The study of factors affected the gene transfer efficiency in chicken embryonic cells by application of lentiviral vectors

Lentivirus-mediated gene transfer is being the one of the attractive method for genetic modification of chicken (S.C. Chapman et al., 2005; C.A. Smith et al., 2009; N.A. Volkova et al., 2013). However, the efficiency of thansgenesis of the chicken embryonic cells has been shown to be relatively low. Therefore, a large number (60,000 to 100,000) of embryonic cells at the start of incubation and the virus preparations with high titers (about 10 9 particles per milliliter) remain one of a crucial problem the researchers are facing with when try to achieve a satisfactory transgene introduction. The aim of the present study was to determine the optimal conditions for production and application of the modified lentiviral vector system of second generation for the transgenesis of chicken embryos. The vector system consisted of three different plasmids: psPAX2, containing gag-pol genes; pLPG, coding envelop glycoprotein G of vesicular stomatitis virus (VVC-G) and pWPXL, the self-inactivated lentiviral vector, carrying eGFP (enhanced green fluorescence protein) gene under control of promoter region of the human elongation factor 1 alpha-encoding gene. To produce the recombinant virus particles and to determine the virus titers we used human cell line 293T. The injections of the virus preparations into the chicken embryos were performed at the different stages: from 20 to 24 hours (group 1) and from 50 to 55 hours (group 2) of incubation. To detect the transgenesis efficiency and the number of the integrated copies of the transgene the total DNA was extracted from embryos on the day 7 of incubation and analyzed for the presence of specific eGFP sequences by real-time PCR. The maximal titers of the virus preparations were produced by the ratio of the psPAX2, pLPG and pWCAG plasmids equal 1:1:3 and were 2.4x10 7 CFU/ml before ultracentrifugation and 6.2x10 8 CFU/ml after concentration by ultracentrifugation. The efficiency of genetic transformation which was evaluated as the part of the transformed cells from the overall number of analyzed cells was 78.0 and 31.0 % in the groups 1 and 2, respectively. It was shown, that the alteration in the ratios between components of the vector system comparing to the standard scheme allows significantly increase the titers of the produced virus preparations. The biological titers of the virus preparations of 10 8 CFU/ml are sufficient to infect up to eighty percent of cells at the earlier stages of embryo development. Presumably, at earlier embryogenesis the cells were infected with more viral particles resulting in different number of the transgene copies integrated into cell genome in the groups 1 and 2. From the obtained results, the efficiency of transgenesis by means of the lentiviral vectors ranged from 30.0 to 34.3 % and varied slightly depending on time after the embryos incubation began. These results indicate that only a part of embryonic cells is usually available for viral infection. Injection of embryos at different intervals of incubation by viral preparations with similar titers produced the populations of embryonic cells with different amounts of vector copies in the cell genome. So the efficiency of hen embryo thransgenesis does not depend on the stage of its development at least for 55 hours of the embryogenesis and can be predictable.