Thus within a range of MOI where interference is insignificant, no replication- 
competent virus were detected in 10 9 packaged virus, infecting greater than 5 x 
10® BHK cells in culture. 
Table 8 
Occurrence of Plaque-forming Units (PFU) in Growth 
Media of Cells Infected with Recombinant SFV" 
SFV4 (PFU) 
lacZ Recombinant Virus Added 
MOI 
0.01 
0.1 
0.5 
5.0 
7.5 
10 
17 
34 
60 
75 
0 
0 
0 
Oh 
O' 
0 
0" 
0 
0 
O' 
0 
1 
0 
+ 
0 
+ 
nd 
nd 
+ 
0 
nd 
0 
3 
+ 
-1- 
+ 
+ 
nd 
nd 
-1- 
0 
nd 
nd 
£ 
4- 
+ 
+ 
+ 
nd 
+ 
nd 
-1- 
nd 
0 
*SFV3-lacZ recombinant SFV stocks packaged with Helper 2 were activated and used to infect 
1 x 10 7 BHK cells (unless otherwise indicated) at various MOI in the presence or absence of 
wild type SFV (virus derived from the SFV4 clone). 
b Repeated 2 times with independent stocks. 
'Repeated 4 times with independent stocks of virus on 1 x ltf cells/experiment. 
d Repeated 4 times. 
TDone with 3 independent stocks using 1 x 10* cells/experiment, 
nd, not determined 
In Vitro Studies With Other Cell Lines 
All of the infections of cultured cells described so far have utilized BHK cells. These 
cells are generally ideal for investigators seeking simply to produce high levels of 
proteins with the SFV/Helper 2 expression system. An important advantage of the SFV 
expression system, however, is that proteins can be produced in a wide variety of other 
cell types, as well. Thus we investigated whether replication-competent virus were 
produced during infection of three additional types of cells. 
[456] 
Recombinant DNA Research, Volume 18 
