FIG. 3.

H5N1 PB1-F2 has no effect on apoptosis enhancement. (A) U937 was challenged with strains PR8, PR8delF2, PH, and PHdelF2 at an MOI of 2. At 12 h postinfection, infected U937 cells were assayed by determining the amount of annexin V-FITC. Infected U937 cells undergo apoptosis, as determined by comparison of infected cells with mock-infected cells. However, the apoptosis level for PH does not differ significantly from that for PHdelF2. (B) The percentages were normalized according to the results for mock infection. Mean values and standard errors from three independent experiments are shown. *, P < 0.05, Student's two-tailed unpaired t test.