A review in Signal Transduction and Targeted Therapy in July 2023 describes adjuvants as “indispensable components of vaccines” that are widely used but not completely understood. Thanks to a greater understanding of the innate immune response, the roles that adjuvants play are becoming “elucidated”. These roles are summarised in the review, followed by an exploration of the “mechanisms, properties, and progress” of “classical” vaccine adjuvants. The authors identify the “development prospects” and the problems presented for the future.  

Immunostimulants or delivery systems 

The review states that there are two clear categories of adjuvants: immunostimulants and delivery systems. 

  • Immunostimulants – danger signal molecules that lead to the maturation and activation of antigen-presenting cells (APCs) by targeting Toll-like receptors (TLRs) and other pattern recognition receptors (PRRs) to encourage production of antigen signals and co-stimulatory signals to enhance the adaptive immune responses. 
  • Delivery systems – carrier materials that facilitate antigen presentation by prolonging the bioavailability of the loaded antigens, as well as targeting antigens to lymph nodes or APCs.  

Used in vaccine development since about 1926, adjuvants’ mechanisms to enhance immune responses are poorly characterised. Specifically, there is “little systematic generalisation and summary of the action mechanisms of adjuvants”, due to “broad definitions and complex mechanisms”. It is therefore “difficult to match and design appropriate adjuvants for specific vaccines”. The review explores the mechanisms of adjuvants and presents a summary of classical adjuvant platforms. 

Aluminium adjuvants 

Aluminium adjuvants are the first licensed for use in human vaccines; the two that are most used are aluminium hydroxide and aluminium phosphate. They generally enhance the production of lgG1 and lgE antibodies by “promoting Th2 cell responses”, yet the mechanisms of action are complex. However, two aspects are well recognised. 

  1. Aluminium adjuvants act as a delivery system, binding closely to the antigens and prolonging the bioavailability of the antigen and increasing antigen presentation. 
  2. Aluminium adjuvants can also be immunostimulants, inducing the production of DAMPs, thus activating PRRs of innate immune pathways. This results in the production of cytokines like IL-and Th2-type responses.  

Thanks to its “widely recognised safety and reliability” aluminium is becoming the “adjuvant of choice” for the industry. Aluminium-containing adjuvants are already used to the prevention and treatment of several diseases, such as diphtheria, tetanus, or meningitis. It is also a candidate adjuvant for newer vaccines, like SARS-CoV-2 vaccines. However, the authors do recognise “some disadvantages”, such as “difficulties in inducing a strong cellular immune response” or the possibility of adverse reactions. Consequently, aluminium adjuvants can be enhanced through improved formulations or preparation as nanoaluminium adjuvants.  

Emulsion adjuvants 

The review presents MF59 and AS03 as “classic oil-in-water emulsion adjuvants. MF59 comprises squalene, Tween 80, and Span 85, and was licensed as an adjuvant for an influenza vaccine in 1997 as the first non-aluminium adjuvant approved for use in human vaccines. It has a “dual function” of antigen delivery and immune stimulation and can prolong antigen interaction with the immune system and increase antigen presentation. It has been “widely used” in human vaccines and has demonstrated “good safety and efficacy”.  

AS03 is another oil-in-water emulsion adjuvant presented in the review; it consists of alpha-tocopherol, squalene, and Tween 80. The effects are similar to those of MF59, with both antigen delivery and immunostimulatory effects. It has been licensed by both the EU and US FDA for use in vaccines, showing “satisfactory safety, reactogenicity, and immunogenicity”. Most recently it has also demonstrated “good clinical benefit” in the development of the COVID-19 vaccine.  

TLR agonist molecule-based adjuvants 

The classical TLR agonist molecule-based adjuvants explored in the review are AS04 and CpG ODN 1018. The former is prepared by aluminium adsorption of TLR4 agonist molecules. As it contains aluminium, it has both immune stimulation and antigen delivery roles. However, it has a greater immunostimulatory function than that of aluminium adjuvants, because it contains a potent immunostimulatory molecule: monophsphoryl lipid A (MPLA). When a vaccine supplemented with AS04 was compared with a vaccine supplemented with just aluminium, it became clear that the vaccine with AS04 “induced higher levels of antibodies”. It is approved for use in human papillomavirus vaccines and HBV vaccines.  

CpG ODN 1018 is a synthetic single-stranded DNA molecule that has been “extensively studied” as a TLR agonist. It “specifically activates” TLR9, triggering the activation of TRF7, leading to the production of pro-inflammatory cytokines and type I interferons. This “ultimately” leads to a strong Th1-type cellular response and cytotoxic T cell production. Consequently, the cellular immune response that is produced is generally better than aluminium adjuvants. Initially approved for use in the HBV vaccines, it is currently being evaluated in clinical trials for COVID-19 vaccines.  

Particulate adjuvant system 

AS01 is the “classical” particulate adjuvant system, a liposomal adjuvant containing the immunostimulant MPLA and an active ingredient extracted from the bark of Quillaja Saponaria: QS-21. It has the dual function of antigen presenting and immune stimulating. AS01 is a component in licensed malaria and zoster vaccines and has also been recently applied to the development of a novel peptide vaccine against tuberculosis.  

Essential, but not without problems 

The review concludes that adjuvants are “essential” but also present problems. For example, they have a “weak ability to enhance vaccine immunity”, and don’t offer long-term immunity. They can be less effective to older populations, and many are only capable of inducing antibody responses, without CD8+ T cell-mediated cellular immunity, a “critical” element of vaccines against viral infectious diseases and cancer vaccines. Finally, as the action mechanisms are “largely unclear”, there is potential for “inappropriate use” or “uncontrollable side effects”. The authors demand novel adjuvants as a response to these problems.  

Thanks to COVID-19 and other infectious diseases presenting public health challenges, the importance of vaccine development has been highlighted.  

“A topic that cannot be ignored in vaccine development is adjuvants, due to their ability to greatly enhance the adaptive immune responses of vaccines.”  
Looking ahead 

However, to encourage greater development within the field, the authors identify some points for attention:  

  • Selecting the right adjuvant is a key but complex issue, as the immune response to a vaccine and adjuvant is “highly dependent on the specific situation”. Thus, the following factors need to be considered for adjuvant selection: 
    • Routes of administration 
    • Type of immune response required 
    • Type of pathogens  
    • Type of antigens 
    • Stage of the diseases 
    • Biological characteristics of the vaccine recipients 
    • Safety and economy of adjuvants 
  • The authors next consider that the combination of immunostimulants and delivery systems are currently used to achieve higher immune activation, and the following issues must be explored: 
    • Through knowledge of the yellow fever virus vaccines, we know that “simultaneous activation of multiple innate receptors is more effective than activation of a single receptor”. Thus, there is a greater need to clarify the signalling interactions between different PRRs to better screen for a more effective combination. 
    • Given the differences of physicochemical characteristics between antigens and PRR agonists, and between distinct PRR agonists, delivery systems should be “more rationally designed” as “flexible and compatible” to deliver multiple vaccine components simultaneously.
  • The classical adjuvant has shown “good biosafety” so far, and mature research and development technologies combine with well-established manufacturing conditions and equipment to provide a “relatively easy platform for the development of new vaccines”. Despite this, classical adjuvants are “limited” in immune stimulation ability. Thus, there is an “urgent” need to identify new adjuvants. As the cost and time of development limits adjuvant conversion, the authors call for performance optimisation and formulation improvement of classical adjuvants: 
    • Surface modification 
    • Granulation 
    • Combination with other adjuvants 

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