Specific hybridization probes demonstrate fewer xenotropic than mink cell focus-forming murine leukemia virus env-related sequences in DNAs from inbred laboratory mice

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.

The Lupus Susceptibility Locus Sgp3 Encodes the Suppressor of Endogenous Retrovirus Expression SNERV

Elevated endogenous retrovirus (ERV) transcription and anti-ERV antibody reactivity are implicated in lupus pathogenesis. Overproduction of non-ecotropic ERV (NEERV) envelope glycoprotein gp70 and resultant nephritis occur in lupus-prone mice, but whether NEERV mis-expression contributes to lupus etiology is unclear. Here we identified suppressor of NEERV (Snerv) 1 and 2, Krüppel-associated box zinc-finger proteins (KRAB-ZFPs) that repressed NEERV by binding the NEERV long terminal repeat to recruit the transcriptional regulator KAP1. Germline Snerv1/Snerv2 deletion increased activating chromatin modifications, transcription, and gp70 expression from NEERV loci. F1 crosses of lupus-prone New Zealand Black (NZB) and 129 mice to Snerv1/Snerv2^-/- mice failed to restore NEERV repression, demonstrating that loss of SNERV underlies the lupus autoantigen gp70 overproduction that promotes nephritis in susceptible mice and that SNERV encodes for Sgp3 (in NZB mice) and Gv-1 loci (in 129 mice). Increased ERV expression in lupus patients inversely correlated with three putative ERV-suppressing KRAB-ZFPs, suggesting that loss of KRAB-ZFP-mediated ERV control may contribute to human lupus pathogenesis.

Recombinant Origins of Pathogenic and Nonpathogenic Mouse Gammaretroviruses with Polytropic Host Range

Ecotropic, xenotropic, and polytropic mouse leukemia viruses (E-, X-, and P-MLVs) exist in mice as infectious viruses and endogenous retroviruses (ERVs) inserted into mouse chromosomes. All three MLV subgroups are linked to leukemogenesis, which involves generation of recombinants with polytropic host range. Although P-MLVs are deemed to be the proximal agents of disease induction, few biologically characterized infectious P-MLVs have been sequenced for comparative analysis. We analyzed the complete genomes of 16 naturally occurring infectious P-MLVs, 12 of which were typed for pathogenic potential. We sought to identify ERV progenitors, recombinational hot spots, and segments that are always replaced, never replaced, or linked to pathogenesis or host range. Each P-MLV has an E-MLV backbone with P- or X-ERV replacements that together cover 100% of the recombinant genomes, with different substitution patterns for X- and P-ERVs. Two segments are always replaced, both coding for envelope (Env) protein segments: the N terminus of the surface subunit and the cytoplasmic tail R peptide. Viral gag gene replacements are influenced by host restriction genes Fv1 and Apobec3 Pathogenic potential maps to the env transmembrane subunit segment encoding the N-heptad repeat (HR1). Molecular dynamics simulations identified three novel interdomain salt bridges in the lymphomagenic virus HR1 that could affect structural stability, entry or sensitivity to host immune responses. The long terminal repeats of lymphomagenic P-MLVs are differentially altered by recombinations, duplications, or mutations. This analysis of the naturally occurring, sometimes pathogenic P-MLV recombinants defines the limits and extent of intersubgroup recombination and identifies specific sequence changes linked to pathogenesis and host interactions.IMPORTANCE During virus-induced leukemogenesis, ecotropic mouse leukemia viruses (MLVs) recombine with nonecotropic endogenous retroviruses (ERVs) to produce polytropic MLVs (P-MLVs). Analysis of 16 P-MLV genomes identified two segments consistently replaced: one at the envelope N terminus that alters receptor choice and one in the R peptide at the envelope C terminus, which is removed during virus assembly. Genome-wide analysis shows that nonecotropic replacements in the progenitor ecotropic MLV genome are more extensive than previously appreciated, covering 100% of the genome; contributions from xenotropic and polytropic ERVs differentially alter the regions responsible for receptor determination or subject to APOBEC3 and Fv1 restriction. All pathogenic viruses had modifications in the regulatory elements in their long terminal repeats and differed in a helical segment of envelope involved in entry and targeted by the host immune system. Virus-induced leukemogenesis thus involves generation of complex recombinants, and specific replacements are linked to pathogenesis and host restrictions.

Origins of the endogenous and infectious laboratory mouse gammaretroviruses

The mouse gammaretroviruses associated with leukemogenesis are found in the classical inbred mouse strains and in house mouse subspecies as infectious exogenous viruses (XRVs) and as endogenous retroviruses (ERVs) inserted into their host genomes. There are three major mouse leukemia virus (MuLV) subgroups in laboratory mice: ecotropic, xenotropic, and polytropic. These MuLV subgroups differ in host range, pathogenicity, receptor usage and subspecies of origin. The MuLV ERVs are recent acquisitions in the mouse genome as demonstrated by the presence of many full-length nondefective MuLV ERVs that produce XRVs, the segregation of these MuLV subgroups into different house mouse subspecies, and by the positional polymorphism of these loci among inbred strains and individual wild mice. While some ecotropic and xenotropic ERVs can produce XRVs directly, others, especially the pathogenic polytropic ERVs, do so only after recombinations that can involve all three ERV subgroups. Here, I describe individual MuLV ERVs found in the laboratory mice, their origins and geographic distribution in wild mouse subspecies, their varying ability to produce infectious virus and the biological consequences of this expression.

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

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

Endogenous gammaretrovirus acquisition in Mus musculus subspecies carrying functional variants of the XPR1 virus receptor

The xenotropic and polytropic mouse leukemia viruses (X-MLVs and P-MLVs, respectively) have different host ranges but use the same functionally polymorphic receptor, XPR1, for entry. Endogenous retroviruses (ERVs) of these 2 gammaretrovirus subtypes are largely segregated in different house mouse subspecies, but both MLV types are found in the classical strains of laboratory mice, which are genetic mosaics of 3 wild mouse subspecies. To describe the subspecies origins of laboratory mouse XP-MLV ERVs and their coevolutionary trajectory with their XPR1 receptor, we screened the house mouse subspecies for known and novel Xpr1 variants and for the individual full-length XP-MLV ERVs found in the sequenced C57BL mouse genome. The 12 X-MLV ERVs predate the origins of laboratory mice; they were all traced to Japanese wild mice and are embedded in the 5% of the laboratory mouse genome derived from the Asian Mus musculus musculus and, in one case, in the <1% derived from M. m. castaneus. While all 31 P-MLV ERVs map to the 95% of the laboratory mouse genome derived from P-MLV-infected M. m. domesticus, no C57BL P-MLV ERVs were found in wild M. m. domesticus. All M. m. domesticus mice carry the fully permissive XPR1 receptor allele, but all of the various restrictive XPR1 receptors, including the X-MLV-restricting laboratory mouse Xpr1(n) and a novel M. m. castaneus allele, originated in X-MLV-infected Asian mice. Thus, P-MLV ERVs show more insertional polymorphism than X-MLVs, and these differences in ERV acquisition and fixation are linked to subspecies-specific and functionally distinct XPR1 receptor variants.

Complete genome sequences of new xenotropic murine leukemia viruses from the senescence-accelerated mouse (SAM): molecular and phylogenetic analyses

Approximately 10% of the mouse genome is constituted by endogenous retroviruses (ERVs), and a number of mouse ERVs remain active. Many copies of endogenous murine leukemia viruses (MuLVs) are detected in the genomes of inbred mouse strains. Some of these MuLVs are transcriptionally active or produce infectious virus particles. Previously, we identified partial env sequences of new xenotropic MuLVs (X-MuLVs) from a senescence-accelerated mouse (SAM) strain. In the present study, we investigated and characterized the complete sequences of the X-MuLVs. The complete genomes and open reading frames (ORFs) of two X-MuLVs, designated xmlv15 and xmlv18 (accession nos. HQ154630 and HQ154631, respectively), were molecularly cloned from the genome of the SAM mice. We confirmed that the xmlv15 and xmlv18 sequences are distinct from all known MuLV genomes and are most similar to DG-75 MuLV. Moreover, we found that common strains of laboratory mice carry our newly identified xmlvs. Additionally, the expression levels of xmlv15-related sequences were much higher in C57BL and ICR mice than in the SAM strains without any stimulators. Our findings suggest that a specific group of endogenous MuLVs is constitutively expressed in the brain and that they may participate in normal functions and/or pathogenic conditions.

The human lung adenocarcinoma cell line EKVX produces an infectious xenotropic murine leukemia virus

The cell lines of the NCI-60 panel represent different cancer types and have been widely utilized for drug screening and molecular target identification. Screening these cell lines for envelope proteins or gene sequences related to xenotropic murine leukemia viruses (X-MLVs) revealed that one cell line, EKVX, was a candidate for production of an infectious gammaretrovirus. The presence of a retrovirus infectious to human cells was confirmed by the cell-free transmission of infection to the human prostate cancer cell line LNCaP. Amplification and sequencing of additional proviral sequences from EKVX confirmed a high degree of similarity to X-MLV. The cell line EKVX was established following passage of the original tumor cells through nude mice, providing a possible source of the X-MLV found in the EKVX cells.

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

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

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.

Common inbred strains of the laboratory mouse that are susceptible to infection by mouse xenotropic gammaretroviruses and the human-derived retrovirus XMRV

Laboratory mouse strains carry endogenous copies of the xenotropic mouse leukemia viruses (X-MLVs), named for their inability to infect cells of the laboratory mouse. This resistance to exogenous infection is due to a nonpermissive variant of the XPR1 gammaretrovirus receptor, a resistance that also limits in vivo expression of germ line X-MLV proviruses capable of producing infectious virus. Because laboratory mice vary widely in their proviral contents and in their virus expression patterns, we screened inbred strains for sequence and functional variants of the XPR1 receptor. We also typed inbred strains and wild mouse species for an endogenous provirus, Bxv1, that is capable of producing infectious X-MLV and that also contributes to the generation of pathogenic recombinant MLVs. We identified the active Bxv1 provirus in many common inbred strains and in some Japanese Mus molossinus mice but in none of the other wild mouse species that carry X-MLVs. Our screening for Xpr1 variants identified the permissive Xpr1(sxv) allele in 7 strains of laboratory mice, including a Bxv1-positive strain, F/St, which is characterized by lifelong X-MLV viremia. Cells from three strains carrying Xpr1(sxv), namely, SWR, SJL, and SIM.R, were shown to be infectable by X-MLV and XMRV; these strains carry different alleles at Fv1 and vary in their sensitivities to specific X/P-MLV isolates and XMRV. Several strains with Xpr1(sxv) lack the active Bxv1 provirus or other endogenous X-MLVs and may provide a useful model system to evaluate the in vivo spread of these gammaretroviruses and their disease potential in their natural host.

Wild mouse variants of envelope genes of xenotropic/polytropic mouse gammaretroviruses and their XPR1 receptors elucidate receptor determinants of virus entry

Mouse xenotropic and polytropic leukemia viruses (XMVs and PMVs) are closely related gammaretroviruses that use the XPR1 receptor for entry. To identify amino acid residues in XPR1 important for virus entry, we tested mouse cells derived from evolutionarily divergent species for susceptibility to prototypical PMVs, XMVs, and the wild mouse isolate CasE#1. CasE#1 has a variant XMV/PMV host range, and sequence analysis of the CasE#1 env gene identifies segments related to PMVs and XMVs. Cells from the Asian mouse species Mus pahari show a unique pattern of susceptibility to these three viruses; these cells are susceptible to XMVs and CasE#1 but are resistant to PMVs, whereas NIH 3T3 cells show the reciprocal pattern, susceptibility to only PMVs. The M. pahari XPR1 gene differs from that of NIH 3T3 in the two extracellular loops (ECLs) previously shown to mediate virus entry (M. Marin, C. S. Tailor, A. Nouri, S. L. Kozak, and D. Kabat, J. Virol. 73:9362-9368, 1999, and N. S. Van Hoeven and A. D. Miller, Retrovirology 2:76, 2005). Using transfected hamster cells expressing chimeric and mutated XPR1s, we demonstrated that the susceptibility differences between NIH 3T3 and M. pahari cells are receptor mediated, that PMV entry requires residues in ECL3, that the CasE#1 entry determinant is in ECL4, and that determinants for XMV entry are in both ECL3 and ECL4. Additional substitutions in ECL3 and ECL4 modulate virus susceptibility and suggest that ECL3 and ECL4 may contribute to the formation of a single virus receptor site. The position of M. pahari at the base of the Mus phylogenetic tree indicates that XPR1-mediated susceptibility to XMVs is the ancestral type in this genus and that the phenotypic variants of mouse XPR1 likely arose in conjunction with exposure to gammaretrovirus infections and coevolutionary adaptations in the viral envelope.

Rmcf2, a xenotropic provirus in the Asian mouse species Mus castaneus, blocks infection by polytropic mouse gammaretroviruses

Cells from the Asian wild mouse species Mus castaneus are resistant to infection by the polytropic host range group of mouse gammaretroviruses. Two factors are responsible for this resistance: a defective XPR1 cell surface receptor for polytropic murine leukemia viruses (P-MLVs), and a resistance factor detectable only in interspecies hybrids between M. castaneus and mice with an XPR1 variant that permits infection by xenotropic MLVs (X-MLVs) as well as P-MLVs. This second novel virus resistance phenotype has been associated with expression of viral Env glycoprotein; Northern blotting with specific hybridization probes identified a spliced X-MLV env message unique to virus-resistant mice. These observations suggest that resistance is due to expression of one or more endogenous X-MLV envelope genes that interfere with infection by exogenous P-MLVs. M. castaneus contains multiple X-MLV proviruses, but serial backcrosses reduced this proviral content and permitted identification of a single proviral env sequence inherited with resistance. The resistance phenotype and the provirus were mapped to the same site on distal chromosome 18. The provirus was shown to be a full-length provirus highly homologous to previously described X-MLVs. Use of viral pseudotypes confirmed that this resistance gene, termed Rmcf2, prevents entry of P-MLVs. Rmcf2 resembles the virus resistance genes Fv4 and Rmcf in that it produces Env glycoprotein but fails to produce infectious virus; the proviruses associated with all three resistance genes have fatal defects. This type of provirus Env-mediated resistance represents an important defense mechanism in wild mouse populations exposed to endemic infections.

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.

Cell surface receptors for gammaretroviruses

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

Characterization of a polytropic murine leukemia virus proviral sequence associated with the virus resistance gene Rmcf of DBA/2 mice

The DBA/2 mouse Rmcf gene is responsible for in vivo and in vitro resistance to infection by the polytropic mink cell focus-forming (MCF) virus subgroup of murine leukemia viruses (MLVs). Previous studies suggested that Rmcf resistance is mediated by expression of an interfering MCF MLV envelope (Env) gene. To characterize this env gene, we examined resistance in crosses between Rmcf(r) DBA/2 mice and Mus castaneus, a species that lacks endogenous MCF env sequences. In backcross progeny, inheritance of Rmcf resistance correlated with inheritance of a specific endogenous MCF virus env-containing 4.6-kb EcoRI fragment. This fragment was present in the DBA/2N substrain with Rmcf-mediated resistance but not in virus-susceptible DBA/2J substrain mice. This fragment contains a provirus with a 5' long terminal repeat and the 5' half of env; the gag and pol genes have been partially deleted. The Env sequence is identical to that of a highly immunogenic viral glycoprotein expressed in the DBA/2 cell line L5178Y and closely resembles the env genes of modified polytropic proviruses. The coding sequence for the full-length Rmcf Env surface subunit was amplified from DNAs from virus-resistant backcross mice and was cloned into an expression vector. NIH 3T3 and BALB 3T3 cells stably transfected with this construct showed significant resistance to infection by MCF MLV but not by amphotropic MLV. This study identifies an Rmcf-linked MCF provirus and indicates that, like the ecotropic virus resistance gene Fv4, Rmcf may mediate resistance through an interference mechanism.

Structure and distribution of endogenous nonecotropic murine leukemia viruses in wild mice

Virtually all of our present understanding of endogenous murine leukemia viruses (MLVs) is based on studies with inbred mice. To develop a better understanding of the interaction between endogenous retroviruses and their hosts, we have carried out a systematic investigation of endogenous nonecotropic MLVs in wild mice. Species studied included four major subspecies of Mus musculus (M. m. castaneus, M. m. musculus, M. m. molossinus, and M. m. domesticus) as well as four common inbred laboratory strains (AKR/J, HRS/J, C3H/HeJ, and C57BL/6J). We determined the detailed distribution of nonecotropic proviruses in the mice by using both env- and long terminal repeat (LTR)-derived oligonucleotide probes specific for the three different groups of endogenous MLVs. The analysis indicated that proviruses that react with all of the specific probes are present in most wild mouse DNAs tested, in numbers varying from 1 or 2 to more than 50. Although in common inbred laboratory strains the linkage of group-specific sequences in env and the LTR of the proviruses is strict, proviruses which combine env and the LTR sequences from different groups were commonly observed in the wild-mouse subspecies. The "recombinant" nonecotropic proviruses in the mouse genomes were amplified by PCR, and their genetic and recombinant natures were determined. These proviruses showed extended genetic variation and provide a valuable probe for study of the evolutionary relationship between MLVs and the murine hosts.

Molecular cloning of Mus dunni endogenous virus: an unusual retrovirus in a new murine viral interference group with a wide host range

Mus dunni endogenous virus (MDEV) is activated from cells of the Asian wild mouse M. dunni (also known as Mus terricolor) in response to treatment with either 5-iodo-2'-deoxyuridine or hydrocortisone. MDEV represents a new murine retrovirus interference group and thus appears to use a different receptor for entry into cells than do other murine retroviruses. Here we show that MDEV is also not in the gibbon ape leukemia virus or RD114 virus interference groups. A retroviral vector with an MDEV pseudotype was capable of efficiently infecting a wide variety of cells from different species, indicating that the MDEV receptor is widely expressed. We isolated a molecular clone of this virus which exhibited no hybridization to any cloned retrovirus examined, suggesting that MDEV has an unusual genome. One copy of a possible retrovirus element that weakly hybridized with MDEV was present in the genomes of laboratory strains of mice, while no such elements were present in other species examined. A virus activated by 5-iodo-2'-deoxyuridine from cells of a BALB/c mouse, however, was not related to MDEV by either hybridization or interference analyses.

Functional dissection of the Moloney murine leukemia virus envelope protein gp70

The envelope protein of Moloney murine leukemia virus (Mo-MLV) is a complex glycoprotein that mediates receptor binding and entry via fusion with cell membranes. By using a series of substitution mutations and truncations in the Mo-MLV external envelope surface protein gp70, we have identified regions important for these processes. Firstly, truncations of gp70 revealed that the minimal continuous receptor-binding region is amino acids 9 to 230, in broad agreement with other studies. Secondly, within this region there are two key basic amino acids, Arg-83 and Arg-95, that are essential for receptor binding and may interact with a negatively charged residue(s) or with the pi electrons of the aromatic ring on a hydrophobic residue(s) in the basic amino acid transporter protein that is the Mo-MLV ecotropic receptor. Finally, we showed that outside the minimal receptor-binding region at amino acids 2 to 8, there is a region that is essential for postbinding fusion events.

A multistep process of leukemogenesis in Moloney murine leukemia virus-infected mice that is modulated by retroviral pseudotyping and interference

Mixed retroviral infections frequently exhibit pseudotyping, in which the genome of one virus is packaged in a virion containing SU proteins encoded by another virus. Infection of mice by Moloney murine leukemia virus (M-MuLV), which induces lymphocytic leukemia, results in a mixed viral infection composed of the inoculated ecotropic M-MuLV and polytropic MuLVs generated by recombination of M-MuLV with endogenous retroviral sequences. In this report, we describe pseudotyping which occurred among the polytropic and ecotropic MuLVs in M-MuLV-infected mice. Infectious center assays of polytropic MuLVs released from splenocytes or thymocytes of infected mice revealed that polytropic MuLVs were extensively pseudotyped within ecotropic virions. Late in the preleukemic stage, a dramatic change in the extent of pseudotyping occurred in thymuses. Starting at about 5 weeks, there was an abrupt increase in the number of thymocytes that released nonpseudotyped polytropic viruses. A parallel increase in thymocytes that released ecotropic M-MuLV packaged within polytropic virions was also observed. Analyses of the clonality of preleukemic thymuses and thymomas suggested that the change in pseudotyping characteristics was not the result of the emergence of tumor cells. Examination of mice infected with M-MuLV, Friend erythroleukemia virus, and a Friend erythroleukemia virus-M-MuLV chimeric virus suggested that the appearance of polytropic virions late in the preleukemic stage correlated with the induction of lymphocytic leukemia. We discuss different ways in which pseudotypic mixing may facilitate leukemogenesis, including a model in which the kinetics of thymic infection, modulated by pseudotyping and viral interference, facilitates a stepwise mechanism of leukemogenesis.

Development of physical forms of unintegrated retroviral DNA in mouse spinal cord tissue during ts1-induced spongiform encephalomyelopathy: elevated levels of a novel single-stranded form in paralyzed mice

ts1 is a murine leukemia virus that causes rapidly evolving hindlimb paralysis in susceptible strains of mice. Following perinatal infection, three physical forms of unintegrated viral DNA were detected in the spinal cord by Southern blot hybridization. Linear and supercoiled closed-circle viral double-stranded DNAs were detected in both the central nervous system and non-central nervous system tissues. An elevated level of a novel minus-sense single-stranded form of viral DNA, which had a very high mobility in agarose gels, was correlated with the onset of symptoms of paralysis. As the severity of paralysis progressed, the level of this single-stranded form increased rapidly, with the highest level in the spinal cords of moribund mice. Since the virulence of a number of cytopathic retroviruses has been associated with the presence of increased amounts of unintegrated viral DNA in the tissues of the infected hosts, this novel form of highly mobile unintegrated single-stranded DNA may have a role in the neuropathogenesis of ts1.

Analysis of retrotransposon families in genomic DNA by two-dimensional restriction mapping: detection of VL30 insertions in mouse thymic lymphoma

Reinsertion of mammalian retrotransposable genetic elements is known to be causally associated with tumourigenesis, typically through mechanisms involving insertional deregulation of cellular protooncogene expression. We report here on the application of a two-dimensional restriction mapping-Southern hybridisation approach for analysis of retrotransposon families of low to moderate genetic complexity, which is particularly suited to pairwise comparisons between DNA samples. By using this method, non-constitutional mink-cell-focus-forming type retro-elements were readily detectable in AKR mouse thymic lymphomas against a background of approx. 30 related elements in control DNA. However, in the WEHI 3B myeloid leukaemia cell line, the resolution of two-dimensional mapping permitted detection of only occasional reinsertions of intracisternal A particle retro-elements (genetic complexity: 10(3)). In analysing the VL30 family of retrotransposon (genetic complexity: 150) we developed a strategy for identifying the known transcriptionally active sub-set of these elements in genomic DNA through the generation of an internal, diagnostic restriction fragment. Moreover, in some cases of thymic lymphoma, several candidate re-insertions of VL30 elements were detected, consistent with a suggested role for retrotransposition of this class of element in lymphomagenesis of retroviral aetiology.

Characterization of epitopes defining two major subclasses of polytropic murine leukemia viruses (MuLVs) which are differentially expressed in mice infected with different ecotropic MuLVs

Polytropic murine leukemia viruses (MuLVs) arise in mice by recombination of ecotropic MuLVs with endogenous retroviral envelope genes and have been implicated in the induction of hematopoietic proliferative diseases. Inbred mouse strains contain many endogenous sequences which are homologous to the polytropic env genes; however, the extent to which particular sequences participate in the generation of the recombinants is unknown. Previous studies have established antigenic heterogeneity among the env genes of polytropic MuLVs, which may reflect recombination with distinct endogenous genes. In the present study, we have examined many polytropic MuLVs and found that nearly all isolates fall into two mutually exclusive antigenic subclasses on the basis of the ability of their SU proteins to react with one of two monoclonal antibodies, termed Hy 7 and MAb 516. Epitope-mapping studies revealed that reactivity to the two antibodies is dependent on the identity of a single amino acid residue encoded in a variable region of the receptor-binding domain of the env gene. This indicated that the two antigenic subclasses of MuLVs arose by recombination with distinct sets of endogenous genes. Evaluation of polytropic MuLVs in mice revealed distinctly different ratios of the two subclasses after inoculation of different ecotropic MuLVs, suggesting that individual ecotropic MuLVs preferentially recombine with distinct sets of endogenous polytropic env genes.

Characterization of replication-competent retroviruses from nonhuman primates with virus-induced T-cell lymphomas and observations regarding the mechanism of oncogenesis

Rapidly progressive T-cell lymphomas were observed in 3 of 10 rhesus monkeys several months after autologous transplantation of enriched bone marrow stem cells that had been transduced with a retroviral vector preparation containing replication-competent virus (R. E. Donahue, S. W. Kessler, D. Bodice, K. McDonagh, C. Dunbar, S. Goodman, B. Agricola, E. Byrne, M. Raffeld, R. Moen, J. Bacher, K. M. Zsebo, and A. W. Nienhuis, J. Exp. Med. 176:1124-1135, 1992). The animals with lymphoma appeared to be tolerant to retroviral antigens in that their sera lacked antibodies reactive with viral proteins and contained 10(4) to 10(5) infectious virus particles per ml. By molecular cloning and DNA sequencing, we have now demonstrated that the serum from one of the monkeys contained a replication-competent retrovirus that arose by recombination between vector and packaging encoding sequences (vector/helper [V/H] recombinant) in the producer clone used for transduction of bone marrow stem cells. Southern blot analysis demonstrated 14 or 25 copies of this genome per cell where present in two animals. The genome of a second replication-competent virus was also recovered by molecular cloning; it arose by recombination involving the genome of the V/H recombinant and endogenous murine retroviral genomes in the producer clone. Twelve copies of this amphotropic virus/mink cell focus-forming virus genome were present in tumor DNA of one animal, but it was not found in tumor DNA of the other two animals with lymphoma. Southern blot analysis of DNA from various tissues demonstrated common insertion site bands in several samples of tumor DNA from one animal, suggesting clonal origin of the lymphoma. Our data are most consistent with a pathogenic mechanism in which chronic productive retroviral infection allowed insertional mutagenesis of critical growth control genes, leading to cell transformation and clonal tumor evolution.

Origins of enhancer sequences of recombinant murine leukemia viruses from spontaneous B- and T-cell lymphomas of CWD mice

Recombinant murine leukemia viruses from the highly leukemic mouse strains AKR, HRS, and C58 usually acquire pathogenic U3 region sequences fro the endogenous xenotropic virus, Bxv-1. However, the majority of tumors from another highly leukemic strain, CWD, contained recombinant viruses that lacked Bxv-1-specific sequences. The nucleotide sequence of the U3 regions of two such CWD recombinants was nearly identical to that of the endogenous ecotropic virus parent Emv-1, but they shared three nucleotide substitutions immediately 3' of the enhancer core. These substitutions were found in recombinant proviruses from about one-third of spontaneous CWD lymphomas as determined by an oligonucleotide hybridization assay of proviral fragments that had been nucleotide substitutions in the CWD viruses were inherited from an endogenous polytropic provirus that is absent in the other highly leukemic strains. On the basis of the results of these and previous studies, we propose that CWD recombinants acquire pathogenic U3 region sequences through recombination with an endogenous polytropic virus or Bxv-1 and that the pathogenicity of these sequences may be related to a sequence motif that is known to bind members of the basic helix-loop-helix class of transcription factors.

Uncoupled expression of Moloney murine leukemia virus envelope polypeptides SU and TM: a functional analysis of the role of TM domains in viral entry

Moloney murine leukemia virus ecotropic envelope expression plasmids were used to demonstrate that the synthesis of the retroviral envelope SU and TM polypeptides can be uncoupled with retention of biologic activity. By substituting a glycosyl-phosphatidylinositol (GPI) membrane anchor for part or all of the retroviral envelope transmembrane protein and creating several deletion variants of the TM subunit, we have begun to dissect the role of the TM protein in envelope function. We show that a GPI-anchored envelope can be incorporated into virions and binds receptor. We found that the envelope cytoplasmic tail, while not required, influences the efficiency of retroviral transduction at some step after membrane fusion (possibly by interacting with core). The membrane-spanning domain of TM is involved in membrane fusion, and this function is distinct from its role as a membrane anchor. As few as eight amino acids of the putative membrane-spanning domain are sufficient to achieve membrane anchoring of envelope but not to mediate membrane fusion. In addition, though not required, the membrane-spanning domain may have some direct role in the incorporation of envelope into virions. Finally, the extracellular domain of TM, besides containing the putative fusion domain and interacting with SU, may influence the synthesis or stability and the glycosylation of envelope, possibly by affecting oligomerization of the complex and proper intracellular transit.

A reverse transcriptase-polymerase chain reaction assay for the detection and quantitation of murine retroviruses

Specific hybridization primers for the PCR assay were developed to detect the presence of the ecotropic, xenotropic, and mink cell focus-forming classes of murine leukemia viruses (MuLVs) in samples derived from cultured cells and cell-free supernatants. The primers, which were tested against reference viruses from all three classes and two subclasses and accurately identified each class present, were used to characterize the endogenous expression of MuLV-related sequences in a number of murine and mink cell lines. Two murine/murine hybridomas were shown to contain expressed retroviral sequences from all three classes. The murine cell lines SC-1, Balb/c 3T3, and NIH 3T3, were found to constitutively express sequences from many of the MuLV classes. These MuLV-related sequences were not expressed in the Mus dunni or mink lung cell lines. When these primers were used in a quantitative PCR assay to determine the retroviral content of hybridoma supernatants, the values were less variable than those obtained by transmission electron microscopy (TEM). This assay can be adapted to detect and quantitate any viral contaminant in cell culture supernatants, ascites fluids, process validation samples, and final products.

Analysis of the functional and host range-determining regions of the murine ectropic and amphotropic retrovirus envelope proteins

A series of Moloney murine leukemia virus (Mo-MuLV) envelope gene constructs were analyzed for biological activity. Three classes of recombinant envelopes were examined: insertions, deletions, and chimeras. Insertion (4 to 5 amino acids) and deletion (31 to 62 amino acids) mutants spanned most of the SU (gp70)-coding region and were all biologically inactive. Radioimmunoprecipitation demonstrated that the mutant envelope proteins were incorrectly processed. The Pr80env envelope precursor proteins failed to obtain the proper posttranslational modifications and were not cleaved into SU (gp70) and TM (p15E), suggesting that disruption of Pr80env structure prevents intracellular transport and processing. To analyze the functional domains of the SU portion of the Env protein, we assembled several chimeric constructs. In these constructs, portions of the ecotropic Mo-MuLV envelope gene were replaced with corresponding sequences from the 4070A amphotropic MuLV envelope. Using a retroviral vector pseudotyping assay, 5 of 12 chimeric envelope proteins were shown to be biologically active. Host range was determined by retroviral vector transduction of the appropriate cell, by viral interference studies, and by the productive infection of Chinese hamster ovary cells expressing the murine ecotropic receptor. These results permit assignment of the amino acids responsible for host range determination. Ecotropic host range is determined by the first 88 amino acids of the Mo-MuLV SU, while the amphotropic host range-determining region spans the first 157 amino acids of the 4070A SU.

Beta cell expression of endogenous xenotropic retrovirus distinguishes diabetes-susceptible NOD/Lt from resistant NON/Lt mice

Endogeneous retroviral expression in beta cells is a feature of prediabetes in nonobese diabetic (NOD) mice. The purpose of this study was to characterize the class-specific pattern of retroviral gene expression in NOD/Lt beta cells versus a related, but diabetes-resistant strain, NON/Lt. Electron microscopic comparison of beta cells from both strains indicated low constitutive expression of the intracisternal type A (IAP) retroviral class. However, NOD beta cells, in contrast to NON beta cells, expressed an additional intracisternal retroviral form resembling a type C particle. Antibodies against both IAP and type C were detected in NOD, with the humoral response to type C, but not IAP, preceding decline in beta cell function. RNA was extracted from freshly isolated islets from NOD and NON males. Comparative Northern blot analysis of total type C retroviral gene expression using a gag-pol DNA probe corroborated expression of endogenous type C proviruses in both NOD and NON islet cells and thymus. Use of class-specific retroviral probes identified the class of expressed endogenous retrovirus distinguishing the two inbred strains. The single ecotropic provirus present in both the NOD and NON genome (Emv-30) was not expressed in islets or thymus of either strain. Comparison of endogenous xenotropic provirus content by Southern blot analysis revealed two unique xenotropic loci (Xmv-65, -66) in NOD; 8.4 and 3.0 kb xenotropic envelope (env) RNA transcripts were detected in NOD, but not NON islets and thymus. NON contained three xenotropic loci common to other inbred strains (Xmv-21, -25, and -28). Both strains were partially characterized for content of recombinant (polytropic and modified polytropic) proviruses. IAP RNA expression was common to both NOD and NON islets and hence could not be specifically associated with the unique intracisternal type C particle found in NOD, but not NON beta cells. In conclusion, this study shows that expression of xenotropic type C but not IAP distinguishes retroviral activity in NOD/Lt versus NON/Lt beta cells. The potential pathogenic role of retroviral gene expression in NOD beta cells is discussed.

Carbon tetrachloride induction of rapid changes in liver nuclear protein factors capable of sequence-specific binding to regulatory elements in the long terminal repeat of polytropic-class endogenous murine leukemia virus-related proviruses

Treatment of mice with hepatic carcinogens, including CCl4, has been shown to rapidly enhance the transcription of endogenous murine leukemia virus-related proviral sequences in the liver. To understand the mechanism for this transcriptional stimulation, we used nuclear protein preparations from mouse livers to perform DNase I protection analyses and identified nuclear protein binding on approximately 20 individual sequences within the regulatory regions of the long terminal repeat (LTR) of a polytropic-class endogenous provirus clone. From 3 to 144 h after treatment with CCl4, the livers of FVB/N mice were analyzed for specific nuclear protein binding to the LTR DNA. Three to nine hours after CCl4 treatment, decreased protection was seen at potential regulatory cis-elements throughout the LTR, including specific sites within the putative negative regulatory element (located 5' of the consensus enhancer sequences) and the 3' terminal portion of the polytropic class-specific enhancer-like inserted sequence element and around the CCAA(C/T) box in the promoter region. In addition, by 3-6 h after treatment, a transient increase in protection activity for the transcription initiation site occurred. The loss of cis-element protection expanded to other binding sites and became most marked by 48 h after treatment. As the regenerating liver recovered, the nuclear protein binding activities for these LTR sequences also recovered, but protection at the TATAA and transcription initiation sites remained deprotected at 144 h after treatment. Nuclear protein protection of other sites, particularly in the conserved LTR enhancer sequences, was minimally affected by CCl4 treatment. Three nuclear protein binding sites that showed rapid CCl4-induced kinetic changes were homologous to the consensus sequence for the binding of the transcription factor families MEF-2, HNF-1, and C/EBP. The complex kinetic changes in factors that may contribute to the rapid and transient induction of endogenous retroviral gene expression in the liver after CCl4 exposure are discussed.

Multiple complex families of endogenous retroviruses are highly conserved in the genus Gallus

We have analyzed the genome of the domestic chicken for the presence of genetic sequences related to the envelope protein-encoding genes of avian sarcoma/leukosis retroviruses to determine the organization, structure, potential functionality, and distribution of such sequences. We have previously identified in the genus Gallus an extensive group of endogenous avian retroviruses termed EAV-0. Southern blot and sequence analysis presented here of EAV-0 elements revealed that the majority of the EAV-0 elements in the domestic chicken genome have large deletions in their env genes. Screening of a line 0 chicken genomic DNA library for potential full-length env gene-containing endogenous elements yielded three provirus clones of a previously unrecognized group of endogenous retroviruses. These three clones, E13, E33, and E51, are more closely related to each other (80% or more sequence identity) than to other avian retroviruses (70% or less sequence identity). The E13 element has a large deletion in env, but the E51 element has full-length and highly divergent SU- and TM-coding domains. Complete sequence analysis of the E51 env gene region revealed a defective SU-coding domain and an intact TM-coding domain. Sequence analysis of the E51, E33, and E13 3' termini revealed highly distinctive long terminal repeats of approximately 360 bp which appear to be the products, in part, of long terminal repeat domain shuffling. Hybridization analysis with E51 and E33 env gene probes indicated that they are members of an extensive group of elements present in all Gallus species, and at least one element, E51, could be shown by polymerase chain reaction amplification and direct sequencing to have integrated prior to Gallus speciation.

The nonobese diabetic scid mouse: model for spontaneous thymomagenesis associated with immunodeficiency

Homozygosity for the severe combined immunodeficiency (scid) mutation results in a block in T- and B-lymphocyte development. An unusually high incidence of spontaneous thymic lymphoma development was observed after transfer of this mutation from the C.B-17 congenic strain background onto the diabetes-susceptible nonobese diabetic (NOD) background. Thymomagenesis in the NOD-scid/scid mouse was associated with expression of an NOD mouse-unique endogenous ecotropic murine leukemia provirus locus (Emv-30, mapped to proximal region of chromosome 11) not expressed in the standard substrain NOD/Lt thymus. All tumors exhibited insertions of ecotropic proviruses, whereas only a subset also exhibited proviral integrations of mink cell focus-forming retrovirus. Neither class of retrovirus was associated with consistent integration into genes previously associated with activation of oncogenesis. We propose that the unusual features of T-cell ontogeny characteristic of the NOD inbred strain synergize with the scid-imparted block in thymocyte development, leading to activation of the NOD-unique Emv-30 to initiate thymomagenesis.

Characterization of the endogenous nonecotropic murine leukemia viruses of NZB/B1NJ and SM/J inbred strains

We characterized 84 endogenous nonecotropic proviruses of NZB/B1NJ and SM/J inbred strains by examining proviral junction fragment segregation in recombinant inbred (RI) and backcross mice. Forty-five proviruses were shared with other laboratory strains, but 28 were unique to NZB/B1NJ or SM/J. Proviral loci were located on 17 of the 19 mouse autosomes and on both sex chromosomes. These markers will facilitate gene mapping in the NXSM RI set and contribute to the pursuit of a more complete map of the mouse genome.

Geographic origin of the Y Chromosomes in ?old? inbred strains of mice

Six distinct Y Chromosomes (Chr) were identified among 39 standard inbred strains of mice with five probes that identified Y Chr-specific restriction fragments on Southern blots. Three Y Chr types, distributed among 31 strains, were of Asian Mus musculus origin. The remaining three Y Chr types, distributed among eight strains, were of M. domesticus origin. The Asian source of the M. musculus Y Chr was confirmed by determining the DNA sequence of 221 bp from an open reading frame within the Sry (sex determining region Y) gene (Gubbay et al., Nature 346: 245-250, 1990) in three inbred strains (C57BL/6J, AKR/J, and SWR/J) and comparing the sequence to the homologous sequences derived from wild caught European and Asian M. musculus males. These data indicate that a minimum of six male mice contributed to the formation of the old inbred strains.

The dispersion of defective endogenous murine retroviral elements suggests retrotransposition-mediated amplification

The dispersion of four replication-defective endogenous proviruses, originally detected in 129 strain mice and shown to have extensive deletions of gag, pol, and env gene regions, was investigated in 13 inbred strains and substrains of mice. Using probes to sequences flanking the integration sites in 129 mice, unique genomic Eco RI fragments were assigned to each of the four endogenous proviral elements. Analyses revealed that certain of these proviral elements are present both in strains closely related to strain 129 (i.e., strains 101 and LP/J) and in more distantly related strains (i.e., strains BALB/cJ, A/J, and C3H/HeJ). In mouse strains lacking proviral integration at a particular locus, the size of the corresponding Eco RI genomic fragment and absence of a characteristic Kpn I site indicated the lack of a residual solitary long terminal repeat. Hybridization of oligonucleotide probes that distinguish the specific deletions present within these elements identified additional analogous proviral integrations at many different sites in all strains investigated. These data indicate that the diversification of these proviral elements found in inbred strains is generated by integration of new copies, rather than excision through homologous recombination. Moreover, the results are consistent with other endogenous retroviruses providing the trans-acting proteins necessary to package the defective viral RNA.

Characterization of endogenous and recombinant proviral elements of a highly tumorigenic AKR cell line

As an approach to evaluating the contribution of classes of endogenous viral sequences to leukemogenesis, a genomic library was prepared from the highly tumorigenic AKR SL12.3 cell line and screened for env-containing proviruses. An extensive battery of virus-derived probes and specific oligonucleotide probes were used to segregate 83 positive clones into related groups. The nonecotropic endogenous retroviruses were identified as members of the polytropic, modified polytropic, or xenotropic groups. At least three unique xenotropic proviruses were detected that differed from the published xenotropic sequence within a variable region of the 5' portion of env. Changes among the xenotropic proviruses included relative insertions and/or deletions that maintain an open reading frame and hence the potential to encode viable envelope gene products. Several recombinant viruses were also detected. Recombination was not random and primarily involved the formation of mink cell focus-inducing class I retroviruses via recombination between polytropic elements and ecotropic virus. One other recombinant was detected which contained ecotropic virus sequences in the 5' region encoding p15 of an otherwise xenotropic provirus. An interesting observation was the finding that certain clones contained more than one provirus within the average 20-kb cloned insert. This would not be expected if integration were totally random. The de novo recombinant proviruses identified here provide a series of potential candidates to be evaluated for their contribution to the tumorigencity of the SL12.3 cell line.

Mechanism of leukemogenesis induced by mink cell focus-forming murine leukemia viruses

The Friend or Moloney mink cell focus-forming (MCF) virus encodes a recombinant-type envelope glycoprotein, gp70, that is closely related to the membrane glycoprotein, gp55, of Friend spleen focus-forming virus (SFFV). We have shown previously that gp55 has the ability to activate cell growth by binding to the cellular receptor for erythropoietin. Here we show that gp70 encoded by either the Friend or Moloney MCF virus also binds to the erythropoietin receptor and that coexpression of the receptor and gp70 in an interleukin-3 (IL-3)-dependent cell line can activate IL-3-independent growth. Furthermore, when the cDNA for the human IL-2 receptor beta chain, which is related by sequence to the erythropoietin receptor, was introduced into this cell line, it became growth factor independent after infection either with SFFV or with one of the two MCF viruses but not with an ecotropic virus. Based on these observations, we propose a mechanism for the early stage of leukemogenesis induced by the MCF-type murine leukemia viruses.

Origin of pathogenic determinants of recombinant murine leukemia viruses: analysis of Bxv-1-related xenotropic viruses from CWD mice

The acquisition of U3 region sequences derived from the endogenous xenotropic provirus Bxv-1 appears to be an important step in the generation of leukemogenic recombinant viruses in AKR, HRS, C58, and some CWD mice. We report here that each of three CWD lymphomas produced infectious xenotropic murine leukemia virus related to Bxv-1. In Southern blot experiments, these proviruses hybridized to probes that were specific for the xenotropic envelope and Bxv-1 U3 region sequences. Nucleotide sequence analysis of a cloned CWD xenotropic provirus, CWM-S-5X, revealed that the envelope gene was closely related to but distinct from those of other known xenotropic viruses. In addition, the U3 region of CWM-S-5X contained a viral enhancer sequence that was identical to that found in MCF 247, a recombinant AKR virus that is thought to contain the Bxv-1 enhancer. Finally, restriction enzyme sites in the CWM-S-5X provirus were analogous to those reported within Bxv-1. These results establish that the virus progeny of Bxv-1 have the potential to donate pathogenic enhancer sequences to recombinant polytropic murine leukemia viruses. Interestingly, the three CWD polytropic viruses that were isolated from the same tumor cells that produced the Bxv-1-like viruses had not incorporated Bxv-1 sequences into the U3 region.

The molecular biology of radiation-induced carcinogenesis: thymic lymphoma, myeloid leukaemia and osteosarcoma

In mice, external X- or gamma-irradiation may induce thymic lymphomas or myeloid leukaemias, while bone-seeking alpha-emitters may induce osteosarcomas and, to a lesser extent, acute myeloid leukaemia. The present paper aims to review briefly some of the experimental data with respect to the molecular mechanisms underlying these radiation-induced carcinogenic processes. Thymic lymphomagenesis proceeds through an indirect mechanism. Recombinant proviruses often occur in the tumour cell DNA, favouring the idea that they might be involved. However, there are indications that they might mediate tumour growth rather than induction. It is plausible that activation of ras oncogenes by somatic point mutations might play a role in the carcinogenic process, although at a yet undetermined stage. Myeloid leukaemogenesis is characterized by a very early, putative initiating event, consisting of non-random rearrangements and/or deletions of chromosome 2. These may be related to deletions in the developmentally important homeobox gene clusters and to rearrangements of the sequences flanking the IL-1 beta gene. Either a gene of the homeobox family or IL-1 beta might be considered as potentially involved in the induction process. Osteosarcomagenesis in mice is often associated with the expression of proviruses, and the tumours often contain somatically acquired proviruses. These viruses may contribute to tumour development by affecting various growth-suppressor genes. Viruses isolated from bone tumours, although non-sarcomagenic, induce osteopetrosis, osteomas and lymphomas upon infection of newborn mice. Osteogenic tumours frequently display amplification of a region on mouse chromosome 15, which encompasses c-myc and Mlvi-1 sequences. Enhanced transcription of various oncogenes is found in individual tumours, but no specificity for osteosarcomas has been identified. In vitro systems of skeletoblast differentiation are being developed to study tumour induction in vitro.

A linkage map of endogenous murine leukemia proviruses

Thirty endogenous proviruses belonging to the modified polytropic (Mpmv) class of murine leukemia virus (MLV) were identified by proviral-cellular DNA junction fragment segregation in several sets of recombinant inbred mice. Twenty-six Mpmv loci were mapped to chromosomal regions by matching proviral strain distribution patterns to those of previously assigned genes. Like other endogenous nonecotropic MLVs, Mpmv loci were present on several chromosomes in all strains examined. We pooled recombinant inbred strain linkage data from 110 MLV loci and selected marker genes in order to construct a chromosomal linkage map. Every mouse chromosome was found to harbor at least one proviral insertion, and several regions contained multiple integrations. However, the overall distribution of the 110 mapped proviruses did not deviate significantly from a random distribution. Because of their polymorphism in inbred strains of mice, and the ability to score as many as 57 proviruses per strain using only three hybridization probes, the nonecotropic MLVs mapped in common strains of mice offer a significant advantage over older methods (e.g., biochemical or individual restriction fragment polymorphisms) as genetic markers. These endogenous insertion elements should also be useful for assessing strain purity, and for studying the relatedness of common and not-so-common inbred strains.

Human cells infected with retrovirus vectors acquire an endogenous murine provirus

A 5.2-kilobase mouse RNA is expressed in human cells following infection with recombinant retroviruses propagated in mouse NIH 3T3 cells as psi-2 pseudotypes. This RNA is transcribed from a defective mink cell focus-forming provirus and copackaged into virions and integrated into human target cell DNA at a frequency comparable to that of the recombinant retrovirus genome.

Inhibition of interleukin-2 production by tumor cell products and by CKS-17, a synthetic retroviral envelope peptide

Tumor cells of all types and species tested have been found to produce, in culture, substances that depress the expression of cell-mediated immunity, in the form of delayed-type hypersensitivity reactions in mouse feet. The factors responsible appear related immunologically to the retroviral envelope protein p15E. We have measured the effects of tumor products and conjugates of a p15E-related peptide, CKS-17, on interleukin-2 (IL-2) production by cultured, mitogen-stimulated EL4 cells; in this system IL-2 production is independent of IL-1. Supernatants of cultures of mouse, human and guinea-pig tumor cells inhibited IL-2 production in a dose-dependent fashion. CKS-17 conjugates, but not control conjugates, also inhibited IL-2 production. Responses to IL-2 of the CTLL line used were less inhibited by tumor products and very slightly inhibited by CKS-17 conjugates. IL-2 receptor density, assayed by flow cytometry, was not inhibited. IL-2 production was inhibited whether the tumor products or CKS-17 conjugates were added early or late in the course of culture of stimulated EL4 cells. Inhibition by CKS-17 conjugates was selective in that IL-2 production was inhibited to a greater degree than general protein synthesis in EL4 cells, and general protein synthesis by fibroblasts was unaffected. Measurement of IL-2 mRNA suggested that inhibition of IL-2 production was mediated post-transcriptionally. Fractionation of six different tumor supernatants on Sephacryl S-300 revealed a single peak of activity with an apparent molecular mass of 18 kDa. Antibodies to CKS-17 conjugates neutralized the inhibitory effect of native tumor products on IL-2 production. Inhibition of IL-2 production, by factors related to p15E, provides a strategically effective means of subversion of host defenses by tumors, and abrogation of this inhibition by means of antibodies might promote host resistance to tumor growth.

Truncated gag products encoded by Gv-1-responsive endogenous retrovirus loci

The conversion of endogenous or exogenous murine retroviruses to a leukemogenic phenotype involves recombination with retroviral sequences present in host genomic DNA. In the 129 Gix+ inbred strain, these endogenous sequences are replication defective but still express retroviral proteins under the apparent transcriptional control of the Gv-1 regulatory locus. To study the protein-coding potential of Gv-1-regulated endogenous retroviral loci, we used oligonucleotide probes directed to env deletion breakpoints identified in previously characterized cDNA clones. Four endogenous retroviral loci were isolated from a library of 129 Gix+ genomic DNA with these probes. Three loci cloned with the env deletion probe del env-1 had virtually identical proviral inserts by restriction analysis. A unique locus was identified and cloned with the del env-2 probe, which must therefore represent a Gv-1-responsive element. Restriction enzyme and nucleotide sequence analyses indicated that the del env-1 and del env-2 loci represented members of the polytropic and modified polytropic classes of endogenous retrovirus, respectively. Despite this divergence, members of both classes contained identical deletions of 19 nucleotides within p30gag and of 1,474 nucleotides from p10gag into the reverse transcriptase-coding region of pol, suggesting that a recombination event had occurred between these proviral sequences prior to insertion within the genome. The del env-1 and del env-2 loci retained coding capacity for truncated gag polyproteins, confirmed by in vitro translation and immunoprecipitation of the protein products. Nucleotide sequence comparison of the untranslated leader (L) regions of the del env-1 and del env-2 loci to a replication-competent ecotropic virus indicated regions that might be important to dispersion of these endogenous retroviral elements throughout the host genome.

Genetic identification of endogenous polytropic proviruses by using recombinant inbred mice

Forty-seven endogenous polytropic murine viruses (Pmv) were identified by examination of proviral-cellular DNA junction fragment segregation in recombinant inbred (RI) mice. Most Pmv loci were found in more than one of the seven RI progenitor strains analyzed, but only four were present in all strains. Chromosomal assignments for 41 Pmv loci were determined by comparing their RI strain distribution patterns with those of known genetic markers. Pmv loci were found dispersed throughout the genome, with chromosomes 1, 3, 4, 5, 7, 11, 12, 15, and 16 each carrying three or more proviruses. Linkage analysis in the AKXD RI set suggested that the gene encoding mink cell focus-forming virus resistance (Rcmfr) of DBA/2J mice is probably not a Pmv provirus. It was also deduced that no single, AKR/J-specific Pmv provirus is required as an env gene donor for thymomagenic mink cell focus-forming viruses. In addition, a Pmv provirus was very closely associated with the albino mutation on chromosome 7.

Negative regulatory element associated with potentially functional promoter and enhancer elements in the long terminal repeats of endogenous murine leukemia virus-related proviral sequences

Three series of recombinant DNA clones were constructed, with the bacterial chloramphenicol acetyltransferase (CAT) gene as a quantitative indicator, to examine the activities of promoter and enhancer sequence elements in the 5' long terminal repeat (LTR) of murine leukemia virus (MuLV)-related proviral sequences isolated from the mouse genome. Transient CAT expression was determined in mouse NIH 3T3, human HT1080, and mink CCL64 cultured cells transfected with the LTR-CAT constructs. The 700-base-pair (bp) LTRs of three polytropic MuLV-related proviral clones and the 750-bp LTRs of four modified polytropic proviral clones, in complete structures either with or without the adjacent downstream sequences, all showed very little or negligible activities for CAT expression, while ecotropic MuLV LTRs were highly active. The MuLV-related LTRs were divided into three portions and examined separately. The 3' portion of the MuLV-related LTRs that contains the CCAAC and TATAA boxes was found to be a functional promoter, being about one-half to one-third as active as the corresponding portion of ecotropic MuLV LTRs. A MboI-Bg/II fragment, representing the distinct 190- to 200-bp inserted segment in the middle, was found to be a potential enhancer, especially when examined in combination with the simian virus 40 promoter in CCL64 cells. A PstI-MboI fragment of the 5' portion, which contains the protein-binding motifs of the enhancer segment as well as the upstream LTR sequences, showed moderate enhancer activities in CCL6 cells but was virtually inactive in NIH 3T3 cells and HT1080 cells; addition of this fragment to the ecotropic LTR-CAT constructs depressed CAT expression. Further analyses using chimeric LTR constructs located the presence of a strong negative regulatory element within the region containing the 5' portion of the enhancer and the immediate upstream sequences in the MuLV-related LTRs.

Defective virus is associated with induction of murine retrovirus-induced immunodeficiency syndrome

C57BL/6 mice infected with a mixture of murine leukemia viruses (MuLV) develop a syndrome characterized by lymphoproliferation and profound immunodeficiency. Analyses of this viral mixture (LP-BM5 MuLV) showed that it includes replication-competent ecotropic and mink cell focus-inducing MuLV and defective viruses with genome sizes of 3.8-6.5 kilobases. The ecotropic and mink cell focus-inducing MuLV biologically cloned from the mixture did not induce disease, whereas viral preparations containing the ecotropic MuLV and 4.8-kilobase defective virus were active. Cells producing the 4.8-kilobase defective virus expressed an unusual gag-encoded polyprotein of Mr 60,000.

Primary virus-induced lymphomas evade T cell immunity by failure to express viral antigens

T lymphoma induction by the mink cell focus-inducing murine leukemia virus MCF 1233 in C57BL/10 and C57BL/6 mice is influenced by a strongly Th-dependent, H-2I-A-restricted antiviral immune response (25). We compared the MHC class I as well as viral env and gag antigenic cell surface profiles of frequent T lymphomas of H-2I-A nonresponder-type mice to that of rare T lymphomas of H-2I-A responder-type mice. Membrane immunofluorescence studies, with a panel of anti-env mAbs (reactive with the highly conserved gp70f epitope, the p15Ec epitope, and the gp70-p15E complex), a polyclonal anti-p30 serum, and anti-H-2 class I mAbs, showed that all 17 nonresponder tumors tested expressed high levels of both env and gag viral proteins, and 15 of these 17 nonresponder tumors expressed high levels of H-2 class I K and D antigens. In contrast, 10 of 11 responder lymphomas lacked env and/or gag determinants. The only responder lymphoma with both strong env and gag expression failed to express H-2K and -D antigens. Preferential loss of env or gag expression did not correlate with H-2 class I allelic specificities. Both responder and nonresponder T lymphoma DNA contained multiple, predominantly MCF-like, newly acquired proviral integrations. Differences in viral antigen cell surface expression were confirmed at cytoplasmic and RNA levels. The amounts of 8.2- and 3.2-kb viral RNA were greatly reduced in two responder lymphomas when compared with four nonresponder lymphomas. In both responder lymphomas, aberrantly sized viral RNA species were found. Upon in vivo passage of these responder lymphomas in either immunocompetent or T cell-deficient nu/nu mice, it was found that various molecular mechanisms may underlie the lack of viral antigen expression at the cell surface of these lymphomas. One lymphoma re-expressed viral antigens when transplanted with nu/nu mice, whereas the other remained stably gag negative. The combined findings indicate that an H-2I-A-regulated antiviral immune response not only strongly reduces T lymphoma incidence, but also forces T lymphomas that still arise to poorly express viral antigens, thus explaining their escape from immunosurveillance.