Improvement of avian leukosis virus (ALV)-based retrovirus vectors by using different cis-acting sequences from ALVs

Genetically engineered birds; pre-CRISPR and CRISPR era

Generating biopharmaceuticals in genetically engineered bioreactors continues to reign supreme. Hence, genetically engineered birds have attracted considerable attention from the biopharmaceutical industry. Fairly recent genome engineering methods have made genome manipulation an easy and affordable task. In this review, we first provide a broad overview of the approaches and main impediments ahead of generating efficient and reliable genetically engineered birds, and various factors that affect the fate of a transgene. This section provides an essential background for the rest of the review, in which we discuss and compare different genome manipulation methods in the pre-CRISPR and CRISPR era in the field of avian genome engineering.

Construction of adeno-associated virus packaging plasmids and cells that directly select for AAV helper functions

Recombinant adeno-associated virus type 2 (rAAV) has promise for use as a gene therapy vector. Potential problems in the production of rAAV stocks are both the limited amount of recombinant virus that is produced by traditional methods and the possibility of wild-type replication competent adeno-associated virus (wtAAV) contamination. The presence of these contaminants is largely dependent upon the helper plasmid used. Whilst wtAAV is not a pathogen, the presence of these contaminants is undesirable as they may affect experiments concerning the biology of rAAV. Additionally as protocols using rAAV with altered tropism are becoming more prevalent, it is important that no recombination be permitted that may cause the creation of a replication competent AAV with modified (targeting) capsids. Many experimental protocols require the generation of large amounts of high titre rAAV stocks. We describe the production of several AAV helper plasmids and cell lines designed to achieve this goal. These plasmids possess split AAV rep and cap genes to eliminate the production of wtAAV and they possess a selection mechanism which is operatively linked to expression from the AAV cap gene. This allows positive selection of those cells expressing the highest level of the structural capsid proteins and therefore those cells which yield the highest amount of rAAV.

Expression of exogenous protein in the egg white of transgenic chickens

Using a replication-deficient retroviral vector based on the avian leukosis virus (ALV), we inserted into the chicken genome a transgene encoding a secreted protein, beta-lactamase, under the control of the ubiquitous cytomegalovirus (CMV) promoter. Biologically active beta-lactamase was secreted into the serum and egg white of four generations of transgenic chickens. The expression levels were similar in successive generations, and expression levels in the magnum of the oviduct were constant over at least 16 months in transgenic hens, indicating that the transgene was stable and not subject to silencing. These results support the potential of the hen as a bioreactor for the production of commercially valuable, biologically active proteins in egg white.

Consistent production of transgenic chickens using replication-deficient retroviral vectors and high-throughput screening procedures

We have developed a novel method of DNA extraction combined with a high-throughput method of gene detection allowing thousands of potentially transgenic chicks to be screened quickly and reliably. By using this method and a replication-deficient retroviral vector based on avian leukosis virus (ALV), we have demonstrated germline transmission of three different transgenes. Several generations of chickens carrying intact transgenes were produced, validating the use of the ALV retroviral vectors for large-scale production of transgenic flocks. Fourth-generation chicks that were nontransgenic, hemizygous, or homozygous for the transgene were identified with the combined genetic screening methods.

Homologous and nonhomologous retroviral recombinations are both involved in the transfer by infectious particles of defective avian leukosis virus-derived transcomplementing genomes

We previously described avian leukosis virus-based packaging cell lines that produce stocks of retroviral vectors in which replication-competent viruses were not detectable. However, following infection of target cells with these retroviral stocks, we recently obtained colonies resulting from the transmission of recombinant genomes. Here, we have analyzed their genetic structure and shown that (i) each of them results from recombination between the packaging- and integration-defective transcomplementing genomes and the retroviral vector; (ii) recombination probably occurred during the reverse transcription step, involving strand switching of the reverse transcription growing point from the infectious retroviral vector to the transcomplementing RNA; and (iii) sequence identity and nonhomologous sequences were both used for the strand switching.

High-titer packaging cells producing recombinant retroviruses resistant to human serum

Novel retroviral protein expression constructs were designed to retain minimal retroviral sequences and to express dominant selectable markers by reinitiation of translation after expression of the viral genes. HT1080 cells were selected as producer cells for their ability to release high-titer viruses that are resistant to inactivation by human serum. Two HT1080-based packaging cell lines which produce Moloney murine leukemia virus cores with envelope glycoproteins of either amphotropic murine leukemia virus (FLYA13 line) or cat endogenous virus RD114 (FLYRD18 line) are described. Direct comparison with previous retroviral packaging systems indicated that 100-fold-higher titers of helper-free recombinant viruses were released by the FLYA13 and FLYRD18 lines.

Germline transmission of exogenous genes in chickens using helper-free ecotropic avian leukosis virus-based vectors

We have used vectors derived from avian leukosis viruses to transduce exogenous genes into early somatic stem cells of chicken embryos. The ecotropic helper cell line, Isolde, was used to generate stocks of NL-B vector carrying the Neo(r) selectable marker and the Escherichia coli lacZ gene. Microinjection of the NL-B vector directly beneath unincubated chicken embryo blastoderms resulted in infection of germline stem cells. One of the 16 male birds hatched (6.25%) from the injected embryos contained vector DNA sequences in its semen. Vector sequences were transmitted to G1 progeny at a frequency of 2.7%. Neo(r) and lacZ genes were transcribed in vitro in chicken embryo fibroblast cultures from transgenic embryos of the G2 progeny.

Structure and expression of endogenous retroviral sequences in the permanent LMH chicken cell line

From DNA mapping data, four endogenous proviral loci have been observed in the chicken permanent cell line LMH. The locus corresponding to endogenous virus (ev) ev1 is present in duplicate whereas the locus corresponding to ev3 is present in one copy. The other loci are probably ev6 and a solitary long terminal repeat. A RNA Northern blot analysis revealed both ev3 and ev6 transcripts but no ev1 transcript was detected. Using avian leukosis virus (ALV)-based vectors, transcomplementing assays were performed. They demonstrate the correct expression and maturation of endogenous env proteins and the absence of production of functional gag and pol components, indicating that these cells are not competent for viral production.

Somatic and germline chicken chimeras obtained from brown and white Leghorns by transfer of early blastodermal cells

Stage X blastodermal cells were isolated from freshly laid unincubated Brown Leghorn chicken eggs. Five hundred cells from Stage X Brown Leghorn embryos were injected into the subgerminal cavity of White Leghorn unincubated embryos exposed to 550 rad of gamma irradiation from a cesium-137 source. Of 712 White Leghorn embryos that were irradiated and injected with Brown Leghorn blastodermal cells, 52 (7.3%) survived to hatching. Somatic chimerism was examined in the melanocyte population and erythroid lineage. The presence of brown feathers indicating donor cell contribution to melanocyte pigmentation was observed in 23 (44%) out of the 52 hatched chicks. Analysis of blood DNA was performed using a probe that revealed an endogenous retroviral gag fragment specific for the donor genome. Three out of these 23 chimeric chickens exhibited the gag-specific fragment. To test germline chimerism, chickens that reached sexual maturity were mated with Brown Leghorns. Three somatically chimeric hens produced Brown Leghorn progeny at a rate of 30.7, 9.2, and 2.9% respectively, thus proving donor cell contribution to the germline differentiation. Chimeric chickens obtained after injection of nonirradiated embryos exhibited a lower extent of chimerism at the feather level and did not show any chimerism in the erythroid lineage and the germline, thus demonstrating the value of the use of compromised recipient embryos to produce chimeras in chickens. Nevertheless, the extent of somatic chimerism could not be used to predict the germline chimerism.

Modifications in the binding domain of avian retrovirus envelope protein to redirect the host range of retroviral vectors

On the basis of theoretical structural and comparative studies of various avian leukosis virus SU (surface) envelope proteins, we have identified four small regions (I, II, III, and IV) in their receptor-binding domains that could potentially be involved in binding to receptors. From the envelope gene of an avian leukosis virus of subgroup A, we have constructed a set of SU mutants in which these regions were replaced by the coding sequence of FLA16, a 16-amino-acid RGD-containing peptide known to be the target for several cellular integrin receptors. Helper-free retroviral particles carrying a neo-lacZ retroviral vector were produced with the mutant envelopes. SU mutants in which regions III and IV were substituted yielded normal levels of envelope precursors but were not detectably processed or incorporated in viral particles. In contrast, substitutions in regions I and II did not affect the processing and the viral incorporation of SU mutants. When FLA16 was inserted in region II, it could be detected with antibodies against FLA16 synthetic peptide, but only when viral particles were deglycosylated. Viral particles with envelopes mutated in region I or II were able to infect avian cells through the subgroup A receptor at levels similar to those of the wild type. When viruses with envelopes containing FLA16 peptide in region II were applied to plastic dishes, they were found to promote binding of mammalian cells resistant to infection by subgroup A avian leukosis viruses but expressing the integrins recognized by FLA16. Deglycosylated helper-free viruses obtained by mild treatment with N-glycosidase F have been used to infect these mammalian cells, and infections have been monitored by neomycin selection. No neomycin-resistant clones could be obtained after infection by viruses with wild-type envelopes. Conversely, colonies were obtained after infection by viruses with envelopes bearing FLA16 in region II, and the genome of the retroviral vector was found correctly integrated in cell DNA of these colonies. By using a blocking peptide containing the minimal adhesive RGD sequence contained in FLA16, we have shown that preincubation of target cells could specifically inhibit infection by viruses with FLA16.

Manipulations of germ-cell populations in the gonad of the fowl

1. Embryos of the domestic fowl have been partially sterilised by injecting the drug busulphan into 24-h incubated eggs. 2. Some of these embryos were injected with primordial germ cells (PGCs) after 55 h of incubation to attempt to repopulate the gonads. 3. Primordial germ cells transfected with a defective retrovirus containing the reporter gene lac Z were shown to settle in these sterilised gonads. 4. Quantitative histology of 6-d embryos showed that busulphan produced 75% sterilisation but that PGCs could repopulate these gonads. 5. The technique of producing such germ line chimaeras is of value in studying cell kinetics, gonad differentiation and the production of transgenics.

Patterns of integration and expression of retroviral, non-replicative vectors in avian embryos: embryo developmental stage and virus subgroup envelope modulate tissue-tropism

We previously demonstrated that Avian Leukemia Viruses (ALV) carrying the v-myc gene specifically induce two types of tumors, cardiomyocytic tumors when the virus is injected before embryonic day 3 (E3), skin tumors when the virus is injected at E3 or E5. Aiming to elucidate the mechanisms which determine this time-dependent change in target, we infected chick and quail embryos at E3 and E5 with replication-deficient, lacZ gene-carrying, ALV-based viruses produced by a packaging cell line. Three constructs driven by 3 different Long Terminal Repeats (LTRs) were tested and yielded similar results. When the constructs were inoculated at E3 and the lacZ gene product revealed 5 days later, around 70% of the embryos carried lacZ+ clones in the heart, around 50% had positive clones in the skin anywhere on the body, while a few embryos displayed clones in internal organs (liver, stomach, lungs). Immunocytological identification of the heart cell type(s) expressing the virus revealed that the only cells infected were cardiomyocytes. When the constructs were inoculated at E5, no lacZ+ clones appeared in the heart but all were located in the cephalic skin. In order to examine the relationship between viral integration and expression, DNA of different organs or tissues from lacZ stained embryos was analyzed by PCR. A tight correlation between integration and expression in the heart and in the skin was revealed in most cases. In contrast, a significant PCR signal was often detected in the liver or the stomach despite weak or absent expression as revealed by lacZ+ clones. We then investigated the influence of envelope glycoprotein subgroups on the tropism of these constructs. The lacZ vector driven by RAV-2 LTRs was packaged as subgroups A, B or E viral particles. The A subgroup, used in the part of the study described above, infects both chick and quail while the B and E subgroups are specific for chick or quail respectively. These B and E subgroups induced lacZ+ clones in the heart (after E3 injection) while no clones or only a few were detected in the skin either after E3 or E5 injection. The following conclusions can be drawn: 1) cardiomyocytes are at E3 the major target for integration and expression of ALV-derived viruses in vivo; 2) targets change rapidly with embryonic age; and 3) tissue-specific infections depend on the envelope subgroup, thus presumably on the presence of the cognate receptor.(ABSTRACT TRUNCATED AT 400 WORDS)

Packaging cells for avian leukosis virus-based vectors with various host ranges

Using our previously described Haydée semipackaging cell line (F. L. Cosset, C. Legras, Y. Chebloune, P. Savatier, P. Thoraval, J. L. Thomas, J. Samarut, V. M. Nigon, and G. Verdier, J. Virol. 64:1070-1078, 1990) which produces avian leukosis virus gag and pol proteins, we have constructed packaging cells with subgroups B, C, and E envelope specificities. This allows us to produce helper-free avian leukosis virus particles carrying the lacZ reporter gene and the A, B, C, or E subgroup specificities. Titers of the recombinant lacZ virus are shown to be dependent upon the type of the env subgroup and the target avian cell.

Newcastle disease virus (NDV) vaccine based on immunization with avian cells expressing the NDV hemagglutinin-neuraminidase glycoprotein

Newcastle disease virus (NDV) is a paramyxovirus that bears two envelope glycoproteins at the virion surface. These proteins, fusion and hemagglutinin-neuraminidase (HN), are involved in the immune response against NDV infection. Recombinant cells constitutively expressing at their surface the HN protein from the velogenic Texas strain were generated by introducing the HN gene with a helper-free AEV-based vector. These recombinant cells were used to immunize chickens by various protocols, and birds were subsequently challenged with a lethal NDV injection. Both NDV protection and serologic response were observed.

Immune response and resistance to Rous sarcoma virus challenge of chickens immunized with cell-associated glycoproteins provided with a recombinant avian leukosis virus

The Rous-associated virus 1 env gene, which encodes the envelope gp85 and gp37 glycoproteins, was isolated and inserted in place of the v-erbB oncogene into an avian erythroblastosis virus-based vector, carrying the neo resistance gene substituted for the v-erbA oncogene, to generate the pNEA recombinant vector. A helper-free virus stock of the pNEA vector was produced on an avian transcomplementing cell line and used to infect primary chicken embryo fibroblasts (CEFs) or quail QT6 cells. These infected cells, selected with G418 (CEF/NEA and QT6/NEA, respectively) were found to be resistant to superinfections with subgroup A retroviruses. The CEF/NEA preparations were used as a cell-associated antigen to inoculate adult chickens by the intravenous route compared with direct inoculations of NEA recombinant helper-free virus used as a cell-free antigen. Chickens injected with the cell-associated antigen (CEF/NEA) exhibited an immune response demonstrated by induction of high titers of neutralizing antibodies and were found to be protected against tumor production after Rous sarcoma virus A challenge. Conversely, no immune response and no protection against Rous sarcoma virus A challenge were observed in chickens directly inoculated with cell-free NEA recombinant virus or in sham-inoculated chickens.