Table 2.

Concentration of virus stocks by ultrafiltration

Virus envelope	Initial titer, cfu/ml	Final titer, cfu/ml	Volume change	Titer increase	% recovery
Experiment 1
Mo-MLV-A	2.0  ×  106	2.8  ×  107	14-fold	14.2-fold	100
VSV-G (SV-GL)	1.8  ×  105	1.5  ×  106	14-fold	8.5-fold	60
Experiment 2
Mo-MLV-A	1.0  ×  107	6.7  ×  107	10-fold	6.7-fold	66
VSV-G (LTR-G)	1.0  ×  107	1.0  ×  108	12-fold	10-fold	83

In experiment 1, 293T cells in 82-mm dishes were transfected with 22 μg of pHIV-neo along with 21 μg of pSV-A-MLV env or 10.5 μg of pSV-GL. Sixty-five hours after transfection, the culture supernatants were collected and filtered through a 0.45-μm filter. In experiment 2, transfection was carried out in 6-well dishes with 5 μg of pHIV-neo and 5 μg of the Env-encoding plasmid per well. Ten-milliliter aliquots of the supernatants were concentrated by using a Centriprep 100 concentrator. Virus titers were determined by measuring the formation of G418-resistant colonies using TE671 cells (experiment 1) or HOS cells (experiment 2).