After five days of cell culture in 12-well plates, macrophages were transfected with 200 nM non-targeting control siRNA (AllStars Negative Control siRNA, Qiagen, Hilden, Germany) and with 100 nM of each of two different P2X 7 receptor siRNAs (Hs_P2RX7_1, Hs_P2RX7_2; Qiagen) by using HiPerFect Transfection Reagent (Qiagen) according to the manufacturer's instruction. After 4 h of siRNA treatment, fresh macrophage media was added to the cells. On the following day, the cells were left unstimulated or infected with influenza A virus for 9 h, after which the cell culture supernatants were collected and total proteins were isolated for ELISA and Western blot analyses, respectively.