Inability of Kaplan radiation leukemia virus to replicate on mouse fibroblasts is conferred by its long terminal repeat

HTLV-I transgenic models: an overview

Human T cell leukemia virus type I (HTLV-I) is the agent of a wide spectrum of human diseases. The mechanisms by which a single virus can cause neurodegenerative disorders as well as leukemia is still a matter of debate. Transgenic mice have been used to assess the contribution of different viral elements in viral tropism as well as on cell transformation in vivo. In particular, transgenic models were generated to study the tissue specificity of expression directed by the viral long terminal repeat and the pathological effects induced by the Tax protein of HTLV-I. These models have led to a description of the cell types able to support the viral expression in vivo, and the use of Tax-transgenic mice has demonstrated that this protein is oncogenic and able to induce muscular atrophy and arthropathies. Finally, these models could provide a useful system to study therapeutic approaches for HTLV-I-associated diseases.

The E47 transcription factor binds to the enhancer sequences of recombinant murine leukemia viruses and influences enhancer function

The genomes of most recombinant murine leukemia viruses (MuLVs) inherit pathogenic U3 region sequences from the endogenous xenotropic provirus Bxv-1. However, the U3 regions of about one-third of recombinant MuLVs from CWD mice, such as CWM-T15, have nonecotropic substitutions that are probably derived from an endogenous polytropic provirus. The CWM-T15 U3 region sequences contain five nucleotide substitutions compared with the less pathogenic sequences of the endogenous ecotropic virus parent, Emv-1. Three of these substitutions are located immediately 3' of the enhancer core, and two form part of an E-box motif that is also found in the Bxv-1 sequence. A series of electromobility shift assays revealed that nuclear extracts from S194 cells and the basic helix-loop-helix transcription factor E47 could distinguish between oligonucleotides that contained the core region sequences of CWM-T15 or Emv-1. The E47 homodimers appeared to bind to the CWM-T15 E-box motif and when expressed at high levels in cells transactivated the CWM-T15 but not the Emv-1 enhancer. Taken together, these results suggest that E47 or related basic helix-loop-helix proteins that are expressed in lymphoid cells bind to and transactivate the CWM-T15 enhancer in vivo. This transactivation may explain why the CWM-T15 and Bxv-1 U3 regions accelerate the onset of lymphoid neoplasms and why related enhancer core region sequences are preferentially incorporated into the genomes of recombinant MuLVs and are found in other leukemogenic mammalian retroviruses.

Mutational analysis of the murine AIDS-defective viral genome reveals a high reversion rate in vivo and a requirement for an intact Pr60gag protein for efficient induction of disease

Pr60gag appears to be the only protein encoded by the murine AIDS (MAIDS)-defective virus. To study the role of Pr60gag or some other sequences of the viral genome in the pathogenicity of the virus, we have generated mutants of the defective viral genome. These mutant defective viruses, prepared as helper-free stocks, were inoculated into susceptible C57BL/6 mice. Mutant Du5H-A virus, which had a stop codon within gag MA(p15), did not induce target cell proliferation or MAIDS. Mutants Du5H-B and -C encoded truncated Pr60gag proteins containing, respectively, MA(p15)-p12 or MA(p15)-p12 and part of CA(p30). These mutants showed a very limited capacity to induce early cell expansion and were poorly pathogenic. Only recombinant (revertant) viruses were recovered from organs of diseased mice inoculated with these two mutants. Mutant Du5H-D was generated by deleting 1.4 kbp of the 3'-end sequences, outside the gag coding region. The levels of RNA and proteins made by this mutant were low. This mutant also reverting frequently but was nevertheless able to induce MAIDS at a low efficiency without reverting. Our results indicate that the Pr60gag protein is necessary and sufficient to induce MAIDS. These data also suggest that the Pr60gag protein needs to be relatively intact to be fully pathogenic. In addition, our study shows a very high reversion rate of some mutants and emphasizes the need to check for the presence of revertant (recombinant) viruses in diseased organs when working with mutants of the MAIDS-defective virus.

Graffi murine leukemia virus: molecular cloning and characterization of the myeloid leukemia-inducing agent

The Graffi murine leukemia virus (MuLV) is a retroviral mixture that induces predominantly myeloid leukemia in several inbred strains of mice. To analyze the viral component responsible for the myeloid leukemogenesis, we cloned several proviruses from a Graffi MuLV-infected cell line. Several infectious molecular clones were obtained that could be classified into two distinct groups of infectious MuLV. Both types of MuLV were nondefective, ecotropic, and NB tropic and induced granulocytic leukemia in BALB/c and NFS mice. Restriction enzyme analysis and molecular hybridization with several MuLV probes on one molecular clone from each group revealed that both groups are closely related to each other but are clearly distinct from all known retroviruses. One component of MuLV, however, induced leukemia with a shorter latency period and harbored a lengthier long terminal repeat. The long terminal repeat of the more leukemogenic component of MuLV had acquired a 60-bp perfect duplication in the U3 region. Analysis of the tumor DNAs with probes for the mouse T-cell receptor and immunoglobulin heavy chain genes revealed frequent rearrangements with one or both probes. This concomitant expression by leukemic cells of markers of different lineages, observed in human leukemias, has been termed "lineage infidelity" and confirms that the latter rearrangements are not restricted to hematopoietic precursors committed to lymphoid differentiation.

Comparative analysis of HTLV-I promoter activities reveals no disease-linked pattern of expression

To investigate the possibility of an association between the type of pathology caused by HTLV-I and the activity of its promoter, we compared the levels of transcription obtained with six LTRs isolated from patients with two different HTLV-I-related diseases: ATL and TSP/HAM. The patients came from different geographical endemic areas. The LTR region was amplified by polymerase chain reaction (PCR) from the DNA of uncultured peripheral blood lymphocytes, and directly cloned upstream of the luciferase reporter gene. Constructs were tested by a transient transfection assay in a variety of cell lines. Although the activities of these LTRs were statistically different in some of the cell lines tested, no correlation could be demonstrated between the promoter activity and the nature of the disease. Thus, the data suggest that the LTR is not a major determinant of the nature of the disease associated with the infection by HTLV-I.

U3 long terminal repeat-mediated induction of intracellular immunity by a murine retrovirus: a novel model of latency for retroviruses

BL/VL3 radiation leukemia virus (RadLV) is a thymotropic, highly leukemogenic murine leukemia virus (MuLV) which is unable to replicate in vitro in mouse fibroblasts. We have previously reported that the U3 long terminal repeat region of its genome is responsible for this block (E. Rassart, Y. Paquette, and P. Jolicoeur, J. Virol. 62:3840-3848, 1988). By using hybrids of permissive and resistant cells infected with BL/VL3 RadLV or fibrotropic MuLV, we found that the resistant phenotype was dominant. Investigation to determine at which step of the virus cycle the block operates revealed that integration, transcription, and translation of the BL/VL3 viral genome occurred at normal levels in nonpermissive cells. The BL/VL3 RadLV Pr65gag proteins made in nonpermissive cells were also myristylated and located at the membrane, and the levels of their cleaved products were similar to those of fibrotropic MuLV. However, processing of BL/VL3 RadLV Pr85env was impaired in nonpermissive cells. Virions were not released into the culture medium of nonpermissive cells, as measured by reverse transcriptase activity and by content in p30 or gp70 protein and as documented by lower levels of budding particles seen by electron microscopy. These results indicate that BL/VL3 RadLV replication is blocked at a late stage of the virus cycle, i.e., at virion assembly. Interestingly, these BL/VL3 RadLV-infected nonpermissive fibroblasts were resistant to superinfection by fibrotropic Moloney MuLV, and this resistance also occurred at a late step of the Moloney virus cycle. Since this block is dominant, it appears that the U3 long terminal repeat region of the BL/VL3 viral genome has the ability to induce a cellular suppressor factor(s), thus bringing intracellular immunity against itself and against other ecotropic MuLVs.

Determinants of thymotropism in Kaplan radiation leukemia virus and nucleotide sequence of its envelope region

Radiation leukemia viruses (RadLVs) are a group of murine leukemia viruses which are induced by radiation and cause T-cell leukemia. Viral clones isolated from the BL/VL3 lymphoid cell line derived from a thymoma show variable tropism and leukemogenic potential. We have constructed chimeric viruses by in vitro recombination between two viruses, a RadLV that is thymotropic and an endogenous ecotropic virus that is nonthymotropic. We show here that, in contrast to thymotropism determinants identified previously, which lie in the long terminal repeat (LTR), it is the envelope region that is responsible for the thymotropism of BL/VL3 RadLV. The nonthymotropic virus which we have rendered thymotropic by transfer of the env region of RadLV in the present study has been shown previously to become thymotropic when the LTR of another thymotropic virus is inserted in its genome. Thus, the LTR and envelope gene may be involved in complementary action to lead to thymotropism.

Tissue expression pattern directed in transgenic mice by the LTR of an HTLV-I provirus isolated from a case of tropical spastic paraparesis

Human T lymphotropic virus type I (HTLV-I) causes adult T cell leukemia/lymphoma and a chronic neurological disease called either tropical spastic paraparesis (TSP) or HTLV-I-associated myelopathy. The different outcomes of this infection could be due to both host and viral factors and it has been proposed that genetic differences could make some HTLV-I strains neurotropic. In this paper, we examined the pattern of tissue-specific expression determined by a long terminal repeat (LTR) obtained from a case of TSP. We constructed transgenic mice in which this LTR controlled the expression of the nlslacZ reporter gene. We observed that in three independent lines of transgenic mice, the reporter gene was expressed predominantly in the central nervous system (CNS), in choroid plexus, and in cells of the hippocampus and cerebellum. Our observations indicate the existence of CNS cells permissive for the expression of HTLV-I and which may be of importance in the pathogenesis of TSP.

The majority of cells infected with the defective murine AIDS virus belong to the B-cell lineage

Murine AIDS (MAIDS) is caused by a defective retrovirus which encodes a gag fusion protein (Pr60gag). We previously reported that this virus induced an oligoclonal proliferation of infected cells and suggested that this cell expansion was an important event in the pathogenesis of MAIDS. To identify these target cells, we constructed novel defective viruses whose genomes could be detected with specific probes. Helper-free stocks of these viruses induced MAIDS. Using in situ hybridization and immunocytochemistry and Southern analysis, we found that most infected cells belong to the B-cell lineage. Transformation of these B cells appears to be the primary event responsible for the development of immunodeficiency. This animal model may be relevant to our understanding of AIDS, of the immunodeficiencies associated with B-cell lymphoproliferative disorders, and of the role of B-cell proliferation and transformation in the effects of superantigens, since Pr60gag appears to be a superantigen.

The specificity of the disease induced by defective murine retroviruses containing abl, fos, or Ha-ras is usually not determined by their LTR

The long terminal repeats (LTR) of the defective murine sarcoma viruses (MSV) containing v-abl, v-Ha-ras, or v-fos were exchanged for LTRs from other retroviruses having different tissue tropism. The new chimeric MSV were found to induce the same diseases as the parental viruses, indicating that sequences outside the LTR, most likely those of the oncogene, are responsible for the disease specificity of these defective MSV.

Determinants of the ability of malignant fibroma virus to induce immune dysfunction and tumor dissemination in vivo

The relationship of virus-induced immunological dysfunction and tumor dissemination was studied using two related tumor-causing leporipoxviruses: malignant fibroma virus (MV) and Shope fibroma virus (SFV). Recombinant viruses, produced by transferring MV's 10.7 kb BamHI C fragment to SFV, replicate in lymphocytes and suppress lymphocyte function in vitro. Those recombinants that replicate in lymphocytes and suppress lymphocyte function in vitro share about 3.5 kb from MV's C fragment. Some recombinants mimic MV in producing immune suppression and disseminated virus infection in vivo. Other recombinants, even some that are highly immunosuppressive in vitro (e.g. R71), only variably induce immune suppression in vivo, and do not cause disseminated disease. A segment of DNA from MV that transfers to Shope fibroma virus almost all of MV's virulence in vivo was identified.

DNA-binding proteins that interact with the long terminal repeat of radiation leukemia virus

We used the electrophoretic mobility shift assay to identify the interactions of nuclear proteins with the long terminal repeat of leukemogenic, thymotropic BL/VL3 radiation leukemia virus (RadLV). In the promoter region, we identified a CCAAT box-binding protein (CBP) that has the same binding characteristics as the CCAAT box-binding protein that binds to the Moloney murine sarcoma virus promoter and most likely represents the CP1 factor. In the upstream enhancer region unique to BL/VL3, we detected several sequence-specific complexes, one with T-lymphocyte extracts but not with fibroblast extracts. This U3 region, UEB, may be important for the T-cell specificity of BL/VL3 RadLV. In the enhancer, which has been uniquely rearranged in this virus, we identified three specific protein-binding sites. Two of them showed characteristics of the LVb and core binding sites previously described for other murine retroviruses. But one binding site, identified as Rad-1, is unique to BL/VL3 RadLV and was found downstream, only 1 nucleotide from the core sequence. Rad-1 has a corelike motif on the minus strand, and the factor that binds to it could be competed by a BL/VL3 core-containing fragment. Moreover, the protein-DNA contacts involve the typical three core Gs separated by one T. These results suggest that Rad-1 binds a factor identical or similar to the core-binding factor. Our data suggest that the LVb, core, and Rad-1 motifs may be sufficient for this enhancer, most likely in association with other U3 long terminal repeat sequences, to promote a high rate of transcription of BL/VL3 RadLV in its specific target cells (thymocytes).

Point mutations in the Moloney murine leukemia virus enhancer identify a lymphoid-specific viral core motif and 1,3-phorbol myristate acetate-inducible element

The transcriptional enhancer of the Moloney murine leukemia virus (MoMLV) is organized as a 75-base-pair repeat, and in each copy of the repeat there are multiple binding sites for nuclear factors. We have introduced point mutations into each of the known nuclear factor-binding sites in the MoMLV enhancer, in both copies of the direct repeat, and have analyzed the transcriptional activity conferred by the mutated enhancers by transient-expression assays in both hematopoietic and nonhematopoietic cell lines. Mutation of individual binding sites in the MoMLV enhancer has moderate effects (less than 2-fold to 20-fold) on transcription in six independent cell lines. Several mutations decreased transcription from the MoMLV enhancer ubiquitously (the leukemia virus factor b site and the glucocorticoid response element), whereas others affected transcription specifically in lymphoid cell lines (core motif) or, more significantly, in fibroblasts (nuclear factor 1 site). The transcriptional activity of the MoMLV enhancer can be induced 8- to 10-fold by 1,3-phorbol myristate acetate in Jurkat T cells. Mutations in any of three adjacent binding sites (leukemia virus factor b and c sites and the core motif) within a 28-base-pair region in the center of the direct repeat sequence of the MoMLV enhancer completely attenuate the response to 1,3-phorbol myristate acetate.