Lecture 10: Recombinant and Synthetic Vaccines III PDF

Lecture 10: Recombinant and synthetic Vaccines III  IMPROVING THE EFFECTIVENESS OF SUBUNIT VACCINES STRATEGIES FOR ADMINISTERING ANTIGEN The production of antibody in response to a vaccine requires the presence of both B cell and T cell epitopes in the vaccine...

Lecture 10: Recombinant and synthetic Vaccines III  IMPROVING THE EFFECTIVENESS OF SUBUNIT VACCINES STRATEGIES FOR ADMINISTERING ANTIGEN The production of antibody in response to a vaccine requires the presence of both B cell and T cell epitopes in the vaccine With the subunit protein vaccines - being a large protein - contains both However, polysaccharides used in vaccination do present a problem - they cannot contain T cell epitopes - attached to carrier proteins to produce conjugate vaccines that can immunize infants  IMPROVING THE EFFECTIVENESS OF SUBUNIT VACCINES The strongest immune responses generally occur when the antigen molecules are present in a concentrated form A pure subunit vaccine almost always produces a weaker response than does the whole pathogen The surface antigens of the hepatitis B and human papilloma viruses were a lucky exception to this rule, because they assembled into particles of a size similar to that of the empty virus particles themselves  IMPROVING THE EFFECTIVENESS OF SUBUNIT VACCINES Various methods have been devised of fixing a large number of antigenic protein molecules on the surface of a particulate carrier All of the subunit vaccines contain, as adjuvants insoluble aluminum salts, which not only maintain a locally high concentration of the immunogen but also may present the antigen as a high- concentration array by adsorbing antigen proteins to their surface One new adjuvant licensed in the United States is an oil-in-water emulsion containing squalene and some detergents, called MF59 Presumably, amphiphilic immunogens become concentrated at the oil/water interface - used in some commercial influenza vaccines IMPROVING THE EFFECTIVENESS OF SUBUNIT VACCINES  IMPROVING THE EFFECTIVENESS OF SUBUNIT VACCINES Another approach to enhance the effect of a subunit antigen takes advantage of the self-assembling feature of the hepatitis B surface antigen fusing the important parts of other subunit antigens to this protein by the recombinant DNA technique Adjuvants that act by interacting with Toll-like receptors are also used In one approach, more than 130 synthetic analogs of the natural adjuvant muramyldipeptide, or MDP, a fragment of the bacterial peptidoglycan, were tested IMPROVING THE EFFECTIVENESS OF SUBUNIT VACCINES L-alanine residue of MDP was replaced by L - threonine, was found to be a potent stimulator of the immune response without the unwanted side effects of MDP  IMPROVING THE EFFECTIVENESS OF SUBUNIT VACCINES Another type of adjuvant that functions by interacting with Toll-like receptors is monophosphoryl lipid A and its derivatives These compounds resemble LPS (or its lipid portion, lipid A) in structure but are not nearly as toxic  USE OF LIVE, ATTENUATED VECTORS The conditions required for a strong immune response - incorporating the subunit antigens into live, attenuated viruses or bacteria Such strategies have both advantages and disadvantages live vaccines - “natural” manner - often confer stronger immunity, sometimes for a longer period, than killed vaccines Furthermore, live vaccines can usually be administered in smaller dosages, because they may, to a limited extent, be able to multiply within the host  Fragments of antigen subunit used as synthetic peptide vaccines Subunit vaccines use only one or a small number of macromolecular components from the pathogenic organism Because only small parts (the epitopes) of these macromolecules are needed for binding to the antibody or to the T cell receptor This approach may be extended even further In many cases, researchers are eliciting immunization responses with nothing more than the small peptide corresponding to the epitope The peptide is first attached to a macromolecular “carrier” protein and then administered to animals.  Fragments of antigen subunit used as synthetic peptide vaccines A peptide vaccine has several advantages Peptides can be made by chemical synthesis Do not require the purification steps necessary for the production of recombinant DNA-based subunit vaccines Such purification is often difficult and expensive Consequently, peptide vaccines tend to be less expensive, purer, and more stable than protein-containing subunit vaccines Although many problems are encountered in using peptides to stimulate B cells, peptides proved to be excellent instruments for generating cellular immunity through the selection of T cell clones  DNA VACCINES In a 1990 experiment, the injection of 100 μg of naked plasmid DNA into mice resulted in the detectable expression of foreign genes cloned downstream from a strong promoter active in animal cells This research, which suggested much potential for gene Injection of plasmid DNA including genes coding for antigens led to the production of immunity in mice Because DNA vaccines are easy to modify in the laboratory - small laboratories and small companies could produce effective new vaccines  DNA VACCINES Steps necessary for the production of recombinant protein vaccines, would not be necessary with DNA vaccines good expression and correct folding of the protein, protein purification, and removal of the potentially toxic contaminants, The antigen gene could be expressed within the APCs Furthermore, the expression of the antigenic protein within the cytoplasm of APCs meant that the immune response would be tilted toward the production of cytotoxic CD8 T cells a result that is difficult to achieve with conventional vaccines  DNA VACCINES Since then, an enormous amount of research has gone into the development of DNA vaccines that are effective in humans Limited success has been achieved, indicated by increases in antibody titer or T cell proliferation, but the general consensus seems to be that DNA vaccines are usually not potent enough for use in humans  DNA VACCINES Current efforts are focused on increasing the potency by improving the method of introduction of DNA - by combining the initial administration of DNA vaccine with subsequent “booster” shots Even when followed by such boosters, however, the antimalarial DNA vaccine was found to have no effect in the recent clinical trial DNA VACCINES  DNA VACCINES Why were DNA vaccines so effective in mice but so disappointing in humans? A major reason appears to be the dosage The initial 1990 gene expression study in mice used 100 μg/mouse and showed that the expression level of the foreign gene goes down tenfold if only 10μgDNA is injected On the basis of human body weight, which is nearly 10,000-fold higher than that of a laboratory mouse, this dosage would translate roughly into 1 g DNA/person It is impossible to inject this much DNA into humans, and besides, manufacturing such a large amount would be quite expensive  DNA VACCINES Thus human trials have so far used between 1 and 3 mg DNA/person Novel approaches seem necessary, if effective DNA vaccines are ever to be available for human use In addition - a very low yet theoretically possible chance that the foreign plasmid DNA -incorporated into the human chromosome Experiments showed - outcome should be extremely rare Nevertheless, any successful vaccine will be administered to millions, even billions, of people, and thus even the unlikely adverse effects must be taken seriously