There is little doubt that the advent of worldwide vaccination programmes has led to dramatic reductions in deaths from fatal viral diseases (Plotkin 1993). Most of the vaccines currently employed to control such infections consist of live, attenuated or killed (inactivated) viruses. Live attenuated viruses confer strong, long-lasting protective humoral and cell-mediated immune (CMI) responses. However, a risk of reversion to virulence remains, especially in immunocompromised recipients (particularly children). Killed vaccines cannot replicate but are weaker immunogens, often requiring multiple booster injections and the co-adminstration of adjuvant to enhance immunogenicity. Furthermore, it is difficult to meet the current requirements from regulatory authorities for exact composition and immune competency with live and killed whole-organism vaccines (Liljeqvist & Stahl 1999).
An alternative approach to this type of vaccination is to use purified subunit immunogens either as vaccines or vaccine components. For example, the predominant immunogenic component of most currently licensed trivalent, inactivated, influenza vaccines (TIVs) is the haemagglu-tinin (HA) polypeptide partially purified from detergent-extracted, inactivated, virions (Kemble & Greenberg 2003), and Phase II trials are underway with purified fusion (F) protein from respiratory syncitial virus (RSV) (Piedra et al. 2003). At the time of writing (February 2007), the results of this trial have not yet been reported.
With the advent of recombinant DNA technology, large quantities of purified viral antigens can be produced for use in immunoprophylaxis. Other approaches to immunization include the use of synthetic peptides representing immunodominant epitopes, the use of anti-idiotypic antibodies and nucleic acid (DNA or RNA) vectors, but these are outside the remit of this review. Having identified suitable protective antigens (see Section 6.3), recombinant immunogens can be produced by isolating the gene for the antigen or antigen fragment, cloning it into a suitable expression vector (usually a plasmid or viral vector), then transducing a suitable host cell. By modification of either the gene insert or plasmid, expressed protein can be secreted into the ambient medium. This greatly simplifies purification and reduces the risk of contamination with potentially oncogenic cellular DNA. The purification technique must be gentle enough to maintain the protein in its native state, thus ensuring that the epitopes involved in the induction of neutralizing antibodies are correctly presented.
The major advantages of recombinant subunits may be summarized as:
• pathogen excluded from vaccine production process;
• no risk of contamination with toxic compounds;
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• low risk of reversion to virulent genotype (ensure that viral vector systems are completely disabled);
• no risk from incomplete inactivation of whole-cell vaccines (assuming recombinant host-cell vaccines are not pathogenic);
• recombinant subunit can be optimized for immunogenicity, either by genetic manipulation prior to transfer into host cell, within the host cell by utilizing the cell's biosynthetic capacity or by the addition of adjuvant molecules to the purified protein by chemical methods;
• genetic fusions may be used to obtain chimaeric antigens. This can have many applications, ranging from simplifying purification to immunopotentiation to increasing half-life;
• a very wide range of delivery systems can be employed to tailor the immune response for the specific pathogen against which the vaccine is targeted.
The disadvantages of recombinant subunits include:
• the possibility of proteolytic degradation;
• problems with stability, aggregation and solubility;
• poor presentation of epitopes due to sub-optimal conformation leading to poor or aberrent immunogenicity (particularly with complex glycoproteins);
• variable therapeutic efficiency (often related to glycosylation);
• addition of adjuvants is often required;
• high production costs;
• lengthy development phase;
• possible risk of contamination by prions, endogenous retroviruses and other adventitious organisms, either through the host cell or medium components (particularly bovine serum).
Despite these caveats, recombinant technologies are currently being employed in both the development of vaccines against viral pathogens for which no vaccines currently exist and to improve upon existing vaccines.
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