Immunology L21 - Introduction To Viruses Viral Classification - Virology 1

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Viruses are:

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[ subcellular genetic elements
[ obligate parasites

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Viruses are not:

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[cells
[self replicating on their own
[motile - they have no energy supply of their own

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Virus Particles = VIRIONS

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•They contain a genome which may be DNA or RNA, single or double stranded
•The genome is packaged in a protective protein coat or capsid
•Sometimes the virion also has an envelope made of cell membrane modified to contain virus proteins

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All kinds of cells are susceptible to their particular viruses

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•bacterial viruses
•plant viruses
•animal viruses
•insect viruses
•fungal viruses
•algal viruses
•protozoal viruses

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Large diversity in size and morphology

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eg Rabies virus (170nm long) and poliovirus (30nm) particles

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Despite their small size (too small to be seen by light beam) we can get very detailed information about their structure by bombarding them with:

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-Electrons
- X-rays

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-Electrons:

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Transmission electron microscopy of virus particles

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- X-rays

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Computer reconstruction of virus from data derived by X-ray crystallography

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X-ray crystallography

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This is important as an aid to:
•understanding virus-receptor interactions
•understanding virus-antibody interactions
•the design of new antivirals

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Capsid Symmetry

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To protect the genome from breakdown by nucleases, the capsid proteins are assembled symmetrically around the nucleic acid to form either:
-an icosahedral capsid
-helical capsid

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icosahedral capsid

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composed of capsomers

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a helical capsid

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enclosed in modified cell membrane envelope)

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Examples of viruses with icosahedral symmetry

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-ADENOVIRUS
-HERPESVIRUS
-PAPILLOMAVIRUS

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ADENOVIRUS

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Adenoviruses that infect humans are usually mild pathogens, and can cause respiratory illness or conjunctivitis

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HERPESVIRUS

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At least 7species are known to infect man, including herpes simplex virus; cytomegalovirus, varicella zoster; and EBV

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PAPILLOMAVIRUS

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Cause benign skin warts in their natural hosts which often regress but human genital warts can become malignant

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Examples of viruses with nucleocapsids of helical symmetry

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-Mumps virus
-Influenza virus

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An exception: Poxviruses

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The poxviruses are the largest and most complex of all viruses. There are >100 different proteins in the virion and the genome is 130 kbp dsDNA (cf. parvoviruses which have 3 proteins and 5kb ssDNA)
-Vaccination against
smallpox (now extinct)
eg Molluscum contagiosum virus Multiple benign nodules in skin

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Some viruses have a double-shelled capsid

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- Inner capsid
- Outer capsid
eg Rotaviruses

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Rotaviruses

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infect the lining of the intestine and cause diarrhoea, especially in children. The virus is very hardy, surviving passage through the gut, and may even survive for long periods in sewage despite stringent treatment.

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Some viruses have a lipid envelope, acquired by budding through the membranes of the host cell

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eg Influenza virus

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Influenza virus

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causes epidemics and pandemics of respiratory disease.
Its envelope is derived from the plasma membrane of the cell
The virus is not very stable in ordinary environmental conditions

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Genomes - viral nucleic acid

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May be
–s.s. (single stranded) DNA
–d.s. (double stranded) DNA
–s.s. RNA
–d.s. RNA
•Sometimes divided into segments, each containing one or two viral genes.
•s.s.RNA can be +ve sense (like mRNA) or -ve sense (complementary to mRNA)

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Classification of viruses - Viruses are classified into Families by

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•Kind of nucleic acid genome
•Strategy of replication (i.e. how is mRNA produced from the genome?)
•Morphology of the virion.

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Classification of viruses - Genera, species and types are distinguished by

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different arrangements of genes, or sizes of proteins, or serological reactions, and often on types of disease produced.

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DNA virus families

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eg
-poxviridae
-adenoviridae
-papovaviridae
-hepadnaviridae
-herpesviridae
-parvoviridae

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RNA virus families

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eg
-paramyxoviridae
-orthomyxoviridae
-coronavirdae
-filoviridae
-reoviridae
-picornaviridae
-caliciviridae
-rhabdoviridae
-areanaviridae
-retroviridae
-togoviridae flaviviridae
-bunyaviridae

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Groupings based on epidemiologic/pathogenic criteria

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These are not the same as taxonomic groupings
- Enteric viruses
-Respiratory viruses
-Arboviruses
-Sexually transmitted viruses
-Hepatitis viruses

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Enteric viruses:

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• replicate primarily in and are localised to the intestinal tract
• acquired by ingestion of material contaminated with faeces
• eg. rotavirus, calicivirus, astrovirus, some adenoviruses

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Respiratory viruses:

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• replicate primarily in and are localised to the respiratory tract
• acquired by inhalation of droplets
• eg. orthomyxovirus, rhinovirus, some paramyxoviruses, coronaviruses & adenoviruses

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Arboviruses:

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• infect insects that ingest vertebrate blood
• replicate in the insect and are transmitted by bite
• eg. orbivirus, most bunyaviruses, flaviviruses & togaviruses

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Sexually transmitted viruses:

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•include some herpesviruses and papillomaviruses that cause lesions in the genital tract
•also certain retroviruses and hepatitis viruses that are transmitted during sexual activity but cause generalized disease

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Hepatitis viruses:

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•principal target organ is the liver
•hepatitis A & E (spread by enteric route) and B, C & D (spread by blood or sexually) - each in a different taxonomic family

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Are viruses alive?

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Virologists refer to “live” viruses as ones that can infect permissive cells. Infectivity must be maintained to grow viruses:
• to study
• from clinical specimens for diagnostic purposes
• to make vaccines

Viruses must be “killed’ or inactivated:
• to avoid disease transmission
• for sterilization and disinfection
• to make inactivated vaccines

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Cultivation of viruses

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•Rather than inoculating healthy hosts, virologists found that viruses could be grown in laboratory animals eg. suckling mice, fertile hens eggs
•Today we mainly use cell cultures - cells derived from humans or animals, grown in semi-synthetic medium in bottles or petri dishes for growth of animal viruses.

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Virus cultivation in chicken eggs - Vaccinia on the chorioallantoic membrane

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Vaccinia produces pocks on the CAM of chick embryos.
These are visible accumulations of leucocytes.

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C.P.E - cytopathic effects

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Cytopathic effects due to poliovirus infection of monkey kidney cells
eg Respiratory syncytial virus

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immunofluorescence

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If antibodies to viral proteins are available, infected cells can be visualised by immunofluorescence

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Infectivity assay

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eg poliovirus plaques in a monolayer cell culture