Sialidase activity and related properties of influenza A2 viruses

Cells persistently infected with newcastle disease virus: I. Properties of mutants isolated from persistently infected L cells

The strain of Newcastle disease virus (NDV(pi)) present in persistently infected L cells differed markedly from the Herts strain (NDV(0)) used to initiate the infection. NDV(pi) produced small plaques (less than 1 mm) in chick embryo cell cultures, whereas the wild type (NDV(0)) produced large plaques (2 to 3 mm). The two viruses differed in a number of additional properties. Whereas 80% of adsorbed NDV(0) eluted from chicken red blood cells at 37 C, only about 20% of NDV(pi) was recovered under similar conditions. There was no significant difference in the neuraminidase content of the two viruses. The infectivity of NDV(0) was stable for 1 hr at 48 C, whereas 99.9% of the infectivity of NDV(pi) was destroyed. The two viruses also differed in lethality for chick embryos; NDV(pi) had significantly reduced lethality for 9-day-old chick embryos when compared to NDV(0). In contrast to NDV(0), which produced an abortive infection in L cells, NDV(pi) not only replicated effectively and destroyed these cells, but also induced significantly higher quantities of interferon than did NDV(0). These data furnished additional evidence for the lack of relationship of interferon production to abortive infection of L cells with NDV(0). In contrast, interferon was found to play a significant role in the maintenance of persistent infection.

STUDIES OF TWO KINDS OF VIRUS PARTICLES WHICH COMPRISE INFLUENZA A2 VIRUS STRAINS : I. CHARACTERIZATION OF STABLE HOMOGENEOUS SUBSTRAINS IN REACTIONS WITH SPECIFIC ANTIBODY, MUCOPROTEIN INHIBITORS, AND ERYTHROCYTES

Two kinds of virus particles have been found in varying proportions in influenza A2 strains isolated during the 1957 pandemic. Pure populations of the different particles were obtained, and these substrains were genetically stable on serial passage in the chick embryo. The two virus particles differ markedly in several biological properties though they are antigenically similar. One kind of particle, designated "+," is relatively sensitive to specific antibody, is highly sensitive to inhibition by serum inhibitors and urinary mucoprotein, fails to elute or elutes very slowly from human erythrocytes, and is capable of agglutinating erythrocytes treated extensively with V. cholerae filtrate. The other particle, designated "-," is relatively insensitive to antibodies and urinary mucoprotein, completely insensitive to serum inhibitors, elutes rapidly from erythrocytes, and can agglutinate erythrocytes treated extensively with V. cholerae filtrate. Both "+" and "-" particles destroy virus receptors on urinary mucoprotein. The relative proportions of these two particles determine the characteristics of parent strains in reactions with specific antibody, mucoprotein inhibitors, and erythrocytes. The "+" and "-" particles with several easily identifiable markers are well suited for genetic studies.

Effect of Ca++ on the stability of influenza virus neuraminidase

The neuraminidases of different strains of influenza virus varied in their stability at 37 degrees C. The enzymes of the strains with N1 neuraminidases were found to be unstable during incubation at 37 degrees C whereas the enzymes of the strains with the N2 neuraminidases were stable. Among the strains with N2 neuraminidases, the enzymes of some strains were inactivated during dialysis at 37 degrees C whereas the enzymes of others were stable. This observed loss of enzyme activity during dialysis at 37 degrees C was not restricted to a single substrate as the same loss of enzyme activity was observed irrespective of the size of the substrate used in the assay. The enzymically inactive neuraminidase was found to be non-antigenic and non-immunogenic. The inactivation of the enzyme could be prevented by the addition of Ca++ but not Mg++. Out results suggest that Ca++ is essential for the stability of the enzyme at 37 degrees C. The results would also suggest that the enzymic, antigenic and immunogenic sites are either the same or very closely situated on the surface of the neuraminidase molecule.

Electron microscope study of ribonucleic acid of myxoviruses

Intact ribonucleic acid (RNA) molecules in an extended form were extracted from purified influenza virus and observed in the electron microscope. For this study, the RNA extraction procedure and the Kleinschmidt protein monolayer technique were modified. The mean lengths of RNA from X7, X7-F1, and WSN strains of influenza virus were found to be 2.69, 2.55, and 2.37 mum, respectively. From these measurements, the corresponding estimated molecular weights would be 2.9, 2.8, and 2.5 x 10(6) daltons. X7 and WSN RNA preparations were exposed to pH 3 to disrupt intact molecules. Histograms of length measurements showed five peaks, which were interpreted to represent the five pieces of RNA reported to exist in the influenza virion. X7 RNA appeared to be more stable than WSN RNA when stored at 4 C. The profiles of histograms of incomplete virus RNA suggest that the high molecular-weight component is missing. In preliminary experiments on Newcastle disease virus RNA, molecules of various lengths were observed.

Functional significance of sialidase during influenza virus multiplication: an electron microscope study

Morphological evidence has been obtained by electron microscopy in support of previous findings that one of the most important functions of sialidase is associated with the release of virus from infected host cells. Highly specific antiserum against fowl plague virus enzyme and specific antiserum against X7 recombinant influenza virus enzyme were shown to influence the morphology of cells infected with their homologous virus. In the presence of enzyme antiserum, an accumulation and aggregation of virus particles were evident on the cell surface and in the extracellular space of infected host cells. The aggregation of virus particles was interpreted to result from the inhibition of the release of virus.