Construction and recovery of viable retroviral genomes carrying a bacterial suppressor transfer RNA gene

Retroviral replicating vectors in cancer

The use of replication-competent viruses for the treatment of cancer is an emerging technology that shows significant promise. Among the various different types of viruses currently being developed as oncolytic agents, retroviral replicating vectors (RRVs) possess unique characteristics that allow highly efficient, non-lytic, and tumor-selective gene transfer. By retaining all of the elements necessary for viral replication, RRVs are capable of transmitting genes via exponential in situ amplification. Their replication-competence also provides a powerful means whereby novel and useful RRV variants can be generated using natural selection. Their stringent requirement for cell division in order to achieve productive infection, and their preferential replication in cells with defective innate immunity, confer a considerable degree of natural specificity for tumors. Furthermore, their ability to integrate stably into the genome of cancer cells, without immediate cytolysis, contributes to long-lasting therapeutic efficacy. Thus, RRVs show much promise as therapeutic agents for cancer and are currently being tested in the clinic. Here we describe experimental methods for their production and quantitation, for adaptive evolution and natural selection to develop novel or improved RRV, and for in vitro and in vivo assessment of the therapeutic efficacy of RRVs carrying prodrug activator genes for treatment of cancer.

Inserting a nuclear targeting signal into a replication-competent Moloney murine leukemia virus affects viral export and is not sufficient for cell cycle-independent infection

The effects of inserting reported nuclear localization signals (NLSs) into the Moloney murine leukemia virus (Mo-MuLV) integrase (IN) protein, within a replication-competent viral construct, were studied. In contrast to the virus harboring IN fused to the simian virus 40 (SV40) large T antigen NLS (SV40 NLS) (J. A. Seamon, M. Adams, S. Sengupta, and M. J. Roth, Virology 274:412-419, 2000), a codon-modified SV40 NLS was stably expressed during viral propagation. Incorporation of the codon-modified SV40 NLS into IN, however, altered the packaging of the Gag-Pol precursor in the virus; viral particles contained decreased levels of reverse transcriptase (RT) and IN. In addition, the virus showed delayed kinetics of viral DNA synthesis upon infection. A panel of infectious MuLVs containing alternative IN-NLS fusions was generated and assayed for cell cycle-independent infection. Viral infection with the NLS-tagged proteins, however, remained dependent on passage of the cells through mitosis. This finding has direct implications for engineering murine-based retroviral vectors for gene therapy.

A replication-competent feline leukemia virus, subgroup A (FeLV-A), tagged with green fluorescent protein reporter exhibits in vitro biological properties similar to those of the parental FeLV-A

We previously established that lymphoid tumors could be induced in cats by intradermal injection of ecotropic feline leukemia virus (FeLV), subgroup A, plasmid DNA. In preparation for in vivo experiments to study the cell-to-cell pathway for the spread of the virus from the site of inoculation, the green fluorescent protein (GFP) transgene fused to an internal ribosome entry site (IRES) was inserted after the last nucleotide of the env gene in the ecotropic FeLV-A Rickard (FRA) provirus. The engineered plasmid was transfected into feline fibroblast cells for production of viruses and determination of GFP expression. The virions produced were highly infectious, and the infected cells could continue to mediate strong expression of GFP after long-term propagation in culture. Similar to parental virus, the transgene-containing ecotropic virus demonstrated recombinogenic activity with endogenous FeLV sequences in feline cells to produce polytropic recombinant FeLV subgroup B-like viruses which also contained the IRES-GFP transgene in the majority of recombinants. To date, the engineered virus has been propagated in cell culture for up to 8 months without diminished GFP expression. This is the first report of a replication-competent FeLV vector with high-level and stable expression of a transgene.

Genomic stability of murine leukemia viruses containing insertions at the Env-3' untranslated region boundary

Retroviruses containing inserts of exogenous sequences frequently eliminate the inserted sequences upon spread in susceptible cells. We have constructed replication-competent murine leukemia virus (MLV) vectors containing internal ribosome entry site (IRES)-transgene cassettes at the env-3' untranslated region boundary in order to examine the effects of insert sequence and size on the loss of inserts during viral replication. A virus containing an insertion of 1.6 kb replicated with greatly attenuated kinetics relative to wild-type virus and lost the inserted sequences in a single infection cycle. In contrast, MLVs containing inserts of 1.15 to 1.30 kb replicated with kinetics only slightly attenuated compared to wild-type MLV and exhibited much greater stability, maintaining their genomic integrity over multiple serial infection cycles. Eventually, multiple species of deletion mutants were detected simultaneously in later infection cycles; once detected, these variants rapidly dominated the population and thereafter appeared to be maintained at a relative equilibrium. Sequence analysis of these variants identified preferred sites of recombination in the parental viruses, including both short direct repeats and inverted repeats. One instance of insert deletion through recombination with an endogenous retrovirus was also observed. When specific sequences involved in these recombination events were eliminated, deletion variants still arose with the same kinetics upon virus passage and by apparently similar mechanisms, although at different locations in the vectors. Our results suggest that while lengthened, insert-containing genomes can be maintained over multiple replication cycles, preferential deletions resulting in loss of the inserted sequences confer a strong selective advantage.

A uniquely stable replication-competent retrovirus vector achieves efficient gene delivery in vitro and in solid tumors

A major obstacle in cancer gene therapy is the limited efficiency of in vivo gene transfer by replication-defective retrovirus vectors in current use. One strategy for circumventing this difficulty would be to use vectors capable of replication within tumor tissues. We have developed a replication-competent retrovirus (RCR) vector derived from murine leukemia virus (MuLV). This vector utilizes a unique design strategy in which an internal ribosome entry site-transgene cassette is positioned between the env gene and the 3' long terminal repeat (LTR). The ability of this vector to replicate and transmit a transgene was examined in culture and in a solid tumor model in vivo. The RCR vector exhibited replication kinetics similar to those of wildtype MuLV and mediated efficient delivery of the transgene throughout an entire population of cells in culture after an initial inoculation with 1 plaque-forming unit (PFU) of vector per 2000 cells. After injection of 6 x 10(3) PFU of vector into established subcutaneous tumors, highly efficient spread of the transgene was observed over a period of 7 weeks, in some cases resulting in spread of the transgene throughout the entire tumor. MuLV-based RCR vectors show significant advantages over standard replication-defective vectors in efficiency of gene delivery both in culture and in vivo. This represents the first example of the use of an RCR vector in an adult mammalian host, and their first application to transduction of solid tumors.

Differential effects of C-terminal molecular tagged integrase on replication competent moloney murine leukemia virus

Moloney murine leukemia virus (M-MuLV) proviruses carrying integrase (IN) protein tagged either with a simian virus 40 (SV40) nuclear localization signal (NLS) or various antigenic epitopes were generated. Hexahistidine (His(6)), hemagluttinin (HA), or two consecutive HA sequences (2XHA) were fused to the C-terminus of IN as antigenic markers. These epitope-tagged IN proteins were stably expressed through multiple rounds of infection. The IN-His(6), IN-HA, and IN-2XHA proteins, purified from virus, could be immunoprecipitated with antibodies against His(6) and HA, respectively. An M-MuLV provirus encoding the SV40 large T antigen NLS fused to IN at the same position as the epitope tags was also passaged through cells. In contrast to the stability of the epitope tags, the SV40 NLS sequence was rapidly mutated by a frameshift mutation that introduced negatively charged amino acids into the basic NLS. The instability of the NLS suggests that the strong nuclear localization of the IN-SV40 NLS may have detrimental effects on virus assembly. These observations have implications for studying nuclear transport properties of M-MuLV and for engineering a murine-based retroviral vector for gene therapy.

Mouse mammary tumor virus carrying a bacterial supF gene has wild-type pathogenicity and enables rapid isolation of proviral integration sites

Mouse mammary tumor virus (MMTV) has frequently been used as an insertional mutagen to identify provirally activated mammary proto-oncogenes. To expedite and facilitate the process of cloning MMTV insertion sites, we have introduced a bacterial supF suppressor tRNA gene into the long terminal repeat (LTR) of MMTV, thus allowing selection of clones containing it in lambda vectors bearing amber mutations. The presence of supF in the LTR should circumvent the screening process for proviral insertion sites, since only those lambda clones with supF-containing proviral-cellular junction fragments should be able to form plaques on a lawn of wild-type Escherichia coli (i.e., lacking supF). The resulting virus (MMTVsupF) induced mammary tumors at the expected rate in infected mice, deleted the appropriate T-cell population by virtue of its superantigen gene, and stably retained the supF gene after passage via the milk to female offspring. To test the selective function of the system, size-selected DNA containing two proviral-cellular junction fragments from an MMTV supF-induced mammary tumor was ligated into lambdagtWES.lambdaB, packaged, and plated on a supF-deficient bacterial host for selection of supF-containing clones. All plaques tested contained the desired cloned fragments, thus demonstrating the utility of this modified provirus for the rapid cloning of MMTV insertion sites.

Expression of heterologous genes from an IRES translational cassette in replication competent murine leukemia virus vectors

We describe replication competent retroviruses capable of expressing heterologous genes during multiple rounds of infection. An internal ribosome entry site (IRES) from encephalomyocarditis virus was inserted in the U3 region of Akv- and SL3-3-murine leukemia viruses (MLV) to direct translation of neo or the enhanced green fluorescence protein gene (EGFP). Akv-MLV's with IRES-neo and IRES-EGFP cassettes replicated with titers of about 10(6) infectious units/ml while SL3-3-MLV with IRES-neo gave about 10(3)-fold lower titers. Interestingly, RNA analysis showed a drastic reduction in the amount of spliced env mRNA for the SL3-3 derived vector relative to the Akv derived vectors, seemingly contributing to its low replication capacity. The EGFP expressing Akv-MLV was genetically stable for multiple rounds of infection; marker-cassette deletion revertants appeared after several replication rounds and these revertants only slowly became dominant in the virus population.

Effects of 3' untranslated region mutations on plus-strand priming during moloney murine leukemia virus replication

A conserved purine-rich motif located near the 3' end of retroviral genomes is involved in the initiation of plus-strand DNA synthesis. We mutated sequences both within and flanking the Moloney murine leukemia virus polypurine tract (PPT) and determined the effects of these alterations on viral DNA synthesis and replication. Our results demonstrated that both changes in highly conserved PPT positions and a mutation that left only the cleavage-proximal half of the PPT intact led to delayed replication and reduced the colony-forming titer of replication defective retroviral vectors. A mutation that altered the cleavage proximal half of the PPT and certain 3' untranslated region mutations upstream of the PPT were incompatible with or severely impaired viral replication. To distinguish defects in plus-strand priming from other replication defects and to assess the relative use of mutant and wild-type PPTs, we examined plus-strand priming from an ectopic, secondary PPT inserted in U3. The results demonstrated that the analyzed mutations within the PPT primarily affected plus-strand priming whereas mutations upstream of the PPT appeared to affect both plus-strand priming and other stages of viral replication.

Sequence analysis of the human DNA flanking sites of human immunodeficiency virus type 1 integration

Human immunodeficiency virus type 1 (HIV-1) tagged with the Escherichia coli supF gene has been used to clone integrated HIV-1 proviruses. Sequence analysis of the 600 to 800 bp of human DNA adjacent to 29 clones revealed a propensity for HIV-1 to integrate near the Alu class of human repetitive elements.

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

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

Tagging the genome of the murine leukemia retrovirus SL3-3 by a bacterial lac operator sequence

The bacterial lactose operator (lacO) was introduced into the PstI site of the long terminal repeat of the SL3-3 murine leukemia virus, generating a virus, SL3-3lacO, that can replicate in NIH3T3 cell cultures. DNA sequences harboring the lacO sequence might be recovered by molecular cloning in Escherichia coli lac+ lacZ+ using bacteriophage lambda or plasmid vectors. The high copy numbers of the lacO sequence titrate out the lac repressor, leading to the induction of the lac operon in the host. We show here that the lacO and the proviral sequences are carried stably together in the genomes of SL3-3lacO-infected cell cultures and in viral particles. This system is designed to facilitate studies on the provirus and the site of viral integration.

Efficient replication, integration, and packaging of retroviral vectors with modified long terminal repeats containing the packaging signal

Retroviral vectors were modified to contain packaging (psi) signals of varying lengths (nucleotides 211-355, 211-565, or 211-1039 of MoMuLV RNA) between the U3-r and U5 sequences of their 5' long terminal repeat (LTR). For the vector MoTN-PR3, containing the full length 211-1039 nucleotide-long psi signal within the 5' LTR, replication, integration, and packaging were almost as efficient as for the original unmodified vector. This result confirmed that the 211-1039 nucleotide-long sequence from the MoMuLV RNA is sufficient and necessary to allow efficient packaging of RNAs. In addition, an important site was revealed where insertion of foreign DNA sequences of up to 829 nucleotides can be made within the 5' LTR, between U3-r and U5 sequences, without affecting viral replication, integration, or packaging.

Ty RNA levels determine the spectrum of retrotransposition events that activate gene expression in Saccharomyces cerevisiae

To learn more about the variety of Ty elements capable of activating gene expression, we characterized 206 spontaneous Ty transpositions that activate the promoterless gene his3 delta 4. Most of the Ty elements appear to be full-length, although a few deleted elements were recovered. Over 95% of the insertions belong to the Ty1 family, and the rest are Ty2 elements. The excessive number of Ty1 transpositions was unexpected because there are only 2-fold more Ty1 than Ty2 elements in the yeast strains used in the selection. However, there is 20-fold more Ty1 than Ty2 RNA present in these yeast strains. This difference in RNA level explains the greater number of Ty1 verses Ty2 transpositions at his3 delta 4, because Ty elements transpose through an RNA intermediate. A similar association between the Ty transcript level and transpositional activation of his3 delta 4 is obtained in cells expressing GAL1-promoted Ty2-H556 or Ty2-917 elements, but only if the element does not contain a marker. Genetically marked Ty2-H556NEO and -917NEO elements transpose into and activate his3 delta 4 with the same efficiency as the previously characterized Ty1-H3NEO element, but are underrepresented relative to the levels of TyNEO transcript. We also found that chromosomal Ty transcripts are even more abundant than previously estimated and comprise about 1% of total cellular RNA.

Construction and properties of replication-competent murine retroviral vectors encoding methotrexate resistance

A series of replication-competent Moloney murine leukemia virus vectors was constructed in which each vector contained a mutant dihydrofolate reductase (DHFR) cDNA insert in the U3 region of the viral long terminal repeat. Two of the resulting viruses, MLV (murine leukemia virus) DHFR*-5 and MLV DHFR*-7, were able to stably transfer methotrexate resistance to infected fibroblast cells upon multiple rounds of virus replication and in the absence of drug selection. Cell lines producing recombinant virus with high titers were established, which indicated that the insert did not grossly interfere with viral replication functions. These vectors should be useful for introducing and expressing foreign genes in vivo in tissues and whole animals in which virus spread is needed for efficient infection.

Improved gene expression upon transfer of the adenosine deaminase minigene outside the transcriptional unit of a retroviral vector

This study describes a type of retroviral vector called double-copy (DC) vector that was designed to improve the expression of transduced genes. The unique feature of DC vectors is that the transduced gene is inserted within the U3 region of the 3' long terminal repeat (LTR). Consequently, in the infected cell the gene is duplicated and transferred to the 5' LTR. The important result is that in its new position the gene is placed outside the retroviral transcriptional unit, eliminating or at least reducing the negative effects of the retroviral transcriptional unit. The utility of the DC vector design was tested by using a 2.1-kilobase-pair (kbp)-long adenosine deaminase (ADA; EC 3.5.4.4) minigene that was inserted into the 3' LTR of the N2 retroviral vector, generating a 2.7-kbp-long chimeric LTR. DNA blot analysis was used to show that the chimeric LTR was faithfully duplicated in cells infected with the corresponding virus, generating two copies of the ADA minigene, one copy in each LTR. Insertion of the ADA minigene into the 3' LTR of the N2 vector led to a 10- to 20-fold increase in ADA transcripts and human ADA isozyme synthesized in NIH 3T3 cells as compared to cells harboring the same vector in which the ADA minigene was inserted between the two LTRs. A similar increase in ADA expression was observed in two human lymphoid cell lines tested, HUT 78 and Raji. These results are consistent with previous observations that upstream promoters exert an inhibitory effect on promoters placed downstream and bear out the predictions used in the design of DC vectors. The use of DC vectors may contribute to the solution of the problems encountered in expressing retrovirally transduced genes in cultured cells and, in particular, when introduced into the live animal.

A nucleoprotein complex mediates the integration of retroviral DNA

The integration of viral DNA into the host genome is an essential step in the retrovirus life cycle. To understand this process better, we have examined the native state of viral DNA in cells acutely infected by murine leukemia virus (MLV), using both a physical assay for viral DNA and a functional assay for integration activity (Brown et al. 1987). The viral DNA and integration activity copurify during velocity sedimentation, gel filtration, and density equilibrium centrifugation, indicating that viral DNA is in a large (approximately 160S) nucleoprotein complex that includes all functions required for integration activity in vitro. Analysis by immunoprecipitation shows that the viral capsid protein is part of the active nucleoprotein complex, but recognition of the complex by only a subset of anti-capsid sera implies that the protein is constrained conformationally. The viral DNA within this structure is accessible to nucleases; the effects of nucleases on the integrity of the complex suggest that the integration-competent particle is derived from and similar to the core of extracellular virions.

Retroviral integration: structure of the initial covalent product and its precursor, and a role for the viral IN protein

An essential step in the life cycle of a retrovirus is the integration of a DNA copy of the viral genome into a host cell chromosome. We have analyzed the structure of the initial covalent product of an in vitro retroviral integration reaction and determined the structure of the ends of the unintegrated linear viral DNA molecules present in vivo in cells infected with murine leukemia virus (MLV). Our results lead to the following conclusions: (i) Circularization of viral DNA plays no role in integration. The direct precursor to the integrated MLV provirus is a linear molecule. (ii) The initial step in the integration reaction is probably a cleavage that removes the terminal 2 bases from each 3' end of the viral DNA. This cleavage depends on a virally encoded protein, IN, that has previously been shown genetically to be required for integration. (iii) The resulting viral 3' ends are joined to target DNA to form the initial recombination intermediate.

Construction and properties of replication-competent murine retroviral vectors encoding methotrexate resistance

A series of replication-competent Moloney murine leukemia virus vectors was constructed in which each vector contained a mutant dihydrofolate reductase (DHFR) cDNA insert in the U3 region of the viral long terminal repeat. Two of the resulting viruses, MLV (murine leukemia virus) DHFR*-5 and MLV DHFR*-7, were able to stably transfer methotrexate resistance to infected fibroblast cells upon multiple rounds of virus replication and in the absence of drug selection. Cell lines producing recombinant virus with high titers were established, which indicated that the insert did not grossly interfere with viral replication functions. These vectors should be useful for introducing and expressing foreign genes in vivo in tissues and whole animals in which virus spread is needed for efficient infection.

Formation of an inverted duplication can be an initial step in gene amplification

We have developed a gene transfer approach to facilitate the identification and isolation of chromosomal regions which are prone to high-frequency gene amplification. Such regions are identified by assaying for transformants which show high-frequency resistance to PALA and/or methotrexate by amplification of a vector containing the genes which encode the enzyme targets of these antiproliferative agents. We identified 2 of 47 transformants which displayed high-frequency amplification of the transfected genes, and in this report we describe the analysis of one of them (L46). Molecular analysis of the integration site in transformant L46 revealed that the donated genes were at the center of an inverted duplication which spanned more than 70 kilobase pairs and consisted largely of host DNA. The data suggest that integration of the transfected sequences generates a submicroscopic molecule containing the inverted duplication and at least 750 kilobases of additional sequences. The donated sequences and the host sequences were readily amplified and lost in exponentially growing cultures in the absence of drug selection, which suggests that the extrachromosomal elements are acentric. In contrast to the instability of this region following gene insertion, the preinsertion site was maintained at single copy level under growth conditions which produced copy number heterogeneity in L46. The implications of our results for mechanisms of genetic instability and mammalian gene amplification are discussed.

Formation of an inverted duplication can be an initial step in gene amplification

We have developed a gene transfer approach to facilitate the identification and isolation of chromosomal regions which are prone to high-frequency gene amplification. Such regions are identified by assaying for transformants which show high-frequency resistance to PALA and/or methotrexate by amplification of a vector containing the genes which encode the enzyme targets of these antiproliferative agents. We identified 2 of 47 transformants which displayed high-frequency amplification of the transfected genes, and in this report we describe the analysis of one of them (L46). Molecular analysis of the integration site in transformant L46 revealed that the donated genes were at the center of an inverted duplication which spanned more than 70 kilobase pairs and consisted largely of host DNA. The data suggest that integration of the transfected sequences generates a submicroscopic molecule containing the inverted duplication and at least 750 kilobases of additional sequences. The donated sequences and the host sequences were readily amplified and lost in exponentially growing cultures in the absence of drug selection, which suggests that the extrachromosomal elements are acentric. In contrast to the instability of this region following gene insertion, the preinsertion site was maintained at single copy level under growth conditions which produced copy number heterogeneity in L46. The implications of our results for mechanisms of genetic instability and mammalian gene amplification are discussed.

Highly preferred targets for retrovirus integration

A central feature of retrovirus replication is integration of the provirus into host cell DNA, but the specificity of this step for cell target sequences has not been clarified. To investigate this issue, we developed a method for screening and comparing large numbers of unselected integration events. Using a replication-competent Rous sarcoma virus containing a bacterial suppressor tRNA gene as a selectable marker, we obtained collections of clones comprising integrated provirus together with host flanking sequences. Hybridization and sequence analysis of the flanking sequence reveals the presence of a number of strongly preferred integration targets. Within these targets, independent integration events occur at sites identical to the base.

An MuLV transmission vector system designed to permit recovery in E. coli of proviral and cellular flanking sequences

We have introduced a bacterial suppressor gene (supF) into the long terminal repeat of a molecular clone of the murine leukemia virus (MuLV) SL3-3. A panel of replication competent virus was derived that replicates to high titers in NIH3T3 cells in culture. The tRNA gene is stably carried in the provirus. The supF and viral sequences are present in equimolar amounts in the RNA genome of the expressed recombinant virus. The proviral sequences containing supF can be recovered by cloning into a lambda vector carrying amber mutations. The DNA sequences in the recovered lambda recombinants show a high degree of stability. The presented system should facilitate the study of the interaction between proviral and cellular sequences flanking the integration site.

Mutants of murine leukemia viruses and retroviral replication

The analysis of retroviral mutants has played a critical role in the development of our understanding of the complex viral life cycle. The most fundamental result of that analysis has been the definition of the replication functions encoded by the viruses. From a biochemical examination of a particular step in the life cycle it is difficult to determine, for example, whether that step is catalyzed by a viral or a host enzyme; but the isolation of a viral mutant defective in that step can firmly establish that a viral function is involved. In this way many facts about the viruses have been established. We know that reverse transcriptase is encoded by the virus; that RNAase H and DNA polymerase activities reside on the same gene product; that processing of many precursor proteins is mediated by a viral proteinase; and that establishment of the integrated provirus requires a viral protein. The list of functions mediated by viral enzymes has largely been defined by the mutants isolated and studied in various laboratories. The second significant result of the studies of viral mutants has been the assignation of the replication functions to particular viral genes, and then more specifically to particular domains of these genes. Mutants and viral variants have been essential in the determination, for example, that the gag protein is the critical gene product for the assembly of a virion particle; that the env protein is the determinant of species specificity of infection; or that the LTR is a major determinant of tissue tropism and leukemogenicity. The subdivisions of functions within a given gene have similarly hinged on mutants. Genetic mapping was needed to establish that P30 is the most important region for assembly; that the proteinase and integrase functions reside, respectively, in the 5' and 3' portions of the pol gene; and that the glycosylated gag protein is dispensable for replication. A third important area of knowledge has depended heavily on viral mutants: the determination of host functions and proteins that interact with viral proteins. Variant viruses with altered or restricted host ranges serve to define differences between pairs of different host cells, and the mapping of the viral mutations serves to define the viral protein important in that interaction with the host. These studies are only in their infancy, but it is clear that substantial efforts will be made to further analyze these host functions.(ABSTRACT TRUNCATED AT 400 WORDS)

Correct integration of retroviral DNA in vitro

We have developed a cell-free system for studying the integration of retroviral DNA. In our assay, amber mutations in a bacteriophage lambda genome that serves as the target for integration are suppressed by integration of an MLV derivative that carries the E. coli supF gene. The structure of the reaction products is that expected from an authentic MLV integration reaction. Linear viral DNA from the cytoplasm of infected cells serves as a precursor, though not necessarily the immediate precursor, to the provirus integrated in vitro. The viral DNA in the infected cell appears to be tightly associated with the enzymatic machinery required for its integration. Supercoiling, chromatin structure, transcription, and replication are not required of the target DNA. Since no high-energy cofactor is necessary, the DNA breakage and joining steps in the integration reaction are probably coupled.

Charons 36 to 40: multi enzyme, high capacity, recombination deficient replacement vectors with polylinkers and polystuffers

New phage lambda based cloning vectors, Charons 36-40, have been constructed which allow cloning of large (up to 24 kb) DNA fragments with up to sixteen cloning enzymes. Several of these could not be used previously with lambda vectors. Clones produced with these vectors can be propagated under recombination deficient conditions. A novel polystuffer method has been developed that permits vector arms to be purified by simple precipitation and which allows reliable identification of clones that have reincorporated any part of the stuffer. Three of the vectors are available with amber mutations in essential genes.

Endogenous retroviruses lead to the expression of a histocompatibility antigen detectable by skin graft rejection

Mov mouse strains differ from their respective, coisogenic partner strains by the embryonic, germ-line introduction of Moloney murine leukemia virus genomes. The possibility that retroviral insertions into the mouse genome resulted in gain or loss mutations at non-H-2 histocompatibility loci was investigated by reciprocal skin grafting between Mov mice and mice from coisogenic, background strains. Two B6-derived and eight 129-derived Mov strains were analyzed. B6 mice rejected skin from the viremic Mov-3 and Mov-14 strains, indicating that these mice had new histocompatibility antigens. No rejections were observed with reciprocal skin grafts exchanged between mice of the 129 background strain and 129-derived Mov strains, one of which (Mov-9) is viremic. To investigate the potential viral origin of the new histocompatibility antigen in Mov-14, lymphocytes from B6 mice primed in vivo with Mov-14 cells or skin were restimulated in vitro with Mov-14 spleen cells and with two retroviral-induced B6 lymphomas, MBL-2 and RBL-5. All three cell types stimulated cytotoxic lymphocytes that lysed Mov-14 Con A lymphoblasts, MBL-2 and RBL-5. The same cytotoxic lymphocytes lysed only lymphoblasts from the viremic Mov-9 strain when tested on cells from 129 and 129 Mov mice. Thus the insertion and expression of exogenous Moloney murine leukemia virus results in the appearance of a new histocompatibility antigen as defined by its stimulation of skin-graft rejection and cytotoxic effector T-cell generation. The non-H-2 histocompatibility antigen identified in this study has been designated H-43 and is encoded by genes mapping to different loci in different Mov strains. These observations suggest that at least a subgroup of non-H-2 histocompatibility antigens is encoded by endogenous retroviruses; the implications of these results for understanding the origin and the identity of non-H-2 histocompatibility antigens are discussed.

Insertion mutagenesis of embryonal carcinoma cells by retroviruses

Mutagenesis was studied in cultured F9 embryonal carcinoma cells infected with a variant of Moloney murine leukemia virus. Proviral insertion induced the inactivation of the hypoxanthine phosphoribosyltransferase locus, and the virus was used to isolate the mutated genes rapidly. Mutagenesis by these methods may be useful for the genetic dissection of the various mammalian cell phenotypes.