7. Vaccine and vaccine development

Question 1

What is immunisation?

Answer 1

Immunisation is an artificial process by which an individual is rendered immune

Question 2

Passive and active immunisation

Answer 2

Passive immunisation – no immune response in recipient
Active immunisation (vaccination) – recipient develops a protective adaptive immune response

Question 3

Effects of immunisation

Answer 3

One of the cheapest and most effective methods of improving survival and reducing morbidity
Estimated reduction in mortality worldwide 3 million/ yr

Question 4

Variolation

Answer 4

Variola =smallpox virus
For variolation, fluid harvested from pustules of recovering individuals and injected under skin of recipient
Crude method of obtaining an ‘inactivated’ vaccine
Documented practice in Far East, Middle East and South Asia from 1000AD
Limited use in UK (1700s)

Question 5

Jenner

Answer 5

Used fluid from cowpox lesions to protect against smallpox infection in 1796; recipient was James Phipps, aged 8

Subsequently experimented with several other children, including his own infant son; published findings in 1798

The first documented use of a live-attenuated vaccine and the birth of modern immunisation

Question 6

Passive immunisation

Answer 6

Immunity conferred without an active host response on behalf of recipient
Passive vaccines are preparations of antibodies taken from hyper-immune donors, either human or animal
Examples:
Immunoglobulin replacement in antibody deficiency
VZV prophylaxis eg during exposure during pregnancy
Anti-toxin therapies eg snake anti-serum
Protection is temporary

Question 7

VZV exposure during pregnancy

Answer 7

VZV during pregnancy can cause fetal complications. In case of exposure, women should contact their GP, Midwife or Virology Dept. Urgent VZV serology is available when required

Question 8

Active immunisation (vaccination)

Answer 8

Immunity conferred in recipient following the generation of an adaptive immune response
General principle is to stimulate an adaptive immune response without causing clinically-apparent infection

Question 9

Herd immunity

Answer 9

To be effective, vaccines need to be administered to targeted cohorts in advance of exposure to the pathogen of interest
Vaccination of sufficient numbers impacts the transmission dynamic so that even unimmunised individuals are at low risk – called herd immunity
As vaccines are given to healthy individuals, the risk-to-benefit ratio requires that vaccines meet high safety standards

Question 10

What prevents the primary infection

Answer 10

Most vaccines work by generating a long-lasting, high-affinity IgG antibody response
These antibodies are sufficient to prevent primary infection
A strong CD4 T cell response is a pre-requisite for this
The most effective vaccines are for diseases where natural exposure results in protective immunity
‘Problem’ diseases are generally those where the immune system cannot eliminate infection or generate long-lasting protective immunity during natural infection
Eg common cold, MTB, HIV, malaria

Question 11

What goes into a vaccine?

Answer 11

Antigen
To stimulate an antigen-specific T and B cell response

Adjuvants
Immune potentiators to increase the immunogenicity of the vaccine

‘Excipients’
Various diluents and additives required for vaccine integrity

Question 12

Classifications of active vaccines

Answer 12

Depends on basis of antigen:
Whole organism - live attenuated or inactivated

Subunit - toxoids, capsular polysaccharides, conjugated polysaccharides, recombinant subunit

Question 13

Live attenuated vaccines

Answer 13

Live but attenuated organisms used
Prolonged culture ex vivo in non-physiological conditions
This selects variants that are adapted to live in culture
These variants are viable in vivo but are no longer able to cause disease
Examples
Measles
Mumps
Rubella
Polio (Sabin)
BCG
Cholera
Zoster
VZV (not routinely used for primary prevention in UK at present)
Live influenza (not routinely used in UK at present)

Question 14

Pros and cons of live vaccines

Answer 14

Replication within host, therefore produces highly effective and durable responses
In case of viral vaccine, intracellular infection leads to good CD8 response
Repeated boosting not required
In some diseases, may get secondary protection of unvaccinated individuals, who are infected with the live-attenuated vaccine strain eg polio

Storage problems, short shelf-life
May revert to wild type
Eg vaccine associated poliomyelitis: around 1 in 750 000 recipients
Immunocompromised recipients may develop clinical disease

Question 15

Varicella-zoster vaccine

Answer 15

Primary infection = chickenpox
Cellular and humoral immunity provide lifelong protection, but viruses establishes permanent infection of sensory ganglia
Viral reactivation=zoster
Particularly elderly, fairly debilitating and may cause long-term neuropathic pain

Live-attenuated VZV, works by induction of anti-VZV antibodies
95% effective at preventing chickenpox
Attenuated virus does establish infection of sensory ganglia, but subsequent zoster is probably rare
3-5% mild post-vaccination varicella infection
Not on UK schedule at present, because:
VZV is a fairly benign childhood infection
Safety concerns based on evidence from other countries
‘Disease shift’ to unvaccinated adults, in whom VZV is less well tolerated
Increase in zoster – probably reduced immune boosting in adults

Question 16

Zoster, immunity and aging

Answer 16

The incidence of zoster increases with age, in parallel with declining cell-mediated immune responses to zoster

Question 17

Zoster vaccine

Answer 17

Similar VZV preparation, but much higher dose
Aims to boost memory T cell responses to VZV
In over 60s, 50% reduction in zoster incidence after vaccination compared to controls; reduced severity and complications amongst vaccinated cases

Question 18

Poliomyelitis

Answer 18

Enterovirus establishes infection in oropharynx and GI tract (alimentary phase)
Spreads to peyers patches then disseminated via lymphatics
Haematogenous spread (viremia phase)
1% of patients develop neurological phase: replication in motor neurones in spinal cord, brainstem and motor cortex, leading to denervation and flaccid paralysis

Question 19

Sabin polio

Answer 19

Sabin oral polio vaccine (OPV) = live-attenuated
Viable virus can be recovered from stool after immunisation
Highly effective, and also establishes some protection in non-immunised population
1 in 750 000 vaccine-associated paralytic polio

Question 20

Salk polio

Answer 20

Salk injected polio vaccine (IPV) = inactivated
Effective, but herd immunity inferior
OPV better suited to endemic areas, where benefits of higher efficacy outweigh risks of vaccine-associated paralysis. UK switched to IPV in 2004

Question 21

Tuberculosis

Answer 21

During primary infection, MTB establishes infection within phago-lysosomes of macrophages. Macrophages present TB antigen to MTB-specific CD4 T cells, which secrete IFN-g – this activates macrophages to encase TB in granuloma.

May be visible as a calcified lesion on plain CXR (Ghon focus)

Most TB thought to be re-activation of this primary infection

Question 22

TB vaccination

Answer 22

Only licensed product is BCG (bacille Calmette-Guerin)
Produced by repeat passage of a non-tuberculus mycobacterium: Mycobacterium bovis
Aims to increase Th1 (IFN-g) cell responses to M bovis, thereby conferring protection against MTB
Given by intradermal injection
80% effective in preventing disseminated TB/ TB meningitis in children; little or no effect on pulmonary TB

Question 23

Killed (inactivated)

Answer 23

Entire organism used, but physical or chemical methods used to destroy viability (eg formaldehyde)
Stimulates B cells, and taken up by antigen-presenting cells to stimulate antigen-specific CD4 T cells
Probably elicit minimal CD8 response, as the vaccine cannot undergo intracellular replication
Responses less robust compared to live-attenuated vaccines
Examples
Hepatitis A
Influenza (standard vaccine – live-attenuated also available but not routinely used)

Question 24

Pros and cons of killed vaccines

Answer 24

No potential for reversion
Safe for immunocompromised
Stable in storage

Mainly CD4/ antibody response
Responses less durable then live vaccines
Generally boosters required
Higher uptake generally required to achieve herd immunity

Question 25

What do influenza vaccines target?

Answer 25

Protective antibody responses largely directed against Haemagglutinin and Neuramidase, and probably mostly work by blocking entry to cells, blocking release of new virions from infected cells and promoting ADCC

Question 26

Difficulties of influenza vaccination

Answer 26

Target antigens prone to mutation (antigenic drift) causing seasonal variation – therefore vaccine produced annually based on predictions CDC provide candidate virus strains to manufacturer; injected into fertilised hens eggs and virus then harvested (inactivated for standard vaccine) More major changes (antigenic shift) occur when viral strains recombine – eg with animal strain, causing pandemic influenza

Question 27

Subunit vaccines

Answer 27

Uses only a critical part of the organism Components may be: purified from the organism or generated by recombinant techniques Protection depends on eliciting CD4 and antibody responses

Question 28

Subunit vaccines: toxoids

Answer 28

Many examples relate to toxin-producing bacteria Corynebacterium diphtheriae Clostridium tetani Bordatella pertussis Toxins are chemically detoxified to ‘toxoids’ Retain immunogenicity Work by stimulating antibody response; antibodies then neutralise the toxin

Question 29

Tetanus vaccine

Answer 29

Pre-formed high-affinity IgG antibodies can neutralise the toxin molecules in the circulation; the immune complexes are then removed via the spleen Anti-toxin can also be given in established cases (passive immunisation)

Question 30

Subunit vaccines: polysaccharide capsules

Answer 30

Thick polysaccharide coats of Streptococcus pneumoniae and Neisseria meningitidis make them resistant to phagocytosis Vaccines for these organisms formed of purified polysaccharide coats Vaccines formed of purified polysaccharide coats; aim to induce IgG antibodies that improve opsonisation Suboptimal as polysaccharides are weakly immunogenic: No protein/ peptide, so no T cell response Stimulate a small population of T-independent B cells Latest vaccines utilise vaccine conjugation to boost responses: protein carrier attached to polysaccharide antigen

Question 31

Vaccine conjugation

Answer 31

Naive B cell expressing surface IgM recognises polysaccharide antigen. Antigen is internalised together with the protein conjugate Conjugate is processed in the class II pathway. Naive B cell presents peptides from the conjugate to a helper T cell with the correct receptor. T cell helps the B cell to perform affinity maturation, but antibody is specific for the polysaccharide and not for the protein conjugate

Question 32

Recombinant protein subunit vaccine

Answer 32

Knowledge of key immunogenic proteins required Proteins expressed in lower organisms Purified to produce vaccine Hepatitis B surface antigen HPV vaccine This approach is increasingly employed in vaccine development

Question 33

HPV vaccine

Answer 33

HPV subtypes 16 and 18 infection major causal factor in cervical carcinoma Vaccine development problematic as HPV is difficult to culture Subunit vaccines are ‘empty virus particles’ that prevent primary infection Quadravalent vaccine covers additional HPV strains (genital warts, penile cancer)

Question 34

Pros and cons of subunit vaccines

Answer 34

Extremely safe Work well where primary infection may be prevented by an antibody response Works when the virus cannot easily be cultured eg HPV and Hep B Development requires detailed knowledge of virology, pathogenesis and immunology Specialised and expensive production Weaker immune responses – boosting often needed and response rate varies

Question 35

Adjuvants

Answer 35

Boost immune response to the antigen Widely used, but mechanism understood only relatively recently Eg alum, lipopolysaccharide Work by binding to pattern-recognition receptors on antigen presenting cells This enhances co-stimulation and cytokine secretion, which ensures a robust T/ B cell response Important field for development in order to improve responses to subunit vaccines Novel adjuvants are toll-like receptor ligands eg CPG repeats

Question 36

Novel approaches: DNA vaccines

Answer 36

Plasmid DNA that encodes vaccine antigen of interest applied; taken up by cells, transcribed and translated Elicits host immune response Mainly performed in mice models Poorly immunogenic to date in human trials

Question 37

Novel approaches: viral vector

Answer 37

Benign virus that can be easily grown in culture engineered to carry genes encoding immunogenic antigens Altered virus used as a live-attenuated vaccine Use restricted to animals to date