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The growing demand of biopharmaceutical products is boosting the market for therapeutic recombinant proteins (r-proteins). More than half of the 140 r-proteins that have gained approval for human therapeutic use are manufactured in mammalian cells that make possible complex post-translational modifications, like glycosylation. In the industry, r-proteins are manufactured by stable gene expression (SGE) following Good Manufacturing Practice (GMP) standards. Transient gene expression (TGE) in mammalian cells is an alternative method for the expression of r-proteins whose main advantages are rapidity and flexibility. TGE involves the expression of the protein of interest from plasmids, which are transfected into the cells with a non-viral DNA transfecting agent, usually polyethylenimine (PEI). DNA is transcribed from the plasmids without the latter having been integrated into the host genome. Despite the recent improvements in r-protein titers by TGE and the scale up of this method to the 100-L scale, TGE remains so far a tool for protein production for research purposes. Therefore, to determine if TGE can be used for manufacturing therapeutic r-protein following GMP standards, different features related to reproducibility and product quality from TGE of a recombinant anti-rhesus D immunoglobulin G in HEK-293E cells using linear PEI were characterized. To test the effects of the size distribution and chemical structure of PEI on TGE, different commercially available PEIs were characterized and their impact on transfection and r-protein expression were assessed. The results showed that both the molecular weight distribution and the chemical structure of the PEI had an effect on TGE. However, the small variations in size distribution and structure between the batches of PEI from the same supplier did not affect significantly the transfection and r-protein production by TGE. Then, the fate of PEI and plasmid DNA in cell culture over time and their removal from the final r-protein during purification were determined and measured. Most of the PEI and pDNA remained in the cell culture medium after transfection. However, both PEI and plasmid DNA, which are additional components in TGE compared to processes based on recombinant cell lines, could be reduced to a concentration below the limit of detection of the assays after Protein A affinity purification of the antibody. To test the reproducibility of a TGE process, 10 batches of transfected cells were run in 500-mL orbitally shaken bottles. The cell specific productivity of the antibody was 20.2 ± 2.6 on average for the 10 batches. The purity and size consistency of the recombinant antibody produced in those 10 batches was demonstrated by gel electrophoresis and size exclusion chromatography. Then, the glycosylation profile of the antibody produced by TGE in HEK-293E cells was determined by mass spectrometry and was reproducible over the 10 batches. Moreover, it was similar to the glycosylation profile of the antibody produced by 3 stable HEK-293E cell lines. Finally, as a proof of concept, TGE was applied for the development of a vaccine against the respiratory syncytial virus. The fusion protein of the respiratory syncytial virus was produced by TGE and used as an antigen for the development of a recombinant vaccine. TGE enabled the rapid evaluation of the candidate vaccine in animal trials. Overall, the results of this study suggest that TGE is a reliable and reproducible method for manufacturing r-proteins of a consistent quality, provided that some particular features, such as reagents quality and process parameters, are well defined and controlled. Since TGE makes possible the rapid production of virtually any protein, even toxic, it could be used to provide GMP approved biopharmaceuticals for clinical trials in a short time, reducing development costs of biological therapeutic products.