Fig. 4.

IAV entry into GnT1-deficient CHO 15B cells is strongly dependent on NA activity in the presence of serum. (A–D) Binding of recombinant soluble trimeric HA to the indicated cell lines in 96-well plates in the absence (black line) or the presence (dotted lines) of 10% serum (x-axis, HA concentration in micrograms). Binding to the parent Pro5 cells (A) or k1 cells (C) was not affected; however, binding to GnT1-deficient lec1 cells (B) and 15B cells (D) was strongly reduced in the presence of serum. (E and F) Entry of luciferase-expressing IAV (with an amount infecting ∼10% of WT CHO cells) was determined at increasing concentrations (40 nM, 160 nM, and 640 nM) of the NA inhibitors zanamivir (E) and oseltamivir carboxylate (F). The NA inhibitors strongly inhibited IAV entry into GnT1-deficient (15B) cells in the presence of serum. (G and H) Addition of the prodrug oseltamivir during entry (G) or oseltamivir carboxylate after entry (H) had no effect on luciferase expression.