Interference between influenza A viruses with a cleavable and a noncleavable hemagglutinin; pH-stability after mixed infection.

The infectivity of influenza A viruses like fowl plague virus (FPV) with a cleaved hemagglutinin (HA) is highly sensitive to treatment at pH 5, while strains like PR 8 or virus N with a noncleaved HA survive under this condition. After double infection of chick embryo cells with FPV and PR 8 or virus N, the yield of virus with the HA gene of FPV is greatly reduced. However, it can now survive treatment at pH 5, and the surviving FPV particles form plaques only in the presence of trypsin, indicating that they were coated by the HA of PR 8 or virus N, depending on the coinfecting virus. The results are discussed with respect to the build-up and maintenance of a large reservoir of nonpathogenic influenza A viruses with noncleavable HA in water fowl.