To explore the feasibility of adenoviral (Ad)-mediated gene transfer to the human placenta, we examined the ability of two recombinant Ad vectors to transduce isolated human cytotrophoblast cells and two choriocarcinoma cell lines (BeWo and JEG-3, which have different potentials to undergo morphological differentiation in response to cAMP). Recombinant Ad efficiently transduced cytotrophoblast cells. However, there was a marked reduction in the transduction efficiency of these vectors after the terminal differentiation of the mononucleate cytotrophoblasts into multinucleate syncytial trophoblast. BeWo and JEG-3 cells were readily transduced with the recombinant Ad, but a striking reduction in transduction efficiency of the Ad vector was observed in BeWo cells following cAMP-stimulated cellular differentiation, which includes cell fusion to form syncytia. In contrast, JEG-3 cells, which are not induced to fuse in the presence of cAMP, did not show a reduced transduction efficiency when exposed to the cyclic nucleotide. Reporter gene copy number increased with Ad-mediated gene transfer into undifferentiated Bewo cells but was low in cells that had been previously exposed to cAMP. In contrast, both undifferentiated and cAMP-treated BeWo cells were capable of expressing a reporter gene when transfected with an Ad-based plasmid. Taken together, these results demonstrate that the reduction in transduction efficiency of the Ad vectors in cAMP-treated BeWo is the result of reduced infectivity rather than of a reduction in the transcription/translation efficiency of the exogenous genes. Our findings demonstrate that recombinant Ad vectors will not be useful for the transfer of genes into differentiated trophoblast cells because these cells are resistant to Ad infection. This may limit the utility of Ad-based vectors for placental gene therapy. However, we have documented that less-differentiated trophoblast cells are susceptible to Ad-mediated gene transfer. Our observations also suggest a mechanism by which differentiated human trophoblast cells resist Ad infection and prevent fetal infection by maternally derived Ad.
