Recombinant Origins of Pathogenic and Nonpathogenic Mouse Gammaretroviruses with Polytropic Host Range

Ecotropic, xenotropic, and polytropic mouse leukemia viruses (E-, X-, and P-MLVs) exist in mice as infectious viruses and endogenous retroviruses (ERVs) inserted into mouse chromosomes. All three MLV subgroups are linked to leukemogenesis, which involves generation of recombinants with polytropic host range. Although P-MLVs are deemed to be the proximal agents of disease induction, few biologically characterized infectious P-MLVs have been sequenced for comparative analysis. We analyzed the complete genomes of 16 naturally occurring infectious P-MLVs, 12 of which were typed for pathogenic potential. We sought to identify ERV progenitors, recombinational hot spots, and segments that are always replaced, never replaced, or linked to pathogenesis or host range. Each P-MLV has an E-MLV backbone with P- or X-ERV replacements that together cover 100% of the recombinant genomes, with different substitution patterns for X- and P-ERVs. Two segments are always replaced, both coding for envelope (Env) protein segments: the N terminus of the surface subunit and the cytoplasmic tail R peptide. Viral gag gene replacements are influenced by host restriction genes Fv1 and Apobec3 Pathogenic potential maps to the env transmembrane subunit segment encoding the N-heptad repeat (HR1). Molecular dynamics simulations identified three novel interdomain salt bridges in the lymphomagenic virus HR1 that could affect structural stability, entry or sensitivity to host immune responses. The long terminal repeats of lymphomagenic P-MLVs are differentially altered by recombinations, duplications, or mutations. This analysis of the naturally occurring, sometimes pathogenic P-MLV recombinants defines the limits and extent of intersubgroup recombination and identifies specific sequence changes linked to pathogenesis and host interactions.IMPORTANCE During virus-induced leukemogenesis, ecotropic mouse leukemia viruses (MLVs) recombine with nonecotropic endogenous retroviruses (ERVs) to produce polytropic MLVs (P-MLVs). Analysis of 16 P-MLV genomes identified two segments consistently replaced: one at the envelope N terminus that alters receptor choice and one in the R peptide at the envelope C terminus, which is removed during virus assembly. Genome-wide analysis shows that nonecotropic replacements in the progenitor ecotropic MLV genome are more extensive than previously appreciated, covering 100% of the genome; contributions from xenotropic and polytropic ERVs differentially alter the regions responsible for receptor determination or subject to APOBEC3 and Fv1 restriction. All pathogenic viruses had modifications in the regulatory elements in their long terminal repeats and differed in a helical segment of envelope involved in entry and targeted by the host immune system. Virus-induced leukemogenesis thus involves generation of complex recombinants, and specific replacements are linked to pathogenesis and host restrictions.

Complete genome sequences of new xenotropic murine leukemia viruses from the senescence-accelerated mouse (SAM): molecular and phylogenetic analyses

Approximately 10% of the mouse genome is constituted by endogenous retroviruses (ERVs), and a number of mouse ERVs remain active. Many copies of endogenous murine leukemia viruses (MuLVs) are detected in the genomes of inbred mouse strains. Some of these MuLVs are transcriptionally active or produce infectious virus particles. Previously, we identified partial env sequences of new xenotropic MuLVs (X-MuLVs) from a senescence-accelerated mouse (SAM) strain. In the present study, we investigated and characterized the complete sequences of the X-MuLVs. The complete genomes and open reading frames (ORFs) of two X-MuLVs, designated xmlv15 and xmlv18 (accession nos. HQ154630 and HQ154631, respectively), were molecularly cloned from the genome of the SAM mice. We confirmed that the xmlv15 and xmlv18 sequences are distinct from all known MuLV genomes and are most similar to DG-75 MuLV. Moreover, we found that common strains of laboratory mice carry our newly identified xmlvs. Additionally, the expression levels of xmlv15-related sequences were much higher in C57BL and ICR mice than in the SAM strains without any stimulators. Our findings suggest that a specific group of endogenous MuLVs is constitutively expressed in the brain and that they may participate in normal functions and/or pathogenic conditions.

Xenotropic and polytropic murine leukemia virus-related sequences are not detected in the majority of patients with chronic fatigue syndrome

XMRV and polytropic MLV-related virus have been controversially associated with chronic fatigue syndrome (CFS). Subsequent reports failed to detect XMRV and MLV-related virus in CFS patients, and the previous results have been interpreted as a massive laboratory contamination by mouse DNA sequences. Among 12 sequential CFS patients, two were positive for XMRV/MLV sequences. In contrast, 40 selected control subjects were negative. CSF patients and controls were negative for mitochondrial mouse-specific DNA sequences. These findings do not confirm the high frequency of MLV-related viruses infection in CFS patients, but also contrast the widespread laboratory contamination previously suggested.

Complete genome sequences of the two viral variants of the Graffi MuLV: Phylogenetic relationship with other murine leukemia retroviruses

A detailed phylogenetic analysis of two variants of the Graffi murine retrovirus, GV-1.2 and GV-1.4, showed that they are closely related to SRS 19-6 and Moloney MuLVs. Two stretches of sequence testify to the divergence between Graffi and SRS 19-6 MuLVs, one corresponding to a recombination event of Graffi MuLV with a xenotropic virus. Moloney MuLV was found more distant, particularly in the GAG region. Our study encompasses every class of MuLVs (ecotropic, amphotropic, xenotropic, polytropic) with some focus on exogenous ecotropic viruses and further adds to previous phylogenetic studies. Graffi, SRS 19-6, Moloney, Friend and Rauscher MuLVs form a cluster that appears to share a common ancestor with the Casitas-amphotropic and -ecotropic MuLVs but are more distant to the Akv-type and xenotropic MuLVs. The analysis also revealed that the ENV region of HEMV, the prototype of the MuLV ancestor, was closely related to the corresponding region of Cas-Br-E.

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.

In vivo interactions of ecotropic and polytropic murine leukemia viruses in mixed retrovirus infections

Mixed retrovirus infections are the rule rather than the exception in mice and other species, including humans. Interactions of retroviruses in mixed infections and their effects on disease induction are poorly understood. Upon infection of mice, ecotropic retroviruses recombine with endogenous proviruses to generate polytropic viruses that utilize different cellular receptors. Interactions among the retroviruses of this mixed infection facilitate disease induction. Using mice infected with defined mixtures of the ecotropic Friend murine leukemia virus (F-MuLV) and different polytropic viruses, we demonstrate several dramatic effects of mixed infections. Remarkably, inoculation of F-MuLV with polytropic MuLVs completely suppressed the generation of new recombinant viruses and dramatically altered disease induction. Co-inoculation of F-MuLV with one polytropic virus significantly lengthened survival times, while inoculation with another polytropic MuLV induced a rapid and severe neurological disease. In both instances, the level of the polytropic MuLV was increased 100- to 1,000-fold, whereas the ecotropic MuLV level remained unchanged. Surprisingly, nearly all of the polytropic MuLV genomes were packaged within F-MuLV virions (pseudotyped) very soon after infection. At this time, only a fractional percentage of cells in the mouse were infected by either virus, indicating that the co-inoculated viruses had infected the same small subpopulation of susceptible cells. The profound amplification of polytropic MuLVs in coinfected mice may be facilitated by pseudotyping or, alternatively, by transactivation of the polytropic virus in the coinfected cells. This study illustrates the complexity of the interactions between components of mixed retrovirus infections and the dramatic effects of these interactions on disease processes.

Linkage of reduced receptor affinity and superinfection to pathogenesis of TR1.3 murine leukemia virus

TR1.3 is a Friend murine leukemia virus (MLV) that induces selective syncytium induction (SI) of brain capillary endothelial cells (BCEC), intracerebral hemorrhage, and death. Syncytium induction by TR1.3 has been mapped to a single tryptophan-to-glycine conversion at position 102 of the envelope glycoprotein (Env102). The mechanism of SI by TR1.3 was examined here in comparison to the non-syncytium-inducing, nonpathogenic MLV FB29, which displays an identical BCEC tropism. Envelope protein expression and stability on both infected cells and viral particles were not statistically different for TR1.3 and FB29. However, affinity measurements derived using purified envelope receptor binding domain (RBD) revealed a reduction of >1 log in the K(D) of TR1.3 RBD relative to FB29 RBD. Whole-virus particles pseudotyped with TR1.3 Env similarly displayed a markedly reduced binding avidity compared to FB29-pseudotyped viral particles. Lastly, decreased receptor affinity of TR1.3 Env correlated with the failure to block superinfection following acute and chronic infection by TR1.3. These results definitively show that acquisition of a SI phenotype can be directly linked to amino acid changes in retroviral Env that decrease receptor affinity, thereby emphasizing the importance of events downstream of receptor binding in the cell fusion process and pathology.

Characterization of hortulanus endogenous murine leukemia virus, an endogenous provirus that encodes an infectious murine leukemia virus of a novel subgroup

Simple retroviruses present a unique opportunity for examining the host-virus relationship. Following exogenous infection and integration into the germ line, copies of these viruses can become fixed within the genome. The resulting endogenous proviral "fossils" represent a record of past retroviral infections and forms. Previous work in our laboratory has been directed at dissecting the extensive nonecotropic murine leukemia virus content of the mouse genome. One such provirus, hortulanus endogenous murine leukemia virus (HEMV), found in a single copy in the genome of Mus spicilegus, was remarkable for characteristics that suggested that it was ancient and related to the hypothetical common ancestor of murine leukemia viruses (MLVs) and other gammaretroviral species. In the present study, we have analyzed its functional properties. Transfection of a molecular clone of the HEMV provirus into mouse-derived cell lines revealed that it is replication competent. Furthermore, host range and interference studies revealed a strictly ecotropic host range and the use of a receptor distinct from those used by other classical MLVs. The identity of nucleotide sequence of the long terminal repeats (LTRs) further suggested that HEMV is a relatively recent insertion into the M. spicilegus genome at the distal end of chromosome 7. Although unique to M. spicilegus, its presence in a homozygous state in three individuals obtained from different regions implies that it has been present long enough to become fixed in this species. Exhaustive phylogenetic analysis of all regions of the HEMV genome supported the previously assigned ancestral position of HEMV relative to other MLV-related viruses. Thus, HEMV is a relatively recent introduction into the Mus germ line but is representative of a relatively ancestral MLV group.

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.

Injury-associated induction of two novel and replication-defective murine retroviral RNAs in the liver of mice

Injury can alter the expression of numerous genes in affected tissues as well as in distant organs. The mouse genome harbors numerous copies of endogenous murine leukemia virus (MuLV)-related retroviral sequences. Mouse liver tissues harvested after burn injury were subjected to RT-PCR analysis to investigate the regulation of MuLV-related sequences using a primer set capable of amplifying the full-length transcript. A doublet of approximately 5-kb was transiently up-regulated at 3 and 6 h after injury. Sequence analyses revealed that these are novel defective endogenous retroviral sequences (MuLV(LI-8) and MuLV(LI-12)), which are predominantly characterized by major deletions in pol and env genes. The MuLV(LI-8) clone is 4.85 kb long and the deduced gag polypeptide sequence was almost identical to a previously reported replication-defective retroviral sequence associated with immunesuppression. In the MuLV(LI-12) clone of 5.06 kb, there were two truncated gag open reading frames (ORFs) and 1 pol ORF fused to the C-terminus of the env p15E. Furthermore, the ORFs for the unique gag p12 presumed to be responsible for the immunesuppression were present in both clones. These novel replication-defective MuLVs may participate in the pathogenesis of distant organs after injury.

Virological properties and nucleotide sequences of Cas-E-type endogenous ecotropic murine leukemia viruses in South Asian wild mice, Mus musculus castaneus

Two types of endogenous ecotropic murine leukemia viruses (MuLVs), termed AKV- and Cas-E-type MuLVs, differ in nucleotide sequence and distribution in wild mouse subspecies. In contrast to AKV-type MuLV, Cas-E-type MuLV is not carried by common laboratory mice. Wild mice of Mus musculus (M. m.) castaneus carry multiple copies of Cas-E-type endogenous MuLV, including the Fv-4(r) gene that is a truncated form of integrated MuLV and functions as a host's resistance gene against ecotropic MuLV infection. Our genetic cross experiments showed that only the Fv-4(r) gene was associated with resistance to ecotropic F-MuLV infection. Because the spontaneous expression of infectious virus was not detected in M. m. castaneus, we generated mice that did not carry the Fv-4(r) gene but did carry a single or a few endogenous MuLV loci. In mice not carrying the Fv-4(r) gene, infectious MuLVs were isolated in association with three of six Cas-E-type endogenous MuLV loci. The isolated viruses showed a weak syncytium-forming activity for XC cells, an interfering property of ecotropic MuLV, and a slight antigenic variation. Two genomic DNAs containing endogenous Cas-E-type MuLV were cloned and partially sequenced. All of the Cas-E-type endogenous MuLVs were closely related, hybrid-type viruses with an ecotropic env gene and a xenotropic long terminal repeat. Duplications and a deletion were found in a restricted region of the hypervariable proline-rich region of Env glycoprotein.

Mus cervicolor murine leukemia virus isolate M813 belongs to a unique receptor interference group

Murine leukemia virus (MuLV) M813 was originally isolated from the Southeast Asian rodent Mus cervicolor. As with the ecotropic MuLVs derived from Mus musculus, its host range is limited to rodent cells. Earlier studies have mapped its receptor to chromosome 2, but it has not been established whether M813 shares a common receptor with any other MuLVs. In this study, we have performed interference assays with M813 and viruses from four interference groups of MuLV. The infection efficiency of M813 was not compromised in cells expressing any one of the other MuLVs, demonstrating that M813 must use a distinct receptor for cell entry. The entire M813 env coding region was molecularly cloned. Sequence analysis revealed high similarity with other MuLVs but with a unique receptor-binding domain. Substitution of M813 env sequences in Moloney MuLV resulted in a replication-competent virus with a host range and interference profile similar to those of the biological clone M813. M813 thus defines a novel receptor interference group of type C MuLVs.

Molecular and phylogenetic analysis of SRS 19-6 murine leukemia virus

The complete nucleotide sequence of the genome of Solid-type Reticulum cell Sarcoma 19-6 murine leukemia virus (SRS 19-6 MuLV) was determined. This virus was isolated in mainland China from laboratory mice that had been separated from western mice since the 1930s. The genome is 8,256 nucleotides in length and exhibits a genetic organization characteristic of replication competent MuLVs. Phylogenies constructed from reverse transcriptase (RT) domains showed that SRS 19-6 MuLV is closely related to other MuLV-related retroviruses; however, it has clearly diverged from previously isolated MuLVs. Comparative sequence analysis of the env sequences indicated that SRS 19-6 MuLV encodes a surface (SU) glycoprotein that is related to other ecotropic MuLVs in the VR-A and VR-B variable regions. However, SRS 19-6 MuLV env glycoprotein was distinct from all other MuLVs (ecotropic and non-ecotropic) in the proline-rich hypervariable region. No evidence for recombination with endogenous MuLV env sequences in generation of SRS 19-6 MuLV was observed. Comparisons of long terminal repeat (LTR) sequences revealed that the GV 1.4 molecular clone of Graffi MuLV contained 96% sequence identity to SRS 19-6 MuLV's LTR with 99% identity when comparisons were restricted to the U3 regions of the two viruses. The consensus enhancer binding motifs contained in the U3 regions of the two viruses were nearly identical. Nevertheless the two viruses have previously been shown to induce distinct patterns of disease. Comparisons between 196 and Graffi GV1.4 MuLVs may provide insights into the mechanisms of disease specificity induced by MuLVs.

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.

Phylogenetic relationship of the complete Rauscher murine leukemia virus genome with other murine leukemia virus genomes

We report the complete nucleotide sequence of the genome of Rauscher murine leukemia virus (R-MuLV), the replication-competent helper virus present in the Rauscher virus complex, and its phylogenetic relationship with other murine leukemia virus genomes. An overall sequence identity of 97.6% was found between R-MuLV and the Friend helper virus (F-MuLV), and the two viruses were closely related on the phylogenetic trees constructed from either gag, pol, or env sequences. Moloney murine leukemia virus (Mo-MuLV) was the next closest relative to R-MuLV and F-MuLV on all trees, followed by Akv and radiation leukemia virus (RadLV). The most distantly related helper virus was Hortulanus murine leukemia virus (Ho-MuLV). Interestingly, Cas-Br-E branched with Mo-MuLV on the gag and pol trees, whereas on the env tree, it revealed the highest degree of relatedness to Ho-MuLV, possibly due to an ancient recombination with an Ho-MuLV ancestor. In summary, a phylogenetic analysis involving various MuLVs has been performed, in which the postulated close relationship between R-MuLV and F-MuLV has been confirmed, consistent with the pathobiology of the two viruses.

Characterization of a novel murine leukemia virus-related subgroup within mammals

The murine leukemia virus (MuLV)-related retroviruses are one of seven genera which together constitute the family Retroviridae. They are widespread as both endogenous and exogenous agents within vertebrates and have been associated with a variety of malignancies and other disorders. We isolated and characterized 12 endogenous representatives of this genus from a number of mammalian hosts. Subsequent sequence analysis revealed that the isolated viruses cluster into two clearly distinct groups. All of the exogenous MuLV-related retroviruses which have been isolated to date, as well as several endogenous examples, fall into the first group, whereas the second group is represented solely by endogenous representatives, including human endogenous retrovirus type E (HERV.E). The two groups are widespread within mammals, with both often present within one animal species. Despite this, there is no evidence to date that recombination between members of the different groups has occurred. Genetic distances and several other properties of the HERV.E genome suggest that if exogenous members of this subgroup exist, they are likely to have biological properties different from those of the other exogenous viruses of this genus. Several of these viruses are known to have been integrated within their hosts' genomes for a long period of time, and a most recent divergence date for the MuLV and HERV.E subgroups can thus be proposed. This date, approximately 30 million years ago, is the most recent date possible, and it is probable that the actual period of time since their divergence is significantly longer.

The amphotropic and ecotropic murine leukemia virus envelope TM subunits are equivalent mediators of direct membrane fusion: implications for the role of the ecotropic envelope and receptor in syncytium formation and viral entry

The murine leukemia virus (MuLV) envelope protein was examined to determine which sequences are responsible for the differences in direct membrane fusion observed with the ecotropic and amphotropic MuLV subtypes. These determinants were studied by utilizing amphotropic-ecotropic chimeric envelope proteins that have switched their host range but retain their original fusion domain (TM subunit). Fusion was tested both in rodent cells and in 293 cells bearing the human homolog of the ecotropic MuLV receptor. The results demonstrate that the amphotropic TM is able to mediate cell-to-cell fusion to an extent equivalent to that mediated by the ecotropic TM, indicating that their fusion domains are equivalent. The "murinized" human homolog of the ecotropic receptor supports syncytium formation as well as the native murine receptor. These findings suggest that interactions between the ecotropic envelope protein and conserved sequences in the ecotropic receptor are the principal determinants of syncytium formation. The relationship of the fusion phenotype to pH-dependent infection and the route of viral entry was examined by studying virions bearing the chimeric envelope proteins. Such virions appear to enter cells via a pathway that is directed by the host range-determining region of their envelope rather than by sequences that confer pH dependence. Therefore, the pH dependence of infection may not reflect the initial steps in viral entry. Thus, it appears that both the syncytium phenotype and the route of viral entry are properties of the viral receptor, the amino-terminal half of the ecotropic envelope protein, or the interaction between the two.

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.

pH-independent murine leukemia virus ecotropic envelope-mediated cell fusion: implications for the role of the R peptide and p12E TM in viral entry

Murine leukemia virus ecotropic and amphotropic envelope expression vectors were genetically engineered to generate truncations of the p15E TM cytoplasmic tail. The ecotropic construct CEET has the entire cytoplasmic tail of TM deleted, while the CEETR construct has only the R peptide portion of the tail deleted, thereby producing a TM subunit (p12E) that is identical to the one found in mature virions. The analogous amphotropic constructs were called CAET and CAETR. These envelopes, as opposed to their p15E TM counterparts, mediate cell-to-cell fusion at neutral pH in both transformed and nontransformed cell lines. Though the TM cytoplasmic domain is not required, its presence appears to augment such cell-to-cell fusion. This envelope-dependent fusion requires the presence of the viral receptor on the cell surface. Ecotropic virions bearing the p12E TM contain wild-type levels of the envelope complex and have near-normal titers. In contrast, virions which lack the cytoplasmic domain of TM (e.g., CEET) have 10- to 100-fold-lower titers but contain normal amounts of envelope. Both of the corresponding amphotropic virions contain normal amounts of envelope but have 10- to 100-fold-lower titers. Using immunofluorescent detection of envelope to monitor the fate of receptor-bound virions, we found that ecotropic murine leukemia virus envelope disappears from the cell surface while amphotropic envelope persists on the cell surface after virus binding. This pattern of immunofluorescence is consistent with the proposed routes of cell entry for these viruses, i.e., by endocytosis and direct fusion, respectively. In this assay, ecotropic virions bearing the genetically engineered p12E TM also appear to be internalized despite the ability of their envelope to mediate fusion at neutral pH in the same target cells. Our results show that direct fusion at neutral pH is a natural consequence of the surface expression of the mature ecotropic envelope and its receptor. We propose that the processing of the R peptide from the envelope TM (p15E) to yield p12E, at the time of virus budding or within virions, renders the envelope competent to fuse.

Clonal heterogeneity of anti-AKR/gross leukemia virus cytotoxic T lymphocytes. Evidence for two distinct antigen systems

AKR/Gross leukemia virus-induced tumor reactive cytotoxic T lymphocyte (CTL) clones were derived from C57BL/6 spleen cells. Analysis of their specificity pattern was performed by using a panel of target cells such as E male G2 and AKR.H-2bSL1 (susceptible tumors to polyclonal anti-AKR/Gross virus CTL), and cl. 18-5 and cl. 18-12 (insusceptible variant sublines derived from AKR.H-2bSL1). Several of these CTL clones were selected for further study. Lysis of Gross cell surface antigen-positive tumor cells by these clones was restricted by the H-2Kb molecule. The cell surface phenotype of these clones was Thy-1.2+, Lyt-2.2+, L3T4-, a phenotype consistent with that of polyclonal anti-AKR/Gross CTL, suggesting that they were of conventional CTL origin. According to their fine specificity pattern, the CTL clones were divided into two major groups (A and B) which were further subdivided into five and three subgroups, respectively. The specificity of group A clones was essentially the same as that of the standard polyclonal CTL population except for a variable level of natural killer-like activity by some of the CTL clones. That is, group A clones did not efficiently lyse the insusceptible variant tumors nor any of Friend-Moloney-Rauscher-positive tumors tested, but they showed strong lytic activity to susceptible tumors and iododeoxyuridine-treated insusceptible variants. Thus, their CTL activity appeared to be strictly directed to Gross cell surface antigen-positive tumors that are susceptible to polyclonal anti-AKR/Gross virus CTL. In contrast, group B clones could lyse both susceptible and insusceptible variant tumors and also a Friend virus-induced tumor (FBL3). Therefore, as defined by these CTL clones, at least two distinct antigenic systems (A and B), each with several antigenic determinants, appeared to be present. Because recent findings suggested that most of the polyclonal anti-AKR/Gross virus CTL activity appeared to be directed to N-ecotropic proviral determinants, we further investigated the nature of these two antigenic systems by use of additional target cells including lipopolysaccharide (LPS)-stimulated spleen cell blasts from AKXL recombinant inbred strains and retrovirus-infected fibroblasts. Group A clones could lyse all LPS blasts derived from AKXL recombinant inbred strains containing the AKV-1 proviral genome, but lysed only very insufficiently or did not lyse AKV-1-negative blasts containing the AKV-3 and/or AKV-4 provirus.(ABSTRACT TRUNCATED AT 400 WORDS)

The four classes of endogenous murine leukemia virus: structural relationships and potential for recombination

The process by which leukemogenic viruses are generated during the lifetime of certain strains of mice is poorly understood. We have therefore set out to define all the murine leukemia virus-related endogenous proviruses of HRS/J mice. We have cloned 34 different proviral fragments and their flanking cellular sequences. These have been characterized by restriction enzyme analysis, by fingerprinting in vitro-synthesized RNA, and by DNA sequencing. We conclude that all the proviruses can be assigned into one of four different classes: the previously characterized ecotropic, xenotropic, and polytropic viruses, as well as a new class we have termed modified polytropic viruses. The xenotropic, polytropic, and modified polytropic classes are closely related to one another, but as a group they differ considerably from the ecotropic class. Sequence analyses show that both polytropic and modified polytropic sequences can contribute env sequences to recombinant viruses.

Different recombinant murine leukemia viruses use different cell surface receptors

Retroviruses can be grouped by viral interference measurements into classes which use common cell surface receptors. We previously tested a large number of isolates of mink cell focus-inducing (MCF) murine leukemia viruses (MuLVs), and reported that all of them share a distinct receptor on NIH/3T3 cells (A. Rein, Virology 120, 251, 1982). We now extend this generalization to several additional recombinant isolates, including two (SL3-2 and GPA-V2) which would not be considered MCFs on the basis of host-range data. We note the superiority of interference tests, based on positive, unambiguous data, over host-range tests for virus classification. We also show that in contrast to the MCFs, which are all derived from ecotropic MuLVs, a recombinant derived from wild mouse amphotropic MuLV (S. Rasheed et al., Int. J. Cancer 29, 345, 1982) uses a unique receptor on NIH/3T3 cells. This suggests that (a) mouse cells contain more than one type of endogenous env sequence; and (b) there is some specificity in the generation of recombinants, since ecotropic MuLVs appear to give rise only to MCFs, while amphotropic MuLV has generated a distinct type of recombinant. It also represents a second case (in addition to the MCFs) in which an env gene recombinant is more pathogenic than its exogenous parent. We also show that xenotropic MuLV does not interfere with MCFs in NZB mouse cells; thus, despite the close homology between MCF and xenotropic env sequences, the gp70 of xenotropic MuLV appears to have no detectable affinity for the MCF receptor.

Endogenous mouse leukemia viruses

The earlier demonstration that genes of the mouse greatly influenced the spontaneous incidence of lymphoma was among the more persistent barriers to general acceptance of a viral etiology of this disease. We now can be fairly certain that some of those mouse genes are the DNA life phase of a class of retrovirus known as murine leukemia virus. These MuLV, although related in sequence to each other, are a collection of viruses that show diverse patterns of host range and tissue tropisms. The host-range properties of MuLV serve as means of classifying them into related families known as ecotropic, xenotropic, and amphotropic, and are probably dictated by determinants on gp70. The preferential abilities to replicate in different tissues, on the other hand, may be dictated by the controlling sequences located at the 3' end of the genome, known as U3. MuLV genomes are located at many different sites in the mouse genome. The viral genomes found at those sites can be induced to be expressed with different efficiencies spontaneously in vivo, by chemicals in vitro, or by DNA transfection. Certain MuLV genomes can also interact to increase expression perhaps by recombination or trans complementation. Although the molecular mechanisms that explain these phenomena are not yet clear, the phenomenon of differential expression has important pathological consequences, particularly in the development of lymphoma. The complex process by which endogenous MuLV induce leukemia appears to involve the expression and interaction of multiple MuLV genomes. It seems apparent that expression of an MCF-like gp70 is an invariant aspect of this process, and that observation suggests that this molecule, like the SFFV gp52, may indeed serve to stimulate cell proliferation. The most common means of expressing such a molecule at elevated levels appears to involve recombining it into an ecotropic genome that replicates with high efficiency. Thus, the viral requirements for leukemogenesis may depend on both efficient and perhaps tissue tropic replication as well as on the expression of a particular gp70.(ABSTRACT TRUNCATED AT 400 WORDS)

Close similarity between endogenous ecotropic virus of Mus musculus molossinus and AKR virus

By using seven different restriction endonucleases, the cleavage patterns of the unintegrated provioral DNA from an ecotropic murine leukemia virus isolated from Mus musculus molossinus were found to be identical to those of AKR virus. An AKR [3H]DNA probe can be completely saturated with M. musculus molossinus and M. musculus castaneus DNAs, although the arrangement of viral sequences in M. musculus molossinus DNA differed from that of AKR virus. These studies indicate that an AKR-type ecotropic virus is present in some wild Asiatic mice.

G(AKSL2): a new cell surface antigen of the mouse related to the dualtropic mink cell focus-inducing class of murine leukemia virus detected by naturally occurring antibody

Normal mouse sera were tested for cytotoxic antibody to surface antigens of cultured monolayer cells infected with AKR-derived ecotropic MuLV, xentropic MuLV, or dualtropic MCF 247 MuLV. Antibody to ecotropic MuLV-infected cells was found in a proportion of C57BL/6, C3Hf/Bi, AKR-Fv-1b, and (C3Hf/Bi X AKR)F1 mice, but not AKR or (AKR X C3Hf/Bi)F1 mice. Antibody to xenotropic MuLV-infected cells was virtually restricted to C57BL/6 mice. Antibody to MCF 247-infected cells was found in all strains tested, including AKR mice. Absorption analysis of (C3Hf/Bi x akr)f1 and AKR-Fv-1b sera with selective reactivity for MCF 247-infected cells showed that these sera recognize distinctive antigens on MCF 247-infected cells that are not present on ecotropic or xenotropic MuLV-infected cells. The transplantable AKR spontaneous leukemia AKSL2 was found to be uniquely sensitive to the cytotoxic action of naturally occurring antibody to MCF 247-related antigens and absorption tests with AKSL2 as the target cell and sera from a single AKR-Fv-1b mouse have permitted the definition of a new MuLV-related cell surface antigen, which has been designated G(AKSL2). Thymocytes from young mice of high leukemia-incidence strains (AKR, C58, and PL) express G(AKSL2), whereas thymocytes from 12 other strains do not. In AKR mice, the antigen is expressed in higher amounts on cells from thymus and bone marrow than on spleen cells. All AKR spontaneous leukemias tested express G(AKSL2), as did three MuLV-induced leukemias arising in G(AKSL2)- strains. Five X-ray-induced leukemias of G(AKSL2)- strains were G(AKSL2)-, as were MuLV+ and MuLV- chemically induced sarcomas. In the limited survey conducted to date, natural antibody to G(AKSL2) has been restricted to strains expressing G(AKSL2) in their normal tissues: AKR, AKR congenic mice AKR-Fv-1b and AKR hybrid mice (C3Hf/Bi x akr)f1 and (C57BL/6 X AKR)F1. In vitro G(AKSL2) induction tests involving MuLV infection of cultured monolayer cells showed that 8 of 12 newly isolated dualtropic MuLV shared the property of G(AKSL2) induction with the prototype MCF MuLV, MCF 247. Of the 12 ecotropic MuLV tested, only the N-tropic MuLV isolated from a leukemia originally induced by Passage A Gross virus induced G(AKSL2). The xenotropic and amphotropic MuLV isolates tested lacked G(AKSL2) inducing activity. Recognition of the g(aksl2) system provides a way to trace the origin and natural history of a class of dualtropic MCF MuLV in the mouse and to determine whether natural antibody to G(AKSL2) plays a role in AKR leukemogenesis.

Leukemogenicity of clonal isolates of murine leukemia viruses

The leukemogenicity of three types of cloned, in vitro grown murine retroviruses was studied. Two Moloney virus clones caused leukemia, as did five clones of the B-tropic endogenous virus of BALB/c mice. Neither of two clones of N-tropic BALB/c virus caused leukemia in Fv-1n/n mice, and the viruses were not recoverable from the animals. The ability to induce leukemia therefore appeared to reside in the genome of at least certain nondefective murine retroviruses.

A method for classification of 5' termini of retroviruses

A new method for the classification of retroviruses is presented. The scheme is based on the length and sequence of a DNA transcript of the 5' end of the genome. The method can be used to detect similarities between distantly related viruses as well as to discriminate between very closely related viruses. The method is applied to viruses isolated from mice, baboons, gibbons, a woolly monkey and chickens.

The effect of helper virus on Abelson virus-induced transformation of lymphoid cells

Abelson murine leukemia virus (A-MuLV)-transformed fibroblast nonproducer cells were used to prepare A-MuLV stocks containing a number of different helper viruses. The oncogenicity of the A-MuLV stocks was tested by animal inoculation and their ability to transform normal mouse bone marrow cells was measured in vitro. All of the A-MuLV stocks transformed fibroblast cells efficiently. However, only A-MuLV stocks prepared with helper viruses that are highly oncogenic were efficient in vivo and in vitro in hematopoietic cell transformation. In addition, inefficient helpers did not establish a stable infection in lymphoid nonproducer cells. Thus, helper virus has a more central role in lymphoid cell transformation than in fibroblast cell transformation.

Segregation of genetic information for a B-tropic leukemia virus with the structural locus for BALB:virus-1

A B-tropic type-C RNA virus isolatable from lymphoreticular tumors of the inbred BALB/c mouse strain has previously been shown to be leukemogenic in its natural host. This virus is not chemically inducible from BALB/c embryo cells or from embryo lines containing segregating inducibility loci for two known endogenous type-C viruses of BALB/c cells. Molecular hybridization and type-specific immunologic assays demonstrate a high degree of genetic homology between the B-tropic leukemia virus and BALB:virus-1, an N-tropic endogenous virus of BALB/c cells. Genetic sequences specific for BALB:virus-1 are shown to segregate with the locus for BALB:virus-1 induction in genetic crosses between BALB/c and the noninducible NIH Swiss strain. Thus, if the information of the B-tropic virus is encoded in the genome of the animal, it must be closely linked to the structural locus for BALB:virus-1. The evidence is consistent with a mechanism by which a small genetic alteration in BALB:virus-1 leads to a virus, whose growth is unrestricted, and subsequently to the development of neoplasia.

Biological and serological characterization of radiation leukemia virus

Radiation leukemia virus, isolated from radiation-induced lymphomas in C57BL/Ka mice and propagated in that strain, is thymotropic and leukemogenic in vivo but replicates poorly, if at all, in mouse and mink fibroblast cultures in vitro. Comparative studies indicate that this naturally occurring virus is distinct from the previously recognized classes of endogenous murine ecotropic and xenotropic C-type viruses which are capable of replication on fibroblasts (fibrotropic) but are neither thymotropic nor leukemogenic. These studies also demonstrate that a differentiation-specific restriction system governing the replication of the murine ecotropic C-type viruses operates in addition to the previously defined Fv-1 and SRV gene restriction systems.

Nucleic acid homology of murine xenotropic type C viruses

Two major subclasses of xenotropic (X-tropic) murine type C viruses can be distinguished by nucleic acid hybridization. The most frequently encountered subclass (MuLV-X-alpha) includes isolates from BALB/c, C57BL/6J, C58/J, AKR/J, CBA/J, and DBA/2J inbred strains and from the Asian feral mouse subspecies Mus musculus molossinus. The other subclass (MuLV-X-beta) consists of viruses isolated from the NIH Swiss and NZB/BINJ strains. Thus, significant polymorphism exists among the endogenous type C virogenes of a single species, Mus musculus. MuLV-X-alpha genes are found in strains that also have endogenous mouse-tropic viruses (either N-tropic, B-tropic, or both), whereas the MuLV-X-beta subclass is restricted to mouse strains from which mouse-tropic viruses have not yet been isolated. The results are consistent with a model which proposes that mouse-tropic endogenous viruses are derived from the MuLV-X-alpha subclass.

Xenotropic viruses: murine leukemia viruses associated with NIH Swiss, NZB, and other mouse strains

Murine leukemia virus activity is present in tissues from NIH Swiss and other mouse strains after cocultivation with nonvirus-yielding rat cells transformed by Harvey sarcoma virus. The resulting pseudotype sarcoma virus has the same type-specific coat as the virus previously isolated from New Zealand black (NZB) mice, and, like the NZB virus, it is unable to infect mouse cells. The results show that this NZB type virus is endogenous in other strains of mice and is xenotropic; that is, it grows only in cells foreign to the host. This is the first clear demonstration that NIH Swiss mice also carry indigenous infectious murine leukemia virus.

Homology between type-C viruses of various species as determined by molecular hybridization

Two strains of feline leukemia virus, two endogenous feline type-C viruses (RD/CCC group), several endogenous and laboratory strains of murine "leukemia" virus, two rat viruses, two primate viruses (woolly monkey and gibbon ape), as well as hamster, pig, and avian type-C viruses were examined for their relatedness to one another by molecular hybridization. The extent of nucleic-acid homology was determined by hybridization of the various viral RNAs to a [(3)H]DNA product synthesized from each virus. Among the murine type-C viruses (Rauscher, Kirsten, AT-124, and endogenous BALB/c virus) a high degree of homology is observed, although the viruses are not identical. The two primate viruses are also closely related to one another. The feline, rat, hamster, and pig endogenous viruses can be readily distinguished from one another and from the murine and primate viruses since their DNA products share very little or no nucleic-acid homology. However, the murine and primate type-C virus groups possess a surprising degree of relatedness. Feline type-C viruses fall into two distinct groups, the feline leukemia virus group and the RD-114/CCC group, with little detectable nucleic-acid homology between them. Infection of feline or rat cells with type-C virus results in production of the endogenous type-C virus of the species along with the infecting virus.

Independent segregation of loci for activation of biologically distinguishable RNA C-type viruses in mouse cells

Two genetic loci for induction of RNA C-type virus in BALB/c mouse embryo cells segregate independently in backcross embryo cell lines. The viruses at these loci are shown to be biologically distinguishable. The first, described previously, codes for activation of a virus that grows preferentially in NIH Swiss embryo cells. The newly detected locus codes for activation of a virus that grows poorly, if at all, in NIH Swiss or BALB/c cells, but replicates well in a rat cell line. Its serologic properties are different from those of viruses of the two major serologic subgroups of murine leukemia virus. The present findings provide support for the hypothesis that the loci for virus induction in BALB/c cells represent genetic information for C-type viruses and that the endogenous viruses are subject to different cellular genetic controls.

A classification of the murine leukemia viruses. Neutralization of pseudotypes of Friend spleen focus-forming virus by type-specific murine antisera

Coinfection of neonatal BALB/c mice with helper-dependent Friend spleen focus-forming virus (SFFV), as contained in the Friend virus (FV) complex, and antigenically distinct Moloney leukemia virus (MolLV) resulted in the recovery of a MolLV pseudotype of SFFV, abbreviated SFFV(MolLV). The antigenic alteration of SFFV was observed by following its neutralization kinetics in vitro by specific Friend or Moloney typing antiserum. Effective pseudotype production was accomplished only when N-tropic LLV-F (the natural helper virus in the FV complex) was inhibited in B-type mice coinfected with an NB-tropic MolLV or other murine leukemia virus (MuLV) preparation. SFFV pseudotypes could not be prepared by using murine viruses other than leukemia viruses. SFFV prepared after two serial passages in the presence of MolLV was effectively neutralized by Moloney antiserum, but not by Friend typing antiserum; therefore, the envelope of the pseudotype virus, SFFV(MolLV), is homogeneous. Pseudotype virus was antigenically stable in the absence of continued mixed infection of BALB/c mice with SFFV(MolLV) and MolLV. However, SFFV(MolLV) was easily converted back to the LLV-F type after only one passage in BALB/c mice coinfected with NB-tropic LLV-F. The antigenic interconversion between LLV-F and MolLV types demonstrated that SFFV is defective with respect to the expression of neutralizable envelope antigens. Analysis of the neutralizable envelope antigens of nine SFFV(MuLV) pseudotypes by a panel of seven typing antisera made possible a "type-specific" SFFV(MuLV) envelope classification. Two major categories have been identified which correspond to the Gross (G) and Friend-Moloney-Rauscher (FMR) subgroups. Further, the FMR subgroup was divided into four types on the basis of distinct neutralization patterns. These results indicated that the specificity observed by cytotoxic G vs. FMR antisera is different from that observed by neutralization kinetics. We therefore suggest that the specific antigens revealed by virus neutralization tests be referred to as type specific.

In vitro and in vivo observations on a murine C-type virus

From 40 discrete mouse tissue culture cell lines examined by electron microscopy or complement fixation, or both, for the presence of detectable virus, one (NCTC 4705), initiated and maintained on chemically defined medium, was chosen for a more extensive study. Virus-like particles (100 to 110 mmu), morphologically similar to previously reported immature and mature C-type leukemia virus particles, were found budding from the plasma membrane and free in the intracellular spaces of cells in tissue culture and in fibrosarcomas resulting from intramuscular implants of these tissue cultures. Complement-fixation tests for group reactive murine leukemia antigens were positive, with titers consistently higher to a broadly reactive anti-serum than to anti-Friend, anti-Moloney, or anti-Rauscher sera. The 4705 virus was neutralized by Gross antiserum, but not by the F-M-R antisera. When injected into DD, BALB/c, or C3H/He newborn mice, the virus thus far has manifested no leukemogenicity, though virus from tumor extracts and tissue culture medium has been shown to be capable of infecting C3H and Swiss mouse embryo tissue cultures and successfully replicating in them. The role of the virus in accelerating or inducing neoplastic transformation in NCTC 4705 is still not known. When it was introduced into NCTC [ill], a non-neoplastic cell line in other respects similar to NCTC 4705, 4823 manifested no signs of neoplastic transformation after harboring the virus more than 300 days in vitro.