Release of viral glycoproteins during Ebola virus infection

Maturation and release of the Ebola virus glycoprotein GP were studied in cells infected with either Ebola or recombinant vaccinia viruses. Significant amounts of GP were found in the culture medium in nonvirion forms. The major form represented the large subunit GP1 that was shed after release of its disulfide linkage to the smaller transmembrane subunit GP2. The minor form were intact GP1,2 complexes incorporated into virosomes. Vector-expressed GP formed spikes morphologically indistinguishable from spikes on virus particles, indicating that spike assembly is independent of other viral proteins. Analysis of a truncation mutant revealed an early and almost complete release of GP1,2 molecules, showing that membrane anchoring is mediated by the carboxy-terminal hydrophobic domain of GP2. We have also compared wild-type virus which requires transcriptional editing for synthesis of full-length GP with a variant that does not depend on editing. Both viruses released comparable amounts of GP1, but the variant expressed only minute amounts of the small, soluble GP which is the expression product of nonedited mRNA species of the GP gene. The abundant shedding of soluble GP1 may play an important role in the immunopathology of Ebola hemorrhagic fever in experimentally and naturally infected hosts.

Characterization of Ebola virus entry by using pseudotyped viruses: identification of receptor-deficient cell lines

Studies analyzing Ebola virus replication have been severely hampered by the extreme pathogenicity of this virus. To permit analysis of the host range and function of the Ebola virus glycoprotein (Ebo-GP), we have developed a system for pseudotyping these glycoproteins into murine leukemia virus (MLV). This pseudotyped virus, MLV(Ebola), can be readily concentrated to titers which exceed 5 x 10(6) infectious units/ml and is effectively neutralized by antibodies specific for Ebo-GP. Analysis of MLV(Ebola) infection revealed that the host range conferred by Ebo-GP is very broad, extending to cells of a variety of species. Notably, all lymphoid cell lines tested were completely resistant to infection; we speculate that this is due to the absence of a cellular receptor for Ebo-GP on B and T cells. The generation of high-titer MLV(Ebola) pseudotypes will be useful for the analysis of immune responses to Ebola virus infection, development of neutralizing antibodies, analysis of glycoprotein function, and isolation of the cellular receptor(s) for the Ebola virus.

Distinct cellular interactions of secreted and transmembrane Ebola virus glycoproteins

The mechanisms by which Ebola virus evades detection and infects cells to cause hemorrhagic fever have not been defined, though its glycoprotein, synthesized in either a secreted or transmembrane form, is likely involved. Here the secreted glycoprotein was found to interact with neutrophils through CD16b, the neutrophil-specific form of the Fc gamma receptor III, whereas the transmembrane glycoprotein was found to interact with endothelial cells but not neutrophils. A murine retroviral vector pseudotyped with the transmembrane glycoprotein preferentially infected endothelial cells. Thus, the secreted glycoprotein inhibits early neutrophil activation, which likely affects the host response to infection, whereas binding of the transmembrane glycoprotein to endothelial cells may contribute to the hemorrhagic symptoms of this disease.

[Changes in certain indicators of hemostasis in rabbits upon administration of Ebola virus preparations]

Changes in some parameters of hemostasis in rabbits insusceptible to Ebola virus (EV) in various periods after reinoculations with live and inactivated virus are described. Challenge with both control protein and live and inactivated EV leads to imbalance in the hemostasis system, which is compensated for in the course of follow-up and does not result in clinically manifest disorders of blood clotting. However, the mechanisms of development of the hemostasis imbalance caused by the control protein and virus preparations were different. In the former case no fibrinogen degradation products were detected in the blood serum, whereas in the latter they appeared in the serum after each reinoculation of the virus. This indicates a peculiar effect of EV on hemostasis.

Haemorrhagic fevers and ecological perturbations

Hemorrhagic fever is a clinical and imprecise definition for several different diseases. Their main common point is to be zoonoses. These diseases are due to several viruses which belong to different families. The Flaviviridae have been known for the longest time. They include the Amaril virus that causes yellow fever and is transported by mosquitoes. Viruses that have come to light more recently belong to three other families: Arenaviridae, Bunyaviridae, and Filoviridae. They are transmitted by rodents (hantaviruses and arenaviruses) or from unknown reservoirs (Ebola Marburg). The primary cause of most outbreaks of hemorrhagic fever viruses is ecological disruption resulting from human activities. The expansion of the world population perturbs ecosystems that were stable a few decades ago and facilitates contacts with animals carrying viruses pathogenic to humans. Another dangerous human activity is the development of hospitals with poor medical hygiene. Lassa, Crimean-Congo or Ebola outbreaks are mainly nosocomial. There are also natural environmental changes: the emergence of Sin Nombre in the U.S. resulted from heavier than usual rain and snow during spring 1993 in the Four Corners. Biological industries also present risks. In 1967, collection of organs from monkeys allowed the discovery in Marburg of a new family of viruses, the Filoviridae. Hemorrhagic fever viruses are cause for worry, and the avenues to reduce their toll are still limited.

Short report: lack of virus replication in arthropods after intrathoracic inoculation of Ebola Reston virus

To evaluate the potential for arthropods to serve as reservoir hosts of Ebola virus, three mosquito species, Aedes albopictus, Aedes taeniorhynchus, and Culex pipiens, and a soft tick, Ornithodoros sonrai, were inoculated with 1O2.5 plaque-forming units of Ebola Reston virus. After incubation at 22 degrees C for 11 days, at least six specimens of each species were triturated and examined for evidence of viral replication by enzyme-linked immunosorbent assay and plaque assay. There was no evidence of viral replication in any of the arthropods tested. Because intrathoracic inoculation bypasses various barriers to viral infection, the lack of replication of Ebola Reston virus in these inoculated arthropods indicates that these mosquito species and soft ticks probably are not involved as natural reservoirs of Ebola virus.

Ebola virus infection: an overview

The current outbreak of the Ebola virus infection in Africa has yet again proven that highly dangerous diseases that are transmitted via the blood-borne route may be endemic in some parts of the world and may emerge as sporadic outbreaks causing worldwide concern. Health care professionals are at the forefront of combatting these diseases and treating infected individuals. Though dental professionals are unlikely to be directly involved in the management of such acute infections, with very high mortality rates, they may encounter patients seeking dental treatment who are either from, or who have recently toured the endemic disease areas. This overview, therefore, is a thumb nail sketch of the Ebola virus infection and its implications for dentistry.

[The morphological changes in Ebola infection in guinea pigs]

Ebola virus reproduction and morphological lesions were investigated in infected guinea pigs by electron microscopy. The liver was found to be the main target organ, whereas in other internal organs the pathological changes were insignificant. Ebola virus reproduction was demonstrated only in cells of the mononuclear phagocyte system.

[The ultrastructural changes in guinea pig organs during the serial passage of the Ebola virus]

Morphological study of internal organs of guinea pigs inoculated with Ebola virus at 2-8 passages was carried out. In the course of these passages the number of infected cells and virus particles in the organs was shown to increase, and the destructive changes in organs became more pronounced. In the 1st-3rd passages Ebola virus replication was observed in macrophagal cells only but beginning from the 4th passage of virus reproduction was found also in hepatocytes, spongiocytes, and fibroblasts.

[Ebola virus reproduction in cell cultures]

Ebola-Zaire virus production in Vero and BGM cells was studied. The CPE developed in both cell cultures. The cell monolayer destruction by 80-90% was seen at a low multiplicity of infection in 7-8 days after virus inoculation. An overlay composition was developed for virus titration using plaque assay. The plaque production was shown to be directly proportional to the virus dose. The curve of Ebola virus production in Vero cell culture fluid was determined. At a multiplicity of infection of 0.01 PFU/cell, the maximum virus titer of 10(6.4) PFU/ml was reached in 7 days postinfection. Specific antisera were generated by inoculation of guinea pigs. Indirect immunofluorescent assay was used for testing of virus-specific antigen and antibody.