Quantification of extracellular and intracellular RNA

A549 cells were infected as described above. At the appropriate time points, 1 mL of cell culture supernatants was harvested and centrifuged at 2,000 rpm for 10 min at 4°C. Extracellular RNA was extracted from the clarified supernatants using the QIAmp viral RNA minikit to quantify extracellular viral copy numbers. Cells were washed with ice-cold stringent wash buffer (1 M NaCl, 50 mM sodium bicarbonate, pH 9.5) for 3 min to remove cell surface-associated viruses. After ice-cold 1× PBS washes, cells were harvested using the Qiagen RNeasy kit to extract intracellular RNA to quantify intracellular viral copy numbers.