Cell specificity of the transcription-factor repertoire used by a lentivirus: motifs important for expression of equine infectious anemia virus in nonmonocytic cells

[Mouse embryonic stem cells - a new cellular system for studying the equine infectious anemia virus in vitro and in vivo]

The complexity of the pathogenesis and insufficient knowledge about the slow retroviral infections, which include equine infectious anemia, necessitates finding an adequate laboratory model for the study of the infection process and immunogenesis to create means of prevention and treatment of diseases. Data about strains and cellular tropism of the virus are discussed. It was shown that mouse embryonic stem cells (ESCS) exhibited unique properties and characteristics. In contrast to fibroblasts and other cell types, these cells can be considered as a new cell system for studying EIAV in vitro and in vivo. Under differentiation-inducing conditions they are able to reproduce in vitro embryogenesis cells and form cells of three germ layers. Differentiation of mouse ESCs in the direction of hematopoiesis could contribute new knowledge and understanding of viral tropism EIAV in vitro. ESC can be returned back to the early pre-implantation embryo. Once in the germ cell environment, they participate in the formation of tissues and organs of the developing fetus. Thus, the adaptation of the mouse ESC to the equine EIAV through genetic transformation makes it possible to get closer to the creation of a laboratory model for the study of the in vivo immune response in the lentiviral infection.

Genetic characterization by composite sequence analysis of a new pathogenic field strain of equine infectious anemia virus from the 2006 outbreak in Ireland

Equine infectious anemia virus (EIAV), the causative agent of equine infectious anaemia (EIA), possesses the least-complex genomic organization of any known extant lentivirus. Despite this relative genetic simplicity, all of the complete genomic sequences published to date are derived from just two viruses, namely the North American EIAVWYOMING (EIAVWY) and Chinese EIAVLIAONING (EIAVLIA) strains. In 2006, an outbreak of EIA occurred in Ireland, apparently as a result of the importation of contaminated horse plasma from Italy and subsequent iatrogenic transmission to foals. This EIA outbreak was characterized by cases of severe, sometimes fatal, disease. To begin to understand the molecular mechanisms underlying this pathogenic phenotype, complete proviral genomic sequences in the form of 12 overlapping PCR-generated fragments were obtained from four of the EIAV-infected animals, including two of the index cases. Sequence analysis of multiple molecular clones produced from each fragment demonstrated the extent of diversity within individual viral genes and permitted construction of consensus whole-genome sequences for each of the four viral isolates. In addition, complete env gene sequences were obtained from 11 animals with differing clinical profiles, despite exposure to a common EIAV source. Although the overall genomic organization of the Irish EIAV isolates was typical of that seen in all other strains, the European viruses possessed ≤80 % nucleotide sequence identity with either EIAVWY or EIAVLIA. Furthermore, phylogenetic analysis suggested that the Irish EIAV isolates developed independently of the North American and Chinese viruses and that they constitute a separate monophyletic group.

Genetic variation in the long terminal repeat associated with the transition of Chinese equine infectious anemia virus from virulence to avirulence

A highly virulent strain of equine infectious anemia virus (EIAV) lost its fatal virulence but retained the desired antigens during serial passage over 130 generations in leukocytes in vitro. We compared the long terminal repeat (LTR) sequences of the different generations and found that three stable genetic variations occurred in the transcriptional start site, the initial base of TAR, and the pre-mRNA cleavage site at the R-U5 boundary, respectively. These three mutations happened at the inflexion of virus pathogenicity loss; therefore, the function of these mutations needs to be further addressed.

Long terminal repeat sequences from virulent and attenuated equine infectious anemia virus demonstrate distinct promoter activities

In the early 1970s, the Chinese Equine Infectious Anemia Virus (EIAV) vaccine, EIAV(DLA), was developed through successive passages of a wild-type virulent virus (EIAV(L)) in donkeys in vivo and then in donkey macrophages in vitro. EIAV attenuation and cell tropism adaptation are associated with changes in both envelope and long terminal repeat (LTR). However, specific LTR changes during Chinese EIAV attenuation have not been demonstrated. In this study, we compared LTR sequences from both virulent and attenuated EIAV strains and documented the diversities of LTR sequence from in vivo and in vitro infections. We found that EIAV LTRs of virulent strains were homologous, while EIAV vaccine have variable LTRs. Interestingly, experimental inoculation of EIAV(DLA) into a horse resulted in a restriction of the LTR variation. Furthermore, LTRs from EIAV(DLA) showed higher Tat transactivated activity than LTRs from virulent strains. By using chimeric clones of wild-type LTR and vaccine LTR, the main difference of activity was mapped to the changes of R region, rather than U3 region.

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.

A tumor necrosis factor receptor family protein serves as a cellular receptor for the macrophage-tropic equine lentivirus

Characterization of cellular receptors for human, simian, and feline immunodeficiency viruses that are tropic for lymphocytes and macrophages have revealed a common theme of a sequential binding of viral envelope proteins with two coreceptors to mediate virus infection of target cells. In contrast to these dual tropic immunodeficiency viruses, the ungulate lentiviruses, including equine infectious anemia virus (EIAV), exclusively infect cells of the monocyte-macrophage lineage to cause progressive degenerative diseases without clinical immunodeficiency. EIAV causes a uniquely dynamic disease that is characterized by recurrent disease episodes including fever, diarrhea, lethargy, anemia, and thrombocytopenia. Although EIAV provides an important animal model for lentivirus disease resulting from macrophage infection, to date there has been no definition of the specific cellular receptor(s) used by the equine lentivirus to infect target cells. In the current study, we have identified and cloned a functional receptor for EIAV, designated equine lentivirus receptor-1 (ELR1), related to the family of TNF receptor (TNFR) proteins. ELR1 was shown to be expressed in various equine cells permissive for EIAV replication in vitro, including monocytes and macrophages. In contrast, EIAV-resistant human, murine, and simian cells were negative for ELR1 expression but became susceptible to virus infection when transduced with a recombinant murine retrovirus expressing the ELR1. Thus, these results identify a specific functional receptor for a macrophagetropic lentivirus and indicate that infection by EIAV may be mediated by a single receptor, in contrast to coreceptors used by the lymphotropic immunodeficiency lentiviruses.

Evolution of the equine infectious anemia virus long terminal repeat during the alteration of cell tropism

Equine infectious anemia virus (EIAV) is a lentivirus with in vivo cell tropism primarily for tissue macrophages; however, in vitro the virus can be adapted to fibroblasts and other cell types. Tropism adaptation is associated with both envelope and long terminal repeat (LTR) changes, and findings strongly suggest that these regions of the genome influence cell tropism and virulence. Furthermore, high levels of genetic variation have been well documented in both of these genomic regions. However, specific EIAV nucleotide or amino acid changes that are responsible for cell tropism changes have not been identified. A study was undertaken with the highly virulent, macrophage-tropic strain of virus EIAV(wyo) to identify LTR changes associated with alterations in cell tropism. We found the stepwise generation of a new transcription factor binding motif within the enhancer that was associated with adaptation of EIAV to endothelial cells and fibroblasts. An LTR that contained the new motif had enhanced transcriptional activity in fibroblasts, whereas the new site did not alter LTR activity in a macrophage cell line. This finding supports a previous prediction that selection for new LTR genetic variants may be a consequence of cell-specific selective pressures. Additional investigations of the EIAV(wyo) LTR were performed in vivo to determine if LTR evolution could be detected over the course of a 3-year infection. Consistent with previous in vivo findings, we observed no changes in the enhancer region of the LTR over that time period, indicating that the EIAV(wyo) LTR was evolutionarily stable in vivo.

PU.1 binding to ets motifs within the equine infectious anemia virus long terminal repeat (LTR) enhancer: regulation of LTR activity and virus replication in macrophages

Binding of the transcription factor PU.1 to its DNA binding motif regulates the expression of a number of B-cell- and myeloid-specific genes. The long terminal repeat (LTR) of macrophage-tropic strains of equine infectious anemia virus (EIAV) contains three PU.1 binding sites, namely an invariant promoter-proximal site as well as two upstream sites. We have previously shown that these sites are important for EIAV LTR activity in primary macrophages (W. Maury, J. Virol. 68:6270-6279, 1994). Since the sequences present in these three binding motifs are not identical, we sought to determine the role of these three sites in EIAV LTR activity. While DNase I footprinting studies indicated that all three sites within the enhancer were bound by recombinant PU.1, reporter gene assays demonstrated that the middle motif was most important for basal levels of LTR activity in macrophages and that the 5' motif had little impact. The impact of the 3' site became evident in Tat transactivation studies, in which the loss of the site reduced Tat-transactivated expression 40-fold. In contrast, elimination of the 5' site had no effect on Tat-mediated activity. Binding studies were performed to determine whether differences in PU.1 binding affinity for the three sites correlated with the relative impact of each site on LTR transcription. While small differences were observed in the binding affinities of the three sites, with the promoter-proximal site having the strongest binding affinity, these differences could not account for the dramatic differences observed in the transcriptional effects. Instead, the promoter-proximal position of the 3' motif appeared to be critical for its transcriptional impact and suggested that the PU.1 sites may serve different roles depending upon the location of the sites within the enhancer. Infectivity studies demonstrated that an LTR containing an enhancer composed of the three PU.1 sites was not sufficient to drive viral replication in macrophages. These findings indicate that while the promoter-proximal PU.1 site is the most critical site for EIAV LTR activity in the presence of Tat, other elements within the enhancer are needed for EIAV replication in macrophages.

Influence of long terminal repeat and env on the virulence phenotype of equine infectious anemia virus

The molecular clones pSPeiav19 and p19/wenv17 of equine infectious anemia virus (EIAV) differ in env and long terminal repeats (LTRs) and produce viruses (EIAV(19) and EIAV(17), respectively) of dramatically different virulence phenotypes. These constructs were used to generate a series of chimeric clones to test the individual contributions of LTR, surface (SU), and transmembrane (TM)/Rev regions to the disease potential of the highly virulent EIAV(17). The LTRs of EIAV(19) and EIAV(17) differ by 16 nucleotides in the transcriptional enhancer region. The two viruses differ by 30 amino acids in SU, by 17 amino acids in TM, and by 8 amino acids in Rev. Results from in vivo infections with chimeric clones indicate that both LTR and env of EIAV(17) are required for the development of severe acute disease. In the context of the EIAV(17) LTR, SU appears to have a greater impact on virulence than does TM. EIAV(17SU), containing only the TM/Rev region from the avirulent parent, induced acute disease in two animals, while a similar infectious dose of EIAV(17TM) (which derives SU from the avirulent parent) did not. Neither EIAV(17SU) nor EIAV(17TM) produced lethal disease when administered at infectious doses that were 6- to 30-fold higher than a lethal dose of the parental EIAV(17). All chimeric clones replicated in primary equine monocyte-derived macrophages, and there was no apparent correlation between macrophage tropism and virulence phenotype.

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.

Characterization of EIAV LTR variability and compartmentalization in various reservoir tissues of long-term inapparent carrier ponies

Dynamic genomic variation resulting in changes in envelope antigenicity has been established as a fundamental mechanism of persistence by equine infectious anemia virus (EIAV), as observed with other lentiviruses, including HIV-1. In addition to the reported changes in envelope sequences, however, certain studies indicate the viral LTR as a second variable EIAV gene, with the enhancer region being designated as hypervariable. These observations have lead to the suggestion that LTR variation may alter viral replication properties to optimize to the microenvironment of particular tissue reservoirs. To test this hypothesis directly, we examined the population of LTR quasispecies contained in various tissues of two inapparent carrier ponies experimentally infected with a reference EIAV biological clone for 18 months. The results of these studies demonstrated that the EIAV LTR is in fact highly conserved with respect to the infecting LTR species after 1.5 years of persistent infection and regardless of the tissue reservoir. Thus, these comprehensive analyses demonstrate for the first time that the EIAV LTR is highly conserved during long-term persistent infection and that the observed variations in viral LTR are associated more with in vitro adaptation to replication in cultured cells rather than in vivo replication in natural target cells.

DH82 cells: a macrophage cell line for the replication and study of equine infectious anemia virus

In vivo, tissue macrophages have been implicated as an important cell for the replication of equine infectious anemia virus (EIAV). Laboratory investigations of EIAV/macrophage interactions, however, have been hampered by the laborious blood monocyte isolation procedures. In addition, adherent equine macrophage cultures generally have poor long-term viability and are resistant to transfection. This report describes an adherent canine macrophage-like cell line, DH82, that supports the replication of EIAV. This cell line was easily transfectable and supported EIAV Tat transactivation of the LTR. Electrophoretic mobility shift assays were carried out to determine which transcription factor binding sites within the LTR enhancer region were bound by DH82 nuclear extracts. It was found that five different motifs were occupied. The ets motifs that are bound by PU.1 in primary macrophage nuclear extracts specifically interacted with DH82 nuclear extracts. In addition, the PEA-2, Lvb and Oct motifs that are occupied by fibroblast nuclear extracts were also bound by DH82 nuclear extracts. Finally, the methylation-dependent binding protein (MDBP) site that is bound by all nuclear extracts investigated to date demonstrated specific interactions with DH82 nuclear extracts. The observation that both macrophage-specific and fibroblast-specific motifs were utilized by DH82 nuclear extracts suggested that both macrophage-adapted and fibroblast-adapted EIAV could replicate in DH82 cells. Indeed, infectivity studies demonstrated that strains of virus that exclusively replicate in macrophages can replicate in DH82 cells and fibroblast-adapted strains of virus can also replicate in these cells. Finally, these cells could be transfected readily with the EIAV molecular clone, pSPeiav19-2, and virus spread was detected within the culture. In conclusion, this study has identified a useful cell line that should facilitate the study of EIAV expression and replication.