Three laboratory strains of spleen focus-forming virus: comparison of their genomes and translational products

The myeloproliferative sarcoma virus causes transformation or erythroid progenitor cells in vitro

The myeloproliferative sarcoma virus induces spleen focus formation in vivo and transforms fibroblasts in vitro. We showed in this study that in vitro infection of spleen or bone marrow cells from susceptible mice with the myeloproliferative sarcoma virus leads to the formation of erythroid bursts. Under optimal conditions erythroid bursts formed in the absence of added erythropoietin, but the addition of as little as 0.05 U of erythropoietin per ml to infected cultures resulted in a significant increase in numbers of erythroid bursts and the proportion of hemoglobinized cells. A comparison of the kinetics of burst formation and the size of the induced bursts with those induced with Friend virus suggested that either sarcoma virus such as the myeloproliferative sarcoma virus or the target cells for the two viruses were not the same. Density characterization and heat lability studies indicated that the increased erythroid proliferation in vitro was a virus-induced event, but the possibility that the induced erythroid burst formation is mediated via interaction with a nonerythroid target cell and subsequent release of a soluble factor cannot be ruled out.

The myeloproliferative sarcoma virus causes transformation or erythroid progenitor cells in vitro

The myeloproliferative sarcoma virus induces spleen focus formation in vivo and transforms fibroblasts in vitro. We showed in this study that in vitro infection of spleen or bone marrow cells from susceptible mice with the myeloproliferative sarcoma virus leads to the formation of erythroid bursts. Under optimal conditions erythroid bursts formed in the absence of added erythropoietin, but the addition of as little as 0.05 U of erythropoietin per ml to infected cultures resulted in a significant increase in numbers of erythroid bursts and the proportion of hemoglobinized cells. A comparison of the kinetics of burst formation and the size of the induced bursts with those induced with Friend virus suggested that either sarcoma virus such as the myeloproliferative sarcoma virus or the target cells for the two viruses were not the same. Density characterization and heat lability studies indicated that the increased erythroid proliferation in vitro was a virus-induced event, but the possibility that the induced erythroid burst formation is mediated via interaction with a nonerythroid target cell and subsequent release of a soluble factor cannot be ruled out.

Neural stem cells as engraftable packaging lines can mediate gene delivery to microglia: evidence from studying retroviral env-related neurodegeneration

The induction of spongiform myeloencephalopathy by murine leukemia viruses is mediated primarily by infection of central nervous system (CNS) microglia. In this regard, we have previously shown that CasBrE-induced disease requires late, rather than early, virus replication events in microglial cells (W. P. Lynch et al., J. Virol. 70:8896-8907, 1996). Furthermore, neurodegeneration requires the presence of unique sequences within the viral env gene. Thus, the neurodegeneration-inducing events could result from microglial expression of retroviral envelope protein alone or from the interaction of envelope protein with other viral structural proteins in the virus assembly and maturation process. To distinguish between these possible mechanisms of disease induction, we engineered the engraftable neural stem cell line C17-2 into packaging/producer cells in order to deliver the neurovirulent CasBrE env gene to endogenous CNS cells. This strategy resulted in significant CasBrE env expression within CNS microglia without the appearance of replication competent virus. CasBrE envelope expression within microglia was accompanied by increased expression of activation markers F4/80 and Mac-1 (CD11b) but failed to induce spongiform neurodegenerative changes. These results suggest that envelope expression alone within microglia is not sufficient to induce neurodegeneration. Rather, microglia-mediated disease appears to require neurovirulent Env protein interaction with other viral proteins during assembly or maturation. More broadly, the results presented here prove the efficacy of a novel method by which neural stem cell biology may be harnessed for genetically manipulating the CNS, not only for studying neurodegeneration but also as a paradigm for the disseminated distribution of retroviral vector-transduced genes.

Erythroleukaemia induction by the Friend spleen focus-forming virus

The Friend spleen focus-forming virus has been a valuable tool for understanding the molecular events involved in the multiple stages of leukaemia. As summarized in Figure 3, the primary effect of SFFV, which occurs within days, is to cause a polyclonal proliferation of erythroid precursor cells that can proliferate in the absence of their normal regulator erythropoietin. This is the direct result of the unique envelope glycoprotein encoded by SFFV, which is transported to the cell surface and apparently interacts with the EpoR or another component of the multimeric EpoR complex, resulting in the constitutive activation of the Epo signal transduction pathway. Within this proliferating population of erythroid cells is a rare cell that has undergone several genetic changes due to the integration of the viral genome in specific sites in the mouse DNA. This leads to the activation of a gene encoding the PU.1 transcription factor, whose high expression in erythroid cells may be the cause of the block in differentiation that is characteristic of SFFV-transformed erythroid cells. SFFV integration can also lead to the inactivation of the p53 tumour supressor gene, giving these cells a growth advantage in the mouse. The disease induced by SFFV in mice is very similar to polycythaemia vera in humans (Golde et al, 1981). The major clinical feature of polycythaemia vera is the continuous expansion of the number of mature red blood cells in the presence of low serum Epo levels. Also, BFU-E and CFU-E from these patients can form in the absence of Epo like the analogous cells from SFFV-infected mice (Casadevall et al, 1982). It is possible that haematopoietic cells from individuals suffering from this disease express a protein similar to the envelope glycoprotein of SFFV that can interact with the EpoR and lead to its constitutive activation. Alternatively, these patients may contain a mutant EpoR gene that is constitutively activated like the mutant EpoR described earlier. As we understand more fully how the SFFV envelope protein constitutively activates te EpoR complex, we can begin to design therapies to counteract its action that can then be applied to treating patients with polycythaemia vera or other human diseases associated with uncontrolled erythropoiesis.

RB virus: a strain of Friend virus that produces a 'Friend virus-like' disease in Fv-2rr mice

RB virus is a newly derived strain of Friend virus that was adapted to produce a 'Friend-like' disease in mice that are genetically resistant to wild-type Friend virus. RB virus was produced by passing high titers of the wild-type Friend virus (Lilly-Steeves polycythemia-producing strain) through adult Fv-2rr mice. Titration of the defective spleen focus-forming virus indicated RB virus infected similar numbers of Fv-2ss or Fv-2rr target cells. Analysis of the spleens from mice infected with RB virus indicated that RB induced the early stage of Friend disease (erythroid proliferation) in both Fv-2rr and Fv-2ss mice. Fv-2ss mice infected with RB virus developed the classical Friend disease within 3 weeks. In contrast, the percentage of Fv-2rr mice developing the 'Friend-like' disease after infection with RB virus never exceeded 60%. The latency period of RBV in Fv-2rr mice was strain dependent. D2.R16 (Fv-2rr) developed the syndrome more rapidly than C57BL/6 (Fv-2rr). RB virus retained the capacity to transform erythroprogenitor cells from both Fv-2ss and Fv-2rr animals. Cells infected with RB virus consistently produced a modified SFFV envelope protein, gp48.

Characterization of cell lines derived from enlarged spleens induced in C57BL/6 mice by the variant BSB strain of Friend erythroleukemia virus

BSB is a variant strain of Friend virus selected for pathogenicity in C57BL/6 mice that are resistant to parental Friend virus strains by virtue of their homozygosity for the recessive Fv-2r allele (Steeves et al., 1970, Int. J. Cancer 5, 346-356). Lines and clones of erythroleukemia cells could readily be established in culture from the enlarged spleens of BSB-infected Fv-2r homozygotes. All lines expressed viral gene products and could be induced to express hemoglobin. Some lines produced infectious virus. In addition to the viral envelope-related proteins (gPr85, gp70, and gp52) detected by precipitation with goat anti-Rauscher gp70 antiserum from tumor cell lines induced by parental Friend virus strains, BSB-induced cell lines also expressed gp80, p52, and gp45 products precipitable with the same antiserum. A rat monoclonal antibody that recognizes an epitope of an amino-terminal region of gp52 (Wolff et al., 1982, J. Virol. 43, 472-481) also precipitated the gp80 and gp45 viral proteins. The data indicate that the BSB strain of Friend virus is oncogenic in Fv-2r homozygotes. Transformation is correlated with the expression of an altered SFFV env-gene product.

Friend virus-specific cytotoxic T lymphocytes recognize both gag and env gene-encoded specificities

We have constructed a series of "synthetic" target cell lines for an analysis of the specificity of anti-Friend virus (FV) CTL. Our results show that murine H-2 genes and individual retroviral genes can be stable expressed in Fisher rat embryo (FRE) cells, and that their products have the potential to form target structures recognized by mouse CTL. Cells expressing H-2Db and either the env or gag genes of one component of FV, helper Friend murine leukemia virus (FMuLV), were lysed by anti-FV CTL and by CTL generated against FMuLV alone. Experiments with Db-transfected FRE clones infected only with the replication-defective spleen focus-forming virus (SFFV) component of FV indicate that the SFFV genome also provides specificities recognized by both anti-FV and anti-FMuLV CTL, thus demonstrating the existence of a crossreactive CTL population. An unexpected finding was that anti-FMuLV CTL, but not anti-FV CTL were also able to lyse FRE clones that expressed H-2Kb in either the presence or absence of FV. The use of heterologous cell lines for the construction of synthetic target cells thus offers a useful approach for the analysis of T cell specificity.

Infection of haematopoietic stem cells in mice with Friend virus induced erythroleukaemia

Animals infected with conventional anaemia (FVA) or polycythemia-inducing (FVP) strains of the Friend virus develop lethal erythroleukaemia. A variant strain, RFV, induces an initially identical disease except that it spontaneously regresses in 50% of infected mice. To determine whether pluripotent stem cells as measured by spleen colony forming units (CFU-s) in leukaemic mice are productively infected with virus and whether their infection influences the outcome of the disease, we tested CFU-s from leukaemic mice for susceptibility to cytotoxicity by monospecific antiviral gp70 antiserum. Spleen CFU-s from progressively leukaemic (FVP, FVA and RFV) mice were productively infected with virus. CFU-s in RFV progressors became infected by 40 days post-virus inoculation. FVA and FVP progressors became infected between 15 and 21 days post virus. Infection of CFU-s was accompanied by an increase in the proportion of replicating (S phase) CFU-s in these populations. Spleen CFU-s from fully regressed RFV regressor mice were uninfected and remained so throughout the course of their disease. Bone marrow CFU-s in both regressors and progressors remained uninfected and were not induced to increased cell cycling.

Heteroduplex analysis of molecular clones of the pathogenic Friend virus complex: Friend murine leukemia virus, Friend mink cell focus-forming virus, and the polycythemia- and anemia-inducing strains of Friend spleen focus-forming virus

The pathogenic Friend virus complex is of considerable interest in that, although members of this group are genetically related, they differ markedly in biochemical and biological properties. Heteroduplex mapping of molecular clones of the Friend virus complex, which includes the replication-competent ecotropic Friend murine leukemia virus (F-MuLV) and mink cell focus-forming virus (F-MCF) and replication-defective polycythemia- and anemia-inducing strains of spleen focus-forming virus (SFFVp and SFFVa, respectively), was employed to provide insight into the molecular basis of their relationships. In heteroduplexes of F-MuLV X F-MCF, a major substitution of 0.89 kilobases in the env gene of F-MCF was discerned. Heteroduplexes of SFFVp X F-MuLV or F-MCF and SFFVa X F-MuLV or F-MCF showed several major deletions in the pol gene region and a single major deletion in the 3' half of the env gene region of SFFVp and SFFVa. A major substitution of 0.89 kilobases was mapped to the 5' end of the env deletion of SFFVp and SFFVa in heteroduplexes with F-MuLV, similar to that seen in F-MuLV X F-MCF heteroduplexes. In contrast, this env gene region was totally homologous in F-MCF X SFFVp or SFFVa and SFFVp X SFFVa heteroduplexes. Our results suggest that (i) both SFFVp and SFFVa lack part of the env gene at its 3' end, corresponding to the p15(E) coding region, (ii) major deletions occur in the pol and env genes which account for the replication defectiveness of SFFVp and SFFVa, (iii) minor substitutions occur in the gag gene region of SFFVa that are not present in SFFVp, F-MuLV, or F-MCF, (iv) a major substitution exists in the gp70 region of the env gene between F-MuLV and F-MCF that probably accounts for the differences in their host range specificities, (v) this substitution in F-MCF is identical to the gp70 part of the gp52 coding region of SFFVp and SFFVa, and (vi) heteroduplexes to F-MCF show unambiguously that no additional large substitutions are present in SFFVp or SFFVa that could account for differences in their leukemogenicity.

Characterization of the env gene and long terminal repeat of molecularly cloned Friend mink cell focus-inducing virus DNA

The highly oncogenic erythroleukemia-inducing Friend mink cell focus-inducing (MCF) virus was molecularly cloned in phage lambda gtWES.lambda B, and the DNA sequences of the env gene and the long terminal repeat were determined. The nucleotide sequences of Friend MCF virus and Friend spleen focus-forming virus were quite homologous, supporting the hypothesis that Friend spleen focus-forming virus might be generated via Friend MCF virus from an ecotropic Friend virus mainly by some deletions. Despite their different pathogenicity, the nucleotide sequences of the env gene of Friend MCF virus and Moloney MCF virus were quite homologous, suggesting that the putative parent sequence for the generation of both MCF viruses and the recombinational mechanism for their generation might be the same. We compare the amino acid sequences in lymphoid leukemia-inducing ecotropic Moloney virus and Moloney MCF virus, and erythroblastic leukemia-inducing ecotropic Friend virus, Friend-MCF virus, and Friend spleen focus-forming virus. The Friend MCF virus long terminal repeat was found to be 550 base pairs long. This contained two copies of the 39-base-pair tandem repeat, whereas the spleen focus-forming virus genome contained a single copy of the same sequence.

Structural analysis of the spleen focus-forming virus envelope gene product

The structure of the envelope gene product (gp52) of the spleen focus-forming virus was analyzed and compared to that of envelope proteins (gp70 and p15E) of another pathogenic Friend virus recombinant, Friend mink cell focus-inducing virus (F-MCF). This has enabled us to confirm and extend previous nucleotide sequence data regarding the make up of specific domains of the SFFV glycoprotein. Amino-terminal 23,000-Da V8 protease fragments from gp52 of the Lilly-Steeves strain of SFFV and from gp70 of a pathogenic F-MCF isolate produced tryptic peptide fingerprints in which the mobilities of the trypsin-generated peptides were identical. The carboxyl-terminal, 21,000 Da, V8 protease fragment of gp52, however, has a unique fingerprint that contained a single highly charged trypsin-generated peptide. This peptide migrated to the same position as a peptide in F-MCF p15E, thus indicating that the p15E-related nucleotide sequences, that follow the large envelope deletion, are translated in the same reading frame as those in the standard p15E. Although R peptide determinants can be detected in F-MCF Pr15E, they could not be detected in gp52. The amino-terminal 23K domain of gp52, like that of MCF gp70, contains two oligosaccharide attachment sites. The other two attachment sites are located within the 21K carboxyl-terminal domain.

Retrovirus transduction: segregation of the viral transforming function and the herpes simplex virus tk gene in infectious Friend spleen focus-forming virus thymidine kinase vectors

A series of deletions and insertions utilizing the herpesvirus thymidine kinase gene (tk) were constructed in the murine retrovirus Friend spleen focus-forming virus (SFFV). In all cases, the coding region for the SFFV-specific glycoprotein (gp55), which is implicated in erythroleukemic transformation, was left intact. These SFFV-TK and SFFV deletion vectors were analyzed for expression of tk and gp55 after DNA-mediated gene transfer. In addition, virus rescued by cotransfection of these vectors with Moloney murine leukemia virus was analyzed for infectious TK-transducing virus, gp55 expression, and erythroleukemia-inducing ability. The experiments demonstrated that deletions or insertions within the intron for the gp55 env gene can interfere with expression of gp55 after both DNA-mediated gene transfer and virus infection. In contrast, the gene transfer efficiency of the tk gene was unaffected in the SFFV-TK vectors, and high-titer infectious TK virus could be recovered. Revertant viruses capable of inducing erythroleukemia and expressing gp55 were generated after cotransfection of the SFFV-TK vectors with murine leukemia virus. The revertant viruses lost both tk sequences and the ability to transduce TK- fibroblasts to a TK+ phenotype. These experiments demonstrate that segregation of the TK and erythroleukemia functions can occur in retrovirus vectors which initially carry both markers.

Retrovirus transduction: segregation of the viral transforming function and the herpes simplex virus tk gene in infectious Friend spleen focus-forming virus thymidine kinase vectors

A series of deletions and insertions utilizing the herpesvirus thymidine kinase gene (tk) were constructed in the murine retrovirus Friend spleen focus-forming virus (SFFV). In all cases, the coding region for the SFFV-specific glycoprotein (gp55), which is implicated in erythroleukemic transformation, was left intact. These SFFV-TK and SFFV deletion vectors were analyzed for expression of tk and gp55 after DNA-mediated gene transfer. In addition, virus rescued by cotransfection of these vectors with Moloney murine leukemia virus was analyzed for infectious TK-transducing virus, gp55 expression, and erythroleukemia-inducing ability. The experiments demonstrated that deletions or insertions within the intron for the gp55 env gene can interfere with expression of gp55 after both DNA-mediated gene transfer and virus infection. In contrast, the gene transfer efficiency of the tk gene was unaffected in the SFFV-TK vectors, and high-titer infectious TK virus could be recovered. Revertant viruses capable of inducing erythroleukemia and expressing gp55 were generated after cotransfection of the SFFV-TK vectors with murine leukemia virus. The revertant viruses lost both tk sequences and the ability to transduce TK- fibroblasts to a TK+ phenotype. These experiments demonstrate that segregation of the TK and erythroleukemia functions can occur in retrovirus vectors which initially carry both markers.

Envelope gene of the Friend spleen focus-forming virus: deletion and insertions in 3' gp70/p15E-encoding region have resulted in unique features in the primary structure of its protein product

A nucleotide sequence was determined for the envelope (env) gene of the polycythemia-inducing strain of the acute leukemia-inducing Friend spleen focus-forming virus (SFFV) and from this the amino acid sequence of its gene product, gp52, was deduced. All major elements of the gene were found to be related to genes of other retroviruses that code for functional glycoproteins. Although the carboxyl terminus of gp52 is encoded by sequences highly related to sequences in its putative parent, ecotropic Friend murine leukemia virus, the majority of the protein (69%), including the amino terminus, is encoded by dualtropic virus-like sequences. Nucleotide sequence comparisons suggest that the nonecotropic region may be more closely related to the 5' substitution in dualtropic mink cell focus-inducing viruses that it is to the 5' end of xenotropic virus env genes. A large deletion and two unique insertions have been located in the env gene of polycythemia-inducing SFFV and may account for some of the unusual structural characteristics, aberrant processing, and pathogenic properties of gp52. As a consequence of the deletion, amino-terminal gp70 and carboxyl-terminal p15E-encoding sequences are juxtaposed and it appears that translation from the p15E region, 3' to the deletion, continues in the standard reading frame used by other retroviruses. Insertions of six base pairs and one base pair at the very 3' end of the gp52-encoding region results in a SFFV-unique amino acid sequence and a premature termination codon.

Cas spleen focus-forming virus. II. Further biological and biochemical characterization

A new isolate of a murine erythroblastosis-inducing spleen focus-forming virus (Cas SFFV), derived from the wild mouse ecotropic murine leukemia virus Cas-Br-M, was further characterized after the production of a nonproducer cell line. When rescued from the nonproducer cells with a helper murine leukemia virus, the Cas SFFV induced rapid splenic enlargement and focus formation when inoculated into adult NFS/N mice. The Cas SFFV nonproducer cell line was also utilized to compare the envelope-related glycoprotein of Cas SFFV with gp52s from three strains of Friend SFFV. Cas SFFV was found to encode a 50,500-dalton glycoprotein (gp50) distinct in size to the envelope-related glycoproteins of the Friend SFFVs. The Cas SFFV was also compared in RNA blot hybridization studies. The genomic viral RNA of Cas SFFV was found to be slightly larger than two polycythemia-inducing strains of Friend SFFV and markedly larger than the anemia-inducing strain. Further comparisons between the SFFVs were made by examining their transforming capabilities in an in vitro erythroid burst assay. The erythroid bursts induced by Cas SFFV and the anemia-inducing strain of Friend SFFV showed similarities in their erythropoietin requirements. This study supports our recent observations that Cas SFFV is biologically similar to the anemia-inducing strain of Friend SFFV yet biochemically distinct from all Friend SFFVs.

Complete nucleotide sequence of an infectious clone of Friend spleen focus-forming provirus: gp55 is an envelope fusion glycoprotein

The Friend spleen focus-forming provirus is 6,296 base pairs (bp) in length. Compared to Moloney murine leukemia virus, it has undergone five major deletions, three substitutions, and a number of minor alterations. Otherwise, these viruses are about 90% homologous. A 16-bp palindrome is found in the region thought to be involved in packaging and dimerization of the RNA genome. Premature termination of translation of the gag polyprotein is attributed to a 13-bp deletion in the p12 region. A substitution of xenotropic env sequences was identified in the 5' region of the env gene; 150 nucleotides 3' to this substitution, a deletion of 585 bp removes the site where the normal env precursor protein is cleaved to form gp70 and p15(E), resulting in a fusion protein of Mr 44,725. Due to these changes, the env product gp55 is expected to have a substantially different conformation on the cell surface compared to either a xenotropic or ecotropic gp70 protein, and may be responsible for the rapid erythroleukemic potential of spleen focus-forming virus.

Complete nucleotide sequence of the gene for the specific glycoprotein (gp55) of Friend spleen focus-forming virus

The complete nucleotide sequence of the gene for the specific glycoprotein (gp55) of the polycythemic strain of Friend spleen focus-forming virus (SFFV) was derived from the cloned SFFV DNA intermediate. The gp55 gene is present within 1.4 kilobases of the 5' side of the 3'long terminal repeat sequence. The open reading frame predicts the primary translation product has a total of 409 amino acids with a Mr of 44,752. Comparisons of the deduced amino acid sequence of gp55 with those of the envelope (env) gene products of murine leukemia viruses (MuLVs) revealed that gp55 is composed of three distinct regions. The amino-terminal 80% of the molecule has a high degree of sequence homology with the amino-terminal portion of the gp70 of the Moloney mink cell focus-forming virus (BALB/Mo-MCFV). This portion of the BALB/Mo-MCFV gp70 is known to be coded for by the acquired xenotropic env-like sequence. The sequence of the following 66 amino acids of gp55 is highly homologous to that of the middle portion of the p15E of Moloney MuLV (Mo-MuLV). The sequence of the Carboxyl-terminal 12 amino acids is specific to gp55 and a comparison of the nucleotide sequence showed that this specific amino acid sequence is due to the presence of seven extra nucleotides compared with the sequence of the Mo-MuLV.

Construction and isolation of a transforming murine retrovirus containing the src gene of Rous sarcoma virus

Recombinant murine retroviruses containing the src gene of the avian retrovirus Rous sarcoma virus were isolated. Such viruses were isolated from cells after transfection with DNAs in which the src gene was inserted into the genome of the amphotropic murine retrovirus 4070A. The isolated viruses had functional gag and pol genes, but they were all env defective since the src gene was inserted in the middle of the env gene coding region. Infectious transforming virus could be isolated only from cells transfected with DNA constructions in which the src gene was in the same polarity as that of a long terminal repeat of the amphotropic viral genome. These recombinant viruses encoded a pp60src protein with a molecular weight similar to that of the Schmidt-Ruppin strain of Rous sarcoma virus. In addition, the src protein(s) of these recombinant viruses was as active as protein kinases in the immune complex protein kinase assay. Intravenous injection of helper-independent Moloney and Friend murine leukemia virus pseudotypes of the src recombinant viruses into 6-week-old NIH Swiss mice resulted in the appearance of splenic foci within 2 weeks, splenomegaly and, later after infection (8 to 10 weeks), anemia. Infectious transforming virus could be recovered from the spleens of diseased animals. Such viruses encoded pp60src but not p21ras or mink cell focus-forming virus-related glycoproteins.

Mouse cells contain two distinct ras gene mRNA species that can be translated into a p21 onc protein

The Kirsten (Ki) and Harvey (Ha) strains of murine sarcoma virus encode a 21,000-dalton protein (p21 ras) which is the product of the transforming gene of these viruses. Normal cells express low levels of p21 ras encoded by cellular genes (Ki-ras and Ha-ras) homologous to the Ki and Ha murine sarcoma virus transformation genes. A bone marrow-derived mouse cell line, 416B, has been shown to express unusually high levels of p21 ras. In this manuscript, we investigated the molecular biology of p21 ras gene expression in 416B and other normal mouse cells. We identified four distinct polyadenylated and polysome-associated RNAs, two related to Ki-ras and two to Ha-ras. The levels in 416B cells of the two Ki-ras RNAs, sized 5.2 and 2.0 kilobases, were both elevated approximately 25-fold over levels found in normal mouse cells; there was no corresponding change in 416B cells in the levels of the two Ha-ras RNAs. We partially purified the two Ki-ras mRNAs and separated them by velocity sedimentation in sucrose density gradients. Both the 5.2- and 2.0-kilobase mRNAs could be translated in vitro into p21 ras. These results show that a cellular onc protein can be translated from two distinct cellular mRNA species.

Identification of a spleen focus-forming virus in erythroleukemic mice infected with a wild-mouse ecotropic murine leukemia virus

An NFS/N mouse inoculated at birth with an ecotropic murine leukemia virus (MuLV) obtained from wild mice (Cas-Br-M MuLV) developed a lymphoma after 18 weeks. An extract prepared from the lymphomatous spleen was inoculated into newborn NFS/N mice, and these mice developed erythroleukemia within 9 weeks. Spleens from the erythroleukemic mice contained ecotropic and mink cell focus-inducing (MCF) MuLVs; however, when these viruses were biologically cloned and reinoculated into newborn NFS/N mice, no erythroleukemia was induced. In contrast, cell-free extracts prepared from the erythroleukemic spleens induced erythroleukemia within 5 weeks. Analysis of cell-free extracts prepared from the erythroleukemic spleens showed that they contained a viral species that induced splenomegaly and spleen focus formation in adult mice, with susceptibility controlled by alleles at the Fv-2 locus. The spleen focus-forming virus coded for a 50,000-dalton protein precipitated by antibodies specific to MCF virus gp70. RNA blot hybridization studies showed the genomic viral RNA to be 7.5 kilobases and to hybridize strongly to a xenotropic or MCF envelope-specific probe but not to hybridize with an ecotropic virus envelope-specific probe. The virus described here appears to be the fourth independent isolate of a MuLV with spleen focus-forming activity.

Glycosylation and intracellular transport of spleen focus-forming virus glycoproteins

We have investigated the pattern of glycosylation of the membrane glycoproteins encoded by a polycythemic strain of spleen focus-forming virus (SFFV). These include a major species designated gp52 and its processed form which is designated gp65. The SFFV glycoproteins were found to be predominantly intracellular, although a portion of gp65 is expressed on the cell surface. gp65 was observed to be highly sialylated and resistant to digestion with endoglycosidase-H (endo-H). In contrast, gp52 was endo-H sensitive and the polyacrylamide gel electrophoresis profile of the endo-H digests suggested the presence of four glycosylation sites. Analysis of tryptic glycopeptides from gp52 by reverse-phase high-performance liquid chromatography also suggested the presence of four glycosylation sites. Glycopeptide analysis of Pronase digests of gp52 revealed two major size classes with molecular weights of 2200 and 1500, which correspond to two of the four oligosaccharide size classes reported previously for MuLV gp70's (M.C. Kemp, N.G. Famulari, P.V. O'Donnell, and R.W. Compans, 1980, J. Virol. 34, 154). Both glycopeptide size classes were sensitive to digestion with endo-H. The glycopeptide profile of gp65 was found to be very heterogeneous and the predominant form was a 2900-dalton size class. In addition a fucosyl glycopeptide of 2500 daltons was observed in gp65, but not in F-MuLV or F-MCF glycoproteins. In the presence of the sodium ionophore monensin, the processing of gp52 to gp65 was inhibited. Instead a smaller protein of about 60,000 daltons was observed, which did not arrive at the cell surface, a situation analogous to the processing and post-translational modification reported for gp52 from anemic isolates of SFFV (S.K. Ruscetti, J.A. Field, and E.M. Scolnick, 1981, Nature (London) 294, 663).

Molecular cloning of biologically active proviral DNA of the anemia-inducing strain of spleen focus-forming virus

Previously, we have molecularly cloned proviral DNA of a polycythemia-inducing strain of the spleen focus-forming virus (SFFVp). In this paper, we report that unintegrated proviral DNA of the anemia-inducing strain of SFFV (SFFVA) has been molecularly cloned into pBR322. This molecularly cloned DNA retains the biological activity of SFFVA, as infectious SFFV can be recovered from the DNA clone by marker rescue using a previously described two-stage cotransfection assay (Linemeyer et al., J. Virol. 35:710-721, 1980). The recovered SFFV retains an important property of the initial SFFVA which distinguishes SFFVA from SFFVP, namely, the ability of SFFVA to cause proliferation of erythroid cells in which hemoglobin synthesis is erythropoietin dependent. By utilizing a marker rescue technique, the splenomegaly and anemia characteristic of SFFVA-induced disease have been traced to a DNA fragment of SFFVA containing sequences coding for the env gene product. gp52. The results suggest that the differences in pathogenicity between SFFVP disease and SFFVA disease are an intrinsic property of the env gene products of these two variants of Friend virus, and future studies with the molecular clones of each strain should allow us to map regions of each env gene responsible for common and distinctive features of the erythroproliferative diseases induced by each virus.

Mouse cells contain two distinct ras gene mRNA species that can be translated into a p21 onc protein

The Kirsten (Ki) and Harvey (Ha) strains of murine sarcoma virus encode a 21,000-dalton protein (p21 ras) which is the product of the transforming gene of these viruses. Normal cells express low levels of p21 ras encoded by cellular genes (Ki-ras and Ha-ras) homologous to the Ki and Ha murine sarcoma virus transformation genes. A bone marrow-derived mouse cell line, 416B, has been shown to express unusually high levels of p21 ras. In this manuscript, we investigated the molecular biology of p21 ras gene expression in 416B and other normal mouse cells. We identified four distinct polyadenylated and polysome-associated RNAs, two related to Ki-ras and two to Ha-ras. The levels in 416B cells of the two Ki-ras RNAs, sized 5.2 and 2.0 kilobases, were both elevated approximately 25-fold over levels found in normal mouse cells; there was no corresponding change in 416B cells in the levels of the two Ha-ras RNAs. We partially purified the two Ki-ras mRNAs and separated them by velocity sedimentation in sucrose density gradients. Both the 5.2- and 2.0-kilobase mRNAs could be translated in vitro into p21 ras. These results show that a cellular onc protein can be translated from two distinct cellular mRNA species.

Monoclonal antibody to spleen focus-forming virus-encoded gp52 provides a probe for the amino-terminal region of retroviral envelope proteins that confers dual tropism and xenotropism

Monoclonal antibodies which recognize a region common to Friend spleen focus-forming virus encoded gp52 and Friend mink cell focus-inducing viral gp70 were isolated. One such antibody from hybridoma 7C10 was tested extensively in immune precipitation and was found to react with a determinant on envelope gp70s of all mink cell focus-inducing, xenotropic, and amphotropic mouse retroviruses tested, but not with envelope gp70s of ecotropic viruses, including Friend, Moloney, and AKR murine leukemia viruses. Monoclonal antibody from hybridoma 7C10 precipitated a 23,000-molecular-weight fragment, derived by V8 protease digestion of Friend mink cell focus-inducing gp70. This 23,000-molecular-weight peptide was determined to derive from the amino terminus of the molecule. These results correlate well with other genetic data which indicate that endogenously acquired sequences of mink cell focus-inducing viruses are found at the 5' end of the envelope gene.

Envelope gene sequences which encode the gp52 protein of spleen focus-forming virus are required for the induction of erythroid cell proliferation

A series of insertion-deletion mutants was constructed in a molecularly cloned DNA copy of the Friend strain of spleen focus-forming virus (SFFV). The mutants were produced by inserting a synthetic oligonucleotide linker containing the recognition sequence of SalI endonuclease into several different locations of the SFFV DNA. Three classes of mutants were isolated: insertion-deletion mutants in the 5' half of the SFFV genome, in the long terminal repeat of the SFFV genome, and in the env gene of the SFFV genome. The env gene mutant has a deletion of sequences shared in common between the env gene of SFFV and the env genes of mink cell focus-inducing murine leukemia viruses. From analyses of the biological activity of the various mutants and a biologically active subgenomic SFFV DNA fragment described herein, we can deduce that the coding sequence encompassing the env gene of SFFV is required for the biological activity. This region, required for the pathogenic phenotype, cannot be larger than 1.5 kilobase pairs, a size only slightly more than that sufficient to encode the nonglycosylated precursor of the gp52 env gene product.

Friend disease in vitro

In long-term marrow cultures, hemopoiesis can be maintained for several months, although erythropoiesis is normally suppressed at the most primitive level of development (the erythroid colony-forming cells). Infection of these cultures with a viral complex combining helper-independent murine leukemia virus (F-MuLV) and a spleen focus-forming virus (SFFVp) results in a productive infection of both the replication defective SFFVp and the F-MuLV. After infection, the cultures show a dramatic elevation in the numbers of late erythroid progenitor cells (CFU-E), many of which will grow in the absence of added erythropoietin, and a transient erythropoietin, independent erythropoiesis, including the production of mature, enucleated erythrocytes. Hemopoiesis eventually declines, with no evidence for the generation of Friend tumor cells. When erythropoiesis is induced in the long-term cultures by addition of anemic mouse serum before infection by polycythemia-inducing Friend virus, the generation of erythropoietin-independent CFU-E and erythrocyte formation is followed by the sustained production (greater than 40 wk) of primitive erythroid cells with low spontaneous levels (less than 5%) of hemoglobinization. Although these cells will produce spleen colonies in irradiated mice and can be cloned in soft-gel media, they do not produce autonomous, permanently growing cell lines in vitro, i.e., they retain a dependency upon the marrow-adherent layer for their continued growth. However, following a further passage on a "virgin" marrow environment, permanent cell lines can be established that are able to grow independently of environmental influences. Thus, this system is the first description of a complete in vitro system for the reproducible production and isolation of Friend virus-induced erythroid cell lines.

Wild mouse ecotropic murine leukemia virus infection of inbred mice: dual-tropic virus expression precedes the onset of paralysis and lymphoma

NFS/N mice inoculated at birth with an ecotropic murine leukemia virus (Cas-Br-MuLV) obtained from wild mice developed hind limb paralysis beginning at 7 weeks of age and nonthymic lymphomas beginning at more than 20 weeks of age. Studies of 1- to 7-week-old Cas-Br-M MuLV-infected mice showed the following: (i) a marked increase in nonecotropic MuLV-related antigens on spleen cells but not thymocytes beginning at 2 weeks; (ii) the appearance of dual-tropic mink cell focus-forming (MCF) MuLV-related gp70 in spleen but not thymus or brain cells at 4 weeks; and (iii) the isolation of infectious MCF MuLV from spleen cells of 7-week-old mice. A role for MCF MuLV in Cas-Br-M MuLV-induced nonthymic lymphomas is indicated by these studies, and a role for recombinant MuLV in neurological disease is considered.

Subcellular localization of the env-related glycoproteins in Friend erythroleukemia cells

A scheme was developed for the subcellular fractionation of murine erythroleukemia cells transformed by Friend leukemia virus. The subcellular localization of the env-related glycoproteins was determined by immune precipitation with antiserum against gp70, the envelope glycoprotein of the helper virus, followed by gel electrophoresis. In cells labeled for 2 h with [35S]methionine, the glycoprotein encoded by the defective spleen focus-forming virus, gp55SFFV, was found primarily in the nuclear fraction and in fractions containing dense cytoplasmic membranes such as endoplasmic reticulum. A similar distribution was noted for gp85env, the precursor to gp70. The concentration of viral glycoproteins in the nuclear fraction could not be accounted for by contamination with endoplasmic reticulum. In pulse-chase experiments, neither glycoprotein underwent major redistribution. However, labeled gp85env disappeared from intracellular membranes with a half-time of 30 min to 1 h, whereas labeled gp55SFFV was stable during a 2-h chase. In plasma membrane preparations with very low levels of contamination with endoplasmic reticulum, gp70 was the major viral env-related glycoprotein detected; a minor amount of gp55SFFV and no gp85env could be detected. The unexpected result of these experiments is the amount of viral glycoproteins found in the nuclear fraction. Presence of viral proteins in the nucleus could be relevant to the mechanism of viral leukemogenesis.

Biological activity of the spleen focus-forming virus is encoded by a molecularly cloned subgenomic fragment of spleen focus-forming virus DNA

A biologically active subgenomic DNA fragment of a polycythemia-inducing strain of the replication-defective spleen focus-forming virus (SFFV) has been molecularly cloned. The SFFV DNA fragment includes 2.0 kilobase pairs (kbp) from the 3' end of SFFV, the long terminal repeat sequences of SFFV, and 0.4 kbp from the 5' end of SFFV. The fragment contains the previously described env-related gene of SFFV. All the properties associated with SFFV can be assigned to this SFFV DNA fragment by using a two-stage DNA transfection assay with infectious helper virus DNA. The virus recovered from the transfection assays can induce erythroblastosis, splenic foci, and polycythemia in infected mice. Fibroblast cultures transfected with the SFFV DNA fragment synthesize gp52, the known intracellular product of the env-related gene of SFFV. gp52 can also be detected in spleens from diseased mice infected with the virus recovered in the two-stage transfection. The results are consistent with the hypothesis that the env-related gene sequences of SFFV and their product gp52 are required for the initiation of SFFV-induced disease.

Viral protein expression in producer and nonproducer clones of friend erythroleukemia cell lines

Erythroleukemia cell lines HFL/d and HFL/b, derived from tumors induced in vivo in BALB/c (H-2d) and congenic BALB.B (H-2b) mice, respectively, by a polycythemia-inducing strain of Friend virus, produced both spleen focus-forming virus (SFFV) and its native NB-tropic helper virus (Friend murine leukemia virus [FMuLV]) during early-passage generations in culture. Eventually each line ceased production of both infectious viruses but retained its tumorigenic potential in syngeneic hosts. Virus-producer and -nonproducer clones of these cell lines were examined for expression of proteins encoded by the SFFV or FMuLV genomes. Lysates of labeled cells were treated with various antiviral sera, and the precipitates were examined by gel electrophoresis. Expression of the FMuLV env gene-encoded precursor protein, gPr84env, was observed in all producer and most nonproducer clones, but the FMuLV gag and pol gene products, Pr65gag and Pr200gag-pol, were uniformly undetectable in nonproducer clones. All HFL/d and HFL/b clones expressed appreciable amounts of the SFFV-encoded envelope protein, gp52, including one exceptional clone which had ceased to express any FMuLV-encoded proteins. The molecular weight of this SFFV-encoded envelope protein was consistently smaller in all HFL/b clones than in HFL/d clones, regardless of their producer or nonproducer status. The virus-nonproducer phenotype thus appears to be due to shutdown of expression of the 5' portion of the FMuLV genome in two independent cell lines.

Spleen focus-forming virus: specific neutralization by antisera to certain gag gene-encoded proteins

Immunization of rats with syngeneic cells infected with spleen focus-forming virus (SFFV) but not with its helper, Friend murine leukemia virus (FMuLV), produces antisera which specifically neutralize SFFV, and not FMuLV, in the Friend virus complex. To determine which SFFV-encoded protein molecule bears the antigen recognized by these neutralizing antibodies, we studied different lots of rat anti-SFFV antiserum by immunoprecipitation and virus neutralization assays. The ability of these sera to neutralize SFFV correlated with the titer of antibodies to p45gag and not with the titer of those to gp52, suggesting that the neutralizing antibodies recognize the p45gag molecule. To verify this specificity for p45gag, we tested antisera to various MuLV gag gene-encoded proteins for neutralization of SFFV. Goat anti-Rauscher murine leukemia virus (RMuLV) p30 and goat anti-RMuLV p10 sera neither precipitated p45gag from SFFV-infected nonproducer cells nor neutralized SFFV. In contrast, goat anti-RMuLV Pr65gag and goat anti-RMuLV p12 sera precipitated p45gag from SFFV-infected cells and also specifically neutralized SFFV in the Friend virus complex. These findings suggest that, unlike the gag proteins coded for by FMuLV, the proteins coded for by defective SFFV are incorporated into the envelope of virions carrying the SFFV genome, but not into the envelope of those carrying the helper FMuLV genome.

Replication-defective Friend murine leukemia virus particles containing uncleaved gag polyproteins and decreased levels of envelope glycoprotein

An erythroleukemia cell clone, 7C, which failed to produce reverse transcriptase-containing virions or infectious virus, was found to produce noninfectious virus particles by gradient banding of [3H]leucine- and [3H]uridine-labeled virions. The RNA from the 7C virus was shown to consist of the normal 70S size component, which converted to 35S upon heat denaturation. In contrast, the 7C virion proteins showed multiple defects. Analysis of the virion proteins by gel electrophoresis demonstrated that the pr65 gag precursor was incorporated into the 7C virus and that the processing of this precursor was severely diminished. Polymerase proteins pr180gag-pol and pr120pol were also detected in virions, and a third possible polymerase protein, p70, was reduced in size compared to its normal counterpart, p80. Incorporation of the viral gp70 glycoprotein into particles was also reduced 10-fold, despite synthesis and incorporation of gp70 into the 7C cell membrane in normal amounts. Pulse-chase analysis of the synthesis of the viral gag and env proteins in 7C cells showed greatly reduced amounts of pr180gag-pol, pr65gag, p80gag, and p42gag, whereas pr90env, gp70, and spleen focus-forming virus-specific gp55 were synthesized and processed normally. These results suggested that at least one defect in 7C virus was impaired cleavage of gag or pol proteins or both, most likely due to a lack of the appropriate viral protease, and that this lack of cleavage might affect incorporation of gp70 into virus particles.

Polycythemia- and anemia-inducing erythroleukemia viruses exhibit differential erythroid transforming effects in vitro

The transforming capabilities of FVA, RLV and FVP have been examined using an in vitro transformation assay. Treatment of bone marrow cells with FVP in vitro led to the formation of hemoglobinized erythroid bursts even when these cells were cultured in methylcellulose for 5 days without added erythropoietin (Epo). A variety of FVA and RLV preparations also produced erythroid bursts without Epo but these bursts contained significantly less hemoglobin than those induced by FVP. When very low levels of Epo were added to cultures of FVA- and RLV-infected cells, the bursts were hemoglobinized, that is, similar to FVP-induced bursts. The burst-inducing agent in FVA preparations was shown to be a virus and not Epo. Spleen focus-forming virus (SFFV) pseudotypes, derived from FVA or FVP, also produced erythroid bursts in vitro, whereas four helper murine leukemia viruses did not. These studies indicated that the SFFV component was essential for erythroid burst transformation and specified the degree of hemoglobinization in the bursts formed.

Plasma membrane glycoproteins encoded by cloned Rauscher and Friend spleen focus-forming viruses

Rauscher spleen focus-forming virus (SFFV) was cloned free of its helper virus into normal rat kidney and mouse fibroblasts, and the resulting nonproducer fibroblast clones were analyzed. Our results suggested that Rauscher SFFV encodes a glycoprotein with an apparent Mr of 54,000 (gp54) that reacts with antisera made to the envelope glycoprotein (gp70) of ecotropic murine leukemia viruses, as well as with a rat antiserum that reacts with the gp70's of dual-tropic mink cell focus-inducing and HIX viruses but not with the gp70's of ecotropic viruses. In these respects and in its tryptic peptide map, Rauscher SFFV-encoded gp54 is nearly identical to the gp55 glycoprotein which we previously reported to be encoded by Friend SFFV (Dresler et al., J. Virol. 30:564--575, 1979). However, gp54 is slightly smaller, and it lacks one methionine-containing tryptic peptide that occurs in gp55. Studies with cytotoxic antiserum in the presence of complement and with a rosetting technique which employed sheep erythrocytes coupled to protein A suggested that the gp54 and gp55 glycoproteins are weakly expressed on the surface membranes of SFFV-infected cells. In addition, the Rauscher SFFV genome also encodes gag polyproteins which appear to be identical to the gag polyproteins encoded by helper Rauscher murine leukemia virus, but differ from the antigenically related polyproteins encoded by some but not all clones of Friend SFFV. Furthermore, the glycosylated gag polyproteins encoded by Rauscher SFFV and by some Friend SFFVs also appear to be expressed on the surface membranes of infected cells. These results suggest that similar env gene recombination and partial deletion events were involved in the independent origins of two different strains of acute erythroleukemia virus.