VIRUS REPLICATION
PETER H. RUSSELL, BVSc, PhD, FRCPath, MRCVS Department of Pathology and Infectious Diseases, The Royal Veterinary College, Royal College Street, London NW1 OTU.
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Objectives Students should be able to:
• describe in outline how to cultivate viruses • list the stages of virus replication in a host cell with reference to how these differ between RNA and DNA viruses. • explain the effects that viruses have on cells.
Viruses grow and kill cells, this results in cell death in vitro and necrotic lesions in-vivo
HOSTS FOR VIRUS CULTIVATION
1) EXPERIMENTAL ANIMALS, 4 uses
2) FERTILE HEN’S EGGS
3) CELL CULTURES
Production of cell cultures:
Continuous cell lines.
VIRUS ISOLATION
GROWTH:
The stages in virus infection are :
• • • • • • • • 1) Attachment 2) Penetration and uncoating 3) Formation of viral messenger RNA 4) Formation of new genomes 5) Formation of new protein 6) Assembly. 7) Release 8) Latency.
The stages in virus infection are :
• • • • • • • • 1) Attachment 2) Penetration and uncoating 3) Formation of viral messenger RNA 4) Formation of new genomes 5) Formation of new protein 6) Assembly. 7) Release 8) Latency.
THE EFFECTS THAT VIRUSES HAVE ON CELLS
Cytopathic effects: cell lysis syncytia transformation.
Syncytia.
Transformation
The cells stop being flat but round up and start dividing uncontrollably to become piles of round cells eg after infection by feline sarcoma virus. Similar cells can also be cultured from some virus-induced tumours e.g. Feline leukaemia virus, Mareks disease of chickens. The virus does not kill the cells.
VIRUS INFECTIVITY
Infectivity is relevant to diagnosis to find out which tissues contain most virus and therefore are best for sampling. 100 infectious units of virus are often used to tests for neutralising antibody in a serum sample.
PRESERVATION OF VIRUS INFECTIVITY
Summary
• Viruses grow in susceptible hosts eg cells, eggs or animals • Viruses utilise the host cells and its enzymes to make several hundred particles overnight. • Certain viruses have their own enzymes because they undergo steps which the host cell does not, eg RNA to RNA • All viruses must attach to host cell receptors. This often determines their host range. • After entry viruses uncoat to release their nucleic acid and are not infectious at this stage. • The internal virus proteins are assembled in discrete areas which appear as inclusion bodies. These are usually in the nucleus for DNA viruses and in the cytoplasm for RNA viruses. • Viruses leave the cell when it dies although some enveloped viruses bud-off living cells