Infectious cell entry mechanism of influenza virus

Uncoating of influenza virus in endosomes

The intracellular uncoating site of influenza virus was studied by measuring the fluorescence intensity of probes conjugated to the virus or the isolated hemagglutinin and also by assaying virus replication under various incubation conditions. Acidification of the viral environment was monitored by the decrease in the fluorescence intensity of fluorescein isothiocyanate, and transport of the virus particles into secondary lysosomes was assayed by the increase in the fluorescence intensity of fluorescein isothiocyanate diphosphate. The intracellular pH was estimated by the ratio of fluorescence intensities excited at two different wavelengths. It was found that the viral environment became acidified to a pH value of 5.1 to 5.2 within 10 min at 37 degrees C or 1 h at 20 degrees C after endocytosis. Addition of ammonium chloride to the medium rapidly raised the pH to 6.7. Transport of the virus particles into the secondary lysosomes was slower and negligibly low during those incubation periods. Virus replication occurred when the cells were incubated for 10 min at 37 degrees C or for 1 h at 20 degrees C, followed by incubation in the presence of ammonium chloride for a total of 12 h. These results indicate the uncoating of influenza virus in endosomes before reaching the secondary lysosomes.

Endosome pH measured in single cells by dual fluorescence flow cytometry: rapid acidification of insulin to pH 6

The acidification of various ligands was measured on a cell by cell basis for cell suspensions by correlated dual fluorescence flow cytometry. Mouse 3T3 cells were incubated with a mixture of fluorescein- and rhodamine-conjugated ligands, and the ratio of fluorescein and rhodamine fluorescence was used as a measure of endosome pH. The calibration of this ratio by both fluorometry and flow cytometry is described. Dual parameter histograms of average endosome pH per cell versus amount of internalization were calculated from this data, for samples in the absence and presence of chloroquine added to neutralize acidic cellular vesicles. The kinetics of acidification of insulin were measured and compared with previous results obtained with the chloroquine ratio technique. Rapid acidification of internalized ligand was observed both for insulin, which was mostly internalized via nonspecific pathways, and for alpha 2-macroglobulin, which was mainly internalized by specific receptor-mediated endocytosis. The average pH observed for internalized insulin was less than pH 6 within 10 min after addition of insulin. At 30 min, the average pH began to decrease to approximately pH 5, presumably because of fusion of endosomes with lysosomes.

An efficient method for introducing defined lipids into the plasma membrane of mammalian cells

An efficient method has been devised to introduce lipid molecules into the plasma membrane of mammalian cells. This method has been applied to fuse lipid vesicles with the apical plasma membrane of Madin-Darby canine kidney cells. The cells were infected with fowl plague or influenza N virus. 4 h after infection, the hemagglutinin (HA) spike glycoprotein of the virus was present in the apical plasma membrane of the cells. Lipid vesicles containing egg phosphatidylcholine, cholesterol, and an HA receptor (ganglioside) were then bound to the cells at 0 degrees C. More than 85% of the vesicles were released by external neuraminidase at 0 degrees C or by simply warming the cells to 37 degrees C for 10 s, probably because of the action of the viral neuraminidase at the cell surface. However, when the cells were warmed to 37 degrees C in a pH 5.3 medium for 30 s, 50% of the bound vesicles could no longer be released by external neuraminidase. This only occurred when the HA protein had been cleaved into its HA1 and HA2 subunits. When we used influenza N virus, whose HA is not cleaved in Madin-Darby canine kidney cells, cleavage with external trypsin was required. The fact that the HA protein has fusogenic properties at low pH only in its cleaved form suggests that fusion of the vesicles with the plasma membrane had taken place. Further confirmation for fusion was obtained using an assay based on the decrease of energy transfer between two fluorescent phospholipids in a vesicle upon fusion of the vesicle with the plasma membrane (Struck, D. K., D. Hoekstra, and R. E. Pagano. 1981. Biochemistry, 20:4093-4099).

The uncoating of paramyxoviruses may not require a low pH mediated step

The effect of lysosomotropic agents, chloroquine and NH4Cl, on the replication of paramyxoviruses was compared with that on the other enveloped RNA viruses whose uncoating is known to be blocked by the agents. Under the conditions where vesicular stomatitis or influenza virus infection was strongly inhibited at an early step, the agents exhibited little inhibitory effect on the progress of infection by Newcastle disease virus and Sendai virus, allowing viral primary transcription to occur and neither inhibiting the virus production significantly nor reducing the number of infected cells. These results are incompatible with the previous report showing that paramyxovirus and the other enveloped RNA viruses may have in common an intracellular step sensitive to lysosomotropic agents (Miller and Lenard, Proc. Nat. Acad. Sci. USA 78, 3605-3609, 1981) and suggest that paramyxovirus uncoating may not require a low pH mediated lysosomal step.

Monoclonal anti-hemagglutinin antibodies detect irreversible antigenic alterations that coincide with the acid activation of influenza virus A/PR/834-mediated hemolysis

Exposure of influenza virus to an acidic environment, which is known to be required for viral fusion and hemolysis, has recently been shown to induce a conformational change in the hemagglutinin molecule. In the present study, we examined the effects of acid incubation on the antigenicity, biological activity, and morphology of influenza virus A/PR/8/34 (H1N1). Incubation of PR8 virus at pH 5 in the absence of erythrocytes resulted in a rapid and irreversible loss of viral hemolytic activity and infectivity. Apart from a less distinct appearance of the viral surface projections and slight damage to the envelope structure, acid incubation did not result in gross morphological changes in the viral architecture. The acid-induced change could be detected in the form of greatly increased or decreased binding of many monoclonal antibodies directed to each of the four major antigenic regions of the hemagglutinin. Triggering of viral hemolytic activity and antigenic alterations was similarly pH dependent. In addition, the different pH dependencies of egg-grown and trypsin-treated MDCK-grown viruses coincided with an analogous pH dependence of the antigenic alterations that were observed with these viruses. These observations are compatible with the idea that some of the anti-hemagglutinin antibodies detect conformational changes in the hemagglutinin which are required for the initiation of fusion and hemolysis.

Hemolysis by liposomes containing influenza virus hemagglutinins

Liposomes containing influenza virus hemagglutinin were reassembled from envelopes solubilized with Nonidet P-40 and were shown to induce hemolysis and cell fusion at low pH.

Hemolytic activity of influenza virus hemagglutinin glycoproteins activated in mildly acidic environments

Hemagglutinin (HA) glycoproteins isolated from influenza virus caused hemolysis and liposome lysis at pH less than 6.0. The pH dependence was similar to that of the parent virus. Hemagglutination and hemolysis titers of HA were comparable with those of virus. The time course of hemolysis by HA was somewhat different from that by virus. HA did not cause fusion of erythrocytes in acidic media, in contrast to virus. Both HA and virus, previously incubated at pH less than 6.0, lost their low-pH-induced hemolytic activity. Isolated HA formed rosette-like structures at neutral pH, and these aggregated in acidic media. Virus also aggregated in acidic media and its envelope became leaky to negative stain. HA previously incubated at pH less than 6.0 became susceptible to trypsin digestion. Both reversible and irreversible structural changes of HA were observed by fluorescence spectroscopy; a reversible change at a pH between neutral and 6.4 and an irreversible one at pH less than 6.0. Bromelain-released HA did not cause hemolysis and liposome lysis in acidic media. The precursor form of HA did not have hemolytic activity in acidic media. The similarity in pH dependence indicates that the structural change in HA induced at pH less than 6.0 is the cause of activation and inactivation of hemolysis, HA and virus aggregation, and trypsin susceptibility. We propose that the hydrophobic NH2-terminal segment of HA2 is exposed during the structural change and interacts with the target membranes, causing a permeability increase and leading to hemolysis and lysis. The virus-induced hemolysis can be ascribed for the most part to envelope fusion activated in acidic media.

Membrane fusion proteins of enveloped animal viruses

In a living cell membrane-bound compartments are continuously either separated or united through fusion reactions, and literally thousands of such reactions take place every minute. The formation of membrane vesicles from pre-existing membranes, and their fusion with specific acceptor membranes, constitute a prerequisite for the transport of most impermeant molecules and macromolecules into the cell by endocytosis, out of the cell by exocytosis, and between the cellular organelles (Palade, 1975; Silverstein, 1978; Evered & Collins, 1982). Less frequent, but equally crucial, are fusion events in fertilization, cell division, polykaryon formation, enucleation, etc. (for reviews see Poste & Nicholson, 1978). Although a great deal is known about the properties and consequences of individual forms of membrane fusion in cellular systems, and about fusion in artificial lipid membranes, the molecular basis for the reactions remain largely unclear.

Changes in the antigenicity of the hemagglutinin molecule of H3 influenza virus at acidic pH

In order to determine the location and biological significance of the acid-induced conformational change in influenza virus hemagglutinin (HA) reported by Skehel et al., monoclonal antibodies were prepared to the molecule before and after treatment at pH 5.0. These antibodies together with monoclonal antibodies to the different antigenic regions of the H3 HA were used in immunoprecipitation and ELISA binding studies to show that antigenic changes accompanied the conformational change in the HA. Treatment at pH 5.2 or less exposed new determinants on the HA while two antigenic regions, located at the tip and interface of the molecule at neutral pH, were lost or modified. Antigenic sites in the loop and hinge regions defined by the available monoclonal antibodies were not affected by the conformational change. Monoclonal antibodies specific for the acid-induced conformation efficiently inhibited hemagglutination of the virus at low pH but were extremely poor inhibitors of virus-induced red blood cell hemolysis at its pH optimum of 5.1. These antibodies were unable to neutralize viral infectivity under neutral or acidic conditions. Antibodies specific for the non-acid-treated HA conformation failed to inhibit hemagglutination at low pH values but were able to both inhibit hemolysis of red blood cells and neutralize virus infectivity. Residual unmodified HA after pH 5.0 treatment could explain the inhibition of hemolysis and infectivity by monoclonal antibodies in each of the different antigenic areas.

[Influenza]

Influenza is the last great uncontrolled plague of mankind. Pandemics and epidemics occur at regular time intervals. The influenza viruses are divided into the types A, B and C and show unique variability of their surface antigens (hemagglutinin and neuraminidase). Influenza viruses of type A show the largest degree of antigenic variation which, in turn, resulted in the definition of a number of subtypes, each comprising many strains. By comparison, influenza viruses of types B and C exhibit much less variation of their surface antigens. As a consequence, no subtypes but many different strains have been recognized. The degree of antigenic variation correlates with the epidemiologic significance of the virus types, type A being the most and type C the least important. Two different kinds of antigenic variation have been recognized: In the case of minor variation of one or both surface antigens, the term "antigenic drift" is employed. Antigenic drift occurs with all three types of virus, it is caused by point mutations which increase the chance of survival of mutants in the diseased host. In addition, influenza A viruses show sudden and complete changes of their surface antigens in regular time intervals, resulting in the appearance of new subtypes. This event is called "antigenic shift". The mechanisms responsible for antigenic shift are poorly understood, only. In addition to the recycling of preceding subtypes, reassortment resulting from double infection of cells with strains of human and animal origin are considered possible explanations. By use of modern DNA recombinant technology, the base sequences of a series of virus genes and, as a consequence, the amino acid sequence of the corresponding antigens have been determined. By means of monoclonal antibodies, the antigenic structure of many influenza antigens has been further elucidated. It can be expected that further research on the molecular basis of antigenic variation could finally result in an understanding of the causal mechanisms. It is an outstanding feature of the epidemiology of influenza A viruses that a family of related strains prevails for a certain period of time and disappears abruptly as a new subtype emerges.(ABSTRACT TRUNCATED AT 400 WORDS)