Leukemia induction by a new strain of Friend mink cell focus-inducing virus: synergistic effect of Friend ecotropic murine leukemia virus.

A new strain of Friend recombinant mink cell focus-inducing retrovirus, FMCF -1-E, was found to induce leukemias in NFS and IRW mice. Although the isolate was obtained from a stock of FMCF -1 ( Troxler et al., J. Exp. Med. 148:639-653, 1978), FMCF -1-E was distinguishable from FMCF -1 by oligonucleotide fingerprinting and antigenic analysis, using monoclonal antibodies. These analyses suggested that FMCF -1-E is a distinct FMCF isolate rather than a simple variant of FMCF -1. After neonatal inoculation, the latency for leukemia induction was 3 to 8 months. A similar long latency was also seen when Friend murine leukemia virus 57 was inoculated into adult (6-week-old) IRW mice. However, sequential inoculation of FMCF -1-E at birth followed by Friend murine leukemia 57 at 6 weeks of age led to a shortened latency period (2.5 to 4 months). Only neonatal inoculation of Friend murine leukemia virus 57 was able to induce a more rapid appearance of leukemia. The leukemia cell type in the majority of cases, regardless of virus inoculation protocol, was erythroid, but occasional myeloid, lymphoid, and mixed leukemias were also observed. In contrast to NFS and IRW mice, BALB/c mice were resistant to leukemia induction by FMCF -1-E and also showed some transient resistance to leukemia induction by Friend murine leukemia virus 57.     

Role of mink cell focus-inducing virus in leukemias induced by Friend ecotropic virus

Recombinant viruses have been implicated in the pathogenesis of murine leukemias induced by a variety of long-latency retroviruses. Neonatal mice of several strains were inoculated with Friend ecotropic virus (F-Eco) and analyzed for the presence of mink cell focus-inducing (MCF) virus or DNA restriction enzyme fragments which were specific for Friend MCF virus (F-MCF). MCF virus was detected within 2 weeks of inoculation in NFS /N mice and at about 2 months after inoculation in BALB/c mice. Both of these strains developed erythroblastosis after inoculation with F-Eco. In contrast, MCF virus was not detected in F-Eco-inoculated C57BL mice. These mice were resistant to erythroblastosis but developed lymphoma or myelogenous leukemia or both at about 5 months after inoculation. Thus, although MCF viruses were associated with F-Eco erythroblastosis in NFS /N and BALB/c mice, they were not necessary for F-Eco-induced lymphoid or myeloid leukemias in C57BL mice. To investigate the association between resistance to erythroblastosis and absence of MCF virus, C57BL mice were inoculated with pseudotypic mixtures of F-Eco plus F-MCF; MCF virus replicated well in these mice, but the mice remained resistant to erythroblastosis. Furthermore, in genetic crosses between C57BL and NFS /N or BALB/c, some mice inherited resistance to F-Eco erythroblastosis without inheriting the C57BL resistance to the generation of MCF viruses. These results indicate that C57BL mice carry a gene for resistance to F-Eco erythroblastosis which is distinct from the C57BL genes which interfere with the generation of MCF viruses.     

The glycoprotein 71 of ecotropic Friend murine leukemia virus. Structure of the oligosaccharides linked to asparagine-12

The glycoprotein from Friend murine leukemia virus was digested with protease from Staphylococcus aureus V8. A glycopeptide comprising the N-terminal glycosylation site (Asn-12) was isolated from the mixture of fragments and analyzed by amino acid sequencing and methylation-capillary gas chromatography-mass spectrometry before and after treatment with sialidase from Vibrio cholerae. Asn-12 was thus found to be substituted by a family of partially sialylated, fucosylated, and intersected glycoprotein N-glycans of the hybrid type.     

Mapping host range-specific oligonucleotides within genomes of the ecotropic and mink cell focus-inducing strains of Moloney murine leukemia virus.

The site of recombination of a mink cell focus-inducing strain (Mo-MuLV83) derived from an ecotropic Moloney murine leukemia virus (Mo-MuLV) was mapped by fingerprint analysis of the large RNase T1-resistant oligonucleotides, employing a two-dimensional gel electrophoresis method. Mo-MuLV83, in contrast to the ecotropic Mo-MuLV, demonstrated a broadened host range, i.e., growth not only on mouse cells but also on mink cells, and recombination involved the env gene function. The genomic RNA of these two viruses shared 42 out of a total of 51 to 53 large T1 oligonucleotides (81%) and possessed a similar subunit size of 36S. Most of these T1 oligonucleotides were mapped in their relative order to the 3' polyadenylic acid end of the viral RNA molecules. There were 10 common oligonucleotides immediately next to the 3' termini. A cluster of 7 (in Mo-MuLV83) or 10 (in Mo-MuLV) unique T1 oligonucleotides were mapped next to the common sequences at the 3' end, and they all appeared concomitantly in a polyadenylic acid-containing RNA fraction with a sedimentation coefficient slightly larger than 18S. Therefore, the env gene of Mo-MuLV was situated at a location approximately 2,000 to 4,000 nucleotides from the 3' end of the genomic RNA, and the gene order of Mo-MuLV appeared to be similar to that of the more rigorously determined avian oncornaviruses. cDNA(SFFV) specific for the xenotropic sequences in the spleen focus-forming virus RNA hybridized to the cluster of unique oligonucleotides of Mo-MuLV83 RNA. This suggests that the loci of recombination involve the homologous env gene region of a xenotropic virus.     

Molecular cloning of Friend mink cell focus-inducing virus: identification of mink cell focus-inducing virus-like messages in normal and transformed cells.

Friend mink cell focus-inducing virus (Fr-MCF) is a leukemogenic murine retrovirus that was isolated from a leukemic NIH Swiss mouse. We molecularly cloned the genome of this virus into pBR322. Restriction enzyme mapping of this cloned Fr-MCF DNA revealed a 1.0-kilobase-pair region in the envelope gene which differs from the restriction map of the Friend ecotropic virus (Friend murine leukemia virus). A 400-base-pair Fr-MCF envelope gene fragment was subcloned from this region and designated pLEK. Probes made with pLEK hybridize to Fr-MCF DNA but do not hybridize to either Friend or amphotropic murine leukemia virus DNA. Polyadenylic acid-selected RNA was prepared from the hematopoietic tissues of normal NIH Swiss mice, Fr-MCF-infected erythroleukemia cells (TP-1), and uninfected chemically transformed T lymphocytes (RB-1). The pLEK probe identified 34S and 22S messages in the TP-1 cells and in the normal hematopoietic tissues. RB-1 cells contain 32S and 26S messages that hybridize to pLEK. However, the pLEK-like RNAs found in the normal hematopoietic tissues and in the RB-1 cells were 400-fold less abundant than the RNAs found in the TP-1 cells.     

An amino-terminal fragment of the Friend murine leukemia virus envelope glycoprotein binds the ecotropic receptor.

Retrovirus entry into cells is mediated by specific binding of the envelope glycoprotein to a cell membrane receptor. Constitutive envelope gene expression prevents infection by interfering with the binding of viruses which recognize the same receptor. We have used this property to investigate the receptor binding capacities of deleted or truncated murine leukemia virus ecotropic envelope glycoproteins. Friend murine leukemia virus envelope glycoproteins bearing internal amino-terminal deletions, or a soluble 245-amino-acid gp70 amino-terminal fragment, were expressed in NIH 3T3 cells. The susceptibility of these cells to ecotropic and amphotropic virus infection was determined. We observed that both membrane-bound and soluble forms of the gp70 245-amino-acid amino-terminal domain induced resistance to ecotropic virus, indicating that this fragment binds the ecotropic receptor. Binding occurs both at the cell surface and in the endoplasmic reticulum, as shown by the use of soluble envelope fragments either secreted in the culture supernatants or retained in the endoplasmic reticulum lumen by a KDEL sequence. These results suggest that the gp70 amino-terminal domain folds into a structure which recognizes the ecotropic receptor regardless of the carboxy-terminal part of the molecule.     

Localization of the intrachain disulfide bonds of the envelope glycoprotein 71 from Friend murine leukemia virus.

Envelope glycoprotein 71 from Friend murine leukemia virus was purified to homogeneity by reversed-phase HPLC. It could be shown that all 20 cysteine residues of the molecule are linked by disulfide bonds. After complete tryptic digestion, peptides containing cystine were identified by comparison of the reversed-phase HPLC profile of the digest with that of a reduced aliquot which had been subjected to affinity chromatography on thiol-Sepharose. The locations of the 10 disulfide bonds were determined by isolation, further digestion and analysis of peptides containing cystine. The first cysteine residue of the sequence (Cys46) was shown to be coupled to the sixth (Cys98), leading to a large loop containing four additional cysteine residues. Computer model building and energy calculations led to the assignment of Cys72 to Cys87 and Cys73 to Cys83. The following four cysteine residues of the sequence also constitute a structural unit, with Cys121 bonded to Cys141 and Cys133 to Cys146, and the last two cysteine residues in the amino-terminal domain of glycoprotein 71 form a small loop (Cys178 to Cys184). The first two cysteine residues of the carboxy-terminal domain produce a very small hydrophobic loop (Cys312-Cys315). Cys361 is bound to Cys373, Cys342 to Cys396 and Cys403 to Cys416. A model for the folding pattern of the viral glycoprotein is proposed.     

Oncogenicity of AKR mink cell focus-inducing murine leukemia virus correlates with induction of chronic phosphatidylinositol signal transduction.

Naturally occurring recombinant murine leukemia viruses (MuLVs), termed mink cell focus-inducing (MCF) viruses, are the proximal leukemogens in spontaneous thymic lymphomas of AKR mice. The mechanism by which these viruses transform lymphocytes is not clear. Previous studies have implicated either integrational activation of proto-oncogenes, chronic autocrine immune stimulation, and/or autocrine stimulation of growth factor receptors (e.g., interleukin 2 receptors) via binding of the viral env glycoprotein (gp70) to these receptors. Any one of these events could also involve activation of second messenger signaling pathways in the cell. We examined whether infection with oncogenic AKR-247 MCF MuLV induced transmembrane signaling cascades in thymocytes of AKR mice. Cyclic AMP levels were not changed, but there was enhanced turnover of phosphatidylinositol phosphates, with concomitant increases in diacyglycerol and inositol 1,4,5-triphosphate. Thus, phospholipase C activity was increased. Protein kinase C activity was also elevated in comparison to that in uninfected thymocytes. The above events occurred in parallel with MCF expression in the thymus and were chronically maintained thereafter. No changes in phospholipid turnover occurred in an organ which did not replicate the MCF virus (spleen) or in thymocytes of AKR mice infected with a thymotropic, nononcogenic MCF virus (AKV-1-C36). Therefore, only the oncogenic MCF virus induced phosphatidylinositol signal transduction. Flow cytometric comparison of cell surface gp70 revealed that AKR-247 MCF virus-infected thymocytes expressed more MCF virus gp70 than did thymocytes from AKV-1-C36 MCF virus-infected mice, suggesting that certain threshold quantities of MCF virus env glycoproteins may be involved in this signaling. This type of signal transduction is not induced by stimulation of the interleukin 2 receptor but is involved in certain oncogene systems (e.g., ras and fms). Its chronic induction by oncogenic MCF MuLV may thus initiate thymocyte transformation.     

Characterization of structural and immunological properties of specific domains of Friend ecotropic and dual-tropic murine leukemia virus gp70s.

A detailed comparison of the gp70 proteins of cloned ecotropic Friend murine leukemia virus (FLV) and dual-tropic Friend mink focus-forming virus (FrMCF) was performed by analyzing the structural and immunological properties of amino- and carboxy-terminal domains of these molecules generated upon controlled trypsinization. The two gp70s gave characteristic fragmentation patterns; the amino-terminal fragments of FrMCF gp70 were smaller than the corresponding fragments of FLV and contained a trypsin site which resulted in a 19,000-dalton amino-terminal fragment not observed for FLV, whereas both molecules yielded an identically sized carboxy-terminal fragment. All amino-terminal fragments of both gp70 molecules contained an endo H-sensitive oligosaccharide chain; for FrMCF, a second endo H-sensitive carbohydrate was present as well at a carboxy-terminal site for approximately 50% of the molecules. Several aspects of the disulfide interactions of the two gp70s were conserved; in both cases the carboxy-terminal fragments were disulfide bonded to p15(E), there were no disulfide bonds between amino- and carboxy-terminal fragments, and the amino-terminal fragments exhibited a significant increase in mobility upon analysis by sodium dodecyl sulfate-polyacrylamide gel electrophoresis under nonreducing conditions. Analysis of the immunoreactivity of the different domains of the proteins by immunoprecipitation of the fragments with antisera prepared against xenotropic murine leukemia virus and feline leukemia virus gp70s indicated major differences in antigenicity for the amino-terminal domains of FLV and FrMCF gp70, whereas the carboxy-terminal domains were immunologically conserved. Similar analyses with antibodies specific for p15(E) and Pr15(E) demonstrate that these components are conserved as well. These data provide direct evidence that p15(E) and the C-terminal gp70 domain of FrMCF gp70 are related to the corresponding regions of the ecotropic FLV parent and indicate that the acquisition of MCF-specific properties is due to the replacement of the ecotropic amino-terminal gp70 domain with sequences related to those of xenotropic gp70s.     

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.     

Possible cell surface receptor for Friend murine leukemia virus isolated with viral envelope glycoprotein complexes.

Water-soluble multimeric complexes of Friend leukemia virus envelope glycoprotein gp85 bind specifically to C57BL/6 mouse spleen leukocytes. Such complexes were used to isolate cell surface receptors for the virus, using an immunoprecipitation technique. The putative rceptor has a molecular weight of 14,000. Mouse H-2 histocompatibility antigens, which are receptors for Semliki Forest virus, are not receptors for Friend leukemia virus.     

Heteroduplex analysis of the sequence relations between the RNAs of mink cell focus-inducing and murine leukemia viruses.

The sequence relationships betwen AKR ecotropic virus and an AKR-derived "mink cell focus-inducing" (MCF) isolate (AKR MCF 247), between Moloney murine leukemia virus (M-MLV) and an M-MLV MCF isolate (M-MLV83), and between AKR and M-MLV were studied by electron microscopic heteroduplex analysis. The MCF-specific sequences were found to map from 1.95 kilobases (kb) to 2.75 kb (+/- 0.15 kb) from the 3' end of the RNAs for both MCF isolates. The major sequence nonhomology regions between AKR and M-MLV lie between 0.9 and 3.5 kb from the 3' end. However, the AKR and M-MLV sequences immediately adjacent to the 1.95- and 2.75-kb junctions with MCF-specific sequences are relatively similar in AKR and M-MLV. Our results suggest that the env gene of MLVs maps from 1 kb to 3 kb from the 3' end of the genomic RNA and that the carboxyl end of the glycoprotein of each MCF strain is similar (or identical) to that of its ecotropic parent.     

Characteristics and contributions of defective, ecotropic, and mink cell focus-inducing viruses involved in a retrovirus-induced immunodeficiency syndrome of mice.

LP-BM5 murine leukemia virus, a derivative of Duplan-Laterjet virus, contains a mixture of replication-competent B-tropic ecotropic and mink cell focus-inducing (MCF) viruses and a defective genome that is the proximal cause of a syndrome, murine AIDS (MAIDS), characterized by lymphoproliferation and immunodeficiency. The defective (BM5d) and ecotropic components of this mixture were molecularly cloned, and complete (BM5d) or partial (ecotropic) nucleotide sequences were determined. BM5d closely resembled the Du5H genome cloned from the Duplan virus, featuring a highly divergent p12 sequence in the gag open reading frame. In MAIDS-sensitive C57BL/6 mice, BM5d was detected in tissues within 2 weeks of infection but was absent from tissues of the MAIDS-resistant strain, A/J, 12 weeks after infection. B-cell-lineage tumors from mice with MAIDS contained and expressed BM5d, and clonal integrations of this genome were variably associated with clonal expansions of B cells in infected mice. Finally, mRNA crosshybridizing with a probe for BM5d was present in spleen but not kidney cells of uninfected B6 mice.     

Sequence analysis of amphotropic and 10A1 murine leukemia viruses: close relationship to mink cell focus-inducing viruses.

Viral interference studies have demonstrated the existence of four distinct murine leukemia virus (MuLV) receptors on NIH 3T3 mouse cells. The four viral interference groups are ecotropic MuLV; mink cell focus inducing virus (MCF); amphotropic MuLV; and 10A1, a recombinant derivative of amphotropic MuLV that uses a unique receptor but also retains affinity for the amphotropic MuLV receptor. We report here that 10A1 infects rat and hamster cells, unlike its amphotropic parent. We isolated an infectious molecular clone of 10A1 and present here the sequences of the env genes and enhancer regions of amphotropic MuLV and 10A1. The deduced amino acid sequences of amphotropic MuLV and 10A1 gp70su are remarkably similar to those of MCF and xenotropic MuLV (for which mouse cells lack receptors), with 64% amino acids identical in the four groups. We generated a consensus from these comparisons. Further, the differences are largely localized to a few discrete regions: (i) amphotropic MuLV has two short insertions relative to MCF, at residues 87 to 92 and 163 to 169, and (ii) amphotropic MuLV and MCF are totally different in a hypervariable region, which is greater than 30% proline, at residues approximately 253 to 304. 10A1 closely resembles amphotropic MuLV in its N terminus but contains an MCF-type hypervariable region. These results suggest the possibility that receptor specificity is localized in these short variable regions and further that the unique receptor specificity of 10A1 is due to the novel combination of amphotropic MuLV and MCF sequences rather than to the presence of any novel sequences. The Env proteins of ecotropic MuLV are far more distantly related to those of the other four groups than the latter are to each other. We also found that the enhancer regions of amphotropic MuLV and 10A1 are nearly identical, although 10A1 is far more leukemogenic than amphotropic MuLV.     

Role of carbohydrate in determining the immunochemical properties of the major glycoprotein (gp71) of Friend murine leukemia virus.

Treatment of Friend leukemia virus gp71 with protease-free glycosidase enzymes results in removal of the major portion of the carbohydrate without affecting the amount of protein present. The digested material migrates as protein of about 60,000 to 65,000 molecular weight on sodium dodecyl sulfatepolyacrylamide gel electrophoresis. Analyses of the serological properties of gp71 after enzyme treatment indicated that the type, group, and interspecies determinants were not destroyed. In contrast, treatment with proteolytic enzymes led to the complete destruction of the gp71 molecule, including the total elimination of its serological reactivity as measured by direct and competition radioimmunoassay and by a serum cytotoxicity assay. We conclude that the carbohydrate portion of gp71 is not of major significance in defining the antigenic determinants of this viral glycoprotein.     

Neurodegenerative disease induced by the wild mouse ecotropic retrovirus is markedly accelerated by long terminal repeat and gag-pol sequences from nondefective Friend murine leukemia virus.

The wild mouse ecotropic retrovirus (WM-E) induces a spongiform neurodegenerative disease in mice after a variable incubation period of 2 months to as long as 1 year. We isolated a molecular clone of WM-E (15-1) which was weakly neurovirulent (incidence, 8%) but was highly leukemogenic (incidence, 45%). Both lymphoid and granulocytic leukemias were observed, and these leukemias were often neuroinvasive. A chimeric virus was constructed containing the env and 3' pol sequences of 15-1 and long terminal repeat (LTR), gag, and 5' pol sequences from a clone of Friend murine leukemia virus (FB29). FB29 has been shown previously to replicate to high levels in the central nervous system (CNS) but is not itself neurovirulent. This finding was confirmed at the DNA level in the current study. Surprisingly, intraperitoneal inoculation of neonatal IRW mice with the chimeric virus (FrCasE) caused an accelerated neurodegenerative disease with an incubation period of only 16 days and was uniformly fatal by 23 days postinoculation. Introduction of the LTR of 15-1 into the FrCasE genome yielded a virus (FrCasEL) with a degree of neurovirulence intermediate between those of 15-1 and FrCasE. No differences were found in the levels of viremia or the relative levels of viral DNA in the spleens of mice inoculated with 15-1, FrCasE, or FrCasEL. However, the levels of viral DNA in the CNS correlated with the relative degrees of neurovirulence of the respective viruses (FrCasE greater than FrCasEL greater than 15-1). Thus, the env and 3' pol sequences of WM-E (15-1) were required for neurovirulence, but elements within the LTR and gag-pol regions of FB29 had a profound influence on the level of CNS infection and the rate of development of neurodegeneration.     

Kinetics of expression of infectious ecotropic, xenotropic, and mink cell focus-forming murine leukemia virus after 5-iododeoxyuridine induction of cells from high- and low-leukemia mouse strains

Endogenous murine leukemia virus (MuLV) was induced with 5-iododeoxyuridine (IdUrd) from the high-leukemia mouse strain AKR and from two low-leukemia strains, C3H/He and BALB/c. A virus-free cell line from strain AKR readily gave rise to infectious, XC-positive MuLV upon treatment with IdUrd, whereas cells from strains C3H/He and BALB/c produced replication-deficient, XC-negative MuLV. IdUrd-induced cells also produced xenotropic and mink cell focus-forming MuLV. Xenotropic virus emerged at a higher titer from both AKR and BALB/c cells during two discrete time intervals, first at day 3 after induction and a second time during spread of the induced ecotropic MuLV. Xenotropic and mink cell focus-forming MuLVs were also produced by IdUrd-induced C3H/He cells but required another round of infection in Sc-1 cells for detection. The in vitro infectivity of endogenous ecotropic MuLV isolated by IdUrd induction from C3H/He cells correlated with pathogenicity in the Fv-1-compatible, leukemia-negative mouse strain NFS/N. Thus, the virulence of endogenous ecotropic MuLV may be an important factor in determining the leukemia incidence in these inbred strains of mice.     

A neutralizable epitope common to the envelope glycoproteins of ecotropic, polytropic, xenotropic, and amphotropic murine leukemia viruses.

An epitope common to all classes of murine leukemia viruses (MuLVs) was detected by reactivity of MuLVs with a rat monoclonal antibody (MAb) termed 83A25. The antibody is of the immunoglobulin G2a isotype and was derived after fusion of NS-1 myeloma cells with spleen cells from a Fischer rat immunized with a Friend polytropic MuLV. The antibody reacted with nearly all members of the ecotropic, polytropic, xenotropic, and amphotropic classes of MuLVs. Unreactive viruses were limited to the Friend ecotropic MuLV, Rauscher MuLV, and certain recombinant derivatives of Friend ecotropic MuLV. The presence of an epitope common to nearly all MuLVs facilitated a direct quantitative focal immunofluorescence assay for MuLVs, including the amphotropic MuLVs for which no direct assay has been previously available. Previously described MAbs which react with all classes of MuLVs have been limited to those which react with virion core or transmembrane proteins. In contrast, protein immunoblot and immunoprecipitation analyses established that the epitope reactive with MAb 83A25 resides in the envelope glycoproteins of the viruses. Structural comparisons of reactive and nonreactive Friend polytropic viruses localized the epitope near the carboxyl terminus of the glycoprotein. The epitope served as a target for neutralization of all classes of MuLV with MAb 83A25. The efficiency of neutralization varied with different MuLV isolates but did not correlate with MuLV interference groups.