Titration of Vectors on Different Cell Lines Before and After Factor Xa Cleavage
| . | Kit-Negative Cells . | Kit-Positive Cells (%) . | ||||||
|---|---|---|---|---|---|---|---|---|
| NIH 3T3 . | A431 . | HMC-1 . | TF-1 . | |||||
| −Xa . | +Xa . | −Xa . | +Xa . | −Xa . | +Xa . | −Xa . | +Xa . | |
| SCFXA | 3.6 × 105 | 5 × 105 | 4.2 × 105 | 6.7 × 105 | 4.2 | 40 | 2 | 38 |
| EXA | 5 × 105 | 5 × 105 | 2 × 102 | 1 × 106 | 40 | 36 | 45 | 46 |
| 4070A | 5.4 × 105 | 5 × 105 | 2.5 × 106 | 2.6 × 106 | 63 | 64 | 77 | 75 |
| . | Kit-Negative Cells . | Kit-Positive Cells (%) . | ||||||
|---|---|---|---|---|---|---|---|---|
| NIH 3T3 . | A431 . | HMC-1 . | TF-1 . | |||||
| −Xa . | +Xa . | −Xa . | +Xa . | −Xa . | +Xa . | −Xa . | +Xa . | |
| SCFXA | 3.6 × 105 | 5 × 105 | 4.2 × 105 | 6.7 × 105 | 4.2 | 40 | 2 | 38 |
| EXA | 5 × 105 | 5 × 105 | 2 × 102 | 1 × 106 | 40 | 36 | 45 | 46 |
| 4070A | 5.4 × 105 | 5 × 105 | 2.5 × 106 | 2.6 × 106 | 63 | 64 | 77 | 75 |
The host-range properties of the amphotropic vectors incorporating the chimeric envelope SCFXA were tested on Kit-expressing and nonexpressing cell lines with EXA and wild-type 4070A vectors as controls. All of these vectors were also tested after treatment with 4 μg/mL factor Xa protease before infection. The titer of SCFXA was reduced considerably compared with that of EXA and 4070A on the Kit-positive cell lines, but was restored by factor Xa protease cleavage. The titer of SCFXA was unaffected by factor Xa cleavage on the Kit-negative cell lines. These data show that infectivity of the SCFA vector on Kit-positive cells is blocked by the display of SCF and can be restored by cleavage of the displayed domain. Titers are shown as enzyme-forming units per mL (adherent cells) or percentage blue cells (suspension cells).