Large-scale preparation and concentration of retrovirus stocks.

For some applications, such as infection of cells in vivo, it is necessary to concentrate retrovirus stocks in order to increase their titer. Because viruses are macromolecular structures, they can be concentrated fairly easily by a relatively short centrifugation step. This unit provides protocols in which the viral particles are either pelleted or centrifuged onto a sucrose density step gradient. An alternate protocol details how virions can be precipitated using polyethylene glycol or ammonium sulfate, and the resulting precipitate collected by centrifugation. After resuspension of the precipitates, the virions can be used directly, or further purified either by sedimentation onto sucrose density gradients, or by column filtration as described here. Perhaps most simply of all, an alternate procedure describes how small volumes of virus stock can be concentrated by centrifugation through a filter that allows only small molecules to pass.