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

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.

Distribution of endogenous gammaretroviruses and variants of the Fv1 restriction gene in individual mouse strains and strain subgroups

Inbred laboratory mouse strains carry endogenous retroviruses (ERVs) classed as ecotropic, xenotropic or polytropic mouse leukemia viruses (E-, X- or P-MLVs). Some of these MLV ERVs produce infectious virus and/or contribute to the generation of intersubgroup recombinants. Analyses of selected mouse strains have linked the appearance of MLVs and virus-induced disease to the strain complement of MLV E-ERVs and to host genes that restrict MLVs, particularly Fv1. Here we screened inbred strain DNAs and genome assemblies to describe the distribution patterns of 45 MLV ERVs and Fv1 alleles in 58 classical inbred strains grouped in two ways: by common ancestry to describe ERV inheritance patterns, and by incidence of MLV-associated lymphomagenesis. Each strain carries a unique set of ERVs, and individual ERVs are present in 5–96% of the strains, often showing lineage-specific distributions. Two ERVs are alternatively present as full-length proviruses or solo long terminal repeats. High disease incidence strains carry the permissive Fv1ⁿ allele, tested strains have highly expressed E-ERVs and most have the Bxv1 X-ERV; these three features are not present together in any low-moderate disease strain. The P-ERVs previously implicated in P-MLV generation are not preferentially found in high leukemia strains, but the three Fv1 alleles that restrict inbred strain E-MLVs are found only in low-moderate leukemia strains. This dataset helps define the genetic basis of strain differences in spontaneous lymphomagenesis, describes the distribution of MLV ERVs in strains with shared ancestry, and should help annotate sequenced strain genomes for these insertionally polymorphic and functionally important proviruses.

Ancient Evolutionary Origin and Positive Selection of the Retroviral Restriction Factor Fv1 in Muroid Rodents

The laboratory mouse Fv1 gene encodes a retroviral restriction factor that mediates resistance to murine leukemia viruses (MLVs). Sequence similarity between Fv1 and the gag protein of the murine endogenous retrovirus L (MuERV-L) family of ERVs suggests that Fv1 was coopted from an ancient provirus. Previous evolutionary studies found Fv1 orthologs only in the genus Mus Here, we describe identification of orthologous Fv1 sequences in several species belonging to multiple families of rodents outside the genus Mus We show that these Fv1 orthologs are in the same region of conserved synteny, between the genes Miip and Mfn2, suggesting a minimum insertion time of 45 million years for the ancient progenitor of Fv1 Our analysis also revealed that Fv1 was not detectable or heavily mutated in some lineages in the superfamily Muroidea, while, in concert with previous findings in the genus Mus, we found strong evidence of positive selection of Fv1 in the African clade in the subfamily Muridae Residues identified as evolving under positive selection include those that have been previously found to be important for restriction of multiple retroviral lineages. Taken together, these findings suggest that the evolutionary origin of Fv1 substantially predates Mus evolution, that the rodent Fv1 has been shaped by lineage-specific differential selection pressures, and that Fv1 has long been evolving under positive selection in the rodent family Muridae, supporting a defensive role that significantly antedates exposure to MLVs.IMPORTANCE Retroviruses have adapted to living in concert with their hosts throughout vertebrate evolution. Over the years, the study of these relationships revealed the presence of host proteins called restriction factors that inhibit retroviral replication in host cells. The first of these restriction factors to be identified, encoded by the Fv1 gene found in mice, was thought to have originated in the genus Mus In this study, we utilized genome database searches and DNA sequencing to identify Fv1 copies in multiple rodent lineages. Our findings suggest a minimum time of insertion into the genome of rodents of 45 million years for the ancestral progenitor of Fv1 While Fv1 is not detectable in some lineages, we also identified full-length orthologs showing signatures of a molecular "arms race" in a family of rodent species indigenous to Africa. This finding suggests that Fv1 in these species has been coevolving with unidentified retroviruses for millions of years.

Infectivity and insertional mutagenesis of endogenous retrovirus in autoimmune NZB and B/W mice

Murine leukaemia virus has been suggested to contribute to both autoimmune disease and leukaemia in the NZB mouse and in the (NZB × NZW) F1 (abbreviated B/W) mouse. However, with apparently only xenotropic but no ecotropic virus constitutively expressed in these mice, few mechanisms could explain the aetiology of either disease in either mouse strain. Because pseudotyped and/or inducible ecotropic virus may play a role, we surveyed the ability of murine leukaemia virus in NZB, NZW and B/W mice to infect and form a provirus. From the spleen of NZB mice, we isolated circular cDNA of xenotropic and polytropic virus, which indicates ongoing infection by these viruses. From a B/W lymphoma, we isolated and determined the complete sequence of a putative ecotropic NZW virus. From B/W mice, we recovered de novo endogenous retroviral integration sites (tags) from the hyperproliferating cells of the spleen and the peritoneum. The tagged genes seemed to be selected to aid cellular proliferation, as several of them are known cancer genes. The insertions are consistent with the idea that endogenous retrovirus contributes to B-cell hyperproliferation and progression to lymphoma in B/W mice.

Postentry restriction of Mason-Pfizer monkey virus in mouse cells

Mason-Pfizer monkey virus (M-PMV) is a prototypical betaretrovirus responsible for simian AIDS (SAIDS) in rhesus macaques. It has been shown previously that mouse cells are resistant to infection by HIV-1 and other primate lentiviruses. However, the susceptibility of mouse cells to primate retroviruses such as M-PMV remains unexplored. In the present study, using single-round green fluorescent protein (GFP) reporter viruses, we showed that various mouse cell lines are unable to support the early stages of M-PMV replication. The block to infection occurs postentry and is independent of the viral envelope. Using quantitative real-time PCR, we showed that the block to infection occurs after reverse transcription but before formation of circular DNA or proviral DNA. Finally, we showed that the M-PMV block in mouse cells is not attributable to the previously characterized mouse restriction factor Fv1. Overall, these findings suggest that mouse cells exhibit a previously uncharacterized block to M-PMV infection.

IMPORTANCE

Here we document a novel postentry restriction to M-PMV infection in mouse cells. The block occurs after reverse transcription but before the formation of circular or proviral DNA and is independent of the previous characterized mouse restriction factor Fv1.

Positive selection and multiple losses of the LINE-1-derived L1TD1 gene in mammals suggest a dual role in genome defense and pluripotency

Mammalian genomes comprise many active and fossilized retroelements. The obligate requirement for retroelement integration affords host genomes an opportunity to 'domesticate' retroelement genes for their own purpose, leading to important innovations in genome defense and placentation. While many such exaptations involve retroviruses, the L1TD1 gene is the only known domesticated gene whose protein-coding sequence is almost entirely derived from a LINE-1 (L1) retroelement. Human L1TD1 has been shown to play an important role in pluripotency maintenance. To investigate how this role was acquired, we traced the origin and evolution of L1TD1. We find that L1TD1 originated in the common ancestor of eutherian mammals, but was lost or pseudogenized multiple times during mammalian evolution. We also find that L1TD1 has evolved under positive selection during primate and mouse evolution, and that one prosimian L1TD1 has 'replenished' itself with a more recent L1 ORF1 from the prosimian genome. These data suggest that L1TD1 has been recurrently selected for functional novelty, perhaps for a role in genome defense. L1TD1 loss is associated with L1 extinction in several megabat lineages, but not in sigmodontine rodents. We hypothesize that L1TD1 could have originally evolved for genome defense against L1 elements. Later, L1TD1 may have become incorporated into pluripotency maintenance in some lineages. Our study highlights the role of retroelement gene domestication in fundamental aspects of mammalian biology, and that such domesticated genes can adopt different functions in different lineages.

Use of IRF-3 and/or IRF-7 knockout mice to study viral pathogenesis: lessons from a murine retrovirus-induced AIDS model

Interferon regulatory factor (IRF) regulation of the type I interferon response has not been extensively explored in murine retroviral infections. IRF-3(-/-) and select IRF-3/7(-/-) mice were resistant to LP-BM5-induced pathogenesis. However, further analyses strongly suggested that resistance could be attributed to strain 129-specific contamination of the known retrovirus resistance gene Fv1. Therefore, caution should be taken when interpreting phenotypes observed in these knockout mice, as strain 129-derived genetic polymorphisms may explain observed differences.

Role of Human TRIM5α in Intrinsic Immunity

Human immunodeficiency virus (HIV) has a very narrow host range. HIV type 1 (HIV-1) does not infect Old World monkeys, such as the rhesus monkey (Rh). Rh TRIM5α was identified as a factor that confers resistance, intrinsic immunity, to HIV-1 infection. Unfortunately, human TRIM5α is almost powerless to restrict HIV-1. However, human TRIM5α potently restricts N-tropic murine leukemia viruses (MLV) but not B-tropic MLV, indicating that human TRIM5α represents the restriction factor previously designated as Ref1. African green monkey TRIM5α represents another restriction factor previously designated as Lv1, which restricts both HIV-1 and simian immunodeficiency virus isolated from macaque (SIVmac) infection. TRIM5 is a member of the tripartite motif family containing RING, B-box2, and coiled-coil domains. The RING domain is frequently found in E3 ubiquitin ligase, and TRIM5α is thought to degrade viral core via ubiquitin–proteasome-dependent and -independent pathways. The alpha isoform of TRIM5 has an additional C-terminal PRYSPRY domain, which is a determinant of species-specific retrovirus restriction by TRIM5α. On the other hand, the target regions of viral capsid protein (CA) are scattered on the surface of core. A single amino acid difference in the surface-exposed loop between α-helices 6 and 7 (L6/7) of HIV type 2 (HIV-2) CA affects viral sensitivity to human TRIM5α and was also shown to be associated with viral load in West African HIV-2 patients, indicating that human TRIM5α is a critical modulator of HIV-2 replication in vivo. Interestingly, L6/7 of CA corresponds to the MLV determinant of sensitivity to mouse factor Fv1, which potently restricts N-tropic MLV. In addition, human genetic polymorphisms also affect antiviral activity of human TRIM5α. Recently, human TRIM5α was shown to activate signaling pathways that lead to activation of NF-κB and AP-1 by interacting with TAK1 complex. TRIM5α is thus involved in control of viral infection in multiple ways.

Long-term infection and vertical transmission of a gammaretrovirus in a foreign host species

Increasing evidence has indicated natural transspecies transmission of gammaretroviruses; however, viral-host interactions after initial xeno-exposure remain poorly understood. Potential association of xenotropic murine leukemia virus-related virus (XMRV) in patients with prostate cancer and chronic fatigue syndrome has attracted broad interests in this topic. Although recent studies have indicated that XMRV is unlikely a human pathogen, further understanding of XMRV xenoinfection would allow in vivo modeling of the initial steps of gammaretroviral interspecies transmission, evolution and dissemination in a new host population. In this study, we monitored the long-term consequences of XMRV infection and its possible vertical transmission in a permissive foreign host, wild-derived Mus pahari mice. One year post-infection, XMRV-infected mice showed no notable pathological changes, while proviral DNA was detected in three out of eight mice. XMRV-infected mice remained seropositive throughout the study although the levels of gp70 Env- and p30 capsid-specific antibodies gradually decreased. When vertical XMRV transmission was assessed, no viremia, humoral immune responses nor endogenization were observed in nine offspring from infected mothers, yet one offspring was found PCR-positive for XMRV-specific sequences. Amplified viral sequences from the offspring showed several mutations, including one amino acid deletion in the receptor binding domain of Env SU. Our results therefore demonstrate long-term asymptomatic infection, low incidence of vertical transmission and limited evolution of XMRV upon transspecies infection of a permissive new host, Mus pahari.

The mouse "xenotropic" gammaretroviruses and their XPR1 receptor

The xenotropic/polytropic subgroup of mouse leukemia viruses (MLVs) all rely on the XPR1 receptor for entry, but these viruses vary in tropism, distribution among wild and laboratory mice, pathogenicity, strategies used for transmission, and sensitivity to host restriction factors. Most, but not all, isolates have typical xenotropic or polytropic host range, and these two MLV tropism types have now been detected in humans as viral sequences or as infectious virus, termed XMRV, or xenotropic murine leukemia virus-related virus. The mouse xenotropic MLVs (X-MLVs) were originally defined by their inability to infect cells of their natural mouse hosts. It is now clear, however, that X-MLVs actually have the broadest host range of the MLVs. Nearly all nonrodent mammals are susceptible to X-MLVs, and all species of wild mice and several common strains of laboratory mice are X-MLV susceptible. The polytropic MLVs, named for their apparent broad host range, show a more limited host range than the X-MLVs in that they fail to infect cells of many mouse species as well as many nonrodent mammals. The co-evolution of these viruses with their receptor and other host factors that affect their replication has produced a heterogeneous group of viruses capable of inducing various diseases, as well as endogenized viral genomes, some of which have been domesticated by their hosts to serve in antiviral defense.

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.

Origin, antiviral function and evidence for positive selection of the gammaretrovirus restriction gene Fv1 in the genus Mus

The Fv1 virus resistance gene is a coopted endogenous retrovirus (ERV) sequence related to the gag gene of the MuERV-L ERV family. Three major Fv1 resistance alleles have been identified in laboratory mice, and they target virus capsid genes to produce characteristic patterns of resistance to mouse leukemia viruses (MLVs). We identified Fv1 in 3 of the 4 Mus subgenera; its absence from Coelomys and 1 of 3 species of Pyromys indicate Fv1 was acquired shortly after the origin of the Mus genus. We sequenced Fv1 genes from 21 mice representative of the major taxonomic groups of Mus. Two lines of evidence indicate that Fv1 has had antiviral function for 7 million years of evolution. First, 2 species of African pygmy mice (subgenus Nannomys) show an Fv1-like MLV resistance, and transduced cells expressing the Nannomys Fv1 gene reproduce this resistance pattern. Second, sequence comparisons suggest that Fv1 has been involved in genetic conflicts throughout Mus evolution. We found evidence for strong positive selection of Fv1 and identified 6 codons that show evidence of positive selection: 3 codons in the C-terminal region including 2 previously shown to contribute to Fv1 restriction in laboratory mice, and 3 codons in a 10-codon segment overlapping the major homology region of Fv1; this segment is known to be involved in capsid multimerization. This analysis suggests that Fv1 has had an antiviral role throughout Mus evolution predating exposure of mice to the MLVs restricted by laboratory mouse Fv1, and suggests a mechanism for Fv1 restriction.

Murine endogenous retroviruses

Up to 10% of the mouse genome is comprised of endogenous retrovirus (ERV) sequences, and most represent the remains of ancient germ line infections. Our knowledge of the three distinct classes of ERVs is inversely correlated with their copy number, and their characterization has benefited from the availability of divergent wild mouse species and subspecies, and from ongoing analysis of the Mus genome sequence. In contrast to human ERVs, which are nearly all extinct, active mouse ERVs can still be found in all three ERV classes. The distribution and diversity of ERVs has been shaped by host-virus interactions over the course of evolution, but ERVs have also been pivotal in shaping the mouse genome by altering host genes through insertional mutagenesis, by adding novel regulatory and coding sequences, and by their co-option by host cells as retroviral resistance genes. We review mechanisms by which an adaptive coexistence has evolved. (Part of a multi-author review).

Novel postentry resistance to AKV ecotropic mouse gammaretroviruses in the African pygmy mouse, Mus minutoides

Cells of Mus minutoides, an African pygmy mouse of the subgenus Nannomys, are susceptible to ecotropic Moloney and Friend mouse leukemia viruses (MLVs) but not to AKV-type MLVs. Transfected MA139 ferret cells expressing the mCAT-1 cell surface receptor, with the minCAT-1 substitutions K222Q and V233L, did not restrict AKV MLV. The resistance of M. minutoides cells to AKV MLV was not relieved by inhibitors of glycosylation or by the introduction of NIH 3T3 mCAT-1. Resistance is thus not mediated by receptor sequence variation, expression level, or glycosylation. M. minutoides cells are also infectible with LacZ pseudotypes having AKV Env and Moloney MLV (MoMLV) Gag proteins, further indicating that AKV Env sequence variations do not contribute to the observed block. The pattern of virus resistance in M. minutoides differs from that of the known variants of the Fv1 postentry resistance gene; M. minutoides is equally resistant to N-, B-, and NR-tropic AKV viruses and is equally susceptible to NR- and NB-tropic Friend MLVs. This novel resistance blocks replication before reverse transcription, whereas Fv1 generally restricts replication after reverse transcription; M. minutoides cells produce 2-long-terminal-repeat viral DNA circles and linear viral DNA after infection with MoMLV but not with AKV MLV. Analysis of MoMLV-AKV MLV chimeras determined that the target of resistance is in the virus capsid gene. Mutagenesis demonstrated that restriction is mediated by two amino acid substitutions, H117L and A110R; substitutions at these sites can also be targeted by the resistance genes Fv1 and TRIM5alpha. M. minutoides cells thus have a novel postentry resistance to AKV MLVs.

Fusion of cyclophilin A to Fv1 enables cyclosporine-sensitive restriction of human and feline immunodeficiency viruses

TRIM5alpha is a potent intracellular antiviral restriction factor governing species-specific retroviral replication. In the New World species owl monkey the coding region for the viral binding B30.2 domain of TRIM5alpha has been replaced by a cyclophilin A (CypA) pseudogene by retrotransposition. The resultant TRIM5-CypA fusion protein restricts human immunodeficiency virus type 1 (HIV-1), as well as feline immunodeficiency virus (FIV), by recruitment of the CypA domain to the incoming viral capsids. Infectivity is rescued by agents such as cyclosporine that disrupt CypA binding to its substrates. Mice encode an antiviral restriction factor called Fv1 (for Friend virus susceptibility gene 1), which is active against murine leukemia virus and related to endogenous gag sequences. Here we show that fusing CypA to Fv1 generates a restriction factor with the antiviral specificity of TRIMCyp but the antiviral properties of Fv1. Like TRIMCyp, Fv1-Cyp restricts HIV-1 and FIV and is sensitive to inhibition by cyclosporine. TRIM5alpha is known to have a short half-life and block infectivity before viral reverse transcription. We show that Fv1-Cyp has a long half-life and blocks after reverse transcription, suggesting that its longer half-life gives the restricted virus the opportunity to synthesize DNA, leading to a later block to infection. This notion is supported by the observation that infectivity of Fv1-Cyp restricted virus can be rescued by cyclosporine for several hours after infection, whereas virus restricted by TRIMCyp is terminally restricted after around 40 min. Intriguingly, the Fv1-Cyp-restricted HIV-1 generates closed circular viral DNA, suggesting that the restricted virus complex enters the nucleus.

Intrinsic immunity: a front-line defense against viral attack

In addition to the conventional innate and acquired immune responses, complex organisms have evolved an array of dominant, constitutively expressed genes that suppress or prevent viral infections. Two major cellular defenses against infection by retroviruses are the Fv1 and TRIM5 class of inhibitors that target incoming retroviral capsids and the APOBEC3 class of cytidine deaminases that hypermutate and destabilize retroviral genomes. Additional, less well characterized activities also inhibit viral replication. Here, the present understanding of these 'intrinsic' immune mechanisms is reviewed and their role in protection from retroviral infection is discussed.

Murine T cells potently restrict human immunodeficiency virus infection

Development of a mouse model for human immunodeficiency virus type 1 (HIV-1) infection has advanced through the progressive identification of host cell factors required for HIV-1 replication. Murine cells lack HIV-1 receptor molecules, do not support efficient viral gene expression, and lack factors necessary for the assembly and release of virions. Many of these blocks have been described using mouse fibroblast cell lines. Here we identify a postentry block to HIV-1 infection in mouse T-cell lines that has not been detected in mouse fibroblasts. While murine fibroblastic lines are comparable to human T-cell lines in permissivity to HIV-1 transduction, infection of murine T cells is 100-fold less efficient. Virus entry occurs efficiently in murine T cells. However, reduced efficiency of the completion of reverse transcription and nuclear transfer of the viral preintegration complex are observed. Although this block has similarities to the restriction of murine retroviruses by Fv1, there is no correlation of HIV-1 susceptibility with cellular Fv1 genotypes. In addition, the block to HIV-1 infection in murine T-cell lines cannot be saturated by a high virus dose. Further studies of this newly identified block may lend insight into the early events of retroviral replication and reveal new targets for antiretroviral interventions.

Retroviral capsid determinants of Fv1 NB and NR tropism

The specificity determinants for susceptibility to resistance by the Fv1 n and b alleles map to amino acid 110 of the murine leukemia virus CA protein. To study the interaction between Fv1 and CA, we examined changes in CA resulting in the loss of susceptibility to Fv1 resistance in naturally occurring NB- and NR-tropic viruses. A variety of amino acid changes affecting Fv1 tropism were identified, at CA positions 82, 92 to 95, 105, 114, and 117, and they all were mapped to the apparent exterior of virion-associated CA. These amino acids may form a binding surface for Fv1.

Characterization of recombinant nonecotropic murine leukemia viruses from the wild mouse species Mus spretus

The wild mouse species most closely related to the common laboratory strains contain proviral env genes of the xenotropic/polytropic subgroup of mouse leukemia viruses (MLVs). To determine if the polytropic proviruses of Mus spretus contain functional genes, we inoculated neonates with Moloney MLV (MoMLV) or amphotropic MLV (A-MLV) and screened for viral recombinants with altered host ranges. Thymus and spleen cells from MoMLV-inoculated mice were plated on Mus dunni cells and mink cells, since these cells do not support the replication of MoMLV, and cells from A-MLV-inoculated mice were plated on ferret cells. All MoMLV-inoculated mice produced ecotropic viruses that resembled their MoMLV progenitor, although some isolates, unlike MoMLV, grew to high titers in M. dunni cells. All of the MoMLV-inoculated mice also produced nonecotropic virus that was infectious for mink cells. Sequencing of three MoMLV- and two A-MLV-derived nonecotropic recombinants confirmed that these viruses contained substantial substitutions that included the regions of env encoding the surface (SU) protein and the 5' end of the transmembrane (TM) protein. The 5' recombination breakpoint for one of the A-MLV recombinants was identified in RNase H. The M. spretus-derived env substitutions were nearly identical to the corresponding regions in prototypical laboratory mouse polytropic proviruses, but the wild mouse infectious viruses had a more restricted host range. The M. spretus proviruses contributing to these recombinants were also sequenced. The seven sequenced proviruses were 99% identical to one another and to the recombinants; only two of the seven had obvious fatal defects. We conclude that the M. spretus proviruses are likely to be recent germ line acquisitions and that they contain functional genes that can contribute to the production of replication-competent virus.

Capsid-dependent and -independent postentry restriction of primate lentivirus tropism in rodent cells

Retrovirus tropism can be restricted by cellular factors such as Fv1, Ref1, and Lv1 that inhibit infection by targeting the incoming viral capsid. Here, we show that rodent cells exhibit differential sensitivity to infection by vesicular stomatitis virus G-pseudotyped lentiviruses and that differences between human immunodeficiency virus type 1 and simian immunodeficiency virus (SIVmac) infectivity are sometimes, but not always, governed by determinants in capsid-p2. In at least one case, resistance to SIVmac infection could be eliminated by saturation of target cells with noninfectious SIVmac particles. However, cross-saturation experiments and analysis of Fv1-null cells engineered to express natural or artificial Fv1 proteins revealed that lentivirus restriction in mouse cells is independent of Fv1. Overall, these findings indicate that novel restriction factors in rodents can modulate sensitivity to specific primate lentiviruses.

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.

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.

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.

A single amino acid change in the murine leukemia virus capsid gene responsible for the Fv1(nr) phenotype

The nr allele at the mouse Fv1 restriction locus governs resistance to B-tropic and some N-tropic murine leukemia viruses (MLVs). Sequence analysis and site-specific mutagenesis of N-tropic MLVs identified a single amino acid difference responsible for this restriction that is distinct from the site that governs N or B tropism. Viruses with other substitutions at this site were evaluated for altered replication patterns.

Fv2 encodes a truncated form of the Stk receptor tyrosine kinase

The Friend virus susceptibility 2 (Fv2) locus encodes a dominant host factor that confers susceptibility to Friend virus-induced erythroleukaemia in mice. We mapped Fv2 to a 1.0-Mb interval that also contained the gene (Ron) encoding the stem cell kinase receptor (Stk). A truncated form of Stk (Sf-stk), which was the most abundant form of Stk in Fv2-sensitive (Fv2ss) erythroid cells, was not expressed in Fv2 resistant (Fv2rr) cells. Enforced expression of Sf-stk conferred susceptibility to Friend disease, whereas targeted disruption of Ron caused resistance. We conclude that the Fv2 locus encodes Ron, and that a naturally expressed, truncated form of Stk confers susceptibility to Friend virus-induced erythroleukaemia.

Positional cloning of the mouse retrovirus restriction gene Fv1

Vertebrate evolution has taken place against a background of constant retrovirus infection, and much of the mammalian genome consists of endogenous retrovirus-like elements. Several host genes have evolved to control retrovirus replication, including Friend-virus-susceptibility-1, Fv1, on mouse chromosome 4 (refs 3, 4). The Fv1 gene acts on murine leukaemia virus at a stage after entry into the target cell but before integration and formation of the provirus. Although restriction is not absolute, Fv1 prevents or delays spontaneous or experimentally induced viral tumours. In vitro, Fv1 restriction leads to an apparent 50-1,000 fold reduction in viral titre. Genetic evidence implicates a direct interaction between the Fv1 gene product and a component of the viral preintegration complex, the capsid protein CA (refs 7-9). We have now cloned Fv1: the gene appears to be derived from the gag region of an endogenous retrovirus unrelated to murine leukaemia virus, implying that the Fv1 protein and its target may share functional similarities despite the absence of nucleotide-sequence homology.

Host-range control of a retroviral disease: Friend erythroleukemia

Since its discovery in 1957, Friend viral erythroleukemia has been the major model for understanding host genetic barriers to retroviral diseases and has facilitated the discovery of many polymorphic leukemia-control genes of mice. Some of these genes limit helper-virus replication, target-cell (erythroblast) pools or immune responses. At least one host gene appears to block the viral oncoprotein.

Deletions in one domain of the Friend virus-encoded membrane glycoprotein overcome host range restrictions for erythroleukemia

Although the Friend virus-encoded membrane glycoprotein (gp55) activates erythropoietin receptors (EpoR) to cause erythroblastosis only in certain inbred strains of mice but not in other species, mutant viruses can overcome aspects of mouse resistance. Thus, mice homozygous for the resistance allele of the Fv-2 gene are unaffected by gp55 but are susceptible to mutant glycoproteins that have partial deletions in their ecotropic domains. These and other results have suggested that proteins coded for by polymorphic Fv-2 alleles might directly or indirectly interact with EpoR and that changes in gp55 can overcome this defense. A new viral mutant with an exceptionally large deletion in its ecotropic domain is now also shown to overcome Fv-2rr resistance. In all cases, the glycoproteins that activate EpoR are processed to cell surfaces as disulfide-bonded dimers. To initiate analysis of nonmurine resistances, we expressed human EpoR and mouse EpoR in the interleukin 3-dependent mouse cell line BaF3 and compared the abilities of Friend virus-encoded glycoproteins to convert these cells to growth factor independence. Human EpoR was activated in these cells by erythropoietin but was resistant to gp55. However, human EpoR was efficiently activated in these cells by the same viral mutants that overcome Fv-2rr resistance in mice. By construction and analysis of human-mouse EpoR chimeras, we obtained evidence that the cytosolic domain of human EpoR contributes to its resistance to gp55 and that this resistance is mediated by accessory cellular factors. Aspects of host resistance in both murine and nonmurine species are targeted specifically against the ecotropic domain of gp55.

Genetic map of the region surrounding the retrovirus restriction locus, Fv1, on mouse chromosome 4

The Friend virus susceptibility-1 (Fv1) gene maps to mouse Chromosome (Chr) 4 close to a cluster of four endogenous murine leukemia viruses (MLVs). To investigate the feasibility of cloning Fv1 by a positional approach, we have performed an extensive genetic analysis of this region of Chr 4. We have typed 368 backcross mice for the four proviruses, Nppa, Lck, and D4Smh6b. Recombinant animals were screened in a hierarchical fashion with a variety of other markers, including Fv1 and the isozyme marker Gpd1. A detailed genetic map of the region surrounding Fv1 was derived. Three markers, Xmv9, Nppa, and Iap3rc11, were identified that showed no recombination with Fv1. By combining backcross and recombinant inbred strain data, we estimated that Xmv9 and Nppa must lie within 0.6 cM of one another and Fv1.

A Friend virus mutant that overcomes Fv-2rr host resistance encodes a small glycoprotein that dimerizes, is processed to cell surfaces, and specifically activates erythropoietin receptors

The env gene of Friend spleen focus-forming virus (SFFV) encodes a membrane glycoprotein (gp55) that is inefficiently (3 to 5%) processed from the rough endoplasmic reticulum to form a larger dimeric plasma membrane derivative (gp55p). Moreover, the SFFV env glycoprotein associates with erythropoietin receptors (EpoR) to cause proliferation of infected erythroblasts [J.-P. Li, A. D. D'Andrea, H. F. Lodish, and D. Baltimore, Nature (London) 343:762-764, 1990]. Interestingly, the mitogenic effect of SFFV is blocked in mice homozygous for the Fv-2r resistance gene, but mutant SFFVs can overcome this resistance. Recent evidence suggested that these mutants contain partial env deletions that truncate the membrane-proximal extracellular domain of the encoded glycoproteins (M. H. Majumdar, C.-L. Cho, M. T. Fox, K. L. Eckner, S. Kozak, D. Kabat, and R. W. Geib, J. Virol. 66:3652-3660, 1992). Mutant BB6, which encodes a gp42 glycoprotein that has a large deletion in this domain, causes erythroblastosis in DBA/2 (Fv-2s) as well as in congenic D2.R (Fv-2r) mice. Analogous to gp55, gp42 is processed inefficiently as a disulfide-bonded dimer to form cell surface gp42p. Retroviral vectors with SFFV and BB6 env genes have no effect on interleukin 3-dependent BaF3 hematopoietic cells, but they cause growth factor independency of BaF3/EpoR cells, a derivative that contains recombinant EpoR. After binding 125I-Epo to surface EpoR on these factor-independent cells and adding the covalent cross-linking reagent disuccinimidyl suberate, complexes that had immunological properties and sizes demonstrating that they consisted of 125I-Epo-gp55p and 125I-Epo-gp42p were isolated from cell lysates. Contrary to a previous report, SFFV or BB6 env glycoproteins did not promiscuously activate other members of the EpoR superfamily. Although the related env glycoproteins encoded by dualtropic murine leukemia viruses formed detectable complexes with EpoR, strong mitogenic signalling did not ensue. Our results indicate that the SFFV and BB6 env glycoproteins specifically activate EpoR; they help to define the glycoprotein properties important for its functions; and they strongly suggest that the Fv-2 leukemia control gene encodes an EpoR-associated regulatory factor.

Fv-1 restriction and its effects on murine leukemia virus integration in vivo and in vitro

We have investigated the mechanisms by which alleles at the mouse Fv-1 locus restrict replication of murine leukemia viruses. Inhibition of productive infection is closely paralleled by reduced accumulation of integrated proviral DNA as well as by reduced levels of linear viral DNA in a cytoplasmic fraction. Nevertheless, viral DNA is present at nearly normal levels in a nuclear fraction, and total amounts of viral DNA are only mildly affected in restrictive infections, suggesting a block in integration to account for reduced levels of proviral DNA. However, integrase (IN)-dependent trimming of 3' ends of viral DNA occurs normally in vivo during restrictive infections, demonstrating that not all IN-mediated events are prevented in vivo. Furthermore, viral integration complexes present in nuclear extracts of infected restrictive cells are fully competent to integrate their DNA into a heterologous target in vitro. Thus, the Fv-1-dependent activity that restricts integration in vivo may be lost in vitro; alternatively, Fv-1 restriction may prevent a step required for integration in vivo that is bypassed in vitro.

HoMuLV: a novel pathogenic ecotropic virus isolated from the European mouse, Mus hortulanus

We isolated a novel infectious murine leukemia virus (HoMuLV) from the wild mouse Mus hortulanus. HoMuLV has an ecotropic virus host range, but the viral DNA fails to hybridize to viral envelope segments specific for the known inbred and wild mouse ecotropic as well as nonecotropic MuLVs. Despite this difference in its env gene, HoMuLV appears to use the same ecotropic cell-surface receptor since it infects only hamster and mouse somatic cell hybrids which contain the Rec-1 ecotropic virus receptor on chromosome 5. Furthermore, HoMuLV does not infect mice carrying the Fv-4r allele which is thought to prevent ecotropic virus infection through an interference mechanism. HoMuLV is NB-tropic and, unlike other infectious MuLVs, does not grow in cells derived from the wild mouse species. M. dunni. Five to ten months after neonatal inoculation with HoMuLV, 72% of female NIH Swiss mice (8/11) contracted lymphoma or erythroid leukemia, but 33% of the inoculated males (5/15) developed erythroid or myelogenous leukemia within 8-16 months. These data suggest that NIH Swiss males and females differ in their susceptibility to HoMuLV-induced disease. Furthermore, NIH Swiss mice were found to be more susceptible to HoMuLV-induced disease than NFS/N mice. Tumors contained infectious MCF virus, which is consistent with the hypothesis that MCF virus may mediate tumorigenesis by HoMuLV.