Cell fusion by various strains of Newcastle disease virus and their virulence

Apoptosis in chicken embryo fibroblasts caused by Newcastle disease virus

The GB strain of Newcastle disease virus (NDV) was used to infect chicken embryo fibroblasts (CEF). At various times after infection, CEF were harvested and processed for DNA extraction, flow cytometry and electron microscopy. Agarose gel electrophoresis of the DNA at 12 h after infection, showed a laddering pattern, indicating fragmentation of cellular DNA. Flow cytometry analysis of the infected cells showed an increase in the population of smaller cells (apoptotic cells). Electron microscopic examination showed extensive cellular necrosis, but also showed some other cells with condensed chromatin and with extensive perinuclear fragmentation of chromatin; apoptotic bodies could also be readily seen. These data suggest that NDV infection of CEF causes apoptosis, in addition to necrosis.

Separate epitopes in the envelope of visna virus are responsible for fusion and neutralization: biological implications for anti-fusion antibodies in limiting virus replication

Visna virus is a lentivirus which causes fusion of infected cells in vitro. Two types of fusion occur. Fusion from without requires no viral replication and a relatively high multiplicity of infection; fusion from within results from the replication of virus in cells. By using fusion from without as an assay, the mechanism of fusion by visna virus was investigated. Immune sera which contained both anti-fusion and neutralizing antibodies interacted with the virus with rapid kinetics in blocking fusion but relatively slow kinetics in the virus neutralization assay. By using visna virus and an antigenic variant, the epitopes responsible for fusion and virus neutralization were shown to be different. Antigenic variation of visna virus resulted in alteration of the neutralization epitope and conservation of the fusion epitope. This suggested that there were two populations of antibodies and that the viral epitopes for fusion and neutralization were separate. These data suggest that visna virus is capable of infecting cells via two pathways: one via the fusion site and the other via the viral epitope which mediates neutralization.

Cell fusion by canine distemper virus-infected cells

AV3 cells (continuous human amnion) infected with the Onderstepoort strain of canine distemper virus produced cell fusion within 2 to 5 hr when added to AV3 cell monolayers. An apparent requirement for intact, infected cells was demonstrated by showing that (i) frozen-and-thawed infected cells failed to induce fusion, (ii) infected cells frozen in the presence of glycerol retained their ability to induce fusion, (iii) infected cells subjected to swelling in hypotonic buffer and homogenization lost their ability to fuse cells, and (iv) semipurified and concentrated virus preparations with infectivity titers as high as 10(7.5) mean tissue culture doses per ml failed to induce fusion within 5 hr. Preparations of intact, infected cells had a mean log(10) ratio of infectivity to fusion activity of 3.6. Treatment with beta-propiolactone rendered the active preparations free from detectable infectivity while they retained their ability to cause cell fusion. Cycloheximide did not block the formation of syncytia in assay cells. This type of cell fusion was neutralized by canine distemper virus immune antisera, and measles virus immune sera showed a slight degree of cross-neutralization. Other cell lines, HEp-2, MA 139 (embryonic ferret lung), MA 104 (embryonic rhesus monkey kidney), and Vero (African green monkey kidney) were also susceptible.

Temperature-dependent inhibition of fusion from without

Fusion from without by Newcastle disease virus is inhibited by incubation at 23 C. The inhibitory effect is exerted at a step after viral attachment and is not reversed by subsequent incubation at higher temperatures.

Hemolytic interaction of Newcastle disease virus and chicken erythrocytes. I. Quantitative comparison procedure

The extent to which erythrocytes are hemolyzed by Newcastle disease virus is a function of the relative concentrations of both virus and erythrocytes. Under proper conditions, the interaction of a single virus particle with an erythrocyte is sufficient to cause lysis. The extent of hemolysis is directly proportional to virus concentration only when the virus-erythrocyte ratio is very low. At the higher virus-erythrocyte ratios usually employed in hemolysis experiments, the extent of hemolysis is proportional to the logarithm of the virus concentration. Thus, quantitative comparisons of hemolytic activities of different virus preparations cannot be made by directly comparing the extent of hemolysis. Relative hemolytic activities must be determined by comparing virus concentrations which yield equivalent amounts of hemolysis (the quantitative comparison procedure).

Virus-erythrocyte interactions

This chapter summarizes the experimental evidence bearing on the nature of virus-erythrocyte reactions characteristic of several taxonomic groups.. Such evidence is culled from (1) the study of conditions necessary for hemagglutination; (2) the examination of specific factors affecting either the cell or the virion to enhance, alter, or abolish the reaction; and (3) the direct physicochemical analysis of cells, viruses, and “receptor analogs.” The hemadsorption phenomenon also provides evidence for virus-erythrocyte interactions, which is based on the attachment of erythrocytes to infected cells in culture having hemagglutinin at their surfaces. This phenomenon reflects the interaction between erythrocytes and viral envelope components. The major virus groups that react with erythrocytes include myxoviruses, paramyxoviruses, pseudomyxoviruses, adenoviruses, arboviruses, reoviruses, enteroviruses, and miscellaneous hemagglutinating viruse (rubella virus, coronaviruses, rhabdoviruses, and oncogenic viruses). The agglutination of erythrocytes by the direct action of viral particles was first described in connection with myxoviruses. This led directly to the discovery of viral neuraminidase—a property unique to myxoviruses and paramyxoviruses. A number of viruses unrelated to myxoviruses have since been shown to agglutinate erythrocytes of various species. The visible result of viral hemagglutination is the “pattern” formed at the bottom of a test tube or well plate by lattices of red cells lightly conjoined by viral hemagglutinin. Hemagglutination serves as a useful direct means of titering intact viral particles or hemagglutinating subunits.

Preliminary analysis of the requirements for fusion from within and fusion from without by Newcastle disease virus

Different strains of Newcastle disease virus vary in their ability to induce cell fusion. When administered to cells at high multiplicities of infection some strains induce cell fusion within three hours. This type of fusion is apparently caused by the virus particles in the inoculum, since it can be induced by noninfectious virus and does not require protein synthesis for induction. It has been designated fusion from without (FFWO). Other strains induce fusion mainly at low multiplicities of infection. This fusion is induced only by infectious virus, and requires protein synthesis for induction. Probably included among these required proteins is a viral antigen which must be available at the cell surface in order for fusion to occur. This type of fusion has been designated fusion from within (FFWI).