Generation of recombinant NDV viruses expressing wild-type and mutant HA genes of H5N1 avian influenza virus.We constructed an infectious clone of NDV LaSota by assembling ten overlapping PCR fragments and inserting the 15,186 nucleotides of NDV LaSota and the sequences of the T7 polymerase promoter, HDV ribozyme, and T7 polymerase terminator into the plasmid pBR322. The wild-type and mutated HA genes of H5N1 AIV were inserted between the P and M genes of the NDV cDNA (Fig. 1A). The resultant recombinant viruses, rLa-H5w and rLa-H5m, were rescued from the cDNA as described in Materials and Methods, and the presence of the HA gene was confirmed by RT-PCR. Expression of influenza virus H5 HA by rLa-H5w and rLa-H5m was confirmed by immunostaining infected BHK-21 cells 28 h p.i. As expected, cells infected with rLa were not stained by chicken antisera to H5 AIV HA protein, but they were positive for immunostaining using chicken polyclonal antisera against NDV (Fig. 1B). In contrast, cells infected with rLa-H5w or rLa-H5m were stained by chicken antisera to H5 AIV HA protein as well as the anti-NDV antiserum. The expression of wild-type and mutant HA protein by the recombinant viruses was also confirmed by Western analysis. As shown in Fig. 1C, the majority of wild-type HA protein expressed by rLa-H5w was cleaved, while large amounts of the mutant HA protein lacking the cleavage site expressed by rLa-H5m was uncleaved.