Infectious Murine Leukemia Virus Research Articles

Molecular cloning of infectious ecotropic murine leukemia virus AK7 from an emv-14-positive AKXL-5 mouse and the resistance of AK7 to recognition by cytotoxic T lymphocytes.

The AKXL-5 recombinant inbred mouse strain is positive for the endogenous ecotropic murine leukemia virus emv-14, the only emv present in its germ line. emv-14 is of particular interest because spleen cells expressing emv-14 virus escape recognition by anti-AKR/Gross virus-specific cytotoxic T lymphocytes. We report here the isolation and characterization of a replication-competent emv clone, pAK7, derived from an AKXL-5 mouse. This clone is novel in that it encodes a variant ecotropic murine leukemia virus that, when expressed in SC.Kb target cells, fails to be recognized efficiently by anti-AKR/Gross virus cytotoxic T lymphocytes. The pAK7 clone can therefore be used to further probe mechanisms of escape from cell-mediated immunity.

Graffi murine leukemia virus: molecular cloning and characterization of the myeloid leukemia-inducing agent.

The Graffi murine leukemia virus (MuLV) is a retroviral mixture that induces predominantly myeloid leukemia in several inbred strains of mice. To analyze the viral component responsible for the myeloid leukemogenesis, we cloned several proviruses from a Graffi MuLV-infected cell line. Several infectious molecular clones were obtained that could be classified into two distinct groups of infectious MuLV. Both types of MuLV were nondefective, ecotropic, and NB tropic and induced granulocytic leukemia in BALB/c and NFS mice. Restriction enzyme analysis and molecular hybridization with several MuLV probes on one molecular clone from each group revealed that both groups are closely related to each other but are clearly distinct from all known retroviruses. One component of MuLV, however, induced leukemia with a shorter latency period and harbored a lengthier long terminal repeat. The long terminal repeat of the more leukemogenic component of MuLV had acquired a 60-bp perfect duplication in the U3 region. Analysis of the tumor DNAs with probes for the mouse T-cell receptor and immunoglobulin heavy chain genes revealed frequent rearrangements with one or both probes. This concomitant expression by leukemic cells of markers of different lineages, observed in human leukemias, has been termed "lineage infidelity" and confirms that the latter rearrangements are not restricted to hematopoietic precursors committed to lymphoid differentiation.

Basic amino acids flanking the zinc finger of Moloney murine leukemia virus nucleocapsid protein NCp10 are critical for virus infectivity.

Nucleocapsid (NC) protein NCp10 of Moloney murine leukemia virus is encoded by the 3' domain of gag and contains a zinc finger surrounded by basic amino acids. During virion assembly, NC protein is necessary for core formation and the NC zinc finger is required for the packaging of the genomic RNA dimer. In vitro NCp10 has RNA-binding and -annealing activities critical for virus infectivity, since NCp10 promotes dimerization of viral RNA containing the Psi packaging element and annealing of replication primer tRNA(Pro) to the initiation site of reverse transcription (primer-binding site). To investigate the role of the basic amino acids flanking the NCp10 zinc finger, neutral residues were substituted for the basic amino acids and the effects of these mutations in vivo on virus assembly and infectivity and in vitro on the RNA-annealing activity of NCp10 were analyzed. Here we report that the substitution of 1 or 2 neutral amino acids for the basic residues did not impair the production of mature virions but that infectivity was either moderately or strongly attenuated. When more than 2 basic residues were replaced by neutral amino acids, viruses were poorly infectious because of a severe defect in genomic RNA dimer packaging and initiation of reverse transcription. In vitro NCp10-derived peptides with similar mutations were chemically synthesized and were found to be either fully or partially active or completely inactive. These data indicate that the basic residues flanking the zinc finger of NCp10 are required for the production of infectious Moloney murine leukemia virus virions.

Genetic control of retroviral disease in aging wild mice.

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

Direct determination of the point mutation rate of a murine retrovirus.

The point mutation rate of a murine leukemia virus (MuLV) genome (AKV) was determined under conditions in which the number of replicative cycles was carefully controlled and the point mutation rate was determined by direct examination of the RNA genomes of progeny viruses. A clonal cell line infected at a low multiplicity of infection (2 x 10(-3)) was derived to provide a source of virus with high genetic homogeneity. Virus stocks from this cell line were used to infect cells at a low multiplicity of infection, and the cells were seeded soon after infection to obtain secondary clonal cell lines. RNase T1-oligonucleotide fingerprinting analyses of virion RNAs from 93 secondary lines revealed only 3 base changes in nearly 130,000 bases analyzed. To obtain an independent assessment of the mutation rate, we directly sequenced virion RNAs by using a series of DNA oligonucleotide primers distributed across the genome. RNA sequencing detected no mutations in over 21,000 bases analyzed. The combined fingerprinting and sequencing analyses yielded a mutation rate for infectious progeny viruses of one base change per 50,000 (2 x 10(-5)) bases per replication cycle. Our results suggest that over 80% of infectious progeny MuLVs may be replicated with complete fidelity and that only a low percentage undergo more than one point mutation during a replication cycle. Previous estimates of retroviral mutation rates suggest that the majority of infectious progeny viruses have undergone one or more point mutations. Recent studies of the mutation rates of marker genes in spleen necrosis virus-based vectors estimate a base substitution rate lower than estimates for infectious avian retroviruses and nearly identical to our determinations with AKV. The differences between mutation rates observed in studies of retroviruses may reflect the imposition of different selective conditions.

Influence of murine leukemia proviral integrations on development of N-methyl-N-nitrosourea-induced thymic lymphomas in AKR mice.

The AKR mouse strain is characterized by a high incidence of spontaneous thymic lymphoma that appears in older animals (greater than 6 months of age) and is associated with novel provirus integrations of ecotropic and recombinant murine leukemia viruses (MuLVs). Treatment of 4- to 6-week-old AKR/J mice with the carcinogen N-methyl-N-nitrosourea (MNU) results in thymic lymphomas that arise as early as 3 to 4 months of age and contain novel somatically acquired MuLV provirus integrations. The AKR/J strain develops MNU-induced lymphoma with a higher incidence and shorter latency than has been observed for other inbred mouse strains. To determine whether provirus integrations of endogenous MuLV account for the enhanced susceptibility of the AKR strain, the incidence and latency of MNU-induced lymphoma development was compared in AKR/J and AKR.Fv-1b mice. The restrictive b allele of the Fv-1 locus restricts integration and replication of endogenous N-tropic MuLV; therefore, AKR-Fv-1b mice have a very low incidence of spontaneous lymphoma. In contrast, AKR.Fv-1b mice develop MNU-induced lymphomas with an incidence and latency similar to those of the AKR/J strain. Furthermore, thymic lymphomas from both strains express an immature CD4-8+ phenotype, indicating neoplastic transformation of the same thymocyte subset. Southern blot analysis confirmed that lymphoma DNA from AKR.Fv-1b mice did not contain somatically acquired provirus integrations. These results demonstrate that provirus integration does not contribute to the predisposition of AKR mice to develop a high incidence of early MNU-induced lymphomas. Nevertheless, MNU treatment stimulated high-level expression of infectious ecotropic MuLV in AKR.Fv-1b as well as in AKR/J mice, suggesting that viral gene products might enhance lymphoma progression.

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.

Evidence against expression of an endogenous murine leukemia virus causing cellular resistance to lysis by activated macrophages.

In previous studies we observed that resistance of murine SV40-transformed fibroblast cell lines to cytolysis by activated macrophages was frequently associated with cellular expression of the gp70 of an endogenous ecotropic murine leukemia virus (MuLV). The work described here was initiated to test directly for a causative relationship between MuLV expression and resistance to lysis by macrophages. Northern blot analysis revealed that macrophage-resistant cells contain full length retroviral RNA. A panel of mAb which distinguish among host-range classes of MuLV detected only a non-recombinant ecotropic gp70 in these cells. The ecotropic MuLV from two independently derived macrophage resistant cells were isolated by limiting dilution cloning on Mus dunii fibroblasts. These viruses were then used to infect macrophage-sensitive cell lines and the resultant MuLV-positive cells tested for sensitivity to macrophage cytolysis. The MuLV-infected lines remained highly sensitive to macrophage lysis despite their high levels of cell surface gp70 and release of infectious MuLV. Thus, although we cannot rule out the possibility that MuLV or a product thereof is necessary for development of macrophage resistance in transformed cells, expression of MuLV per se is not sufficient to create the resistant phenotype.

Establishment and characterization of osteogenic cell lines from a spontaneous murine osteosarcoma

Abstract Five clonal cell lines were established from a spontaneous BALB/c mouse osteosarcoma, and characterized. Four of these lines showed some similarities in morphology, in vitro growth properties, production of collagenous and noncollagenous extracellular matrix proteins and osteogenic differentiation. The cells formed colonies with characteristic differences in size and morphology in soft agar, and osteogenic sarcomas and metastases in syngeneic mice after transplantation. Ultrastructurally, cells in the transplant tumours showed marked osteogenic features. There were no osteoclast-like cells. The fifth cell line had somewhat different characteristics. All five lines expressed infectious endogenous murine leukemia viruses. Increased c- myc protoon-cogene expression was found in one cell line and c- fos expression at different levels in all lines. There was only very low expression of c-Ha- ras and no expression of c-Ki- ras and c- sis. DNA analysis showed the presence of newly acquired proviral genomes integrated at different sites in the cellular DNA. The results show that distinct osteogenic neoplastic subclones can be obtained from a primary mouse osteosarcoma. Although the clones exhibited an appreciable morphological, functional, and molecular diversity they retained the basic pathogenic properties of the tumour from which they were derived.

A nucleotide substitution in the gag N terminus of the endogenous ecotropic DBA/2 virus prevents Pr65gag myristylation and virus replication.

The endogenous ecotropic provirus Emv-3 present in DBA/2 mice is poorly expressed in the animal, as well as in cell cultures. Transfection of proviral DNA into NIH 3T3 cells localized the expression defect to the 5' region of the viral genome, spanning the untranslated region and the N-terminal part of the gag gene. Comparison of the nucleotide sequence of the Emv-3 provirus with the sequence of the highly infectious Akv murine leukemia virus revealed three nucleotide differences within the gag coding region. One of these differences was found in codon 3 of the gag polyprotein, where a Gln codon is seen in Akv and a Pro codon is differences was found in codon 3 of the gag polyprotein, where a Gln codon is seen in Akv and a Pro codon is seen in Emv-3. By site-directed mutagenesis, we showed that the defect of Emv-3 expression indeed is localized to codon 3 of the gag gene. The gag polyprotein of mammalian type C retrovirus contains myristic acid covalently linked to the N-terminal glycine. This myristylation is not seen in the Emv-3-coded gag polyprotein. We showed that the in vitro-mutagenized Emv-3 genome containing a Gln codon at position 3 of the gag gene yields a myristylated gag polyprotein. Thus, it seems most likely that the defect of expression of the Emv-3 provirus is due to the presence of a proline is position 3 of the gag polyprotein, preventing the myristylation.

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-4 r 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.

Inoculation of newborn SWR/J females with an ecotropic murine leukemia virus can produce transgenic mice.

Endogenous ecotropic murine leukemia proviruses that were not present in the parental stock are acquired by the progeny of some SWR/J X RF/J hybrid females. We have made a stock of an ecotropic murine leukemia virus produced by such a hybrid female and inoculated newborn SWR/J females with it. We show that upon crossing of the inoculated females to SWR/J males, some of their progeny acquire ecotropic proviruses. Although most of these proviruses appear to be distributed in somatic tissues in a mosaic way, some are transmitted through the germ line. Thus an exogenous infection is able to mimic the phenomenon observed in SWR/J X RF/J hybrid mice. Available evidence suggests that this infection occurs during oogenesis in the recipient female. Our results document the conversion of an exogenous infectious ecotropic murine leukemia virus to an endogenous provirus without any manipulation of either eggs or embryos.

Effect of the Gv-1 locus on Moloney ecotropic murine leukemia virus induced disease in inbred wild mice.

The nonacute retroviruses induce neoplastic disease following a long latency period. During this latency period, the infectious ecotropic murine leukemia virus (MuLV) spreads throughout the mouse resulting in chronic viremia. As the virus spreads, it recombines with germline copies, or proviruses, of nonecotropic MuLVs to produce infectious mink cell focus-forming (MCF) viruses. These MCF viruses resemble their ecotropic MuLV progenitors but contain novel sequences in their envelope (env) genes which encode the major virion glycoprotein, and in the long terminal repeats (LTRs) which contain transcriptional control signals. These recombinant viruses have an expanded host range and are themselves leukemogenic. The mechanisms involved in the production of MCF viruses are unknown, although it has been established that multiple recombinations occur, and novel viruses have been isolated from preleukemic tissues which may represent intermediate recombinants in this process (Cloyd and Chattopadhyay 1986) . Proviral sequences related to both the xenotropic and MCF host range classes of infectious MuLVs ultimately contribute to the altered env and LTR regions of the MCF virus (Holland et al. 1985; Quint et al. 1984) . Although the specific proviruses involved in this process have not been identified, it has been suggested that the xenotropic provirus Bxv-1 contributes LTR sequences to the leukemogenic MCF MuLV (Hoggan et al. 1986) .KeywordsMurine Leukemia VirusWild MouseProviral SequenceExpand Host RangeGermline CopyThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Generation of infectious Moloney murine leukemia viruses with deletions in the U3 portion of the long terminal repeat.

Deletional analysis within the long terminal repeat (LTR) of Moloney murine leukemia virus (M-MuLV) was performed. By molecular cloning, deletions were made in the vicinity of the XbaI site at -150 base pairs (bp) in the U3 region, between the tandemly repeated enhancers and the TATA box. The effects of the deletions on LTR function were measured in two ways. First, deleted LTRs were fused to the bacterial chloramphenicol acetyltransferase gene and used in transient expression assays. Second, infectious M-MuLVs were generated by transfection of M-MuLV proviruses containing the deleted LTRs, and the relative infectivity of the mutant viruses was assessed by XC-syncytial assay. Most of the deleted LTRs examined showed relatively high promoter activity in the transient chloramphenicol acetyltransferase assays, with values ranging from 20 to 50% of the wild-type M-MuLV LTR. Thus, the sequences between the enhancers and the TATA box were not absolutely required for transient expression. However, infectivity of viruses carrying the same deleted LTRs showed more pronounced effects. Deletion of sequences from -195 to -174 bp reduced infectivity 20- to 100-fold. Deletion of sequences within the region from -174 to -122 bp did not affect infectivity, indicating that this region is dispensable. On the other hand, deletion of sequences from -150 to -40 bp reduced infectivity from 5 to 6 logs, although the magnitude of the reduction partly may have reflected threshold envelope protein requirements for positive XC assays. The reduced infectivity did not appear to result from a failure of proviral DNA synthesis or integration by the mutant. Thus, the infectivity measurements identified three functional domains in the region between the enhancers and the TATA box.

Generation of infectious Moloney murine leukemia viruses with deletions in the U3 portion of the long terminal repeat.

Deletional analysis within the long terminal repeat (LTR) of Moloney murine leukemia virus (M-MuLV) was performed. By molecular cloning, deletions were made in the vicinity of the XbaI site at -150 base pairs (bp) in the U3 region, between the tandemly repeated enhancers and the TATA box. The effects of the deletions on LTR function were measured in two ways. First, deleted LTRs were fused to the bacterial chloramphenicol acetyltransferase gene and used in transient expression assays. Second, infectious M-MuLVs were generated by transfection of M-MuLV proviruses containing the deleted LTRs, and the relative infectivity of the mutant viruses was assessed by XC-syncytial assay. Most of the deleted LTRs examined showed relatively high promoter activity in the transient chloramphenicol acetyltransferase assays, with values ranging from 20 to 50% of the wild-type M-MuLV LTR. Thus, the sequences between the enhancers and the TATA box were not absolutely required for transient expression. However, infectivity of viruses carrying the same deleted LTRs showed more pronounced effects. Deletion of sequences from -195 to -174 bp reduced infectivity 20- to 100-fold. Deletion of sequences within the region from -174 to -122 bp did not affect infectivity, indicating that this region is dispensable. On the other hand, deletion of sequences from -150 to -40 bp reduced infectivity from 5 to 6 logs, although the magnitude of the reduction partly may have reflected threshold envelope protein requirements for positive XC assays. The reduced infectivity did not appear to result from a failure of proviral DNA synthesis or integration by the mutant. Thus, the infectivity measurements identified three functional domains in the region between the enhancers and the TATA box.

Normal expression of polymorphic endogenous retroviral RNA containing segments identical to mink cell focus-forming virus

In the absence of infectious virus, strains of mice express polyadenylated RNA transcripts homologous to the genome of murine leukemia virus. In addition to transcripts consistent with full-length and spliced env retroviral RNAs, several unique RNA species which lack the env sequence accumulate in a tissue-specific manner. These RNA species are presumed to be transcribed from endogenous retroviral sequences that constitute the bulk of the murine leukemia virus-related sequences in the murine genome. To determine the relationship of these RNA transcripts to infectious murine leukemia virus and the precise structural basis of the heterogeneity observed for the env-lacking transcripts, we isolated and sequenced cDNA recombinants representing the RNAs expressed in strain 129 GIX+ mice. Comparisons of the nucleotide sequences demonstrated that the endogenous retroviral transcripts differed in pol, p15E, and R-peptide regions by single nucleotide changes. In contrast, the gp70-coding regions of two cDNA clones derived from epididymis and liver were completely homologous over a 599-nucleotide overlapping sequence. The structures of env-lacking transcripts were examined in two independent cDNA clones, and each was found to contain a different deletion that was potentially mediated by seven-base pair direct repeats in the intact sequence. The extensive sequence homology between cDNAs allowed construction of a cumulative sequence map of the 3' end of an intact endogenous retroviral transcript. A comparison of this sequence with infectious ecotropic and mink cell focus-forming viruses revealed that the endogenous transcripts are highly homologous with the substituted portions of leukemogenic mink cell focus-forming viruses and therefore further define the boundaries of recombination required to generate these viruses.

Neurotropic Cas-BR-E murine leukemia virus harbors several determinants of leukemogenicity mapping in different regions of the genome.

The infectious virus derived from the molecularly cloned genome of the neurotropic ecotropic murine Cas-BR-E retrovirus was previously shown to have retained the ability to induce hind-limb paralysis and leukemia when inoculated into susceptible mice (P. Jolicoeur, N. Nicolaiew, L. DesGroseillers, and E. Rassart, J. Virol. 45:1159-1163, 1983). To map the viral sequences encoding the leukemogenic determinant(s) of this virus, we used chimeric viral genomes constructed in vitro between cloned viral DNAs from the leukemogenic Cas-BR-E murine leukemia virus (MuLV) and from the related nonleukemogenic amphotropic 4070-A MuLV. Infectious chimeric MuLVs, recovered from NIH 3T3 cells microinjected with these DNAs, were inoculated into newborn NIH Swiss, SIM.S, and SWR/J mice to test their leukemogenic potential. We found that each chimeric MuLV, harboring either the long terminal repeat, the gag-pol, or the pol-env region of the Cas-BR-E MuLV genome, was leukemogenic, indicating that this virus harbors several determinants of leukemogenicity mapping in different regions of its genome. This result suggests that the amphotropic 4070-A MuLV has multiple regions along its genome which prevent the expression of its leukemogenic phenotype, and it also shows that substitution of only one of these regions for Cas-BR-E MuLV sequences is sufficient to make it leukemogenic.

Greying with age in mice: relation to expression of murine leukemia viruses

Some strains of C57BL/10 H-2-congenic mice were found to exhibit greying with age, whereas others did not. Two patterns of greying were observed, diffuse greying beginning at 4 to 6 months of age and patterned greying beginning at 4 to 6 weeks. Strains exhibiting either greying pattern expressed high levels of infectious ecotropic and mink cell focus-inducing murine leukemia viruses (MuLV) in tests of thymus and spleen and in cultures from skin or tail biopsies, whereas nongreying strains expressed little virus until late in life. Electron microscopy demonstrated large accumulations of MuLV in grey, but not in black areas, of skin from a mouse with patterned greying. Infectious MuLV was produced spontaneously by embryos of greying, but not of nongreying, mice and pups of nongreying strains fostered on greying mothers turned grey after 3 months. These results suggest that greying with age results from melanocyte dysfunction that occurs subsequent to pre- or early postnatal infection with MuLV.

Abnormalities induced by the mutant gene, lpr. Patterns of disease and expression of murine leukemia viruses in SJL/J mice homozygous and heterozygous for lpr.

SJL/J mice heterozygous or homozygous for the lpr mutation were compared with SJL/J-+/+ mice for longevity, histopathology, antigenic characteristics of lymphocytes and expression of murine leukemia viruses (MuLV). In comparison to +/+ mice, lpr homozygotes had a markedly shortened life span, died with infiltrative pulmonary disease, but little or no renal disease, and expressed high levels of infectious ecotropic MuLV in lymphoid tissues. SJL-lpr/+ mice had a life span intermediate between SJL-+/+ and -lpr/lpr mice, died with lymphomas that histologically resembled the neoplasms of +/+ mice, and sometimes expressed high levels of ecotropic MuLV. The lymphomas of lpr/+ could be transplanted to +/+ recipients in 78% of cases, and continuous in vitro lines were established from some of them. Similar effects on virus expression or lymphoma development were not observed in other strains homozygous or heterozygous for the lpr mutation. These results indicate that the diseases expressed by mice homozygous for the lpr mutation are highly strain-dependent, and that this gene can have an effect in the heterozygous state in SJL mice.

Envelope and long terminal repeat sequences of a cloned infectious NZB xenotropic murine leukemia virus.

An infectious NZB xenotropic murine leukemia virus (MuLV) provirus (NZB was molecularly cloned from the Hirt supernatant of NZB-IU-6-infected mink cells, and the nucleotide sequence of its env gene and long terminal repeat (LTR) was determined. The partial nucleotide sequence previously reported for the env gene of NFS-Th-1 xenotropic proviral DNA (Repaske, et al., J. Virol. 46:204-211, 1983) is identical to that of the infectious NZB xenotropic MuLV DNA reported here. Alignment of nucleotide or deduced amino acid sequences, or both, of xenotropic, mink cell focus-forming, and ecotropic MuLV proviral DNAs in the env region identified sequence differences among the three host range classes of C-type MuLVs. Major differences were confined to the 5' half of env; a high degree of homology was found among the three classes of MuLVs in the 3' half of env. Alignment of the nucleotide sequence of the LTR of NZB xenotropic MuLV with those of the LTRs of NFS-Th-1 xenotropic, mink cell focus-forming, and ecotropic MuLVs revealed extensive homology between the LTRs of xenotropic and MCF247 MuLVs. An inserted 6-base-pair repeat 5' to the TATA box was a unique feature of both NZB and NFS-Th-1 xenotropic LTRs.