Is it true that a cell line cannot be infected with virus again once it was alre - virus infection (Feb/21/2008 )

Hi,

I have a cell line created from primary mouse embryonic fibroblast. I have infected this cell line once with retrovirus already with neomycin as selection marker. I am planning to infected these cells with virus again but with different selection marker. Some people told me that if the cells have been already infected once before, it would be very difficult to infected another time because the cell have a kind of defense mechanism. I would like to know if it is true or not!

Do anyone know how I can know more about the virus infection in tissue culture. Are there any references you can recommend?

thanks
Ch