Cellular expression of murine leukemia virus gp70-related antigen on thymocytes of uninfected mice correlates with Fv-4 gene-controlled resistance to Friend leukemia virus infection

Transcriptome Analyses Implicate Endogenous Retroviruses Involved in the Host Antiviral Immune System through the Interferon Pathway

Human endogenous retroviruses (HERVs) are the remains of ancient retroviruses that invaded our ancestors' germline cell and were integrated into the genome. The expression of HERVs has always been a cause for concern because of its association with various cancers and diseases. However, few previous studies have focused on specific activation of HERVs by viral infections. Our previous study has shown that dengue virus type 2 (DENV-2) infection induces the transcription of a large number of abnormal HERVs loci; therefore, the purpose of this study was to explore the relationship between exogenous viral infection and HERV activation further. In this study, we retrieved and reanalyzed published data on 21 transcriptomes of human cells infected with various viruses. We found that infection with different viruses could induce transcriptional activation of HERV loci. Through the comparative analysis of all viral datasets, we identified 43 key HERV loci that were up-regulated by DENV-2, influenza A virus, influenza B virus, Zika virus, measles virus, and West Nile virus infections. Furthermore, the neighboring genes of these HERVs were simultaneously up-regulated, and almost all such neighboring genes were interferon-stimulated genes (ISGs), which are enriched in the host's antiviral immune response pathways. Our data supported the hypothesis that activation of HERVs, probably via an interferon-mediated mechanism, plays an important role in innate immunity against viral infections.

Mapping Viral Susceptibility Loci in Mice

Genetic alleles that contribute to enhanced susceptibility or resistance to viral infections and virally induced diseases have often been first identified in mice before humans due to the significant advantages of the murine system for genetic studies. Herein we review multiple discoveries that have revealed significant insights into virus-host interactions, all made using genetic mapping tools in mice. Factors that have been identified include innate and adaptive immunity genes that contribute to host defense against pathogenic viruses such as herpes viruses, flaviviruses, retroviruses, and coronaviruses. Understanding the genetic mechanisms that affect infectious disease outcomes will aid the development of personalized treatment and preventive strategies for pathogenic infections.

Ancestral Mutations Acquired in Refrex-1, a Restriction Factor against Feline Retroviruses, during its Cooption and Domestication

Endogenous retroviruses (ERVs) are remnants of ancestral retroviral infections of germ cells. Retroviral endogenization is an adaptation process for the host genome, and ERVs are gradually attenuated or inactivated by mutation. However, some ERVs that have been "domesticated" by their hosts eventually gain physiological functions, such as placentation or viral resistance. We previously reported the discovery of Refrex-1, a soluble antiretroviral factor in domestic cats that specifically inhibits infection by feline leukemia virus subgroup D (FeLV-D), a chimeric virus of FeLV, and a feline ERV, ERV-DC. Refrex-1 is a truncated envelope protein (Env) encoded by both ERV-DC7 and ERV-DC16 proviral loci. Here, we reconstituted ancestral and functional Env from ERV-DC7 and ERV-DC16 envelope genes (env) by inducing reverse mutations. Unexpectedly, ERV-DC7 and ERV-DC16 full-length Env (ERV-DC7 fl and ERV-DC16 fl), reconstructed by removing stop codons, did not produce infectious viral particles. ERV-DC7 fl and ERV-DC16 fl were highly expressed in cells but were not cleaved into surface subunits (SU) and transmembrane subunits, nor were they incorporated into virions. G407R/N427I-A429T and Y431D substitutions within the SU C-terminal domain of ERV-DC7 fl and ERV-DC16 fl, respectively, caused these dysfunctions. The residues glycine 407 and tyrosine 431 are relatively conserved among infectious gammaretroviruses, and their substitution causes the same dysfunctions as the tested retroviruses. Our results reveal that specific mutations within the SU C-terminal domain suppressed Env cleavage and incorporation into virions and indicate that these mutations contributed to the domestication of Refrex-1 through multistep events that occurred in the postintegration period.

IMPORTANCE

Domestic cats are colonized with various exogenous retroviruses (exRVs), such as feline leukemia virus (FeLV), and their genomes contain numerous ERVs, some of which are replication-competent proviruses. The feline hosts, exRVs, and ERVs have complicated genetic interactions and provide an interesting field model for triangular relationships: recombination between FeLV and ERV-DC, which is a feline ERV, generated FeLV-D, a chimeric virus, and FeLV-D is restricted by Refrex-1, an antiretroviral factor corresponding to truncated Env of ERV-DC7 and ERV-DC16. Here, we reconstructed ancestral, functional Env from ERV-DC7 and ERV-DC16 env by inducing reverse mutations to elucidate how Refrex-1 was generated from its ancestor. Our results reveal that they were repeatedly inactivated by mutations preventing Env maturation. Our results provide insights into how ERVs were "domesticated" by their hosts and identify the mutations that mediated these evolutions. Notably, experiments that restore inactivated ERVs might uncover previously unrecognized features or properties of retroviruses.

piRNAs derived from ancient viral processed pseudogenes as transgenerational sequence-specific immune memory in mammals

Endogenous bornavirus-like nucleoprotein elements (EBLNs) are sequences within vertebrate genomes derived from reverse transcription and integration of ancient bornaviral nucleoprotein mRNA via the host retrotransposon machinery. While species with EBLNs appear relatively resistant to bornaviral disease, the nature of this association is unclear. We hypothesized that EBLNs could give rise to antiviral interfering RNA in the form of PIWI-interacting RNAs (piRNAs), a class of small RNA known to silence transposons but not exogenous viruses. We found that in both rodents and primates, which acquired their EBLNs independently some 25-40 million years ago, EBLNs are present within piRNA-generating regions of the genome far more often than expected by chance alone (ℙ = 8 × 10(-3)-6 × 10(-8)). Three of the seven human EBLNs fall within annotated piRNA clusters and two marmoset EBLNs give rise to bona fide piRNAs. In both rats and mice, at least two of the five EBLNs give rise to abundant piRNAs in the male gonad. While no EBLNs are syntenic between rodent and primate, some of the piRNA clusters containing EBLNs are; thus we deduce that EBLNs were integrated into existing piRNA clusters. All true piRNAs derived from EBLNs are antisense relative to the proposed ancient bornaviral nucleoprotein mRNA. These observations are consistent with a role for EBLN-derived piRNA-like RNAs in interfering with ancient bornaviral infection. They raise the hypothesis that retrotransposon-dependent virus-to-host gene flow could engender RNA-mediated, sequence-specific antiviral immune memory in metazoans analogous to the CRISPR/Cas system in prokaryotes.

Refrex-1, a soluble restriction factor against feline endogenous and exogenous retroviruses

The host defense against viral infection is acquired during the coevolution or symbiosis of the host and pathogen. Several cellular factors that restrict retroviral infection have been identified in the hosts. Feline leukemia virus (FeLV) is a gammaretrovirus that is classified into several receptor interference groups, including a novel FeLV-subgroup D (FeLV-D) that we recently identified. FeLV-D is generated by transduction of the env gene of feline endogenous gammaretrovirus of the domestic cat (ERV-DCs) into FeLV. Some ERV-DCs are replication competent viruses which are present and hereditary in cats. We report here the determination of new viral receptor interference groups and the discovery of a soluble antiretroviral factor, termed Refrex-1. Detailed analysis of FeLV-D strains and ERV-DCs showed two receptor interference groups that are distinct from other FeLV subgroups, and Refrex-1 specifically inhibited one of them. Refrex-1 is characterized as a truncated envelope protein of ERV-DC and includes the N-terminal region of surface unit, which is a putative receptor-binding domain, but lacks the transmembrane region. Refrex-1 is efficiently secreted from the cells and appears to cause receptor interference extracellularly. Two variants of Refrex-1 encoded by provirus loci, ERV-DC7 and DC16, are expressed in a broad range of feline tissues. The host retains Refrex-1 as an antiretroviral factor, which may potentially prevent reemergence of the ERVs and the emergence of novel ERV-related viruses in cats. Refrex-1 may have been acquired during endogenization of ERV-DCs and may play an important role in retroviral restriction and antiviral defense in cats.

The AKV murine leukemia virus is restricted and hypermutated by mouse APOBEC3

APOBEC3 proteins are potent restriction factors against retroviral infection in primates. This restriction is accompanied by hypermutations in the retroviral genome that are attributable to the cytidine deaminase activity of the APOBEC3 proteins. Studies of nucleotide sequence diversity among endogenous gammaretroviruses suggest that the evolution of endogenous retroelements could have been shaped by the mutagenic cytidine deaminase activity of APOBEC3. In mice, however, APOBEC3 appears to restrict exogenous murine retroviruses in the absence of detectable levels of deamination. AKV is an endogenous retrovirus that is involved in causing a high incidence of thymic lymphoma in AKR mice. A comparative analysis of several mouse strains revealed a relatively low level of APOBEC3 expression in AKR mice. Here we show that endogenous mouse APOBEC3 restricts AKV infection and that this restriction likely reflects polymorphisms affecting APOBEC3 abundance rather than differences in the APOBEC3 isoforms expressed. We also observe that restriction of AKV by APOBEC3 is accompanied by G-->A hypermutations in the viral genome. Our findings demonstrate that APOBEC3 acts as a restriction factor in rodents affecting the strain tropism of AKV, and they provide good support for the proposal that APOBEC3-mediated hypermutation contributed to the evolution of endogenous rodent retroviral genomes.

Host factors involved in resistance to retroviral infection

Viral replication requires the help of host cell factors, whose species specificity may affect viral tropism. On the other hand, there exist host factors that restrict viral replication. The anti-viral system mediated by some of these restriction factors, which is termed intrinsic immunity and is distinguished from conventional innate and adaptive immunity, has been described as playing an important role in making species-specific barriers against viral infection. Here, we describe the current progress in understanding of such restriction factors against retroviral replication, focusing on TRIM5alpha and APOBEC, whose anti-retroviral effects have recently been recognized. Additionally, we mention cyclophilin A, which is essential for HIV-1 replication in human cells and may affect viral tropism. Understanding of these host factors would contribute to identification of the determinants for viral tropism.

Spontaneous heteromerization of gammaretrovirus envelope proteins: a possible novel mechanism of retrovirus restriction

The env gene of gammaretroviruses encodes a glycoprotein conserved among diverse retroviruses, except for the domains involved in receptor binding. Here we show that pairs of gammaretrovirus envelope proteins (from Friend virus and GALV or xenotropic viruses) assemble into heteromers when coexpressed. This assembly results in a strong inhibition of infectivity. An unrelated envelope protein does not assemble in heteromers with the gammaretrovirus glycoproteins tested and does not affect their infectivity, demonstrating the specificity of the mechanism we describe. We propose that the numerous copies of endogenous retroviral env genes conserved within mammalian genomes act as restriction factors against infectious retroviruses.

Retroviral superinfection resistance

The retroviral phenomenon of superinfection resistance (SIR) defines an interference mechanism that is established after primary infection, preventing the infected cell from being superinfected by a similar type of virus. This review describes our present understanding of the underlying mechanisms of SIR established by three characteristic retroviruses: Murine Leukaemia Virus (MuLV), Foamy Virus (FV), and Human Immunodeficiency Virus (HIV). In addition, SIR is discussed with respect to HIV superinfection of humans. MuLV resistant mice exhibit two genetic resistance traits related to SIR. The cellular Fv4 gene expresses an Env related protein that establishes resistance against MuLV infection. Another mouse gene (Fv1) mediates MuLV resistance by expression of a sequence that is distantly related to Gag and that blocks the viral infection after the reverse transcription step. FVs induce two distinct mechanisms of superinfection resistance. First, expression of the Env protein results in SIR, probably by occupancy of the cellular receptors for FV entry. Second, an increase in the concentration of the viral Bet (Between-env-and-LTR-1-and-2) protein reduces proviral FV gene expression by inhibition of the transcriptional activator protein Tas (Transactivator of spumaviruses). In contrast to SIR in FV and MuLV infection, the underlying mechanism of SIR in HIV-infected cells is poorly understood. CD4 receptor down-modulation, a major characteristic of HIV-infected cells, has been proposed to be the main mechanism of SIR against HIV, but data have been contradictory. Several recent studies report the occurrence of HIV superinfection in humans; an event associated with the generation of recombinant HIV strains and possibly with increased disease progression. The role of SIR in protecting patients from HIV superinfection has not been studied so far. The phenomenon of SIR may also be important in the protection of primates that are vaccinated with live attenuated simian immunodeficiency virus (SIV) against pathogenic SIV variants. As primate models of SIV infection closely resemble HIV infection, a better knowledge of SIR-induced mechanisms could contribute to the development of an HIV vaccine or other antiviral strategies.

DNA-dependent protein kinase enhances DNA damage-induced apoptosis in association with Friend gp70

Friend leukemia virus (FLV) infection strongly enhances gamma-irradiation-induced apoptosis of hematopoietic cells of C3H hosts leading to a lethal anemia. Experiments using p53 knockout mice with the C3H background have clarified that the apoptosis is p53-dependent and would not be associated with changes of cell populations caused by the infection with FLV. In bone marrow cells of FLV + total body irradiation (TBI)-treated C3H mice, the p53 protein was prominently activated to overexpress p21 and bax suggesting that apoptosis-enhancing mechanisms lay upstream of p53 protein in the signaling pathway. Neither of DNA-dependent protein kinase (DNA-PK)-deficient SCID mice nor ataxia telangiectasia mutated (ATM) gene knockout mice with the C3H background exhibited a remarkable enhancement of apoptosis or p53 activation on FLV + TBI-treatment indicating that DNA-PK and ATM were both essential. ATM appeared necessary for introducing DNA damage-induced apoptosis, while DNA-PK enhanced p53-dependent apoptosis under FLV-infection. Surprisingly, viral envelope protein, gp70, was co-precipitated with DNA-PK but not with ATM in FLV + TBI-treated C3H mice. These results indicated that FLV-infection enhances DNA damage-induced apoptosis via p53 activation and that DNA-PK, in association with gp70, might play critical roles in modulating the signaling pathway.

Svi3: A provirus common integration site in c-myc in SL/Kh pre-B lymphomas

Spontaneous pre-B lymphomas in SL/Kh mice occur by somatic acquisition of a provirus genome of endogenous murine leukemia virus (MuLV). Inverse PCR amplification and sequence analyses of a provirus and its host flanking fragment revealed a proviral insertion into c-myc in 3 out of 60 SL/Kh pre-B lymphomas, named Svi3 lymphomas (SL/Kh virus integration site-3). Southern blot analysis revealed that two lymphomas had clonal integration in c-myc exon 1 and the other, in the promoter region. In 2 out of 3 Svi3 lymphomas, a fusion transcript of provirus 3' long terminal repeat and c-myc and a normal full-length c-myc transcript were obtained, but in one Svi3 lymphoma, only the normal transcript was obtained. All three Svi3 lymphomas had increased c-myc expression, producing normal 67-kDa c-Myc protein. Svi3 lymphomas had more mature phenotypes in the steps of early B-cell differentiation than Svi1 lymphomas, in which c-myc expression was indirectly up-regulated by provirus integration into Stat5a.

Restriction factors: a defense against retroviral infection

Susceptibility to retroviral infection is determined, in part, by host genes with antiviral activity. The Fv1 gene, which inhibits murine leukemia virus infection in mice, encodes one such resistance factor, and was long thought to be unique in that it restricts post-entry, pre-integration steps of retroviral replication. However, recent findings suggest the existence of similar restriction factors in primates, including humans. These factors, termed Lv1 and Ref1, can inhibit a range of retroviruses, including human immunodeficiency virus type 1 and its relatives. Fv1, Lv1 and Ref1 target capsid determinants to block infection but can be saturated by incoming virions. Primate- and murine-retrovirus restriction factors have diverse and overlapping specificities, and some variants of Lv1, as well as Ref1, apparently recognize and inhibit infection by widely divergent retroviruses.

Fv-4: identification of the defect in Env and the mechanism of resistance to ecotropic murine leukemia virus

Mice expressing the Fv-4 gene are resistant to infection by ecotropic murine leukemia viruses (MuLVs). The Fv-4 gene encodes an envelope (Env) protein whose putative receptor-binding domain resembles that of ecotropic MuLV Env protein. Resistance to ecotropic MuLVs appears to result from viral interference involving binding of the endogenously expressed Fv-4 env-encoded protein to the ecotropic receptor, although the immune system also plays a role in resistance. The Fv-4 env-encoded protein is processed normally and can be incorporated into virus particles but is unable to promote viral entry. Among the many sequence variations between the transmembrane (TM) subunit of the Fv-4 env-encoded protein and the TM subunits of other MuLV Env proteins, there is a substitution of an arginine residue in the Fv-4 env-encoded protein for a glycine residue (gly-491 in Moloney MuLV Env) that is otherwise conserved in all of the other MuLVs. This residue is present in the MuLV TM fusion peptide sequence. In this study, gly-491 of Moloney MuLV Env has been replaced with other residues and a mutant Env bearing a substitution for gly-487 was also created. G491R recapitulates the Fv-4 Env phenotype in cell culture, indicating that this substitution is sufficient for creation of an Env protein that can establish the interference-mediated resistance to ecotropic viruses produced by the Fv-4 gene. Analysis of the mutant MuLV Env proteins also has implications for an understanding of the role of conserved glycine residues in fusion peptides and for the engineering of organismal resistance to retroviruses.

Virological properties and nucleotide sequences of Cas-E-type endogenous ecotropic murine leukemia viruses in South Asian wild mice, Mus musculus castaneus

Two types of endogenous ecotropic murine leukemia viruses (MuLVs), termed AKV- and Cas-E-type MuLVs, differ in nucleotide sequence and distribution in wild mouse subspecies. In contrast to AKV-type MuLV, Cas-E-type MuLV is not carried by common laboratory mice. Wild mice of Mus musculus (M. m.) castaneus carry multiple copies of Cas-E-type endogenous MuLV, including the Fv-4(r) gene that is a truncated form of integrated MuLV and functions as a host's resistance gene against ecotropic MuLV infection. Our genetic cross experiments showed that only the Fv-4(r) gene was associated with resistance to ecotropic F-MuLV infection. Because the spontaneous expression of infectious virus was not detected in M. m. castaneus, we generated mice that did not carry the Fv-4(r) gene but did carry a single or a few endogenous MuLV loci. In mice not carrying the Fv-4(r) gene, infectious MuLVs were isolated in association with three of six Cas-E-type endogenous MuLV loci. The isolated viruses showed a weak syncytium-forming activity for XC cells, an interfering property of ecotropic MuLV, and a slight antigenic variation. Two genomic DNAs containing endogenous Cas-E-type MuLV were cloned and partially sequenced. All of the Cas-E-type endogenous MuLVs were closely related, hybrid-type viruses with an ecotropic env gene and a xenotropic long terminal repeat. Duplications and a deletion were found in a restricted region of the hypervariable proline-rich region of Env glycoprotein.

Resistance to Friend murine leukemia virus infection conferred by the Fv-4 gene is recessive but appears dominant from the effect of the immune system

Fv-4 is a mouse gene that dominantly confers resistance to infection with Friend murine leukemia virus (F-MuLV) (S. Suzuki, Jpn. J. Exp. Med. 45:473-478, 1975). However, the resistance caused by Fv-4 is recessive in nude mice, which suggests that immunological effects play important roles in this resistance in vivo (K. Higo, Y. Kubo, Y. Iwatani, T. Ono, M. Maeda, H. Hiai, T. Masuda, K. Kuribayashi, F. Zhang, T. Lamin, A. Adachi, and A. Ishimoto, J. Virol. 71:750-754, 1997). To determine the immunological effect on the resistance in vivo, we infected immunologically immature newborn mice homozygous (Fv-4(r/r)) and heterozygous (Fv-4(r/-)) for Fv-4. Although the Fv-4(r/r) mice showed complete resistance to F-MuLV whether infected neonatally or as adolescents, the Fv-4(r/-) mice showed high sensitivity to viral proliferation and disease induction when infected as newborns but complete resistance when infected as adolescent mice. To confirm the immunological effect on the resistance in adolescent mice with the Fv-4(r/r) and Fv-4(r/-) genotypes, we examined the effect of an immunosuppressant drug, FK506, on the resistance. The mice with the Fv-4(r/r) genotype treated with FK506 still showed resistance, but the mice with the Fv-4(r/-) genotype became highly sensitive to F-MuLV infection. Flow cytometric analysis to detect the Fv-4 gene product showed that the Fv-4 gene product was expressed on the cells from newborn and adolescent mice. The Fv-4 gene product was also detected on the cells from the FK506-treated mice as well as on those from untreated mice. However, a quantitative difference in the gene product between the cells with the Fv-4(r/r) and Fv-4(r/-) genotypes was detected by indirect staining for flow cytometry. These results show that the resistance to F-MuLV infection conferred by the Fv-4 gene is originally recessive, but it looks dominant in adolescent mice mainly because of the effect of the immune system.

Protection of retrovirus-induced disease by transplantation of bone marrow cells transduced with MuLV env gene via retrovirus vector

Fv-4 is a mouse gene that dominantly confers resistance to infection by ecotropic murine leukemia virus (MuLV). We have demonstrated previously that bone marrow chimeras in which hematopoietic cells were replaced with cells expressing Fv-4 resistant (Fv-4r) gene product became refractory to Friend leukemia virus (FLV)-induced leukemogenesis. To induce in vivo resistance against retrovirus-induced diseases by retroviral vector-mediated gene transduction, we introduced Fv-4 env gene into bone marrow cells of FLV-susceptible C3H/He (C3H) mice with retroviral vector (pLSF) derived from murine Friend spleen focus forming virus (SFFV) and the cells were transplanted into lethally irradiated C3H mice. After the bone marrow transplantation, Fv-4r gene product was successfully expressed on erythroid and myeloid cells, while lymphoid cells were only weakly expressing Fv-4r gene product. The C3H mice expressing relatively higher amounts of Fv-4r gene product were rendered resistant to FLV-induced erythroleukemia, while mice expressing lower amounts of the Fv-4r gene product were still susceptible. Effective protection of FLV-induced leukemia in these mice suggested that the Fv-4r gene expression by erythroid cells that were the major target of FLV infection might be critical for resisting FLV-induced leukemia. Thus, gene therapy model by transducing Fv-4r env gene using bone marrow transplantation would provide a useful protection model system of retrovirus-induced diseases.

Spontaneous priming for anti-viral envelope cytotoxic T lymphocytes in mice transgenic for a murine leukaemia virus envelope gene (Fv4)

Compared with non-transgenic controls, mice bearing an Fv4 murine retroviral env transgene resist infection and do not become immunosuppressed when inoculated with Friend virus (FV). When immunized with FV antigens in the absence of infectious virus, they make antibodies and cytotoxic lymphocytes (CTL) to FV comparably to non-transgenic controls. Unimmunized transgenic mice were found to have CTL precursors, which could be activated by in vitro stimulation, specific for viral (and self) envelope protein (Env). This "spontaneous priming' for antiviral CTL is surprising because the transgene Env is present on the surface of thymocytes and in serum from before birth. Our experiments demonstrate that T cells reactive with self-thymic and serum antigens sometimes avoid clonal elimination or inactivation.

Susceptibility of nude mice carrying the Fv-4 gene to Friend murine leukemia virus infection

Fv-4 is a mouse gene that dominantly confers resistance to infection with Friend murine leukemia virus (F-MuLV) (S. Suzuki, Jpn. J. Exp. Med. 45:473-478, 1975). Despite complete resistance to ecotropic MuLV infection in mice carrying the Fv-4 gene, it is known that cells carrying the resistance gene in tissue culture do not always show resistance as extensive as that in vivo (H. Yoshikura and T. Odaka, JNCI 61:461-463, 1978). To investigate the immunological effect on resistance in vivo, we introduced the Fv-4 gene into BALB/c nude mice (Fv-4-/- nude[nu/nu]) by mating them with Fv-4 congenic BALB/c mice (Fv-4r/r nude+/+) and examined the susceptibility of the F2 progeny to F-MuLV. All BALB/c nude mice without the Fv-4 gene (Fv-4-/- nude[nu/nu]) were permissive to F-MuLV and developed erythroleukemia within 2 weeks after virus inoculation. The BALB/c nude mice with the Fv-4 gene (Fv-4r/r nude[nu/nu]) did not develop leukemia, and no or little virus was detected in the spleen 7 weeks after virus inoculation. The resistance to F-MuLV was dominant in (Fv-4 congenic BALB/c x BALB/c nude) F1 mice with the Fv-4r/- nude(nu/+) genotype as strictly as in (Fv-4 congenic BALB/c x BALB/c) F1 mice with the Fv-4r/- nude+/+ genotype. However, almost all BALB/c nude mice with the Fv-4r/- nude(nu/nu) genotype developed the disease within 7 weeks, and the virus was detected in all of their spleens even in the mice without leukemia. These results show that the resistance caused by the Fv-4 gene is recessive in nude mice and dominant in BALB/c mice. Some immunological effects, perhaps cell-mediated immunity, may play important roles in the resistance to F-MuLV infection in vivo in addition to the dosage effect of the Fv-4 product.

Establishment of a therapeutic model for retroviral infection using the genetic resistance mechanism of the host

Resistance to retroviral infection is often regulated by multiple genes that control different aspects of the host-virus interaction. Genetically distinct inbred strains of mice differ in their susceptibility to retrovirus and have allowed the identification of several host-resistant loci that regulate the host defense mechanism to retroviral infection. Using the murine retrovirus infection system, a therapeutic model has been developed of retrovirus infection in association with the resistant mechanism of host genes. The most effective result achieved with the model was when using bone marrow transplantation of retrovirus-resistant cells with receptor interference function, which was genetically defined by the Fv-4 resistant gene. The possible application of these findings to the gene therapy of retrovirus-induced disease of humans is discussed.

Murine leukemia virus envelope protein in transgenic-mouse serum blocks infection in vitro

Transgenic mice bearing a murine retroviral envelope transgene (Fv4) have Fv4 gp70env (SU) in their serum in amounts sufficient to block infection by ecotropic virus in vitro. Fv4 Env in serum is derived largely but not exclusively from hematopoietic cells. Tail cells from Fv4 mice and cell lines transduced with the Fv4 env transgene synthesize both components of the envelope protein (gp70 SU and p15E TM) but secrete the gp70 moiety, in the absence of retroviral particles. Blocking of the ecotropic viral receptor by secreted gp70 SU may contribute to resistance to retroviral infection in these mice.

Efficiency of in vivo gene transfer using murine retroviral vectors is strain-dependent in mice

Retroviral vectors can be used to transduce cultured cells at high frequencies, but efficient transduction of target cells in vivo has proved difficult and little is known about the factors that influence the efficiency of retroviral infection. Many commonly used mouse strains harbor endogenous C-type proviruses, some of which are expressed and have circulating antibodies against the viral envelope glycoproteins that cross-react with the Moloney strain of murine leukemia virus (MoMLV), from which most current retroviral vectors are derived. We have investigated the relative efficiency of retroviral-mediated gene transfer into regenerating skeletal muscle of a variety of mouse strains using a MoMLV-based vector. Humoral immune competence and interference between endogenous MLVs and exogenous recombinant MoMLV were observed to affect the efficiency of retroviral-mediated transfection in vivo. Our results indicate that the mouse genetic background and immune status need to be considered when choosing a preclinical model for in vivo retroviral-mediated gene transfer.

Murine retroviral vector that induces long-term expression of HIV-1 envelope protein

A retroviral vector was constructed that induces long-term expression of human immunodeficiency virus type 1 (HIV-1) rev, vpu and env genes. The vector contains the neo gene and a cytomegalovirus (CMV) immediate early promoter followed by HIV-1 sequence. When HeLa cells were infected with viral stocks derived from this vector, about 25% of the resulting G418-resistant clones expressed HIV-1 envelope protein (Env), easily detectable by Western blot analysis, metabolic labelling, and syncytium formation after co-cultivation with HeLa-CD4 cells. In most cases the level of Env expression was higher than in a T cell line (H9) chronically infected with HIV-1. Env-expressing HeLa cell lines also expressed Rev, detected by transfection with a Rev-dependent CAT gene construct, and Vpu, detected by immunoprecipitation with a Vpu-specific antiserum. The 75% of G418-resistant HeLa cell lines that did not express Env were found to contain proviruses that had undergone deletion of env sequences corresponding to a known intron; presumably these cell lines arose as a result of infection with virions derived from spliced RNAs. This vector should be useful for studying non-transient effects of HIV Env, Rev and Vpu in tissue culture, and for the production of Env- and/or Rev-expressing cell lines.

Protection against retroviral diseases after vaccination is conferred by interference to superinfection with attenuated murine leukemia viruses

Cell cultures expressing a retroviral envelope are relatively resistant to superinfection by retroviruses which bear envelopes using the same receptor. We tested whether this phenomenon, known as interference to superinfection, might confer protection against retroviral diseases. Newborn mice first inoculated with the attenuated strain B3 of Friend murine leukemia virus (F-MuLV) were protected against severe early hemolytic anemia and nonacute anemiant erythroleukemia induced by the virulent strain 57 of F-MuLV. Vaccinated animals were also protected as adults against acute polycythemic erythroleukemia induced upon inoculation with the viral complex containing the defective spleen focus-forming virus and F-MuLV 57 as helper virus. Animals were inoculated as newborns, which is known to induce immune tolerance in mice, and the rapid kinetics of protection, incompatible with the delay necessary for the immune response to develop, indicated that protection was not due to an immune mechanism but rather was due to the rapid and long-lasting phenomenon of interference. This result was confirmed by combining parental and envelope chimeric MuLV from different interference groups as vaccinal and challenge viruses. Although efficient protection could be provided by vaccination by interference, we observed that attenuated replication-competent retroviruses from heterologous interference groups might exert deleterious synergistic effects.

Transgenic Fv-4 mice resistant to Friend virus

Fv-4 is a mouse gene that confers resistance to infection with ecotropic retroviruses. A candidate Fv-4 gene was cloned previously and found to resemble the 3' half of a murine leukemia virus (MuLV). To study the effect of this gene in vivo, we generated two transgenic mouse strains carrying the Fv-4 env gene under control of its presumed natural promoter, a cellular sequence unrelated to retroviruses. Transgenic progeny expressed a 3-kb Fv-4 env RNA in all of the organs and tissues examined, as well as an Fv-4 envelope antigen on the surface of thymocytes and spleen cells, similar to mice carrying the natural Fv-4 gene. One of the two transgenic strains (designated Fv4-2) expressed three to nine times as much transgene RNA and protein as the other strain (Fv4-11). When challenged with a Friend virus complex containing up to 10(4) XC PFU of Friend MuLV, Fv4-2 mice were completely resistant to development of splenomegaly and had no detectable ecotropic virus in the spleen or blood, confirming that the cloned Fv-4 gene is responsible for resistance to ecotropic MuLV in vivo. In contrast, Fv4-11 mice were only partially resistant, developing viremia and splenomegaly at the highest inoculum dose but recovering from viremia several weeks after inoculation with 10-fold less virus. The phenotype of recovery from viremia in Fv4-11 mice was unexpected and suggests that low levels of expression of the Fv-4 gene enhance the effectiveness of the immune response.

Genetic control of retroviral disease in aging wild mice

Different populations of wild mice (Mus musculus domesticus) in Los Angeles and Ventura Counties were observed over their lifespan in captivity for expression of infectious murine leukemia virus (MuLV) and murine mammary tumor virus (MMTV) and for the occurrence of cancer and other diseases. In most populations of feral mice these indigenous retroviruses were infrequently expressed and cancer seldom occurred until later in life (> 2 years old). MMTV was found in the milk of about 50% of wild mice, but was associated with only a low incidence (> 1%) of breast cancer after one year of age. By contrast, in several populations, most notably at a squab farm near Lake Casitas (LC), infectious MuLV acquired at birth via milk was highly prevalent, and the infected mice were prone to leukemia and a lower motor neuron paralytic disease after one year of age. These two diseases were both caused by the same infectious (ecotropic) strain of MuLV and were the principal cause of premature death in these aging LC mice. A dominant gene called FV-4R restricting the infection with ecotropic MuLV was found segregating in LC mice. Mice inheriting this FV-4R allele were resistant to the ecotropic MuLV associated lymphoma and paralysis. The FV-4R allele represents a defective endogenous MuLV provirus DNA segment that expresses an ecotropic MuLV envelope-related glycoprotein (gp70) on the cell surface. This FV-4R encoded gp70 presumably occupies the receptor for ecotropic MuLV and blocks entry of the virus. The FV-4R gene was probably acquired by the naturally occurring crossbreeding of LC feral mice with another species of feral mice (Mus castaneus) from Southeast Asia. The FV-4R gp70 does not block entry of the amphotropic MuLV that uses a separate cell surface receptor. Therefore LC mice continued to be susceptible to the highly prevalent but weakly lymphogenic and nonparalytogenic amphotropic strain of MuLV. The study points out the potential of feral populations to reveal genes associated with specific disease resistance.

Analysis of Moloney murine leukemia virus revertants mutated at the gag-pol junction

Among Moloney murine leukemia viruses (Mo-MuLVs) having stop codons other than UAG at the gag-pol junction, Mo-MuLV with UAA, but not with UGA, had a replication disadvantage. Mo-MuLV with a glutamine codon (CAG) at the junction did not replicate. A revertant of this virus consisted of the original virus and a virus with a deletion of the pol region. Protease and Pr65gag encoded by their respective genomes complemented each other.

Construction and characterization of the recombinant Moloney murine leukemia viruses bearing the mouse Fv-4 env gene

A nucleotide sequence of the mouse Fv-4 env gene was completed. Structural comparison revealed a close relationship of Fv-4 to the ecotropic Cas-Br-E murine leukemia virus isolated from a wild mouse in southern California. Various portions of the env gene of Moloney murine leukemia virus were replaced by the corresponding Fv-4 env sequence to construct recombinant murine leukemia virus clones. Infectivity of these recombinants was checked by the S+L- cell focus induction assay and the XC cell syncytium formation assay. Recombinants bearing the following Fv-4 env sequence retained ecotropic infectivity; the AccI-BamHI and BamHI-BalI regions coding for the N- and C-terminal halves of Fv-4 gp70SU, respectively; and the BalI-NcoI region encoding the cleavage site between gp70SU and p15(E)TM of the Fv-4 env. However, when the Fv-4 sequence was substituted for the p15(E)TM-coding NcoI-EcoRV region or the AccI-EcoRV region covering almost the entire env gene, infectivity was undetectable in our assays. The recombinant clone containing the Fv-4 AccI-EcoRV region, i.e., almost the entire Fv-4 env sequence, was introduced with pSV2neo into NIH 3T3 cells, and a G418r cell line named NIH(Fv4)-2 was isolated. The NIH(Fv4)-2 cell released viral particles that contained reverse transcriptase, Fv-4 env molecules as well as the other viral proteins, and viral genomic RNA. However, proviral DNA synthesis was not detected upon inoculation of this virus in NIH 3T3 cells. The loss of infectivity of the recombinant virus bearing the Fv-4 AccI-EcoRV region appeared to be caused by failure in an early step of replication.

Fv-4 resistance gene: a truncated endogenous murine leukemia virus with ecotropic interference properties

Fv-4 is a mouse gene which controls susceptibility to infection by ecotropic murine leukemia virus (MuLV). We previously cloned part of an endogenous MuLV associated with the resistance allele of the Fv-4 gene (Fv-4r). In this report, we describe an extended clone of the Fv-4r allele consisting of a 17-kilobase DNA fragment containing the retroviral sequence and its 5'-flanking sequence. The new DNA clone contains a truncated MuLV with delta pol-env-long terminal repeat sequences but no other MuLV-reactive sequence within 13 kilobases upstream of the truncated MuLV. Transfection of this clone into mouse cells led to transcription of Fv-4 env mRNA, expression of the Fv-4r-specific MuLV envelope protein, and resistance to infection with ecotropic MuLV but not amphotropic and dualtropic MuLVs. Restriction of ecotropic viruses appears to occur at or before viral cDNA synthesis. This result is consistent with a model of receptor interference for Fv-4 restriction. Our data also suggest that the 5' non-MuLV sequence is important for biological function, since a DNA clone which lacks most of the 5'-flanking sequence did not efficiently confer the resistance phenotype.

Reduced yield of infectious pseudorabies virus and herpes simplex virus from cell lines producing viral glycoprotein gp50

Pseudorabies virus (PRV) glycoprotein gp50 is the homolog of herpes simplex virus (HSV) glycoprotein D. Several cell lines that constitutively synthesize gp50 were constructed. Vero cells, HeLa cells, and pig kidney (MVPK) cells that produce gp50 all gave reduced yields of PRV and HSV progeny viruses when compared with the parent cell line or the same cell line transfected to produce a different protein. The reduction in virus yield was greatest at low multiplicities of infection. The Vero and HeLa cells that produce gp50 showed an even greater reduction in HSV yield than in PRV yield. This phenomenon may be an example in a herpesvirus of the interference observed in retroviruses or cross-protection in plant virus systems.

The endogenous mink cell focus-forming (MCF) gp70 linked to the Rmcf gene restricts MCF virus replication in vivo and provides partial resistance to erythroleukemia induced by Friend murine leukemia virus

The Rmcf locus restricts the in vitro replication of mink cell focus-forming (MCF) viruses in cell cultures derived from mice carrying the resistance allele. Previously we reported that in cell cultures from first backcross progeny, this Rmcf-linked restriction segregates with the expression of an endogenous retroviral gp70 serologically related to that of MCF viruses. The current report details the results of genetic studies designed to examine the possible association of this endogenous gp70 with resistance of mice to Friend murine leukemia virus (F-MuLV)-induced erythroleukemia. This env gene segregates as a single dominant trait in (DBA/2 X IRW) X IRW progeny, in which the expression of the gene can be detected by serological techniques. Results indicated that the gp70- progeny developed leukemia at the same rate as the susceptible IRW parent, whereas the tempo of disease among the gp70+ progeny was significantly slower. However, the resistance mediated by this gene was only partial, since most of the gp70+ offspring eventually developed erythroleukemia when followed for 6 mo. This endogenous gp70 also segregated with a restriction to the expression of recombinant MCF viruses after infection with F-MuLV. Since in this study all unlinked genes segregated independently, this is direct evidence that MCF viruses participate in the induction of erythroleukemia.

Identification of two forms of an endogenous murine retroviral env gene linked to the Rmcf locus

The Rmcf gene restricts the replication of recombinant murine mink cell focus-inducing (MCF) viruses in cell cultures derived from mice carrying the resistance allele (Rmcfr) and may play a role in resistance to retrovirus-induced leukemias in vivo. We have characterized the endogenous gp70 expressed by Rmcfr and Rmcfs mice with a panel of type-specific monoclonal antibodies which discriminate xenotropic and MCF gp70. Embryo and tail skin cultures derived from Rmcfr mice (DBA/2 and CBA/N) expressed gp70 bearing a determinant unique to MCF viruses, whereas cultures from Rmcfs mice expressed either no detectable gp70 (NFS/N and IRW) or a gp70 serologically related to a subgroup of xenotropic viruses (C57BL/6, CBA/J, and A/WySn). Studies of progeny embryos derived from a (C57BL/6 X DBA/2) X C57BL/6 backcross established that the Rmcf resistance allele was linked to the expression of the MCF gp70 and that the gene encoding the xenotropic gp70 expressed by C57BL/6 Rmcfs mice was allelic with the MCF gp70 from Rmcfr mice. These data indicate that the Rmcf locus contains an endogenous gp70 gene having two allelic forms, one of which inhibits exogenous MCF infection in vitro by a mechanism of viral interference.

Relationship between the cellular resistance to Friend murine leukemia virus infection and the expression of murine leukemia virus-gp70-related glycoprotein on cell surface of BALB/c-Fv-4wr mice

Fv-4r is a dominant resistance gene which controls resistance of mice to exogenous infection with ecotropic murine leukemia viruses. Cell lines with various degrees of resistance to ecotropic Friend murine leukemia virus infection were established from BALB/c-Fv-4wr mice which are partially congenic with BALB/c. The degree of resistance of these cell lines correlated well to the amount of glycoprotein with mol wt 80 K on cell surface. The resistance of cells was reduced by the treatment of glycosylation inhibitors, tunicamycin and 2-deoxy-D-glucose. The results indicate that the Fv-4 resistance is ascribed to the unique glycoprotein on cell surface.

The pathophysiology of murine retrovirus-induced leukemias

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

Susceptibility of BALB/c mice carrying various DBA/2 genes to development of Friend murine leukemia virus-induced erythroleukemia

Using a series of BALB/c mice congenic for various DBA/2 genes, we were able to establish that DBA/2 mice carry a gene on chromosome 5, at or near the Rmcfr locus, that plays a major role in resistance to early erythroleukemia induced by injection of Friend murine leukemia virus (F-MuLV) into newborn mice. The fact that this gene controls the replication of mink cell focus-inducing (MCF) viruses strengthens the case for these viruses playing a crucial role in the development of erythroleukemia, since failure to replicate MCF viruses results in resistance to early erythroleukemia. The expression of the Rmcfr gene is correlated with the constitutive expression of an MCF virus-related envelope glycoprotein that apparently blocks the receptor for MCF viruses, preventing their spread. Thus, the Rmcfr gene is either a structural gene for this unique protein, which can block the receptor for MCF viruses, or is a regulatory gene that controls expression of such a structural gene. Although the Rmcfr gene is clearly involved in resistance to the early erythroleukemia induced by F-MuLV, it appears to have no effect on the late myeloid, lymphoid or erythroid diseases that appear in DBA/2 and other strains of mice after injection of F-MuLV, consistent with data indicating that replication of MCF viruses is not required for the development of these late diseases. Our studies with congenic and backcross mice also indicate that, in addition to the Rmcfr gene, other genes of DBA/2 origin may contribute to resistance to F-MuLV-induced early erythroleukemia by mechanisms other than blocking the replication of MCF viruses.

Susceptibility of wild mouse cells to exogenous infection with xenotropic leukemia viruses: control by a single dominant locus on chromosome 1

Although xenotropic murine leukemia viruses cannot productively infect cells of laboratory mice, cells from various wild-derived mice can support replication of these viruses. Although the virus-sensitive wild mice generally lack all or most of the xenotropic proviral genes characteristic of inbred strains, susceptibility to exogenous infection is unrelated to inheritance of these sequences. Instead, susceptibility is controlled by a single dominant gene, designated Sxv, which maps to chromosome 1. Sxv is closely linked to, but distinct from Bxv-1, the major locus for induction of xenotropic murine leukemia viruses in laboratory mice. Genetic experiments designed to characterize Sxv show that this gene also controls sensitivity to a wild mouse virus with the interference properties of mink cell focus-forming murine leukemia viruses, and that Sxv-mediated susceptibility to xenotropic murine leukemia viruses is restricted by the mink cell focus-forming virus resistance gene Rmcf. These data, together with genetic mapping of the mink cell focus-forming virus cell surface receptor locus to this same region of chromosome 1, suggest that Sxv may encode a wild mouse variant of the mink cell focus-forming virus receptor that allows penetration by xenotropic murine leukemia viruses.

Characterization of a molecularly cloned retroviral sequence associated with Fv-4 resistance

The murine leukemia virus (MuLV) sequence associated with the resistance allele of the Fv-4 gene (Fv-4r) was molecularly cloned from genomic DNA of uninfected mice carrying this allele. The 5.2-kilobase cloned EcoRI DNA fragment (pFv4) was shown by nucleotide sequencing to contain 3.4 kilobases of a colinear MuLV-related proviral sequence which began in the C-terminal end of the pol region and extended through the env region and the 3' long terminal repeat. Cellular sequences flanked the 3' as well as the 5' ends of the truncated MuLV sequence. Alignment of the N-terminal half of the pFv4 env sequence with ecotropic, mink cell focus-forming, and xenotropic MuLV env sequences established the relatedness of pFv4 and ecotropic MuLV env sequences. A subcloned 700-base pair segment (pFv4env) from the 5' env region of pFv4 was used as an Fv-4-specific probe; it hybridized specifically to the Fv-4r-associated proviral sequence but not to endogenous ecotropic MuLV proviral DNA under high stringency. All Fv-4-resistant mice contained the same retroviral segment associated with the same flanking cellular DNA. Expression of Fv-4r-specific mRNA was demonstrated in the spleens of Fv-4r mice but not Fv-4s mice, supporting the previously proposed resistance model based on interference.

Analysis of wild-derived mice for Fv-1 and Fv-2 murine leukemia virus restriction loci: a novel wild mouse Fv-1 allele responsible for lack of host range restriction

Wild-derived mice originally obtained from Asia, Africa, North America, and Europe were typed for in vitro sensitivity to ecotropic murine leukemia viruses and for susceptibility to Friend virus-induced disease. Cell cultures established from some wild mouse populations were generally less sensitive to exogenous virus than were cell cultures from laboratory mice. Wild mice also differed from inbred strains in their in vitro sensitivity to the host range subgroups defined by restriction at the Fv-1 locus. None of the wild mice showed the Fv-1n or Fv-1b restriction patterns characteristic of most inbred strains, several mice resembled the few inbred strains carrying Fv-1nr, and most differed from laboratory mice in that they did not restrict either N- or B-tropic murine leukemia viruses. Analysis of genetic crosses of Mus spretus and Mus musculus praetextus demonstrated that the nonrestrictive phenotype is controlled by a novel allele at the Fv-1 locus, designated Fv-10. The wild mice were also tested for sensitivity to Friend virus complex-induced erythroblastosis to type for Fv-2. Only M. spretus was resistant to virus-induced splenomegaly and did not restrict replication of Friend virus helper murine leukemia virus. Genetic studies confirmed that this mouse carries the resistance allele at Fv-2.

Identification of a unique erythroleukemia-associated retroviral gp70 expressed during early stages of normal erythroid differentiation

Late in the course of Friend virus (FV)-induced erythroleukemia, leukemic spleen cells express a cell surface retroviral gp70 envelope protein not detected during the early proliferative phase of the disease. Characterization of this gp70 revealed it was unrelated to the input Friend murine leukemia virus (F-MuLV), but antigenically similar to a unique subset of endogenous xenotropic viruses. This gp70 was expressed by murine erythroleukemia cell lines but has not been identified on cell lines of other lineages. A monoclonal antibody (18-6) specifically reactive with this polypeptide was used to examine hematopoietic organs of normal uninoculated mice. This antibody detected a gp70 expressed by a majority of erythroid cells in fetal liver and by a small but significant percentage of normal adult spleen and bone marrow cells. Increased erythropoietic activity induced by treatment of adult mice with phenylhydrazine ( PHZ ) resulted in a seven- to eightfold increase in the frequency of spleen and bone marrow cells expressing this gp70. Peptide map analysis indicated that the 18-6 reactive gp70 expressed by Friend erythroleukemia cells and by cells from normal fetal liver were structurally identical. These results suggested that this unique gp70 was an erythroid-specific differentiation antigen.

A cell membrane "gp70" associated with Fv-4 gene: immunological characterization, and tissue and strain distribution

Fv-4r is a dominant resistance gene which controls susceptibility of mice to exogenous infection with ecotropic murine leukemia virus (MuLV). A MuLV env-gene product related to gp70 on cells from uninfected Fv-4r mice has been found. Expression of this molecule is closely linked to the resistance allele. Antigenic determinants on the Fv-4r gp70 are similar to many ecotropic and some dualtropic MuLV gp70s but different from xenotropic and amphotropic MuLV gp70s. The Fv-4r gp70 molecule also has unique antigenic determinants since it is recognized by Fv-4 specific antisera which do not recognize other cell membrane gp70s or viral gp70s. The Fv-4-specific cell membrane gp70 is detectable on cells from thymus, spleen, lymph node, bone marrow, and embryo fibroblasts from Fv-4r mice, but not from Fv-4s mice. Slight differences in electrophoretic mobility of Fv-4r gp70 are found on different tissues. This unique cell membrane gp70 may be causally related to Fv-4 resistance.

A unique sequence related to the ecotropic murine leukemia virus is associated with the Fv-4 resistance gene

Several strains of laboratory and wild-derived mice from Japan carry the dominant allele at the Fv-4 locus (Fv-4r) that is responsible for resistance to infection by exogenous ecotropic murine leukemia virus. We have used blot hybridization with a probe specific for the ecotropic viral envelope to show that a unique envelope-reactive sequence is present in the Japanese mouse Mus musculus molossinus and in four independently derived partially congeneic strains carrying Fv-4r. Analysis of 31 backcross mice shows complete concordance between inheritance of this fragment and resistance to Friend virus complex-induced erythroblastosis. Inheritance of this sequence also suppresses spontaneous expression of the endogenous ecotropic viruses carried by M. m. molossinus. Restriction enzyme analysis shows that the Fv-4r-associated sequence is different from the full-length ecotropic proviruses of laboratory mice. Infectious virus cannot be induced from mice carrying only the Fv-4r-associated sequence. Examination of other wild-derived mice resistant to Friend virus complex shows that Mus cervicolor cervicolor also contains the Fv-4r sequence. Our data indicate that a unique or incomplete provirus containing ecotropic envelope-related sequences is responsible for Fv-4-mediated resistance to both exogenous and endogenous ecotropic virus in various Asian mice.