Live (attenuated) Flashcards
Attenuation
is the process of losing the virulence of a pathogenic organism. Previously, the organism was attenuated via repeated passage in cells or chick embryos
attenuation is achieved via
mutation of a specific genes in non-human cells; the virus is isolated & grown in human cultured cells, then used to infect monkey cells. The virus acquires many mutations that enable proliferation in monkey cells, but it no longer grows well in human cells, so can be used as a vaccine. If the antigen responsible for inducing protective immunity is known, genetic manipulation allows the creation of a safe, potent vaccine. To attenuate, only 10 genomes need to be changed; this prevents virulence but gives an immune response. Live vaccines require cold chain (refrigeration). Attenuated vaccines are only used in those with mature immune system (aged 2-65 and non- immunosuppressed). Attenuated vaccines generally give a better immune response because it triggers both types of immunity, producing Abs & CTLs, and also gives mucosal immunity.
e.g. Oral polio vaccine (OPV) – oral drop offers lifelong immunity. In addition, the vaccine is shed in faeces, so in areas where faecal contamination is prevalent, it may offer mass immunisation.
Killed (inactivated)
Virus/bacteria grown in culture and the organism can be inactivated physically (heat, radiation) or chemically (formalin). It must be tested for inactivation before use. If vaccine only contains killed microorganisms, no cold chain is required. Therefore, killed vaccines are better in less-developed countries, or hard-to-reach regions. They can be used in children, elderly & immunosuppressed. However, a limitation is that only Abs are produced; no CTLs and no mucosal immunity is produced, so multi-doses are needed (priming, boosting at specific intervals).
e.g. Inactivated poliovirus vaccine (IPV) – two injections protects 90% of the population; three injections protects 99% of the population at $25-50 per person.
Recombinant
Subunit vaccines contain only specific antigens/ epitopes that induce potent & specific protective immune responses. Adverse reactions are rare, the contain specific antigens/epitopes induce protection. Once the key antigens/epitopes identified, subunit vaccines can be manufactured:
1. 2. 3.
Chemical fractionation of microbe
a. purification and stabilisation of key antigens b. chemical linkage of antigens (if needed)
If key epitopes are known
a. Manufacture as peptides, that link amino acids in specific sequence b. Three-dimensional epitopes are created using “Scaffolds”
Recombinant DNA technology
a. Genetic construct coding for antigens/polyepitopes b. Expression as soluble proteins/ glycoproteins
e.g. the gene responsible for Hepatitis B’s Surface Antigen is isolated and introduced into yeast, where it is modified and may be expressed as pure HBsAg.
e.g. pertussis – four protective antigens (pertussis toxin (PT), filamentous haemagglutinin (FHA), pertactin (PRN) and fimbrial antigens were isolated and incorporated into Infanrix vaccine, replacing the whole-cell vaccine. It was as effective, few/ no side effects.
Purified protein
Pathogenic organisms may emit a toxin, which induce powerful Th2-type response (ABs). The toxin can be chemically inactivated using formalin to become toxoid, which is active as a vaccine. Immunisation with toxoids creates memory B-cells which produce nABs upon reinfection, where they bind to the complex and bacterial toxins, inactivating them.
e.g. Humanised IgG mAB, Palivizumab is used to treat severe paediatric Respiratory Syncytial Virus (RSV) infection by passive immunisation; it binds to fusion protein on RSV, preventing the virus from binding to surface receptors on target cells.
Virus-like particle
The particles self-assemble and form a structure similar to the virus, which is recognised by APCs. e.g. Gardasil
Polysaccharide
Encapsulated bacteria (see table) have a slimy layer composed of polysaccharides. This polysaccharide capsule is antigenic, producing an immune response independent of T-cells. It is useful in adults, but provides little protective immunity in children, because they cannot produce a T-cell-independent immune response or lay down memory cells. e.g. Pneumovax 23
Glycoconjugate
Since polysaccharide vaccines are not effective in children, modification of antigens is needed: Chemically-coupling of the polysaccharide capsular antigens (PS) to a carrier protein (e.g. toxoid from tetanus/ diphtheria). e.g. Men ACWY
Route of delivery of vaccines
IM injection
Most vaccines are given via IM injection; however, this is impractical in a number of ways. Firstly, it is painful and therefore unpopular, reducing the uptake. Needles/ syringes are expensive, so the cost is driven up, and it is laborious for mass immunisation. Since most pathogens enter via the mucosa, the immune response may not be optimally stimulated.
Oral/ nasal delivery
Delivery is easier for patient and professional, with fewer delivery-related side effects. Polio eradication campaign success due to simplicity of OPV. Paediatric flu vaccine is given intranasally.
Transdermal delivery
Patches, or dry powder injections using high pressure gun are ideal for areas where sterility cannot be assured, or when mass injections are required.
Route of delivery of vaccines
Effective delivery is based on the following notions:
- efficient encapsulation (trapping) of the active
- successful ‘targeting’ of the active to a ‘specific’ region of the body & pathogen
- successful release of that active in situ
