Xenotropic type C viruses

Naturally Occurring Polymorphisms of the Mouse Gammaretrovirus Receptors CAT-1 and XPR1 Alter Virus Tropism and Pathogenicity

Gammaretroviruses of several different host range subgroups have been isolated from laboratory mice. The ecotropic viruses infect mouse cells and rely on the host CAT-1 receptor. The xenotropic/polytropic viruses, and the related human-derived XMRV, can infect cells of other mammalian species and use the XPR1 receptor for entry. The coevolution of these viruses and their receptors in infected mouse populations provides a good example of how genetic conflicts can drive diversifying selection. Genetic and epigenetic variations in the virus envelope glycoproteins can result in altered host range and pathogenicity, and changes in the virus binding sites of the receptors are responsible for host restrictions that reduce virus entry or block it altogether. These battleground regions are marked by mutational changes that have produced 2 functionally distinct variants of the CAT-1 receptor and 5 variants of the XPR1 receptor in mice, as well as a diverse set of infectious viruses, and several endogenous retroviruses coopted by the host to interfere with entry.

No evidence of murine-like gammaretroviruses in CFS patients previously identified as XMRV-infected

Members of the gammaretroviruses--such as murine leukemia viruses (MLVs), most notably XMRV [xenotropic murine leukemia virus (X-MLV)-related virus--have been reported to be present in the blood of patients with chronic fatigue syndrome (CFS). We evaluated blood samples from 61 patients with CFS from a single clinical practice, 43 of whom had previously been identified as XMRV-positive. Our analysis included polymerase chain reaction and reverse transcription polymerase chain reaction procedures for detection of viral nucleic acids and assays for detection of infectious virus and virus-specific antibodies. We found no evidence of XMRV or other MLVs in these blood samples. In addition, we found that these gammaretroviruses were strongly (X-MLV) or partially (XMRV) susceptible to inactivation by sera from CFS patients and healthy controls, which suggested that establishment of a successful MLV infection in humans would be unlikely. Consistent with previous reports, we detected MLV sequences in commercial laboratory reagents. Our results indicate that previous evidence linking XMRV and MLVs to CFS is likely attributable to laboratory contamination.

The mouse "xenotropic" gammaretroviruses and their XPR1 receptor

The xenotropic/polytropic subgroup of mouse leukemia viruses (MLVs) all rely on the XPR1 receptor for entry, but these viruses vary in tropism, distribution among wild and laboratory mice, pathogenicity, strategies used for transmission, and sensitivity to host restriction factors. Most, but not all, isolates have typical xenotropic or polytropic host range, and these two MLV tropism types have now been detected in humans as viral sequences or as infectious virus, termed XMRV, or xenotropic murine leukemia virus-related virus. The mouse xenotropic MLVs (X-MLVs) were originally defined by their inability to infect cells of their natural mouse hosts. It is now clear, however, that X-MLVs actually have the broadest host range of the MLVs. Nearly all nonrodent mammals are susceptible to X-MLVs, and all species of wild mice and several common strains of laboratory mice are X-MLV susceptible. The polytropic MLVs, named for their apparent broad host range, show a more limited host range than the X-MLVs in that they fail to infect cells of many mouse species as well as many nonrodent mammals. The co-evolution of these viruses with their receptor and other host factors that affect their replication has produced a heterogeneous group of viruses capable of inducing various diseases, as well as endogenized viral genomes, some of which have been domesticated by their hosts to serve in antiviral defense.

Xenotropic murine leukemia virus-related virus prevalence in patients with chronic fatigue syndrome or chronic immunomodulatory conditions

We investigated the prevalence of xenotropic murine leukemia virus-related virus (XMRV) among 293 participants seen at academic hospitals in Boston, Massachusetts. Participants were recruited from the following 5 groups of patients: chronic fatigue syndrome (n = 32), human immunodeficiency virus infection (n = 43), rheumatoid arthritis (n = 97), hematopoietic stem-cell or solid organ transplant (n = 26), or a general cohort of patients presenting for medical care (n = 95). XMRV DNA was not detected in any participant samples. We found no association between XMRV and patients with chronic fatigue syndrome or chronic immunomodulatory conditions.

The source of self: genetic parasites and the origin of adaptive immunity

Stable colonization of the host by viruses (genetic parasites) can alter the systems of host identity and provide immunity against related viruses. To attain the needed stability, some viruses of prokaryotes (P1 phage) use a strategy called an addiction module. The linked protective and destructive gene functions of an addiction module insures both virus persistence but will also destroy cells that interrupt this module and thereby prevent infection by competitors. Previously, I have generalized this concept to also include persistent and lytic states of virus infection, which can be considered as a virus addiction module. Such states often involve defective viruses. In this report, I examine the origin of the adaptive immune system from the perspective of a virus addiction module. The likely role of both endogenous and exogenous retroviruses, DNA viruses, and their defective elements is considered in the origin of all the basal components of adaptive immunity (T-cell receptor, RAG-mediated gene rearrangement, clonal lymphocyte proliferation, antigen surface presentation, apoptosis, and education of immune cells). It is concluded that colonization by viruses and their defectives provides a more coherent explanation for the origin of adaptive immunity.

Molecular and phylogenetic analyses of a new amphotropic murine leukemia virus (MuLV-1313)

BACKGROUND

The amphotropic murine leukemia viruses (MuLV-A's) are naturally occurring, exogenously acquired gammaretroviruses that are indigenous to the Southern California wild mice. These viruses replicate in a wide range of cell types including human cells in vitro and they can cause both hematological and neurological disorders in feral as well as in the inbred laboratory mice. Since MuLV-A's also exhibit discrete interference and neutralization properties, the envelope proteins of these viruses have been extremely useful for studying virus-host cell interactions and as vehicles for transfer of foreign genes into a variety of hosts including human cells. However, the genomic structure of any of the several known MuLV-A's has not been established and the evolutionary relationship of amphotropic retroviruses to the numerous exogenous or endogenous MuLV strains remains elusive. Herein we present a complete genetic structure of a novel amphotropic virus designated MuLV-1313 and demonstrate that this retrovirus together with other MuLV-A's belongs to a distinct molecular, biological and phylogenetic class among the MuLV strains isolated from a large number of the laboratory inbred or feral mice.

RESULTS

The host range of MuLV-1313 is similar to the previously isolated MuLV-A's except that this virus replicates efficiently in mammalian as well as in chicken cells. Compared to ENV proteins of other MuLV-A's (4070A, 1504A and 10A-1), the gp70 protein of MuLV-1313 exhibits differences in its signal peptides and the proline-rich hinge regions. However, the MuLV-1313 envelope protein is totally unrelated to those present in a broad range of murine retroviruses that have been isolated from various inbred and feral mice globally. Genetic analysis of the entire MuLV-1313 genome by dot plot analyses, which compares each nucleotide of one genome with the corresponding nucleotide of another, revealed that the genome of this virus, with the exception of the env gene, is more closely related to the biologically distinct wild mouse ecotropic retrovirus (Cas-Br-E) isolated from another region of the Southern California, than to any of the 15 MuLV strains whose full-length sequences are present in the GenBank. This finding was corroborated by phylogenetic analyses and hierarchical clustering of the entire genomic sequence of MuLV-1313, which also placed all MULV-A's in a genetically distinct category among the large family of retroviruses isolated from numerous mouse strains globally. Likewise, construction of separate dendrograms for each of the Gag, Pol and Env proteins of MuLV-1313 demonstrated that the amphotropic retroviruses belong to a phylogenetically exclusive group of gammaretroviruses compared to all known MuLV strains.

CONCLUSION

The molecular, biological and phylogenetic properties of amphotropic retroviruses including MuLV-1313 are distinct compared to a large family of exogenously- or endogenously-transmitted ecotropic, polytropic and xenotropic MuLV strains of the laboratory and feral mice. Further, both the naturally occurring amphotropic and a biologically discrete ecotropic retrovirus of the Southern California wild mice are more closely related to each other on the evolutionary tree than any other mammalian gammaretrovirus indicating a common origin of these viruses. This is the first report of a complete genomic analysis of a unique group of phylogenetically distinct amphotropic virus.

No evidence for infection of human embryonic stem cells by feeder cell-derived murine leukemia viruses

Until recently, culture and expansion of nondifferentiated human embryonic stem cells (hESCs) depended on coculture with murine embryonic fibroblasts. Because mice are known to harbor a variety of pathogens, such culture conditions implicate the risk of xenozoonoses. Among these pathogens, endogenous retroviruses, including murine leukemia viruses (MuLVs), are of special importance. It is well known that some strains cause pathogenic (e.g., leukemic) effects and that xenotropic, polytropic, and amphotropic MuLVs are able to infect human cells. In view of potential clinical applications of hESC lines, it is therefore imperative to investigate potential infection of hESCs by mouse feeder cell-derived viruses. As a first step towards a comprehensive infection risk assessment, we have analyzed embryonic fibroblasts derived from different mouse strains for expression and release of xenotropic, polytropic, and amphotropic MuLVs. Moreover, several hESC lines have been investigated for expression of specific receptors for xenotropic/polytropic MuLVs, as well as for MuLV infection and expression. Evidence for expression of humantropic MuLVs was found in cultures of mouse embryonic fibroblasts (MEFs). Moreover, expression of specific receptors for xenotropic/ polytropic MuLV on human HEK293 and hESC lines and infection after coculture with an MuLV-producing mink cell line could be demonstrated. In contrast, no evidence of MuLV transmission from MEFs to human HEK293 cells or to the hESC lines I-3, I-6, I-8, and H-9 has been obtained. Our results suggest that recently established hESC lines are free of MuLV infections despite long-term close contact with MEFs.

Cell surface receptors for gammaretroviruses

Evidence obtained during the last few years has greatly extended our understanding of the cell surface receptors that mediate infections of retroviruses and has provided many surprising insights. In contrast to other cell surface components such as lectins or proteoglycans that influence infections indirectly by enhancing virus adsorption onto specific cells, the true receptors induce conformational changes in the viral envelope glycoproteins that are essential for infection. One surprise is that all of the cell surface receptors for gamma-retroviruses are proteins that have multiple transmembrane (TM) sequences, compatible with their identification in known instances as transporters for important solutes. In striking contrast, almost all other animal viruses use receptors that exclusively have single TM sequences, with the sole proven exception we know of being the coreceptors used by lentiviruses. This evidence strongly suggests that virus genera have been prevented because of their previous evolutionary adaptations from switching their specificities between single-TM and multi-TM receptors. This evidence also implies that gamma-retroviruses formed by divergent evolution from a common origin millions of years ago and that individual viruses have occasionally jumped between species (zoonoses) while retaining their commitment to using the orthologous receptor of the new host. Another surprise is that many gamma-retroviruses use not just one receptor but pairs of closely related receptors as alternatives. This appears to have enhanced viral survival by severely limiting the likelihood of host escape mutations. All of the receptors used by gamma-retroviruses contain hypervariable regions that are often heavily glycosylated and that control the viral host range properties, consistent with the idea that these sequences are battlegrounds of virus-host coevolution. However, in contrast to previous assumptions, we propose that gamma-retroviruses have become adapted to recognize conserved sites that are important for the receptor's natural function and that the hypervariable sequences have been elaborated by the hosts as defense bulwarks that surround the conserved viral attachment sites. Previously, it was believed that binding to receptors directly triggers a series of conformational changes in the viral envelope glycoproteins that culminate in fusion of the viral and cellular membranes. However, new evidence suggests that gamma-retroviral association with receptors triggers an obligatory interaction or cross-talk between envelope glycoproteins on the viral surface. If this intermediate step is prevented, infection fails. Conversely, in several circumstances this cross-talk can be induced in the absence of a cell surface receptor for the virus, in which case infection can proceed efficiently. This new evidence strongly implies that the role of cell surface receptors in infections of gamma-retroviruses (and perhaps of other enveloped animal viruses) is more complex and interesting than was previously imagined. Recently, another gammaretroviral receptor with multiple transmembrane sequences was cloned. See Prassolov, Y., Zhang, D., Ivanov, D., Lohler, J., Ross, S.R., and Stocking, C. Sodium-dependent myo-inositol transporter 1 is a receptor for Mus cervicolor M813 murine leukemia virus.

Mink cell focus-forming murine leukemia virus killing of mink cells involves apoptosis and superinfection

Induction of apoptosis by different types of pathogenic retroviruses is an important step in disease development. We have observed that infection of thymic lymphocytes by the mink cell focus-forming murine leukemia virus (MCF MLV) during the preleukemic period resulted in an enhancement of apoptosis of these cells. To further study the ability of MCF MLVs to induce apoptosis and the role of this process in viral pathogenesis, we have developed an in vitro system of virus-induced apoptosis. MCF13 MLV infection of mink epithelial cells resulted in the production of cytopathic foci. In contrast, infection of mink cells with the 4070A amphotropic MLV did not produce any cytopathic effects. Staining of MCF13 MLV-infected cells with propidium iodide and annexin V-fluorescein isothiocyanate indicated that virus-induced cell death was due to apoptosis. At 6 days postinfection, the percentage of apoptotic MCF13 MLV-infected cells was 27% compared with 2 to 3% for mock- or amphotropic MLV-infected cells, representing a 9- to 14-fold difference. Assays for caspase-3 activation confirmed the detection by flow cytometry of apoptosis of MCF13 MLV-infected cells. Large amounts of unintegrated linear viral DNA were detectable by Southern blot analysis during the acute phase of infection, which indicated that MCF13 MLV is able to superinfect mink cells. Unintegrated viral DNA of only the linear form was detectable in thymic lymphocytes isolated from MCF13 MLV-inoculated mice during the preleukemic period. These results indicated that the ability of MCF13 MLV to induce apoptosis is correlated with its ability to superinfect cells and that this occurs as an early step in thymic lymphoma development.

Retroviruses in foreign species and the problem of provirus silencing

Retroviruses are known to integrate in the host cell genome as proviruses, and therefore they are prone to cell-mediated control at the transcriptional and posttranscriptional levels. This plays an important role especially after retrovirus heterotransmission to foreign species, but also to differentiated cells. In addition to host cell-mediated blocks in provirus expression, also so far undefined host specificities, deciding upon the pathogenic manifestation of retrovirus heterotransmission, are in play. In this respect, we discuss especially the occurrence of wasting disease and immunodeficiency syndrome, which we established also in avian species using avian leukosis virus subgroup C (ALV-C) inoculated in mid-embryogenesis in duck or chicken embryos. The problem of provirus downregulation in foreign species or in differentiated cells has been in the recent years approached experimentally. From a series of observations it became apparent that provirus downregulation is mediated by its methylation, especially in the region of proviral enhancer-promoter located in long terminal repeats (LTR). Several strategies have been devised in order to protect the provirus from methylation using LTR modification and/or introducing in the LTR sequence motifs acting as antimethylation tags. In such a way the expression of retroviruses and vectors in foreign species, as well as in differentiated cells, has been significantly improved. The complexity of the mechanisms involved in provirus downregulation and further possibilities to modulate it are discussed.

The restricted cellular host range of human herpesvirus 8

DESIGN

A selection of primary and transformed cell types were evaluated for their susceptibility to infection with human herpesvirus 8 (HHV-8)/Kaposi's sarcoma-associated herpesvirus.

METHODS

Sources of HHV-8 included Kaposi's sarcoma lesion punch biopsies that were either cocultured directly with target cells or that were first cocultured with human lymphocytes to derive HHV-8-containing fluids that were inoculated onto target cells. HHV-8 was also obtained from primary effusion lymphoma-derived cell lines. Techniques to detect infection included the PCR, immunofluorescence assays and in situ hybridization.

RESULTS

Susceptible cells included human umbilical cord blood mononuclear cells (UCMC), adult CD19 B cells, macrophages and certain endothelial cells of human and animal origin, including some that are transformed with human papilloma virus type 16 E6 and E7 genes. The infection of lymphocytes did not yield established lymphoblastoid cell lines (LCL) and virus infection persisted for only 4-7 days. However, long-term HHV-8 infection of UCMC could be achieved by coinfection with Epstein-Barr virus. HHV-8 could also infect UCMC LCL recently derived by Epstein-Barr virus transformation, but long-established LCL could not be infected with HHV-8.

CONCLUSIONS

These data provide further biological evidence in cell culture for the limited cellular host range of HHV-8 to CD19 B cells, macrophages, and certain endothelial cells.

Cloning and characterization of a cell surface receptor for xenotropic and polytropic murine leukemia viruses

Xenotropic and polytropic murine leukemia viruses (X-MLVs and P-MLVs) cross-interfere to various extents in non-mouse species and in wild Asian mice, suggesting that they might use a common receptor for infection. Consistent with this hypothesis, the susceptibility of some wild mice to X-MLVs has been mapped to the P-MLV receptor locus at the distal end of mouse chromosome 1. In this study, we report the isolation and characterization of a cDNA for the human X-MLV cell surface receptor (X-receptor) by using a human T lymphocyte cDNA library in a retroviral vector. The predicted X-receptor contains 696 amino acids with multiple hydrophobic potential membrane-spanning sequences and with weak homologies to the yeast proteins SYG1, of unknown function, and PHO81, which has been implicated in a system that regulates transport of inorganic phosphate. Expression of the X-receptor in Chinese hamster ovary cells, which are substantially resistant to P-MLVs and to X-MLVs, made them susceptible to both of these virus groups. The mouse homologue of the X-receptor was mapped by hybridization to the distal end of chromosome 1 at the same position as the P-MLV receptor gene Rmc1. These results strongly support the hypothesis that a common gene encodes the receptors for X-MLVs and P-MLVs, with the human X-receptor preferentially mediating X-MLV infections and the homologous protein of inbred mice mediating only P-MLV infections. We propose that X-MLVs and P-MLVs comprise a single family of retroviruses that have coevolved in response to diversification in X-receptor genes of the host.

Replication of HIV-1 in a wide variety of animal cells following phenotypic mixing with murine retroviruses

Human T cells co-infected with the human immunodeficiency virus type 1 (HIV-1) and the xenotropic or dual-tropic mouse type C virus (MuLV) give rise, by phenotypic mixing, to progeny virus that can transfer HIV-1 into a wide variety of mammalian and avian cells. Differences in the extent of HIV-1 replication in these animal cells can be observed. Replication is best in human cells, but occurs substantially in cells from many animal species including mink, horse, and bush wallaby. Virus production in murine and avian cells is very limited. These results confirm that the major block to HIV-1 infection of animal cells is at the cellular surface but that intracellular regulation of viral replication is also involved. Moreover, an enhancement of HIV-1 cytopathic effects can be seen in human cells co-infected by MuLV. All these data suggest phenotypically mixed viruses might be useful for developing an animal model system for studying AIDS, and that the pathological expression of HIV-1 could be modified by the presence in cells of other retroviruses. They also indicate a potential mechanism by which HIV strains can be generated with an increased ability to spread in nature.

Retrovirus phylogeny and evolution

The elucidation of complete genomic sequences from a wide variety of retroviruses and retrotransposons has allowed the construction of sequence-based phylogenies that reveal their evolutionary history. True retroviruses, whether exogenous or endogenous, tend to cluster into four major groups. Not only is there no distinction between exogenous and endogenous viruses, but their evolutionary limb lengths on the phylogenetic trees are comparable. This can be taken as evidence favoring a dynamic equilibrium balancing a constant invasion of germlines by infectious retroviruses on the one hand, with subsequent escape of endogenous viruses to alternative hosts on the other. Retroviruses share a common ancestry with a wide variety of retrotransposons and other reverse transcriptase-bearing entities. One of these retrotransposon groups, the Gypsy group, resembles the Moloney mouse group of retroviruses much more closely than it does other retroviruses. The simplest explanation is that the evolutionary rate of the retrotransposon is much slower than the retrovirus rate and that among the retroviruses the Moloney mouse group has been evolving more slowly than the other three groups, leaving the two short-limbed taxa more similar. The alternative explanation that these two groups actually shared a common ancestor more recently than has either with the other retrovirus groups is not supported by residue-by-residue character assessment.

Mouse xenotropic viruses can inhibit cell transformation

Mammalian type C retroviruses can be detected by their induction of foci of cell transformation in S+I - cells. We have noted that certain subtypes of the mouse xenotropic type C retrovirus (MuLV) inhibit this cell alteration. This inhibition, associated with intact virions, is irreversible and gives the infected cells a phenotype of uninfected cells. In comparison to the transformed cells, the inhibited cultures showed primarily a decrease in murine sarcoma virus (MSV) progeny production concomitant with a reduction in the MSV mos RNA expression. No difference in beta-actin RNA production was observed between the inhibited and transformed cultures. This selective effect of mouse xenotropic MuLV on MSV and mos RNA production in these cells focuses attention on the mechanism of transformation in this system.

Amphotropic proviral envelope sequences are absent from the Mus germ line

We derived an amphotropic murine leukemia virus (MuLV) type-specific probe for use in Southern blot hybridizations with cloned and genomic DNAs. A 133-base-pair RsaI-RsaI fragment from the 5' env region of the amphotropic viral isolate 4070A was subcloned into M13mp18 and radiolabeled in vitro. The probe detected the proviral DNAs in mink cells infected with seven different amphotropic MuLV isolates. The probe did not cross hybridize with the DNAs of molecular clones of ecotropic, mink cell focus-forming, or xenotropic MuLVs; nor did it anneal to the proviral DNAs of four xenotropic or six mink cell focus-forming viral isolates grown in mink cells. DNAs of 12 inbred laboratory mouse strains and more than 15 different wild mouse species and subspecies were examined for the presence of endogenous amphotropic env-related fragments. Amphotropic env-related sequences were found only in the DNAs of wild mice trapped in southern California in an area previously shown to harbor mice producing infectious amphotropic virus. Restriction enzyme analyses of DNAs from these mice showed that amphotropic sequences were not present as germ line copies but were the result of congenital or horizontal infection or both in this population. The DNAs of 11 various mammalian and avian species, including both natural predators of mice and squabs from the farms with virus-positive mice, lacked amphotropic envelope-related sequences.

Specific hybridization probes demonstrate fewer xenotropic than mink cell focus-forming murine leukemia virus env-related sequences in DNAs from inbred laboratory mice

We have derived hybridization probes from analogous 100-base-pair segments located within the N-terminal region of gp70 coding sequences which differentiate xenotropic from mink cell focus-forming (MCF)-related murine leukemia virus (MuLV) DNAs. The MCF probe annealed to the integrated proviruses of all six MCF MuLV isolates tested; the xenotropic probe hybridized to the DNAs of all four xenotropic proviral isolates examined. No cross-hybridization was observed, and neither probe reacted with the env segments of amphotropic or ecotropic MuLV DNAs. Southern blot analysis of HindIII- or EcoRI-digested genomic DNAs from a variety of inbred laboratory mice demonstrated the presence of more MCF- than xenotropic MuLV-related segments in every strain tested.

Normal expression of polymorphic endogenous retroviral RNA containing segments identical to mink cell focus-forming virus

In the absence of infectious virus, strains of mice express polyadenylated RNA transcripts homologous to the genome of murine leukemia virus. In addition to transcripts consistent with full-length and spliced env retroviral RNAs, several unique RNA species which lack the env sequence accumulate in a tissue-specific manner. These RNA species are presumed to be transcribed from endogenous retroviral sequences that constitute the bulk of the murine leukemia virus-related sequences in the murine genome. To determine the relationship of these RNA transcripts to infectious murine leukemia virus and the precise structural basis of the heterogeneity observed for the env-lacking transcripts, we isolated and sequenced cDNA recombinants representing the RNAs expressed in strain 129 GIX+ mice. Comparisons of the nucleotide sequences demonstrated that the endogenous retroviral transcripts differed in pol, p15E, and R-peptide regions by single nucleotide changes. In contrast, the gp70-coding regions of two cDNA clones derived from epididymis and liver were completely homologous over a 599-nucleotide overlapping sequence. The structures of env-lacking transcripts were examined in two independent cDNA clones, and each was found to contain a different deletion that was potentially mediated by seven-base pair direct repeats in the intact sequence. The extensive sequence homology between cDNAs allowed construction of a cumulative sequence map of the 3' end of an intact endogenous retroviral transcript. A comparison of this sequence with infectious ecotropic and mink cell focus-forming viruses revealed that the endogenous transcripts are highly homologous with the substituted portions of leukemogenic mink cell focus-forming viruses and therefore further define the boundaries of recombination required to generate these viruses.

AIDS-associated retroviruses (ARV) can productively infect other cells besides human T helper cells

We have examined the host range of AIDS-associated retroviruses (ARV) that are known to infect human T cells of the helper subset. We have observed that the virus cannot infect fibroblast and epithelial cell lines of many different animal species. It is infectious and replicates efficiently in peripheral mononuclear cells (PMC) of chimpanzee and at low levels in baboon and rhesus monkey PMC. Most importantly, it has been found to replicate in established lines of human B cells, monocytes, and promyelocytes. This ability to infect these other cell types appears to be associated, in most cases, with the presence of the Leu 3 T helper cell antigen on the cell surface. Other mechanisms for virus infection, however, may be involved. The results suggest that ARV will be found in other cells of AIDS patients, besides T cells, and that these cells could be the reservoir for continual virus spread in the host. Variations in the replicative ability of ARV isolates in human cells have also been noted; they could reflect potentially important pathogenic differences among these human retroviruses.

Envelope and long terminal repeat sequences of a cloned infectious NZB xenotropic murine leukemia virus

An infectious NZB xenotropic murine leukemia virus (MuLV) provirus (NZB was molecularly cloned from the Hirt supernatant of NZB-IU-6-infected mink cells, and the nucleotide sequence of its env gene and long terminal repeat (LTR) was determined. The partial nucleotide sequence previously reported for the env gene of NFS-Th-1 xenotropic proviral DNA (Repaske, et al., J. Virol. 46:204-211, 1983) is identical to that of the infectious NZB xenotropic MuLV DNA reported here. Alignment of nucleotide or deduced amino acid sequences, or both, of xenotropic, mink cell focus-forming, and ecotropic MuLV proviral DNAs in the env region identified sequence differences among the three host range classes of C-type MuLVs. Major differences were confined to the 5' half of env; a high degree of homology was found among the three classes of MuLVs in the 3' half of env. Alignment of the nucleotide sequence of the LTR of NZB xenotropic MuLV with those of the LTRs of NFS-Th-1 xenotropic, mink cell focus-forming, and ecotropic MuLVs revealed extensive homology between the LTRs of xenotropic and MCF247 MuLVs. An inserted 6-base-pair repeat 5' to the TATA box was a unique feature of both NZB and NFS-Th-1 xenotropic LTRs.

Recovery and inactivation of infectious retroviruses from factor VIII concentration

The ability of infectious retroviruses to withstand the procedures used for factor VIII concentration was investigated. Mouse retroviruses added to human plasma survived these procedures and remained infectious in lyophilised samples of factor VIII. Lyophilised material had to be heated at 68 degrees C for several hours before substantial quantities of infectious virus became inactivated. These findings support the possible role of retroviruses in AIDS, and indicate that factor VIII concentrates must be heated to inactivate these infectious viruses.

Murine xenotropic type C viruses. V. Biologic and structural differences among three cloned retroviruses isolated from kidney cells from one NZB mouse

Three xenotropic retroviruses have been biologically cloned from cells cultured from the kidney of a 3-month-old NZB female mouse. They were obtained by first cocultivating the kidney cells for several weeks with mink, dog, and human cells and then cloning them by endpoint dilution. The cloned viruses differ in their infectivity and replicative ability in a variety of heterologous cell lines. The mink cell line-derived virus (X-NZB/K-Mlc) reaches titers in culture of over 10(8) infectious viruses/ml, and is produced in high titer within 24 hr after infection of mink lung cells. The human and dog cell-derived NZB viruses (X-NZB/K-Huc and X-NZB/K-Dgc) grow to lower titers and are similar in many respects. They differ in their relative ability to replicate in dog and human cells and to transform mink S+L- cells. Peptide mapping studies indicate that the X-NZB/K-Mlc virus has a unique p15(E) protein which distinguishes it from the other two cloned NZB viruses. These results lend further support to the observation that several types of xenotropic virus are present in a mouse strain and that more than one virus can be expressed by one organ of a particular mouse.

Family of middle repetitive DNA sequences in the mouse genome with structural features of solitary retroviral long terminal repeats

Screening of a 129/J mouse genomic library under nonstringent hybridization conditions with a xenotropic virus-like long terminal repeat (LTR) probe revealed a family of sequences resembling insertion elements (IS) with structural features of solitary retroviral LTRs; these are called LTR-IS. They are interspersed among variable flanking regions of mouse DNA and lack any viral structural genes. LTR-IS elements start and end with 11-base-pair inverted repeats and contain signals implicated in RNA polymerase II transcriptional regulation: C-C-A-A-T, T-A-T-A-A-A, and A-A-T-A-A-A. The members of the family are homologous, but not identical, approximately equal to 500-base-pair-long elements with 4-base-pair target-site duplications on both sites of the element. There are 500 LTR-IS per mouse haploid genome.

Freeze-drying is an effective method for preserving infectious type C retroviruses

Conventional freeze-drying techniques using a sucrose stabilizer and gelatin can be employed to preserve the infectivity of retroviruses. Lyophilized virus retains its infectivity even at room temperature for more than one year. A lyophilized virus preparation of Friend leukemia virus kept at 4 degrees C for more than 20 years was found to contain high titers of infectious pathogenic virus. This technique offers an easy economic means for shipping type C viruses and for preserving them for long periods of time in the laboratory without the need for cold storage.

A genetic locus regulates the expression of tissue-specific mRNAs from multiple transcription units

129 GIX- mice, unlike animals of the congeneic partner strain GIX+, do not express significant amounts of the retroviral antigens gp70 and p30. Evidence is presented indicating that the GIX phenotype is specified by a distinct regulatory gene acting on multiple transcription units to control the levels of accumulation of specific mRNA species. The steady-state levels of retroviral-homologous mRNA from the tissues of GIX+ and GIX- mice were examined by blot hybridization using as probes DNA fragments from cloned murine leukemia viruses. RNA potentially encoding viral antigens was reduced or absent in GIX- mice, even though no differences in integrated viral genomes were detected between these congeneic strains by DNA blotting. Tissue-specific patterns of accumulation of these RNA species were detected in brain, epididymis, liver, spleen, and thymus, and several distinct RNA species were found to be coordinately regulated with the GIX phenotype. Measurements of RNA synthesis suggest a major role for transcriptional control in the regulation of some retroviral messages.

Ecotropic murine leukemia virus DNA content of normal and lymphomatous tissues of BXH-2 recombinant inbred mice

BXH-2 recombinant inbred mice spontaneously produce a B-tropic murine leukemia virus (MuLV) beginning early in life and have a high incidence of non-T-cell lymphomas. These traits are not characteristic of the progenitor strains (C57BL/6J and C3H/HeJ) or of 11 other BXH recombinant inbred strains. Since B-tropic virus expression may be causally related to the high incidence of lymphoma in this strain, we have analyzed the ecotropic MuLV DNA content of both normal and lymphomatous tissues of BXH-2 mice. Southern analysis and hybridization with an ecotropic MuLV DNA-specific probe showed that DNA of normal BXH-2 tissues contained both parental N-tropic MuLV proviruses but lacked endogenous B-tropic MuLV DNA sequences. In addition, none of 116 F1 hybrid mice derived from male BXH-2 mice spontaneously produced ecotropic MuLV early in life. These results suggest that the B-tropic virus is horizontally transmitted in BXH-2 mice. Southern analysis of DNA from tumor tissues of 12 BXH-2 mice showed that amplification of ecotropic-specific DNA sequences had occurred in lymphomatous tissues of 3 mice and suggested that these tumors were monoclonal. The number of newly acquired proviruses, which appeared to be structurally nondefective and integrated at different sites, varied from one to three copies. Since lymphomatous tissues from only 3 of 12 mice examined carried additional detectable ecotropic proviruses, these results suggest that amplification of ecotropic MuLV DNA sequences is not required for maintenance of transformation in BXH-2 lymphomas.

Dietary fat affects immune response, production of antiviral factors, and immune complex disease in NZB/NZW mice

Autoimmune-prone (NZB x NZW)F1 (B/W) mice fed three nearly isocaloric diets with varied fat content showed a marked difference in their spontaneous development of immune complex disease and their immune response. Those animals received the diets high in either unsaturated or saturated fats had more severe immune complex nephritis and died earlier than mice on the low-fat diet. Endogenous production of the mouse xenotropic virus was unaffected by dietary fats, but the serum lipoproteins associated with antiviral activity were increased to levels as high as 1:600,000 in the B/W mice on the high-fat diets. These lipoproteins may be partially responsible for the decreased mitogenic response of spleen cells from mice fed the two high-fat diets. The mice receiving a diet high in saturated fats produced substantially higher titers of natural thymocytotoxic autoantibody, an IgM class of antibody, than did the mice maintained either on the high-unsaturated-fat or low-fat diet. In contrast, the mice receiving the diet high in unsaturated fats made significantly greater levels of antibodies to double-stranded DNA, an IgG, than did the mice kept on the two other diets. These results suggest that the type of fat in the diet could affect the serum level of different immunoglobulin classes. The data provide further evidence that the amount of dietary lipids alone can influence cellular and humoral immune responses and the spontaneous development of immune complex disease.

Moloney virus (M-MuLV) leukemogenesis: virus spread, antibody production and antigenic expression in neonatally virus-inoculated young mice

(A X C57BL) and (A X C57L)F1 hybrid mice were inoculated neonatally with M-MuLV. Virus spread, antigenic expression and antibody production were followed during the preleukemic period. M-MuLV was first detectable in the spleen and later in the thymus. Virus spread was faster and the level of viremia higher in A X C57L than in A X C57BL mice. Also, A X C57L mice had no or only low titers of virus neutralizing antibodies, whereas A X C57BL mice had high titers. Anti-MCSA antibodies, reacting with the surface of syngeneic M-MuLV-induced lymphoma cells, were present in a minority of the mice, but disappeared ultimately in all mice. The two groups of mice differed with regard to the length of the preleukemic latency period. High virus load and a low level of virus neutralizing and anti-MCSA antibodies were correlated with an earlier onset of leukemia.

Inhibition of AKR leukemogenesis by SMX-1, a dualtropic murine leukemia virus

Intrathymic injection of SMX-1, a dualtropic murine leukemia virus (MuLV) originally derived from Moloney murine leukemia virus stocks, protects AKR mice from developing MuLV-accelerated leukemia and spontaneous leukemia. Thymuses of SMX-1-injected mice show no change in weight, morphology, or thymocyte size, and quantitative expression of Thy-1 and Lyt-2 differentiation antigens is identical to control mice. The amplified thymic expression of MuLV-related antigens that occurs spontaneously in 6-month-old preleukemic AKR mice or that can be induced in young AKR mice by leukemogenic AKR dualtropic MuLV is prevented by SMX-1. It appears unlikely that the protective effect of SMX-1 is explicable in terms of cross-immunogenicity with transforming MuLV or transformed cells. As SMX-1 persists for long periods after intrathymic injection and does not alter levels of thymic ecotropic MuLV, SMX-1 may interfere with the generation, spread, or leukemogenicity of dualtropic MuLV that form de novo in AKR thymus during the late preleukemic phase. SMX-1 provides a way to probe the events leading to cell transformation in AKR mice.

In vitro construction of a B-tropic virus by recombination: B-tropism is a cryptic phenotype of xenotropic murine retroviruses

A B-tropic virus was isolated in vitro from the progeny of mouse cells doubly infected with N-tropic and xenotropic murine leukemia viruses. Biological and structural evidence is presented suggesting that the phenotypically silent structural marker for B-tropism, expressed by the xenotropic virus p30, was transferred to an N-ecotropic virus via recombination, thus resulting in the expression of a B-ecotropic murine leukemia virus.

Antibody directed against Friend leukemia virus stimulates DNA synthesis in a subpopulation of mouse B lymphocytes

Goat and rat antisera directed against Friend leukemia virus (anti-FLV) were found to be B-lymphocyte mitogens stimulating DNA synthesis in these cells but not in T lymphocytes. Membrane fluorescence microscopy showed that anti-FLV reacts with a subset of B lymphocytes of which the majority express immunoglobulin mu chains. The mitogenic effect was found with all mouse strains tested including 129 and AKR. Absorption experiments with purified viruses indicated that the mitogenic effect is specific for an antigen present in murine leukemia viruses of the FMR subgroup. In absorption experiments with viable cells, the antigen involved in mitogenicity was found to be expressed on Friend erythroleukemia cell lines (4/4) and on myelomas (2/2) but not on normal thymus T lymphomas (0/2) or on rabbit or mink cells infected with BALB/c xenotropic virus. Preincubation of spleen cells with anti-gp70 antiserum inhibited the mitogenic effect of anti-FLV but not of lipopolysaccharide.

Effect of interferon on chronic infection of human cells by xenotropic type-C viruses

Different preparations of human interferon inhibit virus production in human cells chronically infected by a variety of type-C xenotropic viruses. Some of these viruses have been incriminated in the development of leukemia in primates. The characteristics of blocking of viral multiplication are similar to those described for the effect of mouse interferon on ecotropic viruses. The amount of free virus in culture supernatants is strongly decreased while intracellular protein p30 stays unchanged or is slightly increased. On the other hand, the inhibitory effect is reversible. The withdrawal of interferon results in a rapid increase in virus production as detectable in supernatant fluids. In the light of these results it is suggested that human interferon might be useful in the treatment of some blood malignancies suspected of being related to infection with xenotropic type-C viruses.

Azathioprine-induced lymphocytic neoplasms of NZB mice lack ecotropic murine leukaemia virus

NZB mice injected intramuscularly throughout a 6-month period with the immunosuppressant azathioprine (Imuran) developed lymphocytic lymphomas 6--7 months after treatment was initiated. These malignancies were quite distinct from the reticulum-cell neoplasia which occurs spontaneously in the strain, and were readily transplantable to NZB or histocompatible BALB/c recipients. Xenotropic, but not ecotropic murine leukaemia virus (MuLV) was detected in leukaemic tissues of some donor and recipient NZBs when tested in vitro by co-cultivation with permissive cell lines, genome rescue, XC and viral polymerase assays. Virus filtrates prepared from donor leukaemic tissues were non-pathogenic when injected into newborn C3H mice. These results are evidence against a mandatory ecotropic MuLV genome in lymphocytic neoplasia.