Endogenous retroviral sequences are present as an integral part of eukaryot
ic genomes. Although the majority of these sequences are defective, a few c
an produce infectious virus, either spontaneously upon long-term culture or
by treatment with various chemical or other agents. Early, extensive studi
es of retrovirus induction were done in mouse cells; however, similar studi
es have not been done using state-of-the-art vir-us detection assays and wi
th cells of other mammalian species. To investigate induction and detection
of occult retroviruses in cells of different species, especially primate c
ells that are used in production of biologics, we have initially determined
the optimum conditions for retrovirus induction in chemically treated K-BA
LB mouse cells using highly sensitive product-enhanced reverse transcriptas
e (PERT) assays as well as transmission electron microscopy (TEM). Retrovir
us induction was detected at day I post-drug treatment under all test condi
tions but was optimum using 30 mug ml(-1) of 5-iododeoxyuridine (IdU) for 2
4 h. Additionally, the combination of IdU and 5-azacytidine specifically en
hanced activation of type C particles. RT activity was detected by PERT ass
ays in one microliter equivalent of test sample and retroviral particle pro
duction was seen by TEM analysis. The induction of infectious murine leukem
ia retroviruses was confirmed by infectivity assays and correlated with PER
T activity. These results indicate that strategies for detection of occult
viral agents should include optimization of induction conditions using mult
iple viral detection assays to evaluate virus activation. (C) 2001 Elsevier
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