Electron microscopic studies on the structure of rabies virus

Rational design of rabies vaccine formulation for enhanced stability

BACKGROUND/AIM

Vaccines are often lyophilized in order to retain their stability and efficacy for a longer period of time. However, the same lyophilization process may also cause a major degradation of the vaccine, especially during early phases of manufacturing, leading to a loss of potency of the product. Many viral diseases, such as rabies, are acute and fatal unless the vaccine is administered prior to exposure or the onset of symptoms in the case of postexposure treatment.

MATERIALS AND METHODS

We investigated the effect of lyophilization on the stability of the virus structure during rabies vaccine manufacturing using dynamic light scattering and transmission electron microscopy.

RESULTS

Our results indicate that some viruses lose their stability and efficacy in the course of lyophilization if the pH of the cell culture medium is controlled by solvated CO₂ because the structure of the rabies virus is very sensitive to the solution pH: the virus either aggregates or its shape is deformed at low solution pH, whereas at high pH empty capsid shells are formed.

CONCLUSION

Based on our findings, we developed a new formulation for the rabies vaccine that is stable in different buffers owing to the prevention of pH upshift upon lyophilization.

The antiviral activity of antibodies in vitro and in vivo

This chapter discusses in vitro and in vivo antiviral activities of antibody. Since experimentation is far easier in vitro , researchers have been sought to develop in vitro assays that are expected to predict activity in vivo . This could be important in both vaccine design and in passive antibody administration. The proposed mechanisms of in vitro neutralization range from those requiring binding of a single antibody molecule to virus to those requiring substantially complete antibody coating of virus. In vitro, antiviral activity can be separated into activity against virions and activity against infected cells. The activity against virions most often considered is neutralization that can be defined as the loss of infectivity, which ensues when antibody molecule(s) bind to a virus particle, and occurs without the involvement of any other agency. In vivo, it is conventional to distinguish phenomenologically between two types of antibody antiviral activity. One of them is the ability of antibody to protect against infection when it is present before or immediately following infection. Evidence for a number of viruses in vitro indicates that lower antibody concentrations are required to inhibit infection propagated by free virus than are required to inhibit infection propagated by cell-to-cell spread.

Properties of rabies virus (MNIIVP-74 strain) adapted to Japanese quail embryo cell culture

The Pasteur strain of fixed rabies virus was adapted to primary cell cultures of Japanese quail embryos and designated as MNIIVP-74. In the course of adaptation the virus pathogenicity for rabbits by the intracerebral route decreased considerably and the pathogenicity for rabbits and adult white mice by extraneural routes was completely lost. After inoculation of Japanese quail embryo cell cultures, a titer of the virus in the culture fluid at 4 days was 6.25--7.0 lg LD50/ml (by the intracerebral inoculation of adult white mice). Viral antigen could be detected by immunofluorescence in the cytoplasm of approximately 60 per cent of the cells. Virus multiplication was accompanied by intensive interferon production. In cultures of BHK-21/13S cells the titer of the virus reached was 5.75 lg LD50/ml at 24 hours and about 30 per cent of the cells were affected. The MNIIVP-74 virus showed a high immunogenic activity in rabbits, guinea pigs and mice.

An electron microscopic study of the surface structures and hemadsorption on chick embryo cells infected with rabies virus

Characteristic alterations at the surface of chick embryo cells infected with the HF-TC strain of rabies virus and the binding sites of hemadsorption were studied employing both scanning and transmission electron microscopes. The initial alteration of the cell surface structure revealed by scanning electron microscopy was an appearance of elongated and reticulated microvilli on the 2nd day after virus inoculation. On the 3rd day, numerous bullet-shaped virions could be seen budding as single, tetrapod-like structures and as radial projections both from the perikarya and microvilli. Thereafter, elongation of microvilli, formation of numerous blebs in various sizes, disappearance of filopodia, and rounding up of infected cells were observed as characteristic cytopathic effects by rabies virus infection. The attachments of goose erythrocytes to the infected cells occured in two forms. The one was adsorption of erythrocytes to the cell surface involving microvilli and filopodia in the absence of detectable virus, and the other was adsorptio n of erythrocytes to the virus particles budding from cell surface. The former could be seen from the early stage of infection through the end of observation period, while the latter was observed only on and after the 3rd day after virus inoculation. These findings were also confirmed with transmission electron microscopy.

Lipids of rabies virus and BHK-21 cell membranes

The lipid composition of highly purified Flury strain of rabies virus (HEP) propagated in BHK-21 cells in a chemically defined medium was observed to be 6.7% neutral lipids, 15.8% phospholipids, and 1.5% glycolipids. In the virion, phosphatidylethanolamine, phosphatidylcholine, and sphingomyelin were the most abundant phospholipids, accounting for 90% of the total, and the molar ratio of cholesterol to phospholipid was 0.48. Uninfected BHK-21 cell membranes were obtained by nitrogen cavitation techniques and separated by density gradient centrifugation, and the membranes were assayed for purity using 5'-nucleotidase, cytochrome oxidase, and reduced nicotinamide adenine dinucleotide phosphate diaphorase activities. Lipids of the plasma membrane were enriched in cholesterol, phosphatidylcholine, and phosphatidylethanolamine. In contrast, membranes of the endoplasmic reticulum were enriched in phosphatidylcholine, but contained smaller amounts of phosphatidylethanolamine and sphingomyelin. Comparison of the fatty acyl chains of virus and membranes from uninfected cells revealed the virion to have the lowest ratio of C18:1 to C18:0 (1.771), compared with values of about 3.0 for the plasma membrane and endoplasmic reticulum. Total polyenoic fatty acids were enriched in the plasma membrane, whereas the virus contained higher amounts of total saturates than either of the two membrane preparations. Analysis of the polar and neutral lipid fractions as well as the acyl chain analysis suggests the virion has a lipid composition that is intermiediate to that of the plasma membrane and endoplasmic reticulum and is consistent with the view that numerous viral particles are synthesized de novo by not utilizing a preexisting membrane template. From the ratio of cholesterol to phospholipid of 0.48, we calculated that 1.92 X 10(5) molecules of lipid would cover 4.14 X 10(4) nm2 in the form of a bilayer. Considerations of the molecular dimensions of the rabies envelope (total surface area, 5 X 10(4) nm2) as a bilayer suggest that some penetration of lipids by envelope proteins (M and G) is necessary.

Interaction of Clostridium perfringens theta-haemolysin, a contaminant of commercial phospholipase C, with erythrocyte ghost membranes and lipid dispersions. A morphological study

Commercially available preparations of phospholipase C from Clostridium perfringens are commonly contaminated with theta haemolysin, one of a group of bacterial haemolysins called oxygen labile (O-labile) haemolysins. Treatment of erythrocyte ghosts and a mixed lipid dispersion containing cholesterol with commercially available phospholipase C in the absence of Ca-2+ and the presence of phosphate buffer and/or EDTA resulted in the formation and release of ring or arc-shaped structures. Highly purified phospholipase C, free of theta-haemolysin, produced no changes in the morphology of erythrocyte ghosts or lipid dispersions in the presence of phosphate or EDTA, but caused the formation of typical diglyceride droplets in the presence of Ca-2+ in the absence of these inhibitors. Ring structures, identical to those caused by commercial phospholipase C, were formed on addition of highly purified theta-haemolysin to erythrocyte ghost membranes, lipid dispersions containing cholesterol and cholesterol dispersions, but not on treatment of membranes from Micrococcus lysodeikticus. Heat-inactivated O-haemolysin (60 degrees C for 10 min) produced no such effects. The dimensions of rings and arcs displayed heterogeneity. The outside diameters in various preparations varied from approx. 27-58 nm with border thickness of 4.1-7.8 nm.