Characteristics of the organism and its antigens

Characteristics of the family Rhabdoviridae

Rhabdoviridae are enveloped RNA viruses characterized by their shape (Gr. rhabdos, rod), and by the presence of helical nucleocapsids that are enclosed in a lipid envelope bearing surface projections. The genome is a single molecule of negative single-stranded RNA which is noninfectious and is transcribed into five mRNAs, each of which codes for a single protein. The gene order in the genus Vesiculovirus is 3'-N-NS-M-G-L-5', representing respectively the nucleocapsid protein N, the nonstructural protein NS, the matrix protein M, the envelope glycoprotein G, and a large protein L.

About 80 members of the family infect vertebrates, mostly mammals, but also fishes and reptiles, and several of the viruses which infect vertebrates also infect invertebrates. Others in the family infect plants and/or invertebrates. The family contains two genera: the genus Vesiculovirus, derived from the name of the virus causing vesicular stomatitis (VSV), a disease of domestic animals observed in the Americas, and the genus Lyssavirus, named after rabies (Gr. lyssa, madness).

The Indiana serotype of vesicular stomatitis virus is the type species of the vesiculoviruses, whereas rabies virus that of the lyssaviruses. In addition, a number of other viruses of vertebrates, invertebrates, and plants are placed within the family but have not yet been placed in a genus.

Certain rhabdoviruses have been isolated only from blood-sucking arthropods, but some of these are believed to infect vertebrates on the basis of serological surveys.

Characteristics of lyssaviruses and their antigens

Among the Rhabdoviridae, lyssaviruses are of special interest. Rabies has been known for centuries as a disease of humans and of domestic and wild animals and for many years rabies virus was thought to be unique. It is now clear that antigenic variation exists within rabies virus isolates, and the existence of several distinct rabies-related viruses is now recognized.

The genome of rabies virus, which has been completely sequenced, contains 11 932 nucleotides. An infectious virus has been obtained from cDNA. The presence of a pseudogene between the G and L cis-trons implies that rabies virus is evolutionarily intermediate in the Rhabdoviridae family; apart from that, the gene order is similar to that of the Vesiculovirus genus. The helical core of ribonucleoprotein (RNP) contains RNA complexed with about 1325 molecules of nucleoprotein N, 50-70 molecules of transcriptase L; and 700 molecules of phosphopro-tein NS. The nucleocapsid structure is surrounded by a lipid envelope with about 1150 molecules of membrane protein M, through which project the surface spikes of the only transmembrane protein, the glycosylated G protein, present at 1350 molecules.

Purified N protein elicits group-reactive antibodies detectable by complement fixation, immunofluorescence and immunoprecipitation that react with rabies and the rabies-related viruses. The immune response against N protein plays a secondary protective role, notwithstanding the fact that it plays the role of a superantigen.

Figure 1 Drawing of the rabies virus (top) showing surface glycoprotein (G) projections (peplomers) extending from the lipid envelope that surrounds the internal nucleocapsid. Matrix (M) protein is shown lining the viral envelope and interacting with the cytoplasmic domain of the surface glycoprotein. The helical nucleocapsid core (bottom) comprises the ribonucleoprotein RNP (i.e.. the single-stranded RNA genome plus nucleoprotein [N]), phosphoprotein (NS), and virion transcriptase (L) molecules. The membrane 'tail' of the virus represents the frequently observed irregular shape of particles budding from the plasma membrane of the infected cell. Reprinted after modification from Rev Infect Dis 10: (4): S771-S784, 1988, with kind permission of Professor William H. Wunner. Wistar Institute, Philadelphia, USA, and the publisher.

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