Complexation of retroviruses with polymers significantly increases the number of genes transferred to murine embryonic stem cells, but does not raise transgene expression levels.

Embryonic stem cells efficiently silence retrovirus transgene expression. To help to solve this problem, retroviruses have been developed that are more resistant to silencing, such as retroviruses derived from the MSCV (murine-stem-cell virus). A complementary approach to increasing transgene expression might be to increase the number of integrated transgenes. To test this approach, we formed polymer complexes with MSCV-derived ecotropic retroviruses, concentrated them up to 40-fold and transduced two different murine embryonic stem cell lines, with a mouse fibroblast cell line as a control. The number of integrated transgenes increased more than 50-fold in the embryonic stem cell lines, yet, surprisingly, transgene expression did not increase. Interestingly, the embryonic stem cells had significantly fewer integrated transgenes than the mouse fibroblasts, even though transduction conditions were identical, which suggests that embryonic stem cells may restrict a post-binding step of retrovirus transduction.