Figure 3. Role for NA activity and proteases in LTGF-β activation by influenza virus.

(A) rLTGF-β (10 ng/ml) was incubated alone (black bar) or with bNA (30,000 RFU NA activity), purified N4 virus (90,000 RFU NA activity), or purified N4 NA (90,000 RFU NA activity) in the presence or absence of 1× PI cocktail (red bars) or 10 nM oseltamivir carboxylate (NAi, gray bars) for 1 h at 37°C. TGF-β levels were measured by (A) PAI/L assay or (B) ELISA. (C) rLTGF-β (10 ng/ml) was incubated with purified Tk/WI virus (10⁶ TCID₅₀ units/ml) alone or in the presence of increasing concentrations of bestatin (▪, nM), leupeptin (•, µM) or GM1489 (Δ, nM), and TGF-β activity measured by ELISA. Error bars represent standard error of the mean. Asterisk (*) indicates significant inhibition as compared with virus/NA-treated LTGF-β in the absence of inhibitor.