Immunology L22 - Viral Replication - Virology

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Stages in viral replication

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1. Attachment
2. Penetration
3. Uncoating
4a. Genome replication
4b. RNA synthesis
4c. Protein synthesis
5. Assembly
6. Release

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1. Attachment/Adsorption

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Viral attachment protein binds specifically to a receptor on the cell plasma membrane. This interaction defines and limits the host species as well as the type of cell that is infected.
Receptors may be
protein (e.g. ICAM-1 for most rhinoviruses)
carbohydrate (sialic acid for influenza virus)
Note: Receptors molecules are not on a cell for the benefit of the virus but are involved in normal cell processes

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Some viruses use two different receptors on the same host cell,

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for initial attachment, then closer attachment.

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2. Penetration

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After adsorption, the coat of enveloped viruses may fuse with the host cell membrane and release the virus nucleocapsid into the host cytoplasm.
Other viruses may enter the cell by a process of "endocytosis" which involves invagination of the cell membrane to form vesicles in the cell cytoplasm.

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3. Uncoating

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Refers to the release of the viral genome from its protective capsid to enable the nucleic acid to be transported within the cell and transcribed to form new progeny virions.

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Fusion and endocytosis strategies used by viruses

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-uncoating at the PM
-uncoating within endosomes
-uncoating at the nuclear membrane

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uncoating at the PM

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Some viruses uncoat by fusing their envelope with the cell plasma membrane

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uncoating within endosomes &
uncoating at the nuclear membrane

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Other viruses uncoat after endocytosis

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HIV is an example of a virus whose contents is released directly into the cytoplasm

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The hydrophobic fusion region of gp41, once exposed, can initiate fusion of the two membranes

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Togavirus is an example of a virus that enters the cell by the endocytic route

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In some cases, release from the endosome is triggered by the low pH of these vesicles which induces a conformational change in the viral proteins that exposes a fusion region.

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4. Amplification of the viral genome and viral proteins

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• Nucleic acid replication produces new viral genomes for incorporation into progeny virions.
• In general, DNA viruses replicate mainly in the nucleus and RNA viruses mainly in the cytoplasm.
• Messenger RNA (mRNA) is transcribed from viral DNA (or formed directly from some RNA viruses) and codes for viral proteins that are translated by the host cell.
• " Early" proteins are usually non-structural (eg. DNA or RNA polymerases) and later proteins are structural, eg. capsid proteins, ie. building blocks of the virion.

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DNA is a polymer of nucleotides

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One strand is a template for the synthesis of a complementary DNA strand or for the synthesis of mRNA

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Poliovirus

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-Nonstructural proteins such as polymerase enzymes must be made before the genome can be replicated.
-genome is a linear single stranded plus sense RNA

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To commence replication,

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each virus must produce mRNA from its genome, then separate proteins. There are many different strategies for this.

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Translation and protein processing

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-Translation of structural and non-structural proteins is carried out by ribosomes in the host cell cytoplasm.
-Post-translational cleavage of polyproteins usually needs virus-coded proteases.
-Glycosylation of envelope glycoproteins occurs in RER and Golgi vesicles

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5. & 6. Assembly and Release

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Non-enveloped animal viruses:
- All have an icosahedral structure. There are many different strategies for assembly of such structures.
- One simple strategy is the spontaneous assembly of the capsid proteins around the nucleic acid genome.
- The virus particle may require proteolytic cleavage to induce the final conformation in the capsid proteins of the mature infectious virion. This is carried out by host or viral proteases.
- Virions accumulate in the cytoplasm or nucleus and are only released when the cell eventually lyses.

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For many enveloped viruses: release may take place by budding from the cell surface.

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Patches of viral envelope glycoproteins accumulate in the plasma membrane.
Capsid proteins and nucleic acid condense directly adjacent to the cell membrane
The membrane surrounding the nucleocapsid then bulges out and becomes "nipped off" to form the new enveloped virion.
eg influenza virus
measles virus

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Some enveloped viruses

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utilize the cellular secretory pathway to exit the cell. Virus particles bud into Golgi-derived vesicles and are released to the outside of the cell when the transport vesicle fuses with the cell membrane.
eg coronavirus

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The infectious process can be halted by:

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-antibody that blocks uptake or release
-killing the infected cell by cytotoxic T cells, NK cells or Ab-mediated mechanisms
-blocking the replication cycle by specific antiviral drugs
-interferon, an anti-viral cytokine

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Virus-induced changes in cells

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- tumour formation
- lytic infection
- chronic infection
- latent infection

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Virus-induced changes in cells - tumour formation

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eg polyoma, oncogenic retroviruses

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Virus-induced changes in cells - lytic infection

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eg enteroviruses, reoviruses

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Virus-induced changes in cells - chronic infection

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-Persistent infections
eg most retroviruses

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Virus-induced changes in cells - latent
infection

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- Persistent infections
eg herpes simplex, varicella-zoster, Epstein-Barr viruses

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Some of these morphological changes occurring as a result of infection can be seen in cell culture by light microscopy and are referred to as cytopathic effects (CPE)

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-Inclusion bodies
-Syncytia
-Cell transformation

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Inclusion bodies

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-Inclusions represent accumulated virions or unassembled viral components
eg - Adenovirus-infected cells: nuclear inclusions
-Reovirus-infected cells: cytoplasmic inclusions

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Syncytia

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-The fusion protein of many enveloped viruses allows the virion to fuse with the host cell membrane for entry of the infectious genomic material into the cell cytoplasm.
-During replication of the virus, expression of the fusion protein at the cell membrane can result in the fusion of neighbouring cells, and the formation of multi-nucleate cells or syncytia.
eg measles virus

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Cell transformation

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Some viruses encode “oncogenes” whose expression in the virus infected-cell is associated with tumour production.
Most oncogenes code for proteins with growth promoting properties and their expression can lead to uncontrolled proliferation of the infected cell and tumour development. Other viruses cause tumours because their replication affects the cellular version of an oncogene.