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

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.

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.

Sequences in the U5-gag-pol region influence early and late pathogenic effects of Friend and Moloney murine leukemia viruses

Friend replication-competent murine leukemia virus (F-MuLV), clone 57, induces a severe early hemolytic anemia and a later erythroleukemia after inoculation of newborn IRW or ICFW mice, whereas Moloney MuLV (M-MuLV) induces only lymphoid leukemia. We have shown previously that the attenuated hemolytic and erythroleukemogenic abilities of an F-MuLV variant, clone B3, were due mostly to changes in the env gene and long terminal repeat, respectively. For the present study, we derived two constructs exchanging env fragments of F-MuLV 57 and M-MuLV and compared them with two constructs described by Chatis et al. (J. Virol. 52:248-254, 1984) exchanging the U3 region of the long terminal repeat of the same parental viruses. When comparing the hemolytic effect of these constructs with those of the parent, we found that the U5-gag-pol region of F-MuLV was required for development of severe early hemolytic anemia and that, unlike the env of F-MuLV B3, the env of M-MuLV was fully competent in inducing severe early hemolytic anemia when associated with the F-MuLV U5-gag-pol and U3 regions. As expected, induction of erythroleukemia depended on the presence of the F-MuLV U3 region; however, the presence of both the U3 and U5-gag-pol regions of F-MuLV appeared to be synergistic and was associated with a more rapid appearance of erythroleukemia.

Requirement of the single base insertion at the 3' end of the env-related gene of Friend spleen focus-forming virus for pathogenic activity and its effect on localization of the glycoprotein product (gp55)

In order to obtain evidence for the essential role of the single base insertion occurring at the 3' end of the env-related gene of Friend spleen focus-forming virus (SFFV) encoding the leukemogenic glycoprotein (gp55) a mutant SFFV genome was constructed in which the segment of the gp55 gene of the polycythemia-inducing strain of SFFV containing the single base insertion and the 6-base-pair duplication was replaced by the corresponding sequence of the Friend murine leukemia virus env gene. The mutant SFFV-Friend murine leukemia virus complex did not induce symptoms of the erythroproliferative disease in adult DBA/2 mice. During passage through newborn DBA/2 mice, the mutant virus complex invariably gave rise to weakly pathogenic variant SFFVs. All of the variant SFFVs induced in adult DBA/2 mice a transient mild splenomegaly associated with normal or slightly low hematocrit value, and they produced gp55 with a molecular weight similar to that of gp55 of the wild-type SFFV. For the two isolates of variant SFFV, the 3' portion of the viral DNA intermediate containing the 3' portion of the gp55 gene was molecularly cloned. Nucleotide sequences of these biologically active cloned DNAs were determined and showed that the variant SFFV genomes arose from the mutant SFFV genome by regaining the single base insertion, indicating that the single base insertion is essential for the biological activity of gp55. Evidence is presented indicating that the single base insertion which causes a loss of the cytoplasmic domain of the env-related protein is not related to the localization of the further-glycosylated form of gp55 in the plasma membrane but is involved with the release of gp55 from cells.

Biochemical characterization of cell-associated and extracellular products of the Friend spleen focus-forming virus env gene

The mature product of the env gene of Friend spleen focus-forming viruses (F-SFFV) is efficiently released from both leukemia cells and infected fibroblasts. Analyses of the kinetics of env protein synthesis and secretion in NRK cells infected with the Lilly-Steeves strain of SFFVp indicated that this product, gp65, was formed rapidly and remained stably associated with cells for up to 4 hr, at which point it was first detected in supernatant medium. By 12 hr after synthesis, greater than 95% of gp65 was found extracellularly. The release of this component was effectively blocked by 10 mM 1-deoxynojirimycin, an inhibitor of oligosaccharide processing, demonstrating a requirement for processing of high mannose precursor oligosaccharides in the secretion of gp65. Similar oligosaccharide substituents were found on cell-associated and extracellular forms of gp65. Enzymatic deglycosylation experiments demonstrated that in addition to the predicted four N-linked oligosaccharides, gp65 contains O-linked carbohydrates which are resistant to the action of peptide N-Glycanase F, but sensitive to neuraminidase and O-Glycanase. These structures may be related to O-linked oligosaccharides previously found on the env gene products of murine leukemia viruses. Comparison of the sizes of the deglycosylated forms of cell-associated and supernatant gp65 demonstrated that the extracellular molecules are approximately 3 kDa smaller than the cell-associated components. These data suggest the involvement of proteolysis at a C-terminal site in the release of gp65 from the plasma membrane.

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.

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.

Rat cells infected with anemia-inducing Friend leukemia virus contain integrated replication-competent but not defective proviral genomes

The integrated proviral DNA in five murine cell lines transformed by the anemic strain of Friend leukemia virus (FLV-A) was examined by Southern hybridization to a cloned Friend virus (F-MuLV) probe. Kpn I fragments 9 kilobases (kb) and 5.7 kb long were observed for each cell line. However, the number of copies of each fragment in the cell genome varied according to the cell type. As compared to the adherent epithelioid cell lines, the anchorage-independent erythroleukemic cell lines contained more copies of the 5.7-kb fragment than of the 9-kb fragment, suggesting that the former may be biologically significant and perhaps related to the growth of erythroid cells. The presence of Kpn I fragments of the same sizes, albeit in fewer copies, in normal mouse spleen DNA made it difficult to distinguish exogenous virus from endogenous viral sequences. Therefore, rat 3Y1 cells, which contained no murine endogenous viruses, were infected with FLV-A stock virus prepared directly from the spleens of leukemic mice. Only the 9-kb Kpn I fragment, representing replication-competent Friend virus component, was detected in the infected rat cell DNA. No hybridization was observed to a 0.6-kb fragment of the spleen focus-forming virus env gene that is specific for xenotropic and dual-tropic mink cell focus-forming viruses. Since the virus synthesized by the infected rat cells was leukemogenic in adult mice, these data suggest that the wild-type FLV-A is replicative and fully pathogenic in the absence of other competent virus components.

Malignant transformation of erythroid cells in vivo by introduction of a nonreplicating retrovirus vector

DNA from a replication-defective spleen focus-forming virus (SFFV) was reconstructed and transfected into psi-2 cells containing a packaging-defective mutant of Moloney murine leukemia virus. Replication-incompetent retrovirus particles (helper virus-free containing genomes that express the transforming envelope gene of SFFV (gp52) transformed bone marrow cells in vitro and, after direct intravenous introduction of the vector, induced malignant erythroid disease in vivo. Disease induction was dependent on prior treatment of mice with phenylhydrazine, which probably increased the availability of erythroid target cells. Since there was no evidence of virus particle expression in mice with malignant disease, this study demonstrates the acute oncogenic potential of a limited number of erythroid cells expressing SFFV gp52. Direct inoculation of animals with nonreplicating retroviral vectors containing transforming genes may be useful in study the oncogenic effects of such genes.

The mature form of the Friend spleen focus-forming virus envelope protein, gp65, is efficiently secreted from cells

The env genes of Friend spleen focus-forming viruses (F-SFFV) have been implicated in the rapid pathogenicity of these agents. Two env-gene products are detected in SFFV-infected cells: the primary translation product, gp52, and a more highly processed form, gp65. In this communication we demonstrate that gp65 is the major end product of the SFFV env gene, and is efficiently secreted from both erythroleukemia cells and infected fibroblasts. Secretion was observed for the mature env-gene products of both polycythemia- and anemia-inducing strains of SFFV. These results suggest that one function of the point mutation near the 3' end of the env gene, which is invariant in the formation of SFFVs, is to allow secretion of gp65, and that secreted gp65 may be the factor mediating the leukemogenic activity of these viruses.

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.

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.

Fluidity of a retrovirus genome

Comparison of the genomic sequences of the Friend spleen focus-forming virus with other murine retroviral sequences indicated that the spleen focus-forming virus was derived from at least three retroviruses. The 5' end of the virus, from the primer binding site through most of gag, was derived from AKV. The rest of gag and all of pol were of uncertain origin, but were probably derived from the same xenotropic virus that gave rise to the 5' half of env. The remainder was derived from Friend murine leukemia virus. The positions of a 585-base deletion, a 6-base duplication, and a point insertion that leads to a frame shift and premature protein termination in the ecotropic 3' end of env were invariant between three spleen focus-forming virus strains, indicating that they had a single common ancestor. However, the point of crossover between xenotropic viral sequences and Friend murine leukemia virus was different in each strain, and two strains were much more closely related to each other than to the third in the xenotropic region, indicating that these strains had diverged by multiple recombinations. Furthermore, a different nucleotide comprised the single point insertion near the 3' end of env, suggesting that these viruses have an extremely high transition and transversion rate.

A time-course study of MuLV env gene expression in the AKR thymus: qualitative and quantitative analysis of ecotropic and recombinant virus gene products

Low levels of synthesis of two species of MuLV env gene polyprotein (PrENV protein) in thymocytes of 3-month-old AKR mice were identified. Synthesis of PrENV proteins which comigrate with those of ecotropic and recombinant, dualtropic MuLV represented, respectively, 0.03-0.05 and 0.01-0.03% of total cell protein synthesis in these animals. An increase in the rate of synthesis of both PrENV species was observed in animals at 5-6 months of age; ecotropic virus PrENV represented 0.2-0.6% of total protein synthesis and recombinant virus PrENV, 1-2.5% of total protein synthesis in thymocytes of mice of this age. This same increase in env gene expression of both the ecotropic and recombinant virus was induced in 3-month-old animals by intrathymic injection of recombinant MuLV at 4-6 weeks of age. The level of recombinant virus env gene synthesis in thymomas was similar to that observed in thymocytes of 5- to 6-month-old animals and in experimentally injected animals; elevated synthesis of ecotropic virus PrENV protein was detected in 85% of these leukemias. Partial protease digest mapping of the recombinant virus PrENV protein isolated from 23 primary thymomas revealed that the predominant type of recombinant (18/23) expressed in these cells was that of the MCF 69L1/247 type. A notable finding was the identification of expression of variant ecotropic MuLV in these thymomas. Of ten leukemias studied, eight expressed ecotropic virus PrENV proteins which were distinguishable from that of Akv virus. Four unique ecotropic virus PrENV proteins were observed.