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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

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)

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

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

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

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

Define antigen

Toxin or other foreign substance which induces an immune response in the body, particularly the production of antibodies

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

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

Define antigenic drift and antigenic shift

Antigenic drift - gradual accumulation of mutations

Antigenic shift - a sudden change in antigenic type

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)

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

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

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

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

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.

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

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

How much do vaccines cost?

Research ($8 million), early development phase ($18 million) and late development phase ($40 million)

Vaccine development is very slow

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

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

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)

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.

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


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

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

HIV and Zidovudine

Inhibits RNA dependent DNA polymerase and reduces replication of virus

A synthetic pyrimidine analogue substutes fpr 3'Oh of deoxyribose ring.

HIV protease

enzyme that cleaves viral proteins


Two subunits related by twofold axis of symmetry

Single peptide binding site of interface

Crystal structure determined

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.

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.