Heterologous gene expression in avian cells: Potential as a producer of recombinant proteins

We have explored the possibility of using avian cells for the expression of human proteins. We found that various avian cells including quail fibrosar coma cells (QF), duck embryo cells (DE) and primary chicken embryo fibrobla sts (CE) could efficiently be transfected with DNA by calcium phosphate cop recipitation, and that promoters which are transcriptionally active in mamm alian cells also functioned well in these avian cells. Among the promoters we tested, the major immediate early promoter of human cytomegalovirus drov e the highest lever of chtoramphenicol acetyl transferase (CAT) expression, outperforming the SV40 early promoter and the RSV LTR. Using the bacterial CAT gene as a reporter, we found that levels of CAT activity were higher i n QF and DE cells than in mammalian cells such as CHO, HeLa, Vero and 293T cells. We further cloned a sequence encoding human erythropoietin (EPO) and compared its expression in QF and mammalian cells. Consistent with the CAT data, in transient transfection assays, QF cells produced higher levels of EPO than the mammalian cell lines tested. QF cells which can be passaged p ermanently were stably transfected with an EPO expression vector. The subcl oned QF line was able to produce up to 1,700 U/ml EPO from 3 x 10(6) cells in 72 h. Purified QF-produced EPO showed a broad but discrete protein band, ranging from 33 to 41 kD and was as biologically active as CHO-produced EP O. Although a number of factors still remain to be optimized, our results d emonstrate the potential of avian cells such as QF as producers of heterolo gous proteins.