Synergistic effect of human lactoferrin and recombinant murine interferon-gamma on disease progression in mice infected with the polycythemia-inducing strain of the Friend virus complex.

Mice infected with the polycythemia-inducing strain of the Friend virus complex (FVC-P) have been used as a leukemic mouse model. In the present study, purified iron-saturated human lactoferrin (LF) and recombinant murine (rmu) interferon-gamma (IFN-gamma), alone or in combination, were used to influence disease progression in virally infected mice. DBA/2 mice were injected i.v. with FVC-P, and were treated s.c. with 100 micrograms LF at day 7, and/or rmuIFN-gamma at 5 x 10(4) units/day for 3 days beginning at day 6 after viral infection. Mice were assessed for survival, and also 14 days after virus inoculation, the mice were killed and spleen extracts were assessed for spleen focus forming virus (SFFV) titers by spleen focus forming unit (SFFU) assay, SFFV mRNA and genomic DNA expression, and natural killer (NK) cell activity. Treatment with LF or rmuIFN-gamma alone had little or no effect on SFFU numbers or SFFV mRNA or genomic DNA expression. However, dramatically decreased SFFV titers and levels of SFFV mRNA and genomic DNA were observed in mice treated with the combination of LF and rmuIFN-gamma. NK cell activity decreased by FVC-P was returned to normal levels by LF and rmuIFN-gamma. The combined treatment also enhanced the survival rates of FVC-P-infected mice. The results suggest synergistic suppressive effects of LF with rmuIFN-gamma on disease progression in FVC-P-infected mice. This information might be of significance as a potential therapy for patients with leukemia and those infected with retroviruses.

Insertional inactivation of the p53 gene during friend leukemia: a new strategy for identifying tumor suppressor genes.

Rearrangements within the p53 gene are observed in a high proportion of independent erythroleukemic cell lines derived from the spleens of mice infected with Friend leukemia virus. The majority of cells with at least one rearranged p53 allele do not express detectable levels of p53 protein. Here, we show that in 4 out of 19 newly established erythroleukemic cell lines induced by infecting DBA mice with either the anemia (FV-A) or polycythemia (FV-P)-inducing strains of Friend virus, the p53 gene is rearranged as a result of integration of spleen focus-forming virus (SFFV). Integration of SFFV within the p53 gene resulted in inactivation of gene expression as determined by Western blot analysis. The sites of SFFV integration in the p53 gene were found, by Southern blot analysis and the polymerase chain reaction, to be localized in a 1-kbp region between introns 7 and 9. In addition, loss of the normal p53 allele was observed in three of the erythroleukemic cell lines that carried a rearranged p53 gene. Insertion of SFFV in these cell lines resulted in either the appearance of aberrant p53 transcripts or the complete lack of p53 expression. The results presented in this paper demonstrate that retroviral insertions can not only contribute to neoplastic transformation by activating dominant oncogenes but also by inactivating genes that normally function in a negative way to control cell growth. Thus, it may be possible to identify additional genes of this class by characterizing chromosomal sequences that are adjacent to common sites of retroviral integration in tumors.

Transmembrane domain of the envelope gene of a polycythemia-inducing retrovirus determines erythropoietin-independent growth.

Both the polycythemia-inducing and the anemia-inducing strains of Friend spleen focus-forming virus can induce acute erythroleukemia in susceptible adult mice. However, only cells infected with the polycythemia-inducing strain become erythropoietin-independent for proliferation and differentiation. The sequences responsible for the altered erythropoietin responsiveness have previously been localized to a 678-base-pair EcoRI-Cla I fragment at the 3' end of the envelope gene. This region is now further analyzed by dividing it into two fragments by using the Fok I restriction site. Two recombinants were made by replacing either the 558-base-pair EcoRI-Fok I or the 113-base-pair Fok I-Cla I env gene fragments from the anemia-inducing strain of spleen focus-forming virus with sequences derived from the polycythemia-inducing strain. Our results indicate that the 113-base-pair Fok I-Cla I fragment, which encodes primarily the transmembrane domain of the envelope protein, determines erythropoietin-independent growth.

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 52 encoded by the polycythemia-inducing strain of Friend spleen focus-forming virus.

The primary envelope (env) gene product of the polycythemia-inducing variant of Friend spleen focus-forming virus (F-SFFVP), representing a glycoprotein with an apparent Mr of 52,000 (gp52), was isolated from F-SFFVP-infected normal rat kidney cells metabolically labeled with [2-3H]mannose in the presence or absence of glucose. Structures of the oligosaccharides present were determined by micromethylation analysis, acetolysis, and digestion with exoglycosidases. Gp52 radiolabeled in the presence of glucose contains solely oligomannosidic glycans comprising 6 to 9 mannose residues (Man6-9GlcNAc2), some of which carry additional glucose. The structures of the glycans found reflect the typical intermediates of oligosaccharide processing. The glycosylation of gp52 isolated from glucose-deprived cells (-Glc), however, is characterized by increased amounts of Man5-7 species comprising other structural isomers. Only gp52 (-Glc) glycans are, in part, further processed yielding incomplete complex-type oligosaccharides. Our results demonstrate that the limited post-translational processing of the primary F-SFFVP env gene product is neither due to aberrant trimming of its oligomannosidic glycans nor due to transfer of immature lipid-linked oligosaccharide-intermediates as observed in glucose-starved cells.

Modulation of Friend virus infectivity in vivo by administration of purified preparations of human lactoferrin and recombinant murine interleukin-3 to mice.

Purified iron-saturated human milk lactoferrin (LF) and purified recombinant murine interleukin-3 (IL-3) were assessed in vivo for their effects on replication of spleen focus forming viruses (SFFV) in spleens of DBA/2 mice injected with the polycythemia-inducing strain of the Friend virus complex. LF and IL-3, inoculated 2 hr prior to the administration of the polycythemia-inducing strain of the Friend virus complex, respectively decreased and increased the replication of SFFV in mice as assessed by the spleen focus forming unit assay in primary and secondary DBA/2 mice. Since virus infectivity is associated with the DNA synthetic phase of the cell cycle and it has been shown elsewhere that LF decreases and IL-3 increases the percent of hematopoietic progenitor cells in S-phase in vivo, the results suggest that the opposing actions of LF and IL-3 on replication of SFFV may reflect the actions of these molecules on cycling of the target cells for SFFV.

Protective influence of lactoferrin on mice infected with the polycythemia-inducing strain of Friend virus complex.

Purified iron-saturated human lactoferrin (LF) was assessed in vivo for effects on the survival rates of C57BL X DBA/2 f1 (hereafter called BD2F1) (Fv-2sr) mice and titers of spleen focus-forming viruses (SFFV) in BD2F1 and DBA/2 (Fv-2ss) mice inoculated with the polycythemia-inducing strain of the Friend virus complex (FVC-P). LF prolonged the survival rates and decreased the titers of SFFV in mice given FVC-P. Titers of SFFV, assayed 14 days after administration of FVC-P, were measured by the spleen focus-forming unit assay in secondary mouse recipients. Decreases in titers of SFFV were apparent when LF was given in vivo as a single bolus dose of 200 micrograms within 2 h of the Friend virus complex (FVC), or as a total dosage of 200 micrograms given on days 1, 2, 4, 7, 9, and 11 after FVC-P, and to a lesser degree when LF was given as a total dosage of 200 micrograms on days 3, 4, 7, 9, and 11 after FVC-P. No decreases in titers of SFFV were detected when LF was given up to 3 days before or more than 3 days after FVC-P. LF did not appear to be directly inactivating the viruses as it did not inactivate the SFFV or the Friend murine leukemia helper virus in vitro. The results suggest that the protective effect of LF in vivo is probably due to an action on cells responding to the FVC or to an action on cells which influence the cells responding to the FVC or which influence the virus. It has been shown elsewhere that LF decreases the percentage of marrow and spleen hematopoietic progenitor cells that are in DNA synthesis in vivo and this may be the means by which the protective effect of LF is mediated in mice given the FVC.

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.

Genesis of a new spleen focus-forming virus: possible role of MCF viruses.

A Friend mink cell focus-inducing (Fr-MCF) virus isolated from a Friend tumor cell line was able to induce acute erythroleukemia associated with polycythemia when injected as a Friend murine ecotropic leukemia virus (F-MuLV) pseudotype into adult Swiss and ICFW mice. One virus isolate recovered from leukemic cells and designated as FV-F3 presented the following properties: (i) persistence of the same leukemogenic power when propagated in vivo and in vitro; (ii) in vivo spleen focus-forming (SFFV) capacity; (iii) presence of erythropoietin (EPO)-independent CFU-E in leukemic animals; (iv) expression of a 32 RNA specifically recognized by a SFFV probe, in FV-F3 infected cells; and (v) expression in FV-F3-infected cell of polypeptides in the range of gp52 SFFV. Peptide analysis of these products revealed close similarities with the parental MCF virus. These data suggest that a SFFV genome arose by genetic recombinational events involving MCF virus.

Erythroleukemia induction by Friend leukemia virus. A host gene locus controlling early anemia or polycythemia and the rate of proliferation of late erythroid cells.

This report confirms that the Fv-5 locus controls the types of erythropoiesis induced by Friend erythroleukemia virus (FLV) (21) and extends the study to investigate the mode of action of this locus. With the use of FLV obtained by a variety of procedures, we showed that the polycythemia spleen focus-forming component (SFFVp) was responsible for the contrasting changes of hematocrits observed in FV-Pp (polycythemia strain)-infected DBA/2 (Fv-5pp) or CBA (Fv-5aa) mice. These changes in hematocrits were found to be a direct result of the rise in circulating reticulocytes and erythrocytes in DBA/2 mice and a corresponding drop of these erythroid cells in CBA mice 2 wk after infection. Examination of the FV-P-induced cellular changes indicated that dramatic increase in erythropoietin (epo)-independent erythroid precursor (CFU-E*) cells was detected in the spleens and marrow of both strains of mice. The epo responsiveness of the CFU-E in the uninfected and FV-P-infected CBA and DBA/2 mice was also very similar. Similar to FLV-infected DBA/2 mice, the FV-P-infected CBA mice also developed tumorogenic cells (CFU-FV) relatively early after infection (4-6 wk). Study of the physiological and pathological changes in the marrows and spleens of these infected mice indicated that significant differences were found in the spleens of the two strains of mice. The percent of reticulocytes in the spleen cells of CBA mice remained between 10 and 20%, and level of the DBA/2 mice increased to approximately 50%. This higher rate of erythropoiesis was also reflected in the significantly higher rate of uptake of 59Fe in the spleens of the DBA/2 mice. These results suggest that the Fv-5 locus might control the hematocrit levels of these mice by regulating the rates of erythropoiesis in the spleen levels of these mice, probably by affecting the rate of proliferation of an erythroid cell or cells. The erythroid cell(s) affected is likely to be more mature than the erythroid progenitor, CFU-E, as the levels of CFU-E in these two strains of mice were similar. The hypothesis that Fv-5 may control the rates of proliferation of a late erythroid (cell(s) is also supported by the significantly higher spleen weights found in the infected DBA/2 (approximately 2.5 g/spleen) mice than in the CBA (approximately 1 g/spleen) strain.

The relationship between genomic RNAs of polycythaemic forms of spleen focus-forming Friend virus and its helper virus.

We have studied the relationship between Friend spleen focus-forming virus (SFFV) and its helper lymphoid leukaemia virus (LLV) by comparing RNase T1 fingerprints of genomic RNAs. Our data indicate that about 70% of the SFFV sequence is a perfect copy of parts of the helper genome. We conclude that our SFFV and LLV isolates have co-evolved very closely and that SFFV-specific sequences are not identical in different Friend virus isolates.

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.

Anemia- and polycythemia-inducing isolates of Friend spleen focus-forming virus. Biological and molecular evidence for two distinct viral genomes.

Two distinct clones of Friend spleen focus-forming virus (SFFV), differing in their erythroleukemic potential, are described. These isolates have been cloned free of their associated helper viruses and shown to be replication-defective. Both SFFV isolates have been rescued from rat fibroblast nonproducer cell clones with cloned replication-competent viruses, F-MuLVA and F-MuLVP, obtained from the anemia- or polycythemia-inducing isolates of Friend virus complex, respectively. These rescued viruses induce a rapid proliferative disease associated with the appearance of macroscopic spleen foci and splenomegaly. In addition, each is subject to regulation by the W, Steel (Sl), and Fv-2 host gene loci. These two isolates of SFFV can, however, be distinguished by both biological and molecular criteria. Friend SFFVP induces a rapid polycythemia associated with the appearance of large numbers of erythropoietin (EPO)-independent erythroid colony-forming cells in the marrow and spleen. In contrast, SFFVA induces a rapid anemia associated with a progressive decrease in the number of EPO-dependent erythroid colony-forming cells in marrow, and a rapid increase in the number of EPO-dependent erythroid colony-forming cells in spleen. Furthermore, the nature of the disease induced by the two isolates of SFFV is independent of the Friend helper virus: SFFVP, rescued from a nonproducer cell clone with either F-MuLVA or F0MuLVP, induced a polycythemic transformation, whereas SFFVA, rescued with either F-MuLVA or F-MuLVP, induced an anemic transformation. The two Friend SFFV isolates can also be discriminated on the basis of translational products encoded by their gag and env genes: SFFVP encodes the amino-terminal gag-gene protein p15, whereas SFFVA encodes the gag-gene proteins p15, p12, and p30. In addition, the SFFV isolates encode nonidentical 55,000-mol wt env gene-related proteins that can be distinguished by analysis of their methionine-containing tryptic peptides.

Erythroleukemia induction by replication-competent type C viruses cloned from the anemia- and polycythemia-inducing isolates of Friend leukemia virus.

In this study, the biological properties of the replication-competent viruses, F-MuLVA, present in the anemia-inducing isolate of Friend leukemia virus complex (FV-A); and F-MuLVP, present in the polycythemia-inducing isolate of Friend leukemia virus complex (FV-P) have been examined. BALB/c mice infected as newborns with clonal isolates of F-MuLVA or F-MuLVP become anemic and show splenic enlargement characterized by an increased proportion of cells that resemble immature nucleated erythroid cells. In addition, the spleens of these F-MuLVA- or F-MuLVP-infected mice contain a markedly increased proportion of both erythropoietin-dependent erythroid progenitor cells and spectrin-containing erythroid cells. These results suggest that Friend murine leukemia virus (F-MuLV) by itself can induce an erythroleukemic transformation in newborn BALB/c mice similar to that induced by the anemia-inducing spleen focus-forming virus (SFFVA) in newborn or adult mice. Kinetic studies indicated that the alterations in hemopoietic cell populations induced by F-MuLVA or F-MuLVP in newborn BALB/c mice occurred more slowly than the rapid changes observed after infection with FV-A. In addition, adult BALB/c mice were fully susceptible to the erythroleukemic transformation induced by either SFFVA or SFFVP, whereas only newborn mice were susceptible to F-MuLV. Taken together, these results suggest that, although the replication-defective Friend spleen focus-forming viruses appear to be the major determinant of erythroleukemia induction in adults, the replication-competent helper F-MuLV also have erythroleukemic potential when assayed in newborn animals.

Comparison of the biological effects of anemia inducing and polycythemia inducing Friend virus complex.

The comparison of the biological effects of FVP and FVA showed that leukemogenesis appears to be delayed in FVA infected mice as compared to FVP infected animals after injection of comparable quantities of virus as measured in spleen focus forming units. In addition, no CFU-EI, characteristic for FVP induced leukemia, were found in leukemic spleen or bone marrow of FVA infected mice. Since it was possible to distinguish both viruses by their different host ranges, which are helper virus determined, it is suggested that the observed differences, especially the lack of CFU-EI in FVA infected mice, might be due to differences in the helper virus component of the FV complex.

In vitro transformation of mouse bone marrow cells by the polycythemic strain of Friend leukemia virus.

Strains of Friend leukemia virus (FLV) that are associated with polycythemia contain the defective spleen focus-forming virus (SFFV). To determine whether the transforming ability of FLV was affected by the presence of this second agent, DBA/2J mouse bone marrow cells were infected in vitro. Criteria for transformation were the establishment of permanent lines, growth on semisolid agarose, and the production of tumors at the site of inoculation in syngeneic hosts. Two lines of immature hematopoietic cells that grow in suspension originated from the infected cultures. Each has an almost diploid karyotype (38-39 chromosomes) and 3-4 metacentric chromosomes. These transformed cells express gp71 viral envelope glycoprotein and p30 viral core protein antigens. Virus production was measured by reverse transcriptase (RNA-dependent DNA polymerase) activity of the virions released into the medium. The virus, assayed in vivo for infectivity, has SFFV activity but is attenuated for leukemogenicity. The stimulation of hemoglobin synthesis in the cells grown in medium supplemented with dimethyl sulfoxide or hexamethylene bisacetamide indicates that the cells are erythroid in origin. SFFV may have a function analogous to erythropoietin in influencing the process of transformation by FLV.