Identification of key amino acids of the mouse mammary tumor virus superantigen involved in the specific interaction with T-cell receptor V(beta) domains

Mouse mammary tumor virus (MMTV) is a retrovirus encoding a superantigen that is recognized in association with major histocompatibility complex class II by the variable region of the beta chain (V(beta)) of the T-cell receptor. The C-terminal 30 to 40 amino acids of the superantigen of different MMTVs display high sequence variability that correlates with the recognition of particular T-cell receptor V(beta) chains. Interestingly, MMTV(SIM) and mtv-8 superantigens are highly homologous but have nonoverlapping T-cell receptor V(beta) specificities. To determine the importance of these few differences for specific V(beta) interaction, we studied superantigen responses in mice to chimeric and mutant MMTV(SIM) and mtv-8 superantigens expressed by recombinant vaccinia viruses. We show that only a few changes (two to six residues) within the C terminus are necessary to modify superantigen recognition by specific V(beta)s. Thus, the introduction of the MMTV(SIM) residues 314-315 into the mtv-8 superantigen greatly decreased its V(beta)12 reactivity without gain of MMTV(SIM)-specific function. The introduction of MMTV(SIM)-specific residues 289 to 295, however, induced a recognition pattern that was a mixture of MMTV(SIM)- and mtv-8-specific V(beta) reactivities: both weak MMTV(SIM)-specific V(beta)4 and full mtv-8-specific V(beta)11 recognition were observed while V(beta)12 interaction was lost. The combination of the two MMTV(SIM)-specific regions in the mtv-8 superantigen established normal MMTV(SIM)-specific V(beta)4 reactivity and completely abolished mtv-8-specific V(beta)5, -11, and -12 interactions. These new functional superantigens with mixed V(beta) recognition patterns allowed us to precisely delineate sites relevant for molecular interactions between the SIM or mtv-8 superantigen and the T-cell receptor V(beta) domain within the 30 C-terminal residues of the viral superantigen.

Extrafollicular plasmablast B cells play a key role in carrying retroviral infection to peripheral organs

B cells can either differentiate in germinal centers or in extrafollicular compartments of secondary lymphoid organs. Here we show the migration properties of B cells after differentiation in murine peripheral lymph node infected with mouse mammary tumor virus. Naive B cells become activated, infected, and carry integrated retroviral DNA sequences. After production of a retroviral superantigen, the infected B cells receive cognate T cell help and differentiate along the two main differentiation pathways analogous to classical Ag responses. The extrafollicular differentiation peaks on day 6 of mouse mammary tumor virus infection, and the follicular one becomes detectable after day 10. B cells participating in this immune response carry a retroviral DNA marker that can be detected by using semiquantitative PCR. We determined the migration patterns of B cells having taken part in the T cell-B cell interaction from the draining lymph node to different tissues. Waves of immigration and retention of infected cells in secondary lymphoid organs, mammary gland, salivary gland, skin, lung, and liver were observed correlating with the two peaks of B cell differentiation in the draining lymph node. Other organs revealed immigration of infected cells at later time points. The migration properties were correlated with a strong up-regulation of alpha(4)beta(1) integrin expression. These results show the migration properties of B cells during an immune response and demonstrate that a large proportion of extrafolliculary differentiating plasmablasts can escape local cell death and carry the retroviral infection to peripheral organs.

Functional plasticity of the LACK-reactive Vbeta4-Valpha8 CD4(+) T cells normally producing the early IL-4 instructing Th2 cell development and susceptibility to Leishmania major in BALB / c mice

Early production of IL-4 by LACK-reactive Vbeta4-Valpha8 CD4(+) T cells instructs aberrant Th2 cell development and susceptibility to Leishmania major in BALB / c mice. This was demonstrated using Vbeta4(+)-deficient BALB / c mice as a result of chronic infection with MMTV (SIM), a mouse mammary tumor virus expressing a Vbeta4-specific superantigen. The early IL-4 response was absent in these mice which develop a Th1 response to L. major. Here, we studied the functional plasticity of LACK-reactive Vbeta4-Valpha8 CD4(+) T cells using BALB/ c mice inoculated with L. major shortly after infection with MMTV (SIM), i. e. before deletion of Vbeta4(+) cells. These mice fail to produce the early IL-4 response to L. major and instead exhibit an IFN-gamma response that occurs within LACK-reactive Vbeta4-Valpha8 CD4(+) T cells. Neutralization of IFN-gamma restores the production of IL-4 by these cells. These data suggest that the functional properties of LACK-reactive Vbeta4-Valpha8 CD4(+) T cells are not irreversibly fixed.

Gene transfer into stimulated and unstimulated T lymphocytes by HIV-1-derived lentiviral vectors

Genetic modification of T lymphocytes holds great potential for treatments of cancer, T cell disorders and AIDS. While in the past recombinant murine retroviruses were the vectors of choice for gene delivery to T cells, vectors based on lentiviruses can provide additional benefits. Here, we show that VSV-G pseudotyped HIV 1 vector particles delivering the enhanced green fluorescent protein (EGFP) efficiently transduce human T lymphocytes. Transduction efficiency was optimal when infection included centrifugation of cells with concentrated vector supernatant in the presence of Polybrene. In contrast to previous reports describing murine retrovirus-mediated gene transfer to T lymphocytes, fibronectin did not improve the transduction efficiency of the VSVG-pseudotyped HIV-1 particles. Similar gene transfer efficiencies were observed following stimulation of cells with PHA/IL-2 or anti-CD3i/CD28i antibodies, although greater transgene expression was observed in the latter case. Interestingly, production of vectors in the absence of the accessory proteins Vif, Vpr, Vpu and Nef was accompanied by a 50% decrease in transduction efficiency in activated T cells. Transduction of T cells that were not stimulated before infection was achieved. No transduction of non-prestimulated cells was observed with a GAL V-pseudotyped murine retroviral vector. The requirement for accessory proteins in non-prestimulated cells was more pronounced. Our results have implications for lentiviral vector targeting of other cells of the hematopoietic system including stem cells.

Role of dendritic cells in the immune response induced by mouse mammary tumor virus superantigen

After mouse mammary tumor virus (MMTV) infection, B lymphocytes present a superantigen (Sag) and receive help from the unlimited number of CD4(+) T cells expressing Sag-specific T-cell receptor Vbeta elements. The infected B cells divide and differentiate, similarly to what occurs in classical B-cell responses. The amplification of Sag-reactive T cells can be considered a primary immune response. Since B cells are usually not efficient in the activation of naive T cells, we addressed the question of whether professional antigen-presenting cells such as dendritic cells (DCs) are responsible for T-cell priming. We show here, using MMTV(SIM), a viral isolate which requires major histocompatibility complex class II I-E expression to induce a strong Sag response in vivo, that transgenic mice expressing I-E exclusively on DCs (I-EalphaDC tg) reveal a strong Sag response. This Sag response was dependent on the presence of B cells, as indicated by the absence of stimulation in I-EalphaDC tg mice lacking B cells (I-EalphaDC tg muMT(-/-)), even if these B cells lack I-E expression. Furthermore, the involvement of either residual transgene expression by B cells or transfer of I-E from DCs to B cells was excluded by the use of mixed bone marrow chimeras. Our results indicate that after priming by DCs in the context of I-E, the MMTV(SIM) Sag can be recognized on the surface of B cells in the context of I-A. The most likely physiological relevance of the lowering of the antigen threshold required for T-cell/B-cell collaboration after DC priming is to allow B cells with a low affinity for antigen to receive T-cell help in a primary immune response.

Preferential binding of mouse mammary tumor virus to B lymphocytes

Mouse mammary tumor virus (MMTV) has been shown to preferentially infect B lymphocytes in vivo. We have used recombinant envelope-coated fluospheres and highly purified MMTV particles to study the distribution of the viral receptors on fresh mouse lymphocytes. A preferential dose-dependent binding to B lymphocytes was observed which could be competed with neutralizing antibodies. In contrast, T-lymphocyte binding remained at background levels. These results strongly suggest a higher density of viral receptor molecules on B lymphocytes than on T lymphocytes and correlate with the preferential initial infection of B lymphocytes observed in vivo.

B cell response after MMTV infection: extrafollicular plasmablasts represent the main infected population and can transmit viral infection

The immune response to mouse mammary tumor virus (MMTV) relies on the presentation of an MMTV-encoded superantigen by infected B cells to superantigen-specific T cells. The initial extrafollicular B cell differentiation involved the generation of B cells expressing low levels of B220. These B220low B cells corresponded to plasmablasts that expressed high levels of CD43 and syndecan-1 and were CD62 ligand- and IgD-. Viral DNA was detected nearly exclusively in these B220low B cells by PCR, and retroviral type-A particles were observed in their cytoplasm by electron microscopy. An MMTV transmission to the offspring was also achieved after transfer of B220low CD62 ligand- CD43+ plasmablasts into noninfected females. These data suggest that B220low plasmablasts, representing the bulk of infected B cells, are capable of sustaining viral replication and may be involved in the transmission of MMTV.

Differential reactivity of TCR Vbeta10 alleles to a mouse mammary tumor virus superantigen

Mouse mammary tumor virus (MMTV) expresses a superantigen (SAg) which plays a critical role in the viral life cycle. We have recently described the new infectious MMTV (SIM) encoding a Vbeta4-specific SAg in mice with a TCR-Vbeta(b) haplotype. We have now compared the SAg activity of this virus in BALB/c mice harboring the TCR-Vbeta(a), TCR-Vbeta(b) or TCR-Vbeta(c) haplotypes which differ by a central deletion in the TCR-Vbeta(a) and TCR-Vbeta(c) locus and by mutations in some of the remaining Vbeta elements. Injection of MMTV (SIM) led to a strong stimulation of Vbeta4+ CD4+ T cells in TCR-Vbeta(b) mice, but only to a weak stimulation of these cells in TCR-Vbeta(a) or TCR-Vbeta(c) mice. A large increase in the percentage of Vbeta10+ cells was observed among CD4+ T cells in mice with the Vbeta(a) or Vbeta(c), but not the Vbeta(b) TCR-Vbeta haplotype. Vbeta10+ cells dominated the response when Vbeta10(a/c) and Vbeta4 subsets were present together. This is the first report of a viral SAg interacting with murine Vbeta10+ cells. Six amino acid differences between Vbeta10(a/c) and Vbeta10(b) could account for the gain of reactivity of Vbeta10(a/c) to the MMTV(SIM) SAg. No mutations were found in the hypervariable region 4 (HV4) of the TCR. Mutations at positions 22 and 28 introduce into Vbeta10(a/c) the same amino acids which are found at these positions in the MMTV(SIM)-reactive Vbeta4. Tridimensional models indicated that these amino acids lie close to HV4 and are likely to be important for the interaction of the SAg with the TCR.

Role of the immune response induced by superantigens in the pathogenesis of microbial infections

Superantigens (SAgs) are microbial proteins which have potent effects on the immune system. They are presented by major histocompatibility complex (MHC) class II molecules and interact with a large number of T cells expressing specific T cell receptor V beta domains. Encounter of a SAg leads initially to the stimulation and subsequently to the clonal deletion of reactive T cells. SAgs are expressed by a wide variety of microorganisms which use them to exploit the immune system to their own advantage. Bacterial SAgs are exotoxins which are linked to several diseases in humans and animals. A classical example is the toxic shock syndrome in which the massive release of cytokines by SAg-reactive cells is thought to play a major pathogenic role. The best characterized viral SAg is encoded by mouse mammary tumour virus (MMTV) and has proved to have a major influence on the viral life cycle by dramatically increasing the efficiency of viral infection. In this paper, we review the general properties of SAgs and discuss the different types of microorganisms which produce these molecules, with a particular emphasis on the role played by the SAg-induced immune response in the course of microbial infections.

Immune response to mouse mammary tumor virus in mice lacking the alpha/beta interferon or the gamma interferon receptor

Mouse mammary tumor virus (MMTV) is a retrovirus which induces a strong immune response and a dramatic increase in the number of infected cells through the expression of a superantigen (SAg). Many cytokines are likely to be involved in the interaction between MMTV and the immune system. In particular, alpha/beta interferon (IFN-alpha/beta) and gamma interferon (IFN-gamma) exert many antiviral and immunomodulatory activities and play a critical role in other viral infections. In this study, we have investigated the importance of interferons during MMTV infection by using mice with a disrupted IFN-alpha/beta or IFN-gamma receptor gene. We found that the SAg response to MMTV was not modified in IFN-alpha/betaR(0/0) and IFN-gammaR(0/0) mice. This was true both for the early expansion of B and T cells induced by the SAg and for the deletion of SAg-reactive cells at later stages of the infection. In addition, no increase in the amount of proviral DNA was detected in tissues of IFN-alpha/betaR(0/0) and IFN-gammaR(0/0) mice, suggesting that interferons are not essential antiviral defense mechanisms during MMTV infection. In contrast, IFN-gammaR(0/0) mice had increased amounts of IL-4 mRNA and an altered usage of immunoglobulin isotypes with a reduced frequency of IgG2a- and IgG3-producing cells. This was associated with lower titers of virus-specific antibodies in serum early after infection, although efficient titers were reached later.

Retrovirus-induced target cell activation in the early phases of infection: the mouse mammary tumor virus model

Mouse mammary tumor virus (MMTV) infects B lymphocytes and expresses a superantigen on the cell surface after integration of its reverse-transcribed genome. Superantigen-dependent B- and T-cell activation becomes detectable 2 to 3 days after infection. We show here that before this event, B cells undergo a polyclonal activation which does not involve massive proliferation. This first phase of B-cell activation is T cell independent. Moreover, during the first phase of activation, when only a small fraction of B cells is infected by MMTV(SW), viral DNA is detected only in activated B cells. Such a B-cell activation is also seen after injection of murine leukemia virus but not after injection of vaccinia virus, despite the very similar kinetics and intensity of the immune response. Since retroviruses require activated target cells to induce efficient infection, these data suggest that the early polyclonal retrovirus-induced target cell activation might play an important role in the establishment of retroviral infections.

Early neutralizing antibody response against mouse mammary tumor virus: critical role of viral infection and superantigen-reactive T cells

Infectious mouse mammary tumor virus (MMTV) is a retrovirus that expresses a superantigen shortly after infection of B cells. The superantigen first drives the polyclonal activation and proliferation of superantigen-reactive CD4+ T cells, which then induce the infected B cells to proliferate and differentiate. Part of the MMTV-induced B cell response leads to the production of Abs that are specific for the viral envelope protein gp52. Here we show that this Ab response has virus-neutralizing activity and confers protection against superinfection by other MMTV strains in vivo as soon as 4 to 7 days after infection. A protective Ab titer is maintained lifelong. Viral infection as well as the superantigen-induced T-B collaboration are required to generate this rapid and long lasting neutralizing Ab response. Polyclonal or superantigen-independent B cell activation, on the contrary, does not lead to detectable virus neutralization. The early onset of this superantigen-dependent neutralizing response suggests that viral envelope-specific B cells are selectively recruited to form part of the extrafollicular B cell response and are subsequently amplified and maintained by superantigen-reactive Th cells.

Early neutralizing antibody response against mouse mammary tumor virus: critical role of viral infection and superantigen-reactive T cells

Infectious mouse mammary tumor virus (MMTV) is a retrovirus that expresses a superantigen shortly after infection of B cells. The superantigen first drives the polyclonal activation and proliferation of superantigen-reactive CD4+ T cells, which then induce the infected B cells to proliferate and differentiate. Part of the MMTV-induced B cell response leads to the production of Abs that are specific for the viral envelope protein gp52. Here we show that this Ab response has virus-neutralizing activity and confers protection against superinfection by other MMTV strains in vivo as soon as 4 to 7 days after infection. A protective Ab titer is maintained lifelong. Viral infection as well as the superantigen-induced T-B collaboration are required to generate this rapid and long lasting neutralizing Ab response. Polyclonal or superantigen-independent B cell activation, on the contrary, does not lead to detectable virus neutralization. The early onset of this superantigen-dependent neutralizing response suggests that viral envelope-specific B cells are selectively recruited to form part of the extrafollicular B cell response and are subsequently amplified and maintained by superantigen-reactive Th cells.

IL-4 rapidly produced by V beta 4 V alpha 8 CD4+ T cells instructs Th2 development and susceptibility to Leishmania major in BALB/c mice

BALB/c mice develop aberrant T helper 2 (Th2) responses and suffer progressive disease after infection with Leishmania major. These outcomes depend on the production of interleukin-4 (IL-4) early after infection. Here we demonstrate that the burst of IL-4 mRNA, peaking in draining lymph nodes of BALB/c mice 16 hr after infection, occurs within CD4+ T cells that express V beta 4 V alpha 8 T cell receptors. In contrast to control and V beta 6-deficient BALB/c mice, V beta 4-deficient BALB/c mice were resistant to infection, demonstrating the role of these cells in Th2 development. The early IL-4 response was absent in these mice, and T helper 1 responses occurred following infection. Recombinant LACK antigen from L. major induced comparable IL-4 production in V beta 4 V alpha 8 CD4+ cells. Thus, the IL-4 required for Th2 development and susceptibility to L. major is produced by a restricted population of V beta 4 V alpha 8 CD4+ T cells after cognate interaction with a single antigen from this complex organism.

Involvement of the Tpl-2/cot oncogene in MMTV tumorigenesis

We report for the first time a relationship between the Tpl-2/cot oncogene and Mouse Mammary Tumor Virus (MMTV) associated transformation of mammary gland cells. A sub-genomic library generated from a primary mammary gland tumor yielded a novel MMTV integration site which disrupted the Tpl-2/cot proto-oncogene between exons 7 and 8. Comparison of a cell line derived from normal mammary gland (comma-D) and a cell line established from an MMTV induced mammary tumor (GR) demonstrated similar rearrangements within Tpl-2/cot for the GR cells but not in the comma-D cells. These rearrangements in the cell line were accompanied by an increase in the level of Tpl-2/cot specific mRNA. This data suggests that Tpl-2/cot expression may be important in epithelial cell transformation or tumor progression.

A V beta 4-specific superantigen encoded by a new exogenous mouse mammary tumor virus

The superantigen (SAg) expressed by mouse mammary tumor virus (MMTV) has been shown to play an essential role in the course of the viral life cycle. In the present study, we describe a V beta 4-specific SAg encoded by a new exogenous MMTV carried by the SIM mouse strain. This is the first report of a viral or bacterial SAg reacting with mouse V beta 4+ T cells. Injection of MMTV(SIM) into adult BALB/c mice leads to a rapid and strong stimulation of V beta 4+ CD4+ T cells, followed by a slow deletion of these cells. Neonatal exposure to the virus also leads to a progressive deletion of V beta 4+ T cells. In contrast to other strong MMTV SAg, this new SAg requires the presence of major histocompatibility complex class II I-E molecules to be presented efficiently to T cells. Sequence analysis revealed a new predicted amino acid sequence in the C-terminal polymorphic region of this SAg. Furthermore, sequence comparisons to the most closely related SAg with different V beta specificities hint at the specific residues involved in the interaction with the T cell receptor.

In vivo effects of a recombinant vaccinia virus expressing a mouse mammary tumor virus superantigen

Early after infection, the mouse mammary tumor virus (MMTV) expresses a superantigen (SAg) at the surface of B lymphocytes. Interaction with the T-cell receptor Vbeta domain induces a polyclonal proliferative response of the SAg-reactive T cells. Stimulated T cells become anergic and are deleted from the T-cell repertoire. We have used a recombinant vaccinia virus encoding the MMTV(GR) SAg to dissect the effects of the retroviral SAg during an unrelated viral infection. Subcutaneous infection with this recombinant vaccinia virus induces a very rapid increase of Vbeta14 T cells in the draining lymph node. This stimulation does not require a large Plumber of infectious particles and is not strictly dependent on the expression of the major histocompatibility complex class II I-E molecule, as it is required after MMTV(GR) infection. In contrast to MMTV infection during which B cells are infected, we do not observe any clonal deletion of the reactive T cells following the initial stimulation phase. Our data show that contrary to the case with MMTV, macrophages but not B cells are the targets of infection by vaccinia virus in the lymph node, indicating the ability of these cells to present a retroviral SAg. The altered SAg expression in a different target cell observed during recombinant vaccinia virus infection therefore results in significant changes in the SAg response.

The 3' half of the mouse mammary tumor virus orf gene is not sufficient for its superantigen function in transgenic mice

The Mouse Mammary Tumor Virus (MMTV) long terminal repeat contains an open reading frame (orf) of 960 nucleotides encoding a 36 kDa polypeptide with a putative transmembrane domain and five N-glycosylation sites in the N-terminal part of the protein. Transgenic mice bearing either the complete or the 3' terminal half of the orf sequence of MMTV-GR under the control of the SV40 promoter were raised. As shown previously by FACS analysis transgenic mice which express the complete orf gene have a significant deletion of V beta 14 expressing T cells at 6 weeks of age. Here we show that no clonal deletion of V beta 14 bearing T cells takes place in transgenic mice that contain orf sequences from the fifth ATG to the termination codon. The pattern of tissues expressing the truncated transgene was studied by the Polymerase Chain Reaction (PCR) and was very similar to the one obtained in the V beta 14 deleting animals. These data suggest that the amino-terminal portion of the ORF protein (pORF) is required for a superantigen function, while our previous data indicated that determinants from the carboxy-terminus play an important role for TCR V beta specificity.

The mouse mammary tumor virus long terminal repeat encodes a 47 kDa glycoprotein with a short half-life in mammalian cells

The mouse mammary tumor virus proviral DNA contains an open reading frame in the 3' long terminal repeat which can code for a 36 kDa polypeptide with a putative transmembrane sequence and five N-linked glycosylation sites. This gene is known to code for a superantigen which deletes a specific subset of CD4+ T lymphocytes in vivo. The superantigen encoded by the exogenous mouse mammary tumor virus of the GR strain acts specifically on V beta 14 bearing T cells. We produced recombinant vaccinia viruses to express either the complete or a truncated ORF protein after infection of primate cells in culture. The complete ORF gene in mammalian cells leads to the production of a 47 kDa protein which is specifically detected with an anti-ORF-peptide antiserum. The 47 kDa protein can be labeled with D-[2-3H]mannose and its synthesis is inhibited by tunicamycin, an N-linked glycosylation inhibitor, indicating that it is a glycoprotein. The truncated ORF protein beginning at the second ATG of the open reading frame is also modified, but the C-terminal half of ORF, starting at the fifth ATG, has the expected size of the non modified polypeptide. Pulse-chase experiments indicate that the ORF protein has a short half-life of about 1.5-2 hr.

Detection of bcl-2/JH rearrangement in follicular and diffuse lymphoma: concordant results of peripheral blood and bone marrow analysis at diagnosis

The capacity to detect t(14;18) breakpoints in non-Hodgkin's lymphoma (NHL) peripheral blood and bone marrow was studied by DNA PCR. We studied 33 patients with follicular lymphoma (FL) (Working Formulation subtypes B, C, D) and 38 patients with intermediate-grade NHL (subtypes F, G). In the FL subgroup, 86% of the morphologically-positive bone marrow patients had amplifiable t(14;18) breakpoints by PCR. Remarkably, of 19 FL patients with 'negative' bone marrows, 11 (58%) were PCR-positive. In addition, half of the early clinical stage patients (I and II) had detectable breakpoints in their bone marrow DNA. Samples from NHL patients with intermediate-grade disease exhibit the same phenomena but at a considerably lower frequency. Paired peripheral blood and bone marrow samples were available at diagnosis in a subset of 56 patients. The concordance between bone marrow and peripheral blood PCR findings was high, with peripheral blood of 55/56 showing the same PCR results as the corresponding bone marrow.

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.

Expression of the mouse mammary tumor virus ORF gene in cultured cells

We have recently shown that expression vectors harboring the open reading frame of the long terminal repeat region of mouse mammary tumor virus direct the synthesis of a product which acts as a superantigen in transgenic mice. The detection of the ORF protein has been hampered by the extremely low levels of expression observed in these mice, as estimated from the low levels of specific mRNA. To study the properties of the ORF protein, we attempted its expression in different cell types in culture. The experiments performed in yeast show that the ORF gene product is a glycoprotein of approximately 45 kDA. As expected from the derived primary sequence, the unglycosylated product made in the presence of tunicamycin has a molecular weight of 36 kDA. No secretion of the glycosylated protein was observed. Curiously, the full-length molecule was made in lower amounts than a truncated version which contains only the C-terminal half of the protein. Transfection experiments in different mammalian cells suggest that high expression of the ORF protein might have an adverse effect on survival of cells in culture.

Tissue-specific factors and glucocorticoid receptors present in nuclear extracts bind next to each other in the promoter region of mouse mammary tumor virus

Nuclear extracts from different mouse tissues have been used to study the interaction of factors with the steroid-inducible promoter of mouse mammary tumor virus. In addition to the glucocorticoid receptor that interacts with the distal region of the promoter, several tissue-specific proteins were found to bind to the 5' flanking region of the receptor-binding site and to the basal promoter region. The differences in the pattern of protection observed suggest that tissue-specific factors might co-operate with steroid receptors and result in a cell-type-dependent modulation of hormone action.

Cellular and tumoural heterogeneity of EGFR gene amplification in human malignant gliomas

It has been demonstrated that the epidermal growth factor receptor (EGFR) gene, the normal human counterpart of the viral erb-B oncogene is amplified and overexpressed in over 50% of human malignant gliomas (HMGs). In the present study, analysis of the immunohistological staining characteristics of 57 HMGs using an anti-EGFR monoclonal antibody (mab) showed positive staining in 65% of the tumours with large cellular and regional differences in staining pattern and intensity. Screening a smaller number of HMGs with molecular hybridization techniques revealed 10/21 glioblastomas (48%) amplified for the gene; of 11 glioblastomas studied by Northern blot hybridization, 7 tumours with gene amplification showed RNA overexpression, the remaining 4 without amplification did not. Regional differences in DNA levels were observed by Southern blot in 2 tumours: in one particular case, amplification and overexpression were found to be localized to one half of a single HMG, the other half showing neither EFGR gene amplification nor overexpression.

Use of a glucocorticoid-inducible promoter for expression of herpes simplex virus type 1 glycoprotein gC1, a cytotoxic protein in mammalian cells

Abundant expression of herpes simplex virus type 1 glycoprotein gC (gC1) in transfected mammalian cells has not previously been achieved, possibly because gC1 protein is toxic to cells. To approach this problem, the gC1 coding sequence was placed under the control of the weak but inducible glucocorticoid-responsive promoter from the mouse mammary tumor virus (MMTV) long terminal repeat (LTR). As controls to evaluate for gC1 cytotoxicity, the MMTV LTR promoter was used to express glycoprotein gD1, and a strong, constitutive promoter from the Moloney murine sarcoma virus LTR was used to express gC1. L cells were transfected with these constructs, and a clone expressing gC1 from the inducible MMTV LTR promoter was analyzed. In the absence of glucocorticoid (dexamethasone) stimulation, only a low level of gC1 mRNA expression was detected; after overnight stimulation with dexamethasone, transcription increased approximately 200-fold. Abundant gC1 protein that was functionally active in that it bound complement component C3b, was produced. From passages 5 through 26 (70 cell population doublings), the gC1-producing clone became less responsive to overnight dexamethasone stimulation. The block to gC1 expression occurred at the level of transcription and was associated with hypermethylation of the MMTV LTR DNA. Treatment of the clone with 5-aza-2'-deoxycytidine partially reversed the block in gC1 protein production. Late-passage cells assumed a gC1-negative phenotype that appeared to offer a selective growth advantage, which suggested that gC1 was cytotoxic. Several findings support this view: (i) some cells expressing gC1 after overnight stimulation with dexamethasone assumed bizarre, syncytial shapes; (ii) continuous stimulation with dexamethasone for 5 weeks resulted in death of most cells; (iii) cells transfected with gC1 under the control of the strong Moloney murine sarcoma virus promoter assumed bizarre shapes, and stable gC1-expressing clones could not be established; and (iv) cells induced to express gD1 retained a normal appearance after overnight stimulation or 15 weeks of continuous stimulation with dexamethasone. The inducible MMTV LTR promoter is useful for expressing gC1 and may have applications for expressing other cytotoxic proteins.

Dual function of a nuclear factor I binding site in MMTV transcription regulation

Using linker-scanning mutagenesis we had previously identified four elements within the MMTV LTR which are necessary for transcriptional stimulation by glucocorticoid hormones. Two of them overlapped with regions to which the glucocorticoid receptor binds in vitro. The third element contained a NF-I binding site, and the fourth the TATA box. Here we show that mutations that abolish in vitro binding of NF-I had a negative effect also on the basal activity of the MMTV promoter of LTR-containing plasmids stably integrated in Ltk- fibroblasts. The analysis of double mutants altered in the NF-I plus either one of the receptor binding elements further demonstrated that the NF-I site functionally cooperated with the proximal (-120) element, which alone was extremely inefficient in stimulation. The stronger distal (-181/-172) element was independent of NF-I and showed functional cooperativity with the proximal hormone-binding element.

Alterations in the U3 region of the long terminal repeat of an infectious thymotropic type B retrovirus

We isolated and characterized a type B thymotropic retrovirus (DMBA-LV) which is highly related to mouse mammary tumor virus (MMTV) isolates and which induces T-cell thymomas with a high incidence and a very short latent period. Regions of nonhomology between the DMBA-LV genome and the MMTV genome were identified by heteroduplex mapping and nucleotide sequence studies. In the electron microscope heteroduplex mapping studies the EcoRI-generated 5' and 3' fragments of the DMBA-LV genome were compared with the corresponding fragments of the MMTV (C3H and GR) genome isolated from mammary tumors. The results indicated that DMBA-LV contained a region of nonhomologous nucleotide sequences in the 3' half of the U3 region of the long terminal repeat (LTR). Nucleotide sequence studies confirmed these results and showed that in this region 440 nucleotides of the MMTV (C3H) sequences were deleted and substituted with a segment of 122 nucleotides. This substituted segment in the form of a tandem repeat structure contained nucleotide sequences derived exclusively from sequences which flanked the substitution loop. The distal glucocorticoid regulatory element was unaltered, and two additional copies of the distal glucocorticoid regulatory element-binding site were present in the substituted region. The restriction endonuclease map of the reconstructed molecular clone of DMBA-LV was identical to that corresponding to unintegrated linear DMBA-LV DNA present in DMBA-LV-induced tumor cell lines. Since the nucleotide sequences of the LTRs present in four different DMBA-LV proviral copies isolated from a single thymoma were identical, we concluded that they were derived from the same parental virus and that this type B retrovirus containing an alteration in the U3 region of its LTR could induce thymic lymphomas. Thus, DMBA-LV represents the first example of a productively replicating type B retrovirus that contains an LTR modified in the U3 region and that has target cell and disease specificity for T cells.

Prognostic implications of expression of the cellular genes myc, fos, Ha-ras and Ki-ras in colon carcinoma

Messenger RNA levels of the c-fos, c-myc, c-Ha-ras and c-Ki-ras genes were studied in 39 tissue samples obtained from 17 patients undergoing surgery for colon carcinoma and other colon diseases. DNA extracted from the same samples was studied by Southern analysis. The tissues were tumors and grossly normal mucosa from each case and in some instances benign polyps and metastases. Our results indicate: (1) that 50% of cases studied show an increase in expression of at least one of the oncogenes studied; (2) that over-expression is not random, some cases over-expressing several of the genes studied; (3) that the expression pattern of the oncogenes studied varies between primary tumor and metastases; (4) that amplification is a rare event, being limited to one instance in which c-myc was amplified in a metastasis; (5) that cases which exhibit high levels of mRNA in one or more genes studied correlate with biologically aggressive tumors; and (6) that "non-expressors" are at higher risk for local recurrence based on correlations with mucin histochemistry.

Exogenous mouse mammary tumor virus proviral DNA isolated from a kidney adenocarcinoma cell line contains alterations in the U3 region of the long terminal repeat

Mouse mammary tumor virus (MMTV) is a B-type retrovirus which induces predominantly mammary carcinomas after a relatively long latency period. To date, very little is known about the reasons for the strict tissue specificity of MMTV. The BALB/cf/Cd strain of mice, which was infected with milk-borne MMTV (C3H), shows a high incidence of kidney adenocarcinomas, and our data suggest that MMTV might be involved in the formation of these tumors. Newly integrated exogenous MMTV proviruses were found in the genome of transplanted tumor cells as well as in the DNA of a cell line derived from one tumor, but not in normal cells of BALB/cf/Cd mice. The MMTV DNA in these tumor cells was transcribed and viral RNA synthesis was strongly stimulated by glucocorticoid hormones. Viral structural polypeptides, comparable in size and antigenicity to MMTV polypeptides of infected mammary tumor cells were synthesized and processed normally in the cell line and were organized correctly into intracytoplasmic particles. Heteroduplex analysis of the molecularly cloned MMTV proviral DNAs of kidney and mammary tumor origin revealed a high degree of homology in the gag, pol, and env genes. A striking difference, however, was observed in the U3 region of the two LTRs that might relate to the different tissue specificity of the two viruses.

Functional analysis of the glucocorticoid regulatory elements present in the mouse mammary tumor virus long terminal repeat. A synthetic distal binding site can replace the proximal binding domain

Transcription of mouse mammary tumor virus DNA is stimulated by steroid hormones. The DNA sequences involved in this regulation are located in the viral long terminal repeat between positions -200 and -50 with respect to the transcription initiation site. In this region four, one distal and three proximal, in vitro binding sites for the glucocorticoid hormone-receptor complexes have been identified. We have prepared a series of 5' and 3' deletions of this region, using the exonuclease ExoIII. Combination of suitable 5' and 3' fragments enabled us to reconstitute the entire long terminal repeat with small internal deletions. The mutated long terminal repeats linked to the coding region of the Herpes simplex virus thymidine kinase gene were introduced into LTK- aprt- cells by transfection. Transcription from the mouse mammary tumor virus promoter in the presence or absence of hormone was assayed by nuclease S1 mapping. Deletion of the proximal in vitro binding sites resulted in a decrease in hormonal inducibility. When a synthetic oligonucleotide harboring the sequence of the distal in vitro binding site was inserted at the site of the proximal ones, hormone response was restored. This indicated that the distal binding site can replace the proximal ones in their hormone-regulatory function. However, insertion at the same site of an oligonucleotide containing the sequence 5' TGTTCT 3' found in all four binding sites, did not restore the hormone response, indicating that sequences flanking the TGTTCT motif are required for hormone response. Insertion of an unrelated DNA fragment at the site of the proximal binding element deletion completely abolished the hormone response. Analyses of different proximal binding-site deletion and insertion mutants suggested the presence of a transcriptional element located downstream from the most proximal hormone-receptor binding site.

Establishment of two rabbit mammary epithelial cell lines with distinct oncogenic potential and differentiated phenotype after microinjection of transforming genes

The goal of this work was to establish an assay for transformation of epithelial cells. Two epithelial cell lines were obtained after microinjecting transforming genes into primary rabbit mammary secretory cells. The cell lines were analyzed for their oncogenic potential and for the maintenance of a differentiated phenotype. A fully transformed cell line, which retained epithelial cell organization, was obtained by coinjecting simian virus 40 DNA and the activated human c-Ha-ras gene. The proliferation rate of these cells was high, with a doubling time of 16 h. Their growth was anchorage independent, and they had lost contact inhibition. The cells were tumorigenic in nude mice, but had no metastatic potential. Both microinjected DNAs were efficiently transcribed and translated, in contrast to the casein genes, which were expressed in primary cells but not in the transformed cell line. An immortalized cell line established after injection with simian virus 40 DNA alone was characterized by a moderate rate of proliferation with a doubling time of approximately 30 h. The growth of these cells was contact inhibited and anchorage dependent. The cells were not tumorigenic in nude mice. The viral DNA was expressed during early passages, as shown by the presence of the large T antigen in cell nuclei, but not at later passages. A high number of lactogenic hormone receptors were found associated with the cell surface. Despite the presence of these receptors, no induction of genes coding for milk proteins was observed after addition of prolactin. These data demonstrate that this assay system can be used to assess the immortalizing and transforming potential of candidate oncogenes in epithelial cells.

A new site of integration for mouse mammary tumor virus proviral DNA common to BALB/cf(C3H) mammary and kidney adenocarcinomas

The BALB/cf/Cd substrain of mice, developed by inbreeding and selection, shows a 70% incidence of spontaneous kidney adenocarcinomas. Initially foster-nursed on C3H mothers, these mice no longer produce mammary tumors, but there is evidence that mouse mammary tumor virus (MMTV) is involved in the formation of these renal carcinomas. We identified a chromosomal region called int-41, representing a locus interrupted by the integration of an exogenous MMTV provirus in a BALB/c mammary tumor. We found a DNA rearrangement and transcriptional activation in the domain specified by the int-41 probe in one primary kidney adenocarcinoma. A 5.2-kb int-41 specific mRNA was detected in the kidney tumor cell line established from a transplantable renal adenocarcinoma. This mRNA hybridized with MMTV and int-41 specific probes suggesting that it is a hybrid molecule composed of MMTV-LTR sequences covalently linked to host cell RNA. This mRNA was strongly stimulated by the presence of glucocorticoid hormone in the culture medium. Our data are compatible with the hypothesis that the int-41 chromosomal domain is involved in the neoplastic transformation of epithelial cells from different organs.

Hygromycin B phosphotransferase as a selectable marker for DNA transfer experiments with higher eucaryotic cells

The DNA coding sequence for the hygromycin B phosphotransferase gene was placed under the control of the regulatory sequences of a cloned long terminal repeat of Moloney sarcoma virus. This construction allowed direct selection for hygromycin B resistance after transfection of eucaryotic cell lines not naturally resistant to this antibiotic, thus providing another dominant marker for DNA transfer in eucaryotic cells.

[Mechanisms of activation and function of oncogenes]

Transforming genes (oncogenes) were first identified and characterized as genetic elements of tumorviruses. It has been demonstrated that these genes are derived from cellular genes by transduction. The corresponding cellular genes are ubiquitous in nature and have been well conserved during evolution. It is therefore very likely that they are important elements in normal regulation of cell growth, e.g. during development. Several mechanisms of oncogene activation have been observed. Qualitative as well as quantitative changes in oncogene expression lead to cell transformation. According to their mechanism of action, at least 4 functional groups of oncogenes can be distinguished. It has recently been shown that the transformation of a normal cell into a tumor cell proceeds in several steps involving different oncogenes.

Dexamethasone increases the number of RNA polymerase II molecules transcribing integrated mouse mammary tumor virus DNA and flanking mouse sequences

In mouse Ltk- cells that were transfected with recombinant bacteriophage DNA containing a complete proviral copy of an integrated endogenous mouse mammary tumor virus (MMTV) with its flanking cellular sequences, the newly acquired MMTV proviruses were transcribed in a glucocorticoid-responsive fashion. After hormone treatment of selected cell clones in culture we isolated the nuclei, elongated the nascent RNA chains in vitro, and determined the number of RNA polymerase II molecules on the transcribed MMTV DNA as well as on the flanking mouse DNA sequences. We found that the specific increase in the polymerase loading after hormone treatment is proportional to the increase in the amount of stable MMTV mRNA. When the DNA sequences which are responsible for hormone-receptor binding and for the increased MMTV mRNA levels were deleted, no increase in RNA polymerase II loading on MMTV DNA was observed. Nuclear RNA chains which were transcribed in response to hormone treatment were detected not only from the transfected MMTV DNA but also from the mouse DNA sequences adjacent to the 3' end of the provirus.

Nucleotide sequence of the 5' noncoding region and part of the gag gene of mouse mammary tumor virus; identification of the 5' splicing site for subgenomic mRNAs

We have determined the sequence of the first 1371 nucleotides at the 5' end of the genome of mouse mammary tumor virus using molecularly cloned proviral DNA of the GR virus strain. The most likely initiation codon used for the gag gene of mouse mammary tumor virus is the first one, located 312 nucleotides from the 5' end of the viral RNA. The 5' splicing site for the subgenomic mRNA's is located approximately 288 nucleotides downstream from the 5' end of the viral RNA. From the DNA sequence the amino acid sequence of the N-terminal half of the gag precursor protein, including p10 and p21, was deduced (353 amino acids).

Glucocorticoid regulation of mouse mammary tumor virus: identification of a short essential DNA region

Transcription of mouse mammary tumor virus (MMTV) DNA is stimulated by steroid hormones. To determine the DNA sequences involved in this regulation, we constructed a plasmid containing the MMTV long terminal repeat (LTR) in front of the coding region of the herpes simplex thymidine kinase gene, from which the promoter had been removed. Portions of the LTR were removed by the nuclease Ba/31, and the deleted molecules were recloned and tested for transcriptional activity in transfections of Ltk-aprt- cells. Stably transfected cell clones were selected and hormone-dependent transcription from the MMTV promoter was studied by the S1 nuclease mapping method. The results show that DNA sequences between -105 and -204 base pairs upstream from the initiation site of viral transcription are required for glucocorticoid stimulation.

Mouse mammary tumour virus related sequences are present in human DNA

MuMTV-related sequences have been identified in the DNA of human breast cancer cells using the Southern transfer technique and hybridisation with cloned MuMTV DNA under conditions in which partially mismatched sequences form stable hybrids. Hybridisation with cloned fragments of the MuMTV genome showed that the gag-pol region shares the most homology (estimated to be greater than 80%) with the human MuMTV-related sequences, however, DNA fragments partially homologous to the MuMTV LTR, gag ad env regions were also detected. Analysis of several human DNA samples suggests that the majority of the human MuMTV-related sequences are genetically transmitted but additional Eco R1 fragments were detected in the DNA of one out of three breast cancer cell lines, MCF7. These sequences are potential probes for the human MuMTV-related retroviral sequences and will allow their possible role in human breast cancer to be evaluated.

Cloned mouse mammary tumor virus DNA exhibits glucocorticoid-dependent expression in simian virus 40-transformed mink cells

Mink lung epithelial cells were transfected with two cloned mouse mammary tumor virus (MMTV) DNAs, a 9-kilobase clone derived from an unintegrated exogenous viral genome and a 14-kilobase clone containing an integrated endogenous provirus along with cellular flanking sequences. Mink lung cells were chosen because they do not contain endogenous MMTV sequences. On the basis of our observation that simian virus 40 DNA efficiently transforms these cells, we isolated cell clones containing MMTV DNA by using transformation with simian virus 40 DNA as a selective marker in cotransfection experiments. Levels of the 9-kilobase MMTV mRNA representing the entire viral genome and of the spliced 4.4-kilobase mRNA which codes for the viral envelope proteins were glucocorticoid dependent in transformed cells. Expression of low levels of Pr77gag, the precursor of the group-specific viral core proteins, and of gPr73env, the precursor of the viral envelope proteins, was also hormone dependent. We conclude that these cloned MMTV DNAs contain all the information necessary for the synthesis of normal viral RNAs and proteins. These findings also provide further evidence that the DNA sequences involved in the hormone responsiveness of MMTV expression are contained within the viral genome.

Isolation and analysis of genomic DNA clones encoding the third component of mouse complement

A gene library was constructed with DNA from strain A mice by using the phage lambda vector lambda 1059. By screening with cloned cDNA for the third component of mouse complement, C3, four different C3 genomic clones were isolated from this library. Two of the recombinant phages carry insertions of 14 and 18 kilobase pairs, respectively, which together cover one complete copy of the C3 gene and several hundred nucleotides of its 5' and 3' flanking sequences. The distance from the 5' end of the gene, which includes the hexanucleotide T-A-T-A-A-A and a translation initiation codon, to its 3' end as defined by the poly(A) attachment site is 24 kilobase pairs. From the genomic DNA sequence, a signal peptide of 24 amino acid residues is predicted at the NH2 terminus of the initial translation product. The signal peptide and the next two amino acids are encoded by the first exon of this gene.

The region of mouse mammary tumor virus DNA containing the long terminal repeat includes a long coding sequence and signals for hormonally regulated transcription

Starting from a biologically active recombinant DNA clone of exogenous unintegrated GR mouse mammary tumor virus, we have generated three subclones of PstI fragments of 1.45, 1.1, and 2.0 kb in the plasmid vector PBR322. The nucleotide sequence has been determined for the clone of 1.45 kb which includes almost the complete region of the long terminal repeat (LTR) plus an adjacent stretch of unique sequence DNA. A short region of the 2.0 kb clone, containing the beginning of the LTR, has also been sequenced. Starting with the A of an initiation codon outside the LTR, we detected an open reading frame of 960 nucleotides, potentially coding for a protein of 320 amino acids (36K). Two hundred nucleotides downstream from the termination codon, and approximately 25 nucleotides upstream from the presumptive initiation site of viral RNA synthesis, we found a promoter-like sequence. The sequence AGTAAA was detected approximately 15-20 nucleotides upstream from the 3' end of virion RNA and probably serves as a polyadenylation signal. The 1.45 kb PstI fragment has been transfected into Ltk- cells together with a plasmid containing the thymidine kinase gene of herpes simplex virus. The virus-specific RNA synthesis detected in a Tk+ cell clone was strongly stimulated by the addition of dexamethasone.

Sequence determination of the 3' end of mouse mammary tumor virus RNA

70S RNA has been prepared from mouse mammary tumor virus (MMTV) produced by the GR tumor cell line. After denaturation for 3 min at 60 degrees C the RNA was applied to a sucrose gradient and molecules sedimenting between 10-16S were selected and passed over an oligo-dT cellulose column. The poly A containing RNA fraction was used as a template for the synthesis of complementary DNA with reverse transcriptase in the presence of dideoxynucleoside triphosphates. Oligo-nucleotides p(dT)7rG, p(dT)7rA and (p(dT)7rC were tested for their primer activity. Two of the three primers gave readable sequences. This suggests a heterogeneity in the 3' end of the viral RNA, a phenomenon also observed with avian retroviruses. Nucleotides 37-45 from the 3' end are made up of only A's and T's and resemble a Hogness box. This finding could have biological consequences for the transcription of proviral DNA in the unintegrated and integrated state. Instead of the usual AAUAAA sequence present in mRNA's close and in front of the site of polyadenylation we find a sequence AGUAAA.

Mouse complement components C3 and C4. Characterization of their messenger RNA and molecular cloning of complementary DNA for C3

Mouse liver mRNA species which direct the cell-free synthesis of pro-C3 and pro-C4 polypeptides with an apparent molecular weight of 175,000 and 190,000, respectively, were shown to sediment faster than 28 S. By electrophoresis in a denaturing gel the mRNA for C3 was determined to contain approximately 7,500 ribonucleotides. cDNA was synthesized from size-fractionated mouse liver mRNA and cloned in the plasmid pBR 322. Among the cDNA clones recovered three were identified as being complementary to portions of the mRNA for C3.