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Vector titer was determined by G418-resistant colony formation. Amphotropic virus was produced by transient transfection using the producer line HW293-A (unpublished data), and viral supernatants were used to infect NIH3T3 cells. In Table 1, the titers for both experiments are on the order of 105 colony forming units (cfu)/ml. Even at high dilutions of supernatant (1:103), which likely give rise to single-copy infected cells in accordance with the Poisson kinetics of viral transduction, colonies are resistant to as high as 1 mg/ml G418 without substantial loss of titer, indicating efficient expression of the Neo gene from the third ORF.
Fig.2
NIH3T3 cells were infected with various dilutions of viral supernatant, and one day following infection the cells were transfected with the ecdysone-inducible reporter vector pEGSH-luc. The following day, cells were induced for 20 hours with ponA, then assayed for luciferase activity. In Figure 2 (at an MOI of 1.0), a strong induction is achieved, which is reproducible in separate infected populations.
Fig.3
NIH3T3 cells were infected with pFB-ERV supernatants and selected with
600 g/ml G418. Resistant colonies were picked and expanded. In an initial
screen, 24 colonies were transfected with reporter vector, and induced
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