Effects of stoichiometry of retroviral components on virus production

A study was conducted to investigate the effects of increasing the amount o f each retroviral component on vector production. It was found that, while the components of both amphotropic and ecotropic vectors were expressed ind ependently of each other in a transient transfection system, increasing the amount of the gag/gag-pol component resulted in a decrease in virus titres for the amphotropic particles but not ecotropic particles. Analyses of the virus stocks produced indicated that the negative effect on titres was clo sely linked to the availability of envelope proteins for virion incorporati on. The negative effect was not observed for ecotropic particle production in 293T cells, where the ecotropic receptor was absent, but was manifested when production was conducted in 293/12 cells expressing the ecotropic rece ptor. This suggested that the premature interaction between envelope and re ceptor in producer cells could limit the amount of envelope available for v irion incorporation. In designing optimal vector production systems it is e ssential, therefore, to balance the concentration of the vector components and to ensure that there is never an excess of Gag/Gag-Pol.