Many current gene therapy protocols use retroviruses as the vector to package and transfer genes to target cells. Cocultivation of target cells on a layer of irradiated retrovirus producing cells is an efficient method for exposure of target cei Is tohigh tilers of vims. However, this technique has several adverse features, most notably the resulting contamination of target ceils with virus producing cells, and the difficulty of carrying out quality control procedures on the virus stock in the timeframe of a transduction procedure. Retroviral transduction, therefore, is usually carried out by culturing the target cells in the presence of cell free (sterile filtered) media collected over a period of time from virus producing cells. This supernatantcan be stored for relatively long periods of time (>6 months) at -80°C with little loss in titer. The storage of viable virus allows exhaustive quality control of virus batches to be carried out before its use in transduction experiments. This is of obvious benefit in clinical application of retroviral gene transfer technology. The titer resulting from such a harvest can range from 10~2 to 10~6, depending on the type of producing cells and the structure of the retrovirus itself.
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