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

Origins of the endogenous and infectious laboratory mouse gammaretroviruses

The mouse gammaretroviruses associated with leukemogenesis are found in the classical inbred mouse strains and in house mouse subspecies as infectious exogenous viruses (XRVs) and as endogenous retroviruses (ERVs) inserted into their host genomes. There are three major mouse leukemia virus (MuLV) subgroups in laboratory mice: ecotropic, xenotropic, and polytropic. These MuLV subgroups differ in host range, pathogenicity, receptor usage and subspecies of origin. The MuLV ERVs are recent acquisitions in the mouse genome as demonstrated by the presence of many full-length nondefective MuLV ERVs that produce XRVs, the segregation of these MuLV subgroups into different house mouse subspecies, and by the positional polymorphism of these loci among inbred strains and individual wild mice. While some ecotropic and xenotropic ERVs can produce XRVs directly, others, especially the pathogenic polytropic ERVs, do so only after recombinations that can involve all three ERV subgroups. Here, I describe individual MuLV ERVs found in the laboratory mice, their origins and geographic distribution in wild mouse subspecies, their varying ability to produce infectious virus and the biological consequences of this expression.

Suppression of acute anti-friend virus CD8+ T-cell responses by coinfection with lactate dehydrogenase-elevating virus

Friend virus (FV) and lactate dehydrogenase-elevating virus (LDV) are endemic mouse viruses that can cause long-term chronic infections in mice. We found that numerous mouse-passaged FV isolates also contained LDV and that coinfection with LDV delayed FV-specific CD8(+) T-cell responses during acute infection. While LDV did not alter the type of acute pathology induced by FV, which was severe splenomegaly caused by erythroproliferation, the immunosuppression mediated by LDV increased both the severity and the duration of FV infection. Compared to mice infected with FV alone, those coinfected with both FV and LDV had delayed CD8(+) T-cell responses, as measured by FV-specific tetramers. This delayed response accounted for the prolonged and exacerbated acute phase of FV infection. Suppression of FV-specific CD8(+) T-cell responses occurred not only in mice infected concomitantly with LDV but also in mice chronically infected with LDV 8 weeks prior to infection with FV. The LDV-induced suppression was not mediated by T regulatory cells, and no inhibition of the CD4(+) T-cell or antibody responses was observed. Considering that most human adults are carriers of chronically infectious viruses at the time of new virus insults and that coinfections with viruses such as human immunodeficiency virus and hepatitis C virus are currently epidemic, it is of great interest to determine how infection with one virus may impact host responses to a second infection. Coinfection of mice with LDV and FV provides a well-defined, natural host model for such studies.

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.

Role of the transmembrane sequence of spleen focus-forming virus gp55 in erythroleukemogenesis

The membrane glycoprotein encoded by the env gene of either the polycythemia- or anemia-inducing spleen focus-forming virus (SFFVp or SFFVa, respectively) is responsible for the induction of erythroleukemia in mice. It has been shown that the SFFVp glycoprotein, gp55, interacts with the erythropoietin receptor (EPO-R) and promotes EPO-independent proliferation of an EPO-R-expressing hematopoietic cell line, Ba/F3 (Li et al., Nature 343:762, 1990). We show here that when residues within the transmembrane (TM) sequence of an SFFVp gp55 are altered based on the sequences of the anemia-inducing gp55s by a methionine-to-isoleucine (M-I) substitution, a di-leucine deletion (dLL), or both, the resulting mutants display an attenuated phenotype that resembles an SFFVa: they induce milder erythroproliferative disease without polycythemia in vivo and are unable to promote EPO-independent cell proliferation in vitro. The dLL mutation directly interferes with EPO-R binding by decreasing the affinity of gp55 for the receptor. On the other hand, the M-I mutation hampers the full mitogenic activation of EPO-R while having no effect on receptor binding and asserts a dominant negative effect over the wild-type SFFVp gp55. Two other sequence changes within the TM sequence did not affect the biological activities of the SFFVp gp55. These results indicate that the TM sequence of the SFFV env glycoprotein plays a prominent role in SFFV-induced erythroleukemogenesis through its influence on the mitogenic activation of EPO-R.

Spi-1/PU.1 transgenic mice develop multistep erythroleukemias

Insertional mutagenesis of the spi-1 gene is associated with the emergence of malignant proerythroblasts during Friend virus-induced acute erythroleukemia. To determine the role of spi-1/PU.1 in the genesis of leukemia, we generated spi-1 transgenic mice. In one founder line the transgene was overexpressed as an unexpected-size transcript in various mouse tissues. Homozygous transgenic animals gave rise to live-born offspring, but 50% of the animals developed a multistep erythroleukemia within 1.5 to 6 months of birth whereas the remainder survived without evidence of disease. At the onset of the disease, mice became severely anemic. Their hematopoietic tissues were massively invaded with nontumorigenic proerythroblasts that express a high level of Spi-1 protein. These transgenic proerythroblasts are partially blocked in differentiation and strictly dependent on erythropoietin for their proliferation both in vivo and in vitro. A complete but transient regression of the disease was observed after erythrocyte transfusion, suggesting that the constitutive expression of spi-1 is related to the block of the differentiation of erythroid precursors. At relapse, erythropoietin-independent malignant proerythroblasts arose. Growth factor autonomy could be partially explained by the autocrine secretion of erythropoietin; however, other genetic events appear to be necessary to confer the full malignant phenotype. These results reveal that overexpression of spi-1 is essential for malignant erythropoiesis and does not alter other hematopoietic lineages.

Cell surface activation of the erythropoietin receptor by Friend spleen focus-forming virus gp55

The leukemogenic membrane glycoprotein gp55, encoded by Friend spleen focus-forming virus (SFFV), induces erythroid cell proliferation through its interaction with the erythropoietin receptor (EPO-R). There are two forms of gp55 in SFFV-infected cells: an intracellular form (more than 95% of the total protein), which is localized within the endoplasmic reticulum (ER) membranes, and a cell surface form (about 3 to 5%). Because both forms of the viral proteins bind to EPO-R, it is not clear whether the viral protein induces mitogenesis intracellularly or at the cell surface. To address this question, we constructed an EPO-R mutant that contained a 6-amino-acid (DEKKMP) C-terminus ER retention signal. Biochemical and functional analyses with this mutant indicated that it was completely retained in the ER and not expressed at the cell surface. Further analysis showed that the mutant, like the wild-type EPO-R, interacted with SFFV gp55. However, this apparent intracellular interaction between the two proteins failed to induce growth factor-independent proliferation of Ba/F3 cells. Furthermore, spontaneous variants of the ER-retained EPO-R selected on the basis of their ability to induce cell proliferation when coexpressed with gp55 were exclusively expressed at the cell surface. Thus, our results support the hypothesis that the mitogenic activation of the EPO-R by gp55 requires the interaction of the two proteins at the cell surface.

Glycosylation of glycoprotein 55 encoded by the anaemia-inducing strain of Friend spleen focus-forming virus

Normal rat kidney cells, non-productively infected with the anaemia-inducing variant of Friend spleen focus-forming virus (F-SFFVA), were metabolically labelled with [2-3H]mannose. The primary translation product of the viral envelope gene (env), representing a glycoprotein with an apparent molecular M(r) of 55,000 (gp55), was isolated from cell lysates by immunoaffinity chromatography and purified by preparative SDS/PAGE. Radiolabelled oligosaccharides, released from tryptic glycopeptides by treatment with endo-beta-N-acetylglucosaminidase H, were characterized chromatographically, by enzymic digestion and by acetolysis. The results revealed that F-SFFVA gp55 obtained from this source carried predominantly oligomannose type sugar chains with five to nine mannoses. As a characteristic feature, glycans with seven to nine mannoses contained, in part, an additional glucose residue. Although the amount of glucosylated species found was higher in F-SFFVA gp55 (about 25% of total endo-H-sensitive oligosaccharides) than in gp55 of the corresponding polycythaemia-inducing variant (F-SFFVP, 16.3%), the overall glycosylation pattern of the F-SFFVA env product closely resembled that of F-SFFVP gp55 [Strube et al. (1988) J Biol Chem 263:3762-71]. Hence, our results demonstrate that the different intracellular processing and transport of the primary F-SFFVA env product cannot be attributed to aberrant trimming of its oligomannose type glycans.

Mutations in the env gene of friend spleen focus-forming virus overcome Fv-2r-mediated resistance to Friend virus-induced erythroleukemia

Although Fv-2r homozygous mice are resistant to leukemias induced either by an erythropoietin-encoding virus or by wild-type Friend virus (FV) (M. E. Hoatlin, S. L. Kozak, F. Lilly, A. Chakraborti, C. A. Kozak, and D. Kabat, Proc. Natl. Acad. Sci. USA 87:9985-9989, 1990), they are susceptible to some variants of FV (R. A. Steeves, E. A. Mirand, A. Bulba, and P. J. Trudel, Int. J. Cancer 5:349-356, 1970; R. W. Geib, M. B. Seaward, M. L. Stevens, C.-L. Cho, and M. Majumdar, Virus Res. 14:161-174, 1989). To localize the virus gene involved in influencing the host range, we cloned and sequenced the env gene of the BB6 variant of FV (Steeves et al., Int. J. Cancer 5:349-356, 1970). In comparison with the wild-type env gene, the BB6 variant contains a 159-bp deletion that eliminates the membrane-proximal portion of the extracellular domain and 58 point mutations resulting in 13 amino acid changes. Substitution of the variant env gene for the wild-type env gene resulted in a recombinant virus that produced a Friend virus-like disease in Fv-2r homozygotes. Our results identify the spleen focus-forming virus env gene as the viral gene involved in this virus-host interaction. Additionally, they suggest that the product of the Fv-2r gene modifies the interaction between the spleen focus-forming virus envelope protein and the erythropoietin receptor.

The hydrophobic membrane-spanning sequences of the gp52 glycoprotein are required for the pathogenicity of Friend spleen focus-forming virus

Friend spleen focus-forming virus (SFFV) codes for a transport-defective envelope glycoprotein designated gp52, which is responsible for the leukemogenic properties of the virus. gp52 is a monotopic integral membrane protein anchored in the membrane by a stretch of hydrophobic amino acid residues located near the carboxy terminus of the molecule. We have constructed a mutant SFFV envelope gene in which the sequences that code for the hydrophobic membrane-spanning domain have been deleted, and we expressed this gene by using recombinant vaccinia virus vectors or retroviral vectors. The mutant SFFV envelope gene was found to encode a truncated glycoprotein (gp52t) which was also transport defective; a majority of gp52t remained cell associated, while a small proportion of the molecules underwent oligosaccharide processing. The processed form of gp52t was secreted from the cells. Retroviral vectors carrying the mutant SFFV envelope gene were found to be nonpathogenic in adult mice. These results indicate that the hydrophobic membrane-spanning region of gp52 is required for pathogenicity of SFFV and suggest that these sequences may play a role in signal transduction. The results also indicate that the transport defect of SFFV gp52 is due to structural features of the ectodomain of the molecule.

Mechanism of leukemogenesis induced by mink cell focus-forming murine leukemia viruses

The Friend or Moloney mink cell focus-forming (MCF) virus encodes a recombinant-type envelope glycoprotein, gp70, that is closely related to the membrane glycoprotein, gp55, of Friend spleen focus-forming virus (SFFV). We have shown previously that gp55 has the ability to activate cell growth by binding to the cellular receptor for erythropoietin. Here we show that gp70 encoded by either the Friend or Moloney MCF virus also binds to the erythropoietin receptor and that coexpression of the receptor and gp70 in an interleukin-3 (IL-3)-dependent cell line can activate IL-3-independent growth. Furthermore, when the cDNA for the human IL-2 receptor beta chain, which is related by sequence to the erythropoietin receptor, was introduced into this cell line, it became growth factor independent after infection either with SFFV or with one of the two MCF viruses but not with an ecotropic virus. Based on these observations, we propose a mechanism for the early stage of leukemogenesis induced by the MCF-type murine leukemia viruses.

Friend spleen focus-forming virus glycoprotein gp55 interacts with the erythropoietin receptor in the endoplasmic reticulum and affects receptor metabolism

The Friend spleen focus-forming virus envelope glycoprotein, gp55, binds to the murine erythropoietin receptor (EPO-R) and triggers growth activation in the absence of EPO. Interleukin 3-dependent lymphoid cell lines that have been stably transfected with the EPO-R cDNA grow in the presence of EPO or interleukin 3. In these cells, the EPO-R is synthesized as a minor 62-kDa unglycosylated form and a major 64-kDa form carrying one high-mannose N-linked oligosaccharide. A fraction of the 64-kDa form is processed to a 66-kDa species with complex-type sugars. Very little of the EPO-R is expressed on the cell surface and all three forms of EPO-R are degraded rapidly. Cells transfected with both EPO-R and gp55 cDNAs grow in the absence of EPO. Most of the EPO-R associated with gp55 is endoglycosidase H-sensitive, suggesting that the interactions between these proteins occur in the endoplasmic reticulum. Furthermore, the endoglycosidase H-sensitive EPO-R is more stable than in the absence of gp55, a result suggesting that interaction of gp55 with the EPO-R causes it to remain within the rough endoplasmic reticulum. It is possible that gp55 EPO-R complexes within this compartment send a growth-promoting signal to the cell.

Activation of cell growth by binding of Friend spleen focus-forming virus gp55 glycoprotein to the erythropoietin receptor

Friend spleen focus-forming virus (SFFV) is a defective murine C-type retrovirus which causes a multi-stage erythroleukaemia in mice and erythroblastosis in bone marrow cultures. The SFFV env gene encodes a membrane glycoprotein, gp55, which is located on the cell surface and in the rough endoplasmic reticulum and is essential both for the induction of leukaemia in vivo and erythroblast proliferation in vitro. The mechanism by which gp55 causes increased erythroblastosis and ultimately leukaemia is unknown, but a reasonable suggestion is that gp55 can mimic the action of erythropoietin by binding to its receptor (Epo-R), thereby triggering prolonged proliferation of erythroid cells. To test this possibility, we have co-expressed gp55 and the murine Epo-R in a fibroblast cell line. We show here that in such cells, the SFFV glycoprotein binds directly to Epo-R. Furthermore, when an interleukin-3 (IL-3)-dependent lymphoid cell line was co-infected by SFFV and a virus that carries the Epo-R gene, it could grow without IL-3. We suggest that through direct binding to Epo-R, gp55 can stimulate the receptor and by-pass the normal requirement for Epo, causing prolonged proliferation of infected erythroid cells. This could be the first step of leukaemogenesis induced by Friend virus.

Glycosylation of glycoproteins 52 and 65 encoded by the polycythemia-inducing strain of Friend spleen focus-forming virus. Isolation of glycopeptides containing individual glycosylation sites

The primary envelope gene product of the polycythemia-inducing strain of Friend spleen focus-forming virus, glycoprotein 52 (gp52), as well as its processed form, glycoprotein 65 (gp65), were isolated from virus-infected normal rat kidney cells metabolically labeled with [2-3H]mannose. Following digestion with trypsin, glycopeptides containing individual N-glycosylation sites were obtained by gel filtration and subsequent reversed-phase high-performance liquid chromatography. N-terminal amino acid sequencing of the glycopeptides demonstrated that only asparagine residues 11 and 26, located in the N-terminal domains of gp52 and gp65, carry carbohydrate substituents, while the potential N-glycosylation sites in the C-terminal portions of the molecules are not used. Carbohydrates attached were liberated by treatment with endo-beta-N-acetylglucosaminidase H or peptide: N-glycosidase F and characterized by high-performance liquid chromatography. The results demonstrated that gp52 carries similar patterns of oligomannosidic glycans in both positions. In gp65, however, asparagine residue 11 is almost exclusively substituted by complete, fucosylated N-acetyllactosaminic oligosaccharides, whereas asparagine residue 26 carries oligomannosidic or truncated N-acetyllactosaminic glycans.

Carbohydrate structure of glycoprotein 65 encoded by the polycythemia-inducing strain of Friend spleen focus-forming virus

The secondary envelope-gene product, glycoprotein 65 (gp65), of the polycythemia-inducing variant of Friend spleen focus-forming virus (F-SFFVp) was isolated from F-SFFVp-infected normal rat kidney cells cultivated in the presence or absence (-Glc) of glucose. Oligosaccharide side chains present were sequentially liberated by treatment of tryptic glycopeptides with endo-beta-N-acetylglucosaminidase H and peptide N-glycosidase F and fractionated by high-performance liquid chromatography. The glycans were characterized by digestion with exoglycosidases, by chromatographic comparison with oligosaccharide standards and by methylation analysis. The results demonstrate that gp65 contains oligomannosidic, hybrid and N-acetyllactosaminic glycans. The oligomannosidic glycans represent the same partially glucosylated species with six to nine mannose residues present in F-SFFVp gp52, the biosynthetic precursor of gp65 [Strube, K.-H. Schott, H.-H. and Geyer, R. (1988) J. Biol. Chem. 263, 3762-3771]. Oligosaccharides of the hybrid type were found to comprise one sialylated lactosamine unit and three or four alpha-linked mannose residues. Analysis of the N-acetyllactosaminic glycans revealed that gp65 carries fucosylated, partially sialylated bi-antennary, tri-antennary and tetra-antennary oligosaccharides, in addition to incomplete species. The glycosylation of gp65(-Glc) is characterized by the presence of oligomannosidic glycans with five to nine mannose residues, similar hybrid-type species and by increased amounts of incomplete N-acetyllactosaminic oligosaccharides, a decrease in sialylation and the lack of tetra-antennary species.

Molecular biology of Friend viral erythroleukemia

Friend virus clearly provides an important model for understanding the molecular biology of cancer. Moreover, the most important aspects of the erythroleukemia can be caused by a single SFFV infection in the absence of any helper virus. The SFFV env gene encodes a membrane glycoprotein, gp55. This glycoprotein, when expressed on erythroblast surfaces, causes a constitutive mitogenesis. However, SFFV infections only rarely increase the cell's self-renewal capability or abrogate its commitment to differentiate. Therefore, the consequence of infection is initially a polyclonal erythroblastosis. This polyclonal proliferation usually leads to cell differentiation and to recovery unless helper virus is present to cause continuing infection of new erythroblasts. Extremely rare SFFV proviral integrations, however, result in abrogation of the cell's commitment to differentiate and in the concomitant acquisition of cell immortality. These immortalizing proviral integrations occur at only a small number of sites in the mouse genome. Therefore, the mitogenic and immortalizing stages of erythroleukemia are now known to be caused by discrete genetic events--the first involving the SFFV env gene and the second involving the rare proviral integration sites. In early investigations of Friend virus, the first stage always preceded the second stage by at least several weeks. Now it is known that this delay in onset of the second stage is caused solely by statistics. Every SFFV-infected erythroblast is mitogenically activated, yet only rarely does the SFFV proviral integration produce immortality. Both steps in leukemogenesis can be caused simultaneously in an erythroblast by a rare single SFFV proviral integration. There has been an explosion of interest in retroviral env gene-mediated pathogenesis. Such pathogenesis has been recently associated with most of the naturally transmitted retroviral diseases including AIDS. Such pathogenesis involves in different viruses immunosuppression, anemia, neuropathy, and leukemia (Mathes et al. 1978; Simon et al. 1984, 1987; Weiss et al. 1985; Lifson et al. 1986; Riedel et al. 1986; Sitbon et al. 1986; Sodroski et al. 1986; Mitani et al. 1987; Schmidt et al. 1987; Klase et al. 1988; Overbaugh et al. 1988a, b). The shuffling and dynamic env gene rearrangements that have been associated with murine retroviral leukemogenesis have also now been seen in FeLV-FAIDS and HIV (Fisher et al. 1988; Overbaugh et al. 1 t88b; Saag et al. 1988; Tersmette et al. 1988). Friend virus provides an important established example of such env gene pathogenesis. Although we still do not understand precisely how gp55 causes erythroblast mitosis, workers in this field have discovered important clues that may lead to answers.(ABSTRACT TRUNCATED AT 400 WORDS)

A tagged helper-free Friend virus causes clonal erythroblast immortality by specific proviral integration in the cellular genome

A colinear molecular clone of the Lilly-Steeves polycythemia strain of Friend spleen focus-forming virus (SFFV) was modified by inserting a 215-base-pair tag of simian virus 40 DNA into its nonfunctional pol gene region. The DNA was then transfected into psi-2 packaging cells, and helper-free tagged SFFV was recovered in the culture medium. Injection of this helper-free virus into NIH/Swiss mice caused transient mild splenomegaly and formation of spleen foci at 9 to 10 days. Although the vast majority of infected erythroblast clones then differentiated and died out, rare cell clones that were present in only 20 to 30% of the mice grew extensively by 26 to 33 days to form transplantable leukemias. The clonality of these leukemias was established by Southern blot analysis of their DNAs by using several restriction endonucleases and the simian virus 40 tag as a hybridization probe. All transplantable leukemias lacked helper virus contamination and contained a single tagged SFFV provirus that expressed the mitogenic env gene product gp55. The SFFV proviruses in these leukemias also appeared to be integrated into a few tightly clustered sites in the cellular genome. Although the tagged SFFV caused polycythemia during the polyclonal early stage of erythroblastosis, growth of the helper-free clonal erythroleukemias caused severe anemia. These results suggest that a single SFFV can cause mitosis of erythroblasts, and that cell immortalization also occurs when the provirus integrates into a critical site in the host genome. We propose that mice with clonal-stage leukemia become anemic because the immortalizing proviral integrations interfere with the cellular commitment to differentiate.

The spleen focus-forming virus (SFFV) envelope gene, when introduced into mice in the absence of other SFFV genes, induces acute erythroleukemia

Previous studies in our laboratory and others have been consistent with the hypothesis that the envelope (env) gene of the spleen focus-forming virus (SFFV) is the only gene essential for the induction of acute erythroleukemia. However, no studies have been carried out with the SFFV env gene in the complete absence of other SFFV sequences. To accomplish this goal, we isolated the sequences that encode the envelope glycoprotein, gp52, of SFFVA and expressed them in a Moloney murine leukemia virus-based double-expression vector containing the neomycin resistance gene. The method used to produce retrovirus stocks in tissue culture cells affected the expression of the gp52 gene in the vector and the subsequent pathogenicity of the vector in mice. Highly pathogenic virus stocks were obtained by cotransfection of vector and helper virus DNAs into fibroblasts, followed by virus replication and spread through the cell population. Mice infected with this stock developed a rapid erythroid disease that was indistinguishable from that induced by the entire SFFV genome, and the virus stock transformed erythroid cells in vitro. Spleen cells from the diseased mice expressed the SFFV env gene product but not the SFFV gag gene product. As expected, mice given the virus containing the SFFV env gene in the reverse orientation did not express the SFFV env gene product or develop erythroleukemia. This study, therefore, demonstrated (i) that double-expression retroviral vectors can be used under specific conditions to produce viruses expressing high levels of a particular gene and (ii) that incorporation of the SFFV env gene into such a vector in the absence of other SFFV sequences results in a retrovirus which is as pathogenic as the original SFFV.

The membrane glycoprotein of Friend spleen focus-forming virus: evidence that the cell surface component is required for pathogenesis and that it binds to a receptor

The leukemogenic membrane glycoprotein of Friend spleen focus-forming virus (SFFV) has an apparent Mr of 55,000 (gp55), is encoded by a recombinant env gene, and occurs on cell surfaces and in intracellular organelles. There is evidence that the amino-terminal region of gp55 forms a dualtropic-specific domain that is connected to the remainder of the glycoprotein by a proline-rich linker (C. Machida, R. Bestwick, B. Boswell, and D. Kabat, Virology 144:158-172, 1985). Using the colinear form of a cloned polycythemic strain of SFFV proviral DNA, we constructed seven in-phase env mutants by insertion of linkers and by a deletion. The mutagenized SFFVs were transfected into fibroblasts and were rescued by superinfection with a helper murine leukemia virus. Four of the mutants cause erythroblastosis. These include one with a 6-base-pair (bp) insert in the ecotropic-related sequence near the 3' end of the gene, two with a 12- or 18-bp insert in the region that encodes the proline-rich linker, and one with a 6-bp insert in the dualtropic-specific region. The other mutants (RI, Sm1, and Sm2) are nonpathogenic and contain lesions in dualtropic-specific region. The other mutants (RI, Sm1, and Sm2) are nonpathogenic and contain lesions in dualtropic-specific sequences that are highly conserved among strains of SFFV. A pathogenic revertant (RI-rev) was isolated from one mouse that developed erythroblastosis 3 weeks after infection with RI. RI-rev contains a second-site env mutation that affects the same domain as the primary mutation does and that increases the size of the encoded glycoprotein. All pathogenic SFFVs encode glycoproteins that are expressed on cell surfaces, whereas the nonpathogenic glycoproteins are exclusively intracellular. The pathogenic SFFVs also specifically cause a weak interference to superinfection by dualtropic MuLVs. These results are compatible with the multidomain model for the structure of gp55 and suggest that processing of gp55 to plasma membranes is required for pathogenesis. The amino-terminal region of gp55 binds to dualtropic murine leukemia virus receptors, and this interaction is preserved in the SFFV mutants that cause erythroblastosis.

Genetic determinants of neoplastic diseases induced by a subgroup F avian leukosis virus

Two subgroup F avian leukosis viruses, ring-necked pheasant virus (RPV) and RAV-61, were previously shown to induce a high incidence of a fatal proliferative disorder in the lungs of infected chickens. These lung lesions, termed angiosarcomas, appear rapidly (4 to 5 weeks after infection), show no evidence of proto-oncogene activation by proviral integration, and are not induced by avian leukosis viruses belonging to other subgroups. To identify the viral sequences responsible for induction of these tumors, we constructed recombinant viruses by exchanging genomic segments of molecularly cloned RPV with those of a subgroup A leukosis virus, UR2AV. The ability to induce rapid lung tumors segregated only with the env sequences of RPV; the long terminal repeat of RPV was not required. However, recombinants carrying both env and long terminal repeat sequences of RPV induced lung tumors with a shorter latency. In several cases, recombinant viruses exhibited pathogenic properties differing from those of either parental virus. Recombinants carrying the gag-pol region of RPV and the env gene of UR2AV induced a high incidence of a muscle lesion termed infiltrative intramuscular fibromatosis. One recombinant, EU-8, which carries the gag-pol and LTR sequences of RPV, and the env gene of UR2AV, induced lymphoid leukosis after an unusually short latent period. The median time of death from lymphoid leukosis was 6 to 7 weeks after infection with EU-8 compared with approximately 5 months for UR2AV.

Role of a membrane glycoprotein in Friend virus erythroleukemia: nucleotide sequences of nonleukemogenic mutant and spontaneous revertant viruses

We previously isolated spontaneous env gene mutants of Friend spleen focus-forming virus that are nonleukemogenic in adult mice but form leukemogenic revertants in newborns; we found that the revertants contain secondary env mutations. To identify sites in the encoded membrane glycoprotein that are important for its pathogenic function, we molecularly cloned and partially sequenced the env genes of two mutant viruses (clone 63 and clone 4) and one revertant (clone 4REV). Clone 63 contained three noncontiguous point mutations that caused nonconservative amino acid substitutions of Gly-119----Arg-119, Cys-180----Tyr-180, and Gly-203----Arg-203 in the xenotropic-related domain of the env glycoprotein. These substitutions were presumably responsible for the altered electrophoretic and pathogenic properties of the mutant glycoprotein. The presence of these and several other G-A nucleotide substitutions at different sites in one spontaneous mutant provided striking evidence that error-rich proviruses can form during retroviral replication. Clone 4 contained a point mutation that generated a premature termination condon at amino acid residue 304 (Gln-304----Ochre-304). This termination codon was located immediately after the proposed xenotropic-ecotropic recombination site and eliminated the ecotropic-related domain, including the putative membrane anchor of the glycoprotein. Clone 4REV was a true revertant derived from clone 4 in which the premature termination codon had back-mutated to re-form the wild-type sequence. These results confirm an essential role for the env gene in Friend spleen focus-forming virus pathogenesis and suggest that the encoded membrane glycoprotein contains different domains that contribute to its pathogenic function.

The pathophysiology of murine retrovirus-induced leukemias

Murine leukemia viruses (MuLVs) are retroviruses which induce a broad spectrum of hematopoietic malignancies. In contrast to the acutely transforming retroviruses, MuLVs do not contain transduced cellular genes, or oncogenes. Nonetheless, MuLVs can cause leukemias quickly (4 to 6 weeks) and efficiently (up to 100% incidence) in susceptible strains of mice. The molecular basis of MuLV-induced leukemia is not clear. However, the contribution of individual viral genes to leukemogenesis can be assayed by creating novel viruses in vitro using recombinant DNA techniques. These genetically engineered viruses are tested in vivo for their ability to cause leukemia. Leukemogenic MuLVs possess genetic sequences which are not found in nonleukemogenic viruses. These sequences control the histologic type, incidence, and latency of disease induced by individual MuL Vs.

Biological and biochemical differences between variants of spleen focus-forming virus can be localized to a region containing the 3' end of the envelope gene

Two variants of the spleen focus-forming virus (SFFV), SFFVP and SFFVA, induce acute erythroleukemia in mice but differ in their effects on erythroid cells as well as in the posttranslational modification of the product of their envelope genes. To localize the region of the SFFV envelope gene responsible for these differences, we utilized a recombinant virus containing the 3' half of the SFFVP env gene, where the vast majority of the differences between SFFVP and SFFVA reside, and the SFFVP long terminal repeat (LTR) on an SFFVA background. Analysis of the recombinant virus indicates that it is capable of inducing all of the biological effects previously associated with SFFVP, including the ability to proliferate and differentiate without the need for erythropoietin. In addition, the env gene product of the recombinant virus can be detected on the cell surface, a property previously associated only with SFFVP. Although the recombinant virus also contains LTR sequences from SFFVP, we do not believe it is likely that the four LTR nucleotides that are unique to SFFVP are responsible for the biological or biochemical differences observed. These results strengthen the argument that the SFFVP env gene product acts at the cell surface to alter the hormonal requirements for erythroid cell growth and differentiation.

Induction of the early stages of Friend erythroleukemia with helper-free Friend spleen focus-forming virus

The polycythemia-inducing strain of Friend virus (FV-P) causes a multistage erythroleukemia in susceptible mice. FV-P is a complex of two viruses, a replication-competent virus [Friend murine leukemia virus (F-MuLV)] and a replication-defective spleen focus-forming virus (SFFVp). We have addressed directly the role of SFFVp in the induction of the early stages of Friend disease by constructing stocks of SFFVp free of detectable F-MuLV, using a recently described retroviral helper-cell line. These preparations are capable of inducing erythroid bursts (vBFU-E) whose inducibility, kinetics, and responsiveness to erythropoietin suggest that they are very similar, if not identical, to the vBFU-E induced by FV-P. Single injections of helper-free SFFVp had no apparent effects in vivo, although the addition of exogenous helper virus to the inoculum resulted in the induction of classic Friend disease. Increasing the effective titer by giving mice five daily virus injections resulted in the induction of splenomegaly and a large increase in the number of erythroid colony-forming units that were independent of erythropoietin. When the injections were discontinued, the spleens regressed and all the mice survived. When the injections were continued, all the mice died within 25 days of the first injection. These results demonstrate that SFFVp alone can alter the growth characteristics of erythroid progenitors and is directly responsible for the induction of vBFU-E in vitro and the erythroid hyperplasia in vivo. They also demonstrate that the initial polyclonal stage of Friend disease is reversible and can be reproduced by using preparations of SFFVp free of detectable F-MuLV.

Sequence comparisons of the anemia- and polycythemia-inducing strains of Friend spleen focus-forming virus

A nucleotide sequence analysis carried out on the envelope gene of the anemia-inducing strain of the Friend spleen focus-forming virus (F-SFFVA) reveals that its product has some unique features in common with previously described polycythemia-inducing strains of F-SFFV (F-SFFVP). (i) It contains an amino terminus that is highly related to the gp70 of mink cell focus-inducing viruses, (ii) it is a fusion protein containing the amino terminus of gp70 and the carboxy terminus of p15E, and (iii) it lacks the R-peptide normally found at the carboxy end of the p15E region. Although the envelope genes of F-SFFVA and F-SFFVP are quite similar overall, they do show sequence variation, particularly at the 3' end in the p15E-related region. These variations may contribute to previously observed differences in the response of F-SFFVP- and F-SFFVA-infected erythroid cells to regulatory hormone or to differences in the way the envelope glycoproteins are processed. The long terminal repeat regions of F-SFFVA and the Lilly-Steeves strain of F-SFFVP were also sequenced and compared with each other and with a previously published sequence of another F-SFFVP long terminal repeat. The sequences were found to be reasonably similar to each other but different from their ecotropic parent, Friend murine leukemia virus, as a result of a deletion of one copy of the direct tandem repeat in the enhancer regions. The observation that all SFFVS have this common change in the long terminal repeat enhancer region raises the possibility that it is required for pathogenicity.

A 3' end fragment encompassing the transcriptional enhancers of nondefective Friend virus confers erythroleukemogenicity on Moloney leukemia virus

Nondefective Friend helper murine leukemia virus (Fr-MuLV) induces primarily erythroleukemias in NFS mice, whereas Moloney murine leukemia virus (Mo-MuLV) induces T cell lymphomas. Using molecular clones of these two viruses, we constructed a recombinant in which a 0.62-kilobase fragment encompassing the U3 region at the 3' end of the Fr-MuLV genome replaced the corresponding region of Mo-MuLV. The recombinant virus obtained by transfection of this clone, whose genome is derived primarily from Mo-MuLV, induces almost exclusively erythroleukemias in NFS mice. This and the previous result of Chatis et al. (Proc. Natl. Acad. Sci. U.S.A. 80:4408-4411), showing that the reciprocal recombinant whose genome is primarily derived from Fr-MuLV induces almost exclusively lymphomas, argue that a strong determinant of the distinct disease specificities of Fr-MuLV and Mo-MuLV lies in this 3' end 0.62-kilobase fragment which contains the putative virus enhancers. To more precisely define this determinant, we have begun to construct recombinants in which smaller 3' end fragments of the Fr-MuLV and Mo-MuLV genomes are exchanged. Analysis of the first such recombinant showed that Fr-MuLV can be converted to a lymphoma-inducing virus in NFS mice by substitution of a 0.38-kilobase fragment encompassing the virus enhancers in U3 with the corresponding region of the Mo-MuLV genome.

Molecular cloning of biologically active Rauscher spleen focus-forming virus and the sequences of its env gene and long terminal repeat

Rauscher and Friend spleen focus-forming viruses (R- and F-SFFVs) cause similar progressive erythroleukemias dependent upon a virus-encoded membrane glycoprotein. Moreover, these SFFV glycoproteins are immunologically related to each other and to the recombinant-type glycoproteins encoded by the env genes of dual tropic murine leukemia viruses. To better understand these diseases and the viral origins, we isolated a pathogenically active molecular clone of R-SFFV proviral DNA, sequenced its 3'-terminal 2,163-base-pair (bp) region, and compared these sequences with previously determined sequences of F-SFFV. The 516-bp R-SFFV long terminal repeat is highly homologous to those of F-SFFV and Friend murine leukemia virus, although only the latter contains a 65-bp direct repeat in its U3 region. The env gene of R-SFFV encodes a glycoprotein with 408 amino acids that is identical in its basic domain organization to the glycoprotein of F-SFFV. Thus, the junctions between the dual tropic-related and ecotropic sequences occur at the same nucleotide, and both SFFV env genes contain identical 585-bp deletions in their ecotropic domains and single-bp insertions which cause premature terminations at the same amino acid in their ecotropic p15E domains. Consistent with their independent origins, however, the env sequences of R- and F-SFFV are distinctive in both their 5' dual tropic-related and 3' ecotropic-related domains. Furthermore, there are several consistent amino acid differences between the polycythemic F-SFFV sequences and the anemia-inducing R-SFFV sequence. The striking similarities of the independently formed F- and R-SFFV env genes imply that all of the glycoprotein domains arranged in a precise organization may be required for its leukemogenic activity

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.

Expression of xenotropic-like env RNA sequences in normal DBA/2 and NZB mouse tissues

Using a DNA probe prepared from cloned env gene sequences of Friend spleen focus-forming viruses, we detected the differential expression of multiple RNA species in uninfected DBA/2 fibroblasts and in various tissues from adult DBA/2 and NZB mice. The size of the major RNA species detected was estimated to be 24S . The 24S RNA species was enriched in polyadenylate-selected preparations and thus may represent a message for endogenous viral envelope glycoproteins. The viral origin of the 24S RNA was further characterized by its hybridization to DNA probes containing the long terminal repeats of Harvey murine sarcoma virus, mouse mammary tumor virus, or the U3 region of an endogenous xenotropic virus. Although the env-related 24S RNA failed to react with either Harvey murine sarcoma virus or mouse mammary tumor virus long terminal repeat probes, it hybridized well with the xenotropic virus long terminal repeat probe. Therefore, it is likely that the RNA detected with the Friend spleen focus-forming virus env probe reflects transcription of xenotropic envelope sequences in uninfected tissues. Our finding that the level of 24S RNA varied in different organs indicated some tissue specificity in the expression of these xenotropic-like env proteins.

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.

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.

Reduced leukemogenicity caused by mutations in the membrane glycoprotein gene of Rauscher spleen focus-forming virus

We isolated and characterized two spontaneous, weakly leukemogenic mutants of Rauscher spleen focus-forming virus (R-SFFV) that contain mutations in nonoverlapping regions of the membrane envelope (env) glycoprotein gene. As reported previously (M. Ruta and D. Kabat, J. Virol. 35:844-853, 1980), the replication-defective R-SFFV encodes a membrane glycoprotein with an apparent Mr of 54,000 (gp54) which is structurally and immunologically related to the membrane envelope glycoproteins of dual-tropic murine leukemia viruses. Mutant R-SFFV clones 3-25 and 4-3 encode abnormally sized gp54-related glycoproteins with apparent Mrs of 52,000 (gp52) and 45,000 (gp45), respectively. Northern and Southern blot analyses of the mutant R-SFFV nucleic acids indicated that an insertion has occurred in the 3-25 env gene and that a deletion has occurred in the 4-3 env gene. Furthermore, restriction endonuclease analyses and comparisons of the fragmentation patterns of the wild-type and mutant glycoproteins generated by partial proteolysis with Staphylococcus aureus V8 protease indicated that the mutations affect nonoverlapping domains of the envelope glycoprotein (amino-terminal fragment affected in 3-25 glycoprotein and carboxyl-terminal fragment affected in 4-3 glycoprotein). Glycosylation inhibition studies indicated that the reduced size of gp52 is caused at least partly by loss of an asparagine-linked oligosaccharide. In addition, these mutant viruses have dramatically reduced leukemogenicities compared with wild-type R-SFFV. We conclude that the gp54 structural gene is required for initiation or amplification of the splenic erythroblast hyperplasia which characterizes the preleukemic phase of Rauscher disease.

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.

Loss of leukemogenicity caused by mutations in the membrane glycoprotein structural gene of Friend spleen focus-forming virus

Friend virus infection of mice causes progressive leukemogenesis--a rapid splenic erythroblastosis that develops weeks later into a disseminating erythroleukemia. Furthermore, the replication-defective Friend spleen focus-forming virus (F-SFFV) encodes a membrane glycoprotein with an apparent Mr of 55,000 (designated gp55), which is structurally and immunologically related to the membrane envelope glycoproteins of dual tropic murine leukemia viruses. We now have isolated three spontaneous F-SFFV mutants that encode abnormally sized gp55-related glycoproteins with apparent Mrs of 40,000, 54,000, and 58,000, respectively. RNA blot and Southern blot analyses indicate that the mutant nucleic acids do not have substantial deletions or insertions in their glycoprotein gene regions. Protein fragmentation patterns indicate that the mutations affect nonoverlapping domains of the glycoprotein. Furthermore, these mutant glycoproteins seem to be defective in their processing to the plasma membranes. Although transmitted efficiently between cultured cells, the mutants have dramatically reduced leukemogenicities compared with the same titers of wild-type F-SFFV. We conclude that the gp55 structural gene is necessary for initiating the erythroblast proliferative phase of Friend disease and that changes in membranes can be primary causes rather than only secondary consequences of tumor progression.

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.

Role for the 3' end of the genome in determining disease specificity of Friend and Moloney murine leukemia viruses

To probe the genetic basis of disease specificity of nondefective murine type C viruses, we are constructing recombinants in vitro between molecular clones of Friend murine leukemia virus (Fr-MuLV) and Moloney murine leukemia virus (Mo-MuLV). Fr-MuLV induces erythroleukemias when injected into newborn NFS mice, whereas Mo-MuLV almost invariably induces T-cell lymphomas. We find that a recombinant whose genome is derived primarily from Fr-MuLV but which has 621 nucleotides of Mo-MuLV information at its 3' end induces almost exclusively thymic lymphomas. The sequences derived from Mo-MuLV include 99 nucleotides encoding the carboxyl terminus of Prp15E, the origin of DNA +-strand synthesis, all of the U3 region, and 36 nucleotides of the R portion of the long terminal repeat. When the segment of Mo-MuLV was removed and replaced with the comparable segment from Fr-MuLV, the virus was again erythroblastosis-inducing. These results, in conjunction with studies from other laboratories [Laimins, L. A., Khoury, G., Gorman, C., Howard, B. & Gruss, P. (1982) Proc. Natl. Acad. Sci. USA 79, 6453-6457], suggest that transcriptional signals in U3 may determine tissue tropism and hence influence disease specificity ("targeting") of murine leukemia viruses.

Genetic structure of Rauscher spleen focus-forming virus

Rauscher spleen focus-forming virus contains functional gag and pol genes and a partially deleted env gene which is structurally related to the env genes of dual tropic murine leukemia viruses.

Genome structure of mink cell focus-forming murine leukemia virus in epithelial mink lung cells transformed vitro by iododeoxyuridine-induced C3H/MuLV cells

We characterized mink cell focus-forming murine leukemia viruses that were isolated from C3H/MCA-5 cells after induction with 5-iododeoxyuridine in culture. Mink lung epithelial cells malignantly transformed in vitro by induced virus were the source of four molecular clones of mink cell focus-forming virus. CI-1, CI-2, CI-3, and CI-4. Three clones, CI-1, CI-2, and CI-3, had full-length mink cell focus-forming viral genomes, one of which (CI-3) was infectious. In addition, we obtained a defective viral genome (CI-4) which had a deletion in the envelope gene. A comparison between the envelope genes of CI-4 and those of spleen focus-forming virus by heteroduplex mapping showed close homology in the substitution region and defined the deletion as being identical to the p15E deletion of spleen focus-forming virus. The recombinant mink cell focus-forming genomes are not endogenous in C3H/MCA-5 cells and therefore must have been formed in culture after induction by 5-iododeoxyuridine. CI-3, the infectious clone of mink cell focus-forming murine leukemia virus, was dualtropic, and mink cells infected with CI-3 were altered in their response to epidermal growth factor. In the presence of epidermal growth factor at 10 ng/ml, uninfected mink cells retained their epithelial morphology in monolayer culture and did not form colonies in soft agar. In contrast, CI-3 virus-infected mink cells grew with fibroblastic morphology in monolayer culture and showed an increased growth rate in soft agar in the presence of epidermal growth factor.

Large RNase T1-resistant oligonucleotides encoding p15E and the U3 region of the long terminal repeat distinguish two biological classes of mink cell focus-forming type C viruses of inbred mice

We used T1 oligonucleotide maps, in conjunction with available nucleotide sequences of appropriate C-type viruses, to identify regions of the viral genome that distinguish two biological classes of mink cell focus-forming (MCF) viruses described previously by Cloyd et al. (J. Exp. Med. 151:542-522, 1980). We found that leukemogenic MCF viruses from thymus differed from non-leukemogenic MCFs isolated from nonthymic neoplasms in nucleotide sequences encoding Prp15E and the U3 portion of the long terminal repeat (LTR). The thymic isolates possessed recombinant Prp15E genes, with the 5' to mid portion derived from their ecotropic parents and the extreme 3' portion invariably derived from their nonecotropic parents. These viruses probably derived the entire U3 portion of their LTRs from their nonecotropic parents. The nonthymic MCFs appeared to inherit their entire Prp15E coding region from their nonecotropic parents. We failed to detect consistent differences in gp70-coding sequences between the two groups of MCFs, but this may simply reflect limitations of the data. The studies presented here, in conjunction with studies from a number of labs indicating a role for MCF gp70 in leukemogenesis, indicate that three genetic elements, gp70, p15E, and the U3 portion of the LTR, may all play a role in determining the leukemogenic phenotype of type C viruses of high-leukemic inbred mice.

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.

Molecular cloning of a highly leukemogenic, ecotropic retrovirus from an AKR mouse

SL3-3 is a leukemogenic, ecotropic retrovirus produced by a T-cell line derived from a spontaneous lymphoma of an AKR mouse. We have isolated a molecular clone of its DNA provirus from infected NIH 3T3 fibroblasts. Cloned proviral DNA produced infectious virus upon transfection onto NIH 3T3 cells. Virus derived by transfection induced lymphomas at high frequency in AKR/J, C3H(f)/Bi, CBA/J, and NFS/N mice. Heteroduplex and RNase T1 fingerprinting analyses showed that the genomes of SL3-3 and the non-leukemogenic virus, Akv, contain no major substitutions relative to one another and differ by only a few base changes. These results unambiguously show that SL3-3 is a highly leukemogenic virus and that major rearrangements of the genome relative to Akv are not required for virulence.

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.