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A vaccine is any preparation of dead or attenuated pathogens, or their products, that when introduced into the body, stimulates the production of protective antibodies or T-cells without causing the disease
The terms vaccination and vaccine derive from the work of Edward Jenner who, over 200 years ago, showed that inoculating people with material from skin lesions caused by cowpox (L. vaccinus, of cows; vacca, cow) protected them from the highly contagious and frequently fatal disease smallpox
He tested his theory in 1796 by inoculating 8-year-old James Phipps with liquid from cowpox pustule
Subsequent inoculation of the boy with smallpox produced no disease
Since Jenner's time, the term has been retained for any preparation intended to attain the same
Edward Jenner (1749-1823)
However, this approach dates back, well before Jenner’s time in India and China where they performed what is known as variolation Variola virus=smallpox virus
Variolation, a procedure developed in China and India 1000 AD used a live smallpox vaccine to generate immunity
Employing several different techniques ‘well individuals’ were exposed to variolous material from a human
with a milder form of smallpox—presumably in the expectation that this would cause less severe disease in the recipient—an early form of ‘attenuation
Whole organism Live/attenuated vaccines Killed/inactivated vaccines
Toxoids
Peptide vaccines
Recombinant vaccines
DNA vaccines
Live/attenuated vaccines make up the bulk of successful viral vaccines
Are prepared from attenuated strains that are almost or completely devoid of pathogenicity but are still immunogenic
They multiply in the human host and provide continuous antigenic stimulation over a period of time
Use of a related virus from another animal – e.g. the use of cowpox to prevent smallpox
Administration of pathogenic or partially attenuated virus by an unnatural route
the virulence of the virus is often reduced when administered by an unnatural route
immunization of military recruits against adult respiratory distress syndrome using enterically coated live adenovirus type 4, 7 and (21).
Passage of the virus in an "unnatural host" or host cell – the major vaccines used in man and animals
have all been derived this way After repeated passages, the virus is
administered to the natural host The initial passages are made in healthy
animals or in primary cell cultures Examples: the 17D strain of yellow fever (in
mice and then in chick embryos), Polioviruses (in monkey kidney cells) and measles (in chick embryo fibroblasts)
Development of temperature sensitive mutants this method may be used in conjunction with
the above method
The vaccine is injected sc/im, virions enter various cell types (APCs) using receptor-mediated endocytosis
Proteolytic degradation of viral proteins occurs, the peptides produced are then loaded onto MHC I molecules
The complex is displayed on the cell surface
Circulating cytotoxic T cells recognize the complex, become activated and and release cytokines
The cytokines trigger apoptosis (programmed suicide) of the infected cells
Some Tc become memory cells but the basis of this is incompletely understood
Additionally, immature DCs will phagocytose virus vaccine initiating a series of events that leads to the production of plasma cells, neutralizing IgG antibodies and memory B cells
One important advantage of live/ attenuated vaccines is that they are sufficiently immunogenic therefore primary vaccine failure are uncommon and are usually the result of inadequate storage or administration
Several potential safety problems exist with live/attenuated vaccines:
Underattenuation Mutation leading to reversion to virulence Preparation instability Contaminating viruses in cultured cells Heat lability administration to immunocompromized or
pregnant patients may be dangerous
The term killed generally refers to bacterial vaccines whereas inactivated relates to viral vaccines
An inactivated whole organism vaccine uses pathogens which are killed and are no longer capable of replicating within the host
The pathogens are inactivated by heat or chemical means while assuring that the surface antigens are intact
Inactivated vaccines are generally safe, but are not entirely risk free
Organism Method of inactivation
Rabies β-propiolactone
Influenza β-propiolactone
Polio Formaldehyde
Hepatits A formaldehyde
Organism Method of inactivation
Salmonella typhi Heat plus phenol or acetone
Vibrio cholera Heat
Bordetella pertusis Heat or formaldehyde
E.Coli (experimental) Colicin
Yersinia pestis Formaldehyde
Following injection, the whole organism is phagocytosed by immature dendritic cells
Processed peptides will be presented on the cell surface as separate MHC II:antigenic fragment complexes
Th2, each with a TCR for a separate antigenic fragment will be activated
B cells, each with a BCR for a separate antigenic fragment will bind antigens that drain along lymph channels
The separate antigens will be internalized and presented as an MHC II:antigenic fragment
This will lead to linked recognition with the appropriate Th2
Activated Th2 will release IL2, IL4 IL5 and IL6, inducing B-cell activation, differentiation and proliferation with subsequent isotype switch (IgM to IgG) and memory B cell formation.
First, they are safe because they cannot cause the disease they prevent and there is no possibility of reversion to virulence
Second, because the vaccine antigens are not actively multiplying, they cannot spread to unimmunized individuals
Third, they are usually stable and long lasting as they are less susceptible to changes in temperature, humidity and light which can result when vaccines are used out in the community
Fourth, all the antigens associated with infection are present and will result in antibodies being produced against each of them
Contamination by toxins or chemicals Allergic reactions
Surface endotoxins on inactivated pertussis vaccine occasionally induce DTH responses, and influenza virus has been linked to similar reactions, though this may be due more to the immunogenicity of the egg whites in which the virions are raised
Autoimmunity
Also, inactivated vaccines do not always induce protective immunity. Multiple boosters and an adjuvant are
usually necessary for continual antigen exposure
the dead organism is incapable of sustaining itself in the host, and is quickly cleared by the immune sysytem
Furthermore, inactivated vaccines are generally capable of inducing humoral immunity rather than cellular immunity.
A toxoid is a chemically or physically modified toxin that is no longer harmful but retains immunogenicity
Certain pathogens cause disease by secreting an exotoxin: these include tetanus, diphtheria, botulism and
cholera In addition, some infections, for example
pertussis, appear to be partly toxin mediated Specific physical or chemical modification of
the toxins produces a toxoid, which is a vaccine
The principal toxin is tetanospasmin Binds to specific membrane receptors located
only on presynaptic motor nerve cells Internalization and migration of this toxin to
the CNS blocks the metabolism of glycine which is essential for the normal functioning of gama amino butyric acid (GABA) neurons
GABA neurons are inhibitory for motor neurons
Their non-functioning results in excess activity in motor neurons
This gives rise to muscle spasms, a characteristic feature of tetanus
Manufactured by growing a highly toxigenic strain of Clostridium tetani in a semi-synthetic medium
Bacterial growth and subsequent lysis release the toxin into the supernatant
Formaldehyde treatment converts the toxin to a toxoid by altering particular amino acids and inducing minor molecular conformational changes
Ultrafiltration then removes unnecessary proteins residues
The toxoid is physicochemically similar to the native toxin thus inducing cross-reacting antibodies
But, the changes induced by formaldehyde treatment render it non-toxigenic
Upon administration (sc/im) the toxoid molecules are taken up at the vaccination site by immature dendritic cells
Within this cell, they are processed through the endosomal pathway where they are bound to MHC II molecules
The MHC II:toxoid complex then migrates to the cell surface
mature DCs migrate along lymph channels to the draining lymph node
There , they encounter naïve Th 2 cells Identifying and then binding of the MHC
II:toxoid to the specific Th2 receptor then activates the naive T cell, causing it to proliferate
Some toxoid molecules not taken up by DCs pass along lymph channels to the same draining lymph nodes
There, they come into contact with B cells
Binding to the B cell through the specific immunoglobulin receptor that recognizes the toxoid
The toxoid is internalized, processing through the endosomal pathway and presented on the cell surface as an MHC II:toxoid complex as happens in the DCs
These two processes occur in the same part of the lymph node
The B cell with the MHC II:toxoid complex on its surface now comes into contact with the activated Th2 whose receptors are specific for this complex
This process is called linked recognition
The Th2 activates the B cell to become a plasma cell with the production initially of IgM, and then there is an isotype switch to IgG
In addition, a subset of B cells becomes memory cells
The rationale of toxoid vaccination is production of antibodies with enhanced capacity to bind the toxins
They thus form complexes with the toxins preventing then to interact with toxin receptors on the nerve cells (tetanus)
This is referred to as neutralization of the toxins by antibodies
There are three principal advantages: First, they are safe because they cannot
cause the disease they prevent and there is no possibility of reversion to virulence.
Second, because the vaccine antigens are not actively multiplying, they cannot spread to unimmunized individuals.
Third, they are usually stable and long lasting as they are less susceptible to changes in temperature, humidity and light which can result when vaccines are used out in the community
First, they usually need an adjuvant and require several doses (otherwise less immunogenic)
Second, local reactions at the vaccine site are more common—this may be due to the adjuvant or a type III (Arthus) reaction The reaction generally starts as redness and
induration at the injection site several hours after the vaccination and resolve usually within 48–72 h
The reaction results from excess antibody at the site complexing with toxoid molecules activating complement by the classical pathway causing
an acute local inflammatory reaction
A recombinant vaccine contains either a protein or a gene encoding a protein of a pathogen origin that is immunogenic and critical to the pathogen function
The vaccine is produced using recombinant DNA technology
The vaccines based on recombinant proteins are also called subunit vaccines e.g. RTS,S malaria vaccine, passed phase II
now entering phase III
The logic of such vaccines, in simple terms, is as follows: Proteins are generally immunogenic, and
many of them are critical for the pathogenic organism
The genes encoding such proteins can be identified and isolated from a pathogen and expressed in E. coli or some other suitable host for a mass production of the proteins
The proteins of interest are then purified and mixed with suitable stabilizers and adjuvants, if required, and used for immunization
The first step is to identify a protein that is both immunogenic and critical for the pathogen
The gene encoding this protein is then identified and isolated
The gene is integrated into a suitable expression vector and introduced into a suitable host where it expresses the protein in large quantities
The protein is then isolated and purified from the culture system
It is used for the preparation of vaccine
1. Genetically engineered microorganisms, e.g., yeast for the expression of hepatitis B surface antigen (HBsAg) used as vaccine against hepatitis B virus
2. Cultured animal cells, e.g., HBsAg expressed in CHO (Chinese hamster ovary) cell line and C-127 cell line
3. Transgenic plants, e.g., HBsAg, HIV-l (human immunodeficiency virus-I) epitope (in experimental stages)
4. Insect larvae; the gene is integrated into a bacculovirus genome, which is used to infect insect larvae. Often a very high quantity of the recombinant protein is produced .
An alternative application or recombinant technology is the production of hybrid virus vaccines e.g. HBsAg vaccine
Here, vaccinia virus is used as a carrier for genes that encode antigenic proteins of interest
The genes may be derived from organisms which are difficult to grow or inherently dangerous, and the constructs themselves are replication deficient, nonintegrating, stable and relatively easy to prepare
DNA sequence coding for the foreign gene is inserted into the plasmid vector along with a vaccinia virus promoter and vaccinia thymidine kinase sequences
The resultant recombination vector is then introduced into cells infected with vaccinia virus to generate a virus that expresses the foreign gene
The recombinant virus vaccine can then multiply in infected cells and produce the antigens of a wide range of viruses
The genes of several viruses can be inserted, so the potential exists for producing polyvalent live vaccines
Proteins encoded by these genes are appropriately expressed in vivo with respect to glycosylation and secretion
They are processed for major histocompatibility complex (MHC) presentation by the infected cells, thus effectively endowing the host with both humoral immunity and CMI
Small peptide sequences corresponding to important epitopes on a microbial antigen can be synthesized readily economically
Some long ones are also being invented, but are more expensive to manufacture eg. Pf MSP-3 long synthetic peptide (now in
phase II)
Synthetic peptides can be highly immunogenic in their free form provided they contain, in addition to the B cell epitope, T- cell epitopes recognized by T-helper cells
The T-cell epitope must be linked to the B-cell epitope
Such T-cell epitopes can be provided by carrier protein molecules, foreign antigens or within the synthetic peptide molecule itself
The antigens are precisely defined and free from unnecessary components which may be associated with side effects
They are stable and relatively cheap to manufacture
Furthermore, less quality assurance is required Feasible even if the pathogen cannot be
cultivated
Changes due to natural variation of the virus can be readily accommodated
DNA vaccines are usually circular plasmids (supercoiled) that include a gene encoding the target antigen (or antigens) under the transcriptional control of a promoter region active in human cells
With DNA vaccines, the subject is not injected with the antigen but with DNA encoding the antigen
DNA vaccines are composed of a bacterial plasmids
Expression plasmids used in DNA-based vaccination normally contain two units: Antigen expression unit composed of
promoter/enhancer sequences followed by antigen-encoding and polyadenylation sequences
The production unit composed of of bacterial sequences necessary for plasmid amplification and selection
The construction of bacterial plasmids with vaccine inserts is accomplished using recombinant DNA technology
Sometimes DNA sequences encoding costimulatory molecules sequences that target the expressed protein
to specific intracellular locations (e.g., endoplasmic reticulum)
Once constructed, the vaccine plasmid is transformed into bacteria, where bacterial growth produces multiple plasmid copies
The plasmid DNA is then purified from the bacteria, by separating the circular plasmid from the much larger bacterial DNA and other bacterial impurities
This purified DNA acts as the vaccine
The DNA vaccine can be injected into a muscle just as conventional vaccines are
Using ordinary syringe or gene gun
DNA vaccines elicit cell-mediated as well as antibody-mediated immune responses
A plasmid vector that expresses the protein of interest (e.g. viral protein) under the control of an appropriate promoter is injected into the skin or muscle of the the host
After uptake of the plasmid, the protein is produced endogenously
The protein is processed intracellularly into small antigenic peptides by the host proteases and presented to the cell surface with MHC I
Subsequent CD8+ cytotoxic T cells (CTL) are stimulated and they evoke cell-mediated immunity
CTLs inhibit viruses through both cytolysis of infected cells and noncytolysis mechanisms such as cytokine production
The foreign protein can also be presented by the MHC class II pathway by APCs which elicit helper T cells (CD4+) responses
Depending on the the type of CD4+ cell that binds to the complex, B cells are stimulated to produce antibodies production
This is the same manner in which traditional vaccines work
The plasmid is taken up by an antigen-presenting cell (APC) like a dendritic cell
The gene(s) encoding the various components are transcribed and translated
The protein products are degraded into peptides
These are exposed at the cell surface nestled in class I histocompatibility molecules
MHC I-peptide complex serves as a powerful stimulant for the development of cell-mediated immunity
If the plasmid is taken up by other cells (e.g. muscle cells)
The proteins synthesized are released and can be engulfed by antigen-presenting cells (including B cells)
In this case, the proteins are degraded in the class II pathway and presented to helper T cells
These secrete lymphokines that aid B cells to produce antibodies
So far, most of the work on DNA vaccines has been done in mice where they have proved able to protect them against tuberculosis, SARS, smallpox, and other intracellular pathogens
Different DNA vaccines against HIV-1 — the cause of AIDS — are in clinical trials
Delivery of the DNA to cells is still not optimal, particularly in larger animals
The possibility that exists with all gene
therapy, that the vaccine's DNA will be integrated into host chromosomes and will either turn on oncogenes or turn off tumor suppressor genes
Antibiotic resistance?
Extended immuno-stimulation by the foreign antigen could in theory provoke chronic inflammation or autoantibody production
Thousands of individual steps (summary) Recognize the disease and Identify the
etiologic agent Attempt to grow the agent in laboratory Establish an animal model for disease Identify an immunologic correlate for
immunity to the disease- Usually a serum antibody
choose antigen (in laboratory) Prepare candidate vaccine following Good
Manufacturing Practice
Evaluate candidate vaccine for ability to protect animal model
Prepare protocol for human studies Phase I human trials- Safety and
immunogenicity using small group (phase Ia in company area)
Phase II trials- Safety and immunogenicity using relatively larger group
Phase III trial- Efficacy and safety Long and complicated process
Usually takes 10–15 years Many vaccine candidates fail for every success
The vaccine:• Attenuated organisms revert to wild type (e.g. polio types 2,3)• ‘Killed’ organisms not properly killed (has happened with polio)• Inclusion of toxic material (e.g. typhoid, pertussis)• Contamination by animal viruses• Contamination by egg proteins (hypersensitivity)• Cross-reaction with ‘self’ (autoimmunity)
The patient:• Immunodeficiency (attenuated organisms may cause serious/fatal disease)• Local inflammatory reactions, often to the adjuvant• Worsening of disease by increasing immunopathology (risk of therapy) • Hypersensitivity to vaccine (e.g. tetanus)• Interference between vaccines given together (not always) • Induction of inappropriate response (e.g. dengue)
•Public aversion to the risks of adverse effects pushes towards sub-unit vaccines rather than live attenuated
•Pathogens change their antigens (or have distinct life cycle stages)a) makes initial composition of the vaccine components complicatedb) make a successful vaccine redundant eg. antigenic drift/shift in influenza, (HIV)
•Success so far has been where naturally occurring immunity is strong eg childhood viruses BUT this is not the case for HIV, TB, malaria etc –so vaccine has to be even better
•Successes to date mostly with antibody inducing vaccines- not cell mediated!
•Confounding effects in tropical populations- eg AIDS, malnutrition, pre-existing exposure to environmental organisms (eg re BCG)- parasitic diseases (Th1 vs Th2 balances)
Does deworming prior to vaccination enhance BCG vaccine?Elias et al., Clin Exp Immunol. 2001 123: 219-25
albendazole
Summary•Early vaccines were mostly whole dead or live attenuated •These provide their own integrated adjuvant/ antigens•Now the shift is towards defined subunit-based vaccines•This requires you to select which adjuvant/ antigen to include•Immunology is now having a greater impact on vaccine design:-optimized activation (e.g. TLR synergy)-temporary removal of immune-regulation? (e.g. anti-IL10)•Safety, efficacy and memory are the key!
Peter Delves, Seamus Martin, Dennis Burton and Ivan Roitt: Roitt’s Essential immunology, 11th edition 2006 page 287-311
David Baxter. Active and passive immunity, vaccine types, excipients and licensing: Occupational Medicine 2007;57:552–556
Medical immunology 2006: edited by Gabriel Virella. ‑‑ 6th ed
Goering RV, Dockrell HM, Zuckerman M, Wakelin D, Roitt IM, Chiodini PL and Mims C; Mims’ Medical Microbiology 4th Edition, Philadelphia Elsevier (2008) page 519-542
http://www.brown.edu/Courses/Bio_160/Projects1999/vaccineoverview/vaccineoverviewbody.html
http://www.microvet.arizona.edu/Courses/MIC419/Tutorials/vaccines.html
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