Newcastle disease virus and two influenza viruses: Differing effects of acid and temperature on the uptake of infectious virus into bovine and canine kidney cell lines

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Abstract
The entry of 2 influenza viruses was compared to the entry of the Ulster strain of Newcastle disease virus (NDV) by measuring the escape rate of preadsorbed virus from neutralization using a microwell assay. The 2-minute entry of Ulster into Madin-Darby bovine kidney (MDBK) cells increased exponentially over the temperature range for fusion of 30–37° C and was prevented by pH 4.6 which corresponded to the pH within endosomes. The 2-hour entry of both influenza viruses into MDBK cells increased exponentially over the temperature range of 0–30° C with Q10 values similar to Q10 for the endocytosis of horse radish peroxidase and their entry times were reduced to less than 10 minutes at pH 4.6. When Madin-Darby canine kidney (MDCK) cells were used the entry of one influenza virus, NIB-8, took 2 minutes whereas the other, A/Jap/Bel, took 2 hours. The reason for this was unknown. NIB-8 contains genes 1, 2, 3, 7, 8 from A/Puerto Rico/8/34 (H1N1) virus and genes 4, 5, 6 from A/Bangkok/1/79 (H3N2) virus. A/Jap/Bel is a triple reassortant which shares genes 3 and 7 with NIB-8 whereas 1, 2, 4, 5 are from A/Japan/305/57 (H2N2) and genes 6, 8 are from A/Bel/42 (H1N1).