Penetration of influenza examined by means of virus aggregates

Fate of influenza A virion proteins after entry into subcellular fractions of LLC cells and the effect of amantadine

Influenza A virus entry into the host cell was studied by monitoring the fate of virion proteins in subcellular fractions of LLC-RMK2 cells under conditions of low multiplicity of infection in the absence of drugs. Adsorption but not entry could be demonstrated at 4 degrees. Restricted entry into a prelysosomal pool could be demonstrated at 20 degrees. At 37 degrees proteins entered the cell in the same relative distribution as present in virions arguing against the fusion hypothesis and for the endocytosis hypothesis. The hemagglutinin and matrix proteins were readily degraded intracellularly at 37 degrees, but the nucleoprotein was relatively resistant to degradation, also consistent with the latter hypothesis. Amantadine blocked neither entry into the host cell nor transfer between the prelysosomal and lysosomal compartments; however, it appeared to block exit from the prelysosomal and lysosomal pools. The addition of the drug reduced degradation of the matrix protein in these pools but not of the hemagglutinin, consistent with inhibition by amantadine of fusion of virion envelope and vesicle membrane.

Fusion of influenza virus membranes with liposomes at pH 7.5

Influenza virus X-31 (H3N2) membranes fuse with liposomes containing ganglioside GD1a at pH 7.5. Fusion was demonstrated by electron microscopy and also can be measured by counting the labeled virus proteins incorporated into liposomes after bound virus has been removed. Liposomes composed of lipids that have no net charge behave as reported by other investigators and do not fuse with influenza X-31 membranes at neutral pH, but they do fuse at low pH. Therefore, the liposomal composition is a factor in whether liposomes fuse with influenza virus membranes at neutral pH, probably by determining whether binding occurs. The liposomal composition necessary for fusion at neutral pH needs to be individualized for each influenza subtype. To establish that a virus requires low pH for membrane fusion, it is first necessary to establish that fusion does not occur at neutral pH under conditions where adequate binding occurs.

Ammonium chloride and chloroquine inhibit rabies virus infection in neuroblastoma cells. Brief report

Two lysosomotropic agents, ammonium chloride and chloroquine inhibit the infection of murine neuroblastoma cells by rabies virus. The experiments indicate that rabies virus infects the neuronal cells through an endosome pH dependent entry pathway.

Evidence of diffusion artefacts in diaminobenzidine immunocytochemistry revealed during immune electron microscope studies of the early interactions between influenza virus and cells

Anti-haemagglutinin-labelled antibodies have been used to search for influenza entry into cells by fusion of viral and plasma membranes. The plasma membranes of infected cells were stained by immunoperoxidase but not by immunoferritin reagents. It is suggested that the staining obtained with the peroxidase conjugate was due to diffusion of the diaminobenzidine reaction product away from the enzymic site. Immunoferritin labelling provided no evidence for entry of influenza by fusion of viral and plasma membranes under conditions of physiological pH.

Infectious entry pathway of influenza virus in a canine kidney cell line

The entry of fowl plague virus, and avian influenza A virus, into Madin-Darby canine kidney (MDCK) cells was examined both biochemically and morphologically. At low multiplicity and 0 degrees C, viruses bound to the cell surface but were not internalized. Binding was not greatly dependent on the pH of the medium and reached an equilibrium level in 60-90 min. Over 90% of the bound viruses were removed by neuraminidase but not by proteases. When cells with prebound virus were warmed to 37 degrees C, part of the virus became resistant to removal b neuraminidase, with a half-time of 10-15 min. After a brief lag period, degraded viral material was released into the medium. The neuraminidase-resistant virus was capable of infecting the cells and probably did so by an intracellular route, since ammonium chloride, a lysosomotropic agent, blocked both the infection and the degradation of viral protein. When the entry process was observed by electron microscopy, viruses were seen bound primarily to microvilli on the cell surface at 0 degrees C and, after warming at 37 degrees C, were endocytosed in coated pits, coated vesicles, and large smooth-surfaced vacuoles. Viruses were also present in smooth-surfaced invaginations and small smooth-surfaced vesicles at both temperatures. At physiological pH, no fusion of the virus with the plasma membrane was observed. When prebound virus was incubated at a pH of 5.5 or below for 1 min at 37 degrees C, fusion was, however, detected by ferritin immunolabeling. t low multiplicity, 90% of the prebound virus became neuraminidase-resistant and was presumably fused after only 30 s at low pH. These experiments suggest that fowl plague virus enters MDCK cells by endocytosis in coated pits and coated vesicles and is transported to the lysosome where the low pH initiates a fusion reaction ultimately resulting in the transfer of the genome into the cytoplasm. The entry pathway of fowl plague virus thus resembles tht earlier described for Semliki Forest virus.

C-type virus-lymphocyte interactions in developing mouse thymus

The appearance of C-type virus particles in thymus cells of Swiss mouse embryos, 11.5 to 15.5 days post-conception age (PCA), was studied with the electron microscope. In thymic rudiments of all specimens examined, virus particles were seen in epithelial cytoplasm, budding from epithelial cell surfaces and in extracellular spaces. Lymphoid cells were first seen in thymic rudiments of 13.5 days PCA, and did not display virus particles at this stage. At 14.5 days PCA, thymic lymphocytes had localized plasmalemmal thickenings of high electron-density which were adjacent to extracellular virus particles. Viruses appeared to be penetrating thymic lymphocytes by viropexis in embryos of 15.5 days PCA. At this stage, many lymphocytes also had cytoplasmic virus-containing vesicles and viral buds at their surfaces. These observations suggest the possibility that, in embryos, C-type viruses are transmitted horizontally from thymic epithelium to early populations of thymic lymphocytes.