of 6 of 12 patients suffering from both malignancies. A novel mutant of the MMTV ENV gene was identified in these patients. Characterization of the MMTV-like LTR highly variable sag sequences revealed total or nearly total identity to three distinct MMTV proviruses from two different branches of the MMTV phylogenetic tree.

CONCLUSIONS

The presence of MMTV-like ENV and LTR sequences in both the breast tumors and non-Hodgkin's lymphomas of 6 additional patients suggests a possible involvement of these sequences in these two malignancies. MMTV-like LTR sequence homology to different MMTV proviruses revealed the presence of more than one strain of MMTV-like sequences in each individual suggesting the possibility of multiple infections in these patients.

Of mice, cats, and men: Is human breast cancer a Zoonosis?

Mouse mammary tumor virus (MMTV), a member of the betaretroviridae, is the most common cause of breast cancer (BC) in mice. MMTV is transmitted in mice both in the germline as endogenous proviruses and exogenously as infectious virions. Here, we review a variety of evidence accumulated for six decades that has suggested that a human homologue of MMTV may exist. The findings include recent studies from several independent laboratories that have detected sequences very closely related to MMTV in DNA isolated from human BC tumors. Other laboratories, however, have failed to detect the MMTV-related sequences in human DNA samples, and conclusive evidence for a human mammary tumor virus has been elusive. We also reviewed additional studies, suggesting that betaretroviruses are present in a much wider range of species than previously known, including rodents, felines, and primates. The observation that a subset of cats may be infected with a close homologue of MMTV may be of epidemiological significance for human BC. Cats may become infected by MMTV from mice, and in turn may transmit the virus to humans, possibly after selection for variants with an expanded host range.

Lack of immunological or molecular evidence for a role of mouse mammary tumor retrovirus in primary biliary cirrhosis

BACKGROUND & AIMS

Recent observations, including a pilot clinical trial, have suggested that a human mouse mammary tumor virus (MMTV) causes primary biliary cirrhosis (PBC). We attempted to confirm such data.

METHODS

We obtained sera from 101 patients (53 with PBC and 48 controls), fixed liver sections from 10 patients (8 PBC and 2 controls), fresh liver specimens (6 PBC and 6 controls), and fresh peripheral blood lymphocytes (PBLs) (10 PBC and 10 controls). We studied sera for reactivities against 3 different strains of MMTV virions, MMTV(C3H), MMTV(FM), and MMTV(LA), including goat polyclonal antibodies against MMTV virions, gp52, and p27 as positive controls. We stained liver specimens using polyclonal antibodies against MMTV and gp52 and further examined tissue samples and PBLs for specific MMTV genome sequences.

RESULTS

By Western blot analysis, no detectable reactivity in any of the PBC sera against any of the 3 MMTV strains or MMTV gp52 or p27 was observed. However, viral proteins were recognized by our control positive polyclonal antibodies. We note that 13%-60% of PBC sera presented low reactivity against 2 proteins of approximately 57 and 74 kilodaltons. Such reactivity is related to the trace amounts of mitochondrial antigens in the virus preparations derived from murine mammary tumor tissue. No detectable immunohistochemical or molecular evidence for MMTV was found in the liver specimens or PBLs.

CONCLUSIONS

We were unable to recapitulate the data on this specific retroviral etiology of PBC and suggest that such data could be the result of contamination.

A Viral Aetiology for Breast Cancer: Time to Re-Examine the Postulate

Despite decades of research, no aetiologic factor(s) for human breast cancer has been identified and the search for a causal agent has all but been abandoned during the past thirty years. Over 60 years ago, it was demonstrated that breast tumours in mice are caused by an oncornavirus, murine mammary tumour virus (MMTV). Whilst many at that time postulated a similar virus might be the causative agent of human breast cancer, genetic evidence was difficult to obtain primarily because of the occurrence of endogenous human retrovirus (HER) sequences within the human genome that share extensive regions of nucleotide homology with MMTV. Recently, there has been a resurgence of interest in the possibility that a significant proportion of human breast cancers may be caused by viral infections. Two candidate viruses have been proposed, a human retroviral analogue of MMTV (which differs significantly in sequence and characteristics from HERs) and, the Epstein-Barr virus (gamma-herpes virus). These two viruses have been reported to occur in up to 37 and 50% of breast cancer cases, respectively. Here we present the background to the infectious hypothesis for the aetiology of breast cancer and review recent findings.

Human murine mammary tumour virus-like agents are genetically distinct from endogenous retroviruses and are not detectable in breast cancer cell lines or biopsies

It has been reported that a human murine mammary tumour virus (MMTV)-like virus (HMLV), which may be an endogenous human retrovirus (HERV), occurs in the human breast cancer cell lines T47D and MCF-7 and, in 38% of human breast cancer biopsies. As the aetiology of most breast cancers remains unknown, it is important to verify these observations in differing breast cancer populations worldwide. Thus, we sought to determine the genetic relationships between HMLVs, MMTVs, and HERVs, and to investigate the association between HMLVs and breast cancer biopsies from South London, UK. Phylogenetic analyses of the env/pol region indicated that HMLVs are indistinct from MMTVs, and that MMTVS/HMLVs exhibit only low sequence homologies with HERVs. A search of the human genome confirmed that HMLVs are not endogenous. Using MMTV polymerase chain reaction (PCR) primers described previously, we amplified DNA from all cell lines except MCF-7 and from 7 of 44 (16%) breast cancer biopsies. A restriction fragment length polymorphism assay was designed to distinguish between HMLVs and MMTVs, and upon analyses, PCR amplicons appeared to be HMLVs. To confirm these findings, amplicons from the T47D cell line and from four randomly selected breast cancer patients were sequenced. Of 106 DNA sequences obtained, 103 were homologous with a short arm of human chromosome (Chr) 3 (3p13), two with Chr 4, and one with Chr 8. None of the sequences exhibited significant nucleotide homology with MMTVs, HMLVs, or with HERVs (all <50%). Thus, we conclude that (i) HMLVs are integral members of the MMTV family; (ii) MMTVs/HMLVs are genetically distinct from HERVs; (iii) MMTV/HMLV DNA is not present in human breast cancer cell lines or clinical biopsies in our locality.

Sequence and genetic analyses of the 3' terminus and integration sites of the RIII/Sa mouse mammary tumor (MMTV) exogenous provirus

The RIII/Sa mouse mammary tumor virus (MMTV) is one of several exogenous MMTV viruses transmitted to suckling mice through the milk. We report herein the nucleotide sequence of the exogenous RIII/Sa provirus from the central Eco RI site through to the end of the U5 region encoded by the 3' LTR. We also provide a detailed sequence analysis often different 3' terminal exogenous MMTV proviral integration sites within mammary tumor DNA obtained by the technique of genome walking. Using a combination of Southern blotting with 3' end probes and PCR utilizing a unique RIII/Sa specific 3' primer, we confirm that the RIII/Sa provirus integrates multiple times in mouse mammary tumors but that little or no integration occurs in various normal tissues. Southern blotting analyses with 3' end probes also indicate that RIII/Sa mice contain two additional endogenous MMTV loci, mtv-6 and mtv- 17, not previously reported. The combined genetic analyses reported herein distinguish between exogenous proviral integrants and endogenous germline MMTV.

A novel mechanism of resistance to mouse mammary tumor virus infection

Exogenous mouse mammary tumor virus (MMTV) is carried from the gut of suckling pups to the mammary glands by lymphocytes and induces mammary gland tumors. MMTV-induced tumor incidence in inbred mice of different strains ranges from 0 to as high as 100%. For example, mice of the C3H/HeN strain are highly susceptible, whereas mice of the I/LnJ strain are highly resistant. Of the different factors that together determine the susceptibility of mice to development of MMTV-induced mammary tumors, genetic elements play a major role, although very few genes that determine a susceptibility-resistance phenotype have been identified so far. Our data indicate that MMTV fails to infect mammary glands in I/LnJ mice foster nursed on viremic C3H/HeN females, even though the I/LnJ mammary tissue is not refractory to MMTV infection. Lymphocytes from fostered I/LnJ mice contained integrated MMTV proviruses and shed virus but failed to establish infection in the mammary glands of susceptible syngeneic (I x C3H.JK)F(1) females. Based on the susceptible-resistant phenotype distribution in N(2) females, both MMTV mammary gland infection and mammary gland tumor development in I/LnJ mice are controlled by a single locus.

Loss of endogenous mouse mammary tumor virus superantigen increases tumor resistance

From a cross between a tumor-susceptible mouse strain (DBA/2; D) and a tumor-resistant MHC-identical strain (B10.D2; D2) new recombinant inbred mouse strains were established over many generations of inbreeding and tumor resistance selection. Since resistance to the highly metastatic DBA/2 lymphoma variant ESb had an immunologic basis, and the two parental strains differed in endogenous viral superantigens (vSAGs), DNA of three D2 x D recombinant inbred mouse lines was typed for endogenous mouse mammary tumor viruses using mouse mammary tumor virus long terminal repeat- and env gene-specific probes. The resistant D2 x D mice were very similar to the susceptible parental strain D in their Mtv Southern blots, except for the lack of a single band corresponding to Mtv-7, the provirus coding for the strong DBA/2 superantigen Mls-1a. A backcross analysis revealed that Mtv-7-negative F2 mice were significantly more resistant than Mtv-7-positive F2 mice. When Mtv-7 was reintroduced into the resistant lines by crossing them with either CBA/J or BALB/D2.Mls-1a, the mice became again more tumor susceptible. Finally, we demonstrate the ability to transfer immunoresistance and graft-vs-leukemia reactivity from tumor-resistant to tumor-susceptible mice.

A highly sensitive in vitro infection assay to explore early stages of mouse mammary tumor virus infection

Mouse mammary tumor virus (MMTV) infection of adult mice induces a strong response to superantigen (Sag) in their draining lymph nodes, which results from the presentation of Sag by MMTV-infected B cells to Sag-reactive T cells. To date, infection with physiologically relevant doses of MMTV can be detected in vivo only after several days of Sag-mediated T-cell-dependent amplification of infected B cells. Furthermore, no efficient in vitro system of detecting MMTV infection is available. Such a model would allow the dissection of the early phase of infection, the assessment of the contributions of different cell types, and the screening of large panels of molecules for their potential roles in infection and Sag response. For these reasons, we have established an in vitro model for detecting infection which is as sensitive and reproducible as the in vivo model. We found that the viral envelope (Env) protein is crucial for target cell infection but not for presentation of Sag. Furthermore, we show that infection of purified B cells with MMTV induces entry of Sag-responsive T cells into the cell cycle, while other professional antigen-presenting cells, such as dendritic cells, are much less efficient in inducing a response.

The nasal-associated lymphoid tissue of adult mice acts as an entry site for the mouse mammary tumor retrovirus

Mouse mammary tumor virus (MMTV) is a B type retrovirus transmitted to the suckling offspring through milk. MMTV crosses the intestinal barrier of neonates, initially infects the lymphoid cells of the Peyer's patches, and later spreads to all lymphoid organs and to the mammary gland. Adult mice can be infected systemically, but not by oral MMTV administration. In this study, we show that nasal administration of infected milk induces the infection of adult mice. Nasal MMTV infection shared the main features of systemic and neonatal intestinal MMTV infections: deletion of the superantigen (SAg)-reactive T cell subset from the peripheral T cell population, presence of viral DNA in lymphoid cells, and transmission of MMTV from mother to offspring. Viral DNA was restricted to the lungs and nasal-associated lymphoid tissue (NALT) 6 d after nasal infection. Furthermore, SAg-induced T cell proliferation was only detected in NALT. These results demonstrate that MMTV crosses the intact epithelium of the upper respiratory tract of adult mice and infects the lymphoid follicles associated with these structures.

Possibilities of a viral etiology for human breast cancer. A review

Previous studies related mouse mammary tumor virus (MMTV) to human breast cancer. However, the presence of human endogenous retroviruses (HERs) confounded these results. We selected a 660-bp sequence of the MMTV env gene with low homology to HER (or any other known gene) and searched for a sequence homologous to it, using the polymerase chain reaction (PCR). The 660-bp sequence was detected in 131 (39%) of 335 unselected breast cancers, in 2 (6.9%) of 29 fibroadenomas, and in 2 (1.65%) of 121 normal breast specimens. The sequence was not present in normal tissues, or in other human cancers or cell lines. Cloning and sequencing of the 660-bp sequence revealed that it is 95-98% homologous to MMTV env gene, but not the known HERs or other viral or human gene. Southern blot hybridization using labeled cloned sequences demonstrated that the 660-bp sequence was present in very low copy number as a 6-8 kb EcoRI fragment only in breast cancer samples and in some of the human breast cancer cell lines that were positive by PCR. Preliminary experiments using reverse transcriptase (RT)-PCR indicated that expression of the 660-bp sequence can be detected in 65% of the positive tumors. We were also able to identify in breast cancer DNA a segment of 1.6 kb comprising LTR and env gene sequences, which are homologous to MMTV, but not to the HERs. The origin of the MMTV-like sequences in tumor DNA could be the result of integrated MMTV-like sequences derived from a human mammary virus.

Mouse mammary tumor virus and mammary tumorigenesis in wild mice

The current knowledge of the distribution of the mouse mammary tumor virus (MMTV) proviral genomes and the mechanism of mammary tumorigenesis by MMTV in mice, with the main emphasis on Asian feral mice, is reviewed. The relevant earlier discoveries on the mode of MMTV transmission are summarized to provide an outline of the biology of MMTV. Finally, the viral etiology of human breast cancer will be discussed.

Endogenous mouse mammary tumor virus Mtv-17 is involved in Mtv-2-induced tumorigenesis in GR mice

Mtv-2 is an endogenous mouse mammary tumor virus (MMTV) that is responsible for the induction of mammary gland tumors in the high mammary gland tumor-incidence strain GR. GR animals inherit four different endogenous MMTVs in addition to Mtv-2: Mtv-3, Mtv-7, Mtv-8 and Mtv-17. In this study we analyzed the involvement of these nonpathogenic endogenous proviruses in the mammary gland tumors caused by Mtv-2. We showed that Mtv-17 is expressed in the mammary gland of GR mice, efficiently packaged into virions, and shed into milk. DNA isolated from both mammary gland tumors and the nonmalignant mammary gland tissues of GR mice contained amplified copies of both newly acquired Mtv-2 and recombinant proviruses with the env gene derived from Mtv-17. A small percentage of these tumors contained predominantly recombinant viruses. These findings suggest a role for Mtv-17 in the mammary gland tumors induced by Mtv-2 in GR mice.

A novel endogenous mouse mammary tumor virus locus in Asian wild mice and its evolutionary divergency

The divergency of endogenous mouse mammary tumor virus (MMTV) in European mouse strains indicates acquisition of exogenous MMTV into their genomes during evolution. In the present study, we selected two strains of Asian wild mice, Cas-Hmi (Mus musculus castaneus) and Sub-Kjr (M. m. subspecies) to investigate a possible divergency of endogenous MMTVs among Asian mouse strains. Southern blot analysis and polymerase chain reaction (PCR) demonstrated that Cas-Hmi and Sub-Kjr have the full structure of integrated endogenous MMTVs, suggesting that these mice were diverged during evolution after MMTV endemics.

Lack of MMTV superantigen presentation in MHC class II-deficient mice

Mammary tumor viruses (MMTVs) as well as their endogenous counterparts encode superantigens which react with T cells expressing particular T cell receptor V beta chains. Several lines of evidence indicated that MHC class II is required for the functional presentation of these superantigens. Here we provide direct proof that the function of superantigens is abrogated in the absence of MHC class II expression. No deletion of Mls-1-reactive T cells was observed in MHC class II-deficient mice, and splenocytes from these animals did not stimulate Mls-1-reactive T cell hybrids in vitro. Furthermore, the viral spread in MHC class II-deficient mice, maternally infected with MMTV(C3H), was severely reduced. While initial infection in the gut-associated lymphocytes was comparable between MHC class II-deficient and normal mice, the level of infection in the spleen and the mammary tissue was much lower in the deficient animals. Quantitation of proviral DNA in spleen revealed a direct correlation between the magnitude of superantigen stimulation and degree of infection. These experiments document the direct effect of superantigen stimulation on viral amplification.

A novel V beta 2-specific endogenous mouse mammary tumor virus which is capable of producing a milk-borne exogenous virus

We have previously reported new Mtv loci, Mtv-48 and -51, in the Japanese laboratory mouse strains CS and NC. Here we show by backcross analysis that both Mtv-48 and -51 cosegregate with very slow deletion of T cells bearing V beta 2. The nucleotide sequences of the open reading frames in the 3' long terminal repeats of Mtv-48 and -51 were very similar to those of Mtv-DDO, mouse mammary tumor virus C4 [MMTV(C4)], and MMTV(BALB/cV), which encode V beta 2-specific superantigens. Furthermore, backcross female mice carrying Mtv-48 but not Mtv-51 were found to be able to produce milk-borne MMTV(CS), which can vigorously stimulate V beta 2-expressing T cells after local injection in vivo in an I-E-dependent manner. On the other hand, mice carrying Mtv-51 but not Mtv-48 could not produce such an MMTV in milk. The nucleotide sequences of MMTV(CS) open reading frame were completely matched with those of Mtv-48. These results indicate that the provirus Mtv-48 but not Mtv-51 is capable of producing a milk-borne virus of which the superantigen stimulates V beta 2-expressing T cells.

Mouse mammary tumor viruses with functional superantigen genes are selected during in vivo infection

Mouse mammary tumor virus (MMTV) encodes a superantigen that is important for viral infectivity in vivo. To determine whether superantigen function was required for infection by milk-borne MMTV, we created HYB PRO/Cla transgenic mice. These mice produced a full-length, packaged viral RNA with a frameshift mutation that caused premature termination of the superantigen protein. Young HYB PRO/Cla mice showed no deletion of their cognate V beta 14+ T cells, although they shed virus in their milk. The nontransgenic offspring of the HYB PRO/Cla mice were infected with this virus, since transgene-specific viral transcripts were detected in their mammary glands. Surprisingly, these offspring demonstrated the progressive deletion of V beta 14+ T cells characteristic of exogenous MMTV (C3H) infection. Sequence analysis demonstrated that these newly acquired viruses had reconstituted superantigen open reading frames resulting from recombination between the HYB PRO/Cla and endogenous Mtv-1 proviral RNAs. Thus, there is selection during the infection process for MMTVs with functional superantigen genes.

Susceptibility to tumors induced by polyoma virus is conferred by an endogenous mouse mammary tumor virus superantigen

A dominant gene carried in certain inbred mouse strains confers susceptibility to tumors induced by polyoma virus. This gene, designated Pyvs, was defined in crosses between the highly susceptible C3H/BiDa strain and the highly resistant but H-2k-identical C57BR/cdJ strain. The resistance of C57BR/cdJ mice is overcome by irradiation, indicating an immunological basis. In F1 x C57BR/cdJ backcross mice, tumor susceptibility cosegregates with Mtv-7, a mouse mammary tumor provirus carried by the C3H/BiDa strain. This suggests that Pyvs might encode the Mtv-7 superantigen (SAG) and abrogate polyoma tumor immunosurveillance through elimination of T cells bearing specific V beta domains. DNA typing of 110 backcross mice showed no evidence of recombination between Pyvs and Mtv-7. Strongly biased usage of V beta 6 by polyoma virus-specific CD8+ cytotoxic T lymphocytes in C57BR/cdJ mice implicates T cells bearing this Mtv-7 SAG-reactive V beta domain as critical anti-polyoma tumor effector cells in vivo. These results indicate identity between Pyvs and Mtv-7 sag, and demonstrate a novel mechanism of inherited susceptibility to virus-induced tumors based on effects of an endogenous superantigen on the host's T cell repertoire.

A V beta 8.2-specific superantigen from exogenous mouse mammary tumor virus carried by FM mice

A number of endogenous mouse mammary tumor virus (MMTV) proviruses encode superantigen that have the ability to stimulate T cells with a certain T cell receptor (TCR) beta-chain variable region (V beta) and to mediate the V beta-specific clonal deletion. The tumorigenic milk-borne MMTV carried by C3H and GR mice also have superantigenic properties in vivo. In the present study we identified and characterized a novel V beta 8.2-specific superantigen of exogenous MMTV carried by FM mice. The open reading frame (ORF) in the 3' long terminal repeat of the MMTV was cloned by polymerase chain reaction with primers corresponding to conserved regions spanning the ORF coding region. Sequence analysis of the ORF revealed that there is no sequence identical to those in other known MMTV in the carboxy terminus implicated in TCR V beta recognition. Subcutaneous injection of the virus into adult BALB/c mice induced an approximately three- to fourfold enlargement of draining lymph nodes and a substantial increase of V beta 8.2+ CD4+ T cells in the lymph nodes within 6 days. The exposure of newborn BALB/c mice to the virus by foster nursing resulted in a marked deletion of V beta 8.2+ cells both in CD4+ and CD8+ T cells. Thus, a novel milk-borne MMTV in FM mice expresses strong superantigenic properties capable of stimulating V beta 8.2+ T cells. V beta 8.2+ T cells have been demonstrated to be frequently involved in recognition of conventional antigens and responsible for autoimmune diseases such as experimental allergic encephalomyelitis. Therefore, the MMTV (FM) may provide a new mouse model system for inducing immunodeficiency or autoimmune disease by retroviral infection.

Dietary regulation of mammary tumorigenesis in RIII/Sa mice: investigation of a possible mechanism

The effects of caloric restriction on the incidence of mammary tumor development, the levels of the expression of mouse mammary tumor virus (MMTV)- and prolactin-RNA, as well as the levels of serum prolactin, were investigated in virgin RIII/Sa mice, a strain known to display a high incidence of spontaneous mammary tumor development. Of the 54 mice fed a low-calorie (LC; 10 kcal/day) diet containing low fat (LF; 5% corn oil) for a period of 72 weeks, only seven mice were found to develop mammary tumors, an incidence of 13%. By contrast, the cumulative tumor incidence in a similar sized group of mice, fed a high-calorie (HC; 16 kcal/day) low fat-containing diet was 73%. Estimation of the relative levels of MMTV-RNA, as determined by Northern and slot blot hybridizations, in the mammary glands of mice fed LCLF and HCLF diets for 8, 10, 16, 28, and 36 weeks revealed that the LCLF diet-fed mice expressed 4-15-fold less RNA than the HCLF diet-fed mice. Interestingly, however, the LCLF diet did not appear to exert any effect on the expression of prolactin RNA even though it reduced the levels of serum prolactin. We suggest that in RII/Sa mice the modulation of MMTV-induced mammary tumors by dietary calorie is linked to the secretion of serum prolactin which, in turn, affects the replication of MMTV required for mammary cell transformation.

Distribution of mouse mammary tumor virus in Asian wild mice

Several groups of wild mice (Mus musculus) were captured from eight different locations in Asia and bred for several generations in a facility free of any laboratory strains of mice carrying mouse mammary tumor virus (MMTV). The distribution of endogenous MMTV proviral sequences in the liver tissues of these mice was investigated by using Southern blot hybridizations. Four categories of mice were identified. Mice originating from Bogor, Indonesia (Cas-Bgr); He-mei, Taiwan (Cas-Hmi/1); and Malaysia (Cas-Mal) were found to carry an endogenous MMTV provirus consisting of the env, gag-pol, and long terminal repeat sequences. Mice captured from Kojuri, Republic of Korea (Sub-Kjr); Nagoya, Japan (Mol-nag); and three Chinese provinces, Shanghai (Sub-Shh), Beijing (Sub-Bjn), and Jiayuguang (Sub-Jyg/1), appeared to carry defective proviruses. Some mice originating from He-mei (Cas-Hmi/2) and Jiayuguang (Sub-Jyg/2) were found to be completely free of endogenous MMTV. Interestingly, however, the Sub-Jyg/2 mice, after several generations of inbreeding, were found, unlike all of the other subspecies that we examined in the present study, to develop mammary tumors at a high incidence (80 to 90%) with a short period of latency. Electron microscopic examination of the mammary glands and mammary tumors of these mice revealed the presence of numerous intracytoplasmic A, immature, budding, and mature B particles. Furthermore, the mammary tumors were found to contain MMTV proviral sequences. It seems, therefore, that Sub-Jyg/2 mice carry an exogenous MMTV which contributes to their developing mammary tumors.

Therapeutic progress and comparative aspects in chronic myelogenous leukemia (CML): interferon alpha vs. hydroxyurea vs. busulfan and expression of MMTV-related endogenous retroviral sequences in CML. German CML Study Group

In summary, it can be expected that the availability of unrelated donors will increase the number of CML patients that can be treated curatively with allogeneic BMT. Hydroxyurea has replaced busulfan as first line treatment in CML since it prolongs survival. Ongoing randomized studies comparing IFN-based treatment regimens with standard chemotherapy or IFN-monotherapy probably will answer the question whether IFN can cure a small percentage of CML patients and whether this small percentage can be increased by additional chemotherapy. The present attempts to improve prognostic scores and to apply them to early treatment decisions will allow treatment adaptation more individually. The implications of endogenous retroviral sequences expressed in CML cells are not known now, but may be far reaching.

Chemoprevention of mammary tumor virus-induced and chemical carcinogen-induced rodent mammary tumors by natural plant products

The natural plant products turmeric, beta-carotene, catechin, and betel leaf extract were evaluated for their antitumor effects on mammary tumorigenesis in murine mammary tumor expressing C3H (Jax) mice and in Wistar rats treated with the chemical carcinogen 7-12-dimethylbenz(a)anthracene (DMBA). Administration of turmeric through the diet and of beta-carotene, catechin, and betel leaf extract through the drinking water to virgin female C3H mice resulted in decreased tumor incidence and tumor burden. Administering 5% turmeric in the diet from 2 months of age showed suppression of mammary tumor virus-related reverse transcriptase activity and of preneoplastic changes in the mammary glands. Furthermore, feeding turmeric from 6 months of age resulted in a 100% inhibition of mammary tumors. In the DMBA model of rat mammary tumorigenesis, administration of turmeric, catechin, and betel leaf extract resulted in decreased tumor burden and tumor incidence, and a delay in the onset of mammary tumors.

Mls-1 and Mls-2 superantigens do not control susceptibility to collagen-induced arthritis in HI and HII mice

The HI mouse line is sensitive to collagen-induced arthritis (CIA), whereas HII is refractory, although both express the H-2q permissive haplotype. The two lines also share the same T-cell receptor (TcR) gene haplotypes for alpha and beta chains. The distribution of mouse mammary tumour viruses (MMTV), which encode endogenous superantigens (SAg) such as minor leucocyte-stimulating antigens (Mls) known to modulate the available TcR-V beta repertoire, was investigated in the two lines. Mls-1 is present in HI-susceptible mice, while Mls-2 and Mls-2-like SAg are absent in both lines. This suggests that Mls antigens play no significant role in the resistance to CIA. Moreover, HI and HII exhibit close V beta gene usage as assessed by fluorescence staining with 11 V beta-specific monoclonal antibodies (mAb). These results indicate that mechanisms other than clonal deletion based on V beta expression and induced by SAg are involved in the resistance of H-2q-positive mice to experimental arthritis. Yet, a slightly reduced level of V beta 5+ T cells is observed in HII animals which might correlate with the presence of Mtv-6 and Mtv-9 proviruses.

A new infectious mammary tumor virus in the milk of mice implanted with C4 hyperplastic alveolar nodules

We have previously characterized an infectious mouse mammary tumor virus [(MMTV(SW)] which induces a strong superantigen response in vivo. Here we describe the isolation and characterization of MMTV(C4) which was derived from milk of mice implanted with hyperplastic alveolar nodules. MMTV(C4) stimulates V beta 2 expressing T cells after local injection in vivo. Comparison with known open reading frame (orf) sequences revealed high homology to Mtv-6, an endogenous virus interacting with V beta 3-expressing T cells. The carboxyl-terminal amino acids were, however, altered. High homology including the carboxyl-terminal orf amino acids were found with MMTV(C3H-K). We show here that MMTV(C3H-K) has lost its superantigen function. Sequence comparisons permitted the characterization of few key amino acids which could be important for T cell receptor interaction and superantigen processing.

Mammary tumour induction by pituitary grafting in male mice: an animal model for male breast cancer

Isologous anterior pituitary grafting, 4 each, to 3-4-month-old SHN and SLN male mice resulted in an appearance of mammary tumours from 8 months of age and the incidence at 12 months reached 53.8% in each strain. All tumours were diagnosed as type B adenocarcinomas. In association with the results, normal mammary gland growth and mouse mammary tumour virus (MMTV)-gp52 antigen levels in the submaxillary glands were stimulated by the treatment in these strains. The effect of pituitary grafting was much less in GR/A male mice in which no mammary tumours appeared.

Growth of the mammary gland in rhino mouse

The rhino mouse, so called because of its characteristic rhinocerous-like skin cannot raise its young. Although the animal has rudimentary mammary glands, these are thought to be defective due to their impossibility to raise their young. Morphological observations of the female mammary glands of rhino mice (hrrh/hrrh) were carried out and the results were compared with those of their normal litter mates (hrrh/+) at various ages from 1 month to 1 year. No morphological differences in mammary gland development was seen between the rhino mice and their normal litter mates, and normal secretory changes were observed in the rhino mice after delivery. Mouse mammary tumor virus (MMTV) expression and the occurrence of hyperplastic alveolar nodule (HAN) were analogous to these features in phenotypically normal hrrh/+ mice. Further, the sweat glands in the rhino mouse were normal. Thus, anomalies in the rhino mouse appeared to be limited to the skin and pilosebaceous apparatus.

Large scale production and purification of human retrovirus-like particles related to the mouse mammary tumor virus

Human retrovirus-like particles related to mouse mammary tumor virus (MMTV) are secreted in a steroid-dependent manner by the breast cancer cell line T47D. We report the successful large scale production and purification of these particles from culture supernatants of T47D cells and describe the experimental conditions established for this purpose. Thus, mg amounts of particles were produced by large scale culturing of T47D cells in an autoharvesting roller bottle system and purified by differential centrifugation and continuous flow ultracentrifugation on density gradients with a 50% recovery and a 350-fold enrichment.

Characterization of experimental allergic encephalomyelitis-susceptible, Biozzi AB/H (H-2dq1) mice which express H-2Anod: analysis of T-cell receptor expression and the detection of a deletion ligand encoded by Mtv-7

Biozzi (H-2dq1) AB/H mice are marked not only by their high titre antibody responses following immunization with protein antigens but are also susceptible to experimental allergic encephalomyelitis (EAE) induction. The T-cell receptor (TcR) repertoire in this recently characterized strain was analysed. Biozzi AB/H mice were found to express the Thy-1a, Ly-1b, Ly-2b and Ly-5b alleles. Serological typing of the TcR-V beta + peripheral T cells suggested that the AB/H mice belong to the TcR-V beta b haplotype and express a deletion ligand for TcR-V beta 6+ and TcR-V beta 8.1+ T cells. This was confirmed by Southern blot analysis which revealed the presence of Mtv-17, Mtv-23, Mtv-31 (Y chromosome) and notably Mtv-7. Therefore the AB/H mice are Mls-1a. Despite the depletion/absence of the majority of TcR-V beta families in EAE-resistant (BALB/c) x EAE-susceptible (AB/H) F1 mice it was possible to induce EAE in these F1 animals. This suggests either that the deleted TcR-V beta-bearing T cells were not the principal encephalitogenic cells or that the TcR-V beta usage is sufficiently heterogeneous to accommodate such deletion events, in spinal cord-induced EAE.

Characterization of mammary plaques in DDD mice congenic for Mtv-2 gene, DDD/1-Mtv-2/Mtv-2

DDD/1 mice free from exogenous mouse mammary tumor virus (MMTV) do not develop any neoplastic mammary lesions. In GR mice, the expression of Mtv-2, an endogenous proviral MMTV, leads to 100% incidence of mammary ductal hyperplasias and tumors. An Mtv-2 congenic line, DDD/1-Mtv-2/Mtv-2, was established by introducing Mtv-2 from GR into DDD/1 to elucidate its function. Development of mammary plaques (MPQ) characterized by ductal hyperplasias was investigated in 152 congenic females on day 17 to 19 of the first pregnancy. The incidence of MPQ was 48.0% and most MPQ-positive mice (75.3%) had only one MPQ. Generally, MPQ were small in size: the diameter was as small as < or = 3 mm in 77.6% of them. Of 84 MPQ implanted into intact fat pads, 43 (51.2%), 38 (45.2%) and 3 (3.6%) showed undetectable, pregnancy-dependent and autonomous growths; respectively when the hosts underwent pregnancy. Almost all MPQ produced normal-appearing ductal-alveolar outgrowths in mammary epithelium-divested or cleared fat pads of virgins. MPQ implanted into cleared fat pads were very similar to normal mammary glands in the responses to progesterone (P) and estradiol (E) alone or in combination except for association of ductal hyperplasias in 4 of 12 MPQ under E+P treatment. These findings revealed the preneoplastic nature of MPQ. Exogenous MMTV proviruses were demonstrated in all MPQ. The int-2 DNA rearrangement was found in 2 of 10 MPQ but in none of 9 mammary carcinomas and the int-1 DNA rearrangement in none of 10 MPQ but in 5 of 10 carcinomas. It is thus likely that the Mtv-2 gene participates in a very early stage of mammary tumorigenesis not directly but indirectly through insertion mutation of host genes, while the cellular oncogenes, int-2 and int-1, may contribute to preneoplastic transformation of mammary epithelium and progression from preneoplastic to more malignant states, respectively.

Purification of immature cores of mouse mammary tumor virus and immunolocalization of protein domains

The immature capsids of the mouse mammary tumor virus (MMTV), known as intracytoplasmic A particles, have been isolated from murine L1210 leukemia cells. The diameter of the isolated particles was 80 nm as determined by negative staining. Two polypeptides of 77 and 110 kDa were found to be their major polypeptide components, in agreement with the expected sizes of the Gag and Gag-Pro precursor polypeptides of the mature MMTV proteins. Both polypeptides were recognized by antibodies directed toward the matrix (p10) and capsid (p27) proteins of MMTV. Immunogold labeling of p10 on isolated A particles, visualized by negative staining, showed that this protein is located at the surface of the immature capsids, whereas p27 can be detected only in broken or disrupted particles, suggesting that it has an internal location. These observations were confirmed by immunolabeling of both proteins on thin sections of A particle-producing cells. In addition, the viral protease had a more internal position than p27. Since the sequential order of the viral proteins in the Gag precursor is p10-pp21-p27-p14 and that in Gag-Pro is p10-pp21-p27-p30-protease, our results demonstrate the radial organization of the polypeptide precursors forming the intracytoplasmic A particles.

An exogenous mouse mammary tumor virus with properties of Mls-1a (Mtv-7)

The classical minor lymphocyte stimulating (Mls) antigens, which induce a strong primary T cell response in vitro, are closely linked to endogenous copies of mouse mammary tumor viruses (MMTV). Expression of Mls genes leads to clonal deletion of T cell subsets expressing specific T cell receptor (TCR) V beta chains. We describe the isolation and characterization of a new exogenous (infectious) MMTV with biological properties similar to the Mls antigen Mls-1a. In vivo administration of either Mls-1a-expressing B cells or the infectious MMTV (SW) led to an increase of T cells expressing V beta 6 followed by their deletion. Surprisingly, different kinetics of deletion were observed with the exogenous virus depending upon the route of infection. Infection through the mucosa led to a slow deletion of V beta 6+ T cells, whereas deletion was rapid after subcutaneous infection. Sequence analysis of the open reading frames in the 3' long terminal repeat of both this exogenous MMTV (SW) and of Mtv-7 (which is closely linked to Mls-1a) revealed striking similarities, particularly in the COOH terminus, which has been implicated in TCR V beta recognition. The identification of an infectious MMTV with the properties of a strong Mls antigen provides a new, powerful tool to study immunity and tolerance in vivo.

Retrovirus-like particles from the human T47D cell line are related to mouse mammary tumour virus and are of human endogenous origin

Retrovirus-like particles were secreted in a steroid-dependent manner by the human mammary carcinoma cell line T47D. The particles exhibited typical retroviral properties such as their electron microscopic appearance (95 nm in diameter) and occasional budding, sedimentation at 1.14 g/ml, reverse transcriptase activity and genomic RNA. The T47D particles were related to mouse mammary tumour virus (MMTV) as shown by their ultrastructural appearance (B type-like eccentric dense cores and budding), Mg2+ dependence of the reverse transcriptase activity; immunological reactivity with MMTV-directed antibodies (revealing proteins of 63K, 52K, 26K and 18K), and hybridization of particle RNA with MMTV DNA under stringent conditions. Purified particles were able to incorporate deoxynucleoside triphosphates in the absence of an exogenous primer and template, thus indicating the existence of a complete and biochemically functional reverse transcription apparatus (reverse transcriptase, RNA and primer) and the ability to direct endogenous cDNA synthesis. Labelled particle cDNA hybridized strongly to human genomic DNA but not to mouse and cat DNA, thus indicating the human origin of the T47D particles. Furthermore all human DNAs, hybridized with the labelled particle cDNA, showed a uniform hybridization pattern of restriction fragments, indicating the endogenous origin and distribution of the proviral particle DNA in the human genome.

Intracellular amplification of proviral DNA in tissue sections using the polymerase chain reaction

We developed a new method to amplify cell DNA in situ using the polymerase chain reaction (PCR). Proviral sequences of mouse mammary tumor virus (MMTV) contained in cultured cells and tissue sections were amplified intracellularly using a thermal cycler. Two techniques were employed to maintain the localization of the amplified DNA. First, complementary tails at the 5' ends of the oligonucleotide primers resulted in the synthesis of high molecular weight concatamers containing the target sequences. Second, the PCR was carried out in a thin film of agarose solidified over the tissue sections. The specifically amplified and localized DNA was then detected by in situ hybridization (ISH). Our results demonstrate that (a) DNA in tissue sections can serve as the target for the polymerase chain reaction in situ, (b) cell morphology is maintained, and (c) a target of 167 BP can be specifically detected in individual cells. This technique should be generally applicable to amplifying cellular DNA targets in tissue sections for detection in situ.

Effects of energy restriction on mouse mammary tumor virus mRNA levels in mammary glands and uterus and on uterine endometrial hyperplasia and pituitary histology in C3H/SHN F1 mice

We investigated the effects of energy restriction on the pituitary-ovarian axis and on a hormone responsive gene, the mouse mammary tumor virus (MMTV). Female C3H/SHN F1-hybrid mice, known to display a high incidence of mammary tumors, ate an energy-restricted diet (48 kcal/wk) or a control diet (95 kcal/wk) beginning at the time of weaning. By 67 wk of age, 12 of 32 mice in the control group, but none of the 33 mice in the energy-restricted group, had developed mammary tumors. Six tumor-free mice from each group were studied in detail at 67 wk of age. All six tumor-free control mice, but none of the six energy-restricted mice, showed uterine endometrial hyperplasia at autopsy. Mice subjected to energy restriction did not display an estrous cycle. The average levels of MMTV mRNA in mammary glands and uteri were strongly reduced by energy restriction. MMTV mRNA levels in mammary glands from control mice were two orders of magnitude lower than those in mammary tumors. Energy restriction lowered the percentage of pituitary mammatropes and suppressed proliferation of mammatropes with advancing age. Energy restriction thus appeared to inhibit endometrial hyperplasia and to decrease MMTV production at the mRNA level in the mammary glands and in the uterus. These effects may be a consequence of hormonal changes originating at the pituitary-ovarian axis.