Adjuvants

The term adjuvant covers a multitude of potential substances with diverse chemical properties. An adjuvant is a substance that increases the effectiveness of the vaccine (from the Latin ad juvare – to help). Adjuvants may create depots of vaccine, thus prolonging its availability at a particular site, increase uptake of the vaccine by antigen presenting cells or they may be immunostimulatory in their own right. There are many different adjuvants available for experimental vaccines but the only ones universally approved for use in humans are the aluminium salt-based adjuvants including aluminium hydroxide, aluminium phosphate and potassium aluminium sulfate (alum). The probable adjuvant activity of alum is the creation of a vaccine depot at or near the site of immunisation. Other adjuvants used in experimental vaccines are described below.
1. Immune-stimulating Complexes (ISCOMs) and Liposomes
ISCOMs are a mixture of cholesterol, phospholipid and Quil A that form a matrix structure described rather well as being similar in shape to a practice golfball. The proteins are added to these lipid carriers and orientate with their hydrophilic residues exposed. Liposomes containing
protein antigens are prepared by mixing the proteins with a suspension of phospholipids and under appropriate conditions form a complex with the proteins protruding out from the centre. Liposomes and ISCOMs probably fuse with the plasma membrane of cells, permitting the antigen to enter the cell where they enter the cytosolic pathway. They are then degraded and presented with MHC class I molecules to cytotoxic T cells.
A variation on the use of liposomes is the use of virosomes. These are functionally reconstituted viral membranes which retain the cell-binding and fusion properties of the virus. Virosome-encapsulated OVA elicited a strong CTL response in mice with as little as 0.75 mg of OVA when administered by an intramuscular, intraperitoneal or subcutaneous route.
2. Freund-type Adjuvants
Freund’s complete adjuvant (FCA) is the stalwart of experimental vaccines. The adjuvant is a mixture of mineral oils containing heat-killed M. tuberculosis cells together with an emulsifier. This is a very potent adjuvant, stimulating both antibody and cellular immune responses in its own right. This is mainly due to the immunogenic properties of the mycobacterial component. Due to potential formation of granuloma, a mixture of epithelioid macrophages and T cells and the chronic inflammation induced, it is not used in vaccines intended for application in humans. Although it undoubtedly has a major role in immunisation for the purposes of inducing immune responses in experimental animals and in the production of antibodies as reagents, its use in vaccine development may be questionable as the immune responses induced may not be representative of the likely outcomes using preparations approved for use in humans.
3. CpG Oligonucleotides (CpG ODNs)
CpG ODNs are unmethylated nucleotide sequences typically less than 20 nucleotides in length. There are three types of CpG ODNs distinguished by the backbone structure. Type A have a mixed phosphodiester–phosphorothioate backbone, type B have a phosphorothioate backbone and type C also have a phosphorothioate backbone with a TCG dimer at the 50 end.37 These unmethylated sequences bind to one of the pattern recognition receptors, toll-like receptor 9 (TLR 9) expressed in intracellular endosomal compartments of dendritic cells, macrophages and to a lesser extent on other cell types. TLR9 typically binds such sequences derived form bacteria following phagocytosis of bacteria.
TLR9 induces cellular activation of dendritic cells inducing cytokine production and promoting TH1 responses. The promotion of this subset is primarily due to the release of Il-12. The ability to activate dendritic cells and other antigen-presenting cells has generated much interest in CpG ODNs as potential adjuvants for both DNA vaccines to boost the low immunogenicity often observed with such vaccines and also for more classical vaccine preparations. CpG ODNs have been shown both to accelerate and to magnify immune responses, particularly at lower vaccine doses. Differences in the immune responses between type A and B CpG ODNs have been reported. Type B induced Agspecific CTL numbers in human PBMCs whereas type A did not.
Soluble factors induced by type A but not type B increased the cytotoxicity of established CD8+ clones.
Combining CpG ODNs with biphasic vesicles carrying the outer membrane lipoprotein A (OmlA) from Actinobacillus pleuropneumoniae delivered by nasal immunisation induced both systemic and localised antibody responses in pigs, whereas the protein alone or with the CpG ODN did not induce mucosal responses. The use of these immunostimulatory molecules coupled with new DNA-based vaccines offers the prospect of a new era in vaccine design.