Tissue selectivity of murine leukemia virus infection is determined by long terminal repeat sequences

EcoHIV Infection of Primary Murine Brain Cell Cultures to Model HIV Replication and Neuropathogenesis

BACKGROUND

EcoHIV is a chimeric HIV that replicates in mice in CD4+ T cells, macrophages, and microglia (but not in neurons), causing lasting neurocognitive impairment resembling neurocognitive disease in people living with HIV. The present study was designed to develop EcoHIV-susceptible primary mouse brain cultures to investigate the indirect effects of HIV infection on neuronal integrity.

RESULTS

We used two EcoHIV clones encoding EGFP and mouse bone marrow-derived macrophages (BMM), mixed mouse brain cells, or enriched mouse glial cells from two wild-type mouse strains to test EcoHIV replication efficiency, the identity of productively infected cells, and neuronal apoptosis and integrity. EcoHIV replicated efficiently in BMM. In mixed brain cell cultures, EcoHIV targeted microglia but did not cause neuronal apoptosis. Instead, the productive infection of the microglia activated them and impaired synaptophysin expression, dendritic density, and axonal structure in the neurons. EcoHIV replication in the microglia and neuronal structural changes during infection were prevented by culture with an antiretroviral.

CONCLUSIONS

In murine brain cell cultures, EcoHIV replication in the microglia is largely responsible for the aspects of neuronal dysfunction relevant to cognitive disease in infected mice and people living with HIV. These cultures provide a tool for further study of HIV neuropathogenesis and its control.

Efficient Expression of HIV in Immunocompetent Mouse Brain Reveals a Novel Nonneurotoxic Viral Function in Hippocampal Synaptodendritic Injury and Memory Impairment

HIV causes neurodegeneration and dementia in AIDS patients, but its function in milder cognitive impairments in virologically suppressed patients on antiretroviral therapy is unknown. Such patients are immunocompetent, have low peripheral and brain HIV burdens, and show minimal brain neuropathology. Using the model of HIV-related memory impairment in EcoHIV-infected conventional mice, we investigated the neurobiological and cognitive consequences of efficient EcoHIV expression in the mouse brain after intracerebral infection. HIV integrated and persisted in an expressed state in brain tissue, was detectable in brain monocytic cells, and caused neuroinflammatory responses and lasting spatial, working, and associative memory impairment. Systemic antiretroviral treatment prevented direct brain infection and memory dysfunction indicating the requirement for HIV expression in the brain for disease. Similarly inoculated murine leukemia virus used as a control replicated in mouse brain but not in monocytic cells and was cognitively benign, linking the disease to HIV-specific functions. Memory impairment correlated in real time with hippocampal dysfunction shown by defective long-term potentiation in hippocampal slices ex vivo and with diffuse synaptodendritic injury in the hippocampus reflected in significant reduction in microtubule-associated protein 2 and synapsin II staining. In contrast, there was no evidence of overt neuronal loss in this region as determined by neuron-specific nuclear protein quantification, TUNEL assay, and histological observations. Our results reveal a novel capacity of HIV to induce neuronal dysfunction and memory impairment independent of neurotoxicity, distinct from the neurotoxicity of HIV infection in dementia.IMPORTANCE HIV neuropathogenesis has been attributed in large measure to neurotoxicity of viral proteins and inflammatory factors produced by infected monocytic cells in the brain. We show here that HIV expression in mouse brain causes lasting memory impairment by a mechanism involving injury to hippocampal synaptodendritic arbors and neuronal function but not overt neuronal loss in the region. Our results mirror the observation of minimal neurodegeneration in cognitively impaired HIV patients on antiretroviral therapy and demonstrate that HIV is nonneurotoxic in certain brain abnormalities that it causes. If neurons comprising the cognition-related networks survive HIV insult, at least for some time, there is a window of opportunity for disease treatment.

EcoHIV infection of mice establishes latent viral reservoirs in T cells and active viral reservoirs in macrophages that are sufficient for induction of neurocognitive impairment

Suppression of HIV replication by antiretroviral therapy (ART) or host immunity can prevent AIDS but not other HIV-associated conditions including neurocognitive impairment (HIV-NCI). Pathogenesis in HIV-suppressed individuals has been attributed to reservoirs of latent-inducible virus in resting CD4+ T cells. Macrophages are persistently infected with HIV but their role as HIV reservoirs in vivo has not been fully explored. Here we show that infection of conventional mice with chimeric HIV, EcoHIV, reproduces physiological conditions for development of disease in people on ART including immunocompetence, stable suppression of HIV replication, persistence of integrated, replication-competent HIV in T cells and macrophages, and manifestation of learning and memory deficits in behavioral tests, termed here murine HIV-NCI. EcoHIV established latent reservoirs in CD4+ T lymphocytes in chronically-infected mice but could be induced by epigenetic modulators ex vivo and in mice. In contrast, macrophages expressed EcoHIV constitutively in mice for up to 16 months; murine leukemia virus (MLV), the donor of gp80 envelope in EcoHIV, did not infect macrophages. Both EcoHIV and MLV were found in brain tissue of infected mice but only EcoHIV induced NCI. Murine HIV-NCI was prevented by antiretroviral prophylaxis but once established neither persistent EcoHIV infection in mice nor NCI could be reversed by long-acting antiretroviral therapy. EcoHIV-infected, athymic mice were more permissive to virus replication in macrophages than were wild-type mice, suffered cognitive dysfunction, as well as increased numbers of monocytes and macrophages infiltrating the brain. Our results suggest an important role of HIV expressing macrophages in HIV neuropathogenesis in hosts with suppressed HIV replication.

Isolation and characterization of a small antiretroviral molecule affecting HIV-1 capsid morphology

Background

Formation of an HIV-1 particle with a conical core structure is a prerequisite for the subsequent infectivity of the virus particle. We have previously described that glycineamide (G-NH₂) when added to the culture medium of infected cells induces non-infectious HIV-1 particles with aberrant core structures.

Results

Here we demonstrate that it is not G-NH₂ itself but a metabolite thereof that displays antiviral activity. We show that conversion of G-NH₂ to its antiviral metabolite is catalyzed by an enzyme present in bovine and porcine but surprisingly not in human serum. Structure determination by NMR suggested that the active G-NH₂ metabolite was α-hydroxy-glycineamide (α-HGA). Chemically synthesized α-HGA inhibited HIV-1 replication to the same degree as G-NH₂, unlike a number of other synthesized analogues of G-NH₂ which had no effect on HIV-1 replication. Comparisons by capillary electrophoresis and HPLC of the metabolite with the chemically synthesized α-HGA further confirmed that the antiviral G-NH₂-metabolite indeed was α-HGA.

Conclusion

α-HGA has an unusually simple structure and a novel mechanism of antiviral action. Thus, α-HGA could be a lead for new antiviral substances belonging to a new class of anti-HIV drugs, i.e. capsid assembly inhibitors.

Activity of the small modified amino acid alpha-hydroxy glycineamide on in vitro and in vivo human immunodeficiency virus type 1 capsid assembly and infectivity

Upon maturation of the human immunodeficiency virus type 1 (HIV-1) virion, proteolytic cleavage of the Gag precursor protein by the viral protease is followed by morphological changes of the capsid protein p24, which will ultimately transform the virus core from an immature spherical to a mature conical structure. Virion infectivity is critically dependent on the optimal semistability of the capsid cone structure. We have reported earlier that glycineamide (G-NH(2)), when added to the culture medium of infected cells, inhibits HIV-1 replication and that HIV-1 particles with aberrant core structures were formed. Here we show that it is not G-NH(2) itself but a metabolite thereof, alpha-hydroxy-glycineamide (alpha-HGA), that is responsible for the antiviral activity. We show that alpha-HGA inhibits the replication of clinical HIV-1 isolates with acquired resistance to reverse transcriptase and protease inhibitors but has no effect on the replication of any of 10 different RNA and DNA viruses. alpha-HGA affected the ability of the HIV-1 capsid protein to assemble into tubular or core structures in vitro and in vivo, probably by binding to the hinge region between the N- and C-terminal domains of the HIV-1 capsid protein as indicated by matrix-assisted laser desorption ionization-mass spectrometry results. As an antiviral compound, alpha-HGA has an unusually simple structure, a pronounced antiviral specificity, and a novel mechanism of antiviral action. As such, it might prove to be a lead compound for a new class of anti-HIV substances.

Detection and differentiation of re-emerging fowlpox virus (FWPV) strains carrying integrated reticuloendotheliosis virus (FWPV-REV) by real-time PCR

Current strains of fowlpox virus (FWPV) carrying circulating reticuloendotheliosis virus (FWPV-REV) sequence are becoming more pathogenic to poultry. This is evidenced by the fact that vaccination with current available FWPV vaccines provides limited protection against them. To characterize REV insertions in a collection of both older and more recent field isolates, we developed three different types of adjacent oligoprobes and primer sets from specific genomic locations of FWPV and REV: REV-ENV (accession no. K02537, 1382-2260), FWPV-REV integration site (accession no. AF006064, 86-1328), FWPV (accession no. AF198100, 232461-232670), and REV-LTR (accession no. V01204, 305-496). The data indicated that the primers from the REV-ENV region and the TaqMan probes specifically targeted REV-ENV sequences of FWPV-REV strains. Furthermore, the strains were differentiated based on quantitative melting temperature (T(m)) of their amplified products using FRET-based probes. The amplified products were further characterized by sequencing and multiple sequence alignment analysis. The results suggest that integrated REV-ENV sequences are both common and mostly conserved in field isolates. However, the minor variations found within the short-targeted ENV sequence from FWPV-REV strains suggest that these strains could have either undergone periodic point mutational changes or integration with different REV-ENV subtypes.

Duplicated sequence motif in the long terminal repeat of maedi-visna virus extends cell tropism and is associated with neurovirulence

Maedi-visna virus (MVV) is a lentivirus of sheep causing chronic inflammatory disease of the lungs (maedi) and the nervous system (visna). We have previously shown that a duplicated sequence in the long terminal repeat (LTR) of MVV is a determinant of cell tropism. Here, we demonstrate that deletion of a CAAAT sequence from either one of the repeats resulted in poor virus growth in sheep choroid plexus cells. A duplication in the LTR encompassing the CAAAT sequence was found in four neurological field cases that were sequenced, but no duplication was present in the LTRs from seven maedi cases; one maedi isolate was mixed. These results indicate that the duplication in the LTR is associated with neurovirulence.

Precise identification of endogenous proviruses of NFS/N mice participating in recombination with moloney ecotropic murine leukemia virus (MuLV) to generate polytropic MuLVs

Polytropic murine leukemia viruses (MuLVs) are generated by recombination of ecotropic MuLVs with env genes of a family of endogenous proviruses in mice, resulting in viruses with an expanded host range and greater virulence. Inbred mouse strains contain numerous endogenous proviruses that are potential donors of the env gene sequences of polytropic MuLVs; however, the precise identification of those proviruses that participate in recombination has been elusive. Three different structural groups of proviruses in NFS/N mice have been described and different ecotropic MuLVs preferentially recombine with different groups of proviruses. In contrast to other ecotropic MuLVs such as Friend MuLV or Akv that recombine predominantly with a single group of proviruses, Moloney MuLV (M-MuLV) recombines with at least two distinct groups. In this study, we determined that only three endogenous proviruses, two of one group and one of another group, are major participants in recombination with M-MuLV. Furthermore, the distinction between the polytropic MuLVs generated by M-MuLV and other ecotropic MuLVs is the result of recombination with a single endogenous provirus. This provirus exhibits a frameshift mutation in the 3' region of the surface glycoprotein-encoding sequences that is excluded in recombinants with M-MuLV. The sites of recombination between the env genes of M-MuLV and endogenous proviruses were confined to a short region exhibiting maximum homology between the ecotropic and polytropic env sequences and maximum stability of predicted RNA secondary structure. These observations suggest a possible mechanism for the specificity of recombination observed for different ecotropic MuLVs.

A mouse model for study of systemic HIV-1 infection, antiviral immune responses, and neuroinvasiveness

We created a model of HIV-1 infection of conventional mice for investigation of viral replication, control, and pathogenesis. To target HIV-1 to mice, the coding region of gp120 in HIV-1/NL4-3 was replaced with that of gp80 from ecotropic murine leukemia virus, a retrovirus that infects only rodents. The resulting chimeric virus construct, EcoHIV, productively infected murine lymphocytes, but not human lymphocytes, in culture. Adult, immunocompetent mice were readily susceptible to infection by a single inoculation of EcoHIV as shown by detection of virus in splenic lymphocytes, peritoneal macrophages, and the brain. The virus produced in animals was infectious, as shown by passage in culture, and immunogenic, as shown by induction of antibodies to HIV-1 Gag and Tat. A second chimeric virus based on clade D HIV-1/NDK was also highly infectious in mice; it was detected in both spleen and brain 3 wk after tail vein inoculation, and it induced expression of infection response genes, MCP-1, STAT1, IL-1beta, and complement component C3, in brain tissue as determined by quantitative real-time PCR. EcoHIV infection of mice forms a useful model of HIV-1 infection of human beings for convenient and safe investigation of HIV-1 therapy, vaccines, and potentially pathogenesis.

Cellular specificity of HIV-1 replication can be controlled by LTR sequences

Two well-established determinants of retroviral tropism are envelope sequences that regulate entry and LTR sequences that can regulate viral expression in a cell-specific manner. Studies with human immunodeficiency virus-1 (HIV-1) have demonstrated that tropism of this virus maps primarily to variable envelope sequences. Studies have demonstrated that T cell and macrophage-specific transcription factor binding motifs exist in the upstream region of the LTR U3; however, the ability of the core enhancer/promoter proximal elements (two NF-kappaB and three Sp1 sites) to function well in macrophages and T cells have led many to conclude that HIV LTR sequences are not primary determinants of HIV tropism. To determine if cellular specificity could be imparted to HIV by the core enhancer elements, the enhancer/promoter proximal region of the HIV LTR was substituted with motifs that control gene expression in a myeloid-specific manner. The enhancer region from equine infectious anemia virus (EIAV) when substituted for the HIV enhancer/promoter proximal region was found to drive expression in a macrophage-specific manner and was responsive to HIV Tat. The addition of a 5' methylation-dependent binding site (MDBP) and a promoter proximal Sp1 motif increased expression without altering cellular specificity. Spacing between the promoter proximal region and the TATA box was also found to influence LTR activity. Infectivity studies using chimeric LTRs within the context of a dual-tropic infectious molecular clone established that these LTRs directed HIV replication and production of infectious virions in macrophages but not primary T cells or T cell lines. This investigation demonstrates that cellular specificity can be imparted onto HIV-1 replication at the level of viral transcription and not entry.

Antigenic subclasses of polytropic murine leukemia virus (MLV) isolates reflect three distinct groups of endogenous polytropic MLV-related sequences in NFS/N mice

Polytropic murine leukemia viruses (MLVs) are generated by recombination of ecotropic MLVs with members of a family of endogenous proviruses in mice. Previous studies have indicated that polytropic MLV isolates comprise two mutually exclusive antigenic subclasses, each of which is reactive with one of two monoclonal antibodies termed MAb 516 and Hy 7. A major determinant of the epitopes distinguishing the subclasses mapped to a single amino acid difference in the SU protein. Furthermore, distinctly different populations of the polytropic MLV subclasses are generated upon inoculation of different ecotropic MLVs. Here we have characterized the majority of endogenous polytropic MLV-related proviruses of NFS/N mice. Most of the proviruses contain intact sequences encoding the receptor-binding region of the SU protein and could be distinguished by sequence heterogeneity within that region. We found that the endogenous proviruses comprise two major groups that encode the major determinant for Hy 7 or MAb 516 reactivity. The Hy 7-reactive proviruses correspond to previously identified polytropic proviruses, while the 516-reactive proviruses comprise the modified polytropic proviruses as well as a third group of polytropic MLV-related proviruses that exhibit distinct structural features. Phylogenetic analyses indicate that the latter proviruses reflect features of phylogenetic intermediates linking xenotropic MLVs to the polytropic and modified polytropic proviruses. These studies elucidate the relationships of the antigenic subclasses of polytropic MLVs to their endogenous counterparts, identify a new group of endogenous proviruses, and identify distinguishing characteristics of the proviruses that should facilitate a more precise description of their expression in mice and their participation in recombination to generate recombinant viruses.

What retroviruses teach us about the involvement of c-Myc in leukemias and lymphomas

Overexpression of the cellular oncogene c-Myc frequently occurs during induction of leukemias and lymphomas in many species. Retroviruses have enhanced our understanding of the role of c-Myc in such tumors. Leukemias and lymphomas induced by retroviruses activate c-Myc by: (1) use of virally specified proteins that increase c-Myc transcription, (2) transduction and modification of c-Myc to generate a virally encoded form of the gene, v-Myc, and (3) proviral integration in or near c-Myc. Proviral integrations elevate transcription by insertion of retroviral enhancers found in the long terminal repeat (LTR). Studies of the LTR enhancer elements from these retroviruses have revealed the importance of these elements for c-Mycactivation in several cell types. Retroviruses also have been used to identify genes that collaborate with c-Myc during development and progression of leukemias and lymphomas. In these experiments, animals that are transgenic for c-Mycoverexpression (often in combination with the overexpression or deletion of known proto-oncogenes) have been infected with retroviruses that then insertionally activate novel co-operating cellular genes. The retrovirus then acts as a molecular 'tag' for cloning of these genes. This review covers several aspects of c-Myc involvement in retrovirally induced leukemias and lymphomas.

Disruption of hematopoiesis and thymopoiesis in the early premalignant stages of infection with SL3-3 murine leukemia virus

A time course analysis of SL3-3 murine leukemia virus (SL3) infection in thymus and bone marrow of NIH/Swiss mice was performed to assess changes that occur during the early stages of progression to lymphoma. Virus was detectable in thymocytes, bone marrow, and spleen as early as 1 to 2 weeks postinoculation (p.i.). In bone marrow, virus infection was detected predominantly in immature myeloid or granulocytic cells. Flow cytometry revealed significant reductions of the Ter-119(+) and Mac-1(+) populations, and significant expansions of the Gr-1(+) and CD34(+) populations, between 2 and 4 weeks p.i. Analysis of colony-forming potential confirmed these findings. In the thymus, SL3 replication was associated with significant disruption in thymocyte subpopulation distribution between 4 and 7 weeks p.i. A significant thymic regression was observed just prior to the clonal outgrowth of tumor cells. Proviral long terminal repeats (LTRs) with increasing numbers of enhancer repeats were observed to accumulate exclusively in the thymus during the first 8 weeks p.i. Observations were compared to the early stages of infection with a virtually nonpathogenic SL3 mutant, termed SL3DeltaMyb5, which was shown by real-time PCR to be replication competent. Comparison of SL3 with SL3DeltaMyb5 implicated certain premalignant changes in tumorigenesis, including (i) increased proportions of Gr-1(+) and CD34(+) bone marrow progenitors, (ii) a significant increase in the proportion of CD4(-) CD8(-) thymocytes, (iii) thymic regression prior to tumor outgrowth, and (iv) accumulation of LTR enhancer variants. A model in which disrupted bone marrow hematopoiesis and thymopoiesis contribute to the development of lymphoma in the SL3-infected animal is discussed.

Type B leukemogenic virus has a T-cell-specific enhancer that binds AML-1

Type B leukemogenic virus (TBLV) induces rapidly appearing T-cell tumors in mice. TBLV is highly related to mouse mammary tumor virus (MMTV) except that TBLV long terminal repeats (LTRs) have a deletion of negative regulatory elements and a triplication of sequences flanking the deletion. To determine if the LTR triplication represents a viral enhancer element, we inserted the triplication upstream and downstream in either orientation relative to the thymidine kinase promoter linked to the luciferase gene. These experiments showed that upregulation of reporter gene activity by the TBLV triplication was relatively orientation independent, consistent with the activity of eukaryotic enhancer elements. TBLV enhancer activity was observed in T-cell lines but not in fibroblasts, B cells, or mammary cells, suggesting that enhancer function is cell type dependent. To analyze the transcription factor binding sites that are important for TBLV enhancer function, we prepared substitution mutations in a reconstituted C3H MMTV LTR that recapitulates the deletion observed in the TBLV LTR. Transient transfections showed that a single mutation (556M) decreased TBLV enhancer activity at least 20-fold in two different T-cell lines. This mutation greatly diminished AML-1 (recently renamed RUNX1) binding in gel shift assays with a mutant oligonucleotide, whereas AML-1 binding to a wild-type TBLV oligomer was specific, as judged by competition and supershift experiments. The 556 mutation also reduced TBLV enhancer binding of two other protein complexes, called NF-A and NF-B, that did not appear to be related to c-Myb or Ets. AML-1 overexpression in a mammary cell line enhanced expression from the TBLV LTR approximately 30-fold. These data suggest that binding of AML-1 to the TBLV enhancer, likely in combination with other factors, is necessary for optimal enhancer function.

Diversity of the HIV-1 long terminal repeat following mother-to-child transmission

A study of the human immunodeficiency virus Type 1 (HIV-1) 5' long terminal repeat (LTR) was performed to determine the extent of variation found within the LTR from 19 mother-infant pairs in Tanzania and to assess whether the LTR is useful in distinguishing maternal sequences that were transmitted to infants. HIV-1 subtypes A, C, and D as well as intersubtype recombinant LTR sequences were detected in mothers and infants. The LTR subtype was 100% concordant between mothers and their infants. Diversity calculations showed a significant reduction in LTR variation in infants compared to their mothers. However, the overall magnitude of LTR variation was less than that found in the env gene from the same individuals. These data suggest a selective constraint active upon the 5' long terminal repeat that is distinct from immune selective pressure(s) directed against HIV-1 structural genes. Detection of maternal LTR variants that were transmitted to infants may yield important information concerning nonstructural determinants of HIV-1 transmission from mother to infant.

Sequence and insertion sites of murine melanoma-associated retrovirus

We previously showed that B16 melanoma cells produce ecotropic melanoma-associated retrovirus (MelARV) which encodes a melanoma-associated antigen recognized by MM2-9B6 monoclonal antibody. The biological significance of MelARV in melanoma formation remains unknown. We found that infection of normal melanocytes with MelARV resulted in malignant transformation. It is likely that MelARV emerged from the defective Emv-2 provirus, a single copy of ecotropic provirus existing in the genome of C57BL/6 mice. In the present study, we cloned and sequenced the full-length MelARV genome and its insertion sites and we completed sequencing of the Emv-2 provirus. Our data show that MelARV has a typical full-length retroviral genome with high homology (98.54%) to Emv-2, indicating a close relationship between both viruses. MelARV probably emerged as a result of recombination between Emv-2 and an endogenous nonecotropic provirus. Some observed differences in the gag and pol regions of MelARV might account for the restoration of productivity and infectivity of a novel retrovirus that somatically emerged during melanoma formation. MelARV does not contain any oncogene and therefore might induce transformation by insertional mutagenesis. We sequenced two insertion sites of MelARV. The first insertion site represents the 3' coding region of the c-maf proto-oncogene at 67.0 centimorgans (cM) on chromosome 8. The c-maf proto-oncogene encodes a basic leucine zipper protein homologous to c-fos and c-jun. Insertion of MelARV in BL6 melanoma cells resulted in the up-regulation of c-maf. It is noteworthy that the Emv-2 provirus is also inserted into a noncoding region at 61.0 cM on the same chromosome 8. The second insertion site is the 3' noncoding region of the DNA polymerase gamma (PolG) gene on chromosome 7. The expression of PolG was not affected by the MelARV insertion. Further investigation of the biological significance of MelARV in melanoma formation is being undertaken.

Structures of endogenous nonecotropic murine leukemia virus (MLV) long terminal repeats in wild mice: implication for evolution of MLVs

To develop a better understanding of the interaction between retroviruses and their hosts, we have investigated the polymorphism in endogenous murine leukemia proviruses (MLVs). We used genomic libraries of wild mouse DNAs and PCR to analyze genetic variation in the proviruses found in wild mouse species, including Mus musculus (M. m. castaneus, M. m. musculus, M. m. molossinus, and M. m. domesticus), Mus spretus, and Mus spicelegus, as well as some inbred laboratory strains. In this analysis, we detected several unique forms of sequence organization in the U3 regions of the long terminal repeats of these proviruses. The distribution of the proviruses with unique U3 structures demonstrated that xenotropic MLV-related proviruses were present only in M. musculus subspecies, while polytropic MLV-related proviruses were found in both M. musculus and M. spretus. Furthermore, one unique provirus from M. spicelegus was found to be equidistant from ecotropic provirus and nonecotropic provirus by phylogenetic analysis. This provirus, termed HEMV, was thus likely to be related to the common ancestor of these MLVs. Moreover, an ancestral type of polytropic MLV-related provirus was detected in M. spretus species. Despite their "ancestral" phylogenetic position, proviruses of these types are not widespread in mice, implying more-recent spread by infection rather than inheritance. These results imply that recent evolution of these proviruses involved alternating periods of replication as virus and residence in the germ line.

CBF, Myb, and Ets binding sites are important for activity of the core I element of the murine retrovirus SL3-3 in T lymphocytes

Transcriptional enhancers within the long terminal repeats of murine leukemia viruses are major determinants of the pathogenic properties of these viruses. Mutations were introduced into the adjacent binding sites for three transcription factors within the enhancer of the T-cell-lymphomagenic virus SL3-3. The sites that were tested were, in 5'-to-3' order, a binding site for core binding factor (CBF) called core II, a binding site for c-Myb, a site that binds members of the Ets family of factors, and a second CBF binding site called core I. Mutation of each site individually reduced transcriptional activity in T lymphocytes. However, mutation of the Myb and core I binding sites had larger effects than mutation of the Ets or core II site. The relative effects on transcription in T cells paralleled the effects of the same mutations on viral lymphomagenicity, consistent with the idea that the role of these sequences in viral lymphomagenicity is indeed to regulate transcription in T cells. Mutations were also introduced simultaneously into multiple sites in the SL3-3 enhancer. The inhibitory effects of these mutations indicated that the transcription factor in T cells that recognizes the core I element of SL3-3, presumably CBF, needed to synergize with one or more factors bound at the upstream sites to function. This was tested further by generating a multimer construct that contained five tandem core I elements linked to a basal long terminal repeat promoter. This construct was inactive in T cells. However, transcriptional activity was detected with a multimer construct in which the transcription factor binding sites upstream of the core were also present. These results are consistent with the hypothesis that CBF requires heterologous transcription factors bound at nearby sites to function in T cells.

Exchange of viral promoter/enhancer elements with heterologous regulatory sequences generates targeted hybrid long terminal repeat vectors for gene therapy of melanoma

To generate transcriptionally targeted vectors, tissue-specific elements of the human tyrosinase promoter were exchanged with corresponding viral elements in the Moloney murine leukemia virus long terminal repeat (LTR). From these experiments, a vesicular stomatitis virus type G pseudotyped, hybrid LTR vector that contained three tyrosinase enhancer elements and gave high-level, tightly tissue-specific expression at high titers (3 x 10(7) CFU/ml) was constructed.

The feline leukemia virus long terminal repeat contains a potent genetic determinant of T-cell lymphomagenicity

Feline leukemia virus (FeLV) is an important pathogen of domestic cats. The most common type of malignancy associated with FeLV is T-cell lymphoma. SL3-3 (SL3) is a potent T-cell lymphomagenic murine leukemia virus. Transcriptional enhancer sequences within the long terminal repeats (LTRs) of SL3 and other murine retroviruses are crucial genetic determinants of the pathogenicities of these viruses. The LTR enhancer sequences of FeLV contain identical binding sites for some of the transcription factors that are known to affect the lymphomagenicity of SL3. To test whether the FeLV LTR contains a genetic determinant of lymphomagenicity, a recombinant virus that contained the U3 region of a naturally occurring FeLV isolate, LC-FeLV, linked to the remainder of the genome of SL3 was generated. When inoculated into mice, the recombinant virus induced T-cell lymphomas nearly as quickly as SL3. Moreover, the U3 sequences of LC-FeLV were found to have about half as much transcriptional activity in T lymphocytes as the corresponding sequences of SL3. This level of activity was severalfold higher than that of the LTR of weakly leukemogenic Akv virus. Thus, the FeLV LTR contains a potent genetic determinant of T-cell lymphomagenicity. Presumably, it is adapted to be recognized by transcription factors present in T cells of cats, and this yields a relatively high level of transcription that allows the enhancer to drive the requisite steps in the process of lymphomagenesis.

Differential replication of ovine lentivirus in endothelial cells cultured from different tissues

Blood-brain barrier dysfunction has been postulated to be important in the pathogenesis of HIV dementia. This study used an in vitro model of the blood-brain barrier to determine the effects of ovine lentivirus (OvLV) infection on endothelial cells. The replication of two American OvLV isolates and two lcelandic OvLV isolates in pure cultures of endothelial cells isolated from brain was compared to replication in endothelial cells from adipose, lung, and aorta. Inoculation with the two American isolates resulted in 100 times greater reverse transcriptase (RT) activity in supernatant of the microvascular endothelial cells (brain, lung, and adipose) than in the macrovascular endothelial cells (aorta). Conversely, inoculation with the two lcelandic isolates resulted in 100 times higher RT activity in aortic, lung, and adipose endothelial cells than in the brain endothelial cells. Transmission electron microscopy of the brain capillary endothelial cells infected with the American isolates revealed polarized viral budding from the lateral cell membrane and a loss of tight junctions. Replication of OvLV in brain capillary endothelial cells could play a role in the pathogenesis of lentiviral encephalitis by altering blood-brain barrier integrity.

Increased lymphomagenicity and restored disease specificity of AML1 site (core) mutant SL3-3 murine leukemia virus by a second-site enhancer variant evolved in vivo

SL3-3 is a highly T-lymphomagenic murine retrovirus. The major genetic determinant of disease is the transcriptional enhancer, which consists of a repeated region with densely packed binding sites for several transcription factors, including AML1 (also known as core binding factor and polyoma enhancer-binding protein 2) and nuclear factor 1 (NF1). Previously, we examined the enhancer structure of proviruses from murine tumors induced by SL3-3 with mutated AML1 (core) sites and found a few cases of second-site alterations. These consisted of deletions involving the NF1 sites and alterations in overall number of repeat elements, and they conferred increased enhancer strength in transient transcription assays. We have now tested the pathogenicity of a virus harboring one such second-site variant enhancer in inbred NMRI mice. It induced lymphomas with a 100% incidence and a significantly shorter latency than the AML1 mutant it evolved from. The enhancer structure thus represents the selection for a more tumorigenic virus variant during the pathogenic process. Sequencing of provirus from the induced tumors showed the new enhancer variant to be genetically stable. Also, Southern blotting showed that the tumors induced by the variant were T-cell lymphomas, as were the wild-type-induced lymphomas. In contrast, tumors induced by the original core/AML1 site I-II mutant appeared to be of non-T-cell origin and several proviral genomes with altered enhancer regions could be found in the tumors. Moreover, reporter constructs with the new tumor-derived variant could not be transactivated by AML1 in cotransfection experiments as could the wild type. These results emphasize the importance of both core/AML1 site I and site II for the pathogenic potential of SL3-3 and at the same time show that second-site alterations can form a viral variant with a substantial pathogenic potential although both AML1 sites I and II are nonfunctional.

Stability of AML1 (core) site enhancer mutations in T lymphomas induced by attenuated SL3-3 murine leukemia virus mutants

Murine retrovirus SL3-3 is highly T lymphomagenic. Its pathogenic properties are determined by the transcriptional enhancer of the U3 repeat region which shows preferential activity in T cells. Within the U3 repeats, the major determinant of T-cell specificity has been mapped to binding sites for the AML1 transcription factor family (also known as the core binding factor [CBF], polyomavirus enhancer binding protein 2 [PEBP2], and SL3-3 enhancer factor 1 [SEF-1]). SL3-3 viruses with AML1 site mutations have lost a major determinant of T-cell-specific enhancer function but have been found to retain a lymphomagenic potential, although disease induction is slower than for the SL3-3 wild type. To compare the specificities and mechanisms of disease induction of wild-type and mutant viruses, we have examined lymphomas induced by mutant viruses harboring transversions of three consecutive base pairs critical to AML1 site function (B. Hallberg, J. Schmidt, A. Luz, F. S. Pedersen, and T. Grundström. J. Virol. 65:4177-4181, 1991). Our results show that the mutated AML1 sites are genetically stable during lymphomagenesis and that ecotropic provirus numbers in DNA of tumors induced by wild-type and mutant viruses fall within the same range. Moreover, proviruses were found to be integrated at the c-myc locus in similar proportions of wild-type and mutant SL3-3-induced tumors, and the mutated AML1 sites of proviruses at c-myc are unaltered. In some cases, however, including one c-myc-integrated provirus, a single-base pair change was detected in a second, weaker AML1 binding site. By DNA rearrangement analysis of the T-cell receptor beta-locus, tumors induced by the AML1 site mutants are found to be of the T-cell type. Thus, although the AML1 site mutants have weakened T-cell-specific enhancers they are T-lymphomagenic, and wild-type- and mutant-virus-induced tumor DNAs are similar with respect to the number of overall ecotropic and c-myc-integrated clonal proviruses. The SL3-3 wild-type and AML1 site mutant viruses may therefore induce disease by similar mechanisms.

Importance of a c-Myb binding site for lymphomagenesis by the retrovirus SL3-3

All murine leukemia viruses (MuLVs) and related type C retroviruses contain a highly conserved binding site for the Ets family of transcription factors within the enhancer sequences in the viral long terminal repeats (LTRs). The T-cell lymphomagenic MuLV SL3-3 (SL3-3) also contains a c-Myb binding site adjacent to the Ets site. The presence of this Myb site distinguishes SL3 from most other MuLVs. We tested the importance of these two sites for the lymphomagenicity of SL3-3. Mutation of the Ets site had little effect on viral pathogenicity, as it only slightly extended the latency period to disease onset. In contrast, mutation of the Myb site strongly inhibited pathogenicity, as only a minority of the inoculated mice developed tumors in the two mouse strains that were tested. All tumors that were induced by either mutant appeared to be lymphomas, and no evidence for reversion of either mutation was detected. The effects of the Ets and Myb site mutations on transcriptional activity of the SL3 LTR were tested by inserting the viral enhancer sequences into a plasmid containing the promoter region of the c-myc gene linked to a reporter gene. Mutation the Myb site almost eliminated enhancer activity in T lymphocytes, while mutation of the Ets site had smaller effects. Thus, the effects of the enhancer mutations on transcriptional activity in T cells paralleled their effects on viral lymphomagenicity. The absence of the c-Myb site in the LTR enhancer of the weakly lymphomagenic MuLV, Akv, likely contributes to the low pathogenicity of this virus relative to SL3-3. However, Moloney MuLV also lacks the Myb site in its LTR, although it induces T-cell lymphomas with a potency similar to that of SL3-3. Thus, it appears that SL3-3 and Moloney MuLV evolved genetic determinants of T-cell lymphomagenicity that are, at least in part, distinct.

Second-site proviral enhancer alterations in lymphomas induced by enhancer mutants of SL3-3 murine leukemia virus: negative effect of nuclear factor 1 binding site

SL3-3 is a highly T-lymphomagenic murine retrovirus. Previously, mutation of binding sites in the U3 repeat region for the AML1 transcription factor family (also known as core binding factor [CBF], polyomavirus enhancer binding protein 2 [PEBP2], and SL3-3 enhancer factor 1 [SEF1]) were found to strongly reduce the pathogenicity of SL3-3 (B. Hallberg, J. Schmidt, A. Luz, F. S. Pedersen, and T. Grundström, J. Virol. 65:4177-4181, 1991). We have now examined the few cases in which tumors developed harboring proviruses that besides the AML1 (core) site mutations carried second-site alterations in their U3 repeat structures. In three distinct cases we observed the same type of alteration which involved deletions of regions known to contain binding sites for nuclear factor 1 (NF1) and the addition of extra enhancer repeat elements. In transient-expression experiments in T-lymphoid cells, these new U3 regions acted as stronger enhancers than the U3 regions of the original viruses. This suggests that the altered proviruses represent more-pathogenic variants selected for in the process of tumor formation. To analyze the proviral alterations, we generated a series of different enhancer-promoter reporter constructs. These constructs showed that the additional repeat elements are not critical for enhancer strength, whereas the NF1 sites down-regulate the level of transcription in T-lymphoid cells whether or not the AML1 (core) sites are functional. We therefore also tested SL3-3 viruses with mutated NF1 sites. These viruses have unimpaired pathogenic properties and thereby distinguish SL3-3 from Moloney murine leukemia virus.

Mapping of a major osteomagenic determinant of murine leukemia virus RFB-14 to non-long terminal repeat sequences

Certain isolates of murine leukemia viruses (MuLVs) have, apart from a leukemogenic potential, the capability of inducing diseases of nonhematopoietic tissues in susceptible strains of mice. We have reported on the molecular cloning of a bone-tumorigenic virus, RFB-14 MuLV, which was found to induce benign bone tumors, osteomas, with 100% incidence in mice of the CBA/Ca strain (L. Pedersen, W. Behnisch, J. Schmidt, A. Luz, F. S. Pedersen, V. Erfle, and P. G. Strauss, J. Virol. 66:6186-6190, 1992). In order to analyze the bone tumor-inducing phenotype of RFB-14 MuLV, we have studied the pathogenic potential of recombinant viruses between RFB-14 and the nonosteomagenic, highly leukemogenic SL3-3 MuLV. The recombinants were constructed so as to reveal whether a major determinant of osteomagenicity maps to sequences within or outside the long terminal repeats (LTR). Our data show that a major determinant of the osteoma-inducing potential of RFB-14 MuLV maps to the non-LTR region of the genome. Furthermore, we demonstrate that a strong determinant of leukemogenicity is harbored by the non-LTR region of SL3-3 MuLV.

Transcriptional activation of a retrovirus enhancer by CBF (AML1) requires a second factor: evidence for cooperativity with c-Myb

Transcriptional enhancer sequences within the long terminal repeats (LTRs) of murine leukemia viruses are the primary genetic determinants of the tissue specificity and potency of the oncogenic potential of these retroviruses. SL3-3 (SL3) is a murine leukemia virus that induces T-cell lymphomas. The LTR enhancer of this virus contains two binding sites for the transcription factor CBF (also called AML1 and PEBP2) that flank binding sites for c-Myb and the Ets family of factors. Using cotransfection assays in P19 cells, we report here that CBF and c-Myb cooperatively stimulate transcription from the SL3 LTR. By itself, c-Myb had no stimulatory effect on transcription. However, when cotransfected with a cDNA encoding one form of the alpha subunit of CBF called CBFalpha2-451, a level of transactivation higher than that seen with CBFalpha2-451 alone was detected. The negative regulatory domain near the carboxyl terminus of c-Myb did not affect this activity. Electrophoretic mobility shift assays indicated that CBF and c-Myb bind to DNA independently. Therefore, it appears that the cooperative stimulation of transcription by these factors occurs at a step in the process of transcription after the two factors are bound to the enhancer. Sequences near the carboxyl terminus of CBFalpha2-451 were important for cooperativity with c-Myb, consistent with previous reports that this region contains an activation domain. However, CBFalpha2-451 failed to activate transcription from a version of the SL3 LTR in which the enhancer was replaced with five tandem CBF-binding sites. Thus, it appears that transcriptional activation of the SL3 enhancer by CBF requires that an appropriate heterologous transcription factor be bound to a neighboring site in the regulatory sequences.

A putative receptor for Venezuelan equine encephalitis virus from mosquito cells

We have identified a cellular protein from a continuous mosquito cell line (C6/36) that appears to play a significant role in the attachment of Venezuelan equine encephalitis (VEE) virus to these cells. VEE virus bound to a 32-kDa polypeptide present in the C6/36 plasma membrane fraction, and binding to this polypeptide was dose dependent and saturable and competed with homologous and heterologous alphaviruses. These observations suggest that this polypeptide binds virus via a receptor-ligand interaction. The 32-kDa polypeptide was expressed on the surfaces of C6/36 cells, and monoclonal antibodies directed against either this cell polypeptide or the VEE virus E2 glycoprotein, which is thought to be the viral attachment protein, interfered with virus attachment. Collectively, these data provide evidence suggesting that the 32-kDa polypeptide serves as a receptor for VEE virus infection of cells. We have characterized this cell polypeptide as a laminin-binding protein on the basis of its ability to interact directly with laminin as well as its immunologic cross-reactivity with the high-affinity human laminin receptor.

E- vectors: development of novel self-inactivating and self-activating retroviral vectors for safer gene therapy

We have developed novel self-inactivating and self-activating retroviral vectors based on the previously observed high-frequency deletion of direct repeats. We constructed spleen necrosis virus (SNV)-based viral vectors that contained large direct repeats flanking the viral encapsidation sequence (E). A large proportion of the proviruses in the target cells had E and one copy of the direct repeat deleted. Direct repeats of 1,333 and 788 bp were deleted at frequencies of 93 and 85%, respectively. To achieve a 100% deletion efficiency in target cells after ex vivo infection and drug selection, we constructed a self-activating vector that simultaneously deleted E and reconstituted the neomycin phosphotransferase gene. Selection of the target cells for resistance to G418 (a neomycin analog) ensured that all integrated proviruses had E deleted. The proviruses with E deleted were mobilized by a replication-competent virus 267,000-fold less efficiently than proviruses with E. We named these self-inactivating vectors E- (E-minus) vectors. These vectors should increase the safety of retroviral vector-mediated gene therapy by preventing the spread of vector sequences to nontarget cells in the event of coinfection with helper virus. We propose that direct-repeat deletions occur during RNA-dependent DNA synthesis and suggest that template switches occur without a requirement for RNA breaks. The minimum template dissociation frequency was estimated as 8%/100 bp per replication cycle. These vectors demonstrate that large direct repeats and template-switching properties of reverse transcriptase can be utilized to delete any sequence or reconstitute genes during retroviral replication.

Amplified and tissue-directed expression of retroviral vectors using ping-pong techniques

Ping-pong amplification is an efficient process by which helper-free retrovirions replicate in cocultures of cell lines that package retroviruses into distinct host-range envelopes [11]. Transfection of a retroviral vector DNA into these cocultures results in massive virus production, with potentially endless cross-infection between different types of packaging cells. Because the helper-free virus spreads efficiently throughout the coculture, it is unnecessary to use dominant selectable marker genes, and the retroviral vectors can be simplified and optimized for expressing a single gene of interest. The most efficient ping-pong vector, pSFF, derived from the Friend erythroleukemia virus, has been used for high-level expression of several genes that could not be expressed with commonly employed two-gene retroviral vectors. Contrary to previous claims, problems of vector recombination are not inherent to ping-pong methods. Indeed, the pSFF vector has not formed replication-competent recombinants as shown by stringent assays. Here we review these methods, characterize the ping-pong process using the human erythropoietin gene as a model, and describe a new vector (pSFY) designed for enhanced expression in T lymphocytes. Factors that limit tissue-specific expression are reviewed.

Long terminal repeat enhancer core sequences in proviruses adjacent to c-myc in T-cell lymphomas induced by a murine retrovirus

The transcriptional enhancer in the long terminal repeat (LTR) of the T-lymphomagenic retrovirus SL3-3 differs from that of the nonleukemogenic virus Akv at several sites, including a single base pair difference in an element termed the enhancer core. Mutation of this T-A base pair to the C-G C-G sequence found in Akv significantly attenuated the leukemogenicity of SL3-3. Thus, this difference is important for viral leukemogenicity. Since Akv is an endogenous virus, this suggests that the C-G in its core is an adaptation to being minimally pathogenic. Most tumors that occurred in mice inoculated with the mutant virus, called SAA, contained proviruses with reversion or potential suppressor mutations in the enhancer core. We also found that the 72-bp tandem repeats constituting the viral enhancer could vary in number. Most tumors contained mixtures of proviruses with various numbers of 72-bp units, usually between one and four. Variation in repeat number was most likely due to recombination events involving template misalignment during viral replication. Thus, two processes during viral replication, misincorporation and recombination, combined to alter LTR enhancer structure and generate more pathogenic variants from the mutant virus. In SAA-induced tumors, enhancers of proviruses adjacent to c-myc had the largest number of core reversion or suppressor mutations of all of the viral enhancers in those tumors. This observation was consistent with the hypothesis that one function of the LTR enhancers in leukemogenesis is to activate proto-oncogenes such as c-myc.

Basic helix-loop-helix proteins in murine type C retrovirus transcriptional regulation

E boxes, recognition sequences for basic helix-loop-helix (bHLH) transcription factors, are detected in the enhancer and promoter regions of several murine type C retroviruses. Here we show that ALF1, a member of bHLH protein family of transcription factors, in vitro binds with differing affinities to distinct E-box sequences found in the U3 regulatory regions of Friend, Moloney, SL3-3, and Akv murine leukemia viruses (MLVs) as well as Friend spleen focus-forming virus (SFFV). In NIH 3T3 fibroblasts, ALF1 overexpression elevated transcription from the U3 region of Moloney MLV and the complete long terminal repeat regions of Friend SFFV, Akv MLV, and SL3-3 MLV but neither from the U3 region nor from the complete long terminal repeat of Friend MLV. Introduction of mutations in the Akv MLV E boxes showed the E-box cis elements to be required for the function of ALF1 as a transcription factor. ALF1 and the glucocorticoid receptor, with overlapping DNA binding sequences, did not act synergistically with respect to transcriptional trans activation of expression from the Akv MLV promoter-enhancer region. We conclude that ALF1 in vivo may be an important transcription regulator for Akv, SL3-3, and Moloney MLVs as well as for Friend SFFV.

Characterization of epitopes defining two major subclasses of polytropic murine leukemia viruses (MuLVs) which are differentially expressed in mice infected with different ecotropic MuLVs

Polytropic murine leukemia viruses (MuLVs) arise in mice by recombination of ecotropic MuLVs with endogenous retroviral envelope genes and have been implicated in the induction of hematopoietic proliferative diseases. Inbred mouse strains contain many endogenous sequences which are homologous to the polytropic env genes; however, the extent to which particular sequences participate in the generation of the recombinants is unknown. Previous studies have established antigenic heterogeneity among the env genes of polytropic MuLVs, which may reflect recombination with distinct endogenous genes. In the present study, we have examined many polytropic MuLVs and found that nearly all isolates fall into two mutually exclusive antigenic subclasses on the basis of the ability of their SU proteins to react with one of two monoclonal antibodies, termed Hy 7 and MAb 516. Epitope-mapping studies revealed that reactivity to the two antibodies is dependent on the identity of a single amino acid residue encoded in a variable region of the receptor-binding domain of the env gene. This indicated that the two antigenic subclasses of MuLVs arose by recombination with distinct sets of endogenous genes. Evaluation of polytropic MuLVs in mice revealed distinctly different ratios of the two subclasses after inoculation of different ecotropic MuLVs, suggesting that individual ecotropic MuLVs preferentially recombine with distinct sets of endogenous polytropic env genes.

Pathogenic determinants in the U3 region of recombinant murine leukemia viruses isolated from CWD and HRS/J mice

Recombinant murine leukemia viruses (MuLVs) from high-leukemia-incidence mouse strains typically acquire pathogenic U3 region sequences from the genome of the endogenous xenotropic virus, Bxv-1. However, a recombinant virus isolated from a leukemic HRS/J mouse and another from a CWD mouse contained U3 regions that lacked genetic markers of Bxv-1. The U3 regions of both recombinants were derived from the endogenous ecotropic virus Env-1 and had retained a single enhancer element. However, compared with that of Emv-1, the U3 region of each of the recombinant viruses contained five nucleotide substitutions, one of which was shared. To determine the biological significance of these substitutions, chimeric ecotropic viruses that contained the U3 region from one of the two recombinant viruses or from Emv-1 were injected into NIH Swiss mice. All three of the chimeric ecotropic viruses were leukemogenic following a long latency. Despite the presence of an enhancer core motif that is known to contribute to the leukemogenicity of the AKR MuLV SL3-3, the HRS/J virus U3 region induced lymphomas only slightly more rapidly than the allelic Emv-1 sequences. The chimeric virus with the U3 region of the CWD recombinant caused lymphomas more frequently and more rapidly than either of the other two viruses. The results support the hypothesis that one or more of the five nucleotide substitutions in the U3 regions of the recombinants contribute to viral pathogenicity. Comparison of DNA sequences suggests that the pathogenicity of the CWD virus U3 region was related to a sequence motif that is shared with Bxv-1 and is recognized by the basic helix-loop-helix class of transcription factors.

Identification of human T-cell lymphotropic virus type I 21-base-pair repeat-specific and glial cell-specific DNA-protein complexes

The human T-cell lymphotropic virus type I (HTLV-I)-encoded protein, Tax, is capable of trans-activating HTLV-I transcription by interacting with specific sequences in the HTLV-I long terminal repeat (LTR) which comprise an inducible enhancer containing three imperfect tandem repeats of a 21-bp sequence. There is no evidence that purified Tax can bind to DNA in the absence of cellular factors, suggesting that Tax most likely regulates transcription via interaction with cellular factors. Since HTLV-I is a documented agent of adult T-cell leukemia and tropical spastic paraparesis, disorders of the immune and nervous systems, respectively, characterization of cellular factors of lymphoid and neuroglial origin which interact with the 21-bp repeat elements is essential to understanding of the mechanisms involved in basal and Tax-mediated transcription in cells of immune and nervous system origin. Utilizing electrophoretic mobility shift (EMS) analyses, we have detected both 21-bp repeat-specific and glial cell-specific DNA-protein complexes. Several 21-bp repeat-specific DNA-protein complexes were detected when nuclear extracts derived from cells of lymphoid (Jurkat, SupT1, and H9), neuronal (IMR-32 and SK-N-MC), and glial (U-373 MG, Hs683, and U-118) origin were used in reactions with each of the three 21-bp repeat elements. In addition, a glial cell-specific DNA-protein complex was detected when nuclear extracts derived from U-373 MG, Hs683, and U-118 glial cell lines reacted with the promoter-distal and central 21-bp repeat elements. Furthermore, EMS analyses performed with nuclear extracts derived from lymphocytic and glial cell origin and a 223-bp fragment of the HTLV-I long terminal repeat encompassing the three 21-bp repeat elements (designated Tax-responsive elements 1 and 2, TRE-1/-2) have also resulted in the detection of glial cell type-specific DNA-protein complexes. Competition EMS analyses with oligonucleotides containing transcription factor binding site sequences indicate the involvement of a cyclic AMP response element binding protein in the formation of DNA-protein complexes which form with all three 21-bp repeat elements and the glial cell-specific DNA-protein complex as well as the involvement of Sp1 or an Sp1-related factor in the formation of the 21-bp repeat III-specific DNA-protein complexes.

Retroviral vectors containing chimeric promoter/enhancer elements exhibit cell-type-specific gene expression

Retroviral vectors were constructed in which the U3 promoter/enhancer of Moloney murine leukemia (Mo-MLV) was replaced by the corresponding region from five related murine retroviruses--AKR murine leukemia virus (AKV), Harvey murine sarcoma virus (HaMSV), myeloproliferative sarcoma virus (MPSV), SL3-3, and the NZB-xenotropic virus (Xeno). In these vectors the chimeric long terminal repeat (chLTR) drives the expression of the chloramphenicol acetyl transferase (CAT) reporter gene that is followed by an internal SV40 virus early region promoter linked to the neomycin phosphotransferase II (NEO) gene. As an initial measure of the relative promoter/enhancer strength of the chLTR vectors, the murine NIH-3T3 cell line and the human JURKAT cell lines were transfected and assayed for CAT reporter activity. Relative to the MoMLV vector, the HaMSV construct was the most active in NIH-3T3 cells whereas the SL3-3 vector displayed the greatest activity in JURKAT cells. Retroviral vector producer cell populations and cell clones were established for each chLTR vector, and all were capable of yielding high vector titers (> 10(5) G418R cfu/ml on NIH-3T3). Supernatant from these cells was used to transduce both mouse and human cell lines and primary cells. In NIH-3T3 cells and two murine fibrosarcoma cell lines, the HaMSV chLTR vector was slightly more active than the MoMLV chLTR vector. In the human HepG2 and HeLa cell lines, the MPSV chLTR vector was the most active. Data from the human JURKAT T-cell line and a T cell line derived from an ADA-deficient severe combined immunodeficiency (SCID) patient demonstrate that the SL3-3 chLTR is the most active in these lymphoid cell lines. The greatest difference in the comparison of the different chLTR vectors was observed in primary human umbilical vein endothelial cells, where the MoMLV vector produced up to 100 times more CAT activity than the SL3-3 vector. These data suggest that the use of specific promoter/enhancer elements may lead to higher levels of gene expression following retroviral-mediated gene transfer into specific cell types and these observations may be useful in the design of human gene therapy experiments.

Negative regulation of the 5' long terminal repeat (LTR) by the 3' LTR in the murine proviral genome

To assess the influence of the 3' long terminal repeat (LTR) on the promoter/enhancer activity of the 5' LTR, a set of isogenic retroviral vectors differing only in the U3 region of the 3' LTR was constructed. These U3 elements were derived from viruses with different tissue tropism. The 5' LTR originated from Moloney murine leukemia virus and directed the transcription of a reporter gene (chloramphenicol acetyltransferase [CAT] gene), giving rise to plasmids of the general configuration LTR-CAT-LTR'. Following transfection of these chimeric constructs into various cell types, the CAT activity in a given cell line was inversely related to the activity of the downstream U3 region when used in a single-LTR construct in that cell type, indicating negative regulation of the 5' LTR by the chimeric 3' LTR'. Our data indicate that a highly active 3' LTR interferes with gene expression from the 5' LTR. Potential mechanisms for this down-regulation are discussed.

Functional Analysis of the ACTGCTGA Sequence Motif in the Human Immunodeficiency Virus Type-1 Long Terminal Repeat Promoter

The ACTGCTGA sequence (CTG motif) is located immediately upstream of the NF-kappaB enhancer in the human immunodeficiency virus type-1 (HIV-1) long terminal repeat (LTR). We previously reported on the frequent duplication of this motif in HIV-1-infected individuals. In this study we further characterized the role of the CTG element in transcription and its interaction with cellular proteins. We analyzed the biological activity of LTR promoters with dimeric, monomeric or deleted CTG motifs. Our results indicate that LTRs containing the monomeric CTG motif are the most active transcriptional promoters. Furthermore, mutant viruses with dimeric or deleted CTG motif were consistently out-competed by the wild-type virus in co-culture experiments. Gel mobility shift assays were used to identify a nuclear protein of approximately 68 kD that specifically interacts with this DNA sequence. Copyright 1994 S. Karger AG, Basel

Viral determinants of simian immunodeficiency virus (SIV) virulence in rhesus macaques assessed by using attenuated and pathogenic molecular clones of SIVmac

To identify viral determinants of simian immunodeficiency virus (SIV) virulence, two pairs of reciprocal recombinants constructed from a pathogenic (SIVmac239) and a nonpathogenic (SIVmac1A11) molecular clone of SIV were tested in rhesus macaques. A large 6.2-kb fragment containing gag, pol, env, and the regulatory genes from each of the cloned (parental) viruses was exchanged to produce one pair of recombinant viruses (designated SIVmac1A11/239gag-env/1A11 and SIVmac239/1A11gag-env/239 to indicate the genetic origins of the 5'/internal/3' regions, respectively, of the virus). A smaller 1.4-kb fragment containing the external env domain of each of the parental viruses was exchanged to create the second pair (SIVmac1A11/239env/1A11 and SIVmac239/1A11env/239) of recombinant viruses. Each of the two parental and four recombinant viruses was inoculated intravenously into four rhesus macaques, and all 24 animals were viremic by 4 weeks postinoculation (p.i.). Virus could not be isolated from peripheral blood mononuclear cells (PBMC) of any animals infected with SIVmac1A11 after 6 weeks p.i. but was consistently isolated from all macaques inoculated with SIVmac239 for 92 weeks p.i. Virus isolation was variable from animals infected with recombinant viruses; SIVmac1A11/239gag-env/1A11 and SIVmac239/1A11env/239 were isolated most frequently. Animals inoculated with SIVmac239 had 10 to 100 times more virus-infected PBMC than those infected with recombinant viruses. Three animals infected with SIVmac239 died with simian AIDS (SAIDS) during the 2-year observation period after inoculation, and the fourth SIVmac239-infected animal had clinical signs of SAIDS. Two animals infected with recombinant viruses died with SAIDS; one was infected with SIVmac239/1A11gag-env/239, and the other was infected with SIVmac1A11/239gag-env/1A11. The remaining 18 macaques remained healthy by 2 years p.i., and 13 were aviremic. One year after inoculation, peripheral lymph nodes of some of these healthy, aviremic animals harbored infected cells. All animals seroconverted within the first few weeks of infection, and the magnitude of antibody response to SIV was proportional to the levels and duration of viremia. Virus-suppressive PBMC were detected within 2 to 4 weeks p.i. in all animals but tended to decline as viremia disappeared. There was no association of levels of cell-mediated virus-suppressive activity and either virus load or disease progression. Taken together, these results indicate that differences in more than one region of the viral genome are responsible for the lack of virulence of SIVmac1A11.

Expression directed from HIV long terminal repeats in the central nervous system of transgenic mice

Infection with the human immunodeficiency virus (HIV) is frequently accompanied by the AIDS (acquired immunodeficiency syndrome) dementia complex. The role of specific HIV genetic elements in the pathogenesis of central nervous system (CNS) disease is not clear. Transgenic mice were constructed that contained the long terminal repeats (LTRs) of two CNS-derived strains and a T cell tropic strain of HIV-1. Only mice generated with CNS-derived LTRs directed expression in the CNS, particularly in neurons. Thus, some strains of HIV-1 have a selective advantage for gene expression in the brain, and neurons can supply the cellular factors necessary for their transcription.

Natural variants of the HIV-1 long terminal repeat: analysis of promoters with duplicated DNA regulatory motifs

Sequence variation in the long terminal repeat (LTR) region of HIV-1 was analyzed in viral isolates of 17 infected individuals. Two classes of LTR size variants were found. One HIV-1 variant was detected containing an additional binding site for the transcription factor Sp1. Another LTR size variation was observed in four patients in a region just upstream of the NF-kappa B enhancer. This variation was the result of a duplication of a short DNA sequence (CTG-motif). Cell culture experiments demonstrated that the natural variant with four Sp1 sites had a slightly higher promoter activity and viral replication rate than the isogenic control LTR with three Sp1 sites. No positive effect of the duplicated CTG-motif could be detected. In order to measure small differences in virus production more accurately, equal amounts of a size variant and the wild-type plasmid were cotransfected into T-cells. The virus with four Sp1 sites did outgrow the three Sp1 virus in 35 days of culture and CTG-monomer virus outcompeted the CTG-dimer virus in 42 days. Based on these results we estimate a 5-10% difference in virus production of the LTR variants when compared to that of wild-type.

A viral long terminal repeat expressed in CD4+CD8+ precursors is downregulated in mature peripheral CD4-CD8+ or CD4+CD8- T cells

The long terminal repeat from a thymotropic mouse mammary tumor virus variant, DMBA-LV, was used to drive the expression of two reporter genes, murine c-myc and human CD4, in transgenic mice. Expression was observed specifically in thymic immature cells. Expression of c-myc in these cells induced oligoclonal CD4+ CD8+ T-cell thymomas. Expression of human CD4 was restricted to thymic progenitor CD4- CD8- and CD4+ CD8+ T cells and was shut off in mature CD4+ CD8- and CD4- CD8+ T cells, known to be derived from the progenitor double-positive T cells. These results suggest the existence of similar and common factors in CD4+ CD8- and CD4- CD8+ T cells and support a model of differentiation of CD4+ CD8+ T cells through common signal(s) involved in turning off the expression of the CD4 or CD8 gene.

Animal models for anti-AIDS therapy

Primate and non-primate species have been used to study the pathobiology of the simian immunodeficiency virus (SIV) and of the human immunodeficiency virus type 1 (HIV-1), respectively, and to develop new therapeutic regimes. Transgenic mice which express either the entire HIV-1 provirus or subgenomic fragments have been used to analyze viral gene products in vivo and may serve as models for the development of agents targeted to select viral functions. Chimeric mice which were created by transplanting human hematolymphoid cells into mice suffering from congenital severe combined immunodeficiency (scid/scid or so called SCID mice), can be infected with HIV-1 and allow one to study the entire HIV-1 replicative cycle. Type C murine leukemia virus models have been used to develop new prophylactic and therapeutic strategies but their use is restricted to the evaluation of select antiviral drug inhibition, targeted to retroviral genes common to both Lentivirinae and Oncovirinae. The role of various animal model systems in the development of anti-HIV-1 and anti-AIDS therapies is summarized.

A viral long terminal repeat expressed in CD4+CD8+ precursors is downregulated in mature peripheral CD4-CD8+ or CD4+CD8- T cells

The long terminal repeat from a thymotropic mouse mammary tumor virus variant, DMBA-LV, was used to drive the expression of two reporter genes, murine c-myc and human CD4, in transgenic mice. Expression was observed specifically in thymic immature cells. Expression of c-myc in these cells induced oligoclonal CD4+ CD8+ T-cell thymomas. Expression of human CD4 was restricted to thymic progenitor CD4- CD8- and CD4+ CD8+ T cells and was shut off in mature CD4+ CD8- and CD4- CD8+ T cells, known to be derived from the progenitor double-positive T cells. These results suggest the existence of similar and common factors in CD4+ CD8- and CD4- CD8+ T cells and support a model of differentiation of CD4+ CD8+ T cells through common signal(s) involved in turning off the expression of the CD4 or CD8 gene.

Development of antiviral treatment strategies in murine models

Murine models with type C murine leukemia viruses have been used to develop major new prophylactic and therapeutic strategies in vaccination, drug therapy of acute virus exposure and chronic viremia, combination therapy, prevention of maternal transmission, and therapy targeted to the central nervous system. Transgenic mice expressing either the whole human immunodeficiency virus type 1 (HIV-1) provirus or subgenomic sequences allow the in vivo analysis of selected HIV-1 functions. The full replicative cycle of HIV-1 can be studied in human/mouse chimerae which were created by transplanting human hematolymphoid cells into SCID mice. The chimeric SCID mouse models have been used successfully to evaluate anti-HIV-1 drugs. The role of the various murine retrovirus systems in the development of anti-HIV-1 and anti-AIDS therapies is summarized.

Retrovirus-mediated gene expression in mammalian cells

Significant advances have been made in precisely defining the elements in the Moloney murine leukemia virus genome responsible for tissue-restricted expression. This knowledge should lead to improved expression vectors for gene transfer in mammalian cells. In the past year, retrovirus-mediated gene expression in a diverse range of cell types has been reported. These cells have been used to study gene transfer relevant to a range of inherited diseases.

Alteration from T- to B-cell tropism reduces thymic atrophy and cytocidal effects in thymocytes but not neurovirulence induced by ts1, a mutant of Moloney murine leukemia virus TB

The ts1 mutant of Moloney murine leukemia virus TB causes degenerative neurologic and immunologic disease in mice, characterized by development of spongiform encephalomyelopathy resulting in hindlimb paralysis, marked thymic atrophy associated with immunodeficiency, and generalized body wasting. To investigate the pathogenesis of the thymic atrophy caused by ts1, we constructed a chimeric virus, ts1-Cas(NS), in which a major portion of the U3 region of the long terminal repeat of ts1, a T-lymphotropic and neurovirulent murine leukemia virus, was replaced by the corresponding U3 region of Cas-Br-E, a B-lymphotropic and neurovirulent murine leukemia virus. In FVB/N mice, ts1-Cas(NS) induced paralytic and wasting disease with incidence, severity, and latency similar to that induced by ts1, but it failed to cause thymic atrophy as severe as that observed in ts1-infected mice. Furthermore, thymocytes cultured from ts1-Cas(NS)-infected mice died at a much slower rate than those of ts1-infected mice. The U3 substitution in ts1-Cas(NS) specifically diminished the ability of the virus to replicate in the thymus, whereas viral replication in the spinal cord was not significantly affected; thus, neurovirulence was not changed. The correlation of reduced thymic atrophy with decreased thymic viral titers and the decreased ability of ts1-Cas(NS) to cause thymocyte death in mice suggest strongly that the marked thymic atrophy in ts1-infected mice is not an indirect effect occurring secondary to neurodegenerative and wasting disease but is a direct cytopathic effect of high-level viral replication in the thymus.

SL3-3 enhancer factor 1 transcriptional activators are required for tumor formation by SL3-3 murine leukemia virus

The transcriptional enhancers of retroviruses that lack an oncogene are important determinants of their oncogenicity. However, no specific cellular transcriptional activator has yet been found to determine the oncogenicity for any of these viruses. The SL3-3 enhancer factor 1 (SEF1) cellular transcriptional activators are expressed preferentially in T lymphocytes. In the SL3-3 murine leukemia virus enhancer, two different sequences can bind SEF1 activators. We show that mutation of the SEF1 binding sites disrupts the disease potential of SL3-3 murine leukemia virus, implying that SEF1 transcriptional activators are required for tumor induction by SL3-3. The SEF1 site mutations did not appear to affect the pathogenicity of SL3-3 by impairment of virus multiplication, but rather by a specific defect in the ability of neoplastic transformation.