A point mutation in the env gene of a murine leukemia virus induces syncytium formation and neurologic disease

Bovine Leukaemia Virus: Current Epidemiological Circumstance and Future Prospective

Bovine leukaemia virus (BLV) is a deltaretrovirus that is closely related to human T-cell leukaemia virus types 1 and 2 (HTLV-1 and -2). It causes enzootic bovine leukosis (EBL), which is the most important neoplastic disease in cattle. Most BLV-infected cattle are asymptomatic, which potentiates extremely high shedding rates of the virus in many cattle populations. Approximately 30% of them show persistent lymphocytosis that has various clinical outcomes; only a small proportion of animals (less than 5%) exhibit signs of EBL. BLV causes major economic losses in the cattle industry, especially in dairy farms. Direct costs are due to a decrease in animal productivity and in cow longevity; indirect costs are caused by restrictions that are placed on the import of animals and animal products from infected areas. Most European regions have implemented an efficient eradication programme, yet BLV prevalence remains high worldwide. Control of the disease is not feasible because there is no effective vaccine against it. Therefore, detection and early diagnosis of the disease are essential in order to diminish its spreading and the economic losses it causes. This review comprises an overview of bovine leukosis, which highlights the epidemiology of the disease, diagnostic tests that are used and effective control strategies.

Patterns of Coevolutionary Adaptations across Time and Space in Mouse Gammaretroviruses and Three Restrictive Host Factors

The classical laboratory mouse strains are genetic mosaics of three Mus musculus subspecies that occupy distinct regions of Eurasia. These strains and subspecies carry infectious and endogenous mouse leukemia viruses (MLVs) that can be pathogenic and mutagenic. MLVs evolved in concert with restrictive host factors with some under positive selection, including the XPR1 receptor for xenotropic/polytropic MLVs (X/P-MLVs) and the post-entry restriction factor Fv1. Since positive selection marks host-pathogen genetic conflicts, we examined MLVs for counter-adaptations at sites that interact with XPR1, Fv1, and the CAT1 receptor for ecotropic MLVs (E-MLVs). Results describe different co-adaptive evolutionary paths within the ranges occupied by these virus-infected subspecies. The interface of CAT1, and the otherwise variable E-MLV envelopes, is highly conserved; antiviral protection is afforded by the Fv4 restriction factor. XPR1 and X/P-MLVs variants show coordinate geographic distributions, with receptor critical sites in envelope, under positive selection but with little variation in envelope and XPR1 in mice carrying P-ERVs. The major Fv1 target in the viral capsid is under positive selection, and the distribution of Fv1 alleles is subspecies-correlated. These data document adaptive, spatial and temporal, co-evolutionary trajectories at the critical interfaces of MLVs and the host factors that restrict their replication.

Evolution of different antiviral strategies in wild mouse populations exposed to different gammaretroviruses

Laboratory mice carry three host range groups of gammaretroviruses all of which are linked to leukemia induction. Although polytropic mouse leukemia viruses (P-MLVs) are generally recognized as the proximate cause of MLV-induced leukemias in laboratory mice, wild mice that carry only endogenous P-MLVs do not produce infectious virus and are not prone to disease; these mice carry the permissive XPR1 retroviral receptor and an attenuated variant of the retroviral restriction factor, APOBEC3. In contrast, Eurasian mice carrying ecotropic and xenotropic MLVs have evolved multiple restrictive XPR1 variants, other factors that interfere with MLV entry, and more effectively antiviral variants of APOBEC3. These different antiviral restrictions in Mus musculus subspecies suggest that the different virus types found in these natural populations may pose different but largely uncharacterized survival risks in their host subspecies.

Utilization of replication-competent XMRV reporter-viruses reveals severe viral restriction in primary human cells

The gammaretrovirus termed xenotropic murine leukemia virus-related virus (XMRV) was described to be isolated from prostate cancer tissue biopsies and from blood of patients suffering from chronic fatigue syndrome. However, many studies failed to detect XMRV and to verify these disease associations. Data suggesting the contamination of specimens in particular by PCR-based methods and recent reports demonstrating XMRV generation via recombination of two murine leukemia virus precursors raised serious doubts about XMRV being a genuine human pathogen. To elucidate cell tropism of XMRV, we generated replication competent XMRV reporter viruses encoding a green fluorescent protein or a secretable luciferase as tools to analyze virus infection of human cell lines or primary human cells. Transfection of proviral DNAs into LNCaP prostate cancer cells resulted in readily detectably reporter gene expression and production of progeny virus. Inoculation of known XMRV susceptible target cells revealed that these virions were infectious and expressed the reporter gene, allowing for a fast and highly sensitive quantification of XMRV infection. Both reporter viruses were capable of establishing a spreading infection in LNCaP and Raji B cells and could be easily passaged. However, after inoculation of primary human blood cells such as CD4 T cells, macrophages or dendritic cells, infection rates were very low, and a spreading infection was never established. In line with these results we found that supernatants derived from these XMRV infected primary cell types did not contain infectious virus. Thus, although XMRV efficiently replicated in some human cell lines, all tested primary cells were largely refractory to XMRV infection and did not support viral spread. Our results provide further evidence that XMRV is not a human pathogen.

Second site mutation in the virus envelope expands the host range of a cytopathic variant of Moloney murine leukemia virus

Spl574 MLV (murine leukemia virus) is a variant of Moloney ecotropic MLV (MoMLV) that is cytopathic in Mus dunni cells and restricted by other mouse cells. Its host range and cytopathicity are due to a mutation, S82F, at a site critical for binding to the CAT-1 receptor. To identify residues that affect affinity for receptor variants, virus with S82F was passed in restrictive cells. The env genes of the adapted viruses contained 18 novel mutations, including one, E114G, present in 6 of 30 sequenced envs. MoMLV-E114G efficiently infected all mouse cells as well as ecotropic MLV resistant Chinese hamster cells. Virus with E114G and S82F induced large multinucleated syncytia in NIH 3T3 and SC-1 cells as well as M. dunni cells. Inoculation of Mo-S82F,E114G into mice produced lymphomas typical of MoMLV. Residues at env position 114 are thus important determinants of host range, and E114G suppresses host range restriction due to S82F, but does not affect S82F-governed cytopathicity.

A mutant retroviral receptor restricts virus superinfection interference and productive infection

Background

Both cell-free and cell-associated infection routes are important for retroviral dissemination. Regardless of the mechanism, the driving force of retroviral entry is the interaction between the viral envelope and its receptor. To date it remains unclear how decreased affinity of viruses for their receptors affects viral cell-free infection, cell-cell transmission, and spreading kinetics. We have previously characterized a mutant form of the amphotropic murine retrovirus receptor human phosphate transporter 2 (PiT2) wherein the single substitution of a glutamic acid for the lysine residue at position 522 of this receptor is sufficient to render it to function as a gibbon ape leukemia virus (GALV) receptor.

Results

In this study we analyzed the binding affinity of the mutant receptor PiT2K522E and determined that it has a 1000 fold decreased GALV envelope binding affinity compared to the GALV wild type receptor. The decreased affinity does not restrict the initiation of cell-free GALV infection. The diminished binding affinity does, however, correlate with a decrease in the ability of GALV to spread in cells expressing this mutant receptor.

Conclusions

The reduced ability of GALV to subsequently spread among cells expressing PiT2K522E is likely resulted from reduced cell-cell transmission, the decreased ability of PiT2K522E-expressing cells to establish superinfection interference, and attendant cytopathic affects.

Naturally Occurring Polymorphisms of the Mouse Gammaretrovirus Receptors CAT-1 and XPR1 Alter Virus Tropism and Pathogenicity

Gammaretroviruses of several different host range subgroups have been isolated from laboratory mice. The ecotropic viruses infect mouse cells and rely on the host CAT-1 receptor. The xenotropic/polytropic viruses, and the related human-derived XMRV, can infect cells of other mammalian species and use the XPR1 receptor for entry. The coevolution of these viruses and their receptors in infected mouse populations provides a good example of how genetic conflicts can drive diversifying selection. Genetic and epigenetic variations in the virus envelope glycoproteins can result in altered host range and pathogenicity, and changes in the virus binding sites of the receptors are responsible for host restrictions that reduce virus entry or block it altogether. These battleground regions are marked by mutational changes that have produced 2 functionally distinct variants of the CAT-1 receptor and 5 variants of the XPR1 receptor in mice, as well as a diverse set of infectious viruses, and several endogenous retroviruses coopted by the host to interfere with entry.

Innate immunity in the pathogenesis of polytropic retrovirus infection in the central nervous system

Neuroinflammation, including astrogliosis, microgliosis, and the production of proinflammatory cytokines and chemokines is a common response in the central nervous system (CNS) to virus infection, including retrovirus infection. However, the contribution of this innate immune response in disease pathogenesis remains unresolved. Analysis of the neuroinflammatory response to polytropic retrovirus infection in the mouse has provided insight into the potential contribution of the innate immune response to retrovirus-induced neurologic disease. In this model, retroviral pathogenesis correlates with the induction of neuroinflammatory responses including the activation of astrocytes and microglia, as well as the production of proinflammatory cytokines and chemokines. Studies of the neurovirulent determinants of the polytropic envelope protein as well as studies with knockout mice suggest that retroviral pathogenesis in the brain is multifaceted and that cytokine and chemokine production may be only one mechanism of disease pathogenesis. Analysis of the activation of the innate immune response to retrovirus infection in the CNS indicates that toll-like receptor 7 (TLR7) is a contributing factor to retrovirus-induced neuroinflammation, but that other factors can compensate for the lack of TLR7 in inducing both neuroinflammation and neurologic disease.

Neurovirulence of polytropic murine retrovirus is influenced by two separate regions on opposite sides of the envelope protein receptor binding domain

Changes in the envelope proteins of retroviruses can alter the ability of these viruses to infect the central nervous system (CNS) and induce neurological disease. In the present study, nine envelope residues were found to influence neurovirulence of the Friend murine polytropic retrovirus Fr98. When projected on a three-dimensional model, these residues were clustered in two spatially separated groups, one in variable region B of the receptor binding site and the other on the opposite side of the envelope. Further studies indicated a role for these residues in virus replication in the CNS, although the residues did not affect viral entry.

Role of receptor polymorphism and glycosylation in syncytium induction and host range variation of ecotropic mouse gammaretroviruses

Background

We previously identified unusual variants of Moloney and Friend ecotropic mouse gammaretroviruses that have altered host range and are cytopathic in cells of the wild mouse species Mus dunni. Cytopathicity was attributed to different amino acid substitutions at the same critical env residue involved in receptor interaction: S82F in the Moloney variant Spl574, and S84A in the Friend mouse leukemia virus F-S MLV. Because M. dunni cells carry a variant CAT-1 cell surface virus receptor (dCAT-1), we examined the role of this receptor variant in cytopathicity and host range.

Results

We expressed dCAT-1 or mCAT-1 of NIH 3T3 origin in cells that are not normally infectible with ecotropic MLVs and evaluated the transfectants for susceptibility to virus infection and to virus-induced syncytium formation. The dCAT-1 transfectants, but not the mCAT-1 transfectants, were susceptible to virus-induced cytopathicity, and this cytopathic response was accompanied by the accumulation of unintegrated viral DNA. The dCAT-1 transfectants, however, did not also reproduce the relative resistance of M. dunni cells to Moloney MLV, and the mCAT-1 transfectants did not show the relative resistance of NIH 3T3 cells to Spl574. Western analysis, use of glycosylation inhibitors and mutagenesis to remove receptor glycosylation sites identified a possible role for cell-specific glycosylation in the modulation of virus entry.

Conclusion

Virus entry and virus-induced syncytium formation using the CAT-1 receptor are mediated by a small number of critical amino acid residues in receptor and virus Env. Virus entry is modulated by glycosylation of cellular proteins, and this effect is cell and virus-specific.

TR1.3 viral pathogenesis and syncytium formation are linked to Env-Gag cooperation

Infection with murine leukemia virus (MLV) TR1.3 or the related molecular construct W102G causes severe neuropathology in vivo. Infection is causally linked to the development of extensive syncytia in brain capillary endothelial cells (BCEC). These viruses also induce cell fusion of murine cell lines, such as SC-1 and NIH 3T3, which are otherwise resistant to MLV-induced syncytium formation. Although the virulence of these viruses maps within the env gene, the mechanism of fusion enhancement is not fully determined. To this end, we examined the capacity of the syncytium-inducing (SI) TR1.3 and W102G MLVs to overcome the fusion inhibitory activity inherent in the full-length Env cytoplasmic tail. These studies showed that the TR1.3 and W102G Envs did not induce premature cleavage of p2E, nor did they override p2E fusion inhibition. Indeed, in the presence of mutations that disrupt p2E function, the TR1.3 and W102G Envs significantly increased the extent of cell fusion compared to that with the non-syncytium-inducing MLV FB29. Surprisingly, we also observed that TR1.3 and W102G Envs failed to elicit syncytium formation in these in vitro assays. Coexpression of gag-pol with env restored syncytium formation, and accordingly, mutations within gag-pol were used to examine the minimal functional requirements for the SI phenotype. The results indicate that both gag-dependent particle budding and cleavage of p2E are required to activate the SI phenotype of TR1.3 and W102G viruses. Collectively, these data suggest that the TR1.3 and W102G viruses induce cell fusion by the fusion-from-without pathway.

Linkage of reduced receptor affinity and superinfection to pathogenesis of TR1.3 murine leukemia virus

TR1.3 is a Friend murine leukemia virus (MLV) that induces selective syncytium induction (SI) of brain capillary endothelial cells (BCEC), intracerebral hemorrhage, and death. Syncytium induction by TR1.3 has been mapped to a single tryptophan-to-glycine conversion at position 102 of the envelope glycoprotein (Env102). The mechanism of SI by TR1.3 was examined here in comparison to the non-syncytium-inducing, nonpathogenic MLV FB29, which displays an identical BCEC tropism. Envelope protein expression and stability on both infected cells and viral particles were not statistically different for TR1.3 and FB29. However, affinity measurements derived using purified envelope receptor binding domain (RBD) revealed a reduction of >1 log in the K(D) of TR1.3 RBD relative to FB29 RBD. Whole-virus particles pseudotyped with TR1.3 Env similarly displayed a markedly reduced binding avidity compared to FB29-pseudotyped viral particles. Lastly, decreased receptor affinity of TR1.3 Env correlated with the failure to block superinfection following acute and chronic infection by TR1.3. These results definitively show that acquisition of a SI phenotype can be directly linked to amino acid changes in retroviral Env that decrease receptor affinity, thereby emphasizing the importance of events downstream of receptor binding in the cell fusion process and pathology.

Novel host range and cytopathic variant of ecotropic Friend murine leukemia virus

A variant ecotropic Friend murine leukemia virus, F-S MLV, is capable of inducing the formation of large multinucleated syncytia in Mus dunni cells. This cytopathicity resembles that of Spl574 MLV, a novel variant recently isolated from the spleen of a Mus spicilegus mouse neonatally inoculated with Moloney MLV. F-S MLV is an N-tropic Friend MLV that also has the unusual ability to infect hamster cells, which are normally resistant to mouse ecotropic MLVs. Syncytium induction by both F-S MLV and Spl574 is accompanied by the accumulation of large amounts of unintegrated viral DNA, a hallmark of pathogenic retroviruses, but not previously reported for mouse ecotropic gammaretroviruses. Sequencing and site-specific mutagenesis determined that the syncytium-inducing phenotype of F-S MLV can be attributed to a single amino acid substitution (S84A) in the VRA region of the viral env gene. This site corresponds to that of the single substitution previously shown to be responsible for the cytopathicity of Spl574, S82F. The S84A substitution in F-S MLV also contributes to the ability of this virus to infect hamster cells, but Spl574 MLV is unable to infect hamster cells. Because this serine residue is one of the critical amino acids that form the CAT-1 receptor binding site, and because M. dunni and hamster cells have variant CAT-1 receptors, these results suggest that syncytium formation as well as altered host range may be a consequence of altered interaction between virus and receptor.

Disparate regions of envelope protein regulate syncytium formation versus spongiform encephalopathy in neurological disease induced by murine leukemia virus TR

The murine leukemia virus (MLV) TR1.3 provides an excellent model to study the wide range of retrovirus-induced central nervous system (CNS) pathology and disease. TR1.3 rapidly induces thrombotic events in brain microvessels and causes cell-specific syncytium formation of brain capillary endothelial cells (BCEC). A single amino acid substitution, W102G, in the MLV envelope protein (Env) regulates the pathogenic effects. The role of Env in determining this disease phenotype compared to the induction of spongiform encephalomyelitis with a longer latency, as seen in several other MLV and in human retroviruses, was determined by studying in vitro-attenuated TR1.3. Virus cloned from this selection, termed TRM, induced progressive neurological disease characterized by ataxia and paralysis and the appearance of spongiform neurodegeneration throughout the brain stem and spinal cord. This disease was associated with virus replication in both BCEC and highly ramified glial cells. TRM did not induce syncytium formation, either in vivo or in vitro. Sequence and mutational analyses demonstrated that TRM contained a reversion of Env G102W but that neurological disease mapped to the single amino acid substitution Env S159P. The results demonstrate that single nucleotide changes within disparate regions of Env control dramatically different CNS disease patterns.

Neuropathogenic murine leukemia virus TR1.3 induces selective syncytia formation of brain capillary endothelium

Exposure of newborn BALB/c mice to murine leukemia virus (MLV) TR1.3 induces fusion of brain capillary endothelial cells (BCEC), loss of cerebral vessel integrity, hemorrhagic stroke, and death. Although TR1.3 infects endothelial cells in multiple organs, syncytia are only observed in BCEC. To determine if viral and cellular factors are responsible for selective syncytia formation, capillary endothelial cells (CEC) from multiple organs were assayed in vitro for MLV infection and cell fusion. Following incubation with virus, all CEC were infected to an equal extent as determined by expression of MLV envelope and infectious virus production; however, MLV-induced syncytia were only observed in TR1.3-infected BCEC cultures. These in vitro results mirror the in vivo pattern of TR1.3 MLV infection and neuropathology, and definitively show that selective fusion and pathology of BCEC by MLV is determined by properties unique to BCEC as contrasted to other endothelial cell types.

Cell surface receptors for gammaretroviruses

Evidence obtained during the last few years has greatly extended our understanding of the cell surface receptors that mediate infections of retroviruses and has provided many surprising insights. In contrast to other cell surface components such as lectins or proteoglycans that influence infections indirectly by enhancing virus adsorption onto specific cells, the true receptors induce conformational changes in the viral envelope glycoproteins that are essential for infection. One surprise is that all of the cell surface receptors for gamma-retroviruses are proteins that have multiple transmembrane (TM) sequences, compatible with their identification in known instances as transporters for important solutes. In striking contrast, almost all other animal viruses use receptors that exclusively have single TM sequences, with the sole proven exception we know of being the coreceptors used by lentiviruses. This evidence strongly suggests that virus genera have been prevented because of their previous evolutionary adaptations from switching their specificities between single-TM and multi-TM receptors. This evidence also implies that gamma-retroviruses formed by divergent evolution from a common origin millions of years ago and that individual viruses have occasionally jumped between species (zoonoses) while retaining their commitment to using the orthologous receptor of the new host. Another surprise is that many gamma-retroviruses use not just one receptor but pairs of closely related receptors as alternatives. This appears to have enhanced viral survival by severely limiting the likelihood of host escape mutations. All of the receptors used by gamma-retroviruses contain hypervariable regions that are often heavily glycosylated and that control the viral host range properties, consistent with the idea that these sequences are battlegrounds of virus-host coevolution. However, in contrast to previous assumptions, we propose that gamma-retroviruses have become adapted to recognize conserved sites that are important for the receptor's natural function and that the hypervariable sequences have been elaborated by the hosts as defense bulwarks that surround the conserved viral attachment sites. Previously, it was believed that binding to receptors directly triggers a series of conformational changes in the viral envelope glycoproteins that culminate in fusion of the viral and cellular membranes. However, new evidence suggests that gamma-retroviral association with receptors triggers an obligatory interaction or cross-talk between envelope glycoproteins on the viral surface. If this intermediate step is prevented, infection fails. Conversely, in several circumstances this cross-talk can be induced in the absence of a cell surface receptor for the virus, in which case infection can proceed efficiently. This new evidence strongly implies that the role of cell surface receptors in infections of gamma-retroviruses (and perhaps of other enveloped animal viruses) is more complex and interesting than was previously imagined. Recently, another gammaretroviral receptor with multiple transmembrane sequences was cloned. See Prassolov, Y., Zhang, D., Ivanov, D., Lohler, J., Ross, S.R., and Stocking, C. Sodium-dependent myo-inositol transporter 1 is a receptor for Mus cervicolor M813 murine leukemia virus.

G541R within the 4070A TM protein regulates fusion in murine leukemia viruses

The mutation G541R within the ectodomain of TM was isolated in three independent chimeric enveloped murine leukemia virus (MuLV) viral populations originally impaired in viral passage and in wild-type 4070A. Isolation of G541R in multiple populations suggested it played a critical role in viral envelope function. Using a viral vector system, the observed effects of the G541R mutation within MuLV envelope proteins were pleiotropic and included effects on the regulation of SU-TM interactions and membrane fusion. G541R suppresses enhanced cell-cell fusion events attributable to the absence of the R-peptide yet does not adversely affect virus titers. The ability to suppress cell-cell fusion is dependent on the presence of the C terminus of the amphotropic 4070A SU protein. Within the wild-type 4070A envelope background, the mutation results in a decreased level of Env at the cell surface that is mirrored in the virion. The TM mutation alters recognition of the SU C terminus by a monoclonal antibody, suggestive of an altered conformation. The presence of G541R allowed the virus to achieve a balance between cytopathogenicity and replication and restored productive viral entry.

Identification of the receptor binding domain of the mouse mammary tumor virus envelope protein

Mouse mammary tumor virus (MMTV) is a betaretrovirus that infects rodent cells and uses mouse transferrin receptor 1 for cell entry. To characterize the interaction of MMTV with its receptor, we aligned the MMTV envelope surface (SU) protein with that of Friend murine leukemia virus (F-MLV) and identified a putative receptor-binding domain (RBD) that included a receptor binding sequence (RBS) of five amino acids and a heparin-binding domain (HBD). Mutation of the HBD reduced virus infectivity, and soluble heparan sulfate blocked infection of cells by wild-type pseudovirus. Interestingly, some but not all MMTV-like elements found in primary and cultured human breast cancer cell lines, termed h-MTVs, had sequence alterations in the putative RBS. Single substitution of one of the amino acids found in an h-MTV RBS variant in the RBD of MMTV, Phe(40) to Ser, did not alter species tropism but abolished both virus binding to cells and infectivity. Neutralizing anti-SU monoclonal antibodies also recognized a glutathione S-transferase fusion protein that contained the five-amino-acid RBS region from MMTV. The critical Phe(40) residue is located on a surface of the MMTV RBD model that is distant from and may be structurally more rigid than the region of F-MLV RBD that contains its critical binding site residues. This suggests that, in contrast to other murine retroviruses, binding to its receptor may result in few or no changes in MMTV envelope protein conformation.

Generation of novel syncytium-inducing and host range variants of ecotropic moloney murine leukemia virus in Mus spicilegus

Mus spicilegus is an Eastern European wild mouse species that has previously been reported to harbor an unusual infectious ecotropic murine leukemia virus (MLV) and proviral envelope genes of a novel MLV subgroup. In the present study, M. spicilegus neonates were inoculated with Moloney ecotropic MLV (MoMLV). All 17 inoculated mice produced infectious ecotropic virus after 8 to 14 weeks, and two unusual phenotypes distinguished the isolates from MoMLV. First, most of the M. spicilegus isolates grew to equal titers on M. dunni and SC-1 cells, although MoMLV does not efficiently infect M. dunni cells. The deduced amino acid sequence of a representative clone differed from MoMLV by insertion of two serine residues within the VRA of SUenv. Modification of a molecular clone of MoMLV by the addition of these serines produced a virus that grows to high titer in M. dunni cells, establishing a role for these two serine residues in host range. A second unusual phenotype was found in only one of the M. spicilegus isolates, Spl574. Spl574 produces large syncytia of multinucleated giant cells in M. dunni cells, but its replication is restricted in other mouse cell lines. Sequencing and mutagenesis demonstrated that syncytium formation could be attributed to a single amino acid substitution within VRA, S82F. Thus, viruses with altered growth properties are selected during growth in M. spicilegus. The mutations associated with the host range and syncytium-inducing variants map to a key region of VRA known to govern interactions with the cell surface receptor, suggesting that the associated phenotypes may result from altered interactions with the unusual ecotropic virus mCAT1 receptor carried by M. dunni.

The Mus cervicolor MuLV isolate M813 is highly fusogenic and induces a T-cell lymphoma associated with large multinucleated cells

M813 is a type-C murine leukemia virus (MuLV) isolated from the Asian rodent Mus cervicolor. We have recently demonstrated that M813 defines a distinct MuLV receptor interference group. Here we show that M813 rapidly induces fusion of MuLV-expressing fibroblasts from "without," with syncytia being observed within 1 h after exposure to virus. Infection of fibroblasts with MuLV from all tested receptor-interference groups imparts susceptibility to M813-induced fusion, provided the cells also express the M813 receptor. Syncytium induction is also observed in vivo; mice infected with M813 develop a peripheral T-cell lymphoma, which is associated with large multinucleated cells of macrophage origin. A recombinant Moloney MuLV/M813 chimeric virus demonstrated that syncytium induction is a function of the Env SU protein. We postulate that the highly fusogenic property of M813 is attributable to either its unique receptor usage or sequences in the proline-rich domain of the Env protein.

At least four non-env factors that reside in the LTR, in the 5'-non-coding region, in gag and in part of pol affect neuropathogenicity of PVC-441 murine leukemia virus (MuLV)

PVC-441 murine leukemia virus (MuLV) is neuropathogenic in F344 rats. Recently, an infectious DNA clone was isolated and its nucleotide sequence was determined (J. Virol. 72: 3423-3426. 1998). To identify the viral determinants of neuropathogenicity of the molecularly cloned PVC-441 MuLV, chimeras were constructed between PVC-441 MuLV and F-MuLV clones at appropriate restriction enzyme sites that divide the viral genome approximately in LTR-non-coding, gag-, pol-, and env-gene regions. Results indicated that the LTR-non-coding and the gag-gene regions of PVC-441 MuLV affected independently the neuropathogenicity in combination with the env gene region as evidenced clinically and pathologically. Studies on the distribution of vacuolar degeneration suggested that the pons and cervical spinal cord areas were the primary targets and the large brain was the latest target of PVC-441 MuLV. Further studies with chimeric viruses that were formed in the LTR-non-coding and the gag gene regions revealed that at least four factors affected the neuropathogenicity of PVC-441 MuLV. Two factors were found in the U3, and R-U5-5'-non-coding regions, and at least two factors in the gag gene region that contained the N-terminal part of the pol gene. Among these factors, at least two factors seemed to be 'cis-acting' from each other

An envelope glycoprotein of the human endogenous retrovirus HERV-W is expressed in the human placenta and fuses cells expressing the type D mammalian retrovirus receptor

A new human endogenous retrovirus (HERV) family, termed HERV-W, was recently described (J.-L. Blond, F. Besème, L. Duret, O. Bouton, F. Bedin, H. Perron, B. Mandrand, and F. Mallet, J. Virol. 73:1175-1185, 1999). HERV-W mRNAs were found to be specifically expressed in placenta cells, and an env cDNA containing a complete open reading frame was recovered. In cell-cell fusion assays, we demonstrate here that the product of the HERV-W env gene is a highly fusogenic membrane glycoprotein. Transfection of an HERV-W Env expression vector in a panel of cell lines derived from different species resulted in formation of syncytia in primate and pig cells upon interaction with the type D mammalian retrovirus receptor. Moreover, envelope glycoproteins encoded by HERV-W were specifically detected in placenta cells, suggesting that they may play a physiological role during pregnancy and placenta formation.

Activation of a cell entry pathway common to type C mammalian retroviruses by soluble envelope fragments

Mutations that negatively or positively affect the fusion properties of murine leukemia viruses (MLVs) have been found within all subdomains of their SU (surface) and TM (transmembrane) envelope units. Yet, the interrelations between these different regions of the envelope complex during the cell entry process are still elusive. Deletion of the histidine residue of the conserved PHQV motif at the amino terminus of the amphotropic or the ecotropic MLV SU resulted in the AdelH or the MOdelH fusion-defective mutant envelope, respectively. These delH mutant envelopes are incorporated on retroviral particles at normal densities and normally mediate virion binding to cells expressing the retroviral receptors. However, both their cell-cell and virus-cell fusogenicities were fully prevented at an early postbinding stage. We show here that the fusion defect of AdelH or MOdelH envelopes was also almost completely reverted by providing either soluble SU or a polypeptide encompassing the receptor-binding domain (RBD) to the target cells, provided that the integrity of the amino-terminal end of either polypeptide was preserved. Restoration of delH envelope fusogenicity was caused by activation of the target cells via specific interaction of the latter polypeptides with the retrovirus receptor rather than by their association with the delH envelope complexes. Moreover crossactivation of the target cells, leading to fusion activation of AdelH or MOdelH envelopes, was achieved by polypeptides containing various type C mammalian retrovirus RBDs, irrespective of the type of entry-defective glycoprotein that was used for infection. Our results indicate that although they recognize different receptors for binding to the cell surface, type C mammalian retroviruses use a common entry pathway which is activated by a conserved feature of their envelope glycoproteins.

Second-site changes affect viability of amphotropic/ecotropic chimeric enveloped murine leukemia viruses

Chimeras were previously generated between the ecotropic (Moloney-MuLV) and amphotropic (4070A) SU and TM proteins of murine leukemia virus (MuLV). After passage in D17 cells, three chimeras with junctions in the C terminus of SU (AE5, AE6, and AE7), showed improved kinetics of viral spreading, suggesting that they had adapted. Sequencing of the viruses derived from the D17 cell lines revealed second-site changes within the env gene. Changes were detected in the receptor binding domain, the proline-rich region, the C terminus of SU, and the ectodomain of TM. Second-site changes were subcloned into the parental DNA, singly and in combination, and tested for viability. All viruses had maintained their original cloned mutations and junctions. Reconstruction and passage of AE7 or AE6 virus with single point mutations recovered the additional second-site changes identified in the parental population. The AE5 isolate required changes in the VRA, the VRC, the VRB-hinge region, and the C terminus of SU for efficient infection. Passage of virus, including the parental 4070A, in D17 cells resulted in a predominant G100R mutation within the receptor binding domain. Viruses were subjected to titer determination in three cell types, NIH 3T3, canine D17, and 293T. AE6 viruses with changes in the proline-rich region initially adapted for growth on D17 cells could infect all cell types tested. AE6-based chimeras with additional mutations in the C terminus of SU could infect D17 and 293T cells. Infection of NIH 3T3 cells was dependent on the proline-rich mutation. AE7-based chimeras encoding L538Q and G100R were impaired in infecting NIH 3T3 and 293T cells.

Induction of syncytia by neuropathogenic murine leukemia viruses depends on receptor density, host cell determinants, and the intrinsic fusion potential of envelope protein

Infection by the neuropathogenic murine leukemia virus (MLV) TR1.3 results in hemorrhagic disease that correlates directly to in vivo syncytium formation of brain capillary endothelial cells (BCEC). This phenotype maps to amino acid 102 in the envelope (Env) protein of TR1.3. Substitution of glycine (G) for tryptophan (W) at this position (W102G Env) in the nonpathogenic MLV FB29 induces both syncytium formation and neurologic disease in vivo. Using an in vitro gene reporter cell fusion assay, we showed that fusion either with murine NIH 3T3 cells or with nonmurine target cells that expressed receptors at or below endogenous murine levels mirrored that seen in BCEC in vivo. In these instances only TR1.3 and W102G Env induced cell fusion. In contrast, when receptor levels on nonmurine cells were raised above endogenous murine levels, FB29 Env was as fusogenic as the neuropathogenic TR1.3 and W102G Env. These results indicate that TR1.3 Env and W102G Env are intrinsically more fusogenic than FB29 Env, that the induction of fusion requires a threshold number of receptors that is greater for FB29 Env than for TR1.3 or W102G Env, and that receptor density on murine NIH 3T3 cells and BCEC is below the threshold for FB29-dependent fusion. Surprisingly, receptor density on NIH 3T3 cells could not be increased by stable expression of exogenous receptors, and FB29-dependent fusion was not observed in NIH 3T3 cells that transiently expressed elevated receptor numbers. These results suggest that an additional undefined host cell factor(s) may limit both receptor expression and fusion potential in murine cells.

Identification of a receptor-binding pocket on the envelope protein of friend murine leukemia virus

Based on previous structural and functional studies, a potential receptor-binding site composed of residues that form a pocket at one end of the two long antiparallel helices in the receptor-binding domain of Friend 57 murine leukemia virus envelope protein (RBD) has been proposed. To test this hypothesis, directed substitutions for residues in the pocket were introduced and consequences for infection and for receptor binding were measured. Receptor binding was measured initially by a sensitive assay based on coexpression of receptor and RBD in Xenopus oocytes, and the findings were confirmed by using purified proteins. Three residues that are critical for both binding and infection (S84, D86, and W102), with side chains that extend into the pocket, were identified. Moreover, when mCAT-1 was overexpressed, the infectivity of Fr57-MLV carrying pocket substitutions was partially restored. Substitutions for 18 adjacent residues and 11 other previously unexamined surface-exposed residues outside of the RBD pocket had no detectable effect on function. Taken together, these findings support a model in which the RBD pocket interacts directly with mCAT-1 (likely residues, Y235 and E237) and multiple receptor-envelope complexes are required to form the fusion pore.

A proline-rich motif downstream of the receptor binding domain modulates conformation and fusogenicity of murine retroviral envelopes

The entry of retroviruses into cells depends on receptor recognition by the viral envelope surface subunit SU followed by membrane fusion, which is thought to be mediated by a fusion peptide located at the amino terminus of the envelope transmembrane subunit TM. Several fusion determinants have been previously identified in murine leukemia virus (MLV) envelopes, but their functional interrelationships as well as the processes involved in fusion activation upon retroviral receptor recognition remain unelucidated. Despite both structural and functional similarities of their envelope glycoproteins, ecotropic and amphotropic MLVs display two different postbinding properties: (i) while amphotropic MLVs fuse the cells at neutral pH, penetration of ecotropic MLVs is relatively acid pH dependent and (ii) ecotropic envelopes are more efficient than amphotropic envelopes in inducing cell-to-cell fusion and syncytium formation. By exploiting the latter characteristic in the analysis of chimeras of ecotropic and amphotropic MLV envelopes, we show here that substitution of the ecotropic MLV proline-rich region (PRR), located in the SU between the amino-terminal receptor binding domain and the TM-interacting SU carboxy-terminal domains, is sufficient to revert the amphotropic low-fusogenic phenotype into a high-fusogenic one. Furthermore, we have identified potential beta-turns in the PRR that control the stability of SU-TM associations as well as the thresholds required to trigger either cell-to-cell or virus-to-cell fusion. These data, demonstrating that the PRR functions as a signal which induces envelope conformational changes leading to fusion, have enabled us to derive envelopes which can infect cells harboring low levels of available amphotropic receptors.

Role of variable regions A and B in receptor binding domain of amphotropic murine leukemia virus envelope protein

For the amphotropic murine leukemia virus (MuLV), a 208-amino-acid amino-terminal fragment of the surface unit (SU) of the envelope glycoprotein is sufficient to bind to its receptor, Pit2. Within this binding domain, two hypervariable regions, VRA and VRB, have been proposed to be important for receptor recognition. In order to specifically locate residues that are important for the interaction with Pit2, we generated a number of site-specific mutations in both VRA and VRB and analyzed the resulting envelope proteins when expressed on retroviral vectors. Concurrently, we substituted portions of the amphotropic SU with homologous regions from the polytropic MuLV envelope protein. The amphotropic SU was unaffected by most of the point mutations we introduced. In addition, the deletion of eight residues in a region of VRA that was previously suggested to be essential for Pit2 utilization only decreased titer on NIH 3T3 cells by 1 order of magnitude. Although the replacement of the amino-terminal two-thirds of VRA with the polytropic sequence abolished receptor binding, smaller nonoverlapping substitutions did not affect the function of the protein. We were not able to identify a single critical receptor contact point within VRA, and we suggest that the amphotropic receptor binding domain probably makes multiple contacts with the receptor and that the loss of some of these contacts can be tolerated.

Characterization of two avian reoviruses that exhibit strain-specific quantitative differences in their syncytium-inducing and pathogenic capabilities

We previously proposed that the conservation of the nonessential syncytium-inducing phenotype among all reported avian reovirus (ARV) isolates may reflect a mechanism for enhanced virus dissemination in vivo, which in turn could contribute to the natural pathogenicity of ARV. Direct testing of this hypothesis has been hampered by the lack of available virus strains with defined differences in their fusion-inducing capability. We now report on the characterization of two ARV strains, ARV-176 and ARV-138, that exhibited strain-specific differences in their fusogenic properties, which correlated with their pathogenic potential in embryonated eggs. Moreover, both virus strains possessed similar replicative abilities in cell culture, suggesting that the weakly fusogenic ARV-138 virus is specifically inhibited in its syncytium-inducing ability. To test the use of these viruses for reassortant studies aimed at assessing the role of cell fusion in viral pathogenesis, a preliminary genetic analysis was undertaken using a monoreassortant that contained nine genome segments from the parental ARV-138 virus and the S1 genome segment from the highly fusogenic and pathogenic ARV-176 parental virus. The monoreassortant possessed the full fusogenic potential of the ARV-176 parental virus and displayed enhanced embryo pathogenicity, providing the first genetic evidence implicating the ARV S1 genome segment in both syncytium formation and viral pathogenesis.

Characterization of the proline-rich region of murine leukemia virus envelope protein

Mammalian type C retroviral envelope proteins contain a variable proline-rich region (PRR), located between the N-terminal receptor-binding domain and the more highly conserved C-terminal portion of the surface (SU) subunit. We have investigated the role of the PRR in the function of murine leukemia virus (MuLV) envelope protein. In the MuLVs, the PRR contains a highly conserved N-terminal sequence and a hypervariable C-terminal sequence. Despite this variability, the amphotropic PRR could functionally substitute for the ecotropic PRR. The hypervariable region of the PRR was not absolutely required for envelope protein function. However, truncations in this region resulted in decreased levels of both the SU and TM proteins in viral particles and increased amounts of the uncleaved precursor protein, Pr85. In contrast, the N-terminal conserved region was essential for viral infectivity. Deletion of this region prevented the stable incorporation of envelope proteins into viral particles in spite of normal envelope protein processing, wild-type levels of cell surface expression, and a wild-type ability to induce syncytia in an XC cell cocultivation assay. However, higher levels of the SU protein were shed into the supernatant, suggesting a defect in SU-TM interactions. Our data are most consistent with a role for the PRR in stabilizing the overall structure of the protein, thereby affecting the proper processing of Pr85, SU-TM interactions, and the stable incorporation of envelope proteins into viral particles. In addition, we have demonstrated that the PRR can tolerate the insertion of a peptide-binding domain, making this a potentially useful site for constructing targetable retroviral vectors.

Mouse adenovirus type-1 replication is restricted to vascular endothelium in the CNS of susceptible strains of mice

Previous studies have shown that mouse adenovirus type-1 (MAV-1) caused a fatal hemorrhagic encephalitis in certain strains of mice. C57BI/6 mice exhibited 100% mortality when given as little 10(3) plaque-forming units (PFU) of MAV, in contrast to BALB/c mice which were resistant to as many as 10(6) PFU. Susceptible animals died with a flaccid paralysis on the 3rd or 4th day after inoculation. The brains and spinal cords of these animals displayed numerous petechial hemorrhages that were found in virtually all areas of the brain, but were more numerous in white matter. In this paper, immunohistochemistry and electron microscopy were used to identify the viral target of replication within the CNS of susceptible mice. These studies showed that the CNS vascular endothelial cell was the primary site of viral replication within the CNS of mice infected with MAV-1. Characterization of cytokine mRNA levels and disease course in immunodeficient mice revealed that the host immune response played little, if any, role in the pathogenesis of MAV-1 disease in susceptible mice and was not responsible for the resistance of BALB/c mice. These results support the conclusion that disease course and outcome in susceptible and resistant strains of mice were determined primarily by the ability of the virus to replicate within the CNS vascular endothelium.

Studies on the pathology, especially brain lesions, induced by R7, a spontaneous mutant of Moloney murine sarcoma virus 124

We have recently isolated R7, a spontaneous Moloney murine sarcoma virus (MoMuSV) 124 variant. Molecular cloning and sequence analysis showed that, relative to MoMuSV 124, R7 has an extra repeat in each enhancer and a truncated mos gene in frame with the truncated gag coding sequence. This report presents a detailed study on the pathology induced by R7. R7 induced not only sarcomas with well developed angiomatous components but also brain lesions. Brain lesions were observed in all less-than-48-hour-old BALB/c mice inoculated with greater than 2 x 10(5) R7 focus-forming units (FFUs). R7 was detected in all brains examined by day 9 after inoculation, and brain lesions were observed in two of four mice examined by day 14 after inoculation. Light microscopy of brains revealed that approximately 15% of the lesions were unenclosed blood pools of varying sizes containing red blood cells and inflammatory cells spreading into surrounding brain tissues. The remainder of the brain lesions had tumor cells. These lesions ranged from a few enlarged vascular endothelial cells intermixed with blood cells to large circumscribed lesions consisting of well developed tangled masses of vessels surrounded by blood pools. Activated astrocytes surrounded and infiltrated the tumors. In addition, the thymus of R7-infected mice regressed significantly and precipitously due to apoptosis (especially of cortical thymocytes) at the end stage of the disease.

A novel virus in swine is closely related to the human hepatitis E virus

A novel virus, designated swine hepatitis E virus (swine HEV), was identified in pigs. Swine HEV crossreacts with antibody to the human HEV capsid antigen. Swine HEV is a ubiquitous agent and the majority of swine >/=3 months of age in herds from the midwestern United States were seropositive. Young pigs naturally infected by swine HEV were clinically normal but had microscopic evidence of hepatitis, and developed viremia prior to seroconversion. The entire ORFs 2 and 3 were amplified by reverse transcription-PCR from sera of naturally infected pigs. The putative capsid gene (ORF2) of swine HEV shared about 79-80% sequence identity at the nucleotide level and 90-92% identity at the amino acid level with human HEV strains. The small ORF3 of swine HEV had 83-85% nucleotide sequence identity and 77-82% amino acid identity with human HEV strains. Phylogenetic analyses showed that swine HEV is closely related to, but distinct from, human HEV strains. The discovery of swine HEV not only has implications for HEV vaccine development, diagnosis, and biology, but also raises a potential public health concern for zoonosis or xenozoonosis following xenotransplantation with pig organs.

Detection and preliminary characterization of a new rabbit calicivirus related to rabbit hemorrhagic disease virus but nonpathogenic

A new rabbit calicivirus related to the rabbit hemorrhagic disease virus (RHDV) was identified. The new virus contains significant differences from the previously characterized RHDV isolates in terms of pathogenicity, viral titer, tropism, and primary sequence of the structural protein. Cross-protection experiments, antigenic data, and sequence comparisons demonstrate that the new virus is more closely related to RHDV than to the European brown hare syndrome virus, another member of the caliciviruses of the lagomorph group. The existence of a nonpathogenic calicivirus, which we propose to name rabbit calicivirus (RCV), provides an explanation for the early discrepancies found in the course of serological surveys of the rabbit population in European countries.

Modulation of ecotropic murine retroviruses by N-linked glycosylation of the cell surface receptor/amino acid transporter

The cell surface receptor for ecotropic host-range (infection limited to mice or rats) murine leukemia viruses (MuLVs) is the widely expressed system y+ transporter for cationic amino acids (CAT-1). Like other retroviruses, ecotropic MuLV infection eliminates virus-binding sites from cell surfaces and results in complete interference to superinfection. Surprisingly, infection causes only partial (ca 40 to 60%) loss of mouse CAT-1 transporter activity. The NIH/Swiss mouse CAT-1 (mCAT-1) contains 622 amino acids with 14 hydrophobic potential membrane-spanning sequences, and it is known that the third extracellular loop from the amino terminus is required for virus binding. Although loop 3 is hypervariable in different species and mouse strains, consistent with its proposed role in virus-host coevolution, loop 3 sequences of both susceptible and resistant species contain consensus sites for N-linked glycosylation. Both of the consensus sites in loop 3 of mCAT-1 are known to be glycosylated and to contain oligosaccharides with diverse sizes (J. W. Kim and J. M. Cunningham, J. Biol. Chem. 268:16316-16320, 1993). We confirmed by several lines of evidence that N-linked glycosylation occludes a potentially functional virus-binding site in the CAT-1 protein of hamsters, thus contributing to resistance of that species. To study the role of receptor glycosylation in animals susceptible to infection, we eliminated loop 3 glycosylation sites by mutagenesis of an mCAT-1 cDNA clone, and we expressed wild-type and mutant receptors in mink fibroblasts and Xenopus oocytes. These receptors had indistinguishable transport properties, as determined by kinetic and voltage-jump electrophysiological studies of arginine uptake in oocytes and by analyses Of L-[3H]arginine uptake in mink cells. Bindings of ecotropic envelope glycoprotein gp7O to the accessible receptor sites on surfaces of mink cells expressing wild-type or mutant mCAT-1 were not significantly different in kinetics or in equilibrium affinities (i.e., K(D) approximately 3.7 X 10(-10) to 7.5 X 10(-10) M). However, when values were normalized to the same levels of mCAT-1 transporter expression, cells with wild-type glycosylated mCAT-1 had only approximately 50% as many sites for gp70 binding as cells with unglycosylated mCAT-1. Although infection with ecotropic MuLV had no effect on activity of the mink CAT-1 transporter that does not bind virus, it caused partial down-modulation of wild-type mCAT-1 and complete down-modulation of unglycosylated mutant mCAT-1. These results suggest that N-linked glycosylation causes wild-type mCAT-1 heterogeneity and that a significant proportion is inaccessible to virus. In part because only the interactive fraction of mCAT-1 can be down-modulated, infected murine cells conserve an amino acid transport capability that supports their viability.

Characterization of chimeras between the ecotropic Moloney murine leukemia virus and the amphotropic 4070A envelope proteins

A series of 22 chimeric envelope (env) genes were generated between the ecotropic Moloney murine leukemia virus and the amphotropic 4070A isolate. The chimeric envelopes were expressed within the complete, replication-competent provirus and tested for virus viability by transient expression assays. Eleven of the 22 viruses were viable. Five of these chimeric viruses showed an ecotropic host range, and six exhibited an amphotropic host range and viral interference. The host range determinants map to the first half of the surface (SU) protein. The N-terminal 72 amino acids of 4070A (42 of processed SU) are not required for amphotropic receptor usage. Ecotropic and amphotropic viruses differ in their ability to form large, multinucleated syncytia when cocultured with the rat XC cell line. Ecotropic murine leukemia virus forms large syncytia with XC cells, whereas no syncytia are reported for amphotropic virus. All chimeras which contained the N-terminal half of the ecotropic SU protein, encoding the receptor binding domain, formed the large multinucleated syncytia with XC cells.

Mouse adenovirus type 1 causes a fatal hemorrhagic encephalomyelitis in adult C57BL/6 but not BALB/c mice

Mouse adenovirus type 1 (MAV-1) produces a lethal disease in newborn or suckling mice characterized by infectious virus and viral lesions in multiple organs. Previous reports of MAV-1 infection of adult mice generally described serologic evidence of infection without morbidity or mortality. However, our current results demonstrate that MAV-1 causes a fatal illness in adult C57BL/6(B6) mice (50% lethal dose, [LD50], 10(3.0) PFU) but not in adult BALB/c mice at all of the doses tested (LD50, > or = 10(5.0) PFU). Adult (BALB/c x B6)F1 mice were intermediately susceptible (LD50, 10(4.5) PFU). Clinically, the sensitive B6 mice showed symptoms of acute central nervous system (CNS) disease, including tremors, seizures, ataxia, and paralysis. Light microscopic examination of CNS tissue from the B6 animals revealed petechial hemorrhages, edema, neovascularization, and mild inflammation in the brain and spinal cord. Analysis by electron microscopy showed evidence of inflammation, such as activated microglia, as well as swollen astrocytic endfeet and perivascular lipid deposition indicative of blood-brain barrier dysfunction. Outside of the CNS, the only significant pathological findings were foci of cytolysis in the splenic white pulp. Assessment of viral replication from multiple tissues was performed by using RNase protection assays with an antisense MAV-1 early region 1a probe. The greatest amounts of viral mRNA in MAV-1-infected B6 animals were located in the brain and spinal cord. Less viral message was detected in the spleen, lungs, and heart. No viral mRNA was detected in BALB/c mouse tissue, with the exception of low levels in the heart. Viral titers of organ tissues were also determined and were concordant with RNase protection findings on the brain and spinal cord but failed to demonstrate significant infectious virus in additional organs. Our experiments demonstrate that MAV-1 has a striking tropism for the CNS that is strain dependent, and this provides an informative in vivo model for the study of adenoviral pathogenesis.

The amphotropic and ecotropic murine leukemia virus envelope TM subunits are equivalent mediators of direct membrane fusion: implications for the role of the ecotropic envelope and receptor in syncytium formation and viral entry

The murine leukemia virus (MuLV) envelope protein was examined to determine which sequences are responsible for the differences in direct membrane fusion observed with the ecotropic and amphotropic MuLV subtypes. These determinants were studied by utilizing amphotropic-ecotropic chimeric envelope proteins that have switched their host range but retain their original fusion domain (TM subunit). Fusion was tested both in rodent cells and in 293 cells bearing the human homolog of the ecotropic MuLV receptor. The results demonstrate that the amphotropic TM is able to mediate cell-to-cell fusion to an extent equivalent to that mediated by the ecotropic TM, indicating that their fusion domains are equivalent. The "murinized" human homolog of the ecotropic receptor supports syncytium formation as well as the native murine receptor. These findings suggest that interactions between the ecotropic envelope protein and conserved sequences in the ecotropic receptor are the principal determinants of syncytium formation. The relationship of the fusion phenotype to pH-dependent infection and the route of viral entry was examined by studying virions bearing the chimeric envelope proteins. Such virions appear to enter cells via a pathway that is directed by the host range-determining region of their envelope rather than by sequences that confer pH dependence. Therefore, the pH dependence of infection may not reflect the initial steps in viral entry. Thus, it appears that both the syncytium phenotype and the route of viral entry are properties of the viral receptor, the amino-terminal half of the ecotropic envelope protein, or the interaction between the two.