Vaccine adjuvants: Why and how

ABSTRACT

Novel vaccine strategies include the so-called subunit vaccines, which encompass only the part of the pathogen to which immune recognition results in protection. The high purity of these vaccines make adverse events less likely, but it also makes the vaccines less immunogenic and therefore potentially less effective. Vaccine adjuvants that increase and modulate the immunogenicity of the vaccine are therefore added to solve this problem. Besides aluminum salts, which have been used in vaccines for 90 years, a number of novel vaccine adjuvants have been included in licensed vaccines over the last 30 years. Increasing insight into immunological mechanisms and how to manipulate them has replaced empirical with rational design of adjuvants, leading to vaccine adjuvants with increased and customized immunogenicity profiles without compromising vaccine safety.

Why vaccine adjuvants are necessary

Historically, vaccines have been based on live attenuated or killed bacteria and viruses i.e., smallpox, measles, polio and tuberculosis. These vaccines have in most cases proven very effective, as recognized by the eradication of smallpox and reduction in polio cases to below 100 cases of wild polio virus globally in 2015.¹ Although very effective, these vaccines often come with risks of side effects; such as fever, rashes, swelling and in some cases even vaccine derived infections, the latter being the case for one-third of all polio cases worldwide in 2015.¹ Furthermore, large-scale production and ensuring consistency of the vaccine is very challenging, as demonstrated in a number of cases of global shortage of vaccines. One example is the decline in global availability of BCG vaccine against tuberculosis, where 180 million doses were required in 2015 to meet global demands, and only 107 million doses were made available from manufacturers.²

To evade these issues, novel vaccine strategies have evolved where only the protective parts of the microbe are included, i.e. the so-called subunit vaccines. These can be produced in recombinant forms in yeast or bacteria yielding a product of high quantity, to be collected and purified. The currently registered hepatitis B vaccine is based on the hepatitis B surface antigen (HBsAg) produced in yeast cells. The high purity makes vaccine-induced adverse events less likely, but it has also caused the vaccines to become much less immunogenic and thus less effective. There has therefore been a growing need for adding immune-potentiators, also known as vaccine adjuvants, to the vaccines to increase the immunogenicity.

Adjuvants in licensed vaccines

The discovery of vaccine adjuvants dates back to 1925, where Gaston Ramon showed that co-administration of his newly invented diphtheria toxoid together with other compounds such as tapioca, lecithin, agar, starch oil, saponin or breadcrumbs increased antitoxin responses to diphtheria. This was followed up by Glenny and co-workers, who in 1926 demonstrated that diphtheria toxoid precipitated with aluminum salts resulted in significant increase of the immune response to the toxoid.³ During the 1930's aluminum salts were for the first time used as adjuvants in human vaccines against diphtheria, pertussis and tetanus and are now also found in vaccines against hepatitis A and B, Haemophilus influenzae type b, pneumococcus and human papilloma virus. It is worth noting that the addition of an adjuvant in these vaccines is a crucial element for them to be effective and it is well established that aluminum salts facilitate higher and longer-lasting immunity.

Aluminum salts were the only adjuvant in use in licensed vaccines for approximately 60 years. Then, from the early 1990's a number of vaccines were licensed containing novel adjuvants. These include the Fluad® vaccine containing the squalene emulsion based adjuvant MF59⁴ and the Epaxal® and Inflexal® vaccines constituting immunopotentiating reconstituted influenza virosomes (IRIV),⁵ the HPV vaccine Cervarix® containing AS04, which is aluminum hydroxide incorporating immunostimulatory monophosporyl lipid A (MPL-A)⁶ and the Pandemrix® containing AS03, a squalene emulsion including the immunopotentiating α-tochoperol (form of Vitamin E).⁷ The malaria vaccine Mosquirix® from GSK contains AS01E, a liposomal adjuvant containing MPL-A and immunostimulatory saponin QS-21.⁸

There remains a great demand for novel adjuvants tailored to induce T cell immunity and local immunity in mucosal tissues, acting as port of entry for many pathogens. Strong T cell responses are pivotal for fighting many infections such as tuberculosis, dengue, human immunodeficiency virus (HIV), chlamydia, streptococcus, hepatitis C virus and respiratory syncytial virus (RSV). Only a few adjuvant candidates are in clinical development, which have published documentation for induction of strong T cell responses in human. These include the IC31® adjuvant, which consists of a cationic antibacterial peptide linked to a synthetic immunostimulatory oligodeoxynucleotide,⁹ the CAF01 adjuvant, based on cationic liposomes incorporating immunostimulatory trehalose dibehenate (TDB)¹⁰ and the aforementioned AS01E from GSK.¹¹

Designing novel adjuvants

The adjuvant components developed over the last 30 years constitute a diverse group of compounds, which are typically categorized as either immunostimulators or delivery systems.

Immunostimulators

Immunostimulators are substances that induce activation or increasing activity of the immune system, typically through ligation to pattern-recognition receptors (PRRs) on innate immune cells. PRRs are protein based receptors expressed on the surface, in the endosome or in the cytosol of cells of the innate immune system, such as B cells, dendritic cells, macrophages, natural killer cells and neutrophils. The receptors recognize microbial pathogens via so-called pathogen-associated molecular patterns (PAMPs), and in some cases host cell derived components released during cell damage or death, called damage-associated molecular patterns (DAMPs). The different immunostimulators depend on their receptor ligand to induce proinflammatory signaling and adaptive immune responses accordingly. The choice of immunostimulator in a vaccine therefore greatly modulates the adaptive immune response.

Delivery systems

The role of the delivery system is in general terms to ensure the presentation of the necessary amount of the vaccine antigens and immunostimulator to the appropriate cells of the immune system to induce immunity. The resulting immune response is highly dependent on the type of antigen presenting cells that initially encounters the vaccine. It is therefore important to select the delivery system that will facilitate the required cellular targeting once injected into the body.

The delivery systems typically consist of self-assembling macromolecules, such as emulsions (MF59, AS03), liposomes (AS01E, CAF01), polymer micro- and nanoparticles, immunostimulating complexes (ISCOMS), virus like particles or virosomes (IRIV), hydrogels (e.g., Alhydrogel® and Adjuphos®) and cationic peptides (KLKL₅KLK in IC31).

The perfect mix

Most novel adjuvants in vaccines already licensed or in clinical development are composed of both a delivery system and an immunostimulator. There are several clear benefit to this: 1) stabilization of the vaccine formulation, as the immunostimulators are often of a physical nature, that makes them highly unstable in aqueous solution; 2) co-localization of the vaccine antigen and the adjuvant, ensuring activation of the same cells, which have encountered the antigen; 3) delivery of the immunostimulator to cells that display the ligand PRR; 4) delivery of the immunostimulator to the compartment of the cell where its ligand PRR is localized.

The future for vaccine adjuvants

Vaccines are accepted as one of the most effective and safe instruments for ensuring human health and billions of vaccine doses are administered each year, not least to small children. Initially vaccines consisted of whole attenuated or inactivated pathogens, which as previously mentioned are usually highly effective, but also come with a higher risk of side effects. This is one of the primary reasons why newer vaccines are often based on rationally designed and highly purified recombinant protein/peptide antigens, with a much better safety profile, but with reduced immunogenicity. Introduction of adjuvants to increase the immunogenicity is therefore a crucial part of novel vaccine development. This does however, stimulate a debate on the relative safety of vaccine adjuvants. The possibility of vaccine adjuvants to induce unwanted immune responses should not be neglected and is subject to intense research to ensure safety. Fortunately, our increasing insight into immunological mechanisms, and how to manipulate them, has replaced empirical with rational design of adjuvants, leading to more effective yet safe vaccines by increasing and modulating the immunogenicity toward the relevant parts of the pathogen. Hopefully this will ensure strong immunogenicity and safety profiles of these important new vaccines of the future.

Disclosure of potential conflicts of interest

Dennis Christensen is employed by Statens Serum Institut, a nonprofit government research facility, which develops and holds patents on vaccine adjuvants.

Acknowledgments

I thank Frank Follmann for critical reading of the manuscript, and the organizers of the 26^th National Immunisation Conference for Health Care Workers, in Manchester, 2015 for inviting me to contribute to the program.