Discussion

We report in this study the construction a replication-defective retroviral vector with the bicistronic expression and generation of transgenic chicken producing chimeric anti-CD2 antibody by injecting a concentrated retroviral vector into the heart of chicken embryos. A chimeric antibody produced by transgenic chickens remained intact (Fig. 2) and exhibited antigen recognition ability (Fig. 3). In the present case, the bicistronic expression system was used for the expression of light and heavy chain genes; light chain was translated by ribosome scanning mechanism from the cap site and the translation of heavy chain was started from the IRES sequence. As shown in our previous report (Hotta et al., 2004), an optimization of valance in expression level of heavy and light chain is very important for effective assembly of antibody. In this regard, optimization of IRES activity with other IRES sequence possibly increase the productivity (Bonnal et al., 2003) since the translation from EMCV-derived IRES sequence seems to be less efficient comparing to cap-dependent translation (Mizuguchi et al., 2000).

Until now, biopharmaceuticals that could be produced by transgenic avian are limited to a secreted protein with a simple structure (Harvey et al., 2002; Rapp et al., 2003). In this study, we succeeded in the production of protein with a complicated structure such as full antibodies. Thus, we think that transgenic chicken bioreactor for the production of pharmaceutical proteins will be a realistic method in near future.

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