Influenza defective interfering PB2 (DI-PB2), DI-PB1, and DI-PA genes with large internal deletion were generated from the backbone of A/WSN/1933(H1N1) virus using fusion PCR. Each DI-PB2, DI-PB1, and DI-PA consisting of internal deletions were inserted into the phw2000 plasmid (Fig. 1a). When these plasmids of DI-PB2, DI-PB1, and DI-PA were co-transfected or individually transfected into 293T and A549 cells, 7–8 log copies per well of each DIG RNA was detected by RT-qPCR (Fig.
