Pert analysis of endogenous retroviruses induced from K-BALB mouse cells treated with 5-iododeoxyuridine: a potential strategy for detection of inducible retroviruses from vaccine cell substrates.

The activation of an endogenous, infectious retrovirus in a cell substrate that is used for the production of biologics is an important safety concern, especially in the case of live, viral vaccines, where there are minimal purification and inactivation steps in order to preserve high vaccine potency. Extensive analysis has been done to evaluate various chemical agents for the induction of endogenous retroviruses in murine and avian cells; however, similar studies have not been done with cells of other species, especially human and non-human primates, that are used in vaccine production. To develop a strategy for optimal induction and sensitive detection of endogenous, infectious retroviruses in currently used or potential vaccine cell substrates, we have initially investigated the use of a state-of-the-art, highly-sensitive, product-enhanced reverse transcriptase (PERT) assay for evaluating the kinetics of retrovirus induction and replication in 5-iododeoxyuridine (IdU)-treated K-BALB mouse cells, where endogenous retrovirus activation has previously been described. In general, the overall kinetics of virus production were similar to those of previous studies in that two peaks of RT activity were seen on long-term culture of IdU-treated K-BALB cells; however, retrovirus activation was detected earlier under our induction conditions and with greater sensitivity using the PERT assay, where 1-10 virions were detected in 1 microl equivalent of the test sample, without concentration. Furthermore, the PERT activity corresponded to the presence of infectious, murine leukaemia viruses (MuLVs) induced from K-BALB cells. Based upon these results, a strategy is proposed using the PERT assay for detection of inducible, endogenous retroviruses in vaccine cell substrates.