Section I – Vaccination
There is also a fundamental distinction between live and dead vaccines.
Living and nonliving vaccines differ in many important respects, notably safety and effectiveness. Live ones consist of organisms (nearly always viruses) that have been attenuated by growth in unfavourable conditions, forcing them to mutate their genes; mutants that have lost virulence but retain antigenicity are repeatedly selected. Nowadays, mutation is usually ‘site-directed’ by recombi- nant DNA technology. Such organisms, which are essentially new strains, can sometimes regain virulence by back-mutation, and can also cause severe disease in immunocompromised individuals. On the other hand they often induce stronger and better localized immunity, do not often require adjuvants or
‘booster’ injections and provide the possibility of ‘herd’ immunity in that mutated nonvirulent virus could be transferred to nonimmunized individuals in a local community. Moreover, the immunity induced is usually more appro- priate for protection against the pathogenic strain of the organism, e.g. Th1 vs Th2 responses.
Killed organisms or molecules derived for these organisms are used when for some reason stable attenuated organisms cannot be produced. These antigens may however induce weak and/or inappropriate (e.g. antibody vs CTL) responses. Immune memory may be variable or poor, but they are usually safe if properly inactivated. In only one case (polio) is there a choice between effec- tive live and killed vaccines. Recently, it has been shown that the genes for one or more antigens can be inserted into a living vaccine (usually virus) ‘vector’, and experiments are being performed with totally synthetic peptides, the idio- type network, and even DNA itself.
Adjuvants Nonliving vaccines, especially those consisting of small molecules, are not very strong antigens, but can be made stronger by injecting them along with some
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Table 1. Antigen preparations used in vaccines
Type of antigen Examples
Viruses Bacteria
Normal heterologous organism Vaccinia (cowpox)
Living attenuated organism Measles BCG
Mumps Typhoid (new)
Rubella Polio: Sabin Yellow fever Varicella-zoster
Whole killed organism Rabies Pertussis
Polio: Salk Typhoid
Influenza Cholera
Subcellular fragment
Inactivated toxin (toxoid) Diphtheria
Tetanus Cholera (new)
Capsular polysaccharide Meningococcus
Pneumococcus Haemophilus Typhoid (new)
Surface antigen Hepatitis B
other substance such as aluminum hydroxide, aluminum phosphate, calcium phosphate or hen egg albumin; such substances are called adjuvants. The prop- erties of adjuvants should include the following: (i) the ability to enable antigens to be slowly released so as to prolong antigen exposure time to the immune system; (ii) preserve antigen integrity; (iii) target antigen presenting cells; (iv) induce cytotoxic lymphocytes; (v) produce high affinity immune responses; and (vi) have the capacity for selective immune intervention. A variety of microbial, synthetic, and endogenous preparations have adjuvant activity, but at present only aluminum and calcium salts are approved for general use in man.
Combinations of macromolecules (oils and bacterial macromolecules) are commonly used as adjuvants in experimental animals to promote an immune response. The oil in the adjuvants increases retention of the antigen, causes aggregation of the antigen (promoting immunogenicity), and inflammation at the site of inoculation. Inflammation increases the response of macrophages and causes local cytokine production, which can modulate the costimulatory molecules, needed for T cell activation. Microparticles have also been used as adjuvants in the experimental model; these include latex beads and poly (lactide-co-glycolide) microparticles. Adjuvants are now being designed and tested to determine how to selectively drive Th1 or Th2 responses. Some experi- mental adjuvants currently under investigation are shown in Table 2.
Table 2. Experimental adjuvants currently undergoing assessment Experimental, but likely to be approved
Liposomes (small synthetic lipid vesicles)
Muramyl dipeptide, an active component of mycobacterial cell walls
Immune-stimulating complexes (ISCOMS) (e.g. from cholesterol or phospholipids) Bacterial toxins (E. coli, pertussis, cholera)
Experimental only Cytokines: IL-1, IL-2, IFNγ
Slow-release devices; Freunds adjuvant Immune complexes
DNA vaccines A few years ago an exciting discovery was made when it was shown that
‘naked’ cDNA that encoded the hemagglutinin of the flu virus could be inocu- lated into muscle tissue to stimulate both antibody production and a CTL response that was specific for the flu protein. The potential for this is still unknown, but if this can become a routine method of immunization, then the cost of generating and transporting vaccines should be very low. Other uses of recombinant DNA technology are the cloning of defined epitopes into viral or bacterial hosts. Typically well characterized infectious agents such as vaccinia, polio, or Salmonella are used. DNA sequences are cloned into the genome of these agents and are expressed in target structures that are known to be immunogenic for the host. This way the antigen is presented for optimal recog- nition by the host. Inclusion of cytokines with the vaccine vectors may prove to be an efficient method for ensuring the correct cytokine environment to steer the immune response accordingly. DNA vaccines have potentially a number of advantages over traditional methods of vaccination. These include specificity, the induction of potent Th1 and cytotoxic T lymphocyte responses similar to those observed with attenuated vaccines but without the potential to revert to overt infection.
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Advances in molecular virology and bacteriology have provided the immunolo- gist with many new targets for vaccine development. The last 20 years of study of viral and bacterial pathogenesis have identified the components of the immune system that are protective for many infectious agents. The use of defined epitopes that are protective for vaccines is now possible. The idea is that certain parts of an infectious disease causing organism, such as herpes virus glycoprotein D (glyD), stimulate CTL that are protective. If the host is inoculated with the defined peptide of glyD, they develop CTL responses to the epitope and do not have to worry about resulting disease from vaccination with a modified live vaccine. This approach is also possible for protection to infec- tious agents that is provided by antibody. In this scenario, both a B cell epitope (the site that the antibody binds to on the infectious agent) and a T cell epitope (the peptide that binds to the MHC Class II to stimulate the CD4 helper cells) must be present, so as to select the appropriate B cells, and to stimulate the specific T cell help.
Cytokines The effects of cytokines can influence the function of professional antigen presenting cells (APC) enabling these cells with much greater efficiency. Thus, IFNγ and IL-4 causes increased levels of class II molecules to be expressed thereby enhancing their antigen presentation abilities. The use of such effector cytokines is being considered as a useful adjunct in vaccination, as polarization of the immune system to a Th1 or Th2 response may be preferable in some instances, e.g. a Th1 response is the preferred response in tuberculosis whereas a Th2 response is important in protecting against polio. Since Th1 and Th2 responses are mutually inhibitory manipulation of these responses may open up avenues of selective intervention.
Recombinant vaccines
180 Section I – Vaccination
Section I – Vaccination