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30 Cards in this Set
- Front
- Back
The difficulty of treating virus infections |
You have to treat the virus whilst it is in the cell without targeting the cell |
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The two ways of dealing with virus infections |
Vaccination = antibodies generated which stop the virus binding to the cell Antiviral drugs - target a specific component of the virus and stop replication (critical part of cycle) |
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The life cycle of a virus |
1. virus binds to cell surface and enters cell 2. Undergoes uncoating (removing shell) 3. Nucleic acid injected into cytoplasm 4. This makes host produce virus proteins which replicate and built particles 5. Particles combine to form virus |
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What two parts of the immune response should a vaccine elicit |
Humoral response - neutralising antibody that kills free virus (the antibodies in bloodstream bind to virus particle and stop it entering). Relies on immunogenic proteins Cellular response - kills infected cells but leaves problem of virus cell infection (cytotoxic T cells) Trying to generate protective IgG and IgA |
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What the cytotoxic T cells do |
If a vaccine generates cytotoxic killer T cells then the infected cell will have virus proteins on the surface to show it is infected. It can then be detected by the immune system |
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How do you get immunity to a virus? |
Need the development of an immune response to antigens on the virus surface and antigens on the virus infected cell. In most cases response to internal proteins has little effect on humoral immunity to infection |
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Define antigen |
Toxin or other foreign substance which induces an immune response in the body, particularly the production of antibodies |
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Antigenic variation in the influenza virus |
Genome is segmented so individual cells get infected with two different types of influenza. During replication a virus can occur with segments from two different strains e.g. avian on outside but human on the inside. Some viruses have more than one surface protein |
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How the influenza virus vaccination works |
An orthomyxovirus Haemagglutinin attaches the virus to the cell receptor and neuraminidase is involved in release of virus from the cell Haemagglutinin is major target for neutralising antibodies |
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Define antigenic drift and antigenic shift |
Antigenic drift - gradual accumulation of mutations Antigenic shift - a sudden change in antigenic type |
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The different types of vaccines |
Live attenuated - viruses that infect but grow weakly, induce a full immune response but do not induce disease. (most vaccines) Killed - inactivated virus or parts of virus that generate an antibody response without infecting. Special forms of killed virus: split (biochemically fractioned), recombinant and DNA/RNA (inject with nucleic acid which transcribes and translates to make proteins) |
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Three ways a virus may be neutralised |
Binding of antibody to virus - blocks interaction with receptor Aggregation of virus by polyvalent anitbody - clumps viruses together so they can only infect one cell Complement mediated lysis - antibodies can stimulate the protective mechanism in serum |
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The three major sites of viral replication |
Mucosal surface of respiratory tract and GI tract e.g. Rhino, parainfluenza, Infection at mucosal surface followed by spread by blood or neurones to target organs e.g. measles, mumps, Hepatitis A and B Direct infection of bloodstream via needle or bites and then spread to target organ e.g. Hepatitis B |
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The two different types of vaccine for the Polio virus |
Sabin (live attenuated) polio vaccine - virus more suited to a foreign environment and less suited in original host. Gives small polio infection in gut to provide local immunity Salk (dead) virus - used in Western world Poliomyelitis gives loss of function in a limb when virus gets to nervous tissue so nerve that drives proliferal limb cannot function |
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The immunity given by the different types of virus |
Live vaccine - gives local immunity (like natural infection). Stops replication in GI tract stops viral replication. Vaccine virus has antigenic drift so gives life long immunity Dead vaccine - no replication so not gut immunity so cannot eradicate wild type virus. Generates Igg antibody so stops poliomyelitis but doesn't stop you getting infected in first place |
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Problems with the two types of virus |
Live vaccine - 10 cases vaccine associated disease so in western world we use Salk vaccine Dead vaccine - immunity more difficult to establish and boosters needed. Does not stop gut infection if the virus is around. Needs to be maintained every 10-15 years. |
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The different antibodies produced by the vaccines |
Killed vaccine - serum igG in bloodstream makes igM and igA in serum. No nasal or duodenal igA as no low level infection occurs as virus is killed Live vaccine - all components are same in serum but igA produced because low level infection occurs |
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Recombinant vaccines e.g. the HBV vaccine |
Isolated surface antigen gene is removed from the irus and inserted into yeast cells. The yeast cells then produce HBsAG, extract and bottled and then used on large scale culture Need boosters to allow for full immunity |
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How much do vaccines cost? |
Research ($8 million), early development phase ($18 million) and late development phase ($40 million) Vaccine development is very slow |
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Do vaccines work? |
Work for notable examples are in principle for many other diseases Made safely by a variety of methods Cost benefit is clear early on |
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Why is it difficult to control viruses? |
Antibiotics really only effective against bacteria due to selectivity and specificity. Need to produce something that is specific to the virus but not to the host - difficult to distinguish between virus replicative mechanisms and host replicative processes |
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The targets for antiviral drugs |
Viral enzymes - RNA dependent RNA polmerase or RNA dependent DNA polymerase which are critical for virus replication Need to design a compound that blocks the viral enzymes (but first we need to understand them molecularly and biochemically) |
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The influenza replication cycle |
Tamiflu blocks release of virus from the cell Virus sticks to surface of membrane, gets endocytosed, uncoats, RNAs make viral proteins which form new virus particles. These bud out of cell to go and infect another cell. Haemaglutanin binds to RBCs so virus can enter. Neuraminidase cleaves the sugar from glycoproteins so virus can be released. |
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Function of nueraminidase enzyme |
Facilitate the release of new viruses by digesting receptor sialic acid from the surface of infected cells and digest mucins A tetramaric protein composed of beta strands To stop flu you need to use neuraminidase inhibitors that mimic binding of sialic acid to the neuraminidase e.g tamiflu and relenza |
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Herpesviruses |
Use nucleoside analogues (guanosine) - the virus uses its own enzyme for infection. Acyclovir has best therapeutic index of available antiviral agents used to rreat herpesvirus infections. Need virus to activate the adminstered drug which affects vDNA synthesis |
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Chain termination |
A modified nucleooside is incorporated into DNA, the lack of 3-OH (three prime hydroxyl on guanosine) prevents formation of phosphodiester bond. Drugs developed to inhibit viral poymerases have shape that mimics standard nucleotide. Without 3-OH no new bases can be added. RIbavarin does have 3-OH but polymerase makes many errors |
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HIV and Zidovudine |
Inhibits RNA dependent DNA polymerase and reduces replication of virus A synthetic pyrimidine analogue substutes fpr 3'Oh of deoxyribose ring. |
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HIV protease |
enzyme that cleaves viral proteins Homodimer Two subunits related by twofold axis of symmetry Single peptide binding site of interface Crystal structure determined |
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Protease inhibitors |
Drug design based on crystal structure of protease Peptideomimetic - good substrate for pro, high avidity and uncleavable. The peptide mimics compound to block the enzyme Minimal toxicity A competitive inhibitor stops protein from working as central component of drug similiar to amino acid recognition site. |
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Antiviral drug resistance |
High mutation rate and large progeny numbers - make viral evoution in response to selective pressure very fast Secondary compensatory mutations - increase viral fitness can restore growth potential of an attenuated resistant mutant Viruses evolve quickly - difficult to control Huge possibility for error - e.g. targeting of enzyme so viruses become resistant Fitness of drug resistant virus vs wild type in vivo can influence whether drug resistant viruses can proliferate. |